OLIGOCENE-MIOCENE SEDIMENTATION RATES ON THE SOUTHWESTERN COLORADO PLATEAU: ARIDITY-DRIVEN AGGRADATION SUPPORTIVE OF A LATE NEOGENE COLORADO RIVER

Laramide drainage incision on the SW Colorado Plateau was followed by widespread fluvial aggradation throughout Oligocene and Miocene time. The Hualapai Plateau has the best Paleocene to Pliocene stratigraphic record of such events in Arizona during this transition period, which preceded the integration of the Colorado River beginning 5 to 6 Ma ago. There are few Colorado Plateau localities with data on long-term Paleogene fluvial sedimentation rates. The Tertiary section at Peach Springs, AZ, includes three datable horizons, which permit calculation of an average fluvial sedimentation rate for nearly 15 Ma. The horizons include basalts (19-20 Ma), the Peach Spring Tuff (18.7 Ma), an Oligocene ash from the Aquarius Mountains (24.1 Ma), and the Eocene-Oligocene climatic boundary (33.9 Ma). Although the Eocene-Oligocene boundary is not precisely dated in Hualapai Plateau sediments, the published North American evidence for abrupt climatic deterioration includes a relatively short vegetation succession from moist to dry forests (34 Ma), to dry woodlands (33 Ma), to wooded grasslands (32 Ma). This suggests that the prominent color boundary (red to tan) preserved throughout the Hualapai Plateau stratigraphy probably developed approximately 33±1 Ma ago. Following Laramide incision, monoclinal folding and early Basin and Range deformation interrupted and severed the northeast flowing streams and initiated aggradation that filled the preexisting valleys with locally derived gravels. The Buck and Doe Conglomerate, dominated by local Paleozoic limestone clasts, buried local divides and created a low-relief aggradational surface of Miocene age near 1525 m (5000 ft). Miocene volcanism preserved this sedimentary record, which continued through late Miocene time. The Peach Springs sediments accumulated in two stages at similar rates of between 6.9 and 7.3 m/Ma from early Oligocene through early Miocene time (~33 Ma to ~19 Ma). Although these rates represent a single locality, it is a lengthy and unbroken record consistent with the diverse body of evidence that precludes regional post-Laramide erosion until the onset of the predominantly Pliocene incision of the through-flowing Colorado River. An older west-flowing “Colorado River” is refuted by the integrated structural, geomorphic, and sedimentary evidence.