Cenomanian-Turonian Oceanic Anoxic Event: Local Perturbations on a Global Theme

Timothy J. Bralower

A high-resolution nannofossil zonation has been derived for the Cenomanian-Turonian (middle Cretaceous) boundary interval based on the investigation of numerous sections from Europe, North America, and Africa. This biostratigraphy allows accurate correlation of sections from shelf and pelagic environments, and the increased resolution helps solve paleo-oceanographic and sedimentologic problems in this interval.

Boundary facies patterns are variable for numerous reasons. A simultaneous, widespread hiatus occurred in the latest Cenomanian and earliest Turonian in northwest Europe, an indirect result of peak marine transgression. In deep Tethys, produced carbonate was dissolved in undersaturated bottom waters or during early diagenesis. Few boundary sections are characterized by continuous carbonate deposition. An excursion in ^dgr¹³C, measured in many sections, has been associated with the burial of large amounts of marine organic carbon and is a potentially useful stratigraphic marker. This excursion commonly coincides with a facies change, lower ratios of coccolith carbonate to micrite, and poorer nannofossil preservation, and therefore is strongly diagenetically overprinted. Nume ous nannofossil events indicate that the carbon excursion is slightly diachronous between sequences of the Western Interior and that the shift occurred distinctly later in this basin than in northwest Germany.

The data obtained do not invalidate the concept of a Cenomanian-Turonian oceanic anoxic event, but show how local oceanographic perturbations have masked the global scenario. In particular, they indicate that carbon shifts may be basinal and not always oceanic phenomena, a conclusion that can be rationalized with overall sluggish middle Cretaceous circulation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.