Seditentary Models and the Evolution of Early and Middle Triassic Carbonate Platform Margins and Slopes in Southern Guizhou and Western Guangxi

A suite of carbonate rocks were deposited in three depositional environments in southern Guizhou and western Guangxi during the Early and Middle Triassic times:(1)the association of oolitic dolomites, sparry calcarenites, oolitic limestones, sparry intraclastic dolomites, oncolite limestones and sparry grainstones indicates a shoal environment;(2)micritic limestones and laminated micritic limestones show a carbonate ramp environment, and(3)carbonate breccias and carbonate turbidites accompanied by intense slump and large-scale truncation structure represent a carbonate slope environment.
Three types of sedimentary models have also been proposed by the author in terms of the characteristics of platform margins and slopes in southern Guizhou and western Guangxi.
(1)Carbonate ramps(Fig. 12)
This model began to develop during early Early Triassic(Indian).The prevailing micritic limestones without large-scale slump structure and carbonate breccias represent a carbonate ramp envi-ronment.
(2)Steepened ramps(Fig. 13)
The model developed in late Early Triassic(Qlenekian)comprises carbonate breccias which stemmed from debris flow, slump flow, grain flow and turbidity current and micritic limestones with slump structure, suggesting a steepened ramp environment. But it differs from distally steepened ramps(Read, 1982, 1983). The former includes a great deal of breccias and shallow-water debris. The author contends that the steepened ramps in southern Guizhou and western Guangxi lie between distally steepened ramps and rimmed platforms.
(3)Rimmed platforms (Fig. 14)
The model chiefly developed in Middle Triassic(including Anisian and Ladinian)consists of carbonate breccias which were derived largely from debris flow, micritic limestones and laminated micritic iimestones(carbonate turbidites)with large-scale truncation structure, representing a carbonate slope environment. The slopes became steeper in the gradient during the period because of the accretion of the shoals on carbonate platform margins. And the addition of terrigenous turbidites often made the slopes narrower and narrower(Fig. 11)during Middle Triassic.