鄂尔多斯盆地延长组的湖泊风暴沉积

Lacustrine Storm Sediment in Yanchang Formation of Ordos Basin

首次详细论述了鄂尔多斯盆地中部三叠世晚期延长组长二段湖泊相风暴岩的沉积特征，建立了本区湖泊风暴沉积体系模式，并探讨了三叠世末期风暴岩发育的构造与气候背景及其分布规律，重新确定了长二段沉积期的湖泊边界和古地理格局。同时指出风暴岩发育区是良好岩性或构造一岩性圈闭区。

The late Triassic lacustrine in Ordos basin is a large open lake with gentle topography. This paper has firstly reported the lacustrine clastic tempestites in the Triassic Yanchang Formation of the central Ordos basin.The typical tempestite consist of packstones which have a lot of deposition-structural characteristic for differently evolutionary stage of storm. Sedimentary structures of the storm deposits are:a)hummocky crossstratification are characterized by gentlly curved, low angel cross-lamination. Presently most researchers think that powerful dominant or multidirectional oscillatory flow result in the special character which is most useful and best indicator of the storm sedimentary sequences distinguished from the turbidites.,b)basal division,consisting of one or more of the following: sharp basel oriented directional sole marks Fbasal lag of pebbles,or rip-up and deformed resedimented intrabasinal clasts of mudstone. The formation of these structure is related to sediment liquefacion,drainage, reflecting the characteristics of density current. Especially the ercrslonal structure and basal lag of pebbles in turbidite facics are the evidence of storm activity c) planar parallel lamination,d) bloturbated mudstone or fine sandstone. The reservation of escaping structure indicate a quick depositional event. After the storm is over,burrow structure will be developed on the sediment surface,e) lower maturity in texture in sandstone. Lower maturity reflect lower degree of sorting that is a signature of rapid deposit.The typical sequence is concluded. It can be divided into:a)high-discharge form,characterized by block structure, deformed pebble bedding, escaping trace, b ) hlatus, c ) low-discharge, marked by wave-cross ripple mark,burrow trace,lenticular bedding and vein bedding.Storm sedimentary model is given in this paper. We also discuss the structure and paleo-latitude setting of the late Triassic lacustrine tempestites,background of which has been defined as high-level system and was developed in the middle of lake-regressive sequence during the evolution of the basin. The storm deposit transit to deltas or beaches deposits upper and to turbidites offshore. We use the combination of storm facies association and geological characters of adjacent areas to modify the former located sedimentary boundary of the Yanchang lake.We futher indicate storm sedimentary association developed area as prospective lithologic or structrure-lithological trap.