''Molar-tooth'' structures are microcrystalline calcite-filled feature
s of long-debated origin that are common in Mesoproterozoic and early
Neoproterozoic fine-grained, shallow-water carbonates. Ne have constra
ined the environment of their formation from the standpoint of deposit
ional and diagenetic conditions inferred on the basis of petrographic
and geochemical data derived from examples in the Helena Formation, a
calcareous marine interval within the Mesoproterozoic Belt Supergroup.
The collective data set supports an early-diagenetic origin for molar
-tooth structures and implies environmental factors that promoted rapi
d precipitation of calcite into CO2-generated voids in cohesive muds.
We propose that the temporally restricted distribution of molar-tooth
structures in the geologic record reflects a unique combination of the
environmental parameters that control their formation. Of these facto
rs, the most fundamental include CaCO3 saturation and redox conditions
in shallow marine waters that promote prolific carbonate production,
tectonic quiescence and the availability of extensive epicratonic site
s for carbonate accumulation, and the absence of bioturbators.