Arizona Bureau of Geology and Mineral Technology

IELDNOTE~Fall 1987

u.s.Air Force,high-aititude,0.2 photo of part ofthe Grand Canyon regionView is to thesouthwes~ from thewestern edge of the Kaibab Plateau(bottom of photo) toward the west-facingGrand Wash Cliffs, which mark the nonhem edgeof the Basin and Range Province (dark line in center,about 112 inch from top ofphoto). Havasu Canyon, the smaller,branching tribut.aJy that cuts horizontally across the center of thephoto, is incised into the Coconino Plateau. The dark area in the upperright comer is the Shivwits Plateau; Lake Mead isjust beyond i~ but not In view.

Arizona Geology: An Aerial Tour

by Peter L KresanUniversity of Arizona

Figure 2. In Marble Canyon, the ColoradoRiver cuts into the ledgy Supai Formationbeneath the slope.forming Hermit Shale and

16

2

9 64,5

~ • FLAGSTAFF

1i'4 "-" j:>1'oS'/~ 10... (4 ,.~101'~ 4(;

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Figure 1

From the air, the geologic fabric of Arizonais visible as "Iandprints" woven into beautifuland impressive designs. Patterns reflectstructure, rock type, and the geologic historywritten in stone. This photo essay takes you onan annotated journey over Arizona's diverselandscapes. It is by no means a complete tour,but it will introduce you to the incredibly richgeologic heritage of this region, as revealedfrom above. Figure 1 is an index map thatidentifies areas depicted in the photographs;numbers on the map correspond to figurenumbers.

Figure 2 Figure3b

2 FIELDNOTES, Fall 1987

Figure 4

overlying massive cliffs of Coconino Sand­stone, Toroweap Formation, and KaibabUmestone. Sheer Wall Rapids (river mile 14.4from Lee's Ferry) lie at the mouth of TannerWash, which merges with the Colorado River(left, center). The viewis to the south across theMarble platform.

Figures 3a and 3b. The western edge of theColorado Plateau in northwestern Arizona isshown in this U.S. Air Force, high-altitude, U-2_new to the north-northeast. The ColoradoPlateau to the east (right) is structurally simple,with nearly horizontal Paleozoic strata cut bynormal faults. The Grand Wash fault consistsof a system of en-echelon, high-angle, down-to­the-west normal faults. Displacement along thefault varies from a throw ofseveral hundred feetat the Utah border to the north to about 16,000feet (5,000 meters) at the mouth of the GrandCanyon and as much as 20,000 feet (6,000meters) in the Red Lake area to the south(Lucchitta and Young, 1986).

Figure 4. The Pleistocene basalt flow fromSP cinder cone spilled onto a multiple grabenfloor that was partially filled by a flow fromCrater 160, south of SP Crater and in line withSan Francisco Mountain. The overall thicknessof the SP flow ranges from 15 to 60 meters.Crater 160 is a cinder, tuff, and spatter conenoted for the abundance and variety ofxenoliths (Moore and others, 1974).

Figures 5aand 5b. The Mesa Butte fault, SPCrater and flow, and San Francisco Mountainare prominent features in this U.S. Air Force,high-altitude, U-2 photo of the San Franciscovolcanic field north of Flagstaff on the Colo­rado Plateau; the view is to the south. BillWilliams Mountain, Sitgreaves Mountain, andKendrick Peak are silicic to intermediate

_ .. eruptive centers along or near the Mesa Butte_fault. Red Mountain, Mesa Butte, and Shadow

Mountain (not shown on photo) are prominentbasaltic eruptive centers along the fault(Shoemaker and others, 1974).

FlFI nl'lCYrF" Fall 1987

Figure 5a

Figure5b

3

Figure 6a

Figure6b

4

The San Francisco volcanic field on thesouthern margin of the Colorado Plateau iscomposed of Quaternary and upper Tertiarybasaltic rocks erupted from vents marked bycinder cones. A series of intermediate to silici<jAeruptive centers also produced volcanoes o.considerable relief. The highest of thesecomposite volcanoes is the 12,670-footHumphreys Peak

Figures 6a and 6b. Mesozoic sedimentaryrocks predominate in this U.S. Air Force, high­altitude, U-2 view across northeastern Arizonafrom above Cameron. Lying at the northernend of the Mesa Butte fault, Shadow Mountaincinder cone is the northernmost isolatederuptive center of the San Francisco volcanicfield. One of the basalt flows has been dated byK-Ar at 0.62 ± 0.23 m.y. B.P. Flows arepenecontemporaneous with faulting in theunderlying Chinle Formation, the varicoloredTriassic mudstone that forms the PaintedDesert (Condit, 1974).

Unobstructed by topographic barriers anddriven by strong, year-round, west-southwestwind, sand climbs the Red Rock Cliffs andforms a pattern of linear dunes, which streakacross the Moenkopi Plateau (Breed andothers, 1984). The entire Glen Canyon Groupis exposed in the Echo Cliffs. Coal Can­yon" is incised into units of the Jurassic SanRafael Group and Upper Cretaceous coal­bearing Dakota Sandstone. CretaceousDakota Sandstone, Mancos Shale, and MesaVerde Group are widely exposed in BlackMesa, the location of Arizona's major cresources.

Figure 7. About 200 meters of erosionexposed the plumbing of the approximately30-m.y.-old Agathla Peak diatreme nearKayenta in northeastern Arizona. The volcanicneck chiefly consists of breccia with branchingdikes and sheets of igneous rock There areseveral hundred well-exposed diatremes inArizona, most of which are in the Navajo andHopi volcanic fields. The edge of Black Mesa

Figure

DN()TES. Fall 1987

Figure 8

cuts across the horizon in this low-altitude,aerial oblique view to the south (Fitzsimmons,1973; Sheridan, 1984).

a Figure 8. Sculptured from mostly Permian• sedimentary rocks involved in the Monument

upwarp, these spires, buttes, and mesasconstitute Monument Valley north of Kayentain northeastern Arizona. This low-altitude, aerialoblique view pans north across the TotemPoles and Spearhead Mesa to the Mittens area.De Chelly Sandstone forms the magnificentcliffs. The slopes beneath the massive cliffs areOrgan Rock Shale. The very uppermost capsof many of the buttes north of the Totem Polesare composed of thin remnants of MoenkopiFormation overlain bythe Shinarump Memberof the Chinle Formation; both are of Triassicage (Baars, 1973).

Figure 9. Entrenched meanders of the LittleColorado River form a spectacular gorge westof Cameron. Arizona State Highway 64 and theslope of the Grand View monocline cut acrossthe upper right (west) corner of this aerial viewto the south. Permian Kaibab Limestone is therimrock, underlain by Toroweap Formationand Coconino Sandstone deep in the gorge.

Figure 10. The erosionally ernbayed Mogol­lon Rim at Oak Creek Canyon near Sedonamarks the southern edge of the ColoradoPlateau. Oak Creek Canyon in the upper left(west) corner extends north toward SanFrancisco Mountain near Flagstaff. The light-

._.... colored cliffs in the foreground are Permian.sedimentary rocks of the Supai Group,

- Coconino Sandstone, Toroweap Formation,and Kaibab Limestone. Late Tertiary flows atthe southern margin of the San Francisco

AELDNOTES, Fall 1987

Figure 9

volcanic field cap the Permian sedimentaryrocks along the rim. On the east side of OakCreek Canyon and exposed in Munds Canyon(cutting diagonally across the upper center ofthe photo), the flows form a significantlythickersequence. One interpretation is that the flowsand underlying late Tertiary gravels filled alarge erosional reentrant carved into the

Figure 10

ancestral plateau, one margin of which wasmarked by the preexistent Oak Creek fault(Peirce and Nations, 1986).

Figure 11. Weaver's Needle, a flow remnantin the Superstition Mountains east of Phoenix,is shown in this aerial view to the north. Thehighly jointed rock in the foreground is the 22-

5

Figure 11

to 25-m.y.-old Superstition welded tuff. Thespire is composed of glassy latitic lava.Between these two units is a stratified laharicbreccia. Threevolcanic centers are identified inthe western part of this volcanic field.Remnants of middle Tertiary ash-flow sheetsblanket a large area of central and southeast­ern Arizona (Sheridan, 1987; oral commun.,1987).

Figure 12. Picacho Peak, a prominentlandmark in southeastern Arizona, is a tilted(dipping to the left or northeast) and faultedremnant of a lower Miocene volcanic complex.Thevolcanic rocks are potassium rich becauseof postvolcanic alteration and are locallyinterlayered with conglomerate (Shafiqullahand others, 1976).

Interstate Highway 10 cuts diagonallyacrossthe center of view. The Tortolita Mountains arein the background and the Santa CatalinaMountains are on the horizon. This aerial viewis to the southeast.

The Picacho Peak rocks are a remnant ofthe deformed upper plate of a detachmentfault that separates them from lower-platerocks exposed in the Picacho Mountains to thenorth (left, but not in view).

Figure 13. This view is to the northeastalong the Santa Catalina core complex nearTucson. The relatively planar slope along thesouthern (right) flank of the mountainexpresses the dip slope of Tertiary myloniticfoliation. The mylonitic fabric was formed bytop-to-the-southwest shear.

Pusch Ridge in the foreground is composedof layers of the early Tertiary leucocraticWilderness granite and darker layers ofmylonites derived from the 1A-b.y.-old OracleGranite. Further along Pusch Ridge, AlamoCanyon marks the gradational lower boundaryof the mylonitic zone. The Catalina detach­ment fault follows a sinuous trace along thesouthern (right) edge of the mountain frontand has a shallow dip to the south and west.

Mount Lemmon in the upper left at the crestof the range consists of Cambrian andPrecambrian metasedimentary rocks that dipto the northeast. The Pinaleno Mountains (lefthorizon) are another metamorphic corecomplex. The Galiuro Mountains between theSanta Catalina and Pinaleno Mountainscontain middle Tertiary volcanic rocks.

Figure 14. The incised Madera Canyonalluvial fan skirts the west flank of the Santa RitaMountains south of Tucson. This aerial view isto the east. Sonoita basin is in the upper leftcorner and the Huachuca Mountains are onthe horizon just left of center.

The lighter, less dissected slope is consid­ered to be middle Pleistocene. The moredissected center portion of the fan is older. Thedark lines that cut the younger left edge of thefan are Quaternary fault scarps. Based onscarp morphology and soils, the youngestrupture may have occurred 8,000 to 15,000years ago (Ely and Baker, 1985, p. 55).

Figure 15. This aerial view is to the south-

Figure 12

east across the community of Green Valley,south of Tucson. The Pima-Mission open-pitcopper mine and tailings piles sit perched onthe broad bajada surrounding the SierritaMountains. The copper is disseminated inskarn associated with a Laramide porphyriticintrusive. The Madera Canyon alluvial fan at thebase ofthe Santa Rita Mountains is in the upperright (south) corner. The Huachuca Mountainsare visible in the center of the horizon.

Figure 16. In this aerial view toward thenorth along the southeastern Arizona­southwestern New Mexico border, the SanBernardino Valleyjoins the southern end of theSan Simon Valley. This valley is bounded byrelatively uplifted mountain blocks of thePerilla-Pedregosa-Chiricahua Mountains on thewest (left) and the Peloncillo Mountains on theeast (right).

Paramore Crater (center) with the tuff ring isone of five steam-blast explosion features in the

Figure 13

6

Figure 14

AELDNOTES, Fall 1987

Figure 15 Figure 16

San Bernardino volcanic field. The alkali olivinebasalts in this field contain large amounts ofxenolithic material, including lherzolite andpyroxenite nodules. San Bernardino basaltshave been radiometrically dated as latePliocene or Pleistocene (Lynch, 1978).

Figure 17. This low-altitude aerial view to thesouth shows exposures of the light-colored,lower Tertiary Gunnery Range granite beingexhumed from beneath a faulted cover of 16­to 17-my.-old dark andesites of the CabezaPrieta Mountains in southwestern Arizona. Inthe foreground, the granite outcrops in Albasin. Cabeza Prieta Peak is the andesite-_apped peak just below the horizon in theupper right corner. The broad shield of thePinacate volcanic field in northwestern Sonora,Mexico is in the center of the horizon (Shafiqul­lah and others, 1980).

Figure 18. The Pinacate volcanic field innorthwestern Sonora, Mexico is one of theyoungest ahd most spectacular lava fields inNorth America. This low-altitude aerial view tothe north shows the central peaks area.Pinacate Peak (left of center) is the highestpoint.

The volcanic complex consists of a smallshield-type volcano with some 500 eruptivecenters, including more than 300 cindercones, along its flanks. Volcanic activity hasbeen continuous but episodic during the last1.2 my. and consisted of two distinct periodsof volcanism. FIrst, the Volcan Santa Claratrachyte shield volcano was built by successiveeruptions through a central summit ventcomplex. Second, smaller, individual basalticcones and lava flows were erupted on theslopes of Santa Clara and extended out ontothe desert surrounding the central ventcomplex (Lynch, 1981).

Acknowledgments

~ The author thanks John Sumner, whoseWalents as a pilot made these photos possible;

Dan Lynch andTad Nichols for their assistance

(continued on page 10)

Figure 17

Figure 18

Fall 1987 7

Geologic Highlights of the Phoenix Region

by Stephen J. ReynoldsArizona Geological Survey

The Phoenix region contains a diverse andspectacular array of geologic features, most ofwhich are easily accessible by short trips in apassenger car. Such features vary fromunusual divergent Holocene terraces along theSalt River to Proterozoic volcanic rocks thatrepresent the initial construction of thecontinental crust of the region. In addition, thearea contains classic examples of middleTertiary volcanic centers, detachment faults,and metamorphic core complexes. This articlesummarizes the geologic history of Arizonaand briefly discusses some of the moreinteresting and accessible geologic featuresnear Phoenix.

GEOLOGIC HISTORY OF ARIZONA

Arizona encompasses parts of threegeologic-physiographic provinces: the Colo­rado Plateau, Basin and Range Province, andintervening Transition Zone (Figure 1). TheColorado Plateau in northeastern Arizona ischaracterized by flat-lying Paleozoic andMesozoic sedimentary rocks that form broadplateaus and mesas, locally interrupted bydeeply incised canyons cut by drainages of theColorado River system. In the bottom of theGrand Canyon, the Paleozoic and Mesozoicrocks have been eroded, exposing the under­lying Proterozoic basement The topographyofthe Colorado Plateau commonly mimicsregional structural features, especially upliftsbounded by monoclinal folds or by faults. Thehighest topographic features in the region arepartially eroded late Cenozoic volcanoes thatform San Francisco Mountain near Flagstaffand the White Mountains of east-centralArizona.

The Basin and Range Province in southernand western Arizona contains a series of north­to northwest-trending mountain ranges sepa­rated by broad alluvial valleys. The rangesinclude a diversity of Proterozoic to Cenozoicrocks with complex structural and metamor­phic histories. The intervening basins containvariable thicknesses of upper Cenozoic clasticdetritus and locally thick, nonmarine evaporitedeposits; they are the primary ground-waterreservoirs of the region.

The Transition Zone, as the name implies,has geologic and physiographic characteris­tics intermediate between those of the Colo­rado Plateau and Basin and Range Province. Inthis region, pre-Miocene erosion has strippedaway much of the Paleozoic and Mesozoicsedimentary cover, leaving wide expanses ofexposed Proterozoic crystalline rocks. TheProterozoic rocks are commonly overlain bymiddle to upper Cenozoic sedimentary andbasaltic rocks. The structural continuity ofthese rocks is disrupted by northwest-trendinglate Cenozoic basins that are narrow analogs ofthose in the Basin and Range Province.

8

The geologic history of Arizona began withan Early to Middle Proterozoic (1.8 to 1.4 b.y.ago) period of crustal construction andeventual stabilizationvia volcanism, sedimenta­tion, deformation, metamorphism, and pluton­ism. During Late Proterozoic (1.4 to 1.1 b.y.ago) and Paleozoic time, carbonate and clasticrocks accumulated in a stable cratonicenvironment close to sea level.

In Late Triassic to Early Jurassic time,southern Arizona became tectonically active inresponse to subduction and deformationalong the southwestern margin of NorthAmerica. A pulse of volcanism and associatedplutonism occurred in southern Arizonaduring Early to Middle Jurassic time and wasfollowed by a period of continental to shallow­marine sedimentation during Late Jurassic tomid-Cretaceous time. The Colorado Plateauremained tectonically stable during this time,but periodically received influxes of orogenicdetritus from the south. In Late Cretaceous toearly Tertiary (Laramide) time, intense com­pressional deformation caused folding andbasement-involved thrusting in southernArizona and formed monoclines as Paleozoicand Mesozoic strata were draped over reacti­vated Proterozoic faults inthe Colorado Plateauand Transition Zone. Deformation was accom­panied by locally extreme metamorphism andby granitic plutonism associated with largeporphyry-type copper deposits.

Laramide tectonism was followed by aperiod of widespread early Tertiary uplift anderosion that denuded southwestern Arizona ofall shallow-level rocks. This erosion wassucceeded by a tremendous outpouring ofvolcanic rocks during middle Tertiary time (38to 15 m.y. ago). Volcanism was accompaniedby granitic plutonism and by crustal extensionthat formed regional, gently dipping normalfaults, termed detachment faults, and asso­ciated mylonitic fabrics within metamorphiccore complexes. Subsequent to middleTertiarytectonism, late Cenozoic blockfaultingformed many, but not all, of the present-daybasins and ranges. Some present-day basinsare relics from the middle Tertiary period ofextensional faulting. Faulting was concurrentwith late Cenozoic, fundamentally basalticvolcanism in the Transition Zone and along themargins of the Colorado Plateau. In lateTertiary and Quaternary time, previouslyunconnected basins with interior drainagebecame integrated into the Colorado and GilaRiver drainage system.

GEOLOGIC SETTING OF PHOENIX

Phoenix is in the Basin and Range Province,50 kilometers southwest of the TransitionZone. Uke most of central Arizona, the Phoenixregion lacks Paleozoic and Mesozoic rocks,which were eroded from the area beforemiddle Tertiarytime (Figure 1). Most mountainranges in the area consist of Proterozoic

crystalline rocks overlain by middle Tertiaryvolcanic and sedimentary rocks. Early andmiddle Tertiary granitic plutons are exposed inthe South and White Tank Mountains southand west of Phoenix. These same two moun­tain ranges are termed metamorphic corecomplexes because they contain myloniticfabrics formed by middle Tertiary ductileshearing along a regional, gently dippingnormal fault (or detachment fault). This faultdips gently to the northeast off the South andWhite Tank Mountains, projecting beneathrocks exposed closer to Phoenix. Rocks abovethe detachment fault, such as those atCamelback Mountain, Papago Park, andTempe Butte, were broken into tilted faultblocks as they were transported to the north­east relative to rocks beneath the fault

After middle Tertiary detachment faulting,the Phoenix region was the site of late Tertiarynormal faulting that created deep basins filledwith several kilometers of nonmarine clasticrocks and evaporite deposits (Peirce, 1976).Examples are the Luke basin west of Phoenixand the Higley basin near Chandler. ByPliocene time, the previously closed basinsbecame integrated with the Salt and Gila Riverdrainage network

GEOLOGIC FEATURESNEAR PHOENIX

Camelback Mountain, Papago Park,Tempe Butte

Camelback Mountain, in the northeasternpart of metropolitan Phoenix, is an erosionalremnant of a large fault block tilted during themiddle Tertiary episode of crustal extension.The mountain is composed of Proterozoicgranite, overlain to the southwest by west­dipping, middle Tertiary sedimentary brecciaand other clastic rocks (Cordy, 1978). Thebasal contact of sedimentary rocks is generallya fault that probably formed while the bedswere tilted to the west. The middle Tertiarysedimentary rocks are well exposed near EchoPark and along residential roads on the southside of the "Camels Head," where they are cutby spectacular clastic dikes of remobilized faultgouge.

Similar middle Tertiary sedimentary brec­cias and clastic rocks form the reddish buttesand hills at Papago Park, near the City ofPhoenix Zoo (Pewe and others, 1987). Here,the sedimentary rocks are in depositionalcontact with underlying Proterozoic granite(such as on the east side of Hole in the Rocknorth of the zoo) and dip moderately to steeplyto the southwest. Large, well-exposed sheets ofmegabreccia, which represent shclUe,reclandslide blocks of Proterozoic granitemetarhyolite, are present within the tiltedsequence of sedimentary breccias northwestof the zoo (Reynolds and Uster, 1987).

FlELDNOTES, Fall 1987

9

Hgure 1. Generalized geologic map of the Phoenix area, centml Arizona

GRANITIC ROCKS (Proterozoic)

METAMORPHIC ROCKS (Proterozoic)

CONTACT

FAULT

CONCLUSION

10 0 10 20 MILES'~=~~--~'---'

REFERENCES

Capps, R. C, Reynolds, S. J., Kortemeier, K. C, and Scott,E A, 1986, Geologic map of the northeastern Hiero­glyphic Mountains, central Arizona: Arizona Bureau ofGeologyand Mineral TechnologyOpen-File Report 86-1 0,16 p., scale 1:24,000.

MYLONITIC FABRIC (Tertiary)

Tertiaryvolcanic rocks and overlying conglom­erate and volcanic-sedimentary breccia (Sheri­dan, 1987). The volcanic rocks, like many inthe region, are commonly tilted and cut bysteeply to gently dipping normal faults. Theregion contains one or more volcanic centersand is best accessed via the Apache Trail(Arizona State Highway 88), a scenic, winding,partly unpaved road that passes from ApacheJunction through Tortilla Flat to RooseveltLake.

The Phoenix region is endowed with adiverse assemblage of rocktypes and geologicfeatures that are, partly because of the aridclimate, very well exposed. Many spectacularexposures are within short walking distancefrom paved roads. Examining a small selectionof these exposures provides an appreciation ofthe general geologic framework of centralArizona. It is an opportunity that should not bemissed.

-SURFICIAL DEPOSITS(Quaternary and Late Tertiary)

BASALT (Late Tertiary)

VOLCANIC AND SEDIMENTARYROCKS (Middle Tertiary)

GRANITIC ROCKS(Tertiary and ~retaceous)

mountain, contains well-traveled hiking trailsthat traverse a sequence of Proterozoicmetavolcanic and metasedimentary rocks,including andalusite-bearing and quartz·kyanite rocks (Thorpe and Burt, 1978). Thepeculiar silica- and alumina-rich compositionof these rocks probably reflects hydrothermalalteration by acidic fluids that leached allmobile elements, such as potassium andsodium, either before or during regionalmetamorphism.

Salt River Area

The Salt River, one of the major rivers inArizona, flows westward across the Phoenixbasin from the high country to the east. TheSalt River is flanked by well-preserved terracesthat are close in elevation near Tempe, butdiverge in elevation upriver to the east (Pewe,1987). South of the Salt River, near Chandler,are excellent examples of earth cracks: largeopen fissures formed by differential subsi­dence due to ground-water withdrawal.

Superstition and Goldfield Mountains

The main geologic attraction in thesemountain ranges, besides the famed LostDutchman mine, is a thicksequence of middle

r+++++3++++++

White Tank Mountains

_ The White Tank Mountains west of Phoenixare geologically very similar to the SouthMountains in that they contain mylonitic fabricand superimposed breccia formed below aneast-dipping detachment zone (Rehrig andReynolds, 1980). The mylonitic fabrics cutProterozoic metamorphic rocks and Tertiaryintrusive rocks and are well exposed nearpaved roads in a regional park on the east sideof the range.

A stratigraphically higher sequence of finergrained reddish clastic rocks is present on thenorth flank of Tempe Butte near Arizona StateUniversity (Pewe and others, 1987). These

Arocks contain well-preserved sedimentary.,structures, including small folds formed during

sedimentation, and are capped bya 17-m.y.-oldvolcanic flow.

Spectacular Proterozoic and middle Tertiarygeology forms the scenery around LakePleasant and the Hieroglyphic Mountainsnorthwest of Phoenix. Exposed on the westshore of Lake Pleasant and to the north alongthe dirt road to Castle Hot Springs are east­dipping middle Tertiaryvolcanic rocks overlainby conglomerate and sedimentary brecciadeposited in half grabens formed duringmiddle Tertiary crustal extension (Capps andothers, 1986). Silicic flow breccias composethe high, sheer walls of Crater Canyon, directIywest of the Castle Hot Springs resort. The largeconstruction project downstream from LakePleasant is the planned New Waddell Dam,part of the federally funded Central ArizonaProject.

The South Mountains, easily accessed viaCentral Avenue and other roads south ofPhoenix, are a simple, but elegant example ofa middle Tertiary detachment zone or meta­morphic core complex (Reynolds, 1985;Reynolds and Uster, 1987). The eastern half ofthe range is composed of a middle Tertiarygranodioritic to granitic pluton, whereas thewestern half consists of Proterozoic gneiss.Both rock types are overprinted by a gently tomoderately dipping mylonitic fabric andyounger breccia. The mylonitic fabric andsuperimposed breccia were formed by top-to­the-northeast normal displacement along agently dipping shear zone, or detachmentzone, that progressively evolved from ductile tobrittle levels of the crust The detachment faultand associated breccia zone, exposed in theeastern and southern South Mountains,projects beneath and probably truncates thetilted fault blocks ofTertiary sedimentaryrocksat Papago Park, Tempe Butte, and vicinity.

South Mountains

Lake Pleasant Area

Phoenix Mountains

Phoenix Mountains consist of isolatedmountains in northern metropolitan

Peak, the most prominent

Cordy, G. E, 1978, Environmental geology of the ParadiseValley quadrangle, Maricopa County, Arizona; pt. II:Tempe, Arizona State University, MS. Thesis, 89 p.

Peirce, H W, 1976, Tectonic significance of Basin andRange thick evaporite deposits, in Wilt, J. C, and Jenney,J. P., eds., Tectonic digest: Arizona Geological SocietyDigest, v. 10, p. 325-339.

Pewe, T. L., 1987, Terraces of the lower Salt River Valley inrelation to the late Cenozoic history of the Phoenix basin,Arizona, in Davis, G. H., and VandenDolder, E M, eds.,Geologic diversity of Arizona and its margins; excursionsto choice areas: Arizona Bureau of geology and MineralTechnology Special Paper 5, p. 221·226.

Pewe, T. L., Wellendorf, C S., and Bales, J. T., 1987,Environmental geology of the Tempe quadrangle,Maricopa County, Arizona: Arizona Bureau of Geologyand Mineral Technology Folio Series No.2, scale1:24,000.

Rehrig, W A, and Reynolds, S. J., 1980, Geologic andgeochronologic reconnaissance of a northwest·trendingzone of metamorphic core complexes in southern andwestern Arizona, in Crittenden, M D., Jr., and others, eds.,Cordilleran metarnorphic core complexes: GeologicalSociety of America Mernoir 153, p. 131-157.

ReYnolds, S. J., 1985, Geology of the South Mountains,central Arizona: Arizona Bureau of Geology and MineralTechnology Bulletin 195,61 p.

Reynolds, S. J., and Uster, G. S., 1987, Field guide to lower·and upper·plate rocks of the South Mountains detach·rnent zone, Arizona, in Davis, G. H., and VandenDolder,E M, eds., Geologic diversity of Arizona and its margins;excursions to choice areas: Arizona Bureau of Geologyand Mineral Technology Special Paper 5, p. 244·248.

Sheridan, M E, 1987, Caldera structures along the ApacheTrail in the Superstition Mountains, Arizona, in Davis, G.H., and VandenDolder, E M, eds., Geologic diversity ofArizona and its margins; excursions to choice areas:Arizona Bureau of Geology and Mineral TechnologySpecial Paper 5, p. 238·243.

Thorpe, D. G., and Burt, D. M., 1978, Precambrianmetavolcanic rocks of the Squaw Peak area, MaricopaCounty, Arizona, in Burt, D. M, and Pewe, T. L., eds.,Guidebook to the geology of central Arizona: ArizonaBureau ofGeologyand MineralTechnologySpecial Paper2, p. 101·106.

10

(continued from page 7)

with the photography; Ivo Lucchitta at the U.S.Geological Survey in Flagstaff, who kindlyprovided the U-2 photos; and colleagues in theDepartment of Geosciences, University ofArizona, for their support.

References

Baars, D. L., 1973, Perrnianland; the rocks of MonumentValley, In James, H L., ed., Guidebook of MonumentValley and vicinity, Arizona and Utah: New MexicoGeological Society Guidebook, 24th Field Conference, p.68-71.

Breed, C S., McCauley, J. E, Breed, W. J., McCauley, C K.,and Cotera, A S., Jr., 1984, Eolian (wind·formed)landscapes, in Smiley, T. L., and others, eds., Landscapesof Arizona; the geological story: Lanham, Md., UniversityPress of America, p. 359413.

Condit, CD., 1974, Geology of Shadow Mountain, Arizona,in Karistrom, T. N. V., and others, eds., Geology ofnorthern Arizona, with notes on archaeology andpaleoclimate; pt. 2, area studies and field guides:Geological Society of America, p. 454463.

Ely, L L., and Baker, V. R, 1985, Geomorphic surfaces in theTucson basin, Arizona, a field guidebook: Dept. ofGeosciences, University of Arizona, and NationalAeronautics and Space Administration, 149 p.

Fitzsirnmons, J. P., 1973, Tertiary igneous rocks of theNavajo country, Arizona, New Mexico, and Utah, inJames, H L, ed., Guidebook of Monument Valley andvicinity, Arizona and Utah: NewMexico Geological SocietyGuidebook, 24th Field Conference, p. 106·109.

Lucchitta, Ivo, and Young, R A, 1986, Structure andgeomorphic character ofwestern Colorado Plateau in theGrand Canyon·Lake Mead region, in Nations, J. D., andothers, eds., Geology of central and northern Arizona:Geological Society of America, Rocky Mountain SectionMeeting, Guidebook, p. 159·176.

Lynch, D. J., II, 1978, The San Bernardino volcanic field ofsoutheastern Arizona, in Callender, J. E, and others, eds.,Land of Cochise, southeastern Arizona: New Mexico

Geological Society Guidebook, 29th Field Conference, p.261·268.

__ 1981, Genesis and geochronology of alkalinevolcanism in the Pinacate volcanic field, northwesternSonora, Mexico: Tucson, University of Arizona, Ph.D.Dissertation, 248 p.

Moore, R B., Ulrich, G. E, and Wolfe, E W, 1974, Field guideto the geology of the eastern and northern San Franciscovolcanic field, Arizona, in Karlstrom, T. N. V., and others,eds., Geology of northern Arizona, with notes onarchaeologyand paleoclimate; pt 2, area studies and fieldguides: Geological Society of America, p. 495-520.

Peirce, H. W., and Nations, J. D., 1986, Tectonic andpaleogeographic significance of Tertiary rocks of thesouthern Colorado Plateau and Transition Zone, inNations, J. D., and others, eds., Geology of central andnorthern Arizona: Geological Society of America, RockyMountain Section Meeting, Guidebook, p. 111·124.

Shafiqullah, M, Damon, P. E, Lynch, D. J., Reynolds, S. J.,Rehrig, W A, and Raymond, R H, 1980, K·Ar geochro·nology and geologic history of southwestern Arizona andadjacent areas, in Jenney, J. P., and Stone, Claudia, eds.,Studies in westem Arizona: Arizona Geological SocietyDigest, v. 12, p. 201·260.

Shafiqullah, M, Lynch, D. J., and Damon, P. E, 1976,Geology, geochronology, and geochemistry of thePicacho Peak area, Pinal County, Arizona, in Wilt, J. C,and Jenney, J. E, eds., Tectonic digest: Arizona GeologicalSociety Digest, v. 10, p. 305-324.

Sherid<j/l, M E, 1984, Volcanic landforms, In Smiley, T. L.,and others, eds., Landscapes of Arizona; the geologicalstory: Lanham, Md., University Press of America, p. 79·109.

__ 1987, Caldera structures along the Apache Trail inthe Superstition Mountains, Arizona, in Davis, G. H., andVandenDolder, E M, eds., Geologic diversity of Arizonaand its margins; excursions to choice areas: ArizonaBureau ofGeologyand MineraITechnology Special Paper5, p. 238-243.

Shoemaker, E M., Squires, R L., and Abrams, M J., 1974,The Bright Angel and Mesa Butte Fault systems ofnorthern Arizona, in Karistrom, T. N. V., and others, eds.,Geology of northern Arizona, with notes on archaeologyand paleoclimate; pt. 1, regional studies: GeologicalSociety of America, p. 355-391.

AELDNOTES, Fall 1987

---------- - - --~--~

Water: Lifeblood of Civilizationin the Salt River Valley

by James P. Lombard(Jniversity of Arizona

In the Sonoran Desert of southern Arizona,a dependable water supply is difficult tomaintain. Rainfall is scarce and unpredictable:the Phoenix area receives an average of lessthan 10 inches peryear (Sellers and Hill, 1974).Most streams are ephemeral. Prehistoriccommunities in the Salt River Valley dependedon canals to convey water from the seasonallyvariable Salt River to fields and houses (Masse,1981). Today ground water is pumped toaugment the supply from the dammed Saltand Verde Rivers, and even part of theColorado River has been rerouted to satisfy agrowing thirst. For more than 2,000 years, theinhabitants of the Salt River Valley havemanaged to maintain a constant water supply;how they accomplished this feat is a fascinat­ing story.

Geographic Setting and Climate

The Salt River Valley is in the Basin andRange province just south of the mountainousterrain of the Transition Zone. The valley flooris a broad area of gently sloping alluvial soils

M"1...at are well suited for irrigated agriculture and.1e construction of gravity-fed canals. Thick

sediments containing much of the Salt RiverValley's ground water have accumulatedbetween the partially buried bedrock moun­tains and hills that protrude from the valleyfloor.

The Salt River, joined by the Verde River eastof Phoenix, flows westward until it meets theGila River southwest of the city. Today the SaltRiver rarely flows through Phoenix exceptduring large floods because much of its wateris diverted upstream for irrigation and anyremaining river water evaporates or quicklysoaks into the sediments of the valley floor(University of Arizona, 1972).

The headwaters of the Salt River are in theMogollon Rim region, where springs andprecipitation runoff supply numerous streamsthat converge to form the river. Becauseprecipitation in this area is seasonal, the SaltRiver provided a dependable water supply onlyduring certain months, until storage damswere built to contain the flow. Using tree-ringdata from both live trees and archaeologicalsites in the upper Salt and Verde drainagebasins, scientists have determined seasonalstreamflow in the Salt River from 740 to 1370AD. and from 1800 to 1979AD. (Graybill andothers, 1987). This reconstruction has allowedthem to assess the difficulties that earlyinhabitants of the valley may have had inmaintaining their principal water supply.

Prehistoric Use of the Salt River

It is believed that the Hohokam peoplemigrated from Mexic;o into the Salt River Valleyabout 300 B.C. and lived there until about 1450AD., when their culture inexplicably declined(Haury, 1976). The Hohokam were riverinefarmers who used an extensive network of

hundreds of miles of hand-dug canals (Figure1) to divert water from the Salt River forirrigation and for settlements as far as 10 milesfrom the river. The Hohokam were evidentlyadapted to seasonality of the Salt Riverbecause they built the most complex urbancivilization outside of Mexico in pre-Columbiantime, with an estimated population of 50,000to 60,000 (Haury, 1976).

Despite their impressive lengths, Hohokamcanals were technologically simple andrelatively inefficient Canal intake structures onthe river's edge were not permanent andprobably could not withstand floods. Thecanals were rarely lined with clays to preventwater loss. The lack of techniques for canalconstruction across steeper terrain limited thearea of land that could be irrigated within thevalley (Graybill and others, 1987). The recon­structed streamflow record of the Salt Riversuggests that the Hohokam spent consider­able energy repairing or rebuilding intakes andother parts of their canals after floods (Graybilland others, 1987). Tree-ring data indicate thatin the middle of the 14th century the Salt Riverflooded heavily, which may have causedirreversible damage to canal systems and beena major factor in the decline of the Hohokamcivilization (Graybill and others, 1987).

Historic Settlement ofthe Salt River Valley

Several thousand Pima individuals livedclose to the banks of the Salt River when

11

RoodBaseline

LEGEND

PREHISTORIC CANAL ~

INTERSTATE HIGHWAY -­

OTHER ROAD

Scale 0,--,---''----'-~_.L1 ~f Miles-

10

Prehlslnrir canal network In the Salt River Valley (adapted (rom Midvale, 1968).

Tolleson

Spanish missionaries arrived in the late 17thcentury. The Pima used irrigated agriculture tocultivate the same types of crops that theHohokam raised. Although they did not buildor maintain long canals, evidence suggeststhat they may have used the portions of theHohokam canals that were closest to the river.The small abandoned cities of the Hohokamwere replaced by the scattered farmingsettlements of the Pima. Pima oral traditionmaintains that the Hohokam were theirancestors, but includes no record ofthe causesof the Hohokam decline (Haury, 1976).

New settlers arrived in the Salt River Valleyafter the establishment of Fort McDowell in1865. Large-scale irrigation farming began in1867 after completion of the Swilling IrrigationCanal Company ditch, which occupied por­tions of the prehistoric canal network (John­son, 1982). At first, the settlers essentially usedprehistoric irrigation technology, but they soonconstructed sturdy intakes on the river usingconcrete and built canals across steeper terrainto areas unfarmed by the Hohokam (Graybilland others, 1987). Even with improved canalsystems, however, farmers experiencedenough difficul1y coping with dry years that bythe turn of the century they lobbied Congressto build a large dam on the Salt River to storewater and generate power. Roosevelt Damwascompleted in 1911, followed byother dams onthe Salt River; these solved the water needs ofvalley residents, at least for a time.

An unusual problem occurred in the early

1920's when the fields in some parts of thevalley became waterlogged from too muchirrigation. Large water wells were drilled for thefirst time in the Salt River Valley to lower thewater table by pumping the excess water andpiping it further to the west to irrigate more land(University of Arizona, 1972). This was thebeginning of the ever-increasing use of largewells to pump ground water for irrigation in thevalley, a practice that has depleted the ground­water supply (University of Arizona, 1972). Asa result, the land hassubsided in several areasand the resulting earth fissures pose problemsto current and future development (Schu­mann and Genualdi, '1986).

Current and Future Water Sources

With a population of 1.8 million, Phoenixandits suburbs constitute the largest desertmetropolitan area in the United States (RandMcNally, 1987). Because of skyrocketingurban growth in the Salt River Valley, waterdemand is expected to double during the next40 years (Undsey, 1987). To ftIl this increasingneed, the valley's water supply from the SaltRiver and diminishing ground-water reserves isbeing augmented by water from the ColoradoRiver. The Central Arizona Project (CAP), amultibillion-dollar system of canals, pumps,and aqueducts, carries the water from LakeHavasu on Arizona's western border to Phoe­nix and will soon service Tucson when theproject is completed. This extra water, how-

ever, will sustain growth in the Phoenixmetropolitan area only until the turn of thecentury, at which time Phoenix will require yetanother source. ~ity planner:' in Mesa, Scotts.,""dale, and Phoerux are explonng other SOUfQkW

outside the city limits. The City of Phoenixrecently bought 14,000 acres of farmland 100miles west of the city to use as a source ofground water for several hundred thousandnew residents sometime early in the 21stcentury (Undsey, 1987).

References

Graybill, D. A, Gregory, D. A, Nials, F. L., Fish, S. K., Gasser,R E, and Szuter, C. R, 1987, The 1982-1984 excavationsat Las Colinas; environment and subsistence: Tucson,Arizona State Museum Archaeological Series 162 [inpress I.

Haury, E. W., 1976, The Hohokam; desert farmers andcraftsmen: Tucson, University of Arizona Press, 412 p.

Johnson, G. W.,Jr., 1982, Phoenix, Valley of the Sun: Tulsa,Continental Heritage Press, Inc., 240 p.

Undsey, Robert, 1987, Booming cities buy up "waterranches" in the Southwest: New York Times, August 16,sec. E, p. 5, col. 1.

Masse, W. B., 1981, Prehistoric irrigation systems in the SaltRiver Valley, Arizona: Science, v. 214, p. 408415.

Midvale, Frank, 1968, Prehistoric irrigation in the Salt RiverValley, Arizona: The Kiva, v. 34, p. 28-34.

Rand McNally, 1987, Commercial atlas and marketing guide(l18th ed.): New York, Rand McNally and Co., p. 125.

Schumann, H. H., and Genualdi, R B., 1986, Landsubsidence, earth fissures, and water-level change insouthern Arizona: Arizona Bureau of Geologyand MineralTechnology Map 23, scale 1:1,000,000.

Sellers, W. D., and Hill, R H., 1974, Arizona climate, 1931­1972 (2d ed.): Tucson, University ofArizona Press, 616 p.

University of Arizona, 1972, Arizona, its people andresources: Tucson, University of Arizona Press, 411 p.

,---------- Fieldnotes ----------..:

Publications of the Arizona Geological Survey

Staffmembers of the Arizona Geological Survey (Geological SurveyBranch of the Arizona Bureau of Geology and Mineral Technology)conduct research, do geologic mapping, and collect and compiledata. The results of these efforts are published (as bulletins, maps, etc.)or are placed on open file. Reldnotes is the survey's quarterlynewsletter; subscriptions are free to U.S. residents. A complete pricelist of survey publications is available on request.

Vol. 17, No.3 Fall 1987State of Arizona ....••..•.......... _•..........•Governor Evan MechamUniversity of Arizona . . . . . . • . . . . . . . . . . . . • . • . . _..... President Henry KomerBureau of Geology & Mineral Technology

Acting Director .•..•... _... _•..............•.. Edgar J. McCullough, Jr.State Geologist & Assistant Director,

Geological Survey Branch _. _......•..........•......Larry D. FellowsAssistant Director,

MineralTechnology Branch ...........•..... _...•.....J. Brent HiskeyEditor ...•.•.........•.•.........•.........Evelyn M. VandenDolderIllustrators. . • . . . • . . . . . . . . . . . . . . . . . . . . . •Peter F. Corrao, Sherry F. Garner

The Bureau of Geology and Mineral Technology is a division of the University of Arizona.

Arizona Bureau of Geologyand Mineral Technology

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