THE LATE CENOZOIC EVOLUTION OF THE ENIGMATIC UPPER ARKANSAS RIVER, COLORADO

The Arkansas River drains much of the eastern flank of the Colorado Rockies. For its first 200 km, the river flows through the Rockies, before emerging onto the Colorado Piedmont section of the Great Plains at Cañon City. The mountain section of the river consists of two segments. For its first 125 km, it is a southward-flowing axial stream in the northern grabens of the Rio Grande Rift. At Salida, Colorado, the river abruptly turns eastward and flows ~ 75 km through a series of canyons cut into Proterozoic crystalline rocks, before emerging onto the Piedmont at the eastern range front. Stratigraphic studies in basins along the course of the river indicate that integration of the upper, rift-axis, segment of the river with the portion down valley from Salida, and the attendant cutting of the canyons, are Neogene phenomena, possibly Plio-Pleistocene.

The Neogene evolution the river has been influenced both by localized, rift-related, tectonism and by local base-level drop resulting from deep (> 1 km) Neogene erosion on the Colorado piedmont. Establishment of the modern course of the river may reflect either (1) blockage of an original southward drainage of the river by uplift of the Poncha Pass block south of Salida, resulting in eastward spillover and canyon cutting, or (2) headward erosion of the canyon section of the river, driven by base-level lowering on the Great Plains, leading to capture of the upper, Rio Grande Rift, portion of the drainage.

Oligocene volcanic rocks preserved on both sides of the river provide a datum from which river incision may be measured, and provenance studies of preserved fluvial gravels in the canyons help constrain the temporal relationship between canyon cutting and drainage integration. At least half, possibly as much as 90%, of the 700 - 800 m of Neogene canyon cutting preceded integration of the upper section of the drainage, suggesting that base-level drop at the mountain front was the primary driving force in canyon cutting and likely played a major role in driving the integration of upper and lower sections of the river.