PATTERNS IN BIOCLASTIC ACCUMULATION THROUGH THE PHANEROZOIC - CHANGESIN INPUT OR IN DESTRUCTION

Evolutionary changes in the ecology and diversity of organisms that pr oduce and destroy calcareous skeletons suggest that bioclastic concent rations themselves might have changed in nature through the Phanerozoi c. Empirical data from marine siliciclastic records of Ordovician-Silu rian, Jurassic, and Neogene ages indicate a significant increase in th e thickness of densely packed bioclastic concentrations over geologic time, from a primarily thin-bedded brachiopod-dominated record in the Ordovician-Silurian to a mollusk-dominated record with many more and t hicker shell beds in the Neogene. Jurassic shell beds vary in thicknes s with the Paleozoic or modern affinities of the chief constituents, s uggesting, along with other evidence, that the Phanerozoic increase wa s determined neither by diagenesis nor by a shift in taphonomic condit ions on the sea floor but rather by the evolution of bioclast producer s, namely, groups with (1) more durable low-organic skeletons, (2) gre ater ecological success in high-energy habitats, and (3) on the basis of indirect evidence, higher rates of carbonate production. These resu lts suggest that (1) reproductive and metabolic output has increased i n benthic communities over time and (2) the scale of time averaging in benthic assemblages has increased owing to greater hard-part durabili ty of modern groups.