Award Date

12-1-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Stephen M. Rowland

Second Committee Member

Ganqing Jiang

Third Committee Member

Douglas Sims

Fourth Committee Member

Minghua Ren

Fifth Committee Member

Dennis Bazylinski

Number of Pages

214

Abstract

The Ediacaran to Cambrian Deep Spring Formation consists of mixed carbonate-siliciclastic strata which contain an increasingly complex and biogeographically important biota. Past investigations of the Deep Spring Formation at Mt. Dunfee, Nevada, explored the highly diverse microbialite reefs consisting of a wide range of stromatolite morphologies which exerted significant control on local sedimentation and topography. Early investigations also documented the biomineralizing metazoan Cloudina (an Ediacaran index fossil). However, recent exploration of the area has resulted in the discovery of several new metazoan fossil communities consisting of a diverse assemblage of Ediacaran soft-tissue tubicolous vermiforms (tube fossils) similar to Cloudina. The focus of this dissertation is on the geomicrobiological processes at work in the Deep Spring Formation and elsewhere which result in the preservation of soft-tissues (Ch. 2), the dissolution/precipitation of feldspars within stromatolites (Ch. 3), and the early formation of modern desert varnish (Ch. 4). Chapter 1 of my dissertation examines the stratigraphic and paleontologic context of the Deep Spring Formation. In this chapter, I summarize the current understanding of the paleontological transition at the Ediacaran-Cambrian boundary in the western United States. The Death Valley and White-Inyo-Esmeralda regions of California and Nevada contain some of the most fossiliferous terminal Neoproterozoic to early Phanerozoic strata in the world. Within these strata, the Cambrian explosion is in full display, resplendent with diverse trace and body fossils. The Wood Canyon and Deep Spring Formations, within which the boundary is contained, are comprised of mixed carbonate and siliciclastic lithologies—allowing for a more in-depth understanding of this critical transition in the history of life. Here, I briefly review our current state of understanding of life contained within these rocks. This chapter has been published (Strange and Rowland, 2017) Chapter 2 consists of a manuscript I submitted to the journal Geobiology describing the microbial-related mineralization pathways responsible for preserving the upper fossil horizon in the Deep Spring Formation at Mt. Dunfee, Nevada. The manuscript was recently rejected, with an offer to resubmit after reviewers’ comments have been addressed. In this manuscript, I describe a new mineralization pathway to exceptional fossil preservation which involves the metabolic activities of an iron-oxidizing bacterial community. This model expands on previous microbial/redox zonation models of exceptional preservation and requires rapid emplacement of organic matter into the microaerophilic sedimentary environment. Within these redox conditions, iron-oxidation by bacteria produce abundant Fe (III) from porewater sourced Fe (II), resulting in the precipitation of early Fe (III) mineral phases with the quick stabilization in the form of goethite and iron-rich aluminosilicates. I have named this taphonomic pathway “ferrumation”. Ferrumation incorporates well into the microbial/redox zonation model of Schiffbauer et al. (2014) due to the occurrence of pyritization within the fossil assemblage. Chapter 3 moves away from iron-oxidizing bacteria to the mineral producing capabilities of the consortia of microbes involved in stromatolite formation. Feldspar assemblages found within stromatolite laminae differ between stromatolite localities and from the surrounding interstromatolite zone between columnar stromatolites. This suggests that microbial activities may be influencing their dissolution and precipitation. Textural associations between feldspars, quartz, and calcite are similar to the micrographic textures seen in some igneous rocks which result from the co-stability of feldspar and quartz in a magma chamber. These highly unusual feldspar textures within Deep Spring stromatolites led to the hypothesis that microbial activity resulted in the dissolution of detrital feldspars (mostly orthoclase) captured by the sticky surface of the stromatolite. The capture of dissolved ions from detrital orthoclase by organic molecules become released into the closed system of a mineralizing microbialite and precipitate out as authigenic albite, quartz, Ca-plagioclase, and a K-rich aluminosilicate, or their poorly crystalline precursors. This pattern of feldspar-quartz-calcite associations has been found at the Ediacaran-Cambrian Mount Dunfee section and the Molly Gibson Mine section of the Deep Spring Formation, in Miocene stromatolites from the Duero Basin, Spain, and also in a stromatolite from the Flinders Range, Australia. This research is significant because it could lead to the ability to isolate and date authigenic albite grains using the Ar/Ar method, potentially opening the utilization of stromatolites as abundant repositories of syndepositional authigenic minerals for geochronology. Chapter four departs from the Ediacaran-Cambrian boundary but continues with the theme of microbe-mineral interactions. This chapter details the earliest phases of desert varnish formation on ephemeral wash sediments from Nelson, Nevada. I show that the early development of incipient desert varnish occurs through small “microdots”. These are likely associated with Fe- and Mn-oxidizing microbial communities on the surface of the sediment grains. The Fe-oxide composition of some microdots and the mixed-composition of more advanced desert varnish suggests that desert varnish may accumulate through the cyclic deposition of layers during periods of habitability of each microbial community. Desert varnish microdots are shown to have progressive mineralization from smooth surfaces to micronodules, while advanced development becomes botryoidal. The previously reported “microstromatolitic” growth habit of some desert varnish seems to correspond with the microdot morphology of the incipient varnish found on these grains.

Keywords

Albite; Cloudinomorph; Desert Varnish; Hyolithid; Stromatolite; Taphonomy

Disciplines

Biology | Geochemistry | Geology | Paleontology

File Format

pdf

File Size

16000 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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