Oxygen-depleted and ferruginous seawater composition imprinted in Early Cretaceous Fe-rich Al-glauconites in marginal marine deposits

Abstract

The composition of marine authigenic iron silicates bears subtle evidence of paleo-oceanographic and paleoclimatic conditions. This study focuses on the compositionally unique glauconite within the Early Cretaceous Pariwar and Habur formations in the Jaisalmer Basin and discusses its implications. The Al-glauconite is enriched in TFe2O3 (> 18 wt%), and is considered as Fe-rich Al-glauconite. The X-ray diffraction parameters, the micro-texture, and the K2O content reveal an evolved to highly evolved structure of the glauconite. It is abundant in shallow subtidal and tidal- and wave-influenced shoreface facies, and it forms either by the alteration of fecal pellets or as bioclast infillings. Micropaleontological and ichnological proxies confirm the glauconitization in an oxygen-depleted, shallow marine depositional setting. Small-sized (< 1 cm) fodinichnial, pascichnial, domichnial, and repichnial traces, and agglutinated foraminifera (Bathysiphon) corroborate the oxygen-depleted seawater. The landward expansion of the oxygen minimum zone, leading to oxygen depletion in coastal settings, facilitates the glauconitization in a marginal marine setting. Kaolinite and Fe-Al-smectite react in the presence of excess Fe, forming Fe-rich Al-glauconite on the seafloor. Therefore, the compositionally unique glauconite reflects atypical seawater chemistry with increased elemental influx of Al, Fe, and Mg related to intense continental weathering, kaolinite substrate, and mobilization of Fe2+ in oxygen-depleted seawater.