Enhanced chemical weathering of the continents promoted fast Late Cretaceous nearshore glaucony formation: implications from the Danubian Cretaceous Group, Germany

Abstract

Widespread, geologically fast (<500 kyr) shallow-marine glaucony formation during the Cretaceous greenhouse world has no recent analogue. A detailed investigation from the Upper Turonian Danubian Cretaceous Group (southern Germany) links authigenic marine glaucony precipitation (Großberg Formation) to the intense chemical weathering in the catchment of a supplying river system (Seugast Member, Roding Formation). Nine glauconitic facies types within the Großberg Formation reflect a mixed shallow-marine depositional environment with land-derived siliciclastics, marine-carbonates and authigenic glaucony. X-ray diffraction analyses confirm these authigenic green grains and matrix as glauconitic minerals. The very high values of the modified chemical index of alteration (mean CIX between 86 and 92) demonstrate that the catchment area of the Seugast River was deeply chemically weathered and leached. In comparison to the hydrochemistry of modern tropical streams, massive Late Cretaceous nearshore glaucony formation was promoted by the riverine flux from the deeply chemically weathered, wet-continents amply supplying essential elements (Fe, Si, K and Al), fuelling shallow-marine glauconitization. The sequence stratigraphic calibration of the shallow-marine to continental transect of the Großberg Formation and the Seugast Member, corresponding to the Late Turonian depositional sequence DS Tu 5, further demonstrates that Late Cretaceous glaucony formation was a fast process (<500 kyr). Supplementary material: Table S1 provides contents of total carbon (TC), total inorganic carbon (TIC), total organic carbon (TOC), major elements and Zr as well as element/Al ratios, calculated excess K (K xs ), silicate K (K sil or K 2 O sil ), molar element oxides and CIX values, and is available at https://doi.org/10.6084/m9.figshare.c.6297404