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Beaver Creek Geology

Geologic Setting

The Beaver Creek Watershed is located within the Colorado Plateau physiographic province, at the southern edge of the Plateau. The watershed is one of many that dissect the Mogollon Rim, the landform that separates the Colorado Plateau from the topographically lower Verde Valley . The Mogollon Rim has been interpreted by Thompson (1968) to be at least a partly erosional escarpment (steep cliff) that formed following the uplift of the Colorado Plateau, which began approximately 70 million years ago and continued episodically through about 20 million years ago. The Mogollon Slope is the regional landform that makes the transition from the Rim to the Valley. The regional dip of the Mogollon Slope is generally toward the northeast in most places; however, the Mormon Mountain anticline (a flexure, or bend, in the earth's crust) causes the slope to dip toward the southwest in the Beaver Creek area (Baker, 1982). Accordingly, Beaver Creek flows generally west-southwest along the slope until it reaches the southeast-flowing Verde River, of which it is a major tributary.

The Mormon Mountain anticline also serves as a watershed boundary between the regional Colorado River and Verde River Watersheds. The Verde Valley lies within Arizona's Transition Zone, a region characterized by extension (stretching) of the earth's crust along low-angle faults that expose older, deep-crustal rocks. This Zone transitions into the Basin and Range Province, a region dominated by crustal extension along high-angle faults, resulting in alternating basins and ranges. Extension in the Transition Zone and the Basin and Range Province occurred contemporaneously with later uplift of the Colorado Plateau during the late Tertiary Period. It is not known why the Plateau remained relatively intact amidst this tectonic activity.

Limestone beds, presumably of the Kaibab Formation (beige), above either upper Toroweap Formation or Coconino Sandstone (red-brown), Wet Beaver Creek Canyon


  The rocks in the Beaver Creek watershed include volcanic and sedimentary rocks that vary in age (see Geologic Map of the Beaver Creek Watershed). The sedimentary rocks are a combination of Paleozoic, Mesozoic, and Tertiary ages, but among the most notable are the Tertiary volcanic rocks (click here to view Geologic Time Scale). The Tertiary volcanics overlie older, late Paleozoic fractured--and, therefore, porous and permeable--units that serve as both water conduits and as underground water reservoirs. The variable porosity and permeability of the volcanic units determine whether precipitation and runoff infiltrate the regional water table, which is estimated to be anywhere from 300m to 600m below the surface (the exact depth varies from one location to the next).


The structural grain, or dominant trend of structures, in the Beaver Creek area is northwest-southeast. Thompson (1968) inferred that the northeast trend of Beaver Creek and adjacent creeks suggests a secondary, northeast-southwest structural trend, but he noted that definitive evidence for such a trend is obscured by the overlying volcanic rocks. Thompson also noted that several northwest-trending faults dissect the Beaver Creek drainage. Furthermore, he interpreted the Mogollon Rim escarpment to be at least partly erosional, since he was not able to observe distinct fault surfaces along the Rim. This implies that structural control is present, but is not always the rule in this area.

Northern Arizona University         Merriam-Powell Center for Environmental Research         Rocky Mountain Research Station         MAB