Description:
The accompanying geologic maps depict the general geology of a two-mile
wide strip centered about the Salt River, starting just below Roosevelt
Dam and extending westward 36 miles to just below Granite Reef Dam. The
geology was accomplished from October 1980 through April 1981, with the
aid of about 25 hours of helicopter logistical support time. Discussions
and/or field assistance by Mike Pryor, Joe Bailey, Dick Raymond, Paul Damon,
and Tom McGarvin are greatly appreciated.
Geologic Summary
The lower Salt River below Roosevelt Dam has cut a gorge which exposes
much of the geology of the northern Superstition volcanic field. The
silicic field of late Oligocene (?) and Miocene age is one of the half
dozen or so found in southern and western Arizona. The volcanics extend
continuously from around Apache Lake Resort westward to Stewart Mountain
Dam, and discontinuously farther west to the Mt. McDowell area, around
Granite Reef Dam. The Superstition volcanics were deposited on Precambrian
crystalline rocks, dominated by a biotite quartz monzonite which age
dates at about 1400 m.y., and which is probably equivalent to the Ruin
and Oracle granites further east and south. The northern Superstition
volcanics are dominated by silicic pryoclastics, plugs, ash flows,
agglomerates, and late stage intrusive domes. Rocks of andesitic
composition are subordinate in volume although important in the
eastern part of the area.