1 College of Geosciences,China University of Petroleum(Beijing),Beijing 102249; 2 State Key Laboratory of Petroleum Resource and Prospecting,China University of Petroleum(Beijing),Beijing 102249
Abstract：To eveluate the paleoceanographic information recorded in the stratigraphic record, carbon and oxygen isotope analyses were conducted for 83 carbonate rock samples from the Longwangmiao Formation of the Lower Cambrian along with outcrop study and thin section analysis. The results (excluding invalid samples)show that the δ13C values of carbonate rocks in Longwangmiao Formation vary from -4.300‰ to 2.694‰ with the mean value of -0.031‰ and the δ18O values vary between -9.880‰ and -0.100‰ with the average of -7.396‰. The vertical variation of δ13C shows a decrease trend followed by an increase trend from bottom to top of the studied interval. The carbon isotopic composition is characterized by negative values with frequent occurrences of positive or negative excursions at the lower to middle part of the formation and positive values with small fluctuation at the other parts. The reconstruction of palaeo-ocean environments shows that marine environment was developed around the eastern Chongqing area during the Lower Cambrian Longwangmiao stage. The salinity of seawater was low during the Early Longwangmiao stage and high during the later stage. Sea water temperature mainly ranged between 20,℃ and 30,℃,suggesting a warm to hot subtropical climate. Three stages of regression-transgression cycles through Longwangmiao stage were identified. When sea level rises,increased marine productivity and fast burial of organic carbon result in an increase of δ13C value. In the case of dropping sea level, the δ13C value decreases. During the middle to late Longwangmiao stage,the negative shift of δ13C value,low seawater level and the high salinity provide favorable conditions for the development of dolomites.
Yang Xueqi,Zhong Dakang,Ren Ying et al. Characteristics and significance of carbon and oxygen isotopes of the Cambrian Longwangmiao Formation, eastern Chongqing[J]. JOPC, 2017, 19(5): 865-878.
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