Masters Theses

Date of Award

5-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Linda C. Kah

Committee Members

Steve G. Driese, Julie K. Bartley

Abstract

Molar-tooth structure (MT) is an unusual Precambrian carbonate fabric characterized by variously shaped voids filled with uniform, equant microspar. A worldwide distribution of MT in Meso- and early Neoproterozoic rocks may be related to secular changes in substrate rheology and ocean chemistry through the Proterozoic. Unfortunately, the use of MT as a tool for understanding environmental evolution is limited because its genesis is still poorly understood. Recently, however, Furniss et al. (1998) showed experimentally that gas generated by decaying organic matter within unconsolidated mud can reproduce MT crack morphologies. A gas expansion hypothesis allows MT morphologies to be modeled as an interaction between the expansive force of the gas and the cohesion of the surrounding substrate. Three scenarios arise: 1) if the expansive force is significantly less than substrate cohesion, a blob morphology should result; 2) if the expansive force exceeds substrate cohesion, a ribbon morphology should result; and 3) if expansive force is significantly greater than the substrate cohesion, gas should diffuse randomly through the sediment pore space leaving no distinct void.

Evidence for each scenario is observed in MT from the 1.4 Ga Helena Formation, Belt Surpergroup, Montana. Within similar, fine-grained carbonate substrates, an array of MT morphologies are behaviorally consistent with increasing gas production. However, the greatest variation in MT morphology occurs when cracks interact with sediments of differing grain size and, presumably, initial cohesion. MT cracks are observed to track or diffuse within coarse-grained layers, consistent with their greater permeability. In rare cases, coarse-grained material in observed to have collapsed into underlying cracks prior to precipitation of void-filling MT cement. Other MT structures become horizontal underneath and between microbial lamina, suggesting greater cohesiveness of these layers. These petrographic relationships are significant in that they support MT genesis by gas expansion, are inconsistent with other models of MT genesis, and suggest a strong rheologic control on MT morphologies.

Cathodoluminescence reveals MT microspar to have dully-luminescent cores with more brightly luminescent polygonal overgrowths. Experimentally produced vaterite, precipitated in the presence of dissolved organic molecules, are similar in size and morphology to the cores of MT microspar, suggesting that precipitation of MT microspar may have been aided by the presence of dissolved organics, perhaps linked with gas production by decay of organic material within the host substrate. In this scenario, migration of dissolved organic molecules from the surrounding substrate came in contact with supersaturated Mesoproterozoic seawater within MT voids, resulting in the precipitation of MT microspar. The presence of MT microspar as a first-generation cement within oolitic grainstones suggests that migration of dissolved organics from the underlying substrate could induce precipitation, even at the sediment-water interface.

Restriction of MT structures to the Meso- and early Neoproterozoic was likely related to a secular decrease of carbonate saturation through the Proterozoic. Whereas higher saturation and more rapid lithification of older substrates inhibited void formation, lower saturation in the Neoproterozoic likely prevented the rapid cementation necessary to preserve gas-produced voids. A critical level of carbonate saturation during the Mesoproterozoic and early Neoproterozoic, however, limited immediate substrate lithification, allowing void production, but remained high enough to allow environmental catalysts to initiate precipitation of MT microspar within voids and pore space.

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