Study On The Late Paleozoic Sequence Stratigraphy Of Shanxi Area

Shanxi area was frequently experienced sea-level changes in the late Paleozoic. After a long time of exposure from the late early Paleozoic, the area was began to accept the late Paleozoic sediments since the late Carboniferous. Due to the ancient structural movement and the imbalance in the North and South, there was a great difference in lithology and lithofacies in the space and time.. During the late Paleozoic, except for the norther most area, Shanxi province was deposited by the narine and terrestrial coal-bearing sediments.Based on the detailed observation and description of 5 measured sections and more than 20 assistant sections, by the analysis of sedimentary fasiec, the division and correlation of sequence stratigraphy have been made, and the fallowing results have been concluded.(1)According to the lithology, sedimentary structure and fossi assemblages, the late Carboniferous and the early Permin stratigraphy can be divided into three depositional systems including carbonate platform, clastic barrier lagoon and shallow delta, and from which 9 sedimentary facies and a number of sedimentary sub-facies have been recognized. These facies are open platform, restricted platform, barrier island, lagoon, tidal flat, peat swamp, former delta, delta front and delta plain. Among which, the clastic barrier lagoon system was developed the whole area, but the shallow delta depositional system is mainly distributed in the northern part of Shanxi, and in Shanxi Formation and the bottom of Taiyuan Formation in the middle and south-east Shanxi Province.(2)By the analysis of facies sequence configuration and the division of sedimentary cycles, combining to the recognization of regional unconformity and transform interfaces, detailed division of sequence stratigraphy for those more than 20 sections has been made, and a second order sequence and 8 third order sequences have been divided, and among which, the lower three sequences are attributed to Benxi Formation, the upper 5 sequences are belong to Taiyuan Formation. Each of these sequences is made up of transgressive and highstand systems tracts, and none of them has a lowstand system tract.(3)By the trace and correlation of the 6 cross sections, the distributional framework and the regional changes of the third order sequences have been revealed. The sequences number of Benxi Formation are complete in the middle part of Shanxi Province, and where the sedimentary thickness is big, the sea water is relatively deeper, but to the southeast and the north Shanxi Province, the sequence number is gradually incomplete, and the sedimentary thickness is become thinner accordingly. The sequence number of Taiyuan Formation is generally complete in the whole area, but the characteristic of the paleogeograhy of Taiyuan Formation is similar to that of Benxi Formation, it was relatively a low-lying area and with bigger sedimentary thickness, and also became thinner to the north and the south part of Shanxi Province.(4) By the analysis of coal accumulating pattern, it is found that the coal beds are developed in both of the transgressive tracts and the highstand tracts, but the important coal beds are mainly concentrated in the highstand tracts, that is because of the quick transgression and slow regression.