Sedimentary Environment of Early Cambrian Maidiping Formation Shale: A Case Study of the Northern Yunnan-Guizhou Region

During the depositional period of the Maidiping Formation in the northern Yunnan–Guizhou region, intense tectonic–sedimentary evolution occurred, accompanied by significant changes in the paleo-marine environment. However, knowledge gaps remain, including insufficient understanding of the lateral distribution of sedimentary facies of the Maidiping Formation. Systematically conducting sedimentary facies analysis to identify sedimentary system types, spatial distribution patterns, and the main controlling factors of evolution is of great significance for revealing the development and distribution of organic-rich and phosphorus-rich sedimentary systems. Based on more than 400 samples from 10 outcrop sections and 13 well cores, we carried out thin-section identification, scanning electron microscopy (SEM), XRD whole-rock analysis, and total organic carbon (TOC) content analysis. Combined with drilling and well-logging data, we systematically conducted stratigraphic division, rock type identification, sedimentary structure and paleontological feature analysis, and research on the spatiotemporal distribution of sedimentary facies. The key insights were as follows: (1) Fourteen lithofacies types across five major types—phosphorite, mudstone, siliceous rock, siltstone, and carbonate rock—were identified in the Maidiping Formation of the northern Yunnan–Guizhou region, establishing a refined lithofacies classification system. Formation thickness is controlled by paleogeomorphology, with the Changning Rift Trough, B1-Z1 well area, and Leibo Region as depocenters. This study systematically constructs the first integrated “lithofacies–formation thickness–paleogeomorphology” response model for the Maidiping Formation in this region, providing constraints for the migration of depositional centers and the restoration of provenance areas. (2) Sedimentary structures including horizontal bedding, massive bedding, graded bedding, banded structures, and lenticular structures, as well as nodular siliceous–clayey nodules, phosphatic nodules, and Syngenetic sutures, were identified. A complete storm-induced depositional sequence was identified in the Maidiping Formation for the first time, confirming frequent storm events during the early Early Cambrian. The paleontological fossils are dominated by radiolarians and archaeocyathids; radiolarian abundance was positively correlated with water depth, whereas the presence of archaeocyathids indicates shallow-water environments. A dual-index “paleontology–sedimentary structure” discrimination method for distinguishing deep- from shallow-water environments was established. (3) Sedimentary evolution was controlled by a paleogeomorphic pattern of “high in the west and low in the east, with two depressions flanking a paleo-uplift.” From west to east, tidal flat, lagoon, shoal, and shelf sedimentary systems developed sequentially. This reveals the controlling effect of the paleogeomorphic framework on the spatial differentiation of the sedimentary system of the Maidiping Formation. These findings provide a sedimentological basis for phosphate mineralization prediction and shale gas exploration and development in the Maidiping Formation in the northern Yunnan–Guizhou region.