Abstract

Molar-tooth (MT) is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform, equant microspar. MT is both abundant and widespread in Mesoproterozoic and early Neoproterozoic strata, where void-filling microspar comprises up to 90% of individual beds and 5–25% of preserved carbonate. The temporal restriction of this fabric suggests a potential link between MT formation and the biogeochemical evolution of marine environments. Detailed petrographic relationships among MT crack morphology, distribution of MT microspar, and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated Proterozoic seawater.

Laboratory experiments have shown that gas generated within unconsolidated mud can reproduce a variety of MT crack morphologies, yet current gas expansion and migration models do not explicitly consider the role of substrate variability in determining morphologies of MT cracks. A detailed petrographic examination of MT structures from the Mesoproterozoic Belt Supergroup, Montana, permits interpretation of the microscale relationship between crack morphology and lithologic, and potentially rheologic, variability of the surrounding substrate by tracing the distribution of petrographically distinctive MT microspar. Observations of lateral offset of MT cracks at bedding planes or within coarser-grained siltstone or sandstone layers, termination of cracks beneath clay- or organic-rich horizons, grain collapse into underlying MT cracks, and the presence of MT microspar as a pore-filling precipitate suggest that grain size, substrate lithology, and substrate cohesion all play critical roles in the development of MT cracks. By contrast, the presence of a wide range of MT crack morphologies within petrographically homogeneous substrates, and an apparent relationship between crack diameter and sinuosity, suggest that the void-forming process itself also played a role in determining the final morphology of MT cracks. Together, these petrographic observations are used to define a model of microscale gas–sediment interactions that can be used to interpret crack morphology in terms of gas pressure and the strength of sedimentary substrates.

The presence of characteristic, void-filling microspar is integral to preservation of MT structures. Cathodoluminescence (CL) identification of this characteristic microspar within MT voids, in pore space of coarse-grained facies, and interstitially within fine-grained facies adjacent to MT voids suggests that MT voids and cement share a common genesis. Because microspar cores are similar in size and morphology to vaterite precipitated experimentally in the presence of a variety of dissolved organic molecules, we suggest that precipitation of MT microspar was intimately linked with gas production during organic decomposition within the host substrate. In this scenario, gas production would result in pore fluids with elevated concentrations of dissolved organic molecules, which would initiate precipitation of MT microspar when the pore fluids come in contact with supersaturated Proterozoic seawater. Restriction of MT largely to Mesoproterozoic and early Neoproterozoic strata likely reflects a critical level of carbonate saturation that limited early substrate lithification, thereby allowing void production but remained high enough that organic catalysts were able to initiate precipitation of MT microspar.