浙西下寒武统大陈岭组沉积异常及其海啸成因初探

Preliminary Study on Abnormal Sediments and Their Tsunami Origin of the Lower Cambrian the Dachenling Formation in Western Zhejiang

前人对浙西下寒武统大陈岭组沉积相分析一直存在半深水相和浅水相两种观点。大陈岭组化石极少,仅中上部产营漂浮或游泳生活的节头虫类Arthicocephalus,Arthricocephalites和Changaspis,不见底栖类化石。对大陈岭组碳酸盐岩碳氧同位素分析显示,对应于产半深水相生物化石的层位存在碳同位素正漂移,表明此处水体较深;但该组大量发育碳酸盐岩台坪相的沉积标志—鸟眼构造,与较深水相生物和碳同位素正漂移的解释结果相悖。作者在对浙西开化、常山、江山寒武系野外调查时发现大陈岭组存在一套地震—海啸沉积岩系,且海啸岩的产出层位与江山较深水相化石、碳同位素正漂移层位相当。研究认为,海啸作用可以使不同环境的沉积物相互混合,造成沉积异常,浙西下寒武统大陈岭期沉积异常是海啸事件的反映,海啸作用把较深水生物和沉积物带到浅水区沉积下来,并造成碳氧同位素短暂、急剧的正漂移。

There have been two views of deep-water and shallow-water facies with regard to the Dachenling Formation of Lower Cambrian in Western Zhejiang.Fossils are very rare in the Dachenling Formation.There are only a few fossils including floating and nektonic Arthicocephalus,Arthricocephalites and Changaspis in the middle to upper part of the Dachenling Formation,but without benthic fossils.Carbon and oxygen isotope analysis were conducted for the research.The result shows that there is a carbon isotope positive excursion for the interval including deep-water fossils,which is not consistent with bird's eye structure,a sedimentary feature of shallow water carbonate platform facies.Our field investigation shows that there is a suite of seismic-tsunami deposits within the Dachenling Formation of Lower Cambrian in Kaihua,Changshan and Jiangshan of Zhejiang.In addition,the interval including tsunami deposits corresponds to the deep-water fossils and carbon isotope positive excursion.Deposits from various environments can be mixed together by tsunami events,which would result in abnormal sedimentary rocks.Thus,abnormal sediments within the Dachenling Formation of Lower Cambrian in western Zhejiang could be interpreted as a signature of tsunami events.The fossils and sediments of the deep-water zone were brought into the shallow-water zone and then deposited by the tsunami events,which also resulted in a brief,sharp positive excursion of carbon and oxygen isotope curves of the coeval sediments.