Genesis of Sinian-Cambrian Unconformity in the Northwestern Tarim Basin: Evidence from Sedimentology

Key words: 
• Sinian-Cambrian /
• Qigebrak Formation /
• Unconformity /
• Genetic mechanism /
• Aksu area

Abstract: Sinian-Cambrian parallel unconformity is widely developed in Aksu area of the NW Tarim Basin, and there is still controversy over its formation mechanism at present. On the basis of sedimentological research, the genesis of the above unconformity is studied through detailed field observation and microscopic identification, aiming at the sedimentary facies and karst phenomenon at the top of the Upper Sinian Qigebrak Formation. The results show that platform margin and slope facies are developed at the top of Qigebrak Formation in Aksu area. Platform margin includes grain beach and microbial reef, consisting of thick- to massive bedded dolo-oolite, medium- to thick- bedded intraclast dolopackstone and massive dolostromatolite reef. Lithofacies of slope include: dolobreccia, thin- to medium- bedded turbidite and thin- bedded dolowackestone. In the late Sinian, the sedimentary water bodies in the eastern Aksu area were deeper, and the degree of karst development was significantly stronger than that in the western areas, which was dominated by horizontal phreatic zone vetically. In addition, in the early Cambrian, the Yuertusi Formation maintained the same sedimentary trend as the top of the Qigebrak Formation, and both showed stratigraphic sequences of deepening water bodies from west to east. This unconformity was formed by tilting movement induced by tectonic activity at the platform margin in the late Sinian, which resulted in the uplift of the western part of Aksu area. Meanwhile, this movement led to atmospheric freshwater hydrological system flowing eastward, resulting in strong karstification in the eastern platform margin zone. This study could provide guidance for oil and gas explorations of the Sinian dolomite reservoirs in the region.