Sequence-Stratigraphic Frameworks And The Global Correlation Of Third-Order Sea Level Changes For The Permo-Carboniferous Of The Dianqiangui Basin And Its Adjacent Areas

The Dianqiangui Basin belonging to a passive-continental-margin rift basin was formed in the south of Guizhou province, the west of Guangxi province and the south-east of Yunnan province after the Caledonian orogeny. The forming process of the Dianqiangui Basin goes through three periods. It began in Devonian, persisted in Carboniferous, and became fiercer in Permian. Controlled by syn-depositional fault-zones, varieties of isolated carbonate platforms, large and small, are developed in the background of a deep-water basin, namely, an inter-platform basin. And a special paleogeographical pattern marked by "platform-basin-hill-trough"was formed in the Dianqiangui Basin and its adjacent areas. On basis of the detail division of third-order sequences in different palaeogeographical settings as well as the tracking and correlation of third-order sequences in space, 12 third-order sequences could be discerned in the strata of Permo-Carboniferous in the Dianqiangui Basin and its adjacent areas. Among 25 third-order sequences (SQ1 to SQ25) that are identified in the strata from Devonian to Permian, 12 Permo-Carboniferous third-order sequences are from SQ14 to SQ25. On the basis of the two kinds of facies-changing surfaces and the two kinds of diachronisms in stratigraphic records, Permo-Carboniferous sequence-stratigraphic frameworks in the Dianqiangui Basin and its adjacent areas can be established. And sequence-stratigraphic frameworks clearly demonstrate two elementary features of third-order sequences: regularity of facies-succession in space and synchronism of environmental changes in time. These features are clearly illustrated in the series of penal diagrams of sequence-stratigraphic framework and the corresponding palaeogeographical maps. Carbonate rocks chiefly constitute the Permo-Carboniferous strata in the study area. In this set of strata, three sets of coal measures are formed in the attached platform and two sets of sponge reef are developed in the margin of attached platform and on the isolated platform, which make up a particular sequence succession of that is greatly different to the sequence succession in Euramerica. Similar to the division of third-order sequences in North America by Busch et al. (1985), among 12 third-order sequences the strata from Carboniferous to Permian Chuanshanian epoch can be divided into 6 third-order sequences (SQ14-SQ19). The forming duration for these 6 third-order sequences is longer than 10 Ma. This feature might be the result of third-order sea-level changes in the convergence period of the Pangea marked by the relative tectonic-stable period. Ultimately, the number of 12 Permo-Carboniferous third-order sequences in the study area is few than the 50 more third-order sequences in Euramerica discerned by Ross and Ross (1985). This clear discrepancy shows that third-order sea-level changes controlled not only by the global sea-level changes but also by the regional tectonic movements. Therefore, it could be concluded that it is very difficulty to establish a globally isochronous sequence-stratigraphic framework demonstrated by third-order sea-level changes. This work is a complex systems engneering.