File:Table Mesa (Mancos Shale, Upper Cretaceous; San Juan County, New Mexico, USA) 2 (8281640744).jpg

Table Mesa in the Cretaceous of New Mexico, USA. (looking ~NNW)

Northwestern New Mexico's Table Mesa is a moderately large, flat-topped hill bordered by subvertical cliffs and sloping talus piles. Mesas are flat-topped due to the presence of rocks relatively resistant to weathering and erosion. The bedrock at Table Mesa consists of fine-grained to coarse-grained siliciclastics of the Mancos Shale, a moderately widespread, marine unit deposited during the Late Cretaceous. In New Mexico, the Mancos Shale Formation is divided into three parts: 1) lower Mancos Shale - dominated by fine-grained siliciclastics; 2) Gallup Sandstone - a coarse-grained siliciclastic unit; 3) upper Mancos Shale - dominated by fine-grained siliciclastics.

Table Mesa's lower cliffs (partly obscured by talus slopes) are dominated by shales, with minor interbedded siltstones and sandstones. The upper cliffs are dominated by relatively hard sandstones, which weather and erode slowly to form the flat-topped mesa. The coarsening-upward sequence shown here represents a minor regression during a time of global sea level highstand.

Stratigraphy: upper Mancos Shale Formation, Coniacian to Santonian to Campanian Stages, middle Upper Cretaceous