COLORADO PLATEAU AND GRAND CANYON: CRETACEOUS AND OLIGOCENE LANDSCAPE EROSION IN A HIGH RAINFALL ENVIRONMENT DURING SEVIER, LARAMIDE, AND CORDILLERAN OROGENIES

The Colorado Plateau landscape is subdivided into six genetically interrelated, geomorphic subprovinces. Two subprovinces, Southern and Central, were eroded by an enormous north-flowing Southern Basin and Range river system; it developed in response to episodes of uplift and northward tilting here identified with Early Cretaceous Sevier and Late Cretaceous Laramide orogenies. Maintained by very high rainfall, this river system eroded two superimposed regional step-bench landscapes. Planar erosion surfaces above stepped escarpments trace into unconformities in and bounding Early and Late Cretaceous strata in the Central Plateau. Following Laramide erosion of the Mogollon Rim, Grand Wash Cliffs, and Grand Canyon, a waning Southern Basin and Range river system deposited a thick Eocene fluvial and lacustrine blanket from Arizona far into Wyoming.

Three subprovinces, Northern, Eastern and Western, were eroded by a south-flowing, post-Eocene Colorado River system, developed in response to Oligocene uplift and southward tilting assigned here to the ‘Cordilleran’ orogeny. Long single-step escarpments and canyons (e.g., Book Cliffs, Mesa Verde, Desolation Canyon) characterize this landscape. In the Central Plateau, Oligocene Colorado River canyons (e.g., Glen Canyon) are entrenched beneath the Cretaceous step-bench landscape. Soft Eocene deposits blanketing the Central and Southern Plateau were rapidly eroded as the Colorado River excavated Eocene deposits from the Grand Canyon and found its way to the Basin and Range. The sixth subprovince (San Juan Basin), not in the path of either ancient river system, was less strongly eroded.

Widespread volcanic activity, coinciding with and following the Cordilleran orogeny, possibly was responsible for development and maintenance of Miocene-Pliocene aridity. On the Plateau, it was a time when storm-generated flash floods were responsible for erosion and aggradation, and attenuated rivers flowed in the subsurface through gravel fill.