Carbonate-rich megabeds within a Triassic siliciclastic deep-water system, West Qinling orogenic belt, Central China: Character, processes and implications

Abstract

Deep-water megabeds are a particular type of sediment gravity flow deposit that are anomalously thick and often of distinctive composition compared to the deep-water strata within which they are embedded. Pure siliciclastic or carbonate megabeds have been widely reported from deep-marine systems. Less documented are carbonate-rich mixed megabeds with abundant carbonate clasts in a siliciclastic matrix, which are embedded in siliciclastic deep-water systems. Here, such examples are reported from outcrops of the Lower Triassic in the West Qinling orogenic belt, central China, with a focus on the character, processes and implications of these carbonate-rich megabeds. Based on regional geology and characteristics of the encasing siliciclastic turbidites and autochthonous micritic limestones, these megabeds are inferred to have been deposited in a deep marine trough. The megabeds are thick (1 to ca 10 m) compared to surrounding beds (commonly less than 1 m), and are of mixed composition, comprising both siliciclastic grains and shallow-water carbonate clasts. These megabeds are commonly characterised by a distinctive bipartite or tripartite vertical succession of facies. A complete (tripartite) sequence consists of a basal clast-supported conglomeratic division (Division I), an intermediate matrix-supported conglomeratic division (Division II), and an upper normally graded and/or laminated sandy division (Division III). These divisions are interpreted to be deposited from evolving debris flows transitioning to turbidity currents during a single flow event, and are the result of flow deceleration and dilution. The megabeds show variability over very short lateral distances (several tens to a few hundred metres), possibly related to surface relief on the debritic portion of the deposit. A new depositional model is proposed for the mixed deep-water system, with frequent siliciclastic turbidite deposition within this elongate basin from axially flowing turbidity currents, and episodic deposition from laterally-supplied carbonate-rich megaflows that eroded and incorporated the substrate during transport.