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Ancient rip current records and their implications: an example from the Cretaceous Ukra Member, Kutch, India
Journal of Palaeogeography ( IF 2.5 ) Pub Date : 2020-04-23 , DOI: 10.1186/s42501-020-00060-2
Subir Sarkar , Amlan Koner

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 coastline, possibly because of the mega cusps indented upon it. Frequency and intensity of storms enhanced during periods of global warming caused the transgression of the Early Cretaceous Ukra Sea.

中文翻译:

古代裂隙流记录及其影响:以印度 Kutch 的白垩纪 Ukra 段为例

在印度 Kutch 盆地下白垩统 Ukra 段海侵系统域 (TST) 的细粒硅质碎屑陆架沉积物中,富含颗粒或坚固化石或两者兼而有之的砾岩斑块分选不良,间隙内的红色泥质基质突出。硅质碎屑陆架沉积物与砾岩的显着横向延伸形成对比。这些化石属于低多样性的定居双壳类动物群,可以追溯到岸面相组合。陆架砂岩几乎总是由波浪结构雕刻而成,尤其是丘状的交叉层理,而砾岩中的纹理表明沉积物沉降通常来自悬浮液。聚集体的质地变化反映了下载前流动粘度的直接变化。从砾岩中获得的洋流结构记录了近海古洋流,与 TST 中与岸平行的古洋流相反。丘状交叉分层 (HCS) 床被解释为季节性风暴沉积物,而砾岩斑块则被视为由更长周期的波浪引起的离岸流沉积物。与砾岩交替出现的富含海绿石的页岩很可能是天气好的产物。砾岩在占据整个向上细化 TST 下部的粗粒岸面沉积物中或在向上粗化和消耗海绿石的高位系统域 (HST) 中均无法识别。与 TST 不同的是,HST 主要受潮汐印记,具有海岸正常的古海流方向。海侵期间波浪的增强和潮汐的减弱似乎有利于产生强烈的离岸流,这可能对海岸和优选在离岸流通道颈部生长的贝壳岸造成了严重破坏。TST 基底部分的岸面沉积物中的快速横向相转变表明海岸线的不规则性增强,可能是因为其上有巨大的尖头。在全球变暖期间增强的风暴频率和强度导致了早白垩世乌克拉海的海侵。TST 基底部分的岸面沉积物中的快速横向相转变表明海岸线的不规则性增强,可能是因为其上有巨大的尖头。在全球变暖期间增强的风暴频率和强度导致了早白垩世乌克拉海的海侵。TST 底部的岸面沉积物的快速横向相转变表明海岸线的不规则性增强,可能是因为其上有巨大的尖头。在全球变暖期间增强的风暴频率和强度导致了早白垩世乌克拉海的海侵。
更新日期:2020-04-23
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