COMPARISON OF LATE-EARLY CAMBRIAN ARCHAEOCYATHAN REEFS FROM NEVADA, U.S.A. AND THE WESTERN HUBEI DISTRICT, CHINA

Coeval archaeocyathan-calcimicrobial reefs occur in the Upper Harkless Formation in Esmeralda County, Nevada and the Tianheban Formation near the town of Yichang in China. These two formations are late Botomian to early Toyonian based on trilobite and archaeocyath fauna data. The Upper Harkless is middle Bonnia-Olenellus Zone using Laurentian terminology while the Tianheban is Megapalaeolenus Zone using Chinese terminology. These two biostratigraphic zones are approximately coeval.

A comparison of these formations reveals several noteworthy similarities and differences. The Upper Harkless reefs are found in several different localities with one to three reefs at each locality. All are patch reefs that range from 0.57 to 1.3 meters thick and 0.72 to 22 meters wide. Only one reef is identified in the Tianheban Formation, near the town of Yichang. The Tianheban reef is 2.1 meters thick and 1.8 meters wide. ‘Irregular’ archaeocyaths dominate both localities; four genera of ‘irregulars’ and one ‘regular’ genus occur in the Nevadan reefs, and only one genus of archaeocyath, an ‘irregular’, is present within the Chinese reef. Both localities contain the genus Archaeocyathus. Surprisingly, the volumetric percentage of archaeocyaths within the Tianheban reefs is far greater (33 %), than in the Upper Harkless reefs (14 %), even though the Tianheban contains only one genus. In conjunction with archaeocyaths, calcimicrobes are important in the reef formation, Renalcis in the Nevadan reefs and Renalcis, Epiphyton, and Girvanella in the Chinese reef. A high diversity of other organisms is present at both localities, such as trilobites, echinoderms, and brachiopods.

The Upper Harkless reefs are underlain and overlain by packstone conglomerate. The underlying contact is conformable while the overlying contact is erosional. These reefs contain a high percentage of primary crypts with fibrous cements and detrital quartz, indicating a high-energy depositional environment. In contrast, the Tianheban reef contains a miniscule amount of quartz, and primary crypts with fibrous cements are not conspicuous. This reef’s overall fabric is pseudofenestral. Underlying and overlying the reef are oncolite beds with burrow-mottled limestone flanking the reef. All of which indicate a low energy environment.