Glauconite authigenesis during the warm climatic events of Paleogene: Case studies from shallow marine sections of Western India

摘要

Glauconite forms abundantly within the Paleogene warm climatic intervals. However, the role of warm climate on glauconitization is yet to be explored. Glauconitic shales are ubiquitous in transgressive shallow marine deposits of Cambay, Kutch, Jaisalmer, and Barmer basins at the western margin of India. Although the glauconite is most abundant in upper Paleocene-lower Eocene sedimentary deposits in these basins, it also occurs within the middle Eocene and upper Oligocene successions. High-resolution biostratigraphic data, integrated with carbon isotope signatures, demarcate the Paleogene hyperthermal events and reveal an exceptionally high abundance of glauconite corresponding to the warming events. Glauconite occurs as pellets and infillings within the pores of bioclasts, although they differ in chemical composition. The glauconite pellets show variable K2O content, ranging from 4 to 8 wt with moderately high Fe2O3 (>20 wt), representing the entire maturation spectrum. Glauconite marking the Paleocene-Eocene transitional sediments is distinctive by the high Al2O3 content (>10 wt ), while those within the middle Eocene and Late Oligocene show considerably low Al2O3 (<8 wt). The glauconitic shales often show minor bioturbation and are rich in rectilinear benthic foraminifera, indicative of the oxygen-depleted bottom-water conditions. The unusual composition of upper Paleocene-lower Eocene glauconites relates to their formation within kaolinite substrates during the extremely warm climatic interval. Contrary to this, during the middle Eocene and Late Oligocene, the waning phase of Paleogene warm climatic conditions, glauconite formed by the initial authigenic precipitation of Fe-smectite/ Fe-Al-smectite and its subsequent maturation. The warm climatic condition enhanced the precipitation and runoff, which supplied enhanced nutrients including K, Fe, Al, Si, and Mg into the shallow marine environment, facilitating prolific organic growth and enriching the seawater with cations. The decomposition of organic matter might have resulted in an oxygen-depleted bottom water condition, which was suitable for the mobility and fixation of iron into the glauconite structure. The glauconite formed abundantly during hyperthermal events because of the convergence of favorable factors such as rapid transgression, reduced sedimentation rate, warm seawater condition, enhanced continental weathering, and enhanced supply of nutrients favoring dysoxic shallow shelves. However, rapid and extreme hyperthermal events such as PETM inhibits glauconite formation.