Sequence stratigraphy, paleogeography, and coal accumulation in a lowland alluvial plain, coastal plain, and shallow-marine setting: Upper Carboniferous–Permian of the Anyang–Hebi coalfield, Henan Province, North China

Abstract

To study the stratal stacking patterns and controls on coal accumulation in a lowland alluvial plain, coastal plain, and shallow-marine setting, the depositional facies, sequence stratigraphy, and paleogeography of the Carboniferous–Permian strata in the Anyang-Hebi (Anhe) coalfield, Henan Province, northern China were analyzed based on borehole cores. Ten facies were grouped into four facies associations, which represent alluvial plain, delta plain, delta front and prodelta, and offshore marine depositional environments. Ten sequences (S1 to S10, in ascending order) were defined on the basis of vertical facies stacking patterns. Sequences S1–S3, of Moscovian to early Asselian age, are composed mainly of offshore marine carbonate and mudstone, prodelta and delta front mudstone and sandstone, and delta plain sandstone, mudstone and coal. Sequences S4–S9, late Asselian to Wuchiapingian in age, consist of delta plain sandstone, mudstone and coal. Sequence S10, corresponding to the Changhsingian stage, is composed mainly of alluvial plain sandstone and mudstone. From S1 to S10, the Anhe coalfield experienced an environmental evolution from offshore marine, prodelta and delta front through delta plain to alluvial plain environments, an evolution from a low-accommodation to a high-accommodation setting, and a paleoclimatic change from humid to semi-arid conditions. Coal accumulation was controlled by a combination of relative sea-level fluctuation, paleoclimate and paleogeography. The cyclothemic nature of S2, including presence of parasequences, and the significant sea-level fall interpreted at the base of S4 may be related to Gondwanan ice dynamics in the late Carboniferous–early Permian. The thick coals are concentrated in the transgressive systems tracts of S2 and S4. The results of this study provide new insights into the environmental evolution of Carboniferous–Permian lowland alluvial plain, coastal plain and shallow-marine successions of the northern subtropical paleolatitudes and the controls on coal deposits in North China.