GAS BUBBLE AND EXPANSION CRACK ORIGIN OF MOLAR-TOOTH CALCITE STRUCTURES IN THE MIDDLE PROTEROZOIC BELT SUPERGROUP, WESTERN MONTANA

''Molar-tooth'' structures in tile Belt Supergroup are complexes of in terconnecting, thin sheets and small spheroids, composed of uniform, b locky 5-15 pm calcite crystals that cut fine grained clayey carbonate layers, Bauerman (1885) likened tile network of sheets to the corrugat ed surface of an elephant molar tooth, ''Molar-tooth'' structure descr ibed here comes from the Chamberlain and Helena formations of the Midd le Proterozoic Belt Supergroup, but the structure occurs in other silt y; clayey limestone and dolomite of the Belt and in Middle and Late Pr oterozoic rocks elsewhere in the world, Previous investigators have ap plied the term ''molar-tooth'' to several kinds of structures and attr ibuted them to a variety of physical, biological, and chemical process es, We adhere to Bauerman's (1885) original citation of ''molar-tooth' ' and limit our definition to blobs, vertical ribbons, and horizontal ribbons (O'Connor 1967, 1972; Eby 1977), Observations of molar-tooth s amples suggest to us that ribbons formed as open voids through which g as and water flowed before being filled by fine, equant sparry calcite crystals, We tested the gas hypothesis by experimenting with mixtures of plaster of Paris, clay, water, sugar, and yeast in glass aquaria, Yeast metabolized the sugar, producing CO2 gas that first formed bubbl es in the mud slurry, nearly identical in shape to Belt ''molar-tooth' ' blobs, The bubbles rose to the surface and pulled water behind them, stiffening the mud, We later sealed off the mud surface with plaster, The confined gas within the stiffened mud produced vertical and horiz ontal expansion cracks that were nearly identical in form to vertical and horizontal ''molar-tooth'' ribbon shapes, Finally, we precipitated fine equant crystals of calcium carbonate, identical to ''molar-tooth '' calcite, by mixing solutions of Na2CO3 and CaCl2. The close replica tion of ''molar-tooth'' blobs and ribbons by biogenic gas convincingly explains the origin of the blob and ribbon forms, This explanation sa tisfies all the constraints of the data. We propose that some ribbons in tile Chamberlain and Helena formations formed as gas cracks in wate r-saturated mud within a meter of the depositional surface, and were p artially filled with fine calcite spar before compaction, Other cracks in tile mud were first compacted and later filled by calcite, Still o ther ''molar-tooth'' structures probably formed deeper in the sediment column, Patterns of ''molar-tooth'' structure are commonly repeated w ithin sedimentary cycles up to tens of meters thick, indicating that t hey may have formed during periods of lowstand between depositional cy cles, Biogenic gas generated in Belt sediment probably included H2S, a s evidenced by microcrystalline pyrite in ''molar-tooth'' calcite, CO2 , and methane, Rapid calcite precipitation in the gas voids is probabl y necessary for ''molar-tooth'' preservation and may be a secular feat ure of the Middle and Late Proterozoic.