Late Cenozoic Longmenshan Slip Sedimentary Characteristics And Evolution Of The Foreland Basin

The present Longmenshan foreland basin lies between the Longmenshan thrust belt and the Longquanshan fault. The Dayi conglomerate at the bottom of the Chengdu basin, although people of the past having studied a certain degree, hasn' t same sugest. In this paper, it benefits the study of relationship between sedimentary basin and mountain uplift. This paper links sedimentary petrology with structural geology, so is the other branch subjects, penetrates into researching the Pliocene-Pleistocene sediments of the present Longmenshan strike-slip foreland basin, aims at studying tentativly the evolvement history and mechanism. Following progresses are made through the project:1) The Dayi conglomerate grows in the Cenozoic Longmenshan foreland basin bottom, whose thickness becomes thin from west to east. The Dayi conglomerate is angular unconformity with the Yaan conglomerate and down layer. Neotectonic movement is marked with the fault and the fold in the Dayi conglomerate. The Dayi conglomerate geologic time is ~4. 6 Ma in Pliocene to ~0. 82Ma in Pleistocene. It shows that its sediment is continuous in the interval, and tectonic event doesn' t occur.2) Analyses and researches systematicly the lithofacies, sedimentary facies and depositional environment of the Dayi conglomerate in the Cenozoic Longmenshan foreland basin. This paper analyzes detailed six coarse elastics lithofacies, five mid-elastics lithofacies and three fine elastics lithofacies. Establishes the association of three depositional environment types including alluvial fan and river.3) According to the Dayi conglomerate' s gravel composition and direction when water flowed during geological age, comeback the Paleogeograph of the Longmenshan foreland basin, speculates that the Dayi conglomerate' material source is located in the Songpan-Ganzi fold belt to the east, the Pengguan fault to the west of the western Sichuan plateau region.4) Analyses boundary faults of the Cenozoic Longmenshan foreland basin, and thinks the basin boundary faults, being active fault, are thrust and dextral strike-slip. So the Longmenshan foreland basin is dextral strike-slip foreland basin. It isn' t symmetry. The west is cliff and the east is gentle. The filling inside the west of basin is thick and the east is thin. Sedimentary center and depositional center of the basin are inconformity for dextral, and continuously move to the east.5) Quantitative analysis to the Dayi conglomerate in the Cenozoic Longmenshanforeland basin has been carried out. The results indicate that the dextral strike-slip rate is 1.02 mm/a, the average sedimentary rate is 0.143 mm/a, and the average decline rate is 0. 3mm/a.6) Discusses the developing history of the Longmenshan strike-slip foreland basin. I think: In the Pliocene-Pleistocene, evolvement of the Longmenshan strike-slip foreland basin is controlled by thrust loading uplift mechanism, and after the Pleistocene, it is controlled by erosion unloading uplift mechanism.