Late oligocene to quaternary tectonic evolution of the extra andean basins of the pampean plain, Argentina

Journal of South American Earth Sciences 94 (2019) 102207

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Late oligocene to quaternary tectonic evolution of the extra andean basins of the pampean plain, Argentina

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Alicia Folgueraa,∗, Marcelo Zárateb a Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino, Colectora de Avenida General Paz 5445, edificio 25, piso 2, oficina 201, San Martín, Buenos Aires, 1650, Argentina b Instituto de Ciencias de la Tierra y Ambientales de La Pampa, Universidad Nacional de La Pampa-Conicet, Avenida Uruguay 151, 6300, Santa Rosa, La Pampa, Argentina

A R T I C LE I N FO

A B S T R A C T

Keywords: Basin analysis Cenozoic Dynamic topography Extra-andean foreland Pampean plain

The extra-Andean Pampean plain of central Argentina shows a complex evolution history. Three basins were identified in the subsurface (Salado, Macachín and Laboulaye) generated during the opening of the South Atlantic Ocean in the Mesozoic with a continuous subsidence history from the Oligocene to the Quaternary. The purpose of this contribution is to analyze the Cenozoic sedimentary filling of the Pampean plain basins with the goal of interpreting the tectonic evolution and understand the subsidence history. The study was based on the correlation and integration of subsurface data. Isopach maps were constructed from boreholes logs for each stratigraphic unit to infer depositional patterns, temporal and spatial thickness distributions, and possible depocenter migrations. The data was integrated in order to analyze the subsidence history of the basins and evaluate the potential mechanisms of post-rift tectonic reactivation of the passive margin. The Cenozoic sedimentary record was divided into three intervals for purposes of analysis with contrasting distribution and mechanisms. The late Oligocene-early Miocene (∼27-19 My) interval started before and continued during the earliest stage of the Andean orogeny; the second interval (early Miocene-middle Miocene 1915 My) that coexisted with the initial stages of Andean uplift and the third interval (middle Miocene-Quaternary, 15 My-Quaternary) developed during the last stages of the Andean orogeny. The subsidence during the late Oligocene-early Miocene was geographically restricted to the pre-existent basins, so an extensional reactivation of the Mesozoic depocenters is proposed. It is considered to be associated with an Oligocene generalized extension responsible for basin development before the beginning of the Andean orogeny, which later shifted into transpressional conditions. The early Miocene sedimentation (19-15 My) is here interpreted as the result of subsidence during the beginning of the Andean orogeny due to a less common but lengthy process that may lead to foreland rifting or transtension in the three analyzed basins. The middle Miocene-Quaternary (15 My-Present) interval is characterized by a blanket-like subsidence resulting in monotonous thicknesses with reduced-scale accumulation, both in the basins and over the relative highs. This sedimentation is interpreted as the result of accumulation in an extensive distal foreland basin with the foredeep located in the Alvear basin, the forebulge in the Pampa Central block, and the backbulge in the Laboulaye, Macachín and Salado basins. Located far east from the Andean tectonic loads, this distal foreland basin cannot be solely explained by short-wave subsidence patterns. Thus, dynamic subsidence is here proposed as an additional mechanism.

1. Introduction The distal extra-Andean region of central Argentina, the Pampean plain, (∼34-38°S, 67-57° W) has been usually regarded as a monotonous and homogeneous domain (Fig. 1). The general knowledge of its geological framework began with the peopling of the territory. The

∗

Dirección General de Minas e Hidrogeología created in the early 20th century started a regional geological and hydrogeological survey. Hence, the subsurface of the Pampean plain was surveyed for both hydrogeological purposes (Windhausen, 1931) and hydrocarbons exploration through several techniques, including drilling, which led to the identification and description of the main stratigraphic and

Corresponding author. Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino. SEGEMAR, 1405, Buenos Aires, Argentina. E-mail addresses: [email protected], [email protected] (A. Folguera), [email protected] (M. Zárate).

https://doi.org/10.1016/j.jsames.2019.05.023 Received 14 December 2018; Received in revised form 17 May 2019; Accepted 17 May 2019 Available online 25 May 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. The central region of Argentina and surrounding areas and the morphostructural units.

basins linked to the Andean dynamics? Is there a correlation of major episodes/events in the Andes and the different basin stages at the Pampean plain? What was/were the mechanism/s involved in the basins subsidence? Then, the main purpose of this contribution is to analyze the Cenozoic sedimentary filling of the Pampean plain basins, interpreting their tectonic evolution in relation to the Andean development, and understanding their subsidence history.

structural features, mainly in the offshore sector. Far from being a vast area with a common origin, the Pampean plain shows a considerable subsurface complexity in terms of its history and evolution (Chebli et al., 1999). A remarkable feature is the occurrence of a series of basins (Salado, Macachín, and Laboulaye) and highs (blocks) (Fig. 1). Concerning this, Ramos (1996) stated that the crustal heterogeneity is responsible for the complex evolution of the Salado basin, while the location of the Macachín and Laboulaye basins are related particularly to the suture of the Córdoba terrane during the Pampia and the Rio de la Plata cratons collision (Ramos, 1988). Hence, a homogeneous lithospheric behavior should not be expected in a place where the substrate is highly heterogeneous (Jordan, 1995). While the Paleozoic-Mesozoic record of the basins was extensively analyzed and discussed, the Cenozoic filling, particularly during the Neogene-Quaternary, was the object of general stratigraphic-sedimentological descriptions (lithology, depositional environments). Consequently, the possible causes and factors controlling the sedimentary dynamics during this time interval were not fully considered. The Cenozoic record of the Salado, Macachín and Laboulaye basins consisting of continental sedimentary sequences with interbedded marine deposits is characterized by an almost continuous history of subsidence (Chebli et al., 1999), explained as the result of the evolution of rift basins generated by the Atlantic opening during the passive margin stage (Tavella and Wright, 1996). Historically, the sedimentary filling was regarded as affected by lithospheric thinning and normal faulting initiated in the Jurassic, without evidence of subsequent contractional tectonics (Chebli et al., 1999). Passive margins have been traditionally considered stable with tectonically induced subsidence at an initial stage, and the influence of younger thermal adjustments (McKenzie, 1978). Nevertheless, Cobbold et al. (2001) demonstrated that tectonics has been an active process in South American passive margin through most of the Andean cycle. In this regard, deformation generated by passive margin tectonics has not been previously analyzed in Northern Patagonia. Moreover, the Cenozoic tectonic evolution of the Pampean plain basins has not been interpreted in a regionally integrated geological context that considers the Andean dynamics and the opening of the Atlantic Ocean. So, this work is structured through the following questions: Is the Oligocene-Quaternary evolution of the distal

2. Methodology Given that a great number of the stratigraphic units described in the Pampean plain basins are known from oil and groundwater boreholes, our analysis was based on the correlation and integration of subsurface data. Isopach maps were constructed from boreholes logs for each stratigraphic unit of the Salado and Macachin basins; the purpose was to infer depositional patterns, temporal and spatial thickness distributions, and possible depocenter migrations. The isopach maps of the Olivos Fm, Paraná Fm, and Puelches Fm were derived from outcrop measurements and borehole data (YPF, Ministerio de Agricultura, Ministerio de Industria y Comercio de la Nación, Ministerio de Obras Públicas, Secretaría de Industria, Administración Provincial del Agua de La Pampa). On the basis of the compiled information, basins geometry was reconstructed and interpreted. Finally, the data was integrated in order to analyze the subsidence history of the basins and to evaluate the potential mechanisms of post-rift tectonic reactivation of the passive margin. A stratigraphic chart summarizing previous schemes of several authors was included to better understand the stratigraphic relationships and correlation between the analyzed basins and the neighboring areas (Fig. 2). 3. Geological setting and sedimentary record of the basins 3.1. Salado basin The Salado basin is developed along a NW-SE axis. It is limited by the Polonio high northwards, which acts as a boundary with the Pelotas basin, and the Tandil high southwards (Raggio et al., 2012) that 2

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Fig. 2. Integrated stratigraphic chart. Based on Forasiepi et al. (2014); Marengo (2006); Bravard (1858); Yrigoyen (1975); Bracaccini (1980). The Ensenada and Buenos Aires Fms by Riggi et al. (1986) corresponds to the upper 28 m of the Pampeano Fm in the area of La Plata.

thickness increases from Buenos Aires city surroundings (223 m) towards the basin axis (800 m). Marengo (2006) proposed a different stratigraphic scheme for the Salado basin, considering the Olivos and Chaco Fms as equivalent units, and defining the Litoral Group, comprising the Chaco Fm (Russo et al., 1979), the Laguna Paiva Fm (Stappenbeck, 1926), and the Paraná Fm (Bravard, 1858); the Chaco Fm involves most of the Litoral Group thickness. The Chaco Fm is characterized by two continental wedges, the Palermo Member and the San Francisco Member. The older Palermo Member underlies the Laguna Paiva Fm, in an apparent concordant stratigraphic relation. It is composed of whitish-brownish silty-shaly sandstones with frequent carbonate concretions, gypsum crystals and abundant volcanic ashes; the mineral assemblage is dominated by volcaniclastic material (Marengo, 2006). The early Miocene San Francisco Member (Marengo, 2006), interbedded between the Laguna Paiva Fm and the Paraná Fm, consists of finer grained sediments dominated by brownish sandy shales with a mineral composition similar to the Palermo Member, and high content of gypsum and carbonate concretions. Marengo (2006) correlated the Laguna Paiva Fm with the early Miocene “Patagoniense” (among other names), a marine unit reported in several sedimentary basins of Patagonia (Cuitiño et al., 2015). On the basis of the information obtained from boreholes drilled by Gas del Estado, a stratigraphic log was sketched out after Bracaccini (1980) comprising the Cenozoic sequence in Conesa (Salado basin) (Fig. 3). The Olivos Fm, deposited on top of Cretaceous basaltic rocks, can be subdivided into three intervals. The lowest interval consists of loosely compacted fine sands with largely hyaline, well rounded but poorly sorted grains (Bracaccini, 1980). At the top, evaporites up to 1.5 m thick are present dominated by gypsum and anhydrite, in turn covered by pinkish siltstones. The second interval consists of whitish quartzitic sandstones with conglomeratic sand levels at the base, and gypsum and anhydrite intercalations; a shaly level occurs at the top. The third interval is made up of poorly consolidated whitish quartzitic

represents the NE part of the Positivo Bonaerense (Buenos Aires High) (Fig. 1). The origin of the basin is related to the Gondwana breakup and the opening of the South Atlantic Ocean in the Mesozoic. The northern sector of the Argentinean continental shelf, where the Salado basin extends, was identified as a passive margin characterized by structures parallel to NW-SE sutures of Proterozoic age (Ramos, 1996). In this regard, the fabric of deformation of the Río de la Plata Craton, as well as the structures of the Salado and Punta Del Este basins are similar (Raggio et al., 2012). The sedimentary filling of the basin (7000 m thick) consists of three main sections that correlate with the most important changes that occurred in the South Atlantic region (Welsink, 2010). The late Jurassicearly Cretaceous rift section is dominated by a thick succession (> 1200 m) of red fluvial conglomerates and sandstones (Ucha et al., 2004). The Aptian-Campanian post rift-sag stage records the beginning of a fluvial environment consisting of sands with lacustrine levels towards the base. The filling of the basin continues throughout the Cenozoic until the present with interbedded continental and marine deposits (> 1600 m); it was interpreted as a passive margin stage due to the expansion of the South Atlantic ocean floor (Raggio et al., 2012). The Cenozoic sedimentary record starts with deposits formally known as Olivos Fm (Groeber, l96l) or Mioceno Rojo (Groeber, 1945), of late Oligocene-Miocene age (Yrigoyen, 1975) which includes two main sections (see stratigraphic chart for stratigraphic correlation and equivalence with other stratigraphic units, Fig. 2). The dominant reddish sandy lower section includes a basal conglomerate with sandy-silty matrix and abundant gypsum; the upper section is shaly (Auge, 2004), mostly comprising pinkish and whitish, coarse grained quartzitic sands partly conglomeratic with quartzite clasts (Bracaccini, 1980). The depositional environment is continental with the upper section dominated by aeolian, and lacustrine/fluvial processes (Auge, 2004). The abundance of gypsum throughout the column is interpreted as the result of strong arid conditions during the deposition (Auge, 2004). The 3

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sandstones with calcareous cement, and very thin interfingered levels of shales. The Paraná Fm unconformably overlies the Olivos Fm. It comprises a marine succession with a maximum thickness of 600 m, and up to 100 m in the central sector of the Pampean plain. Auge (2004) reported an erosive unconformity between both units in the area of Buenos Aires city; in marginal areas of the basin, the stratigraphic contact with the underlying units is a transitional paraconcordance (Carretero, 2010). Recently, Del Río et al. (2018) performed Sr datings in marine deposits of the Paraná Fm, bracketing the Paraná Fm between 11.9 and 6 My (mid-late Miocene). According to these ages, the upper part of the Paraná Fm is synchronic with the late Miocene (∼10–6.8 My) Andean volcanoclastic Cerro Azul Fm (Visconti, 2007). The basin was filled with Andean continental aeolian deposits when the regression of the Paraná Fm occurred. North of the Tandilia range, the Paraná Fm is covered (subtle angular unconformity) by quartz sand deposits identified as Arenas Puelches or Puelches Fm (Bracaccini, 1980). The deposits are dominantly composed of poorly consolidated, yellowish to whitish, medium and fine quartz sands with graded stratification; the grain size increases towards the base, where coarse sands and gravels are dominant (Santa Cruz, 1972). According to this author the sands were deposited by a NNE-SSW fluvial channels system transporting sediments derived from the Brazilian craton. The sedimentary environment of the Puelches Fm was interpreted as a late Pliocene-early Pleistocene prodelta, localized to the SW of the present Paraná River delta (Auge, 2004). A tentative age of ∼3.2 Ma has been also assigned to the unit (Zárate, 2005 and references therein). The Puelches Fm is covered by the Pampeano Fm (late Pliocene/ early Pleistocene?-mid/late Pleistocene) and the Late PleistoceneHolocene sedimentary units (Zárate, 2005 and references therein). The Pampeano Fm, also referred to as the Pampean sediments, includes the Preensenadense, Ensenadense and Bonaerense units sensu Ameghino (1889), comprising a thickness of approximately 30 m in the area of Buenos Aires city, and up to 60 m in the Salado basin. According to Gonzaléz Bonorino (1965), the Puelches/Pampeano stratigraphic contact is an abrupt to transitional boundary without a sedimentary hiatus. Considering the mineralogical composition, the Pampeano Formation was divided into two stratigraphic sections by Gonzaléz Bonorino (1965). The lower section is characterized by the abundance of quartz, montmorillonite associated with kaolinite, and almost none volcanic glass. The upper section is made up of abundant illite and plagioclase in the sand/silt fraction. Gonzaléz Bonorino (1965) stated that the sudden mineralogical passage results from the replacement of the former Paraná drainage by the Pampean drainage, roughly coinciding, according to the author, with the stratigraphic boundary between the Preensenadense/Ensenadense (sensu Ameghino, 1889). The composition of the Ensenadense is volcanoclastic derived from Andean sources (e.g. Riggi et al., 1986) consisting of loessial sediments modified by pedogenesis with levels of paleosols and carbonate accumulations. 3.2. Macachín basin The Macachín basin is located in the central western part of the Pampean plain (∼35°30′-38° S; 63-64° W) with a NNW-SSE axial trend, and long and narrow basin geometry. The sedimentary filling ranges from the late Cretaceous to the Neogene, covered by Quaternary aeolian deposits. De Elorriaga and Camilletti (1999) interpreted the basin as a hemi graben of a rift system developed on continental crust, and generated by an E-W extensional strain related to the opening of the Atlantic Ocean. According to the authors, the interpretation of seismic sections allows differentiating three seismostratigraphic units: the prerift stage including granites, micaceous gneisses, quartzites and quartzitic sandstones of probable Precambrian and Paleozoic age; the syn-rift stage (Cretaceous sediments) composed of up to 4000 m of siltstones and fine sandstones with shaly interbedded levels in the southern sector

Fig. 3. Schematic profile of the Cenozoic sequence in the San Nicolás, Conesa and Pergamino areas.

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of sedimentation were found between the Salado and Macachin-Laboulaye basins. The deposition of the Olivos Fm occurred under subsidence conditions that reactivated the Salado basin allowing the accumulation of a remarkable sedimentary thickness (800 m), as well as the Macachín and Laboulaye basins, although at a much-reduced scale according to the lower sedimentary thicknesses recorded. Several mechanisms can be proposed to generate the subsidence of the basins. At a regional scale, northwards of the study area at the latitude of the Pampean ranges (29°-31°S), the Litoral Group (equivalent to Olivos and Paraná Fms) was interpreted as filling a distal foreland basin associated with the Andean orogeny (Astini et al., 2014). Dávila et al. (2010) analyzed the Pampean foreland along the modern flat-slab segment of the southern Central Andes between 31° and 33°S, and concluded that the magnitude and distribution of the sedimentary record cannot be solved with flexural models, but to longer wavelength driven by the flat subduction dynamics. In the Patagonian Andes, Encinas et al. (2017) inferred that an extensional regional event along with crustal thinning (∼26-23 My) related to the reorganization of southeastern Pacific plates, was likely involved in the subsidence mechanisms that favored the early Miocene Patagoniense transgression. This event coincided with a major tectonic reconfiguration of the Andean margin occurred in the late Oligocene (about 25 Ma) by the breakup of the Farallón Plate, generating a change in convergence to an orthogonal direction (Pardo Casas and Molnar, 1987; Somoza, 1998; Somoza and Ghidella, 2012). The southern Central Andes (36°S) were under extension at least since 27 My ago (Folguera et al., 2010, Álvarez Cerimedo et al., 2013). In this context, the Palauco Fm, situated in the Principal Cordillera of Mendoza and northern Neuquén was interpreted as an Oligocene-Miocene extensional basin developed in the zone comprehended between the Andean foreland and the orogenic front (Suárez and Emparán, 1995; Radic, 2010). This basin is partially coetaneous with the Cura Mallín basin, located within the orogenic belt at the same latitudes. In this regional context, we propose that the late Oligocene-early Miocene sedimentation (27-20 My) of the Salado, Macachin and Laboulaye basins was related to the same generalized extension episode recorded from the central Argentinean-Chilean Andes to the foreland region (Jordan et al., 2001; Burns et al., 2006). The early Miocene sedimentation (19-15 My) is here interpreted as the result of subsidence during the beginning of the Andean orogeny due to a less common but lengthy process that may have led to foreland rifting or transtension in the three analyzed basins. Marine sedimentation dominates including isolated continental episodes with a main supply coming from the Brazilian craton (Marengo, 2006; Bracaccini, 1980). In this regard, Molnar and Tapponnier (1975) and Sengör (1976) proposed that in a collisional context, continental indentation could transmit far-field stresses that could drive foreland extension and or transtension at a high angle in relation to the orogenic front. These intraplate rift basins induced by continental accretions are defined as impactogenes (Sengör, 1976). More recently, Gianni et al. (2015) considered that Andean-type margins could also induce this mechanism. In subduction settings, changes in plate convergence (velocity and direction) and local modifications of slab geometry (shallowing) seem to be effective triggers to induce discrete amounts of foreland rifting and transtension (Gianni et al., 2015). The sub-parallel orientation of the basins in relation to the imposed contractional stressfield, along with an initial lateral unconfined state, may represent a key factor to could facilitate their extensional reactivation (Gianni et al., 2015). In addition, Cogné et al. (2013) interpreted the Taubaté basin, located in the southeast of Brazil (22°30’-23°S) as the result of Andean Paleogene compression affecting transtensionally the pre-existent basement fabric. The 15 My-Quaternary sedimentation developed under a compressive regime during the Andean orogeny. Its record begins with marine deposits corresponding to a generalized transgression, grading to continental environments towards the Pliocene that continued until the

of the basin (Salso, 1966; Zambrano, 1974); and according to De Elorriaga and Camilletti (1999) a post Cretaceous tectonic inversion attributed to the Andean phase associated with gentle folding of the Cretaceous sediments in the proximity of faults. This third seismostratigrahic unit in fact involves the Oligocene Olivos Fm, not included for this time frame by De Elorriaga and Camilletti (1999). Salso (1966) identified a 200 m thick sequence assigned to the Oligocene, composed of conglomeratic sandstones cemented with anhydrite. Correlative whitish quartzitic conglomeratic sandstones are reported at the > 300 m Maza N°1 borehole (Adolfo Alsina, Buenos Aires province) (Fig. 1). Gypsum and anhydrite are present throughout the stratigraphic column derived from ground water with high salt and carbonate concentration (De Elorriaga, 2010). The occurrence of similar levels was reported at Epecuén by Chebli et al. (1999). The Paraná Fm is also present in the Macachin basin, overlying the continental deposits of the Olivos Fm. According to Salso (1966), the unit is covered by a 290 m thick succession described as ’Pampeano’, including Neogene and Quaternary sediments, dominantly composed of silts and sands with frequent calcium carbonate accumulations (tosca) in the uppermost part. 3.3. Laboulaye basin The Laboulaye basin recognized through seismic information and boreholes data (Padula, 1972), and later by some other geophysical analyses (Kostadinoff et al., 2001), is situated in the western central part of the Pampean plain, north of the Macachín basin (Fig. 1). It is made up of a series of connected asymmetrical rifts that coincide with the suture of the Río de la Plata and the Pampia cratons, noticeable by the change in the magnetic fabric (Chernicoff et al., 2009). Leal et al. (2003) inferred the location of the NE trend suture between both terrains in the proximity of the western limit of Buenos Aires province, being the southern extension of the Transbrazilian lineament. Kostadinoff et al. (2001) proposed the continuity of the Laboulaye depocenter in the Macachín basin. Sedimentary and volcanic rocks tentatively assigned to the Triassic-Cretaceous (Laboulaye Fm) were reached by boreholes below a 200 m thick Cenozoic cover. Paleozoic sedimentary rocks are reported below 500 m (Zambrano, 1980, Gregori et al., 2009). On the basis of boreholes data (Guardia Vieja, Laboulaye localities), Yrigoyen (1975) interpreted the occurrence of the Olivos Fm with a thickness varying from 96 up to 200 m. The Olivos Fm overlies marine sediments (Mariano Boedo sensu Yrigoyen, 1975; Laguna Paiva sensu Marengo, 2006), and is covered by 26 m of the Paraná Fm. 4. Basin analysis and tectonic evolution The sedimentary record of the basins can be subdivided into three intervals (∼27-19 My, 19-15 My and ∼15 My -Quaternary) for the purposes of analysis. The sedimentation during the 27-19 My interval which begins under continental conditions and ends with marine environments, takes place before and during the Andean orogeny. This interval is made up of deposits grouped into the Olivos Fm; in the Salado basin it rests on Cretaceous-Paleogene marine deposits exceeding the limits of the Mesozoic sedimentation. Further South (Arroyo del Azul basin, Buenos Aires province, Fig. 1) these deposits overlie lower Paleozoic rocks (Carretero, 2010). The composition of the Olivos Fm suggests a source from the southern Brazil craton (Santa Cruz, 1972), by a drainage system (paleo-fluvial system of the Paraná-Uruguay rivers) that transported and accumulated sediments at the end of the Oligocene with a secondary supply of Andean volcanism, and a minor contribution from the Tandilia range. The Olivos Fm isopach map (Fig. 4) suggests a wide depocenter (thickness of up to 800 m) during the Oligocene-mid Miocene with the axis in the Salado Basin through the localities of Dolores-General Madariaga (Buenos Aires province). In addition, up to 200 m of sediments were accumulated in the Macachin and Laboulaye basins. No evidences 5

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Fig. 4. Isopach map of the Olivos Fm. The location and name of the boreholes are indicated in the map.

(late Miocene) diastrophic phases. This marine regression was synchronic with the beginning of volcaniclastic continental sedimentation (Cerro Azul Fm, 200 m thick) accumulated across the highest topographic areas (Pampa Central Block, Positivo Bonaerense) (Fig. 6). The depocenter of the Puelches Fm (average thickness of ∼26 m) is situated along the Dolores-General Madariaga, with a NW-SE axis through the Salado basin, reaching a thickness of 75 m (Santa Cruz, 1972) (Fig. 7). A similar thickness is also recorded in the area of Chivilcoy and Saladillo. The Puelches Fm pinches out at the NE margin of the Positivo Bonaerense. The subsidence during the Pliocene is much more reduced in comparison with the Oligocene-Miocene subsidence stage as it is suggested by a sediment thickness of a few dozens of meters. While the source area of the Puelches Fm is the Brazilian craton (Santa Cruz, 1972), most of the sediments of the Pampean Fm, and the late Pleistocene-Holocene units are of Andean origin. The compressive Andean regime at these latitudes is interpreted as the result of low angle subduction that generated the last deformation

present. The isopach map of the Paraná Fm (Fig. 5) indicates a depocenter in the Salado basin with a thickness of up to 600 m that decreases to the west, towards La Pampa Central block. Southwards, the Positivo Bonaerense including the Tandilia range constituted a positive area during the deposition of the Paraná Fm. Hernández et al. (2005) considered that the marine ingression of the Paraná Fm, between 15 and 13 My, was the result of tectonics and eustatic processes across ancient accommodation zones of the South American plate during a sea level high stand. At the same time, the eastern migration of the Andean magmatic arc resulting from the Miocene shallowing of the oceanic plate through the Pampean segment (Jordan et al. 1983) promoted a subsidence acceleration favoring the marine transgression in the basins where the maximum thicknesses of the Paraná Fm are found. Later, Astini et al. (2014) concluded that marine ingressions were not only a product of high eustatic sea levels but also were favored by other global events. Accordingly, they hypothesized that dynamic subsidence stages could have been involved associated with peak plate convergence rates, related to the Pehuenche (Oligocene-middle Miocene) and Quechua 6

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Fig. 5. Isopach map of the Paraná Fm The location and name of the boreholes are indicated in the map.

Salado, Macachin and Laboulaye basins, a reinterpretation of the basin's geometry is plausible. The foredeep is therefore inferred to be located in the San Rafael block and the Alvear basin, dominated by alluvial fan and braided river deposits (e.g. Aisol Fm). In the San Rafael block, continental Neogene sedimentation began circa 19 My (Friasean stageage) (Aisol Fm; Forasiepi et al., 2014). Recently, the lower section of this unit was assigned to the middle Miocene (Friasean or Colloncurean stage-age) and the middle section to the late Miocene (Huayquerian) (Forasiepi et al., 2014) on the basis of vertebrate fossils and radiometric datings. On the other hand, the forebulge is interpreted to be located in La Pampa Central block (Nivière et al., 2013; Folguera and Zárate, 2018) characterized by a low subsidence rate, with a sediment thickness of ∼200 m. Fine grained subparallel sequences dominate including paleosol levels that indicate an episodic sedimentation process. The backbulge, depocenter located between the forebulge and the craton comprises the Salado, Macachín and Laboulaye basins, and involves the sedimentary record of the Paraná Fm, the Puelches Fm, the Pampean Fm, and the late Pleistocene-Holocene sedimentary units.

stage of the Malargüe fold and thrust belt and the exhumation of the Bloque de San Rafael to the east (San Rafael block) (Ramos and Folguera, 2005; Ramos and Kay, 2006). A later extensional event related to rollback of the subducted slab was documented by Ramos and Folguera (2005), that explains profuse Quaternary mafic eruptions in the Payenia area (Ramos and Kay, 2006; Ramos and Folguera, 2005; Burd et al., 2008). The 15 My-Quaternary sedimentation is interpreted as the accumulation in an extensive distal foreland basin. Chase et al. (2009) based in the South America geoid anomaly postulated the existence of a flexural forebulge with an amplitude of up to 500 m or even less, located 300–500 km away from the Andean orogenic front. On this subject, Folguera (2011) proposed a preliminary sedimentary model at 37°S considering the basin geometry, sedimentary facies, ages and sequence thicknesses. Later on, a compatible model with a flexural geometry of the lithosphere in the La Pampa High (37°S-65°W) was presented by Nivière et al. (2013). However, if considering the sedimentary facies, ages and thickness of the sequences hosted in the 7

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Fig. 6. Isopach map of the Paraná Fm regression and the Cerro Azul Fm. The location and name of the boreholes are indicated in the map.

orogenic loading and dynamic subsidence, taking place at different scales. Cavallotto and Violante (2005) described evidences of Pliocene deformation in the Río de La Plata. Similarly, a deformation stage has been inferred at ∼3 My in the Positivo Bonaerense (Folguera et al. 2014). This structuring event could have reactivated old weakness lines (fractures), such as the Transbrazilian lineament and other minor structures of the Río de la Plata, and the Salado basins. This late deformations in the foreland zone could be related to compressive stresses transmitted from the subduction zone at these latitudes (e.g., the collision at 3.6 My of the Mocha plateau; Folguera and Ramos, 2009). In the light of these studies, it is interpreted that the Umbral de Martin García (Martin García High) (Cingolani, 2005) which is the northeastern border of the Salado basin, could have been uplifted or reactivated in this stage (∼3 My), constituting since then a natural barrier to the Brazilian fluvial system (paleo Paraná-Uruguay rivers). Additionally, other horizontal tectonic forces need to be incorporated into the analysis to understand distal deformations in these

Catuneanu (2004) proposed that the relation between the rates of dynamic loading and flexural loading is the factor controlling the base level, and the stratigraphic architecture in the foreland basins. Flament et al. (2015) analyzed the Miocene topographic evolution of South America, concluding that the observed subsidence cannot be explained only by lithospheric deformation, suggesting the combined effects of mantle fluxes and flexure to explain shallow marine transgressions. Analyzing the subsidence mechanisms acting in the mid MioceneQuaternary time period, it is evident that the geometry of the deposits as well as their distribution and sedimentary thicknesses of hundreds of meters cannot be solely explained by tectonic loadings. DeCelles and Giles (1996) suggested that thicknesses of hundreds of meters accumulated in backbulge areas are greater than those expected if flexural subsidence were the only acting mechanism. Therefore, other mechanisms need to be considered such as dynamic subsidence (Pardo et al., 2002). Following this interpretation, we propose that since the late Miocene the accommodation space in the Salado, Macachín and Laboulaye basins, was generated by flexural tectonism related to both 8

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Fig. 7. Isopaquic map of the Puelches Fm (modified from Auge, 2004). The location and name of the boreholes are indicated in the map.

extensional reactivation of the Mesozoic rift systems. In the middle Miocene-Pliocene dynamic subsidence was prevalent, resulting in sediment accumulation not only at the basins but over the relative highs with continuous thicknesses, producing a generalized, relatively homogeneous, small scale subsidence (Fig. 8). During this stage, the synorogenic sedimentation associated with the Andean uplift started about 19 My ago. A major geomorphological change in the fluvial systems reaching the Pampean plain (Paraná and Uruguay system) by ∼3 My is interpreted as a response to a mild reactivation of some topographic barriers in the passive margin. Although geochronological dating is still lacking in order to constraint this event, the general coincidence of this episode of deformation in the study region with the Diaguita deformational phase in the Andes (Yrigoyen, 1976, 1979) led us to suggest that they might have a common origin. Finally, it must be emphasized that the extra Andean basins of the Pampean plain since the early Miocene were highly sensitive to the stress fields imposed from the Andean subduction zone and the mid-Atlantic ridge (Brunetto et al. 2017).

foreland basins, such as the Atlantic ridge push, which may trigger reactivations in passive margins (Cloetingh et al., 1990; Ransome and de Wit, 1992; Boldreel and Andersen, 1998; Hudec and Jackson, 2002; Mosar et al., 2002). In this regard, geological evidences of tectonic postrift reactivation in onshore Cenozoic basins of the continental margin of southeast Brazil were reported and attributed to compression in the South American plate generated by the Andean subduction zone and the mid-Atlantic ridge-push (Cogné et al., 2011).

5. Conclusions Subsidence was continuous, although with different magnitude, in the Salado, Macachin and Laboulaye basins of the Pampean plain since the late Oligocene to the present. The Macachin and Laboulaye basins structurally controlled by the Rio de la Plata/Pampia suture suffered contrastingly milder reactivations than the Salado basin. The late Oligocene-early Miocene subsidence stage gave way to sedimentation concentrated in preexisting basins basically as a result of the 9

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Fig. 8. Extra-Andean foreland basin evolution proposed.

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Acknowledgments

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