BIOGEOGRAPHIC EVOLUTION THROUGH THE PERMIAN-TRISSIC BOUNDARY IN SOUTH CHINA

After comparison of conodont zones and related ash beds between fifteen regional sections an isochronous stratigraphic framework is established in South Chins. In this framework the sequence boundaries (SB) which comprise unconformities and corresponding conformities is recognized by arrangements of conodont zones and related ash beds. The unconformities erase the C. meishanensis conodont zone in platform interior setting and the C. taylorea conodont zone in slope setting, thus the C. yini zone that underlies the SB can think as a highstand system tract (HST) and the C. meishanensis zone as a regressive system tract (RST). The conformities occur between C. meishanensis zone and C. taylorea zone and coincide with ash bed 3. Above the SB the C. taylorea zone rests on conformities and the C. microcuspidata, H. parvus, I. staeschei and I. isarcica zones are onlap on unconformities in turn. C. taylorea zone is recognized as the lowstand system tract (LST) and C. microcuspidata, H. parvus, I. staeschei and I. isarcica zones make up the transgressive system tract (TST) that can be further subdivided into four intervals by four regionally isochronous ash beds (A4, A5, A6 and A7). Generally seven isochronous stratigraphic units can be distinguished around the PTB, including the HST, the RST, the LST as well as the interval 1, the interval 2, the interval 3 and the interval 4 of TST in ascending order.

Biogeographic environments and ecosystems of every stratigraphic unit are reconstructed by performing study of spatio-temporal distributions of microfacies, fossils and bioclasts. The evolution of biogeography and ecosystem shows a gap in the platform interior and slope settings. This gap divides seven stratigraphic units into two evolutionary cycles. The lower evolutionary cycle comprises the HST, the RST and the LST and appears as offlap stratigraphic framework that is response to eustantic sea level falling. Permian faunas in the LST are in danger due to the maximal sea level drop. The upper evolutionary cycle comprises four intervals of the TST and is response to eustatic sea level rising. The evolution of ecosystem in four intervals shows, in turn, a resurrection of Permian faunas in interval1, a major extinction after resurrection, a transition in interval 2+3, a last extinction after transition and a disaster interval in interval 4.