Mesoproterozoic and Neoproterozoic shallow marine carbonates commonly contain complex networks of sheet-and bleb-like structures made up of microcrystalline calcite. These “molar-tooth” structures have been attributed to a diverse array of physical, biologic and chemical processes. Of the numerous origins proposed, one that attributes molar-tooth to early calcite precipitation into gas-generated voids best explains the full range of these enigmatic structures. This petrographically based model is examined further from the standpoint of depositional and early diagenetic conditions inferred on the basis of isotopic, geochemical and petrographic study of the Helena Formation, a calcareous interval within the Mesoproterozoic Belt Supergroup. In the context of these paleoenvironmental constraints, molar-tooth structures are interpreted to reflect rapid calcite precipitation from highly supersaturated marine pore fluids into CO₂-generated voids during the early diagenetic oxidation of organic matter.

Considered on a larger scale, the spatially widespread, yet temporally restricted occurrence of molar-tooth structures infers environmental factors unique to the Mesoproterozoic and Neoproterozoic that promoted extensive early diagenetic calcite precipitation in shallow marine environments. The widespread appearance of molar-tooth structures in the Mesoproterozoic may reflect a change in ocean chemistry relative to the Archean, when calcite precipitation from a CaCO₃-supersaturated ocean may have been inhibited by dissolved species that limited crystal nuclei formation. Their absence from Phanerozoic successions may reflect the appearance of bioturbating metazoans and a decrease in the calcium carbonate saturation state of shallow marine waters associated with the appearance of CaCO₃-secreting metazoans and, in the latter part of the Phanerozoic the onset of deep sea biogenic carbonate production.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah