Changing depositional environments reflected by foraminifera in a transgressive-regressive sequence of the Lower Cretaceous on Spitsbergen

Abstract

The Boreal Valanginian to Barremian foraminiferal succession of the Rurikfjellet Formation in central Spitsbergen is applied to stratigraphic zonation, analyses of depositional conditions and developing a sequence stratigraphic model. The study is based on the Janusfjellet section where the formation is 204 m thick, and represents a depth range from upper bathyal to prodelta top. It contains a continuous benthic foraminiferal succession permitting distinction of five assemblage zones. The assemblages are almost entirely agglutinated and reflect calcium carbonate depletion, which appears as an overall feature of Mesozoic Boreal seas superimposed on other, but more local restricting factors such as salinity and food supply. Close taxonomic similarities within the Boreal Realm are revealed by comparison of the Janusfjellet foraminiferal faunas to those of other pan-Arctic sites. The lower half of the Janusfjellet section contains a significant faunal component comparable to modern bathyal faunas and Late Cretaceous to Paleogene flysch-type assemblages indicating upper bathyal to offshore shelf depths. In the upper half of the section increased diversities are created by enhanced food supply in middle shelf setting. This is succeeded by increasing deltaic impact leading to hyposaline environments reflected by low species diversities. Changing foraminiferal biofacies trends through the Janusfjellet section are portrayed in the framework of the third order transgressive–regressive Wimanfjellet Sequence. Its transgressive systems tract was deposited mainly at upper bathyal depths with maximum flooding surface in the lower (Valanginian) part of the Rurikfjellet Formation. The regressive systems tract reveals an upward shoaling succession from upper bathyal conditions, through marine shelf to hyposaline prodelta setting.