Ostracode biostratigraphy and carbon and oxygen isotopic studies across the Paleocene/Eocene boundary in the subsurface of Jaisalmer Basin, Rajasthan, India

Abstract

The ostracode fauna across the Paleocene/Eocene boundary has been studied at 5 m intervals between 690–810 m in the Ghotaru Fort Well (GTF Well-A), Jaisalmer Basin, Rajasthan. Ostracodes and foraminifera have been utilized to provide the biostratigraphic framework. Four ostracode zones designated as POS-I to POS-IV in the Late Paleocene and one zone EOS-I in the earliest Early Eocene age sediments have been recognized. Quantitative ostracode and qualitative foraminiferal data were used to interpret the paleoenvironment across the Paleocene/Eocene boundary. Occurrence of the benthic foraminiferaDiscocyclina seunesi andMiscellanea miscella at 735–740 m depth in this well suggests a latest Late Paleocene age. The frequency and diversity of ostracodes at this depth is low. The ostracodes recorded near the top of the Paleocene are:Anommatocytkere indica, Echinocythereis contexta, Phyrocythere alfuraihi andSchizocythere prolata. This ostracode assemblage along with benthic foraminifera suggests that sediments were deposited in an inner neritic environment. This is followed by an earliest Early Eocene age transgression marked by the extinction of preceding Late Paleocene ostracodes and advent of a new ostracode suite represented byAlocopocythere abstracta in the 725–730 m interval. In the 720–725 m interval, however, there is a turnover to an entirely new suite of ostracodes represented byParagrenocythere reticulospinosa, Phalcocythere sentosa, Hornibrookella avadheshi, H. jaisalmerensis, Alocopocythere abstracta, Gyrocytbere parvicarinata, Schizocythere appendiculata, Bairdoppilata kalakotensis, Uroleberis khoslai, Loxoconchella ? guhai, Cytherella jaisalmerensis andC. tawaica. This ostracode assemblage is indicative of Early Eocene age and deposition in an inner neritic environment. The Paleocene/Eocene boundary in well GTF-A is recognised on the basis of ostracodes and foraminifera at 735–740 m, which in turn coincides with an overall sea level fall attributed to global warming, and this is further corroborated by a negative carbon excursion (δ¹³C=−13.63) across the Paleocene/Eocene boundary.