Structural control, seismic expression, and types of reefs of rifting margins in the late Permian Changxing Age, Sichuan Basin, China

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A total of 3 intracratonic rifts were developed during the Changxing period in the Sichuan Basin, and organic reefs developed in the periphery areas of the rifts. Outcrops, drilling, and 2D and 3D seismic data covering the entire basin were used to study the structure of the periphery areas of the intracratonic rift formed in the Late Permian Changxing Period, and seismic responses for the organic reefs were also used. The organic reefs in the periphery areas of the rifts can be divided into two categories: accretionary platform margin reefs and progradational platform margin reefs. The external morphology and internal structure of the organic reefs are described. In addition, the organic reef genesis sites in the periphery areas of the rifts in the Changxing period are analyzed for both the tectonic aspects and depositional background. Studies suggest that transfer structures were developed on both sides of the rifts in the Changxing period. The superimposed degree of the fault systems in the equidirectional transitional zones and the equidirectional accommodation zones had important impacts on the topography and structural relief of the rift. The differential tectonic subsidence, the uplifting and subsidence of the sea level, and environmental impacts (monsoon and ocean currents) were major factors controlling the evolution, external morphology, and internal structure of the organic reefs on the periphery areas of the rifts. The synsedimentary activities of the basement rifts controlled the variations in growing spaces and thicknesses of the organic reefs. The structural segmentation effect caused by the transfer structures that developed on both sides of the rifts caused progradation in the margin of the platform. Sea level and environmental changes also affected the time-space migration and internal sedimentary texture of the organic reefs as well as the vertical superposition pattern of the reefs in the Changxing Formation.