Foraminifera from Upper Jurassic - Lower Cretaceous hydrocarbon seep carbonates and adjacent shales in the Slottsmøya Member, central Spitsbergen

Abstract

Benthic foraminifera have been extracted from both hydrocarbon seep carbonate structures and adjacent shales of the upper Slottsmøya Member (Agardhfjellet Formation) in central Spitsbergen. The assemblages contain both agglutinated and calcareous taxa, although the shale assemblages are virtually agglutinated. The fossil-bearing hydrocarbon seep bodies are the first indication of the existence of cold seeps in Spitsbergen.

The preservation of the agglutinated tests from the seep carbonates is excellent. This is due to authigenic precipitation of carbonate, which led to early cementation of the sediment, protecting the foraminiferal tests from compaction. The taxa composing the assemblages are closely related to previously recorded species from the dark shales of the Slottsmøya Member as well as from age equivalent deposits of Arctic Russia and other areas of the Boreal Realm. Some new occurrences are also observed.

The relatively high diversity mixed (calcareous-agglutinated) assemblages of the seeps indicate environmental conditions close to those of a normal marine shelf, while the low diversity agglutinated assemblages of the surrounding shales suggest hypoxic conditions. Thus, the hydrocarbon seeps created patches of environments where assemblages with a dominant normal marine aspect could exist on the generally hypoxic basin floor.

The foraminifera date the seep carbonate bodies as Late Volgian-Late Ryazanian; an age determination that is in part divergent from ammonite datings previously obtained from these carbonates. Early Volgian-Earliest Ryazanian age is proposed for the succession of shales sampled.

This study presents the first systematic report of foraminifera from hydrocarbon seep structures developed at the Jurassic-Cretaceous transition and the first comparison of their faunas with assemblages in the adjacent shales and siltstones.