Molar tooth structure (MTS) is well-preserved in the Neoarchean (2.6 Ga) Monteville Formation, Transvaal Supergroup, South Africa. Monteville MTS is unusual because it pre-dates other examples by 900 My and has a siliciclastic host-rock. The Monteville Formation is a mixed carbonate-siliclastic ramp; MTS occurs in environments shallower than storm wave base and deeper than areas swept by tidal currents. The host-rock for MTS is a muscovite/illite shale with intercalated carbonate grainstone lenses. MTS microsparite forms veins in the shale, and similar microsparite cements grains in the lenses. Rarely, vein-fills are stratified. Microspar nucleated on, and partially coated, crack walls; distinct layers of microspar with minor shale then filled the cracks from their base. Rarely, mm- to cm-scale clasts appear within veins. These clasts consist of shale with a microspar coating and were derived from the crack wall.
Monteville MTS has implications for the origin of MTS. The complete absence of lime mud in the depositional environment demonstrates that MTS veins are not recrystallized injections of fluidized lime mud, but rather the microsparite precipitated from a fluid in an open vein. This fluid permeated available pore space, including pores between sand grains.
Models for MTS that are consistent with these observations include crack generation due to physical dewatering during compaction/seismicity, chemical dewatering during synaeresis, and gas expansion during organic decay. However, precipitation of the microspar requires substantial fluid transport of calcium to the cracks. Using the [Ca2+] of modern seawater (10-2 m), and a spar density of 2.7g/cm3, an absolute minimum of 2700 mL of fluid are required to precipitate 1cm3 of MTS. With typical MTS veins ~25cm3, ~60 L of fluid are required. Thus, while any of the above-mentioned processes may have produced cracks, it seems unlikely that they could force enough fluid through the cracks to rapidly and fully cement them. Changing pore pressures due to water waves passing overhead may have generated this fluid movement. This observation may explain the correlation of MTS with environments above wave base.
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Dawn Y. Sumner
Department of Geology
University of California
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