Characteristics And Genesis Of Mesozoic And Cenozoic Circular,Centimeter-Scaled Carbonate Particles

Carbonate particles types can be integrated to reflect a variety of deposition and diagenesis features to give sustained attention of many scholars. Up to now, there have been few contrast research data of Mesozoicz and Cenozoic centimeter-scaled carbonate particles. This study includes the characteristics and genesis of Mesozoic and Cenozoic circular, centimeter-scale carbonate particles as rhodoids from Huangliu Formation in Well Xike-1 in Xisha Island, giant oolitics and spheroids from Lower Triassic Feixianguan Formation in the northwest Sichuan, oncoids from Late Triassic Carnian Heiligkreuz F m. in Dolomitess(Italy), whose diameter range mainly from 0.1 to 0.5cm. And with the comparative analysis of Paleozoic oncolites, it's revealed that their morphology, formation mechanism are speculated to be related to special paleoclimatic conditions, ecosystems, and water chemistry of paleo-oceans through its structural characteristics, particulate component, sedimentary environment. In addition, the traces of micro-organisms found in the part of the particles indicate that microbes also played a crucial role.The previous studies concluded that rhodoids are the special biological bone structure of Cenozoic reef-building organisms. Rhodoids are commonly found from Miocene Huangliu Formation in Well Xike-1 located in Xisha Islands, South C hina Sea and its forming environment and development is closely related to reef flat facies with reletively high-energy. Different forms of Cenozoic rhodoids in Xike-1 Well(e.g. round, oval, disc-shaped, etc.) reflect the energy intensity o f the reaction of water, and the dry paleoclimatic conditions facilitate its growth. It's believed that oncoids from Late Triassic Carnian Heiligkreuz F m in Dolomitess, Italy are the cross between giant ooids and oncolites and it's speculated that they yielded in the lagoon facies with turbulent hydrodynamic conditio ns and relatively high waterbody energy environment. The irregular laminae of the Carnian oncoids in Dolomites Area indicate microbes might be involved for its formation conditions. Giant ooids and spheroids from Lower Triassic Feixianguan Formation in the northwest Sichuan, upper Yangtze Region deposited in a special period, i.e., during the slow biological recovery period shortly after the mass extinction at P-T boundary. Meanwhile, lack of metazoan, prosperous algal, carbonate supersaturation of ancient seawater, frequent storms, extreme weather conditions, ctc. promoted the generation of giant ooids. Spheroids in the Early Triassic Moenkopi group were discovered and proposed by Pruss and Payne(2009) in the western United States virgin limestone, while this circular, centimeter-scaled carbonate particles without concentric laminae have not been reported in C hina from the publications. Here the spheroids found in the Lower Triassic Feixianguan formation from Baimiaozi section, Beibei, Chongqing is the first study case in china. Both can be comparatively studied and the similar causes from Moenkopi formation are mentioned. O ncolites from middle Devonian Jinbaoshi Formation in Ganxi Section in Beichuan, Sichuan are studied to compare and it's revealed that these oncolites yielded in the relatively low-energy environment of the lagoon facies. The concentric laminae of the Devonian oncolites are irregular and a large number of Girvanella gathered in these centimeter-scaled particles reflecting the significant microbial origin. The comprehensive study indicates that the formation of giant ooids and spheroids should be combined with both inorganic cohesion and biochemical effection. Whether the concentric laminae developed is related to the hydrodynamic conditions. It shows a clear pattern of concentric laminae of giant ooids grew in the environment with higher energy than that of few significant concentric laminae in the spheroids. However, both of them are interlayered between the oolitic facies, reflecting the intergrowth of giant ooids and spheroids occurred with the similar hadrochemical conditions from the normal oolitic limestone. The role of microorganisms is discussed to form laminae with both construction and damaging effection. Based on the previous research of microbialite, it's discussed whether the giant oolitic limestone, oncoid limestone, spheroid limestone should be involved in microbialite and it's considered that they are special microbialite.