Description:
The Clark Ranch 7½' Quadrangle and the western half of the Rhodes Peak 7½' Quadrangle include part of
the San Pedro River, flanking valley fill, and bedrock on the east side of the valley that forms part of the
Galiuro Mountains range front (Fig. 1). Production of this new geologic map continues the Arizona
Geological Survey mapping program of the San Pedro River valley. This mapping was done under the
joint State-Federal STATEMAP program, as specified in the National Geologic Mapping Act of 1992,
and was jointly funded by the Arizona Geological Survey and the U.S. Geological Survey under
STATEMAP Program Contract award number 07HQAG0110. Mapping was compiled digitally using
ESRI ArcGIS software. Parts of the Clark Ranch and Rhodes Peak Quadrangles were mapped previously
by Creasey et al. (1961), Dickinson (1987, 1991, 1998), Marsh (2001), and Skotnicki (undated)). Studies
of Cenozoic sedimentary strata in the Clark Ranch 7½' Quadrangle include those by Blake (1902, 1903),
Bollaert (1952, 1953, 1962), Heindl (1963), Agenbroad (1967), Krieger et al. (1974), Ladd (1975),
Scarborough and Wilt (1979), Utley (1980), Lindsay (1984), Dickinson (1988), Shenk (1990), and
Dickinson (2003). The regional geochronology of the Oligo-Miocene volcanic rocks is outlined by
Dickinson and Shafiqullah (1989). Studies of the Copper Canyon mineral district include those by Marsh
(2001), Creasey et al., (1981), Guthrie (1994), Guthrie and Moore (1978), Hausen et al. (1988), Walker
(1979) and Eichenlaub and Seedorff (2008).