黔南桂西早中三叠世碳酸盐台地边缘和斜坡沉积模式及其演化

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

摘要:

黔南桂西地区(东经105°-107°,北纬24°20'-26°40')地跨扬子准地台和加里东褶皱带(华南褶皱带)两个一级大地构造单元。属于南盘江沉积盆地的北部或西北边缘。区内早、中三叠世碳酸盐地层出露良好、层序完整,为台地边缘和斜坡沉积。详细研究该区碳酸盐台地边缘和斜坡沉积,对了解台地边缘特征和演化、海平面变化以及边缘的构造作用等有着重要的理论意义,为确定三叠糸南盘江沉积盆地的西北或北部边缘的构造性质提供可靠的沉积学依据。

Abstract:

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.