Geochemical and depositional environment of an Upper Cretaceous greensand giant (Münsterland Cretaceous Basin, Germany)

Abstract

Based on integrated geochemical, mineralogical and stratigraphic-sedimentological analyses of core section KB 4507/1001 Essen-Bedingrade from the Münsterland Cretaceous Basin (northern Germany), the geochemical and depositional environment of a Late Cretaceous greensand giant has been elucidated. The >100-m-thick Cretaceous succession comprises the Essen Greensand (Cenomanian), Büren (lower Turonian), Duisburg (middle Turonian to lower Coniacian) and Emscher formations (middle to upper Coniacian), essentially consisting of alternating greensand and marl lithofacies. X-ray diffraction analyses showed that the abundant green grains undoubtedly constitute glauconitic minerals with high-order, 1M-type layer stacking. The depositional environment can be characterized as a shallow-marine setting in which nearshore greensands interfingered with mid-shelf marls. Inorganic geochemical analyses normalized to Al and compared to average shale (AS) show that most element/Al (E/Al) ratios are higher than AS values, including the chemical index of alteration (CIX). This suggest that an intensely chemically weathered wet hinterland provided for a constant supply of essential elements required for nearshore glaucony authigenesis under variably reducing or oxidizing conditions and slightly increased palaeoproductivity. The leaching of paleosols and swamp-like coastal wetlands during transgressive phases related to eustatic early Late Cretaceous sea-level changes was an important source for trace metals and nutrients while the considerable influx of terrestrial organic matter suggests a significant input of K related to plant decay. In a nutshell, our new integrated data from the Münsterland Cretaceous Basin provide important novel insights into the formative processes of authigenic glauconitic minerals during warm-humid climate phases of Earth history.