Abstract: Paleobiogeography and Preservation of Middle Cretaceous Calcareous Nannofossil Assemblages in Atlantic Ocean--Paleo-oceanographic Implications

Peter H. Roth, Jay L. Bowdler

The middle Cretaceous was a time of widespread anaerobism and low carbonate sedimentation in the Atlantic Ocean. A study of coccolith assemblages contributes to our understanding of paleo-oceanographic conditions.

Anoxic sediments of middle Cretaceous age in the Atlantic and Indian Oceans indicate low oxygen concentrations in deep and/or bottom waters. This probably led to denitrification and a resultant drop in fertility of surface waters. A large amount of organic matter was preserved in the sediments because it escaped oxidation at the sediment-water interface. Bacterial breakdown of part of the organic matter resulted in elevated carbon dioxide concentrations in interstitial waters and concomitant calcite dissolution. Therefore, coccolith preservation is generally poor in sediments with high organic carbon content. Major factors affecting the distribution of coccoliths in middle Cretaceous sediments are: (1) dissolution, increasing with depth and organic content of the sediment; (2) neritic effect of being nearshore; (3) latitude; and (4) upwelling.

A Q-mode cluster analysis of coccolith counts in Deep Sea Drilling Project samples from two time slices (Zone NC 7, late Aptian, and Zone NC 10, late Albian) helped to define distinctive assemblages. The clusters for both time slices can be grouped roughly into oceanic and neritic assemblages. Each of these larger groups can be broken down further into well, moderately, and poorly preserved oceanic or neritic assemblages, depending on the degree of dissolution that has occurred. Neritic assemblages are either characteristic of western continental margins, shallow plateaus, restricted basins, or upwelling regions from the eastern North Atlantic. Middle- and low-latitude coccolith assemblages are similar. High-latitude assemblages are quite distinctive. Paleogeographic distribution of c ccolith assemblages thus helps to delineate regions of unusual paleo-oceanographic conditions.

AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah