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Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin
Journal Pre-proof Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin J.I. Isola, J.P. Ormazabal, G. Flores, S. Arismendi, M. Druet, A. Muñoz, J.L. del Río, S.D.A. Etienot, M.P. Gomez Ballesteros, S. Principi, N.D. Bolatti, A.A. Tassone PII:
S0264-8172(20)30029-5
DOI:
https://doi.org/10.1016/j.marpetgeo.2020.104246
Reference:
JMPG 104246
To appear in:
Marine and Petroleum Geology
Received Date: 1 April 2019 Revised Date:
13 January 2020
Accepted Date: 14 January 2020
Please cite this article as: Isola, J.I., Ormazabal, J.P., Flores, G., Arismendi, S., Druet, M., Muñoz, A., Río, J.L.d., Etienot, S.D.A., Gomez Ballesteros, M.P., Principi, S., Bolatti, N.D., Tassone, A.A., Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/j.marpetgeo.2020.104246. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin
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J.I. Isola 1, J.P. Ormazabal1, G. Flores2, S. Arismendi2, M. Druet3, A. Muñoz4, J.L. del Río5,
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S.D.A. Etienot1, M.P. Gomez Ballesteros6, S. Principi 1,7, N.D. Bolatti2, A.A. Tassone1
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Exactas y Naturales. Ciudad Universitaria, Pabellón 2, CABA, C1428EGA, Argentina
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36390, Vigo, Spain.
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Abstract
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Analysis of high-resolution multi-beam bathymetry, 2D multi-channel seismic, and high-
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resolution seismic sub-bottom profiles revealed the presence of widespread fluid escape
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features in the middle slope of the Patagonian Continental Margin. On the sea-bottom,
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these features correspond to pockmarks and mud volcanoes, whereas in the sub-surface
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they are represented in the seismic records by several acoustic anomalies such as
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chimneys, acoustic blanking, enhanced reflectors, and reverse-phase enhanced reflectors
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among others. Some of these acoustic signatures can be traced to syn-rift deposits that fill
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a deep re-activated and inverted graben. Analysis of pockmarks elongation and pockmarks
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alignments show a good correlation with inverted normal faults, suggesting that faults and
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fractures have influenced the pockmark’s shape and might have acted as pathways for
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upward fluid migration. Some of the acoustic anomalies associated with pockmarks are
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interpreted as evidence of gas. The gas observed in the seismic data seems to be
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thermogenic, and the seismic data suggests a deep origin associated with over-pressured
CONICET – Universidad de Buenos Aires. IGeBA, Dpto. de Cs. Geológicas, Fac. de Cs.
YPF S.A. Buenos Aires, Argentina. Instituto Geológico y Minero de España, Tres Cantos, Madrid, Spain. Tragsa-SGM, Julian Camarillo 6b, 28037 Madrid, Spain. Instituto Español de Oceanografía, Centro Oceanográfico de Vigo, Subida a Radio Faro 50,
Instituto Español de Oceanografía, Corazón de María 8, 28002, Madrid, Spain
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syn-rift deposits. Two morphometrically different sets of pockmarks were identified: an
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older set hosted by Miocene aged rocks, and a younger set hosted by Quaternary
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deposits. Different potential triggers are discussed for the genesis of the Miocene
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pockmarks in relation to the seismostratigraphy, structural geology and regional tectonics.
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It is concluded that, tectonic activity associated with the Neogene inversion of the graben
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faults, due to Andean compression, is the most likely cause for the formation these
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pockmarks. The presence of gas charged sediment and young pockmarks also suggest that
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after the Middle Miocene tectonic climax, a more recent pulse of release of fluids
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occurred.
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1 Introduction
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The study of structures associated with fluid release in passive margins has had an
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increasing attention throughout the last decades because of its importance in
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understanding the fluid flow systems in sedimentary basins (Anka et al., 2014; Brown et
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al., 2017; Gay et al., 2007; Hartwig et al., 2012; Pinet et al., 2008). Moreover, it also plays a
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crucial role in ocean processes of high scientific interest such as: greenhouse gases release
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by dissociation of gas-hydrates (Davy et al., 2010; Kennett et al., 2013); geo-hazards
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associated with the presence of gas in shallow sediments (Sun et al., 2017); and enhanced
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development of chemosynthetic communities near cold seeps (Dando et al., 1991; Levin
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and Mendoza, 2007).
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The hydrocarbons expelled to the hydrosphere and atmosphere can be produced by
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either bacterial activity within the first hundred meters of the sedimentary record
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(biogenic origin) or by long-term cracking of complex long-chain hydrocarbons in the deep
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old rocks (thermogenic origin; Floodgate and Judd, 1992). Due to the hydrocarbon release,
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seafloor features such as pockmarks, mud volcanoes and carbonate mounds are created
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(King and Maclean, 1970; Hovland and Judd 1988; Judd and Hovland 1992), whereas the
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flow and entrapment of hydrocarbons within the sediment creates a series of changes in
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the acoustic properties of the host rock which can be identified on seismic data
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(Ligtenberg, 2005; Løseth et al., 2009; Judd and Hovland 2007). The recognition and
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characterization of these superficial and sub-superficial features provide key information
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about the components of the plumbing system which allows the migration of
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hydrocarbons from the deep sub-surface to the seafloor (Talukder 2012).
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Even though the presence of gas in marine sediment is a world-wide known phenomenon
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(e.g. Baristeas et al., 2012; Brothers et al., 2014; Brown et al., 2017; Bünz et al., 2003;
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Dandapath et al., 2010; Dimitrov and Woodside, 2003; Gay et al., 2007; Jané et al., 2010;
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Sun et al., 2011), it has been rarely reported in the western South Atlantic Ocean (Anka et
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al., 2014; de Mahiques et al., 2017; Portilho-Ramos et al., 2018; Schattner et al., 2016;
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Sumida et al., 2004), and most of the studies dealing with the presence of hydrocarbons in
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the subaqueous sediments of the Argentinean Continental Margin (ACM) correspond to
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coastal areas (Aliotta et al. 2011; Bravo et al., 2018; Parker and Paterlini 1990).
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Recent acoustic expeditions on the middle slope of the Patagonian Continental Margin
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(PCM) revealed the presence of pockmarks and fluid-charged shallow sediments in Nágera
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and Perito Moreno terraces (Muñoz et al., 2012 and 2013), but no further investigation
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was addressed in specific contributions. The objective of this paper is to unveil the genesis
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of the pockmarks and characterize the main components of the associated plumbing
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system in order to determine timing, controls, and sources for the leakage of fluids. To
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achieve this, an integrated analysis of multi-beam high resolution bathymetry,
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multichannel 2-D seismic lines and ultra-high-resolution sub-bottom profiles was carried
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out.
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2 Physical Settings
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The study area is located on the southernmost part of the western South Atlantic Ocean in
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the continental slope of the Argentine Continental Margin (ACM; Fig. 1), in an area named
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Patagonian Continental Margin (PCM; Fig. 2). The depths of the study area vary from 600
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to 1500 mbsl (meters below sea level), and are located between latitudes 46° 30’ – 47° 40’
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S and longitudes 60°15’- 59°10’ W (Fig. 3).
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2.1 Tectonic evolution of the foreland and offoff-shore regions of Central Patagonia
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The PCM comprises the Argentine offshore area located south of the Colorado River outlet
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and its off-shore continuation along the named Colorado Fracture Zone (CFZ; Fig. 2). To
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the south of the study area the main depocenter is the North Malvinas Basin (NMB),
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located on the Malvinas Plateau (Becker et al., 2012; Richards et al., 1996). The area of
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study includes part of a northern subsidiary branch of NMB (Fig. 1; Becker et al., 2012).
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Several wells drilled by oil companies in the 90’s on the main depocenter of the NMB have
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shown that the basin filling started in the Upper Jurassic (Richardson and Underhill, 2002).
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To the west and southwest lie the San Jorge, Valdez-Rawson and San Julian Basins (Fig. 1).
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These depocenters are genetically linked to the break-up of the Gondwana super-
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continent and were developed during Jurassic and Cretaceous periods (Nürnberg and
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Müller 1991; Ramos et al. 1996).
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All the off-shore basins of the PCM are comprised in what Gianni et al., (2015), tectonically
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defines as the distal Patagonian broken foreland basin. This broken foreland basin exhibits
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the easternmost deformation associated with three main Patagonian Andean tectonic
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pulses. The first of these pulses corresponds to the earliest episode of Andean build-up
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occurred during early Cretaceous (Folguera and Iannizzotto, 2004; Ghiglione et al., 2013;
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Savignano et al., 2016; Suárez et al., 2009). This event was dated in sin-depositional units
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in the northern and austral Patagonian Andes, obtaining ages between 118 and 121 My
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(Ghiglione et al., 2013, Suárez et al., 2009). In the off-shore basins, this compressional
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phase is also recognized in seismic data as a transpressive inversion of the listric normal
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faults associated with the Mesozoic extensional basins. During Late Cretaceous
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(Maastrichtiano – Daniano) a second compressive pulse affected the Patagonian area,
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which was stronger in central Patagonia due to collision of oceanic ridges. This event is
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recorded in San Julian and Rawson basins with a prominent angular unconformity that
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separates Cretaceous and Tertiary deposits (Continanzia et al., 2011; Micucci et al., 2011).
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A tectonically quiet time spam characterized the Patagonian Foreland basin between
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Paleocene and Early Miocene. During Early to Late Miocene, another phase of
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deformation affected the Patagonian Andes. In on-shore Patagonia, the Jurassic and
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Cretaceous basements were exhumated producing folding in Miocene-Oligocene deposits.
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The age of this deformation was dated by Bilmes et al., (2013), and they constrained the
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peak of the contractional episode between 19 and 14.8 My, although evidence close to
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the Andes can be can be traced up to 11.8 My (Bucher et al., 2019; López et al., 2019),.
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During this last compressional pulse, non-deformed areas separated the onshore broken
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foreland from the distal broken foreland basin where the strain was mainly focused by the
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inversion of the offshore Mesozoic basins affecting the Tertiary record (Continanzia et al.,
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2011; Gianni et al., 2015; Micucci et al., 2011).
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2.2 2.2 ACM continental slope Cenozoic seismic stratigra stratigraphy
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The Cenozoic stratigraphy of the ACM lies above a prominent erosive surface (AR3) which
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truncates the uppermost cretaceous record (Gruetzner et al., 2011, 2012, 2016; Hinz et
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al., 1999). This erosive surface is present in the slope and shelf of the ACM and is also
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named Pedro Luro equivalent (Franke et al., 2007; Fryklund et al. 1996). According to wells
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drilled in Rawson and Colorado Basin, the age of this unconformity corresponds to the
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Cretaceous - Paleogene boundary (Bushnell et al., 2000; Cavallotto et al., 2011;
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Continanzia et al., 2011).
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The Paleogene - Neogene record of the ACM is characterized by three regional
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unconformities linked to world scale climatic-oceanographic changes. The Eocene-
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Oligocene boundary is marked by the conspicuous unconformity AR4, which represents
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the restructuring of the global circulation following theDrake gateway opening (Cavallotto
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et al., 2011; Gruetzner et al., 2016; Hinz et al., 1999; Preu et al., 2012, 2013; Violante et
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al., 2010). Above it, lies a contourite depositional system (CDS; Hernández-Molina et al.,
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2009; Hernández-Molina et al., 2010) that covers all the ACM and is segmented by two
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younger unconformities: AR5, linked to the Climatic Optimum of the Mid Miocene
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(Gruetzner et al., 2011), and H2/AR6 (Ewing and Lonardi 1971; Gruetzner et al., 2016)
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associated with enhanced influence of North Atlantic Deep Water in the South Atlantic
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Ocean after the Panamá isthmus closure (Preu et al., 2012). Wells constraints indicate that
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the age of this unconformity corresponds to the Miocene - Pliocene boundary (Bushnell et
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al., 2000).
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2.3 2.3 Oceanographic and morpholog morphological hological settings
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Nowadays the superficial circulation of the PCM is characterized by a northern branch of
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the Antarctic Circumpolar Current named the Malvinas current (Fig. 1). At depths beneath
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the 500 mbsf, the continental slope of PCM is influenced by the circulation of 4 northward
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flow bottom water masses (Tsuchiya et al., 1994): Antarctic Intermediate Water (AIW
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~500-1000 m), Upper Circumpolar Deep Water (UCDW ~1000-2000 m), Deep Circumpolar
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Deep Water (DCDW 2000 – 3500 m) and Antarctic Bottom Water (AABW >3500 m). The
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vigorous activity of these water masses has contributed since the Eocene-Oligocene
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boundary to the genesis of big sedimentary accumulations in all the ACM, forming a
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complex CDS (Hernández-Molina et al., 2010). In the PCM, the CDS mainly consists of 4
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contouritic terraces (Fig. 2), from west to east: Nágera Terrace (NT), Perito Moreno
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Terrace (PMT), Piedra Buena Terrace and Valentín Feilberg Terrace (Hernández-Molina et
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al., 2010). The evolution of each of these contouritic terraces is linked to the activity of a
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north-sourced Antarctic sourced bottom current (Hernández-Molina et al., 2010). NT is
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swept by the circulation of AIW, while PMT is under the influence of UCDW. The AIW is a
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cold-water mass with relatively lowsalinity, born in the polar front (Talley 1996; Piola and
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Matano 2001). The UCDW is also a north flowing current originated in the Antarctic region
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and it is characterized by an oxygen minimum and higher relative salinity in comparison
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with AIW (Tsuchiya et al., 1994).
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3 Materials Materials and Methods
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Multibeam bathymetry, ultra-high resolution parasound seismic profiles and 2D
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multichannel seismic data (Fig. 3) were implemented in this study to analyze and
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characterize the distribution of pockmarks, fluid escape pathways and accumulations in
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the shallow and deep sub-surface of NT and PMT.
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3.1 High Resolution Bathymetry
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In the years 2007 and 2008 high resolution multibeam bathymetry was collected over NT
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and PMT using a Simrad EM-302 echosounder on board of the R/V Miguel Oliver (details
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of acquisition parameters are given in Muñoz et al., 2013). In this study we analyze over
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10.000 km2 of that multibeam data in water depths that range from 600 to 1350 mbsl.
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Also, 69 km2 of the bathymetry used in this work was acquired on board of the
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Argentinian R/V Austral with the Kongsber EM-122 (location in Fig. 2). These data were
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processed with CARIS.
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3.2 Parasound P70 subsub-bottom profiles
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Ultrahigh resolution seismic data were acquired in 2017 in the frame of the Argentine
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national initiative Pampa Azul during the cruises “Batimetría Área Marina Protegida
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Namucurá” and “YTEC-GTGM 2”. All the seismic profiles were acquired with sub-bottom
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profiler Parasound System DS3 (P70) on board of the Argentinean R/V Austral.
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The Parasound System DS3 (P70) is a hull-mounted parametric echo sounder developed
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by the company Teledyne Reson Gmbh. Its operation is based on the acoustic parametric
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effect. Such effect is generated when two high energy signals of slightly different
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frequency are pinged simultaneously causing the creation of harmonics at two
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frequencies: a low frequency corresponding to the difference between the two primary
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frequencies; and a high frequency corresponding to the addition of the two primary
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frequencies. In the Parasound system, the parametric effect is achieved by one fixed
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primary frequency of 18 Khz, that distributes the energy within a beam of 4.5° through a
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transducer of approximately 1m length, and a variable primary frequency that can be set
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up to operate between 18.5 and 24 Khz.
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The Parasound system records three signals separately: the fixed primary high frequency
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signal (18 Khz; PHF), the secondary low frequency signal (difference between the primary
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fixed and primary variable frequencies; 0.5 to 6 Khz; SLF) and the secondary high
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frequency (addition between the primary fixed and primary variable frequencies; 36.5 to
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42 Khz; SHF).
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During the acquisition cruise, the primary variable high frequency signal was set up at 22
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Khz, giving as a result a SLF of 4 Khz. SLF and PHF raw data were stored in PS3 format. The
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Parasound profiles showed in this paper correspond to the SLF record. The acquisition and
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conversion of the profiles to SEG-Y format was carried out with the software Parastore
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property of TELEDYNE. These profiles were later imported and interpreted in ©Kingdom
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software.
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2.3 Multichannel 2D seismic
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Over 500 kilometers of 2D multichannel seismic records (MCS) were analyzed in this work.
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All the MCS were provided by the Argentine Secretary of Energy in SEG-Y format. The
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visualization and interpretation of the seismic data were carried out using
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software and OpendTect.
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The YMN-95 seismic lines consists on Kirchoff time migrated 2D seismic lines (PSTM).
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During the acquisition of the YMN-95 seismic lines two different streamers were
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alternated depending on the depth of water and geological settings: a shorter one with a
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length of 4600 m and 368 channels, and a longer one with a length of 6000 m with 480
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channels. The distance between channels in both streamers was 12.5m, the distance
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between shots 25 m, and the distance between Common Depth Points (CDP) was 6.25 m.
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The resulting nominal folds were 92 for the short streamer and 120 for the long streamer.
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The sampling interval was 2msec and the seismic record depth was 8193 msec. The
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seismic lines were processed in YPF SA offices using the software Echos. The processing
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included spherical divergence correction and further deconvolution and amplitude
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balance.
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The Line as1800 was also provided by Argentine Secretary of Energy in SEG-Y format. For
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this seismic profile a Pseudo-Relief Attribute (TecVA, Bulhões and de Amorim, 2005) was
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calculated. This attribute allowed a better identification of structures.
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The MCS used in this paper follow the American standard for polarity – impedance
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(Asveth et al., 2000). This standard implies that an increase in the acoustic impedance, or
©Kingdom
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a “hard event”, is represented as positive peak in the wiggle, or as a blue-yellow-blue
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reflection loop in the kingdom’s pallet “RSA col_amp_flipped.CLB”.
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2.4 Mapping of pockmarks geometry and alignments
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2.4.1 Pockmark geometry
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To visualize and interpret the bathymetric data, QGIS Software was used. Also with QGIS,
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54 pockmarks were manually mapped, and 6 morphometric attributes were measured for
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each pockmark: area, perimeter, long axis, short axis, long axis azimuth and shape index
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(Table 2). The ellipticity was calculated to evaluate whether pockmarks were elliptical or
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circular. Pockmarks with ratios higher than 1.1 were considered elliptical. Finally, rose
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diagrams were built for: pockmarks long axis, and canyons’ thalweg, pockmarks’
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alignments, ridges crest and faults.
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2.4.1 Pockmark alignment
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The method used to define pockmarks alignments was based on the procedure proposed
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by Paulsen and Wilson (2010) for volcanic vents and edifices. It is well-known that
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elongated pockmarks and pockmarks alignments can be associated with deep seated
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faults (Jané et al., 2010; Pilcher and Argent, 2007; Roy et al., 2015). Normally when this
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type of structural control is present on pockmarks, the elongation direction and the strike
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of alignments are sub-parallel or parallel to the azimuth of faults. Following that premise,
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the first step used to constraint possible alignment directions was to determine trends in
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the orientations of pockmark’s elongation, and look for alignments on those directions.
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The pockmarks included in each alignment were defined by visual inspection. After select
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a trend pockmarks a lineal orthogonal regression was done considering the center of the
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ellipse that best fits the pockmark in UTM-20 X and Y coordinates. In pockmarks 23 and
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31, when pockmarks coalescence is evident, two centers were defined using the two
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depocenters identified. Besides the initial considerations, the orientation of alignments
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was defined following some extra criteria, as it is the minimum of 3 features with the
244
same orientation to define an alignment.
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The equation of the best fit line calculated trough orthogonal regression and the residuals
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value for each point were calculated using the R Studio Statistical Software. Using these
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parameters 4 degrees of confidence were defined (Table 3) based on the criteria
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stablished by Paulsen and Wilson, (2010) and later modified by Bonini and Mazzarini
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(2010) for mud volcanoes and applied by Maestro et al., (2019) in the study of submarine
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mud volcanoes.
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4 Results and Interpretation
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4.1 Seismostratigrapy
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The seismostratigraphic interpretation of the MCS and the SBP data allowed the
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identification of 6 mayor unconformities in the study area. These unconformities were
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interpreted as sequence boundaries, and 7 seismic units were defined.
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Given that no well was ever drilled on the PCM, a straightforward age calibration for the
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seismic units was not possible, and the age model used in this work was elaborated by
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comparing the main characteristics of the interpreted seismic units with regional
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seismostratigraphic charts published in previous contributions for the ACM, PCM (Ewing
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and Lonardi 1971; Gruetzner et al., 2011, 2012, 2016; Hernández-Molina et al., 2009; Hinz
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et al., 1999; Isola et al., 2017; Preu et al., 2012; Violante et al., 2010) and North Malvinas
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Basin (Becker et al. 2012, Richards and Fannin, 1997; Richardson and Underhill, 2002).
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The complete seismic record was divided in 2 mega-sequences, syn-rift and post-rift
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deposits. The Basement and pre-rift couldn’t be separated due to the poor quality of MCS
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in deep strata, and both are included in the seismic basement here called Unit 0.
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4.1 4.1.1 SynSyn-rift deposits and Graben structure
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The geometry of a major graben located in the south-east of the study area is clearly
268
displayed in YMN-04 and YMN-06 seismic records (Figs. 4 and 5). The graben has 50 km
269
length and 35 km width, and is controlled by a series of synthetic and antithetic NW
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striking normal faults, that in some cases, show signs of re-activation (Fig. 5). In the middle
271
of the graben an intra-graben high separates two depocenters. The northeastern
272
depocenter host the thickest sedimentary record, and has up to 2 sec TWT thick, while the
273
thickness of the southwestern depocenter is less than 1 sec TWT (Fig. 4).
274
The infill of the graben is characterized by a wedge shaped syn-rift deposit (Unit SR). This
275
deposits are truncated on its top by a flat erosive surface represented by the reflector AR1
276
(Fig. 5), of Hauterivian minimum age, interpreted as the break-up unconformity generated
277
after the opening of the South Atlantic Ocean (Franke et al., 2007; Hinz et al., 1999).
278
Fault reactivation and inversion is more noticeable on the northwestern part of Fault 1
279
(Line YMN 95-06; Fig 5), where the compression was strong enough to generate a
280
considerable vertical displacement in reflectors of Units 1 and 2. In the southeastern part
281
of Fault 1 signs of reactivation are not appreciable on the MCS (Fig. 4).
282
Becker et al., (2012) interpreted a series of grabens near the study area and confirmed
283
Richards et al., (1996) hypothesis of a northward continuation of the NMB. We followed
284
their seismostratigraphic concept, and correlated the seismic Unit SR to the late syn-rift
285
unit deposited in north Malvinas Graben during Tithonian to Berriasian (Becker et al.
286
2012; Richards and Fannin, 1997; Richardson and Underhill, 2002).
287
4.1.2 Cenozoic postpost-rift deposits
288
The post-rift seismic-stratigraphy of the ACM has been studied and refined over the last
289
45 years (Ewing and Lonardi 1971; Gruetzner et al., 2011, 2012, 2016; Hernández-Molina
290
et al., 2009; Hinz et al., 1999; Isola et al., 2017; Preu et al., 2012; Violante et al., 2010). As
291
a result of those studies, a general seismostratigraphic scheme for the entire ACM was
292
developed. In this work, we follow the nomenclature for the unconformities of Violante
293
et al. (2010) and Preu et al. (2012): AR1, AR3, AR4, AR5, H2, and N.
294
In Fig. 6, a key seismic section shows the regional seismic architecture of the Cenozoic
295
strata of NT and PMT. The Cenozoic record is segmented in 5 seismic units (1 to 5;
296
equivalent to seismic units defined by Muñoz et al. 2013) bounded by 5 regional
297
discontinuities (Table 1).
298
Several authors have recognized in the Argentine and Uruguayan Continental Margins two
299
post-rift megasequences, the Cretaceous post-rift and the Cenozoic post-rift (Conti et al.,
300
2017; Gruetzner et al., 2011, 2012, 2016; Hinz et al., 1999; Preu et al. 2012; Violante et al.,
301
2010). In the study area, the Cretaceous post-rift is absent, and above the break-up
302
unconformity there is a hiatus of ~60 Ma represented by the absence of reflectors
303
between AR1 and AR3 (Fig. 6).
304
Unit 1, onlaping on horizon AR3, has a thickness that range from 0.1 s TWT in the shelf to
305
0.75 s TWT in the continental slope, and is characterized by a divergent internal acoustic
306
configuration composed of continuous medium amplitude reflectors. This unit represents
307
a Paleogene episode of thermal subsidence (Gruetzner et al., 2011, 2012, 2016; Isola et
308
al., 2017; Preu et al., 2012; Violante et al., 2010). This unit is affected in some areas by the
309
reactivation and inversion of the graben faults (Fig. 5). Its uppermost part is truncated by
310
reflector AR4 (Eocene-Oligocene). This horizon is a widespread erosive surface recognized
311
in all the ACM, and represents a period of enhanced activity of ocean currents associated
312
with the opening of the Drake Passage and the onset of Antarctic bottom waters current
313
(Gruetzner et al., 2016; Hernández-Molina et al., 2009).
314
Unit 2 is characterized by medium amplitude continuous reflectors with an internal
315
acoustic structure that ranges from sigmoidal clinoforms to parallel stratification towards
316
the ocean basin (Fig. 6 and Fig. 7 A-E). The age of this unit is Oligocene-Mid-Miocene and
317
is interpreted as a deltaic progradation associated with a lowstand sea level (Isola et al.,
318
2017; Preu et al., 2012). The top of this unit is the erosive surface represented by reflector
319
AR5, of middle Miocene age, associated with the Miocene climatic optimum (Gruetzner et
320
al., 2011).
321
Unit 3 is well imaged in parasound seismic profiles (Fig 7), and is characterized by
322
reflectors with medium to high amplitude and parallel internal structure. It is bounded on
323
its top by reflector H2 (Miocene-Pliocene; Preu et al., 2012; Violante et al., 2010).
324
Unit 4 is characterized by prograding clinoforms that onlap above horizon H2 in the
325
continental shelf, and has a thickness of up to 500 ms TWT in the outer shelf. In the
326
continental slope, this unit is well imaged in parasound seismic profiles by its high
327
amplitude reflectors in comparison to the underlying Unit 3. In some areas of the
328
continental slope, another prominent unconformity was also identified above Unit 4, the
329
reflector N. This horizon is interpreted as the base of the transgressive-regressive events
330
associated with the Pliocene – Quaternary glacieustatic boundary (Violante et al., 2010).
331
Above this unit lays the youngest deposits of the terraces, Unit 5, associated with the
332
Quaternary glacioesustatic variations. This unit, as well as Units 3 and 4, is affected by
333
faulting (Fig. 7D).
334
For more detailed description of the seismic units and main unconformities age
335
constraints for the PCM, readers are referred to, e.g. Gruetzner et al., 2011; Hernández-
336
Molina et al., 2009; Isola et al., 2017; Muñoz et al., 2013.
337
4.2 4.2 Seismic evidence of fluid flow
338
The interaction between an acoustic signal and gas charged sediment generates
339
detectable signatures that can be recognized in the seismic record in a variety of acoustics
340
features (Ligtenberg, 2005; Løseth et al., 2009; Judd and Hovland 2007). The analysis of
341
our seismoacoustic database allows the identification of several of these features such as
342
gas chimneys, acoustic turbidity, vertical discontinuities, lateral decrease in amplitude and
343
enhanced reflectors. Acoustic turbidity is generated by the attenuation of the acoustic
344
energy caused by the interaction with gas bubbles, and is recognized as a zone of chaotic
345
reflections (Tóth et al., 2014). Gas chimneys are vertical acoustic blankings or turbid zones
346
associated with previous or on-going focused fluid migration. Enhanced reflectors with
347
reverse phase are coherent reflections with markedly high amplitude that could well be
348
caused by the large impedance contrast between the gas bearing sediment and water
349
bearing sediment. Vertical discontinuities (even tough not completely understood) are
350
believed to be the result of gas migration through low permeable sediment (Judd and
351
Hovland 2007; Ligtenberg, 2005; Løseth et al., 2009).
352
The description of acoustic indicators provided below is focused in three main regions
353
localized in Fig. 3A, Irregular Depressed Area (IDA), Pockmark Area (PA) and Mud-
354
Volcanoes Area (MVA).
355
4.2. 4.2.1 2.1 Parasound profiles
356
The analysis of almost 350 km of very high resolution Parasound seismic profiles allows
357
the identification of several acoustic anomalies that may be associated with the presence
358
of gas in the shallow sub-surface.
359
Beneath the IDA, columnar disturbances are common and were interpreted as gas
360
chimneys, meanwhile patches in the seismic record with the acoustic signal obliterated,
361
were interpreted as acoustic blanking (Fig. 7A and C). Gas chimneys are found in the
362
profiles located in the west part of the IDA, and are often associated with buried
363
pockmarks (Fig. 7C). They have lengths between 500 and 1000 meters, cutting through
364
Units 2, 3 and 4, and are in some cases rooted in Unit 1. Acoustic blankings are more
365
frequent in the southwest side of the IDA and are occasionally accompanied by
366
diapirism/doming (Fig. 7A and C).
367
In the PA, acoustic blanking appears as curtains distorting the seismic reflectors of units 1
368
and 2, and the lower reflectors of unit 3 (Fig. 7E and F). The length of these features varies
369
between 1500 and 2000 m and are often associated with enhanced reflectors and gas
370
chimneys (Fig. 7 E and F).
371
In the MVA, Unit 2 is outcropping or subcroping and is folded due to diapirism (Fig. 7B).
372
Within this unit, seismic chimneys and kilometric sized acoustic blankings are frequent
373
(Fig. 7B). The seismic chimneys can be traced to a reflector located ~100 msec TWT that
374
simulates the shape of the sea bottom (Fig. 7B). Given that the MVA presents favorable
375
pressure and temperature conditions for gas hydrate development, we infer that this
376
bottom simulation reflector (BSR) could be related to the boundary of gas hydrates and
377
free gas.
378
4.2.2 Multichannel Seismic Records
379
The MVA is located above an inverted hemi-graben (Fig 8). The inversion of the graben
380
affects the Neogene record and is possibly the cause of the folding shown in Unit 2. Within
381
Unit 2, some sinuous and vertical features with diffuse and chaotic internal acoustic
382
structure, interpreted as gas chimneys, were recognized (Fig. 9). The gas chimneys can be
383
traced from Unit 1 to the seafloor, where they end up as reflectors with enhanced
384
amplitude (Fig. 9). Inside the diapiric structures, several enhanced reflectors and bright
385
spots with lateral polarity inversion are identified (Fig. 9). The amplitude linked to the
386
reverse polarity reflectors is -37973, while the seafloor reflector in the same wavelet is
387
33722. Bright spots may well be associated with the presence of gas pockets (Ligtenberg,
388
2005), although further analysis is needed to rule out other possible explanations for
389
these reflections (Asveth et al., 2000).
390
In the PA and in the boundary between NT and PMT, anomalous acoustic patterns are
391
common. Anomalies consist in closely spaced vertical discontinuities within Unit 1 in zones
392
close to faults (Figs. 11 and 12). The appearance of this pattern is very similar to the
393
“Vertical Discontinuities Zones” defined by Løseth et al., (2009). According to Ewing and
394
Lonardi (1971) and Muñoz et al., (2013), Unit 1 is an Eocene lutite. This lithology provides
395
a poorly permeable medium, and gas interaction with the acoustic signal would result in
396
vertical discontinuity zones (Løseth et al., 2009). The dime zones and the presence of
397
vertical discontinuities, likely related to hydrofacturing, are interpreted as evidence of
398
fluid migration. The top of the vertical discontinuity zones is often associated with high
399
amplitude reflectors close to the boundary between units 1 and 2 (Figs. 11 and 12). The
400
polarity of the upper high amplitude reflector is opposite of the seafloor polarity and its
401
amplitude is higher than the amplitude of the seafloor reflection (anomaly amplitude: -
402
42280dB, seafloor amplitude: 22280 dB; Fig. 12).
403
In the western flank of the IDA, seismic units 2, 3 and 4 are truncated, evidencing the
404
erosive genesis of this giant depression (Fig. 13). Three chimneys recognized in the
405
western part of the IDA, rooted in Unit 1, reach the seafloor with high amplitude reflectors
406
below small tributary channels (Fig. 13; same feature was imaged with parasound profiles
407
in Fig. 7A). The bottom of the IDA almost reaches reflector AR4 and, on the relicts of Unit
408
2, enhanced reflectors with phase reversal are recognized. In the eastern flank of the IDA
409
Unit 2 thickness reach up to 150 msec TWT and acoustic indicators of gas like chimneys,
410
enhanced reflectors, and reversed phase enhanced reflectors are often identified.
411
Eastwards, the acoustic anomalies disappear.
412
4.3 4.3 Geomorphology
413
The relief of the study area is characterized by two gently dipping contouritic terraces
414
delimited by a scarp that loses sharpness southwards, and two canyons that cross-cut the
415
terraces with west-east direction. In the northern part of the study area it is also imaged a
416
bathymetric low and a ridge (IDA and R in Fig. 3A). Southwards, mud volcanoes and
417
elliptical pockmarks are scattered throughout the seafloor. The morphology and
418
morphometrical characteristics of these features are given below.
419
For a detailed description of the main morphological features of NT and PMT readers are
420
referred to the contributions of, e.g., Lastras et al., (2011), López-Martínez et al., (2011),
421
and Muñoz et al. (2012 and 2013).
422
4.3.1 Distribution, morphometry and structural control of pockmarks and canyons
423
Iglesias et al., (2010) propose the truncation of reflectors as a valid criterion to
424
differentiate proper pockmarks (erosive crater-like depressions originated by ascending
425
fluids) from pockforms (non-erosive crater-like depressions formed by a variety of
426
different processes). As truncated reflectors are commonly encountered in the flanks of
427
the studied depressions (Fig. 7F), and acoustic evidence of sub-surface fluid mobilization
428
in the study area is widespread, we classified the studied depressions as proper
429
pockmarks.
430
The pockmarks are located between 750 and 1200 mbsl. Most of them (~80 %) are
431
concentrated above the main graben and the remainder lie near canyon 1 and the IDA
432
(Fig. 15). All pockmarks show U-shaped cross-sections and the southern and western
433
flanks are usually steeper than the northern and eastern ones (Fig. 16).
434
Three different plan view pockmark shape could be distinguished: elliptical (Fig. 16A), sub-
435
circular (Fig. 16B), and coalescent pockmarks (Fig. 16C). The length of unitary pockmarks
436
longer axis varies between 584 m and 1899 m with an average of 1269 m, while
437
coalescent pockmarks have a longer axis that reach a length of up to 4078 m.
438
The 91% of those pockmarks where an elongation direction could be defined, show a long
439
axis/short axis ratio bigger than 1.1. The analysis of those directions in rose diagrams
440
indicates that the elongation is not random and two preferential strikes, ~45° and ~135°,
441
were identified.
442
Following the criteria adopted by Bonini et al., (2016) and Maestro et al., (2019) 6
443
alignments with reliability degree ≥ C were defined in 25 pockmarks. These alignments
444
can be divided in two sets of three alignments each oriented in ~NW and ~NE (table 4 and
445
Fig. 15).
446
The rose diagram built for the canyons indicates that canyons are in some areas deflected
447
from the bathymetric gradient in the pockmarks elongation directions (Fig. 15C).
448
4.3 4.3.2 Mud Volcanoes, Volcanoes, young pockmarks and ridges idges
449
Mud volcanoes were recognized by means of high resolution bathymetry (Fig. 17A) and
450
are associated with diapirism and doming of the seismic Unit 2 (Fig. 7B and Fig. 8). These
451
features are restricted to the south west part of the study area between depths ranging
452
from 500 to 650 mbsl. They have approximately 35 m high and their slopes have a
453
gradient that varies between 1.5° and 2°.
454
Besides the elongated kilometric-sized pockmarks described above, another set of smaller
455
crater-like depressions also interpreted as pockmarks, are identified in the bathymetry.
456
These pockmarks occur within unit 4 at depths ranging from 500 to 700m and have long
457
axis length of up to 500m. Even though these are not as many as the old mega-pockmarks,
458
a potential alignment with the highest degree of confidence stablish in this work could be
459
defined (Fig. 17A). Also, 2 directions of elongation were defined in 9 of these pockmarks;
460
one of the directions is similar to the calculated alignment (Fig. 17A).
461
In the northern part of the study area, a ~20 km long ridge surrounded by scours (Figs. 3A
462
and 17B) was identified. This ridge is colonized by cold water corals (Muñoz et al. 2012,
463
Ríos et al. 2011). The height difference between the base of the ridge and the highest
464
point of it is ~450m. It is worth to note that the strike of this feature matches the normal
465
inverted faults azimuth (Fig. 15C). Another ridge is shown in Fig. 10; the relation of this
466
positive relief with basement structures is clearly imaged by the seismic line YMN95-05.
467
5 Discussion
468
5.1 5.1 Age of Compressional Structures
469
The distal Patagonian broken foreland basin, where the study area is located, has been
470
affected by 3 major phases of Andean compression. The first two pulses took place during
471
the Cretaceous period (Gianni et al., 2015; Savignano et al., 2016), and the youngest
472
corresponds to early-middle Miocene (Bilmes et al., 2013; Bucher et al., 2019). The latter
473
is represented on land by widespread folding and reverse and strike-slip faults affecting
474
Oligocene and Miocene rocks (Bilmes et al., 2013; Bulcher et al., 2019; Cobbold et al.,
475
2007; Diraison et al., 1998; Ramos, 1989). Whereas in the distal broken foreland basin the
476
deformation was focalized in the edges of the Mesozoic extensional basins (Gianni et al.,
477
2015). Evidence of such compressional event was reported close to the study area in
478
Rawson and San Julian basins (Continanzia et al., 2011; Micucci et al., 2011).
479
Given that the inverted faults studied in this work crosscut Units 1 and 2 (Paleocene to
480
Middle Miocene), and faulting, folding and diapirism within Unit 2 is widespread (Figs. 5, 7
481
A-E, 8, 9 and 10), we ascribe the genesis of these compressional features to the last peak
482
of Andean compression occurred in Central Patagonia from the Early to Middle Miocene
483
(Gianni et al., 2015).
484
Within the Plio-Quaternary units, compressional structures are restricted to just a few
485
areas and represented by small vertical faults and slightly sin-depositational curved
486
reflectors close to the mud volcanoes (Figs. 7 B-D and 9). Such scarce amount of
487
compressional evidence indicates that tectonic activity in the area decreased after
488
deposition of Unit 2. The top of the diapirs were in some cases eroded and covered by a
489
veneer of Quaternary deposits (Fig. 7B). These young deposits gently onlap on the flanks
490
of the diapirs suggesting that during the deposition the uplifiting of the diapirs was
491
relatively slow.
492
It is worth noting that the deformed areas are located above deep inverted normal faults,
493
whereas the areas between grabens show scarce or no signs of deformation. This
494
indicates that the compressional stress was focused in the grabens’ faults.
495
5.2 5.2 Pockmarks Pockmarks Morphometry, Morphometry, Distribution and Origin
496
5.2 5.2.1 Pockmarks Pockmarks morphology, morphology, age and distribution
497
According to Judd and Hovland (2007), the original shape of pockmarks is a circular crater-
498
like depression. Subsequently, this shape can be modified by the influence of exogenic
499
agents like ocean currents (Andresen et al., 2008; Bøe et al., 1998), gravity triggered mass
500
movements, and turbidity currents (Brothers et al., 2014; Çifi̧ et al., 2003). The effect of
501
these processes can be also conditioned by shallow faults and fracture networks (Jané et
502
al., 2010; Hovland and Judd 1988). Directional statistical analysis of the long axis direction
503
provides clues about the origin of the elongation of pockmarks (Michel et al., 2017).
504
Whereas pockmark alignments are associated with factors controlling their formation, as
505
deep seated faults, gas hydrate dissociation minimum depth, and buried canyons among
506
others (Brothers et al., 2014; Jané et al., 2010; Pilcher and Argent, 2007).
507
In this study, three exogenic processes were taken into consideration to analyze their
508
potential influence on pockmarks’ shape: erosion by bottom currents, slumping, and
509
turbidity currents along gradient direction. The bottom currents that interact with the
510
seafloor in the study area are two, AIW in NT and UCDW in PMT; both of them flow
511
northwards (Hernández-Molina et al., 2009; Piola and Matano 2001). The general
512
bathymetric gradient dips mostly eastwards (Fig. 15C), and is relatively smooth on the
513
terraces, characterized by values ranging from 0.5° to 1.5°.
514
The morphometrical analysis of the elliptical pockmarks revealed 2 main preferential
515
directions of elongation: NE and NW (Fig. 15C). Those strikes match neither ocean
516
currents nor gradient directions, although the NW direction is similar to the azimuth of
517
the faults identified in the MCS, suggesting a relation between faults and pockmarks’
518
elongation. Also, three alignments with ~NW strike and reliability degree ≥ C were defined
519
on the NW elongated pockmarks (Table 4).
520
The NE direction of pockmarks’ elongation admits no simple explanation; it matches
521
neither: ocean currents directions nor gradient direction, or the strike of the inverted
522
normal faults. Even though its origin is unclear, the fact that most of NE elongated
523
pockmarks are located above an inverted graben suggests that it may also have a
524
structural origin, and may, somehow, be related to fractures or sub-seismic faults
525
associated with the inversion of the graben. Furthermore, the alignment analysis allows us
526
to identify 3 alignments with a reliability grade ≥ B in ~NE direction. There is, however, no
527
sub-superficial geophysical data available that corroborate this hypothesis.
528
The fact that two independent variables, as elongation and alignment, show consistent
529
results is a strong indicator that the elongation could be controlled by the same factor
530
controlling alignment. As it was explained earlier in the text, elongation of pockmarks can
531
be produced after they are created, but alignments are controlled by a factor which acts
532
at the moment of pockmark formation, as deep-seated faults.
533
In the cases studied by Jané et al. (2010) and Roy et al. (2014), even when structural
534
control on pockmarks genesis is evident, faults do not reach seafloor. To explain how blind
535
faults can control pockmarks distribution and morphology they proposed that the fluids
536
were expelled through the fault planes from a deep source rock and then, in the more
537
permeable shallow strata, vertical gradual diffusive flow would be the responsible
538
mechanism for the upward migration of gas. This mechanism would generate a vertical
539
path for the migration of gas above the faults and thus, the development of pockmarks
540
will concentrate above the faults. In this sense, we believe that the main graben’s faults
541
work as conduits for the fluids migration from the syn-rift deposits to the Cenozoic strata.
542
Then, inside the more permeable shallow strata, fluids would migrate through diffusive
543
fluid or small faults and fractures (Figs. 7F, 11 and 12). Once pockmarks are formed two
544
mechanisms are envisioned for their elongation: 1) pockmarks that form aligned above
545
faults traces are then enlarged by the erosive agents until they coalesce with each other in
546
the directions of faults, creating an elliptical pockmark elongated along the strike of the
547
deep fault (see pockmark 23 and 13 in Fig. 15); 2)tectonic activity associated with the
548
inversion of the faults created small fractures in the younger units (see small
549
faults/fractures in Figs 7F and 12), these smaller faults could have similar direction than
550
the main blind faults and provide a weaker rheological behavior in specific directions for
551
the erosive agents to enlarge the pockmark.
552
Even though acoustic evidence of fluid flow occurs in all the study area, the incidence of
553
pockmarks is constrained just to depths up to 1200 m and most of them appear on the
554
outcrops of Unit 2 (Fig. 15B, 7C and 7F). As the pockmarks have approximately the same
555
age as the sediments on which they formed, we infer that the studied pockmarks were
556
formed coeval to the last stages of deposition of Unit 2. By then, during the Middle
557
Miocene, the rheological behavior of the superficial deposits in deeper areas, where
558
Eocene rocks outcropped or subcropped (Unit 1 in Fig. 6), did not allow the creation of
559
pockmarks; areas with soft sediments are a necessary condition for pockmarks formation.
560
The low depositional rates and the strong bottom circulation that have characterized this
561
area since the Middle-Miocene could have preserved the pockmarks regardless there was
562
or not an active plumbing system beneath (Hammer et al., 2009; Pau et al., 2014).
563
In summary, mega-pockmarks were formed during the last stages of the deposition of
564
Unit 2 (Early to Middle Miocene). As regards their distribution, we have identified three
565
controls: the presence of source of fluid, presence of migration pathways beneath
566
(inverted normal faults), and presence soft sediments on the seafloor.
567
5.2 5.2.2 Origin of fluids
568
Since the pioneer studies of King and Maclean (1970), the genesis of pockmarks has been
569
consensually attributed to the erosive effect of ascending fluids. The fluid expelled from
570
the host rock to the seafloor can be either water or gas. The latter generates acoustic
571
anomalies detectable in seismic data (Ligtenberg, 2005; Løseth et al., 2009; Judd and
572
Hovland 2007).
573
Compressional wave attenuation caused by the presence of gas in sediment is a common
574
phenomenon observed around the globe. In high frequency seismic methods (>1Khz), like
575
the Parasound D70, even a concentration of volume of 0.0002% can cause a significant
576
attenuation of the acoustic wave (Tóth et al., 2014). The most common seismic features
577
associated with this effect are acoustic blanking and acoustic turbidy (Tóth et al., 2014).
578
These indicators were found in our data base and in the high frequency seismic data
579
presented in previous contributions in the area (Lastras et al., 2011; López-Martínez et al.,
580
2011; Muñoz et al., 2013). We interpret that these features are caused by the presence of
581
gas in the shallow strata (<150m).
582
The deep acoustic anomalies described in section 4.2.2 admit no simple explanation. In
583
MCS, the presence of anomalous acoustic patterns, as isolated reverse polarity bright
584
spots and HARP, may well indicate the presence of gas. However, the same anomalies can
585
be also caused by: over-pressured sands/shales, coal deposits, and salt domes (Asveth et
586
al., 2013). The presence of salt domes is discarded given that there is no evidence of
587
Mesozoic evaporites along the ACM (Ramos 1996). Similarly, the presence of coal in this
588
area is highly unlikely considering that well data show that the Cenozoic sedimentary
589
record of the PCM is dominated by marine facies (Continanzia et al., 2011; Micucci et al.,
590
2011), and significant coal deposits cannot be formed in such environments.
591
Overpressured sands/shales can, however, be an alternative explanation for the HARP
592
identified within the folds and diapirs of Unit 2 (Fig. 10). Given that a more definitive
593
analysis, as AVO, cannot be performed, and well information of shallow shale/sand
594
acoustic behavior lacks, an unequivocal interpretation of the deeper acoustic anomalies
595
cannot be reached in this work.
596
Sub-surface hydrocarbons can have two different origins: thermogenic and biogenic (Judd
597
and Hovland 2007). Thermogenic fluids are generated in deep old rocks with high organic
598
content and are the consequence of long-term cracking of complex long-chain
599
hydrocarbons. Meanwhile, biogenic fluids (mostly methane) are produced by rapid
600
bacterial degradation of organic material inside shallow sediments in geological
601
environments with high depositional rates. The seismostratigraphic interpretation carried
602
out in this work shows that, in the study area, the Plio-Quaternary depositional rates are
603
rather slow, and the thickness of the sedimentary record deposited since Pliocene (units 4
604
and 5) in NT is between 0 and 40 msec (Figs. 6 and 7). In PMT the Plio-Quaternary units
605
are absent or represented by isolated drifts or just a veneer of coarse sediment (Muñoz et
606
al., 2012 and 2013; Ewing and Lonardi 1971). These sedimentary rates make the
607
hypothesis of a biogenic source for the gas related anomalies recognized in the SBP
608
records unlikely. On the other hand, multiple oil producers rocks were discovered in the
609
North Malvinas Basin (Farrimond et al., 2015; MacAulay, 2015; Richardson and Underhill,
610
2002). One of these is the Barremian syn-rift lacustrine succession of the North Malvinas
611
Graben located ~100 km southwards the study area. Total organic carbon between 2 and
612
12 % has been recorded in the late syn-rift and early post-rift successions of the Sea Lion
613
Field located in the North Malvinas Graben (Farrimond et al., 2015). Pockmarks related
614
with acoustic anomalies, similar to the ones recognized in this work, were imagen by 3D
615
seismic data (Brown et al., 2017).
616
The hypothesis of a thermogenic source for the fluids involved in the generation of the
617
pockmarks seems plausible, considering that: syn-rift deposits of a northern branch of the
618
NMB are located beneath the study area; 80% of the pockmarks are located above the
619
main graben; pockmarks' shape and distribution suggest a relation with basement faults
620
that can work as conduits for gas migration; and some of the acoustic indicators can be
621
traced to the top of the syn-rift deposits (Figs. 4, 5, 8, 11 and 12).
622
A final consideration regarding the fluid involved in the formation of pockmarks is the
623
presence of groundwater venting. Even though water venting wouldn’t explain the
624
presence of gas related acoustic anomalies identified in the SBP, Hübscher and Borowski
625
(2006) have proved that even when hydrocarbons are present in the system, artisan
626
groundwater flux can be the main factor that controls pockmarks formation. The presence
627
of a gas-free fluid involved in the creation of pockmarks is often associated with
628
dewatering features as polygonal faults (Andresen and Huuse, 2011), or a artisan water
629
reservoir (Chenrai and Huuse, 2017; Hübscher and Borowski, 2006). However, such
630
features are absent in our data base and have not been reported for the study area. Thus,
631
we believe that the role of water in the genesis of these pockmarks –if any- was not
632
important.
633
5.3 5.3 Patagonian plumbing system: system: Trigger mechanism and role of tectonics
634
The identification of a deep sourced fluid involved in the genesis of mega-pockmarks
635
suggests that over-pressured fluids ruled the plumbing system. Several external processes
636
can trigger the release of gas in passive margin by excess of pore pressure. The most
637
common factors include: overburden erosion (Naudts et al., 2006), sea-level changes
638
(Talukder, 2012), overloading by quick burial (Sun et al., 2012), and compressional
639
tectonics (Judd and Hovland 2007; Kopf, 2002; Plaza-Faverola et al., 2015).
640
In the study area, the lack of erosive features capable to generate a decrease in
641
lithoestatic pressure big enough to overcome the release of overpressured fluids (Brown
642
et al., 2017; Naudts et al., 2006), pose overburden erosion as an unlikely trigger
643
mechanism. In the same way, with the low depositional rates that characterize the study
644
area, it is not likely that the excess of pore pressure is linked to quick burial. Conversely,
645
several evidences indicate that tectonic deformation and seismic activity associated with
646
the inversion of graben normal faults was the more likely cause for the buildup of
647
overpressure and the subsequent release of fluids: the relation between inverted normal
648
faults and pockmarks’ distribution and alignments (Fig. 15); the fact that most of the
649
pockmarks are located above the inverted main graben; the presence of faulting,
650
diapirism and folding within the unit hosting the pockmarks; and last but not least the
651
periodicity indicated by buried pockmarks inside different stratigraphic levels of this unit.
652
Even though the PCM is located on a typical passive margin since ~65 Ma ago, previous
653
contributions and the evidence discussed in section 5.1 demonstrated that the PCM was
654
affected by Andean tectonic compressive pulses. Its effect is strong enough to generate
655
faults that cross-cut the Cenozoic post-rift strata (Figs. 5 and 10), and condition the
656
development of kilometric sized seafloor features, such as submarine canyons and ridges
657
(Fig. 10; Rosello et al. 2005). Therefore, it seems plausible that an increase in Andean
658
tectonic activity could generate changes in the stress conditions in the pore charged with
659
gas near faults. This would contribute to the increase in pore pressure, triggering the
660
release of fluids from the deep sequences to the Cenozoic strata. In the same way, the
661
inversion of faults will fracture the overlying rocks, creating paths for deep fluids
662
migration.
663
5.4 5.4 Conceptual model, model, timing and tectonic controls on fluid leakage
664
The seismostratigraphic interpretation and the topics discussed above enable the
665
elaboration of a conceptual evolutionary model that explains the release of fluids in the
666
study area since the Eocene-Oligocene boundary to recent times (Fig. 18).
667
The evidence of fluids migration through Unit 1 (Fig. 8, 11 and 12) and the lack of
668
pockmarks inside or in the outcrops of this unit, suggest that when gas migrated for the
669
first time Unit 1 was already consolidated, and the absence of soft sediments did not allow
670
the genesis of pockmarks. This observation led us to infer that during Paleocene and
671
Eocene the plumbing system was not active. Andean activity in Patagonia during this
672
period was characterized by relatively small trench-normal velocities values (<3 cm/yr;
673
Maloney et al., 2013). These values resulted on no-deformation of the broken foreland
674
basin (Gianni et al., 2015). We infer that the lack of tectonic activity during this period in
675
the Patagonian Andes precluded the release of fluids.
676
Maloney et al., (2013) have shown that from Early to Middle Miocene the trench normal
677
convergence velocity was significantly faster, resulting in intensification of Andean
678
tectonic activity, and re-activation and inversion of the distal broken foreland basin
679
(Gianni et al. 2015). This tectonic pulse is appreciable in our MCS, where basement faults
680
cut through units 1 and 2 (Figs. 5, 8 and 10), and in the SBP data where fluidification,
681
folding, diapirism and faulting are identified within Unit 2 (Fig. 7). As previously discussed,
682
this tectonic scenario provides the plumbing system of both, overpressure conditions in
683
pore charged with fluids and pathways for upward migration of fluids. We speculate that
684
during this period considerable amounts of deep fluids were expelled from the syn-rift
685
deposits to the hydrosphere. This assumption is supported by the mega-pockmarks
686
identified within and in the outcrops of Unit 2 (Figs. 4, 7 and 14 B; Muñoz et al. 2013 their
687
Fig. 11B). Even though the release of fluids during this period was widespread in all the
688
study area, pockmarks were only created where Unit 2 was being deposited and soft
689
sediment was present. In deeper areas, hard Eocene rocky outcrops precluded the
690
formation of pockmarks (Fig. 18).
691
The obliquity between the normal re-activated faults and the direction of the stress (~ W-
692
E; Cobbold et al., 2007) resulted in compression-transpression. The faults inverted during
693
this period cut through units 1 and 2 vertically, and created positive ridges, which are an
694
indicator of a high seismic activity.
695
From Mid-Miocene to the present the seafloor of the study area was under the influence
696
of a very dynamic deep ocean circulation (Hernández-Molina et al., 2009). The strong
697
interaction between the seafloor and bottom water masses resulted in elongation, and
698
coalescence of the pockmarks (Fig. 18).
699
From Mid-Miocene to the Holocene, the trench normal convergence velocity diminished
700
to values around 4 cm/yr (Maloney et al. 2013). During this period the tectonic activity in
701
the area was considerably reduced. However, the presence of young circular pockmarks
702
and chimneys affecting Plio-Quaternary units (Fig. 7) indicates that even after the Middle
703
Miocene tectonic climax, the plumbing system was still active. What triggered the Plio-
704
Quaternary release of fluids is beyond the scope of this work, although the occurrence of
705
modern seismicity close to the study area (inset map in Fig. 1), would provide a potential
706
trigger for modern activity. Plio-Quaternary pockmarks and mud volcanoes, also, appear
707
to be related with an inverted hemi-graben (Fig. 8) and aligned and elongated in similar
708
directions than older pockmarks (Fig. 17A). However, the recorded earthquakes close to
709
this area are all of Magnitude <5 in Richter scale and external earthquakes should have a
710
higher magnitude to perturb a distal plumbing system (Bonini et al., 2016; Manga et al.,
711
2009). Also, the fact that young circular pockmarks occur at the theoretical minimum
712
depth of hydrate formation pose gas hydrate dissociation linked to sea level changes as a
713
plausible trigger for the Plio-Quaternary activity of the plumbing system.
714 715
6 Conclusions •
In this study, we present the first integrated analysis of high-resolution multibeam
716
bathymetry, MCS lines and Parasound seismic data which illustrate the relation
717
between gas releasing features and the deep structure and stratigraphy of the
718
PCM.
719
•
The Cenozoic pos-trift record is interrupted by 5 unconformities associated with
720
global scale climatic oceanographic changes, and lies above Mesozoic half-grabens
721
with evidence of Neogene fault reactivation and inversion.
722
•
The morphometrical analysis of mega-pockmarks shows two preferential
723
elongation and alignment directions which in some cases coincide with the
724
azimuth of mapped faults, suggesting faults as the main pathway for upward
725
migration of gas.
726
•
The acoustic evidence of gas in the SBP data indicates that at least part of the fluid
727
involved in the genesis of pockmarks is gas. The gas seems to be thermogenic, and
728
the correlation with deep faults suggests a deep origin associated with over-
729
pressured syn-rift deposits.
730
•
Seismicity linked to Neogene reactivation and inversion of Mesozoic graben faults
731
is the more likely trigger for the release of gas from the syn-rift deposits to the
732
Cenozoic post-rift. This is supported by: the fact that the mega-pockmarks and the
733
compressional structures are hosted in the same seismic unit, the close relation
734
between pockmarks’ distribution and morphometry with faults; the alignment of
735
the geomorphological features with faults; and the periodicity of fluid seepage
736
during Miocene, indicated by the presence of buried pockmarks
737
•
The inversion of faults is consequence of Andean compression. Following our
738
hypothesis, the peaks of Andean activity could modulate the release of gas. This
739
assumption should have two main consequences during enhanced activity of
740
Andean compression: 1) the increased amount of greenhouse gases are released
741
to the hydrosphere, and 2) the increasing failure of seal rocks provoke off-shore
742
basins to expel hydrocarbons.
743
•
We believe that gas hydrate must have some influence on the recent modulation
744
of gas release. Further 3D seismic should be acquired to better constrain the effect
745
of this mechanism.
746
Acknowledgments
747
This work has been made within the framework of a collaboration between YPF
748
technology S.A. (Y-TEC) and the Consejo Nacional de Investigaciones Científicas y Técnicas
749
(CONICET). The authors thank to YPF S.A. for their constant support. The authors are also
750
especially grateful to the Servicio de Hidrografía Naval (SHN; Argentine Hydrographic
751
Survey) and the captain, officials, and crew of the R/V Austral. The Spanish Oceanographic
752
Institute (IEO) is also acknowledged for providing the bathymetric data used in this
753
contribution, and the Secretary of Energy of Argentina is thanked for providing the
754
multichannel seismic data interpreted in this work. Thanks also to María Cerredo for her
755
fruitful comments on a first version of the manuscript. Matías Zylbersztejn is thanked for
756
his assistance on the elaboration of deming regressions. Finally, associated editor
757
Domenico Chiarella, David Iacopini and an anonymous reviewer are thanked for their
758
work and suggestions that really improves the quality of this original manuscript.
759
This contribution is framed within the Pampa Azul project, which is funded by the Ministry
760
of Science Technology and Productive Innovation of Argentina (MINCyT).
761
The seismic package Kindgom Suite® (v. 8.8) was used in this study.
762
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1134
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1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155
1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236
Table 1. Compilation of the Cenozoic seismostratigrpahic schemes published for the MCA and PCM.
1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266
Table 2: Morphological parameters of mega-pockmarks.
1267
1268 1269
Table 3: Reliability assessment system for vent alignments modified from Paulse and Wilson (2010).
1270 1271
Table 4: Calculated alignments for elliptical mega-pockmarks.
1272
1273 1274 1275 1276 1277 1278 1279 1280
Fig. 1. A: Tectonic setting of the western South Atlantic (after Franke et al. 2007; MFZ= Malvinas Fracture Zone and CFZ= Colorado Fracture Zone) and locations of the earthquakes with magnitude >4.4 recorded in western South Atlantic Ocean since 1970 according to IRIS data base (https://www.iris.edu/hq/; blue dots). B: Bathymetric map of the ACM (after Smith and Sandwell, 1997), contour lines every 1000 m (black lines), superficial water circulation on the shelf (thin black arrows), main superficial circulation pattern of the slope and basin of the ACM (thick color arrows), and main sedimentary basins (dash black-outlined grey polygons) of the Patagonian Continental Margin after Ramos and Turic 1996 and Becker et al. 2012; SJGB = San Jorge Gulf Basin; SJB = San Julian Basin; NMB = North Malvinas Basin; VRB = Valdez-Rawson Basin; NENMB = Northern extension of North Malvinas Basin.
1281 1282 1283 1284 1285 1286 1287
1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300
Fig. 2. Main morphosedimentary features of the PCM (after Hernández-Molina et al., 2010 and Muñoz et al., 2013), Fracture Zone in blue dashed lines (after Franke et al., 2007; MFZ: Malvinas Fracture Zone; CFZ: Colorado Fracture Zone) and bottom water masses with white thick arrows (After Piola and Matano 2001; AIW: Antarctic Intermediate Water; UCDW: Upper Circumpolar Deep Water; LCDW: Lower Circumpolar Deep Water; AABC: Antarctic Bottom Current). In the black box the area covered with high resolution bathymetry is indicated.
1301 1302 1303 1304 1305 1306
1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319
Fig. 3. A: Shaded relief map of the study area. Bathymetric contours every 50 m (thin black lines) and every 250 m (thick black lines), pockmarks (white arrows) and main geomorphological features (labels), are shown. Below bathymetric profile across the study area, location is indicated with the thick black line. B: Slope grid of the study area with the location of the seismic profiles and bathymetric details. Location in Fig 2.
1320 1321 1322 1323 1324 1325 1326 1327
Fig. 4. Un-interpreted (A) and interpreted (B) seismic line YMN 95-04 showing the basement (U0), main graben, syn-rift (SR) and post-rift deposits. See location of seismic profile in Fig 3B. Un-interpreted (C) and interpreted (D) seismic line as1800 processed with TecVA (Bulhões and de Amorim, 2005) showing the basement (U0), main graben, syn-rift (SR) and post-rift deposits. See location of seismic profile in Fig 3B
1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351
Fig. 5. Un-interpreted (A) and interpreted (B) 3D cross lines YMN95-06 an YMN95-04 showing the 3D geometry of the northeastern depocenter of the main graben, the graben infill (SR), seismic basement (U0) and cenozoic post-rift units (U1 and U2). See location in Fig. 3B.
1352 1353 1354 1355 1356 1357 1358
Fig. 6. Un-interpreted (A) and interpreted (B) seismic line YMN95-02 showing the seismic basement (U0) and the Cenozoic seismic stratigraphy (U1, U2, U3, and U4) of Nágera and Perito Moreno Terraces following the seismoestratigraphic concept from (Isola et al., 2017). See location of seismic section in Fig. 2.
1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374
Fig. 7. A. Parasound seismic profile across the irregular depressed area indicated in Fig. 3. Several acoustic indicators of gas/fluid mobilizations such as chimneys (CH), acoustic blanking (AB) and diaprisim (D) are indicated B. Parasound seismic profile across Nágera Terrace located at depths ranging between 600 and 750 mbsl. Diapirism (D) inside Unit 2 as well as mud volcanoes (MV) on the seafloor were recognized associated with the CHs and AB. Also, a bottom simulating reflector, possibly associated with the presence of gas-hydrates was recognized. C. Parasound seismic profiles across the irregular depressed area showing several acoustic indicators of gas/fluid mobilizations such as CH, AB and D. Buried pockmarks (BP) inside unit 2 are frequent in the western flank of the irregular depressed area. D. Parasound seismic profile cross-cutting Nágera Terrace. Vertical transpressional faults were identified affecting the Plio-Quaternary record. E. Seismic section located on the boundary between Nágera and Perito Moreno Terraces. A kilometric sized AB is observed in the seismic profile bounded on its top by a high amplitude reflector (ER). F. Seismic reflection profile of Nágera Terrace crossing three mega-pockmarks, AB and CH are frequent, it can also be observed the presence of truncated reflectors (TR) on the mega-pockmark walls indicating the erosive genesis of these features. See locations of parasound seismic profiles in Fig. 3B.
1375 1376 1377 1378 1379
Fig. 8. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-04 showing the seismic basement (U0), synrift deposits (SR) and Cenozoic seismic units (U1, U2, U3 and U4) across the mud volcanoes area. The graben master fault shows evidence of reactivation and inversion and affects the Cenozoic strata. Diapiric structures are developed inside Unit 2. See location of seismic section in Fig. 3B.
1380
1381 1382 1383 1384 1385 1386
Fig. 9. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-04 showing Cenozoic seismic units (U1, U2, and U3) across the mud volcanoes area. It can be observed within unit 2 the presence of diapiric structures and liquefaction. Within the diapiric structures high amplitude reflectors with reversed polarity (HARP), enhanced reflectors (ER) and chimneys were identified. See location of seismic profile in Fig. 3B.
1387 1388 1389 1390
Fig. 10. Un-interpreted (A) and interpreted (B) part of seismic line YMN95-05 showing seismic basement (U0), syn-rift deposits (SR) and Cenozoic seismic units across Perito Moreno Terrace. A ~1 sec TWT thick half-graben is controlled by an inverted normal fault. See location of seismic section in Fig. 3B.
1391 1392 1393 1394 1395
Fig. 11. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-06 showing syn-rift and Cenozoic seismic units across PMT. Above the fault, dim zones (DZ) and vertical discontinuity zones (VD) are identified within Unit 1. In the boundary between Units 1 and 2 enhanced reflectors (ER) are also recognized. Within Unit 2 pull-up reflectors that might be linked with diapirism (D) are also depicted.
1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410
Fig. 12. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-06 showing syn-rift and Cenozoic seismic units across Perito Moreno Terrace. Above the fault, dim zones (DZ) and vertical discontinuity zones (VD) are identified within Unit 1. In the boundary between Units 1 and 2 enhanced reflectors are also recognized. Inside Unit 2 chimneys (CH) are also depicted.
1411 1412 1413 1414 1415 1416 1417 1418 1419
Fig. 13. Un-interpreted (A) and interpreted (B) part of seismic line YMN95-03 crossing trough the irregular depressed area showing the seismic basement (U0) and the Cenozoic units (U1, U2, U3 and U4) . Inside the Cenozoic record, acoustic indicators of gas are identified. Enhanced reflectors (ER), Vertical discontinuities zone (VD), gas chimney (GC), high amplitude reflectors with phase reversal (HARPR) are also depicted. See location of seismic section in Fig. 3B.
1420 1421 1422 1423
Fig. 14: best fit line calculated through orthogonal regression analysis for the 8 alignments recognized on elongated pockmarks.
1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434
Fig. 15. A. Slope grid of the area indicated in Fig.3B. Pockmarks’ name is also indicated. B. Map depicting pockmarks, pockmarks longer axis, canyons’ thalweg, calculated alignments, outcropping/subcropping units, and basement faults recognized in the multi-channel seismic record. The strike of faults 1-4 was determined with lines YMN95-04, YMN95-06 and As1800, while faults 5 and 6 were only recognized on YMN 4 and As1800. C. From left to right: rose diagram of the directions of the main physical settings of the study area, including gradient and ocean currents directions; rose diagram of pockmarks longer axis direction indicated the three groups recognized according to their azimuth; rose diagram of faults and calculated alignments; rose diagram of ridges’ crest; and rose diagram of canyons’ thalweg.
1435 1436 1437 1438 1439 1440 1441 1442 1443 1444
Fig. 16. Detail of different types of pockmarks imaged in the gradient grid with depth profiles and maximum calculated slope values in the flanks of the pockmarks. A. Elliptical pockmark. B. Sub-circular unit pockmark. C. Coalescent pockmarks. See location of details in Fig. 3B.
1445 1446 1447 1448 1449 1450
Fig. 17. A. Mud Volcano and pockmarks imaged in the slope grid with rose diagrams of different features. B. Ridge imaged in the slope grid and below the bathymetric cross-section. Location in Fig. 3B.
1451 1452
Fig. 18. Conceptual evolutionary model for the release of fluids in the study area since the Oligocene to the Holocene.
Highlights • •
Morphometrical analysis of a pockmark field was carried on. Mesozoic inverted grabens were identified beneath and close the pockmark field.
• •
The inversion of the grabens is associated with an Andean Neogene compressional event. A relation between the grabens’ faults and the elongation and alignments of pockmarks was identified in some cases. Faults have possibly worked as conduits for the upward migration of fluids. The origin of the pockmarks seems to be caused by expulsion of gas. The expulsion of fluids was enhanced during the Miocene, possible caused by the Andean Neogene compressional event. A 3D evolutionary model is proposed to explain timing, pathways, triggers and sources for the release of gas.
• • • •
S0264-8172(20)30029-5
DOI:
https://doi.org/10.1016/j.marpetgeo.2020.104246
Reference:
JMPG 104246
To appear in:
Marine and Petroleum Geology
Received Date: 1 April 2019 Revised Date:
13 January 2020
Accepted Date: 14 January 2020
Please cite this article as: Isola, J.I., Ormazabal, J.P., Flores, G., Arismendi, S., Druet, M., Muñoz, A., Río, J.L.d., Etienot, S.D.A., Gomez Ballesteros, M.P., Principi, S., Bolatti, N.D., Tassone, A.A., Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin, Marine and Petroleum Geology (2020), doi: https://doi.org/10.1016/j.marpetgeo.2020.104246. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Tectonic and structural controls on Neogene fluid release in the Patagonian Continental Margin
1 2 3
J.I. Isola 1, J.P. Ormazabal1, G. Flores2, S. Arismendi2, M. Druet3, A. Muñoz4, J.L. del Río5,
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S.D.A. Etienot1, M.P. Gomez Ballesteros6, S. Principi 1,7, N.D. Bolatti2, A.A. Tassone1
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Exactas y Naturales. Ciudad Universitaria, Pabellón 2, CABA, C1428EGA, Argentina
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36390, Vigo, Spain.
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Abstract
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Analysis of high-resolution multi-beam bathymetry, 2D multi-channel seismic, and high-
15
resolution seismic sub-bottom profiles revealed the presence of widespread fluid escape
16
features in the middle slope of the Patagonian Continental Margin. On the sea-bottom,
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these features correspond to pockmarks and mud volcanoes, whereas in the sub-surface
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they are represented in the seismic records by several acoustic anomalies such as
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chimneys, acoustic blanking, enhanced reflectors, and reverse-phase enhanced reflectors
20
among others. Some of these acoustic signatures can be traced to syn-rift deposits that fill
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a deep re-activated and inverted graben. Analysis of pockmarks elongation and pockmarks
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alignments show a good correlation with inverted normal faults, suggesting that faults and
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fractures have influenced the pockmark’s shape and might have acted as pathways for
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upward fluid migration. Some of the acoustic anomalies associated with pockmarks are
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interpreted as evidence of gas. The gas observed in the seismic data seems to be
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thermogenic, and the seismic data suggests a deep origin associated with over-pressured
CONICET – Universidad de Buenos Aires. IGeBA, Dpto. de Cs. Geológicas, Fac. de Cs.
YPF S.A. Buenos Aires, Argentina. Instituto Geológico y Minero de España, Tres Cantos, Madrid, Spain. Tragsa-SGM, Julian Camarillo 6b, 28037 Madrid, Spain. Instituto Español de Oceanografía, Centro Oceanográfico de Vigo, Subida a Radio Faro 50,
Instituto Español de Oceanografía, Corazón de María 8, 28002, Madrid, Spain
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syn-rift deposits. Two morphometrically different sets of pockmarks were identified: an
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older set hosted by Miocene aged rocks, and a younger set hosted by Quaternary
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deposits. Different potential triggers are discussed for the genesis of the Miocene
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pockmarks in relation to the seismostratigraphy, structural geology and regional tectonics.
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It is concluded that, tectonic activity associated with the Neogene inversion of the graben
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faults, due to Andean compression, is the most likely cause for the formation these
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pockmarks. The presence of gas charged sediment and young pockmarks also suggest that
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after the Middle Miocene tectonic climax, a more recent pulse of release of fluids
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occurred.
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1 Introduction
37
The study of structures associated with fluid release in passive margins has had an
38
increasing attention throughout the last decades because of its importance in
39
understanding the fluid flow systems in sedimentary basins (Anka et al., 2014; Brown et
40
al., 2017; Gay et al., 2007; Hartwig et al., 2012; Pinet et al., 2008). Moreover, it also plays a
41
crucial role in ocean processes of high scientific interest such as: greenhouse gases release
42
by dissociation of gas-hydrates (Davy et al., 2010; Kennett et al., 2013); geo-hazards
43
associated with the presence of gas in shallow sediments (Sun et al., 2017); and enhanced
44
development of chemosynthetic communities near cold seeps (Dando et al., 1991; Levin
45
and Mendoza, 2007).
46
The hydrocarbons expelled to the hydrosphere and atmosphere can be produced by
47
either bacterial activity within the first hundred meters of the sedimentary record
48
(biogenic origin) or by long-term cracking of complex long-chain hydrocarbons in the deep
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old rocks (thermogenic origin; Floodgate and Judd, 1992). Due to the hydrocarbon release,
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seafloor features such as pockmarks, mud volcanoes and carbonate mounds are created
51
(King and Maclean, 1970; Hovland and Judd 1988; Judd and Hovland 1992), whereas the
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flow and entrapment of hydrocarbons within the sediment creates a series of changes in
53
the acoustic properties of the host rock which can be identified on seismic data
54
(Ligtenberg, 2005; Løseth et al., 2009; Judd and Hovland 2007). The recognition and
55
characterization of these superficial and sub-superficial features provide key information
56
about the components of the plumbing system which allows the migration of
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hydrocarbons from the deep sub-surface to the seafloor (Talukder 2012).
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Even though the presence of gas in marine sediment is a world-wide known phenomenon
59
(e.g. Baristeas et al., 2012; Brothers et al., 2014; Brown et al., 2017; Bünz et al., 2003;
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Dandapath et al., 2010; Dimitrov and Woodside, 2003; Gay et al., 2007; Jané et al., 2010;
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Sun et al., 2011), it has been rarely reported in the western South Atlantic Ocean (Anka et
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al., 2014; de Mahiques et al., 2017; Portilho-Ramos et al., 2018; Schattner et al., 2016;
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Sumida et al., 2004), and most of the studies dealing with the presence of hydrocarbons in
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the subaqueous sediments of the Argentinean Continental Margin (ACM) correspond to
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coastal areas (Aliotta et al. 2011; Bravo et al., 2018; Parker and Paterlini 1990).
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Recent acoustic expeditions on the middle slope of the Patagonian Continental Margin
67
(PCM) revealed the presence of pockmarks and fluid-charged shallow sediments in Nágera
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and Perito Moreno terraces (Muñoz et al., 2012 and 2013), but no further investigation
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was addressed in specific contributions. The objective of this paper is to unveil the genesis
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of the pockmarks and characterize the main components of the associated plumbing
71
system in order to determine timing, controls, and sources for the leakage of fluids. To
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achieve this, an integrated analysis of multi-beam high resolution bathymetry,
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multichannel 2-D seismic lines and ultra-high-resolution sub-bottom profiles was carried
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out.
75
2 Physical Settings
76
The study area is located on the southernmost part of the western South Atlantic Ocean in
77
the continental slope of the Argentine Continental Margin (ACM; Fig. 1), in an area named
78
Patagonian Continental Margin (PCM; Fig. 2). The depths of the study area vary from 600
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to 1500 mbsl (meters below sea level), and are located between latitudes 46° 30’ – 47° 40’
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S and longitudes 60°15’- 59°10’ W (Fig. 3).
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2.1 Tectonic evolution of the foreland and offoff-shore regions of Central Patagonia
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The PCM comprises the Argentine offshore area located south of the Colorado River outlet
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and its off-shore continuation along the named Colorado Fracture Zone (CFZ; Fig. 2). To
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the south of the study area the main depocenter is the North Malvinas Basin (NMB),
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located on the Malvinas Plateau (Becker et al., 2012; Richards et al., 1996). The area of
86
study includes part of a northern subsidiary branch of NMB (Fig. 1; Becker et al., 2012).
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Several wells drilled by oil companies in the 90’s on the main depocenter of the NMB have
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shown that the basin filling started in the Upper Jurassic (Richardson and Underhill, 2002).
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To the west and southwest lie the San Jorge, Valdez-Rawson and San Julian Basins (Fig. 1).
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These depocenters are genetically linked to the break-up of the Gondwana super-
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continent and were developed during Jurassic and Cretaceous periods (Nürnberg and
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Müller 1991; Ramos et al. 1996).
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All the off-shore basins of the PCM are comprised in what Gianni et al., (2015), tectonically
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defines as the distal Patagonian broken foreland basin. This broken foreland basin exhibits
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the easternmost deformation associated with three main Patagonian Andean tectonic
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pulses. The first of these pulses corresponds to the earliest episode of Andean build-up
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occurred during early Cretaceous (Folguera and Iannizzotto, 2004; Ghiglione et al., 2013;
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Savignano et al., 2016; Suárez et al., 2009). This event was dated in sin-depositional units
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in the northern and austral Patagonian Andes, obtaining ages between 118 and 121 My
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(Ghiglione et al., 2013, Suárez et al., 2009). In the off-shore basins, this compressional
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phase is also recognized in seismic data as a transpressive inversion of the listric normal
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faults associated with the Mesozoic extensional basins. During Late Cretaceous
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(Maastrichtiano – Daniano) a second compressive pulse affected the Patagonian area,
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which was stronger in central Patagonia due to collision of oceanic ridges. This event is
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recorded in San Julian and Rawson basins with a prominent angular unconformity that
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separates Cretaceous and Tertiary deposits (Continanzia et al., 2011; Micucci et al., 2011).
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A tectonically quiet time spam characterized the Patagonian Foreland basin between
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Paleocene and Early Miocene. During Early to Late Miocene, another phase of
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deformation affected the Patagonian Andes. In on-shore Patagonia, the Jurassic and
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Cretaceous basements were exhumated producing folding in Miocene-Oligocene deposits.
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The age of this deformation was dated by Bilmes et al., (2013), and they constrained the
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peak of the contractional episode between 19 and 14.8 My, although evidence close to
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the Andes can be can be traced up to 11.8 My (Bucher et al., 2019; López et al., 2019),.
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During this last compressional pulse, non-deformed areas separated the onshore broken
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foreland from the distal broken foreland basin where the strain was mainly focused by the
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inversion of the offshore Mesozoic basins affecting the Tertiary record (Continanzia et al.,
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2011; Gianni et al., 2015; Micucci et al., 2011).
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2.2 2.2 ACM continental slope Cenozoic seismic stratigra stratigraphy
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The Cenozoic stratigraphy of the ACM lies above a prominent erosive surface (AR3) which
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truncates the uppermost cretaceous record (Gruetzner et al., 2011, 2012, 2016; Hinz et
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al., 1999). This erosive surface is present in the slope and shelf of the ACM and is also
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named Pedro Luro equivalent (Franke et al., 2007; Fryklund et al. 1996). According to wells
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drilled in Rawson and Colorado Basin, the age of this unconformity corresponds to the
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Cretaceous - Paleogene boundary (Bushnell et al., 2000; Cavallotto et al., 2011;
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Continanzia et al., 2011).
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The Paleogene - Neogene record of the ACM is characterized by three regional
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unconformities linked to world scale climatic-oceanographic changes. The Eocene-
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Oligocene boundary is marked by the conspicuous unconformity AR4, which represents
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the restructuring of the global circulation following theDrake gateway opening (Cavallotto
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et al., 2011; Gruetzner et al., 2016; Hinz et al., 1999; Preu et al., 2012, 2013; Violante et
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al., 2010). Above it, lies a contourite depositional system (CDS; Hernández-Molina et al.,
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2009; Hernández-Molina et al., 2010) that covers all the ACM and is segmented by two
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younger unconformities: AR5, linked to the Climatic Optimum of the Mid Miocene
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(Gruetzner et al., 2011), and H2/AR6 (Ewing and Lonardi 1971; Gruetzner et al., 2016)
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associated with enhanced influence of North Atlantic Deep Water in the South Atlantic
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Ocean after the Panamá isthmus closure (Preu et al., 2012). Wells constraints indicate that
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the age of this unconformity corresponds to the Miocene - Pliocene boundary (Bushnell et
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al., 2000).
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2.3 2.3 Oceanographic and morpholog morphological hological settings
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Nowadays the superficial circulation of the PCM is characterized by a northern branch of
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the Antarctic Circumpolar Current named the Malvinas current (Fig. 1). At depths beneath
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the 500 mbsf, the continental slope of PCM is influenced by the circulation of 4 northward
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flow bottom water masses (Tsuchiya et al., 1994): Antarctic Intermediate Water (AIW
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~500-1000 m), Upper Circumpolar Deep Water (UCDW ~1000-2000 m), Deep Circumpolar
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Deep Water (DCDW 2000 – 3500 m) and Antarctic Bottom Water (AABW >3500 m). The
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vigorous activity of these water masses has contributed since the Eocene-Oligocene
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boundary to the genesis of big sedimentary accumulations in all the ACM, forming a
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complex CDS (Hernández-Molina et al., 2010). In the PCM, the CDS mainly consists of 4
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contouritic terraces (Fig. 2), from west to east: Nágera Terrace (NT), Perito Moreno
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Terrace (PMT), Piedra Buena Terrace and Valentín Feilberg Terrace (Hernández-Molina et
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al., 2010). The evolution of each of these contouritic terraces is linked to the activity of a
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north-sourced Antarctic sourced bottom current (Hernández-Molina et al., 2010). NT is
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swept by the circulation of AIW, while PMT is under the influence of UCDW. The AIW is a
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cold-water mass with relatively lowsalinity, born in the polar front (Talley 1996; Piola and
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Matano 2001). The UCDW is also a north flowing current originated in the Antarctic region
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and it is characterized by an oxygen minimum and higher relative salinity in comparison
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with AIW (Tsuchiya et al., 1994).
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3 Materials Materials and Methods
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Multibeam bathymetry, ultra-high resolution parasound seismic profiles and 2D
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multichannel seismic data (Fig. 3) were implemented in this study to analyze and
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characterize the distribution of pockmarks, fluid escape pathways and accumulations in
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the shallow and deep sub-surface of NT and PMT.
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3.1 High Resolution Bathymetry
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In the years 2007 and 2008 high resolution multibeam bathymetry was collected over NT
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and PMT using a Simrad EM-302 echosounder on board of the R/V Miguel Oliver (details
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of acquisition parameters are given in Muñoz et al., 2013). In this study we analyze over
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10.000 km2 of that multibeam data in water depths that range from 600 to 1350 mbsl.
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Also, 69 km2 of the bathymetry used in this work was acquired on board of the
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Argentinian R/V Austral with the Kongsber EM-122 (location in Fig. 2). These data were
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processed with CARIS.
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3.2 Parasound P70 subsub-bottom profiles
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Ultrahigh resolution seismic data were acquired in 2017 in the frame of the Argentine
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national initiative Pampa Azul during the cruises “Batimetría Área Marina Protegida
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Namucurá” and “YTEC-GTGM 2”. All the seismic profiles were acquired with sub-bottom
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profiler Parasound System DS3 (P70) on board of the Argentinean R/V Austral.
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The Parasound System DS3 (P70) is a hull-mounted parametric echo sounder developed
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by the company Teledyne Reson Gmbh. Its operation is based on the acoustic parametric
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effect. Such effect is generated when two high energy signals of slightly different
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frequency are pinged simultaneously causing the creation of harmonics at two
180
frequencies: a low frequency corresponding to the difference between the two primary
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frequencies; and a high frequency corresponding to the addition of the two primary
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frequencies. In the Parasound system, the parametric effect is achieved by one fixed
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primary frequency of 18 Khz, that distributes the energy within a beam of 4.5° through a
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transducer of approximately 1m length, and a variable primary frequency that can be set
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up to operate between 18.5 and 24 Khz.
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The Parasound system records three signals separately: the fixed primary high frequency
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signal (18 Khz; PHF), the secondary low frequency signal (difference between the primary
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fixed and primary variable frequencies; 0.5 to 6 Khz; SLF) and the secondary high
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frequency (addition between the primary fixed and primary variable frequencies; 36.5 to
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42 Khz; SHF).
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During the acquisition cruise, the primary variable high frequency signal was set up at 22
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Khz, giving as a result a SLF of 4 Khz. SLF and PHF raw data were stored in PS3 format. The
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Parasound profiles showed in this paper correspond to the SLF record. The acquisition and
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conversion of the profiles to SEG-Y format was carried out with the software Parastore
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property of TELEDYNE. These profiles were later imported and interpreted in ©Kingdom
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software.
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2.3 Multichannel 2D seismic
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Over 500 kilometers of 2D multichannel seismic records (MCS) were analyzed in this work.
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All the MCS were provided by the Argentine Secretary of Energy in SEG-Y format. The
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visualization and interpretation of the seismic data were carried out using
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software and OpendTect.
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The YMN-95 seismic lines consists on Kirchoff time migrated 2D seismic lines (PSTM).
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During the acquisition of the YMN-95 seismic lines two different streamers were
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alternated depending on the depth of water and geological settings: a shorter one with a
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length of 4600 m and 368 channels, and a longer one with a length of 6000 m with 480
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channels. The distance between channels in both streamers was 12.5m, the distance
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between shots 25 m, and the distance between Common Depth Points (CDP) was 6.25 m.
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The resulting nominal folds were 92 for the short streamer and 120 for the long streamer.
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The sampling interval was 2msec and the seismic record depth was 8193 msec. The
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seismic lines were processed in YPF SA offices using the software Echos. The processing
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included spherical divergence correction and further deconvolution and amplitude
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balance.
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The Line as1800 was also provided by Argentine Secretary of Energy in SEG-Y format. For
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this seismic profile a Pseudo-Relief Attribute (TecVA, Bulhões and de Amorim, 2005) was
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calculated. This attribute allowed a better identification of structures.
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The MCS used in this paper follow the American standard for polarity – impedance
217
(Asveth et al., 2000). This standard implies that an increase in the acoustic impedance, or
©Kingdom
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a “hard event”, is represented as positive peak in the wiggle, or as a blue-yellow-blue
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reflection loop in the kingdom’s pallet “RSA col_amp_flipped.CLB”.
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2.4 Mapping of pockmarks geometry and alignments
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2.4.1 Pockmark geometry
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To visualize and interpret the bathymetric data, QGIS Software was used. Also with QGIS,
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54 pockmarks were manually mapped, and 6 morphometric attributes were measured for
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each pockmark: area, perimeter, long axis, short axis, long axis azimuth and shape index
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(Table 2). The ellipticity was calculated to evaluate whether pockmarks were elliptical or
226
circular. Pockmarks with ratios higher than 1.1 were considered elliptical. Finally, rose
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diagrams were built for: pockmarks long axis, and canyons’ thalweg, pockmarks’
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alignments, ridges crest and faults.
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2.4.1 Pockmark alignment
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The method used to define pockmarks alignments was based on the procedure proposed
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by Paulsen and Wilson (2010) for volcanic vents and edifices. It is well-known that
232
elongated pockmarks and pockmarks alignments can be associated with deep seated
233
faults (Jané et al., 2010; Pilcher and Argent, 2007; Roy et al., 2015). Normally when this
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type of structural control is present on pockmarks, the elongation direction and the strike
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of alignments are sub-parallel or parallel to the azimuth of faults. Following that premise,
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the first step used to constraint possible alignment directions was to determine trends in
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the orientations of pockmark’s elongation, and look for alignments on those directions.
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The pockmarks included in each alignment were defined by visual inspection. After select
239
a trend pockmarks a lineal orthogonal regression was done considering the center of the
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ellipse that best fits the pockmark in UTM-20 X and Y coordinates. In pockmarks 23 and
241
31, when pockmarks coalescence is evident, two centers were defined using the two
242
depocenters identified. Besides the initial considerations, the orientation of alignments
243
was defined following some extra criteria, as it is the minimum of 3 features with the
244
same orientation to define an alignment.
245
The equation of the best fit line calculated trough orthogonal regression and the residuals
246
value for each point were calculated using the R Studio Statistical Software. Using these
247
parameters 4 degrees of confidence were defined (Table 3) based on the criteria
248
stablished by Paulsen and Wilson, (2010) and later modified by Bonini and Mazzarini
249
(2010) for mud volcanoes and applied by Maestro et al., (2019) in the study of submarine
250
mud volcanoes.
251
4 Results and Interpretation
252
4.1 Seismostratigrapy
253
The seismostratigraphic interpretation of the MCS and the SBP data allowed the
254
identification of 6 mayor unconformities in the study area. These unconformities were
255
interpreted as sequence boundaries, and 7 seismic units were defined.
256
Given that no well was ever drilled on the PCM, a straightforward age calibration for the
257
seismic units was not possible, and the age model used in this work was elaborated by
258
comparing the main characteristics of the interpreted seismic units with regional
259
seismostratigraphic charts published in previous contributions for the ACM, PCM (Ewing
260
and Lonardi 1971; Gruetzner et al., 2011, 2012, 2016; Hernández-Molina et al., 2009; Hinz
261
et al., 1999; Isola et al., 2017; Preu et al., 2012; Violante et al., 2010) and North Malvinas
262
Basin (Becker et al. 2012, Richards and Fannin, 1997; Richardson and Underhill, 2002).
263
The complete seismic record was divided in 2 mega-sequences, syn-rift and post-rift
264
deposits. The Basement and pre-rift couldn’t be separated due to the poor quality of MCS
265
in deep strata, and both are included in the seismic basement here called Unit 0.
266
4.1 4.1.1 SynSyn-rift deposits and Graben structure
267
The geometry of a major graben located in the south-east of the study area is clearly
268
displayed in YMN-04 and YMN-06 seismic records (Figs. 4 and 5). The graben has 50 km
269
length and 35 km width, and is controlled by a series of synthetic and antithetic NW
270
striking normal faults, that in some cases, show signs of re-activation (Fig. 5). In the middle
271
of the graben an intra-graben high separates two depocenters. The northeastern
272
depocenter host the thickest sedimentary record, and has up to 2 sec TWT thick, while the
273
thickness of the southwestern depocenter is less than 1 sec TWT (Fig. 4).
274
The infill of the graben is characterized by a wedge shaped syn-rift deposit (Unit SR). This
275
deposits are truncated on its top by a flat erosive surface represented by the reflector AR1
276
(Fig. 5), of Hauterivian minimum age, interpreted as the break-up unconformity generated
277
after the opening of the South Atlantic Ocean (Franke et al., 2007; Hinz et al., 1999).
278
Fault reactivation and inversion is more noticeable on the northwestern part of Fault 1
279
(Line YMN 95-06; Fig 5), where the compression was strong enough to generate a
280
considerable vertical displacement in reflectors of Units 1 and 2. In the southeastern part
281
of Fault 1 signs of reactivation are not appreciable on the MCS (Fig. 4).
282
Becker et al., (2012) interpreted a series of grabens near the study area and confirmed
283
Richards et al., (1996) hypothesis of a northward continuation of the NMB. We followed
284
their seismostratigraphic concept, and correlated the seismic Unit SR to the late syn-rift
285
unit deposited in north Malvinas Graben during Tithonian to Berriasian (Becker et al.
286
2012; Richards and Fannin, 1997; Richardson and Underhill, 2002).
287
4.1.2 Cenozoic postpost-rift deposits
288
The post-rift seismic-stratigraphy of the ACM has been studied and refined over the last
289
45 years (Ewing and Lonardi 1971; Gruetzner et al., 2011, 2012, 2016; Hernández-Molina
290
et al., 2009; Hinz et al., 1999; Isola et al., 2017; Preu et al., 2012; Violante et al., 2010). As
291
a result of those studies, a general seismostratigraphic scheme for the entire ACM was
292
developed. In this work, we follow the nomenclature for the unconformities of Violante
293
et al. (2010) and Preu et al. (2012): AR1, AR3, AR4, AR5, H2, and N.
294
In Fig. 6, a key seismic section shows the regional seismic architecture of the Cenozoic
295
strata of NT and PMT. The Cenozoic record is segmented in 5 seismic units (1 to 5;
296
equivalent to seismic units defined by Muñoz et al. 2013) bounded by 5 regional
297
discontinuities (Table 1).
298
Several authors have recognized in the Argentine and Uruguayan Continental Margins two
299
post-rift megasequences, the Cretaceous post-rift and the Cenozoic post-rift (Conti et al.,
300
2017; Gruetzner et al., 2011, 2012, 2016; Hinz et al., 1999; Preu et al. 2012; Violante et al.,
301
2010). In the study area, the Cretaceous post-rift is absent, and above the break-up
302
unconformity there is a hiatus of ~60 Ma represented by the absence of reflectors
303
between AR1 and AR3 (Fig. 6).
304
Unit 1, onlaping on horizon AR3, has a thickness that range from 0.1 s TWT in the shelf to
305
0.75 s TWT in the continental slope, and is characterized by a divergent internal acoustic
306
configuration composed of continuous medium amplitude reflectors. This unit represents
307
a Paleogene episode of thermal subsidence (Gruetzner et al., 2011, 2012, 2016; Isola et
308
al., 2017; Preu et al., 2012; Violante et al., 2010). This unit is affected in some areas by the
309
reactivation and inversion of the graben faults (Fig. 5). Its uppermost part is truncated by
310
reflector AR4 (Eocene-Oligocene). This horizon is a widespread erosive surface recognized
311
in all the ACM, and represents a period of enhanced activity of ocean currents associated
312
with the opening of the Drake Passage and the onset of Antarctic bottom waters current
313
(Gruetzner et al., 2016; Hernández-Molina et al., 2009).
314
Unit 2 is characterized by medium amplitude continuous reflectors with an internal
315
acoustic structure that ranges from sigmoidal clinoforms to parallel stratification towards
316
the ocean basin (Fig. 6 and Fig. 7 A-E). The age of this unit is Oligocene-Mid-Miocene and
317
is interpreted as a deltaic progradation associated with a lowstand sea level (Isola et al.,
318
2017; Preu et al., 2012). The top of this unit is the erosive surface represented by reflector
319
AR5, of middle Miocene age, associated with the Miocene climatic optimum (Gruetzner et
320
al., 2011).
321
Unit 3 is well imaged in parasound seismic profiles (Fig 7), and is characterized by
322
reflectors with medium to high amplitude and parallel internal structure. It is bounded on
323
its top by reflector H2 (Miocene-Pliocene; Preu et al., 2012; Violante et al., 2010).
324
Unit 4 is characterized by prograding clinoforms that onlap above horizon H2 in the
325
continental shelf, and has a thickness of up to 500 ms TWT in the outer shelf. In the
326
continental slope, this unit is well imaged in parasound seismic profiles by its high
327
amplitude reflectors in comparison to the underlying Unit 3. In some areas of the
328
continental slope, another prominent unconformity was also identified above Unit 4, the
329
reflector N. This horizon is interpreted as the base of the transgressive-regressive events
330
associated with the Pliocene – Quaternary glacieustatic boundary (Violante et al., 2010).
331
Above this unit lays the youngest deposits of the terraces, Unit 5, associated with the
332
Quaternary glacioesustatic variations. This unit, as well as Units 3 and 4, is affected by
333
faulting (Fig. 7D).
334
For more detailed description of the seismic units and main unconformities age
335
constraints for the PCM, readers are referred to, e.g. Gruetzner et al., 2011; Hernández-
336
Molina et al., 2009; Isola et al., 2017; Muñoz et al., 2013.
337
4.2 4.2 Seismic evidence of fluid flow
338
The interaction between an acoustic signal and gas charged sediment generates
339
detectable signatures that can be recognized in the seismic record in a variety of acoustics
340
features (Ligtenberg, 2005; Løseth et al., 2009; Judd and Hovland 2007). The analysis of
341
our seismoacoustic database allows the identification of several of these features such as
342
gas chimneys, acoustic turbidity, vertical discontinuities, lateral decrease in amplitude and
343
enhanced reflectors. Acoustic turbidity is generated by the attenuation of the acoustic
344
energy caused by the interaction with gas bubbles, and is recognized as a zone of chaotic
345
reflections (Tóth et al., 2014). Gas chimneys are vertical acoustic blankings or turbid zones
346
associated with previous or on-going focused fluid migration. Enhanced reflectors with
347
reverse phase are coherent reflections with markedly high amplitude that could well be
348
caused by the large impedance contrast between the gas bearing sediment and water
349
bearing sediment. Vertical discontinuities (even tough not completely understood) are
350
believed to be the result of gas migration through low permeable sediment (Judd and
351
Hovland 2007; Ligtenberg, 2005; Løseth et al., 2009).
352
The description of acoustic indicators provided below is focused in three main regions
353
localized in Fig. 3A, Irregular Depressed Area (IDA), Pockmark Area (PA) and Mud-
354
Volcanoes Area (MVA).
355
4.2. 4.2.1 2.1 Parasound profiles
356
The analysis of almost 350 km of very high resolution Parasound seismic profiles allows
357
the identification of several acoustic anomalies that may be associated with the presence
358
of gas in the shallow sub-surface.
359
Beneath the IDA, columnar disturbances are common and were interpreted as gas
360
chimneys, meanwhile patches in the seismic record with the acoustic signal obliterated,
361
were interpreted as acoustic blanking (Fig. 7A and C). Gas chimneys are found in the
362
profiles located in the west part of the IDA, and are often associated with buried
363
pockmarks (Fig. 7C). They have lengths between 500 and 1000 meters, cutting through
364
Units 2, 3 and 4, and are in some cases rooted in Unit 1. Acoustic blankings are more
365
frequent in the southwest side of the IDA and are occasionally accompanied by
366
diapirism/doming (Fig. 7A and C).
367
In the PA, acoustic blanking appears as curtains distorting the seismic reflectors of units 1
368
and 2, and the lower reflectors of unit 3 (Fig. 7E and F). The length of these features varies
369
between 1500 and 2000 m and are often associated with enhanced reflectors and gas
370
chimneys (Fig. 7 E and F).
371
In the MVA, Unit 2 is outcropping or subcroping and is folded due to diapirism (Fig. 7B).
372
Within this unit, seismic chimneys and kilometric sized acoustic blankings are frequent
373
(Fig. 7B). The seismic chimneys can be traced to a reflector located ~100 msec TWT that
374
simulates the shape of the sea bottom (Fig. 7B). Given that the MVA presents favorable
375
pressure and temperature conditions for gas hydrate development, we infer that this
376
bottom simulation reflector (BSR) could be related to the boundary of gas hydrates and
377
free gas.
378
4.2.2 Multichannel Seismic Records
379
The MVA is located above an inverted hemi-graben (Fig 8). The inversion of the graben
380
affects the Neogene record and is possibly the cause of the folding shown in Unit 2. Within
381
Unit 2, some sinuous and vertical features with diffuse and chaotic internal acoustic
382
structure, interpreted as gas chimneys, were recognized (Fig. 9). The gas chimneys can be
383
traced from Unit 1 to the seafloor, where they end up as reflectors with enhanced
384
amplitude (Fig. 9). Inside the diapiric structures, several enhanced reflectors and bright
385
spots with lateral polarity inversion are identified (Fig. 9). The amplitude linked to the
386
reverse polarity reflectors is -37973, while the seafloor reflector in the same wavelet is
387
33722. Bright spots may well be associated with the presence of gas pockets (Ligtenberg,
388
2005), although further analysis is needed to rule out other possible explanations for
389
these reflections (Asveth et al., 2000).
390
In the PA and in the boundary between NT and PMT, anomalous acoustic patterns are
391
common. Anomalies consist in closely spaced vertical discontinuities within Unit 1 in zones
392
close to faults (Figs. 11 and 12). The appearance of this pattern is very similar to the
393
“Vertical Discontinuities Zones” defined by Løseth et al., (2009). According to Ewing and
394
Lonardi (1971) and Muñoz et al., (2013), Unit 1 is an Eocene lutite. This lithology provides
395
a poorly permeable medium, and gas interaction with the acoustic signal would result in
396
vertical discontinuity zones (Løseth et al., 2009). The dime zones and the presence of
397
vertical discontinuities, likely related to hydrofacturing, are interpreted as evidence of
398
fluid migration. The top of the vertical discontinuity zones is often associated with high
399
amplitude reflectors close to the boundary between units 1 and 2 (Figs. 11 and 12). The
400
polarity of the upper high amplitude reflector is opposite of the seafloor polarity and its
401
amplitude is higher than the amplitude of the seafloor reflection (anomaly amplitude: -
402
42280dB, seafloor amplitude: 22280 dB; Fig. 12).
403
In the western flank of the IDA, seismic units 2, 3 and 4 are truncated, evidencing the
404
erosive genesis of this giant depression (Fig. 13). Three chimneys recognized in the
405
western part of the IDA, rooted in Unit 1, reach the seafloor with high amplitude reflectors
406
below small tributary channels (Fig. 13; same feature was imaged with parasound profiles
407
in Fig. 7A). The bottom of the IDA almost reaches reflector AR4 and, on the relicts of Unit
408
2, enhanced reflectors with phase reversal are recognized. In the eastern flank of the IDA
409
Unit 2 thickness reach up to 150 msec TWT and acoustic indicators of gas like chimneys,
410
enhanced reflectors, and reversed phase enhanced reflectors are often identified.
411
Eastwards, the acoustic anomalies disappear.
412
4.3 4.3 Geomorphology
413
The relief of the study area is characterized by two gently dipping contouritic terraces
414
delimited by a scarp that loses sharpness southwards, and two canyons that cross-cut the
415
terraces with west-east direction. In the northern part of the study area it is also imaged a
416
bathymetric low and a ridge (IDA and R in Fig. 3A). Southwards, mud volcanoes and
417
elliptical pockmarks are scattered throughout the seafloor. The morphology and
418
morphometrical characteristics of these features are given below.
419
For a detailed description of the main morphological features of NT and PMT readers are
420
referred to the contributions of, e.g., Lastras et al., (2011), López-Martínez et al., (2011),
421
and Muñoz et al. (2012 and 2013).
422
4.3.1 Distribution, morphometry and structural control of pockmarks and canyons
423
Iglesias et al., (2010) propose the truncation of reflectors as a valid criterion to
424
differentiate proper pockmarks (erosive crater-like depressions originated by ascending
425
fluids) from pockforms (non-erosive crater-like depressions formed by a variety of
426
different processes). As truncated reflectors are commonly encountered in the flanks of
427
the studied depressions (Fig. 7F), and acoustic evidence of sub-surface fluid mobilization
428
in the study area is widespread, we classified the studied depressions as proper
429
pockmarks.
430
The pockmarks are located between 750 and 1200 mbsl. Most of them (~80 %) are
431
concentrated above the main graben and the remainder lie near canyon 1 and the IDA
432
(Fig. 15). All pockmarks show U-shaped cross-sections and the southern and western
433
flanks are usually steeper than the northern and eastern ones (Fig. 16).
434
Three different plan view pockmark shape could be distinguished: elliptical (Fig. 16A), sub-
435
circular (Fig. 16B), and coalescent pockmarks (Fig. 16C). The length of unitary pockmarks
436
longer axis varies between 584 m and 1899 m with an average of 1269 m, while
437
coalescent pockmarks have a longer axis that reach a length of up to 4078 m.
438
The 91% of those pockmarks where an elongation direction could be defined, show a long
439
axis/short axis ratio bigger than 1.1. The analysis of those directions in rose diagrams
440
indicates that the elongation is not random and two preferential strikes, ~45° and ~135°,
441
were identified.
442
Following the criteria adopted by Bonini et al., (2016) and Maestro et al., (2019) 6
443
alignments with reliability degree ≥ C were defined in 25 pockmarks. These alignments
444
can be divided in two sets of three alignments each oriented in ~NW and ~NE (table 4 and
445
Fig. 15).
446
The rose diagram built for the canyons indicates that canyons are in some areas deflected
447
from the bathymetric gradient in the pockmarks elongation directions (Fig. 15C).
448
4.3 4.3.2 Mud Volcanoes, Volcanoes, young pockmarks and ridges idges
449
Mud volcanoes were recognized by means of high resolution bathymetry (Fig. 17A) and
450
are associated with diapirism and doming of the seismic Unit 2 (Fig. 7B and Fig. 8). These
451
features are restricted to the south west part of the study area between depths ranging
452
from 500 to 650 mbsl. They have approximately 35 m high and their slopes have a
453
gradient that varies between 1.5° and 2°.
454
Besides the elongated kilometric-sized pockmarks described above, another set of smaller
455
crater-like depressions also interpreted as pockmarks, are identified in the bathymetry.
456
These pockmarks occur within unit 4 at depths ranging from 500 to 700m and have long
457
axis length of up to 500m. Even though these are not as many as the old mega-pockmarks,
458
a potential alignment with the highest degree of confidence stablish in this work could be
459
defined (Fig. 17A). Also, 2 directions of elongation were defined in 9 of these pockmarks;
460
one of the directions is similar to the calculated alignment (Fig. 17A).
461
In the northern part of the study area, a ~20 km long ridge surrounded by scours (Figs. 3A
462
and 17B) was identified. This ridge is colonized by cold water corals (Muñoz et al. 2012,
463
Ríos et al. 2011). The height difference between the base of the ridge and the highest
464
point of it is ~450m. It is worth to note that the strike of this feature matches the normal
465
inverted faults azimuth (Fig. 15C). Another ridge is shown in Fig. 10; the relation of this
466
positive relief with basement structures is clearly imaged by the seismic line YMN95-05.
467
5 Discussion
468
5.1 5.1 Age of Compressional Structures
469
The distal Patagonian broken foreland basin, where the study area is located, has been
470
affected by 3 major phases of Andean compression. The first two pulses took place during
471
the Cretaceous period (Gianni et al., 2015; Savignano et al., 2016), and the youngest
472
corresponds to early-middle Miocene (Bilmes et al., 2013; Bucher et al., 2019). The latter
473
is represented on land by widespread folding and reverse and strike-slip faults affecting
474
Oligocene and Miocene rocks (Bilmes et al., 2013; Bulcher et al., 2019; Cobbold et al.,
475
2007; Diraison et al., 1998; Ramos, 1989). Whereas in the distal broken foreland basin the
476
deformation was focalized in the edges of the Mesozoic extensional basins (Gianni et al.,
477
2015). Evidence of such compressional event was reported close to the study area in
478
Rawson and San Julian basins (Continanzia et al., 2011; Micucci et al., 2011).
479
Given that the inverted faults studied in this work crosscut Units 1 and 2 (Paleocene to
480
Middle Miocene), and faulting, folding and diapirism within Unit 2 is widespread (Figs. 5, 7
481
A-E, 8, 9 and 10), we ascribe the genesis of these compressional features to the last peak
482
of Andean compression occurred in Central Patagonia from the Early to Middle Miocene
483
(Gianni et al., 2015).
484
Within the Plio-Quaternary units, compressional structures are restricted to just a few
485
areas and represented by small vertical faults and slightly sin-depositational curved
486
reflectors close to the mud volcanoes (Figs. 7 B-D and 9). Such scarce amount of
487
compressional evidence indicates that tectonic activity in the area decreased after
488
deposition of Unit 2. The top of the diapirs were in some cases eroded and covered by a
489
veneer of Quaternary deposits (Fig. 7B). These young deposits gently onlap on the flanks
490
of the diapirs suggesting that during the deposition the uplifiting of the diapirs was
491
relatively slow.
492
It is worth noting that the deformed areas are located above deep inverted normal faults,
493
whereas the areas between grabens show scarce or no signs of deformation. This
494
indicates that the compressional stress was focused in the grabens’ faults.
495
5.2 5.2 Pockmarks Pockmarks Morphometry, Morphometry, Distribution and Origin
496
5.2 5.2.1 Pockmarks Pockmarks morphology, morphology, age and distribution
497
According to Judd and Hovland (2007), the original shape of pockmarks is a circular crater-
498
like depression. Subsequently, this shape can be modified by the influence of exogenic
499
agents like ocean currents (Andresen et al., 2008; Bøe et al., 1998), gravity triggered mass
500
movements, and turbidity currents (Brothers et al., 2014; Çifi̧ et al., 2003). The effect of
501
these processes can be also conditioned by shallow faults and fracture networks (Jané et
502
al., 2010; Hovland and Judd 1988). Directional statistical analysis of the long axis direction
503
provides clues about the origin of the elongation of pockmarks (Michel et al., 2017).
504
Whereas pockmark alignments are associated with factors controlling their formation, as
505
deep seated faults, gas hydrate dissociation minimum depth, and buried canyons among
506
others (Brothers et al., 2014; Jané et al., 2010; Pilcher and Argent, 2007).
507
In this study, three exogenic processes were taken into consideration to analyze their
508
potential influence on pockmarks’ shape: erosion by bottom currents, slumping, and
509
turbidity currents along gradient direction. The bottom currents that interact with the
510
seafloor in the study area are two, AIW in NT and UCDW in PMT; both of them flow
511
northwards (Hernández-Molina et al., 2009; Piola and Matano 2001). The general
512
bathymetric gradient dips mostly eastwards (Fig. 15C), and is relatively smooth on the
513
terraces, characterized by values ranging from 0.5° to 1.5°.
514
The morphometrical analysis of the elliptical pockmarks revealed 2 main preferential
515
directions of elongation: NE and NW (Fig. 15C). Those strikes match neither ocean
516
currents nor gradient directions, although the NW direction is similar to the azimuth of
517
the faults identified in the MCS, suggesting a relation between faults and pockmarks’
518
elongation. Also, three alignments with ~NW strike and reliability degree ≥ C were defined
519
on the NW elongated pockmarks (Table 4).
520
The NE direction of pockmarks’ elongation admits no simple explanation; it matches
521
neither: ocean currents directions nor gradient direction, or the strike of the inverted
522
normal faults. Even though its origin is unclear, the fact that most of NE elongated
523
pockmarks are located above an inverted graben suggests that it may also have a
524
structural origin, and may, somehow, be related to fractures or sub-seismic faults
525
associated with the inversion of the graben. Furthermore, the alignment analysis allows us
526
to identify 3 alignments with a reliability grade ≥ B in ~NE direction. There is, however, no
527
sub-superficial geophysical data available that corroborate this hypothesis.
528
The fact that two independent variables, as elongation and alignment, show consistent
529
results is a strong indicator that the elongation could be controlled by the same factor
530
controlling alignment. As it was explained earlier in the text, elongation of pockmarks can
531
be produced after they are created, but alignments are controlled by a factor which acts
532
at the moment of pockmark formation, as deep-seated faults.
533
In the cases studied by Jané et al. (2010) and Roy et al. (2014), even when structural
534
control on pockmarks genesis is evident, faults do not reach seafloor. To explain how blind
535
faults can control pockmarks distribution and morphology they proposed that the fluids
536
were expelled through the fault planes from a deep source rock and then, in the more
537
permeable shallow strata, vertical gradual diffusive flow would be the responsible
538
mechanism for the upward migration of gas. This mechanism would generate a vertical
539
path for the migration of gas above the faults and thus, the development of pockmarks
540
will concentrate above the faults. In this sense, we believe that the main graben’s faults
541
work as conduits for the fluids migration from the syn-rift deposits to the Cenozoic strata.
542
Then, inside the more permeable shallow strata, fluids would migrate through diffusive
543
fluid or small faults and fractures (Figs. 7F, 11 and 12). Once pockmarks are formed two
544
mechanisms are envisioned for their elongation: 1) pockmarks that form aligned above
545
faults traces are then enlarged by the erosive agents until they coalesce with each other in
546
the directions of faults, creating an elliptical pockmark elongated along the strike of the
547
deep fault (see pockmark 23 and 13 in Fig. 15); 2)tectonic activity associated with the
548
inversion of the faults created small fractures in the younger units (see small
549
faults/fractures in Figs 7F and 12), these smaller faults could have similar direction than
550
the main blind faults and provide a weaker rheological behavior in specific directions for
551
the erosive agents to enlarge the pockmark.
552
Even though acoustic evidence of fluid flow occurs in all the study area, the incidence of
553
pockmarks is constrained just to depths up to 1200 m and most of them appear on the
554
outcrops of Unit 2 (Fig. 15B, 7C and 7F). As the pockmarks have approximately the same
555
age as the sediments on which they formed, we infer that the studied pockmarks were
556
formed coeval to the last stages of deposition of Unit 2. By then, during the Middle
557
Miocene, the rheological behavior of the superficial deposits in deeper areas, where
558
Eocene rocks outcropped or subcropped (Unit 1 in Fig. 6), did not allow the creation of
559
pockmarks; areas with soft sediments are a necessary condition for pockmarks formation.
560
The low depositional rates and the strong bottom circulation that have characterized this
561
area since the Middle-Miocene could have preserved the pockmarks regardless there was
562
or not an active plumbing system beneath (Hammer et al., 2009; Pau et al., 2014).
563
In summary, mega-pockmarks were formed during the last stages of the deposition of
564
Unit 2 (Early to Middle Miocene). As regards their distribution, we have identified three
565
controls: the presence of source of fluid, presence of migration pathways beneath
566
(inverted normal faults), and presence soft sediments on the seafloor.
567
5.2 5.2.2 Origin of fluids
568
Since the pioneer studies of King and Maclean (1970), the genesis of pockmarks has been
569
consensually attributed to the erosive effect of ascending fluids. The fluid expelled from
570
the host rock to the seafloor can be either water or gas. The latter generates acoustic
571
anomalies detectable in seismic data (Ligtenberg, 2005; Løseth et al., 2009; Judd and
572
Hovland 2007).
573
Compressional wave attenuation caused by the presence of gas in sediment is a common
574
phenomenon observed around the globe. In high frequency seismic methods (>1Khz), like
575
the Parasound D70, even a concentration of volume of 0.0002% can cause a significant
576
attenuation of the acoustic wave (Tóth et al., 2014). The most common seismic features
577
associated with this effect are acoustic blanking and acoustic turbidy (Tóth et al., 2014).
578
These indicators were found in our data base and in the high frequency seismic data
579
presented in previous contributions in the area (Lastras et al., 2011; López-Martínez et al.,
580
2011; Muñoz et al., 2013). We interpret that these features are caused by the presence of
581
gas in the shallow strata (<150m).
582
The deep acoustic anomalies described in section 4.2.2 admit no simple explanation. In
583
MCS, the presence of anomalous acoustic patterns, as isolated reverse polarity bright
584
spots and HARP, may well indicate the presence of gas. However, the same anomalies can
585
be also caused by: over-pressured sands/shales, coal deposits, and salt domes (Asveth et
586
al., 2013). The presence of salt domes is discarded given that there is no evidence of
587
Mesozoic evaporites along the ACM (Ramos 1996). Similarly, the presence of coal in this
588
area is highly unlikely considering that well data show that the Cenozoic sedimentary
589
record of the PCM is dominated by marine facies (Continanzia et al., 2011; Micucci et al.,
590
2011), and significant coal deposits cannot be formed in such environments.
591
Overpressured sands/shales can, however, be an alternative explanation for the HARP
592
identified within the folds and diapirs of Unit 2 (Fig. 10). Given that a more definitive
593
analysis, as AVO, cannot be performed, and well information of shallow shale/sand
594
acoustic behavior lacks, an unequivocal interpretation of the deeper acoustic anomalies
595
cannot be reached in this work.
596
Sub-surface hydrocarbons can have two different origins: thermogenic and biogenic (Judd
597
and Hovland 2007). Thermogenic fluids are generated in deep old rocks with high organic
598
content and are the consequence of long-term cracking of complex long-chain
599
hydrocarbons. Meanwhile, biogenic fluids (mostly methane) are produced by rapid
600
bacterial degradation of organic material inside shallow sediments in geological
601
environments with high depositional rates. The seismostratigraphic interpretation carried
602
out in this work shows that, in the study area, the Plio-Quaternary depositional rates are
603
rather slow, and the thickness of the sedimentary record deposited since Pliocene (units 4
604
and 5) in NT is between 0 and 40 msec (Figs. 6 and 7). In PMT the Plio-Quaternary units
605
are absent or represented by isolated drifts or just a veneer of coarse sediment (Muñoz et
606
al., 2012 and 2013; Ewing and Lonardi 1971). These sedimentary rates make the
607
hypothesis of a biogenic source for the gas related anomalies recognized in the SBP
608
records unlikely. On the other hand, multiple oil producers rocks were discovered in the
609
North Malvinas Basin (Farrimond et al., 2015; MacAulay, 2015; Richardson and Underhill,
610
2002). One of these is the Barremian syn-rift lacustrine succession of the North Malvinas
611
Graben located ~100 km southwards the study area. Total organic carbon between 2 and
612
12 % has been recorded in the late syn-rift and early post-rift successions of the Sea Lion
613
Field located in the North Malvinas Graben (Farrimond et al., 2015). Pockmarks related
614
with acoustic anomalies, similar to the ones recognized in this work, were imagen by 3D
615
seismic data (Brown et al., 2017).
616
The hypothesis of a thermogenic source for the fluids involved in the generation of the
617
pockmarks seems plausible, considering that: syn-rift deposits of a northern branch of the
618
NMB are located beneath the study area; 80% of the pockmarks are located above the
619
main graben; pockmarks' shape and distribution suggest a relation with basement faults
620
that can work as conduits for gas migration; and some of the acoustic indicators can be
621
traced to the top of the syn-rift deposits (Figs. 4, 5, 8, 11 and 12).
622
A final consideration regarding the fluid involved in the formation of pockmarks is the
623
presence of groundwater venting. Even though water venting wouldn’t explain the
624
presence of gas related acoustic anomalies identified in the SBP, Hübscher and Borowski
625
(2006) have proved that even when hydrocarbons are present in the system, artisan
626
groundwater flux can be the main factor that controls pockmarks formation. The presence
627
of a gas-free fluid involved in the creation of pockmarks is often associated with
628
dewatering features as polygonal faults (Andresen and Huuse, 2011), or a artisan water
629
reservoir (Chenrai and Huuse, 2017; Hübscher and Borowski, 2006). However, such
630
features are absent in our data base and have not been reported for the study area. Thus,
631
we believe that the role of water in the genesis of these pockmarks –if any- was not
632
important.
633
5.3 5.3 Patagonian plumbing system: system: Trigger mechanism and role of tectonics
634
The identification of a deep sourced fluid involved in the genesis of mega-pockmarks
635
suggests that over-pressured fluids ruled the plumbing system. Several external processes
636
can trigger the release of gas in passive margin by excess of pore pressure. The most
637
common factors include: overburden erosion (Naudts et al., 2006), sea-level changes
638
(Talukder, 2012), overloading by quick burial (Sun et al., 2012), and compressional
639
tectonics (Judd and Hovland 2007; Kopf, 2002; Plaza-Faverola et al., 2015).
640
In the study area, the lack of erosive features capable to generate a decrease in
641
lithoestatic pressure big enough to overcome the release of overpressured fluids (Brown
642
et al., 2017; Naudts et al., 2006), pose overburden erosion as an unlikely trigger
643
mechanism. In the same way, with the low depositional rates that characterize the study
644
area, it is not likely that the excess of pore pressure is linked to quick burial. Conversely,
645
several evidences indicate that tectonic deformation and seismic activity associated with
646
the inversion of graben normal faults was the more likely cause for the buildup of
647
overpressure and the subsequent release of fluids: the relation between inverted normal
648
faults and pockmarks’ distribution and alignments (Fig. 15); the fact that most of the
649
pockmarks are located above the inverted main graben; the presence of faulting,
650
diapirism and folding within the unit hosting the pockmarks; and last but not least the
651
periodicity indicated by buried pockmarks inside different stratigraphic levels of this unit.
652
Even though the PCM is located on a typical passive margin since ~65 Ma ago, previous
653
contributions and the evidence discussed in section 5.1 demonstrated that the PCM was
654
affected by Andean tectonic compressive pulses. Its effect is strong enough to generate
655
faults that cross-cut the Cenozoic post-rift strata (Figs. 5 and 10), and condition the
656
development of kilometric sized seafloor features, such as submarine canyons and ridges
657
(Fig. 10; Rosello et al. 2005). Therefore, it seems plausible that an increase in Andean
658
tectonic activity could generate changes in the stress conditions in the pore charged with
659
gas near faults. This would contribute to the increase in pore pressure, triggering the
660
release of fluids from the deep sequences to the Cenozoic strata. In the same way, the
661
inversion of faults will fracture the overlying rocks, creating paths for deep fluids
662
migration.
663
5.4 5.4 Conceptual model, model, timing and tectonic controls on fluid leakage
664
The seismostratigraphic interpretation and the topics discussed above enable the
665
elaboration of a conceptual evolutionary model that explains the release of fluids in the
666
study area since the Eocene-Oligocene boundary to recent times (Fig. 18).
667
The evidence of fluids migration through Unit 1 (Fig. 8, 11 and 12) and the lack of
668
pockmarks inside or in the outcrops of this unit, suggest that when gas migrated for the
669
first time Unit 1 was already consolidated, and the absence of soft sediments did not allow
670
the genesis of pockmarks. This observation led us to infer that during Paleocene and
671
Eocene the plumbing system was not active. Andean activity in Patagonia during this
672
period was characterized by relatively small trench-normal velocities values (<3 cm/yr;
673
Maloney et al., 2013). These values resulted on no-deformation of the broken foreland
674
basin (Gianni et al., 2015). We infer that the lack of tectonic activity during this period in
675
the Patagonian Andes precluded the release of fluids.
676
Maloney et al., (2013) have shown that from Early to Middle Miocene the trench normal
677
convergence velocity was significantly faster, resulting in intensification of Andean
678
tectonic activity, and re-activation and inversion of the distal broken foreland basin
679
(Gianni et al. 2015). This tectonic pulse is appreciable in our MCS, where basement faults
680
cut through units 1 and 2 (Figs. 5, 8 and 10), and in the SBP data where fluidification,
681
folding, diapirism and faulting are identified within Unit 2 (Fig. 7). As previously discussed,
682
this tectonic scenario provides the plumbing system of both, overpressure conditions in
683
pore charged with fluids and pathways for upward migration of fluids. We speculate that
684
during this period considerable amounts of deep fluids were expelled from the syn-rift
685
deposits to the hydrosphere. This assumption is supported by the mega-pockmarks
686
identified within and in the outcrops of Unit 2 (Figs. 4, 7 and 14 B; Muñoz et al. 2013 their
687
Fig. 11B). Even though the release of fluids during this period was widespread in all the
688
study area, pockmarks were only created where Unit 2 was being deposited and soft
689
sediment was present. In deeper areas, hard Eocene rocky outcrops precluded the
690
formation of pockmarks (Fig. 18).
691
The obliquity between the normal re-activated faults and the direction of the stress (~ W-
692
E; Cobbold et al., 2007) resulted in compression-transpression. The faults inverted during
693
this period cut through units 1 and 2 vertically, and created positive ridges, which are an
694
indicator of a high seismic activity.
695
From Mid-Miocene to the present the seafloor of the study area was under the influence
696
of a very dynamic deep ocean circulation (Hernández-Molina et al., 2009). The strong
697
interaction between the seafloor and bottom water masses resulted in elongation, and
698
coalescence of the pockmarks (Fig. 18).
699
From Mid-Miocene to the Holocene, the trench normal convergence velocity diminished
700
to values around 4 cm/yr (Maloney et al. 2013). During this period the tectonic activity in
701
the area was considerably reduced. However, the presence of young circular pockmarks
702
and chimneys affecting Plio-Quaternary units (Fig. 7) indicates that even after the Middle
703
Miocene tectonic climax, the plumbing system was still active. What triggered the Plio-
704
Quaternary release of fluids is beyond the scope of this work, although the occurrence of
705
modern seismicity close to the study area (inset map in Fig. 1), would provide a potential
706
trigger for modern activity. Plio-Quaternary pockmarks and mud volcanoes, also, appear
707
to be related with an inverted hemi-graben (Fig. 8) and aligned and elongated in similar
708
directions than older pockmarks (Fig. 17A). However, the recorded earthquakes close to
709
this area are all of Magnitude <5 in Richter scale and external earthquakes should have a
710
higher magnitude to perturb a distal plumbing system (Bonini et al., 2016; Manga et al.,
711
2009). Also, the fact that young circular pockmarks occur at the theoretical minimum
712
depth of hydrate formation pose gas hydrate dissociation linked to sea level changes as a
713
plausible trigger for the Plio-Quaternary activity of the plumbing system.
714 715
6 Conclusions •
In this study, we present the first integrated analysis of high-resolution multibeam
716
bathymetry, MCS lines and Parasound seismic data which illustrate the relation
717
between gas releasing features and the deep structure and stratigraphy of the
718
PCM.
719
•
The Cenozoic pos-trift record is interrupted by 5 unconformities associated with
720
global scale climatic oceanographic changes, and lies above Mesozoic half-grabens
721
with evidence of Neogene fault reactivation and inversion.
722
•
The morphometrical analysis of mega-pockmarks shows two preferential
723
elongation and alignment directions which in some cases coincide with the
724
azimuth of mapped faults, suggesting faults as the main pathway for upward
725
migration of gas.
726
•
The acoustic evidence of gas in the SBP data indicates that at least part of the fluid
727
involved in the genesis of pockmarks is gas. The gas seems to be thermogenic, and
728
the correlation with deep faults suggests a deep origin associated with over-
729
pressured syn-rift deposits.
730
•
Seismicity linked to Neogene reactivation and inversion of Mesozoic graben faults
731
is the more likely trigger for the release of gas from the syn-rift deposits to the
732
Cenozoic post-rift. This is supported by: the fact that the mega-pockmarks and the
733
compressional structures are hosted in the same seismic unit, the close relation
734
between pockmarks’ distribution and morphometry with faults; the alignment of
735
the geomorphological features with faults; and the periodicity of fluid seepage
736
during Miocene, indicated by the presence of buried pockmarks
737
•
The inversion of faults is consequence of Andean compression. Following our
738
hypothesis, the peaks of Andean activity could modulate the release of gas. This
739
assumption should have two main consequences during enhanced activity of
740
Andean compression: 1) the increased amount of greenhouse gases are released
741
to the hydrosphere, and 2) the increasing failure of seal rocks provoke off-shore
742
basins to expel hydrocarbons.
743
•
We believe that gas hydrate must have some influence on the recent modulation
744
of gas release. Further 3D seismic should be acquired to better constrain the effect
745
of this mechanism.
746
Acknowledgments
747
This work has been made within the framework of a collaboration between YPF
748
technology S.A. (Y-TEC) and the Consejo Nacional de Investigaciones Científicas y Técnicas
749
(CONICET). The authors thank to YPF S.A. for their constant support. The authors are also
750
especially grateful to the Servicio de Hidrografía Naval (SHN; Argentine Hydrographic
751
Survey) and the captain, officials, and crew of the R/V Austral. The Spanish Oceanographic
752
Institute (IEO) is also acknowledged for providing the bathymetric data used in this
753
contribution, and the Secretary of Energy of Argentina is thanked for providing the
754
multichannel seismic data interpreted in this work. Thanks also to María Cerredo for her
755
fruitful comments on a first version of the manuscript. Matías Zylbersztejn is thanked for
756
his assistance on the elaboration of deming regressions. Finally, associated editor
757
Domenico Chiarella, David Iacopini and an anonymous reviewer are thanked for their
758
work and suggestions that really improves the quality of this original manuscript.
759
This contribution is framed within the Pampa Azul project, which is funded by the Ministry
760
of Science Technology and Productive Innovation of Argentina (MINCyT).
761
The seismic package Kindgom Suite® (v. 8.8) was used in this study.
762
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1134
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1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155
1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236
Table 1. Compilation of the Cenozoic seismostratigrpahic schemes published for the MCA and PCM.
1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266
Table 2: Morphological parameters of mega-pockmarks.
1267
1268 1269
Table 3: Reliability assessment system for vent alignments modified from Paulse and Wilson (2010).
1270 1271
Table 4: Calculated alignments for elliptical mega-pockmarks.
1272
1273 1274 1275 1276 1277 1278 1279 1280
Fig. 1. A: Tectonic setting of the western South Atlantic (after Franke et al. 2007; MFZ= Malvinas Fracture Zone and CFZ= Colorado Fracture Zone) and locations of the earthquakes with magnitude >4.4 recorded in western South Atlantic Ocean since 1970 according to IRIS data base (https://www.iris.edu/hq/; blue dots). B: Bathymetric map of the ACM (after Smith and Sandwell, 1997), contour lines every 1000 m (black lines), superficial water circulation on the shelf (thin black arrows), main superficial circulation pattern of the slope and basin of the ACM (thick color arrows), and main sedimentary basins (dash black-outlined grey polygons) of the Patagonian Continental Margin after Ramos and Turic 1996 and Becker et al. 2012; SJGB = San Jorge Gulf Basin; SJB = San Julian Basin; NMB = North Malvinas Basin; VRB = Valdez-Rawson Basin; NENMB = Northern extension of North Malvinas Basin.
1281 1282 1283 1284 1285 1286 1287
1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300
Fig. 2. Main morphosedimentary features of the PCM (after Hernández-Molina et al., 2010 and Muñoz et al., 2013), Fracture Zone in blue dashed lines (after Franke et al., 2007; MFZ: Malvinas Fracture Zone; CFZ: Colorado Fracture Zone) and bottom water masses with white thick arrows (After Piola and Matano 2001; AIW: Antarctic Intermediate Water; UCDW: Upper Circumpolar Deep Water; LCDW: Lower Circumpolar Deep Water; AABC: Antarctic Bottom Current). In the black box the area covered with high resolution bathymetry is indicated.
1301 1302 1303 1304 1305 1306
1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319
Fig. 3. A: Shaded relief map of the study area. Bathymetric contours every 50 m (thin black lines) and every 250 m (thick black lines), pockmarks (white arrows) and main geomorphological features (labels), are shown. Below bathymetric profile across the study area, location is indicated with the thick black line. B: Slope grid of the study area with the location of the seismic profiles and bathymetric details. Location in Fig 2.
1320 1321 1322 1323 1324 1325 1326 1327
Fig. 4. Un-interpreted (A) and interpreted (B) seismic line YMN 95-04 showing the basement (U0), main graben, syn-rift (SR) and post-rift deposits. See location of seismic profile in Fig 3B. Un-interpreted (C) and interpreted (D) seismic line as1800 processed with TecVA (Bulhões and de Amorim, 2005) showing the basement (U0), main graben, syn-rift (SR) and post-rift deposits. See location of seismic profile in Fig 3B
1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351
Fig. 5. Un-interpreted (A) and interpreted (B) 3D cross lines YMN95-06 an YMN95-04 showing the 3D geometry of the northeastern depocenter of the main graben, the graben infill (SR), seismic basement (U0) and cenozoic post-rift units (U1 and U2). See location in Fig. 3B.
1352 1353 1354 1355 1356 1357 1358
Fig. 6. Un-interpreted (A) and interpreted (B) seismic line YMN95-02 showing the seismic basement (U0) and the Cenozoic seismic stratigraphy (U1, U2, U3, and U4) of Nágera and Perito Moreno Terraces following the seismoestratigraphic concept from (Isola et al., 2017). See location of seismic section in Fig. 2.
1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374
Fig. 7. A. Parasound seismic profile across the irregular depressed area indicated in Fig. 3. Several acoustic indicators of gas/fluid mobilizations such as chimneys (CH), acoustic blanking (AB) and diaprisim (D) are indicated B. Parasound seismic profile across Nágera Terrace located at depths ranging between 600 and 750 mbsl. Diapirism (D) inside Unit 2 as well as mud volcanoes (MV) on the seafloor were recognized associated with the CHs and AB. Also, a bottom simulating reflector, possibly associated with the presence of gas-hydrates was recognized. C. Parasound seismic profiles across the irregular depressed area showing several acoustic indicators of gas/fluid mobilizations such as CH, AB and D. Buried pockmarks (BP) inside unit 2 are frequent in the western flank of the irregular depressed area. D. Parasound seismic profile cross-cutting Nágera Terrace. Vertical transpressional faults were identified affecting the Plio-Quaternary record. E. Seismic section located on the boundary between Nágera and Perito Moreno Terraces. A kilometric sized AB is observed in the seismic profile bounded on its top by a high amplitude reflector (ER). F. Seismic reflection profile of Nágera Terrace crossing three mega-pockmarks, AB and CH are frequent, it can also be observed the presence of truncated reflectors (TR) on the mega-pockmark walls indicating the erosive genesis of these features. See locations of parasound seismic profiles in Fig. 3B.
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Fig. 8. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-04 showing the seismic basement (U0), synrift deposits (SR) and Cenozoic seismic units (U1, U2, U3 and U4) across the mud volcanoes area. The graben master fault shows evidence of reactivation and inversion and affects the Cenozoic strata. Diapiric structures are developed inside Unit 2. See location of seismic section in Fig. 3B.
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Fig. 9. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-04 showing Cenozoic seismic units (U1, U2, and U3) across the mud volcanoes area. It can be observed within unit 2 the presence of diapiric structures and liquefaction. Within the diapiric structures high amplitude reflectors with reversed polarity (HARP), enhanced reflectors (ER) and chimneys were identified. See location of seismic profile in Fig. 3B.
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Fig. 10. Un-interpreted (A) and interpreted (B) part of seismic line YMN95-05 showing seismic basement (U0), syn-rift deposits (SR) and Cenozoic seismic units across Perito Moreno Terrace. A ~1 sec TWT thick half-graben is controlled by an inverted normal fault. See location of seismic section in Fig. 3B.
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Fig. 11. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-06 showing syn-rift and Cenozoic seismic units across PMT. Above the fault, dim zones (DZ) and vertical discontinuity zones (VD) are identified within Unit 1. In the boundary between Units 1 and 2 enhanced reflectors (ER) are also recognized. Within Unit 2 pull-up reflectors that might be linked with diapirism (D) are also depicted.
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Fig. 12. Un-interpreted (A) and interpreted (B) section of seismic line YMN95-06 showing syn-rift and Cenozoic seismic units across Perito Moreno Terrace. Above the fault, dim zones (DZ) and vertical discontinuity zones (VD) are identified within Unit 1. In the boundary between Units 1 and 2 enhanced reflectors are also recognized. Inside Unit 2 chimneys (CH) are also depicted.
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Fig. 13. Un-interpreted (A) and interpreted (B) part of seismic line YMN95-03 crossing trough the irregular depressed area showing the seismic basement (U0) and the Cenozoic units (U1, U2, U3 and U4) . Inside the Cenozoic record, acoustic indicators of gas are identified. Enhanced reflectors (ER), Vertical discontinuities zone (VD), gas chimney (GC), high amplitude reflectors with phase reversal (HARPR) are also depicted. See location of seismic section in Fig. 3B.
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Fig. 14: best fit line calculated through orthogonal regression analysis for the 8 alignments recognized on elongated pockmarks.
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Fig. 15. A. Slope grid of the area indicated in Fig.3B. Pockmarks’ name is also indicated. B. Map depicting pockmarks, pockmarks longer axis, canyons’ thalweg, calculated alignments, outcropping/subcropping units, and basement faults recognized in the multi-channel seismic record. The strike of faults 1-4 was determined with lines YMN95-04, YMN95-06 and As1800, while faults 5 and 6 were only recognized on YMN 4 and As1800. C. From left to right: rose diagram of the directions of the main physical settings of the study area, including gradient and ocean currents directions; rose diagram of pockmarks longer axis direction indicated the three groups recognized according to their azimuth; rose diagram of faults and calculated alignments; rose diagram of ridges’ crest; and rose diagram of canyons’ thalweg.
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Fig. 16. Detail of different types of pockmarks imaged in the gradient grid with depth profiles and maximum calculated slope values in the flanks of the pockmarks. A. Elliptical pockmark. B. Sub-circular unit pockmark. C. Coalescent pockmarks. See location of details in Fig. 3B.
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Fig. 17. A. Mud Volcano and pockmarks imaged in the slope grid with rose diagrams of different features. B. Ridge imaged in the slope grid and below the bathymetric cross-section. Location in Fig. 3B.
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Fig. 18. Conceptual evolutionary model for the release of fluids in the study area since the Oligocene to the Holocene.
Highlights • •
Morphometrical analysis of a pockmark field was carried on. Mesozoic inverted grabens were identified beneath and close the pockmark field.
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The inversion of the grabens is associated with an Andean Neogene compressional event. A relation between the grabens’ faults and the elongation and alignments of pockmarks was identified in some cases. Faults have possibly worked as conduits for the upward migration of fluids. The origin of the pockmarks seems to be caused by expulsion of gas. The expulsion of fluids was enhanced during the Miocene, possible caused by the Andean Neogene compressional event. A 3D evolutionary model is proposed to explain timing, pathways, triggers and sources for the release of gas.
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