Analysis Of Provenance And Basin Type Of The Cambrian In The Micangshan Area

The Micangshan area in the northern margin of the Yangtze plate is located at the junction of the South Qinling orogenic belt and Sichuan Basin.The particular stratigraphic framework of the study area provides a good example for the Cambrian tectonic setting.A lot of studies have been carried on Cambrian strata,however,different recent results come into contradictions.The first is temporal and spatial distribution of sedimentary system and the direction of source.The other is tectonic evolution reflected by provenance.Therefore,this paper use the sedimentology to further discuss the Cambrian sediment source,tectonic setting and re-evaluate the tectonic evolution of the northern margin of the Yangtze.Based on field geological observations and measurements,this study disscussed the Cambrian sedimentological features and the erosion process of the source area through various methods(paleocurrent analysis,heavy mineral analysis,detrital composition of sandstones,and zircon U-Pb dating,etc.).Combining with previous research results,comprehensive analysis of the sedimentary evolution in the Micangshan area and the basin-mountain topography were conducted to determine the basin type.The Cambrian sedimentary sequences were characterized as follow.The Guojiaba Formation was deposited in shore,shallow shelf and submarine fan.The Xiannüdong Formation was in tidal flat and carbonate slope environments.The facies of Yanwangbian Formation is the delta-alluvial fan.During the forming period of Kongmingdong Formation,terrestrial material significantly decreased,and the sedimentary environment was coastal shore and tidal flat.The Douposi Formation was deposited in coastal shore and tidal flat.During the Cambrian,sea water in the study area gradually became shallow,and regression occurred towards northeast and southeast.Based on paleocurrent restoration,heavy mineral analysis,and detrital zircon U-Pb dating,the Cambrian sources in the Micangshan area can be determined.The provenance restored by paleocurrent mainly have two directions,namely,southwest and northwest.The heavy minerals analysis shows that the parent rocks are mainly magmatic rocks,along with minor metamorphic rocks.The detrital zircons U-Pb dating in the southwestern provenance area shows that the peak ages range in783-849 Ma,and the secondary ages range in 524-622 Ma.The main U-Pb ages in the northwestern source area range from 741 Ma to 870 Ma.The detrital composition of sandstone reflects tectonic settings of the source area which is supplied from the mixed zone into mixed and dissected arc zone.Combined with previous studies on the provenance analysis,this paper believes that the Kangdian Oldland in the southwest and the northwestern Motianling Oldland are the main provenance areas of the Cambrian sediments.Meanwhile,deep ocean face,shallow marine face,and continental face assemblages during the Cambrian indicate a compressional setting.The paleocurrent exhibits bidirectional source.Litharenites composed of quartz and chert occurred in study area.The provenance rocks are mainly volcanic origin.Therefore,the Cambrian basin in the Micangshan area is foreland basin.