Facies distribution and retreat of Middle Triassic platform margin, Guizhou province, south China

ABSTRACTThe southern margin of the vast Yangtze platform in central Guizhou Province, China, retreated during the Anisian (early Middle Triassic) by shedding skeletal debris and boundstone blocks into the margin of the adjacent basin. Anisian platform deposits are shoaling‐up cycles that commonly terminated in subaerial exposure. Platform‐margin facies are obscured by massive dolomitization and mechanical erosion. Distal basin deposits are terrigenous mudstone and siltstone. At the basin margin a wedge of mixed carbonate and terrigenous rocks consists of (A) thin‐bedded dolostone and limestone, (B) lime breccia with thin‐bedded mudstone, and (C) lime mudstone with breccia. Blocks within the breccias indicate that the shelf margin contained extensive boundstone formed by ‘Tubiphytes,’encrusting organisms, and early marine cement. Interspersed thin beds of skeletal packstone represent unlithified skeletal debris at the platform margin.The profile of the shelf margin from detailed mapping indicates 1.7–2.7 km of platform‐margin retreat during deposition of a basin‐margin wedge 250 m thick. Intertonguing of various basin‐margin facies reflects alternating minor episodes of advance and retreat of the margin. Near‐parallelism of the tongues suggests low relief at the platform margin. An upward stratigraphic progression to more distal, carbonate‐free, terrigenous basin facies indicates a cessation of carbonate production on the platform owing to emergence during early Anisian time. Retreat may have occurred entirely by collapse of blocks less than 100 m wide and 30 m thick, the largest observed dimensions. A re‐entrant in the margin 7 km wide and 10 km deep could also reflect collapse. Retreat occurred along 175 km of the platform margin. The lack of platform‐margin facies along this front suggests 3–7 km of retreat and a total area of 875 km2.The Anisian platform‐margin retreat in Guizhou is one of relatively few examples of platform‐margin retreat in the geological record.