Palaeoecology and palaeoenvironments of the Middle Jurassic to lowermost Cretaceous Agardhfjellet Formation (Bathonian–Ryazanian), Spitsbergen, Svalbard

We describe the invertebrate assemblages in the Middle Jurassic to lowermost Cretaceous of the Agardhfjellet Formation present in the DH2 rock-core material of Central Spitsbergen (Svalbard). Previous studies of the Agardhfjellet Formation do not accurately reflect the distribution of invertebrates throughout the unit as they were limited to sampling discontinuous intervals at outcrop. The rock-core material shows the benthic bivalve fauna to reflect dysoxic, but not anoxic environments for the Oxfordian–Lower Kimmeridgian interval with sporadic monospecific assemblages of epifaunal bivalves, and more favourable conditions in the Volgian, with major increases in abundance and diversity of Hartwellia sp. assemblages. Overall, the new information from cores shows that abundance, diversity and stratigraphic continuity of the fossil record in the Upper Jurassic of Spitsbergen are considerably higher than indicated in outcrop studies. The inferred life positions and feeding habits of the benthic fauna refine our understanding of the depositional environments of the Agardhfjellet Formation. The pattern of occurrence of the bivalve genera is correlated with published studies of Arctic localities in East Greenland and northern Siberia and shows similarities in palaeoecology with the former but not the latter. Ammonite biostratigraphy is used as a tool to date bivalve assemblage overturning events to help identify similar changes in other sections.


Introduction
The Middle Jurassic to lowermost Cretaceous Agardhfjellet Formation of Spitsbergen is dominated by organic-rich shales and may be considered as a partial time equivalent of the Kimmeridge Clay Formation of Great Britain, the Draupne Formation of the Norwegian Sea, and the Hekkingen Formation of the Barents Sea, all of which are important petroleum source rocks. The Agardhfjellet Formation has been the focus of many previous palaeontological studies. These include papers on vertebrates (Ginsburg & Janvier, 1974;Hurum et al., 2012;Knutsen et al., 2012;Roberts et al., 2014Roberts et al., , 2017Delsett et al., 2015Delsett et al., , 2017, palynology and microfossils (Bjaerke, 1980;Århus, 1988;Nagy et al., 1990Nagy et al., , 2009; Nagy invertebrate fossils are hard to find at outcrop in the majority of the formation.
In this study, we provide data on bivalve and ammonite faunas, as well as other invertebrate fossils, from the Agardhfjellet Formation in rock cores and from field studies in Central Spitsbergen between 2013 and 2016. The rock cores proved to be an excellent source of invertebrate fossils and revealed a far richer fauna than previously assumed. We use this invertebrate fauna, together with previous geochemical and sedimentological studies, to provide an integrated palaeoecological and palaeoenvironmental analysis of the Agardhfjellet Formation, and, using ammonites for correlation, compare this to other High Boreal sections in East Greenland and northern Siberia.

Geological Background
The Agardhfjellet Formation is 250 metres thick at the Janusfjellet section ( Fig. 1) and consists of four members: the Oppdalen Member (Bathonian to Oxfordian), the Lardyfjellet Member (Lower Kimmeridgian), the Oppdalssåta Member (Upper Kimmeridgian) and the Slottsmøya Member (Volgian to Ryazanian), with considerable lateral thickness variations between the members (Fig. 2). The succession consists mainly of dark, organic-rich shales (TOC between 2 and 12%; Koevoets et al. (2016)), interbedded with siltstones, sandstones and concretionary carbonates (Dypvik, 1984;Dypvik et al., 1991b;Koevoets et al., 2018). The sediments were deposited in a shallow-marine, dysoxic environment, where there was periodic oxygenation, according to Koevoets et al. (2018) and Collignon & Hammer (2012), based on major and trace element analysis. The only true anoxic interval in the Agardhfjellet Formation is situated in the Lardyfjellet Member, where shales are finely laminated and with little or no benthic invertebrate fauna or bioturbation.

Materials and methods
In November-December of 2007, the DH2 rock core was drilled as part of the UNIS CO2 LAB project, researching the feasibility of CO2 storage on Spitsbergen in the Triassic-Lower Jurassic sandstones (Ogata et al., 2014;Sand et al., 2014). The complete 250 metre-thick, Middle Jurassic-Lowermost Cretaceous Agardhfjellet Formation sequence was fully recovered in the well. The well is located at Hotellneset, on the western shore of Adventfjorden, along the Longyearbyen airport road, Spitsbergen (Braathen et al., 2012) (Fig. 1). Drilling was performed with an ONRAM 1400 rig belonging to Store Norske Spitsbergen Kulkompani (SNSK), which allowed for continuous full-coring by a wireline system using telescope drilling (Braathen et al., 2012). The core is kept in wooden, metre-long boxes each containing up to 5 metres of core-material and stored in a dry, cool facility in Endalen, Spitsbergen.
Complete and fragmentary macrofossils were noted whenever they occurred on core breakage surfaces, the number of which was variable depending on lithology. Some lithologies formed continuous metres of core and were broken at up to ten levels per metre to increase fossil specimen recovery. The main fossil groups are ammonites, belemnites, bivalves, brachiopods, gastropods, onychites (cephalopod hooks) and teleost remains. The specimens of each group per metre of core-section were counted (Fig. 2). Other macrofossils, such as scaphopods and wood fragments, were also noted in the stratigraphic column (Fig. 2).
Palaeoecological information on the identified invertebrate fauna, mainly bivalves, was obtained using the publications of Stanley (1970) and Cox et al. (1969), and earlier works on invertebrate fauna across the Boreal and Sub-boreal realms of the Upper Jurassic by Fürsich (1982Fürsich ( , 1984, Birkenmajer et al. (1982), Kelly (1984), Kopik & Wierzbowski (1988), Wignall (1990) and Hryniewicz et al. (2014). Of particular interest in this study were the life positions (infaunal, semi-infaunal or epifaunal) and feeding habits (filter or deposit feeding). Figure 3 shows a compilation of palaeoecological data for each species of bivalve throughout the stratigraphy.
Selected pieces of core were imaged for cryptic bioturbation using a Nikon Metrology XT H 225 ST microfocus CT scanner at the Natural History Museum, University of Oslo, at 200 kV, 0.5 s exposure, 3000 rotations, 0.25 mm tin filter.

Oppdalen Member
The Oppdalen Member (Bathonian-Lower Oxfordian) is the least fossiliferous interval of the Agardhfjellet Formation (Figs. 3 & 4). Consisting mainly of pyrite-rich, silty shales and muddy siltstones as part of a transgressive sequence ending in laminated black shale, it is believed to represent an outer-shelf deposit (Koevoets et al., 2018). Figure 3 shows low faunal abundances for the Oppdalen Member. Nektic fossils, such as the ammonites Cadoceras sp. (Fig. 5G) and Kepplerites (Seymourites) aff. svalbardensis and onychites, are more common throughout the member than benthic fossils. Onychites become even more abundant towards the top of the member. Bivalves and gastropods become somewhat more numerous in the Lower Oxfordian, but the bivalve fauna consists only of Grammatodon cf. pictum (Fig. 6H, I) (Milashevich, 1881;Gerasimov, 1955) and Mesosaccella sp. (Fig. 7E) (Duff, 1978;Hryniewicz et al., 2014).
These semi-infaunal/shallow infaunal bivalves lived close to the sediment-water interface and are indicative of at least periodic oxygenation. The semi-infaunal, byssally   attached G. cf. pictum (Stanley, 1970) specimens were found in siltstones to fine-grained sandstones. These siltstones and sandstones were deposited near the storm wave base, under periodically dysoxic conditions (Koevoets et al., 2018). G. cf. pictum was well adapted to moderately energetic conditions, feeding on suspended organic matter (Stanley, 1970). Mesosaccella sp. was encountered in the uppermost Oppdalen Member, where dysoxic/ anoxic silty mudstones/black shales dominate (Koevoets et al., 2018). This occurrence is expected, as Mesosaccella is associated with dysoxic conditions in other formations (Wignall, 1990;Kenig et al., 2004). Being infaunal deposit feeders, Mesosaccella specimens spent most of their lives buried. Hodges (2000) suggests that Mesosaccella could have had a short siphon for respiration and/or feeding from the sediment-water interface, and which could have been used as a snorkel during periodic deoxygenation events. If conditions become too challenging it is possible that individuals would have risen to the substrate surface to extend their siphons into oxygenated waters above.    DH2 535.48 m,Slottsmøya Member,middle Volgian (PMO 228.777). (A2) Magnification of dentition of underlying valve (PMO 228.777

Lardyfjellet Member
The coarsening-upward, regressional Lardyfjellet Member (Lower Kimmeridgian-Upper Kimmeridgian) consists of mostly laminated black shales and siltstones, which are moderately fossiliferous in the lower half, with increasing specimen abundance towards the upper part of the member. Koevoets et al. (2018) describe the environment as transitioning from outer shelf dysoxic/ anoxic to the distal part of a delta front, implying an increase in oxygenation and energy levels. Fossils become particularly abundant at 675-655 metres in the member, as the ammonites Amoebites spp. (Fig. 5H, J & L) and Rasenia cymodoce (Koevoets et al., 2016) are encountered at several levels, with up to five specimens per metre core (Fig. 2). Bivalves are common, as are fish remains, and a few scaphopods occur. Curiously, the onychite abundance decreases in the same interval. This could be a result of preservational differences, as onychites are made of chitin, which was preserved better under oxygen-depleted conditions (Durska & Dembicz, 2015). The bivalve occurrences in the Lardyfjellet Member can be divided into two assemblages: Buchia cf. concentrica ( Fig. 7F-H) dominated the middle part of the member, transitioning into an Astarte sp. dominated assemblage in the uppermost, more silt-dominated part of the section.
Species in the genus Buchia were opportunistic epifaunal, epi-byssate suspension-feeders, commonly found in shallow to deep-marine facies (Fürsich, 1984;Wignall, 1990). A proposed life position for B. keyserlingi was given by Zakharov (1981, fig. 78). The abundance of Buchia cf. concentrica is highest at 671-661 m. In this interval, the strata consist of black shales devoid of bioturbation, TOC around 10% (Koevoets et al., 2016), and V / (V + Ni) ratios indicative of dysoxic to anoxic conditions (Koevoets et al., 2018). These circumstances are in agreement with the epifaunal/epi-byssate lifestyle of Buchia and with the findings of Zell & Stinnesbeck (2015) that buchiids lived an epifaunal lifestyle, and were able to survive both high and low oxygen conditions. An opportunistic species such as B. cf. concentrica could take advantage of changing conditions in the Lardyfjellet Member as the coastline prograded, supplying the bottom water with more nutrients and increasing oxygenation. In the Buchia dominated bivalve assemblage, other species occurring in order of abundance were Astarte sp. (Fig.  6N, O) and Oxytoma sp. (Fig. 7N, O). The latter was an epibyssate filter feeder, living on hard substrates such as shells (Stanley, 1970;Hryniewicz et al., 2014).
Most species of Astarte were shallow infaunal filter feeders (Stanley, 1970) tolerating moderately energetic environments. The sediments in which these specimens were found were deposited in a distal prodelta environment where energy levels were relatively low, but higher than in the underlying interval, TOC was very high and oxygenation of the bottom water was relatively low (Koevoets et al., 2018). The siltstones of the Lardyfjellet At 724.3 m in DH2, microfocus CT imaging revealed a dense ichnofabric of pyritised, thread-like filaments with inclinations varying from horizontal to vertical (Fig. 4). Similar fabrics were seen at 711.4 m and 668.3 m. These trace fossils are referred to the ichnogenus Trichichnus, which has recently been interpreted as remains of intrasediment, sulphide-oxidising bacterial mats (Kędzierski et al., 2015). In modern sediments, these bacterial mats form primarily in areas with high organic content and poor, but not zero, oxygenation.
An outer shelf, low to moderately energetic environment seems to agree with the bivalve taxa found in the Oppdalen Member. The order of occurrences of the bivalves can be explained by decreasing energy levels and dysoxia. It does not explain their rarity, as black shales are commonly known for well preserved and common invertebrate faunas (Wignall, 1990;Kenig et al., 2004). The specimen rarity could have been a result of persistent anoxic conditions, with only few and short oxygenated intervals. This is in agreement with the rarity of other benthic groups, such as gastropods.   The transition from a low hydro-energetic environment (laminated shales) to a moderate hydro-energetic environment (siltstones) is accompanied by a change from assemblages dominated by epifaunal bivalves to infaunal bivalves. The overall increase in diversity of invertebrate fauna can be an indication of more favourable living conditions, such as an increase in nutrient supply in the water column.

Oppdalssåta Member
The Oppdalssåta Member (Upper Kimmeridgian-?Volgian) is characterised by thick tabular beds of finegrained sandstones, containing cross-stratification and commonly capped with shell beds (Dypvik et al., 1991b;Mørk et al., 1999;Koevoets et al., 2018). It is a series of coarsening-upward sequences in a slowly retrograding system (Koevoets et al., 2018). The transgression resulted in finer-grained sediments in every coarsening-upward (CU) unit. Bivalves are less common in the sandstones and only one genus (Astarte) is found in these sediments. However, after flooding of the basin and the deposition of siltstones/shales in the upper part of the member, infaunal bivalve genera such as Hartwellia and Modiolus (Strimodiolus) are relatively common, and epifaunal genera, such as Oxytoma, appear repeatedly. In the Oppdalssåta Member, the nektic invertebrate fauna decreases drastically: ammonites all but disappear, and onychites are absent until the appearance of the lowenergetic deposits in the final CU unit of the member. Teleost fish respond in a similar fashion. The only group profiting from the higher energetic conditions seems to be the gastropods. The gastropods preserved in this part of the section resemble procerithids, belonging to the superfamily Cerithioidea, which are often associated with higher energetic, oxygenated, bottom-water conditions (Allmon, 1988;Anderson et al., 2017). In this interval they have also been observed in the field in large numbers locally in fine-grained sandstone beds.
Hartwellia first occurs in the Upper Kimmeridgian and increases in abundance throughout the Oppdalsåta Member. Hartwellia sp. (Fig. 5A-F) was most likely a shallow infaunal suspension feeder, living close to the sediment-water interface, based on shell morphology (Stanley, 1970;Fürsich, 1982). Most specimens are entire and articulated, and some are in life position, indicating there was little to no transportation after death. Hartwellia sp. was found in the distal part of a retrograding prodelta system (Koevoets et al., 2018). The presence of the semiinfaunal Modiolus (Strimodiolus) sp. (Fig. 6D-G) and epifaunal Oxytoma sp. (Stanley, 1970;Fürsich, 1982) are also indicative of lower energetic environments. Astarte cf. rouillieri replaces Astarte sp. in this member and occurs in fine-grained sandstone beds, i.e., a coarser sediment than its Lardyfjellet Member counterpart, which occurs in siltstones. As infaunal filter feeders, many of the species within the genus Astarte were adaptable to moderately energetic environments (Stanley, 1970).
An alternation of a slightly higher energetic environment with low-energetic shales and siltstones (Fig. 3) coincides with the changes in abundances of the infaunal Hartwellia sp., and the presence or absence of Modiolus (Strimodiolus) sp. and Oxytoma sp. (semi-infaunal and epifaunal, respectively). The siltstones and shales bear the majority of the specimens and only few specimens, generally unidentifiable, occur in the sandstones. The interpretation of the coarsening-upward sequences as transitioning from outer shelf to transition zone/prodelta to lower middle shoreface/delta front can explain the alternating invertebrate abundances. Alternatively, the paucity of specimens in the sands could have taphonomic explanations, including a high sedimentation rate causing dilution of specimens.
The Hartwellia assemblage is found in an alternating oxic-dysoxic, regressive, outer-shelf marine environment (Koevoets et al., 2018). Modiolus (Strimodiolus) sp. is believed to have been an endobyssate, low-level (bottom 5 cm of the water column) suspension feeder (Stanley, 1970;Fürsich, 1982). Specimens belonging to these taxa were found in muddy siltstones typical for outer-shelf marine deposits (Koevoets et al., 2018). According to Collignon & Hammer (2012) the bottom waters of the Slottsmøya Member were periodically oxygenated, as a result of a sudden incoming of reworked sediments (siltstones to very fine sandstones) deposited as parts of turbidites or storm deposits. This conclusion is confirmed by element analysis of Koevoets et al. (2018) where V / (V + Ni) and U/Th ratios are used as oxygenation proxies.
There is a remarkable increase in onychites specimens and teleost remains in the uppermost part of the Slottsmøya Member, in the interval 493-500 m in DH2 (Fig. 2). This increase is followed by a sudden peak of abundance of lingulid brachiopods, and the onychite and teleost remains disappear. During this interval the numbers of bivalves and ammonites drop drastically, and the lithology changes from siltstone to laminated black shale. Just below this interval, infaunal bivalves become very rare and are replaced by epifaunal species such as Buchia sp., Oxytoma cf. octavia, Pseudolimea sp., and the infaunal deposit feeder Dacromya sp. All these changes point to a sudden deepening of the basin and increased input of organic matter, creating more anoxic bottom waters.
Buchia sp. was most likely an opportunistic epi-byssate filter feeder, like B. cf. concentrica, which fits with a sudden decrease in energy levels and the early onset of increased oxic conditions. O. octavia and Pseudolimea sp. were filter feeders (Stanley, 1970) that lived byssally attached to loose shells or hard substrates, such as authigenic carbonate crusts from the hydrocarbon seeps  which occur at this level . O. octavia most likely lived in low-energy environments, based on their thin shells . The specimens in this study were collected from an interval that belongs to a transgressional sequence (Koevoets et al., 2018), just below a maximum flooding surface. The depositional environment of this interval is assigned to the outer-shelf marine realm (Koevoets et al., 2018), which fits the findings of Hryniewicz et al. (2014) on the low-energy environment habitat for O. octavia.
The high mud percentage of the siltstones containing Dacromya indicates a low-energy deposit. The presence of this burrowing bivalve indicates a periodic oxygenation of the seafloor and the top layers of the substrate. The TOC content of this interval is 2-5% (Koevoets et al., 2016), potentially a good food source for a deposit-feeding species (Stanley, 1970).
Other specimens of H. elegans were encountered a couple of metres below in DH5R (Fig. 9A) and at Janusfjellet just above the glauconitic sandstone of the Oppdalssåta Member (Figs. 5I & 9B).
The ammonite zonation in the Slottsmøya Member based on the specimens from the DH2 and DH5R core is difficult, because no identifiable ammonites were found in the lower part of the member. The first identified ammonite in the DH2 core, Pavlovia cf. iatriensis at 543 m (Fig. 8A), indicates a middle Volgian age, as supported by finds of Dorsoplanites sp. at 543 m in DH2 and 488 m in DH5R (Fig. 9J, K). In the field, a sideritic bed, informally referred to as the Dorsoplanites bed, correlating to c. 536 m in DH2, is rich in fossils. From this bed, Dorsoplanites sp. (Fig. 9D) and Epivirgatites cf. sokolovi (Fig. 8B-D), of the middle Volgian, were recovered.

Bivalves
Buchiid bivalves have been used in the Boreal Upper Jurassic and Lower Cretaceous for biostratigraphical zonation, complementing the established ammonite zones to increase coverage (Table 2). Since in this study we were only able to identify one buchiid to species level, we were not able to compare the results for the entire formation. The presence of Buchia cf. concentrica in the Lardyfjellet Member corresponds with the finds of B. concentrica in East Greenland and Northern Siberia (Surlyk & Zakharov, 1982;Zakharov, 1987;Rogov & Zakharov, 2009;Shurygin et al., 2011), The bivalve diversity in Spitsbergen is low compared with other Arctic areas, such as Milne Land, East Greenland (Fürsich, 1984) and northern Siberia (Zakharov, 1966). When comparing the bivalve assemblages and genera of other Arctic sections to this study, the succession in Milne Land stands out (Fürsich, 1982(Fürsich, , 1984. The species richness found in Spitsbergen is significantly lower and the depositional environment of the Agardhfjellet Formation appears to be more distal to the coast than the section in Milne Land, but the trends and presence of genera through the stratigraphy seem to follow a similar sequence (Fürsich, 1982(Fürsich, , 1984. As in the Agardhfjellet Formation, the Kap Leslie Formation has a base consisting of coarse-grained sandstone roughly following a fining-upward succession from the Bathonian to the Kimmeridgian (Fig. 10) (Fürsich, 1984;Birkelund & Callomon, 1985). The Kosmocerasdal Member (Upper Callovian-Upper Oxfordian) is dominated by semiinfaunal/epifaunal bivalves (G. keyserlingii, A. sollae and C. broenlundi) (Fürsich, 1984), although specimen abundances are very low. This fauna is similar to that in the Oppdalen Member. The overlying Aldinger Elv Member (Upper Oxfordian) and Bays Elv Member (Upper Oxfordian-Lower Kimmeridgian) consist of sandstones and specimen abundances in these units are even lower, but still dominated by semi-infaunal/epifaunal assemblages (Fürsich, 1982;Birkelund et al., 1984). These sandstones are interpreted as shallow-water near-shore sandbars wedged in between fine-grained sediment bodies (Fürsich & Heinberg, 1983). Similar sandstone bodies are not present in Spitsbergen as the location was more distal and laminated black shales were deposited instead (Koevoets et al., 2018). The lack of distinguishable bivalves in the Agardhfjellet Formation prevents detailed comparison with the Milne Land section fauna for this time interval.   (Birkelund et al., 1984). At this point in the East Greenland section anoxic conditions became widespread (Fürsich, 1984). Black shales containing epifaunal Buchia and infaunal Astarte bivalve faunas are very similar to those of the Lardyfjellet Member, which was also deposited in a dysoxic environment (Koevoets et al., 2018). The major difference in lithology in this interval is the incoming of sandstones of the Oppdalssåta Member in the Upper Kimmeridgian.
In the Volgian Krebsedal, Pernaryggen, Astartedal and Hennigryggen members of East Greenland, the abundance and diversity of bivalves increase drastically (Fürsich, 1982), similar to the trend seen in the Slottsmøya Member in Spitsbergen. As in Spitsbergen, the Milne Land Volgian age siltstones are dominated by infaunal and semi-infaunal bivalves, such as Grammatodon, Pleuromya, Astarte and Hartwellia (Fürsich, 1982). The most commonly found bivalve in Spitsbergen in the Slottsmøya Member is Hartwellia, which is represented by four different species in Milne Land (Fürsich, 1982). Like the Svalbard Slottsmøya Member, the East Greenland Krebsedal, Pernaryggen and Astartedal members consist mainly of shallow shelf siltstones deposited beneath the fair-weather wave base (Fürsich, 1984). The depositional environment and the benthic faunal assemblages of the Volgian stage in East Greenland are interpreted in terms of a shallowing-upward sequence ending in deltaic and lagoonal settings at the end of the middle Volgian in the Hennigryggen Member (Fürsich, 1984). This shallowingupward sequence also seems to be reflected in the lower to middle Volgian of the Slottsmøya Member but in a more distal setting (Koevoets et al., 2018).
To compare the Sverdrup Basin (Canadian Arctic) invertebrate fauna to other Arctic studies more data need to be collected in the future, as previous research mainly focuses on ammonite and Buchia zonation (Frebold, 1961;Jeletzky, 1966;Christie & McMillan, 1994).
The northern Siberian sections on the Nordvik Peninsula do not show the degree of similarity with Spitsbergen seen in the East Greenland faunas. The abundance and diversity of genera in northern Siberia are highest in the Oxfordian and Kimmeridgian (roughly 18 genera) and lowest in the Volgian (4 genera) (Zakharov et al., 2014). The bivalve assemblages were dominated by epifaunal and semi-infaunal genera throughout the Oxfordian to Volgian (Zakharov, 1966) and feeding habits became less diverse toward the end of the Volgian (Zakharov et al., 2014). Deposit feeders were particularly rare in the late Volgian, if present at all. The remaining filter-feeding bivalves were represented by a single genus (Buchia), which is believed to be a result of environmental circumstances, such as deepening of the sea causing low oxygen levels in both bottom waters and substrate (Zakharov et al., 2014). Considering the stratigraphic succession and the difference in bivalve assemblages compared to this study, it appears that the strata in the Nordvik sections were deposited in a marine basin that was isolated from the area in which the Agardhfjellet Formation was deposited, or with very different environmental conditions.
The changes in the less diverse bivalve assemblages in the Agardhfjellet Formation seem to mimic the changes in the more proximal settings of Milne Land, suggesting that these areas were connected oceanographically during the Middle Jurassic to lowermost Cretaceous time period. Assemblages and lithology show an overall transgression and starvation of the basins with widespread undisturbed, laminated, black shale deposits, followed by shallowing-up and more hospitable circumstances causing rapid benthic invertebrate colonisation across the basins (Parker, 1967;Birkelund et al., 1984;Koevoets et al., 2016).

Lingulid brachiopods
A noteworthy bed is found in the DH2 core at 481-484 m, in the uppermost part of the Slottsmøya Member, where there are abundant specimens of Lingularia sp. (Fig. 8J-L). This lingulate brachiopod genus is also found in hydrocarbon seeps of the member (Holmer & Nakrem, 2012). A similar bed is present in the lowermost part of the Hennigryggen Member of East Greenland (Fürsich, 1984). The lack of any other benthic invertebrates in these beds signals a significant environmental change.
U.V. Figure 10. Stratigraphical logs of the Bathonian to Valanginian successions of Spitsbergen (DH2 core based on Koevoets et. al., submitted), Milne Land, East Greenland (based on (Fürsich, 1982(Fürsich, , 1984, Sverdrup Basin, Arctic Canada (Embry, 1993) and the Nordvik Peninsula area (composite section), Northern Siberia (Nikitenko et al., 2013(Nikitenko et al., , 2015 Conclusions At outcrop, invertebrate fossils from the shales in the Middle Jurassic to lowermost Cretaceous Agardhfjellet Formation are largely limited to moulds and many are heavily fragmented by frost-wedging in the Arctic environment. In contrast, rock cores drilled as part of the CO 2 storage project show that invertebrates in this formation were more abundant and diverse than previously thought. The ammonite stratigraphy provides a good correlation between this and other Arctic sections. The succession of bivalve assemblages and the accompanying preferred life position and feeding habits fit well with the oxygenation and depositional environments proposed by Koevoets et al. (2018) as a distal response to prograding and retrograding of the coastline.
The Oppdalen Member, with a sparse invertebrate fauna, represents a transition to outer-shelf environments where nektic fauna dominates and benthic fauna consists of rare epifaunal/semi-infaunal bivalve occurrences. The fining-upward successions of the Lardyfjellet and lower Oppdalssåta members are interpreted as a forwardstepping coastline/delta as they shallow from outer-shelf nektic and epifaunal dominated benthic fauna, to the transitional zones where the benthic fauna is dominated by infaunal bivalves as bottom waters become more oxygenated, to relatively proximal sandstones of a distal delta front with extremely low bivalve abundances as a result of the higher energetic environment. The upper Oppdalssåta Member and Slottsmøya Member are interpreted as part of a backstepping coastline/delta sequence, ending in more distal deposits belonging to the transitional zone and eventually outer shelf before shallowing up again in the uppermost Slottsmøya Member. The increased abundance and diversity of the invertebrate fauna and bivalve assemblages dominated by the infaunal Hartwellia sp. for the majority of the Slottsmøya Member is indicative of shallow-shelf conditions.
The sequence of regression and transgression in the Agardhfjellet Formation is similar to the sequences observed in the more proximal East Greenland sections. With similar bivalve genera and assemblage overturns at the same ages, these localities appeared to be connected. In contrast, the northern Siberian Nordvik sections show different bivalve genera, overturning moments and sequence stratigraphy.