Lithofacies and depositional dynamics of golden Oolite (Bathonian), Kachchh Mainland, Gujarat (India)

The Golden Oolite Member of the Patcham Formation consisting of 84 in thick alternate sequence of limestones and mudstone are well exposed in the Jhura Dome, Kachchh Mainland, Gujarat. Petrographic study of limestones reveals four types of microfacies: oolitic fossiliferous grainstone (A(1)); fossiliferous intraclastic grainstone (A(2)); sandy fossiliferous grainstone (A(3)); and pebbly fossiliferous grainstone (A(4)). The microfacies normally form microfacies assemblages with calcareous mudstone (B I) and are stacked vertically in ascending order as A(1)-A(3), A(1)-B-1, A(4)-B-1 and A(2)-B-1. The assemblage (A(1)-A(3)) is characterised by interbedding of moderately to thickly bedded, hard and compact, golden coloured oolitic fossiliferous grainstone and sandy fossiliferous grainstone exhibiting small scale low angle planar cross beddings. It contains well preserved bioclast. Assemlage (A(1)-B-1) is distinguished by rhythmic alternations of earthy, concretionary calcareous mudstone and moderately to very thickly bedded golden coloured oolitic fossiliferous grainstone showing ripple bedding, abundant bioclast and reworked intraclasts, whereas assemblage (A(4)-B-1) exhibits rhythmic alternations of bioturbated, earthy, concretionary calcareous mudstone and moderately to thickly bedded pebbly fossiliferous grainstone. Assemblage (A(2)-B-1) is characterised by earthy,bioturbated,calcareous mudstone containing thin uneven beds of fossiliferous intraclastic grainstone having micritic intraclast and microfossils. The study of lithofacies suggests two main depositional processes for the formation of golden oolite: (1) The high energy physical sedimentation from current flows during transgression characterized by irregular to sharp nature of basal contact of each cycle, abundance of well preserved bioclasts and reworked intraclasts and large scale ripple bedding; (2) Settling of fines from suspension during fair-weather period as distinguished by homogenous fine grained interbeds of mudstone in the sequence. The transition of facies from A(1)-A(3) to A(2)-B-1 marks deepening upward event during Bathonian period from shallow inner shelf to calcareous mud dominated outer shelf. The energy condition was very high during deposition of the lower and middle part (A(1)-A(3) and A(1)-B-1 assemblage) whereas low to very low as revealed by abundance of bioturbated calcareous mudstone (B-1) with episodic interruption of moderate to high energy storm event depositing A4 (pebbly fossiliferous grainstone) microfacies during the upper part (A(4)-B-1 and A(2)-B-1 assemblage) of the sequence. (C) 2004 Elsevier Ltd. All rights reserved.