Controlling effects of the Mid-Permian multistage slope-break zones on paleogeomorphology and large-scale shoals in the Sichuan Basin, SW China

Abstract

This study reconstructed the paleo-uplift and depression pattern within the sequence stratigraphic framework of the Mid-Permian Maokou Formation, Sichuan Basin, investigated its tectono-sedimentary mechanisms and its control on paleogeomorphology and large-sale shoals based on analysis of outcrops, loggings and seismic data. The results show that the Maokou Formation comprises two third-order sequences, six fourth-order sequences (SSQ1–SSQ6), and four distinct slope-break zones developing progressively from north to south. Slope-break zones I–III in the northern basin, controlled by synsedimentary extensional faults, exhibited a NE-trending linear distribution with gradual southeastward migration. In contrast, slope-break zone IV in the southern basin displayed an arcuate distribution along the Emeishan Large Igneous Province (ELIP). The evolutions of these multistage slope-break zones governed the Mid-Permian paleogeomorphy in the Sichuan Basin transformations from a giant, north-dipping gentle slope (higher in the southwest than in the northeast) in the early-stage (SSQ1–SSQ2) to a platform (south)-basin (north) pattern in the middle-stage (SSQ3–SSQ5). Ultimately, a further depression zone developed in the southwestern basin during the late-stage (SSQ6), forming a paleo-uplift bounded by two depressions. The developments of the Mid-Permian paleogeomorphic configuration reflected the combined control by the rapid subduction of the Mianlüe Ocean and the episodic eruptions of the Emeishan mantle plume (or hot spots), which jointly facilitated the formation of extensive high-energy shoal facies belts along slope-break zones and around paleo-volcanic uplifts.