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Paleoenvironmental conditions during the paleocene–eocene transition imprinted within the glauconitic giral member of the barmer basin, India
Journal
Minerals
Date Issued
2022-01-01
Author(s)
Choudhury, Tathagata Roy
Banerjee, Santanu
Khanolkar, Sonal
Meena, Sher Singh
Abstract
The roughly 6 m thick limestone–green shale alternation within the lignite‐bearing Giral Member of the Barmer Basin corresponds to a marine flooding event immediately after the Paleo-cene–Eocene transition. A detailed characterization of the glauconite using Electron Probe Micro Analyzer (EPMA), X‐Ray Diffraction (XRD), Mössbauer and Field Emission Gun‐Scanning Electron Microscope (FEG‐SEM) reveals its origin in the backdrop of prevailing warm climatic conditions. The glauconite pellets vary from fine silt‐sized to coarse sand‐sized pellets, often reaching ~60% of the rock by volume. Mineralogical investigation reveals a ‘nascent’ to ‘slightly evolved’ character of the marginal marine‐originated glauconite showing considerable interstratification. The chemical composition of the glauconite is unusual with a high Al2O3 (>10 wt%) and moderately high Fe2O3(to-tal) contents (>15 wt%). While the K2O content of these glauconites is low, the interlayer sites are atypically rich in Na2O, frequently occupying ~33% of the total interlayer sites. The Mössbauer spec-trum indicates 10% of the total iron is in ferrous form. High tetrahedral Al3+ of these glauconites suggests a high‐alumina substrate that transformed to glauconite by octahedral Al‐for‐Fe substitu-tion followed by the addition of K into the interlayer structure. The unusually high Na2O suggests the possibility of a soda‐rich pore water formed by the dissolution of alkaline volcanic minerals. The Giral glauconite formation could have been a part of the major contributors in the Fe‐seques-tration cycle in the Early Eocene shelves. Warm climate during the Early Eocene time favored the glauconitization because of the enhanced supply of Fe, Al, and Si and proliferation of an oxygen-depleted depositional environment.
Subjects