Molar-Tooth Carbonates: Shallow Subtidal Facies of the Mid- to Late Proterozoic: Discussion

Abstract

From field work in northern Canada and many other examples documented in the literature, James et al. (1998) highlighted the potential use of molar-tooth structures (i.e., a series of spar-filled cracks) as a stratigraphic indicator for the Mesoproterozoic and Neoproterozoic. These authors also stated that all known occurrences where molar-tooth structures are volumetrically significant are from shallow-water carbonates. Although these are comprehensive statements, considering that most molar-tooth structures reported so far are from shallow-water Mesoproterozoic and Neoproterozoic deposits, I am concerned about such proposed temporal and environmental constrictions, which are not supported by the field data, summarized in the following. Detailed field work in the Grajau Basin, northern Brazil, revealed that structures similar to the ones reported by James et al. (1998) are volumetrically widespread in the Codo Formation. This is a lacustrine lithostratigraphic succession formed during the early stages of the South Atlantic rifting in the Aptian (e.g., Petri and Fulfaro 1983). The sedimentary features described here occur in lime mudstone to grainstone, ostracodal grainstone, and crystalline limestone interbedded with shales strongly affected by penecontemporaneous deformation, as indicated by a variety of complex convolute folds, small faults, fissures, and bed collapses. The molar-tooth structures occur throughout these beds, although they substantially increase in volume downward, where evidence for soft-sediment deformation diminishes. These …