Guerrero-Morelos Platform drowning at the Cenomanian–Turonian boundary, Huitziltepec area, Guerrero State, southern Mexico

Facies successions in three stratigraphic sections (Barranca del Tigre, Axaxacoalco, and Zotoltitlán, Guerrero State, southern Mexico) that comprise middle Cenomanian to lower Turonian rocks of the central part of the Guerrero-Morelos Platform, indicate the drowning of some parts of the platform near the Cenomanian-Turonian boundary. In the western part of the study area (Barranca del Tigre section), Cenomanian shallow-marine limestones (mainly subtidal facies) with abundant benthic fauna (mainly foraminifera) pass upwards to a 7-m-interval of Turonian open-marine nodular limestones with few benthic organisms and then to dark-grey and black laminated pelagic limestones and marls. In the west-central part (Axaxacoalco section), Cenomanian restricted shallow-marine limestones (intertidal to subtidal facies) change abruptly upwards to Turonian black and dark-grey laminated pelagic limestones and marls with only pelagic fauna (calcisphaerulids, planktonic foraminifera, and radiolaria). In the eastern part of the study area (Zotoltitlán section), Cenomanian restricted shallow-marine limestones (mainly intertidal facies) are overlain by 45 m of open-marine limestones showing up-section a rapid decrease in benthic diversity until only calcisphaerulids and echinoderm fragments occur. Overlying these rocks, there are 65 m of nodular shaly limestones in thick strata with renewed benthic fauna, 80 m of shaly and silty limestones intercalated with claystones and some siltstones, and finally, dark grey/black laminated pelagic limestones and marls. The facies successions in the stratigraphic sections suggest the progressive drowning of the Guerrero-Morelos Platform around the Cenomanian-Turonian boundary. We attribute the drowning of some parts of the platform to the occurrence of the Cenomanian-Turonian anoxic event. The impingement of anoxic waters over the platform could produce the drastic reduction of the carbonate producing benthos observed in the stratigraphic sections and therefore a reduction in carbonate accumulation rates. Subsidence and the late Cenomanian-earliest Turonian sea-level rise were then able to drown the platform. This occurred first in the western part, where subtidal conditions dominated and an irreversible drowning occurred, allowing the deposition of organic-rich pelagic sediments over pre-existing shallow-marine carbonates. At the same time, the eastern part, where intertidal conditions dominated, changed to open-marine conditions, shallow first and deep later. Here, a temporal restoration of shallow open-marine carbonate sedimentation resulted, but eventually this region was also drowned, probably by the interplay of terrigenous-clastic supply to the platform and a new impingement of anoxic waters.Copyright 1997 Academic Press Limited