Abstract: The Middle and Late Devonian stratigraphic sequences in Guilin,Guangxi record two major en-vironmental cycles in a vast of epicontinental seas.The first cycle began with a transgressive phase in the Effelian that reached its maximum in the early and middle Givetian,and was followed by a regressive phase during the late Givetian.The transgressive phase of the second cycle reached its peak in the Frasnian,and was followed by a regressive phase during the early Famennian.In gener-al,the greatest water depth and most open marine conditions were attained during the first cycle,and seas were shallowest and most restricted toward the end of the second cycle.
The bioevents took place mainly during the transgressive maximum of the first cycle,while their direct results were displayed fully during the second cycle.Many taxa,which were prosperous during the Givetian and Frasnian,ended their living action at the very onset of the Famennian.The changes in environments are correlated with the rise and fall of the global sea level.The mass extinc-tion took place even more clearly in the tropical zone during the Devonian.