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ABSTRACT: Katrol Formation, Kutch, India: Varying Interactions Between Eustasy and Basin Subsidence

Pradip K. Bose, Ashoke Seth, Subir Sarkar

Local tectonics, which control sedimentation pattern, eustasy, and sedimentation rate, are additional determinants for ultimate stratigraphy. The marine terrigenous Katrol Formation (Kimmeridgian-Tithonian), Kutch, India, illustrates their interplay at the passive margin of the Indian plate at its northwestern end. Recurrence of thick black shale is the signal of three major events of basin subsidence. Deepening was accompanied by ocean-bottom current activity, volcanic outpourings, and igneous intrusions, followed by deposition of tempestites (tsunami products?) and turbidites. Abundant synsedimentary extensional faults of various scales,

their episodic reactivations, footwall collapse, detachment of laterally contiguous blocks, intrabasinal mass flows of unconsolidated sediments that disturbed and extruded undersurface sediments due to seismic seiches, truncated submarine fans--all attest to the instability of the depositional platform. Gradual relaxation in the rate of basin subsidence enabled the ongoing global eustatic fall, coupled with accelerated sedimentation, to leave its record in the development of three regressive subsequences. Despite mass wasting of nearshore sediments into the offshore through storm and turbidity current transport, the global trend of regression eventually controlled the stratigraphic scenario. Regressive phases when sufficiently prolonged led to the establishment of tidal and beach regi es. Stenohaline nektonic ammonites dwindled and suffered mass extinction at the end of the Tithonian when global regression attained its acme. The record of enfaunal activity, however, presents reverse trends, decreasing with intensification of local tectonism. The whole gamut of sedimentologic, paleontologic, and stratigraphic history of the Katrol Formation is apparently calibrated against the changing balance between global and local tectonics.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990