缓坡及其构造背景——以中国南方早寒武世龙王庙期扬子碳酸盐缓坡为例

CARBONATE RAMPS AND THEIR TECTONIC CONTROLS,WITH AN EXAMPLE FROM THE LONGWANGMIAOIAN(EARLY CAMBRIAN)YANGZI CARBONATE RAMP IN SOUTH CHINA

引言自Ahr(1973)提出第一个碳酸盐缓坡模式以来,沉积学家通过对现代或古代地层沉积相分析,相继发现了一系列各种类型的现代或古代的碳酸盐缓坡(Read,1980,1985,1989;Ahr,1989;Grotzinger,1989;Bechstadt,1989;Wright,1986;Gawthorpe,1986)。尽管这些缓坡各自都具有与陆架碳酸盐台地截然不同的共同特征,但它们在沉积学上都各具特色,特别是它们都是在某一特定的构造背景(如热沉降、前陆负荷绕曲或海平面上升)下开始生长发育的。我国南方早古生代初期扬子碳酸盐缓坡具有浅水缓坡带宽广,具远岸加积障壁;深水缓坡具加积碳酸盐建隆等特征。它是继晚前寒武纪我国南方大陆裂谷作用之后。

A ramp is a shallow, gently dipping platform (slopes typically less than 1°) that lacks a marked break on slopes. Controls on the development of the ramps are: (1) stable tectonic settings; (2) thermal subsidence; (3) flexure of the loaded foreland basin, and (4) sea level fluctuations. The Yangzi ramp was formed on a passive continental margin, and its development was controlled mainly by thermal subsidence. Five distinct geomorpho-sedimentary facies units may be recognized: (1) nearshore tidal flat; (2) shallow-water subtidal ramp; (3) shallow-water ramp buildups; (4) deep-water ramp, and (5) deep-water ramp buildups.