MOLAR-TOOTH STRUCTURE IN PROTEROZOIC CARBONATE ROCKS - ORIGIN FROM SYNSEDIMENTARY EARTHQUAKES, AND IMPLICATIONS FOR THE NATURE AND EVOLUTION OF BASINS AND MARINE SEDIMENT

Proterozoic argillaceous lime mudstones commonly contain molar-tooth s tructure, a curious synsedimentary feature consisting of crumpled, gen erally vertically oriented veins filled with calcite microspar. Long e nigmatic, these veins are interpreted here as earthquake-induced defor mation structures. A model is proposed whereby violent ground motion c aused shrinkage, dewatering, and fissuring of the colloidal sea-floor sediment of clay and lime mud, A slurry of equant particles of lime mu d, segregated from the clay platelets trapped in the matrix, was expel led into these voids. The host bed underwent as much as 80% compaction as well as shear, causing the lime mud-filled fissures to be ptygmati cally folded and shingled both vertically and horizontally, and in pla ces pulled apart. This lime mud lithified rapidly by high-Mg calcite g rain growth, such that subsequent earthquakes shattered the folded vei ns. In the Belt basin of western North America, tsunamis from differen t 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 structur e is a signature of basin tectonism recorded specifically in argillace ous Lime mudstones deposited in low-energy settings above the thermocl ine in supersaturated marine water. Syneresis cracks are the counterpa rt in terrigenous facies, in that silt and sand were injected into the shrinkage fissures. This process points to the importance of seismica lly induced compaction under negligible burial. Smectite might have ma de up a significant proportion of the sediment making it more conduciv e to shrinkage. The absence of molar-tooth structure in Phanerozoic st rata is ascribed mainly to changes in sediment rheology brought about by increased organic binding due to diversification of the microbiota.