Latest Oligocene time (∼25 m.y. ago) is established as a minimum age for the regional northeast-flowing drainage system in west-central Arizona that existed prior to the formation of the Grand Canyon. The amounts of erosion and fluvial deposition that occurred before eruption of the oldest dated volcanic rocks suggest a Paleocene to Eocene age span for the uplift and deformation in southwestern Arizona that initiated this drainage pattern. Recent refinements and revisions of the Tertiary stratigraphy in southwestern Utah indicate that the regional drainage from northern Arizona ended in basins like that represented by the Claron Formation and associated fluviolacustrine sedimentary rocks in southern Utah. This period of erosion and north-to northeast-flowing drainage is separated from the Grand Canyon erosion cycle by at least 15 m.y. of block faulting and interior basin deposition, followed by gradual capture and integration of the existing drainage by the Colorado River.
- Geological Society of America