Abstract
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 show 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 the understanding of the depositional environments of the Agardhfjellet Formation. The occurrence pattern 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.
Highlights
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
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
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)
Summary
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. Birkenmajer et al (1982) collected bivalve specimens from Myklegardfjellet and Agardhfjellet, on the east coast of Spitsbergen, from the harder lithologies of the formation (e.g., sandstones, siltstones, siderite layers and carbonate concretions). 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. The Oppdalssåta Member comprises three smaller coarsening-upward units, grading from shales at their bases to fine-grained sandstones (Upper Kimmeridgian–?Volgian), capped with glauconitic, sideritic and carbonate-cemented beds containing wood fragments, belemnites, gastropods, bivalves and ammonites. The Late Bathonian–Early Oxfordian transgressional phase, Early Kimmeridgian–Early Volgian regressional phase, and early Volgian to Ryazanian transgressional phase are recognised in East Greenland (Surlyk, 1991) and the Sverdrup Basin (Embry, 1993)
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