Stratigraphy and Palaeoenvironments of the Jurassic Rocks of Kachchh Field Guide

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Language: English

The Jurassic sedimentary succession of Kachchh represents the fill of a small rift basin situated at the western margin of the Indian Plate adjacent to the Malagasy Gulf. Jurassic rocks are exposed in three areas, the so-called lsland Belt, the Kachchh Mainland, and the Wagad Uplift. Rifting started in the Late Triassic documented by red beds and alluvial fans in cores from the Banni Plain. During the Early and early Middle Jurassic marine and non-marine conditions alternated. The corresponding sediments are exposed in the Island Belt of northern Kachchh. By Bathonian times and until the end of the Jurassic shallow marine facies prevailed. During the Bathonian, probably as result of a comparatively warm and arid climate, carbonates were widespread. In contrast, the remaining time slices are characterised by siliciclastic sediments. They range from silty clays in the low-energy, offshore areas to cross-bedded sandstones or ferruginous oolites in high-energy, shallow-water areas. Both carbonates and siliciclastics were deposited on ramps with a depth gradient towards the west and from the island Belt in the North to the Kachchh Mainland in the South. Sediment input was mainly from the north and from the east. The basin was subdivided by several E-W trending faults into sub-basins which corresponded to half grabens with somewhat differing sedimentation histories. The by far most interesting lithostratigraphic unit of the basin is the Oxfordian Dhosa Dolite member, a highly condensed unit that can be traced across the Kachchh Mainland for more than 100 km and that documents both offshore conditions and at least two phases of subaerial exposure followed by a major depositional gap comprising the Late Oxfordian and Early Kimmeridgian. Towards the end of the Jurassic rifting ceased and the basin became gradually filled with sediment from the east. The Kachchh Basin is well known for its rich macro invertebrate faunas, particularly ammonites, bivalves, brachiopods, and corals which
belong to the Ethiopean faunal province.

This field guide, prepared on the occasion of the 9th International Congress on the Jurassic System in Jaipur, India, attempts to give an overview of the present state of knowledge on the geology of the area. It starts with a general description of Kachchh, ranging from geographic aspects to its inhabitants, food and history. This is followed by an overview on the Jurassic geology including the lithofacies, various aspects of stratigraphy, faunal and floral composition, a chapter on palaeoecology, and concludes with a summary of the palaeoenvironmental evolution of the basin. The third section contains descriptions of 13 localities which allow to trace the development of the basin from the early Middle Jurassic to the Early Cretaceous.

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