STRATIGRAPHY OF THE MIDDLE CAMBRIAN SUSAN DUSTER LIMESTONE MEMBER, PIOCHE SHALE, EASTERN NEVADA

The Lower and Middle Cambrian Pioche Shale is a clastic-dominated, inner shelf facies with several prominent limestone units. One of these is the Susan Duster Limestone Member, which is the focus of this NSF RUI-funded undergraduate study. A half-dozen sections were investigated in the type region of the Pioche Shale of eastern Nevada.

The Susan Duster Limestone Member is 4.5-6.8 meters thick, and consists of a basal two meter thick ledge, overlain by a two to three meter thick shale and nodular limestone interval. This is capped by a silty, nodular carbonate of variable thickness, up to two meters. The lower limestone ledge is composed of a basal half meter of high-angle, cross-bedded coquinites occurring with limestone pebble beds, overlain by thin, repetitive, fining-upward bioclastic beds.

Two early Middle Cambrian trilobite faunas occur within the Susan Duster Limestone Member. The low diversity, ptychopariid-dominated fauna of the Amecephalus arrojosensis Biozone occurs in the basal meter, in a facies dominated by trilobite, articulate brachiopod, and hyolithid coquinites, and white sparry calcite. The overlying higher diversity, corynexochid-dominated Plagiura-Poliella Biozone faunas occur in the gray, thin-bedded to nodular carbonates.

The facies and faunas of the Susan Duster Limestone Member are consistent with a deepening upward pattern. This relatively thin limestone sequence has a wide regional extent across the paleoshelf and appears to have been deposited during a relatively rapid marine transgression over a low-relief erosional surface.