FACIES DISTRIBUTION AND RETREAT OF MIDDLE TRIASSIC PLATFORM MARGIN, GUIZHOU PROVINCE, SOUTH CHINA

The southern margin of the vast Yangtze platform in central Guizhou Pr ovince, 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 bas in deposits are terrigenous mudstone and siltstone. At the basin margi n a wedge of mixed carbonate and terrigenous rocks consists of (A) thi n-bedded dolostone and limestone, (B) lime breccia with thin-bedded mu dstone, 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. Inters persed thin beds of skeletal packstone represent unlithified skeletal debris at the platform margin. The profile of the shelf margin from de tailed mapping indicates 1.7-2.7 km of platform-margin retreat during deposition of a basin-margin wedge 250 m thick. Intertonguing of vario us basin-margin facies reflects alternating minor episodes of advance and retreat of the margin. Near-parallelism of the tongues suggests lo w relief at the platform margin. An upward stratigraphic progression t o more distal, carbonate-free, terrigenous basin facies indicates a ce ssation of carbonate production on the platform owing to emergence dur ing early Anisian time. Retreat may have occurred entirely by collapse of blocks less than 100 m wide and 30 m thick, the largest observed d imensions. A re-entrant in the margin 7 km wide and 10 km deep could a lso reflect collapse. Retreat occurred along 175 km of the platform ma rgin. The lack of platform-margin facies along this front suggests 3-7 km of retreat and a total area of 875 km(2). The Anisian platform-mar gin retreat in Guizhou is one of relatively few examples of platform-m argin retreat in the geological record.