IV    Report Title

Prepared in Cooperation with the Arizona Department of Water Resources

Hydrogeologic Framework of the Middle San Pedro

Watershed, Southeastern Arizona

Scientific Investigations Report 2010–5126

U.S. Department of the Interior

U.S. Geological Survey
COVER:

View of the Dragoon Mountains looking eastward. Photo by Candice B. Adkins.
Hydrogeologic Framework of the Middle San 

Pedro Watershed, Southeastern Arizona

By Jesse E. Dickinson, Jeffrey R. Kennedy, D.R. Pool, Jeffrey T. Cordova, 

John T. Parker, J.P. Macy, and Blakemore Thomas

Prepared in Cooperation with the Arizona Department of Water Resources

Scientific Investigations Report 2010–5126

U.S. Department of the Interior

U.S. Geological Survey
Cataloging-in-publication data are on file with the Library of Congress (URL http://www.loc.gov/).

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia 2010

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1−888−ASK−USGS (1-888-275-8747)

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov/

Telephone: 1-888-ASK-USGS (1-888-275-8747)

Any use of trade, product, or firm names is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright 

owners to reproduce any copyrighted material contained within this report.

Suggested citation: 

Dickinson, J.E., Kennedy, J.R., Pool, D.R., Cordova, J.T., Parker, J.T., Macy, J.P., and Thomas, B., 2010, Hydrogeologic 

framework of the middle San Pedro watershed, southeastern Arizona: U.S. Geological Survey Scientific Investigations 

Report 2010–5126, 36 p.

This report and any updates to it are available online at:

http://pubs.usgs.gov/sir/2010/5126/
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Physical Setting.....................................................................................................................................3

Physiography................................................................................................................................3

Acknowledgments................................................................................................................................3

Methods of Investigation..............................................................................................................................5

Hydrogeologic Setting...................................................................................................................................5

Upper and Lower Basin Fill Depositional History............................................................................6

Miocene-Pliocene Lower Basin Fill..........................................................................................6

Pliocene-Pleistocene Upper Basin Fill.....................................................................................8

Late Pleistocene–Holocene Stream Alluvium .......................................................................8

Hydrogeologic Framework...........................................................................................................................9

Bedrock ..................................................................................................................................................9

Pre-Basin-and-Range Sedimentary Rocks ......................................................................................9

Basin Fill ...............................................................................................................................................10

Lower Basin Fill..........................................................................................................................10

Upper Basin Fill ..........................................................................................................................10

Stream Alluvium..................................................................................................................................10

Basin Fill Textural Distribution..........................................................................................................10

Airborne TEM Surveys..............................................................................................................10

Subsurface Electrical Properties............................................................................................11

Spatial Distribution of Electrical Resistivity ..........................................................................11

Inferred Basin Fill Lithology from Electrical Resistivity Distribution .................................15

Bedrock Depth and Structures Affecting Groundwater Flow.....................................................15

Gravity Data Collection and Basin Gravity Anomaly............................................................15

Subsurface Density ...................................................................................................................18

Subsurface Density Modeling .................................................................................................18

Density Model Results ..............................................................................................................20

Distribution of Holocene Stream Alluvium .....................................................................................23

Hydrogeologic Framework Model ...................................................................................................23

Construction of the Hydrogeologic Framework Model .......................................................23

Modeled Hydrogeologic Units........................................................................................23

Data Inputs.........................................................................................................................24

Gridding of Hydrogeologic-Unit Horizons..............................................................................25

Building the Model ....................................................................................................................25

Description of the Hydrogeologic Framework Model .........................................................25

Bedrock ..............................................................................................................................25

Coarse-Grained Lower Basin Fill ...................................................................................25

Fine-Grained Lower Basin Fill.........................................................................................26

Interbedded Lower Basin Fill..........................................................................................30

Hydrologic Properties of Hydrogeologic Units .....................................................................30
iv

Aquifer-Test Data..............................................................................................................30

Specific-Capacity Data....................................................................................................30

Saturated Thickness of Coarse-Grained Lower Basin Fill.........................................30

Incorporating the Hydrogeologic Framework Model into a Groundwater-Flow 

Model .........................................................................................................................................32

Summary......................................................................................................................................32

References Cited..........................................................................................................................................33

Figures

1. Map showing the middle San Pedro watershed, in southeastern Arizona. ...................................4

2. Generalized geologic map of the middle San Pedro watershed, southeastern Arizona..............6

3. Generalized east-west section and stratigraphic units in the middle San Pedro watershed, 

southeastern Arizona. ..............................................................................................................................9

4. Airborne transient electromagnetic (TEM) survey flight lines and interpolated resistivity 

values from one-dimensional resistivity models at a depth of 200 m in the middle San Pedro 

watershed, southeastern Arizona........................................................................................................12

5. Locations of wells with geophysical logs and drill logs used in construction of the 

hydrogeologic framework model and the extent of the hydrogeologic framework model........13

6. Depths of the top of the lower basin fill and fine-grained and coarse-grained facies in the 

middle San Pedro watershed indicated by electrical resistivity and sonic velocity logs from 

four wells..................................................................................................................................................14

7. Section A–A’ (fig.2 ) showing an stratigraphic interpretation on the basis of geophysical and 

lithologic data. .........................................................................................................................................16

8. Section B–B’ (fig. 2 ) showing an stratigraphic interpretation on the basis of geophysical and 

lithologic data. .........................................................................................................................................17

9. Sites of gravity measurements in the middle San Pedro watershed and derived gravity 

anomaly used to estimate bedrock altitude. ......................................................................................19

10. Subsurface density profiles from wells and the gravity density function used to assign 

density of layers in the subsurface density model for the middle San Pedro watershed, 

southeast Arizona. .................................................................................................................................20

11. Section C–C’ (fig. 9) showing the layers in the subsurface density model and 

gravity values..........................................................................................................................................21

12. Bedrock altitude determined from subsurface density model in the middle San Pedro 

watershed. ..............................................................................................................................................22

13. Oblique view of a 3D block diagram of the hydrogeologic framework model of the 

middle San Pedro watershed, southeastern Arizona. .....................................................................26

14. Extent and saturated thickness of the coarse-grained lower basin fill hydrogeologic unit 

(HGU) in the hydrogeologic framework model (HFM) of the middle San Pedro watershed, 

southeastern Arizona. ...........................................................................................................................27

15. Extent and thickness of the fine-grained lower basin fill hydrogeologic unit (HGU) 

constructed by the hydrogeologic framework model (HFM) of the middle San Pedro 

watershed, southeastern Arizona.......................................................................................................28

16. Extent and thickness of the interbedded lower basin fill hydrogeologic unit (HGU) 

represented in the hydrogeologic framework model (HFM) of the middle San Pedro 

watershed, southeastern Arizona.......................................................................................................29
v

Tables

1. Physical properties and ages of the stratigraphic units and hydrogeologic units in the middle

 San Pedro watershed, southeastern Arizona...................................................................................7

2. Data sources for hydrogeologic units represented by the hydrogeologic 

 framework model.................................................................................................................................24

3. Available specific capacity data for wells in the middle San Pedro watershed, 

 southeastern Arizona..........................................................................................................................31
vi

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.004047 square kilometer (km2

)

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year 

(hm3

/yr)

foot per year (ft/yr) 0.3048 meter per year (m/yr)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Specific capacity

gallon per minute per foot 

[(gal/min)/ft)]

 0.2070 liter per second per meter 

[(L/s)/m]

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1988 (NGVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

Elevation, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, meter 

squared per day (m2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L)
Hydrogeologic Framework of the Middle San Pedro 

Watershed, Southeastern Arizona

By Jesse E. Dickinson, Jeffrey R. Kennedy, D.R. Pool, Jeffrey T. Cordova, John T. Parker, J.P. Macy, and 

Blakemore Thomas

Abstract

Water managers in rural Arizona are under increasing 

pressure to provide sustainable supplies of water despite rapid 

population growth and demands for environmental protection. 

This report describes the results of a study of the hydrogeologic framework of the middle San Pedro watershed. The components of this report include: (1) a description of the geologic 

setting and depositional history of basin fill sediments that 

form the primary aquifer system, (2) updated bedrock altitudes 

underlying basin fill sediments calculated using a subsurface 

density model of gravity data, (3) delineation of hydrogeologic 

units in the basin fill using lithologic descriptions in driller’s 

logs and models of airborne electrical resistivity data, (4) 

a digital three-dimensional (3D) hydrogeologic framework 

model (HFM) that represents spatial extents and thicknesses 

of the hydrogeologic units (HGUs), and (5) description of the 

hydrologic properties of the HGUs. The lithologic interpretations based on geophysical data and unit thickness and extent 

of the HGUs included in the HFM define potential configurations of hydraulic zones and parameters that can be incorporated in groundwater-flow models.

The hydrogeologic framework comprises permeable and 

impermeable stratigraphic units: (1) bedrock, (2) sedimentary 

rocks predating basin-and-range deformation, (3) lower basin 

fill, (4) upper basin fill, and (5) stream alluvium. The bedrock 

unit includes Proterozoic to Cretaceous crystalline rocks, sedimentary rocks, and limestone that are relatively impermeable 

and poor aquifers, except for saturated portions of limestone. 

The pre-basin-and-range sediments underlie the lower basin 

fill but are relatively impermeable owing to cementation. 

However, they may be an important water-bearing unit where 

fractured. Alluvium of the lower basin fill, the main waterbearing unit, was deposited in the structural trough between 

the uplifted ridges of bedrock and (or) pre-basin-and-range 

sediments. Alluvium of the upper basin fill may be more permeable than the lower basin fill, but it is generally unsaturated 

in the study area.

The lower basin fill stratigraphic unit was delineated into 

three HGUs on the basis of lithologic descriptions in driller’s 

logs and one-dimensional (1D) electrical models of airborne 

transient electromagnetic (TEM) surveys. The interbedded 

lower basin fill (ILBF) HGU represents an upper sequence 

having resistivity values between 5 and 40 ohm-m identified 

as interbedded sand, gravel, and clay in driller’s logs. Below 

this upper sequence, fine-grained lower basin fill (FLBF) HGU 

represents a thick silt and clay sequence having resistivity values between 5 and 20 ohm-m. Within the coarse-grained lower 

basin fill (CLBF) HGU, which underlies the silt and clay of the 

FLBF, the resistivity values on logs and 1D models increase to 

several hundred ohm-m and are highly variable within sand and 

gravel layers. These sequences match distinct resistivity and 

lithologic layers identified by geophysical logs in the adjacent 

Sierra Vista subwatershed, suggesting that these sequences are 

laterally continuous within both the Benson and Sierra Vista 

subwatersheds in the Upper San Pedro Basin.

A subsurface density model based on gravity data was 

constructed to identify the top of bedrock and structures that 

may affect regional groundwater flow. The subsurface density model contains six layers having uniform density values, 

which are assigned on the basis of geophysical logs. The 

density values for the layers range between 1.65 g/cm3

 for 

unsaturated sediments near the land surface and 2.67 g/cm3

for bedrock. Major features include three subbasins within 

the study area, the Huachuca City subbasin, the Tombstone 

subbasin, and the Benson subbasin, which have no expression in surface topography or lithology. Bedrock altitudes 

from the subsurface density model defined top altitudes of 

the bedrock HGU.

The HFM includes the following HGUs in ascending 

stratigraphic order: (1) bedrock and pre-basin-and-range sediments, (2) CLBF, (3), FLBF, and (4) ILBF. Data for the model 

includes digital elevation models, lithology from drill logs, 

geophysical borehole logs, one-dimensional layered profiles 

from electrical-resistivity models, and bedrock altitudes from 

subsurface density models. The saturated thickness of the 

CLBF varies in relation to the depth to underlying bedrock and 

to the presence of the overlying FLBF and IBLF that thicken 

in the basin center. Three areas where the saturated thickness 

of the CLBF is about 1,000 m are south of Redington, northwest of Benson, and southeast of St. David. The thickness is 

about 600 m in the southern portion of the study area near the 
2 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Whetstone Mountains. Thin areas of the CLBF are largely the 

result of shallow bedrock that underlies much of the San Pedro 

River south of Benson and at The Narrows. The extent of the 

FLBF is limited to the central north-south-axis of the Benson 

subarea, and the thickness increases northward to about 300 m 

south of The Narrows. The ILBF is up to 100 m thick and is a 

transition between the CLBF and FLBF. 

Introduction

Water managers in rural Arizona are under increasing 

pressure to provide sustainable supplies of water despite rapid 

population growth and demands for environmental protection. 

In the middle San Pedro watershed in southeastern Arizona 

(fig. 1), groundwater is the primary source of water for 

municipal, domestic, industrial, and agricultural use. Increased 

groundwater pumping could have several undesirable consequences, such as loss of stored groundwater and declining 

water levels, which can lead to increased costs of pumping 

in existing wells, deepening of existing wells, installation of 

new wells, decreased quality of drinking water, and reduction of flow and riparian habitat along the San Pedro River. 

Benson is the largest city in the area (fig. 1) and had a population of about 4,900 in 2005 (U.S. Census Bureau, 2006). The 

population of the area has grown moderately by a factor of 

about 1.5 during the past 20 years (Arizona Department of 

Water Resources, 2005; U.S. Census Bureau, 2006), but it is 

expected to increase more rapidly as residential developments 

are constructed for retirement communities or for satellite 

communities of nearby Tucson, Arizona.

Several entities, including the State of Arizona, the city 

of Benson, Cochise County, the public, and other interested 

parties, are concerned about the future availability and sustainability of the water supply in the middle San Pedro watershed. 

In 2005, the U.S. Geological Survey (USGS) began an investigation of the hydrology of the middle San Pedro watershed in 

cooperation with the Arizona Department of Water Resources 

(ADWR). This investigation is part of the Rural Watershed 

Initiative (RWI), a program established by the State of Arizona 

and managed by ADWR that focuses on water-supply issues in 

rural areas and encourages participation in local partnerships. 

Other RWI projects that were begun in 2005 by the USGS 

include investigations of the hydrology of Detrital, Mojave, 

and Hualapai basins in Mohave County in northwestern Arizona and of the Willcox and Douglas Basins in southeastern 

Arizona. RWI projects that were begun in 1999 by the USGS 

include studies of the hydrology of the Coconino Plateau in 

northern Arizona (Bills and others, 2007), the Mogollon Highlands in central Arizona (Parker and others, 2005), and the 

upper and middle Verde watershed in central Arizona (Blasch 

and others, 2006). 

Effective management of the groundwater supply in the 

middle San Pedro watershed needs to be guided by a comprehensive understanding of the regional aquifers, surface-water 

bodies, and recharge and discharge processes. To obtain this 

understanding, the USGS and ADWR identified three work 

elements: 

1. Refine the hydrogeologic framework;

2. Characterize the groundwater and surface-water system, 

recharge and discharge processes, and water quality in 

selected areas; and

3. Develop a predictive numerical groundwater-flow model 

for examining the potential groundwater and surfacewater responses to changes in recharge and discharge.

This report presents the results of element 1. A hydrogeologic framework model (HFM) of the middle San Pedro 

watershed was developed to represent a new conceptual model 

of the spatial relations, extents, and thicknesses of hydrogeologic units (HGUs) that compose the aquifer system. The 

HGUs represent unconsolidated sediments and consolidated 

rocks that are significant for groundwater flow and occurrence. 

The HGUs are spatially extensive, and each HGU represents 

a unit that has similar geologic and water-bearing properties. 

The HFM provides a mechanism to systematically compile 

and visualize the geometry and spatial extent of the HGUs. 

The HGUs would be used in element 3 to develop a numerical 

groundwater-flow model of the middle San Pedro watershed.

Purpose and Scope

This report presents a synthesis by the USGS, in cooperation with the Arizona Department of Water Resources, of 

new and existing hydrogeologic and geophysical data for the 

middle San Pedro watershed in southeastern Arizona. The 

purpose of the report is to describe features of the hydrogeologic framework of the middle San Pedro watershed that are 

most important for developing groundwater-flow models that 

can be used to help manage water resources in the area. This 

report includes: (1) a description of the hydrogeologic setting 

and depositional history of basin fill sediments that form the 

primary aquifer system, (2) description of the stratigraphic and 

hydrogeologic units (HGUs) that compose the hydrogeologic 

framework, (3) updated bedrock altitudes underlying basin fill 

sediments using gravity and subsurface density models, (4) 

delineation of the stratigraphic units into HGUs in the basin 

fill on the basis of hydrologic and geophysical properties, and 

(5) a digital three-dimensional (3D) hydrogeologic framework 

model (HFM) that represents spatial extents and thicknesses of 

the HGUs.

Descriptions of the hydrogeologic framework include a 

review of the hydrologic and geophysical characteristics of the 

stratigraphic units within the aquifer from previous reports, 

a summary of existing well logs and geophysical logs, and a 

new 3D digital HFM of the aquifer system that describes the 

spatial extent and thicknesses of the HGUs. Discussion of 
Introduction 3

bedrock altitudes includes a summary of interpretations of new 

and existing gravity data, bedrock densities from geophysical 

logs, major faults and areas of shallow bedrock, and the results 

of a subsurface density model of the gravity data. Descriptions of the distribution of fine- and coarse-grained basin fill 

alluvium in the basin fill include a summary of subsurface 

electrical properties from logs and an airborne transient electromagnetic survey.

This report is the first of three planned USGS reports 

on the hydrology of the middle San Pedro watershed, and 

describes the results of the first work element identified by 

ADWR and the USGS. The second planned report would 

describe the groundwater and surface water system, water 

budget, and water quality in selected areas (second element). 

The third planned report would describe a numerical groundwater-flow model that simulates new conceptualizations of the 

groundwater-flow system (third element).

Physical Setting

The middle San Pedro watershed comprises the northern 

portion of the Upper San Pedro Basin and the southern portion 

of the Lower San Pedro Basin (fig. 1). The Upper and Lower 

San Pedro Basins are northwest-southeast trending structural 

basins in Cochise, Pima, and Graham Counties in southeastern 

Arizona. The Upper and Lower San Pedro Basins are separated near the streamflow-gaging station San Pedro River near 

Benson, Arizona (09471800) (fig. 1), at areas of exposed bedrock at The Narrows, north of Benson (fig. 2). The southern 

boundary of the study area coincides with the northern boundary of the Sierra Vista subwatershed and the streamflowgaging station near Tombstone (San Pedro River near Tombstone, Arizona, 09471550). The northern boundary of the 

study area coincides with the streamflow-gaging station near 

Redington (San Pedro River at Redington Bridge, near Redington, 09472050). The western and eastern boundaries coincide with the surface-water divides of the Upper and Lower 

San Pedro Basins. For this report, the portion of the study area 

within the Upper San Pedro Basin is called the Benson subarea 

and the area within the Lower San Pedro Basin is called the 

Narrows-Redington subarea.

The San Pedro Riparian National Conservation Area 

(SPRNCA) extends from the Sierra Vista subwatershed to 

the southern part of the study area. The SPRNCA is managed 

by the Bureau of Land Management and was established in 

1988 by Congress to protect the riparian ecosystem along the 

San Pedro River. The SPRNCA extends about 60 km, from 

the international border north to St. David. The northernmost 

14 km of the SPRNCA are within the Benson subarea.

Physiography

The middle San Pedro watershed is within the Basin 

and Range province of Fenneman (1931) and is characterized 

by rugged, subparallel mountain ranges trending generally 

north-northwest that are separated by eroded desert floor 

underlain by Cenozoic fill (Dickinson, 1991; Anderson and 

others, 1992). The area of the middle San Pedro watershed 

is about 3,500 km2 and it lies in a north-trending structural 

trough that extends from south of the international border 

with Mexico to several km beyond the confluence of the Gila 

River. About half the land surface is bedrock of the mountains 

that form the east and west sides of the basin and in isolated 

outcrops near the intermittent San Pedro River that roughly 

bisects the basin (fig. 2). The study area is about 110 km 

long in the north-south direction, and sedimentary deposits 

that compose the basin fill and valley floor average about 16 

km wide and range from about 1 to 25 km wide. The valley 

floor, which is predominantly an erosional surface, generally 

slopes upward both to the east and west from the San Pedro 

River. Elevations of the valley floor range from about 880 to 

1,500 m, and elevations of the surrounding mountains range 

from about 1,500 to 2,650 m. 

The San Pedro River (fig. 1) drains the study area and 

flows from its origins in Mexico north to the Gila River north 

of the study area. The river is intermittent from the southern 

boundary of the study area to St. David (about 21 km) and is 

ephemeral from St. David to the northern boundary at Redington (about 84 km). All the tributaries to the river are ephemeral, incised less than 10 m, and generally flow at right angles 

to the river. In the Benson subarea, the river valley is about 

40–50 km wide and the basin slopes moderately upward to 

mountain elevations of about 2,000 m on the west and 2,100 m 

on the east. In the Narrows-Redington subarea, the river valley 

is about 30–40 km wide and the basin slopes steeply upward 

to mountain elevations of about 2,600 m on the west and 2,300 

m on the east.

Acknowledgments

The authors thank many landowners that permitted 

land access and installation of hydrologic monitoring equipment. This study would lack essential data without their help 

and support. The authors are grateful to Leslie Graser, Bill 

Remick, Tom Whitmer, and John Fortune of the Arizona 

Department of Water Resources for their technical guidance 

throughout the study. Ross Groom and Laura Davis of Petros 

Eikon Inc. modeled the airborne TEM data collected by Fugro 

Airborne Surveys. Ben K. Sternberg and the Geophysics Field 

Camp of The University of Arizona Department of Mining 

and Geological Engineering conducted geophysical surveys 

near The Narrows, Pomerene, and St. David. The Community 

Watershed Alliance provided a forum for discussing technical 

aspects of the study. Barbara Clark and David Harris of The 

Nature Conservancy helped obtain land access for installation 

of monitoring equipment. Kathleen McHugh, Eric Roudebush, and Leo Leonhart of Hargis+Associates, Inc., provided 

well inventories and geophysical surveys and logs. Mark 

McGowan, John Whittier, Daniel Weber, and Mark Cross of 

Errol L. Montgomery & Associates provided aquifer-test data 
4 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Redington

Benson

St. David

80

82

90

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

 

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

32o15’

10

THE NARROWS

San

R

vi er

P

de

ro

GRAHAM CO

COCHISE CO

ESIHCOC OC

OC AMIP

Cascabel

Pomerene

San Pedro River 

near Benson

(09471800)

San Pedro River

near Tombstone

(09471500)

RINCON

MOUNTAINS

HAPPY

VALLEY

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS JOHNNY LYON HILLS

ALLEN

FLAT

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

UPPER SAN PEDRO BASIN

LOWER SAN PEDRO BASIN

2640

2335

2640 Elevation, m

U.S. Geological Survey streamflow-gaging station

Boundary of the city of Benson

Boundary of the San Pedro Riparian National Conservation Area

Boundary between the Benson subarea and the Sierra Vista Subwatershed

Boundary between the Benson subarea and the Narrows-Redington subarea

EXPLANATION

1863

2264

1746

2350

2265

2050

2560

1910

BENSON SUBAREA

SIERRA VISTA SUBWATERSHED

BENSON SUBAREA

NARROWS-REDINGTON

SUBAREA

Arizona

UPPER SAN PEDRO

BASIN (shown in white)

United States

Mexico

BENSON

SUBWATERSHED

SIERRA VISTA

SUBWATERSHED

Tucson

Figure 1. Map showing the middle San Pedro watershed, in southeastern Arizona. The 

boundary between the Benson and Narrows-Redington subareas, which passes through The 

Narrows, coincides with the boundary between the Upper and Lower San Pedro Basins.
Hydrogeologic Setting 5

and geophysical well logs and helped to obtain land access. 

Nathan Dieterich of the Bureau of Land Management provided 

drill logs for wells in the SPRNCA.

Methods of Investigation

The hydrogeologic framework for the middle San Pedro 

watershed was investigated using a combination of analysis 

and modeling of existing and new geologic and geophysical data to infer the spatial extent and hydraulic properties of 

hydrogeologic units. An extensive literature search of published and unpublished hydrogeologic information was used 

to develop initial conceptual models and identify data needs. 

Preexisting data are available from drill logs for about 1,350 

wells, geophysical logs for 4 wells (Daniel Weber, Errol L. 

Montgomery & Associates, written commun., 2009), and from 

many regional geologic studies (Melton, 1965; Gray, 1967; 

Scarborough and Peirce, 1978; Shafiqullah and others, 1980; 

Drewes, 1981; Haynes, 1987; Menges and Pearthree, 1989; 

Scarborough, 1989; Dickinson, 1991; Hereford, 1993; Smith, 

1994; Saltus and Jachens, 1995; Dickinson, 2003) and hydrogeologic studies (Brown and others, 1966; Freethey, 1982; 

Anderson and others, 1992; Jahnke, 1994; Corell and others, 1996; Pool and Coes, 1999; Goode and Maddock, 2000; 

Leenhouts and others, 2006; Coes and Pool, 2007; Pool and 

Dickinson, 2007).

Data needs included geophysical data for modeling subsurface structures that affect groundwater flow, depth to bedrock and thickness of saturated sediments, extents and depths 

of silt and clay layers within coarse-grained basin fill, and 

the extent and thickness of stream alluvium along the San 

Pedro River. To address these data needs, new data collection 

included airborne transient electromagnetic (TEM) surveys, 

surface and borehole geophysical surveys, gravity surveys, 

examination of drill cuttings, and basic geologic reconnaissance mapping. Geophysical data types collected for mapping subsurface structures, silt and clay layers, and stream 

alluvium include seismic refraction, magnetic soundings, 

controlled-source audiomagnetotellurics (CSAMT), borehole 

electrical resistivity and sonic velocity, airborne and groundbased TEM, and relative gravity measurements. CSAMT and 

airborne, ground-based, and borehole TEM methods measure 

electrical resistivity of the subsurface, which is a measure of 

the ability of the earth to resist the flow of electrical current. Electrical-resistivity values obtained from airborne 

TEM surveys were used to delineate areas of saturated silt 

and clay from saturated sand and gravel and to identify the 

extent of thick sequences of silt and clay layers within the 

basin fill along the main axis of the Benson subarea. Seismic 

refraction data were collected to delineate subsurface layers 

and structures that transmit pressure waves at faster velocities than overlying sediments. Seismic refraction was used 

to locate shallow crystalline rocks near bedrock outcrops at 

The Narrows. Relative gravity measures differences in the 

acceleration of gravity related to variations of the depth of 

crystalline and sedimentary rocks having higher density than 

overlying sediments. Relative gravity methods were used 

to constrain subsurface-density models of Bouguer gravity 

anomalies to identify bedrock structures and depths to the 

bedrock that underlies less dense basin fill sediments.

The HFM was constructed to describe spatial relations 

between bedrock and permeable and less permeable basin fill 

sediments. The HFM was developed using the results of the 

geologic and geophysical modeling and synthesizes the data 

into a single framework. The HFM was constructed using 

ESRI® GIS and Earthvision® software and data from geologic maps, driller reports, geophysical surveys, and subsurface density models.

Hydrogeologic Setting

Geologic exposures and relations in the study area 

reveal a complex tectonic and depositional history that is 

described by previous investigations (fig. 2) (Scarborough 

and Peirce, 1978; Shafiqullah and others, 1980; Drewes, 

1981; Dickinson, 1991). The present geography of southeastern Arizona is mainly the result of extensional forces of 

the basin-and-range disturbance during Oligocene through 

Pliocene time (Scarborough and Peirce, 1978; Shafiqullah 

and others, 1980). Steeply dipping north- to northwest-trending faults resulted in fault-block mountains that are separated 

by structural troughs. The basin-fill alluvium that has been 

deposited in the structural troughs forms the major aquifers 

of southeastern Arizona and the middle San Pedro watershed 

(Anderson and others, 1992).

The hydrogeology is typical of many alluvial basins in 

the southwestern United States (fig. 3, table 1) (Anderson 

and others, 1992) and has been described by several reports 

(Brown and others, 1966; Roeske and Werrell, 1973; Pool 

and Coes, 1999; Coes and Pool, 2007; Pool and Dickinson, 

2007). Groundwater flows through the basin fill aquifer from 

recharge areas near the mountains to perennial reaches of 

the San Pedro River, where it discharges to the stream and 

is transpired by phreatophytes (Pool and Coes, 1999). Rates 

and directions of groundwater flow are dependent on rates 

and distributions of recharge, discharge, and distributions of 

aquifer hydraulic properties. The volume of water in storage can be estimated by the distribution of specific yield 

(drainable porosity), which can be described as the portion 

of water stored in pores that can be drained owing to reduced 

hydraulic head. Specific yield distributions depend on the silt 

and clay content of the alluvial aquifer, because water drains 

readily from pores spaces in coarse-grained deposits of sand 

and gravel but tends to be retained within the small pore 

spaces between grains of silt and clay. Therefore, estimates 

of the volume of stored groundwater depend not only on 

extents of the basin fill aquifer, but also depend on the distribution of fine-grained deposits of silt and clay.
6 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Upper and Lower Basin Fill Depositional History

The depositional history and resulting facies distributions 

within the upper and lower basin fill are primary controlling factors in the hydrologic properties of the hydrogeologic 

framework. The middle San Pedro watershed underwent 

episodes of deposition of basin fill alluvium separated by long 

intervals of erosion, channel incision, and soil formation (Morrison, 1985). The basin fill corresponding to these episodes is 

subdivided into Miocene–Pliocene lower basin fill, Pliocene– Pleistocene upper basin fill, and Holocene stream alluvium.

80

82

90

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

EXPLANATION

Map units—geology modified from

Hirschberg and Pitts (2000)

and Dickinson (2002)

Upper and lower basin fill

Includes St. David Formation

and Quiburis Formation

Stream alluvium and alluvial fans

Pre-Basin and Range

sedimentary rocks

Includes San Manuel Formation, 

Pantano Formation, Mineta Formation

Proterozoic–Cretaceous bedrock

Oligocene–Miocene

Miocene–Pleistocene

Pleistocene–Holocene

Metamorphic rocks

Sedimentary rocks

Volcanic rocks

Granitic rocks

Fault

Limestone

Sedimentary and volcanic rocks

Line of geologic section

A A’ (shown on figures 7 and 8)

B’

B

A

A’

Sa

n

Pedro

River

Benson

St. David

WHETSTONE

MOUNTAINS

RINCON

MOUNTAINS

SANTA CATALINA MOUNTAINS

GALIURO

MOUNTAINS

DRAGOON

MOUNTAINS

Redington dington GRAHAM CO

COCHISE CO

COCHISE CO

PIMA CO

LITTLE

DRAGOON

MOUNTAINS

JOHNNY LYONS

HILLS

WINCHESTER

MOUNTAINS

Cascabel

Pomerene

BENSON FAULT

MARTINEZ RANCH FAULT

CATALINA CORE COMPLEX

Figure 2. Generalized geologic map of the middle San Pedro watershed, southeastern Arizona.

Miocene-Pliocene Lower Basin Fill

The lower basin fill was deposited upon bedrock and 

older Tertiary sediments during the basin-and-range disturbance beginning in the mid to late Miocene (Scarborough and 

Peirce, 1978). Deposition occurred during the tectonically 

active period of basin-and-range disturbance into closed basins 

flanked by fault-block mountains. Deposition was followed 

by an extended period of erosion that removed an unknown 

volume of fill and caused retreat and pedimentation of the 

adjacent mountain fronts. The erosional event was regionally 
Table 1. Physical properties and ages of the stratigraphic units and hydrogeologic units in the middle San Pedro watershed, southeastern Arizona[--, not included]

Stratigraphic 

Unit Age Geologic units

Hydrogeologic 

Unit in 

Hydrogeologic 

Framework Model

Lithologic description

Thickness, 

in 

meters

Resistivity, 

in 

ohm-meters

Sonic velocmeterper se

Stream alluvium Holocene

Quaternary

Holocene alluvium -- Sand and gravel Less than 20 10–50 500–1

Upper basin fill Pliocene- Pleistocene

Tertiary

Upper divisions of St. 

David Formation of Gray (1967)

-- Clay, silt, sand, 

and gravel

Less than 200 10–100 1,500

2,000

Lower basin fill Miocene- Pliocene

Lower division of St. 

David Formation of Gray (1967), Quiburis Formation

Fine-grained lower 

basin fill Clay, siltstone, silt Less than 300

less than 

10–30

1,500

2,000

Interbedded lower 

basin fill

Clay, silt, sand, 

and gravel Less than 300 less than 10–30 1,500

5,000

Coarse-grained 

lower basin fill

Sand and gravel Less than 700 Sand and 

gravel 

30–70

2,000

5,000

pre-basin and 

range 

sediments

OligoceneMiocene

San Manuel 

Formation, Pantano Formation, Mineta Formation

--

Siltstone and 

conglomerate

Greater 

than 3000 10–50 3,000

5,000 

Bedrock

Cretaceous Mesozoic Bisbee Group

Bedrock

Siltstone, 

mudstone, conglomerate

Unapplicable

10–50 3,000

5,000

Paleozoic

Horquilla Formation, 

Escabrosa Limestone, Martin Formation, Abrigo Formation, Bolsa Quartzite

Limestone, 

calcareous sandstone

Often greater 

than 100

3,000

5,000

Proterozoic Pinal Schist

Crystalline and 

metamorphic rocks

Often greater 

than 100

3,000

5,000
8 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

time-transgressive, occurring in southeastern Arizona over as 

much as 5 m.y. from the end of the Miocene to the beginning 

of the Pleistocene (Melton, 1965; Morrison, 1985). The upper 

sections of the lower basin fill in the basin center of the Benson subarea are represented by the lowest fine-grained division 

of the St. David Formation, which is dominated by red clays 

(Gray, 1967). In the Narrows-Redington subarea, the lower 

basin fill is represented by the Quiburis Formation, which is 

exposed owing to Pliocene-Quaternary dissection by the San 

Pedro River (Ladd, 1975; Dickinson, 2003). The Quiburis 

Formation consists of fluviolacustrine sandy silts that increase 

in coarseness to alluvial fan and braidplain facies toward the 

valley margins and abundant diatomite beds that increase in 

thickness northward. Unlike the St. David Formation, the 

Quiburis Formation contains few clay beds in the study area, 

but it contains extensive lacustrine facies downstream north of 

the study area (Dickinson, 2003).

Pliocene-Pleistocene Upper Basin Fill

Upper basin fill represents the depositional period after 

the integration of the Gila River drainage system across 

southeastern Arizona. The upper basin fill primarily is represented in the study area by the middle and upper divisions 

of the St. David Formation (Gray, 1967), which occupies a 

40-km reach of the San Pedro River from The Narrows to 

about 8 km south of the town of St. David. The St. David 

Formation is not present downstream from The Narrows 

(Gray, 1967). The St. David Formation is well exposed in the 

Benson-St. David area, where it forms prominent terraces 

as much as 200 m above the channel of the San Pedro River 

(Gray, 1967). In the middle division of the St. David Formation, red beds are interlayered with white, chalky freshwater 

limestones, green clays, brown silts, and light gray pyroclastic units. The upper division is characterized by brown, 

tan, or orange silts, silty clays, and fine sands separated by 

paleosols and caliche. Although coarser grained sediments 

become more prevalent in the upper division, interlayered 

beds of brown and red clays are not uncommon. Clay-size 

particles are prevalent through the St. David Formation and 

predominant in the lower and middle divisions, where they 

constitute 80 percent of the former and 50–60 percent of the 

latter. The upper division contains about 25–35 percent claysized particles (Gray, 1967).

The end of St. David Formation deposition marked the cessation of a lacustrine-fluvial environment and the integration the 

San Pedro River with the Gila River drainage system. The drop 

in base level and the establishment of a through-flowing stream 

initiated a period of extensive erosion and channel incision that 

included removal of much of the upper member of the St. David 

Formation (Melton, 1965; Gray, 1967). A mid-Pleistocene 

erosional surface is mantled with alluvial gravels, called the 

“gravel wash” by Gray (1967) and “pediment gravels” by Morrison (1985) that is as much as 30 m thick where it fills channels 

cut into the St. David Formation.

Late Pleistocene-Holocene Stream Alluvium 

Most work on the late Pleistocene–Holocene alluvial 

history of the San Pedro River has been conducted upstream 

from the town of St. David, mainly above the southern boundary of the study area. Working in Curry Draw west of Lewis 

Springs, Haynes (1981) used radiocarbon dating to define 

the chronology of a sequence of alluvial deposits that record 

a number of episodes of channel aggradation and degradation from more than 45,000 years ago to historical times. The 

oldest alluvium, which is older than the 45,000-year range of 

carbon-14 dating, was deposited in channels incised into the 

St. David Formation. Subsequent erosion of this unit led to 

formation of numerous springs along San Pedro River tributaries that formed clay and marl marsh deposits between 30,000 

and 13,000 years ago. Desiccation of the marsh deposits 

between 13,000 and 11,000 years led to their erosion, followed 

by deposition of channel sands. Then, between 11,000 and 

9,000 years ago, deposits of a black, organic-rich clay filled 

the channels again, preserving some artifacts of early hunters 

belonging to the Clovis culture (Haynes, 1981). 

The first period of Holocene channel entrenchment 

occurred about 7,000 years ago, as recorded in poorly sorted 

alluvium filling deep channels. That episode was followed by 

at least four more episodes of channel filling and entrenchment before the historical arroyo formation that began in the 

1880s (Haynes, 1981). Whether these episodes of Holocene 

arroyo formation and burial occurred more or less synchronously throughout the Upper San Pedro Basin has not been 

established. Hereford (1993) investigated historical channel 

processes in the San Pedro River from the community of Hereford to Clifford Wash near the southern boundary of the study 

area. He identified a preentrenchment alluvium that was inset 

into the St. David Formation. He believed this to be correlative 

with the McCool alluvium described by Haynes (1981, 1987) 

that was deposited over a period of time beginning about A.D. 

1450 and continuing until the formation of the modern arroyo. 

The timing of onset of arroyo formation is not precisely 

known. Within the reach studied by Hereford (1993), channel 

incision began sometime after 1890 and had migrated almost 

30 km upstream less than 18 years later. Within the middle San 

Pedro watershed, the modern channel is entrenched as much 

as 6 m in the reach north of The Narrows. Arroyo inception in 

the San Pedro River most likely began in the late 19th or early 

20th century, as it did in other entrenched streams throughout 

the western United States (Cooke and Reeves, 1976).

Postentrenchment alluvium includes all channel, floodplain, and terrace deposits inset into the arroyo of the San 

Pedro River. Working upstream from St. David, Hereford 

(1993) described four postentrenchment depositional units 

inset 1–10 m below the preentrenchment surface. These units 

included two terraces and two flood plains above the active 

channel and below the top of the arroyo walls that bound 

the postentrenchment channel system. Judging from aerial 

photographs, most postentrenchment deposits appear to have 

formed after 1937; however, the highest inset terrace had 
Hydrogeologic Framework 9

been emplaced, stabilized, and become vegetated by that time 

and an incipient, lower, younger terrace was visible. The two 

flood-plain units are present in a number of places above the 

active channel. The older flood plain is inset about 1 m below 

the lowest terrace, and deposits are as much as 2 m thick. The 

younger flood plain is inset beneath the older one less than 

1 m. Both flood-plain units are composed mainly of mediumto coarse-grained sand with lenses of granule to pebble-size 

gravel. The older flood plain was not visible in 1937 aerial 

photographs but does appear in 1955 aerial photos, and the 

younger flood plain was first visible in aerial photos of 1970 

(Hereford, 1993). One or more of these depositional units may 

be missing in various reaches along the San Pedro River. No 

detailed study of postentrenchment alluvium has been conducted in that part of the middle San Pedro watershed downstream from St. David.

Hydrogeologic Framework

A conceptual model of the hydrogeologic framework of 

the San Pedro Basin comprises stratigraphic units and structural features that influence groundwater flow. The stratigraphic 

units consist of undifferentiated bedrock, pre-basin-and-range 

sediments, lower basin fill, upper basin fill, and stream alluvium. The 3D framework model includes a division of the 

lower basin fill into coarse- and fine-grained HGUs. The spatial 

distribution of the fine- and coarse-grained HGUs is inferred 

from models of the subsurface electrical resistivity distribution 

Upper basin fill

Lower basin fill

Pre- and post-entrenchment stream alluvium

Upper and lower basin fill sand and gravel

Upper and lower basin fill silt and clay

Pre-Basin and Range sedimentary rocks

Sedimentary rock

Crystalline rock Bedrock

Piezometric surface

Stratigraphic units

Line separating upper and lower basin fill

EXPLANATION

WEST EAST

}

Figure 3. Generalized east-west section and stratigraphic units in the middle San Pedro watershed, southeastern Arizona. 

The east-west width of the watershed is about 40 km in the Benson subarea and 15 km in the Narrows-Redington subarea. Total 

thickness of the basin fill sediments is about 1,000 m or less.

measured during the airborne TEM survey. The pre-basin-andrange sediments, however, are not included as an HGU owing 

to insufficient subsurface data needed to define the spatial 

extent of the sediments. Structural features in the hydrogeologic framework include areas of shallow bedrock that results 

in relatively thin basin fill and areas of deep bedrock associated 

with thick basin fill. The subsurface features were defined by 

the subsurface density models of the gravity data.

Bedrock

Crystalline rocks, limestone, and clastic rocks ranging in 

age from Precambrian to Cenozoic form the mountains and 

underlying bedrock that bound the basin (figs. 2, 3) (Drewes, 

1981; Dickinson, 1991). These rocks have been subjected to 

extensive tectonic deformation, including mid-Tertiary lowangle extensional tectonics that resulted in extensive faulting 

and rotation (Eberly and Stanley, 1978; Scarborough and 

Peirce, 1978; Shafiqullah and others, 1980; Dickinson, 1991). 

The undifferentiated bedrock is not an important aquifer, with 

the exception of the Paleozoic limestone, which is locally an 

important aquifer. 

Pre-Basin-and-Range Sedimentary Rocks

Tertiary sedimentary rocks deposited before the basinand-range disturbance may be minor aquifers along the 

margins of the basins and underlie the basin-fill sediments. 

The sedimentary rocks include the Mineta Formation (Clay, 
10 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

1970), Pantano Formation (Brown and others, 1966; Finnell, 

1970; Balcer, 1984), San Manuel Formation (Dickinson, 

1991), and Teran basin beds (also called the Mineta Formation 

by Grover (1984)). Except for sediments of the San Manuel 

Formation, these fluvial and lacustrine strata were deposited in 

local basins before mid-Tertiary volcanism and were derived 

from the uplifted Catalina-Rincon metamorphic core complex 

to the west of the Narrows-Redington subarea (Scarborough, 

1989). The San Manuel Formation is exposed in three areas 

near Cascabel and contains volcaniclastics mainly derived 

from the Galiuro Volcanics. Pre-basin-and-range sediments 

have been subjected to mid-Tertiary extensional tectonics 

related to the basin-and-range disturbance (Eberly and Stanley, 

1978; Scarborough and Peirce, 1978; Shafiqullah and others, 

1980; Dickinson, 1991). Hydrologic properties of these units 

are generally unknown owing to paucity of data, but they are 

likely to be relatively impermeable. In the Upper San Pedro 

Basin near Sierra Vista, the Pantano Formation may be an 

important water-bearing unit where fractured, but it is generally of low permeability owing to cementation (Pool and 

Dickinson, 2007). 

Basin Fill

Basin fill comprises a sequence of unconsolidated to 

moderately well consolidated alluvial sediments of Late 

Tertiary and Quaternary age derived from erosion of uplifted 

mountain blocks. The basin fill aquifer is greater than 1,000 m 

thick in some areas and is bounded laterally and at depth by 

relatively impermeable crystalline rocks of pre-Tertiary age, 

Paleozoic limestone, Mesozoic sandstone and mudstone, and 

Tertiary prebasin sediments (Pool and Coes, 1999). Basin 

fill was deposited during and following the waning phases 

of mid-Tertiary extensional tectonics. Drainage patterns and 

lateral extents of depositional systems indicate gradations 

from a coarse-grained facies of sand and gravel at the basin 

margins and at depth to a fine-grained facies of silt and clay 

in the basin center. The basin fill can be divided into lower 

and upper parts on the basis of geologic logs, drill logs, and 

physical-property variations that are evident in geophysical 

logs (Pool and Coes, 1999; table 1). The lower basin fill forms 

the primary aquifer and the upper basin fill is unsaturated in 

most areas.

Lower Basin Fill

Sediments of the lower basin fill generally include poorly 

cemented gravel and sand at the basin margins and silt, clay, 

and some evaporates at the basin center (fig. 3). Lower basin 

fill is distinguished from upper basin fill in well logs by 

greater consolidation, higher density, higher sonic velocity, 

and fewer fine-grained interbeds. The coarse-grained facies of 

lower basin fill is commonly described in drill logs as conglomerate or decomposed granite, whereas the fine-grained 

facies of lower basin fill includes gypsum and few sand and 

gravel interbeds. The fine-grained facies is represented by 

the lowest division of the St. David Formation (Gray, 1967), 

and forms a confining bed that divides the basin fill into deep 

and shallow aquifers in the central part of the study area. 

The confining bed varies considerably in thickness and areal 

extent—ranging from more than 300 m in thickness near the 

basin center to nonexistent along the basin margins. Thus, 

groundwater flow is complex in the study area, and the flow 

characteristics are substantially affected by the location and 

properties of the confining bed.

Upper Basin Fill

The upper basin fill unit lies unconformably upon the 

lower basin fill and typically contains a variety of alluvial fan 

and fluviolacustrine sediments and sand and gravel interbeds. 

The upper basin fill is considerably thinner than the lower 

basin fill. The upper basin fill lies above the water table across 

much of the basin. However, local areas of saturated upper 

basin fill are hydrologically important because the deposit can 

be much more permeable than lower basin fill. 

Stream Alluvium

A narrow and thin stringer of highly permeable stream 

alluvium is incised into the basin fill along the major stream 

channels (fig. 3) (Hereford, 1993; Pool and Coes, 1999). The 

stream alluvium is an important local aquifer that drains the 

basin fill aquifer, receives streamflow infiltration, and stores 

water that supports riparian vegetation during periods lacking 

runoff (Pool and Coes, 1999).

Basin Fill Textural Distribution

Airborne TEM Surveys 

Airborne TEM surveys were used to identify the boundary between basin-fill sediments and impermeable crystalline 

rocks, and to map thick fine-grained deposits within coarsergrained deposits in the basin fill (fig. 4). The airborne TEM 

surveys were flown by Fugro Airborne Surveys in 2006 using 

the GEOTEM system (Annan and Lockwood, 1991). In the 

Benson subarea, the surveys consisted of 489 km of approximately north-south lines (25 degrees west of north, parallel 

to the long axis of the basin), 111 km of east-west cross lines, 

and 2 lines (27 km) at 10 degrees east of north (fig. 4). Near 

The Narrows, the surveys consisted of 245 km of north-south 

lines. Depth of investigation of the surveys was 100 to 300 m. 

The GEOTEM system consists of a transmitter on an airplane 

towing a 3-coil receiver. The system transmits a periodic (30 

Hz) magnetic field with a waveform that is a half-sine current 
Hydrogeologic Framework 11

pulse of 4.045 msec. The transmitter is offset from the receiver 

approximately 132 m horizontally and 39 m vertically, which 

theoretically provides better depth resolution than a colocated 

transmitter and receiver. 

One-dimensional (1D) models of electrical resistivity versus depth derived from the airborne and ground-based TEM 

data were developed by Petros Eikon in Toronto, Canada, to 

map subsurface electrical resistivity (Dickinson and others, 

in press). The 1D resistivity models were calibrated at several 

locations on the basis of drill-log control and ground-based 

TEM surveys using EMIGMA software from Petros Eikon, 

Inc. The 1D models comprise six horizontal layers having 

uniform electrical resistivity.

Subsurface Electrical Properties

Electrical properties of the subsurface are known from 

borehole geophysical logs and previous electrical surveys in 

the nearby Sierra Vista subwatershed (Pool and Coes, 1999; 

Fleming and others, 2002; Wynn, 2006) (fig. 5, table 1). Relations between lithology and electrical properties from two 

geophysical logs near Benson were the basis for the lithologic 

interpretation of GEOTEM surveys. Unsaturated sediments can 

be highly resistive—greater than 100 ohm-m in many cases. 

Saturated basin fill tends to be much less resistive, ranging 

from less than 10 ohm-m for clay-rich intervals to more than 

50 ohm-m for sand and gravel intervals that include minor clay 

(fig. 6, wells D-17-20 31AAB and D-17-20 33DCC). Older 

rocks such as limestone and relatively impermeable granite and 

metamorphics are also generally more resistive than saturated 

alluvial sediments—greater than 100 ohm-m. However, lowpermeability sedimentary rocks of Mesozoic and older Tertiary 

age can be electrically indistinguishable from basin fill. Surface 

geology, borehole geologic logs, or other geophysical information are needed to distinguish basin fill from these older finegrained sedimentary rocks. Regardless of the inability to distinguish older sediments from basin fill using available data, low 

resistivity values are indicative of poor aquifers or poor quality 

water that is not suitable for most supply needs. Groundwater 

quality in the study area is generally good, less than 500 mg/L 

total dissolved solids (TDS), with the exception of the extreme 

southwest part of the area, where some marginal water quality, 

about 1,500 mg/L TDS, has been identified (Coes and others, 

1999). Therefore, low resistivity values of 10 ohm-m or less 

generally indicate fine-grained sediments and poor aquifers in 

the study area, or high salinity in the southwestern part.

Spatial Distribution of Electrical Resistivity

Two-dimensional (2D) grids of the resistivity values (250 

m by 250 m) generated from the 1D models along flight lines 

using a 2D natural neighbor interpolation scheme show an 

estimated spatial distribution of resistivity between flight lines 

(fig. 4). Inputs for the interpolation were resistivity values 

from each 1D model within 10-m thick vertical intervals that 

span the entire TEM survey. The resistivity value assigned 

to each cell in the 2D grid was calculated as the arithmetic 

mean of resistivity values within each 10-m interval. For 

horizontal positions having multiple resistivity model data at 

10-m intervals, the arithmetic means of the modeled resistivity values were inputs to the interpolation. Three-dimensional 

(3D) resistivity distributions were constructed by vertically 

stacking each 2D resistivity grid. Data inputs for each interpolation consist of resistivity values from each 1D model within 

10-m thick vertical intervals that span the entire TEM survey 

from altitudes of -100 m to 1,390 m (vertical datum NAVD 

88). Interpolations were limited to resistivity values within the 

altitude of saturated materials on the basis of water levels measured in wells in 2006. A rectangular gap near Benson was left 

out of the interpolation because the TEM survey was not done 

in this area in order to avoid conductive manmade structures 

that distort the electrical response.

Interpolated resistivity values from the 1D models vary 

within broad and continuous areas of low and high values that 

generally align along the regional trend of mountain ranges 

and basins (fig. 4). Resistivity values are mainly restricted to 

basin fill, but some values are available for bounding bedrock 

outcrops. Lower values range from less than 10 ohm-m near 

the central north-south axis of the basin, and larger values 

greater than 500 ohm-m occur near bedrock outcrops at the 

basin margins and at The Narrows (fig. 4). 

Areas of low resistivity values, from less than 10 to 30 

ohm-m, between altitudes of 990 and 1,000 m (fig. 4) are 

common along three major areas. Such an area underlies 

much of the lowest land-surface elevations within the central 

part of the basin and extends northward from areas of high 

resistivity near GEOTEM east-west line 38040 to another 

area of high resistivity south of The Narrows. This central 

area may be continuous across the data gap near Benson. A 

region of low resistivity in the southwestern part of the study 

area, visible at the western end of line 38030, coincides 

with an area having salinity greater than 500 ppm, which 

may contribute to lower electrical resistivity in saturated 

sediments. Another area of low resistivity was mapped at 

the western end of line 38060 (fig. 4). This area is at higher 

land-surface elevations and may indicate conductive bedrock 

near the land surface; it appears to indicate a feature that is 

discontinuous from the central area. 

Areas with larger resistivity values are near small mapped 

outcrops of Paleozoic sedimentary and limestone rocks that are 

surrounded by basin fill, south of line 38030 (fig. 4). Resistivity 

values between 30 and 100 ohm-m occur near and to the east 

of the small outcrops. Additional areas with values of 30-100 

ohm-m occur within a broad area between the large data gap 

near the cities of Benson and Pomerene and the Dragoon 

Mountains, and also along the Whetstone Mountains. High 

salinity measured at two wells (1,250 and 651 mg/L) in the 

southwestern areas may contribute to low resistivity values. 
12 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

EXPLANATION

Greater than 500

100−500

50−100

30−50

20−30

10−20

Less than 10

Interpolated electrical resistivity values

from 1-D models at depth of 200 m, in ohm-m

38060

Airborne TEM flight line and flight line number

Map units—geology modified from

Hirschberg and Pitts (2000)

and Dickinson (2002)

Upper and lower basin fill

Includes St. David Formation

and Quiburis Formation

Stream alluvium and alluvial fans

Pre-Basin and Range

sedimentary rocks

Includes San Manuel Formation, 

Pantano Formation, Mineta Formation

Proterozoic–Cretaceous bedrock

Oligocene–Miocene

Miocene–Pleistocene

Pleistocene–Holocene

Metamorphic rocks

Sedimentary rocks

Volcanic rocks

Granitic rocks

Limestone

Sedimentary and volcanic rocks

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

80

82

90

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

Sa

n

Pedro

Rive

Benson r

St. David

WHETSTONE

MOUNTAINS

RINCON

MOUNTAINS

SANTA 

CATALINA MOUNTAINS

GALIURO

MOUNTAINS

DRAGOON

MOUNTAINS

Redington dington GRAHAM CO

COCHISE CO

COCHISE CO

PIMA CO

LITTLE

DRAGOON

MOUNTAINS

JOHNNY LYONS

HILLS

WINCHESTER

MOUNTAINS

Cascabel

Pomerene

THE

NARROWS

ALLEN

FLAT

30010

30020

30030

30100

30100

30110

30120

30130

30140

30150

30160

30060

30070

30080

30090

30050

30040

30010

28010

20010

28030

20020

20030

20040

20050

20060

20070

20080

20090

28030

28020

30020

30030

30100

38030

38040

38050

30110

30120

30130

30140

30150

30160

30060

30070

30080

38010

38020

30090

30050

30040

38030

38040

38050

38060

18040

18030

 Figure 4. Airborne transient electromagnetic survey (TEM) flight lines and interpolated resistivity values from 1D 

resistivity models at a depth of 200 m. Electrical resistivity values are generally less than 10 ohm-m in the central parts of 

the basin and greater than 100 ohm-m along the basin margins and in areas of shallow bedrock. The survey omitted an 

area of nearly 100 km2

 surrounding the city of Benson.
Hydrogeologic Framework 13

MOUNTAIN

R

S

INCON

MOUNTAINS

WHETSTONE

Redington

Benson

St. David

80

82

90

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

THE

NARROWS

DRAGOON

MOUNTAINS

San

R

vi er

P

de

r

o

GRAHAM CO

COCHISE CO

ESIHCOC OC

OC AMIP

WINCHESTER

MOUNTAINS

GALIURO

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

Cascabel

Pomerene

o

D-17-20 31AAB

D-17-20 33DCC

D-20-21 06DDA1

D-19-21 29ACB1,2

0–20

20–60

60–100

100–200

200–300

and 300–815

and

Well having lithology log.

Larger circles represent wells

also having a geophysical log

Well depth, in m

EXPLANATION

Extent of the hydrogeologic 

framework model

 Figure 5. Locations of wells with geophysical logs and drill logs used in construction of the 

hydrogeologic framework model and the extent of the hydrogeologic framework model.
14 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona DEPTH, IN METERS

D-17-20 31AAB

SONIC VELOCITY,

IN METERS 

PER SECOND

0 5,000

RESISTIVITY, IN OHM-METERS

0

0

50 100 150 200

100

200

300

400

0 50 0 5,000

DEPTH, IN METERS

D-17-20 33DCC1

RESISTIVITY, 

IN OHMMETERS

0

100

200

300

400

SONIC VELOCITY,

IN METERS 

PER SECOND

DEPTH, IN METERS

D-20-21 06DDA1

0 5,000

SONIC VELOCITY,

IN METERS

PER SECOND

RESISTIVITY, 

IN OHMMETERS

RESISTIVITY, 

IN OHMMETERS

50

0

100

200

0

DEPTH, IN METERS

D-19-21 29ACB1,2

SONIC VELOCITY,

IN METERS

PER SECOND

0 50

0

100

200

0 3,000

Fine to coarse 

Fine to coarse 

Fine Coarse

Fine Coarse

Fine Coarse

Coarse Fine

EXPLANATION

Lower basin fill

Upper basin fill 8-inch electrode separation

Resistivity logs

16-inch electrode separation

64-inch electrode separation

Sonic velocity log

Figure 6. Depths of the top of the lower basin fill and fine-grained and coarse-grained facies in the middle San Pedro 

watershed indicated by electrical resistivity and sonic velocity logs from four wells.
Hydrogeologic Framework 15

Inferred Basin Fill Lithology from Electrical 

Resistivity Distribution

Inferred lithology using resistivity and drill logs (figs. 7 

and 8, panels A and B) ranges from crystalline bedrock and 

limestone on the basin margins to a thick sequence of basin 

fill, more than 300 m, including an interval of silt and clay 

as thick as 200 m, underlain by sand and gravel in the deepest part of the basin. The top and lateral distribution of the 

fine-grained facies in the basin center is indicated by a zone 

of resistivity values of 10 ohm-m or less and by drill logs. The 

fine-grained facies is bounded at the top and lateral margins 

by an interval of 10–30 ohm-m material that likely represents 

a gradation from the fine-grained facies to the surrounding 

coarse-grained facies of basin fill. A significant wedge of 

medium-grained basin fill is mapped by a wedge of material 

having resistivity of 10–50 ohm-m and overlies the finegrained basin fill east of the San Pedro River (fig. 7). Interpolated resistivity values compare favorably with the electric 

log at the well D-17-20 33DCC, where the saturated zone has 

values of 20–30 ohm-m in logs for wells D-17-20 31AAB and 

D-17-20 33DCC (fig. 6). A zone of 10–20 ohm-m resistivity at 

depths of about 170 to 210 m in the electric log likely represents the margin of the fine-grained facies that is evident as the 

thick interval of 10 ohm-m or less resistivity immediately east 

of the well and at the basin center.

Bedrock Depth and Structures Affecting 

Groundwater Flow

Subsurface geologic features, such as buried faults and 

areas of shallow bedrock, can result in spatially varying hydrologic flow properties of the subsurface. Faults can be a barrier 

to groundwater flow because of (1) juxtaposition of high- and 

low-permeability materials and (2) low-permeability material 

(fault gouge) in the fault zone owing to pulverization of rocks 

or deposition of minerals along the fault. Areas of shallow 

bedrock can affect groundwater flow because thin, permeable 

sediments overlying the bedrock are less transmissive than 

areas of thicker sediments of equivalent permeability. The 

amount of groundwater flow generally is reduced, resulting 

in increased groundwater levels on the upgradient side of the 

shallow bedrock.

Bedrock depth and subsurface structures in the middle 

San Pedro watershed have been investigated both in regional 

gravity studies and in more focused local studies. Regional 

studies include that of Oppenheimer and Sumner (1980), who 

constructed 2D profiles across alluvial basins throughout 

Arizona and then interpolated between profiles to produce a 

3D depth-to-bedrock map. The study assumed that the average sediment density in a particular basin varies as a function 

of the gravity anomaly; larger anomalies were inferred to 

correspond with deeper basins and higher average densities. 

Density values ranged from about 2.04 g/cm3

 to 2.42 g/cm3. 

Saltus and Jachens (1995) constructed a depth-to-bedrock 

map across the entire Basin and Range Province using constant densities of 2.02 g/cm3

, 2.12 g/cm3, 2.32 g/cm3, and 2.42 

g/cm3 for depth intervals of 0–200 m, 200–600 m, 600–1,200 

m, and greater than 1,200 m, respectively. The authors also 

accounted for low densities associated with Cenozoic volcanic rocks. Gettings and Houser (2000) developed subsurface 

models for the Sierra Vista area that included the mid-Tertiary 

Pantano Formation.

Likely the first gravity study to focus on the middle San 

Pedro watershed was that of Spangler (1968), who collected 

data at 360 stations in the Walnut Gulch Experimental Watershed. Spangler (1968) modeled a 1,000-m-thick sequence of 

alluvium in the Tombstone subbasin along a single two-dimensional profile using an alluvium density of 2.27 g/cm3

. Bankey 

and Kleinkopf (1982) collected data at 112 gravity stations in 

the Whetstone Mountains to evaluate mineral resource potential. No interpretation of the data was published, but those 

data are incorporated in the present study. Halvorson (1984) 

collected gravity data at 273 new stations in the San Pedro 

Valley. 2D models of the data were constructed using the same 

anomaly-density relationship presented by Oppenheimer and 

Sumner (1980). Halvorson (1984) modeled bedrock depths of 

about 800 m for the Huachuca City subbasin and about 700 m 

for the Tombstone subbasin. Schwartz (1990) measured gravity values at a station spacing of about 1 mile throughout the 

rugged, 214-km2 Galiuro Mountains. 2D models were constructed using field-measured density values of 2.43 g/cm3

 for 

volcanic rock and 2.16 g/cm3 for alluvium. Schwartz (1990) 

investigated the internal structure of the Galiuro volcanic 

complex, and alluvium thickness in the basins bounding the 

mountain range was not estimated.

Gravity Data Collection and Basin Gravity 

Anomaly

Existing and newly collected gravity data were used to 

constrain the subsurface density model. New gravity data were 

collected at 266 stations in the study area (fig. 9) using Lacoste 

and Romberg D-127 and D-209 relative gravity meters. Existing gravity data were obtained from the Pan-American Center 

for Earth and Environmental Studies (PACES) dataset (PACES, 

2008), which in turn is based largely on the National Oceanic 

and Atmospheric Administration (NOAA) National Geophysical Data Center (NGDC) Land and Marine Gravity Data compilation (Dater and others, 1999). Individual gravity surveys of 

the Whetstone Mountains (Bankey and Kleinkopf, 1982), San 

Pedro Valley (Halvorson, 1984), Galiuro Mountains (Schwartz, 

1990), and Walnut Gulch Experimental Watershed (Spangler, 1968) contributed to most of the data available through 

PACES. Other sources of data obtained through PACES/NGDC 

data are from the Defense Mapping Agency Aerospace Center 

and the Arizona Gravity Data Base maintained at the University of Arizona in the 1970’s and 1980’s.
16 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

800

900

1,000

1,100

1,200

1,300

1,400

ALTITUDE, METERS

0 5,000 10,000 15,000 20,000 25,000 30,000

DISTANCE, METERS

Resistivity,

in ohm-m

Depth, in m

Sonic velocity,

in m/s

0 50

0 5,000

100

0

200

300

400

?

?

?

?

?

?

? ?

100

0

200

300

400

Depth, in m

C. Hydrogeologic units in hydrogeologic framework model

Layer 2

Layer 5 Layer 6

Layer 4

Layer 3

Layer 6

Layer 2

Layer 5 Layer 6

Layer 4

Layer 3

Layer 6

D. Groundwater model layering based on hydrogeologic units

San Pedro

River

Fine and coarse

streambed alluvium

layer 1

Fine and coarse

streambed alluvium Upper basin fill

Lower basin fill

800

900

1,000

1,100

1,200

1,300

1,400

1,500

0 5,000 10,000 15,000 20,000 25,000 30,000

DISTANCE, METERS

0 5,000 10,000 15,000 20,000 25,000 30,000

DISTANCE, METERS

ALTITUDE, METERS

Crystalline

bedrock

Crystalline and limestone bedrock

Crystalline and limestone bedrock

Coarse

Coarse

Coarse

Coarse

Coarse

Fine

Fine

Fine

Fine

800

900

1,000

1,100

1,200

1,300

1,400

1,500

ALTITUDE, METERS

800

900

1,000

1,100

1,200

1,300

1,400

1,500

ALTITUDE, METERS

D-17-20 33DCC

EXPLANATION

Boundary between upper and lower basin fill

Extent of interpolated electrical resistivity

Piezometric surface inferred from water levels

in wells along section A-A’

Cased interval in well

Screened interval in well

A. Interpolated 1D resistivity models

B. Hydrogeologic units based on interpolated resistivity and lithology from driller’s logs

Interpolated electrical resistivity

from 1D models, in ohm-m

100-500

50-100

30-50

20-30

10-20

Less than 10

D-17-20 33DCC

0 5,000 10,000 15,000 20,000 25,000 30,000

DISTANCE, METERS

Interbedded

fine and coarse

Interbedded

fine and coarse

Interbedded

fine and coarse

A A’

A A’

D-17-20 33DCC

A A’

A A’

 Figure 7. Section A–A’ (fig.2 ) showing a stratigraphic interpretation on the basis of geophysical and 

lithologic data. A, Interpolated one-dimensional electrical-resistivity models from the land surface to 

200 m depth in the center of the section, and 300 m elsewhere. B, Interpreted hydrogeologic units using 

electrical resistivity and lithology from driller’s logs. C, Section within the digital hydrogeologic framework 

model. D, Possible configuration of hydrogeologic zones and layers for a numerical groundwaterflow model. The interpolated resistivity values and the resistivity log at well D-17-20 33DCC indicate 

similar sequences of resistivity values. The sonic velocity log at well D-17-20 33DCC and the lithologic 

descriptions from driller’s logs indicate a possible boundary between the upper and lower basin fill. 
Hydrogeologic Framework 17

EXPLANATION

Extent of interpolated electrical resistivity

Piezometric surface inferred from water levels in

wells along section B-B’

Cased interval in well

Screened interval in well

Interpolated electrical resistivity

from 1D models, in ohm-m

100-500

50-100

30-50

20-30

10-20

Less than 10

INTERBEDDED

FINE AND COARSE

Crystalline bedrock Crystalline, sedimentary,

and limestone bedrock

Crystalline, sedimentary,

and limestone bedrock Crystalline

bedrock

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000

700

800

900

1,000

1,100

1,200

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000

Interbedded

fine and coarse

Narrows-Redington The Narrows Benson subarea

subarea

Narrows-Redington The Narrows Benson subarea

subarea

Narrows-Redington The Narrows Benson subarea

subarea

Narrows-Redington The Narrows Benson subarea

subarea

No electrical

resistivity survey

No electrical

resistivity survey

DISTANCE, METERS

ALTITUDE, METERS

700

800

900

1,000

1,100

1,200

ALTITUDE, METERS

700

800

900

1,000

1,100

1,200

ALTITUDE, METERS

DISTANCE, METERS

DISTANCE, METERS

B B’

B B’

DISTANCE, METERS

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000

700

800

900

1,000

1,100

1,200

ALTITUDE, METERS

Fine

Fine

Fine

Coarse

Interbedded

fine and coarse

Layer 2

Layer 6 Layer 6 Layer 6 Layer 6

Layer 6 Layer 6

Layer 3 Layer 3

Layer 4 Layer 4

Layer 5 Layer 5

Layer 2

B B’

B B’

Coarse

Coarse

Fine

Fine

Interbedded

fine and coarse

C. Hydrogeologic units in hydrogeologic framework model

D. Groundwater model layering based on hydrogeologic units

A. Interpolated 1D resistivity models

B. Hydrogeologic units based on interpolated resistivity and lithology from driller’s logs

Figure 8. Section B–B’ (fig.2 ) showing a stratigraphic interpretation on the basis of geophysical 

and lithologic data. A, Interpolated one-dimensional electrical-resistivity models from the land 

surface to 200 m depth in the center of the section, and 300 m elsewhere. B, Interpreted lithology 

using electrical resistivity. C, Section within the digital hydrogeologic framework model. D, Possible 

configuration of hydrogeologic zones and layers for a numerical groundwater-flow model.
18 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

In compiling the initial NGDC gravity dataset (later 

incorporated into the PACES dataset), quality control procedures were maintained by NOAA to remove outliers and poor 

data. All data were made consistent with the IGSN71 gravity 

datum. To verify the compatibility of the PACES data with 

data newly collected for this study, an A10 absolute gravimeter 

by Micro-g Lacoste, Inc. was used to establish new absolute gravity benchmarks near gravity stations in the PACES 

dataset. Although direct comparison is difficult because the 

PACES and A10 stations are not exactly coincident (the A10 

stations were located on existing benchmarks for ease of measurement), the measurements agree to within 1 mGal. Data 

retrieved from PACES in NAD27 and NGVD29 elevation 

datums were converted to NAD83 and GRS80 datums. Only 

the measured gravity values from PACES were used; free-air, 

Bouguer, and terrain corrections were performed independently for this study.

The complete Bouguer gravity anomaly (CBA) represents the departure from the expected gravitational field at a 

given latitude, land surface elevation, and surrounding terrain, 

assuming a constant subsurface density. Differences from the 

expected gravitational field are caused primarily by heterogeneities in subsurface density. To calculate the CBA, gravity 

data were corrected using the International Gravity Formula 

based on the GRS80 ellipsoid, a free-air correction with a 

vertical gradient of 0.3086 mGal/m, a Bouguer slab correction 

with a density of 2.67 g/cm3

, and a Bullard B spherical cap 

correction (LaFehr, 1991). Terrain corrections were carried to 

a distance of 167 km from each station using a combination 

of methods described by Kane (1962) and Nagy (1966). The 

CBA data were gridded using a minimum curvature algorithm.

The CBA includes the cumulative effect of all subsurface density heterogeneities. To isolate anomaly features 

caused only by low-density basin fill, density contrasts 

caused by isostatic compensation and nonuniform bedrock 

density were removed. Isostatic compensation was modeled using the Airy root model (Simpson and others, 1983), 

assuming a depth of sea level compensation of 30 km and 

a Moho density contrast of 0.33 g/cm3. The gravitational 

response of the isostatic compensation, ranging from about 

–100 mGal in the northwest corner of the study area to –123 

mGal in the southeast corner, was subtracted from the CBA 

to create the isostatic residual anomaly.

To remove the effect of nonuniform bedrock density, a 

gridded basement anomaly for the Basin and Range Province, 

representing the gravitational effect of pre-Cenozoic formations and calculated using an iterative procedure from gravity 

stations located on pre-Cenozoic outcrops (Saltus and Jachens, 

1995), was subtracted from the isostatic residual anomaly to 

create the basin gravity anomaly (fig. 9). Because the procedure to produce the basement gravity anomaly is dependent 

on an estimation of the depth-density relation in Cenozoic 

sedimentary fill, the basin gravity anomaly would differ somewhat if a different depth-density relation were used. However, 

2D modeling of the isostatic residual anomaly suggests that 

the influence of the depth-density relation is minimal and the 

basement gravity surface would differ only slightly if a different relation were used. A number of Cenozoic rock formations in the study area are not incorporated in calculating the 

basement gravity anomaly. These include volcanic rocks in the 

Galiuro Mountains, volcanic rocks east of Tombstone (but not 

Cretaceous volcanic rocks west and north of Tombstone), and 

crystalline rocks in the Dragoon Mountains. 

The basin gravity anomaly (fig. 9) includes the effects of 

both low-density basin fill deposits and low-density Cenozoic 

volcanic rocks, but it also reflects errors in data collection and 

assumptions made in the previous steps. Areas of exposed 

basement rocks are uniformly near zero mGal, indicating that 

the density used for these areas (2.67 g/cm3

) in the Bouguer 

and terrain corrections was appropriate. Areas mapped as 

basin fill alluvium show negative gravity anomalies.

 Subsurface Density

Subsurface density contrasts between bedrock and basin 

fill sediments were estimated using borehole gamma-gamma 

density logs and sonic velocity logs (fig. 10). Density was 

estimated from the sonic velocity logs by using a polynomial 

expression of the Nafe-Drake curve (Brocher, 2005). The 

borehole gamma-gamma density logs are from three wells in 

the Sierra Vista subwatershed, and the sonic velocity logs are 

from two wells about 6 km southwest of Benson and two wells 

near the San Pedro River 25 km south of Benson. 

Density values increase at greater depths in logged intervals in upper basin fill and are nearly uniform in lower basin 

fill (fig. 10). The lower basin fill is at about 30 m depth in wells 

D-19-21 29ACB2 and D-20-21 06DDA1. Above 30 m, density 

values increase rapidly with greater depth, varying from less 

than 1.5 g/cm3 at a depth of 12 m to 2.2 g/cm3 at a depth of 30 

m. Density values are uniformly about 2.2 g/cm3 below 30-m 

depth. Similarly, density values in logged intervals of wells 

D-17-20 31BAA and D-17-20 33DCC are nearly uniform, 2.2 to 

2.3 g/cm3, below depths of 140 m and 160 m, respectively. Density values in logged intervals in wells TW3, D-21-20 11BCD, 

and D-21-20 13CBB1 increase throughout the interval, varying 

from 2.1 g/cm3 to 2.3 g/cm3 between depths of 65 m and 170 m. 

Based on these data, a variable depth-density relationship was 

calculated for the upper basin fill (fig. 10). 

Subsurface Density Modeling

A 3D subsurface density model was constructed to 

simulate the basin gravity anomaly. To interpret the gravity 

anomaly, a surface-based Fourier transform method (Parker, 

1972) was used to estimate depth to bedrock through iterative 

inverse modeling. In this method, one or more surfaces, rather 

than discrete volumes, are used to denote density contrasts. 

This method is well suited for modeling sedimentary basin 

depths in the Basin and Range Province, where the contrast 

between basin fill and basement rocks is distinct and continuous (Saltus and Jachens, 1995). The gravitational response of 
Hydrogeologic Framework 19

Site of gravity measurement

Line of section showing gravity values and 

subsurface density distribution

Basin gravity anomaly,

in milligals

EXPLANATION

-10.8

-60.3

C C’

Fault

10

90

82

C

C

C’

St. David

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

GRAHAM CO

COCHISE CO

HUACHUCA CITY

SUBBASIN

TOMBSTONE

SUBBASIN

BENSON

SUBBASIN

Sa

n

Pedro

River

St. David

WHETSTONE

MOUNTAINS

RINCON

MOUNTAINS

SANTA 

CATALINA MOUNTAINS

GALIURO

MOUNTAINS

DRAGOON

MOUNTAINS

COCHISE CO

PIMA CO

LITTLE

DRAGOON

MOUNTAINS

JOHNNY LYONS

HILLS

Cascabel

Pomerene

WINCHESTER

MOUNTAINS

Benson

Redington dington

MARTINEZ RANCH FAULT CATALINA CORE COMPLEX

Figure 9. Sites of gravity measurements and derived gravity anomaly used to estimate 

bedrock altitude in the middle San Pedro watershed, southeastern Arizona. Section C–C’ is 

shown on figure 11.
20 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Figure 10. Subsurface density profiles from wells and the 

gravity density function used to assign density of layers in the 

subsurface density model for the middle San Pedro watershed, 

southeastern Arizona.

0

50

100

150

200

250

DEPTH, IN METERS

300

350

400

1.0 1.5 2.0 2.5

DENSITY, IN GRAMS PER CUBIC CENTIMETER

EXPLANATION

TW3 

Calculated density in wells in the Sierra Vista Subwatershed

Calculated density in wells in the Benson subarea

D-21-20 11BCD

D-21-20 13CBB1

D-17-20 31BAA

D-17-20 33DCC 

D-19-21 29ACB2

D-20-21 06DDA1

Gravity density function

subsurface density variations is calculated on a level surface; 

for comparison of modeled and measured gravity, the measured gravity must also be on a level surface. Therefore, the 

final residual was upward continued to a constant elevation of 

3,000 m, above the highest topography in the study area.

The subsurface density model includes six layers having 

uniform density values, which are assigned on the basis of 

geophysical logs (figs. 10, 11). The density of sediments is 

1.65 g/cm3 from the land surface to 20 m depth. Between 20 m 

and 25 m, density is 1.9 g/cm3. From 25 m to 110 m, density is 

2.1 g/cm3. From 110 m to 250 m, density is 2.2 g/cm3. Density 

is 2.3 g/cm3 below 250 m. The density of lower basin fill is 

assumed to be constant at 2.3 g/cm3. Density of bedrock is 

2.67 g/cm3. Not all layers are present throughout the model. 

The elevation of the top of lower basin fill is fixed, based on 

geophysical and driller’s logs. Where lower basin fill is closer 

to the surface than 250 m, it supersedes the presence of overlying layers. This occurs primarily at the basin margins, but also 

where bedrock is near the surface south of Benson. The upper 

two layers are generally present throughout the domain. 

These density values are slightly less, particularly near 

the surface, than those used in a previous study of depth-tobedrock immediately south of the present study (Gettings and 

Houser, 2000), which used density values based primarily on a 

gamma-gamma log from well D-21-20 13CBB1. Lower density values used in this report result in more shallow bedrock 

depths than those described by Gettings and Houser (2000). 

Density values used in other studies (Oppenheimer and Sumner, 1980; Saltus and Jachens, 1995) are similar to those used 

in this report.

Lithologic logs from thirty-five wells were used to constrain the bedrock depths in the subsurface density model (fig. 

12). Of the 35 wells, 21 penetrated bedrock. The remaining 

logs provide information about the minimum depth to bedrock. The deepest bedrock is 10 km north of Benson at ADWR 

well no. 55-568567, a mineral exploration well that penetrated 

granite at 742-m depth. Bedrock was not penetrated within 

512 m of the land surface at ADWR well no. 55-211774, about 

15 km south of Benson. 

Density Model Results

Major structural features indicated by the density model 

include a north-south trending gravity low in the southern part 

of the study area referred to as the Huachuca City subbasin 

(fig. 12; Gettings and Houser, 2000); a northwest-southeast 

trending low to the southeast called the Tombstone subbasin; a 

nearly circular low to the north of Benson in the central part of 

the study area called the Benson anomaly; and a north-south 

trending low along the San Pedro River to the north of The 

Narrows. The Huachuca City and Tombstone subbasins are 

separated by a gravity high near the San Pedro River (figs. 9, 

12). This gravity high continues to the southeast, connecting 

with exposed bedrock in the Tombstone volcanic field and 

Mule Mountains near Bisbee. The gravity high and outcrops of 

Cretaceous crystalline and sedimentary rocks at the center of 

the Benson subarea about 12 km south of St. David (Shipman 

and Ferguson, 2006) indicate that a buried fault-block bisects 

the Benson subarea in a southeast-northwest direction. 

The basin gravity anomaly in the Benson area reflects the 

complicated geology at the southeast corner of the Catalina 

core complex (figs. 9, 12). The circular Benson subbasin is 

only about 15 km across, while the east-west extent of surficial 

alluvium is about 30 km, from the Martinez Ranch Fault in the 

west to the Little Dragoon Mountains in the east (fig. 12). The 

Martinez Ranch Fault juxtaposes Lower Cretaceous Bisbee 

Group on the west against mostly basin fill deposits to the east 

(Dickinson, 1991). The lack of a prominent gravity anomaly to 

the east of this fault indicates either a thin sequence of basin fill 
Hydrogeologic Framework 21

Measured

 Comparison between measured and simulated gravity

 Density values in the subsurface density model

Simulated

A

BGRAVITY, IN MILLIGALS

DISTANCE, IN METERS

ALTITUDE, IN METERS

0 10,000 20,000 30,000 40,000 50,000

0

500

1,000

1,500

2,000

-36

-34

-32

-30

-28

-26

-24

2.67 g/cm3

2.2 g/cm3

2.2 g/cm3

2.1 G/CM3 1.9 G/CM3

2.3 g/cm3 2.3 g/cm3

2.67 g/cm3

2.2 g/cm3

2.2 g/cm3

2.1 g/cm3 1.9 g/cm3

2.3 g/cm3 2.3 g/cm3

DISTANCE, IN METERS

0 10,000 20,000 30,000 40,000 50,000

C C’

C C’

Figure 11. Section C–C’ (fig. 9) showing the layers in the subsurface density model and gravity values. A, 

Comparison between measured and simulated gravity. B, Density values in the subsurface density model. The 

layers in the model represent bedrock (density of 2.67 g/cm3

), lower basin fill (density of 2.3 g/cm3), and upper 

basin fill (density values from 2.2 g/cm3

 to 1.9 g/cm3).

or a thick sequence of higher density Pantano Formation, or a 

combination of both. The Pantano Formation is exposed outside of the study area about 15 km east of Benson and underlies 

the lower basin fill in much of the Sierra Vista subwatershed 

(Brown and others, 1966; Pool and Coes, 1999), suggesting 

that the Pantano Formation may underlie the lower basin fill 

in the middle San Pedro watershed. The Benson fault to the 

southwest of the Benson anomaly, on the northern end of the 

Whetstone Mountains, does not appear to be a basin-bounding 

fault. The gravity high in this area may be indicative of buried 

intrusive rocks (Gettings, 1996). In contrast, the east side of the 

Whetstone Mountains and the southwest side of the Dragoon 

Mountains display steep horizontal gravity gradients, indicative of basin-bounding faults. The presumed fault bounding the 

west side of the Tombstone subbasin, if traced further north, 

coincides neatly with the west edge of the Benson subbasin. 

Lack of knowledge of the presence or absence of midTertiary Pantano Formation in the study area is the biggest 

obstacle to accurately calculating basin depths. A well log 

from the Century Petroleum-Colglazier #1 oil exploration 

well, about 12 km west of Benson, indicates multiple occurrences of “red beds” at depths of 18 m to 147 m. Red beds in a 

deep exploratory well in the Tucson basin have been identified 

as Pantano Formation (Dickinson, 1991). Many other driller’s 

logs in the Benson and St. David areas also mention red beds, 

although these probably indicate the prominent red beds of 

the lower division of the St. David Formation. The density 

of indurated parts of the Pantano Formation was estimated 
22 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

-1,213 – -1,000

Altitude of top of bedrock, in m

(contours dashed where uncertain)

Well that penetrates bedrock

Top value is depth to bedrock, in meters

Bottom value is depth to bedrock in subsurface density model, in m

Well that does not penetrate bedrock

Top value is well depth, in meters

Bottom value is depth to bedrock in subsurface density model, in m

EXPLANATION

-1,000 – -800

-800 – -600

-600 – -400

-400 – -200

-200 – 0

0 – 200 sea level

200 – 400

400 – 600

600 – 800

800 – 1,000

1,000 – 1,200

1,200 – 1,400

!

149

276

304

95

MOUNTAINS

WHETSTONE

Redington

Benson

St. David

80

82

90

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

DRAGOON

MOUNTAINS

GRAHAM CO

COCHISE CO

ESIHCOC OC

OC AMIP

RINCON

MOUNTAINS

San

Pedro

River

CATALINA CORE COMPLEX

SANTA 

CATALINA MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

!

!

!

!

! ! !

!

!

!

!

! !

!

!

! !

!

!

! !!

42

166

36

216

198

158

512

408 121

72

56

423

228

790

67

380

149

276

304

95

304

596 59

142

260

255 2

105

234

459 231

589

304

316

150

204

130

359

426

195

3

0

359

86

310

116

188

119

36

151

213

732

15

63

109

152 742

919

249

267

131

119

106

1019

180

1640

219

0

344

296

42

166

36

216

198

158

512

408 121

72

56

423

228

790

67

380

149

276

304

95

304

596 59

142

260

255 2

105

234

459 231

589

304

316

150

204

130

359

426

195

3

0

359

86

310

116

188

119

36

151

213

732

15

63

109

152 742

919

249

267

131

119

106

1019

180

1640

219

0

344

296

HUACHUCA

SUBBASIN

TOMBSTONE

SUBBASIN

BENSON

SUBBASIN

THE NARROWS

Figure 12. Bedrock altitude determined from subsurface density model in the middle San Pedro 

watershed, southeastern Arizona. Locations of wells used to constrain the model are shown; these 

wells provide either an altitude at which bedrock was penetrated or a maximum bedrock altitude if the 

well did not penetrate bedrock.
Hydrogeologic Framework 23

at 2.35 g/cm3 (Gettings and Houser, 2000), between those 

of basin fill and basement rock. Although omitted from the 

subsurface density model, the Pantano Formation, if present 

in large thicknesses, would cause basin depths to be underestimated by the gravity method.

Bedrock depths are uncertain at a prominent gravity low 

in the northwestern part of the Narrows-Redington subarea. 

Interpretation of this gravity low is complicated by insufficient descriptions of subsurface lithology from drill logs and 

uncertain thickness of late Oligocene volcanic rocks in the 

Galiuro Mountains, both items of information needed for the 

subsurface density model. The Oligocene volcanic rocks have 

a density of about 2.43 g/cm3 (Schwartz, 1990), between those 

of the basin fill and crystalline bedrock. Therefore, the source 

of these gravity lows could be either thick sequences of volcanic rocks, a lesser amount of basin fill, or some combination 

of volcanics and basin fill. In general, the presence of volcanic 

rocks having intermediate density (between basin fill alluvium 

and crystalline bedrock), which are not accounted for in the 

model, results in underestimates of the depth of the interface 

between basin fill alluvium and bedrock.

Distribution of Holocene Stream Alluvium

Targeted surface surveys were done to characterize the 

geophysical and hydrologic properties of stream alluvium 

near the San Pedro River. Seismic refraction, magnetic, TEM, 

and CSAMT surveys by The University of Arizona Department of Mining and Geological Engineering Geophysics Field 

Methods class were done near The Narrows in the Three Links 

Farm, near Pomerene, and near St. David. Surveys near The 

Narrows were done to characterize buried structures in areas 

of shallow bedrock. Surveys near Pomerene and St. David 

were done to estimate the thickness and extent of streambed 

alluvium near the San Pedro River. Results indicated that 

streambed alluvium thickness varies along the length of the 

San Pedro River but is generally less than 10 m. 

Hydrogeologic Framework Model

A three-dimensional hydrogeologic framework model 

(HFM) was constructed to represent the HGUs and major 

structures in the middle San Pedro watershed. The HFM 

provides a mechanism to systematically compile and visualize 

the stratigraphy of the HGUs. The HFM comprises HGUs that 

are spatially extensive and have similar geologic and waterbearing properties. HGUs in the HFM are the most important 

units with regard to regional groundwater flow.

Construction of the Hydrogeologic Framework 

Model

Methods for the construction of the HFM are based on 

those used by Faunt and others (2004) and Dickinson and 

others (2006). The rocks and sediments were divided into 

four HGUs on the basis of physical characteristics and waterbearing properties. The HFM represents the HGUs by using 

a vertical sequence of three-dimensional (3D) volumes. Each 

volume represents the physical spatial extent and thickness of 

the rocks and sediments contained in each HGU (table 2). 

 Several software packages were selected for various parts of the HFM construction process. The HFM was 

constructed by using Dynamic Graphic’s EarthVision® 

software. EarthVision® represents stratigraphic and structural 

relations of sedimentary basins, including deposition, erosion, and unconformities, as well as truncation of units and 

faulting. Spatial data, such as digital elevation, outcrop, and 

borehole information, were compiled and manipulated using 

Environmental Science Research Institute’s (ESRI) ArcGIS® 

geographic information systems (GIS) software. Modeled bedrock surfaces were exported from Geosoft’s Oasis Montaj® 

software and Golden Software’s Surfer® software. Electrical-resistivity models were exported from Petros Eikon’s 

EMIGMA® software.

The HFM is formed by a vertical stack of mostly horizontal 3D grids that represent the spatial extent and thickness 

of the HGUs. After data were imported from other software 

packages, grids representing the HGUs were constructed 

using EarthVision®. The grid for each HGU is oriented along 

principal north-south directions. The datum of the HFM grid is 

the Universal Transverse Mercator (UTM) projection Zone 12, 

North American Datum 1983 (NAD 83). The north and south 

boundaries of all grids are at UTM northing 3,595,000 m 

and 3,495,000 m, respectively. The west and east boundaries of all grids are at UTM easting 533,000 m and 606,000 m 

respectively. Grids for each HGU have uniform horizontal 

discretization in the east-west and north-south directions of 

100 m; they are divided into 731 nodes in the east-west direction and 1,001 nodes in the north-south direction. The vertical 

discretization ranges from 0 m to the maximum thickness of 

each HGU (several thousand meters for the bedrock HGU). 

Software constraints require that the HFM grid be constructed 

for a bounding rectangle, but the HGU grids are truncated at 

any lateral or vertical boundary to represent realistic spatial 

extents that match the extents of units described on geologic 

maps, well logs, or geophysical models. The altitude of the 

HFM extends from 500 m below to 2,500 m above the vertical 

datum (altitude of 0 m, NGVD 29) to encompass nearly all 

of the aquifer units, structural troughs, and mountains in the 

study area. 

Modeled Hydrogeologic Units

The bedrock HGU represents pre-Tertiary bedrock and 

pre-basin-and-range sediments that form the mountains and 

underlie the basin fill units. The bedrock HGU represents 

rocks that are likely to be poorly water bearing. Exceptions are 

porous limestone rocks that were not modeled separately from 

other bedrock because the limestone is difficult to represent 

structurally by the HFM. The top of the bedrock HGU is 
24 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Table 2. Data sources for hydrogeologic units represented by the hydrogeologic framework model. 

[Data sources are used to define the top of a unit, and the bottom of a unit is defined by the top of the underlying unit. The units are listed in stratigraphic 

stacking order in the hydrogeologic framework model. --, not included]

Hydrogeologic Unit Topographic data Borehole lithology Borehole 

geophysics

Electrical resistivity 

models

Subsurface density 

models

Interbedded lower 

basin fill

--- X X X ---

Fine-grained lower 

basin fill

--- X X X ---

Coarse-grained lower 

basin fill

--- X X X ---

Bedrock X X X --- X

defined by results from the subsurface density model. Overlying the bedrock is the coarse-grained lower basin fill (CLBF) 

HGU, which represents the permeable sands and gravels of 

the lower basin fill that underlie fine-grained basin fill along 

the central axis of the basin and along the margins. Next is 

the fine-grained lower basin fill (FLBF) HGU, which represents thick sequences of silt and clay sediments deposited in 

lacustrine facies exposed along the central axis. Along the 

flanks and overlying some areas of the FLBF is the interbedded lower basin fill (ILBF) HGU, comprising clay and silt 

interbeds in sand and gravels, which represents sediments that 

become more coarse outward from the central axis of the basin 

to the margins.

Data Inputs

Data inputs were used to define the vertical sequence, top 

elevations, and extents of the HGUs. The input data were compiled and selected in order to build an HFM that represents 

the conceptual model of the hydrogeology developed in this 

study. Necessary data include land surface elevation, extent 

of exposed geologic units from maps, stratigraphic relations 

of HGUs from vertical sections, drill logs containing depths 

of units below the surface, geophysical models of electrical 

properties that can be related to lithology, depth to bedrock, 

and hydrologically important subsurface structures inferred 

from subsurface density models, electrical-resistivity models, 

and maps of geologic structures. 

Topographic Data.—Digital Elevation Models (DEMs) 

from the 7.5 minute National Elevation Data (NED) DEMs 

were used to constrain the top surface of the uppermost 

HGUs. Data were downloaded from the U.S. Geological 

Survey Seamless Server (accessed August, 2007) in UTM 

projection Zone 12, North American Datum 1983 (NAD83). 

Elevation data were manipulated into 100-m grid resolution. 

Topographic data were used to assign elevations to the top of 

driller’s logs for wells without elevation data.

Surface Geology.—Data from geologic maps were used 

to identify the extent of hydrogeologic units on the land 

surface. In the Benson subarea, the main sources of data were 

the 1:125,000-scale geologic map of southeastern Arizona 

by Drewes and others (2002) and the 1:500,000-scale geologic map of Arizona containing many sources and compiled 

by Hirschberg and Pitts (2000). In the Narrows-Redington 

subarea, the main sources of data were the 1:125,000-scale 

map by Dickinson and others (2002) and the 1:24,000-scale 

map compiled by Bolm and others (2002). The altitudes of 

geologic units were computed by integrating the geologic map 

with a DEM. GIS tools were used to create and export x,y,z 

coordinate locations for outcrops and exposures of each HGU, 

which constrained HGU positions within the HFM. Table 2 

lists geologic units in the geologic reports and maps and the 

corresponding HGUs contained in the HFM.

Borehole Lithology Data.—Lithology data from 1,350 

wells were compiled and manipulated to obtain lithologic 

contacts between HGUs (fig. 5). Lithology logs of cuttings and 

core material provided the majority of information to delineate 

the spatial extents and thicknesses of shallow water-bearing 

stream alluvium near the San Pedro River and the thicknesses 

of the fine-grained sections of the lower basin fill at the center 

of the basin. Data were assembled from the Ground Water Site 

Inventory databases of the USGS and ADWR and from the 

ADWR 55 registry database. The lithology logs are widely 

distributed across the basin, and the densest concentrations 

of these logs are within several kilometers of the San Pedro 

River and near Benson, St. David, and Pomerene (fig. 5). Tops 

and bottoms of vertical lithology intervals were digitized and 

assigned altitudes on the basis of the elevation of the well at 

the land surface and depth along the well log. The vertical 

lithology intervals were assigned to an HGU on the basis of 

texture and location within the known extent of the HGU.

Borehole Geophysical Data.—Direct-current resistivity 

logs, sonic velocity, and drill logs are available for two wells, 

D-17-20 31AAB and D-17-20 33DCC, which are about 2 km 
Hydrogeologic Framework 25

apart on the west side of the basin near Benson (fig. 5). The 

logs were digitized and used to pick top and bottom surfaces 

of several HGUs on the basis of electrical resistivity and 

density. 

Airborne Transient Electromagnetic Data.—Airborne 

TEM data were used to identify the extent of silt and clay 

layers in the basin fill and to infer areas of shallow bedrock. 

Each modeled TEM section line was exported to EMIGMA® 

as a set of data points with x,y,z coordinates and a value for 

electrical resistivity. The modeled TEM data were imported 

into Earthvision® as a three-dimensional dataset. The electrical resistivity values were used to infer the lithology of 

sediments on the basis of previous descriptions of the geophysical properties of basin fill sediments (table 1). The tops 

of the fine-grained and medium-grained HGUs were digitized 

using uppermost altitudes of resistivity-model layers with 

resistivity values between 0 to 10 ohm-m and 10 to 20 ohm-m, 

respectively. 

Subsurface Density Models and Structures.—Grids for 

the tops of modeled bedrock from subsurface density models 

were used to define the top of relatively impermeable rocks 

and consolidated sediments. Mapped faults are not defined in 

the HFM, but are represented by steep vertical gradients in the 

depth-to-bedrock grids that serve as approximations for vertical offsets in bedrock. The grids were exported from Oasis 

Montaj® and Surfer® software as x,y,z coordinates and used 

to define the top of the bedrock HGU. Topographic data for 

exposed rocks associated with subsurface bedrock were joined 

with bedrock surface grids in order to create a continuous 

surface that represents the bottom zone of the HFM.

Gridding of Hydrogeologic-Unit Horizons

The grids for the top surfaces of the HGUs were computed 

from data for the top and bottom surfaces of the HGUs (see 

the “Data Inputs” section). The minimum-tension algorithm 

available in EarthVision® was used to calculate each grid. The 

minimum-tension algorithm calculates a smooth surface that 

closely fits the input data values by using biharmonic-cubic 

spline interpolation techniques (Dynamic Graphics Inc., 1999). 

This algorithm produces a trend surface in areas of sparse data 

that accurately represents the data points. However, in some 

areas with no data, the algorithms can produce grid elevations 

that are higher than the land surface, so each grid was constrained by the topographic surface from DEMs. 

The accuracy of the grid in representing the top of HGUs 

depends on the available data and the spatial complexity of the 

HGU. For example, because of their relatively simple geometry, sedimentary HGUs that were deposited relatively flat 

with planar beds can be represented more accurately with only 

a few data points, whereas faulted and folded rocks with more 

complex geometries are more difficult to represent even with 

a large number of data points. HGU horizons near the surface, such as the top of fine-grained lower basin fill (FLBF), 

were relatively well defined by numerous well-distributed 

data. Lack of data along the basin margins resulted in poorly 

defined top elevations for the coarse-grained lower basin fill 

(CLBF) at the margins, but numerous wells penetrate the top 

of the CLBF underlying the FLBF at the basin center. In areas 

of sparse data for the CLBF, top elevations were assumed to 

be similar to top altitudes of the fine-grained lower basin fill 

unit exposed at the basin center. In general, the stratigraphically lower HGUs are less well defined than more shallow 

units because data for the lower HGUs are sparse. In areas of 

sparse data, gridded surfaces were enhanced by using specified 

“control points” to ensure that the surfaces followed structural 

trends and honored data.

Building the Model

The hydrogeologic framework model (HFM) was constructed in the software Earthvision® by stacking the grids 

that represent the top altitudes of the HGUs into a threedimensional volume (fig. 13). The grids are stacked in an order 

that produces reasonable approximations for the extents and 

thicknesses (table 2). The lowest HGU is the bedrock HGU. 

Above the bedrock is the grid that represents the top of the 

CLBF. The FLBF overlies the coarse-grained lower basin fill 

unit, and the grid for the ILBF is overlain on the top of both 

the FLBF and CLBF. Finally, to ensure that the top of the 

highest unit conforms to the land surface, a grid based on the 

land surface DEM truncates the uppermost surface. 

Description of the Hydrogeologic Framework 

Model

The HGUs as they are depicted in the HFM are described 

below. The HFM characterizes the extent and saturated thickness of the CLBF (fig. 14), and the extent and thickness of the 

FLBF (fig. 15) and the ILBF (fig. 16) HGUs.

Bedrock

The bedrock HGU forms the base of the HFM at an arbitrary depth of –500 m. The unit represents all pre-Tertiary rocks 

and sediments predating basin-and-range deformation that form 

the mountains that bound the basin and underlie the basin fill. 

The top of the unit conforms to the land surface along mountains and to altitudes estimated from the subsurface density 

models in the middle San Pedro watershed (fig. 13). In addition, 

the unit is exposed in areas of bedrock outcrops in the southern 

portion of the study area and at The Narrows. 

Coarse-Grained Lower Basin Fill

The CLBF generally exists in all areas where bedrock is 

not exposed at the surface (fig. 14). Most wells in the basin 

penetrate to the top of the unit, and a large number of lithologic logs were available to define the top surface near Benson, Pomerene, and St. David (fig. 5). The wells range from 

100 m to 350 m in depth and extend through the fine-grained 
26 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

UTM NORTHING,

IN METERS

UTM EASTING,

IN METERS

ALTITUDE,

IN METERS

2,500

540,000

550,000

560,000

570,000

580,000

590,000

600,000 3,500,000

3,520,000

3,540,000

3,560,000

3,580,000

0

DRAGOON

MOUNTAINS

TOMBSTONE HILLS

Fine-grained

lower basin fill

Coarse-grained

lower basin fill

Interbedded

lower basin fill

WHETSTONE

MOUNTAINS

GALIURO MOUNTAINS

WINCHESTER

MOUNTAINS

North

RINCON MOUNTAINS

San Pedro RiverSan Pedro River

Vertical exaggeration x 4

Horizontal and vertical scales vary spatially

100-meter grid based on Universal Transverse Mercator

projection, Zone 12. Shaded-relief base from 1:24,000-scale 

Digital Elevation Model

CATALINA

MOUNTAINS

Fine-grained lower basin fill

Interbedded lower basin fill

Coarse-grained lower basin fill

Bedrock

Extent of a volume of the coarse-grained lower basin fill

HGU removed to reveal the other hydrogeologic units

EXPLANATION

Hydrogeologic units

Figure 13. Oblique view of a 3D block diagram of the hydrogeologic framework model of the middle San Pedro watershed, 

southeastern Arizona. The colors represent 3D volumes of the bedrock and fine, interbedded, and coarse lower basin fill hydrogeologic 

units. The bedrock and fine and interbedded lower basin fill hydrogeologic units are revealed after removing an arbitrary rectangular 

volume of the coarse lower basin fill. The dashed lines represent the extent of the removed volume.

and interbedded facies of the lower basin fill, which can have 

a combined thickness of as much as 300 m (figs. 15, 16). The 

unit is also well defined by geophysical logs for wells D-17-20 

17AAB and D-17-20 33DCC near Benson (fig. 6). 

The top of the CLBF is highest along the basin margins 

as defined by the geophysical logs, and lowest where overlain by fine-grained and interbedded lower basin fill facies 

at the basin center. The top of the unit represents the top of 

coarser-grained facies, which were lowest in the center of the 

basin because of increased deposition of fine-grained facies 

as the lacustrine and playa facies began to dominate in the 

basin center. The top of the unit slopes gradually down to the 

north, similar to the present topography, and it slopes most 

in the center of the basin because the fine and interbedded 

facies become thicker to the north.

Fine-Grained Lower Basin Fill

The FLBF follows the central north-south axis of the 

basin in the Benson subarea (fig. 15). The fine-grained 

facies exist in the Narrows-Redington subarea but are not 

included in the HGU there owing to paucity of data. Drill 

logs in the Narrows-Redington subarea are of insufficient 

quantity and quality to identify the facies, and the airborne 

TEM survey did not include all of the subarea. In the Benson 

subarea, the extent and thickness of the FLBF is defined 

by many drill logs that penetrate through the unit into the 

underlying CLBF (figs. 7 and 8, panel C). In areas without 

drill logs, the extent of the FLBF was primarily defined by 

electrically conductive areas having electrical resistivity 

of less than 10 ohm-m, as determined by interpolated 1D 
Hydrogeologic Framework 27

0 - 100

100 - 200

200 - 300

300 - 400

400 - 500

500 - 600

600 - 700

700 -800

800 - 900

900 - 1,000

greater than 1,000

EXPLANATION

Saturated thickness of the

coarse-grained lower basin fill

HGU in the HFM, in meters

Extent of the coarse-grained

lower basin fill HGU

Line of geologic section

A A’

SANTA CATALINA MOUNTAINS

MOUNTAIN

R

S

INCON

MOUNTAINS

WHETSTONE

Redington

Benson

St. David

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

DRAGOON

MOUNTAINS

San

R

vi

er

P

de

ro

GRAHAM CO

COCHISE CO

ESIHCOC OC

OC AMIP

10

80

82

90

HUACHUCA CITY

SUBBASIN

BENSON

SUBBASIN

TOMBSTONE

SUBBASIN

Cascabel

Pomerene

WINCHESTER

GALIURO

MOUNTAINS

MOUNTAINS

B’

B

A

A’

Figure 14. Extent and saturated thickness of the coarse-grained lower basin fill hydrogeologic unit (HGU) 

in the hydrogeologic framework model (HFM) of the middle San Pedro watershed, southeastern Arizona.
28 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

SANTA CATALINA MOUNTAINS

MOUNTAIN

R

S

INCON

MOUNTAINS

WHETSTONE

Redington

Benson St. David

80

82

90

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

DRAGOON

MOUNTAINS

R

vi

er

P

de

ro

GRAHAM CO

COCHISE CO

ESIHCOC OC

OC AMIP

EXPLANATION

0 - 50

50 - 100

100 - 150

150 - 200

200 - 250

250 - 300

Greater than 300

Thickness of the fine-grained

lower basin fill HGU

in the HFM, in meters

Extent of interpolated electrical resistivity

values between 0 to 10 ohm-m

within 200 m of the land surface

Line of geologic section A A’

Cascabel

Pomerene

WINCHESTER

MOUNTAINS

GALIURO

MOUNTAINS

San

Pedro

River

B’

B

A

A’

Figure 15. Extent and thickness of the fine-grained lower basin fill hydrogeologic unit (HGU) constructed by the 

hydrogeologic framework model (HFM) of the middle San Pedro watershed, southeastern Arizona. The extent and 

thickness were determined from drill logs and one-dimensional models of electrical resistivity.
Hydrogeologic Framework 29

SANTA CATALINA MOUNTAINS

MOUNTAIN

R

S

INCON

MOUNTAINS

WHETSTONE

Redington

Benson St. David

80

82

90

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

DRAGOON

MOUNTAINS

GRAHAM CO

COCHISE CO

ESIHCOC OC

OC AMIP

EXPLANATION

0 - 50

50 - 100

100 - 150

150 - 200

200 - 250

250 - 300

Greater than 300

A A’ Line of geologic section

San

Pedro

River

WINCHESTER

MOUNTAINS

GALIURO

MOUNTAINS

Cascabel

Pomerene

Thickness of the interbedded

lower basin fill HGU

in the HFM, in meters

Extent of interpolated electrical resistivity

values between 10 to 20 ohm-m

within 200 m of the land surface

B’

B

A

A’

Figure 16. Extent and thickness of the interbedded lower basin fill hydrogeologic unit (HGU) represented in the 

hydrogeologic framework model (HFM) of the middle San Pedro watershed, southeastern Arizona. The extent and thickness 

were determined from drill logs and one-dimensional models of electrical resistivity.
30 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

electrical-resistivity models. The thickness was defined both 

by the top and bottom of electrically conductive areas along 

airborne TEM sections and by lithologic descriptions in drill 

logs. The thickness increases from south to north. Thin areas 

are near high bedrock along the southern areas (near airborne 

TEM section line 38030, fig. 4), and the thickness increases 

to about 300 m between Pomerene and The Narrows. The 

thickest areas correspond to the playa depocenter of the St. 

David Formation described by Smith (1994). In the Benson 

subarea, the FLBF underlies much of the San Pedro River, 

which may result in complicated groundwater/surface water 

interactions that require investigation with numerical groundwater-flow models. 

Interbedded Lower Basin Fill

The interbedded lower basin fill unit (ILBF) represents 

interbedded fine-grained sediments within sands and gravels 

between the fine-grained and coarse-grained lower basin fill 

units (fig. 16). The interbedded lower basin fill unit is present 

along the outer margins of the fine-grained lower basin fill 

unit and commonly at similar altitudes (figs. 7 and 8, panel 

C). The unit is generally less than 100 m thick. Near St. 

David and south toward areas of high bedrock the unit underlies the San Pedro River. 

Hydrologic Properties of Hydrogeologic Units

Knowledge of the hydrologic properties of the HGUs 

that control the rates and direction of groundwater flow and 

responses to stresses is necessary to construct accurate and 

reliable groundwater-flow models. The most important properties for groundwater models are saturated thickness, horizontal 

and vertical hydraulic conductivity, aquifer thickness, specific 

yield, and specific storage. 

Hydrologic properties of basin fill aquifers in Arizona 

have been well characterized by previous studies (Anderson 

and Freethey, 1995). Previous values of the hydrologic properties for the Upper and Lower San Pedro Basins estimated by 

aquifer tests and specific capacity are summarized here to 

describe the ranges of hydrologic properties of the HGUs. 

Aquifer-Test Data

Published aquifer-test data are available for wells operated by the city of Benson, in planned areas of future development east of the Whetstone Mountains, and near the Apache 

Nitrogen Superfund Site (Hargis + Associates Inc., 2008). 

Transmissivity estimates for four wells that penetrate the 

coarse facies of the lower basin fill range from 24 to 1,600 

m2

/d (Daniel Weber, Errol L. Montgomery & Associates, 

written commun., 2009). Storativity (the product of specific 

storage and aquifer thickness) and specific yield values for 

the four wells range from 0.0003 to 0.0007 and from 0.001 

to 0.02, respectively. Values of transmissivity, T, can be 

converted to hydraulic conductivity, K, using the relation

K = T/b ,where b is the assumed aquifer thickness. In cases 

where the perforated interval of a well penetrates the full saturated thickness of an aquifer and flow is generally horizontal 

near the well, the length of the perforated interval can approximate the thickness b. The approximated horizontal hydraulic 

conductivity values of the wells in the coarse-grained facies of 

the lower basin fill range from 0.14 to 13 m/d, and the ratios 

between horizontal and vertical hydraulic conductivity range 

from 70:1 to 15:1. The aquifer test in the stream alluvium near 

the Apache Nitrogen Superfund Site indicated a horizontal 

hydraulic conductivity of 90 m/d and specific yield in the 

range of 0.03 to 0.05. 

Specific-Capacity Data

In the absence of aquifer test data, potential groundwater 

declines owing to pumping can be inferred by well productivity. Well productivity can be expressed in terms of the specific

capacity Cs, as CS = Q ⁄Δhw , where Q is the pumping rate 

and Δhw is the drawdown in the well. The specific capacity 

is dependent on well construction and efficiency and only 

reflects aquifer properties near the well. Specific capacity values (table 3) are primarily available for the coarsegrained facies of the lower basin fill and have a maximum of 

1,640 m3/d/m at a well in the Narrows-Redington subarea. 

The specific capacity values are consistent with the expected 

hydraulic properties of the coarse-grained facies of the lower 

basin fill.

Saturated Thickness of Coarse-Grained Lower Basin Fill

The saturated thickness of the coarse-grained lower basin 

fill (CLBF) largely governs the amount of groundwater flow 

in the regional aquifer (fig. 14). The saturated thickness varies 

from about 1,000 m to near zero as the unit thins along mountain fronts. In unconfined areas, the saturated thickness was 

determined by subtracting the water table elevation from the 

bottom of the CLBF determined from drill logs and the subsurface density model. In confined areas, the saturated thickness was calculated by subtracting the bottom of the confining 

FLBF from the bottom of the CLBF. The water table elevation 

was estimated by interpolation of water-level data measured 

by ADWR (Arizona Department of Water Resources, 2009) 

during October–November 2006. The bottom elevation and 

extent of the FLBF was determined from calculations by the 

hydrogeologic framework model using drill log data. The 

extent of the FLBF was determined by the extent of the areas 

with electrical resistivity less than 20 ohm-m simulated by the 

1D electrical-resistivity models. The greatest saturated thickness of the CLBF is in subbasin areas estimated by the subsurface density models. The thickest area is in the NarrowsRedington subarea south of Redington. Other thick areas are in 

the Benson subbasin, southeast of St. David in the Tombstone 

subbasin, and in the Huachuca City subbasin (fig. 14). The 

saturated thickness thins to less than 400 m in confined areas 

underlying the FLBF in areas of high bedrock near St. David.
Hydrogeologic Framework 31

Table 3. Available specific capacity data for wells in the middle San Pedro watershed, southeastern Arizona.

[CLBF, coarse-grained lower basin fill; SA, stream alluvium]

Local well name Well depth, 

meters

Principal

hydrogeologic 

unit

Discharge 

rate (m3

/min)

Static depth 

to 

water, m

Production 

depth to 

water, m

Specific 

capacity 

(m3

/d/m)

Data source

D-12-18 13DBC 45.7 CLBF 2.4 18.6 20.7 1,600 ADWR

D-18-21 16BCD2 25.0 SA 1.9 16.3 36.1 140 ADWR

D-18-21 16BDB5 167.6 CLBF 0.2 1.5 11.2 30 ADWR

D-18-21 06ADA3 30.2 SA 1.9 10.7 19.8 300 ADWR

D-17-21 33CBC 161.5 CLBF 1.6 9.9 23.3 170 ADWR

D-17-21 33CBC 161.5 CLBF 1.6 9.9 23.3 170 ADWR

D-17-21 33CBC 161.5 CLBF 1.6 9.9 23.3 170 ADWR

D-17-21 33CBC 161.5 CLBF 1.6 9.9 23.3 170 ADWR

D-17-21 33CBC 161.5 CLBF 1.6 9.9 23.3 170 ADWR

D-17-21 33CCB2 111.6 CLBF 1.0 6.3 58.7 30 ADWR

D-17-21 32ADA 176.8 CLBF 1.9 21.5 47.2 110 ADWR

D-17-21 32ADA 176.8 CLBF 1.9 21.5 47.2 110 ADWR

D-17-20 22BDA 286.5 CLBF 1.0 23.5 68.9 32 ADWR

D-17-20 22BDA 286.5 CLBF 1.0 23.5 68.9 32 ADWR

D-17-20 19BBB 304.8 CLBF 1.8 177.0 180.1 840 ADWR

D-17-20 17CCC 310.9 CLBF 1.8 142.3 152.7 250 ADWR

D-17-20 17CCC 310.9 CLBF 1.8 142.3 152.7 250 ADWR

D-17-20 18BBB 305.7 CLBF 0.7 142.5 145.6 300 ADWR

D-17-20 09ADC 304.8 CLBF 1.0 7.5 65.0 25 ADWR

D-17-20 09ADC 304.8 CLBF 1.0 7.5 65.0 25 ADWR

D-16-19 36DBD 305.7 CLBF 1.1 64.1 67.5 470 ADWR

D-16-19 36CAD 277.4 CLBF 1.1 66.1 69.9 420 ADWR

D-16-20 34DBD 203.0 CLBF 0.3 16.1 38.2 20 ADWR

D-16-20 34ACB 47.5 CLBF 1.9 23.8 32.3 320 ADWR

D-16-20 34ACA1 35.7 SA 1.1 20.7 29.3 180 ADWR

D-16-19 36CAD 277.4 CLBF 1.1 66.0 71.0 320 Montgomery 

and 

Associates

D-17-20 17CCC 310.9 CLBF 1.8 141.5 152.5 240 Montgomery 

and

 Associates

D-17-20 19BBB 304.8 CLBF 1.7 176.8 180.2 720 Montgomery 

and 

Associates

D-17-20 31BAA 402.3 CLBF 0.2 69.6 85.2 20 Montgomery 

and 

Associates
32 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Incorporating the Hydrogeologic Framework 

Model into a Groundwater-Flow Model

The spatial relations between the HGUs in the HFM 

describe hydrogeologic zones that define the spatial extent of 

permeable and impermeable zones. This information is needed 

by groundwater-flow models such as MODFLOW (Harbaugh, 

2005) that use a finite-difference scheme to approximate 

governing equations within a discretized flow domain. Vertical discretization is commonly referred to as the layers of the 

model, which are often related to horizontal stratigraphic layers. The model can incorporate hydrogeologic zones within a 

single or multiple model layers, and a hydrogeologic zone can 

cover portions of a layer to account for horizontal variations in 

permeability within that layer.

Hydrogeologic zones created from the lithologic zones 

(figs. 7 and 8, panel D) are indicated as model layers that 

overlap the lithologic zones (figs. 7 and 8, panels B and C). 

This configuration includes generally horizontal layers that 

comprise coarse sediments near the basin margin and fine 

sediments at the basin center. Layer 6 represents pre-Tertiary 

granite, limestone, sandstone, siltstone, and mudstone and 

sediments predating basin-and-range deformation along the 

margin of the basin fill. Layer 6 is generally impermeable 

except for local instances of limestone and other permeable 

sedimentary rocks. Layers 3, 4, and 5 represent saturated 

lower basin fill sediments, in which layer 5 represents highly 

permeable sand and gravel that overlie bedrock and underlie 

clay and silt. Layer 2 represents saturated portions of upper 

basin fill along the basin margins. Layer 1 represents saturated 

fine and coarse sediments within the streambed of the San 

Pedro River, which have been described by previous studies 

(for example, Pool and Coes, 1999). 

Inferred lithologies allow for delineation of permeable 

and impermeable zones within layers 3, 4, and 5, which can 

be used to simulate important features of the groundwaterflow system. Such features include confined groundwater 

conditions within portions of layers 4 and 5 that underlie the 

fine-grained hydrogeologic zones within layers 3 and 4. Additionally, impermeable fine-grained sediments west of the San 

Pedro River may restrict horizontal groundwater flow from 

areas of high hydraulic head at the west to areas of lower head 

near the San Pedro River.

Summary

This report documents the results of the investigation 

of the hydrogeologic framework of the middle San Pedro 

watershed in southeastern Arizona. The overall investigation 

includes three planned work elements:

1. Refine the hydrogeologic framework;

2. Characterize the groundwater and surface-water 

system, recharge and discharge processes, and water 

quality in selected areas; and

3. Develop a predictive numerical groundwater-flow 

model for examining the potential groundwater and 

surface-water responses to changes in recharge and 

discharge.

This report describes existing data, the development of 

a hydrogeologic conceptual model, extensive geophysical 

surveys to delineate sediment textures and lithology of the 

basin fill and to quantify the thickness of basin fill deposits in 

order to describe hydrologic properties of the aquifer system, 

and a data management system to incorporate these data into 

groundwater models. The next two reports would describe the 

results of work elements 2 and 3.

A hydrogeologic framework model (HFM) was constructed for the purpose of delineating the physical extent 

and thickness of hydrogeologic units (HGUs) that fit within 

a three-dimensional volume. The stacking order of the HGUs 

from the model bottom to the land surface are bedrock, coarsegrained lower basin fill, fine-grained lower basin fill, and 

interbedded lower basin fill. The bedrock HGU represents preTertiary crystalline and sedimentary rocks and mid-Tertiary 

sediments predating the basin fill and having low permeability. The top of the bedrock HGU is defined by drill logs that 

penetrate bedrock and by the top of dense bedrock determined 

by subsurface density models. The coarse-grained lower basin 

fill unit represents gravel and sand facies of the lower basin fill 

at the basin margins and at the center of the basin under finegrained facies. The fine-grained lower basin fill unit represents 

clay and silt sequences as much as 300 m thick at the center of 

the basin that are commonly exposed at the center of the basin. 

The interbedded lower basin fill unit represents interbeds of 

clay and silt within sand and gravel along the margins of the 

fine-grained facies.

HGU spatial extents and thickness simulated by the HFM 

can be incorporated into groundwater-flow models to define 

hydrogeologic zones and layers. The saturated thickness of 

the coarse-grained lower basin fill varies in relation to the 

depth to underlying impermeable bedrock and the thickness 

of overlying silt and clay. Three areas where the thickness is 

about 1,000 m are south of Redington, northwest of Benson, 

and southeast of St. David. The thickness is about 600 m at the 

southern portion of the study area near the Whetstone Mountains. Thin areas are largely controlled by shallow bedrock that 

underlies much of the San Pedro River south of Benson and at 

The Narrows. The extent of the fine-grained lower basin fill is 

limited to the central north-south axis of the Benson subarea, 

and the thickness increases northward to about 300 m south of 

The Narrows. The interbedded lower basin fill unit is as much 

as 100 m thick and is a transition between the coarse-grained 

and fine-grained lower basin fill units along the margin of the 

fine-grained facies. 

One-dimensional electrical models of airborne TEM 

surveys were used to map important lithologic and hydraulic 

property distributions where drill log data are scarce or of 

uncertain quality. Electrical and geologic properties from two 

geophysical well logs were closely related to the 1D resistivity 
References Cited 33

distributions and were the basis for defining electrical resistivity values of lithology throughout the alluvial basin. The 

two geophysical logs and 1D resistivity models identified an 

upper sequence of highly variable resistivity values identified as interbedded sand, gravel, and clay. Below the upper 

sequence, the resistivity logs and 1D models range between 5 

and 20 ohm-m within a silt and clay sequence. Underlying the 

silt and clay to the bottom of the boreholes, the resistivity logs 

and 1D models increase to several hundred ohm-m and are 

highly variable within sand and gravel layers. These sequences 

matched distinct resistivity and lithologic layers identified by 

geophysical logs in the adjacent Sierra Vista subwatershed, 

suggesting that these sequences are laterally continuous within 

both the Benson and Sierra Vista subwatersheds in the Upper 

San Pedro Basin.

Significant electrical resistivity zones mapped from 1D 

models of airborne TEM surveys correspond with hydrologically significant lithologic zones (Dickinson and others, in 

press). Within basin fill sediments, this includes fine-grained 

zones of silt and clay of poor permeability, medium-grained 

zones of sand, silt, gravel, and clay, and coarse-grained zones 

of sand, gravel, and conglomerate. Lithologic zones of crystalline and limestone bedrock underlie the basin fill sediments. 

New data on spatial extents and thicknesses of the lithologic 

zones reduces the uncertainty of the extents of high- and 

low-permeability zones. Improved definition of the hydrologic 

zones allows for use of a groundwater model to examine other 

uncertainties.

A subsurface density model was constructed to identify 

the top of bedrock and structures that may affect regional 

groundwater flow. Major features include exposed bedrock 

at The Narrows, a north-south trending gravity low in the 

southern part of the study area referred to as the Huachuca 

City subbasin, a northwest-southeast trending low to the 

southeast called the Tombstone subbasin, a nearly circular low 

to the north of Benson in the central part of the study area, 

and a north-south trending low along the San Pedro River to 

the north. The Huachuca City and Tombstone subbasins to the 

south are located at the east and west margins of the San Pedro 

Basin, separated by a gravity high along the present-day San 

Pedro River. This gravity high continues to the southeast and 

connects with exposed bedrock in the Tombstone volcanic 

field and Mule Mountains near Bisbee. 

References Cited

Anderson, T.W., and Freethey, G.W., 1995, Simulation of 

ground-water flow in alluvial basins in south-central Arizona and parts of adjacent states: U.S. Geological Survey 

Professional Paper 1406-D, 78 p.

Anderson, T.W., Freethey, G.W., and Tucci, P., 1992, Geohydrology and water resources of alluvial basins in southcentral Arizona and parts of adjacent states: U.S. Geological 

Survey Professional Paper 1406-B, 67 p.

Annan, A.P., and Lockwood, R., 1991, An application of 

airborne GEOTEM* in Australian conditions: Exploration 

Geophysics, v. 22, no. 1, p. 5–12.

Arizona Department of Water Resources, 2005, Groundwater 

resources of the Upper San Pedro Basin, Arizona: Arizona 

Department of Water Resources Technical report to the 

Upper San Pedro Basin AMA review report, 85 p.

Arizona Department of Water Resources, 2009, Groundwater site inventory database: Arizona Department of Water 

Resources (also available at http://gisweb.azwater.gov/

waterresourcedata/).

Balcer, R.A., 1984, Stratigraphy and depositional history of 

the Pantano Formation (Oligocene-early Miocene), Pima 

County, Arizona: Tucson, Arizona, University of Arizona, 

unpublished M.S. thesis, 107 p.

Bankey, V., and Kleinkopf, M.D., 1982, Principal facts for 

gravity stations in the Whetstone Roadless Area, Arizona: 

U.S. Geological Survey Open-File Report 82–738, 14 p.

Bills, D.J., Flynn, M.E., and Monroe, S.A., 2007, Hydrogeology of the Coconino Plateau and adjacent areas, Coconino 

and Yavapai Counties, Arizona: U.S. Geological Survey 

Scientific Investigations Report 2005-5222, 101 p.

Blasch, K.W., Hoffmann, J.P., Graser, L.F., Bryson, J.R., and 

Flint, A.L., 2006, Hydrogeology of the upper and middle 

Verde River watersheds, central Arizona: U.S. Geological 

Survey Scientific Investigations Report 2005–5198, 115 p.

Bolm, K.S., Lewis, T., Hirschberg, D.M., Pitts, G.S., and 

Dickinson, W.R., 2002, Spatial digital database for the geology of the San Pedro River Basin in Cochise, Gila, Graham, 

Pima, and Pinal Counties, Arizona: U.S. Geological Survey 

Open-File Report 02-393, digital database, version 1.0, 

http://geopubs.wr.usgs.gov/open-file/of02-393/.

Brocher, T.M., 2005, Empirical relations between elastic 

wavespeeds and density in the earth's crust: Bulletin of the 

Seismological Society of America, v. 95, no. 6, 

p. 2081–2092.

Brown, S.G., Davidson, E.S., Kister, L.R., and Thomsen, 

B.W., 1966, Water resources of Fort Huachuca Military 

Reservation, southeastern Arizona: U.S. Geological Survey 

Water Supply Paper 1819-D, 57 p.

Clay, D.W., 1970, Stratigraphy and petrology of the Mineta 

Formation, Pima and Cochise Counties, Arizona: Tucson, 

Arizona, University of Arizona, Ph.D. dissertation, 183 p.
34 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Coes, A.L., and Pool, D.R., 2007, Ephemeral-stream channel 

and basin-floor infiltration and recharge in the Sierra Vista 

subwatershed of the Upper San Pedro Basin, southeastern 

Arizona, in Stonestrom, D.A., Constantz, J., Ferre, T.P.A., 

and Leake, S.A., eds., Ground-water recharge in the arid 

and semiarid southwestern United States: U.S. Geological 

Survey Professional Paper 1703-J, p. 253–311.

Coes, A.L., Gellenbeck, D.J., and Towne, D.C., 1999, Groundwater quality in the Sierra Vista subbasin, Arizona, 1996-97: 

U.S. Geological Survey Water-Resources Investigations 

Report 99-4056, 50 p.

Cooke, R.U., and Reeves, R.W., 1976, Arroyos and environmental change in the American South-West: Oxford, 

Clarendon Press, 213 p.

Corell, S.W., Corkhill, F., Lovvik, D., and Putnam, F., 1996, 

A groundwater flow model of the Sierra Vista Subwatershed of the Upper San Pedro Basin-southeastern Arizona: 

Arizona Department of Water Resources Model Report 10, 

107 p.

Dater, D., Metzger, D., and Hittelman, A., 1999, Land and 

marine gravity CD-ROMs: Gravity 1999 edition on 2 

CD-ROMs, National Geophysical Data Center, Boulder, 

Colorado (also available at http://www.ngdc.noaa.gov/mgg/

gravity/1999cd.html).

Dickinson, J.E., Land, M., Faunt, C.C., Leake, S.A., Reichard, 

E.G., Fleming, J.B., and Pool, D.R., 2006, Hydrogeologic 

framework refinement, ground-water flow and storage, 

water-chemistry analyses, and water-budget components 

of the Yuma area, southwestern Arizona and southeastern 

California: U.S. Geological Survey Scientific Investigations 

Report 2006-5135, 88 p.

Dickinson, J.E., Pool, D.R., Groom, R.W., and Davis, L.J., 

in press, Inference of lithologic distributions in an alluvial 

aquifer using airborne transient electromagnetic surveys: 

Geophysics.

Dickinson, W.R., 1991, Tectonic setting of faulted Tertiary 

strata associated with the Catalina core complex in southern 

Arizona: Geological Society of America Special Paper 264, 

106 p.

Dickinson, W.R., 2002, Spatial digital database of the geologic 

map of Catalina Core Complex and San Pedro Trough, 

Pima, Pinal, Gila, Graham, and Cochise counties, Arizona 

[digital database by Hirschberg, D.M., Pitts, G.S., and 

Bolm, K.S.]: U.S. Geological Survey Open-File Report 

2002-365, 25 p.

Dickinson, W.R., 2003, Depositional facies of the Quibruis 

Formation, basin fill of the San Pedro trough, southeastern 

Arizona Basin and Range Province, in Raynolds, R.G., and 

Flores, R.M., eds., Cenozoic Systems of the Rocky Mountain Region: Denver, Colorado, Society for Sedimentary 

Geology, p. 157–181.

Drewes, H., 2002, Spatial digital database for the tectonic 

map of Southeast Arizona [digital database by Fields, R.A., 

Hirschberg, D.M., and Bolm, K.S.]: U.S. Geological Survey 

Geologic Investigations Series I-1109, 38 p.

Drewes, H.D., 1981, Tectonics of southeastern Arizona: U.S. 

Geological Survey Professional Paper 1144, 96 p.

Dynamic Graphics Inc., 1999, Getting started with Earthvision 

5: Alameda, California, Dynamic Graphics, Inc., 130 p.

Eberly, L.D., and Stanley, T.B., 1978, Cenozoic stratigraphy 

and geologic history of southwestern Arizona: Geological 

Society of America Bulletin, v. 89, no. 6, p. 921–940.

Faunt, C.C., Sweetkind, D.S., and Belcher, W.R., 2004, Threedimensional hydrogeologic framework model, in Belcher, 

W.R., ed., Chapter E of Death Valley regional ground-water 

flow system, Nevada and California—hydrogeologic framework and transient ground-water flow model: U.S. Geological Survey Scientific Investigations Report 2004-5205, 89 p.

Fenneman, N.M., 1931, Physiography of western United 

States (1st ed.): New York, McGraw-Hill, 534 p.

Finnell, T.L., 1970, Pantano Formation, in Cohee, G.V., Bates, 

R.G., and Wright, W.B., eds., Changes in stratigraphic 

nomenclature by the U.S. Geological Survey, 1968, 

p. 35–36.

Fleming, J.B., Pool, D.R., and Leenhouts, J.M., 2002, Electrical surveys to define distributions of silt and clay layers near 

the San Pedro River, Sierra Vista subwatershed of the Upper 

San Pedro River Basin, in Symposium on the Application 

of Geophysics to Environmental and Engineering Problems 

15, Las Vegas, Nevada, February 2002, Proceedings: The 

Environmental and Engineering Geophysical Society, 

p. 1–12.

Freethey, G.W., 1982, Hydrologic analysis of the Upper San 

Pedro Basin from the Mexico-United States international 

boundary to Fairbank, Arizona: U.S. Geological Survey 

Open-File Report 82-752, 64 p.

Gettings, M.E., 1996, Aeromagnetic, radiometric, and gravity data for Coronado National Forest, in du Bray, E.A., 

ed., Mineral resource potential and geology of Coronado 

National Forest, Southeastern Arizona and Southwestern 

New Mexico: U.S. Geological Survey Bulletin 2083-D, 

p. 70–101.
References Cited 35

Gettings, M.E., and Houser, B.B., 2000, Depth to bedrock in 

the upper San Pedro Valley, Cochise County, southeastern 

Arizona: U.S. Geological Survey Open-File Report 2000-

138, http://pubs.usgs.gov/of/2000/of00-138/.

Goode, T.C., and Maddock, T.I., 2000, Simulation of groundwater conditions in the Upper San Pedro Basin for the 

evaluation of alternative futures: The University of Arizona 

HWR No. 00-030, 113 p.

Gray, R.S., 1967, Petrography of the Upper Cenezoic nonmarine sediments in the San Pedro Valley, Arizona: Journal 

of Sedimentary Petrology, v. 37, no. 3, p. 774–3790.

Grover, J.A., 1984, Petrology, depositional environments and 

structural development of the Mineta Formation, Teran 

Basin, Cochise County, Arizona: Sedimentary Geology, v. 

38, no. 1-4, p. 87–105.

Halvorson, P.H.F., 1984, An exploration gravity survey in the 

San Pedro Valley, southeastern Arizona: Tucson, Arizona, 

The University of Arizona, M.S. thesis, 70 p.

Harbaugh, A.W., 2005, MODFLOW-2005; the U.S. Geological Survey modular ground-water model—the groundwater flow process: U.S. Geological Survey Techniques 

and Methods 6-A16, 253 p.

Hargis + Associates Inc., 2008, Northern area performance 

assessment model, Apache Powder Superfund Site, 

Cochise County, Arizona: prepared for Apache Powder 

Superfund Site, Cochise County, Arizona, 28 p.

Haynes, C.V., 1981, Geochronology and paleoenvironments 

of the Murray Springs Clovis site, Arizona: National Geographic Society Research Reports, v. 13, p. 243–251.

Haynes, C.V., 1987, Curry Draw, Cochise County, Arizona; a 

late Quaternary stratigraphic record of Pleistocene extinction and paleo–Indian activities, in Davis, G.H., and VandenHolder, E.M., eds., Geologic diversity of Arizona and 

its margins; excursions to choice areas: Tucson, Arizona 

Bureau of Geology and Mineral Technology Special Paper 

5, p. 23–28.

Hereford, R., 1993, Entrenchment and widening of the Upper 

San Pedro River, Arizona: Geological Society of America 

Special Paper 282, 46 p.

Hirschberg, D.M., and Pitts, G.S., 2000, Digital geologic 

map of Arizona; a digital database derived from the 1983 

printing of the Wilson, Moore, and Cooper 1:500,000-scale 

map: U.S. Geological Survey Open-File Report 2000-409, 

67 p.

Jahnke, P., 1994, Modeling of groundwater flow and surface/

groundwater interaction for the San Pedro River Basin 

from Fairbank to Redington, Arizona: Tucson, The University of Arizona, M.S. thesis.

Kane, M.F., 1962, A Comprehensive system of terrain corrections using a digital computer: Geophysics, v. 27, no. 4, 

p. 445–3462.

Ladd, T.W., 1975, Stratigraphy and petrology of the Quiburis 

Formation near Mammoth, Pinal County, Arizona: Tucson, 

Arizona, The University of Arizona, M.S. thesis, 102 p.

LaFehr, T.R., 1991, An exact solution for the gravity curvature (Bullard B) correction: Geophysics, v. 56, no. 8, 

p. 1179–1184.

Leenhouts, J.M., Stromberg, J.C., and Scott, R.L., 2006, 

Hydrologic requirements of and consumptive ground-water 

use by riparian vegetation along the San Pedro River, 

Arizona: U.S. Geological Survey Scientific Investigations 

Report 2005-5163, 154 p.

Melton, M.A., 1965, The geomorphic and paleoclimatic 

significance of alluvial deposits in southern Arizona: The 

Journal of Geology, v. 73, no. 1, p. 1–38.

Menges, C.M., and Pearthree, P.A., 1989, Late Cenozoic 

tectonism in Arizona and its impact on regional landscape 

evolution, in Jenny, J.P., and Reynolds, S.J., eds., Geologic 

evolution of Arizona: Tucson, Arizona, Arizona Geological 

Society, p. 649–680.

Morrison, R.B., 1985, Pliocene/Quaternary geology, geomorphology, and tectonics of Arizona: Geological Society of 

America Special Paper 203, p. 123–3146.

Nagy, D., 1966, The gravitational attraction of a right rectangular prism: Geophysics, v. 31, no. 2, p. 362–371.

Oppenheimer, J.M., and Sumner, J.S., 1980, Depth-to-bedrock map, Basin and Range province, Arizona: University 

of Arizona, Department of Geosciences, Laboratory of 

Geophysics, scale 1:1,000,000.

PACES, 2008, Gravity and magnetic dataset repository: PanAmerican Center for Earth and Environmental Studies 

(PACES), http://irpsrvgis00.utep.edu/repositorywebsite/.

Parker, J.T.C., Steinkampf, W.C., and Flynn, M.E., 2005, 

Hydrogeology of the Mogollon highlands, central Arizona: 

U.S. Geological Survey Scientific Investigations Report 

2004-5294, 87 p.

Parker, R.L., 1972, The rapid calculation of potential anomalies: Geophysical Journal of the Royal Astronomical Society, v. 31, no. 4, p. 447–455.
36 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Pool, D.R., and Coes, A.L., 1999, Hydrogeologic investigations of the Sierra Vista subwatershed of the upper San 

Pedro Basin, Cochise County, southeast Arizona: U.S. 

Geological Survey Water-Resources Investigations Report 

99-4197, 41 p.

Pool, D.R., and Dickinson, J.E., 2007, Ground-water flow 

model of the Sierra Vista subwatershed and Sonoran portions 

of the upper San Pedro basin, southeastern Arizona, United 

States, and northern Sonora, Mexico: U.S. Geological Survey Scientific Investigations Report 2006-5228, 48 p.

Roeske, R.H., and Werrell, W.L., 1973, Hydrologic conditions 

in the San Pedro River valley, Arizona, 1971: Arizona Water Commission Bulletin 4, 76 p.

Saltus, R.W., and Jachens, R.C., 1995, Gravity and basindepth maps of the Basin and Range Province, western 

United States: U.S. Geological Survey Geophysical Investigations GP-1012, scale 1:2,500,000.

Scarborough, R.B., 1989, Cenozoic erosion and sedimentation in Arizona, in Jenny, J.P., and Reynolds, S.J., eds., 

Geologic evolution of Arizona: Arizona Geological Society 

Digest 17, p. 515–538.

Scarborough, R.B., and Peirce, H.W., 1978, Late Cenozoic 

basins of Arizona, in Callender, J.F., Wilt, J.C., and Clemons, R.E., eds., Land of Cochise, Southeastern Arizona: 

New Mexico Geological Society Guidebook, 29th field 

conference, p. 253–259.

Schwartz, K.L., 1990, A geohydrological investigation of volcanic rocks using the gravity method; Galiuro mountains, 

Graham, Pinal, and Cochise Counties, Arizona: Tucson, 

Arizona, The University of Arizona, M.S. thesis, 138 p.

Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., 

Rehrig, W.A., and Raymond, R.H., 1980, K–Ar geochronology and geologic history of southwestern Arizona and 

adjacent areas, in Jenny, J.P., and Stone, C., eds., Studies 

in western Arizona: Tucson, Arizona, Arizona Geological 

Society Digest 12, p. 201–260.

Shipman, T.C., and Ferguson, C.A., 2006, Geologic map of 

the Land 7.5’ Quadrangle, Cochise County, Arizona: Arizona Geological Survey Digital Geologic Map 49, scale 

1:24,000.

Simpson, R.W., Jachens, R.C., and Blakely, R.J., 1983, AIRYROOT; a FORTRAN program for calculating the gravitational attraction of an airy isostatic root out to 166.7 km: 

U.S. Geological Survey Open-File Report 83-883, 66 p.

Smith, G.A., 1994, Climatic influences on continental deposition during late-stage filling of an extensional basin, southeastern Arizona: Geological Society of America Bulletin, v. 

106, no. 9, p. 1212-1228.

Spangler, D.P., 1968, Application of the gravity survey 

method to watershed hydrology: Ground Water, v. 6, no. 6, 

p. 21–26.

U.S. Census Bureau, 2006, Annual estimates of the population for incorporated places in Arizona: U.S. Census Bureau, accessed January 6th, at http://www.census.gov/prod/

www/abs/decennial/index.htm.

Wynn, J., 2006, Mapping ground water in three dimensions—

an analysis of airborne geophysical surveys of the Upper San 

Pedro River Basin, Cochise County, Southeastern Arizona: 

U.S. Geological Survey Professional Paper 1674, 33 p.
References Cited 37

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication May 21, 2010

Text edited by Peter H. Stauffer

Layout and design by Judy Weathers
38 Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona

Dickinson and others—Hydrogeologic Framework of the Middle San Pedro Watershed, Southeastern Arizona—Scientific Investigations Report 2010–5126