Biostratigraphy and environmental changes across the Cenomanian±Turonian boundary, southern Mexico Noemi Aguilera-Franco a, * , Ulises Herna Ândez-Romano b , Peter A. Allison a a T.H. Huxley School of Environment, Earth Science and Engineering, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK b Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 6AB, UK Received 31 August 1998; revised 31 May 1999; accepted 31 January 2001 Abstract 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 Pseu- dorhapydionina dubia, Whiteinella archaeocretacea,and Helvetoglobotruncana helvetica zoneshavebeencharacterisedinthisinterval.The disappearance of the zonal marker and of most benthic foraminifers de®nes 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 scarcityofbothbenthicandplanktonicforaminifers,includingthezonalmarker.Thetopofthiszoneismarkedbythe®rstappearanceof H. helvetica.Somechangesinthefossilassemblagesre¯ectthesuccessivestagesofthedrowningoftheplatform.Othersseemtobelinkedto 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 deeperopenmarineconditionswiththesubsequentrelativesea-levelrisecouldhavedeliveredthe®nalblowtothosegroupsthatsurvivedthe 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. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Shallow-marine; Biostratigraphy; Foraminifers Resumen La integracio Ân de bioestratigrafõ Âa de foraminõ Âferos bento Ânicos y plancto Ânicos permitio  correlacionar secciones del sur de Me Âxico que varõ Âan de facies marinas-someras en el Cenomaniano superior a facies pela Âgicas en el Turoniano inferior y medio. Las calizas de facies somerasysemirestringidasdelCenomanianocarecendeforaminõ Âferosplancto Ânicos,mientrasquelasfaciespela ÂgicasdelTuronianocasino contienen fauna bento Ânica. En estas rocas se caracterizaron las biozonas de Pseudorhapydionina dubia, Whiteinella archaeocretacea y Helvetoglobotruncana helvetica. La desaparicio Ândelmarcadorzonalydelamayorõ Âa de los foraminõ Âferos bento Ânicosde®nelacimadela zona de P. dubia.Lazonade W. archaeocretacea comprende la transicio Ân de condiciones someras semirestringidas a ambientes marinos abiertosyma Âsprofundos.Unacaracterõ Âsticadeestazonaeslaescase Âzdeforaminõ Âferostantobento Ânicoscomoplancto Ânicos,incluyendoel marcadorzonal.Lacimadeestazonasede®neconlaprimeraaparicio Ânde H. helvetica.Algunoscambiosenlaasociacio Ânfo Âsilre¯ejanlas etapassucesivasdelahogamientodelaplataforma.Otrosparecenestarrelacionadosafactoresglobales,locuallesasignaunpotencialpara correlaciones ma Âs amplias. La plataforma del Cenomaniano tuvo su ma Âximo desarrollo en la zona de P. dubia. Las condiciones adversas para los productores de carbonato y para el crecimiento de la plataforma) empezaron en el Cenomaniano ma Âs tardõ Âo, cuando extensas areas de la plataforma Journal of South American Earth Sciences 14 2001) 237±255 0895-9811/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII:S0895-981101)00014-1 www.elsevier.nl/locate/jsames * Corresponding author. Present address: Go ÂmezFarõ ÂasN.16,Col.SantaFe Â, Mexico City DF 01210, Mexico. Tel.: 152-55707339; fax: 152-55707339. E-mail address: naguiler@imp.mx N. Aguilera-Franco).