~rizona qeologyVol. 35, No.3FALL 2005 published by the 74rizona geological Survey

THE STATE AGENCY FOR GEOLOGIC INFORMATION

MISSION

To inftrm and advise the publicabout the geologic chal'acter ofArizona in order to increaseunderstanding and encouragepru­dent development of the State'sland, wate/; mineral, and energyresources.

ACTIVITIES

DRILL HOLES IN THE LUKE SALT BODY

PENETRATE UNDERLYING FAULT

Jon E. Spencer, Senior GeologistSteven L. Rauzi, Oil and Gas Administrator

Arizona Geological Survey

PUBLIC INFORMATION

Inftnn the public by answeringinquiries, preparing and sellingmaps and reports, maintaining alibrary, databases, and a website,giving talks, and leadingfieldtrips.

GEOLOGIC MAPPINGMap and deSC1"ibe the origin andcharacter of /wk units and theirweathering products.

HAZARDS ANDLIMITATIONS

Investigate geologic hazards andlimitations such as earthquakes,land subsidence, flooding, and rocksolution that may affict the healthand welfare ofthepublic or impactland and Tesource management.

ENERGY ANDMINERAL RESOURCES

Describe the oTigin, distribution,and character of metallic, non­metallic, and energy Tesources andidentify areas that have potentialftrfuture discoveries.

OIL AND GASCONSERVATION

COMMISSION

Assist in canying out the Tules,oTde,.s, and policies established bythe Commission, which "egulatesthe drillingftr and production ofoil, gas, helium, carbon dioxide,andgeothermal Tesources.

T he logo of the Arizona GeologicalSurvey includes a small geologicmap that shows Arizona divided

into three regions. The northeast region isthe Colorado Plateau, a land of flat-lyingto gently inclined, layered sedimentaryrocks such as seen in the walls of theGrand Canyon, and of young volcanicrocks like those seen around Flagstaff andbetween Show Low and Springerville. Thesouthwest region is the Basin and Rangeprovince, an area of numerous smallmountain ranges separated by flat to gen­tly sloping valley floors that are typicallyunderlain by thick deposits of sand andgravel. Phoenix and Tucson are within theBasin and Range province. Between thetwo regions is the Transition Zone, anorthwest-trending region that has somesimilarities to the other two but is distinc­tive in having widespread exposures ofveryold bedrock.

The Basin and Range provinceobtained its distinctive topography large­ly between about 30 and 10 million yearsago when the Earth's crust broke apartinto numerous fault blocks and extendedin a northeast-southwest direction. Thefaults that slipped during this period ofcrustal extension dip into the earth atmoderatd.td1 gentle angles. Where suchfault: are'presbhly active in southwesternNorth America, for example along theWasatch front 'near Salt Lake City andalong the east side- of the Sierra Nevada,they are usually found at the foot of eachmountain range where faults of this typedip under the adjacent basin. The rocks

below the dipping fault ramp are dis­placed upward to uplift the mountainranges, while rocks above the fault rampare displaced downward to make thebasins. This type of fault is called a "nor­mal" fault. The Pitaicachi fault in Sonora,south ofDo~glas, for example, is an activenormal fault that produced a magnitude7.2 earthquake in 1887 (DuBois andSmith, 1980).

Most of Arizona's normal faults havebeen inactive for so long that they areburied and can only be inferred based onindirect geologic evidence. So it is alwaysinteresting to geologists when someonedrills a hole that penetrates one of thesefaults, especially when it appears to be alarge fault (Figure 1). It is even more inter­esting when several drill holes penetrate aburied fault because the dip of the fault canthen be determined from the fault depthsin the different drill holes. Such a discov­ery happened recently west of Phoenix.

Between downtown Phoenix and theWhite Tank Mountains to the west is abroad, flat area that is crossed by the AguaFria River and includes west Phoenix,Luke Air Force Base, and the communitiesof Glendale, Peoria, and Litchfield Park.Beneath this region is a deep sedimentarybasin called the Luke basin that containsan enormous body of salt (Figure 2). Theextent and geometry of this salt body isonly approximately known, but it isthought to underlie at least 100 km2 (40mi2 ) with an average thickness of perhapsa kilometer (0.6 mi). The geologic age ofthe salt is probably younger than about 15

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Of the four wells drilled, the one near thecenter of the basin penetrated 263 m of finegrained sediments (mostly sand and silt) under­lain by 1300 m of salt, and did not reach thebottom of the salt body (Figure 3). Three otherholes on the western margin of the salt bodypenetrated hundreds of meters of salt, clay, silt,and minor anhydrite and then passed abruptlyinto metamorphic rocks without passingthrough sand and gravel. (Anhydrite is calciumsulfate [CaSO ], which is similar to gypsum[CaSO4 . 2H/)] and is also formed by evapo­ration of lake waters). The underlying meta­morphic rocks are severely altered, with nobiotite or hornblende (both common blackminerals containing iron and magnesium) pres­ent for a distance of more than 80 to 100 mbelow the top of the bedrock, but much chloritehad replaced the dark minerals. In one of thewells, the bedrock was cored over about 3 m,and the cored sample looks identical (Figure 4)to the chlorite-altered, crushed but cementedrock that is seen below large-displacement, gen­tly dipping normal faults known as detachmentfaults (e.g., Reynolds, 1985).

Because of the abruptness of the boundarybetween salt and be'drock, and because of thecrushed and altered state of the bedrock direct­ly beneath the boundary, the boundary is inter­preted as a fault. And because it places unmeta­morphosed sediments over metamorphosedbedrock, the fault is interpreted as a normalfault. The depth to the fault in the differentwells indicates that, if the fault is planar, it dips12° toward N64°E, and so is a gently dippingnormal fault (Figure 3). The chloritic brecciabelow the fault is characteristic of normal faultzones that have accommodated at least 10-15km (6-10 miles) of displacement, and so this

fault is probably a type of large-displacement, gently dip­ping normal fault called a detachment fault. Such a faulthad been thought to exist in the area based on the geologyof the White Tank Mountains, and had been given thename White Tank detachment fault (Kruger and others,1998; Ferguson and others, 2004).

Millions ofyears ago, a large, commonly dry lake westof what is now Phoenix received river water intermittent­ly and repeatedly dried to form a salt-pan playa (alsoknown as a "salar"). An active fault along its west side keptthe basin deep enough to trap lake water and sediment.Repeated earthquakes gradually uplifted the White TankMountains and sent seismic waves rippling across thewhite surface of the salar. As the fault became inactive,the lake filled with sediments and eventually spilled overand integrated with the regional drainage system that wesee today. The Luke salt body is thus a relict of an earliertime when the basin and range landscape of southern andwestern Arizona was actively forming.

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Figure 1. SunCor #1-2 drill rig located over the central area of the Lukesalt body, February 2001.

million years and older than about 2 million years. Thesalt was deposited in a closed basin containing a lake thatwas probably dry most of the time. It is not known whatriver system entered the lake.

Man-made caverns in the Luke salt body are used forstorage of liquefied petroleum gas (LPG, specificallypropane and butane). Each cavern was made by pumpingfresh water down a drill hole into the salt where the saltwas dissolved and the resulting brine was pumped back tothe surface. This process gradually created the largeunderground caverns that are now used to store LPG.Copper Eagle Gas Storage, LLC, recently drilled fourholes into the Luke salt body to evaluate the possibility ofdeveloping a new underground LPG storage facility. Thecompany was specifically interested in identifying porousand permeable conglomerate or coarse sand beneath thesalt so that they could pump the brine derived from saltdissolution into a deeper geologic reservoir and therebydispose of it.

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To Phoenix10

~~.~~~~~~4Mlles6 Kilometers

"0(\Jo

0:::t(\Jif)>,o

Luke Salt Body (buried)

"0(\Jo

0:::if):::l~

U

---------+---------(10_-~-----+-----

60

(J) /Sun City

.S19c: ......,:J ~0~ ()~

(lJ

.S:c: :;;s

~ )6.2"-~ Northern

1-2

Figure 2. Map showing the approximate subsurface outline of the Luke salt body, the location of the drill holes discussed in text, and the loca­tion of the cross section shown in Figure 3. The area is between the White Tank Mountains on the west and downtown Phoenix on the east.

EAST

km10

silt, sand,and gravel

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buried fault anhydrite, silt,

Drill hole and sand Drill hole#1-12 #1-2

~~C-+---:;~~~~~~~~O-::;;.... -1

salt _--:-:::~.:--d-g~;ve\1---..;:..:.- sana ~~------ -2------ -----

--: - - - - - - - - bedrock?--- -3

15 20 25

granitic and metamorphic rocks

WEST

Figure 3. Cross section along cross-section line shown in Figure 2. This shows how rocks would be distributed within the subsurface if wecould cut open the earth and reveal a vertical surface.

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Figure 4. Picture of crushed, altered, and cemented bedrock from rock core taken just below the base of the over­lying salt and fine-grained sediments of the Luke basin. Crushing and fracturing is interpreted as a result of faultmovements on a nearby fault surface. The dark greenish gray color from the mineral chlorite, and the fracture-fill­ing reddish hematite (iron oxide), silica, and calcite are interpreted as a result of hot water moving through thecrushed and fractured rocks. The white and grayish white are quartz and feldspar that have been fractured andcrushed but have not been altered to other minerals. Core is from 1638 meters depth (5373 feet) in the Suncor #1­24 drill hole (aGCC #909).

REFERENCES CITED

DuBois, S.M., and Smith, A.W., 1980, The 1887 earthquake in San Bernardino Valley, Sonora:Historic accounts and intensity patterns in Arizona: Arizona Bureau of Geology and MineralTechnology Special Paper 3, 112 p.

Ferguson, C.A., Spencer, J.E., Pearthree, EA., Youberg, A., and Field, J.J., 2004, Geologic map of theWagner Wash Well 71/21 O1Iadrangle, Maricopa County, Arizona: Arizona Geological Survey DigitalGeologic Map 38 (DGM-38), v. 1.0, 7 p., scale 1:24,000.

Kruger,J.M., Faulds,J.E., Reynolds, S.j., and Okaya, D.A., 1998, Seismic reflection evidence for detach­ment polarity beneath a major accommodation zone, west-central Arizona, in Faulds, J.E., and Stewart,J.H., eds., Accommodation Zones and Transfer Zones: The Regional Segmentation of the Basin andRange Province: Boulder, Colorado, Geological Society of America Special Paper 323, p. 89-113.

Rauzi, S.L., 2002, Arizona has Salt!: Arizona Geological Survey Circular 30, 36 p.

Reynolds, S.j., 1985, Geology of the South Mountains, central Arizona: Arizona Bureau ofGeology andMineral Technology Bulletin 195, 61 p., 1 sheet, scale 1:24,000.

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