Greensands and the unique Neuburger Siliceous Earth: Late Cretaceous continent–shelf interactions in the Danubian Cretaceous Basin revealed (Bavaria, SE-Germany)

The Late Cretaceous epoch was characterized by extreme greenhouse climates and widespread glaucony formation in marine shallow water settings. In the Danubian Cretaceous Basin (DCB, Bavaria, SE-Germany), contemporaneous and closely-spaced shallow-marine accumulation of glauconitic strata in the eastern and the glaucony-free Neuburger Siliceous Earth in the western parts of the basin during Cenomanian–early Turonian times represents an unsolved phenomenon with great potential to unravel important aspects of the poorly understood mechanisms of Late Cretaceous glaucony formation.

An integrated sedimentologic, stratigraphic, mineralogic and geochemical approach reveals that the striking small-scale lithofacies differences can be attributed to the geological structure of the hinterland and the nature of elemental influx: in the eastern DCB, deeply chemically weathered granites and gneisses of the Bohemian Massif were leached due to the warm climate and high precipitation rates, and elements crucial for glaucony formation (K, Fe, Si, Al) were amply supplied by rivers, fueling a shallow marine greensand giant (Regensburg Formation). In the western DCB, a hinterland consisting of karstified Jurassic carbonates devoid of elements necessary for glaucony formation and a lack of significant fluvial input repressed the shallow marine glaucony factory. Furthermore, the originally rather marly sediment of the Neuburg Siliceous Earth of the Wellheim Formation was affected by early diagenetic silicification due to a combination of inflowing silica-rich groundwater from karst aquifers and dissolution of biogenic siliceous components. Additionally, our geochemical data provide the first evidence of a redox-sensitive trace metal drawdown during Oceanic Anoxic Event 2 in shallow water/coastal settings in Germany.