Foraminiferal morphogroups in dysoxic shelf deposits from the Jurassic of Spitsbergen

Abstract

Analysis of benthic foraminiferal assemblages was performed in Bathonian to Kimmeridgian deposits through a section covering the lower half of the Agardhfjellet Formation in central Spitsbergen. The section consists mainly of organic-rich shales, which contain low-diversity agglutinated assemblages. In this foraminiferal succession five morphogroups were differentiated according to shell architecture (general shape, mode of coiling and number of chambers), integrated with the supposed microhabitat (epifaunal, shallow infaunal and deep infaunal) and feeding strategy (suspension-feeder, herbivore, bacterivore, etc.). The environmental evolution of the analysed section is interpreted by using the stratigraphic distribution of morphogroups, combined with species diversities and sedimentary data, in a sequence stratigraphic framework. The section comprises two depositional sequences, which demonstrate that species diversity and relative frequency of morphogroups are correlative with transgressive–regressive trends controlling depth and oxygenation of the water column. In both sequences, the maximum flooding interval is characterized by increased organic carbon content, dominance of the epifaunal morphogroups and reduced species diversity: features reflecting the increased degree of stagnation separating the transgressive phase from the regressive phase.The participation of J. Nagy in this research has been supported by the Statoil/Hydro VISTA programme. The contribution of M. Reolid and F.J. Rodríguez-Tovar has been supported by the projects CGL2005-06636-C0201 and CGL2005-0316/BTE, the EMMI group (RNM-178, Junta de Andalucía) and the Acción Integrada 30.AI.PO.1300 (University of Granada–University of Oslo). A grant of the Universidad de Jaén financed M. Reolid’s short stay at the University of Oslo