Identifier

etd-04142014-155552

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

Silicified sedimentary rocks from the 3.4 Ga Kromberg Formation of the Barberton Greenstone Belt in South Africa contain laminated structures that have been identified as possible stromatolites in the field. Morphological evaluation and a variety of chemical analyses are presented here, in an effort to describe the samples in a sedimentary context and consider biogenicity of these laminated forms. Two major types of laminated structures were identified in the field – domical laminates and flat-laminated samples with little to no synoptic relief. The domical sample presents the best morphological evidence for biogenicity. There are several characteristics that suggest the deposition must be biologically mediated: dome slopes are greater than 40º and their crests have thickened laminae, varied fine-grained sand bimodal depositional patterns appear within the domes, with a high degree of laminae inheritance from the base of the sample to the top. The flat-laminated samples, while lacking domical morphology, do show high levels of lamina cohesion, mineralogic deposits in individual lamina, and, in most cases, a high degree of laminae inheritance. Raman spectroscopy indicates that the laminae in the domical and flat-laminated samples are carbonaceous, with strong disordered and ordered carbon peaks appropriate for indigenous carbon in these greenschist facies. Although the carbonaceous matter is less than 1% of the rock, samples from the lower K1 Member of the Kromberg Formation were analyzed for δ13C, and the values range from -29‰ to -39‰, which is consistent with the isotopic signatures of autotrophic microbes. Rare earth element (REE) analyses indicate that the depositional environment was marine and anoxic. With all the evidence taken together, the author suggests it is more plausible for the domical sample to be biogenic. Additionally, it is likely that the flat-laminated samples are also biogenic, even though there is no strong resemblance to modern stromatolites. However, they do resemble modern microbial mats, further supporting a biogenic interpretation.

Date

2014

Document Availability at the Time of Submission

Secure the entire work for patent and/or proprietary purposes for a period of one year. Student has submitted appropriate documentation which states: During this period the copyright owner also agrees not to exercise her/his ownership rights, including public use in works, without prior authorization from LSU. At the end of the one year period, either we or LSU may request an automatic extension for one additional year. At the end of the one year secure period (or its extension, if such is requested), the work will be released for access worldwide.

Committee Chair

Byerly, Gary

DOI

10.31390/gradschool_theses.1507

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