Architecture and evolution of calciclastic marginal slope fans of the Ordovician carbonate platform in the Yijianfang outcrop of the Bachu area, west Tarim Basin

摘要

The Ordovician carbonate platform at the Yijianfang outcrop of the Bachu uplift region in the western Tarim Basin contains four types of genetic facies associations developed in the calciclastic slope-fan depositional system: an olistostrome zone, fan channels, lobes, and a marginal slope. The olistostrome zone is characterized by olistoliths and slump fans, whereas the fan channels and lobes are further divided into proximal and distal facies. The marginal slope deposits constitute the background sedimentation in which the calciclastic slope fans are intercalated. From proximal to distal parts of the fan channels and lobes, their scale gradually becomes smaller, and the size and sorting of grains become finer and better, respectively. Analysis of the stratigraphic framework indicates that the fans formed in the lower strata of the Upper Ordovician Lianglitage Formation in four high-frequency sequences (i.e., Pssl-Pss4). Field paleocurrent measurements indicate northeast-southwest depositional strike for the early platform margin of the Lianglitage Formation. Sediments in the calciclastic slope fans were derived from the platform margin, and evolution of the calciclastic slope fans was generally progradational from Pss1 to Pss2 and then continuously retrogradational from Pss2 to Pss4. The calciclastic slope fans in the outcrop area are not reservoir-prone rocks, but interpretation on these fans can provide useful information about potential hydrocarbon reservoirs along the platform margin. The P-wave velocity, S-wave velocity, and density variations in each genetic facies may be used to identify the subsurface calciclastic slope-fan depositional system.