Development of the Middle Triassic Kobbe Formation shelf-margin prism and transgressive-regressive cycles on the shelf (Hammerfest Basin, SW Barents Sea)

Marine and Petroleum Geology 111 (2020) 868–885

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Research paper

Development of the Middle Triassic Kobbe Formation shelf-margin prism and transgressive-regressive cycles on the shelf (Hammerfest Basin, SW Barents Sea)

T

Valentina Marzia Rossia,∗, Snorre Olaussenb, Ivan Nicola Stainec, Matteo Gennaroc a

Department of Earth Science, University of Bergen, Allégaten 41, 5007 Bergen, Norway UNIS, The University Centre in Svalbard (UNIS), N-9171 Longyearbyen, Norway c Vår Energi, Vestre Svanholmen, 4313 Sandnes, Norway b

ARTICLE INFO

ABSTRACT

Keywords: Shelf margin Triassic Barents Sea Kobbe Formation Trangressive deposits Estuarine valley fill Subsidence variability Troms-Finnmark Fault Complex

The development and evolution of the middle Triassic shelf-margin prism of the southwestern Barents Sea has been so far under-researched. In particular, no attention has been paid to the thick transgressive packages developed at the top of the clastic prism. This study focuses on the Anisian-Early Ladinian Kobbe Formation, and it is based on a seismic and well log dataset from the Hammerfest Basin and adjacent Loppa High, as well as on a process-based investigation of available cores. 2D seismic lines show the development of shelf margin clinoforms up to 300 m thick, and shelf-edge trajectory analysis shows intervals with flat trajectories and intervals with ascending trajectories. The Kobbe Formation shelfmargin prism grew through repeated cross-shelf (regressive-transgressive) transits of deltaic systems. Mapping of the shelf-edge locations shows that the shelf-margin prism prograded ca. 30–50 Km in a north-westward direction, and localized protrusions of the shelf edges at different times can indicate cross-shelf sediment supply fairways. The farthest basinward extent of the sedimentary prism appears to have been controlled, at least in part, by the topographic relief of the paleo-Loppa High, as the bottomsets pinch out against this synsedimentary structural high. On the shelf, during regressive phases, the shoreline prograded from the south and from the east-southeast. This work recognizes, for the first time, the presence of thick transgressive packages within the Kobbe Formation. The transgressive sequence at the top of the Kobbe Formation is up to 50 m thick, and it is interpreted as stacked estuarine valley fills close to the Troms-Finnmark fault complex. In the Goliat area, the presence of an expanded sedimentary sequence, compared to adjacent areas, suggests a tectonic control in this area. Tectonic activity may have also influenced the development of stacked estuarine valleys in the same location, therefore controlling sand storage on the shelf.

1. Introduction The Barents Sea is a frontier basin with areas, like the Hammerfest Basin, of proven hydrocarbon potential. Even though hydrocarbon exploration focused on Triassic and Jurassic sediments started in 1980's (Olaussen et al., 2010; Paterson and Mangerud, 2017), there are still several aspects regarding the depositional history and characteristics of the sedimentary systems that are poorly understood. During the Triassic, the Barents Sea was an intra-cratonic sea located at a paleolatitude of approximately 44°–49° (Cocks and Torsvik, 2006; Klausen et al., 2015). At this time, a series of shelf margin clinoforms infilled the basin, prograding mainly from the east and south-east, and from the south, fed respectively by the Uralide Orogen and by the Fennoscandian

∗

Shield (Eide et al., 2017; Fleming et al., 2016; Glørstad-Clark et al., 2010; Klausen et al., 2015). The interaction between these two systems is of particular interest, since the Caledonian source (derived from the south) is characterized by much better reservoir qualities than the Uralian source (Fleming et al., 2016). In shelf-margin successions, the shelf-margin clinoforms are of a much larger scale than the shoreline systems developed on the shelf (100s of meters vs 10s of meters). On the topsets of the shelf-margin clinoforms, shoreline, coastal plain, estuarine and shelf environments are present, whereas on the foresets and bottomsets deep-water slope and basin floor environments are present (Helland-Hansen and Hampson, 2009; Johannessen and Steel, 2005; Olariu and Steel, 2009; Patruno and Helland-Hansen, 2018).

Corresponding author. Current address: Department of Earth and Environmental Science, University of Pavia, Via Adolfo Ferrata, 1, I-27100, Pavia, Italy. E-mail address: [email protected] (V.M. Rossi).

https://doi.org/10.1016/j.marpetgeo.2019.08.043 Received 17 April 2019; Received in revised form 26 July 2019; Accepted 22 August 2019 Available online 26 August 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Stratigraphy of the Triassic succession in the southwestern Barents Sea (based on Mørk et al., 1999; Gradstein et al., 2012; Vigran et al., 2014; Rossi et al., 2019) and bathymetric chart of the Arctic Ocean; yellow polygon highlights the study area (map from Jakobsson et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

This work focuses on the Middle Triassic Kobbe Formation (Fig. 1), and it is based on a seismic, well log and core dataset. We used this subsurface dataset to (1) characterize the growth of the Kobbe Formation shelf margin prism in the southwestern Barents Sea, (2) identify depositional systems occurring on the shelf, (3) characterize through facies analysis previously unrecognized, thick transgressive packages occurring close to the Troms-Finnmark Fault Complex, and (4) provide new insights into the shelf to deep-water evolution and subsidence variability in the southwestern Barents Sea.

dated as Anisian to Early Ladinian (Glørstad-Clark et al., 2010; Klausen et al., 2017; Paterson and Mangerud, 2017) and it overlies, after a regional transgression, the shelf-margin clinoforms of the Klappmyss Formation. The maximum thickness of the Kobbe Formation shelf-margin clinoforms is 500 m (based on undecompacted clinoform heights; Glørstad-Clark et al., 2010). The sediment delivered to the basin was sourced from the east (Uralian orogen and Kara Sea; Eide et al., 2017), and was mainly immature and mud-rich. However, sediments sourced from the south (Fennoscandia) were characterized by higher maturity and coarser grain sizes (Eide et al., 2017; Fleming et al., 2016; Henriksen et al., 2011). These deposits appear to be located close to the TFFC (Eide et al., 2017; Fleming et al., 2016; Rossi et al., 2019), as exemplified by the Goliat and Nucula discoveries (Ohm et al., 2008).

2. Geological setting The Hammerfest Basin is a prolific hydrocarbon basin in the Norwegian Barents Sea (Figs. 1 and 2), containing important fields such as Goliat and Snøhvit (Mulrooney et al., 2017; Olaussen et al., 2010; Ostanin et al., 2013; Paterson and Mangerud, 2017). The Hammerfest Basin is bordered to the South by the Troms-Finnmark Fault Complex (TFFC; Gabrielsen, 1984; Mulrooney et al., 2017) and to the North by the Loppa High (Fig. 2). The TFFC is a basement involved fault system, characterized by episodes of extension in the Late Paleozoic (Carboniferous to Permian rift) and Late Mesozoic (Late Jurassic to Early Cretaceous rifting), and by inversion during the Early Cretaceous (Faleide et al., 1984; Gabrielsen and Færseth, 1988, 1989; Gabrielsen, 1984; Mulrooney et al., 2017). The paleo-Loppa High was a synsedimentary high in the Late Paleozoic and Early to Middle Triassic, whereas it acted as a depocenter in the Late Triassic (Faleide et al., 1984; Glørstad-Clark et al., 2010, 2011). During the Triassic, the Barents Sea was infilled by mainly northwestward-migrating shelf-margin clinoforms (e.g. GlørstadClark et al., 2011; Glørstad-Clark et al., 2010; Rossi et al., 2019). The Kobbe Formation represents one of these sedimentary prisms, it is

3. Dataset and methodology This study is based on well logs, core data, and 2D and 3D seismic. The cores and well logs are from wells 7120/12-1, 7120/12-2 (Alke area), 7122/7-3, 7122/7-4S, 7122/7-5, 7122/7-6 (Goliat area), 7122/ 6-2 (Tornerose area), 7120/9-2 (Albatross area) in the Hammerfest Basin and wells 7121/1-1R and 7120/1-1 on the Loppa High (Fig. 2). Cores have been logged at 1:25 scale and grain size, sorting, thickness of beds and laminae, sedimentary structures and bioturbation index have been recorded. Well logs have been used for correlation, using the top Kobbe Fm. as datum on the shelf. We correlated regressive-transgressive cycles using maximum flooding surfaces (MFS) and maximum regressive surfaces (MRS) (Carvajal and Steel, 2009; Galloway, 1989; Gomez-Veroiza and Steel, 2010, 2017; Zhang et al., 2016). These surfaces mark basinward and landward migration of the facies belts above them, respectively. The regressive half-cycles develop from the MFS to 869

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Fig. 2. Simplified regional structure map of the study area in the southwestern Barents Sea (see Fig. 1), showing the location of the seismic lines and well log correlations. Green circles represent wells. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 3. Seismic line (uninterpreted and interpreted) showing the occurrence of the first shelf-edge breaks in the Kobbe Fm. is influenced by the underlying topography (i.e., the last break in slope in the Klappmyss Fm. shelf margin). The seismic line is flattened at the base of the Klappmyss Formation. 2D seismic survey ST88-13. See Fig. 2 for location of the seismic line.

the MRS, whereas the transgressive half-cycles develop from the MRS up to the MFS. Seismic lines (2D seismic surveys BARE05, ST88-13, ST88-23, IKU84, NBR, NH8707, TNGS83, TR82R1, D-6-85) have been used to map clinoforms of different scales and the position of shelf edges and bottomset terminations of large-scale shelf-margin clinoforms. Shelf edge trajectory is described according to Helland-Hansen and Hampson (2009). Clinoforms that are in the order of tens of meters high are defined as delta-scale clinoforms, whereas clinoforms that are hundreds of meters high are defined as shelf-margin clinoforms (shelfedge clinoforms; Patruno and Helland-Hansen, 2018).

4. Large scale patterns of the Kobbe Fm 4.1. Seismically-imaged clinoforms 2D seismic images clearly show the development of the Kobbe Fm. shelf-margin prism; the first recognizable shelf-edge breaks of the Kobbe Fm. are influenced by the pre-existing topography (Fig. 3). The (undecompacted) shelf-margin clinoform thickness is ca. 200–300 m. Shelf-edge trajectory analysis reveals the occurrence of intervals with ascending trajectories and intervals with flat 870

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Fig. 4. Shelf-edge trajectory of the Kobbe shelfmargin prism. Seismic lines (2D seismic survey ST8813) show the top and bottom of the Kobbe Formation (white lines). White circles represent the shelf-break positions; blue circles represent bottomsets pinch out positions; yellow lines highlight the clinoforms. The seismic lines are flattened at the base of the Kobbe Formation. See Fig. 2 for location of the seismic lines. A) The SE-NW oriented seismic line (uninterpreted and interpreted) shows a shelf-edge trajectory that is flat and then ascending; in this line it is clear that the bottomsets close against the paleo Loppa High, demonstrating some interaction between the high and the sedimentary infill of the basin. B) The SE-NW oriented seismic line (uninterpreted and interpreted) shows a shelf-edge trajectory that is slightly ascending, flat and then ascending. The interaction between bottomsets and the paleo Loppa High occurs farther to the NW. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5. Uninterpreted and interpreted small-scale clinoforms (delta scale) within the shelf of the Kobbe Fm, highlighted in white. Inset shows zoomed-in detail of the clinoforms. 2D seismic survey BARE 05. See Fig. 2 for location of the seismic line.

trajectories (Fig. 4). In more detail, the lower part of the succession is mainly progradational, whereas the upper part is strongly aggradational (Fig. 4). The Kobbe Formation shelf-margin prism grew through repeated cross-shelf (regressive-transgressive) transits of deltaic systems, as commonly seen in many other examples (Carvajal and Steel, 2006; Chen et al., 2017; Johannessen and Steel, 2005;

Olariu and Steel, 2009; Zhang et al., 2016). These transits are imaged in seismic as the “tramline” character of the topset reflectors (see Chen et al., 2017). However, in some areas, it is possible to image small-scale clinoforms, in the order of several tens of meters thick, which can be interpreted as delta-scale clinoforms on the shelf (Fig. 5). 871

872

1.3

FA1

1.1

1.2

-

3.1

3.2

FA2

FA3

4.2

FA4

4.1

Facies

Facies associations

Poorly sorted coarse- and very coarse-grained sandstones and matrix-supported or clast-supported conglomerates (10–70 cm). Coaly, shale and carbonate clasts. Sharp erosive base. Cross-bedded upper very fine- to medium-grained sandstones. Moderate to good sorting. Stacked, fining upward units a few meters thick. Planar, tangential or trough cross-bedding, with double mud or organic drapes along the foresets. In places, the sandstone units form clean, apparently structureless units lacking mud drapes with very uniform grain size and faint cross-strata. Bidirectional paleocurrents (from image logs). Medium- to very fine-grained sandstone bodies, with sharp and erosive base (marked by granules, mudpebble conglomerates and carbonaceous fragments). Bioclasts. Low-angle laminations, ripple crosslamination and cross-stratification (marked by clay or carbonaceous drapes), apparent opposite-dipping foresets. Carbonaceous fragments and fluid muds. Mottled greenish and brownish calcareous finegrained sandstone to siltstone. Rhizoliths (up to 70 cm long), cm-scale and mm-scale calcite nodules and concretions (including pisoliths). In places massive micritic and micro-fractured light brown-yellow limestone. 10–40 cm thick coarse- to medium- and fine-grained sandstones (or reworked calcite nodules) with sharp, erosive bases. Heterolithic deposits and shaly lower very finegrained sandstones with organic and abundant coaly fragments. Wavy-lenticular bedding, mud drapes, softsediment deformed laminae, plane parallel laminations with small cross-lamination, rhythmic mud laminae. Root traces and bioturbation. Dark gray to greenish muds and coaly shales with carbonate clasts and nodules, coal and plant fragments, sparse pyrite. Structureless, but there can be very thin laminae of very fine-grained sandstones and siltstones, starved ripples. Very fine-grained sandstone beds up to 70 cm thick. Root traces, bioturbation.

Coarsening upward unit, characterized in the lower part by highly bioturbated mudstones and siltstones intercalated with lower very fine-grained sandstones. Mudstone drapes, organic fragments and clay chips. Amalgamated, lower fine-grained sandstones with low-angle lamination and hummocky crossstratification. Sharp base (wave ravinment surface)

Description of the sedimentary structures

Table 1 List of facies and facies associations.

1 to 5 Palaeophycus, Phycosiphon, Planolites, Skolithos, Macaronichnus, Diplocraterion, Bergaueria, Siphonichnus

0 to 5 Palaeophycus, Planolites, Diplocraterion, Teichichnus, Phycosiphon,

1 to 5 Skolithos, Phycosiphon, Palaeophycus, Teichichnus, Bergaueria, Planolites, Teichichnus, Asterosoma, Diplocraterion, root traces

Up to 7 m

Up to 5 m

2 to 5 Teichichnus, Planolites, Asterosoma, Palaeophycus, Phycosiphon, Rizhocorallium, Siphonichnus, Macaronichnus

BI

up to 15 m

Up to 3 m

Thickness in core

Tide-influenced distributary channels Active, tide-influence distributary channels in the lower delta plain. Recognized within regressive half cycles. Significant river flow, but they were also influenced by strong tidal currents. Bay/marsh deposits and Paleosols Restricted, low-energy coastal plain environment, such as a bay or marsh. The thin sandstones occurring within the muddy sediments are interpretd to have originated from overbank flooding and crevasse splays. FA 1.3 is interpreted as calcrete paleosol profiles.

Serrate pattern, usually finingupward (but it can contain punctuated coarsening-upward intervals). Not possible to distinguish between coastal-plain and floodplain-alluvial plain deposits

Shoreface and open marine deposits Deposition dominated by storm-wave processes. Highly bioturbated mudstones, siltstones and finegrained sandstones can represent deposition during fair-weather conditions, or a more distal environment (e.g., lower shoreface). The high diversity/high intensity of bioturbation also points to a relatively open marine environment Tide-influenced estuarine valley fill Tide-influenced, estuarine valley fill. Deposition controlled by persistent strong tractional currents, both unidirectional and bidirectional. The crossstratified sandstones are likely the result of migrating channelized compound dunes and bars, in an (innermiddle) estaurine environment. The base of the channels is marked by erosional surfaces and coarser grain sizes.

Interpretation of the depositional processes and environments

Bell or symmetrical shape and finingupward trend

Single or multiple, bell, blocky or symmetrical pattern, and in general by a fining-upward trend above a sharp base

Funnel or blocky pattern, with a coarsening-upward trend

Well log signature

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Fig. 6. Selected wells of the Kobbe Formation. Black bars in the side of the Gamma Ray (GrN) track represent the position of the cores. The top (green line) and bottom (pink line) of the Kobbe Formation are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 873

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Fig. 7. Examples of fine-grained and heterolithic deposits of FA 1.1 (wells 7122/7-3 and 7122/7-6) and FA 1.2 (well 7122/7-4S). Trace fossils: Bergaueria (B), Phycosiphon (Ph), Palaeophycus (P), Teichichnus (T), Planolites (Pl).

Skolithos, Phycosiphon and small vertical and horizontal traces) and the Bioturbation Index (BI) is variable (1–5). FA 1.2 is up to 3 m thick, and characterized by heterolithic deposits and shaly lower very fine-grained sandstones with organic and coaly fragments (Fig. 7). The main sedimentary structures include wavylenticular bedding (especially in 7122/7-5), mud drapes (especially on cross-laminae), soft-sediment deformed laminae, plane parallel laminations with small cross-lamination, rhythmic mud laminae (especially 7122/7-5). There is also presence of starved ripples, root traces, and loading structures of sandstone layers. BI can be variable, and it can reach up to 5 (Phycosiphon, Palaeophycus, Teichichnus, Bergaueria). Within finer fractions (green mudstones) there can be siderite nodules. FA 1.3 is up to 5 m thick, and it is characterized by alternating greenish and brownish beds, or mottled greenish and brownish calcareous finegrained sandstone to siltstone (Fig. 8). Otherwise, FA 1.3 shows many of the characteristics of FA 1.1. Trace fossils include Palaeophycus, Planolites, Phycosiphon, Asterosoma, Teichichnus, Taenidium and unidentified small vertical and horizontal traces. Rhizoliths (up to 70 cm long), cm-scale and mm-scale calcite nodules and concretions (including pisoliths) are in places very common throughout the sediments (Fig. 8). FA 1.3 mudstones and silty mudstones are in places eroded and overlain by 10–40 cm thick coarse-to medium- and fine-grained sandstones with sharp, erosive bases, characterized by tabular cross-lamination. Some of these coarser beds are marked at the base by a clast-supported conglomeratic layer, composed by sub-angular calcite nodules. A well-developed example of FA 1.3 is found in well 7122/7-6 (Fig. 8), where a ca 2 m-thick fining upward unit occurs. This unit is characterized by alternating brownish and greenish beds, passing upwards into mottled very fine-grained sandstones, in turn overlain by reddish very fine-grained sandstone and siltstone with calcilte nodules and calcite-coated rhizolith (Fig. 8D). The sediments are also characterized by a distinct upward increase in calcite content, and the top of the unit is characterized by massive micritic and micro-fractured light brown-yellow limestone (Fig. 8E). In well logs, FA 1 is characterized by a serrate pattern, usually fining-upward (but it can contain punctuated coarsening-upward intervals). However, in well logs it is not possible to distinguish between FA 1 and other coastal-plain, floodplain and alluvial-plain deposits. Interpretation - Overall FA 1 is interpreted to represent a restricted, lowenergy coastal plain environment, such as a bay or marsh, based on the

5. Core and well log data 5.1. Facies associations The facies associations (Table 1) are based on sedimentological descriptions of cores (Fig. 6) from wells 7122/7-3, 7122/7-4S, 7122/7-5, 7122/7-6 and 7120/12-1. All cored intervals belong to the topmost part of the Kobbe Formation, and they were here calibrated to the well logs. In intervals where core information is absent, well log pattern (blocky, serrate, bell, symmetrical and funnel), sandstone content, stacking patterns, as well as stratigraphic position within specific segments of the shelf margin profile were used to make an interpretation of the broad depositional environment as commonly used and outlined by Carvajal and Steel (2012); Zhang et al. (2016). The floodplain and coastal plain deposits (dissected by fluvial channels) present in the clinoform topsets are usually characterized by a serrate, blocky and bell-shaped well-log pattern (see also Carvajal and Steel, 2012). Shallow-marine, sand-rich deposits are defined based on their funnel or blocky pattern, with a coarsening-upward trend. The shale-prone shelf and deep-water slope deposits are characterized by a serrate and spiky log pattern (see also Carvajal and Steel, 2012). Gravity flow deposits towards the base of slope are recognized based on their blocky to slightly serrate log pattern, and by a fining upward trend (see also Carvajal and Steel, 2009; Carvajal and Steel, 2006). It is important to keep in mind that the amount of core data available is limited, and therefore environmental interpretations have an inherent degree of uncertainty. 5.1.1. Facies association 1 (FA 1) – bay/marsh deposits and paleosols FA 1.1 is up to 4 m thick, and characterized by dark gray to greenish muds, with carbonate clasts and nodules and sparse framboidal pyrite (Fig. 7). The deposits can be characterized by either a broad coarseningupward trend or by a uniform grain-size trend. The sediments are mostly structureless, but there can be very thin laminae of very finegrained sandstones and siltstones or finely laminated mudstones with plane parallel laminations and occasional current ripples (including starved ripples). Lower very fine-grained sandstone beds up to 70 cm thick with current ripple cross-lamination can also be present in places. Coal fragments, wood and plant fragments are common and there can be intervals of carbonaceous black mudstone. Root traces can be present. The sediments are burrowed (Palaeophycus, Planolites, Techichnus, 874

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Fig. 8. Examples of fine-grained deposits of FA 1.3. Note the frequent color changes, presence of rhizholits and calcite nodules. A) Core photos of well 7122/7-5. B) Core photos of well 7122/7-6 (from 1803 m to 1808 m). C) Sedimentological profile of FA 1.3 in well 7122/7-6. D) Detail of calcite-coated rhizolith. E) Detail of micro-fractured limestone in the topmost part of the calcrete paleosol profile (hardpan). Trace fossils: Teichichnus (T), Taenidium (Ta). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

muddy nature of the deposits, presence of bioturbation and root traces, and sedimentary structures. Root traces point towards a terrestrial influence (Wei et al., 2018), but the mud drapes, wavy-lenticular bedding and rhythmic mud laminae are indicative of tidal influence. The thin sandstones occurring within the muddy sediments are interpretd to have originated from overbank flooding and crevasse splays (e.g., Chen et al., 2014). FA 1.3 is also interpreted to have originally formed in a restricted, low-energy environment. However, these deposits were later exposed and subject to pedogenic activity (as suggested by mottling and long rhizholits), and are therefore interpreted as paleosols. The multicoloured bedding and mottling within the soil profiles indicate changing ground water level and shift in redox conditions (Pimentel et al., 1996). Where calcite nodules, micro-fractured limestone, and calcite-

coated rhizoliths are present (e.g. in well 7122/7-6), the deposits are interpreted as well-developed pedogenic calcrete (e.g. Alonso-Zarza and Wright, 2010; Gómez-Gras and Alonso-Zarza, 2003). The fact that calcrete nodules are present as lags at the base of coarser-grained beds also points at a prolonged period of exposure characterized mainly by non-deposition and erosion, in a likely semi-arid climate (Gómez-Gras and Alonso-Zarza, 2003). Facies similar to FA 1 have been described from the modern and Late Pliocene Orinoco Delta, where there is a high proportion of muddy floodbasins and muddy brackish-water embayments between distributaries and along the estuary margin (Chen et al., 2014; Warne et al., 2002a, 2002b), which are common during both regressive and transgressive phases (see Chen et al., 2017). 875

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Fig. 9. Example of tide-influenced distributary channels of FA 2; f.m. indicates fluid mud. Trace fossils: Phycosiphon (Ph), Teichichnus (T).

fragments (Fig. 9). Occasional bioclastic beds also occur. The grain-size ranges from medium to very fine, and the trend is overall slightly finingupwards, but irregular. The dominant sedimentary structures are lowangle laminations, ripple cross-lamination and cross-stratification. Crossstrata toesets and foresets usually are marked by clay or carbonaceous drapes, and they can display apparent opposite-dipping foresets (Fig. 9). In few places it is possible to observe water escape structures in the upper part of the foresets. Ripple cross-lamination is usually unidirectional, with drapes along the foresets and occasionally climbing ripple crosslamination. However, wave ripple and combined flow ripple cross-lamination is also present. Carbonaceous fragments are ubiquitous. Finergrained intervals are composed of bioturbated mudstones and siltstones and thick, dark unbioturbated mudstones. Paleocurrent information has been obtained from cross-strata visible through image logs. Paleocurrents are directed towards the NW, but in the upper part of the core there are two main paleocurrent directions, one towards ENE and SE, and one towards SW-NE. BI is variable. It tends to be low in the sand-rich strata, and higher in the finer-grained intervals or where carbonaceous debris concentrations are high. The burrows are usually small, horizontal forms, and rare small vertical forms. Trace fossils include Palaeophycus, Planolites, Diplocraterion, Teichichnus, Phycosiphon. In well logs, FA 2 is characterized by a bell or symmetrical shape and fining-upward trend. Interpretation – FA 2 is interpreted as active, tide-influence distributary channels in the lower delta plain based on facies characteristics and association to FA 1 (in particular FA 1.3). FA 2 has been recognized within a regressive half cycle, and also the well log pattern (bell and fining-upward) is indicative of channels in a delta plain setting (Zhang et al., 2016). These channels experienced significant river flow, but they were also influenced by strong tidal currents (e.g. Dalrymple and Choi, 2007). Tidal influence is recognized by the presence of clay and carbonaceous drapes along the foresets and by opposite-dipping foresets (Nio and Yang, 1991; Peng et al., 2018; Willis, 2005). Dark, unbioturbated mudstones are interpreted as fluid muds (e.g. Dalrymple and Choi, 2007; Ichaso and Dalrymple, 2009; Longhitano et al., 2012;

Fig. 10. Examples of tide-influenced to tide-dominated cross-stratified sandstones of FA 3.1. FA 3.1 cross-strata from well 7122/7-3. Thick solid white lines highlight the base of individual stacked cross-stratified units. Red rectangle (in inset) shows the UV photograph, in which the double mud drapes in the crossstrata toeset are clearly visible. Trace fossils: Phycosiphon (Ph), Planolites (Pl). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

5.1.2. Facies association 2 (FA 2) – tide-influenced distributary channels FA 2 is vertically associated to FA 1 and it is characterized by sandbodies up to 7 m thick. The sandstone bodies have sharp, erosive bases, marked by granules, mud-pebble conglomerates and carbonaceous 876

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Fig. 11. Examples of tide-influenced to tide-dominated cross-stratified sandstones of FA 3.1. Stacked cross-stratified units in well 7122/7-4S. Black arrows point at double mud drapes. The bottom part is characterized by very homogenous cross-strata. Trace fossils: Phycosiphon (Ph), Palaeophycus (P), Siphonichnus (Si).

is moderate to good. The grain size trend is variable and it can be uniform, slightly fining-upwards or slightly coarsening-upwards. In some cases, the sandstone bodies appear to be composed of stacked, fining upward sub-units a few meters thick (Fig. 10). There are variable concentrations of coaly and mud rip-up clasts, and the sediments are quite rich in white mica. In several places, deformation bands occur. The main sedimentary structures are planar, tangential or trough cross-bedding, very often with double mud or organic drapes along the foresets (Fig. 10). The cross-laminae can be highlighted by mud or coaly rip-up clasts, especially in the toesets, but they can also be highlighted by shell fragments. Dark, homogenous, 2–3 mm thick mudstones laminae are also present along the foresets and toesets. In places, the sandstone units form clean, apparently structureless units lacking mud drapes with very uniform grain size and faint cross-strata (Fig. 11). Cross-strata are in the order of 10–30 cm thick, and they display locally apparent opposite-dipping foresets. Locally flaser bedding and soft-sediment deformation are also present. Paleocurrent information has been obtained from cross-strata visible through image logs (Fig. 13). The paleocurrent analysis shows a marked bidirectionality in the cross-strata within the sandy packages. The main paleocurrent directions are towards NE and SW, SE and NW. BI is variable (1–5), with presence of vertical and horizontal burrows (some burrows have mud lining). In some intervals, burrows are concentrated in the more mud-rich layers, in the upper parts of individual, stacked, sub-units, or at the base of cross-strata. Trace fossils include Phycosiphon, Palaeophycus, Skolithos, Planolites, Macaronichnus, Diplocraterion, Siphonichnus, Bergaueria. FA 3.2 is characterized by poorly sorted coarse- and very coarsegrained sandstones and matrix-supported or clast-supported conglomerates (Fig. 12). The deposits range in thickness from 10 cm to 70 cm. FA 3.2 beds contain coaly, shale and carbonate clasts. The base is sharp and in places scouring into underlying deposits. The coarsest grained beds can be separated by mudstone intervals. Strata are normally graded and in places carbonate-cemented. Coaly and plant fragments are sparse throughout (Fig. 12), and in places in the topmost part of the beds there are dark, unbioturbated mudstone laminae. In well logs, FA 3 is characterized by a single or multiple, bell, blocky or symmetrical pattern, and in general by a fining-upward trend above a sharp base (see also similar examples in Zhang et al., 2016). Interpretation – Overall, FA 3 is interpreted as tide-influenced, estuarine valley fill based on facies characteristics, stratigraphic position and well log pattern. The facies characteristics of FA 3 indicate deposition controlled by persistent strong tractional currents, both

Fig. 12. Examples of FA 3.2. A) Note the scoured base. Inset shows basal conglomerate (UV photograph). Red arrows point at large clasts sparse within the sandy matrix. (B) Scoured base and clast-supported conglomerate in the bottom part of the core; scoured base and mud-clasts conglomerate in the upper part. Red arrows point at large clasts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Wei et al., 2018). Wave-generated structures can be produced by waves penetrating the channels during high tides or storms (Choi, 2011; Dalrymple, 2010). Additionally, the presence of low diversity, small size but high numbers of trace fossils in the bioturbated intervals is indicative of brackish-water conditions (Dalrymple and Choi, 2007). 5.1.3. Facies association 3 (FA 3) – tide-influenced estuarine valley fill FA 3.1 is characterized by cross-bedded upper very fine-to mediumgrained sandstones up to 15 m thick, and it is vertically associated to FA 1.1, FA 1.2 and FA 4.1. The sandstone bodies are characterized by sharp bases (locally erosional surfaces), marked by coarser grain sizes and associated in places with coal fragments (Figs. 10 and 11), shell fragments and reworked carbonate clasts. The sorting of the sandstone units 877

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Fig. 13. Sedimentary log of the topmost interval of the Kobbe Fm. in wells 7122/7-3 and 7122/7-4S, with paleocurrent information derived from image logsimage logs (paleocurrent information is derived from cross-strata, after dip removal). The paleocurrents show a marked bidirectionality of the cross-strata (red boxes). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

unidirectional and bidirectional (as shown by paleocurrent data; Fig. 13). The cross-stratified sandstones are likely the result of migrating channelized compound dunes and bars, in an (inner-middle) estaurine environment (Chen et al., 2014; Longhitano et al., 2012; Wei et al., 2018). The base of the channels is marked by erosional surfaces and coarser grain sizes (FA 3.2). Slack-water periods are identified by the presence of mud drapes on the foresets and toesets of cross-strata (Nio and Yang, 1991; Wei et al., 2018), but where absent it could indicate the presence of a cleaner water column or more intense reworking and winnowing of mud by tides and waves (Dalrymple and

Choi, 2007; Wei et al., 2018). Dark and homogeneous mudstone laminae are interpreted as fluid muds (Ichaso and Dalrymple, 2009). 5.1.4. Facies association 4 (FA 4) – shoreface and open marine deposits FA 4.1 is a sand-rich unit, 3 m thick and characterized by a uniform grain size trend, above a sharp base overlying FA 3.1. FA 4.1 is characterized by amalgamated, lower fine-grained sandstones (Fig. 14A) with low-angle lamination and hummocky cross-stratification (HCS). FA 4.2 is a ca. 5 m thick, coarsening upward unit, characterized in the lower part by highly bioturbated mudstones and siltstones intercalated with lower very 878

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Fig. 14. Core photos of FA 4. A) FA 4.1: amalgamated fine-grained sandstones with low-angle laminae and HCS (red inset shows UV photograph). B) FA 4.2: coarsening upward unit with highly bioturbated deposits. Trace fossils: Asterosoma (As), Teichichnus (T), Palaeophycus (P), Siphonichnus (Si), Rizhocorallium (R), Macaronichnus (M). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 15. Map of the locations of shelf edges and bottomset terminations, superimposed to the thickness map of the Kobbe Formation. Brown areas represent fields and discoveries (source: NPD factmaps). Purple circles represent delta clinoforms break; orange circles represent the first occurrence of shelf breaks in the Kobbe Fm; green circles represent the last shelf breaks of the Kobbe Fm; yellow circles represent shelf breaks in intermediate positions during the progradation of the Kobbe Fm; white circles represent shelf break positions of uncertain interpretation; blue circles represent bottomset terminations; light blue circles represent bottomset terminations of uncertain interpretation. Landward of each shelf break, shelf and coastal plain environments occur. Basinward of each shelf break, slope and basin floor environments occur. Gray lines show structural elements of the southwestern Barents Sea. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

fine-grained sandstones, and characterized in the upper part by finegrained sandstones (Fig. 14B). Shell fragments are abundant, and sparse within the sediments. In this well, throughout the deposits, there is presence of mudstone drapes, organic fragments and clay chips. Flaser and wavy bedding occur in places. FA 4.2 is characterized by high bioturbation

intensity and diversity. The paleocurrent spread is high (ca. 150°; Fig. 13 left hand-side). In FA 4, BI ranges from 1 to 5, and trace fossils include Teichichnus, Planolites, Asterosoma, Palaeophycus, Phycosiphon, Rizhocorallium, Siphonichnus, Macaronichnus. In well logs, FA 4 is characterized by a funnel or blocky pattern, with a coarsening-upward trend. 879

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Fig. 16. Correlation from shelf to basin floor, from the Goliat area to the Loppa High area. On the shelf there are at least five regressive and five transgressive cycles bound by maximum flooding surfaces and maximum regressive surfaces. See Fig. 2 for location of the correlation.

the fluvial fairway is characterized by a wider shelf, a shelf protrusion (10s–100s of kilometers wide), and a marked break in slope at the shelf edge (i.e., an accentuated shelf break), compared to the areas off-axis. For these reasons, the morphology of ancient shelves can be useful to identify cross-shelf supply fairways (Olariu and Steel, 2009). The superimposition of the locations of shelf breaks and bottomset terminations with the Kobbe Fm. thickness map shows that the area of maximum sediment thickness is located between the shelf edge and the slope, as it is expected for shelf margin successions. The final bottomset terminations of the Kobbe Fm. (blue circles in Fig. 15) have a highly irregular shape in plan view. These irregular bottomset terminations, combined with seismic images (Fig. 4), imply that the farthest basinward extent of the sedimentary prism was controlled, at least in part, by the topographic relief of the paleo-Loppa High, as the bottomsets terminated against this synsedimentary structural high (Fig. 4).

Interpretation – The suite of sedimentary structures exhibited in FA 4 records deposition dominated by storm-wave processes. Highly bioturbated mudstones, siltstones and fine-grained sandstones can represent deposition during fair-weather conditions, or a more distal environment (e.g., lower shoreface). The high diversity/high intensity of bioturbation also points to a relatively open marine environment. FA 4 is interpreted as shoreface deposits. In well 7122/7-3, FA 4.1 is underlain by an erosive surface and it overlies FA 3. It is therefore interpreted to represent wave-dominated deposits overlying the main estuarine infill above the wave ravinment, as transgression continues. On the contrary, in well 7120/12-1, FA 4.2 is part of a regressive sequence. 6. Results and discussion 6.1. Kobbe Formation shelf-margin evolution

6.2. Shelf to deep-water evolution, sediment partitioning, and subsidence variability

The basinward growth of the Kobbe Fm. shelf-margin prism is shown by the migration of the shelf-slope break. As seen in 2D seismic lines (Fig. 3), the first occurrence of the Kobbe Fm. shelf break is influenced by the underlying topography (i.e., the pre-existing shelf edge). The Kobbe Fm. shelf margin, in the SW Barents Sea, migrated ca. 30–50 Km in a north-westward direction. The plan-view map of shelf edges position highlights the presence of marked protrusions of the shelf edge (Fig. 15), that could indicate supply fairways. Sediment supply can influence shelf morphology, as the areas in front of large river fairways are of relatively high sediment-supply compared to areas lateral to the river mouths (Olariu and Steel, 2009). Based on a dataset of modern shelves, Olariu and Steel (2009) showed that in the majority of cases the area in front of

In the Kobbe Fm. shelf-margin prism, delta, coastal plain, estuarine, shelf, and deep-water slope and basin floor environments are present (Figs. 16–18). On the shelf, during the regressive transits of deltaic systems, deltaic and coastal plain deposits prograded, potentially reaching the shelf edge position. During the retrogradation of the system, transgressive deposits formed on the shelf (including estuaries and lagoon/ barrier islands), as commonly seen in the topset stratigraphy of shelf margins (Chen et al., 2017; Plink-Bjorklund and Steel, 2006; Pontén and Plink-Björklund, 2009; Zhang et al., 2016). Beyond the shelf edge, deposition is, in general, dominated by sediment gravity flows (Olariu and 880

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Fig. 17. Fence diagram correlation from the Goliat area to the Loppa High area. On the shelf there are at least five regressive and five transgressive cycles bound by maximum flooding surfaces and maximum regressive surfaces. Legend is the same as Fig. 16. See Fig. 2 for location of the correlation.

Fig. 18. Correlation from Alke area to Tornerose area, showing regressive-transgressive cycles on the shelf. Note the presence of an expanded succession in Goliat area. Legend is the same as Fig. 16. See Fig. 2 for location of the correlation.

Steel, 2009). The core and well log analyses presented in this work show the cross-shelf regressive-transgressive transits, and match well with the mapped position of the shelf breaks on seismic. In particular, our analysis recognizes, for the first time, the presence of thick transgressive packages within the Kobbe Formation. The best-developed transgressive deposits occur in the topmost part of the formation (Fig. 19), and are composed of FA 3 and FA 4. The infilling of the fluvial/estuarine sequence is characterized by: 1) basal (fluvial) erosional surface; 2) fluvial-dominated deposits; 3) tide-influenced (estuarine) deposits; 4) wave-dominated deposits. The wave-dominated deposits of FA 4 occur on top of the tideinfluenced deposits of FA 3. This stacking pattern records the landward

movement of the facies belts during transgression, so that the wave dominated deposits (occurring at or outside the estuary mouth) overly the tide-influenced inner estuarine deposits (see also Plink-Bjorklund and Steel, 2006). Below the basal erosional surface, paleosols have been recognized from the core analysis (FA 1.3). These deposits can form in floodplain environments, but because of their occurrence right below the basal erosional surface of the valley fill succession, they could also represent, in some cases, the interfluve areas of this unconformity. In coastal plain settings, it is difficult to recognize stratigraphic surfaces dipping more steeply than the sub-horizontal shelf, even in outcrop studies (see Plink-Bjorklund and Steel, 2006). In these cases, incised 881

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Fig. 19. Correlation detail of the Goliat area; the main facies associations recognized in each core are indicated. Through core facies analysis and well log correlation, a main estuarine valley in the topmost interval of the Kobbe Formation has been recognized in this study. Two stacked estuarine valley-fill sequences are recognized, with the youngest one capped in most of the wells by shoreface deposits lying above a wave ravinement surface.

(estuarine) valley fills, implying a transgression punctuated by short term regressions (Fig. 19). The regressive-transgressive cycles within the Kobbe Fm. (Fig. 20), with the development of prograding deltas and valleys during regressive phases, and estuaries during transgressive phases, suggests that the sediment sink is dependent on this cyclicity. During regressive phases, the sink is the shelf, shelf edge and slope, whereas during transgressive phases the main sediment sink is on the shelf within the estuaries. The correlation from Alke area to Tornerose area (Fig. 18) parallels the TFFC, and shows along strike variabilities. The regressive-transgressive sequences are visible, with the development of a thick transgressive sequence at the top of the Kobbe Formation. However, this correlation highlights for the first time two important aspects. The first one is the presence of thick estuarine packages in Alke and Goliat areas. The second aspect is that in Goliat the sedimentary succession is expanded (with the presence of an additional deltaic sequence at the base of the Kobbe Fm.) with respect to adjacent areas. These observations can be explained considering the location of these areas. Compared to the Tornerose area, Alke and Goliat are located right next to the TFFC. In particular, Goliat position is on a bend, formed by the intersection of fault segments with different orientations within the TFFC (Mulrooney et al., 2018). We therefore hypothesize that transgressions and development of estuarine valleys are related to (or at least enhanced by) subtle synsedimentary fault activity occurring at times. Even small movements along the faults could have been enough to create irregularities in the coastline and embayments (see Wei et al., 2018). This would have enhanced the thickness of the transgressive deposits in certain locations closer either to the bounding fault or to bends within it. This hypothesis is supported by the presence of a thicker sedimentary succession in Goliat. Similarly, Wei et al. (2018) have linked the presence of thick (estuarine) sandstone units in the Brent Delta to subsidence and syn-sedimentary faulting along the Viking Graben bounding faults during Brent Delta progradation. Therefore, it is not unreasonable to postulate that the TFFC created local embayments and subsidence that affected sand distribution.

valleys are recognized by the occurrence of unconformities, facies variations, and landward or lateral onlap of the valley fills (Plink-Bjorklund and Steel, 2006). It is therefore quite likely that such valleys would be very difficult to identify in seismic datasets with a low resolution. However, the valleys are recognized in cores and through well logs correlation, and they should be linked, in a source-to-sink perspective, to incisions at the shelf edge and bypass of sediment to the deep water (see Plink-Bjorklund and Steel, 2006). The well log correlations (Figs. 16–18) show that the shelf succession is characterized by at least five regressive-transgressive sequences. The correlation from Goliat area to the Loppa High (Figs. 16 and 17) shows that in the regressive phases, deltaic systems prograded from the south (sourced from the Fennoscandian shield) and from the E-SE (sourced from the Urals). The character of the well logs changes markedly from the Goliat area (sand-rich signature) to wells 7120/9-2 and 7121/1-1R (less sand-rich units, even though they are closer to the shelf edge). Fleming et al. (2016) have shown, through a zircon provenance study, that the Goliat area is characterized by a Caledonian source, and that the Bjarmeland Platform and Nordkapp Basin areas (see Fig. 2) are characterized by a pure Uralian source. For these reasons, we hypothesize that the change from predominantly Caledonian sediments (in the southern part of the Hammerfest Basin), to a mixed (Uralian) sediment composition happens around the central-northern part of the Hammerfest Basin (Albatross-Snøhvit area). However, to conclusively prove this, more analyses (such as zircon provenance) are needed. Nonetheless, these considerations can have important implications for the petroleum system of this area, as the Caledonian source provides reservoirs of much higher quality than the Uralian source (Eide et al., 2017; Fleming et al., 2016). During the transgressive phases, estuarine, barrier-lagoon and open marine systems developed. The well log correlations (Figs. 16–18) show that the transgressive sequence in the topmost part of the Kobbe Fm. is significantly thicker than the ones occurring farther down in the stratigraphy. In the former, thanks to the presence of cores, a valley fill sequence has been interpreted in this study (Fig. 19). The well log stacking pattern of this topmost sequence suggests that there might be two or more stacked 882

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Fig. 20. Paleogeographic maps of the study area with superimposed structural elements (gray lines). A) Paleogeographic map of a typical regressive half cycle. B) Paleogeographic map of a typical transgressive half cycle with the presence of estuarine environments with purely Caledonian provenance close to the TFFC. Tide-influence within the estuarine valleys is supported also by the presence of bidirectional paleocurrents (inset map). Faults in the inset map are based on Mulrooney et al. (2018).

7. Conclusions

synsedimentary structural high.

• The core analysis, combined with image log data for palaeocurrent

In the southwestern Barents Sea, the evolution of the Middle Triassic Kobbe Formation shelf-margin succession has been studied through the integration of seismic, well log and core data. The main results can be summarized as follow:

•

• Seismic data show two different scales of clinoforms: shelf-margin • •

clinoforms (undecompacted thickness of 200–300 m) and delta-scale clinoforms on the shelf (several 10s of meters thick). The shelf-margin prism prograded ca. 30–50 Km in a north-westward direction, and localized protrusions of the shelf edges can indicate the location of cross-shelf sediment supply fairways. The influence of the paleo-Loppa High on the progradation of the Kobbe Formation shelf margin is evidenced by the highly irregular shape of the final bottomset terminations, and it is confirmed by seismic images showing the bottomsets pinching out against this

•

883

information, recognizes four facies associations: bay/marsh deposits and paleosols, tide-influenced distributary channels, tideinfluenced estuarine valley fill and, shoreface and open marine deposits. On the shelf, during the regressive transits of deltaic systems the shoreline prograded from the south and from the east-southeast. During the retrogradation of the system, transgressive deposits formed on the shelf (including estuaries). This work recognizes, for the first time, the presence of thick transgressive packages within the Kobbe Formation. The transgressive sequence at the top of the Kobbe Formation is up to 50 m thick, and it is interpreted as stacked estuarine valley fills. It is characterized at its base by an erosive surface underlain by calcrete paleosols, followed by fluvial deposits, tide-influenced (estuarine) deposits, and wavedominated deposits above the wave ravinment surface.

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• The Troms-Finnmark Fault Complex appears to have controlled sub-

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sidence variability during the deposition of the Kobbe Formation, as evidenced by an expanded sedimentary sequence in the Goliat area. Subtle synsedimentary fault activity could have been enough to create irregularities in the coastline and embayments, enhancing the thickness of the transgressive deposits in certain locations closer either to the bounding fault or to bends within it.

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