上扬子地区早三叠世异常碳酸盐岩的分类与特征

Classification and characters of the Early Triassic anomalous carbonate rocks in Upper Yangtze Area

上扬子地区晚二叠世晚期及早三叠世早期异常碳酸盐岩可以按成因分为3大类:A类—生物成因的异常碳酸盐岩,B类—非生物成因(化学及水动力成因)的异常碳酸盐岩,C类—生物、水动力及化学等复合成因的异常碳酸盐岩;还可进一步识别出8个具体类别,包括微生物岩、角砾状灰岩、扁平砾屑灰岩、条带状灰岩、薄板状、纹层状泥质灰岩、鲕粒碳酸盐岩及a、b型蠕虫状灰岩。近乎极端的古海洋环境和古气候条件是P/T转换时期以错时相为主的异常碳酸盐岩得以发育并作为早三叠世重要沉积类型的基础;P/T生物大灭绝事件发生以后,底栖动物对海相沉积层表面的破坏几乎停止,海洋洋流近乎停滞,与此同时表层海水饱受季风与飓风风暴的侵扰,分布在生物礁或浅水碳酸盐岩台地之上的透光带菌藻生态系统生长为具有一定抗浪能力的微生物岩,频繁的风暴对海底的影响使得同生期弱固结的碳酸盐软泥被打碎而形成同生砾屑,一定的古地形坡度又加剧了风暴为诱因的碳酸盐沉积物重力流的发育;此外,碳酸钙过饱和的海水可以使得碳酸盐直接化学沉淀于海底,同时巨型季风带来的黏土和粉砂与碳酸盐化学沉积共生,这一系列的过程发生于早三叠世早期特殊的古环境中,造成了上扬子地区多种多样的异常碳酸盐岩沉积。

The anomalous carbonates of the late period of Late Permian and Early Triassic in the Upper Yangtze Area can be divided into three main types by genesis：A—biogenic anomalous carbonate rocks,B—abiogenetic（chemical and hydrodynamic）anomalous carbonate rocks,C—multi-genetic anomalous carbonate rocks.Eight specific sorts of the anomalous carbonate rocks can be further distinguished,including microbiolite,brecciform limestone,flat-pebble limestone,ribbon limestone,thin-bedded or laminated argillaceous limestone,oolitic limestone and 2 kinds of vermiculate limestone（Type a and Type b）.The extreme palaeoceanic environment and paleoclimatic conditions are the basis of the development of the anomalous carbonate rocks（mainly anachronistic facies）around P/T boundary.After the P/T mass extinction,the destruction of benthonic animals living on the surface of marine sediments was nearly stopped and the ocean currents were almost stagnated,meanwhile the surface layer of the sea water suffered the disturbing of megamonsoon and cyclonic storms,the algo-fungus ecosystem of euphotic-zone grew and formed the wave-resistance microbiolites on the biohermal facies or on the shallow water carbonate platform.Seafloor was often affected by frequent storms which tear up the unconsolidated syngenetic carbonate ooze and thus the syngenetic pebble formed.At the same time,the particular gradient of the paleotopography intensified the development of carbonate gravity flows which might also be triggered by the storms;moreover the calcium carbonate-supersaturated sea water allowed the carbonate directly precipitating in the forms of chemical precipitation on the seafloor and the clays brought by the megamonsoon deposited together with the carbonate chemical precipitation.All the mentioned process happened at the earliest Early Triassic specific palaeoenvironment,which resulted in the appearance of the diverse anomalous carbonate deposition in the Upper Yangzte Area.