Basin-range transition and depositional records:an example from the Chuxiong foreland basin in Yunnan

The Chuxiong Basin located in central Yunnan on the southwestern margin of the Yangtze landmass is a Mesozoic peripheral foreland basin,which is bounded on the southwest by the Honghe fault,on the west by the Chenghai fault and on the east by the Puduhe fault.Tectonically,three units maybe distinguished for the basin.They are:(1)western thrusting zone;(2)central depression zone,and (3)eastern uplifted zone.In terms of sedimentary facies,sequence stratigraphy and palaeogeographic evolution in combination with tectonic evolution of the Ailaoshan orogenic zone,the basin shows a depositional evolution from the Palaeozoic passive continental margin to the Mesozoic foreland basin.Several phases of sediments have been identified in the basin.For example,during the Carnian (Late Triassic) and the early and middle Norian (Late Triassic),the foreland flysch sediments were accentuated.while during the late Norian (Late Triassic) to the Paleocene,the foreland molasse sediments were highlighted.The molasse sediments consist of two categories:marine and continental.The latter are characterized by long time duration,widespread occurrence and greater thickness.On the whole,the basin may have experienced four stages of evolution:basin formation,rapid subsidence, inversion and filling,and final termination.The upward growth and cratonward overburden of the over-thrusted orogenic wedges led to the fomiation of sudden deepening- and fining-upward sequences during the early stage and shallowing- and coarsening-upward sequences in the basin during the late stage.In other words,there are gradations from the early flysch sediments to the late molasse sediments,and from deep and narrow basal relief during the early stage to wide and shallow basal relief during the late stage.The centres of subsidence and deposition of the basin once in front of the thrusting orogenic wedges gradually migrated northeastwards (cratonwards).All the data on palaeocurrents,petrography, lithogeochemistry indicate multiple sources of the detritus in the basin,i.e.the detritus were derived dominantly from the ancient Ailaoshan orogenic zone,and subordinately from the eastern uplifted zone.