WOODS, ADAM D., MARIA MUTTI, and DAVID J. BOTTJER
University of Southern California, Los Angeles, CA

Abstract: Extensive Growth of Inorganic Carbonate Cements During the Triassic is Linked to Local Anoxia and Shifting Global Oceanic Conditions

The synsedimentary precipitation of crystalline calcium carbonate masses directly onto the seafloor, and the growth of large volumes of cements in marginal settings, has generally been thought to occur only in the Proterozoic, when seawater was highly oversaturated with respect to calcium carbonate. However, the Late Permian to the Late Triassic was a time of atypically extensive carbonate precipitation as the result of a combination of local environmental conditions coupled with shifting oceanic conditions which were unique to the Phanerozoic.

An analysis of Lower to Upper Triassic carbonate facies reveals that the volume of carbonate precipitation was variable, and decreases through the Triassic. Additionally, large volumes of seafloor cements occur primarily in settings associated with anoxic waters. An analysis of the Lower Triassic Union Wash Formation of east-central California, for example, reveals that paleoxygenation and seafloor precipitate abundance are closely related. Our data suggests that calcium carbonate supersaturation was caused by the upwelling of anoxic, alkaline deep waters, which mixed with surface waters, resulting in CO₂ degassing and carbonate growth. Another example is the extensive occurrence of calcium carbonate cements on the margins of the Ladinian carbonate banks in the Alps which are associated with coeval anoxic basins. The conditions which apparently led to extensive carbonate precipitation during the Triassic apparently disappeared by the beginning of the Jurassic, as anoxic basins are not associated with concomitant voluminous carbonate precipitation.

These observations indicate that both anoxia and unique Triassic ocean chemistry were necessary for extensive carbonate growth. However, it remains unknown how the extent of anoxia (global versus local) affected cement textures and growth dynamics. It is therefore necessary to gain a better understanding of the relationship between anoxia and carbonate precipitation and its temporal variability, in order to evaluate hypotheses concerning Triassic ocean chemistry.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas