Sedimentary Characteristics And Mountain-basin Coupling Of Triassic Lanping-simao Baisn In Southern Yunnan

Lanping-Simao basin is the major tectonic basin during the Mesozoic andCenozoic period in Yunnan, which is situated in the southern part of Three Riverstectonic belt. The geotectonic position is in the southeast of Tethys-Himalaya tectonicdomain between the Gondwanaland and the ancient Eurasia，the east side is JinshaRiver–Ailao Mountain nappe structure belt and west side is lancang river fault zone.The basin’s geotectonic position is significant, aslo with complex geology background,especially the nature of Triassic prototype basin and mountain-basin coupling iscontroversial.The principal research area is the southeast edge of Lanping-Simao basin. Basedon the theoretical direction of sedimentology and structural geology, with the fieldgeological survey and experimental analysis indoor, author carries on a intensivestudy on lithological association, sediment source, tentonic characteristics andmountain-basin coupling of Triassic depositional basin. According above results, weconfirm the nature of Triassic prototype basin and sedimentary tectonic evolution inbasin.The study is based on the observing outcrop and dates analysis, the researchcontent involves the Triassic strata’s features、thickness、distribution and sedimentaryfacies features, sedimentary filling pattern and sedimentary setting are sorted out. Thisthesis indicates that the lower part of Waigucun Formation mainly deposits in deltafacies, the middle and upper parts are mainly braided plain fan deposits and river plaindeposits. The Sanhedong Formation is mainly carbonate platform deposit, with tidalsedimentary locally. The Waluba Formation and Maichuqing Formation togetherformed a whole deltaic depositional system.The results of the characteristic of gravel constituent, the characteristic of debrisconstituent and Dickinson triangular diagram shows that the sediment source was fromorogenic zone in east. Comparative study about characteristics of detrital zircons,shows that the sediments belong to the same provenance area, but some certaindifferences still exist in local. In addition, Associated with the detrital zirconchronology results, an early-middle Triassic island arc can be suggested in the east of study area. Accordingly, the nature of Trassic basin is a back-arc foreland basin.The study zone belongs to the foreland thrust belt in the southwest of AilaoMountain nappe structure belt, the thrust uplift tectonic movement has lasted fromPaleogene. The initial activity time is at about64Ma, the activity is divided into fourstages:40.3Ma�'35.5Ma�'32Ma�'22.5Ma. From the east to west, the ages arebecoming younger. It indicates that the thrust uplift tectonic activity which is the firstexpansion expanded in turn from east to west.The tectono-sedimentary evolution of the basin in study zone started from lateTriassic epoch. In the early Waigucun Formation’s sedimentary period, the orogenicbelt uplifted rapidly, the deltaic deposit in the bottom of foreland basin had beendenudated, because of the later thrust uplift tectonic movement; in the middle and lateperiod, Ailao Mountain belt’s thrust activity occurred once again from east to west,the foreland thrust belt expanded towards into the foreland basin. The basin wasmainly filled with alluvial fan-braid river deposits. The Sanhedong Formation’ssedimentary period was structural stable phase, carbonate platform deposit was filledin the basin. During the Waluba-Maichuqing Formation’s sedimentary period, AilaoMountain belt’s uplift activity which is more active than before was alive again. TheWaluba Formation and Maichuqing Formation together formed a whole deltaicdepositional system. The Waigucun and Maichuqing Formation’s sedimentary periodsare apparent transition points of sediment source.