A PALEOECOLOGICAL STUDY OF EARLY CAMBRIAN STROMATOLITIC, THROMBOLITIC, AND ARCHAEOCYATHAN REEFS ON THE YANGTZE PLATFORM, SOUTHERN SHAANXI PROVINCE, CHINA: CYCLES, TRENDS, AND ASSOCIATIONS

An extensive section of Early Cambrian strata containing several cycles of microbial-boundstone, packages of siltstone/mudstone, and packages of oolite is located near the village of Fucheng in Shaanxi Province, China. The Xiannudong Formation illustrates the interactions between adjacent facies and it consists of at least, five shallowing upward cycles. The age of the Xiannudong Formation is Qiongzhusian Stage (Chiungchussuan), Eoredlichian-Wutingaspis biozone, which correlates to the lower-mid Botomian Stage of Siberia and upper-Nevadella biozone of Laurentia.

The Xiannudong is 67.3 meters thick in the Fucheng Section and is divided into 13 units (A-M). Five of these units contain stromatolitic and thrombolitic reefs and mats, some of which contain archaeocyaths as a minor constituent. The peri-reefal lithology is typically archaeocyath-trilobite-ooid floatstone and rudstone, and rarely grainstone and packstone. This indicates that archaeocyathan reefs were present nearby on the Yangtze platform. The upper most unit (M) contains the only in-situ archaeocyathan reefs in this section, which occur among microbial reefs. Overlying most of the microbial reefs are packages of silt/mudstones of varying thicknesses that contain abundant burrows and wave ripple marks. Two packages of oolite directly overlie microbial boundstone, with the uppermost oolite (M) containing bi-directional cross-beds trending NNE/SSW.

Preliminary data suggest that ‘irregular’ archaeocyaths are more common in the microbial reefs, while ‘regular’ archaeocyaths are major constituents of surrounding floatstone and rudstone. An overall trend towards increasing ‘irregular’ archaeocyaths upsection is noted in the floatstone and rudstone, and by the presence of abundant irregular archaeocyaths in the in-situ reefs of unit M.

In general, the lithostratigraphy of the Xiannudong Formation indicates a close association between microbial reefs, adjacent migrating ooid shoals, and neighboring archaeocyathan reefs; a relationship that resembles the Bahamas. Each of the five shallowing upward sequences causes a facies shift towards lower energy. At the beginning of each new cycle, the microbes re-established themselves within the new high-energy environment.