An exploration gravity survey in the San Pedro Valley, southeastern Arizona
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Authors Halvorson, Phyllis Heather Fett
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/557989 AN EXPLORATION GRAVITY SURVEY
IN THE SAN PEDRO VALLEY,
SOUTHEASTERN ARIZONA
b y
Phyllis Heather Fett Halverson
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fullfillment of the Requirements For the Degree of
MASTER OF SCIENCE WITH A MAJOR IN GEOPHYSICS
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 8 4 Call No. BINDING INSTRUCTIONS INTERUBRARY INSTRUCTIONS — OQ a I ,, >—» >—— , , i I r STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of require ments for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotations from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate School when in his judg ment the proposed use of the material is in the interests of scholarship. In o.U other instances, however, permission must be obtained from the author.
SIGNED: Zvl/i CdOx
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
\ J. S. SUMNER Date Professor of Geophysics ACKNOWLEDGMENTS
I would like to thank my advisor, Dr. John S. Sumner, for initiating this project and encouraging me to develop a complete and
descriptive final product. I would also like to thank the other members of my thesis committee, Drs. Randall M. Richardson and Robert F.
Butler, who have also taken the time to review this thesis.
A very dynamic lady, H. R. Hauck, has edited and typed the
thesis. Richard S. Brokaw drafted my figures and maps and is
responsible for their professional look. The data collection in the
northern San Pedro Valley was organized and carried out by Kurt
Kretvix, with the help of Paul Boissevain, Siki Sene, and Randy Himes.
Boissevain and Himes also helped me process the data from the northern
San Pedro Valley.
I would like to thank John D. Fett of Earth Science and
Engineering, Inc., of Hemet, California, for supervising me while I
gathered new data in the southern San Pedro Valley. Mr. Fett also
assisted in obtaining additional gravity data in the southern San Pedro
Valley that had been gathered by Earth Science and Engineering em
ployees for Tenneco. I would like to thank Tenneco, Englewood,
Colorado, for allowing roe to use these gravity data. Simple Bouguer
anomaly values for my data were calculated by Earth Science and Engi
neering .
I would like to thank Donald R. Pool for accompanying us occa
sionally in the field and more importantly for finding the pertinent
iii iv maps, well logs, and reports at the U.S. Geological Survey Water Re sources Division. Dr. Denis L. Norton and his daughter Debbie, surveyed the line south of Knob Hill for me. Dr. Norton also had numerous interesting chats with me and provided references on the
geology of the Dragoon Mountains while I was developing the
depth-to-bedrock map and the interpretation section of this thesis.
Stanley G. Davis reviewed the data processing programs thoroughly and brought discrepancies between the various programs to my attention.
I would like to thank my parents for providing me with a
pleasant abode for the duration of my stay in Tucson. I would also
like to thank my father, John D. Fett, for the ample loans that made
finishing this thesis possible. I would like to thank my friends and
loved ones for being such great amigos.
This research was financed in part by the U.S. Department of
the Interior, as authorized by the Water Research and Development Act
of 1978 (P.L. 95-567). This research was also supported by Project
A-108-ARIZ. TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS...... vii
LIST OF T A B L E S ...... viii
A B S T R A C T ...... ix
1. INTRODUCTION...... 1
2. DATA COLLECTION...... 7
3. DATA PRO CESSING ...... 11
Input P a r a m e te r s ...... 11 Data R e d u c t io n ...... 12 Southern San Pedro Valley Da ta ...... 12 N orthern San Pedro Valley Da ta ...... 12 G ravity C o r r e c tio n s...... 13 T heoretical S ea-level G r a v i t y ...... 13 Free-air Anomaly ...... 14 B ouguer G ravity A n o m a ly ...... 16 T errain C o r r e c tio n s...... 17 Regional C o r r e c tio n ...... 18 Arizona G ravity Data B a se...... 18
4. PLOTTING AND CONTOURING D A T A ...... 20
P osting Stations and D a t a ...... 20 Contouring D a t a ...... 20
5. GENERAL GEOLOGY OF THE SAN PEDRO VALLEY...... 22
P re-B asin and Range G e o lo g y ...... 22 Basin and Range D is tu r b a n c e ...... 24 Basin Alluvial F i l l ...... 25 General G eom orp h ology...... 27
6. H Y D R O LO G Y ...... 29
Upper San Pedro H ydrologic B a sin ...... 29 Lower San Pedro H ydrologic B a sin ...... 30
v vi
TABLE OF CONTENTS—C ontinued
Page
7. DEPTH-TO-BEDROCK MODELING...... 32
Preliminary Depth Estimates...... 32 Two-dim ensional M o d e lin g ...... 36 Three-dimensional M odeling ...... 38
8. INTERPRETATION...... 39
Lower San Pedro H ydrologic B a sin ...... 39 Upper San Pedro H ydrologic B a sin ...... 44 Volume of Ground W ater...... 57
9. SUMMARY AND CONCLUSIONS...... 61
REFERENCES...... 64 LIST OF ILLUSTRATIONS
Figure Page
1. Map of the San Pedro Valley showing its principal fe a tu r e s ...... 2
2. Benson base station two (BN2)...... 10
3. Gravity station location map, San Pedro Valley, A r iz o n a ...... in pocket
4. Depth to bedrock versus anomalous gravity value for quick estimate pu rp ose ...... 35
5. Complete Bouguer gravity anomaly map, San Pedro Valley, Arizona ...... in pocket
6. Residual Bouguer gravity anomaly map, San Pedro Valley, Arizona ...... in pocket
7. Depth-to-bedrock map, San Pedro Valley, Arizona . . in pocket
8. D ep th -to -b e drock profile of line 1 ...... 41
9. Depth-to-bedrock profile of line 2 ...... 42
10. Depth-to-bedrock profile of line 3 ...... 46
11. Depth-to-bedrock profile of line 4 ...... 47
12. D ep th -to-b ed rock profile of line 5 ...... 50
13. D ep th -to-b ed rock profile of line 6 ...... 53
14. D ep th -to-b ed rock profile of line 7 ...... 55
15. D ep th -to-b ed rock profile of lin e 8 ...... 56
vii LIST OF TABLES
Table Page
1. Density contrast between alluvium and bedrock at a given d e p th ...... 33
2. Quick estimate table for finding depth to bedrock in the San Pedro V alley ...... 34
3. ITMOD variable d en sity s c a l e ...... 37
4. Cross-sectional areas and distances between centroids for the depth-to-bedrock profiles that were modeled b y u sin g ITMOD de n s it ie s ...... 60
viii ABSTRACT
To increase the amount of available gravity data in the San
Pedro Valley, southeastern Arizona, and to constrain models for the San
Pedro subsurface geologic structure, 273 new gravity stations were read in the valley and privately owned data from 283 gravity stations were recently donated to this thesis project and thus to the Arizona Gravity
Data B ase.
To obtain more accurate knowledge of the thickness and extent of the water-bearing alluvium in the San Pedro Basin, two-dimensional models of the best gravity profiles were generated. Well data and general knowledge of the geology in the San Pedro Valley were used to help constrain the variables involved in modeling the gravity data. It appears that approximately 50 % 10^ acre-ft of water underlies the northern portion between Winkelman and the Narrows, north of Benson and that 90 x 10^ acre-ft of water south of the Narrows to the Mexican border, for a total of 140 x 10^ acre-ft more or less.
ix 1
CHAPTER 1
INTRODUCTION
The purpose of this thesis was to increase the amount of avail able gravity data for the San Pedro Valley in southeastern Arizona.
From gravity data and borehole depths to bedrock, the density differ ence between the bedrock and alluvium can be estimated and the inter vening depths to bedrock can be calculated. Thickness of the water bearing alluvium can be estimated. Basin structure can also be made more apparent when the bedrock depths are plotted.
The San Pedro Valley is located in southeastern Arizona. The ephemeral San Pedro River flows northward. For mapping purposes, the division between the southern and northern San Pedro Valleys is at lat 32° N.; the southern San Pedro Valley and northern San Pedro
Valley are located in the Nogales and Tucson 1 % 2-degree quadrangles, respectively. Hydrologically and physiographically, the basin is divided into the upper San Pedro River Basin, which corresponds to the south ern San Pedro Valley, and the lower San Pedro River Basin, which cor responds to the northern San Pedro Valley. The Narrows, which di vides the upper from the lower San Pedro Basin, is located several miles north of lat 32° N. Figure 1 shows the principal features of the
San Pedro Valley.
1 2
10 0 10 20 30 40 50 KM
Figure 1. Map of the San Pedro Valley showing its principal featu res 3
A total of 121 new gravity stations were observed by me in the southern San Pedro Valley (upper San Pedro hydrologic basin). Also several stations from the following data sets were reoccupied.
Previous gravity work in the San Pedro Valley incorporates the work performed for Tenneco from 1974 to 1976 by. Earth Science and
Engineering of Hemet, California, who observed gravity at over 250 sites in the Charleston subbasin and at about a score of sites in the
Benson subbasin; thus a total of 283 stations were observed for . / f
Tenneco in the upper San Pedro River basin. Tenneco has contributed these data to this thesis project and thus to the Arizona Gravity Base.
The original data collection for 152 new stations in the lower
San Pedro Basin was organized by Kretvix (1981). A few of these stations were reoccupied under my supervision, and the data were pro cessed under my supervision.
Hansen (1983) observed the gravity at 16 stations in an alluvial valley in the Black Hills in the lower San Pedro Basin. The Black Hills are considered to be hydrologic bedrock, but Hansen calculated alluvial thicknesses approaching 800 ft. This alluvial fill is primarily Tertiary
Quiburis Formation.
Bittson (1976) analyzed data from the Cienega Creek area. His data between the Whetstone Mountains and the Huachuca Mountains were used for this study of the San Pedro Valley. Bittson also did a depth-to-bedrock profile that extended from the Huachuca to the Whet stone Mountains. This profile was used to control depths on my own depth-to-bedrock map between the Mustang and Huachuca Mountains. 4
Spangler (1969) did detailed gravity work for his geophysical study of the hydrology of The Walnut Gulch Experimental Watershed at
Tombstone, Arizona. Spangler's data were used for my study of the
San Pedro Valley. The data in the northeastern corner of Spangler's thesis area were helpful in discerning the basin structure between the
Tombstone Hills and the Dragoon Mountains. A slightly different special wavelength was notable between the contoured data on bedrock in the Tombstone Hills and contoured data on the alluvial fill. The lateral variations in the density of the near-surface rocks correlated with short-wavelength anomalies for the bedrock stations. The variations in the thickness of the alluvial fill correlated with longer wavelength anomalies over alluvium.
The gravity data from the above sources were combined with the data that I collected. The complete data set was plotted, hand contoured, and interpreted. I developed depth-to-bedrock profiles for several lines of new data. I also used depths from profiles that had been developed for previous studies of southern Arizona. Residual aeromagnetic and geologic maps were essential for developing a realistic interpretation of the complete data set.
The Laboratory of Geophysics of the University of Arizona and the Defense Mapping Agency have also contributed data to the Arizona
Gravity Data Base. These gravity stations were regional and were par ticularly helpful in areas where other data were sparse.
Oppenheimer (1980) organized the preparation of a depth-to- bedrock map for southern Arizona. She used both well and gravity data. The major differences between her map and the one prepared for 5 this thesis are due to the lack of gravity data available to Oppen- heimer. Her interpretation was used to map some of the areas where new data were not available. The differences between our interpre tations of depth to bedrock in areas adjacent to volcanics are also due to the different methods we used to model profiles that end in volcan ics.
Aiken and Sumner (1974) discussed potentially favorable areas for petroleum exploration in southeastern Arizona. Aeromagnetic data of Sauck and Sumner (1970) were used to determine the type of bed rock beneath the alluvial fill. Their aeromagnetic data were also used to interpret the thickness of Paleozoic sedimentary rocks. Gravity data were used in conjunction with the aeromagnetic data to determine the thickness of the alluvial fill. Thus, both gravity and aeromagnetic data were used to distinguish between alluvial fill and sedimentary rocks and between sedimentary and igneous rocks. A depth-to-density bedrock profile was modeled from gravity data for the upper San Pedro Valley.
The profile extended from Cienega Arroyo, through Benson, to Texas
Canyon. Density differences of -0.5 g/cc and -0.3 g/cc were used.
The calculated depths are similar to the corresponding depths calculated and mapped in this survey of the San Pedro Valley. Either density difference could be appropriate near Benson, but the -0.3-g/cc density
difference is more appropriate for the thicker alluvial sequence in the
southwestern section of the Benson subbasin.
Sauck and Sumner (1970) developed an aeromagnetic map of
southeastern Arizona. The subtle changes in the aeromagnetic
anomalies were useful in determining the type of bedrock beneath the 6 alluvium. For example, igneous granite contains more magnetic minerals than limestone, so a subtle spacial shift to a higher magnetic anomaly could mean that bedrock has changed from limestone beneath the magnetic low to igneous rocks beneath the magnetic high. If bedrock remains homogeneous, the subtle changes in the aeromagnetic anomaly
should reflect changes in thickness of the alluvium. Stronger anomalies
are interesting because they generally mean a strong concentration of
magnetic minerals near or at surface. Drewes (1980) included an
interpretation of the strong aeromagnetic anomalies in his tectonic map
of southeastern Arizona. His interpretation correlated the strong anom
alies with quartz monzonite or granodiorite stocks similar to those in the
Whetstone Mountains and the Tombstone Hills. His map was not used
for my study as a tectonic map but as a geologic map for interpretation
of the San Pedro Valley gravity anomalies.
Hayes and Raup (1968) mapped the geology of the Mustang and
Huachuca Mountains. Their map was particularly useful for interpreting
gravity anomalies along the edge of the Huachuca Mountains and within
the Mustang Mountains.
A progress report on this research was previously submitted as
a technical completion report to the U.S. Department of the Interior
(Sumner and Halvorson, 1983). CHAPTER 2
DATA COLLECTION
All field observations of gravity in the San Pedro Valley area were made with LaCoste & Romberg, Inc. gravimeters. The early data collection from 1974 through 1976 was made by Earth Science and
Engineering, Inc. of Hemet, California for Tenneco with gravimeter No.
D-3. This is a "microgal" meter with each small scale division of the nulling dial equal to approximately one raicrogal. The scale constant of the gravimeter is 0.102 667 milligals per scale division; This value has been checked against 15 other LaCoste & Romberg Model G and Model D gravimeters on the Pines to Palms calibration network south of Palm
Desert, California, and against more than 10 LaCoste & Romberg Model
G gravimeters on the Mount Hamilton calibration network east of San
Jose, California. The dial instrument calibration curve of several Model
D gravimeters is straight enough so that single calibration factors were used for the entire range of these meters.
Gravity observations made for this thesis study in 1982 and
1983 were made with the LaCoste & Romberg, Inc. gravimeter No.
G-575. A Model G gravimeter has approximately 10 microgals per small division of the nulling dial. Cursory comparisons between the Model
No. D-3 observations and the Model No. G-575 observations indicated that the calibration of the Model No. G-575 was compatible with that of
7 8 the Model No. D-3. Model No. G-575, which was constructed in 1979, appears to be in very good condition, and its levels and sensitivity were checked before and after the field observations.
Field conditions were good during the period when observations were made with the Model No. G-575 gravimeter.
Stations locations were obtained by several methods. Most regional stations were located by use of U.S. Geological Survey 7£-
minute quadrangle maps. Stations were selected where bench-mark or
spot elevations were indicated on the maps. Spot elevations were
generally at cultural features such as road intersections, windmills,
tanks, section corners, and fence intersections.
Some stations were located near Arizona State Highway survey
and bench-mark monuments. Many of these were not shown on the
U.S. Geological Survey maps because they were established subsequent
to the publication of the maps, but they were easy to locate on the
maps because they were by the side of a highway at a crossing of a
major drainage. The actual gravity stations were not on the bridge or
culvert where the monument was located but were on nearby level
ground where the local topography would not cause a serious topo
graphic anomaly. The difference in elevation between the station and
the monument was estimated by hand level.
Locations and elevations of a few lines of closely spaced stations
were surveyed with a theodolite. All others were located from land
marks on U.S. Geological Survey maps, and their elevations taken from
the maps, or in the case of Arizona Highway survey monuments, from
the elevation stamped on the monument. 9
A gravity base was established on the sidewalk 15 inches northwest of the north edge of the entrance to the Benson Post Office
(condemned in 1983), on the northeast corner of 5th Street and
Huachuca Street (fig. 2). Several ties were made with the Model No.
G575 gravimeter from the University of Arizona's primary gravity base in the basement of the Geoscience Building to the new Benson base.
Ties were also made from the Benson base to the gravity bases used in the 1974 and 1976 gravity surveys made by Earth Science and Engineer ing, Inc. and to the local gravity base in Tombstone used by Spangler in 1969.
All new gravity observations made from Interstate Freeway 1-10 to the south were made with the Model No. G-575 gravimeter. The gravimeter was first read at the Benson Post Office base, the field observations were made, and within 4 to 8 hours the Benson base was reoccupied. \0
U S. 80 CHAPTER 3
DATA PROCESSING
Input Parameters
The latitudes and longitudes of most of the gravity stations were obtained by carefully scaling from the U.S. Geological Survey
7i-minute maps. For stations along the few surveyed lines of closely spaced stations, the latitude and longitude of a central station was scaled from the map and latitudes and longitudes of surrounding stations were calculated from the theodolite survey data.
Elevations for most stations are the spot elevations or bench mark elevations stated on the U.S. Geological Survey maps or those stamped on the Arizona Highway survey monuments.
Greenwich mean time is calculated by adding 7 hours to Arizona local standard time. Arizona does not use daylight savings time. Dates for readings taken after 1700 hours Arizona time (2400 hours mean time) were changed to the following date (the appropriate Greenwich date).
The absolute value of gravity at a base station is either input for data processing or is associated with the base station number. The letter and number of the gravimeter used are also input, and the asso ciated calibration factors are inherent parts of the programs used to process the data.
11 12
Data Reduction
Southern San Pedro Valley Data
Gravity data for the southern San Pedro Valley were reduced by computer with program GRAV3 of Earth Science and Engineering,
Inc. This program converts the gravimeter readings by the appropriate meter calibration factors. It also corrects for earth tides by using a subroutine based on the Longman (1959) tidal attraction computation and an earth compliance factor of 1.16 (Jachens, 1971). Meter drifts between base station observations were assumed to be linear. Free-air and simple Bouguer anomalies for a specific gravity of 2.67 g/cc are also computed. These values, together with station name and number, latitude, longitude, and elevation, are automatically punched onto IBM data cards for entry into the University of Arizona computer system.
Northern San Pedro Valley Data
Gravity data collected in the northern San Pedro Valley were processed with the University of Arizona's gravity reduction programs.
Program XGFORM.FIO (Maclnnes, 1982) is a program for changing free- formatted data (data formatted in the proper order with each value separated by commas) into data in the proper format to enter program
XGRAV (Himes, 1982).
Program XGRAV does the basic data processing (Lysonski,
1980). First, it changes the dial reading to milligals by using a calibration factor that varies with the gravimeter's scale reading. Then the tidal correction is calculated by using an earth compliance factor of 13
1.20. The tidal correction is subtracted from the observed value of gravity. Finally, the free-air and Bouguer corrections are calculated and subtracted from the observed gravity value. The final output from program XGRAV is the free-air anomaly and the simple Bouguer anom aly.
The free-air and Bouguer corrections will be described in more detail as will the terrain and regional corrections, which are done by separate programs that were applied to data from both the northern and southern San Pedro Valleys.
Gravity Corrections
Theoretical Sea-level Gravity
The theoretical sea-level gravity includes a correction for latitude because the earth rotates and is oblate. Theoretical sea-level
gravity (j*) can be expressed by the first-order form of Clairaut's
equation (Aiken, 1976):
£ = g0(l + B2sin2
where gQ = gravity at the equator
Bg = 2m - (3K/2a2) = (3m/2) - earth flattening (f)
m = centripetal acceleration
K = -(A + B )/2
a = equatorial radius
A and B are principal moments of inertia 14
The Clairaut model for the gravity of the earth was developed in 1777 (Garland, 1971). The 1930 International Gravity Formula (IGF) was based on Clairaut's model. The 1930 IGF was used with the
Potsdam datum. The Geodetic Reference System 1967 (GRS 67) was
described by Woollard (1979) and refined from more accurate satellite
data by Jacobs (1974, p. 108). The GRS 67 is compatible with the
International Gravity Standardization Net 1971 (IGSN 71) datum. The
change in constants between the 1930 IGF and the GRS 67 were dis
cussed by Lysonski (1980) and are seen from a comparison of the two
gravity formulas:
1. 1930 International Gravity Formula (IGF):
gQ = 978,049(1 + 0.005 288 4 sin2* - 0.000 005 9 sin22 2. 1967 Geodetic Reference System (GRS 67) formula: 4 gQ = 978 031.846(1 + 0.005 889 5 sin2* + 0.000 023 462 sin *). Lysonski .(1980) converted the data in the Arizona Gravity Data Base from Potsdam datum values to IGSN 71 datum values. The conver sion was done for Arizona data by subtracting 13.700 mgal from the Potsdam datum value of observed gravity if the base station of the original Potsdam datum reading was unknown. The base stations of this survey are tied into the International Gravity Standardization Net of 1971. Free-air Anomaly The free-air correction removes the elevation effect on gravity observations made above sea level, The free-air gravity anomaly (8p^) is 15 SFA = So * S + gF where: gQ = observed gravity ^ = theoretical sea-level gravity gp = free-air correction. The free-air correction (gp) is (Dobrin, 1976): „ 280hs . 8p Rl where: gy = value of gravity at mean sea level hg = height above mean sea level R' = mean radius of the earth Aiken (1976) used a trigonometrically rearranged version for XGRAV: gp = (0.308 77 + 0.000 44 sin2<|>r)hs + 0.072 x 10~6 hg2 where the latitude ^ is in radians and the height above mean sea level is in meters. However, the data in the Arizona Gravity Data Base have been recalculated by using the equation Plouff (1977) used in the terrain correction program. The difference is a sign change in the second and third terms such that gp = (0.308 77 - 0.000 44 sin2<^)hg - 0.072 x 10"6 hg The free-air correction can be derived from the expression -2So IE 2oi2 3 z R' 16 where w is angular velocity, or by expanding the equation for the gravity effect of a spherical mass. Free-air gravity anomalies generally follow local variations in elevation. Bouguer Gravity Anomaly The Bouguer reduction removes the effect of lateral and vertical variations in mass due to differing station elevations. The simple Bouguer correction is g g = -ZirGph = -0 .0 1 2 777 6ph (m gal/ft) or gg = -0.041 921 4 ph (mgal/m) where: p = density of the slab G = International Gravity constant (6.67 x 10 ^ cgs) h = station elevation or slab thickness. / The simple Bouguer gravity anomaly, adds both the free-air and Bouguer correction to the observed gravity and subtracts the theoret ical gravity: g BA = Bo + gF + g B " 4 The curvature correction for "curving" the Bouguer slab around the spheroidal earth is (Aiken, 1976): gc = h(h(1.27 x 10"15h - 3.282 x IQ-8) + 4.462 x l(f4) The curvature correction is subtracted from the simple Bouguer gravity anomaly before the terrain correction is added to obtain the complete 17 Bouguer anomaly, and then the regional correction is added to obtain the residual Bouguer gravity anomaly. Terrain Corrections The effect of topography is to create an upward gravitational attraction from hills above the station or a lack of downward gravita tional attraction due to a lack of mass in the valleys below the station. The terrain correction compensates for the attraction of all the mass that would have to be added to the valleys below and removed from the hills above to give a perfectly flat topography with equivalent station elevations (Dobrin, 1976). Transparent Hammer charts are used to calculate local terrain corrections. The charts consist of concentric circles with radiating lines. The average elevation within each compartment is estimated. Then, by using terrain correction tables that correspond to the charts, the actual terrain correction can be calculated (Dobrin, 1976). Programs TERR99 and TERMAP are, respectively, the formatting program and the job control program that pulls the proper "map files" from program TERAN and starts TERAN (Lysonski, 1980; Plouff, 1977). Program TERAN corrects for the terrain effects that are outside the 2.6-km radius of the hand-calculated terrain corrections and within a 167-km radius. Average elevations for 1 minute by 1 minute compart ments of latitude and longitude are stored in blocks of 225 on magnetic tape. The blocks of 225 are stored according to 15-minute topographic map names, unless 7i-minute maps are available. If 7£-minute maps are available, the map name of the northwest quarter of the 15-minute area 18 is used. Program TERR99.F10 is executed on the DEC 10 computer and the title line at the end of the output data should be deleted before running TERMAP. TERMAP.CDC is sent to the Cyber 175 computer with the data set from TERR99. Regional Correction Program FRPTS.CDC (Aiken, 1976; Lysonski, 1980) is a high-pass Fourier series filter for eliminating the regional gravity anomaly due to regional variations in the crust thickness. The output from FRPTS is called the residual Bouguer anomaly. The filter consists of a two-harmonic trend surface of the regional gravity for Arizona. Other filters or methods could be used to obtain the regional anomaly, but the above filter is believed to be a good estimate and is consistent with other work from The University of Arizona. Arizona Gravity Data Base The Arizona Gravity Data Base (AGDB) is the University of Arizona's storage system on computer tape for gravity data. The origi nal record format for the AGDB was discussed by Schmidt (1976). The data were sorted on latitude and longitude in ascending order. Then an index sequential file was created for the sorted records. Succeeding records may be added using the original index (Schmidt, 1976). The symbolic structure of the data base key is DDMMHHLLLMMHHTT, where the first six numbers are the latitude, the next seven numbers are longitude, and the last two letters or numbers are a special code. As of 1983, the AGDB tape included data that were collected and processed prior to June 1980. The tape was a 9-track tape that 19 was made on the DEC 10 using Back Up. The Save Set files were con veniently divided into 1° % 2° quadrangles. Insertion of new data re quires editing of the pertinent files, then making a new tape from the old tape and the edited files. A quick-sort program (Gentry, 1981) can be used to sort data in areas where new data are to be inserted. CHAPTER 4 PLOTTING AND CONTOURING DATA Computer-aided mapping greatly speeds the portrayal of the data. However, contouring routines are not totally satisfactory and frequent manual corrections are necessary. Posting Stations and Data Program XPLOT is the job-control program for POSTR.CDC (Lysonski, 1980). POSTR.CDC is a program for plotting station points and writing their corresponding gravity values next to the station location. A Lambert conformal conic projection is used for gridding the station points. The output from POSTR.CDC is sent to the Calcomp Plotter to be plotted at the specified scale. During field work the plotting routine was used to plot existing data or data that had been recently collected. In this way, the inves tigator can visually identify areas where more coverage is necessary. Final plots that include the new data were made at a smaller scale that was easy to hand contour. Figure 3 (in pocket) shows station locations at a scale of 1:250,000. Contouring Data Program SURFII (Sampson, 1978), which stands for Surface II, is a plotting and contouring package that unfortunately proved unsatis factory for final contoured plots. SURFII was used to contour the data 20 21 in the southern San Pedro Valley while field work was still in progress. This contoured map of the data was helpful in locating basin edges and other large-scale geological structures. Machine contouring is especially useful in revealing erroneous gravity values in a new data set. The final maps were hand contoured. Hand contouring has the advantage that geological controls can be incorporated into the contour in g . t CHAPTER 5 GENERAL GEOLOGY OF THE SAN PEDRO VALLEY The San Pedro Valley is a complex rift structure of the Basin and Range Province. Bounding faults and thrusts (Drewes, 1980) con trol the landforms and subsurface features, but subsequent alluvial deposition has largely obscured these structures from a detailed geo logic inspection. The subsurface hydrologic environment is controlled by the geologic structure and lithology, but to an unknown extent. The basement rock in the San Pedro Valley is well mineralized. The region has been heavily prospected in the past; however, the geology is still not well understood, especially outside the mining dis tr ic ts. Pre-Basin and Range Geology Precambrian rocks in the mountains surrounding the San Pedro Valley consist primarily of granites and metamorphosed granites. Lower Precambrian rocks such as the Apache Group on the eastern edge of the Santa Catalina Mountains are also exposed (Agenbroad, 1967). Paleozoic rocks consist primarily of sedimentary rocks, most of which are limestones with lesser amounts of quartzite, shale, and dolo mite. Epeirogenic seas covered the San Pedro Valley sporadically until the Mississippian Period (Hayes, 1978; Schumacher, 1978). Then the San Pedro became the outer San Pedro shelf on the western edge of the Pedrogosa Basin (Armstrong and Mamet, 1978; Ross, 1978). 22 23 Mesozoic rocks consist of intrusive rocks, volcanics, Cretaceous subaerial clastic rocks, rhyolitic tuffs, and lower Cretaceous marine limestones of the Bisbee Group. The Mogollon topographic high and an offshore island arc to its southwest stretched diagonally from the west ern edge of Arizona through southeastern Arizona (Coney, 1978). A long oceanic trench was to the southwest and the Chinle Basin was to the northeast (Coney, 1978). By the end of the Jurassic Period, the Mohave-Sonoran mega-shear (Coney, 1978) had completed its strike-slip motion, and by the Cretaceous Period, the Bisbee sea crept into the Sonoran embayment (Gilluly, 1956) as the arc rotated to the south (Coney, 1978). The Laramide orogeny resulted in northeast-southwest-directed compression, which caused folding, thrust faulting, and basement up lift. Intermediate to silicic volcanism and plutonism accompanied the compression. Many plutons with copper and molybdenum mineralization were emplaced during the Laramide orogeny (Shafiqullah and others, 1978). From the end of the Laramide orogeny (50 m.y. B.P.) to the Oligocene Epoch (38 m.y. B.P.) was a quiescent period. Detritus from the mountainous areas accumulated to form the Pantano Formation. The Oligocene Epoch consisted of three stages (Shafiqullah and others, 1978). The first stage was a period of heating and crustal melting with intermediate to silicic volcanism, granitic pluton emplace ment, and evolving cauldron complexes (Shafiqullah and others, 1978). The second stage was a transitional period between magmatism and rift ing. The third stage was the Basin and Range disturbance, during 24 which the thin brittle crust rifted and mantle-related basaltic volcanism became prevalent. Basin and Range Disturbance The major uplift of the Basin and Range disturbance occurred during the early to middle Miocene time (22.5 to 12 m .y. B.P.) (Melton, 1965). The disturbance has continued to occur from mid-Miocene to the present (Shafiqullah, 1978). The resulting mountains trend north- northwest and are several kilometers wide. The basins are alluvial filled with both pre-Oligocene detritus and more recent Basin and Range alluvium . Menges (1981) discussed fault-movement mechanics in detail. The basins consist of a network of subgrabens that have shifted along transverse structures. During the Basin and Range disturbance, pre existing fault zones reactivated as secondary structures. The San Pedro Valley can be subdivided at lat 32° N. into the southern (or upper hydrologic) San Pedro and the northern (or lower hydrologic) San Pedro Valleys. The Narrows, which divides the upper from the lower hydrologic basins, is located in the northern San Pedro Valley (i.e., just north of lat 32° N. and Benson, Arizona). The northern part of the southern San Pedro Valley is partially divided from the southern part by the Tombstone Hills and outcrops that extend outward from the Tombstone Hills. The southern part of the southern San Pedro Valley is a long, narrow graben, which has a fairly deep layer of sediments lying on the bedrock. 25 Basin Alluvial Fill The oldest fill in the basins is pre-Basin and Range in age and consists of formations such as the Pantano and Minetta Formations. The Pantano Formation reaches a maximum observed thickness of 6,400 ft and contains clay stone, volcanic, fanglomerate and mudflow units (Finnell, 1970). The San Manuel Formation predates the present struc tural trough but is still a member of the Gila Group (Agenbroad, 1967). The San Manuel Formation is a reddish-brown pebble-to-boulder conglomerate composed primarily of Oracle Granite. It is middle Tertiary in age and occurs in the northern San Pedro Valley. The Quiburis Formation consists of fine-grained deposits, named the "Redington member" by Agenbroad (1967), which grade into coarse grained deposits, named the Tres Alamos member by Agenbroad (1967) and is similar to Montgomery's (1963) Tres Alamos member. The St. David formation (Gray, 1965) is upper Pliocene to middle Pleistocene in age and is exposed near Benson and St. David in the southern San Pedro Valley. The St. David formation is fluviolac- ustrine and can be divided into three major divisions (Gray, 1965): 1. Below 3,800 feet, red clay, red mudstone, and minor small sand lenses predominate. 2. Above 2,800 feet, red clay, limestone, green clay, tuffaceous units, and brown silt predominate, with limestone lenses in the silt and nodules in the clay. 3. Light-brown to grayish-orange silt, silty clay, and fine sand are separated by paleosol and caliche units. Fossiliferous limestone forms resistant ledges. 26 The "granite wash" alluvium of Gray (1965) is a reddish-orange to reddish-brown, fine- to coarse-grained gravel alluvium, consisting mainly of granitic fragments. The "granite wash" is present in both the northern and southern San Pedro Valleys. It lies unconformably on the St. David formation in the southern San Pedro Valley. The Sacaton Formation (Heindl, 1963) in the northern San Pedro Valley is of Pleistocene age. The Sacaton Formation is a gray to reddish-gray gravel cap, which has a composition the reflects the m ountains. In the southern San Pedro Valley, the gravel alluvium is re worked granite wash, quartzitic and volcanic rock fragments with minor amounts of limestone, sandstone, and metamorphic rock fragments (Gray, 1965). Pleistocene lacustrine units in the northern San Pedro Valley are white and are a sequence of basal conglomerate, siltstone, mud stone, and fresh-water limestone deposited in local lakes (Agenbroad, 1967). Former flood-plain and cienega deposits were deposited in Pleistocene and Holocene time to form the Aravaipa surface (Bryan 1926). Melton (1965) dated these sediments in the southern San Pedro Valley as 2500 B.P. to 200 B.P. Agenbroad (1967) discovered at least three levels of aboriginal hearths at least as old as Cochise Culture in age in the northern San Pedro Valley. Before 100 B.P., the channel of the San Pedro River was not well defined (Bryan, 1926; Antevs, 1952; Hastings, 1959; Gray, 1965). Erosion and arroyo cutting were caused by many factors, the most 27 important of which are probably overgrazing and a change to a more arid climate with a shift in the rainfall pattern and intensity. General Geomorphology A report on the interpretation of Landsat imagery in south- central Arizona was done by Calvo (1981) to aid in ground-water exploration. The most important ground-water reservoirs are located between the lower basin-bounding edges of the marginal pediments, including the inner graben (Calvo, 1981). Braided stream deposits of the bajada facies and distal portions of alluvial fans have a relatively high hydraulic conductivity and are found in the basin interior. Very fine grained playa and alluvial plain deposits can obstruct water movement or create artesian conditions, or both (Calvo, 1981). Evaporites in the playa deposits may decrease water quality by imparting a high total dissolved solids content. The basin-bounding faults are hidden beneath pediment surfaces. Generally, the pediment edge may be placed not very far basinward of the line connecting inselbergs and outermost promontories along the mountain-front segments (Calvo, 1981). Recharge occurs along axial drainage ways near younger sur faces, along the bedrock—alluvium contact at the mountain front, and through permeable reaches of stream channels during major flow events. Aquifer recharge can take place along faults as ground-water falls are created across the fault. On satellite imagery, the faults may be de tected as lineaments in the surficial material (Calvo, 1981). 28 Springs and seeps often occur where a fault juxtaposes a water-bearing facies against an impermeable facies. Associated vege tation then forms a lineament pattern in the satellite imagery (Calvo, 1981). Also, if permeable materials plug or cement the fault zone, ground-water may collect on the up slope side of the fault, thus raising the local water levels (Calvo, 1981). Infiltration through the poorly sorted fan deposits is minimal. Dissected fans and fan complexes have a yellowish signature on the false composite Landsat print. This signature is due to red relict soil capping and high clay content (Calvo, 1981). CHAPTER 6 HYDROLOGY In the past the San Pedro River seems to have been a perennial stream, but ground-water withdrawal has now changed the river to only occasional flows. Upper San Pedro Hydrologic Basin The upper San Pedro hydrologic basin (southern San Pedro Valley) is the drainage basin of the north-flowing San Pedro River between the International Border and the Narrows. The San Pedro River drains an area of 2,500 mi2 above the Narrows (Heindl, 1952b). The Tombstone Hills jut into the valley and divide the upper San Pedro Valley into two subbasins. Heindl called the upper subbasin the Charleston subbasin and the lower subbasin the Benson subbasin. North and south of the Tombstone Hills is a flood plain, which is about a quarter to H miles wide (Heindl, 1952b). Holocene alluvium, which supplies shallow ground water to wells, consists of unconsolidated sands and gravels, ranging from 10 to about 120 feet in thickness. Deep wells have penetrated artesian aquifers in both subbasins. The Palominas-Hereford artesian area is about 10 miles long and at least one mile wide (Heindl, 1952b). The Holocene alluvium overlies gypsum, which overlies clay beds of older alluvium containing at least seven sand or gravel members (Heindl, 1952b). The St. David-Pomerene artesian area is wider, especially near St. David, Arizona. A shallow 29 30 artesian zone is encountered at 250 feet, and a deeper artesian zone extends from 600 to 1,400 feet (Heindl, 1952b). The water quality of the two aquifers is similar. Most of the recharge is along the mountain fronts and in sur face flow. The gradient of the water table is about 13 feet per mile (Heindl, 1952b). Most of the discharge is from wells, both artesian and nonartesian, and from evapotranspiration. Lower San Pedro Hydrologic Basin The lower San Pedro hydrologic basin (northern San Pedro Valley) is the drainage basin of the San Pedro River between the Narrows and the mouth of the river near Winkelman, Arizona. The lower San Pedro basin is about 65 miles long and 15 to 30 miles wide, with an area of approximately 1,550 square miles (Heindl, 1952a). The San Pedro River has a fairly steep gradient of approximately 18 feet per mile between the Narrows and Reding ton and approximately 22 feet per mile between Redington and Winkelman (Heindl, 1952a). The average width of the flood plain of the San Pedro River is about half a mile but is greater where large tributaries enter the river. The Holocene alluvium is from 60 to 150 feet thick and is underlain by clay or tightly cemented conglomerate of the older alluvial fill (Heindl, 1952a). Irrigation water is drawn from the Holocene alluvium. Domes tic and stock wells obtain water from Holocene alluvium, older alluvium, or volcanics below the older alluvium. Water in mines is pumped from fault zones in volcanic rocks and rocks of the crystalline and metamorphic complex. Wells in the center of the basin southeast of Mammoth, Arizona, yield artesian water from the older alluvial fill. CHAPTER 7 DEPTH-TO-BEDROCK MODELING Preliminary Depth Estimates The simple Bouguer anomalies were plotted and contoured. When the regional correction was removed from the Bouguer anomaly, a quick estimate of depth to bedrock could be made (Fett, 1983). The estimate assumes that lateral changes in depth to bedrock are so gradual that they can be neglected and that density contrasts change with depth exponentially. Cordell (1973) compared data from Athy (1930), Hedberg (1936), and Howell, Heintz, and Berry (1966) and concluded that the effect of a systematic decrease in density contrast with depth can be simulated by using an exponential density contrast-depth function: Ap(z) = ApQe *z where z is positive vertically downward in units of length, X is in reciprocal length units, and ApQ represents density contrast at the surface. Cordell applied the method to a practical example in the San Jacinto Valley, Riverside County, California, that had been investigated by Fett (1968). Fett used seismic refraction data to determine depths from the surface in the center of the San Jacinto graben to the water table, to the contact between alluvium and the top of Tertiary clastic rocks, and to crystalline basement. Fett derived a five-component 32 33 density model that gave rise to a synthetic gravity anomaly comparable to the actual gravity anomaly within the constraints of the seismic refraction data. Cordell applied the exponential density contrast-depth function to simulate Fett's five-component density model with a ApQ value of 0.55 g/cc and two different X values. Later, as Fett (1983) did more gravity surveys in California and the Basin and Range pro vince , he developed ApQ and X values that seemed to be most appropri ate for modeling gravity anomalies in the alluvial-filled basins of the southwestern United States. The density of the unsaturated allu vial fill is extremely low. Fett's (1983) density-contrast model is likely to vary the most from the actual density contrast directly above the water table. Table 1 shows the variation of the density contrast be tween bedrock and alluvium with depth according to the model. Table 1. Density contrast between alluvium and bedrock at a given depth — From Fett (1983) Depth Density Contrast (feet) (g/cc) 0 0 .7 100 0 .6 1,000 0 .4 2,000 0 .3 3,000 0.22 34 Cordell (1973) evaluated the vertical component of the anomalous gravitational force £ for the case of an infinite (Bouguer slab having constant thickness h, 2ttyA p o (1 ) ■^infinite slab I where y is the gravitational constant. When the density-contrast model in Table 1 is applied as an exponential function to the above equation, the quick estimate table for finding depth to bedrock (Table 2) can be calculated (Fett, 1983). The values in Table 2 may be plotted as in Figure 4. Table 2. Quick estimate table for finding depth to bedrock in the San Pedro Valley. — From Fett (1983) Depth to Bedrock Anomaly (feet) (mgal) 0 0.0 100 0.83 500 3.67 1.000 6.58 1.500 8.97 2.000 11.05 2.500 12.87 3.000 14.37 35 Anomaly Anomaly (Mgato) 1600 2000 2600 Depth to Bedrock (feet) Figure 4. Depth to bedrock versus anomalous gravity values for quick estimate purposes 36 To use this quick estimate table, the residual Bouguer anomaly should be shifted such that the anomaly at the stations on bedrock equal zero. Then the depth to bedrock can be estimated by following the anomalous gravity value to the depth vs. anomaly curve and read ing the corresponding depth-to-bedrock estimate. This is strictly a quick estimate method because a one-dimensional equation was used to develop Figure 4 and the basin-and-range valleys are at least a two- dimensional problem. Topography and outcrop boundaries were taken into considera tion during preliminary contouring of the simple Bouguer values such that final contours would be as realistic as possible. Oppenheimer's (1980) depth-to-bedrock map correlated fairly well with the simple Bouguer anomaly contours and the estimated depths. The southern end of the southern San Pedro Basin is evidently deeper than 3,000 feet. Two-dimensional Modeling Two computer programs are available for two-dimensional model ing of gravity data. The first program, ITMOD (West, 1971), uses Bott's (I960) method. The regional anomaly is chosen such that the Bouguer anomaly equals zero at both ends of the profile. Both ends of the profile are then assumed to be on bedrock. The profile is divided into prisms, and the Bouguer anomaly value at the center of each prism is calculated. The formula for the two-dimensional body with a rectangular cross section is used (Morris and Sultzbach, 1967). Program ITMOD.F10 iterates until the difference between the calculated 37 and observed gravity values are within a specified range or a given number of iterations is completed. The second program, TPLT.F10 (Maclnnes, 1982; Brundage, 1983), is a Talwani (Taiwan! and Ewing, 1960; Taiwan!, 1965) two-dimensional modeling program originally written as a subroutine for a three-dimensional program (Barnett, 1976; Maclnnes, 1983). Program TPLT.F10 is currently a forward modeling program, but the original subroutine, 2DINVS.CDC, is an inverse modeling program. Programs ITMOD and TPLT are both run on the DEC 10 com puter and displayed using interactive graphics on the Tektronix 4010. Subroutine 2DINVS.CDC is run on the Cyber 175 and displayed on the Calcomp Plotter. The variable density-contrast scale shown in Table 3 is incorporated in ITMOD and is used if densities are not specified. Note that the density contrasts are negative. Table 3. ITMOD variable d en sity sca le, From Oppenheimer (1980, p . 29). Residual Gravity Range Density Contrast 0.0 to - 1.5 -0.64 —1,5 to - 5.0 -0.39 -5.0 to -10.0 -0.35 -10.0 to -30.0 -0.30 -30.0 to -35.0 -0.27 <-35. -0.25 38 The density-contrast scale in Table 3 was developed by Oppen- heiraer (1980). Previous density studies, including bulk density measurements and borehole density logs, were used to deduce density values. In the method of vertical prisms, the average density of each prism accounts for the fact that density increases with depth. An unsaturated layer near the surface is accounted for in the average density of each prism. A density value based on the residual gravity value is assigned for each prism. Before Oppenheimer's work, either layers of density values or only one density value was assigned to a profile. If one density value is used, the ends of a basin tend to be too deep and the middle of a basin tends to be too shallow in the modeled profile. Three-dimensional Modeling Maclnnes (1983) developed an inverse three-dimensional model ing program that is based on Barnett's (1976) scheme. A finite-element grid was developed for the modeled basin, density remains constant, and depth is allowed to vary. I did not use an inverse three- dimensional modeling program to model the San Pedro Valley. However, future work in the valley could include a three-dimensional model of at least the southern San Pedro Valley. CHAPTER 7 INTERPRETATION The complete Bouguer gravity anomaly map (Fig. 5, in pocket) and the residual Bouguer gravity anomaly map (Fig. 6, in pocket) were made of the San Pedro Basin by hand contouring both new data and data stored in the Arizona Gravity Data Base. Geology was taken into consideration during the contouring process. The depth-to-bedrock map (Fig. 7, in pocket) was made from the two Bouguer anomaly maps, depth profiles modeled u sin g ITMOD and ITMOD d en sities, local geology, and well data provided by the Water Resource Division of the U.S. Geological Survey or plotted on Drewes's (1980) map. Bedrock in the basins is considered to be those rocks with densities averaging 2.67 glee. However, the bedrock outcrop bound aries are based on hydrological bedrock; therefore, discrepancies may occur at bedrock outcrop boundaries where hydrological bedrock has a density less than 2.67 glee. Lower San Pedro Hydrologic Basin In the lower San Pedro hydrologic basin, the eastern edge of the valley is bounded by volcanics in the Galiuro Mountains. The density of the volcanics is similar to the density of the Tertiary sedi mentary rocks and poorly sorted indurated basin fill. The density difference scales used to model the thickness of alluvium are more appropriate for profiles that compare alluvium to igneous or sedimentary 39 40 bedrocks such as limestone. For this reason, line 1 (Fig. 8) and line 2 (Fig. 9) were modeled such that bedrock was assumed to occur at the west end of each line and near the middle of each line where bedrock also occurred. The eastern ends of the lines were allowed to vary in density difference and depth in the same manner as the portions of the lines that extended over alluvium. However, the volcanics are con sidered to be hydrological bedrock where they crop out on the depth-to-bedrock map. North of line 1, near Mammoth, the complete Bouguer anomaly increases by 10 mgal fairly rapidly on a steady gradient. Drillers' logs from th ree w ells near Mammoth show that unconsolidated sedim ents are present to depths of at least 800 feet and unconsolidated sediments and semi-consolidated Tertiary sediments are present to depths of approxi mately 2,000 feet. The presence of a strong magnetic saddle on the magnetic anomaly map of Arizona north of the strong Bouguer anomaly gradient indicates that a magnetic rock such as diorite bounds and underlies the sediments in the basin east of the Black Hills (Sauck and Sumner, 1970). Older Tertiary alluvium, which is considered to be hydrological bedrock, fills the basin west of the Black Hills. The depths to bedrock south of the strong Bouguer anomaly gradient and north of the Narrows are controlled by the depth profile of line 1 (Fig. 8) and the corresponding Bouguer anomaly contour lines. Density bedrock outcrops on the western edge of this middle section of the lower San Pedro hydrologic basin and volcanic rocks make up most of the Galiuro Mountains on the eastern side of the basin. Line 1 crosses a fairly dense cluster of stations in the lower hydrologic TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet) (Mgals) -6000 -2000 -4000 iue . et-obdok rfl o ln 1 — Program — 1, line of profile Depth-to-bedrock 8. Figure Residual Anomaly Residual ITMOD Densities 41 42 Figure 9. Depth-to-bedrock profile of line 2. — Program ITMOD.FIO (West, 1971) was used to model the profile with the variable density scale developed by Oppenheimer (1980). 43 basin; therefore, a deeper portion of the lower basin may be expected to exist either north or south of this cluster. However, from the avail able data, the deepest portion of the lower basin seemed to occur east of Reding ton beneath line 1. The 4,000 feet of alluvial fill probably consists of Quaternary and Tertiary alluvial fill and Tertiary volcanics. The depth to bedrock where line 1 crosses the bedrock boundary of the Galiuro volcanics is on the order of 2,000 feet. Line 2 crosses the Martinez Ranch fault, then north of Beacon Hill at the south end of the Rincon Mountains, several alluvial fans east-northeast of Beacon Hill, the Narrows, Johnny Lyon Hills, Allen Flat and ends in rhyolitic volcanics on the western edge of the Win chester Mountains. Line 2 follows a Tucson Electric Power Company power line. The data from this line were extremely helpful because the line crosses the most critical features at the boundary between the upper and lower San Pedro hydrologic basins and Allen Flat. The dif ference between Oppenheimer's (1981) depth estimate (1,600+ feet) and my depth estimate (6,000+ feet) in Allen Flat may be partially due to the sparseness of the data from which Oppenheimer made her estimate and partially due to the difference in methods used to calculate depth profiles adjacent to volcanic bedrock. Oppenheimer used what she termed the "repeating prism method" for modeling profiles that cross volcanic bedrock boundaries. I used quartz monzonite and granodiorite in the southern Rincon Mountains and sedimentary rocks in the Johnny Lyon Hills as bedrock for estimating depths along line 2. Also, the Johnny Lyon granodiorite is exposed north and west of the sedimentary rocks in the Johnny Lyon Hills (Cooper and Silver, 1964). In the 44 Winchester Mountains, the Threelinks Conglomerate is overlain by Galiuro Volcanics, both of middle Tertiary age (Cooper and Silver, 1964). Once again, the depth to bedrock where line 2 crossed the Galiuro volcanics bedrock boundary was on the order of 2,000 feet. Drewes’s (1980) map of southeastern Arizona includes some very useful well data and cross sections that were made with the aid of well data. In Allen Flat the DH-8 well passed through 2,183 feet of gravel, sand, and silt that is Holocene to Miocene in age. DH-8 bottomed in alluvium; however, DH-9 passed through 3,600 feet of similar alluvium into a rock that has been only tentatively identified. Drewes's cross section of Allen Flat contradicted the well information by placing first volcanics and than Paleozoic sedimentary rocks beneath the alluvium. The gravity data across Allen Flat tend to correlate better with Drewes's interpretation of his cross section. Outcrops in the Johnny Lyon Hills, Little Dragoon Mountains, Steele Hills, and the southern Winchester Mountains were evidently the justification for Drewes's interpretation. Upper San Pedro Hydrologic Basin Wells in Benson, Pomerene, Dragoon Wash, and St. David passed through at least 1,000 feet of alluvium (Drewes, 1980). Wells west and south of St. David passed through 700 to 600 feet, respec tively, of alluvium (Drewes, 1980). A well near Highway 86, south of Mescal and west of Benson, passed through only 120 feet of alluvium before entering the Bisbee Group (Drewes, 1980). It therefore may be concluded that the alluvium north of Whetstone Mountains and south 45 of Mescal is a shallow pediment. A well at Mescal passed through 250 feet of alluvium (Drewes, 1980). The Benson fault caused the Bisbee Group to surface along the northernmost edge of the Whetstone Mountains (Drewes, 1980). Sauck and Sumner (1970) produced a residual aeromagnetic map of southeastern Arizona at a scale of 1:250,000. This is the same scale as that for the San Pedro Valley maps (Figs. 3, 5-7, in pocket). In the southern San Pedro Valley a few strong magnetic highs correlated directly with Lower Cretaceous quartz monzonite or granodiorite stocks that are possibly mineralized (Drewes, 1980). Tombstone is located on one of these Lower Cretaceous stocks, and one of the stocks is located at the southwestern edge of the Tombstone Hills (Drewes, 1980). Such stocks also occur at the southern end and southeastern edge of the Whetstone Mountains. Possible near-surface occurrences of these stocks were shown as dashed outcrops on Drewes's (1980) map where residual aeromagnetic highs from Sauck and Sumner’s (1970) aeromagnetic map had the same character as the residual aeromagnetic high in the southern end of the Whetstone Mountains. The gravity anomalies for these areas show that bedrock is fairly close to the surface. One inferred stock is three miles east of St. David, another is two miles west of Fairbank, and a third is 5 miles east of Huachuca Vista and 4 miles northwest of Charleston Station at the southern tip of volcanic outcrops west of the Tombstone Hills (Drewes, 1980). Line 3 (Fig. 10) and line 4 (Fig. 11) are very similar except for a graben structure 1,000 to 2,000 feet deeper beneath the eastern half of line 4 that extends to the southeast of line 4. In the Dragoon TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet) (Mgals) -2000 -3000 -1000 iue 0 Dpht-erc poie f ie . Program — 3. line of profile Depth-to-bedrock 10. Figure eiul Anomaly Residual ITMOD Densities I . I ■ i i i i I i * . . . I I i I i 46 TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet) (Mgals) -2000 -3000 -1000 iue 1 Dpht-erc poie f ie . Program — 4. line of profile Depth-to-bedrock 11. Figure eiul Anomaly Residual ITMOD Densities 47 48 Mountains, Paleozoic and Lower Cretaceous sedimentary rocks predomi nate north of Stronghold Canyon, with Precambrian Pinal Schist and granite on the western edge of the sedimentary rocks (Drewes, 1980). These sedimentary rocks are evidently beneath a shallow pediment west of the Dragoon Mountains between Dragoon Wash and Stronghold Can yon. Satellite photographs show an arcuate area of a different tonal shade, representing differing ground cover and geomorphology extend ing west along the probable location of the shallow pediment (NASA, 1972). Lines 3 and 4 end slightly north of Stronghold Canyon. The Stronghold granite of Tertiary age adjacent to Stronghold Canyon extends south for approximately 5 miles. The same rock types present north of the Stronghold granite are also present south of the the granite, with outcrops extending north into the Stronghold granite in the central highest portion of the Dragoon Mountains (Drewes, 1980). Klein (1983) published a contoured residual gravity map of the Dragoon Mountains. The complete Bouguer anomaly value and the residual Bouguer anomaly value for bedrock at the end of lines 3 and 4, adjacent to Knob Hill, are similar to the bedrock values at the end of an east-west extension of line 4 that ended on Stronghold granite south of Knob Hill. The aeromagnetic anomaly east of St. David is between lines 3 and 4. A ridge or subsurface horst that trends north down the center of the Benson subbasin was interpreted from the higher complete Bouguer and residual Bouguer anomalies. An outcrop of Paleozoic sedi mentary rocks is located approximately 6 miles south and one or two miles west of St. David (Drewes, 1980). Cretaceous (or Tertiary) volcanics and sedimentary rocks crop out south of the Paleozoic 49 sedimentary rocks (Drewes, 1980). The two other aeromagnetic anomalies are also south of the Paleozoic sedimentary rocks (Sauck and Sumner, 1976b). The western half of the Benson subbasin was estimated to be 2,200 and 2,600 feet deep beneath lines 3 and 4, respectively. The western half of the subbasin becomes progressively deeper toward the south and was estimated to be from 6,000 to 9,000 feet deep beneath line 5 (Fig. 12). In the depth profile of the eastern half of line 5, Spangler's (1969) data were not taken into consideration and the profile was therefore drastically oversimplified along the eastern half. How ever, the complete Bouguer anomaly map and the depth-to-bedrock map were made using Spangler's (1969) data, Bittson's (1976) data, and other data in the Arizona Gravity Data Base for the San Pedro Valley. Bittson (1976) described the geology of the Whetstone, Mustang, and Huachuca Mountains fairly completely. The Whetstone Mountains are a southwest-tilted fault block. The Precambrian basement complex is exposed along the southern side of the Benson fault on the northern flank and on the eastern flank of the Whetstone Mountains. This Precambrian basement complex is overlain by approximately 6,500 feet of Paleozoic sedimentary rocks and 9,000 feet of Cretaceous sedi mentary and volcanic rocks. A strong characteristic aeromagnetic anomaly, very similar to the three in the San Pedro Basin, is centered over a Lower Cretaceous granodiorite or quartz monzonite stock at the southern end of the Whet stone Mountains. A general east-west trend may be used to correlate the rock types in the Whetstones with the rock types found in the small TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet) (Mgals) -2500 -5000 -7500 iue 2 Dpht-erc poie f ie . Program — 5. line of profile Depth-to-bedrock 12. Figure eiul Anomaly Residual ITMOD Densities 50 51 outcrops in the center of the Benson subbasin. An east-west structure is evident through the southern end of the Whetstone Mountains and the Pantano Formation surfaces at the western edge of this general struc ture. A large inlier is present on the corresponding eastern edge of the Whetstone Mountains. It is possible that a portion of the sediments in the deep basin adjacent to the southeastern edge of the Whetstone Mountains consists of the Pantano Formation. Finnell (1980) stated that the Pantano Formation can reach a thickness of 6,400 feet. Balcer (1983) estimated that a thickness of 4,000 feet could be present near the Huachuca Mountains. The western end of line 5 is located near Paleozoic sedimentary rocks at the southernmost tip of the Whetstone Mountains along Highway 82. The Mustang Mountains are characterized by extensive normal faulting with subordinate amounts of thrust faulting (Bittson, 1976). Hayes and Raup (1968) mapped Paleozoic sedimentary rocks throughout the Mustang Mountains, overlain by Trias sic and Jurassic volcanic and sedimentary rocks. Bittson (1976) modeled a profile that extended from the Huachuca Mountains to the Mustang and Whetstone Mountains. He estimated that the depth to bedrock between the Huachuca and Mustang Mountains approaches 3,000 feet (Bittson, 1976). The Pantano Forma tion crops out along the northern flank of the Huachuca Mountains (Drewes, 1980; Hayes and Raup, 1968). Bittson (1976) found that a density contrast as small as -0.15 g/cc may result for Cenozoic valley fill that consists predominantly of sediments of the Pantano Formation. This means that the deeper estimates of depth to bedrock with smaller density contrasts may be more appropriate where the Pantano Formation 52 is suspected to be a significant portion of the valley fill. Outcrops of Pantano Formation are also present off the eastern flank of the Hua- chuca Mountains (Drewes, 1980: Hayes and Raup, 1968). The eastern flank of the Huachuca Mountains is Precambrian granite, which is overlain by Paleozoic and Cretaceous sedimentary rocks and Jurassic volcanic rocks (Hayes and Raup, 1968). The Paleozoic sedimentary for mations generally trend northwestward and have moderate to steep dips toward the southwest (Bittson, 1976). Pool (1983) plotted the locations for wells in the upper San Pedro hydrologic basin and annotated the wells in the Fort Huachuca area with depths to the top of the Pantano Formation and lithological summaries of the Pantano Formation in each well. Depth to top of the Pantano Formation in the wells was interpreted by Davidson (1973). The top of the Pantano Formation is at a depth of approximately 600 feet in the area east of Huachuca Vista and southwest of the volcanics in the Tombstone Hills. Near Sierra Vista and south of Huachuca Vista, the top of the Pantano Formation is 400 to 500 feet below the surface. West and south of these wells, the Pantano Formation is even closer to the surface, sometimes cropping out (Drewes, 1980; Hayes and Raup, 1968). Line 6 (Fig. 13) extended from the sediments adjacent to the eastern flank of the Huachuca Mountains to the Mule Mountains. The western end of line 6 is not quite on bedrock, therefore the depth esti mates may be progressively shallower than the actual depths to bedrock toward the western end of line 6. Also a smaller density contrast be tween the Pantano Formation and bedrock than that estimated could TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet) (Mgale) -2000 -4000 iue 3 Dpht-erc poie f ie . Program — 6. line of profile Depthrto-bedrock 13. Figure eiul Anomaly Residual ITMOD Densities i i i i i i I i i i I i i 53 54 possibly have resulted in depth estimates that are too shallow along the western end of line 6. Spangler (1969) surveyed the Tombstone Hills in detail. The eastern half of the Benson subbasin is bounded on the south by the Tombstone Hills, a large Jurassic granite or quartz monzonite stock at the southernmost end of the Dragoon Mountains, and Tertiary volcanics between the Tombstone Hills and the Dragoon Mountains. The Charleston subbasin is fairly shallow between the Tombstone Hills and the Mule Mountains. The depths along line 6 directly south of the Tombstone Hills were less than 500 feet and were zero along the northern edge of the Mule Mountains. The western ends of lines 7 (Fig. 14) and 8 (Fig. 15) were based very near bedrock on the eastern flank of the Huachuca Moun tains. The east ends of lines 7 and 8 were located on bedrock in the northwestern edge of the Mule Mountains and near bedrock on the southeastern flank of the Mule Mountains, respectively. The north eastern section of line 7 crosses the Mule Mountains where no data were available. For modeling purposes a midpoint between the two north- easternmost data points was fabricated. Line 8 delineates a large graben, at least 4,000 feet deep, in the Charleston subbasin, which trends northwesterly parallel to the Huachuca and Mule Mountains. The deepest portion of the Charleston subbasin trends northwesterly parallel to the Huachuca and Mule Moun tains. The deepest portion of the Charleston subbasin north of the Mexican border is filled with at least 9,000 feet and perhaps as much as 14,000 feet of sediments. These depths were based on depth profiles TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet x 104) (Mgals) iue 4 Dpht-erc poie f ie . Program — 7. line of profile Depth-to-bedrock 14. Figure eiul Anomaly Residual ITMOD Densities till! I i i i i I i 55 TO.1 (et 17) a ue t mdl h poie ih h variable the with profile the model to used was 1971) (West, ITMOD.F10 est sae eeoe b Opnemr (1980). Oppenheimer by developed scale density Depth (feet x 104) (Mgale) iue 5 Dpht-erc poie f ie . Program — 8. line of profile Depth-to-bedrock 15. Figure eiul Anomaly Residual ITMOD Densities i i i i I i i 56 57 for lines 7 and 8 and on the contoured complete and residual Bouguer anomaly maps. Four thousand feet of Pantano Formation may account for most of the sediments that are deeper than the general level of the large basin-and-range graben structure. Because the Pantano Forma tion here is theoretically at a depth greater than in the Fort Huachuca area, the density estimates used for calculating depth to bedrock are more appropriate. The Mule Mountains are a northeast-tilted fault block trending northwest and comprise Paleozoic and Mesozoic sedimentary rocks, with the exception of Escabrosa Ridge, which is centered along a northwest-trending outcrop of Precambrian Pinal Schist (Drewes, 1980). The northeastern half of the Mule Mountains comprises the Bisbee Group. The tolerance for errors in the depth-to-bedrock estimate in creases with depth. Therefore, errors of 10 to 15 percent are reasonable for depth of 1,000 feet, but errors of 15 to 20 percent may be possible for depths that approach 8,000 feet. Errors may even be greater for one- or two-point depth anomalies as in those in the western end of line 2. Oppenheimer (1980) modeled the basins in southeastern Arizona and used the same density-contrast model and modeling program I used. Oppenheimer and Sumner (1981) stated that depth-to-bedrock estimates may vary by 30 percent. This is a conservative estimate that reflects the regional nature of their map. Volume of Ground Water The volume of alluvial fill was multiplied by a reasonable storage coefficient to calculate the total amount of water stored in the San Pedro Valley. To calculate the volume of alluvial fill, the 58 cross-sectional area of each depth to bedrock profile was estimated by Simpson's rule. Then, by dividing the cross sections of the profiles into rectangles and right triangles, the centroid of each rectangle or triangle was estimated. The centroid for a triangular mass is one-third its altitude above its base (Meriam, 1978). The centroid of the total cross-sectional area is found by summing the products of the individual areas and their respective centroids, then by dividing by the total area. This is done separately for the x and z components. EAx ZAz m and z = ZA ZA The horizontal distance between the centroids of adjacent profiles can be measured in plan view. Then, the Pythagorean theorem was used to calculate the total distance from the horizontal distance and the difference between the centroid depths. Actually, the difference between centroid depths did not change the total volume calculation enough to be noticed after rounding off was done. The average of the two areas was multiplied by the distance between the centroids to obtain the approximate volume between the two profiles. The total volume of the valley was the sum of the volumes between the profiles. The final volume in acre-feet was calculated by using the conversion factor of 1 acre-ft = 43,560 ft3. A storage coefficient of 15 percent may be appropriate for Qua ternary alluvium. However, the total volume of sediments includes Tertiary alluvial fill such as Pantano Formation. Oppenheimer's (1980) 59 conservative estimate of 10 percent for a storage coefficient therefore seemed more appropriate. The western half of line 1 extends west of the San Pedro Valley; therefore, only the eastern half of line 1 was used to calculate the volume of alluvial fill and the volume of water stored in the lower San Pedro hydrologic basin. The volume of alluvial fill and water stored at Allen Flat was calculated separately from the portion of the lower basin that extends north and south of the Narrows. The equation for the volume of a cone was used for the Allen Flat calculations: (1/3) (nr2a). A radius of 20,000 feet and a depth of 7,000 feet were used as the dimensions of th e cone. Table 4 is a compilation of the cross-sectional areas of each depth-to-bedrock profile and the distances between their centroids. The cross-sectional area of Allen Flat is shown separately from the rest of line 2. The centroid for line 2, excluding Allen Flat, was also calculated. The volume of alluvial fill in the upper San Pedro hydrologic basin from the Mexican border to the Narrows is approximately 900 % 10^ acre-ft. If a storage coefficient of 0.1 is used, approximately 90 x 10^ acre-ft of water is stored in the upper San Pedro hydrologic basin. The volume of alluvial fill in the lower San Pedro hydrologic basin, excluding Allen Flat, is approximately 400 x 10^ acre-ft. If a storage coefficient of 0.1 is used, approximately 40 x 10^ acre-ft of water is stored in the lower San Pedro hydrologic basin, excluding Allen Flat. The volume of alluvial fill in Allen Flat is approximately 70 x 10^ acre-ft. The total amount of water stored in 60 the lower San Pedro hydrologic basin, including Allen Flat was calculated to be a little less than 50 x 10^ acre-ft by using a storage coefficient of 0.1. Thus the total amount of water stored in the San Pedro Basin is approximately 140 x 10^. acre-ft. Table 4. Cross-sectional areas and distances between centroids for the depth-to-bedrock profiles that were modeled by using ITMOD d en sities Distance between Line Cross-section area, ft3 Centroids, ft 1 77.6 x 10^ (excluding west of bed 221,700 to Winkelman rock in center of line) 2 114.5 (excluding Allen Flat) 121,400 234.5 (for Allen Flat) 72,000 3 83.0 19,500 4 100.3 67,700 5 194.3 61,200 6 99.2 48,200 7 257.8 16,300 8 242.6 16,300 to Mexican Border CHAPTER 9 SUMMARY AND CONCLUSIONS LaCoste & Romberg gravimeters were used for the gravity- survey . A new gravity base station was established at the Benson Post Office. In 1983, the Post Office was condemned, but it still stands. All gravity maps and data have been calculated using IGSN 71 base station values. The complete Bouguer anomaly values have been cor rected for terrain effects. The residual Bouguer anomaly has had the regional gravity anomaly subtracted from the complete Bouguer anomaly using a two-harmonic trend surface of the regional gravity for Arizona. Residual aeromagnetic data, well data, and geological maps were used along with the gravity data to develop the interpretation. The basin boundaries and hydrologic bedrock outcrop boundaries for my maps were taken from the U.S. Geological Survey's Water Resources Division map (n.d.) of the San Pedro basins. Igneous and sedimentary rocks were considered to be density bedrock for depth-to-bedrock models. Mesozoic- and Cenozoic-age volcanic rocks and Cenozoic-age sediments were not considered to be density bedrock. However, Mesozoic- and Tertiary-age volcanic rocks and Tertiary-age sediments were considered to be hydrologic bedrock. The new gravity data have been used along with older data to develop a more refined model of the subsurface geologic structure of the San Pedro Valley. Depth-to-bedrock profiles that cross the San 61 62 Pedro Valley have been modeled from the gravity data. Bott's (1960) method was used to do two-dimensional modeling. The San Pedro River flows north. Therefore, the upper San Pedro hydrologic basin is south of the lower San Pedro hydrologic basin. The upper basin is divided by the Tombstone Hills into the Charleston subbasin in the south and the Benson subbasin in the north (Heindl, 1952b). The Charleston subbasin was estimated to reach a depth of 9,000 feet. The Pantano Formation probably makes up at least 4,000 feet of the deeper alluvial fill in the Charleston subbasin. The Benson subbasin has a shallow artesian aquifer near St. David, and both subbasins have deeper artesian aquifers. The current model for Allen Flat, based on new data, resulted in an estimated maximum depth of 8,000 feet. Allen Flat is bounded on the north and east by volcanics. Most of the lower San Pedro basin is also bounded on the east by volcanics in the Galiuro Mountains. Bed rock gravity values in the Johnny Lyon Hills, Rincon Mountains, and Santa Catalina Mountains were used to estimate the regional trend for modeling profiles in the lower San Pedro basin. The alluvial thickness along profiles that cross the San Pedro Valley have been used to estimate the amount of ground water stored in the upper and lower San Pedro hydrologic basins. A storage coefficient of 0.1 was used in the calculations. The total amount of water stored in the upper basin was calculated to be 90 x 10^ acre-feet. The total amount of water stored in the lower basin, including Allen Flat, was calculated to be 50 x 10^ acre-feet. 63 Land subsidence in the Benson area has been determined to be the result of collapsible soils (soils subject to hydrocompaction) in the upper 40 feet. Several soils investigations have been performed for several buildings in Benson, including the Benson City Hall and the Methodist Church (Fett, 1983). Because subsidence in similar basins in southeastern Arizona has been shown to be due to ground-water with drawal, significant overdraft of the San Pedro ground-water basins may result in similar subsidence. The density of the gravity stations in the southern San Pedro basin was such that a three-dimension subsurface model could be developed. Maclnnes's (1983) gravity inversion program is a three- dimensional modeling program based on a scheme developed by Barnett (1976). An exponential function could be used for the density-contrast model. Refinements to the exponential density-contrast function are probably necessary for values directly above the water table. Although my work has added considerable information on the subsurface structure of the San Pedro Valley, more work should be done as urbanization increases. 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(eds.), Land of Cochise, New Mexico Geological Society Guidebook, 29th Field Conference: Socorro, New Mexico, p. 175-182. Shafiqullah, M., Damon, P. E., Lynch, D. J., Kuck, P. H., and Rehrig, W. A., 1978, Mid-Tertiary magmatism in southeastern Arizona, in Callender, J. F., Wilt, J. C ., and Clemens, R. E. (eds.), Land of Cochise, New Mexico Geological Society Guidebook, 29th Field Conference: Socorro, New Mexico, p. 231-242. Seff, Philip, 1964, Petrography and origin of the 111 Ranch cherts, Graham County, Arizona: Tucson, Arizona Geological Society Digest v. 7, p. 115-121. Spangler, D. P ., 1969, A geophysical study of the hydrogeology of the Walnut Gulch Experimental Watershed, Tombstone, Arizona: unpublished Ph.D. thesis, University of Arizona, Tucson, 103 P- Sumner, J. S ., and Halverson, P. F ., 1983, Geophysical exploration surveys for ground water in the San Pedro Basin, Arizona, Research Project Technical Completion Report A-108-ARIZ: Tucson, University of Arizona, Department of Geosciences. Sumner, J. S ., Schmidt, J. S ., and Aiken, C. L. V., 1976, Free-air gravity anomaly map of Arizona: Tucson, Arizona Geological Society Digest, v. 10, p. 7-12. Sumner, J. S ., and Schnepfe, 1966, Underground gravity surveying at Bisbee, Arizona, in Mining Geophysics, Vol. 1, Case histories: Tulsa, Oklahoma, Society of Exploration Geophysicists, p. 243-251. Talwani, M., 1965, Computation with the help of a digital computer of magnetic anomalies caused by bodies of arbitrary shape: Geo physics, v. 30, p. 797-817. Talwani, M., and Ewing, M., 1960, Rapid computation of gravitational attraction of three-dimensional bodies of arbitrary shape: Geo physics, v. 25, p. 203-225. 70 West, R. E ., 1971, Iterative two-dimensional gravity modeling program: Laboratory report, Laboratory of Geophysics, Department of Geosciences, University of Arizona, Tucson. Woollard, G. P ., 1979, The new gravity system—changes in interna tional gravity base values and anomaly values: Geophysics, v. 44, p. 1352-1366. v *• U/ <3_ , J» 3 9001 01716 5666 GRAVITY STATION LOCATION MAP SAN PEDRO VALLEY, ARIZONA P.H.F. Halvorson, 1984 Transverse Mercator Projection 33° 00' Scale 1:250 000 N V/1 n k e I m a n 0 5 10 1 20 miles V & 5 0 30 kilometers %.•: H M M_ A % (v* A* z / c | Z fj ) Gravity station \Q Station interval less than one-half mile MN >< 45' Bedrock outcrop boundaries as defined by . • / the Water Resources Division, USGS L approximate i Basin boundaries as defined by mean declination , ( the Water Resources Division, USGS n VV; • V % \ # r: e • * fe % 30' % • • » . e * e • V V _ o V \ z - • in • s • s—#— 9 V- - X T V: 15' w A 'W • 1 \ *' JT-I * / NacmBws of, the . j V San Pedro % . f: >• • v ) # # | x V . 32° €3 z 00' \ e# Womerene ## ^ v z av:; 1 N \ • • • • / Zay V #• •• • * «•* % /•: f "33 is8e * • *v* • e • # • • 6 ,:. L o . • •• 2 . " • . * ...... # c.’V ** : * ; .* •- > f c / > . 3 4/ tv • * VT r * • % . e • e<# .» -• • • • • • • • • • Xb e e 5k • m■•‘‘v;: Tombst % <• •* • in . * X .s * * X . ^ •• •• • • %V\ • • . . x X • .** * * * • •- • \ • . • • •• # u 4 * • t •• V Hubc • #> • • . • • • * . • * » sierra # e •r. V is la 31° 30' % • • # ## » # # • • I • #J ••• • UNITED STA" Mexico” 111° 0 0 'W 45' 30' 110° 0 0 'W 109° 4 5 'W P.H.F. Halvorson, M.S. Thesis, Geosciences, University of Arizona, 1984 Figure 3: Gravity Station Location Map, San Pedro Valley, Arizona COMPLETE BOUGUER GRAVITY ANOMALY MAP SAN PEDRO VALLEY, ARIZONA (IGSN 71) P.H.F. Halvorson, 1984 33° 00' Transverse Mercator Projection N Scale 1:250 000 20 miles 30 kilometers 4 M U III I is/ Complete Bouguer gravity anomaly contour, \ dashed where less certain. Contour interval equals 2 milligals. MN 45' Bedrock outcrop boundaries as defined by the Water Resources Division, USGS approximate Basin boundaries as defined by mean declination f j j i j the Water Resources Division, USGS Wi /I'P- 30' 15' 32° 00 ' N 45' 31° 30' N 111° o o 'w 45' 30' 15' 110° o o 'w 109° 4 5 'W Figure 5: Complete Bouguer Gravity Anomaly Map, San Pedro Valley, Arizona P.H.F. Halvorson, M.S. Thesis, Geosciences, University of Arizona, 1984 RESIDUAL BOUGUER GRAVITY ANOMALY MAP SAN PEDRO VALLEY, ARIZONA (IGSN 71) P.H.F. Halverson, 1984 33° 00' Transverse Mercator Projection N Scale 1:250 000 20 miles 30 kilometers Residual Bouguer gravity anomaly contour, dashed where less certain. Contour interval equals 2 milligals. MN 45' Bedrock outcrop boundaries as defined by the Water Resources Division, USGS approximate Basin boundaries as defined by mean declination the Water Resources Division, USGS V//'v.y / //// / / / / ^ //// / / # «o 30' 15' 32° 00' N 45' 31° 30' N T UNITED STATES MEXICO 111° 0 0 'w 45' 30' 15' 110° 00'W 109° 45'W Figure 6: Residual Bouguer Gravity Anomaly Map, San Pedro Valley, Arizona P.H.F. Halvorson, M.S. Thesis, Geosciences, University of Arizona, 1984 V DEPTH-TO-BEDROCK MAP SAN PEDRO VALLEY, ARIZONA P.H.F. Halvorson, 1984 Transverse Mercator Projection Scale 1:250 000 33° 0 5 10 15 20 miles 00' N 5 0 30 kilometers M M % o \ \ \ \ I Bedrock in the basins is considered to be those rocks with densities greater x y than 2.67 g/cc. However, the bedrock outcrop boundaries are based on hydrologic bedrock. Therefore, discrepancies may occur at bedrock outcrop boundaries, particularly where volcanics are exposed or inlets are filled with pre-basin and range material that is considered semi-permeable to impermeable. Depth-to-bedrock contour, dashed where less certain. Contour interval equals 1000 feet. 30' 15' 32° 00' N 31° 30' N UNITED STATES MEXICO 111%00'W 45' 30' 15' 110° 00'W 109° 4 5 'W Figure 7: Depth-to-Bedrock Map, San Pedro Valley, Arizona P.H.F. Halvorson, M.S. Thesis, Geosciences, University of Arizona, 1984