Publication Date

2019

Document Type

Honors Project

Degree Name

Bachelor of Arts

Department

Geosciences

Advisors

Sarah B. Pruss

Keywords

Lowoer Cambrian, Archaoecyath, Carbon-Isotope, Fossils-Cambrian, Benthic, Nevada, Western US, Paleontology, Paleoecology-Cambrian, Stratigraphy-Cambrian, Harkless Formation

Abstract

The first calcifying sponges of the Cambrian, the archaeocyaths, produced reefs that altered evolutionary diversity and abundance both in reefs and in surrounding benthic communities. The archaeocyathan sponges became extinct toward the end of the early Cambrian interval; their rise and fall is preserved in sections of the Poleta and Harkless Formations from the western United States. The extinction, which has been documented globally in coeval strata, has been placed in the uppermost Harkless, after a brief period of abundant reef-building in the Poleta and upper Harkless. Although the extinction of the archaeocyaths is well documented, the effect of this extinction on benthic communities living on and around the patch reefs has been less well-studied; these organisms may have been affected by the same environmental/ecological factors that caused the reef disappearance. Point counts of 178 thin sections from non-reef environments of the Poleta, Harkless and Mule Springs formations were completed and analyzed to compare skeletal production in benthic communities before, during and after the extinction. Before the extinction, in the upper Poleta and basal upper Harkless, samples contain an average of 12.4% skeletal material (range 0-25%), while after the disappearance of archaeocyaths, samples drop to an average of 1.8% skeletal material (range 0-13%). Pre-extinction facies are mainly grainstone containing a diverse assemblage of echinoderms, trilobites, brachiopods and Salterella. Post-extinction facies are dominated by oolite and oncolite, and skeletal diversity is much lower, composed of only rare echinoderms and trilobites. Overall, the abundance and diversity of skeletal organisms in benthic environments appears to decrease across the interval of the early Cambrian archaeocyath extinction in western United States sections.

Rights

©2019 Rhiannon Zoe Nolan. Access limited to the Smith College community and other researchers while on campus. Smith College community members also may access from off-campus using a Smith College log-in. Other off-campus researchers may request a copy through Interlibrary Loan for personal use.

Language

English

Comments

35 pages : color illustrations. Includes bibliographical references (pages 30-35)

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