楚雄前陆盆地系统的构造单元及沉积标识

楚雄盆地是一个中生代周缘前陆盆地。地表及深部地质资料显示,盆地内部以渔泡江-沙桥断裂为界可划分为西部推覆带和东部前陆带。本文运用前陆盆地系统新理论,结合沉积、构造和地化标识,认为西部推覆带是典型的楔顶沉积,而不是前人划分的前渊沉积,东部前陆带上经历了递进式前陆盆地系统沉积作用,早期典型的前渊沉积可能消减于西部推覆带之下。

Tectonic Units and Sedimentary Signatures in the Chuxiong Foreland Basin System, Yunnan Province

The Chuxiong Basin in centeral Yunnan is a Mesozoic oil-bearingperipheral foreland basin, which was developed mainly during the Late Triassic, and passed gradually into an intracontinental downwarped basin during Jurassic period. The integration of geological and geophysical data shows that the basin is separated by the YupaojiangShaqiao 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 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 ocurred, 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 presentday 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 thust 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.