Master of Science (MS)
Geology and Geophysics
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.
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Shircliff, Corey E., "Evaluation of Stromatolites from the 3.4 Ga Kromberg Formation, Barberton Greenstone Belt, South Africa" (2014). LSU Master's Theses. 1507.