Abstract：In order to clarify the sedimentary-palaeogeography characteristics and sequence framework of the Triassic in Awati sag,Tarim Basin,based on the research ideas of terrigenous sequence stratigraphy and lake-basin sedimentology,and using basic data consisting of drilling core and seismic data,this paper puts forward new understanding on sequence division,sediment provenance,and sedimentary-palaeogeography distribution and evolution. In this study,the Ohobluk Formation and Karamay Formation of the Triassic in Awati sag are divided into seven third-order sequences,among which the Lower Triassic Ohobluk Formation is divided into five(SQ1-SQ5)and the Middle Triassic Karamay Formation is divided into two sequences(SQ6-SQ7). Each sequence is composed of the lowstand system tract(LST),transgressive system tract(TST),and highstand system tract(HST). It is suggested that the Triassic in Awati sag is characterized by fan deltas,braided river deltas,and shore-shallow lacustrine facies controlled by multiple sources. There are four major sources around the sag and the sediment supply is mainly from the southwestern Bachu Uplift and the western Keping Fault Uplift,then from the northern Tabei Uplift and the southeastern Tazhong Uplift. The braided river delta plain subfacies are mainly distributed at the southern and southwestern edge of the sag,mainly composed of coarse grained sandstone and sandy gravel sandbodies of braided distributary channel microfacies. The center of the lake basin is located in the eastern part of the sag. Within the sag,there are braided delta front subfacies and lacustrine facies,mainly composed of underwater braided distributary channels and beach bar sandbodies. During the Triassic,the lake basin experienced stages of early initiation,intermediate growth and peak,and finally filling and extinction. The early stage of the Lower Triassic(SQ1-SQ2 period),corresponding to the development of the lower part of the Ohobluk Formation,is the initiation stage of the Awati lake basin in the Triassic. Sediments mainly came from the Bachu Uplift in the south and the Keping Fault Uplift in the west. The sedimentary facies were distributed in an asymmetric belt from south to north,with the development of braided delta plain and front subfacies and shore-shallow lacustrine subfacies. Moreover,during that stages,the lake basin is shallow in the south and deep in the north,shallow in the west and deep in the east. The middle to late stage of the Lower Triassic(SQ3-SQ5 period),corresponding to the development of the upper member of the Ohobluk Formation,is the growth and peak period of lake basin. With the continuous progradation of sources from the southwest and west towards the basin,fan delta facies were formed. At the same time,the lake basin expanded to the north and northeast,forming a braided delta plain and front subfacies that continuously migrated from south to north,while the northeast is a shore-shallow to semi-deep lacustrine subfacies. The early-middle period of the Middle Triassic(SQ6-SQ7 period),corresponding to the development of the Karamay Formation,is the filling and extinction period of the lake basin. The migration of the subsidence center to the northeast region resulted in the continuous progradation of the sources,causing the lake basin to rapidly shrink and eventually died out.
JIA Jinhua. Sequence characteristics and sedimentary palaeogeography evolution of the Triassic in Awati sag,Tarim Basin[J]. JOPC, 2024, 26(4): 763-778.
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