Migration of the carbonate ramp and sponge buildup driven by the orogenic wedge advance in the early stage (Carnian) of the Longmen Shan foreland basin, China

The marine Carnian the Maantang Fm. overlies a flexural forebulge unconformity and records the initial establishment, drowning and migration of a carbonate ramp and sponge buildup along the forebulge margin of the Longmen Shan foreland basin. The Maantang Fm. is of wedge-shaped geometry, and is composed of oolitic and bioclastic limestones, siliceous sponge reef and shale in an upward-fining succession. The formation shows the establishment and drowning of a distal margin carbonate ramp and sponge buildup, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics. The formation also shows the transition from shale cratonward into carbonate rock southeastward. The sponge reefs and shoal were deposited on a carbonate ramp on the distal margin of the early foreland basin. The growth rate of sponge reefs is 0.04 mm/yr, equivalent to the rate of relative sea level rise of 0.01-0.05 mm/yr. The sponge buildup and oolitic shoal are divided into seven zones southeastward on the carbonate ramp along the basal unconformity. Their migration rate of 18 mm/yr from NW to SE coincides with the estimated orogenic wedge advance rate (5-15 mm/yr, Li et al., 2003), a clear indication that the advancing wedge controlled the migration rate of foreland oolitic shoal-siliceous sponge reef. We have inferred that the tectonic load of the Longmen Shan orogenic wedge led to flexural subsidence and rising relative sea level in the foreland basin located at the western margin of the Yangtze Craton, driving the growth and subsequent drowning of the oolitic shoal-sponge buildup in the early stage of the foreland basin. We propose that the drowning and migration processes were the sedimentary response to the orogenic wedge advance toward the Yangtze Craton, and to the rapid closure of the Carnian Songpan-Ganzi remnant ocean basin.