Devonian Sequence Stratigraphy and Sea-Level Change in Longmenshan Mountains——An Example from the Guixi Section in Beichuan County, Sichuan Province

The Caledonian orogeny that occurred in the transitionalperiod from Silurian to Devonian led to convergence of the Yangtze Plate and the Huaxia Plate and foamed the uniform South China Plate.Because of the eastward expanding of the northern branch of the Paleotethys, during Devonian, the Longmenshan area had become thepassivemargin of the northwestern South China Plate.On this relatively tectonically stable background, the stable Devonian Guixi section could be approximately subdivided into 18 thirdorder sedimentary sequences(DS₁-DS₁₈).The sequence stratigraphic subdivision indicates the Longmenshan area underwent an evolutionary process of sedimentation from siliciclastic sediments of transgression (DS₁-DS₉)to the mixed carbonate-siliciclastic sedimentsof highstandperiod (DS₁₀-DS₁₂), and then to the sedimentationof ramp carbonate of regression period (DS₁₃-DS₁₈),which therefo reformed a complex but regular sequence stratigraphic succession.For the Guixi section, 18 third-rder sequences can further be grouped into 2 second-order tectonic sequences corresponding to two highstands of transgression.The characteristics of the sedimentary succession and sea-level changes could be correlated with the Devonian Dianqiangui area.But the Devonian Guixi section is much thicker and the number of sequences is more than that in the Dianqiangui area in which 13 third-order sequences can only be discerned.Variable fades fabrics of different stages are considered to be in response to the different patterns of sea-level changes and the paleogeographic backgrounds.