Sedimentary And Environment Analysis Of The Mesozoic Strata In The Southwest Margin Of Chuxiong Basin

The study area is located in the eastern side of Ailaoshan orogenic belt and the southwestern margin of Chuxiong basin. Previous studies were main in Chuxiong Basin, but the combination of rock and sedimentary characteristics of Mesozoic strata in the Chuxiong basin was too simple. Moreover, the material source of sediments, sedimentary facies and the evolution of sedimentary environment are still controversial. This paper is based on stratigraphic section measurement instruments, field geological survey sampling, rock microscopic identification, scanning electron microscopy, isotope analysis, and it focused on the combination of rock, sedimentary characteristics and material sources of the triassic Yunnanyi group on Mesozoic strata. Luojiadashan group, Baitutian group and Jurassic Fengjiahe formation have made a detailed study, combing with regional geological data and the results of previous studies, analyzing and discussed the sedimentary facies, sedimentary environment and basin-mountain coupling of Chuxiong basin Mesozoic strata.The lower and upper of triassic Yunnanyi formation is composed by clastic sediments of the deep sea, and it mainly including mudstone, muddy siltstone and sandstone, minor cherts, shale, conglomerate and fine-grained sandstone; the middle is a water carbonate deposition, microcrystalline limestone and micritic limestone. Containing abundant fossils, ammonites harvesting Celtidae, Isculitidae(save the Section typical suture) and Badiotella? sp. Luojiadashan group is mainly composed of fine-grained sandstone, siltstone, muddy siltstone, silty mudstone, mudstone interbedded output, the overall rhythm is good, less conglomerate and shale content. Also, there is visible seam thickness of 0.3m where around a lot in the Luojiadashan group appeared on the segment. Baitutian group only exposed the upper section, with the underlying Luojiadashan group in fault contact with the overlying Fengjiahe formation conformably contacts. Mainly composed of fine-grained sandstone, siltstone, argillaceous siltstone, mudstone, silty mudstone, conglomerate less content. Lower Jurassic Fengjiahe formation is a huge purple deposition, mainly dominated by fine-grained sandstone, siltstone, shale and siltstone, mudstone, a small conglomerate, conglomerate color, mostly yellow-green or yellow-white, as was source region handling, recycling deposition.The result from the discriminatory analysis of gravel components, roundness in the formation, debris components in sandstone and Dickinson triangle diagram shows that the source of sediments in the study area are mainly from the west of orogenic belts. By Geochronology experimental analysis, it shows that the era of the formation of Chuxiong basin is not earlier than late Triassic. In other words, an early-middle triassic island arc exists in the western part of the study area and the provenance area consistently provided provenance to the basin from proterozoic to middle triassic.The sedimentary basement of Chuxiong basin consists of permian and the old paleozoic strata, and it started depositing from the lower Yunnanyi group. Meanwhile, because of the provenance area of ailaoshan orogenic belts was just exposed in ground, the whole basin showed under-compensation state. In the mid-term of upper Yunnanyi group, due to the provenance area of orogenic belts structure trended to stabilization, this Yunnanyi group appeared carbonate marine environment. In the period of upper Yunnanyi group and lower Luojiadashan group, the activity of orogenic belts and basin subsidence had intensified, and the formation of the deep sea-bathyal turbidite fan deposition phase. Luojiadashan began during the middle of the mountains, basin subsidence weakened by the deep-water basin into a shallow shelf, Luojiadashan segment, and Baitutian group coastal delta plain. A single overall shallowing-upward cycles, showing from early to late, from dark to light, from the deep sea-shallow-shore, delta plain-continental general evolutionary trend of large lakes.