METAZOAN REEFS FROM THE UPPER CAMBRIAN OF THE ARROW CANYON RANGE, CLARK COUNTY, NEVADA

A sponge-microbialite reef horizon occurs in the upper Cambrian (Millardan) strata of the Arrow Canyon Range, Clark County, Nevada. This horizon represents one of the few metazoan reefs occurring between the extinction of the Archaeocyathids at the end of the early Cambrian and the proliferation of Calathium-sponge-microbialite reefs in the late early Ordovician.

Good exposures of the reefs occur discontinuously on the western slopes of the Arrow Canyon Range. Uneven dolomitization obscures some reef structures while creating patches of well-preserved reef rock that are up to 15 m thick by 50 m wide. Some dolomitic intervals between reef mounds reveal faint reef-like textures and structures, therefore the shape and extent of the original reef(s) is uncertain. The interval may have extended into the overlying dolostone and the original reef may have formed distinct mounds larger than 50 m wide, or a single continuous biostrome over the 8 km exposure.

The reef(s) show an ecological succession from basal-domal stromatolites (0.5 m wide, 1 m tall) to digitate stromatolites (5-20 cm wide, 10-20 cm high), a core of sponge-thromolite reef (3-7 m thick), and a cap layer of columnar stromatolites (20 cm w., 1-2 m h.) with attached and detrital sponges between the columns. Sponges in the core have thin walls (1-2 mm), and are broadly conical to bowl-like in shape, (5-10 cm w/h). They form a closely packed framework that is in-situ with pendent microbialites; Renalcis and Epiphyton.

Age determination is currently supported by 4 conodont collections from strata underlying the reef through the reef core. Strata below the reef have Proconodontus muelleri and are assigned to that zone. Eoconodontus notchpeakensis occurs in the stromatolitic base and indicates the E. notchpeakensis Subzone of the Eoconodontus Zone. Palaeobotryllus taylori, also from the Eoconodontus Zone, occurs in the core.

After the demise of Archaeocyathid reefs, metazoan reefs were rare in the middle and late Cambrian. They increased in abundance during the early Ordovician, and in faunal diversity during the late early and middle Ordovician. The Arrow Canyon Range reef(s) resemble Archaeocyathid reefs in structure and microbiota, and the sponges are similar to those of late early Ordovician reefs, providing a link between these two important times of reef evolution.