SEQUENCE BIOSTRATIGRAPHY OF VERTEBRATE-BEARING EARLY TERTIARY STRATA OF GUJARAT, INDIA

Abstract

In India, the early Tertiary sequences (late Paleocene-Eocene) are well developed on the western peninsular margin in the Kutch (also spelt Kachchh) and Cambay basins of Gujarat state. This interval, marked by the Paleocene-Eocene Thermal maximum or PETM, and the Early Eocene Climatic Optimum or EECO, is critical in understanding the biotic response of India-Asia collision. In Gujarat, this interval includes a widespread episode of lignite formation during the early Eocene, represented by the Naredi Formation of Kutch and the Cambay Shale Formation of District Surat. A number of open cast lignite mines in districts Kutch and Surat offer an excellent opportunity to study near-shore, lagoonal, foraminifer-bearing successions both in a regional sub-continental framework and in the context of global (eustatic) sea level stratigraphy. In recent years, these lignite deposits and the stratigraphically adjacent formations have attracted global attention because of their rich and diverse vertebrate fauna, especially marine and terrestrial mammals. These mammal faunas are of great importance from the evolutionary and biogeographic viewpoints, and hence a well-constrained chronostratigraphic framework needs to be developed for these sequences, using the sequence stratigraphic approach. This method involves division of sedimentary record into distinct packages, where each package is interpreted to represent a cyclic change in the sea level and / or sediment supply. Biostratigraphy helps in age assignment to different sequence stratigraphic surfaces and in building a precise chronostratigraphic framework for the different sequences. Sequence biostratigraphy is a new approach, which is designed to integrate biostratigraphic information into the stratigraphic analytical process before sequences are mapped. The Naredi Formation of Kutch is important in constraining the timing of earliest marine sedimentation in the Kutch Mainland. Investigations carried out as part of the present doctoral work have led to the discovery of a significant assemblage of late Paleocene planktic foraminifers from the lower part of type Naredi Formation in the Kakdi river cliffs.This assemblage, part of which is illustrated in this dissertation, not only helps to establish the age of the earliest marine transgression in the Kutch Mainland, but also identifies a potentially important section for delineating the P-E boundary for the first time in India. Supportive evidence for this comes from the preliminary carbon isotope data which shows a negative shift near the base of the section. The type Naredi Formation exposed in the Kakdi river section represents a third order depositional sequence. The lower sequence boundary is marked by the Deccan Traps and the upper sequences boundary by extensive lateritic clays. In the present study, two fourth order and eight fifth order cycles have been identified. The subsurface equivalent of the Naredi Formation (also called Panandhro Formation of Lakhpat Formation) is exposed in an open pit lignite mine near Panandhro which has yielded marine mammals (fossil whales) in recent years. Unlike previous interpretations, this study shows that the vertebrate-yielding strata in the Panandhro Lignite Mine and those in the lower part of the Harudi Formation are time equivalents (i.e. middle Eocene, Lutetian). Two long-known vertebrate-bearing sections of the middle Eocene Harudi Formation have also been investigated as part of this work: the type Harudi Formation exposed along the Rato Nala, and the Babia Hill. The lower part of the Harudi Formation proved barren in age diagnostic fossils and the upper part yielded a rich assemblage of planktic foraminifers. Sequence stratigraphic considerations show a break in sedimentation near the top of the Harudi Formation and that the whale-bearing strata belong to an older cycle of sea level change and are of early middle Eocene (late Lutetian) in age, not Bartonian, as advocated by some workers. It is proposed that this whale-yielding interval corresponds to the Domanda Formation of Sulaiman Range, Pakistan. In recent years a diverse assemblage of terrestrial mammals including the artiodactyls, perissodactyls and primates (APP taxa) has been discovered in the Cambay Shale of Vastan Lignite Mine of Surat. The precise age of the Vastan mammal fauna is of considerable importance in testing the current Out-of-lndia hypothesis. This study shows that previous age assignment (ca. 52 Ma) based on the larger benthic foraminifer Nummulites burdigalensis, needs to be revised because the species in question pertains to Nummulites globulus, indicating a 11 slightly older age (ca. 53.5 Ma). Ongoing work on dinoflagellates has revealed the presence of an age diagnostic taxon Muratodinium fimbriatum (latest Sparnacianearly Ypresian), supporting the new age assigned here. Further, based on the field criteria and lithofacies variation, two fourth sequences (systems tracts) and six fifth order cycles (parasequences) have been identified in the Vastan sequence. Of the two major lignite seams, the lower one formed in a Transgressive Systems Tract (TST) and the upper one in the early Highstand Systems Tract (HST).