Deciphering Late Cretaceous (Cenomanian to Campanian) coastline dynamics in the southwestern Münsterland (northwest Germany) by using calcareous nannofossils: Eustasy vs local tectonics

Abstract

The Late Cretaceous global sea level maximum is documented by the widespread occurrence of chalks and marls. These carbonates, which contain rich and diverse calcareous nannofossil assemblages, were deposited in pelagic to hemipelagic environments. Previous studies of the biostratigraphy and ecology of calcareous nannofossils have therefore often focused on sections from open oceanic settings, including DSDP and ODP sites. Marine nearshore sediments of Late Cretaceous age have recently been cored by drilling activities in the western part of the Münsterland. Six wells, positioned about 5–20 km north of the former coastline, penetrated Cenomanian to Campanian sedimentary rocks. Lithologically these strata are composed of sand-rich, glauconitic marls and glauconitic limestones, which unexpectedly yielded well-preserved and diverse calcareous nannofossil assemblages. The light-microscopic analysis of 330 rock samples provided the following results: 1) A detailed, biostratigraphic zonation based on calcareous nannofossils has been established for shallow-water successions. The interval studied covers biozones UC0-1a (lowermost Cenomanian) to UC14a (lower Campanian). 2) The δ13C bulk carbonate curve obtained from one of the cores correlates well with the standard curve of the English Chalk facies. The following isotope events were observed: CTBE (late Cenomanian), White Fall (middle Coniacian), Haven Brow and Horseshoe Bay (middle Santonian), Buckle (middle to upper Santonian), Hawks Brow (upper Santonian) and SCBE (lower Campanian). 3) The biostratigraphic data suggest a heterochronous onset of the Late Cretaceous transgression in the study area, related to local variations in topography. 4) By correlating the six cores stratigraphically, we recognise that certain sedimentation patterns are controlled by eustasy and others by local tectonics. The Cenomanian to lower Turonian successions suggest a gradual flooding, where cores with condensation and hiatuses reflect local topographic highs. For the northernmost core, positioned furthest from the palaeocoast, an early to middle Turonian phase of tectonic uplift has been identified.