Evolution of the Triassic Yanchang Lake, Ordos Basin, China

As a multi- composite basin developed on the stable Paleozoic craton of north China, the Triassic Yanchang Formation of the Ordos basin, divided into 10 reservoir units of Chang10 to Chang1 from base to top, was regarded by many researchers as deposits of a big sag lake basin with unified subsidence and uplift, in which south and north delta systems were developed symmetrically around the fixed lake basin center. While based on new data of heave / light minerals, distribution of dark mudstones, seismic profiles, it is suggested that taking Chang 7 age as the turning point, the depocenter of the Yanchang Lake migrated at least 30 km southwestwardly from the Wuqi-Fuxian area of the north Shaanxi province in the middle Triassic to the Huachi-Huangling area of the east Gansu province in the late Triassic. Controlled by the lake evolution, the delta systems of this basin developed unsymmetrically. The southwest delta systems extended more basinward in middle Triassic than that during late Triassic, however the northeast delta systems has the opposition condition. The SW to NE compression caused by the intensive late Triassic western Qinglin orogenic movement as well as the tilt of the northeast part of the basin, which was affected by the vertical uplift of Yingshan region, may be the origin of the conversion of the south and the north depositional system of the Ordos basin during middle-late Triassic.