Contrastive Research About Giant Ooids Of Early Cambrian And Early Triassic In Upper Yangtz Area

Giant ooids are commonly seen in Precambrian and Early Triassic, while they are hardly reported in Cambrian. Recently, we found giant ooids limestone about 35 cm in the Lower Cambrian Tianheban Formation at the Shiliu Section in Shizhu County, Chongqing Municipalitie, SW China,which could provide the latest material for the study of giant ooids.To research the genetic mechanism of giant ooids,we analyze structure characteristics, particle composition, sedimentary environment, ancient climate characteristics, ancient ecological background and the ancient seawater chemistry of this two giant ooids,which occur in eraly Cambrian and Early Triassic. The formation of giant oolite must satisfy the following conditions: nucleus which can be embraced,and could be algal pellet, bioclastic and oolite; In terms of climatic conditions, significantly higher CO2 concentration in atmosphere could cause two effects:on the one hand, it forced frequent and strong storm which could provide strong hydrodynamic conditions for the forming of giand ooids;on the other hand, Under the influence of frequent and strong storm,the seawater would roll continuously and led to a high concentration of dissolved CO2 in the atmosphere to form the sea water’s chemical property of supersaturated CO2. In terms of microbe,extremely poor sedimentary environment led to the lack of metazoon and the prosperity of microbes.and the seawater of supersaturated CO2 provide favorable conditions for the survival of microbe,meanwhile a large number of microbial metabolic activity also influence the local microenvironment of the giant oolite—deposition of CaCO3 around the nucleus at a high speed. In term of sedimentary environment,although the sedimentary environment which had doposited giant ooids of early Cambrian is inner-middle ramp,and the sedimentary environment of early Triassic is open-platform,but these two sedimentary environment show that thier depth is shallow and can be affected by the frequent and strong storm. In a word,the active involvement of microorganisms,extreme paleoclimate background and favorable sedimentary environment contribute to the forming of giant ooids.In addition,based on study of the genetic mechanism of giant ooids in the two eras,we also discussed related problems about giant ooids. It seems that giant ooid of early Triassic and "anachronistic facies" on the sedimentary period is completely consistent,but the distribution range and frequency of giant oolite in the early Triassic are far inferior to other "anachronistic facies" deposition. Thus it can be seen that whether the giant oolite can be as a new kind of early Triassic "anachronistic facies" deposition need to do further analysis and research.Meanwhile,microbe paly a crucial role in the forming of giant oolite,which not only constitute the main components of rocks,but also through its physiological activities such as changing the chemical property of the water medium in sedimentary environment to promote the deposition of giant oolite limestone.Therefore,we hold opinion that giant ooids should belong to Microbialite.