Lithofacies characteristics and genetic mechanism of hybrid sedimentary rocks in Da'anzhai member, Sichuan Basin

Abstract

The Jurassic Da'anzhai Member in the Sichuan Basin is a stratigraphic unit characterized by widespread mudstone and hybrid sedimentation. It comprises various lithofacies types and sedimentary structures, including massive structure, laminated structure, mud rip-up clasts and erosion surfaces. By means of detailed analysis of core, logging, thin sections, scanning electron microscope, XRD and XRF data, six lithofacies types are identified in the Da'anzhai Member, and the genetic mechanisms of these lithofacies such as sandy debris flow, turbidity current, bioclastic flow and still water sedimentation are analyzed based on the characteristics of lithofacies. A complete third-order cycle was identified in the LA 1 and RA 1 well based on the variation in the value of gamma ray, bioclastic content as well as lithofacies characteristics. The paleo-sedimentary environment of Da'anzhai Member of Sichuan Basin is reconstructed on the basis of geochemical proxies such as Sr/Cu, Ca/(Ca + Fe), V/(V + Ni), V/Cr, Ti, Al, Al/(Al + Fe) and Fe/Mn. Different sedimentary environments in the Da'anzhai Member controlled the characteristics of lithofacies distribution and the lake level (water depth) primarily controlled the scale and frequency of the development of hybrid sedimentation and mudstone. During the early and late sedimentary periods of the Da'anzhai Member, the climate was arid, water salinity was high, reducing conditions were weak, and the water body was deep. Sediments such as sandy and bioclastic materials in shallow areas were transported to deeper areas through gravity flows under the influence of sudden events like earthquakes, floods or storms and interbedded with still water mudstones. In the middle period of the Da'anzhai Member, the climate was humid, water salinity was low, reducing conditions were strong, and the water body remained deep. A large amount of deep-water mudstone developed in this period, with gravity flow deposits appearing as thin layers. Finally, the comprehensive sedimentary model of the study area is completed based on this characteristics.