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

Highlights

• Glauconite formed abundantly during the Paleogene.

• Abundance of glauconite corresponds to hyperthermal events.

• Glauconite formed on shallow suboxic shelves.

• Rapid transgression and abundant supply of nutrients facilitated glauconitzation.

• Glauconite corresponding to hyperthermal events show unique geochemistry.

Abstract

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.

Introduction

Iron is a limiting nutrient for marine productivity as it has a major impact on the evolution of the ocean geochemistry and life forms throughout the earth's history (Baldermann et al., 2015; Taylor and Macquaker, 2011). Authigenic iron-bearing minerals show a wide compositional spectrum ranging from Proterozoic hematite-magnetite rich Banded Iron Formation (BIF) to iron-bearing silicates including glauconite, ferric illite, berthierine/chamosite, and celadonite as well as iron oxides (Bekker et al., 2014; Planavsky et al., 2011; Tounekti et al., 2021). Phanerozoic iron formations, especially oolitic ironstones, have received considerable attention regarding their stratigraphic and environmental implications and temporal variation (Van Houten and Arthur, 1989; Van Houten and Purucker, 1984). In contrast, glauconites and other iron-bearing clays have received insignificant attention (Banerjee et al., 2020). The widespread glauconite formation, especially during the Phanerozoic time, deserves considerable focus for its potential to be used as a geochemical archive for understanding oceanographic parameters as well as ocean geochemistry and elemental cycling (Mänd et al., 2021). Although reports on the temporal distribution of glauconite reveal a non-uniform distribution, with peak abundances of glauconite during the Cretaceous and Paleogene, the controlling factors for this distribution are still poorly understood (Banerjee et al., 2016, Banerjee et al., 2020; Bansal et al., 2020b; Roy Choudhury et al., 2021). While recent studies suggest that glauconite prefers to be associated with deposits of warm climatic intervals in Paleogene (Roy Choudhury et al., 2021, Roy Choudhury et al., 2022), the role of warm Paleogene climate on its formation is still poorly understood. In addition, the overall warm Paleogene climate is further superimposed by several short-lived, extreme climatic conditions known as hyperthermal events, including Paleocene-Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM2), Eocene Thermal Maximum 3 (ETM3), Early Eocene Climatic Optimum (EECO), Middle Eocene Climatic Optimum (MECO) and Late Oligocene Warming Event (LOWE) (Zachos et al., 2001). However, very few studies have highlighted the effects of hyperthermal events on authigenic glauconite and other Fe-silicate formation. The lack of biostratigraphic resolution is a major challenge to link the glauconitization to short-lived warm climatic events. The formation of abundant glauconite during the Paleogene across the globe needs to be understood using an integrated approach, including sedimentology, biostratigraphy and geochemistry. The stratigraphically expanded Paleogene successions from shallow shelves in western India, across four sedimentary basins prompt us to explore the connection between warming events and glauconite formation.

Paleogene sedimentary basins in western India, viz. Kutch, Cambay, Jaisalmer, and Barmer basins, record the transgressive deposits at the eastern Tethyan domain. These shallow marine basins show thick piles of sediments with economically viable lignite, oil, and gas deposits. Although the biotic assemblages of the basins have received considerable attention, the abundance of glauconite within these sedimentary basins is poorly examined. The biostratigraphy of the glauconitic succession is poorly constrained. Although glauconites were reported in the context of sequence architecture, the climatic significance of glauconites was not explored. A few studies indicated the hyperthermal events within the Paleogene deposits (Clementz et al., 2011; Khanolkar et al., 2021; Khozyem et al., 2021; Samanta et al., 2013a). However, the relationship between glauconite occurrence and hyperthermal events was not explored. The objectives of this study are to a) establish the relationship between Paleogene hyperthermal events and glauconite deposits, b) highlight the impact of hyperthermal events on the geochemistry of glauconites, and c) discuss the factors that promote glauconitization during warming events. In this study, we have compiled the available biostratigraphic and carbon isotopic data from four sedimentary basins in western India to understand the origin of glauconite in the backdrop of Paleogene hyperthermal events.