Morphology of Molar-Tooth Structures in Precambrian Carbonates: Influence of Substrate Rheology and Implications for Genesis

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 resu...

A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars

“…In most cases on Earth, the cracks are filled by other sediments, however, in one very particular instance known as "molar tooth structure" spindle--shaped cracks and related subspherical voids are filled with mineral (calcite) cement. The origin of subaqueous cracks on Earth is poorly understood, though the involvement of gas in generating the cracks and voids represented by molar tooth structure is likely (43,47). As with the hollow nodules, a number of processes could generate gases in Martian sediments.…”

Sedimentation patterns during the Precambrian: A unique record?

“…The majority opinion, however, does not question its primary carbonate composition, considers it not as a replacement product but as crack-filling cement; these researchers, nevertheless differ widely with regard to the mechanism for crack generation. Tectonic fracturing (Daly, 1912;Cowan and James, 1992;Smith and Winston, 1997;Bishop and Sumner, 2006), earthquake induced dewatering (Fairchild et al, 1997;Pratt, 1998b), desiccation or synaeresis (Bell, 1966;Horodyski, 1976Horodyski, , 1983Young and Long, 1977;Hofmann, 1985;Beukes, 1987;Knoll and Swett, 1990;Calver and Baillie, 1990;Demicco and Hardie, 1994;Liu et al, 2005) and fluid pressure (Dix and Mullins, 1987;Desrochers and Al-Aasm, 1993;Mozley and Burns, 1993;Furniss et al, 1994Furniss et al, , 1997James et al, 1998;Kuznetsov, 2003;Pope et al, 2003;Marshall and Anglin, 2004;Pollock et al, 2006) have all been suggested in this respect.…”

“…Meng and Ge (2002) contemplated a relation between MTS and prevalent bioforms transitional to Phanerozoic multicellular forms, and suggested that related micro-bioelectromagnetic waves encouraged CaCO 3 precipitation within cracks induced by CO 2 overpressuring. Pollock et al (2006) proposed intimate links between crack formation and concomitant microspar precipitation and the decomposition of sedimentary organic matter in the presence of supersaturated seawater; they produced a variety of crack morphologies within unconsolidated mud under gas pressure in the laboratory. Proliferation of sulphate reducing bacteria and methanogens in the Proterozoic time period could have promoted carbonate precipitation within cracks (e.g., Vasconcelas and McKenzie, 1997;Wright, 1999;van Lith et al, 2003;Roberts et al, 2004;Wright and Oren, 2005;Wacey, 2004, 2005).…”

Reinterpreting a Proterozoic Enigma:Conophyton‐JacutophytonStromatolites of the Mesoproterozoic Atar Group, Mauritania

“…Similarly, stratigraphically higher siliciclastic intervals represent lowstand deposition following craton-wide fl ooding and development of the Atar Formation Conophyton-Jacutophyton biostromes ). Disappearance of fl at-clast breccias and fl at-laminated microbial carbonate interbeds in stratigraphically higher siliciclastic intervals and their replacement by small (<2 m thick) discontinuous bioherms of irregularly branching columnar stromatolites (Tungussia confusa; BertrandSarfati & Moussine-Pouchkine, 1999), as well as rare intervals of 'molar-tooth' carbonate (Furniss et al, 1998;James et al, 1998;Pollock et al, 2006) suggest shallow-marine deposition and overall greater accommodation space across the platform.…”