Molar-tooth structure in Proterozoic carbonate rocks: Origin from synsedimentary earthquakes, and implications for the nature and evolution of basins and marine sediment

摘要

Proterozoic argillaceous lime mudstones commonly contain molar-tooth structure, a curious synsedimentary feature consisting of crumpled, generally vertically oriented veins filled with calcite microspar. Long enigmatic, these veins are interpreted here as earthquake-induced deformation structures. A model is proposed whereby violent ground motion caused shrinkage, dewatering, and fissuring of the colloidal sea-floor sediment of clay and lime mud. A slurry of equant particles of lime mud, segregated from the clay platelets trapped in the matrix, was expelled into these voids. The host bed underwent as much as 80% compaction as well as shear, causing the lime mud-filled fissures to be ptygmatically folded and shingled both vertically and horizontally, and in places pulled apart. This lime mud lithified rapidly by high-Mg calcite grain growth, such that subsequent earthquakes shattered the folded veins. In the Belt basin of western North America, tsunamis from different events created currents that transported ooids and coarse sand from shoals into deeper water, and scoured deeply into the now-consolidated sediment, leaving behind a lag of vein fragments. Molar-tooth structure is a signature of basin tectonism recorded specifically in argillaceous lime mudstones deposited in low-energy settings above the thermocline in supersaturated marine water. Syneresis cracks are the counterpart in terrigenous facies, in that silt and sand were injected into the shrinkage fissures. This process points to the importance of seismically induced compaction under negligible burial. Smectite might have made up a significant proportion of the sediment, making it more conducive to shrinkage. The absence of molar-tooth structure in Phanerozoic strata is ascribed mainly to changes in sediment rheology brought about by increased organic binding due to diversification of the microbiota.