Sequence-Stratigraphic Framework of the Early to Middle Triassic and Evolution of Sedimentary-Facies and Paleogeography in the Nanpanjiang Basin and Its Adjacent Areas: Discussion on Evolutionary Process from the Dianqiangui Basin to the Nanpanjia

A large-scale drowning event occurred in transitional period between Permian and Triassic and resulted in demise of some carbonate platforms in Nanpanjiang and its adjacent areas. The research areas experienced an evolutionary process of sedimentary basin from Dianqiangui to Nanpanjiang in Triassic. Accordingly, Triassic period is a differential period of sedimentary facies in research areas, which are marked by spatial changes from connective platforms to turbidity basin with development of several isolated platforms. More particularly, oolitic banks of early epoch are developed in some isolated platforms such as in Jingxi platform and in Nanning platform in southern part of Nanpanjiang Basin. Same as limestones of reef and beach in margin of attached platform, limestones of oolitic banks in isolated platforms came to front. Despite differences in time-span and architecture of facies-succession of third-order sedimentary sequences, process of third-order relative sea-level changes reflected by sedimentary-facies succession is generally synchronous. Therefore, six third-order sequences could be discerned in Early and Middle Triassic in Nanpanjiang Basin. With two types of facies-changing surfaces and two types of diachronisms in stratigraphic records as key elements, framework of sequence stratigraphy from Early to Middle Triassic in Nanpanjiang could be constructed. The sequence-stratigraphic framework and relevant paleo-geographical features of Early to Middle Triassic reveal formational and evolutionary process of Nanpanjiang Basin, and paleogeographical features of Late Triassic reflect dying of Nanpanjiang Basin. In research areas, Early Triassic can be grouped into three third-order sequences, SQ1, SQ2 and SQ3. The forming process of SQ1 represents a building process of a ramp carbonate platform not only in attached platform but also in isolated platform. The forming process of SQ2 and SQ3 reflects an evolutionary process of carbonate platform from ramp type to rimmed-shelf type. Correspondingly, oolitic-bank limestones constitute HSTs from SQ1 to SQ3 in background of an isolated platform. A succession from non-compensatory sediments to compensatory sediments of turbidity basin is formed in settings of a deep-water basin. Three third-order sequences from SQ4 to SQ6 of Middle Triassic reflect three obvious changing processes in deepening-shoaling of sedimentary environment. Consequently, turbidity rock-series are formed in setting of a basin; reef limestones or bank limestones are developed in margin of attached platform, and bioherm limestones are formed in isolated platforms. In transitional period between Early and Middle Triassic, in concomitance with strong volcanic activity, there was a large-scope fall of third-order sea level changes, which generated unconformity of first episode of Indosinian movement-the top boundary of SQ3. With rise of crust caused by Indosinian movement in east part of research area, the Xianggui Basin is defined by Zhao et al. (1996) was gradually closed, thus a unified turbidity basin i. e, the Nanpanjiang Basin defined by Enos et al. (1998) was formed in Ladinian age in Dianqiangui area.