Department of Geology
Eastern Washington University

Recent Masters theses in Geology

MIDDLE CAMBRIAN BIOSTRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE PIOCHE
SHALE NEAR DELAMAR, NEVADA

Julie D. Eddy

Abstract

Middle Cambrian rocks of the Pioche Shale from a section in the Delamar
Range of Nevada were studied to document the biostratigraphy and interpret
depositional environment.  Lithofacies include 1) a fissile shale and
ribbon/nodular lime mudstone lithofacies; 2) a packstone/grainstone
lithofacies; and 3) a coarsening-upward clastic lithofacies.  Overall, the
stratigraphic sequence reflects lateral migrations of shallow, inner-shelf
depositional settings.  Repetitious sedimentary patterns are apparent from
stratigraphic relationships.  Coarsening-upward intervals of shale,
siltstone, and sandstone reflect marine regressions, while overlying
carbonate units reflect a return to deeper water settings during a
transgression.  Transgressive and regressive sequences were likely produced
by regional sea level changes brought about by tectonic subsidence,
eustatic sea level changes, or changes in sediment source.

Trilobites of the "Plagiura-Poliella" and Albertella assemblage zones are
recognized.  Faunas within these zones are further subdivided into informal
faunules, which include a Poliella faunule, a Kochaspid faunule, an
Albertella-Mexicella faunule, and a Zacanthoidid faunule.

The trilobite fauna of the Middle Cambrian part of the Pioche Shale in the
Delamar Mountains includes 36 species belonging to at least 19 genera,
which are described and illustrated.  One new taxon is recognized,
Zeugospinella delamarensis n. gen. n. sp.