Abstract

As much as 2,350 m (7,710 ft) of Upper Cretaceous rocks once covered much of the southern Colorado Plateau and adjacent areas. Deformation, uplift, and erosion in Tertiary time removed these rocks in most of the area. Preservation in basinal areas, however, is sufficient to reconstruct the depositional history of these rocks and to map the shorelines of the major transgressions and regressions of the Western Interior seaway that occurred in the area. Most of the deposition took place during regressions and, in ascending order, consisted of offshore pelitic deposits, coastal, deltaic and interdeltaic deposits, and alluvial plain deposits. Most of the regressive deposits rose significantly in stratigraphic position as they prograded, generally to the northeast, as the result of basin subsidence or sea-level rise. At least 1,555 m (5,100 ft) of relative sea-level rise (or subsidence) was necessary to accommodate the total Upper Cretaceous section.

Five major transgressive-regressive cycles are recognized. These cycles, which range in age from Cenomanian to early Maestrichtian, are designated T1-R1 to T5-R5. The R2 regression, which is the Gallup regression, appears to be unique to New Mexico and northeasternmost Arizona. The initial Cenomanian-early Turonian (Greenhorn) transgression was the most extensive throughout the area; each succeeding cycle did not transgress as far landward as the preceding cycle, indicating an overall filling of that part of the Western Interior seaway. Although the major cycles are correlatable in the southern Colorado Plateau area, faunal evidence indicates that some of the cycles are diachronous, being slightly younger to the northwest.