Comparative Study of Fan-deltas,Braided-river Deltas and Meandering-river Deltas of Mesozoic Erathem in Kuche Depression,Tarim Basin

Key words: 
• fan-delta /
• braided-river delta /
• meandering-river delta /
• Mesozoic Erathem /
• Kuche Depression /
• Tarim Basin

Abstract: Kuche Depression,situated in the northern part of Tarim Basin, is a "dustpan”shaped depression with steep slope in north side and gentle slope in the south side in Mesozoic Era.There is a set of hugely thick,alluvial—lake classic deposition system in it.Three types of deltas and they are recognized:fan-deltas,braided-river deltas and meandering-river deltas and they are very abundant in the lake margin.Their characters are distinct and distinctions are clear:(1)fan-deltas,formed where an alluvial fan is deposited directly into lake from an adjacent highland, are alternatively deposited by gravity flows sediments,formed by gusty、momentary and catastrophic event,and tractive current sediments,formed between catastrophic events,gravity current sediments are dominant in fan-deltas.Delta plain subfacies are similar to alluvial fan facies,and channel bars take the form of lentiform and their thickness is small and various.(2)braided-river deltas,formed where a braided fluvial system progrades into a lake,are deposited by river tractive current sediments,controlled by torrential flood in general,and are seasonal sediments,of which the delta plain subfacies are similar to braided river facies,channel bars are abundant,overall,sand bodies take the form of layered bodies,but internally,they are composed of many finer-upward lenticular sand bodies that pile up each other,cross-beddings are abundant,and especially,exceedingly rich lateral accretion cross-beddings are the distinct character of braided-river deltas.The rock types of this kind delta are mostly grain-supported sandstones and conglomerates.(3)meandering-river deltas,formed where a meandering fluvial system progrades into a lake,are deposited by river tractive current sediments that are correspondingly continuous river production,of which the delta plain subfacies are similar to meandering river facies,and channel bars take the form of layered bodies,with abundant cross seddings.Certainly,there is close inherent relationship in three types of deltas,not only coexist at the same term,but also transform along with geologic development.