Middle Cenomanian Unconformity in East-Central Mexico
Brian A. Smith
The Valles platform in east-central Mexico consists of a thick sequence (2,000 m) of shallow-water carbonates deposited during the Albian and early Cenomanian. Rudistid bivalves are the principal organisms that formed the reefs that grew on the platform margin. The center of the platform was a shallow lagoon with rudists and miliolids as the dominant fossil types.
During the middle Cenomanian, a global drop in sea level exposed the top of the Valles platform subaerially. Although the lowstand lasted only 4 m.y., the associated hiatus across the platform had, in places, a duration of as much as 20 m.y. This variation indicates that uplift and deformation of the platform were also responsible for the unconformity.
The Upper Cretaceous rocks deposited on the platform are mostly deep-water limestones and shales. However, shallow-water limestones were being deposited at the same time. Such a pattern of sedimentation cannot be explained by a fluctuating sea level. Folding and faulting of the platform rocks caused more topographic relief than the original depositional surface of the top of the platform. As the platform began subsiding or sea level started to rise during the Turonian, the lower parts of the platform were drowned and both deep- and shallow-water carbonates were deposited. Sea level continued to rise throughout the Late Cretaceous and covered more of the deformed platform. It was not until near the end of the Cretaceous that the entire platform was covered by the sea.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.