鄂尔多斯盆地中—晚三叠世事件沉积对印支运动Ⅰ幕的指示

摘要

Episode I of the Indosinian movement at the end of the Middle Triassic is expressed by unconformity between the Middle Triassic and the Late Triassic in the eastern part of North China plate, but it is difficult to recognize in the western Ordos basin due to the sequential deposition of Triassic stratum. Based on observation of the drilling cores of the Yanchang Formation, this paper analyzed the development features and time series of the event sediments, such as sublacustrine fan, seismites, tuff and kerogenous shale. Analytical results indicate that their development climaxes concentrate at the initial stage of Chang-7 gas reservoir formation, coupling with the time of episode I of the Indosinian movement. This is an important indicator for episode I of the Indosinian movement in Ordos basin. It also implies that the boundary between Middle Triassic and Upper Triassic may be roughly equivalent to that between Chang-7 section and Chang-8 section. In the planar distribution, they are characterized by weakening trend from southwest to northeast, suggesting that these event depositions were response to Indosinian Qinling orogeny. The genetic relations of these event sediments and the Dickinson triangular diagram reveal that the Indosinian Qinling collisional orogenic process led to the unprecedented changes of Ordos basin and the genetic dynamic resulted mainly from the collisional suture between the Yangtze plate and the North China plate during the process of the Paleo-Tethys closure.%中三叠世末的印支运动Ⅰ幕在华北板块东部明显地表现为中、上三叠统间的不整合,而西部鄂尔多斯盆地的三叠系内部一般为连续沉积,使得印支运动Ⅰ幕难以识别.本文通过观察延长组的钻井岩芯,分析了湖底扇、震积岩、凝灰岩和油页岩等事件沉积体的发育特征及时间序列,显示出它们发育的高潮期皆为长7油层组沉积初期,时间上与印支运动Ⅰ幕相耦合,是鄂尔多斯盆地印支运动Ⅰ幕的重要标志,暗示了鄂尔多斯盆地中、上三叠统的界线可能大致相当于长7与长8油层组间的界线；平面分布上,都具有从西南部向东北部减少减弱的特点,表明这些事件沉积主要是印支期的秦岭造山作用的沉积响应.事件沉积的成因联系和Dickinson三角图解表明,印支期的秦岭碰撞造山过程导致了鄂尔多斯盆地发生前所未有的变格,盆地的成因动力主要来自古特提斯洋关闭过程中的扬子板块与华北板块间的碰撞缝合作用.