PETROGRAPHIC EXAMINATION OF MOLAR-TOOTH PODS FROM THE BELT SUPERGROUP, MONTANA

Molar-tooth (MT) is an enigmatic carbonate fabric composed of a network of inconsistently oriented, sharply bounded cracks and spheroidal voids that are filled with uniform calcite microspar, mostly lack detrital infill, and commonly show compaction of matrix around cemented structures.

Initial classification of MT structures documented three distinct morphologies: ribbons, blobs, and pods (O’Connor, 1972). Ribbons represent sheet-like structures that range from vertical to horizontal, are commonly ptygmatically folded, and typically show compaction of matrix around folded and fractured structures. Blobs represent features that are spheroidal in three dimensions, and occasionally show protrusion of incipient sheet-like fractures. The range of morphology of ribbon and blob-like textures, the character of the microspar infill, and the interaction of MT structures with the surrounding substrate has commonly been attributed to the exsolution of gasses within the shallow subsurface, which trigger both formation of initial voids and the rapid precipitation of microspar within these structures (Furniss et al., 1998; Pollock et al., 2006). The global abundance of MT structures in Mesoproterozoic and early Neoproterozoic age carbonate rocks, with infrequent occurrences in older and younger Proterozoic strata, has further been used to infer a link between the formation of MT and the biogeochemical evolution of Proterozoic oceans.

Here we address a little-studied morphology of MT: molar-tooth pods. O’Connor (1972) described pods in the Belt Supergroup, Montana, as irregular ellipsoidal to cylindrical features that occur in the deepest-water portions of Belt cycles and occassionally coalesce into horizontal lenses. Although the relationship between pods and more typical MT structures is uncertain, O’Connor (1972) suggested that pods represent concretionary features formed around MT structures. Here we examine a range of MT cracks, blebs, and pods via standard light microscopy and cathodoluminescence to place constraints on the composition of carbonate within MT pods, and to better constrain their potential origin and relationship to more traditional MT structures.