Sedimentary characteristics and geological significance of tempestites in the Upper Cambrian Xixiangchi Formation, Chengkou area, northern margin of the Yangtze Platform

The Upper Cambrian Xixiangchi Formation has been considered as a potential target for hydrocarbon exploration in the Sichuan Basin, but little is known about its sedimentary facies and controls on reservoir quality. The tempestite deposition found in the Xixiangchi Formation at the northern margin of the Yangtze Platform may provide some important constraints for that. Through field survey and thin section analysis, we present a detailed description on the tempestite deposition of the Upper Cambrian Xixiangchi Formation in Chengkou area on the northern margin of the Yangtze Platform, and further discuss the significance for paleogeography and implications for hydrocarbon reservoir in the northeastern Sichuan Basin. Many diagnostic sedimentary structures can be recognized in these Upper Cambrian tempestites of the Xixiangchi Formation in Chengkou area, such as basal scour-and-fill structures, rip-up clasts, hummocky cross stratification (HCS), graded bedding, and so on. Five types of tempestite sequences were recognized in terms of variant assemblies of storm-induced sedimentary structures, with a gradual transition from Type 1 at the bottom to Type 5 on the top. According to tempestite deposition, lithological association and sedimentary sequence, combined with the classical mode of tempestites developed in shallow water carbonate environments, we suggested that the sedimentary environment of tempestites of the Upper Cambrian Xixiangchi Formation in Chengkou area were dominated by middle ramp, and evolved from inner ramp at the bottom to mid-outer ramp on the top, with a deepening-upward trend. Integrated with regional geological background, it is speculated that the Upper Cambrian Xixiangchi Formation on the northern margin of the Yangtze Platform is dominated by mid-outer ramp facies to the northeast of Chengkou, and inner-ramp facies to the southwest. This implies that there were many geologic advantages for high quality shoal reservoirs in the Upper Cambrian Xixiangchi Formation developed in the northeastern Sichuan Basin to the southwest of Chengkou, worthy to be explored in the future.