Glaucony formation during warm phases of Earth history: new insight from Upper Cretaceous greensand giants

Massive and (geologically) fast shallow-marine glaucony formation was a widespread phenomenon during the Cretaceous greenhouse world that has no recent analogue. Based on several integrated case studies from different basins around the Mid-European Island, we intend to better understand the geochemical and depositional constraints on Late Cretaceous glaucony formation. X-ray diffraction analyses showed that the abundant green grains and matrix of all sites definitely constitute glauconitic minerals with high-order, 1M-type layer stacking. Inorganic geochemical analyses, normalized to Al and compared to average shale (AS), demonstrate that most element/Al ratios of greensand deposits are higher than AS values, including the chemical index of alteration (CIX). These observations suggest an intense chemical weathering of the exposed hinterlands, resulting in a continuous supply of essential elements required for glaucony authigenesis in nearshore settings under variably reducing and/or oxidizing redox conditions, associated with somewhat increased palaeo-productivities. The leaching of palaeosols and swampy coastal low-/wetlands during major transgressions related to eustatic Late Cretaceous sea-level rises was an important process for trace metal and nutrient mobilization. Furthermore, the significant influx of terrestrial organic matter from the densely vegetated continents suggests a significant impact of plant-decay-related potassium to glaucony maturation. In a nutshell, our new integrated data provide novel insights into the anactualistic formative processes of authigenic glauconitic minerals during greenhouse phases of Earth history.