Molar-Tooth Structure Derangement Syndrome-A Cure

Pratt, Brian

Molar-tooth structure is a common feature in carbonate rocks mainly, but not exclusively, of Proterozoic age. It consists mostly of upright to tilted, vertically and often horizontally crumpled, veins composed typically of calcite microspar. It lacked a satisfactory explanation for over 130 years since it was fi rst observed. Detailed petrography and the sedimentological context of molar-tooth structure then showed that it is a seismically induced soft-sediment deformation feature, whereby equant, fi ne silt-sized calcite mud was segregated from the muddy matrix during a dewatering event and injected into vein-shaped features. In some cases, silt was also injected, and in intervals where the veins cross silt and fi ne sand layers, some of the siliciclastic grains were incorporated. Cementation by grain growth was rapid, and subsequent ground motion led to brittle failure of the veins. Strong currents occasionally exhumed them, producing intraclastic rudstone. The veins were never gasor water-fi lled voids that subsequently became occluded by calcite cement. The puzzle of molar-tooth structure was solved 25 years ago, and in part it is the carbonate analog of "syneresis" cracks that are also injection features, that is, small-scale dikes, or dikelets, emanating from sand and silt interbeds in mudstone.