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Distribution, Composition, and Paleoecology of Middle Triassic Carbonate Reefs of the Nanpanjiang Basin, Guizhou, Guangxi, and Yunnan, South China

Christensen, Shannon and Daniel Lehrmann
Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI

The Nanpanjiang Basin occurs in the southern margin of the Yangtze microplate. Middle Triassic reefs rim the basin along the Yangtze Platform (YP) in a continuous belt up to 600m thick and 1.5 km wide that extends 530km from Guizhou to Yunnan. Reefs also occur in isolated platforms (IP) as substantial platform-rimming structures and “patch” reefs with up to 750m thickness and 400m of depositional relief.

Anisian reefs of IP consist of delicate Tubiphytes framestone-cementstone generally containing little framework (~ 10%) and large volumes of microbial crusts and marine cement (up to 90%). Anisian reefs of YP have greater diversity with the addition of minor scleractinian corals, sphinctozoan sponges, bryozoans, and serpulids. Ladinian reefs of the YP and northernmost IP have greater biodiversity and more complex paleoecologic structure with 2-3 frame builders on a local scale, and contain lesser volumes of marine cement. Frame participants include solenoporacean algae, sphinctozoans, corals, Tubiphytes and bryozoans. When Tubiphytes is not the primary frame it takes on a binding role. Additional encrusters include microbial crusts, serpulids, Bacinella, Ladinella, foraminifera and bryozoa. Dwelling organisms are similar to those found in the Anisian reefs.

Tubiphytes reefs of the Nanpanjiang basin began in the Earliest Anisian or in the latest Scythian—substantially earlier than European examples. Middle Triassic reefs of south China developed vast structures and substantial relief in contrast to European counterparts. Framework in these reefs is seldom robust enough to be considered “wave-resistant” thus cements and microbial crusts largely contributed to their rigidity.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004