Facies mosaic in the Ghuneri Member (Jurassic) of the Bhuj Formation, western Kutch, India

Abstract

In marginal marine environments facies mosaics become complicated through time as and when the regular autocyclic changes are frequently intervened by allocyclic changes. The facies mosaic of the marginal marine Tithonian Ghuneri Member (476 m thick) exposed in western Kutch, India, is likewise fairly complex with frequent lateral as well as vertical alternations between six facies. The basic motif, built up of autocyclic facies transitions of the bar-lagoon spectrum in response to shift of the shoreline, is time and again interfered with by allocyclic changes caused by storm surges. These storm surges overtopped the bar and laid their sediments over the indigenous facies as washover. The autocyclic facies association is composed of three facies, namely: (1) bar; (2) lagoon-margin; and (3) lagoon-interior. The allocyclic facies association comprises: (4) proximal storm washover; (5) distal storm washover; and (6) proximal grain flow (of insignificant occurrence) facies. Statistical analysis indicates that the vertical facies transitions between these six facies are fairly unpredictable. Elimination of the products of instantaneous deposition (storm washover facies) from the same data base, however, renders the sequence of congruous autocyclic facies predictable.