CONTROLS ON THE INITIATION AND GROWTH OF A MIDDLE TRIASSIC (ANISIAN) REEF COMPLEX ON THE GREAT BANK OF GUIZHOU, GUIZHOU PROVINCE, CHINA

A platform-to-basin transition on the Great Bank of Guizhou is exposed on the steeply-dipping limb of a faulted syncline. A reef complex approximately 800 m thick and over 1 km wide occurs on the platform margin overlying Lower Triassic oolite shoals and rare molluscan shell banks. Ooids from the Lower Triassic platform margin are unusually large – up to 1 cm in diameter. Reef-derived grains first occur in the uppermost Lower Triassic on the basin margin making the reef complex the oldest known from the Mesozoic Era. The oldest comparable reefs known from western Tethys are Bithynian-Pelsonian (Anisian). The reef framework consists primarily of Tubiphytes - mm-scale cylindrical, branching micritic structures containing axial microspar-filled tubes ca. 100 μm wide. Tubiphytes commonly contains 500 μm calcified spheres containing several smaller (ca. 50 μm) calcified sphere attached to their inner walls. We interpret these structures as algal sporangia. We further interpret the micrite-and-tube structure of Tubiphytes to reflect microbially-induced micritic precipitation around an otherwise soft-bodied alga. The microbial interpretation of the micritic portion of Tubiphytes tubes is further supported by the common occurrence of irregularly distributed ~10 μm filaments within the micrite that may reflect the presence of filamentous bacteria. The Tubiphytes framework was reinforced by penecontemporaneous clotted micritic cement. Early diagenetic fibrous cements occlude nearly all of the remaining void space. A low diversity metazoan and algal community also occurs within the reef complex but these organisms did not contribute significantly to the reef framework or to the accretion of the reef complex. Rather, reef development resulted largely from the stabilization and cementation of platform margin sediments by algae and associated microbial mats indicating a decoupling of the return of cemented reefs on platform margins from the recovery of framework-building metazoans following the end-Permian extinction. The importance of Tubiphytes framework in Anisian reefs may reflect a physiological adjustment of algae and microbial communities to environmental change more rapid than the evolutionary response of metazoans.