The logo of the Arizona Geological
Survey is a small geologic map that
includes Arizona divided into
three regions. The northeast region is the
Colorado Plateau, a land of flat-lying to
gently inclined, layered sedimentary rocks
such as seen in the walls of the Grand
Canyon, and of young volcanic rocks like
those seen around Flagstaff and between
Show Low and Springerville. The south-west
region is the Basin and Range
province, an area of numerous small
mountain ranges separated by flat to gen-tly
sloping valley floors that are typically
underlain by thick deposits of sand and
gravel. Phoenix and Tucson are within the
Basin and Range province. Between the
two regions is the Transition Zone, a
northwest-trending region that has some
similarities to the other two but is distinc-tive
in having widespread exposures of very
old bedrock.
The Basin and Range province
obtained its distinctive topography large-ly
between about 30 and 10 million years
ago when the Earth’s crust broke apart
into numerous fault blocks and extended
in a northeast-southwest direction. The
faults that slipped during this period of
crustal extension dip into the earth at
moderate to gentle angles. Where such
faults are presently active in southwestern
North America, for example along the
Wasatch front near Salt Lake City and
along the east side of the Sierra Nevada,
they are usually found at the foot of each
mountain range where faults of this type
dip under the adjacent basin. The rocks
below the dipping fault ramp are dis-placed
upward to uplift the mountain
ranges, while rocks above the fault ramp
are displaced downward to make the
basins. This type of fault is called a “nor-mal”
fault. The Pitaicachi fault in Sonora,
south of Douglas, for example, is an active
normal fault that produced a magnitude
7.2 earthquake in 1887 (DuBois and
Smith, 1980).
Most of Arizona’s normal faults have
been inactive for so long that they are
buried and can only be inferred based on
indirect geologic evidence. So it is always
interesting to geologists when someone
drills a hole that penetrates one of these
faults, especially when it appears to be a
large fault (Figure 1). It is even more inter-esting
when several drill holes penetrate a
buried fault because the dip of the fault can
then be determined from the fault depths
in the different drill holes. Such a discov-ery
happened recently west of Phoenix.
Between downtown Phoenix and the
White Tank Mountains to the west is a
broad, flat area that is crossed by the Agua
Fria River and includes west Phoenix,
Luke Air Force Base, and the communities
of Glendale, Peoria, and Litchfield Park.
Beneath this region is a deep sedimentary
basin called the Luke basin that contains
an enormous body of salt (Figure 2). The
extent and geometry of this salt body is
only approximately known, but it is
thought to underlie at least 100 km2 (40
mi2 ) with an average thickness of perhaps
a kilometer (0.6 mi). The geologic age of
the salt is probably younger than about 15
MISSION
To inform and advise the public
about the geologic character of
Arizona in order to increase
understanding and encourage pru-dent
development of the State’s
land, water, mineral, and energy
resources.
ACTIVITIES
PUBLIC INFORMATION
Inform the public by answering
inquiries, preparing and selling
maps and reports, maintaining a
library, databases, and a website,
giving talks, and leading fieldtrips.
GEOLOGIC MAPPING
Map and describe the origin and
character of rock units and their
weathering products.
HAZARDS AND
LIMITATIONS
Investigate geologic hazards and
limitations such as earthquakes,
land subsidence, flooding, and rock
solution that may affect the health
and welfare of the public or impact
land and resource management.
ENERGY AND
MINERAL RESOURCES
Describe the origin, distribution,
and character of metallic, non-metallic,
and energy resources and
identify areas that have potential
for future discoveries.
OIL AND GAS
CONSERVATION
COMMISSION
Assist in carrying out the rules,
orders, and policies established by
the Commission, which regulates
the drilling for and production of
oil, gas, helium, carbon dioxide,
and geothermal resources.
THE STATE AGENCY FOR GEOLOGIC INFORMATION
Vol. 35, No. 3
FALL 2005
DRILL HOLES INTHE LUKE SALT BODY
PENETRATE UNDERLYING FAULT
ARIZONA
GEOLOGI CAL
SURVEY
Jon E. Spencer, Senior Geologist
Steven L. Rauzi, Oil and Gas Administrator
Arizona Geological Survey
million years and older than about 2 million years. The
salt was deposited in a closed basin containing a lake that
was probably dry most of the time. It is not known what
river system entered the lake.
Man-made caverns in the Luke salt body are used for
storage of liquefied petroleum gas (LPG, specifically
propane and butane). Each cavern was made by pumping
fresh water down a drill hole into the salt where the salt
was dissolved and the resulting brine was pumped back to
the surface. This process gradually created the large
underground caverns that are now used to store LPG.
Copper Eagle Gas Storage, LLC, recently drilled four
holes into the Luke salt body to evaluate the possibility of
developing a new underground LPG storage facility. The
company was specifically interested in identifying porous
and permeable conglomerate or coarse sand beneath the
salt so that they could pump the brine derived from salt
dissolution into a deeper geologic reservoir and thereby
dispose of it.
Of the four wells drilled, the one near the
center of the basin penetrated 263 m of fine
grained sediments (mostly sand and silt) under-lain
by 1300 m of salt, and did not reach the
bottom of the salt body (Figure 3). Three other
holes on the western margin of the salt body
penetrated hundreds of meters of salt, clay, silt,
and minor anhydrite and then passed abruptly
into metamorphic rocks without passing
through sand and gravel. (Anhydrite is calcium
sulfate [CaSO4], which is similar to gypsum
[CaSO4 · 2H2O] and is also formed by evapo-ration
of lake waters). The underlying meta-morphic
rocks are severely altered, with no
biotite or hornblende (both common black
minerals containing iron and magnesium) pres-ent
for a distance of more than 80 to 100 m
below the top of the bedrock, but much chlorite
had replaced the dark minerals. In one of the
wells, the bedrock was cored over about 3 m,
and the cored sample looks identical (Figure 4)
to the chlorite-altered, crushed but cemented
rock that is seen below large-displacement, gen-tly
dipping normal faults known as detachment
faults (e.g., Reynolds, 1985).
Because of the abruptness of the boundary
between salt and bedrock, and because of the
crushed and altered state of the bedrock direct-ly
beneath the boundary, the boundary is inter-preted
as a fault. And because it places unmeta-morphosed
sediments over metamorphosed
bedrock, the fault is interpreted as a normal
fault. The depth to the fault in the different
wells indicates that, if the fault is planar, it dips
12° toward N64°E, and so is a gently dipping
normal fault (Figure 3). The chloritic breccia
below the fault is characteristic of normal fault
zones that have accommodated at least 10-15
km (6-10 miles) of displacement, and so this
fault is probably a type of large-displacement, gently dip-ping
normal fault called a detachment fault. Such a fault
had been thought to exist in the area based on the geology
of the White Tank Mountains, and had been given the
name White Tank detachment fault (Kruger and others,
1998; Ferguson and others, 2004).
Millions of years ago, a large, commonly dry lake west
of what is now Phoenix received river water intermittent-ly
and repeatedly dried to form a salt-pan playa (also
known as a “salar”). An active fault along its west side kept
the basin deep enough to trap lake water and sediment.
Repeated earthquakes gradually uplifted the White Tank
Mountains and sent seismic waves rippling across the
white surface of the salar. As the fault became inactive,
the lake filled with sediments and eventually spilled over
and integrated with the regional drainage system that we
see today. The Luke salt body is thus a relict of an earlier
time when the basin and range landscape of southern and
western Arizona was actively forming.
2
Figure 1. SunCor #1-2 drill rig located over the central area of the Luke
salt body, February 2001.
3
Outline of saltOutline salt
White Tank Mountains
N
6 Kilometers
Figure 2. Map showing the approximate subsurface outline of the Luke salt body, the location of the drill holes discussed in text, and the loca-tion
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.
Figure 3. Cross section along cross-section line shown in Figure 2. This shows how rocks would be distributed within the subsurface if we
could cut open the earth and reveal a vertical surface.
4
Figure 4. Picture of crushed, altered, and cemented bedrock from rock core taken just below the base of the over-lying
salt and fine-grained sediments of the Luke basin. Crushing and fracturing is interpreted as a result of fault
movements on a nearby fault surface. The dark greenish gray color from the mineral chlorite, and the fracture-fill-ing
reddish hematite (iron oxide), silica, and calcite are interpreted as a result of hot water moving through the
crushed and fractured rocks. The white and grayish white are quartz and feldspar that have been fractured and
crushed but have not been altered to other minerals. Core is from 1638 meters depth (5373 feet) in the Suncor #1-
24 drill hole (OGCC #909).
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 Mineral
Technology Special Paper 3, 112 p.
Ferguson, C.A., Spencer, J.E., Pearthree, P.A., Youberg, A., and Field, J.J., 2004, Geologic map of the
Wagner Wash Well 7 1/2' Quadrangle,Maricopa County, Arizona: Arizona Geological Survey Digital
Geologic 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 detach-ment
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 and
Range 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 of Geology and
Mineral Technology Bulletin 195, 61 p., 1 sheet, scale 1:24,000.
REFERENCES CITED
5
The summer of 2005 was a busy one for earth
fissures in the news. Heavy rain in early
August on the north side of the San Tan
Mountains produced flooding that affected broad
areas of an unincorporated area known as Chandler
Heights, south of the town of Queen Creek. Some of
the flood water found its way into an old fissure sys-tem,
parts of which had been covered-over for devel-opment.
As water flowed through the fissure and
under the capping of dirt, the surface collapsed into
the underlying void and the old fissure was reborn.
That particular fissure was first reported in U.S.
Geological Survey (USGS) Circular 466 in 1962,
although it formed a few years before that. A fissure
opening in the Chandler Heights area was not news
to geologists, but was to residents who moved into
the area who were unaware of earth fissures, or who
were warned of their existence but were unaware of
what a fissure actually looked like.
Some recent newspaper articles have given the
impression that (1) the Chandler Heights area is the
only place in Arizona with earth fissures, (2) the
AZGS publication that covers the Chandler Heights
area, OFR 94-11, is Arizona’s only earth-fissure map,
and (3) no earth fissures have been mapped since.
First, earth fissures are present in four counties, not
just the Chandler Heights area. By far the largest
number is in Pinal County. Maricopa and Cochise
Counties have perhaps one-tenth the number of fis-sures
that Pinal County has, and Pima County has
only half a dozen or so. Second, there are dozens of
reports and maps showing earth fissures from AZGS,
USGS, U.S. Bureau of Reclamation, and other scien-tific
publications and university theses. In fact, a com-prehensive
bibliography of references about subsi-dence
and earth fissures in Arizona is 21 pages long!
Third, many reports showing earth fissures have been
produced since 1994. These include AZGS open-file
reports OFR 95-6, OFR 97-19, OFR 99-26, OFR
01-10, and OFR 04-01, and a 2001 ASU thesis by
Hugh Larkin.
Before the late 1990s, AZGS did not have
Global Positioning System technology and did not
produce digital versions of its maps using
Geographic Information System software. Mapping
was done using topographic maps or aerial photos to
determine fissure locations. Many USGS topograph-ic
maps are from the 1960s or 1970s and do not show
more recent roads and other developments, so fis-sures
were located on maps without the use of clear-ly
identifiable landmarks that are now taken for
granted. Aerial photos used for making most of the
earth-fissure maps in the 1990s (OFRs 94-11, 95-6,
and 97-19) were poster-sized blueline copies with
low resolution and poor print quality. Lines on pub-lished
maps were drawn by hand with ink on mylar,
with the geologist locating the fissures on the base
map as best as could be done with the materials and
technology available at the time. Additional error
was introduced during reproduction, where a single
mylar was generated by overlaying the mylar with
hand-drafted lines over a base map. This old style of
map generation typically introduces uncertainty of at
least several tens of feet, and simply digitizing old
maps will produce a digital map with as much or
more error than the original. As a result of this situ-ation,
AZGS earth fissure maps were never intended
to be used for site-specific engineering purposes.
Whether or not new fissures have opened in an
area in the past 10 or 20 years, the old fissures could
still be active in that they can continue to open and
could still damage structures. This is especially true if
some of the cracks were due in part to, or are now
controlled by, cycles of near-surface desiccation of
clays in the sediment. If the cracks are "active" in the
sense that they still occasionally reopen, that means
they must be regarded as hazardous and should not
be ignored. For both earth fissures originating from
groundwater pumping and giant desiccation cracks,
the only proper mitigation is to know exactly where
they are so development can be planned to avoid
placing structures near or on top of them. Both
types of fissures can reactivate after years of dorman-cy.
For the Chandler Heights fissures specifically, the
one that reopened this past summer did not neces-sarily
reflect new activity in the sense that the fissure
was growing or changing, but rather that the fissure
has not been filled completely, so there is still void
space underground that surface material can wash
into. In other words, that fissure system is still
“active” because it is still filling, not because the walls
of the fissure are moving apart.
To order earth fissure publications and maps call
the AZGS Publication Sales Office at (520) 770-
3500 or visit us at 416 W. Congress St. Suite 100,
Tucson, Arizona.
Ray Harris, Reasearch Geologist
Arizona Geological Survey
ARIZONA GEOLOGICAL SURVEY
416 West Congress, Suite 100
Tucson, AZ 85701
(520) 770-3500
Return Service Requested
NON PROFIT ORG.
U.S. POSTAGE
PAID
TUCSON,ARIZONA
PERMIT NO. 3088
Shipping & Handling CHARGES
In the United States
Less than $2.00, add $2.50
2.01 - 10.00, add 6.00
10.01 - 20.00, add 7.50
20.01 - 30.00, add 8.75
30.01 - 40.00, add 9.50
40.01 - 50.00, add 11.00
50.01 - 100.00, add 13.00
101.01 - 200.00, add 17.00
More than 200.01 - call
Other countries, request price quotation
Shipping and handling charges include insurance.
For rolled maps, add $1.00 for a mailing tube.
You may purchase publications at the AZGS office or by mail. Address
mail orders to AZGS Publications, 416 W. Congress St., Suite 100, Tucson,
AZ 85701. Orders are shipped by UPS, which requires a street address for
delivery. All mail orders must be prepaid by a check or money order payable in
U.S. dollars to the Arizona Geological Survey or by Master Card or VISA. Do
not send cash. Add 7.6% sales tax to the publication cost for orders purchased
or mailed in Arizona. Order by publication number and add these shipping and
handling charges to your total order:
PUBLICATION ORDERING INFORMATION
Copyright © 2005
STATE OF ARIZONA
Janet Na p o l i t a n o, Gover nor
ARIZONA GEOLOGICAL SURVEY
OFFICE OF THE DIRECTOR
Rose Ellen McDonnell, Assistant Director of
Administration
GEOLOGISTS
Jon E. Spencer Senior Geologist
Thomas G. McGarvin Geologist II
Stevan Gyetvai Geologist II
Philip A. Pearthree Research Geologist
Steven L. Rauzi Oil and Gas Administrator
Richard A. Trapp Information Technology Manager
CONTRACTED GEOLOGISTS
Charles A. Ferguson
Raymond C. Harris
Stephen M. Richard
Todd C. Shipman
Ann M. Youberg
SUPPORT STAFF
Mary N. Andrade Business Manager
Maricella M. Moreno Publication Sales
A. Marie Madero Secretary
ARIZONA GEOLOGY is published four times per
year by the Arizona Geological Survey (AZGS), an
executive branch agency of the State of Arizona. Please
make comments, subscription requests, and address
changes to the AZGS at 416 W. Congress Street, Suite
100, Tucson, AZ 85701. Phone: (520) 770-3500. Visit
our website at:
www.azgs.az.gov
The Arizona Geological Survey website (www.azgs.az.gov)
provides geologic information (including geologic hazards),
lists publications and maps for sale, posts the most recent
Arizona Geology issues, has staff listing, and much more.
VISIT AZGS WEBSITE
