The Middle-Late Triassic Event Sediments in Ordos Basin: Indicators for Episode I of the Indosinian Movement

Abstract

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.