During the Carnian, the Hanwang area in the northwestern Sichuan Basin (South China) was characterized by shallow water carbonate sedimentation that underwent an abrupt demise associated to a sudden input of ter- rigenous sediments. This major facies change was considered to be the expression of the onset of the Carnian Humid Episode, a most remarkable environmental crisis in Late Triassic that is well recognized in northwestern Tethys margins and coincides with a major global perturbation of the carbon stable isotope record. However, the lack of detailed biostratigraphic constraints have so far prevented a precise dating of the carbonate platform demise in western Sichuan Basin.
In this work, the Qingyan Gou (HWQ) section, cropping out in Hanwang, was investigated for its facies and microfacies, carbonate carbon and oxygen isotopes on brachiopods, microbial grains and bulk matrix. Facies analysis shows a marine transgression from inner ramp oolitic shoal to middle ramp siliceous sponge mound, then overlain by shale and calcareous siltstone with interbedded silty mudstones. Refined biostratigraphic data from HWQ and Guanyin Ya (HWG) sections imply that the demise of sponge mounds occur in the late Tuvalian or later.
A negative carbon isotope perturbation was found in the bulk matrix immediately below the sponge mounds demise, but it was not found on the isotope record from brachiopods. This suggests that the negative shift in the bulk carbonate was probably related to diagenesis. Given the late Tuvalian (last substage of Carnian) age at- tributed to the demise of the sponge mounds and the absence of a carbon isotopic excursion, we infer that the carbonate platform crisis and strong terrigenous input in Hanwang cannot be related to the onset of the Carnian Humid Episode. These the demise of the carbonate platform and the facies deepening trend could be rather due to the interplay between accelerating subsidence rates, environmental chants and enhanced siliciclastic input related to the formation of a foreland basin during Indosinian orogenesis.
