PALEOGENE LARGER FORAMINIFERAL BIOSTRATIGRAPHY OF PARTS OF WESTERN INDIAN SHELF

Abstract

The Paleogene of western Indian continental shelf is geographically so extensive that it could be subdivided into several distinct sedimentary basins, each having its own geological history and evolution through time. The western continental shelf extends from the Rajasthan Basin through Kachchh and parts of Saurashtra and Cambay Basin to the western offshore basins, right down to the Kerala - Konkan area, with some remarkable similarities. A major similarity is the Deccan Traps basement (also referred to ―technical basement,‖ in hydrocarbon exploration parlance), over which fairly extensive carbonate - shale deposition took place in somewhat varying intensity. The hydrocarbon provinces of these sedimentary basins are distinct, with a few commonalities i.e. deposition of extensive Paleogene carbonate / marl / shale sequence over most of the western Indian continental shelf. These sequences are predominated by larger benthic foraminifera occurring together with the other fossil groups. In the present study, detailed biostratigraphic work on the taxonomic standardization of selected foraminiferal species with the help of morphometric analysis and foraminiferal biozonation was undertaken on the Paleogene outcrops from two main western Indian sections i.e. Berwali River section (District Kachchh) of the Kachchh Basin and the Tapti River section of the Cambay Basin in the Surat and Bharuch districts; both in the western Indian state of Gujarat. Both these regions provide excellent outcrops of Tertiary formations, 7 with prolific larger foraminifera, encompassing almost all the epochs of the Tertiary era. Vast outcrops with enormous shelf carbonate facies embedded with both mega and microfossils occur in all the major river or road sections in Kachchh, parts of south Cambay Basin as well as in the neighbouring Jaisalmer Basin of the Rajasthan state in the western India. Besides the outcrops, selected samples from the exploratory wells drilled for the hydrocarbon exploration in Cambay and western offshore basins, have also yielded diagnostic foraminifera useful in proposing larger foraminiferal bio-zonation for western shelf. In the south-western Kachchh, Paleocene - Eocene outcrops are located in the vicinity of Kakdi - Doriwali river section and road sections on way to Lakhpat/ Narayansarovar. The Berwali river area shows the outcrops of rocks belonging to Middle Eocene, Late Eocene and Oligocene epochs. Further south, Miocene and younger formations successively outcrop in Berwali and other adjoining river sections; dipping and younging towards the present day coast. The thickness of all the outcropping formations greatly increases towards the basinal (Offshore) area. In the south Cambay basin, Paleogene outcrops in Amrawati and Tapti River / road sections; Vastan-Tarkeshwar mines, in Surat – Ankleshwar area are common. The late Eocene carbonates are better exposed here and provide an ample opportunity to collect and study larger foraminifera. The study area was mapped with the help of Brunton compass at 1:50,000 scale, during several field studies undertaken since 1985 onwards. The foraminifera and associated fauna were studied for biostratigraphic determinations. The 8 conclusions drawn from the present work provide insights into geological history of the basins, taxonomic standardization of important species, larger foraminifer zonation applicable to western Indian shelf; stratigraphic range of important species, correlation with the SBZ scheme and Indian equivalents to SBZ scheme. A brief account of paleoecological interpretation and generalized hydrocarbon prospects of the region is also given. It is important to point out that the morphological as well as biometric (measurable) changes in the tropical Tethyan larger foraminiferal fauna (especially nummulitids) have been recorded and compared with their temperate counterparts in this study and a monograph was published during the year 2009. Elsewhere, similar studies have been mainly conducted in the European basins by Prof. Hans Schaub; Prof. Alphonso Blondeau (on the larger foraminifer Nummulites populations of Swiss Alps and Paris basins, respectively); Dr. Andrew Racey and possibly a few more scientists. These studies point to the need to develop locally applicable biostratigraphic zonation on the lines of Shallow Benthic Zonation (Serra Kiel et al., 1995). It has been frequently observed that many forms characterizing Serra Kiel et al. (1995) S.B. Zones do not frequent in Indian material and hence creation of Indian equivalents to the Serra Kiel et al. (1995) scheme was immensely needed. This scheme is a widely acceptable biostratigraphic tool for regional correlations in Indian sedimentary basins and serves as the link between the Mediterranean and Far-Eastern 9 Tethyan species. The present work supplements, earlier bio-chronostratigraphic investigations carried out by previous workers. The present thesis is divided into six chapters. The first chapter deals mainly with the location, area of study, previous work, objectives and scope of the work, methodology and the sampling and field related aspects. The second chapter describes geology and tectonic setting, stratigraphy and geological history of the Kachchh and Cambay basins. The third chapter discusses systematic paleontology, as well as microfossil / sample preparation techniques and biostratigraphic data management software utilized in the present thesis. The fourth chapter deals with regional biostratigraphy, systematics, biometric analysis, taxonomic description of species recorded, foraminiferal distribution in the study areas, Tethyan shallow benthic zones, Indian biochronostratigraphic stages and their relationship with the shallow benthic zonation and modified larger foraminiferal biozonation on western Indian shelf. The fifth chapter briefly discusses paleoenvironments of the study areas and generalized hydrocarbon prospects. The sixth chapter comprises the thesis summary and conclusions along with the check lists of foraminifera & proposed biozonation. A list of the publications based on the results presented here is also included at the end of the thesis.