ABSTRACT

Molar tooth is a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age. This sedimentary structure appears to be limited in time and space. Our field studies in northern Canada and a survey of literature on Precambrian carbonates worldwide indicates that such features are localized to mid/inner ramp and shallow platform paleoenvironments of Mesoproterozoic and Neoproterozoic age (ca. 1500-650 Ma) with only sporadic occurrences in older and younger strata. As such they are equivalent in terms of texture, location, and abundance to the ubiquitous shallow subtidal, fossiliferous limestones of the Phanerozoic. The amount of synsedimentary calcite spar in molar tooth, in excess of 50% of individual beds and 5-25% of entire formations, is further roughly equivalent to the amount of calcite in calcareous benthic invertebrates in these younger rocks. Thus, there is an evolving trend in such facies through geologic time; Archean-Paleoproterozoic = massive seafloor precipitates; Mesoproterozoic-Neoproterozoic = molar-tooth mudstones and grainstones; Phanerozoic = burrowed and fossiliferous limestones.