PLANKTOGENIC EUSTATIC CONTROL ON CRATONIC OCEANIC CARBONATE ACCUMULATION

Data on mass-age distributions and latitudinal limits of Mesozoic-Cenozoic shelf carbonate, on Cenozoic deepening of the CCD in all major ocean basins, and on the abundance of pelagic chalk in epicontinental carbonate sequences, indicate that the primary locus of global limestone deposition has gradually shifted from shallow cratonic to deep oceanic settings since Jurassic time. This change is generally coincident with progressive emergence of continents during global regression as well as with taxonomic diversification of calcareous plankton, and implicates one or both processes as the primary control on carbonate sediment partitioning between shallow and deep marine reservoirs. Widespread pelagic carbonate does not occur with Phanerozoic ophiolite sequences until the late Mesozoic and generally corresponds to origins of planktonic foraminifera and diversification of coccolithophorids. Similarly, cratonic chalk is largely restricted to Cretaceous sections. Correlation of Mesozoic-Cenozoic positions of the CCD and global sea level, lack of carbonate in pre-Mesozoic lowstand ophiolites, and the restricted presence of epicontinental chalk in highstand sequences imply that patterns of carbonate deposition primarily reflect subequal but nonconcurrent influences of biological and eustatic processes in determining sites of carbonate accumulation. Available data suggest deepening of the global CCD throughout much of the Phanerozoic in response to panktic evolution, with shorter-term change correlative with variation in continental freeboard. The descending CCD intersected relatively narrow ridge crests about 300 m.y. ago, and significant deep-sea pelagic carbonate accumulation began concurrent with increased plate spreading about 150 m.y. thereafter. Continued biologic-eustatic control on cratonic-to-abyssal carbonate transfer suggests near cessation of platform carbonate accumulation in about the next 100 m.y. and eventual destruction of the cratonic carbonate reservoir via subduction.