Palaeogeographic and Sedimentological Characteristics of the Triassic Chang 8, Ordos Basin, China

Key words: 
• palaeogeography /
• shallow lacustrine delta /
• Chang 8 /
• Yanchang Formation /
• Triassic

Abstract: Data of seismic and bore hole show that the palaeotopography of the Triassic Chang 8 , in the lower part of the Yanchang Formation, Ordos basin, was possibly not "steep to the south while gentle to the north" style as the previous studies suggested, and large areas of bathyal or abyssal  areas  were also impossible in this age. The bed form of the lake was very gentle on the basin scale with the slope angle no more than 0.1° , or 2 m/km. The climate was possibly a little dry and the mildly brackish water with depth no more than 10m was the dominant media of the sediments. Vertical or high angel burrows and root systems, coal seams or coaly mudstones and plant fossils of Neocalamites are commonly inclusions either at the edge or in the center of the basin. Shallow lacustrine delta with the following characters were well developed in this basin at that time: 1) detrital components mostly  mediumfine graded lithic arkose and feldspathic litharenite, 2）wide delta front with 80~100 km in length and uncovered frequently to the air, 3) sandstones mainly the subaqueous distributary channel deposits, 4) mouthbar and the gravity flow deposits being infrequent, 5) topsets, foresets and bottomsets of the Gilbert Delta not developed.
  Obvious difference of the slopes of the southern part and the northern part of the basin emerged in the early Chang 7, tightly following the Chang 8 in age. Triggered by the early Indosinian movement and the uplift of the west Qinling Mountains at the southwestern margin of the Ordos basin, the base of this basin was bended over unsymmetrically and the palaeotopography of the "steep to the south while gentle to the north" was generated. And from this time (Chang 7) to the end of the late Triassic slopebreaks and deltas with three elements of topsets, foresets and bottomsets were well developed in the basin.