MOLAR-TOOTH STRUCTURE: CARBONATE MUD DIKELETS GENERATED BY SYNSEDIMENTARY EARTHQUAKES

Molar-tooth structure is a synsedimentary feature in calcareous mudstones of mainly—but not exclusively—Precambrian age. It consists of striking arrays of closely spaced, upright veins, locally with horizontal sheets and spheroids. In plan view veins range from linear to reticulate. They are vertically to obliquely oriented, discontinuous, typically crumpled to squashed and even smeared, and often brecciated. The matrix is usually deformed; vein fragments are shingled or dispersed. These characteristics are evidence for variably directed compressional, tensile and shear stresses imposed sometimes repeatedly during the discrete rheological evolution of both components, beginning with liquefaction and plastic behavior and ending with brittle failure. Locally molar-tooth structure consists of micrite. However, usually it is composed of inclusion-free microspar crystals up to ~15 µm across. In some cases these crystals are seen to consist of calcite cement overgrowths on well-sorted rhombs and multi-faceted euhedra. These cores may be the original carbonate mud particles or a product of synsedimentary recrystallization. Depending on the composition of the host mudstone, silt and fine sand may be incorporated. Some recent explanations insist on precipitation of carbonate mud in voids created by gas generated by oxidation of organic matter. However, this mechanism cannot account for vein patterns, the concomitant sand and silt injection, and the complex and dynamic nature of the stresses indicated. The famous Pratt hypothesis, formulated in the early 1990s, calls upon cyclic loading during earthquake-induced shaking which caused elevated fluid pressure and loss of shear strength. During abrupt dewatering, veins were jacked open by the injection of granular lime mud segregated from the matrix, and deformed by continued shaking. Vein fills quickly cemented and thus were prone to brittle failure during subsequent events, with the matrix becoming progressively stiffer each time. Molar-tooth structure opens a critical window onto the nature of Precambrian muddy sediments. Its near-absence in the Phanerozoic testifies to dramatic changes wrought by the evolution of microbial and multicellular life in the seas.