The Recognition of the Chuxiong Foreland Basin System in Yunnan, China

The Chuxiong Basin in central Yunnan,locates in the west edge of Yangtze platform to the southeast of Tibet and is a Mesozoic oil\|bearing peripheral foreland basin, which developed mainly during the Late Triassic, and passed gradually into an intracontinental downwarped basin during Jurassic time. The integration of geological and geophysical data shows that the basin is separated by the Yupaojiang\|Shaqiao fault into two parts. According to the classic models, the Triassic sediments in the western thrust zone are generally interpreted as the foredeep sediments, while the eastern part is believed to be a foreland bulge and an intracontinental depression. However the authors in the present paper argued, on the basis of structure and texture, distribution, stacking pattern and sediment provenance in combination with the geochemical and geophysical data, that the Triassic sediments in the western thrust zone should be assigned to the wedge\|top sediments, whereas the foreland area in the eastern part recorded progressive deposition in the foreland basin system. The earlier foredeep sediments might have been consumed under the nappe zone in the west rather than cropped out on the surface. (1) Viewed from the horizontal distribution of sedimentary facies, although the strata in the western thrust zone have undergone multiple phases of tectonic deformation, the earlier Triassic sediments are still well preserved. On the other hand, relatively weak tectonic deformation of the strata in the foreland area in the east once occurred, and the early strata were apparently consumed under the thrust zone in the west. (2) The stacking patterns of the sediments indicate that the Middle and Lower Triassic strata in the basin should be ascribed to the typical passive continental marginal sediments, which are overlain unconformably by the Carnian or Norian (Upper Triassic) strata with a depositional break during the Ladinian—Carnian, implying that the Chuxiong foreland basin might originate during the early Carnian. Before this period, the whole basin was once in the passive continental marginal area, where stable carbonate deposits prevailed. (3) The Upper Triassic strata may be divided into four second\|order sequences altogether: four for the wedge\|top sediments in the west; three for the eastern part, and one or two for the margins of the basin. The isochronous surfaces in individual sequences can be regionally correlated. (4) The evidences from structure and texture, composition, palaeocurrent direction and geochemical signature have revealed that the Upper Triassic sediments from the wedge\|top sediments in the western nappe zone are characterized by low compositional and textural maturity, and progressively thinned sediment thickness toward the western orogenic zone as the source area. The present\|day foreland area in the east only accepted the post\|Norian deposits with high compositional and textural maturity. The principal source of detritus lay to the Xikang\|Yunnan oldland in the east. During the late Late Triassic, the thrust zone overthrusted upon the thick rigid lithospheric crust, giving rise to weak warping and slow subsidence of the basin. For this reason, no typical foredeep sediments are observed.