Chalk palaeoenvironments and depositional model, Valhall-Hod fields, southern Norwegian North Sea

Abstract

Abstract Palaeoenvironmental analysis in the North Sea chalk is hampered by the prevalence of widespread allochthonous units, intervals of massive reworking of microfossils and an often monotonous lithology of massive chalks of uncertain depositional history. Most difficult of all is devising precise regional biostratigraphic correlations in a complex section with large, but very subtly expressed, disconformities and condensed zones. Facies change quickly both laterally and vertically. Without very precise regional correlations, it is impossible to formulate any detailed palaeoenvironmental model. Through a multidisciplinary palaeontological approach, employing graphic correlation methodology and a composite standard database, a highly accurate and detailed regional chronostratigraphy was generated. Biofacies recognized within the section were then scaled in absolute time, allowing for more reliable recognition of facies trends at any one time horizon. Biofacies were formulated based upon recovered benthic foraminifera, planktic foraminifera, calcispheres, siliceous microfossils and macrofossil debris. Biofacies mapping for several higher-order sequences recognized in the Upper Cretaceous chalks has allowed recognition of four major periods of deposition: (1) early Turonian-early Santonian high pelagic productivity and rapid deposition of chalks over a relatively continuous ridge from the Valhall to Hod fields; (2) early Campanian-earliest Maastrichtian deep-water autochthonous chalk deposition off-structure with structural crests mantled by submarine hardgrounds; (3) very heterogeneous autochthonous and allochthonous middle Maastrichtian chalk deposition controlled by local grabens formed from crestal collapse; and (4) widespread sediment flows and redeposition of sediments with crestal shoaling and winnowing in the latest Maastrichtian culminating in crestal erosion of Cretaceous chalks in the early Danian.