Sedimentary Response of Late Triassic Structures,Ordos Basin

The late Triassic Yanchang Formation, Ordos Basin,experienced three large-scale tectonic movements during the early, middle and late sedimentary periods. In this paper,these movements and their sedimentary response are studied. The early Indo-China movement before Yanchang Formation’ s deposition is a basis for not only lake-basin formation but also whole Yanchang Formation’s deposition. The lake-basin in the early period was in a state of undercompensation and two sedimentary facies of both alluvial fan and fluvial might widely be found. And then,one movement during the end of Yanchang 8 Member made the whole lake-basin went down rapidly; and Yanchang 7 Member, a set of source rock,formed. Meanwhile,there was developed a lacustine-delatic deposition,and depocenter migrated and sedimentary pattern changed till the lake-basin deposition disappeared. During the end of Yanchang Formation, all strata uplifted and were erosive from an E-W trending by the Indo-China movement,especially for the top; furthermore, the early Jurassic landform with very various reliefs formed;at the same time,the petrophysical properties of upper Yanchang Formation became better due to fresh-water leaching.