MOLAR-TOOTH CRACK FORMATION AND THE PROTEROZOIC MARINE SUBSTRATE: INSIGHTS FROM THE BELT SUPERGROUP, MONTANA AND THE ATAR GROUP, MAURITANIA

Molar-tooth (MT) structures comprise an enigmatic carbonate fabric found primarily within Proterozoic subtidal to intertidal deposits. The fabric consists of variously shaped cracks and voids that formed in unlithified shallow substrates, and were filled rapidly with a characteristic equant microspar. The temporal restriction of this fabric in the geologic record, combined with its relationship to a dynamic marine carbonate system, makes MT carbonate a potentially valuable indicator of Proterozoic environmental conditions.

In the Belt Supergroup, Montana, thin MT cracks (<2 mm wide) are typically more sinuous than thicker cracks, leading workers to hypothesize a link between cohesive strength of the substrate and MT crack morphology. For models that include crack formation by either gas escape or wave-induced tension, more cohesive, clay-rich substrates are expected to produce broad, straight cracks because higher gas pressures would be required to initiate substrate failure. By contrast, less cohesive and more variable substrates should result in smaller, more irregular cracks.

New observations from the Atar Group, Mauritania, provide a mixed assessment of substrate cohesion models. Here, MT cracks form predominantly within intervals of high clay content. As predicted, cracks within clay-rich substrates appear broader than those found in more carbonate-rich strata of the Belt Supergroup, and cracks often deflect or disappear when they intersect coarser-grained strata. Three observations, however, suggest that MT within the Atar Group may allow for refinement of MT genesis models: (1) the width of Atar Group cracks does not vary substantially between substrate lithologies; (2) there is no clear relationship between crack width and sinuosity; and (3) homogenous green shales often contain alternating sets of thick, horizontal MT and thin, vertical MT. These observations reaffirm that the complexity of MT cracks represents a dynamic interplay of substrate rheology and tensional forces within the substrate, and indicate that a better understanding of MT behavior in shaly substrates may be critical in testing current models of MT genesis.