楚雄盆地性质与沉积层序演化

The nature of the Chuxiong Basin and evolution of sedimentary sequences

关键词:  楚雄前陆盆地  潮控三角洲  水下舌形体  关键界面和洪泛体类型

Key words:  Chuxiong foreland basin  tidal delta  subaqueous lobe  key boundary and marineflooding body

摘要:

晚三叠世楚雄盆地是扬子西南缘的前陆盆地,其西侧的前陆挠曲盆地盆底变形导致海平面相对上升,从而在卡尼期和瑞替期有3次海泛,并向东侧的前陆盆地隆起带和克拉通边缘推进,沉积了前陆碳酸盐缓坡和暗色泥页岩海泛物。由于海泛体的推进和海退下超的双重作用,构筑了楚雄三角洲与水下砂砾质舌形体组成的复合沉积体系。
前陆盆地三角洲复合沉积体发生在海平面的主体下降阶段,与被动大陆边缘层序迥然有别,对比标志也各异。复合沉积体系的特征由于海退下超面掩盖了海侵面,因而凝缩层不发育,其时空展布则呈进积型迁移,沉积体系域多为楔形体和朵状体。层序对比的标志:一是据3个关键界面的特征,包括暴露不整合面、海泛上超面和洪泛下超面;二是水下舌形体和洪泛体的性质和类型,依此划分了3个准二级层序。

Abstract:

The Chuxiong Basin was a foreland basin on the southwestern margin of the Yangtze platform during the Late Triassic.The basin-floor deformation of the foreland upwarped basin in the western part once led to a relative rise in sea level,and thus to three-phase marine flood- ings during the Camian and Rhaetian.Then these marine floodings prograded gradually toward the uplifted zone and eratonie margin in the eastern part,resulting in the deposition of the flooding sediments of carbonate ramps and dark mudstones and shales.The progradation of the marine-flooding bodies and coastal downlap in turn allow the formation of the composite depositional systems consisting of the Chuxiong delta and subaqueous sandy and gravelly lobes.
The delta complexes in the foreland basin appeared during the falling phases of sea level, and are quite different from the passive continental marginal sequences.In the ease of the conposite depositional systems,the condensed sections are less developed due to the covering of the transgressive surfaces by the dowrdap surfaces.These depositional systems are well represented by wedges and lobes.The sequence correlation is based on such indicators as three key boundaries,exposed unconformities,marine-flooding onlaps and downlaps plus the features and types of subaqucous lobes and flooding bodies.Three sub-second-order sequences have been identified just by these indicators.