Langenheim, others—Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data—U.S. Geological Survey Scientific Investigations Report 2005–5278

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Geophysical Framework Based on Analysis of Aeromagnetic 

and Gravity Data, Upper and Middle Verde River Watershed, 

Yavapai County, Arizona

By V.E. Langenheim, Ed DeWitt, and Laurie Wirt

Scientific Investigations Report 2005–5278

U.S. Department of the Interior

U.S. Geological Survey 

Prepared in cooperation with the 

Arizona Department of Water Resources

Yavapai County, Arizona, provided funding for this report. The views or findings in this report do not 

necessarily represent those of Yavapai County.

Analysis of aeromagnetic and gravity data provides new insights into the geometry of geologic structures, such as 

faults, basins, and paleochannels, of the upper and middle Verde River watershed.
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey Information Services

Box 25286, Denver Federal Center

Denver, CO 80225-0046

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

http://pubs.usgs.gov/sir/2005/5278/

For additional information write to:

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Cataloging-in-Publication data are on file with the Library of Congress

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, December 8, 2005

Text edited by Peter H. Stauffer

Layout and design by Stephen L. Scott

FRONT COVER

Block diagrams looking northwest across the Verde Valley, Arizona. Left block shows the surface topography, 

with interpreted faults shown by red dashes, buried basalt flows by brown dashes, and the Verde River and 

major springs in dark blue. Right block shows the basement topographic surface of the same area. 
Contents

Abstract ------------------------------------------------------------------------------------------------------- 1

Introduction---------------------------------------------------------------------------------------------------- 1

Geologic setting----------------------------------------------------------------------------------------------- 2

Rock types ----------------------------------------------------------------------------------------------- 2

Structure ------------------------------------------------------------------------------------------------- 4

Aeromagnetic and gravity data ----------------------------------------------------------------------------- 5

Filtering techniques ------------------------------------------------------------------------------------------ 5

Wavelength separation -------------------------------------------------------------------------------- 5

Geophysical boundaries ------------------------------------------------------------------------------- 7

Physical properties ------------------------------------------------------------------------------------------- 8

Geophysical anomalies------------------------------------------------------------------------------------ 12

Depth to basement----------------------------------------------------------------------------------------- 14

Method ------------------------------------------------------------------------------------------------ 14

Estimating uncertainties in basin depth ----------------------------------------------------------- 15

Depth to basement results -------------------------------------------------------------------------- 17

Big Chino Valley -------------------------------------------------------------------------------- 17

Verde Valley------------------------------------------------------------------------------------- 19

Little Chino and Lonesome Valleys, including upper Agua Fria Basin ------------------- 19

Williamson Valley ------------------------------------------------------------------------------ 20

Distribution of Tertiary volcanic rocks in the subsurface --------------------------------------------- 20

Faults and fractures---------------------------------------------------------------------------------------- 21

Conclusions and recommendations --------------------------------------------------------------------- 22

Acknowledgments ----------------------------------------------------------------------------------------- 22

References-------------------------------------------------------------------------------------------------- 22

Plate

[In pocket]

1. Interpretive map of upper and middle Verde River watershed, Yavapai County, Arizona: Geology 

overlaid on shaded-relief topography with magnetic contours and boundaries

Figures

 1. Index map of the study area----------------------------------------------------------------------------- 2

 2. Simplified geologic map of the study area ------------------------------------------------------------ 3

 3. Aeromagnetic anomaly map (reduced to pole) of the study area---------------------------------- 6

 4. Isostatic gravity anomaly map of the study area----------------------------------------------------- 6

 5. Maps showing aeromagnetic data match-filtered to enhance very shallow, deeper, and 

 deep magnetic sources---------------------------------------------------------------------------------- 7

 6. Map showing first vertical derivative of the aeromagnetic field in the study area-------------- 8 

 7. Map showing reduced-to-pole aeromagnetic data filtered by subtraction of an upward 

continued field -------------------------------------------------------------------------------------------- 9 

 8. Residual isostatic gravity map of the study area--------------------------------------------------- 10 

 9. Aeromagnetic maps of the study area derived by upward continuation and by reduction 

to pole --------------------------------------------------------------------------------------------------- 11

III
10. Plot of average densities of Quaternary and Tertiary sedimentary rocks against depth------ 12

11. Schematic representation of basin-bedrock separation------------------------------------------ 14

12. Maps showing basin thicknesses based on density-depth function and on borehole 

 gravity --------------------------------------------------------------------------------------------------- 16

13. Map showing bedrock gravity field for the study area -------------------------------------------- 17

14. Map showing minimum basin thicknesses from wells minus predicted basin 

 thicknesses --------------------------------------------------------------------------------------------- 18

Tables

 1. Densities and magnetic susceptibilities of hand samples collected for this study ------------ 12

 2. Density-depth functions for basin-filling deposits ------------------------------------------------- 15

IV


Abstract

Analysis of aeromagnetic and gravity data provides new 

insights into the geometry of geologic structures of the upper 

and middle Verde River watershed, Yavapai County, Arizona. 

Magnetic anomalies reveal hidden volcanic rocks lying at 

shallow depths beneath the ground surface in Williamson, 

Little Chino, Big Chino, and Verde Valleys in the upper Verde 

River watershed. Concentrations of shallowly buried volcanic 

plugs or centers are located down-gradient of springs (Del 

Rio) and perennial flow (Williamson Valley), suggesting that 

these volcanic centers or plugs may retard ground-water flow. 

Magnetic data also map paleochannels that were filled with 

basalt (6 to 4 Ma) as Big Chino Valley formed and subsided 

during late Tertiary. The magnetic data reveal a predominantly 

northeast- to north-striking structural grain within Proterozoic 

basement rocks, in contrast to the generally northwest strike 

of late Tertiary faults in this region. The magnetic grain may 

serve as a proxy for fracturing and faulting, an important 

source of permeability in these generally impermeable rocks.

Magnetic and gravity data also delineate exposed and 

concealed faults within the study area. The Big Chino Fault 

and Verde Fault Zone have the largest amounts of vertical 

throw of the faults in the study area on the basis of gravity, 

magnetic, and limited well data. These faults bound deep (1-2 

km) basins in Big Chino and Verde Valleys. The geophysical 

data also reveal concealed faults in Williamson Valley that 

bound a previously undiscovered basin with approximately 

1 km of Cenozoic fill inferred from inversion of gravity data. 

Little Chino and Lonesome Valleys, including the upper reach 

of the Agua Fria Basin, are characterized by basin fill that has 

an irregular distribution, with local, north- to northwest-striking pockets of thicker sediment, but nowhere exceeds 1 km of 

thickness. A 15- to 20-km-long northwest-striking magnetic 

lineament that passes through Page Springs in Verde Valley 

can be used to project a mapped fault 5-10 km northwest and 

southeast of its mapped trace. The colocation of the lineament, mapped fault, and Page Springs suggests structural 

influence on the location of this large spring.

Introduction

Yavapai County, Arizona, is one of the fastest growing 

rural counties in the United States, with population growth 

concentrated in Verde Valley, the Prescott area, and Sedona 

(fig. 1). Rapid population growth has led to a concurrent 

increase in water-resources development. Although the 

Verde River provides surface water to water-rights holders in 

Verde Valley and Phoenix, ground water is the major source 

of most public and domestic water supplies in Yavapai 

County, according to the Arizona Department of Water 

Resources (2000). In order to improve understanding of the 

geologic framework and its effect on ground-water flow, the 

county contracted the U.S. Geological Survey to conduct 

geophysical and geologic studies to aid construction of a 

model for ground-water flow in the region.

The goal of this study is to improve understanding of the 

geologic framework of the upper Verde River region (fig. 1) 

from analysis of geophysical data. This work builds upon earlier geohydrologic studies (Twenter and Metzger, 1963; OwenJoyce and Bell, 1983; Water Resources Associates, 1989; 

Ostenaa and others, 1993) and geologic studies (Krieger, 

1965; Ed DeWitt and others, unpub. data) that focused on 

geologic mapping and water well information. This study 

concentrates on projecting surficial structures and stratigraphy 

into the subsurface using geophysical data. This includes a 

more quantitative and detailed interpretation of aeromagnetic 

and gravity data collected by the U.S. Geological Survey in 

1999-2001 than that outlined in Langenheim and others (2000, 

2002) and expands the area of interpretation beyond that presented in Langenheim and others (2005).

The isostatic gravity anomaly data reflect density variations in the upper and middle crust. Interpretation of these 

anomalies provides information on the depth of Cenozoic 

basins and on the nature of pre-Cenozoic basement rocks. 

The aeromagnetic data are sensitive to the distribution of 

magnetic rocks, primarily those containing magnetite. In 

Yavapai County, these rocks are Tertiary volcanic rocks and 

certain rock types within the Proterozoic crystalline basement. In particular, aeromagnetic anomalies mark abrupt 

spatial contrasts in magnetization that can be attributed to 

lithologic boundaries, often caused by faulting and fracturing 

of these rocks. These two geophysical datasets, in concert 

with information from geologic mapping, wells, and other 

types of geophysical data, are effective tools in defining 

hidden structures important to ground-water studies, such as 

the configuration and structural fabric of basement and and 

the distribution of volcanic rocks concealed beneath Tertiary 

sedimentary deposits.

Geophysical Framework Based on Analysis of Aeromagnetic 

and Gravity Data, Upper and Middle Verde River Watershed, 

Yavapai County, Arizona

By V.E. Langenheim, Ed DeWitt, and Laurie Wirt
 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

Geologic Setting

The study area lies within the transition zone between 

the Colorado Plateau and Basin and Range physiographic 

provinces and includes much of Yavapai County (fig. 1). 

The transition zone exhibits geologic features common to 

both provinces. For example, regional projections indicate 

that Paleozoic strata are nearly continuous across the boundary from the Colorado Plateau to the transition zone, with a 

regional dip of <10o northward toward the plateau (Twenter and Metzger, 1963). This regional dip is interrupted by 

steeper dips associated with monoclines (Krieger, 1965). The 

transition zone is also characterized by a series of Tertiary 

and Quaternary north- and northwest-striking normal faults 

(Menges and Pearthree, 1983; Reynolds, 1988), which are 

characteristic of the Basin and Range province to the south.

Rock Types

Rocks exposed in the study area consist of four main 

types and ages. Early and Middle Proterozoic plutonic and 

metamorphic rocks constitute the basement of the mountains 

and valleys in the region. Basement exposures are abundant in 

the Bradshaw and Juniper Mountains and the Black Hills (fig. 

2). Distribution of basement rocks is important because the 

plutonic and metamorphic units typically have very low permeability (except through interconnected fractures) and thus 

are generally poor aquifers.

Paleozoic sedimentary rocks overlie the basement and 

are exposed throughout the area, except where removed by 

erosion during Tertiary (maybe Cretaceous?) time or where 

buried by younger rocks or alluvium. Rock types include 

sandstone, limestone, dolomite, and minor shale, which 

0 10 20 KILOMETERS

112 ̊45 ́ 112 ̊30 ́ 112 ̊15 ́ 112 ̊ 111 ̊45 ́

34 ̊30 ́

34 ̊45 ́

35 ̊

 875

1050

1225

1575

1750

1925

2100

2275

2450

2625

ELEVATION, IN METERS

1400

Flagstaff

Sedona

Camp

Verde

Prescott

Jerome

Prescott

Valley

Chino

Valley

Cottonwood

Ashfork

Williams

PS

DR

UVR

Lake

Montezuma VERDE VALLEY

BIG CHINO VALLEY

LONESOME VALLEY

BLACK

HILLS

BRADSHAW

MTNS

WILLIAMSON

VALLEY

Verde

River

COLORADO

PLATEAUS

GM

SB

ALT

89

69

89

96

89

169

260

ALT

89

179

260

17

17

17

40 40

69

89

HC

Perkinsville

MOGOLLON

RIM

SL

Cornville

Paulden

Drake

Cherry

CV-2

CV-1

CV-3

WV-1

WV-3

CVR1

CVR2

CVR3 Sycamore Canyon

519687

578737

72-3

73-12

614259

AZ#1

AZ#2

10bca

587410

HM

GH

ARIZONA

Clarkdale

BIG BLACK MESA

Oak Creek

MW

LITTLE CHINO

VALLEY

upper

Agua

Fria 

basin

HiM

Partridge Creek

area of

aeromagnetic survey

COLORADO

PLATEAUS

TRANSITION ZONE

BASIN AND

RANGE PROVINCE

Figure 1. Index map of the study area. Red line outlines area of new aeromagnetic data (Langenheim and others, 2000; 2002). Blue dots denote 

springs. Towns and cities in magenta lettering. Magenta-rimmed white circles are selected deep wells that reach pre-Cenozoic bedrock; 

yellow circles are deep wells that did not reach basement. Abbreviations: DR, Del Rio Springs; GH, Glassford Hill; GM, Granite Mountain; HC, 

Hell Canyon; HiM, Hickey Mountain; HM, House Mountain; MW, Montezuma Well; PS, Page Springs; SB, Sullivan Buttes, SL, Sullivan Lake, UVR, 

Upper Verde springs.


together constitute what is termed the regional carbonate 

aquifer. 

Tertiary volcanic rocks are locally abundant, both in 

mountain ranges and within the basins. Depending on composition and texture, volcanic rocks can have a wide range 

of permeability and either transmit water readily or retard its 

flow. Tertiary to Quaternary sediments and rocks are abundant 

in the valleys and form the basin-fill aquifers. These rocks can 

be important aquifers and include fanglomerate, varied alluvial 

deposits, and playa deposits, all of which interfinger, creating 

locally complex depositional patterns and highly heterogeneous permeabilities.

Proterozoic igneous and metamorphic rocks are exposed 

in the mountain ranges that lie along the southern (Bradshaw Mountains) and western (Juniper and Santa Maria 

Mountains) fringes of the study area (Anderson and Blacet, 

1972a; fig. 2). Proterozoic rocks also are extensively 

exposed in the Black Hills in the south-central part of the 

study area. Isolated Proterozoic metasedimentary, metavolcanic and igneous rocks also are exposed in the area (for 

example, 2 km east of Del Rio Springs, 5 km west of Sullivan Buttes, where the Verde Fault intersects the Verde River, 

and the outcrops at the northern end of the Big Chino Fault; 

figs. 1, 2). These basement rocks are compositionally very 

heterogeneous, ranging from mafic (gabbro and metabasalt) 

to felsic (metarhyolite, alaskite, and quartzite). Intruding 

these basement rocks are Lower Cretaceous tonalite stocks 

in the Bradshaw Mountains, one of which crops out along 

the southern border of the geophysical survey 5 km west of 

Highway 69. A possible buried stock is suggested by boulders of granodiorite in sedimentary rocks along the Verde 

Fault, east of Cherry (Ed DeWitt and others, unpub. data). 

These Cretaceous stocks are included with the Proterozoic 

rocks as part of the basement.

The Proterozoic rocks are overlain by Cambrian to Permian sedimentary rocks throughout much of the area, although 

the Paleozoic rocks have been stripped away in much of Lonesome Valley, the upper part of the Agua Fria Basin, the Brad112 ̊45 ́ 112 ̊30 ́ 112 ̊15 ́ 112 ̊ 111 ̊45 ́

34 ̊30 ́

34 ̊45 ́

35 ̊

35 ̊15 ́

Tb

Xto

Verde

BIG

CHINO VALLEY

BLACK

HILLS

BRADSHAW

MTNS

WILLIAMSON

VALLEY

COLORADO

PLATEAUS

MOGOLLON

RIM

Sedona

Prescott

Flagstaff

LONESOME

VALLEY 

Quaternary deposits

Tertiary Verde Formation

latite

Paleozoic and Mesozoic

sedimentary rocks

Crystalline basement rocks

VF

BCF

BWCF

OF

RRF

VF

Big Black Mesa

SANTA

MARIA

MTNS

JUNIPER

MTNS

SB

PS

DR

VERDE VALLEY

LEGEND

Tertiary sedimentary rocks

basalt

Quaternary and Tertiary sedimentary 

deposits

Tertiary volcanic rocks

tonalite/granodiorite

gabbro

metasedimentary

Cambrian-Triassic

Cretaceous pluton

metavolcanics River

bedrock well

well identifications shown on fig. 1

basin well

BCw-PC

FAULT ABBREVIATIONS

BWCF, Bear Wallow Canyon Fault

BCF, Big Chino Fault

BCw-PC, scarp near intersection

 of Big Chino and Pine 

 washes

CRF, Cathedral Rock Fault

RRF, Railroad Fault

OF, Orchard Fault

SF, Sheepshead Fault

VF, Verde Fault

CRF

SF

MW

LITTLE CHINO

VALLEY

0 10 20 KILOMETERS

Figure 2. Simplified geologic map of the study area (from Ed DeWitt and others, unpub. data). Geology outside of thick dotted blue line from 

Reynolds (1988) and Richard and Kneale (1998). See figure 1 for explanation of abbreviations. Dark blue line outlines aeromagnetic survey 

boundary. Springs are blue dots. Heavy gray lines are highways. Solid red lines are faults from Twenter and Metzger (1963), Owens-Joyce and 

Bell (1983), Pearthree (1998), and Ed DeWitt and others (unpub. data). Dashed red lines are inferred faults from analysis of gravity data from this 

study; dashed brown lines are faults located by well and magnetic data.

Geologic Setting
 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

shaw Mountains, and parts of the Black Hills (Krieger, 1965). 

The Paleozoic sequence is as thick as 1,330 m (4,375 feet) 

in the northern parts of the study area. Triassic sedimentary 

rocks (Moenkopi Formation) are present in the far north-central part of the area, along the Mogollon Rim near Sycamore 

Canyon (fig. 1).

A major unconformity and long period of erosion separate 

the Proterozoic, Paleozoic, and Mesozoic rocks from Tertiary 

and Quaternary sedimentary and volcanic rocks. Quaternary 

deposits include alluvial fans, colluvium covering hill slopes, 

floodplain terraces, and stream gravels. In the centers of the 

valleys, these Quaternary deposits are commonly less than 15 

m (50 ft) thick. Thick alluvial fan deposits are prevalent along 

the valley margins. For example, some of the largest fans 

along Big Black Mesa are at least 150 m (500 ft) thick (Ostenaa and others, 1993). 

Tertiary (Oligocene? to Pliocene) sedimentary rocks are 

extensively exposed in the valley areas. Along the margins 

of Lonesome Valley, Tertiary fanglomerate, mud flow deposits, and tuffaceous deposits are exposed; towards the interior 

of the basin, these rocks also include channel gravel, sand, 

silt, clay, and marl. Tertiary sedimentary rocks both underlie, 

overlie, and, in places, are interbedded with Tertiary volcanic 

rocks. 

Tertiary volcanic rocks consist of two main lithologies, 

lati-andesite and basalt. Lati-andesite, ranging in age from 27 

to 21 Ma, is extensively exposed at Sullivan Buttes (Tyner, 

1984). The lati-andesite forms a volcanic field that overlies a 

dissected surface underlain by rocks ranging in age from the 

Early Proterozoic Mazatzal Group through the Mississippian 

to Permian Supai Group to Tertiary gravels (Krieger, 1965; 

Ed DeWitt and others, unpub. data). The lateral extent of the 

volcanic field is larger than suggested by surficial deposits. 

Parts of the volcanic field are buried beneath the northern part 

of Little Chino and Lonesome Valleys and occupy a considerable part of the southeastern part of Big Chino Valley, as 

indicated by interpretation of aeromagnetic data (Langenheim 

and others, 2000; Langenheim and others, 2005). Outcrops of 

lati-andesite and related rocks to the west of Big Chino Valley, 

in the Camp Wood area (Ash, 1997), suggest continuity of 

the volcanic field across much of Williamson Valley and parts 

of southern Big Chino Valley (Ed DeWitt and others, unpub. 

data).

Tertiary basalt is exposed in many places throughout 

the study area and underlies much of Lonesome and Verde 

Valleys. The basalts are divided into two main sequences 

(McKee and Anderson, 1971). The older sequence, the 

Hickey Formation, consists of 15 to 10 Ma basalt flows and 

minor interbedded sedimentary rocks as much as 300 m 

(1,000 ft) thick. The interbedded sedimentary rocks were 

derived from the Basin and Range Province to the southwest 

and include distinctive clasts of Proterozoic basement rocks 

exposed in the Bradshaw Mountains. The younger sequence 

consists of 7 to 4 Ma basalt flows and interbedded conglomeratic sedimentary rocks derived from the Colorado Plateau to 

the north, including the so-called “ramp” basalts that flowed 

into Verde Valley and basalts that flowed into the northern and 

southern ends of Big Chino Valley. Interbedded sedimentary 

rocks constitute a significant part of the total thickness of this 

sequence.

The thickness of the volcanic rocks in the Hickey Formation varies from 0 to 400 m in the Black Hills (Anderson and 

Creasey, 1958). Well-log data in Lonesome Valley suggest that 

the upper surface of the volcanic rocks (“malpais” or “malapais” in driller’s logs and inferred to be basalt) slopes gently 

to the southwest from the northeast side of the basin (Krieger, 

1965). The maximum thickness of the basalt penetrated in 

wells in Lonesome Valley is 150 m (500 ft).

In Verde Valley, Tertiary volcanic rocks of the Hickey 

Formation are overlain by the lakebed deposits of the Miocene 

to Pliocene Verde Formation (Jenkins, 1923; Lehner, 1958;

Twenter and Metzger, 1963; Nations and others, 1981; Holm 

and others, 1998; Leighty, 1998). These rocks were deposited 

in a valley similar in size to the present Verde Valley and consist of limestone, mudstone, sandstone, and, locally, evaporite 

deposits. The exposed thickness of the Verde Formation is 

about 480 m, but the thickness of the entire formation may 

exceed 950 m (Nations and others, 1981; Ed DeWitt and 

others, unpub. data). Playa deposits constitute much of the 

sedimentary sequence underlying Big Chino Valley (Ostenaa 

and others, 1993; DeWitt and others, 2005), in contrast with 

the coarser grained deposits found in deep drill holes in Williamson Valley.

Structure

The most intensely and complexly deformed rocks in the 

area are the Proterozoic rocks (Anderson and Creasey, 1958; 

Anderson and Blacet, 1972a, b; Anderson, 1989; DeWitt, 

1987, 1989; Wilson, 1939). These rocks have been regionally 

metamorphosed during Early Proterozoic time to greenschist 

facies and deformed, and they possess a northeast-striking 

foliation that dips steeply. Zones of high strain are locally 

apparent in the metamorphic rocks. This deformation did not 

affect the Paleozoic sequence. 

Paleozoic rocks generally dip less than 10o to the northeast in the study area, except near monoclines, such as the 

Limestone Canyon monocline on Big Black Mesa. The local 

steeper dip of Paleozoic strata on Big Black Mesa and in the 

area north of the Verde River is probably the result of deformation related to monocline formation. This deformation 

resulted in a series of basement-cored blocks that are tilted 

gently to the northeast and are bounded by the northwest-striking monoclines. Because of the resulting differences in structural level, Paleozoic rocks were stripped from the basement 

in the southern part of the study area. The age of these monoclines is probably Late Cretaceous to early Tertiary, judging 

from their similarity to Laramide monoclines on the Colorado 

Plateau (Davis, 1978).

Oligocene (?) to Miocene gravels exposed in the headwaters area of the Verde River are evidence for a northeast-slop-


ing paleoerosion surface (Elston and Young, 1991). Remnants 

of these gravels exposed elsewhere suggest that this erosional 

surface developed before the older sequence of basalts (or 

latites) erupted. Later extensional faulting resulted in largescale downdropping of Tertiary volcanic and sedimentary 

rocks in Verde and Big Chino Valleys starting at about 11 to 

8 Ma (Elston and others, 1974; Elston and Young, 1991; Ed 

DeWitt and others, unpub. data). Faulting probably continued 

to 1 Ma. Presumably, faulting also occurred west of the Black 

Hills in Lonesome Valley at about the same time. Basalts 

flowed over the resultant escarpments and into valleys in 

selected places.

Several faults in the transition zone may show evidence 

of multiple ages of movement. Shoemaker and others (1978) 

describe several northeast- and northwest-striking fault systems in northwestern Arizona that are Tertiary in age and 

parallel the Precambrian structural grain. A fault system 

identified by Krieger (1965) and Shoemaker and others (1978) 

is the northwest-striking Big Chino Fault, exposed along the 

southwestern edge of Big Black Mesa (figs. 1, 2). This fault 

clearly shows evidence of movement as young as Quaternary 

(Menges and Pearthree, 1983), with scarps 8 to 10 m high in 

alluvial deposits; the latest movement on the fault is older than 

Holocene (Menges and Pearthree, 1983; Pearthree, 1998). 

The Big Chino Fault may have been responsible for the February 4, 1976, Chino Valley earthquake (Mb

 4.9), analysis of 

which indicated normal movement along a northwest-trending 

fault plane dipping 40o to the southwest (Eberhart-Phillips and 

others, 1981).

Another large fault system, the Verde Fault Zone, shows 

evidence of Quaternary movement (Pearthree, 1998). It forms 

a northwest-striking zone of faults along the western margin 

of Verde Valley, progressively downdropping to the east. 

Anderson and Creasey (1958) suggested that as much as 300 

m of offset on this fault may have occurred during the Precambrian, although this early faulting is not related to the present 

basin. Most of the offset took place during the late Miocene 

and Pliocene (Bressler and Butler, 1978; Nations and others, 

1981), displacing Tertiary rocks as much as 1,000 m (Twenter 

and Metzger, 1963). Evidence of Quaternary offset is limited 

to an 8-km-long section of fault scarps in the southern part of 

the fault zone, called the Camp Verde Fault by Menges and 

Pearthree (1983).

Faults can serve as conduits or barriers to ground-water 

movement. On a regional scale, northwest-striking fractures 

throughout the Colorado Plateaus area in northern Arizona 

tend to be open to fluid flow (Thorstenson and Beard, 1998; 

L.S. Beard, oral commun., 1999). Twenter and Metzger 

(1963) argue that the Verde Fault Zone acts as a barrier to 

ground-water flow and directs ground water to the southeast 

along its fractures and joints. Well yields in the middle Verde 

Basin are generally improved by the presence of solution 

cavities along fractures in the Redwall Limestone and Martin 

Limestone (Owen-Joyce and Bell, 1983, p. 20). These limestone units are exposed along the margins of the Big Chino 

Basin and the canyon walls of the upper Verde River (fig. 2).

Aeromagnetic and Gravity Data

Details of the processing techniques of the high-resolution 

aeromagnetic data collected for the study are given in Langenheim and others (2000; 2002). Flight lines were oriented 

east-west, spaced 150-300 m (0.093-0.186 mi) apart, and flown 

at an average terrain clearance of 350 m. Significant deviations from the average terrain clearance were as much as 1,087 

m over Verde Valley and 700 m over the area east of Granite 

Mountain north of Prescott (Langenheim and others, 2002). 

North-south control lines were spaced 3.0 km (1.83 mi) apart. 

Total flight distance was 27,291 km (16,958 mi). To shift 

anomalies over their respective sources, the magnetic data were 

reduced to the pole (fig. 3; Blakely, 1996). We assume in this 

study that there are no large areas with directions of remanent 

magnetization significantly different from the present Earth 

field direction and that therefore application of the reduction to 

pole transformation to the aeromagnetic data is justified.

About 3,300 gravity stations (Frank, 1984; Smith, 1984; 

Water Resources Associates, 1989; Dater and others, 1999; 

Langenheim and others, 2000, 2002) were used to produce 

an isostatic gravity anomaly map of the region (fig. 4); 862 of 

those stations were collected as part of this study. The data 

were reduced using a density of 2.67 g/cm3. The isostatic 

gravity data reflect density variations within the middle and 

upper crust (Simpson and others, 1986). Details on the processing of the gravity data are given in Langenheim and others 

(2000; 2002). Gravity stations are nonuniformly distributed 

in the region. Station spacing is on average 1 station per 2 

km2, although locally it is as low as 1 station per 10 km2, even 

within parts of Big Chino and Little Chino Valleys.

Filtering Techniques

Magnetic and gravity anomalies are produced by a variety of sources of variable size and depth. Superposition of 

anomalies from multiple sources can result in interpretational 

ambiguities. For example, both Proterozoic crystalline and 

Tertiary volcanic rock types contain magnetic units, but they 

may be characterized by anomalies of differing wavelengths 

(or characteristic length). Shallow sources typically cause 

shortwavelength anomalies, whereas deep sources cause 

long wavelength anomalies. Generally, the Tertiary volcanic 

rocks, comparatively thinner and shallower than Proterozoic 

crystalline rock, should produce shorter wavelength anomalies. Several analytical techniques were applied to the geophysical data to distinguish particular anomaly characteristics, 

such as wavelength or trend.

Wavelength Separation

To emphasize both short-wavelength anomalies caused 

by shallow sources (for example, Tertiary volcanic rock) and 

Filtering Techniques
 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

-33

-30

-27

-24

-21

-18

-15

-12

 -9

 -6

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Figure 3. Aeromagnetic anomaly map (reduced to pole) of the study area. White lines mark boundaries of 

high-resolution aeromagnetic data collected for this study. Gray lines are highways; blue line is Verde River.

Figure 4. Isostatic gravity anomaly map of the study area. Blue line marks extent of high-resolution aeromagnetic 

data. Gray lines are highways. Dark green circles are sites of gravity measurements collected for this study; 

smaller blue dots are sites of previously collected data. Red lines are faults from Ed DeWitt and others (unpub. data). 

Dashed brown lines are faults inferred from well, magnetic, and gravity data. Dashed orange line is extent of finegrained basin fill in Big Chino Valley from Schwab (1995).
Filtering Techniques 

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MAGNETIC ANOMALY, IN nT MAGNETIC ANOMALY, IN nT MAGNETIC ANOMALY, IN nT

a

A

B

C

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Figure 5. Match-filtered aeromagnetic data. Dark blue line is extent of high-resolution aeromagnetic data. Pale blue lines are gravity lows; pink lines are gravity highs. 

Red lines, faults from Ed DeWitt and others (unpub. data); dashed brown lines, faults 

inferred from geophysical and well data. A, Data filtered to enhance very shallow 

magnetic sources (<1 km). B, Data filtered to enhance deeper sources (~2-3 km). C, 

Data filtered to enhance deep sources (~10 km). Feature “a” is discussed in the text.

long-wavelength anomalies (for example, Proterozoic 

crystalline rock), a match filter was applied (Phillips, 

2001). Match filtering separates the data into different 

wavelength components by modeling the observed 

spectra as a sum of distinct equivalent source layers 

at increasing depths (see Phillips, 2001). Figure 5 

shows the resulting separated fields produced by the 

dipole equivalent-source layers at depths of 0.76, 

2.93, and 12.24 km, associated with increasingly 

deeper sources. Another method, the first vertical 

derivative of the magnetic data (fig. 6) suppresses 

longer-wavelength trends caused by more deeply 

buried magnetic rock types (Blakely, 1996).

A third method to sharpen the effects of nearsurface sources involves removal of a regional field 

derived by upward continuation. Analytically continuing the magnetic or gravity field upward by a 

small interval (100 m for the magnetic data, fig. 7; 

1 km for the gravity data because of the nonuniform 

distribution of gravity stations, fig. 8) generates a 

regional or smoothed field. By transforming the 

magnetic or gravity data measured on one surface 

to those that would be measured on a higher surface 

(farther from the source), upward continuation tends 

to smooth the data by attenuating short-wavelength 

anomalies (Dobrin and Savit, 1988). This smoothed, 

upward-continued field is then subtracted from the 

unfiltered field to produce a residual field. The 

unfiltered and residual fields (figs. 3, 5, 6, and 7 

for the magnetic field, figs. 4 and 8 for the gravity 

field) illustrate the effectiveness of this approach to 

highlight subtle geologic features. To emphasize the 

smoothing effect of thick basin fill or the significant 

deviation from the average terrain clearance in Verde 

Valley on the wavelength of observed magnetic 

anomalies, the data were upward continued 750 m, 

which was the maximum deviation from the average 

terrain clearance. Comparing this smoothed map to 

the reduced-to-pole magnetic map (fig. 9) shows that 

short-wavelength anomalies are greatly attenuated 

and illustrates the effectiveness of wavelength separation to determine the depths of magnetic sources.

Geophysical Boundaries

To help delineate structural trends and gradients 

expressed in the gravity field, a computer algorithm 

was used to locate the maximum horizontal gravity gradient (Cordell and Grauch, 1985; Blakely and 

Simpson, 1986). Concealed basin faults in Verde and 

Williamson Valleys were mapped using horizontal gradients in the gravity field (dashed red lines on fig. 2). 

Gradient maxima occur approximately over steeply 

dipping contacts that separate rocks of contrasting densities. For moderate to steep dips (45° to vertical), the 
 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

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INSET

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Figure 6. Shaded-relief map showing the first vertical derivative of the aeromagnetic field in the study area. Brown 

outlines indicate where Tertiary volcanic rocks are exposed; pink and purple outlines show where Proterozoic rocks 

are exposed. Small magenta crosses are wells that reach Tertiary volcanic rocks. Blue line is the Verde River. 

Smooth character of the field outside the extent of high-resolution data (white lines) may reflect lack of resolution from 

older data. Inset shows area north of Clarkdale (black outline) with mapped faults (red) and inferred faults (yellow) 

from subtle magnetic anomalies primarily over Paleozoic sedimentary rocks.

horizontal displacement of a gradient maximum from the top 

edge of an offset horizontal layer is always less than, or equal 

to, the depth to the top of the source (Grauch and Cordell, 

1987). Magnetization boundaries (plate 1) were calculated 

in a similar way as described in Blakely and Simpson (1986), 

using the magnetic potential field produced from the residual 

magnetic field shown in figure 7. Concealed basin faults in 

Lonesome Valley and the upper Agua Fria Basin were mapped 

using a combination of the magnetic boundaries and well data.

Physical Properties

Magnetic and gravity data reflect the surface and subsurface distribution of magnetization and density, respectively. 

The magnetization (emu/cm3) of a rock is the sum of remanent and induced components. The remanent component is 

not easily measured in the field and will be discussed later. 

The induced component depends on magnetic susceptibility 

(cgs unit), which is easily measured in the field. Magnetic 

susceptibility and density information of exposed rock types 

is critical to determining the sources of gravity and magnetic 

anomalies. Table 1 summarizes the magnetic susceptibility and density data of various rock types collected for this 

study.

The most magnetic rock types in the study area, in general, are the Tertiary volcanic rocks, which have an average 

magnetic susceptibility of 0.87x10-3 cgs units. The lati-andesites are in general slightly less magnetic than the basalts, 

although they exhibit a wider range in magnetic properties, 

from 0.04x10-3 cgs units for oxidized lati-andesites to 5.04x10-3

cgs units for a Tertiary hornblende-bearing latite. The most 


magnetic Tertiary basalt sample (4.0x10-3 cgs units) is a magnetite-rich basalt, tentatively assigned to the Hickey Basalt.

Proterozoic rocks, although slightly less magnetic in 

general than the Tertiary volcanic rocks, include the highest magnetic susceptibility readings (6.37x10-3 cgs units) 

for the study area. These rocks are very heterogeneous, as 

indicated by standard deviations that exceed the average susceptibility values for the four main rock types. In general, 

the metasedimentary rocks are not magnetic, except for an 

iron-rich metachert that forms a minor lithologic constituent 

exposed south of the towns of Prescott and Prescott Valley 

(Krieger, 1965) and metagraywacke exposed on the western side of Sullivan Buttes. Metavolcanic rocks, gabbros, 

and some of the other intrusive rocks can be quite magnetic 

and therefore can produce prominent magnetic anomalies. 

Granodiorites and granites show a wide range in susceptibility, although individual granodiorite and granite plutons tend 

to be characterized by narrower ranges in susceptibility. For 

example, the Prescott Granodiorite, exposed near Prescott, has 

a range of 0.04 to 1.27x10-3 cgs units for eight samples (mean 

of 0.81x10-3). The Minnehaha Granodiorite, exposed in the 

southern Bradshaw Mountains, on the other hand, is less magnetic, showing a range in susceptibility of 0.0008 to 0.028x10-3 

cgs units for 5 samples (mean of 0.004x10-3).

Limited physical property data for Tertiary sedimentary 

rocks (21 samples) suggest that these rocks can produce measurable magnetic anomalies, although their average susceptibility is only 0.15x10-3 cgs units, considerably less than that of 

the Tertiary volcanic rocks or Proterozoic rocks. Detritus from 

the volcanic rocks probably is responsible for the magnetic 

properties of the Tertiary sedimentary rocks. The most magnetic Tertiary sedimentary samples are volcaniclastic rocks 

Physical Properties

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Figure 7. Reduced-to-pole aeromagnetic data for the study area filtered to enhance shallow magnetic sources by subtraction of 

an upward continued field (100 m). White lines mark extent of new high-resolution aeromagnetic surveys. Blue line is Verde River. 

Thick gray lines are major highways. Red-rimmed white circles are deep wells that reach pre-Cenozoic bedrock; blue-rimmed 

gray circles are wells deeper than 300 m that did not. Solid red lines are faults from Ed DeWitt and others (unpub. data); dashed 

red lines are faults inferred from geophysical and well data. Blue dots are springs. For explanation of c1, c2, e1, e2, e3, and vc, 

see text.
10 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

Remanent magnetization could be significant for Cretaceous 

plutonic rocks, as shown for Mesozoic plutonic rocks elsewhere in Arizona (Gettings, 2002); in the study area, however, 

these rocks do not cause significant anomalies (plate 1).

The Tertiary basalts in this region typically include lava 

flows that individually may have a uniform direction of magnetization. The basalts are of normal or reversed polarity, as 

indicated by the magnetostratigraphic study by McKee and 

Elston (1980) in the Verde Valley area. Two basalt flows 

exposed near Sullivan Lake in and near the Verde River 

Canyon were collected for paleomagnetic measurements and 

found to have reversed directions, confirming interpretations 

of short-wavelength, negative anomalies there (Langenheim 

and others, 2005). Natural remanent magnetizations in these 

basalts are high, ranging from 10-3 to 10-1 emu/cm3, yielding Koenisberger ratios (Q, the ratio of remanent to induced 

immediately beneath Tertiary basalt (?) sampled at depths of 

740 to 766 ft (226 to 233 m) in WV-3 in Williamson Valley 

(fig. 1).

The magnetic properties of the Paleozoic sedimentary 

rocks (table 1) are very weak. The result is low-amplitude 

magnetic anomalies generally undetectable by airborne surveys.

Magnetic susceptibility contributes to one component of 

the total magnetization of a rock. The other component, the 

remanent magnetization, is a function of the direction and 

strength of the Earth’s magnetic field when the rock acquired 

the magnetization. Remanent magnetization can be an important component of the magnetization of the Tertiary volcanic 

rocks, but it is unlikely to contribute to the magnetization of 

the other magnetic rock types in the study area because of 

the great age and coarse grain size of the Proterozoic rocks. 

-5.0

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Figure 8. Residual isostatic gravity map of the study area (derived from removal of the smoothed field from upward continuation of the gravity 

field by 1 km). Colored areas within map show gravity values controlled by gravity stations within 2.5 km of a gravity measurement. Purple 

line marks extent of high-resolution aeromagnetic data. Thin blue line shows the course of the Verde River. Gray lines are highways. Rows of 

dark blue circles are density boundaries from horizontal gravity method. Solid red lines, faults from Ed DeWitt and others (unpub. data); brown 

dashed and red dashed lines, faults inferred from well data and gravity gradients, respectively. Orange dashed line is extent of fine-grained 

deposit in Big Chino Valley, from Schwab (1995). Light blue dots are springs.
Physical Properties 11

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PS

Figure 9. Aeromagnetic maps of the study area, upward continued 750 m (A) and reduced to pole (B). White lines mark 

boundaries of high-resolution aeromagnetic data collected for this study. Gray lines are highways; blue line is Verde 

River. Red lines are faults from Ed DeWitt and others (unpub. data); brown dashed lines are faults inferred from geophysical and well data. PS marks location of Page Springs.
12 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

intensity of the remanent magnetization is about 1x10-3 to 

3x10-3 emu/cm3. This information supports the interpretation 

that many of the circular magnetic lows in figures 3, 5, 6 and 7 

may be caused by reversely magnetized lati-andesite plugs.

The density measurements for this study (table 1) are 

consistent with earlier data (Frank, 1984; Cunion, 1985; Langenheim and others, 2005). Proterozoic rocks are dense (average value of 2.76 g/cm3

) but exhibit a wide range in values. 

For instance, gabbro and metavolcanic rocks can be as dense 

as 3.09 and 3.15 g/cm3, respectively; however, representative density values of aplites and pegmatites are substantially 

lower (2.59 g/cm3). The metasedimentary and granitic rocks 

are characterized by densities intermediate to these values. 

The densities of the Paleozoic rocks are indistinguishable from 

those of the Proterozoic granitic rocks, although the carbonate 

formations (Martin Formation and Redwall Limestone; average 2.72 g/cm3

) are denser than the clastic formations (Tapeats 

Sandstone, Hermit Shale, or Supai Group rocks; average 2.53 

g/cm3). Similarly, the Tertiary basalts, with an average density 

of 2.69 g/cm3, may be difficult to distinguish from the preCenozoic rocks. The lati-andesites are less dense (average 2.55 

g/cm3), although densities of vesicular basalts and some of the 

lati-andesites in the study area are as low (2.16 g/cm3) as those 

of the Tertiary sedimentary rock samples.

Only 10 direct density measurements of the Tertiary 

sedimentary sequence were made in the study area; these 

measurements are undoubtedly biased towards higher densities because of the difficulty in obtaining a hand sample in 

unconsolidated materials. The Tertiary sedimentary samples 

are significantly less dense (~2.49 g/cm3) than most of the 

other rock types. The average density of the Verde Formation, 

based on 21 measurements (Smith, 1984), is 2.16 g/cm3 (dry) 

and 2.41 g/cm3 (saturated) and suggests that our average density is indeed skewed toward higher values. No measurements 

were made on Quaternary sedimentary deposits for this study. 

Because of the difficulty of obtaining direct density measurements on Quaternary and Tertiary sedimentary rocks, one must 

rely on indirect information previously discussed at length 

by Langenheim and others (2005) and summarized in figure 

10. The most direct measure of the density of the sedimentary 

sequence comes from borehole gravity surveys outside the 

study area, such as those compiled for the State of Arizona 

(Tucci and others, 1982, their figure 3) and those compiled for 

the Basin and Range province (Jachens and Moring, 1990).

Geophysical Anomalies

The aeromagnetic data show distinctive magnetic 

anomaly patterns associated with distribution of Proterozoic 

basement and Tertiary volcanic rocks. The anomalies over 

Proterozoic exposures contrast with those over exposures of 

Tertiary volcanic rocks, which produce distinctive high-amplitude, short-wavelength magnetic anomalies. The anomalies 

produced by buried Tertiary volcanic rocks are particularly 

Table 1. Densities and magnetic susceptibilities of hand samples collected for study.

2.00 2.10 2.20 2.30 2.40 0

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AVERAGE DENSITY OF QUATERNARY AND TERTIARY SEDIMENTARY ROCKS

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DEPTH

(m)

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 Basin & Range v

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Figure 10. Average densities of Quaternary and Tertiary sedimentary 

rocks derived by various methods, modified from Langenheim and 

others (2005): r, derived from electrical resistivity; v, derived from 

seismic velocity; b, derived from borehole gravity of Tucci and others 

(1982; their figure 3). Dashed line, density-depth function derived from 

borehole gravity results of Basin and Range Province (Jachens and 

Moring, 1990).

magnetization) of 1.25 to 400. The magnetic remanence of 

two lati-andesites exposed in the Sullivan Buttes area was also 

measured. The results indicated reversed directions (declinations (D) of 149° and 182° and inclinations (I) of –47° and 

–58°; the present-day direction is D=13° and I=61°), and the 
13

evident in much of Lonesome Valley and Little Chino Valley; 

anomalies produced by outcrops of Tertiary volcanic rocks 

are obvious in the Black Hills, Sullivan Buttes, and the area 

around Paulden. Many of the Tertiary outcrops of lati-andesite 

coincide with very strong circular magnetic lows indicative 

of volcanic plugs. Tertiary basalts generally produce a wormlike magnetic anomaly pattern. Anomalies associated with 

Proterozoic rocks tend to be smoother and less noisy than their 

counterparts over Tertiary volcanic rocks.

The magnetic anomaly map of the area (fig. 3) can be 

divided into two main sections, east and west of the Black 

Hills. East of the Black Hills, distinctive high-amplitude, 

short-wavelength magnetic anomalies, closely associated with 

exposures of Tertiary volcanic rocks, contrast with smooth, 

generally low magnetic values present over Verde Valley 

(plate 1). The subdued nature of those anomalies is probably caused by two factors: (1) the increased flight elevation 

(height of the magnetic sensor) above the ground surface 

in this region and (2) the substantial thickness of relatively 

nonmagnetic Verde Formation in the Verde Valley. Volcanic 

rocks may be present throughout the subsurface in Verde 

Valley, but because of the attenuation of these anomalies, it is 

not possible to map their distribution with confidence using 

unfiltered aeromagnetic data. The amount of smoothing is 

exemplified in the area around Page Springs (PS on fig. 9); 

the magnetic signature of volcanic rocks at depths (or distances) greater than 750 m is indistinguishable from that of 

Proterozoic basement. Even where filtered data are used, as 

in the vertical derivative map (fig. 6), wells that penetrated 

volcanic rocks in Verde Valley coincide with only very subtle 

anomalies.

The dominant anomaly trends in the Verde Valley and 

areas east of the Black Hills are northwest, with subordinate 

northeast and north-northeast trends. The northwest trends 

are parallel to the dominant young faults mapped on the west 

side of the valley. North- to northeast-striking anomalies are 

associated with Proterozoic basement rocks. Directly west 

of the Black Hills is a linear north-striking magnetic high, 40 

km long, that extends from the town of Prescott Valley north 

towards Perkinsville. Its source resides within the Proterozoic 

basement, because the magnetic high can be traced onto scattered outcrops of Proterozoic gabbro (plate 1).

In the Bradshaw Mountains and Black Hills, anomalies 

caused by Proterozoic basement rocks typically strike north 

to northeast, parallel to steep foliations mapped in these rocks 

(Ed DeWitt and others, unpub. data). Intense magnetic highs 

coincide with exposures of gabbro and metavolcanic rocks 

(plate 1). Lower magnetic values coincide with exposures 

of some of the felsic plutonic rocks, such as the Prescott 

Granodiorite in the southwest corner of the survey area, and 

the Cherry Tonalite (of Ed DeWitt and others, unpub. data) 

in the Black Hills, and metasedimentary rocks such as the 

quartzite of the Mazatzal Group southeast of Del Rio Springs. 

Superposed on the low magnetic values over the felsic plutonic rocks are anomalies (amplitudes generally <100 nT) that 

strike north to northeast. In the Black Hills, these north- to 

northeast-striking anomalies over the Cherry Tonalite contrast 

with a northwest fabric associated with scattered exposures of 

gabbro and with the areas to the east in Verde Valley.

The magnetic field over the alluvial deposits of Lonesome Valley, Little Chino Valley, and upper Agua Fria Basin 

shows many short-wavelength, northwest- and north-trending 

anomalies. Because alluvium is commonly weakly magnetic, 

many of these anomalies probably originate from volcanic 

rocks concealed beneath the surface. Some anomalies can in 

places be traced to outcrops such as Glassford Hill, west of the 

town of Prescott Valley. The irregular texture of the magnetic 

field over these alluvial deposits contrasts with the relatively 

smooth magnetic character of much of Big Chino Valley, parts 

of Williamson Valley, and part of Little Chino Valley east of 

Granite Mountain. The smooth character of the magnetic field 

over Big Chino and Williamson Valleys can be attributed to a 

greater depth to magnetic basement because of basin fill that 

is at least 600 m thick (Langenheim and others, 2005; this 

study). The smooth character of magnetic anomalies in the 

Little Chino Valley adjacent to Granite Mountain is mostly 

caused by the increased height of as much as 700 m of the 

magnetic sensor above the ground surface in this region (Langenheim and others, 2002, their fig. 4)

Extensive outcrops of the Paleozoic sedimentary sequence 

(thickness 0-200 m) generally produce a relatively featureless magnetic field. The first derivative map (fig. 6) shows 

the generally smooth magnetic character associated with the 

Paleozoic sequence north of Hickey Mountain and in the area 

around Sedona. Surprisingly, similar rocks on Big Black 

Mesa do not show a similar smooth character. Instead, largeamplitude, relatively short-wavelength magnetic highs are 

present over these weakly magnetic sedimentary rocks. The 

source of these anomalies is thus likely in the Proterozoic 

basement and concealed by Paleozoic units. Several small 

exposures of Proterozoic granitic rocks along the Big Chino 

Fault (Krieger, 1967) are the most likely source of the Big 

Black Mesa aeromagnetic high. This is supported by measured magnetic susceptibilities of the Chino Valley Granite 

as high as 2.07x10-3 (cgs units). The high is truncated by the 

Big Chino Fault on its southwestern margin (Langenheim and 

others, 2000).

Although Paleozoic rocks near Sedona and north of 

Hickey Mountain are characterized by smooth anomalies, 

some of these anomalies have considerable amplitude. For 

example, the area around Sedona is part of a broad magnetic 

high, with an amplitude of >250 nT (fig. 3). Oil test wells in 

the Sedona area encountered Proterozoic granite beneath 300 

to 500 m of Paleozoic sedimentary rocks (Peirce and Scurlock, 

1972). The long-wavelength nature of the anomaly suggests 

that the source of the anomaly is buried deeper than 300 to 

500 m; see, for example, the prominent magnetic highs at 

matched-filter depths of 2.93 and 12.24 km in the Sedona area 

(fig. 5). This area also coincides with a large positive gravity anomaly (fig. 4), suggesting that a more mafic rock type 

of Proterozoic age may be the source of both the gravity and 

long-wavelength magnetic positive anomalies.

Geophysical Anomalies
14 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

The gravity field is dominated by gravity highs along the 

northeastern part of the study area, with lower gravity values 

in the southwestern part of the study area (fig. 4). Superposed 

on this regional field are local gravity lows in the valley areas. 

Big Chino Valley is characterized by a gravity low, bounded 

on the northeast by the Big Chino Fault. The deepest part of 

the basin, as suggested by the lowest gravity values within 

the valley, is near Bureau of Reclamation drillhole CV-3 (fig. 

4). The Big Chino gravity low narrows to the northwest and 

becomes less prominent. To the southeast, gravity values 

increase towards Sullivan Lake, corresponding with shallower 

basin deposits and Proterozoic outcrops east of Sullivan Lake.

Little Chino Valley is characterized by higher gravity 

values than those in Big Chino Valley, suggesting that the 

basin fill beneath Little Chino Valley is less than 1 km thick 

(Langenheim and others, 2000). South of the town of Chino 

Valley is an east-west striking gravity gradient that is bordered 

by a large gravity low to the south. This low could be caused 

by a deep basin centered near the intersection of Highway 89 

and alternate route 89, but a more likely explanation is a thick 

stock of Prescott Granodiorite (Cunion, 1985). The gravity 

low extends over Proterozoic Prescott Granodiorite and Dells 

Granite. These rocks are less dense (averages of 2.67 and 2.61 

g/cm3, respectively) than the some of the more mafic metavolcanic rocks and gabbro within Proterozoic basement (2.81 and 

2.93 g/cm3 for metavolcanic and mafic igneous rocks, respectively; see table 1). This low may mask more subtle gravity 

lows caused by locally thick (as much as 200 to 300 m) accumulations of basin fill. Local gravity lows highlighted in the 

residual gravity map (fig. 8) may reflect these local subbasins.

A northwest-trending gravity low over Williamson Valley 

(fig. 4) may reflect a relatively deep (1 to 2 km) sedimentary 

basin, a low-density pluton (Langenheim and others, 2000), 

or a combination of the two. The gravity low also coincides 

with relatively low magnetic values (fig. 3), especially for the 

shallow matched-filter depth (fig. 5), and a relatively smooth 

magnetic field (figs. 6, 7). The basin interpretation is preferred 

here because the gravity low does not extend across Proterozoic outcrops exposed to the east of the gravity low. A drillhole completed for the city of Prescott, WV-1 (fig. 1), supports 

the basin interpretation; the drillhole penetrated 460 m (1,500 

ft) of Tertiary clastic fill without encountering Proterozoic 

basement rocks (T. Merrifield, oral commun., 2002; figs. 4, 8).

Like Williamson Valley, Verde Valley is characterized 

by a series of northwest-trending gravity lows. The lows, 

approximately 35 km long and 8-10 km wide, lie within the 

southwestern half of the valley. South of a point 4 km north 

of the old Cherry Road, the western margin of the Verde 

Valley gravity lows is nearly coincident with the central and 

southern parts of the Verde Fault Zone. The western margin 

of the gravity lows lies 1-2 km east of the mapped trace of the 

northern part of the fault zone. The northeastern margin of the 

gravity lows also has a very linear northwest-southeast trend, 

suggestive of a fault origin. Near the intersection of I-17 and 

Hwy 260, the eastern edge of the gravity low changes to a 

more southerly strike for a distance of about 10 km, before 

reverting to a southeasterly strike south of Camp Verde. The 

lowest gravity values are northwest of this last change in strike 

of the eastern margin. A smaller gravity low, about 8 km long, 

lies to the south of the step and is enhanced in the filtered version of the gravity field (fig. 8). The southern extent of this 

gravity low, outside the boundary of the new aeromagnetic 

data, is poorly defined by gravity data. Another gravity low, 

located about 10 km west of Camp Verde, is on the footwall 

side of the Verde Fault. This low could reflect lower density 

Proterozoic basement rocks or a basin concealed beneath Tertiary basalt flows. Because the gravity field is affected both by 

changes in thickness of the Cenozoic deposits and by density 

variations in the underlying Paleozoic and Proterozoic rocks, 

a method described below was used in attempting to separate 

these two sources.

Depth to Basement 

Method

In this section, depth to bedrock is calculated beneath the 

study area in order to define the shape of the basins beneath 

valleys within the study area and to determine the geometry 

of bounding and internal faults. Bedrock is here defined to be 

rocks of pre-Cenozoic age.

The method used in this study to estimate the thickness of 

Cenozoic rocks (rocks above bedrock) is a modified version of 

an iterative method developed by Jachens and Moring (1990) 

that incorporates drill hole and other geophysical data (fig. 

11). The inversion procedure allows the density of bedrock 

to vary horizontally as needed, whereas the density of basinfilling deposits is specified by a predetermined density-depth 

Observed gravity

Basin gravity

Bedrock gravity

Bedrock Iteration 1 of

basin thickness

Iteration 1

of bedrock

gravity

Observed gravity

Basin

Final iteration

of bedrock

gravity

Final iteration of

of basin 

thickness

Figure 11. Schematic representation of basin-bedrock separation 

using iterative method of Jachens and Moring (1990).
15

relationship. Two density-depth functions listed in table 2 were 

used to provide two end-member models.

In this iterative approach (see fig. 11), a first approximation of the bedrock gravity field is derived from gravity measurements made on exposed pre-Cenozoic rocks or bedrock, 

augmented by appropriate bedrock gravity values calculated 

at sites where depth to bedrock is known. This approximation, which ignores the gravity effects of nearby basins, is 

subtracted from the observed gravity, which provides the first 

approximation of the basin gravity field. Repeating the process using the specified density-depth relation, the thickness 

of the basin-fill deposits is calculated. The gravitational effect 

of this first approximation of the basin-fill layer is computed at 

each known bedrock station. This effect is, in turn, subtracted 

from the first approximation of the bedrock gravity field and 

the process is repeated until successive iterations produce no 

substantial changes in the bedrock gravity field.

The inversion presented here does not take into account 

lateral variations in the density distribution of the Cenozoic 

deposits, which may be an important source of error in this 

region, particularly in areas underlain by thick, dense basalt 

flows or complex, intertonguing Cenozoic rocks of differing densities. Because of the wide density range of the local 

Cenozoic volcanic rocks and their limited thickness (basalts 

are generally less than 100 m thick), the same density-depth 

relationship was assumed for Cenozoic volcanic rocks as 

for the Cenozoic sedimentary deposits. One might consider 

including the basalts with the pre-Cenozoic bedrock. However, the difficulty of distinguishing dense basalt from lowerdensity lati-andesite or vesicular basalt in driller’s logs, and 

the presence of gravels beneath and interbedded with both the 

basalts and lati-andesites, made this approach intractable.

The inversion method has been shown to be effective in 

determining the general configuration of the pre-Cenozoic 

bedrock surface in Nevada (Phelps and others, 1999). They 

showed that the model bedrock surface of Yucca Flat (Nevada 

Test Site, northwest of Las Vegas, Nevada) was a reasonable 

approximation of the true surface by comparing calculated 

basin depths with densely spaced drill hole data. The general 

shape of the basin did not change significantly with additional 

well control. Furthermore, it appears that lateral variations 

in basin density, unless abrupt, do not change the overall 

modeled general shape of the basin. Although the method is 

a good tool at predicting the shapes of basins, it can be less 

effective in estimating the thickness of basin fill, especially in 

basins having thick basalt flows or in areas of poor well control. Sources of error in the depth-to-basement calculations 

are discussed below.

Estimating Uncertainties in Basin Depth

Two basin models (fig. 12) were created using two different density-depth functions (table 2). Figure 12A shows the 

basin model obtained using a density-depth function based on 

the Basin and Range density-depth function of Jachens and 

Moring (1990); figure 12B is the basin model obtained using 

a density-depth function based on borehole measurements in 

Arizona (Tucci and others, 1982). Both models use the same 

bedrock gravity values and well control, and thus the shapes 

of the basins are similar, differing only in the predicted basin 

thickness. The inversion using the Basin and Range densitydepth function is preferred because of the lack of geologic 

evidence around the margins of the basins for the greater 

basin-fill thickness predicted by the Tucci inversion. In addition, the Tucci inversion consistently overestimated basin-fill 

thickness at wells that did penetrate basement in Big Chino 

and Williamson Valleys (Langenheim and others, 2005). 

Figure 13 shows the bedrock gravity from the inversion using 

the Basin and Range density-depth function.

The gravity anomaly values used in the inversion are relative to the reduction density of 2.67 g/cm3

. If the topography 

consists of rocks that differ from the reduction density, errors 

in the basin-fill thickness can be introduced. If we recalculate basin-fill thickness using gravity data reduced to a higher 

reduction density (2.76 g/cm3, the average Proterozoic density), the error from this source of uncertainty is less than 100 

m, except for the Verde Valley, where the error is as high as 

150 m. 

The inclusion of wells that intersected bedrock greatly 

improves the fit of the predicted basin-fill thickness to the 

actual thickness found at these wells (fig. 12C). The basin 

thickness predicted by the inversion, where constrained by 

well data, is within 50 m of the real thickness at these points 

(average is 1 m), thus providing a sense of the maximum 

error where the model is well constrained. Without wells 

used as constraints, the average misfit is 16 m, with the total 

basin thickness at one well as much as 1.1 km less than that 

predicted by the inversion. This well is in the middle of Big 

Chino Valley and illustrates the importance of having independent control near the center of the basin. Estimation of the 

absolute magnitude of basin thickness in areally large basins 

is hampered because most of the wells that reach bedrock are 

fairly close to bedrock outcrops. No deep wells in the center 

of Verde Valley penetrate bedrock.

Another way to validate the models is to compare the 

predicted basin thickness with the minimum thickness of 

Cenozoic deposits found in wells that did not bottom in preCenozoic rock. In most of the study area, the basin thickness 

Depth to Basement

Depth Density-depth function

Range Based on Arizona borehole Based on Basin & Range

 (m) gravity (Tucci and others, 1982) (Jachens and Moring, 1990)

0-100 -0.67 -0.65

100-200 -0.47 -0.65

200-600 -0.37 -0.55

600-1200 -0.25 -0.35

> 1200 -0.25 -0.25

Table 2. Density-depth functions for basin-filling deposits

[Numbers represent density contrast, in g/cm3, relative to underlying pre-Cenozoic bedrock]
16 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

BR

112 ̊45 ́ 112 ̊30 ́ 112 ̊15 ́ 112 ̊ 111 ̊45 ́

34 ̊30 ́

34 ̊45 ́

35 ̊

 0

 100

 200

 300

 400

 500

 600

 700

 800

 900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

THICKNESS OF BASIN FILL, IN METERS

WV-3

CV-1

A

0 10 20 KILOMETERS

BR

112 ̊45 ́ 112 ̊30 ́ 112 ̊15 ́ 112 ̊ 111 ̊45 ́

34 ̊30 ́

34 ̊45 ́

35 ̊

B

C

WV-3

 0

 100

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 300

 400

 500

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1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

THICKNESS OF BASIN FILL, IN METERS

0 10 20 KILOMETERS

0

50

100

150

-60-30

0

3060

including wells that reach

bedrock as constraints

Basin thickness at well minus 

predicted thickness, in meters

N=342

ave=-1

min=-51

max=48

std dev= 11

0

10

20

30

40

50

Frequency

-1200

-900

-600

-300

0

300

N=342

ave=16

min=-1186

max=287

std dev= 129

Frequency

not including wells that reach

bedrock as constraints

Figure 12. Basin-thickness models based on (A) Basin and Range density-depth function (Jachens and Moring, 1990) and (B) Tucci and others 

(1982) borehole gravity. Yellow circles are gravity stations measured on bedrock; magenta-rimmed white dots are wells that reach bedrock and 

were used in the gravity inversion; orange dots are additional inferred bedrock gravity information. Light gray areas are Paleozoic sedimentary 

rocks; dark gray are Proterozoic basement rocks. BR is bedrock ridge. C, Comparison of the predicted basin thickness with actual basin thicknesses encountered in wells for both an inversion not using the constraints of wells that reached bedrock (top) and for the inversion shown in A 

(bottom).
17

predicted by the models is generally greater than that found in 

these wells (fig. 14; purple areas are where model thickness 

is supported by these wells). Misfits at only a few wells are 

more than the maximum error (50 m) for the well-constrained 

inversion. Most of the large misfits (labeled “g” on fig. 14) 

coincide with steep gravity gradients. If the basin thickness 

is changing abruptly, the inversion may not be capable of 

resolving the actual thickness within the grid interval (300 

m); this is a likely explanation for the misfits in Big Chino 

and Verde Valleys. For Little Chino Valley, the largest misfit 

coincides not only with a steep gravity gradient, but also with 

a large positive gravity anomaly in the western part of the 

valley (figs. 4 and 14). Without well control to constrain the 

bedrock gravity, the modeling process will not show a basin in 

the vicinity of the gravity high. For Verde Valley, the absence 

of wells penetrating bedrock near the center of valley adds 

even more uncertainty to the basin depth results because of 

the uncertainty in the bedrock gravity field. Another source of 

uncertainty for Verde Valley is potentially thick volcanic flows 

in the downdropped block of the Verde Fault Zone, adjacent to 

the western margin of Verde Valley. For example, basalt of the 

Hickey Formation is as much as 400 m thick on the upthrown 

block in the Black Hills.

Depth to Basement Results

Big Chino Valley

The results presented here build upon, and extend farther 

north of, those of Langenheim and others (2005). The incorporation of the Chino Valley Ranch (CVR) wells as constraints 

for the inversion confirms that the pre-Cenozoic bedrock 

gravity beneath Big Chino Valley is relatively featureless (fig. 

13), as inferred by Langenheim and others (2005). The lack 

of significant variations in bedrock gravity suggests that there 

may be similar basement rock types beneath Big Chino Valley 

and Big Black Mesa. The aeromagnetic data suggest that 

the magnetic basement beneath Big Black Mesa does extend 

southwest of the Big Chino Fault (figs. 3 and 5), at least as far 

southeast as the scarp near the intersection of Big Chino Wash 

and Pine Creek (BCw-PC on fig. 2). Large changes in bedrock density are not anticipated beneath the eastern part of Big 

Chino Valley, suggesting that at least the shape of the basin 

inferred from the gravity inversion should not change significantly with additional data.

A narrow (3-4 km), deep basin underlies the northeastern part of Big Chino Valley and extends approximately 20 

Depth to Basement

112 ̊45 ́ 112 ̊30 ́ 112 ̊15 ́ 112 ̊ 111 ̊45 ́

34 ̊30 ́

34 ̊45 ́

35 ̊

CVR-1

CVR-2

CVR-3

0 10 20 KILOMETERS

-27

-24

-21

-18

-15

-12

 -9

 -6

 -3

 0

 3

 6

 9

 12

 15

 18

 21

 24

 27

 30

 BEDROCK GRAVITY ANOMALY, IN mGal

Figure 13. Shaded-relief bedrock gravity field for the study area. White diamonds with red rims are wells that reach 

bedrock. Yellow diamonds are gravity stations measured on bedrock. Large orange dots are gravity stations very 

close to bedrock or are points where we inferred the bedrock gravity value to constrain the inversion. Solid red 

lines, faults from Ed DeWitt and others (unpub. data). Brown dashed lines and red dashed lines, faults inferred from 

well data and gravity gradients, respectively.
18 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

km northwest of drillhole CV-1 (fig. 12). The northeastern 

margin of the basin is steep and well defined by the Big Chino 

Fault. The southwestern boundary of the basin is less well 

defined, partly because of the relative paucity of gravity measurements. The southwestern boundary appears to be less 

steep and is most likely formed by a number of faults, none 

mapped at the surface. One set of inferred faults parallels 

and is 3-4 km southwest of the Big Chino Fault (as suggested 

by Ostenaa and others, 1993). Another set of faults farther 

west strikes more northerly and parallels the gravity gradient 

that marks the western edge of the gravity low (fig. 4). The 

northern margin of the deep basin is at the intersection of these 

inferred northerly-striking western margin faults with the Big 

Chino Fault. South of the Big Chino Fault and aligned with a 

marked change in its strike is a bedrock ridge (BR on fig. 12) 

that separates the deep basin to the southeast from a shallower, 

wider basin to the northwest. The southern margin of the deep 

basin may also be fault-bounded by multiple concealed faults 

that parallel the scarp near Big Chino Wash (BCw-PC on fig. 

2). Decreasing basin thickness at the southeast margin of the 

basin corresponds to a decrease in displacement on the Big 

Chino Fault, which terminates in a series of horsetail splays 

north of Paulden (Krieger, 1965). 

The lateral extent of fine-grained sediments in Big Chino 

Valley, as delineated by water-well logs (Schwab, 1995) coincides well with the outline of the gravity low (Langenheim 

and others, 2005). The fine-grained carbonate sediments were 

apparently deposited in a lacustrine or playa environment, 

related to interior drainage in Big Chino Valley. This interior drainage was attributed to damming of the valley by 6-4 

Ma basalt flows east of Paulden (Ostenaa and others, 1993). 

However, the presence of fine-grained material throughout 

the depth interval in well CV-3 (fig. 1) suggests that deposition of this material predates eruption of the basalts. The 

interior drainage may have been facilitated by downwarping 

of the basin by the Big Chino Fault during the late Tertiary and 

Quaternary (Menges and Pearthree, 1983; Ostenaa and others, 

1993; DeWitt and others, 2005). The bedrock ridge along 

112 ̊45 ́ 112 ̊30 ́ 112 ̊15 ́ 112 ̊ 111 ̊45 ́

34 ̊30 ́

34 ̊45 ́

35 ̊

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THICKNESS OF BASIN FILL, IN METERS

g

g

g

g g

0 10 20 KILOMETERS

Figure 14. Minimum basin thicknesses from wells (red crosses) minus predicted basin thicknesses. Differences greater than 0 indicate that 

the predicted basin surface is too shallow. Lines of blue circles are gravity gradients; “g” denotes gravity gradient that may contribute to error 

in basin-thickness estimate. Brown lines are faults.
19

the northwestern margin of the deep basin may have isolated 

these deposits from those in the shallower, broader basin to the 

northwest.

Verde Valley

The basin defined by the gravity inversion for Verde 

Valley is probably not as well-defined as that for Big Chino 

Valley because of large variations in bedrock gravity observed 

around the edges of Verde Valley (fig. 13). These bedrock 

gravity variations must be extrapolated across an area of basin 

deposits as large as 25 by 50 km without any independent well 

control on the depth to bedrock. To help constrain the bedrock 

gravity, aeromagnetic data filtered to enhance sources within 

the bedrock were compared to the gravity field. A strong gravity gradient along the course of the Verde River correlates with 

the edge of a magnetic block (“a” on fig. 5C), suggesting that 

dense, magnetic basement is present beneath much of the eastern part of Verde Valley. Accordingly, several inferred bedrock gravity values in this area were added to the inversion.

The low bedrock gravity values west of the Verde 

Fault Zone at the latitude of Camp Verde are the result of 

low gravity values measured on the Cherry Tonalite (of Ed 

DeWitt and others, unpub. data), part of the bedrock in the 

southern Black Hills (fig. 13). These values are as much as 

16 mGal lower than bedrock values measured on Proterozoic 

metavolcanic and gabbroic rocks in the Black Hills. The 

average density of the Cherry Tonalite is 2.72 g/cm3, which 

provides a density contrast of 0.10 g/cm3 relative to the average density of Proterozoic metavolcanic rocks (table 1) and 

would suggest that the Cherry Tonalite is about 10 km thick. 

If the density of the Cherry Tonalite (of Ed DeWitt and 

others, unpub. data) is less, as suggested by numerous felsic 

dikes in this area, the tonalite would be thinner.

Another potential source for the low gravity values 

beneath Verde Valley south and east of Cherry is Cretaceous 

granodiorite, which has an average density of 2.67 g/cm3. 

Boulders of granodiorite within the sedimentary rocks at 

the base of the Hickey Formation exposed along the Verde 

Fault may be compatible with a buried stock (Ed DeWitt and 

others, unpub. data). The buried stock would extend southeastward toward I-17 and the area of the low bedrock gravity 

values (fig. 13). Despite the lack of direct well information 

to constrain the bedrock gravity, the combination of geologic 

inference and aeromagnetic data helps constrain some of the 

regional bedrock gravity variations beneath the valley.

The basin configuration beneath Verde Valley is complex 

(fig. 12). West of the Verde River is a northwest-trending 

alignment of subbasins, whose western margins are the Verde 

fault zone (fig. 12). East of the Verde River are relatively 

gentle undulations of the bedrock surface. A deep basin predicted by both inversions lies northwest of Page Springs and 

extends eastward beneath Hickey basalt. The inversion most 

likely overestimates the magnitude of the basin-fill thickness in that area because the maximum thickness of basal 

sedimentary rocks in the Hickey Formation is about 50 m 

(Ed DeWitt and others, unpub. data). An alternate explanation could be lower density Proterozoic rocks in the bedrock. 

With the data available, it is not possible to rule out either 

explanation.

The basins west of the Verde River are segmented by 

transverse structures. One transverse structure coincides with 

a 4-km left step in the trace of the Verde Fault Zone approximately 10 km northwest of Camp Verde (fig. 12). North of 

the step, the western margin of the basin deviates from the 

mapped trace of the Verde Fault Zone for 12 km, until it 

rejoins the mapped fault trace near Cottonwood. Another 

transverse structure is located near the southern outskirts of 

Camp Verde. Here the transverse structure coincides with a 

3-4-km right step in the eastern margin of the basins. The step 

is aligned with a more northerly strike in the Verde Fault Zone. 

The extreme northwestern and southeastern terminations of 

the basins west of the Verde River are both gradual, mimicking the myriad splays of the fault zone at the southeastern terminus of the basin and decreasing displacement of the Verde 

Fault Zone at the northwestern terminus.

Verde and Big Chino Valleys are both relatively large, 

elongate valleys underlain by locally deep basins formed by 

prominent late Tertiary normal faults. Both valleys contain 

sedimentary deposits that formed in a lacustrine or playa environment. Tertiary volcanic rocks presumably dammed both 

valleys at their southern margins, contributing to lacustrine 

environments. The basin configuration beneath these valleys 

contrasts with that beneath Little Chino, Lonesome, and Williamson Valleys and the upper part of Agua Fria Basin discussed below.

Little Chino and Lonesome Valleys, including 

upper Agua Fria Basin

Although the inversion results in these areas are hindered 

by large bedrock variations (in particular the large gravity low 

associated with the Dells Granite and Prescott Granodiorite), 

Little Chino and Lonesome Valleys have more wells that constrain the bedrock gravity (particularly in Lonesome Valley) 

than Verde Valley (fig. 13). The bedrock gravity (fig. 13) 

shows a strong north-south–trending gradient along the eastern 

margin of the valleys, consistent with the strong north-south 

magnetic grain shown in the aeromagnetic data filtered for 

deeper sources (fig. 5B, C).

Basin inversion results for the Little Chino and Lonesome 

Valleys show no major, northwest-striking, deep basins. This 

result is not surprising, given the absence of prominent young 

normal faults in the valleys and that known faults have less 

than 200 m of displacement (Ed DeWitt and others, unpub. 

data). The basin configuration beneath these valleys is irregular compared to that of Big Chino and Verde Valleys. In general, thicker basin fill of inferred late Miocene age is located 

along the northern margin of Little Chino Valley, and basin 

Depth to Basement
20 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

fill of middle to late Miocene age is present in the upper Agua 

Fria Basin. These pockets of thicker sediment are elongated 

in two major directions: north and northwest. Those basins 

elongated in a north direction may appear deeper according to 

the gravity inversion than their true depths because of northsouth bedrock density variations. Some of the north-trending 

basins may be real because some of the Tertiary faults also 

strike north. Northwest elongation of basins suggests some 

structural control on the basin configuration beneath these valleys, because the regional strike of major late Tertiary normal 

faults is northwest.

Williamson Valley

Williamson Valley contains few deep wells that provide 

control for the gravity inversion, in contrast to Lonesome 

Valley, its neighbor to the east. However, the combination 

of a gravity low and a magnetic low that interrupts the northstriking magnetic grain of this valley suggests that the inferred 

deep, northwest-striking basin in Williamson Valley is not an 

artifact of bedrock gravity variations. The deep basin interpretation is further supported by a well that did not penetrate 

bedrock at a total depth of 460 m (WV-1, fig. 1) in the center 

of the gravity low.

The presence of this basin is surprising, given the absence 

of any prominent late Tertiary normal faults in the area of Williamson Valley. It may have formed earlier, in middle to late 

Miocene time, than the fault-defined basins beneath Big Chino 

and Verde Valleys. The sediments found in both Williamson 

Valley drill holes include coarse-grained to conglomeratic 

clastic sediments deposited in an energetic environment. This 

suggests that the shallow bedrock ridge north of WV-3 (fig. 

12) marks the boundary between the lacustrine environment of 

Big Chino Valley and the coarser grained, alluvial and fluvial 

deposits of Williamson Valley.

Distribution of Tertiary Volcanic Rocks 

in the Subsurface

As described earlier, magnetic data are effective for 

mapping the distribution of buried Tertiary volcanic rocks 

because volcanic rock produces distinctive anomaly patterns. 

The distribution of buried lati-andesite plugs in Little Chino 

Valley was mapped by Langenheim and others (2005) and 

correlated with exposed Tertiary lati-andesite by the semicircular, intense magnetic lows they produce. Ground-based 

magnetic data over one of the inferred plugs near Del Rio 

Springs confirms the presence of a negative magnetic anomaly (University of Arizona Field Geophysics Class, 2003). 

Comparing the amplitudes of the anomaly measured at two 

different heights predicts a depth of 260 m (Peters, 1949) to 

the top of the plug. Coincident electromagnetic data indicate 

a resistive body, interpreted as an intrusive body or plug, at 

this depth.

Our study expands the mapping of buried plugs into Williamson Valley, Lonesome Valley, and upper Big Chino Valley. 

Semicircular magnetic lows in areas covered by young sedimentary deposits that are probably Tertiary lati-andesite plugs 

are annotated with “p” on plate 1. Some of the semicircular 

magnetic highs (“p+”) could also be caused by lati-andesite 

plugs that are normally polarized or could be caused by semicircular topographically high remnants of normally polarized 

basalt (“b?” on plate 1).

The subsurface distribution of Tertiary basalt was also 

mapped in southern Big Chino Valley (Langenheim and 

others, 2005). Exposed Tertiary basalt produces a complicated 

(“wormlike”) magnetic anomaly pattern. In the area east 

of Paulden and directly north of the upper part of the Verde 

River, exposed basalt produces high-frequency magnetic 

anomalies. These magnetic anomalies can be traced beneath 

sedimentary deposits as much as 10 km northwest of the basalt 

outcrops (plate 1). These flows, derived from eruptive centers 

on the Colorado Plateau, flowed over the Mogollon Rim and 

into Big Chino Valley and the present-day area of the upper 

Verde River east of Paulden from about 6 to 4 Ma (McKee and 

Anderson, 1971; Ed DeWitt and others, unpub. data). Local 

sources, such as the cinder cone northeast of Paulden (unit Tcy 

of Ed DeWitt and others, unpub. data) may also have contributed flows that fed into Big Chino Valley. The new aeromagnetic data suggest that some reversely magnetized basalt may 

have flowed from the northeast down paleocanyons in the area 

near Paulden and north of the upper part of the Verde River. A 

narrow (< 1 km wide), prominent magnetic low (“c1” on fig. 

7) that is parallel to and 5 km east of highway 89 can be traced 

onto the negative magnetic anomalies associated with exposed 

Tertiary basalt (plate 1). Southeast of this prominent, irregular 

magnetic low is an area with a smooth magnetic field that suggests thin Cenozoic sedimentary deposits lying directly on the 

Paleozoic sedimentary sequence.

The new aeromagnetic data also delineate the extent 

of buried basalt flows in the northwestern end of Big Chino 

Valley. The pronounced magnetic low (“c2” on fig. 7) coincides with a narrow outcrop of basalt flanked by exposures 

of weakly magnetic Cambrian Tapeats Sandstone in the 

northeast corner of Big Chino Valley (plate 1). The low can 

be traced across the Big Chino Fault, where the low changes 

strike to the south and heads into Big Chino Valley. Here the 

low bisects an east-striking magnetic high (figs. 3, 7) caused 

by concealed Proterozoic basement and is present only along 

the western edge of a bedrock ridge identified by the gravity 

inversion. The sinuous low, well expressed in the first vertical 

derivative map (fig. 6), can be traced nearly 10 km beneath the 

valley alluvial deposits. Water wells confirm the presence of 

at least two basalt flows at depths of 150-180 m (500-600 feet) 

near the magnetic low (Ostenaa and others, 1993; Arizona 

Department of Water Resources, 2003). 

In Williamson Valley a buried volcanic center (“vc” on 

fig. 7) that covers several square kilometers is delineated by 
21

very pronounced magnetic highs about 1 km south of drillhole WV-3. The steep gradients suggest that the source is 

not deeply buried. This volcanic center produces an anomaly 

pattern different from that of the lati-andesite plugs in that the 

anomaly is compound, lobate, and positive (plate 1) rather 

than singular, semicircular, and negative. A possible source 

for these magnetic anomalies was probably sampled by drillhole WV-3, which intersected 15 m (50 ft) of basalt at a depth 

of 216 m (710 ft). A likely source is therefore either Hickey 

Basalt or 6-4 Ma basalt, although the complex, lobate anomaly 

pattern does not rule out the presence of lati-andesite. 

This inferred volcanic center is one of many buried volcanic centers in northern Williamson Valley, and there are several inferred buried lati-andesite plugs that extend west across 

Williamson Valley from Sullivan Buttes. Similar positive 

anomalies are located over sedimentary deposits in Lonesome 

Valley east of Highway 89. These anomalies bear some semblance to the positive anomalies over Glassford Hill (GH on 

fig. 1), which is composed of Hickey Formation basalt. The 

anomalies buried beneath the alluvial deposits of Lonesome 

Valley appear to be elongated in a northwest direction, perhaps 

reflecting feeder dikes to the volcanic centers.

Mapping the subsurface distribution of Tertiary volcanic 

rocks in Verde Valley is hampered by the necessarily increased 

height of the magnetic sensor above the ground surface. The 

intense magnetic patterns associated with exposed volcanic 

rocks of the Hickey Formation at House Mountain, however, 

can be traced as far west as Cornville. In the northern part of 

Verde Valley, about 10 km northwest of Cottonwood, subtle 

magnetic highs coincide with outcrops of 6-4 Ma basalt in the 

Verde River channel. These very subtle magnetic features can 

be traced another 5-10 km to the southwest and may reflect 

the subsurface distribution of Tertiary basalt in the Verde Formation. Alternatively, given the north-northeast strike of the 

anomalies, the source could reside in the Precambrian basement rocks. Helicopter-mounted or ground-based magnetic 

data in this area would help resolve the depth (and thus the 

age) of the source.

Langenheim and others (2005) argued that some of the 

volcanic plugs are likely resistant to ground-water flow, on 

the basis of the relatively unfractured and impermeable nature 

of intrusive centers compared to the fractured and permeable 

nature of volcanic flows and aprons of ejected material. A 

concentration of these plugs is downgradient from Del Rio 

Springs in Little Chino Valley. A similar explanation may 

apply to the spring southeast of WV-3 in Williamson Wash, 

which is upgradient from the concentration of buried volcanic 

centers in northern Williamson Valley.

Faults and Fractures

Abrupt, linear changes in magnetization and density are 

often the result of faulting or fracturing. Because of this, 

linear magnetic and gravity boundaries can be used to estimate the distribution of faults and/or large fracture systems. 

One of the most prominent magnetic and density boundaries in the region of this study coincides with one of the most 

prominent faults in the area, the Big Chino Fault. The nearcolocation of the mapped surface trace of the fault with the 

magnetic and density boundaries indicates that the Big Chino 

Fault is steeply dipping. Other prominent magnetic and density boundaries coincide, in part, with mapped faults, which 

locally project into areas that are covered by surficial deposits. 

Because of this relation, the geophysical boundaries can be 

used to map extensions of the faults, such as the Verde Fault 

Zone northwest of Interstate 17, where it is beneath young 

sedimentary deposits. Other gravity and magnetic lineaments 

do not coincide with mapped faults, but can be used to infer 

the locations of concealed faults, such as the basin-bounding 

faults beneath Williamson Valley and faults forming the eastern margin of the basins along the Verde Fault Zone.

Below we discuss magnetic and gravity lineaments that 

may mark the locations of potential faults and fractures that 

influence groundwater pathways in the area. Northwest-striking anomalies west and northwest of exposed 6-4 Ma basalt 

in the Paulden area are most likely caused by faulting-related 

relief on the upper surface of the buried basalt or by relief on 

the bottom surface in fault-controlled paleochannels. Mapped 

faults in Little Chino Valley cut across many magnetic boundaries, suggesting that their displacements are small. We 

include the interpreted lineaments from an earlier study (Langenheim and others, 2005) and expand the area of analysis to 

include the new aeromagnetic survey. Many of the lineaments 

are produced by physical property variations in the Proterozoic basement that parallel faults and folds mapped in outcrop. 

The structural grain may serve as a proxy for fracturing, an 

important source of permeability in these generally impermeable basement rocks. In some cases, the Precambrian structural grain may have influenced subsequent faulting.

The Bear Wallow Canyon Fault, an east-west striking 

fault that bisects Sedona, offsets Permian sedimentary rocks. 

The maximum displacement is 52 m (170 ft) with offset down 

to the south (Levings, 1980, p. 7). A change in magnetic signature coincides with the fault, suggesting that Precambrian 

basement rocks are a controlling factor. The magnetic data can 

be used to extend the Bear Wallow Canyon Fault west of its 

mapped extent into the northern part of Verde Valley, where 

it curves to the southwest. This fault appears to disrupt the 

regional ground-water flow (Owen Joyce and Bell, 1983, p. 

18). The Cathedral Rock Fault, a northwest-striking fault that 

merges into the Bear Wallow Canyon Fault, coincides with the 

edge of a magnetic block (fig. 5B, C), where it offsets weakly 

magnetic sedimentary rocks as much as 244 m (800 ft, Twenter and Metzger, 1963).

South of the Bear Wallow Canyon Fault are pronounced 

northwest-striking magnetic gradients (figs. 3 and 7). One 

of these gradients coincides with the trace of the Sheepshead 

Fault (Ed DeWitt and others, unpub. data). The Sheepshead 

Fault served as a growth fault during deposition of the Hickey 

Formation and older sediment of the Verde Formation, but 

not for the beds at the ground surface. The aeromagnetic data 

Faults and Fractures
22 Geophysical Framework Based on Analysis of Aeromagnetic and Gravity Data

can be used to project the concealed trace of this normal fault 

beneath the Verde Formation 5 km southward of its mapped 

location (plate 1). Parallel to and northeast of the Sheepshead 

Fault is another magnetic lineament. Part of this lineament 

coincides with a normal fault mapped through Page Springs 

(Ed DeWitt and others, unpub. data), suggesting that structure 

plays a role in the location of this large spring. The magnetic 

data can be used to project this structure another 5-10 km 

northwest and southeast of its mapped trace.

Prominent in the eastern part of Verde Valley are northeast-striking magnetic anomalies (fig. 3). Some of these 

anomalies have gentle to moderate gradients that indicate 

sources within the Proterozoic basement. One such anomaly 

is in the northern part of the Valley near Sedona (figs. 3 and 

5C). Three narrow anomalies with steep gradients occur 

between Dry Beaver Creek and Lake Montezuma (fig. 7). The 

width and steep gradients of these anomalies suggest that the 

sources are either exposed or only shallowly buried. Modeling of the southernmost anomaly near Montezuma Well 

indicates that a source at 300 m depth (approximately the top 

of Proterozoic basement) would have to be no more than 1 

meter wide and have unreasonably high magnetizations (1 

emu/cc) to reproduce the width and amplitude of the observed 

anomaly. The southernmost anomaly projects northeast 

towards outcrops of Paleozoic sedimentary rocks, but does not 

continue onto these outcrops. It corresponds with outcrops of 

the 6-4 Ma “ramp” basalts that are considered to be the likely 

source. The linear nature of the anomalies suggests either 

fault-controlled channels or dikes. Both of these geologic 

scenarios have drawbacks. The 6-4 Ma basalts do not appear 

to fill paleochannels into the Verde Formation, and dikes are 

not likely because virtually all of the dikes feeding Tertiary 

basalt flows in this area strike northwest, perpendicular to the 

direction of Basin and Range extension. Tertiary dikes, however, may have been influenced by the preexisting Proterozoic 

structural grain, as imaged by northeast-striking anomalies in 

the basement gravity and filtered magnetic data (figs. 5B, C, 

and 13).

North of Clarkdale a magnetic grain striking northnortheast to north-northwest is pervasive (figs. 3 and 7). The 

anomalies that cause this grain coincide in part with mapped 

faults that offset the Paleozoic sedimentary sequence. From 

depth estimates based on the anomaly gradients, we infer that 

these anomalies are caused by magnetization variations in the 

Proterozoic basement. Superposed on these anomalies are 

very subtle features that appear on the residual and vertical 

derivative maps (figs. 6 and 7). The gradients and amplitudes 

of these subtle anomalies suggest that weakly magnetic Paleozoic rocks are the cause.

Despite difficulties in attributing the source of magnetic 

and gravity lineaments to rock type and age, these data are 

effective in mapping structure concealed beneath sedimentary 

cover. Inferred major lineaments are shown on plate 1. The 

relationship of ground-water flow and these lineaments, if 

any, may be determined by acquiring additional data, such as 

hydrologic data from existing and new wells.

Conclusions and Recommendations

The aeromagnetic and gravity data provide new insights on 

the distribution of and structures associated with Tertiary volcanic rock and Proterozoic basement buried beneath younger 

sedimentary rocks and sediment in the upper and middle Verde 

River watershed. Some interpreted geologic structures may act 

as potential pathways or barriers for ground-water movement. 

Of particular interest are the shallowly concealed lati-andesite 

plugs in Williamson, southern Big Chino, and northern Little 

Chino Valleys, manifested in the aeromagnetic data as semicircular magnetic lows. The magnetic data and limited well 

and resistivity data indicate that these plugs lie as much as 

200-300 m beneath valley fill. The plugs may be resistant to 

ground-water flow, on the basis of the the relatively unfractured 

and impermeable nature of intrusive centers compared to the 

fractured and permeable nature of volcanic flows and aprons of 

ejected material. Additional volcanic centers of Hickey Formation age in Williamson and Lonesome Valleys may also influence the movement of ground water.

The gravity data provide information on the thickness 

of basin fill in Big Chino, Verde, Lonesome, and Williamson 

Valleys. Abrupt changes in basin-fill thickness are used to 

locate faults that are buried beneath the surficial deposits in 

Verde and Williamson Valleys. Additional constraints, such as 

wells that penetrate the entire basin sequence in both valleys 

and more detailed gravity, electric, or seismic surveys, would 

improve the basin-modeling efforts and reduce uncertainty in 

the interpretations presented here. Such wells could also test 

whether the structures identified here influence ground-water 

movement. For example, do the northwest-striking fractures 

inferred from geologic and geophysical data tend to be more 

open to fluid flow, as proposed for similarly oriented fractures 

throughout the Colorado Plateau (Thorstenson and Beard, 

1998)?

Acknowledgments

We would like to thank Yavapai County for financial support. We appreciate the helpful comments of reviewers Mark 

Gettings and Edwin McKee.

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