The Lower Triassic cyclic deposition in Chaohu, Anhui Province, China

The Lower Triassic strata developed in Chaohu, Anhui Province of east central China, including the Yinkeng, Helongshan and Nanlinghu formations is about 268 m thick and consists of carbonate rocks such as micrite limestone and nodular limestone as well as shale and argillaceous mudstone. The deficiency of sedimentary structures and bioclastic particles and presence of pyrite and high ratios of V/(V + Ni) suggest that the Lower Triassic strata formed in a quiet, open-marine basin environment bordering carbonate slope and lower slope.Within the Lower Triassic, the cyclic deposits include two orders of thickening- and coarsening-upward successions, respectively called basic unit (BU) and basic sequence (BS), which represent shallowing-upward cycles. Five basic units are recognized and they are mostly composed of shale, argillaceous mudstone, calcareous mudstone and nodular limestone and overlying nodular limestone and micrite limestone, about 0.25–1.4 m thick. The shale and mudstone mainly formed in a deep water basin, and the nodular limestone and micrite limestone formed on the rise of basin margin or the base of slope. Thus, the basic units are interpreted to reflect cyclic sedimentation from the deep water basin to the rise of basin margin or from the rise to base of slope. The basic units are combined with medium- to thick-bedded micrite limestones and thus form four types of basic sequences, average 6.5–14.62 m thick. Likewise, they are interpreted to be shallowing-upward cycles with long period, for example from the deep water of basin, through the rise of basin margin, to the base of slope, farther, reaching to lower slope.The various orders of cycle in the Lower Triassic are laterally very extensive, regionally, which suggests that the fluctuation of sea level was likely a control on the origin of the cycles. These small-scale transgressive–regressive cycles possibly reflect Milankovitch cyclicity.The associations of basic sequences constitute two transgressive–regressive cycles which comprise strata of Yinkeng–Helongshan and Nanlinghu stages, respectively, which is similar to the carbon isotope cycles of the Lower Triassic. Tectonic activities can control the origin of the cycles. The collision of South China Block and North China Block in the Lower Middle Triassic may have resulted in the development of the two cycles, and caused the shallowing-upward sequence of the Lower Triassic from deep water basin to slope.