Ancient rip current records and their implications: an example from the Cretaceous Ukra Member, Kutch, India

作     者：Subir Sarkar Amlan Koner Subir Sarkar;Amlan Koner

作者机构：Department of Geological SciencesJadavpur UniversityKolkata 700032India 

出 版 物：《Journal of Palaeogeography》 (古地理学报（英文版）)

年 卷 期：2020年第9卷第2期

页      面：181-197页

核心收录：

学科分类：070904[理学-构造地质学] 070903[理学-古生物学与地层学(含：古人类学)] 0709[理学-地质学] 07[理学] 

基　　金：the Council of Scientific and Industrial Research (CSIR) Government of India sponsoring the Project No.24(0336)/14/EMR-Ⅱ the Department of Science and Technology (DST) Government of India sponsoring an Inspire Fellowship 

主　　题：Rip current fan Ukra Member TST shelf Longshore current Shore-normal flux Mass flow deposits Global warming Ukra Sea 

摘      要：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 trangressive systems tract(TST) of the Lower Cretaceous Ukra Member, Kutch Basin, India. The siliciclastic shelf sediments contrast 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 that the sediment settling was generally from suspensions. Textural variations in conglomerates reflect an immediate variation in flow viscosity prior to the downloading. The current structures obtained from the conglomerates record offshoreward palaeocurrent, in contrast to the shore-parallel palaeocurrent in the TST. The hummocky cross-stratified(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 conglomerates is probable fair-weather products. The conglomerates could not be recognized either in the coarse-grained shoreface deposits occupying the lower part of the overall fining-upward TST or in the coarsening-upward and glauconite-depleted highstand systems tract(HST). In contrast to the TST, the HST is dominantly tide-imprinted, having shore-normal palaeocurrent direction. It appears that intensification of waves and weakening of tides during transgression favored strong rip currents generation, which had presumably caused severe damage to the sea coast and to the shell banks growing 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 coast