CHARACTERISTICS AND ORIGIN OF THE DOLOSTONES OF THE EDIACARAN DENGYING FORMATION IN THE EASTERN SICHUAN BASIN, SOUTH CHINA

Abstract

The Ediacaran Dengying Formation dolostones were the crucial carrier of the co-evolution between the biota and environment before the Cambrian Explosion and the solid mineral and petroleum resources. Based on the petrological and mineralogical analyses, the Dengying Formation at the Liaojiacao Section consists of micritic, fine-crystalline and medium-crystalline dolostones and algal-bonded or algal-clastic dolostones. Moreover, the micritic and algal-bonded dolostones mostly retain the original sedimentary structure with widespread microbial mats, algal spots, algal traces, and other microbial fossils. Recrystallization, dissolution, and other diagenetic alterations occurred in the fine-crystalline and medium-crystalline dolostones as well as algal-clastic dolostones, and their original sedimentary structures had been significantly altered. Furthermore, in situ geochemical analysis shows that the micritic and algal-bonded dolostones of the Dengying Formation, which retain the original sedimentary structure, have Mg/Ca ratios close to the ideal primary dolomite; Na, Fe, Mn, and Sr concentrations located in the distribution range of calcites and dolomites formed in a normal marine environment, negative Ce anomaies and positive Eu anomaies with the patterns of light rare earth element depletion and medium and heavy rare earth element enrichment. In addition, the whole-rock stable isotopic analyses show that the δ13Cdol values of dolostones that retain the original sedimentary structure are consistent with the distribution range of the original marine carbonate rocks, but the δ18Odol values are slightly lower than those of the original marine carbonate rocks. Therefore, we infer that the dolostones of the Dengying Formation at the Liaojiacao Section with original sedimentary structure are the products of mimetic dolomitization and formed in an environment with extensive microbial activity, calcite as the preexisting mineral, normal salinity, and oxidized seawater as the dolomitization fluid. Moreover, the fine- and medium-crystalline dolostones were the results of recrystallization that retained the original sedimentary structures during diagenesis.