Biological Survey of Ironwood Forest National Monument

Legend and literature references for Silverbell - West Silverbell Mountains geologic map

(abstracted from Sawyer 1996):

af - manmade fills related to Silverbell copper mine operations of Asarco, Inc.

Qc - colluvium (Holocene) heterogeneous unconsolidated deposits of soil-rock materials, that cover bedrock at base of hills.

Qy2 - late Holocene unconsolidated alluvium in and adjacent to stream and wash bottoms.

Qye - Holocene age sand dune fields, exact boundaries undefined because of reworking.

Qy - Holocene age alluvial deposits on lower bajada slopes, with weak soil development

Qygr - latest Pleistocene or Holocene mudflow complex, with a light-colored rocky pavement covering that is unvarnished, producing a very light tone on aerial photos. Originated on the western flank of the Silver Bell Mountains. Rocks are granite and light-colored volcanics.

Qly - alluvial deposits of Holocene and Pleistocene age, consisting of coalescing distal alluvial fan and stream deposits, forming bajada surfaces in the map area.

Ql - late Pleistocene deposits and soils on lower bajada slopes, often with indications that both sedimentation and stripping have occurred.

Qm - deposits overlying mountain front pediments, usually with remnants of paleargid soils (thick calcretes, etc), indicating that stripping has occurred.

Qaf - fluvial and mass-flow deposits forming recent fan-shaped accumulations at the mouth of valleys, adjoining mountain fronts. This map unit tends to show discrete fan deposits.

Qab - Quaternary deposits on bajada mid-slopes, with minor soil development.

Qao - older alluvial fan and terrace deposits, of Pleistocene age. Fluvial and mass-flow deposits forming relict older surfaces and low terraces adjoining the mountain fronts. Often mantle shallow-buried bedrock pediments, with bedrock exposures in wash bottoms.

Tsy - gravels, sandstones, siltstones that overlie Cenozoic volcanic rocks, thus of middle-late Miocene age, variable colors, often gray-orange. Related to upper basin fill beds elsewhere, such as the Tinaja beds of the eastern Sierrita Mts.

Tb - mid-Cenozoic basalt flows

Ta - mid-Cenozoic andesite flows

Tv - mid-Cenozoic volcanic rocks, including rhyolites, dacites, andesites and related rocks, sometimes with thin sedimentary units. Ages generally 30-20 m.y.

Tso - older Cenozoic sedimentary rocks, east of Ragged Top dome and poorly exposed as windows through older alluvial fan deposits. Colors of light brown to reddish brown. Variable age deposits represented, including some syn-mid Cenozoic volcanic sediments, and prevolcanic units, potentially including Eocene-Paleocene beds. Some may be temporal equivalents of Pantano Formation east of Tucson.

Kv - Complex volcanic assemblage of Laramide age (75-65 Ma), including caldera flows, infill units, multiple granitic stocks and east-west trending granite porphyry dikes. See Sawyer (1996) for details. One flow may correlate with the Cat Mountain rhyolite of the Tucson Mountains. One area shown on the map contains complex multiple megabreccia slide blocks of Paleozoic sediments, interpreted by Sawyer as landslide blocks into a collapsing caldera depression. This volcanic-plutonic assemblage responsible for metallization at Silver Bell mine, which produced 2.08 million tons of ore with grades of 6.46% Cu, 0.08% Pb, 0.98% Zn, and 6.8 grams/ton Ag (Titley and Anthony, 1989, p. 494).

Kg - various granitic plutons of Laramide age, related to the volcanic center of the Silverbell Mountains. For descriptions and names of all plutons see Sawyer (1996). They are mapped separately here, even if they abut. Their ages are contemporaneous with units of Kv, above.

Kdi - small plugs of even-grained diorite, Laramide age, minor associate of the numerous granitic plutons. Seen in southern West Silver Bell Mountains, west of the main mine.

Ka - possible lateral equivalent of the Amole Arkose of the Tucson Mountains, including arkoses, mudstones, thin-bedded dark gray pelecypod-bearing and laminar-bedded limestones with fetid odors. Colors include pale reddish brown to grayish orange to white. Some pebbley conglomerates clasts are volcanic. See Sawyer (1996) for details.

Mzg - Mesozoic granites, undated, possibly Laramide or Jurassic age. Found in West Silver Bell Mountains.

Mzr - Mesozoic rhyolite flows and dike masses, possibly equivalent to ash flow sequence remnants found rarely in the Tucson Mountains [Brown Mountain], or in the southern Waterman Mountains, where an unpublished U/Pb zircon date by U of A geologists of 172 Ma forms a maximum age for bright-colored redbeds found there, possible equivalents of the Tucson Mountains 'Recreation Redbeds'.

Mzsv - Mesozoic sedimentary and volcanic rocks (undivided), with local abundances of andesites, volcaniclastic sediments, arkoses, typically grayish-olive colors. Some are above possible Amole Arkose equivalent rocks. In the West Silver Bell Mountains these rocks were folded and deformed prior to the deposition of the Confidence Peak tuff. The package includes some arkosic sandstones conformably underlying the Confidence Peak tuff.

Pzs - sedimentary rocks of Paleozoic - Permian age, likely including limestones, quartzites and siltstones of Scherrer, Concha, and Rainvalley Formation.s. Seen in SW end of West Silver Bell Mountains. In one case limestone pods strung out along a fault zone.

Ya - Apache Group of middle Proterozoic age, seen to unconformably overlie Yg north of the Ragged Top fault east of Ragged Top peak. Consists here of arkosic sandstone, siltstones, and thin basal conglomerates typical of the Scanlon Conglomerate member (lacking red chert fragments). Similar to exposures in the nearby Slate Mountains. Age dates from surrounding region suggests deposition at 1.4-1.1 Ga.

Yg - middle Proterozoic granite, partially chloritized, and equivalent in age to the Oracle-Ruin granite suite (1.4 Ga) of the region. Contains numerous pods, small plutons and dikes of dark-colored diabase, not mapped separately here, but differentiated by Sawyer. The diabase dates at near 1.1 Ga, younger than both Yg and Ya units. The granite lacks tectonic fabric, and in places is K-spar porphyritic. A common lead model age from these exposures determined on potash feldspar yields a 1.38 Ga age (Sawyer, 1996).

Xs - early Proterozoic quartz-muscovite schist, light bluish gray color, strongly foliated and folded. Derived from pelitic parent rocks. Many small outcrops on the east side of the Silver Bell range south of the Ragged Top fault. Part of the Pinal Schist suite.


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