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

Key words: 
• tsunami /
• abnormal sediments /
• seismic event /
• Dachenling Formation /
• Cambrian

Abstract: 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.