Sequence paleogeography and coal accumulation in epicontinental basin

Abstract

Based on an investigation of the Benxi–Shanxi formations in the southwestern Shandong province, we studied the forming mechanisms of key stratigraphic boundaries in an epicontinental basin and the coal-accumulating characteristics in the paleogeographic background of isochronous stratigraphic frameworks. In total, we sequenced three types of stratigraphic boundaries (i.e., regional unconformities, the shifting surface of transgressions caused by basin basement subsidence, and fluvial erosional surfaces including exposed surfaces as well as correlative conformities, which are caused by forced regression) and proposed a model for coal-accumulation. The results show that in the high accommodation space of the barrier coast system in the lower Benxi–Taiyuan formations, the base level was higher than sedimentary boundaries, parasequences were bounded by a marine flooding surface and formed during the semi-cycle of base level falling that was influenced by high-frequency fluctuations of base level, and coal seams were thin with multistoried patterns. In this region, the coal accumulating center was located in the transitional zone of the barrier island and tidal-flat. In the low accommodation space of the fluvial delta in the upper Shanxi formation, the base level coincided with sedimentary boundaries, parasequences formed during the semi-cycle of base level rising were bounded by erosional surfaces and a marine flooding surface, and the coal accumulating center was located in the lower deltaic plain and interdistributary lacustrine environment. In this region, the surplus accommodation space was relatively high.