MIOCENE DRAINAGE HISTORY OF WESTERN GRAND CANYON, AZ

The Colorado River (CR) conforms to elements of a partially exhumed Laramide cuestaform landscape west of the Hurricane fault. Runoff from the Hualapai Plateau (2500 sq km) prior to accelerated Plio-Pleistocene canyon incision could only flow westward after northward Laramide outlets parallel to the Hurricane fault zone were blocked in Miocene time, following Basin and Range extension and structural lowering of the western plateau margin. There is no wide Esplanade surface south of the CR, where the river formed on a Laramide surface that bevels Cambrian through Mississippian carbonates and is covered with a veneer of Tertiary sediments and Miocene volcanic flows (19-15 Ma). An unresolved issue is how and where the evolving drainage west of the Hurricane fault exited the Hualapai Plateau in Miocene time. The local Miocene Hualapai Plateau drainage must have included significant runoff contributed from larger plateaus west and south of the Kaibab Uplift (Cataract and Kanab Creek basins), surfaces extensively covered by Laramide sediments until at least 14 Ma. Miocene events in this region left no explicit sedimentary record, but would logically have initiated the westward course of the modern CR into the Grand Wash Trough along the 450-meter Laramide scarp separating the Shivwits and Hualapai Plateaus. Remnants of the Miocene Muddy Creek Formation (11 to 6 Ma) of western Lake Mead reveal no recognized facies considered to be compatible with the existence of a significant perennial inflow from an integrated upper CR. However, the Laramide physiography implies that ephemeral runoff from local plateau surfaces had to begin reshaping the landscape following Basin and Range extension (16-9 Ma). It is probable that Miocene aridity slowed fluvial erosion and produced a minimal sedimentary record, possibly buried under younger sediments in the Grand Wash Trough, or removed during erosion of Muddy Creek strata directly adjacent to the plateau margin. The current constraints on CR chronology leave unresolved how the final integration of the upper and lower CR was achieved from middle to late Miocene time. However, previously postulated southerly and northerly exits for mid-Tertiary CR drainage are unrealistic. The Neogene climatic and exhumation histories are key to understanding the CR drainage integration.