Preliminary Geophysical Framework of the Upper and Middle Verde River Watershed, Yavapai County, Arizona

Analysis of aeromagnetic and gravity data provides new insights into the
geometry of geologic structures of the upper and middle Verde River watershed,
Yavapai County, Arizona. Magnetic anomalies reveal hidden volcanic rocks lying
at shallow depths beneath the ground surface in Williamson, Little Chino, Big
Chino and Verde Valleys in the upper Verde River watershed, Yavapai County,
Arizona. Concentrations of shallowly buried volcanic plugs or centers are
located down-gradient of springs (Del Rio) and perennial flow (Williamson
Valley), suggesting that these volcanic centers or plugs may retard
ground-water flow. Magnetic data also map paleo-channels filled with 4-6 Ma
basalt as Big Chino Valley formed and subsided during late Tertiary time. The
magnetic data reveal a predominantly northeast- to north-striking structural
grain within Proterozoic basement rocks, in contrast to the generally northwest
strike of late Tertiary faults in this region. The magnetic grain may serve as
a proxy for fracturing and faulting, an important source of permeability in
these generally impermeable rocks. Magnetic and gravity data also delineate
exposed and concealed faults within the study area. The Big Chino fault and
Verde fault zone have the largest amounts of vertical throw of any fault in the
study area based on gravity, magnetic, and limited well data. These faults
bound deep (1-2 km) basins in Big Chino and Verde Valleys. The geophysical data
also reveal concealed faults in Williamson Valley that bound a previously
undiscovered basin with approximately 1 km of Cenozoic fill inferred from
inversion of gravity data. Little Chino and Lonesome Valleys, including the
upper reach of the Agua Fria basin, are characterized by basin fill which
nowhere exceeds 1 km and has a more irregular distribution with local, north-
to northwest-striking pockets of thicker sediment. A 15- to 20-km-long
northwest-striking magnetic lineament that passes through Page Springs in Verde
Valley can be used to project a fault 5-10 km northwest and southeast of its
mapped trace. The colocation of the lineament, mapped fault, and the spring
suggests structural influence on the location of this large spring.

[Summary provided by the USGS.]