RELATIVE SEA-LEVEL RISES AND ORGANIC REEF DEPOSITION:AN EXAMPLE FROM THE LATE PERMIAN SPONGE REEFS IN SOUTHWESTERN CHINA

Based on the palaeogeographic location, the Late Permian organic reefs in southwestern China may be divided into five categories. The reef-building organisms include framework-building, binding and reef-attached organisms according to their different functions. The framework-building organisms consist of cylindrical or massive calcareous sponges and Tabulozoa characteristic of native burial; the binding organisms comprise blue-green algae, Tubiphytes and Cystoporata, and the abundant benthonic reef-attached organisms are composed mainly of brachiopods, foraminifera, echinoderms, Tubiphytes, Fenestella and nautiloids. The relative sea-level rises controlled by synsedimentary faulting during the Late Permian are responsible for the formation of the sponge reefs from southwestern China. From early to late, the relative sea-level rises underwent four stages of evolution. The first is a stable sea-level stage when bioclastic wackestone and packstone are formed. The second is a rising sea-level stage when framestone and bafflestone (20-300 m thick) are developed. The reef bodies are vertically accretionary, with a growth rate in equilibrium with a rising rate of sea level. The third stage is the alternation of a stable sea-level stage and a pulsatory rise stage when the shoal reef facies are deposited. The last one is a rapidly rising sea-level stage when the formation of the incipiently drowned platform facies and drowned platform facies leads to the disappearance of the reef bodies. The differences in time, time limits, stages and amplitudes of the relative sea-level rises in southern and northern parts of the study area during the Late Permian indicate that the sea-level rises are attributed to synsedimentary faulting within the Yangtze plate rather than to the global marine incursion. The synsedimentary faulting accompanied by tensional faulting result not only in the uplift of submarine topography, thus contributing to the growth of the reef-building organisms, but also in the relative sea-level rises which have consequences for the continuous growth of the reef bodies in the study area.