Tectonic Retreat and Drowning of Carbonate Platform Margins: Triassic Yangtze Platform, South China

Minzoni, Marcello¹ (1) University of Kansas, Lawrence, KS

Geologic mapping, field observations, and photogeology reveal that extensive faulting and differential block-subsidence caused margin backstepping and final drowning of the Triassic Yangtze platform, Guizhou, south China.

Seismic-scale geometry and facies architecture of platform-to-basin transition exposed along faulted synclines record repeated pulses of margin aggradation and fault-controlled backstepping prior to final drowning. Extensive collapse and scalloping of the margin due to oversteepening accompanied periods of rapid aggradation. Intervals of reduced subsidence and fault inactivity allowed for progradation of the margin. Termination was accompanied by renewed, extensive faulting and slight tilting of the Yangtze platform margin and by development and subsequent tectonic destruction of pinnacle reefs on faulted blocks. A distinct flower structure near the platform margin indicates an important strike-slip component to faulting and probably resulted from reactivation of older (Devonian?) faults. After drowning, the Yangtze platform became a deep-water plateau and was draped by pelagic limestones and black shale.

Platform termination in south China documents the fundamental role of faulting in terminal growth and drowning of a vast, attached carbonate platform. Faulting and catastrophic collapse of productive margins, associated with or induced by episodically rapid tectonic subsidence, can prove lethal to otherwise healthy carbonate systems. Architecture and stratigraphy of the final stage of the Yangtze platform present an analog for the response of platform margins to tectonic foundering in inversed, active basins. This example provides predictive criteria applicable to exploration for reservoir-quality facies in tectonically drowned carbonate platforms.