Sedimentary Record And Provenance Analysis Of Late Triassic-Middle Jurassic Basin In Northeastern Guangxi

A Late Triassic to Middle Jurassic intracontinental basin in northeastern Guangxi Province,China was developed after the Indosinian tectonic movement.The sedimentary environment,provenance and associated variation characteristics of the Late Triassic-Middle Jurassic strata are of great significance for understanding the formation and evolution of the basin as well as the dynamic processes.Accordingly,this paper applied stratigraphy and detrital zircon U-Pb geochronological data to systematically study the sedimentary facies and detrital sources of the Late Triassic to Middle Jurassic strata in the Northeast Guangxi Basin,and reveals the evolution and tectonic setting of the Basin.The analytical results of stratigraphic sequence and sedimentary facies show that the lower and middle parts of Upper Triassic-Lower Jurassic Tiantang Formation in the basin were formed in an alluvial fan environment under arid and semi-arid climatic conditions,whereas the upper part of the Tiantang Formation is the product of fluvial facies indicating a transition from alluvial fan to fluvial facies.Bivalve fossils were identified in the Lower Jurassic Daling Formation,which is mainly deposited by fluvio-lacustrine facies,along with alluvial fan in some parts of the basin.The Middle Jurassic Shiti Formation is mainly composed of sandstone and mudstone,with plant fossils and coal seams,and were dominantly deposited in fluviallacustrine environments,along with marshy facies in some places.The results of detrital zircon provenance analysis show that sandstone samples from the upper part of the Upper Permian Leping Formation in Northeast Guangxi are dominated by the Grenvillian(peaking at 971 Ma)and Caledonian-age(peaking at 466 Ma)detrital zircons,indicating that the detrital zircons were mainly derived from Yunkai,Wuyishan and Nanling areas.Sandstones of the Upper Triassic-Lower Jurassic Tiantiang Formation unconformably overlain the Leping Formation mainly contains Paleoproterozoic,Indosinian and early Yanshanian-age zircons,which indicates that the detritus were mainly from the southern margin of the Yangtze Plate,Wuyishan-Nanling region and the eastern segment of Nanling from southern Jiangxi to northern Guangdong to western Fujian.The detrital zircons of Grenvillian,Caledonian and Indosinian in age appear in sandstones of the Lower Jurassic Daling Formation,of which the Indosinian-age zircons are dominant,indicating that the provenance area may be Yunkai-Wuyishan area and the Miaoershan-Yuechengling regions in the southern margin of the Yangtze plate.The age peaks of detrital zircons in the Middle Jurassic Shiti Formation include Caledonian and Indosinian-age zircons,as well as 765 Ma zircons,indicating that the Grenvillian magmatic rocks in Yunkai area may also provide the sediments.The main provenance areas may be Yunkai-Wuyishan area(corresponding to Caledonian and Indosinian zircons),and the secondary provenance area may be Yunkai area(corresponding to Grenvillian zircons)and Wuyishan-Nanling area(corresponding to Palaeoproterozoic zircons).The change in sedimentary provenance(sandstone compositions and detrital zircon)of the Northeast Guangxi Basin is related to the switch from the compressive stress during the Indosinian stage to the extension during the Early Yanshanian stage,corresponding to changes in the tectonic configuration of the southeastern South China during the Mesozoic.