281 / 2019-07-31 18:06:33

Rip-current deposit in the early Cretaceous Ukra Member, Kutch and their implications

Rip current fans, Cretaceous Ukra Member, TST Shelf, Longshore current, Shore-normal flux, Mass flow deposits, Global warming

Poorly-sorted conglomerate patches rich in granules or sturdy fossils or both, and reddish mud matrix within the interstices stand out amidst fine-grained siliciclastic shelf sediments of the Ukra Trangressive Systems Tract (TST). The latter contrasts the conglomerates with their remarkable lateral extension. The fossils belong to a low-diversity group of sedentary bivalves that can be traced into the shoreface facies assemblage. The shelf sandstones are almost always sculpted by wave structures, especially hummocky cross-stratification, while textures in the conglomerates suggest sediment settling generally en masse from suspension. Textural varaiations in the latter reflect variation in flow viscosity immediately prior to downloading. The few current structures obtained from the conglomerates record offshoreward paleocurrent, in contrast to the shore-parallel paleocurrent in the TST. The HCS beds are interpreted as seasonal storm deposits, while the conglomerate patches are taken as rip current deposits induced by waves of much longer periods. The glauconite-rich shale that alternates with them is probable fair-weather products.
The conglomerates could not be recognized either in the coarser grained shoreface deposits occupying the lower part of the overall fining upward TST or in the coarsening upward and glauconite-depleted High Stand Systems Tract (HST). In contrast to the TST, the HST is dominantly tide-imprinted having shore-normal paleocurrent direction. It appears that intensification of waves and weakening of tides during transgression favored strong rip currents that presumably caused severe damage to the sea coast and to the shell banks that grew preferably at the necks of the rip-current channels. Rapid lateral facies transitions in the shoreface deposits at the basal part of the TST suggest enhanced irregularity in the coastline, possibly because of megacusps indented upon it. Frequency and intensity of storms enhanced during period of global warming that caused the transgression of the Early Cretaceous Ukra sea.