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CONTOUR INTERVAL 40 FEET

LOCATION MAP FOR DGM-20showing location of the Desert Peak 7 1/2' quadrangle

Marana

MAP AREA

Desert Peak (DGM-20)

N

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Tucson

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Youberg1Ferguson2Ferguson-Photo Interpretation3Richard4Johnson5Maher6Gilbert7Banks and others, 19778

Additional surficial mapping byYouberg throughout the map area.

MAPPING RESPONSIBILITIESAND DATA SOURCES

Geologic map of the Desert Peak 7½' Quadrangle,southeastern Pinal County, Arizona

byAnn Youberg, Charles A. Ferguson, Stephen M. Richard,

Brad J. Johnson, David J. Maher, and Wyatt G. Gilbert

November, 2002Arizona Geological Survey Digital Geologic Map 20

citation for this map:Youberg, A., Ferguson, C.A., Richard, S.R., Johnson, B.J., Maher, D.J., and Gilbert, W.G., 2002, Geological map of the Desert Peak 7½' Quadrangle, southeastern Pinal

County, Arizona: Tucson, Arizona Geological Survey Digital Geologic Map 20 (DGM-20), 1 CD-ROM with 1 Adobe PDF file (1 sheet, layout scale 1:24,000).

Not to be reproduced for commercial purposes

Research supported by the Arizona Geological Survey and the U.S. Geological Survey, National Cooperative Geologic Mapping Program,under USGS award #01HQAG0098. The views and conclusions contained in this map are those of the authors and should not be interpreted

as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

Arizona Geological SurveyDigital Geological Map 20, Sheet 1 of 1

Arizona Geological SurveyDigital Geological Map 20, Sheet 1 of 1

Topographic base from Desert Peak, Arizona, quadrangle; Provisional edition 1988.Transverse Mercator projection; Universal Transverse Mercator grid: UTM Zone 12; 1927 North American Datum; Clark 1866 spheriod.

INTRODUCTION This digital geologic map and report describes the bedrock and surficial geology of the Desert Peak 7 ½' Quadrangle. The study area is located approximately 40 km northwest of the Tucson metropolitan area. It includes the extreme western edge of the Tortolita Mountains, Owl Head Buttes, and the piedmont areas extending west to, and including, Desert Peak. Elevations in the study range from approximately 3100 feet in the Tortolita Mountains to 2000 feet on the western piedmonts. Land use in the area includes state trust lands, federal lands, ranches and, increasingly, single-family suburban homes. The rate of development in this area is increasing, underscoring the need to understand the geologic setting. Mapping was conducted between October 2001 and May 2002 as part of continuing efforts by the Arizona Geological Survey (AZGS) to map the geology of the Phoenix - Tucson urban corridor. This map is part of a concurrent effort encompassing several adjacent quadrangles including Fortified Peak (DGM 18), Durham Hills (DGM 19), Oro Valley (DGM 21), Chief Butte (DGM 22), North of Oracle (DGM 23), and the Tortolita Mountains including the northern portion of Ruelas Canyon (DGM 26). The primary product of this mapping is a 1:24,000 layout scale geologic map. Bedrock mapping consists almost entirely of new mapping but locally incorporates past mapping by Banks and others (1977). The surficial geologic mapping builds on and complements previous efforts in the Tucson area (McKittrick, 1988; Jackson, 1989; Jackson, 1990; Pearthree and others, 1992; Field and Pearthree, 1993; Klawon and others, 1999; Pearthree and Biggs, 1999; Skotnicki, 1999; Pearthree and Youberg, 2000; Skotnicki, 2000; Youberg and Helmick, 2001). This mapping is part of the STATEMAP Program of the U.S. Geological Survey, administered under contract #01HQAG0098, and jointly supported by the AZGS and the U.S. Geological Survey National Cooperative Mapping Program. Bedrock geology is dominated by the upper part of a detachment fault system, although the actual fault and shear zone are exposed only along the eastern edge of the map area. A pair of low-angle, southwest-striking normal faults offset a prominent mylonitic shear zone, the Carpas Wash shear zone in the southern half of sections 19 and 20. Kinematic indicators along the shear zone and along the low-angle faults are consistently top-to-the-southwest directed. The hanging wall block consists of Proterozoic Oracle Granite and Pinal Schist overlain by at least 2km of mafic to intermediate volcanic rocks. The volcanic rocks are preserved in a series of three oppositely dipping fault blocks, with the western edge of the westernmost (west-tilted) block preserved in the northeast part of this

map area. The large pedimented area in the north-central part of the map area is believed to be underlain by gently dipping volcaniclastic sedimentary rocks which are probably younger than the volcanic succession. The footwall block is represented by a complex suite of Tertiary plutonic and metaplutonic rocks that are preserved at Desert Peak in the northwest and in the Tortolita Mountains in the southeast. The bounding Carpas Wash shear zone and associated low-angle normal faults are believed to track westerly across the southern half of the map area and then turn sharply to the north and then northeasterly across the northwestern part of the map area and join with a north- to northeast-striking, gently east-dipping detachment fault in the northerly adjacent Durham Hills quadrangle.

ACKNOWLEDGMENTS Cathy McGuire of the Natural Resources Conservation Service made available large-format, 1:24,000 scale, color aerial photographs of the study area. Philip Pearthree and Steve DeLong provided valuable field support and guidance. Tim Orr assisted with the map layout and final digital information products.

CLIMATE Two weather stations near the map area have operated during intervals over the past century and provide climatological data for the study area. The weather station at Red Rock (Red Rock 6 SSW), west of the study site, has records from 1893 to 1973. The station at Cortaro (Cortaro 3 SW), south of the study site, has records from 1948 to 1976. Throughout this region, most of the annual precipitation (50-60%) falls during the summer monsoon from June to September. Late summer rainfall occurs as heavy thunderstorms when moist air sweeps northwards from the Gulf of California and the Gulf of Mexico. Occasional intense late summer to early fall precipitation occurs in this region as a result of incursions of moist air derived from dissipating tropical storms in the Pacific Ocean. Winter precipitation generally is caused by cyclonic storms originating in the Pacific. It is usually less intense and may be more prolonged, and therefore infiltrates into the soil more deeply than summer rainfall (summarized from Sellers and Hill, 1974). Freezing temperatures are common during most winters, but snow is uncommon and not persistent. Climate records from these stations illustrate the relatively warm and dry climate of the lower elevations in the Sonoran Desert. Cortaro, at about 2280 ft asl, has an average annual precipitation of 11.2 in. while Red Rock, at about

1860 ft asl, has an average annual precipitation of 9.8 in. (Western Regional Climate Center, 2002). The average maximum temperature for Cortaro is 85.7°F, with an average high temperature of 102.7° F in July and 67.0 ° F in January. Temperature data from Red Rock are typically within a degree of Cortaro. These conditions reflect those typically found on the piedmonts through out the map area. The higher, mountainous areas of Desert Peak, Owl Head Buttes and Tortolita Mountains will be cooler and, possibly, wetter.

METHODOLOGY Quaternary surficial geology was mapped and described by Youberg. Bedrock geology of Desert Peak was mapped and described by Richard. Bedrock geology of Owl Head Buttes and the Tortolita Mountains was mapped by Ferguson, Johnson, Mahr, and Gilbert, and compiled and described by Ferguson. Bedrock geology was mapped by direct field interpretation. Surficial geology was mapped using aerial photograph interpretation and direct field observation. Aerial photographs included 1984, 1:30,000 scale, USGS color photographs, supplemented by large-format, 1983, 1:24,000 scale, high altitude, false color photos provided by NRCS. The physical characteristics of Quaternary alluvial surfaces (channels, alluvial fans, floodplains, stream terraces) evident on aerial photographs and in the field were used to differentiate their associated deposits by age. Surficial deposits of this quadrangle were then correlated with similar deposits in this region in order to roughly estimate their ages. Alluvial surfaces of similar age have a distinctive appearance and soil characteristics because they have undergone similar post-depositional modifications. Terraces and alluvial fans that are less than a few thousand years old still retain clear evidence of the original depositional topography, such as bars of gravel deposits, swales (troughlike depressions) where low flows passed between bars, and distributary channel networks, which are characteristic of active alluvial fans. Young alluvial surfaces have little rock varnish on surface clasts, little soil development, and are minimally dissected. Very old fan surfaces, in contrast, have been isolated from substantial fluvial deposition or reworking for hundreds of thousands of years. These surfaces are characterized by strongly developed soils with clay-rich argillic horizons and cemented calcium-carbonate horizons, well-developed tributary stream networks that are entrenched 1 to 10 m below the fan surface, and strongly developed varnish on surface rocks. The ages of alluvial surfaces in the southwestern United States may be roughly estimated based on these surface characteristics, especially soil development (Gile and others, 1981; Bull, 1991).

REFERENCESBanks, N.G., 1980, Geology of a zone of metamorphic core complexes in southeastern Arizona, in Crittenden, M.D., Jr., Coney, P.J., and Davis, G.H., eds., Cordilleran metamorphic core complexes: Geological Society of America Memoir 153, p177-215.Banks, N.G., Dockter, R.D., Briskey, J.A., Davis, G.H., Keith, S.B., Budden, R.T., Kiven, C.W., and Anderson, Phillip, 1977, Reconnaissance geologic map of the Tortolita Mountains quadrangle, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map MF-864, 1 sheet, scale 1:62,500.Banks, N.G., McKee, E.H., Keith, S.B., Shafiqullah, M., and Damon, P.E., 1978, Radiometric and chemical data for rocks of the Tortolita Mountains 15' quadrangle, Pinal County, Arizona: Isochron/West, no. 22, p. 17-22.Bull, W.B., 1991, Geomorphic Response to Climatic Change, New York: Oxford University Press, 326 p.Creasey, S.C., Banks, N.G., Ashley, R.P., and Theodore, T.G., 1977, Middle Tertiary plutonism in the Santa Catalina and Tortolita Mountains, Arizona: U.S. Geological Survey Journal of Research, v. 5, no. 6, p. 705-717.Dickinson, W.R., and Shafiqullah, M., 1989, K-Ar and F-T ages for syntectonic mid-Tertiary volcanosedimentary sequences associated with the Catalina core complex and San Pedro trough in southern Arizona: Isochron/West, no. 52, p. 15-27.Field, J.J., and Pearthree, P.A., 1993, Surficial geologic maps of the northern Avra Valley - Desert Peak area, Pinal and Pima counties, southern Arizona: Arizona Geological Survey Open-File Report 93-13, 11 p., 9 sheets, scale 1:24,000.Gile, L.H., Hawley, J.W., and Grossman, R.B., 1981, Soils and geomorphology in the basin and range area of southern New Mexico - guidebook of the Desert Project: New Mexico Bureau of Mines and Mineral Resources Memoir 39, 222 p.

Jackson, G.W., 1989, Surficial geologic maps of the northeastern, southeastern, and southwestern portions of the Tucson metropolitan area: Arizona Geological Survey Open-File Report 89-2, 6 p., 7 sheets, scale 1:24,000.Jackson, G.W., 1990, Quaternary geologic map of the Corona de Tucson 7.5' quadrangle, Arizona: Arizona Geological Survey Open-File Report 90-3, 6 p., scale 1:24,000.Klawon, J.E., Dickinson, W.R., and Pearthree, P.A., 1999, Surficial geology and geologic hazards of the northern Tucson basin, Tucson North and Sabino Canyon 7 ½' quadrangles: Arizona Geological Survey Open-File Report 99-21, 21 p. 1 sheet, scale 1:24,000.Machette, M.N., 1985, Calcic soils of the southwestern United States: in Weide, D.L., ed., Soils and Quaternary Geology of the Southwestern United States: Geological Society of America Special Paper 203, p. 1-21.McKittrick, M.A., 1988, Surficial geologic maps of the Tucson metropolitan area: Arizona Geological Survey Open-File Report 88-18, 7 p., 12 sheets, scale 1:24,000.Menges, C.M., and McFadden, L.D., 1981, Evidence for a latest Miocene to Pliocene transition from Basin-Range tectonic to post-tectonic landscape evolution in southeastern Arizona: Arizona Geological Society Digest 13, p. 151-160.Menges, C.M., and Pearthree, P.A., 1989, Late Cenozoic tectonism and landscape evolution in Arizona, in Jenney, J., and Reynolds, S.J., eds., Geologic Evolution of Arizona: Arizona Geological Society Digest 17, p. 649-680.Orr, T.R., DeLong, S.B., Spencer, J.E., and Richard, S.R., 2002 , Geologic map of the Fortified Peak 7½' quadrangle, southeastern Pinal County, Arizona: Arizona Geological Survey Digital Geologic Map 18 (DGM-18), 1 CD-ROM with 1 Adobe PDF file (1sheet, layout scale 1:24,000).

Pearthree, P.A., and Biggs, T.H., 1999, Surficial geology and geologic hazards of the Tucson Mountains: Arizona Geological Survey Open-File Report 99-22, 12 p., 2 sheets, scale 1:24,000.Pearthree, P.A., and Calvo, S.S., 1987, The Santa Rita fault zone: Evidence for large magnitude earthquakes with very long recurrence intervals, Basin and Range province of southeastern Arizona: Bull. Seis. Soc. Amer., v. 77, p. 97-116.Pearthree, P.A., Demsey, K.A., Onken, Jill, Vincent, K.R., and House, P.K., 1992, Geomorphic assessment of flood-prone areas on the southern piedmont of the Tortolita Mounstins, Pima County, Arizona: Arizona Geological Survey Open-File Report 91-11, 31 p., scale 1:12,000 and 1:24,000, 4 sheets.Pearthree, P.A., and Youberg, A., 2000, Surficial geology and geologic hazards of the Green Valley-Sahuarita Area, Pima County, Arizona: Arizona Geological Survey Open-File Report 99-22, 12 p., 2 sheets, scale 1:24,000.Skotnicki, S.J., 1999, Geologic map of the Ninetysix Hills, Pinal County, Arizona: Arizona Geological Survey Open-File Report 99-20, 4 plates, scale 1:24,000, with 14 p. text.Skotnicki, S.J., 2000, Geologic map of the Oracle 7.5' quadrangle and the eastern third of the Tortolita Mountains 7.5' quadrangle, Pima County, Arizona, Arizona Geological Survey Open-File Report 00-04, 19 p., scale 1:24,000, text and sheet.Sellers, W.D., and Hill, R.H., 1974, Arizona Climate, 1931-1972: Tucson, University of Arizona Press, 616 p.Western Regional Climate Center, 2002, Arizona climate summaries, in Western U.S. historical climate summaries, WRCC Web Page, Desert Research Institute, University of Nevada.

Slaty cleavageX35Schistosity¦7

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Mylonitic stretching lineation with down-dip sense of shearÜMylonitic lineation with up-dip sense of shearß

Mineral lineationç

Lineations

Solid where accurate, dashed where approximately locatedDepositional or intrusive contact

Cartographic contactUsed to close map unit polygons where actual contact location is unknownMarker Bed

Contacts

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Dashed where approximately located; dotted where concealed; arrow indicates dip, diamond head indicates slickenside lineation

Normal fault

BBBBBBBBBBBBBBBBB Buried shear zone

Dikes

Felsic dikes (Map Unit Ttm)V V V V V V V V

Mafic dikes (Map Unit Yd)

Mafic dikes (Map Unit Tmd)

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BeddingInclined planar beddingo11

Apparent dip of beddingô5

STRUCTURE SYMBOLS

Tertiary map unitsTcy Younger Conglomerate (Miocene)

Thin- to thick-bedded, pebbly sandstone, pebble-cobble sandy conglomerate and sandstone, typically in tabular-planar sets. Clasts are rounded to sub-angular and dominated by schist and granite with between 0 - 20% mafic volcanics. (>50m thick)Younger tuff (Miocene) Phenocryst-poor, white nonwelded tuff containing sparse lithic lapilli of granite and schist. (0-5m thick)

Tty

Other Quaternary and Late Tertiary map unitsQc Talus and colluvium (< 2 Ma)

Qc consists of gravitly-transported, unconsolidated talus and colluvium on steep hillslopes where it is sufficiently thick to obscure underlying bedrock. Deposits consists of angular to subangular boulders to fine sediments, composed mainly of adjacent bedrock lithologies. Vegetation is generally not found on these deposits due to the frequent movement of material.

Tsa Arkosic sandstone (Miocene) Dark red, thin- to medium-bedded, tabular-planar, plane-bedded, internally laminated, medium-grained arkosic sandstone with trains of sub-rounded, vesicular, mafic phenocryst-porphyritic basaltic lava pebbles. (>20m thick)

Tdi Hypabyssal dacite (Miocene) Phenocryst-poor, plagioclase- and biotite-phyric dacitic hypabyssal bodies intrude Tertiary volcanic rocks in the northern part of the map area. The unit is petrographically, and geochemically identical to phenocryst-poor, dacitic lava flows that occur just to the east of the map area, and within the Cloudburst volcanics in the Fortified Peak area (Orr and others, 2002). Compositionally, the hypabyssal dacite unit ranges from trachyte to rhyolite. A biotite K-Ar date of 26.7 + 0.5 Ma was obtained from this unit from the Chief Butte area (Banks and others, 1978).

Ta Andesite lava (Miocene) A composite unit of mafic to intermediate lava flows characterized by phenocryst assemblages dominated by plagioclase with subordinate pyroxene and olivine phenocrysts. Plagioclase phenocrysts range in size from less than 1mm to 6mm. These flows are differentiated from the basaltic andesite lava unit (Tb) by phenocryst assemblage (ferromagnesian phases subordinate to plagioclase in both size and abundance), and from the dacitic lava and hypabyssal dacite unit by the lack of biotite phenocrysts. Two main flow units are recognized in the Chief Butte area. Quartz xenocrysts up to 2cm are present near the base of the flows, and the upper flow is overlain in some areas by the welded rhyolitic tuff unit (Tt). Compositionally, the andesite lava unit ranges from trachyandesite to trachyte. A feldspar concentrate K-Ar date of 24.0 +/- 0.6 Ma (Dickinson and Shafiqullah, 1989) was obtained for his unit from the southern part of the Owl Head Buttes area. (0-120 meters thick)

Tb Basaltic andesite lava (Miocene) A composite unit of amalgamated mafic lava flows characterized by phenocryst assemblages of 2-15% pyroxene + olivine (ranging in size from 0.3mm to 4mm), and subordinate plagioclase (typically less than 1.5mm). The unit is differentiated from the andesite lava unit (Ta) by its subordinate abundance and size of the plagioclase phenocrysts relative its ferromagnesian phases. The matrix of the lavas is typically microcrystalline, but locally aphanitic, and ranges from vesicular to massive. Individual flow units range between 2 to 20m thick. The flow tops are defined by reddish scoriaceous zones that are commonly infiltrated with red volcaniclastic sandstone, and inmany areas, thin intervals of volcaniclastic sandstone occur between the flows. Compositionally, the lavas range from true basalt to trachybasalt and basaltic trachyandesite. A whole rock K-Ar date of 21.0 + 0.5 Ma (Jennison, 1976) was obtained for this unit from a sample in the Owl Head Buttes area. (0-600 meters thick)

QoQoÕ

Early Pleistocene alluvium (~750 ka to 2 Ma) Moderately to deeply dissected relict alluvial fans remnants in the Owl Head Buttes area. Qo surfaces are highly eroded, rounded ridges with up to 10 m of active channel incision. Qo deposits consist of cobbles, boulders, and sand and finer clasts with soil development similar to Qmo soils. Qo surfaces vary from red where the argillic horizon is preserved, to light colored where carbonate-rich fragments derived from the underlying petrocalcic horizon litter the surface. QoÕ is a thin veneer of well-preserved Qo alluvium over bedrock pediments. Qo surfaces record the highest levels of aggradation in the area and are probably correlative with other high, remnant surfaces found at various locations throughout southern Arizona (Menges and McFadden, 1981; Pearthree and Calvo, 1987; Menges and Pearthree, 1989; Skotnicki, 2000). Qo surfaces support sparse vegetation including palo verde, mesquite, ocotillo, cholla and prickly pear cacti.

Qm

Qm£QmÕ

QmÛ

Middle Pleistocene alluvium (~130 to 750 ka) Qm consists of moderately to highly dissected relict alluvial fans and terraces with strong soil development found throughout the map area. Well-preserved, planar Qm surfaces are smooth with loosely to moderately packed gravel and cobble lags; surface color is reddish brown; rock varnish on surface clasts is typically orange or dark brown. More eroded, rounded Qm surfaces are characterized by scattered cobble lags with moderate to strong varnish and broad ridge-like topography. Qm surfaces have a distinctive dark red color on color aerial photos, reflecting reddening of the surface soil and surface clasts. Soils typically contain reddened, clay argillic horizons, with obvious clay skins and subangular to angular blocky structure. Underlying soil carbonate development is typically stage II-III, with abundant carbonate through at least 1 m of the soil profile; indurated petrocalcic horizons are rare. Qm alluvium derived from granitic rocks tend to be smooth gravel surfaces with little to no varnish on surface clasts, and lesser amounts of carbonate accumulation through the soil profiles. QmÕ consists of thin, discontinuous pockets of Qm alluvium over bedrock pediments. Outcrops of bedrock are common in these areas. Qm£ surfaces are found on the lower western piedmonts. They are typically incised two to four meters and form isolated, well rounded ridges with coarse gravel and cobble lags. These surfaces are typically higher and more incised than surrounding Ql surfaces, with coarser lag and redder soil. However, soil development is not as strong as a typical Qm soil. QmÛ are dark, well rounded alluvial fans found along the base of Owl Head Buttes. The lag on these fans are mainly composed of volcanic clasts. Qm surfaces generally support mesquite, palo verde, ironwood, creosote, cholla, prickly pear, and sometimes ocotillo.

QmoQmoÕ

Middle to Early Pleistocene alluvium (~500 ka to 1 Ma) Moderately to deeply dissected relict alluvial fans. Qmo forms broadly rounded ridges that are higher than adjacent Qm surfaces but not as high or eroded as Qo surfaces. Tributary drainage networks are incised 3 to 6 m, increasing towards the mountains. Qmo soils are very well developed with a distinct dark red (5-2.5 YR), heavy clay argillic horizon and subangular blocky to prismatic structure. Carbonate accumulations are 2-4 m thick and range from stage III - V. QmoÕ consists of thin, discontinuous pockets of Qmop alluvium over bedrock pediments. Qmo surfaces support sparse vegetation including palo verde, mesquite, ocotillo, cholla and prickly pear cacti.

Ql Late Pleistocene alluvium (~10 to 130 ka) Broadly rounded to rounded and moderately dissected relict alluvial fans and terraces. Surfaces are 1 to 2 m of active channels with relict bar and swale morphology preserved. Deposits consist of gravel, cobbles, and finer-grained sediments. Ql surfaces commonly have loose, open lags of pebbles and cobbles; surface clasts may exhibit weak rock varnish. Ql soils are brown to strong brown (7.5YR), weakly to moderately developed, small to moderate subangular blocky strucure and stage I-II calcium carbonate accumulation. Vegetation includes triangle leaf bursage, creosote bush, cholla, prickly pear, barrel cacti, saguaros, ironwoods, and palo verde trees.

Tcd Dacite clast conglomerate - breccia (Miocene) Dominantly monomict dacite clast conglomerate and breccia, typically thick-bedded to very thick-bedded or massive with poorly defined bedding. Clasts are angular to sub-rounded, very poorly-sorted and suspended in a sandy volcaniclastic matrix. Clast-supported and matrix-supported units are both present. The unit occurs in at least two sequences interbedded with mafic lava flows to the north of the Owl Head Buttes. The sequences become thinner and finer grained to the north where they grade into polymict conglomerate and sandy conglomerate units. The polymict varieties contain up to 30% clasts of andesitic lava, basaltic lava and coarse-grained, equigranular to potassium feldspar porphyritic granite. To the south, the sequences become thicker as they grade into monomict lava breccia associated with dacite lava flows. (0-120 meters thick)

Td Dacite lava (Miocene) 5-10% plagioclase-porphyritic, biotite-phyric dacite lava. The unit occurs as two massive to autobrecciated flows that make up the northern Owl Head Buttes, and which grade to the north into monomict dacite clast breccia and conglomerate. Compositionally, the dacite lava unit ranges from trachyte to rhyolite. (0-200 meters thick)

Tt Welded rhyolitic tuff (Miocene) Moderately welded rhyolitic ash-flow tuff containing approximately 10% phenocrysts of plagioclase, sanidine, quartz, and biotite with up to 10% lithic fragments of mafic to intermediate volcanic rocks. Color ranges from light peach to red. In some areas, a thin-bedded nonwelded tuff interval less than a few meters thick occurs at the base of the unit. The unit is identical petrographically to a welded rhyolitic tuff in the Cloudburst volcanics of the Fortified Peak area (Orr and others, 2002). (0-15 meters thick)

Tf Granite of Fresnal Canyon, composite unit (Tertiary) A mixed body of medium- to coarse-grained leucogranite with abundant (>5%) pegmatite. The leucogranite ranges from muscovite-garnet bearing to muscovite-biotite and magnetite, + /-garnet bearing. Pegmatite, which is typically an alkali feldspar granite with minor muscovite, garnet and rare biotite, occurs as irregular dikes and pods with overlapping cross-cutting relationships with the leucogranite. The unit was previously know as the quartz monzonite of Samaniego Ridge (Banks and other, 1977), and the pluton of Cottonwood Canyon (Banks, 1980).

Tmd Mafic dikes (Mid-Tertiary) Fine- to medium-grained diorite, variably foliated. Forms northeast-striking dikes in the southwestern Tortolita Mountains.

Ttm Granite of Tortolita Mountains (Tertiary) Medium-grained, equigranular granite containing 5-12% biotite. The granite occurs as a small pluton in the southeastern Tortolita Mountains, as a swarm of dikes that invade the pluton of Wild Burro Canyon northwest of Wild Burro Canyon, and sparse dikes in the southeast corner of the map area. This unit was previously referred to as the quartz monzonite of Tortolita Mountains by Banks (1980), and it correlates with the Tgf unit of Skotnicki (2000). A K-Ar biotite cooling age of 22.7 +/- 0.7 Ma was btained from this unit near the center of its pluton in the southeastern Tortolita Mountains (Creasy and others, 1977).

Tgp Granite porphyry dikes (Mid-Tertiary) Medium-grained syenogranite to alkali feldspar granite with less than or equal to 5% biotite. Dikes ranging in thickness up to 50 meters but generally less than 15 meters thick. The dikes are consistently northwest-striking and nearly vertical. The dikes are also characteristically unaffected by the ductile deformation that is pervasive in all host rocks. Texturally, the dikes change from potassium feldspar porphyritic along margins to medium-grained, equigranular in the core. Graphic texture and myrmekitic texture is also very common in these dikes. A K-Ar biotite cooling age of 24.0 +/- 0.5 Ma (Banks and others, 1978) was obtained from a dike of this unit near Bass Spring in the central Tortolita Mountains.

Tf-Tgd Granite of Fresnal Canyon with less than 65% pluton of Chirreon Wash enclaves (Tertiary) A composite unit consisting of pluton of Chirreon Wash intruded by abundant dikes of the granite of Fresnal Canyon.

Tgd-Tf Pluton of Chirreon Wash with between 5-35% granite of Fresnal Canyon dikes (Tertiary) A composite unit consisting of the pluton of Chirreon Wash intruded by abundant dikes of the granite of Fresnal Canyon.

Tgd Pluton of Chirreon Wash (Tertiary) An extensive pluton consisting of a main phase of medium-grained, equigranular granodiorite containing 15-30% ferromagnesian minerals dominated by subequal amounts of biotite and hornblende, and minor clinopyroxene, and opaque minerals. The pluton is pervasively weakly foliated with a consistent moderate northwesterly dip. To the southeast, zones with minor potassium feldspar porphyritic texture are present. Subordinate mafic and leucocratic phases are present in the east in the vicinity of Derrio Canyon and near the head of Wild Burro and Cochie canyons. The unit was previously referred to as the granodiorite of Derrio Canyon by Skotnicki (2000). A K-Ar biotite cooling age of 25.1 +/- 0.9 Ma was obtained from this unit on the eastern piedmont of the Tortolita Mountains (Banks and others, 1978).

TXg Aplite and granitoid of Desert Peak (Tertiary, Cretaceous or Proterozoic) Mixed aplite and granitoid. Country rock for aplite dikes is very poorly exposed, generally appears to be fine-grained heterogranular granitoid, containing 2-3% biotite. The rock typically contains a weak grain-shape fabric, generally with lineation stronger than foliation. The granitoid phase in the outcrops at the southern end of Desert Peak has light-colored granitoid bands 2-5 cm thick, spaced 20-100 cm that give the outcrop a vaguely gneissic appearance. The granitoid phase is distinctily lighter colored than the granodiorite phase of the Aplitic granitoid and granodiorite of Desert Peak unit. Aplite dikes are the most prominent component of the unit, and dominate the outcrop and float, making estimation of the relative abundance of aplite and granite difficult to estimate. Contacts are gradational, interleaved.

TXgd Granodiorite of Desert Peak (Tertiary, Cretaceous or Proterozoic) Medium gray, texturally variable granodiorite with less than 25% aplite, pegmatite and felsite dikes. The most common phase contains plagioclase up to 5-7 millimeter in diameter, with 10% biotite in 1 millimeter diameter flakes, 2-3% hornblende in 1-2 millimeter long prisms, 25-30% anhedral quartz. Rock ranges from fine-grained (1 millimeter grain size) to coarse, porphyritic granodiorite with 1 cm diameter white feldspar (plagioclase?). Texture of rock is granoblastic, heteroblastic, looks like recrystallized igneous texture.

TXga Aplitic granitoid and granodiorite of Desert Peak (Tertiary, Cretaceous or Proterozoic) Similar to the Aplite and granitoid of Desert Peak, but the host rock for the aplite dikes is the granodiorite of Desert Peak. Contact with the Aplite and granitoid unit is gradational, interleaved.

TXgg Mixed gneiss and granodiorite of Desert Peak (Tertiary, Cretaceous or Proterozoic) Heterogeneous unit of meta-granodiorite and gneiss intruded by felsite, aplite, and pegmatite dikes. Granodiorite phase is lithologically indistinguishable from granodiorite of Desert Peak unit (see description of that unit). Granodiorite irregularly intrudes fine-grained, medium gray equigranular quartz-feldspar-biotite psammitic semi-schist. Semi-schist may have Pinal schist protolith, but contains 5-7% biotite and no muscovite. Aplite and pegmatite dikes cut gneissic foliation, felsite dikes cut both. The aplite locally grades to pegmatite and to white felsite. The white felsite has a creamy aphanitic groundmass with sparse 1 millimeter quartz or feldspar crystals. Aplite or pegmatite dikes are locally subconcordant to the gneissic foliation, giving the rock a strongly banded appearance. This unit is distinguished from the granodiorite of Desert Peak because is is more heterogeneous, and from the aplite and granodiorite of Desert Peak unit by the presence of >25% gneissic component. Contacts between these units are interleaved, gradational.

QyÈ Modern river channel deposits (< ~1 ka) Active channel deposits composed of well-sorted sand and gravel to poorly-sorted sand, gravel and cobbles in the steeper channels. Channels are generally incised less than 1 m below adjacent terraces and bars, but locally incision may be as much as 2 m. Channel morphologies generally consist of a single thread high flow channel or multi-threaded low flow channels with gravel bars. Channels are extremely flood prone and are subject to deep, high velocity in moderate to large flow events, and severe lateral bank erosion.

Qy Holocene alluvium, undifferentiated (<~10 ka) Includes QyÈ, Qy£, and/or Qy¢ deposits. Used where not possible, at this scale, to map surfaces separately. Unit Qy consists of smaller incised drainages in the mountains and on the upper piedmonts.

Qy¢ Holocene alluvium (~2 to 10 ka) Low terraces found at scattered locations along incised drainages and broad alluvial fans on the western edge of map area. Qy¢ terraces are 1 to 2 m above adjacent active channels and less subject to inundation than adjacent Qy£ terraces. Qy¢ surfaces are generally planar but fans may be incised up to 2 m. Deposits typically are sandy but locally have unvarnished, open, fine gravel lags. Soil development is minimal with weak subangular blocky structure and no to weak (stage 1) carbonate accumulation (see Machette, 1985, for description of stages of calcium carbonate accumulation in soils). Yellow brown (10YR) soil color is similar to original fluvial deposits. Qy¢ surfaces mainly support creosote bush and cholla.

Qly Holocene to Late Pleistocene alluvium (~2 to 130 ka) Terraces and broadly rounded alluvial fan surfaces less than 2 m above active channels. Qly is composed of mixed late Pleistocene (Ql) and Holocene (Qy¢ and Qy£) alluvium. Qly areas are covered by a thin veneer of light brown, fine-grained Holocene alluvium and scattered fine gravel lag, with reddened late Pleistocene alluvium exposed in patches on low ridges, in roads and wash cut banks. Drainage networks are a mix of distributary channel networks on the lower piedmonts and tributary channels on the upper piedmonts. Vegetation includes triangle leaf bursage, creosote bush, cholla, prickly pear, barrel cacti, saguaros, ironwoods, and palo verde trees.

Qy£ Late Holocene alluvium (< ~2 ka) Channels, low terraces, and small alluvial fans composed of gravel, sand, and silt, with cobbles and boulders recently deposited by modern drainages. Includes QyÈ where not mapped separately. Near mountain fronts, channel sediment is generally sand and gravel, but may include gravel, cobbles and boulders; terraces typically are mantled with sand and finer sediment. On piedmont areas, young deposits are mainly sand and silt, with some gravels and cobbles in channels. Channels generally are incised less than 1 m below adjacent terraces and fans, but locally incision may be as much as 2 m. Channel morphologies consist of a single-thread high flow channel or multi-threaded low flow channels with gravel bars adjacent to low flow channels. Downstream branching distributary channel patterns - small, discontinuous, well-defined channels alternating with broad expansion reaches where channels are very small and poorly defined - are typically on the western piedmont. Local relief varies from fairly smooth channel bottoms to the undulating bar-and-swale topography that is characteristic of coarser deposits. Channels are flood prone and may be subject to deep, high velocity flows in moderate to large flow events. Potential lateral bank erosion is severe. Flood flows may significantly change channel morphology and flow paths. Terraces have planar surfaces, but small channels are common. Soil development associated with Qy£ deposits is minimal. Vegetation along Qy£ channels includes ironwood, mesquite and palo verde, along with several shrub species.

Quaternary and Late Tertiary map unitsUnconsolidated deposits

Xp Pinal Schist (Paleoproterozoic) Pinal Schist consists of thin-bedded to laminated siltstone and sandstone intebedded with mudstone. These rocks are metamorphosed to slate and phyllite, but in proximity to intrusive contacts with the Oracle Granite, the rock is metamorphosed to medium- to coarse-grained, locally porphyroblastic sericitic schist and psammitic schist. Rectangular porphyroblasts up to 1cm are totally replaced by fine-grained aggregates of quartz, sericite, and chlorite. Slate and phyllite locally preserve primary sedimentary structures such as ripple cross-lamination, graded bedding and flute molds, suggesting that the rocks are part of a turbidite succession.

Early Proterozoic map units

Yd Diabase (Mesoproterozoic) Mafic dikes and pods displaying diabase texture or interpreted to be associated with nearby mafic dike with diabase texture. This unit is only recognized north of the Carpas Wash shear zone.

Yo Oracle Granite (Mesoproterozoic) Medium- to coarse-grained, potassium feldspar porphyritic, biotite granite. This map unit includes fairly extensive zones of medium- to coarse-grained, equigranular granite with biotite and muscovite in the vicinity of intrusive contacts with the Pinal Schist.

Middle Proterozoic map units

Pzq Quartzite (Paleozoic) Thin- to thick-bedded or banded gray vitreous quartzite with sparse interbeds of calc-silicate schist and rusty sericitic schist.

Paleozoic map units

GEOLOGIC MAP UNITS