Glaucony formation during warm phases of Earth history: Lessons learnt from Upper Cretaceous greensand giants

Widespread, apparently rapid nearshore glaucony authigenesis was a conspicuous process during the (Late) Cretaceous green−/hothouse that has no analogue today. Based on detailed case studies from different depositional domains around the Mid-European Island, we aim to gain a deeper understanding of the stratigraphic, environmental and geochemical constraints on the formation of Upper Cretaceous greensand giants. SEM-EDX and XRD analyses demonstrate that the rock-forming green grains from all studied basins consist of glauconitic minerals with high-order, 1 M-type layer stacking. Inorganic geochemical patterns, using palaeoenvironmentally significant major and trace elements normalized to Al, show that numerous element/aluminum (E/Al) ratios of greensand deposits, including the modified chemical index of alteration (CIX), are considerably higher than average shale values. In contrast, contemporaneous continental deposits exhibit low E/Al ratios and extremely high CIX values. Furthermore, abundant plant remains indicate substantial influx of terrestrial organic matter. These findings indicate strong chemical weathering of densely vegetated hinterlands under warm-humid conditions, causing a continuous supply of essential elements required for nearshore glaucony authigenesis. The decay of terrestrial organic matter favored this process in two independent ways, i.e., ensuring slightly reducing conditions of porewaters and providing plant-decay-related potassium for rapid glaucony maturation. Reworking of swampy coastal lowlands during major Late Cretaceous shoreline retrogradation was another crucial process for trace metal mobilization. In short, our integrated data provide insight into the anactualistic process of glaucony neoformation during hothouse phases of Earth history that, as part of the reverse weathering reaction, could be a significant factor in future global change scenarios.