STRATIGRAPHIC SETTING OF AN UPPER CAMBRIAN METAZOAN REEF BETWEEN THE NOPAH FORMATION TO GOODWIN FORMATION TRANSITION IN SOUTHERN NEVADA

The Upper Cambrian in southern Nevada marks the occurrence of an early anthaspidellid sponge-microbialite reef that presages the rise of the Paleozoic reef building metazoans during the Ordovician. Anthaspidellids and microbialites of similar age have been identified in Texas and Colorado and may be related. In southern Nevada this reef occurs as part of an unusual 35-40 m interval above the dark gray dolostone of the Nopah Formation and below the light gray Goodwin Limestone and presumably separated from each by unconformity.

Hummocky cross-bedded deposits rich in reddish brown dolomitized ooids define the lower 2/3rd of this interval, while the upper 3rd is blue-gray reef rock. The brown color of this interval forms an easily recognized band informally named the brown oolite. The brown oolite interval is exposed in the Arrow Canyon Range, 50 km to the north in the Delamar Range and 30 km to the west in the Sheep Range. Conodonts collected from the Arrow Canyon Range show that the interval ranges in age from P. muelleri zone to E. notchpeakensis sub-zone. To the west and south, across the right-lateral Las Vegas Shear and presumably up depositional dip, the brown oolite and reefs are not present, and the conodont barren upper Nopah is unconformably overlain by the C. proavus zone rocks of the basal Goodwin. While the micobialite reef is present wherever the brown oolite is found, it only contains sponges along the 10 km outcrop in the Arrow Canyon Range.

The HERB event, a carbon isotopic excursion found within the P. muelleri to E. notchpeakensis interval, is associated with microbialites, especially in the western U.S. The microbialite deposits that are consistently associated with the HERB event suggest that the Arrow Canyon Range sponge reefs were localized patches found within a more extensive microbialite reef complex; the sponges may have been sensitive to local environmental conditions. Other patches of sponge reef might be associated with the HERB event elsewhere. Conodont biostratigraphy and isotope work in progress will test this age assertion and help to identify unconformities. A refined understanding of the stratigraphy of this unit may lead to a better understanding of the environmental conditions that lead to the development of sponge reefs.