Eocene foraminiferal biofacies in Kutch Basin (India) in context of palaeoclimate and palaeoecology

Sonal Khanolkar,Pratul Kumar Saraswati

doi:10.1186/s42501-019-0038-2

Abstract

The Eocene Epoch passed through multiple hyperthermal events and recorded highest temperatures in the Cenozoic. Very few studies from Eocene palaeotropical sites have recorded changes in shallow marine foraminiferal assemblages. The present study investigates the foraminiferal biofacies of shallow marine successions from a palaeotropical site in western India (Kutch Basin) to understand the palaeoclimate and its impact on the ecology of foraminifera. The sections were biostratigraphically constrained using planktic and larger benthic foraminifera. Four biofacies are recognized by detrended correspondence analysis of the sample-wise distribution of foraminifera. Low diversity and dwarfed foraminifera characterize Bulimina–Chiloguembelina biofacies (SBZ5/6–SBZ10), corresponding to the interval of Paleocene–Eocene Thermal Maxima (PETM) and Eocene Thermal Maxima 2 (ETM 2). Rectilinear benthic foraminifera and biserial and triserial planktic foraminifera, typical of high runoff, upwelling or eutrophic conditions, are dominant taxa in this biofacies. The specialist taxa increased significantly in Asterigerina–Cibicides biofacies, corresponding to SBZ11 (Early Eocene Climatic Optimum, EECO), and the environment became oligotrophic. The Jenkinsina–Brizalina biofacies (E12) is distinguished by foraminiferal assemblage ecologically like that of Bulimina–Chiloguembelina biofacies. It is characterized by high abundance of rectilinear benthic foraminifera and bloom of triserial planktic foraminifera, suggesting eutrophy and high runoff at the initiation of Middle Eocene Climatic Optimum (MECO). The foraminifera were more diverse and abundant in Cibicides–Nonion biofacies. The highly diverse larger benthic foraminiferal assemblage in this biofacies, signify warm and clear-water oligotrophic sea that promoted the development of platform carbonate in Kutch Basin and other basins in western India. The EECO and MECO did not have an adverse impact on shallow marine foraminifera, and particularly the larger benthic foraminifera attained high diversity, high abundance, larger size and wider latitudinal distribution in the middle Eocene.

Highlights

The palaeotropical, Eocene, shallow marine succession is well exposed in Kutch Basin of western India (Biswas 1992; Khanolkar and Saraswati 2015)
The Cenozoic rocks of Kutch Basin have been explored for oil in the offshore field in the past decade, and numerous papers based on the exposed onland sections were published in reference to lithostratigraphy, biostratigraphy and sequence stratigraphy
The Eocene Epoch is punctuated by various hyperthermal events including the Paleocene–Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM 2), Eocene Thermal Maximum 3 (ETM 3), Early Eocene Climatic Optimum (EECO) and Middle Eocene Climatic Optimum (MECO) (Zachos et al 2001; Whidden and Jones 2012), and it would be crucial to study this interval with respect to the changes in foraminiferal assemblages

Summary

Introduction

The palaeotropical, Eocene, shallow marine succession is well exposed in Kutch Basin of western India (Biswas 1992; Khanolkar and Saraswati 2015). It is a highly fossiliferous, mixed carbonate–siliciclastic succession comprising well-preserved foraminifera of early Eocene (shallow benthic zones: SBZ5/6 to SBZ11) and middle Eocene (planktic zone: E11 to E13). The distribution of foraminifera across the Eocene succession has not been detailed for statistical analysis to document the changes in foraminiferal biofacies. The carbon isotope excursions (CIE) corresponding to the early and middle Eocene hyperthermal events have lately been established in Kutch Basin and the adjacent Cambay Basin (Clementz et al 2011; Samanta et al 2013; Saraswati et al 2013; Khanolkar et al 2017), and the oxygen isotopic data of foraminiferal carbonates of

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