Benthic foraminifera from the lower jurassic transgressive mudstones of the south-western barents sea-a possible high-latitude expression of the global Pliensbachian-Toarcian turnover?

Abstract

The Lower Jurassic section of two commercial wells (7119/12-1 and 7119/12-2), drilled in the Hammerfest Basin, was analysed for foraminiferal assemblages in order to reconstruct palaeoenvironmental changes. Foraminifera occurred singularly in transgressive mudstone intervals of the Stø Formation in well 7119/12-1. High-energy marginal marine and offshore sandstones and freshwater sand- and mudstones were barren of foraminifera. Three distinct foraminiferal assemblages could be distinguished. They contain exclusively agglutinated taxa and have extremely low species diversities, indicating restricted environmental conditions. The main restricting factor was low salinity in strongly delta-influenced waters. Foraminiferal assemblage 1 is developed at the base of the Stø Formation and suggests transgressive, extremely hyposaline lagoonal conditions. Assemblages 2 and 3 occur in hyposaline prodelta shelf mudstones of the Stø Formation and reveal increased diversities in major local transgressive phases. The foraminiferal assemblages indicate Pliensbachian-Toarcian ages. The mudstones of the Stø Formation are regarded as a local expression of the global Pliensbachian-Toarcian turnover recording regional transgressive developments in the Toarcian. © 2014S. Hess et al.