Search

The Cenomanian-Turonian succession of southern Mexico is characterised by an abrupt change from shallow marine to pelagic facies. The drowning of the platform coincides with the widely documented Cenomanian-Turonian Oceanic Anoxic Event (CTOAE). A proper understanding of the drowning event and the effects of the OAE requires, as an essential first step, the construction of a detailed stratigraphic framework. This has been achieved and utilises sedimentological data as well as a combination of benthic and planktonic biostratigraphic schemes. The onset of the drowning and the OAE are marked by a number of palacoecological changes. These are the result of a series of conspicuous changes in diversity and abundance of the fossil assemblage, as well as the disappearance or appearance of one or more taxa. Some of them are probably the result of drastic irreversible changes in local environmental conditions (platform wide), and reflect the successive stages of the drowning of the platform. Others have equivalents in other parts of the world, and are probably linked to global palaeoceanographic changes. The identified sequence of bioevents occurs within 20-30m of the section, starting with: 1) the disappearance of Cenomanian benthic foraminifers and dasycladacean algae; 2) an interval with abundant calcisphaerulids; 3) the first appearance of hippuritid nidists; 4) an abundance peak of gymnocodiacean and udoteacean algae; 5) an abundance peak of calcisphaerulids, echinoids and roveacrinids; 6) diversification and increase in abundance of whiteincllids; 7) the first appearance of Helvetoglobotnmcana helvética accompanied by a diversification of keeled planktonic foraminifers. Shell beds and abnipt changes from shallow (supratidal-intertidal) to deep (subtidal and/or pelagic) facies were useful to recognise stratigraphic levels for local correlation. Graphic correlation shows that most of the Cenomanian-Turonian sections include hiatuses or condensed intervals that hamper a fine correlation. Nevertheless, a composite succession of events allows a relative dating where the biostratigraphic control is poor. The deposition of laminated, non-bioturbated sediments devoid of benthic fauna in the upper Whiteinella archaeocretacea and lower HeJvetoglobotruncana helvética Zones is probably associated with the establishment of low oxygen conditions. Preliminary carbon isotope data suggest a link between the drowning of the platform and the CTOAE. The drowning of the uppermost Cenomanian platform is likely to have been caused by subsidence. Other contributing factors were nutrient excess, sea level, terrigenous-clastic material, low oxygen levels and temperature stratification.