Abstract

Oligocene and Miocene sedimentary rocks of Chino and Verde valleys record changes in drainage patterns along the southern edge of the Colorado Plateau. Strata of the Paulden and Beavertail Butte formations represent the first deposits of a southeast-flowing drainage system. Following the development of the Mogollon Rim, a significant deflection in drainage direction occurred, mainly from north- to southeast-directed flow and also a significant change in sediment sources. It is apparent that the Mogollon Rim formed prior to 15 Ma and that the topography of the rim was similar to that of the present day rim. Oligo-Miocene sedimentary deposits represent a period of aggradation that followed an extensive period of erosion that formed the Mogollon Rim.

The Beavertail Butte formation is an informal name given to conglomerates deposited near Sedona, Arizona, approximately 500–1000 meters below the present rim. Capping basalt flows from House Mountain volcano were dated at 13–15 Ma and brackets the upper age of these sedimentary rocks. Deposits of the Beavertail Butte formation consist of basal conglomerate, middle mudstone, and upper cong-lomerate unit. A change of provenance is recorded between the upper (Precambrian terrains to the west) and basal (Paleozoic strata of the Mogollon Rim) conglomerate, probably recording the development and expression of the Mogollon Rim. The upper conglomerate unit represents southeast-direct transport and possibly integration of stream systems.

The Paulden formation is probably older than the upper conglomerate of the Beavertail Butte formation. The Paulden formation outcrops to the northwest of Sedona and probably represents a similar drainage pattern as the Beavertail Butte formation. The Paulden formation is overlain by the 22–26 Ma Sullivan Buttes Latite. Clasts from the overlying Sullivan Buttes Latite are found in the upper conglomerate of the Beavertail Butte formation. This change in drainage direction correlates with the inception of Basin and Range extension in southern Arizona. These two formations help to document evolving drainage patterns of the Transition zone during a period of significant tectonic evolution.