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Analysis of pre-extinction archaeocyathan reefs of the lower Cambrian Harkless formation, southwest Nevada
Sara B. Pruss
Bachelor of Arts
Archaeocyathan, Cambrian, Harkless formation, Reefs, Sponge reefs
Archaeocyaths were the first calcifying metazoans to construct reefs, spanning the Lower Cambrian before going extinct. The focus of this study is an archaeocyathan reef environment preserved in the Harkless Formation in Southwest Nevada. This locality is significant in that it records the last pre-extinction archaeocyath reefs in the western United States. This research specifically aims to understand this pre extinction interval through the lens of paleoecology and diagenetic history. This was done by collecting 31 rock samples from three patch reefs and their surrounding grainstone and packstone facies. From these samples, thin sections were prepared, imaged, and point counted. Additionally, 9 samples from reef and grainstone facies were placed in acid to extract insoluble residue that could then be sieved and picked for small shelly fossils. Figures were then made to better visualize the data. Across the reef environment, four species of Archaeocyaths were identified. Archaeocyathans and the calcimicrobe Renalcis were found to be confined to the patch reefs, never appearing in the surrounding facies. Renalcis was shown to be the dominant carbonate producer making up the patch reefs. In the grainstone reef flanking and channel environments, a diversity of metazoan life was found, including larger fossils of trilobites, echinoders, brachiopods, and Salterella, as well as small shelly fossil hexactinellid and chancelloriid sponge spicules, trilobite spines, echinoderm plates, and mollusks. Additionally, one occurrence of a coralomorph was discovered in the packstone facies below the reefs. Fossil remains were found to display a range of preservation styles. Larger fossil fragments were overwhelmingly preserved by micrite and sparite cement, with the exception of echinoderm plates, which appeared in thin section to be replaced or coated by glauconite cement. Small shelly fossils exhibited a range of small shelly style preservation, namely glauconite and apatite mineral replacement, as well as iron oxide mineral replacement. These different preservation ii styles reflect the environments these fossils were preserved in. Glauconite and apatite indicate oscillating redox conditions during early diagenesis, supporting in situ mineralization of the fossils, sand grains, and cement displaying this style of preservation. Fossil and grain size also played a role in this, since only particles of a certain size can be preserved in this manner. This mineralization would have occurred after transport and deposition, since packstone and grainstone facies indicate moderate to high energy environments. Small shelly fossils were shown to display differences in preservation style that reflected differences in size, morphology, and sample collection site. This study found similar small shelly fossil assemblages to other Archaeocyath reef environments from this time interval, and serves to provide the first comprehensive record of small shelly fossil assemblages from the Harkless locality. These findings support the assertion that this pre-extinction archaeocyathan reef environment hosted a diversity of complex marine calcifying metazoans, preserved through an array of dynamic taphonomic and diagenetic processes comparable to those seen in correlative reef environments found globally.
©2019 Olivia Renton Leadbetter. 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.
Leadbetter, Olivia Renton, "Analysis of pre-extinction archaeocyathan reefs of the lower Cambrian Harkless formation, southwest Nevada" (2019). Honors Project, Smith College, Northampton, MA.
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v, 39 pages : color illustrations. Includes bibliographical references (pages 37-39)