Sediment Production and Evolution of Proterozoic Carbonate Platforms

John P. Grotzinger

The development of early Proterozoic platforms was broadly similar to the growth of Phanerozoic platforms, although in detail, early Proterozoic platforms differ significantly from Phanerozoic and even middle to late Proterozoic platforms. Specific unresolved problems include the sources and mechanisms of early Proterozoic carbonate production, environmental versus biologic control on stromatolite morphology/microfabric, and temporal restriction of unique facies.

Major sites of early Proterozoic carbonate production include stromatolite reefs, tidal flats, and early cementation of most facies. Carbonate production was prolific, marine cements were locally precipitated directly on the sea floor, and widespread sheets of tufa (precipitated crusts including microdigitate stromatolites) formed on tidal flats. Cyanobacteria probably induced carbonate precipitation during CO₂ degassing related to photosynthesis.

The decline of microdigitate stromatolites and other nonstromatolitic forms of tufa at the end of the early Proterozoic is interpreted to reflect a long-term evolution of Precambrian seawater and perhaps atmospheric pCO₂. Other important features of early Proterozoic platforms include the absence of bedded (massive) gypsum before 1.2 Ga, the occurrence of abundant halite casts in many sequences with rare or absent gypsum/anhydrite casts, the absence of important Mississippi Valley-type lead-zinc deposits, the abundance of tidal-flat chert, and ubiquitous fine-grained dolomite. Collectively these observations suggest that early Proterozoic seawater alkalinity was increased, possibly due to much higher values of total inorganic carbon in seawater such that HCO₃ ^- exceeded 2Ca⁺⁺; this may have resulted from higher atmospheric pCO₂ relative to younger Proterozoic and Phanerozoic pCO₂, although other possibilities exist. During evaporation of seawater on tidal flats, tufas would be precipitated using most if not all Ca⁺⁺ so that continued evaporation would result in precipitation of halite and bypassing of the gypsum field. Dolomitizing solutions could be produced by precipitation of calcium carbonate alone. This model is consistent with theoretical arguments for elevated Precambrian atmospheric pCO₂.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.