Sedimentary facies and palaeogeography of the middle-upper Yangtze area during the Early Cambrian (Terreneuvian-Series 2)

Constrained by regional tectonics, sea-level changes, upwelling oceanic currents and palaeoclimates, the sedimentary environments in the middle-upper Yangtze area appeared as the neritic shelf during the Early Cambrian (Terreneuvian-Series). During the Meishucunian (Early Cambrian), the study area was covered by tidal-flat clastic rocks, phosphorites, dolostones and siliceous rocks; shelf siliceous rocks with phosphatic nodules and mudstones, and shelf deep-water basin carbonaceous shales and siliceous rocks. During the Qiongzhusian, the shelf deep-water basin expanded toward the northwestern part due to constant transgressions. The adjacent areas were still occupied by the shallow siliciclastic sea. During the Canglangpuan, the study area was developed into the mixed siliciclastic-carbonate shelf environments where the carbonaceous shales were accentuated. Finally till the Longwangmiaoan, the study area was graded into the carbonate platforms. The above-mentioned sedimentary facies and palaeogeographic evolution during the Early Cambrian shows the persistence of the deep-water depositional area along the southeastern margin of the Yangtze block, where the dominant carbonaceous shales and siliceous rocks are favourable for the deposition of the marine source rocks and the accumulation of hydrocarbons. Meanwhile, the reservoir rocks and wide-spread gypsum horizons were also formed during these periods. It follows that the emphasis of future hydrocarbon exploration should be placed upon the Early Palaeozoic marine strata, especially the target strata in the lower part of the gypsum horizons in the middle-upper Yangtze area.