Sedimentology and sequence stratigraphy of lacustrine deep-water fine-grained sedimentary rocks: The Lower Jurassic Dongyuemiao Formation in the Sichuan Basin, Western China

Abstract

Despite the sedimentology of deep-water fine-grained sedimentary rocks from marine environments having received highlighted attention in recent years, their counterparts from the lacustrine environments are rarely reported. Based on a complete sedimentological, petrographic, and geochemical analysis of three continuous cored wells of the Lower Jurrasic Dongyuemiao Formation, Sichuan Basin, West China, this study illustrates the range of lithofacies, cyclicities, and transportation and deposition mechanisms of this lacustrine deep-water succession. The aim of this research is to establish a synthesis depositional model for lacustrine deep-water fine-grained sedimentary systems in the Sichuan Basin. The results indicate that seven lithofacies were developed in the Dongyuemiao Formation and they were stacked in a repeated pattern to constitute ca 10–20 m thick composite cycle sets. A typical composite cycle set comprises an upper and lower unit. Specifically, the lower unit is characterized by basal relatively coarse-grained detrital-rich lithofacies interpreted to record gravity flows that grade upward into relatively fine-grained detrital-lean lithofacies interpreted as suspension deposits from the water column. The upper unit is stacked in a reverse pattern and characterized by basal relatively fine-grained detrital-lean lithofacies that grade upward into relatively coarse-grained detrital-rich lithofacies. Such cyclicities in the Dongyuemiao Formation are associated with third-order lake level variations. Specifically, the relatively coarse-grained detrital-rich lithofacies is interpreted to be deposited from low-density turbidity currents, likely hyperpycnal turbidity flows, during maximum lake level regression when terrigenous fluvial and delta systems developed in the basin margin. The relatively fine-grained detrital-lean lithofacies is interpreted to be deposited from suspension fallout during maximum lake level transgression. This cyclicity pattern and frequent occurrence of sediment gravity flow deposits in the Dongyuemiao Formation suggests a dynamic hydrographic condition in the deep-water lacustrine environment of the Sichuan Basin.