Biostratigraphy and environmental changes across the Cenomanian-Turonian boundary, southern Mexico

Integrated benthic and planktonic foraminiferal biostratigraphy has been used to correlate sections that include the transition from upper Cenomanian shallow-marine facies to lower-middle Turonian pelagic facies in southern Mexico, The Cenomanian shallow-marine, semi-restricted limestones lack planktonic foraminifers, while the Turonian pelagic limestones are almost devoid of benthic fauna, The Pseudorhapydionina dubia, Whiteinella archaeocretacea, and Helvetoglobotruncana helvetica zones have been characterised in this interval. The disappearance of the zonal marker and of most benthic foraminifers defines the top of the P, dubia zone, The W, archaeocretacea zone comprises the transition from shallow semi-restricted conditions to open-marine, deeper environments, A characteristic of this zone is the scarcity of both benthic and planktonic foraminifers, including the zonal marker, The top of this zone is marked by the first appearance of H. helvetica. Some changes in the fossil assemblages reflect the successive stages of the drowning of the platform. Others seem to be linked to global factors and have potential for wider correlations. The Cenomanian platform had its maximum development in the P, dubia zone. Adverse conditions for carbonate producers (and for platform growth) started in the latest Cenomanian, when extensive areas of the platform underwent subaerial exposure, Habitat reduction might have been an important factor for the extinction of most large benthic foraminifers at the end of the P, dubia zone, The change to deeper open marine conditions with the subsequent relative sea-level rise could have delivered the final blow to those groups that survived the habitat reduction. These organisms did not survive for long, into the W. archaeocretacea zone, Some areas deepened to pelagic conditions after a short period of grainstone deposition in which molluscs were the dominant bioclasts. In the areas where shallow-marine deposition continued, a sharp increase in algal remains indicates higher nutrient levels and mesotrophic conditions. Fully eutrophic conditions are represented in overlying facies dominated by echinoids, calcisphaerulids, and roveacrinids. This degradation of the environment further abated carbonate production and contributed to the drowning of the whole platform. (C) 2001 Elsevier Science Ltd, All rights reserved.