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The Cenomanian–Turonian succession of southern Mexico is characterized 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 utilizes sedimentological data as well as a combination of benthic and planktonic biostratigraphic schemes.Deposition of the Cenomanian–Turonian sedimentary rocks of the Guerrero–Morelos basin was controlled by tectonic and oceanographic factors resulting in depositional environments ranging from a semi-restricted shelf, ramp, pelagic and prodelta deposits. Facies analysis indicates that shallow marine limestones of the Morelos Formation (lower-upper Cenomanian) were deposited in intertidal–shallow supratidal and subtidal environments in a semi-restricted shelf. Peloidal-bioclastic packstone–wackestones with minor grainstones are the predominant texture of these rocks. Abundant large benthic foraminifers, calcareous algae (dasycladacean) and mollusks (gastropods and rudists) characterize the fossil assemblage.The Cuautla Formation (uppermost Cenomanian–Turonian) represents sedimentation on a low-energy, wave-dominated, carbonate ramp. The inner ramp accumulated bioclastic banks and shoals composed of peloidal-benthic foraminifer-grainstone, calcareous red and green algae, rudists and minor solitary corals. The middle ramp is represented by nodular packstones with a diverse assemblage of echinoderms, green and red algae, bryozoan, rudists, solitary corals, roveacrinids, calcisphaerulids, and non-keeled planktonic foraminifers. The outer ramp is dominated by argillaceous wackestone–packstone characterized by calcisphaerulids, roveacrinids, and non-keeled planktonic foraminifers. An increase in terrigenous-clastic material towards the eastern part of the area indicates progradation of a deltaic system while the Mexcala Formation (uppermost Cenomanian–Turonian) was deposited in a pelagic setting.The drowning of the platform is at the contact between the Morelos and Cuautla or Mexcala formations and is dated as latest Cenomanian. The drowning is a hiatus in most sections and it began before the end of the Cenomanian by a minimum of 150 ky if the top of the Morelos is not eroded. [Copyright &y& Elsevier]