Microfacies Analysis of Transgressive Condensed Sequence: a Study from the Oxfordian of Kachchh Basin, Gujarat

1-9m thick Dhosa Oolite Member of Chan Formation exposed in Kachchh Mainland, western India, represents a condensed transgressive sequence of the Oxfordtan age. It is composed of hard and compact, fossilifeious, sandy/conglomeratic oolitic limestone interbedded with friable, thinly bedded calcareous siltstone/fine sandstone Field&petrographic studies revealed six microfacies namely, (i)sandy/pebbly ironstone(A₁), (ii)conglomeratic fossililerous ironstone (A₂), (iii)conglomeratic fossiliferous oolitic packstone(B₁), (iv) sandy oolitic packstone (B₂), (v) siltstone (C₁), (vi) fine grained sandstone (C₂). These microfacies are grouped in to three microfacies assemblages namely (i)sandy oolitic Packstone-Siltstone assemblage (B₂-C₁), (ii) Siltstone-Fine grained sandstone assemblage (C₁ - C₂) and (iii) conglomeratic fossiliferous oolitic packstone - fine grained sandstone assemblage (C₂-B₁).

The study of microfacies suggests that the deposition of the Dhosa Oolite Member took place in an open marine shallow shelf environment with clear proximal-to'distal trend from southeast to northwest. Two transgressive pulses were responsible for its formation. An earlier slow and frequently interrupted transgression together with subsidence is lecordedby abrupt change in clastic grain size, mixing of two lithologies and abundance of siliciclastics in each microfacies Later, rapid transgression is suggested by the extremely reduced thickness, erosive to irregular contacts, development of intraformatjonal conglomerates, multi phase reworked sediments, an increase in the abundance of carbonates and ammonites and negligible sediment input Lateral vanation in the microfacies and their assemblages together with variable extent of marine mega fauna (ammonites) from southeast to northwest marks the depositional slope ot the basin towards northwest during studied interval. The mixed lithologies in this condensed sequence together with the bioturbated nature of sediments suggest that the rate of sedimentation was extremely slow.