U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1761

Geophysical Interpretations of the Southern Española Basin, 

New Mexico, That Contribute to Understanding Its

Hydrogeologic Framework 

Prepared in cooperation with the New Mexico Office of the State Engineer

Geophysical Interpretations of the 

Southern Española Basin, New Mexico, 

That Contribute to Understanding Its 

Hydrogeologic Framework

By V.J.S. Grauch, Jeffrey D. Phillips, Daniel J. Koning, Peggy S. Johnson,

and Viki Bankey

Prepared in cooperation with the New Mexico Office of the State Engineer

Professional Paper 1761

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Grauch, V.J.S., Phillips, J.D., Koning, D.J., Johnson, P.S., and Bankey, Viki, 2009, Geophysical interpretations of the 

southern Española Basin, New Mexico, that contribute to understanding its hydrogeologic framework: 

U.S. Geological Survey Professional Paper 1761, 88 p.
iii

Contents

Executive Summary.......................................................................................................................................1

Introduction.....................................................................................................................................................3

Objectives, Approach, and Hydrogeologic Significance ........................................................................3

Elements of the Basin Floor ................................................................................................................5

Elements of Basin Fill ...........................................................................................................................9

Structure.................................................................................................................................................9

Geologic Setting.............................................................................................................................................9

Data Types and Descriptions .....................................................................................................................12

Physical Properties.............................................................................................................................12

Aeromagnetic Data.............................................................................................................................13

Gravity Data .........................................................................................................................................18

Other Data............................................................................................................................................18

Methods of Data Analysis ..........................................................................................................................20

Edge Detection....................................................................................................................................20

Magnetic Anomaly Separation ........................................................................................................21

Magnetic Depth Estimation ..............................................................................................................21

Model Construction.....................................................................................................................................24

Gravity Inversion.................................................................................................................................24

Profile Models .....................................................................................................................................27

Integrated Three-Dimensional Model.............................................................................................31

Interpreted Elements of the Basin Floor..................................................................................................35

Precambrian Basement.....................................................................................................................35

Pre-Rift Sedimentary Section ...........................................................................................................41

Intrusions..............................................................................................................................................44

Older Volcanic Rocks .........................................................................................................................50

Interpreted Elements of Basin Fill.............................................................................................................52

Santa Fe Group Sediments................................................................................................................52

Younger Volcanic Rocks ....................................................................................................................57

Interpreted Structure ..................................................................................................................................57

Summary........................................................................................................................................................65

Description of Digital Products .................................................................................................................66

Acknowledgments.......................................................................................................................................66

References Cited..........................................................................................................................................67

Appendix 1. Well Data Used to Constrain Three-Dimensional Integrated Model and 

Gravity Inversion.............................................................................................................................77

Appendix 2. Well Data Used to Guide Three-Dimensional Integrated Model................................82

Appendix 3. Magnetic Depth Estimation Methods Used in This Study...........................................84

Appendix 4. Other Point-Located Data .................................................................................................86
iv

Plates

1. Composite of Selected Geophysical Interpretations at 1:100,000 Scale (in pocket and 

on CD)

2. Geologic Units Inferred to Directly Underlie the Santa Fe Group at 1:100,000 Scale

(on CD)

Figures

1–3. Maps showing:

 1. Española Basin and areas used for various analyses ....................................................4

 2. Key basin elements addressed by geophysical interpretation......................................6

 3. Generalized geology and geography of the study area ................................................11

4. Diagrams showing magnetic property information for geologic units in study area......14

5–9. Maps showing:

 5. Coverage of high-resolution aeromagnetic surveys .....................................................17

 6. Isostatic residual gravity and station locations for area of gravity analyses ...........19

 7. First vertical derivative of reduced-to-pole aeromagnetic data .................................22

 8. Broad magnetic anomalies enhanced by lowpass filters ............................................23

 9. Data coverage and types of independent constraining data used for gravity

inversion................................................................................................................................26

10. Diagram showing simplified density-depth functions developed from borehole

density logs in the Albuquerque and Española Basins ........................................................27

11–13. Maps showing:

 11. Elevation of base of basin fill derived from gravity inversion ......................................28

 12. Bedrock gravity map derived from gravity inversion ....................................................29

 13. Section lines and data used to construct three-dimensional integrated model ......30

14. Two sections demonstrating three-dimensional model and forms of input .....................34

15. Aeromagnetic map showing labels on interpreted sources of selected features ..........36

16. Isostatic residual gravity map for study area ........................................................................37

17. Two-dimensional gravity and magnetic model for Tesuque profile ...................................38

18. Geologic map and aeromagnetic image of southeastern part of study area...................39

19. Geologic map and aeromagnetic image of northeastern part of study area ...................40

20. Two-dimensional gravity and magnetic model for Pojoaque profile .................................42

21. Sections through three-dimensional model showing general relations between

three modeled surfaces.............................................................................................................43

22. Two-dimensional gravity and magnetic model for Alamo Creek profile ...........................45

23–29. Maps showing:

 23. Aeromagnetic image, interpretations, and geology of southern part of

study area .............................................................................................................................46

 24. First vertical derivative image, interpretations, and thickness of

Santa Fe Group in southern part of study area ..............................................................48
v

 25. Elevation of base of Santa Fe Group, derived from three-dimensional model..........54

 26. Isopach thickness of Santa Fe Group, derived from three-dimensional model........55

 27. Comparison of bases of Ancha and underlying Tesuque Formations in southern

part of study area ................................................................................................................56

 28. Demonstration of horizontal-gradient magnitude method used to interpret

 faults from magnetic data .................................................................................................59

 29. Geologically mapped and geophysically interpreted faults in study area.................60

30. Two-dimensional gravity and magnetic model for Seton Village profile...........................61

31. Map showing geologically mapped and geophysically interpreted faults on base of 

Santa Fe Group surface.............................................................................................................63

32. Perspective view of basin model in relation to topography ................................................64

Tables

1. Interpretation approach and constraints for key basin elements........................................7

2. Densities of geologic units........................................................................................................15

3. Bodies used in two-dimensional geophysical profile models.............................................32

4. Digital georegistered files .........................................................................................................67
vi

Abbreviations Used in This Report

A/m amperes per meter

cgs centimeter, gram, second

ft foot, feet

ft/mi feet per mile

ft/sec feet per second

kg/m3 kilograms per cubic meter

km kilometer, kilometers

lb/ft3 pounds per cubic foot

m meter, meters

m/km meters per kilometer

m/s meters per second

mi mile, miles

nT nanotesla

2D two dimensional

3D three dimensional

CD compact disk

HGM horizontal-gradient magnitude 

MT magnetotelluric

N structural index

NMBGMR New Mexico Bureau of Geology and Mineral Resources

ohm-m ohm-meter

SAGE Summer of Applied Geophysical Experience 

SI Système International

Vertical coordinate information is referenced to the North American Vertical Datum of 1929 

(NAVD27).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD27).

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

Conversion Factors

Multiply By To obtain

kg/m3 0.001 g/cm3

km 0.621 mi

magnetic susceptibility (SI) 1/(4π) = 0.079577 magnetic susceptibility (cgs)

m/s 3.2808 ft/sec
Executive Summary

The southern Española Basin consists of a

westward- and northward-thickening wedge of rift fill, 

composed primarily of Santa Fe Group sediments, that 

serves as the principal aquifer for the city of Santa Fe 

and surrounding areas. Detailed aeromagnetic surveys 

were flown to better understand ground-water resources 

in this aquifer. This report presents a synthesis of interpretations derived from the aeromagnetic data combined 

with gravity data and other constraints. The interpretations were accomplished using qualitative interpretation, state-of-art data analysis techniques, and two- and 

three-dimensional modeling. The results depict the presence of and depth to many geologic features that have 

hydrogeologic significance, such as shallow faults, different types of igneous units, and basement rocks. The 

results are presented as map interpretations, geophysical 

profile models, and a digital surface that represents the 

base of Santa Fe Group sediments. The following are 

key points derived from our geophysical interpretations 

that contribute to an improved understanding of southern 

Española Basin’s hydrogeologic framework.

• The Santa Fe Group aquifer thickens 

substantially northward across a curvilinear 

structural feature located south of Santa Fe, 

called the Rancho Viejo hinge zone. South 

of the hinge zone, Santa Fe Group sediments 

overlie the Santa Fe platform and average 

about 75 meters (250 feet) thick. The platform 

is composed of generally low permeability 

volcaniclastic and volcanic rocks of the Espinaso 

Formation and Cieneguilla volcanic complex. 

North of the hinge zone, Santa Fe Group 

sediments thicken markedly by about 130–

150 meters per kilometer (700–800 feet per mile) 

and ultimately reach thicknesses greater than 

3 kilometers (10,000 feet) under the Pajarito 

Plateau west of Española.

• The Santa Fe platform has local irregularities that 

produce variations in sediment thickness at the 

base of the Santa Fe Group. These variations are 

not apparent at the surface. A large, northwestelongated paleovalley containing as much as 

90–150 meters (300–500 feet) of Santa Fe Group 

sediments is interpreted to be buried in the 

vicinity of Eldorado at Santa Fe.

• As indicated by the aeromagnetic analysis, in 

three large areas the low permeability bedrock 

of the Santa Fe platform is completely missing. 

In these areas, there may be a direct hydrologic 

connection between the Santa Fe Group and 

older, pre-Santa Fe Group sedimentary aquifers.

• Older, pre-Santa Fe Group sedimentary rocks 

are thickest under the Santa Fe Platform and 

decrease to a thin veneer of volcanic rocks and 

limestone overlying crystalline basement north 

of the Rancho Viejo hinge zone. Thus, deep 

sandstone aquifers are possible under the platform but are unlikely below the Santa Fe Group 

in the deeper parts of the basin.

• North of Santa Fe, geophysical modeling supports geologic information showing that the 

basin floor dips generally westward; dips are 

moderate (7°–20°) at the mountain front and 

again about 10–12 km (6.3–7.5 miles) west of 

the front and are shallow (~5°) in between. The 

Santa Fe Group aquifer thickens westward in 

concert with these dips.

• A northerly regional fabric in the basement 

rocks of the mountain front, expressed in the 

aeromagnetic data, may influence groundwater 

flow within the mountain block. This fabric does 

not extend more than 1–5 kilometers (0.6–

3.1 miles) west of the mountain front. Thus, 

the basement in the subsurface is expected to 

Geophysical Interpretations of the Southern Española 

Basin, New Mexico, That Contribute to Understanding

Its Hydrogeologic Framework 

By V.J.S. Grauch, Jeffrey D. Phillips, Daniel J. Koning,1 Peggy S. Johnson,1 and Viki Bankey

1

 New Mexico Bureau of Geology and Mineral Resources
2 Geophysical Interpretations of the Southern Española Basin, New Mexico

have much different lithologic characteristics 

and fracturing than is observed in the exposed 

basement rocks. 

• Aeromagnetic interpretations suggest that 

numerous northerly striking faults offset the 

basin fill, more than can be recognized at 

the surface. Moreover, the magnitudes of the 

observed aeromagnetic anomalies indicate 

the faults likely juxtapose at least 30 meters 

(~100 feet) of sediments that have contrasting 

bulk magnetic properties. These inferences and 

measured magnetic susceptibilities suggest that 

(1) significant thicknesses of sediments with 

differing grain size or volcanic-clast content are 

juxtaposed at these faults; (2) the juxtapositions 

may produce permeability contrasts that 

compartmentalize Santa Fe Group aquifers at 

depth; and (3) the potential for these permeability 

contrasts is greater than previously recognized. 

Conversely, the absence of aeromagnetic 

expression associated with mapped faults that 

offset magnetic sediments south of Chimayó 

suggests that the sediment type, when considered 

in bulk, does not differ appreciably across these 

faults.

• Many of the numerous mapped and geophysically inferred faults can be organized 

into two main fault systems (Agua Fria and 

Barrancos), which traverse north to northnorthwest through the central part of the study 

area near the communities of Eldorado at Santa 

Fe, Santa Fe, Pojoaque, and Española. The faults 

in these systems typically involve basement 

rocks. The faults may serve as partial barriers, 

compartmentalize aquifers, or possibly control 

upwelling of deep ground water from fractured 

basement rocks. The Agua Fria fault system 

includes the San Isidro Crossing fault (exposed 

near the road of the same name at the Santa Fe 

River), which coincides with a large drop in the 

water table. 

• A north-northeast-trending bedrock high, 

indicated by gravity and electrical geophysical 

models, underlies the Cerros del Rio volcanic 

field. Its steep eastern side may be represented by 

east-down faults or an east-dipping monocline. It 

may form a major obstacle to westerly groundwater flow.

• Basalts of the Cerros del Rio volcanic field, 

included in the uppermost sections of the basin 

fill, are generally limited in lateral extent to the 

area of exposure within the study area, on the 

basis of their aeromagnetic signatures. However, 

9 kilometers (5.5 miles) directly west of Santa Fe 

the basalts are interpreted to underlie sediments 

of the Ancha Formation at shallow depth. Thus, 

drilling adjacent to the volcanic field should not 

encounter particularly thick basalts except in this 

local area.

• Fracture-dominated granitic rocks in the Cerrillos 

Hills area form a massive body at depth, on the 

basis of a strong signature in the aeromagnetic 

data. The bulk of this intrusive complex has fairly 

steep sides, encompasses the area of the hills, and 

extends outside the hills, mostly to the southeast. 

The geophysical interpretations also offer insights 

into the geologic history of the southern Española Basin. 

The following are some key points.

• A sharp decrease in thickness of the pre-rift 

sedimentary section inferred from gravity 

analysis and seismic information follows a 

40-kilometer (25-mile)-long northwest trend in 

the southwestern Española Basin. This trend may 

mark the southwestern extent of the Laramide 

uplift previously hypothesized for the area to the 

northeast. 

• The Rancho Viejo hinge zone may represent a 

long-lived crustal weakness that underwent a 

reversal of motion from Laramide time (north 

side up) to Rio Grande rift time (north side 

down). As the north side went down, strata on the 

Santa Fe Platform (the south side) were generally 

eroded. The north-side-down tilting was completed before deposition of the Ancha Formation 

in Pliocene time.

• Near the Rancho Viejo hinge zone, threedimensional modeling suggests that the late 

Eocene–Oligocene Espinaso Formation thickened, and aeromagnetic data delineate associated 

volcanic vents. This coincidence suggests that 

magmatism was concentrated along the hinge 

zone at this time.

• A marked contrast in regional fabrics within the 

Precambrian basement, apparent in the aeromagnetic data, occurs at a semilinear north-south 

trend at the mountain front. Comparison with 

gravity data suggests the contrast is not associated with structural relief, so it likely developed 

before rifting began.

• The north-northeast-trending bedrock high under 

the Cerros del Rio volcanic field may have 

eroded less during Laramide time than adjacent 

rocks to the east. This interpretation suggests 

that the modern bedrock high was a structural 

low in Laramide time that was later inverted to a 

structural high in rift time.
Objectives, Approach, and Hydrogeologic Significance 3

Introduction

Basin-fill aquifers within the Española Basin and 

adjoining Santa Fe embayment (fig. 1) are the primary 

ground-water resource of the City of Santa Fe, Española, 

six Pueblo nations, and surrounding suburban areas. A 

better understanding of the subsurface geologic controls on 

ground-water flow and storage is needed to better define and 

manage this important ground-water resource. Geophysical 

investigations using aeromagnetic and gravity data are 

valuable for improving this understanding. Aeromagnetic 

data, acquired from aircraft, measure the subtle variations in 

the Earth’s magnetic field due to differences in the naturally 

occurring magnetic properties of the underlying rocks and 

sediments. Gravity data, typically acquired from the ground 

surface, measure small variations in the Earth’s gravity field 

due to differences in bulk densities of the underlying rocks 

and sediments. Although both aeromagnetic and gravity 

data are generally insensitive to water, differences in the 

magnetic properties and densities that they detect can be 

used to infer many aspects of ground-water-bearing rocks 

(the “hydrogeologic framework”). In the Española Basin, 

aeromagnetic and gravity data are used to interpret variations 

in thickness of basin-fill sediments (the Santa Fe Group 

aquifer), define major structures and composition of the basin 

floor, map the distribution of intrusions and volcanic rocks, 

and locate concealed intrabasin faults. 

The impetus for using aeromagnetic and gravity methods for ground-water studies in the southern Española Basin 

came from a multi-agency study of the Middle Rio Grande 

Basin during the late 1990s (Bartolino and Cole, 2002) that 

encompassed the Albuquerque and Santo Domingo Basins 

to the south. In particular, the aeromagnetic method (previously considered unconventional for ground-water studies) 

proved successful in the Middle Rio Grande Basin. It efficiently located subsurface hydrogeologic features, knowledge 

of which ultimately improved the ground-water-flow model 

for the basin. In anticipation of future ground-water studies 

to the north, the New Mexico Office of the State Engineer 

augmented funding to extend the high-resolution aeromagnetic 

coverage into the southern Española Basin in 1998 (U.S. Geological Survey and others, 1999). Subsequent interpretations 

of these data provided useful new information on the depth 

and lateral extent of Precambrian basement rocks, buried 

intrusive and volcanic rocks, and shallow faults (Grauch and 

Bankey, 2003; Phillips and Grauch, 2004). This knowledge 

improved the understanding of aquifer thickness and depths to 

potential aquifers, and it allowed better prediction of groundwater storage, flow, and quality and better informed choices 

of drilling sites. The aeromagnetic interpretations benefitted 

ground-water studies by providing new information used in 

(1) follow-up drilling and studies by the New Mexico Office 

of the State Engineer, (2) development of water-use zoning 

maps for the County of Santa Fe, (3) construction of regional 

ground-water flow models for the City and County of Santa 

Fe, and (4) the work of hydrology consultant firms in support 

of existing and new subdivisions. Because of these benefits, an 

additional helicopter survey was flown in 2005 to expand the 

coverage of high-resolution aeromagnetic data for the front of 

the Santa Fe Mountains (Bankey and others, 2006). Funding 

was provided by the U.S. Geological Survey, the New Mexico 

Office of the State Engineer, and Santa Fe County.

In addition to the new aeromagnetic coverage, recent new 

work by others in the basin has provided motivation to update 

and expand the scope of the earlier aeromagnetic interpretations of Grauch and Bankey (2003) and Phillips and Grauch 

(2004). A major objective has been construction of a digital 

surface representing the base of Santa Fe Group sediments—

the base of the basin aquifer system. For this effort, gravity 

modeling, constrained by lithologic logs from wells and independent geophysical data, has been invaluable for resolving 

ambiguities in magnetic depth estimates encountered in the 

earlier work, and it provides most of the information for the 

deeper parts of the basin. 

The organization of this report progresses from introductory material to data descriptions, methods, model construction, and interpreted features arranged by geologic age, and 

then to a summary. However, the topics are interrelated and 

they overlap owing to the iterative nature of the geophysical 

interpretation. For example, as one element in the basin is 

understood from one geophysical approach, a new understanding of other elements develops or a new approach becomes 

apparent. Moreover, one commonly must understand the geophysical expression of the subsurface geology below an aquifer of interest before the overlying units can be interpreted. 

Therefore, final interpretations and all approaches that led to 

the conclusions are difficult to discuss in a linear fashion, and 

the geologic units of the least interest hydrogeologically must 

be discussed first. For this reason, the next section—the geophysical objectives and approach and the relevance of these 

objectives to the hydrogeologic framework—is provided to 

help place the following sections in context. 

Objectives, Approach, and 

Hydrogeologic Significance 

The objectives of the geophysical interpretations are to 

characterize and image basin elements that are important for 

understanding the hydrogeologic framework, especially where 

those elements are concealed below the ground. The key elements amenable to geophysical interpretation in the southern 

Española Basin can be divided into geologic units composing basin fill, geologic units composing the basin floor, and 

structure (fig. 2). For this study, geologic units are grouped 

into interpretative packages on the basis of similar geophysical expression and representation of key basin elements. 

Basin-fill units constitute the most important aquifers in the 

basin, whereas basin-floor units are generally less permeable 

and form the base of the basin aquifers. Overall basin shape 
4 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 1. Española Basin and areas used for various analyses. Processed Landsat satellite imagery is from 

Sawyer (2004). Overall topographic relief within the study area is 1,145 m (3,750 ft).

0 10 20 30 KILOMETERS

107°00' 105°30'

EXPLANATION

Limit of preserved Neogene deposits

Española Basin

Aeromagnetic data area

Study area

3D model area

Gravity inversion area

35°15'

36°30'

Taos

Santa

Fe

Bernalillo

Los

Alamos

Española

Abiquiu

SANTA FE MOUNTAINS

Santa Fe

Embayment

Valles/Toledo

Calderas

SANTO 

DOMINGO 

BASIN

SAN

LUIS

BASIN

ESPAÑOLA

BASIN

ALBUQUERQUE 

BASIN

Cerros

del Rio

volcanic

field

Cerrillos

Hills

Black Mesa

Ortiz

Mountains 

SANGRE DE CRISTO MOUNTAINS

Jemez volcanic field

JEMEZ MOUNTAINS

NEW

MEXICO

COLORADO

TEXAS

MEXICO

RIO GRANDE

RIFT

Rio Grande

0 10 20 30 MILES
Objectives, Approach, and Hydrogeologic Significance 5

and irregularities on the top of the basin floor, resulting from 

faulting or the erosion of ancient landscapes, may bear on the 

hydrogeology of the overlying basin fill because they affect 

factors such as sediment type, aquifer thickness, and stratal 

dip. In addition, faults that cut basin fill can act as partial barriers to fluid flow and juxtapose units of differing permeabilities, which effectively compartmentalize the aquifers.

A different geophysical approach is required to characterize and image each basin element (table 1). The final interpretations take the form of lines on a map (map interpretations) 

and on digital surfaces derived from three-dimensional (3D) 

integrated modeling. Interim interpreted products are twodimensional (2D) geophysical profile models that were incorporated into the 3D model. Map interpretations, cross sections, 

and contour maps derived from the surfaces are shown for 

selected areas in the figures cited in subsequent sections. In 

plate 1 they are plotted together with contours from a digital 

surface for the whole study area. Digital surfaces related to the 

model surface of the Santa Fe Group, in georegistered format, 

can be downloaded from the compact disk (CD) in the pocket.

The following sections summarize the hydrogeologic 

importance of the key basin elements and what aspects of 

these elements are determined from the geophysical interpretations in this report. The basin floor is discussed first, because 

it must be understood before aspects of the overlying basin 

fill can be addressed. Mapped geologic units that are included 

in these categories are discussed in the section entitled 

Geologic Setting.

Elements of the Basin Floor

We generally categorize components of the basin floor as 

Precambrian basement, pre-rift sedimentary section, Tertiary 

intrusions, and Tertiary older volcanic rocks. Not all of these 

units are present throughout the study area. In particular, the 

pre-rift sedimentary section is thickest at the southern end of 

the basin, but it was variably removed before deposition of the 

Santa Fe Group and thins considerably to the north (fig. 2). 

Moreover, some of the Tertiary intrusions are confined to the 

vicinity of the Cerrillos Hills, and some are inferred to lie 

within the pile of older volcanic rocks. 

Interpreting the nature of and depth to the Precambrian 

basement is important for understanding the hydrogeologic 

framework for several reasons: (1) it closely underlies and 

thus helps define the base of the basin-fill aquifers throughout 

most of the area north of the Santa Fe River; (2) it is the main 

mountain-block aquifer, dominated by flow through fractures; 

(3) it directly underlies the limestone aquifers southeast of 

Santa Fe and thus helps define the base of this aquifer; and 

(4) its internal structure may control the lateral or vertical 

flow of ground water and influence recharge. Aeromagnetic 

data and filtered maps were used to qualitatively characterize overall differences in the lithology and structure of the 

Precambrian basement, especially at the mountain front. In 

some areas, strong aeromagnetic signatures of the Precambrian 

basement show elongate structural and lithologic trends that 

may help predict ground-water flow or patterns of recharge 

from the mountains. In other places, the Precambrian basement is effectively invisible in the aeromagnetic data because 

of its very low magnetic susceptibilities. Thus, gravity data 

were used to constrain the overall form of the basement 

surface, despite sparse data in the northern part of the area. 

Qualitative concepts of the shape and nature of the basement 

were developed using joint gravity and magnetic modeling in 

two dimensions and limited integrated modeling of subsurface 

information in three dimensions. 

The pre-rift sedimentary section contains Paleozoic 

through Eocene mostly clastic sedimentary units deposited in 

a variety of environments. At the top of the uneroded section is 

an Eocene sandstone-dominated sequence that may locally be 

important for water storage (Finch, 2008). At the base of the 

section, a sequence of Paleozoic limestones locally produces 

water. Below the Eocene sandstones, the low-permeability 

Mesozoic Mancos Shale is as much as 700 meters (m) thick 

in the area of the Cerrillos Hills (Sawyer and Minor, 2006a). 

Although none of the clastic units in the pre-rift section has 

overall permeabilities and yields as high as those of the Santa 

Fe Group sediments (Lewis and West, 1995), they may be 

important where the overlying Santa Fe Group is thin or where 

uplift has brought the limestones near the surface, such as near 

the mountain front southeast of Santa Fe. The thickness of 

the pre-rift section is interpreted by using limited seismic and 

well-log information, supplemented by indirect inferences that 

can be made from gravity data analysis. The pre-rift section is 

considered as one package, because gravity and aeromagnetic 

data cannot distinguish individual units. Inferences about the 

presence of individual units underlying the Santa Fe Group 

were made in a few local areas, where well data exist.

Tertiary intrusions are important in the hydrogeologic 

framework because they represent a fracture-dominated rather 

than a porous medium. Dikes and sills emanating from the 

bodies may locally divert or obstruct ground-water flow paths 

and may influence ground-water quality. Small intrusions are 

located under the Cerros del Rio volcanic field and within the 

sequence of Tertiary older volcanic rocks. A larger intrusive complex, which is associated with turquoise and metal 

deposits (Giles, 1995), is located in the vicinity of the Cerrillos Hills (fig. 2). Inspection and analysis of the aeromagnetic 

data provides the best definition of the overall lateral extents 

of these intrusions at depth. No attempt was made to estimate 

depths to the top of the Cerrillos intrusion from the geophysical data because (1) in many places the intrusive rocks cannot 

be distinguished from overlying, magnetic, Tertiary older 

volcanic rocks; (2) weakly magnetic pre-rift sedimentary rocks 

cannot be distinguished from overlying Santa Fe Group sediments; and (3) gravity data and subsurface information from 

other sources are not adequate. 

The Tertiary older volcanic rock package is an important element of the hydrogeologic framework because (1) the 

rocks directly underlie Santa Fe Group sediments in most of 

the southern part of the study area; (2) they generally form 
Figure 2. Key basin elements addressed by geophysical interpretations and their roles in the geologic and hydrogeologic frameworks.

Pajarito-Embudo fault system

0

5,000

10,000

ELEVATION,

IN FEET

–5,000

0 0 10 KILOMETER

N

BASIN FLOOR

Basin Element

BASIN FILL AND FAULTS

Hydrogeologic Importance

Normal faults—Ballon downthrown side

Santa Fe Group sediments

Tertiary younger volcanic rocks

Generally unsaturated in the Cerros del Rio volcanic field;  contain perched water under the Pajarito PlateauPrimary basin aquifers; fill thickens toward Parajito Plateau 

Potential partial barriers to flow; may compartmentalize basin  aquifers; in places form structural relief on basin floor that  produces variations in aquifer thickness that may redirect  groundwater flow 

EXPLANATION

Tertiary older volcanic rocksTertiary intrusive rocksPre-rift sedimentary rocks

Precambrian basement

Generally form a low-permeability base below  Santa Fe Group aquifers 

Form a fracture-dominated aquifer rather than a  porous one

Low- to moderate-permeability aquifers and aquitards;  Paleozoic limestones at base are locally productive 

Mountain front fracture-dominated aquifer; recharge  area in the Santa Fe Mountains; forms base of basin floor

PAJARITO PLATEAU

Cerros del Riovolcanic field Cerrillos

Hills Española

Santa Fe Santa Fe

SANTA FE MOUNTAINS 

VERTICAL EXAGGERATION = 3

La Bajada fault

Rio Grande

Santa Fe River

SOUTHWEST NORTHEAST

Barrancos fault system

Agua Fria fault system

Black Mesa
Objectives, Approach, and Hydrogeologic Significance 7

Table 1. Interpretation approach and constraints for key basin elements. —Continued

[Approaches and data constraints involved in geophysical interpretation of aspects of basin elements illustrated in figure 2. Methods described more fully in 

the sections entitled “Methods of Data Analysis” and “Model Construction.” 2D, two-dimensional; 3D, three-dimensional]

Basin element Interpreted aspect Interpretation approach and constraints

Elements of basin fill

Tertiary younger 

volcanic rocks

[units Qbt, QTv, fig. 3].

Lateral extent of Cerros 

del Rio volcanic field.

Interpret magnetic maps by recognizing anomaly patterns. Exact boundaries determined by 

the horizontal gradient magnitude of magnetic data.

Isolated volcanic vents Interpret magnetic maps by recognizing circular anomalies. Boundaries determined by 

the horizontal gradient magnitude of magnetic data. More detailed interpretations of 

geologic unit and age can be found in Thompson and others (2006) and Grauch and 

others (2006).

Region of thick volcanic 

rocks and numerous 

vents.

Interpretation derived from Grauch and Bankey (2003), who inspected magnetic anomaly 

patterns in comparison with mapped vents and used magnetic depth estimation techniques on profile data to indicate areas of thick magnetic sources. 

Areas of thin basalt Interpretation derived from Grauch and Bankey (2003), who used magnetic depth estimation techniques to indicate areas underlain by thin magnetic sources. 

Santa Fe Group 

sediments

[units QTsf, Tsf, fig. 3].

Elevation of base surface 3D integrated model developed from grid-based magnetic depth estimates, basin-fill 

thickness results from gravity inversion, well information, geologic cross sections, 

2D geophysical models, magnetotelluric pseudoboreholes derived from 2D resistivity 

models, and depth picks from seismic horizons. Modeled in conjunction with surfaces 

representing the tops of Tertiary older volcanic rocks, pre-rift sedimentary section, and 

Precambrian basement. 

Thickness Subtract interpreted Santa Fe Group base elevation surface from a topographic surface. 

Differences in magnetic

lithology.

Interpret maps by inspecting correlations between the shapes of magnetic anomalies with 

topography in areas where Santa Fe Group is mapped.

Elements of basin floor

Tertiary older 

volcanic rocks

[units Tomv, Te, fig. 3].

Elevation of top surface 3D integrated model developed from grid-based magnetic depth estimates, basin fill 

thickness results from gravity inversion, well information, geologic cross sections, and 

isolated depth picks from seismic horizons in deep parts of the basin. Modeled in conjunction with surfaces representing the base of Santa Fe Group sediments and the tops of 

pre-rift sedimentary section and Precambrian basement. This surface is included in the 

2D geophysical models, but it is not well constrained except near the mountain front. 

Elevation of base surface 3D integrated model using isolated depth picks from seismic horizons and limited well 

information. In some places, the surface was interpreted to be parallel to grid-based 

magnetic depth estimates that presumably show internal structure. 

Lateral extent underneath 

Santa Fe Group 

sediments.

Interpret maps by recognizing unique anomaly patterns. Incorporated into the 3D integrated 

model from map interpretations, well information, and grid-based magnetic depth estimates in relation to basement interpretations.

Thickness Subtract the interpreted base from the interpreted top surface of Tertiary older volcanic 

rocks.

Tertiary intrusions

[units Ti, fig. 3; inferred 

vents related to Te].

General lateral extent of

 Cerrillos intrusion.

Interpret magnetic anomalies in relation to mapped intrusive rocks. Boundaries mapped 

using the horizontal gradient magnitudes of magnetic and pseudogravity data.

Locations of buried volcanic

vents probably associated 

with Espinaso Formation.

Recognize and interpret circular negative magnetic anomalies on maps. Boundaries 

mapped using the horizontal gradient magnitude of magnetic data. Use magnetic depth 

estimates to interpret depths of burial and geologic affiliation, supported by magnetic 

property measurements.
8 Geophysical Interpretations of the Southern Española Basin, New Mexico

Table 1. Interpretation approach and constraints for key basin elements. —Continued

[Approaches and data constraints involved in geophysical interpretation of aspects of basin elements illustrated in figure 2. Methods described more fully in 

the sections entitled “Methods of Data Analysis” and “Model Construction.” 2D, two-dimensional; 3D, three-dimensional]

Basin element Interpreted aspect Interpretation approach and constraints

Elements of basin floor—Continued

Pre-rift sedimentary section

[units Tgd, cgu, 

hap, fig. 3].

General differences in total

thickness.

Interpret trends in the bedrock gravity map resulting from gravity inversion. Inspect 2D 

geophysical models constrained by gravity data, limited information from magnetotelluric pseudoboreholes derived from 2D resistivity models, and deep wells.

Buried bedrock high 3D integrated model developed from basin fill thickness results from gravity inversion, 

2D geophysical models, magnetotelluric pseudoboreholes derived from 2D resistivity 

models, and depth picks from seismic horizons. 

Differences in magnetic

lithology.

Inspect magnetic maps in relation to mapped contacts in areas where pre-rift sedimentary 

rocks are exposed.

Precambrian basement

[unit pe, fig. 3].

Elevation of basement 

surface.

2D geophysical models and profile-based magnetic depth estimates. 3D integrated model 

developed from 2D geophysical models, depth picks from seismic horizons and well 

information. Additional information near the mountain fronts provided by geologic cross 

sections, grid-based magnetic depth estimates, and basin fill thickness results from 

gravity inversion. Modeled in conjunction with surfaces representing the base of 

Santa Fe Group sediments and the tops of Tertiary older volcanic rocks and pre-rift 

sedimentary section. 

Regional magnetic 

character.

In mountain areas, inspect patterns in magnetic and first-vertical-derivative maps in relation to topography and mapped geology. In basin areas, inspect patterns in magnetic and 

lowpass-filtered maps in relation to gravity data.

Major intrabasement 

boundaries.

Inspection of magnetic maps in relation to gravity maps to determine magnetic anomaly 

differences that are unrelated to modern structural relief.

Structure and miscellaneous

Normal faults Fault traces based on 

magnetic expression.

Interpretations from Grauch and Hudson (2007), who used horizontal-gradient magnitudes 

of magnetic data after local trends were removed within a moving data window. As part 

of the procedure, linear trends solely related to topography are discarded. Locations cannot be mapped near Santa Fe, where there is too much interference from anthropogenic 

sources.

Fault traces based on 

gravity expression.

Map interpretation derived from the horizontal-gradient magnitude of gravity data. Poor 

data coverage precluded these interpretations in much of the study area.

Major fault systems Determined by inspection of geophysically interpreted and geologically mapped faults and 

other structures in comparison to seismic reflection records.

Basin geometry Structural relief on the 

basin floor.

Derived from relief on interpreted base of Santa Fe Group surface.

Miscellaneous Linear features of undetermined origin.Lines drawn along crests of linear magnetic anomalies on maps that may represent normal 

or strike-slip faults; linear contacts, such as the strike of dipping strata; other linear 

geologic features, such as narrow paleovalleys or narrow structural blocks; or regional 

metamorphic fabric of Precambrian basement.
Geologic Setting 9

a relatively impermeable base (Spiegel and Baldwin, 1963), 

although permeabilities may be moderate in local areas; 

(3) they contain information about faults that may influence 

the overlying Santa Fe Group sediments; (4) their absence 

indicates areas where the overlying Santa Fe Group and underlying pre-rift section have hydraulic connection; and (5) they 

likely contribute locally to poor ground-water quality. The 

Tertiary older volcanic rocks are one of the most important 

targets of the geophysical interpretations owing to their unique 

aeromagnetic signature and their position at the top of the 

basin floor. Using magnetic depth analysis and gravity modeling to locate the top of these rocks is equivalent to locating the 

depth to the base of the overlying Santa Fe Group; it places 

important constraints on maximum aquifer thickness. In addition, interpretations were derived from aeromagnetic maps to 

delineate the limits of these rocks in plan view and constrain 

their configuration in the 3D model. 

Elements of Basin Fill

We generally categorize geologic units that compose the 

basin fill as Santa Fe Group sediments and Tertiary younger 

volcanic rocks. Santa Fe Group sediments form the primary 

aquifers for domestic and agricultural water supplies in the 

southern Española Basin. The aeromagnetic and gravity data 

can provide information about the maximum thickness of 

this sediment and suggest where major lithologic contrasts 

occur. To determine the Santa Fe Group’s maximum thickness, 

information from analysis and modeling of the gravity and 

aeromagnetic data were integrated with other types of subsurface information and with geologic cross sections to develop 

a 3D-model surface representing the base of Santa Fe Group 

sediments. Documentation of variations in basin-fill thickness 

provides water-storage constraints for regional ground-water 

flow models and identifies potential structures at the base 

of the section. Major contrasts in lithology, as represented 

by different magnetic properties, suggest the locations of 

potential contrasts in aquifer properties that can influence 

ground-water flow. Because the geophysical data cannot 

readily distinguish between the formations or other lithostratigraphic divisions of the Santa Fe Group, the sediments are 

considered as a collective package. 

In the study area, Tertiary younger volcanic rocks 

generally lie at or near the top of basin fill and are much 

thinner relative to the bulk of the sediments that compose the 

Santa Fe Group. The volcanic rocks are generally unsaturated 

in the Cerros del Rio volcanic field (Sawyer and Minor, 

2006b), but under the Pajarito Plateau they form perched 

aquifers that provide horizontal flow paths (Robinson and 

others, 2005). The younger volcanic rocks have strong 

signatures in the aeromagnetic data, but the complexity 

of these signatures limits interpretations to qualitative 

determinations of thickness in the Cerros del Rio volcanic 

field and mapping their presence under shallow sediment 

cover. More extensive use of the aeromagnetic data to interpret 

volcanostratigraphy and map units in the Cerros del Rio 

volcanic field are presented in Grauch and others (2006) and 

Thompson and others (2006). 

Structure

Faults and basement structures are important elements 

of the hydrogeologic framework for several reasons. First, 

faults that offset basin aquifers may partition aquifers 

if the sediments on either side have different hydraulic 

characteristics. Second, ubiquitous, impermeable, clay-rich 

fault zones in the northern Albuquerque and Española Basins 

suggest that intrabasin faults in these basins commonly are 

partial barriers to flow (Minor and Hudson, 2006; Caine and 

others, 2008). Finally, basement structures may affect lateral 

or vertical flow at depth and can control connectivity between 

adjacent basins. Faults that cut the basin fill are expressed 

in the aeromagnetic data throughout much of the study area, 

but a more comprehensive view of their patterns and extent 

is gained by combining information from geologic mapping 

and limited seismic reflection data. Integrating these data 

sets allows identification of two major deformation zones 

that extend the length of the study area, which we call the 

Barrancos and the Agua Fria fault systems (fig. 2). The 

Agua Fria and Barrancos fault systems are the subjects of 

geophysical interpretation presented in this report. Two other 

previously known fault systems included in the 3D modeling 

commonly serve as convenient boundaries in regional groundwater models. The Pajarito-Embudo fault system forms the 

master system of normal faults that bounds the west-dipping 

half graben of the Española Basin. The La Bajada fault, which 

forms a topographic escarpment on the southwest side of the 

Cerros del Rio volcanic field, is commonly viewed as the 

boundary between the Española and Santo Domingo Basins 

(Minor, 2006). 

Geologic Setting

The Española Basin is one of a series of basins 

composing the Rio Grande rift, which formed during crustal 

extension that was active from late Oligocene to the present. 

It is located at a major right step in the Rio Grande rift and 

has the general form of a west-dipping half graben. It adjoins 

the east-dipping San Luis Basin on the northeast and the 

transitional Santo Domingo and east-dipping Albuquerque 

Basins to the southwest (fig. 1). The Rio Grande flows through 

these basins. The river and its tributaries drain the primary 

watershed of the area. The boundary of the Española Basin has 

been defined in a variety of ways in the literature (Baltz, 1978; 

Kelley, 1978; Manley, 1979; Wilkins, 1998); we use the limit 

of Neogene units because that limit encompasses a convenient 

area for gravity modeling. “Santa Fe embayment” is a name 

introduced informally by Kelley (1952, 1978) to indicate a 

tectonic element surrounded by the southward protrusion of 
10 Geophysical Interpretations of the Southern Española Basin, New Mexico

the Española Basin margin east of the Santo Domingo Basin 

(fig. 1). However, we use this term solely to indicate the 

general locality, following its common usage. 

This study focuses on the southern Española Basin and 

the Santa Fe embayment; it provides only limited evaluation 

of the area south of the basin margins. The southern Española 

Basin is bounded on the east by the Santa Fe Mountains and 

on the west by the Jemez Mountains (fig. 1). The Santa Fe 

Mountains are composed primarily of Precambrian basement 

rocks (unit pe on fig. 3), with limited exposures of Paleozoic 

rocks (Read and others, 2003, 2004). Precambrian basement 

rocks are predominantly granitic with subordinate schist, 

gneiss, and amphibolite. The rocks are highly brecciated in 

many localities (Spiegel and Baldwin, 1963; Read and others, 

2004). 

Pre-rift sedimentary units of the basin floor are exposed 

outside of the basin in the southern part of the study area 

(fig. 3). These units include Paleozoic to Eocene sedimentary rocks (units hap, cgu, Tgd) and Eocene to Oligocene 

igneous and volcaniclastic rocks (units Ti through Tomv). 

Paleozoic-Mesozoic rocks (unit cgu) form a sedimentary 

section as much as 2,000 m (6,560 feet (ft)) thick that generally correlates with sections in the Colorado Plateau (Sawyer 

and Minor, 2006a). Unconformably overlying the PaleozoicMesozoic section are the late Paleocene-Eocene Galisteo and 

Diamond Tail Formations (unit Tgd), which accumulated in 

basins at the southern and southwestern margins of the present 

Española Basin in the late stages of the Laramide orogeny 

(Cather, 1992, 2004; Lucas and others, 1997). The entire prerift sedimentary section is variably eroded within the basin 

(fig. 2). Only the basal, limestone-dominated portion of the 

Paleozoic section (unit hap) is preserved in isolated exposures 

in the Santa Fe Mountains (fig. 3) and in the subsurface north 

of the Santa Fe River (Myer and Smith, 2006). Northeast of 

Pojoaque, the Mesozoic-Paleozoic section may be totally 

absent in the subsurface, on the basis of well data and geologic 

inference (Koning, Nyman, and others, 2005). 

Eocene and Oligocene intrusions related to a porphyry 

belt (the Ortiz porphyry belt) are represented by monzonite 

and monzonite porphyry rocks exposed in the Cerrillos Hills 

(unit Ti, fig. 3). Laccolithic sills that emanated from the intrusive rocks domed overlying sedimentary deposits (Maynard 

and others, 2002); the sills have been encountered in boreholes 

(Grant, 1999). Intrusive episodes of several different ages and 

rock types are recognized as part of this intrusion (Maynard 

and others, 2002).

At the top of the basin floor, the late Eocene and Oligocene Espinaso Formation (unit Te) and the Cieneguilla volcanic complex (unit Tomv) overlie the Cerrillos intrusion and the 

pre-rift sedimentary section (fig. 3). The Espinaso Formation 

is a heterogeneous unit composed primarily of volcaniclastic 

sandstones, conglomerates, boulder conglomerates, near-vent 

volcaniclastic breccias, and minor intermediate-composition 

lava flows (Kautz and others, 1981; Smith and others, 1991; 

Thompson and others, 2006). Speigel and Baldwin (1963) 

describe rocks related to the Espinaso Formation as a series 

of latite flows and breccias. Kautz and others (1981) and 

Erskine and Smith (1993) interpret the Espinaso Formation as 

an extensive apron of volcaniclastic debris as much as 400 m 

thick that accumulated next to volcanic centers associated with 

the emplacement of the Ortiz porphyry belt. The overlying 

Cieneguilla volcanic complex is composed of basanite and 

nephelenite lava flows and interbedded sediments, as much as 

200 m (660 ft) thick. The term Cieneguilla basanite replaces 

the previously used term Cieneguilla limburgite (Sawyer and 

others, 2002). Ages range from 36–29 Ma for the Espinaso 

Formation (Sawyer and others, 2002; Baldridge and others, 

1980; Sauer, 1999) to 24–27 Ma (probably 25–26 Ma) for the 

Cieneguilla basanite (Baldridge and others, 1980; Kautz and 

others, 1981; Koning and Hallett, 2001). 

Basin-fill sediments that formed in response to extension 

of the Rio Grande rift from Oligocene to Pleistocene time 

are collectively placed in the Santa Fe Group. These clastic 

sediments form important basin aquifers for ground water in 

the region. The primary aquifers in the Santa Fe area are the 

Oligocene-Miocene Tesuque Formation and the PlioPleistocene Ancha Formation (fig. 3). The Tesuque Formation, 

which represents the bulk of the basin fill north of Interstate 

25, is composed primarily of pink to tan sandstone and 

silty-clayey sandstone with minor conglomerate, siltstone, 

and claystone (Spiegel and Baldwin, 1963; Borchert and 

others, 2003; Koning and Maldonado, 2003; Read and others, 

2005; Koning, Connell, and others, 2005). The Tesuque 

Formation can be subdivided into lithostratigraphic units 

deposited in unique paleolandscape positions in the Española 

Basin having different sediment source areas, as follows. 

Sediment deposited on the alluvial slope (or piedmont slope) 

along the foot of the Sangre de Cristo Mountains and on 

the basin floor to the west has been called lithosomes A and 

B, respectively, by Cavazza (1986). Lithosome S applies to 

reddish, sandy sediment with a slightly higher proportion of 

quartzite and Paleozoic detritus compared with lithosome 

A, which was deposited by an ancestral Santa Fe River as 

an alluvial fan near Santa Fe (Koning, Read, and others, 

2005). Volcaniclastic, alluvial fan sediment that was eroded 

from paleohighlands near the source vents of the Espinaso 

Formation and Cieneguilla basanite forms a subsurface 

unit southwest of Santa Fe called lithosome E (Koning and 

Johnson, 2006). 

Coarser grained basin fill that postdates about 13 Ma has 

been subdivided into various members of the Chamita and 

Tesuque Formations (Koning and Aby, 2005; Koning, Connell, 

and others, 2005). The Cuarteles and Cejita Members of the 

Tesuque Formation east of the Rio Grande are similar to the 

underlying lithosome A and B units of the Tesuque Formation, 

respectively, in their source areas and general composition. 

In the lower Tesuque Formation, the volcaniclastic Bishops 

Lodge Member has recently been interpreted as a distal 

equivalent of the Espinaso Formation (Smith, 2000; Read 

and others, 2003). The late Pliocene to early Pleistocene 

Ancha Formation is composed of poorly consolidated sand, 

gravel, and clayey-silty sand, fining to the west. It forms a 
Geologic Setting 11

Figure 3. Generalized geology and geography of the study area. Geologic contacts are generalized from Read and others 

(2004), Minor (2006), New Mexico Bureau of Geology and Mineral Resources (2003), Koning (2005b), and Koning, Nyman, 

and others (2005). The irregular outline of the extent of mapped Santa Fe Group units (units QTsf and Tsf) is used for location 

purposes in many subsequent figures.

Seton Village fault

Barrancos Monocline

La Bajada fault

Cochiti Cone fault

Tijeras-Cañoncito fault

Pajarito-Embudo fault system

San Is di ro Crossing fault

25

84

285

84

68

502

Cerrillos

Hills

Gallina Arroyo

Pajarito

Plateau

Cerros del Rio

volcanic field

Rio

Grande

Rio

Grande

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

Quaternary valley-fill alluvium 

 and landslides 

Late Eocene and Oligocene 

 Espinaso Formation

Mesozoic-Paleozoic clastic sedimentary 

 rocks, undivided

Precambrian basement rocks, undivided

EXPLANATION

Pleistocene Bandelier Tuff, erupted 

 from the Valles and Toledo calderas

Late Eocene and Oligocene 

 intrusive rocks

Late Paleocene and Eocene clastic 

 rocks of the Galisteo and Diamond 

 Tail Formations

Pennsylvanian sedimentary rocks 

 of the Alamitos and La Pasada 

 Formations, mostly limestone

Major mapped fault or fault zone—Ball

 on downthrown side; dashed where inferred

Aeromagnetic data area

pe

hap

Tgd

Ti

Te

Tomv Early Tertiary mafic volcanic 

 rocks (Cieneguilla volcanic complex 

 and undivided)

Tsf Oligocene-Miocene Santa Fe 

 Group sediments (Tesuque and 

 Chamita Formations)

QTv Plio-Pleistocene basalts and 

 andesites of the Cerros del Rio 

 volcanic field and Black Mesa

QTsf Plio-Pleistocene Santa Fe Group 

 (Ancha Formation, Tuerto Gravel, 

 Puye Formation, and Sierra 

 Ladrones Formation)

Qbt

Qal

cgu

cgu

cgu

cgu

cgu

106°00'

Santa Fe River

pe

hap

pe

pe

pe

hap

Tgd

Tgd

Tgd

Tgd

Tgd

Ti

Ti

Ti

Ti

Ti

Ti

Ti

Ti

Ti

Te

Te

Te Te

Te

Te Tgd

Tomv

Tomv

Tomv

QTv

QTv

QTv QTv

QTv

QTv

QTv

Qbt

Qbt

QTsf

QTsf

QTsf

QTsf QTsf

QTsf

QTsf

QTsf

Tsf

Tsf

Tsf

Tsf

Tsf

Tsf

Tsf

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Qal

Ti

Española

Santa Fe

Pojoaque

La Cienega

Eldorado

at Santa Fe

Galisteo

Seton

Village

Cundiyo
12 Geophysical Interpretations of the Southern Española Basin, New Mexico

thin (10–90 m (33–295 ft)) blanket over older units (Koning, 

Connell, and others, 2002). West of the Rio Grande, the 

gravelly Puye Formation occupies a stratigraphic level similar 

to that of the Ancha Formation, and it is overlain by basaltic 

rocks and tuff of Pliocene-Quaternary age (Kelley, 1978; 

Dethier, 2003, Dethier and Koning, 2007).

The basin fill also includes Plio-Pleistocene basalts of the 

Cerros del Rio volcanic field and the silicic volcanic rocks that 

erupted from the Valles and Toledo calderas in the late stages 

of the Jemez volcanic field. In the study area, these rocks overlie the bulk of the Santa Fe Group sediments, as determined 

by examination of well and geophysical data from the Pajarito 

Plateau (Broxton and Vaniman, 2005; Los Alamos National 

Laboratory, 2006) and the Cerros del Rio volcanic field 

(Minor, 2006). They generally overlie the Plio-Pleistocene 

Santa Fe Group sediments on the west side of the study area, 

but locally the two units interfinger. Speigel and Baldwin 

(1963) included these volcanic rocks in the definition of the 

Santa Fe Group. We distinguish them as Tertiary younger volcanic rocks in this report because of their contrasting physical 

properties with the Santa Fe Group sediments. 

The southern Española Basin is structurally bounded on 

the northwest by the Pajarito-Embudo fault system and on 

the southwest by the La Bajada fault (fig. 2). On the Pajarito 

Plateau, the Pajarito-Embudo fault system is represented by 

the northerly striking Pajarito fault that bounds the west-tilted 

half graben of the eastern Española Basin (Golombek, 1983). 

On the southeast side of Black Mesa (fig. 1), the fault system 

is represented by the northeast-striking Embudo transfer fault, 

which extends northeastward outside of the study area. The 

Embudo fault transfers strain between the west-tilted Española 

and east-tilted San Luis Basins (Muehlberger, 1979; Faulds 

and Varga, 1998) (fig. 1). The Pajarito fault links with the 

Embudo fault at a complexly faulted area in the northwest 

part of the study area. For simplicity, we consider these faults 

together as part of the Pajarito-Embudo fault system (fig. 3). 

The La Bajada fault is a major down-to-the-west normal fault 

that is commonly considered as the boundary between the 

Española and Santo Domingo Basins (Minor, 2006) (fig. 1). 

North of the Santa Fe River where the La Bajada fault curves 

to the west, regional gravity gradients suggest that most of the 

down-to-the-west displacement is accommodated instead by 

the north-south Cochiti Cone fault (fig. 3; Minor and others, 

2006). Thus, for purposes of this report, the basin-bounding 

fault system in this area is considered as a combination of 

the La Bajada and Cochiti Cone faults. Numerous normal 

faults are exposed in the Tesuque Formation (unit Tsf) in 

the middle of the basin; whereas very few faults deform the 

over-lying Ancha Formation (unit QTsf) on the south (fig. 3). 

The concealed Seton Village fault (fig. 3) south of Santa Fe 

was previously inferred from geophysical evidence (Speigel 

and Baldwin, 1963). Many more concealed faults are revealed 

by the geophysical interpretations in this report and are 

introduced in subsequent sections. 

Data Types and Descriptions

Magnetic and gravity data are measurements that show 

variations in the Earth’s magnetic and gravity fields, respectively. Aeromagnetic data are magnetic data that are collected 

from aircraft. Once known field variations are removed, the 

remaining measured variations provide clues to the geology 

that underlies the surface. These clues are based on an understanding of the behavior of subtle fields produced by differences in the distribution and nature of the physical properties 

of rocks and sediments. For magnetic data, the applicable 

physical property is magnetization, which is determined by 

naturally occurring magnetic minerals in rocks and sediments. 

For gravity data, the applicable physical property is bulk 

density, which is the overall mass per unit volume of rocks, 

sediments, and their pore spaces. 

Aeromagnetic and gravity interpretation involves indirect 

investigation of physical properties to infer the geology of 

the subsurface. To minimize ambiguities and subjectivity, a 

variety of additional data are commonly considered during 

analysis and interpretation. For this study, these additional data 

included measurements of physical properties, geologic mapping, subsurface well lithology, seismic-reflection geophysical 

data, and deep-looking electrical geophysical models. 

Physical Properties

The physical property germane to aeromagnetic interpretation is total magnetization, which is the vector sum of 

remanent and induced components. The induced magnetization is defined as the magnetic field spontaneously produced 

in a volume of rock by the action of an applied field (Bates 

and Jackson, 1984). The ratio of the induced magnetization 

to the applied field (the present-day Earth’s magnetic field in 

this case) is defined as magnetic susceptibility. A dimensionless number in the SI system, magnetic susceptibility is mainly 

a function of the quantity of magnetic minerals (commonly 

magnetite). Rock types typically containing the greatest abundance of magnetic minerals are mafic and ultramafic igneous 

and metamorphic rocks. Sedimentary rocks and sediments 

typically contain the least abundance (Reynolds and others, 

1990). The remanent component, commonly called remanent 

magnetization, represents the vector sum of all permanent 

magnetizations held by the magnetic minerals, which have 

fixed directions irrespective of applied magnetic fields (Bates 

and Jackson, 1984). An important contribution to the remanent 

magnetization is acquired at the time the rock forms, which 

results in a record of the direction of the Earth’s magnetic field 

at the time of formation. Remanent magnetization can be large 

but highly variable in magnitude in volcanic and metamorphic rocks but is commonly negligible in sedimentary rocks 

and sediments (Reynolds and others, 1990). Traditionally, 

magnetic susceptibility is reported rather than its associated 

vector quantity, induced magnetization. In contrast, remanent 
Data Types and Descriptions 13

magnetization is commonly reported as intensity in amperes 

per meter (A/m in Système International (SI)) with a given 

orientation (declination and inclination).

Reconnaissance measurements of magnetic susceptibility and limited information about remanent magnetization are 

available for many geologic units in the Española Basin and 

nearby areas (fig. 4). These measurements guide the interpretations but are not intended to represent a rigorous study of rock 

magnetic properties. The large variability of these properties 

is readily apparent within individual units and between units 

(fig. 4); note that magnetic susceptibility is presented on a log 

scale. In general, the volcanic rocks have the greatest magnetic 

susceptibilities, whereas the sedimentary rocks have the least, 

as expected. Precambrian basement rocks have the greatest 

variability. Total magnetizations for the volcanic rocks are 

likely very large because remanent components are typically 

much greater than the induced components in these rocks. The 

opposite is true for the sedimentary rocks, giving rise to low 

total magnetizations. 

The physical property germane to gravity interpretation 

is bulk density. Bulk density correlates fairly well with rock 

type, but it is also affected by porosity and cementation related 

to diagenesis, compaction, and degree of fracturing. Density 

measurements from single samples and from borehole density 

logs are available for the Española and nearby Albuquerque 

Basins (fig. 1, table 2). Several deep oil exploration wells 

in the Española Basin, and more in the Albuquerque Basin, 

provide valuable information about the densities of geologic 

units. This information is especially important for basin-fill 

sediments, which become denser with depth owing to compaction. The results show that the Santa Fe Group is considerably 

less dense compared with the other rock types (differences 

greater than 140 kilograms per cubic meter (kg/m3

)), even to 

depths of 3.75 kilometers (km) (12,300 ft). This relation and 

the small overall volume of volcanic rocks compared with the 

volume of sediments in the study area allow robust application 

of a gravity inversion technique (discussed in a subsequent 

section) to separate the gravity effect of sediments from that of 

other rock types.

Because robust geophysical interpretation relies on good 

contrast between physical properties, the physical properties 

ascribed to rocks in the study area generally formed the basis 

for dividing the geologic units into interpretative packages 

(table 1). For example, the low densities of the Santa Fe 

Group sediments set them apart from all other rock types, 

although individual formations within the Santa Fe Group 

cannot be distinguished. On the other hand, the Tertiary 

younger volcanic rocks are considered separately from the 

Santa Fe Group sediments for map interpretation and modeling in the study area, but they are lumped with the sediments 

for the basin-scale gravity inversion, where their volume is 

small compared with that of fill in the whole basin. Tertiary 

older volcanic rocks are considered as a package because the 

Cieneguilla volcanic complex (unit Tomv, fig. 3) closely overlies the Espinaso Formation (unit Te) and they have similar 

densities and magnetic properties. In contrast, Precambrian 

basement is discussed as one package despite its wide variation in magnetic properties and a density similar to that of the 

Paleozoic limestones. Precambrian rocks that have very low 

magnetic susceptibilities cannot be distinguished by density 

or magnetic properties from Paleozoic limestones. On the 

other hand, they are easily distinguished in seismic-reflection 

records which, combined with well data, constrain the models 

in limited areas.

Aeromagnetic Data 

Aeromagnetic surveys measure the total intensity of the 

Earth’s magnetic field from an aircraft as it follows a regular 

pattern of flight lines. The collected data are processed to 

remove time-varying external fields and are corrected for 

noise from aircraft movements. The effects of Earth’s primary 

magnetic field are removed to produce “magnetic anomaly 

data” or “magnetic data” that isolate subtle variations related 

to geology. The processed data are gridded at an interval 

commensurate with the flight-line spacing before they are 

displayed on magnetic anomaly maps. High-resolution 

surveys are flown closer to the ground and with narrower line 

spacing than conventional aeromagnetic surveys. The low 

flight path allows better detection of weak magnetic sources 

near the surface. The narrow line spacing increases sampling 

of the magnetic field and thus provides better definition of 

sources with limited lateral extent, and it improves the overall 

resolution of details in map view. 

Four surveys provide high-resolution aeromagnetic data 

for the southern Española Basin study area (fig. 5). All but the 

Santa Fe East survey (which used a helicopter) were flown 

using fixed-wing aircraft. The Sandoval–Santa Fe survey, 

which covers most of the area, was flown using east-west 

lines spaced 150 m (500 ft) apart and a magnetometer height 

nominally 150 m (500 ft) above ground (U.S. Geological 

Survey and others, 1999). The Cochiti survey was flown 

with 400 m (1,310 ft) line spacing at a nominal 73 m 

(240 ft) above ground and the Pajarito Plateau survey with 

100–200 m (330–660 ft) line spacing at a nominal 75 m 

(246 ft) above ground. For the latter two surveys, the wide 

line spacing relative to the ground clearance was not optimal 

for aeromagnetic map interpretation, but it was required 

because the primary purpose of both surveys was to collect 

electromagnetic geophysical data (Deszcz-Pan and others, 

2000; Sweeney and others, 2002; Cole and others, 2006; 

Baldridge and others, 2007). The Santa Fe East survey was 

flown with 200 m (660 ft) line spacing at a nominal 150 m 

(500 ft) above ground (Bankey and others, 2006). To minimize 

analytical problems caused by regions that lacked data, areas 

not covered by these four surveys were filled with data from 

a regional grid compiled for the state of New Mexico (Kucks 

and others, 2001).

After flight-line data from the four surveys were 

processed to remove Earth’s field variations and reduce 

ordinary data-acquisition errors, the data were analytically 
14 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 4. Magnetic-property information for geologic units in the study area. Diagrams show magnetic susceptibility (SI) 

measured for volume on a logarithmic scale; symbols indicate available information about the ratio of remanent to induced 

magnetization and direction (see explanation). Different symbols reflect different data sources, sampling procedures, and 

types of reported data. The data set primarily represents reconnaissance measurements, which should not be considered 

statistically robust.

0.01 0.1 1 10 100 1,000

MAGNETIC SUSCEPTIBILITY (SI) × 10–3 S A N T A F E G R O U P S E D I M E N T S VOLCANIC ROCKS

BASIN FILL

Basalts of the middle

 eruptive phase, Cerros del 

 Rio volcanic field

Basalts of the early

 eruptive phase, 

 Cerros del Rio 

 volcanic field

Basalt, undivided

Stream terrace deposits

Ancha Formation

Puye Formation

Cuarteles Member,

 Tesuque Formation

Lithosome A, 

 Tesuque Formation

Lithosome B, 

 Tesuque Formation

Mixed Lithosomes B and

 S, Tesuque Formation

Lithosome E, 

 Tesuque Formation

Bishops Lodge Member,

 Tesuque Formation

Tesuque Formation,

 undivided

0.01 0.1 1 10 100 1,000

MAGNETIC SUSCEPTIBILITY (SI) × 10–3

P R E C A M B R I A N 

B A S E M E N T R O C K S

BASIN FLOOR

INTRUSIVE

ROCKS

TERTIARY OLDER 

VOLCANIC ROCKS

SEDIMENTARY

ROCKS

Basalt, Cieneguilla

 equivalent(?)

Basanite, Cieneguilla

 volcanic complex

Latite flow, Espinaso

 Formation(?)

Volcaniclastic rocks,

 Espinaso Formation

Intrusive rocks,

 Cerrillos Intrusion

Eocene Galisteo

 Formation

Cretaceous shales 

 and sandstones

Paleozoic limestone

Megacrystic granitoid

Pink granitic gneiss

Biotite-rich granitic

 gneiss

Strongly foliated amphib-

 olite and mafic schist

Quartz-feldspar-biotite-

 magnetite schist

EXPLANATION

Site average (circle) generally using six to eight 

 samples per site from Cerros del Rio volcanic field

 (Thompson and others, 2006). Remanent component 

 of normal (red) or reversed (blue) polarity greater 

 than induced component

Laboratory sample value (triangle) or average 

 and range (triangle with bars) (Spiegel and Baldwin, 

 1963). Remanent component of unknown polarity 

 greater than (upright triangle) or less than 

 (inverted triangle) the induced component

Typical value (box) or mid value and range (box 

 with bars) from reconnaissance measurements 

 of outcrops (V.J. S. Grauch, unpub. data, 1999, 

 2006–2007 and V.J.S. Grauch and D.J. Koning, 

 unpub. data, 2007). Remanent magnetization 

 not measured 

Average and range (octagon and bars) of sites in northern 

 Albuquerque Basin (Hudson and others, 2008). Induced 

 component greater than remanent component 

Average value and range (diamond with bars) from 

 reconnaissance site measurements (8–10 samples per site) 

 (M.R. Hudson, unpub. data, 2002) and traverse sampling 

 (three samples taken every 2–10 m for 100–150 m distance) 

 (M.R. Hudson and V.J.S. Grauch, unpub. data, 2006). 

 Remanent magnetization not measured 

Magnetic susceptibility range estimated by multiplying 

 3.47 times 3–5%, the volume fraction of magnetite in thin 

 section (Spiegel and Baldwin, 1963), following empirical 

 relations reported by Hansen and others (2005) 

Mafic gneiss

Muscovite gneiss

Gneiss and schist
Data Types and Descriptions 15

Table 2. Densities of geologic units.—Continued

[Measured and estimated bulk densities of geologic units in the Española Basin and equivalent units in the Albuquerque Basin and vicinity to the south. ft, 

feet; m, meter; N/A, not applicable; NR, not reported]

Geologic unit 

or rock type

Bulk density1

(kilograms/cubic meter)

Number of 

samples or 

wells2

Data source

Minimum Maximum Mean

Basin fill volcanic rocks

Basalt 2,540 2,810 2,660 3 Laboratory samples (Spiegel and Baldwin, 1963).

Volcanic rocks of the 

Jemez volcanic field.

N/A N/A 2,350 N/A Estimated from gravity modeling of Valles Caldera 

(Nowell, 1996).

Basin fill sedimentary units

Ancha Formation of the Santa 

Fe Group.

N/A N/A 1,700 N/A Estimated from geophysical analysis (Spiegel and 

Baldwin, 1963).

Tesuque Formation of the 

Santa Fe Group.

N/A N/A 2,100 N/A Estimated (unexplained analysis by Speigel and 

Baldwin, 1963).

Santa Fe Group, undivided, 

0–1,250 m (0–4,100 ft) 

depth.

2,090 2,260 2,170 12 Borehole density logs from Albuquerque and 

Española Basins.

Santa Fe Group, undivided, 

1,250–2750 m 

(4,100–9,020 ft) depth.

2,230 2,400 2,300 3 Borehole density logs from Albuquerque and 

Española Basins.

Santa Fe Group, undivided, 

2,750–3,750 m 

(9,020–12,300 ft) depth.

2,300 2,450 2,380 1 Borehole density log from Albuquerque Basin.

Santa Fe Group, undivided, 

3,750–4,250 m 

(12,300–13,940 ft) depth.

2,500 2,580 2,540 1 Borehole density log from Albuquerque Basin.

Basin floor volcanic rocks

Cieneguilla volcanic rocks NR NR 3,080 2 Laboratory samples (limburgite of Spiegel and 

Baldwin, 1963).

Cieneguilla volcanic rocks 2,350 2,640 2,480 2 Borehole density logs from the Española Basin.

Flow latite 2,690 2,690 2,690 1 Laboratory sample (Spiegel and Baldwin, 1963).

Brecciated latite 2,180 2,180 2,180 1 Laboratory sample (Spiegel and Baldwin, 1963).

Espinaso Formation 2,320 2,540 2,430 3 Borehole density logs from Albuquerque and 

Española Basins.

Basin floor intrusive rocks

Intermediate-composition 

intrusive rocks.

2,470 2,500 2,485 2 Laboratory samples (andesite dike and monzonite 

of Spiegel and Baldwin, 1963).

Basin floor sedimentary units

Eocene Galisteo Formation 

(sandstone).

2,400 2,450 2,425 6 Laboratory samples (Spiegel and Baldwin, 1963).

Eocene and Oligocene clastic 

rocks, undivided,

 0–1,750 m 

(0–5,740 ft) depth.

2,380 2,390 2,390 2 Borehole density logs of Oligocene and Eocene 

clastic rocks from the Albuquerque and 

Española Basins.
16 Geophysical Interpretations of the Southern Española Basin, New Mexico

Table 2. Densities of geologic units.—Continued

[Measured and estimated bulk densities of geologic units in the Española Basin and equivalent units in the Albuquerque Basin and vicinity to the south. ft, 

feet; m, meter; N/A, not applicable; NR, not reported]

Geologic unit 

or rock type

Bulk density1

(kilograms/cubic meter)

Number of 

samples or 

wells2

Data source

Minimum Maximum Mean

Basin floor sedimentary units—Continued

Eocene and Oligocene clastic 

rocks, undivided,

1,750–3,250 m 

(5,740–10,660 ft) depth.

2,500 2,560 2,530 2 Borehole density logs of Oligocene and Eocene 

clastic rocks from the Albuquerque and Española Basins.

Eocene and Oligocene clastic 

rocks, undivided,

4,750–6,500 m 

(15,580–21,330 ft) depth.

2,610 2,670 2,630 1 Borehole density log of Oligocene clastic rocks 

from the Albuquerque Basin.

Cretaceous Mancos Shale NR NR 2,590 8 Laboratory samples (Spiegel and Baldwin, 1963).

Jurassic Wingate and Entrada 

sandstones.

2,640 2,640 2,640 1 Laboratory sample (Spiegel and Baldwin, 1963).

Mesozoic sedimentary rocks, 

undivided, 0–2,500 m 

(0–8,200 ft) depth.

2,400 2,500 2,440 10 Borehole density logs from Albuquerque and 

Española Basins.

Mesozoic sedimentary rocks, 

undivided, 2,500–3,250 m 

(8,200–10,660 ft) depth.

2,500 2,550 2,530 3 Borehole density logs from Albuquerque and 

Española Basins.

Mesozoic sedimentary rocks, 

undivided, 3,250–4,500 m 

(10,660–14,760 ft) depth.

2,509 2,670 2,620 1 Borehole density log from Albuquerque Basin.

Permian sedimentary rocks, 

undivided, 0–3,250 m 

(0–10,660 ft) depth.

2,510 2,660 2,600 10 Borehole density logs from Albuquerque and 

Española Basins.

Pennsylvanian sedimentary 

rocks, undivided, 

0–3,250 m (0–10,660 ft) 

depth.

2,550 2,690 2,630 9 Borehole density logs from Albuquerque and 

Española Basins.

Precambrian basement rocks

Precambrian crystalline rocks 2,490 2,990 2,690 5 Laboratory samples (Spiegel and Baldwin, 1963).

Precambrian crystalline rocks 2,570 2,700 2,620 5 Borehole density logs from Albuquerque and 

Española Basins.

1

For samples measured in the laboratory, range and average bulk densities are reported for the number of samples measured. For all but densities for basin 

floor volcanic rocks obtained from borehole density logs, typical densities throughout 250-m (820-ft)-depth intervals were estimated by inspection of each borehole log and were assigned to the appropriate geologic unit. Then, for each geologic unit within the indicated overall depth range, densities were averaged for 

all applicable 250-m (820-ft)-depth intervals from all boreholes. Overall depth ranges were determined by inspection of major differences in density with depth. 

Where multiple samples were unavailable or densities were estimated, a single value is listed as the mean. 

2

Number of laboratory samples measured or number of wells for which borehole density logs were inspected for the geologic unit and depth range specified. 

Number of samples is not applicable for bulk densities that were estimated by geophysical modeling.
Data Types and Descriptions 17

Figure 5. Aeromagnetic map showing coverage of high-resolution surveys. Reduced-to-pole aeromagnetic data are 

shown after final data processing and merging of data from individual surveys (pink lines). Image is color-shaded relief with 

illumination from the east. The Pajarito Plateau survey covers a larger area than shown here; it extends as much as 5 km 

northwest of its boundary on this map.

Rio

Grande

25

25

285

84

285

84

285

68

502

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Cerrillos

Hills

La Cienega

Cochiti

Survey

Pajarito

Plateau

Survey

Sandoval–

Santa Fe

Survey

Santa Fe 

East

Survey

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

–1,200

0

–800

–400

400

800

1,200

1,600

2,000

2,400

2,800

NANOTESLAS

EXPLANATION

Boundary of individual 

 high-resolution survey—Represents 

 a merge boundary between 

 overlapping surveys where they adjoin 
18 Geophysical Interpretations of the Southern Española Basin, New Mexico

continued from the variable observation surfaces to a 

common surface draped 100 m (330 ft) above ground and 

digitally merged with regional magnetic data into a 50-m 

(160-ft)-interval grid, following procedures described in 

Sweeney and others (2002) and Grauch and Hudson (2007). 

The gridded data were then transformed to reduced-topole data, a standard geophysical operation applied to data 

collected in areas of high latitude (Blakely, 1995). Reductionto-pole transformations correct for shifts of anomalies away 

from the centers of their magnetic sources; these shifts are 

an effect of the oblique orientation of the measured magnetic 

field with respect to Earth’s surface. If not corrected, positive 

aeromagnetic anomalies for the study area would be offset to 

the southwest of their causative sources and partial rings of 

negative anomalies would occur on the northeast. A related 

technique, called the pseudogravity transformation (or 

computation of the magnetic potential), also minimizes these 

shifts while enhancing the broad features of the data. The 

pseudogravity transformation is used in this report solely as 

an interim step during application of interpretative techniques 

(discussed below). The reduced-to-pole and pseudogravity 

transformations are best utilized in areas where the total 

magnetizations of sources are generally collinear (within 25°) 

with Earth’s main field (Bath, 1968). General collinearity is a 

reasonable approximation for most of the study area because 

of the Neogene age of volcanic units, which are the ones most 

likely to have consistently strong remanent magnetization. 

The reduced-to-pole magnetic data for the central Rio 

Grande rift incorporate the assumption that Earth’s field has 

declination=11° and inclination=63°.

Figure 5 is a color shaded-relief image of the final, 

reduced-to-pole aeromagnetic data. This image uses color to 

display the broad variations in the data and shading (analogous 

to illumination by the sun) to enhance many subtle details 

obliquely oriented to the direction of illumination. 

Gravity Data

Gravity data throughout the Española Basin and 

surrounding region come from Kucks and others (2001), 

from unpublished data collected by an educational group 

called Summer of Applied Geophysical Experience (SAGE) 

(Jiracek and others, 2008) during the period 1997–2002 

(Shawn Biehler, University of California, Riverside, written 

commun., 2002), and from a web database maintained by the 

Pan-American Center for Earth and Environmental Studies 

(http://research.utep.edu/Default.aspx?tabid=37229) 

accessed in 2006. The third database contains decades of data 

collected by numerous previous workers. Data from these 

three sources were merged, reconciled for discrepancies and 

duplicate stations, and edited to remove station data with large 

discrepancies compared with data at surrounding stations. 

Standard Bouguer corrections were applied using a reduction 

density of 2,670 kg/m3

. Terrain corrections were applied 

within a 167 km (104 miles (mi)) radius to the data at each 

station using digital terrain data with a resolution as fine as 

30 m. The station data were then interpolated onto a grid at a 

250 m interval.

In order to focus on density variations within the upper 

crust, an isostatic regional field was removed from the terraincorrected Bouguer gravity values by using parameters established for New Mexico by Heywood (1992). The resulting 

“isostatic residual gravity map” (fig. 6) generally isolates the 

gravity effects produced by sources within the upper 10 km of 

the crust (Simpson and others, 1986). 

Reliability of the features depicted on the isostatic gravity 

residual map is highly dependent on station spacing, which 

is unevenly distributed across the region (fig. 6). Resolution 

of anomalies is excellent in areas south of Santa Fe, where 

station spacing is 1 km (0.625 mi) or less. However, gaps in 

data coverage in the northern part of the study area can span 

8 km (5 mi) or more. In these areas, only gradients orthogonal to profile data are fairly well defined. These data may not 

properly resolve irregular boundaries or the expression of local 

variations underlying the regional gradients. 

Other Data 

Additional data incorporated into interpretations of 

the gravity and aeromagnetic data include geologic cross 

sections, lithologic picks in well data, electrical resistivity 

models from magnetotelluric (MT) soundings, and interpreted 

picks of seismic-reflection horizons. Geologic cross sections 

and geologic concepts come from several sources, including 

published maps and cross sections (Koning and Maldonado, 

2003; New Mexico Bureau of Geology and Mineral 

Resources, 2003; Koning, 2005b; Koning, Nyman, and 

others, 2005; Read and others, 2005; Shroba and others, 2005; 

Sawyer, Minor, Grauch, and others, 2006; Sawyer, Minor, 

Thompson, and others, 2006), unpublished cross sections 

(Daniel Koning, New Mexico Bureau of Geology and Mineral 

Resources, 2008), and consultation with other geologists 

who have worked in the area (David Sawyer, Scott Minor, 

Jonathan Caine, and Ren Thompson, U.S. Geological Survey, 

and Adam Read, New Mexico Bureau of Geology and Mineral 

Resources). 

Interpretations of borehole logs and cuttings are available from water wells and exploration wells, mostly from the 

southern part of the study area. Lithologic picks in more than 

a hundred of these wells were compiled by Peggy Johnson 

and Daniel Koning at New Mexico Bureau of Geology and 

Mineral Resources (NMBGMR). These picks and those from 

other sources are listed in appendixes 1 and 2. Many more 

wells exist in the basin with excellent lithologic information 

pertinent to the internal stratigraphy of the Santa Fe Group 

(for example, Frost, 2006; Los Alamos National Laboratory, 2006), but they are not included here because they have 

limited use in the deeper parts of the basin. Although the 

NMBGMR database includes much lithologic information, 

only a few lithologic picks were used as constraints for gravity 
Data Types and Descriptions 19

Figure 6. Isostatic residual gravity map of the area of gravity analysis. 

–35 –30 –10–20 0 10 20 30 40 MILLIGALS

0 5 10 KILOMETERS

0 5 10 MILES

35°30’

36°00'

107°00' 106°30' 106°00' 105°30'

Santa Fe

EXPLANATION

Gravity station

Aeromagnetic data area

Valles and Toledo calderas—General outline

Española Basin—Limit of Neogene deposits
20 Geophysical Interpretations of the Southern Española Basin, New Mexico

inversion and 3D modeling: the top of Tertiary older volcanic 

rocks (Espinaso Formation and overlying Cieneguilla volcanic 

complex if present), the top of the pre-rift sedimentary section 

(lower Tertiary through Paleozoic sedimentary section), and 

the top of Precambrian basement (appendix 1). 

Magnetotelluric data are available for the southwestern 

part of the study area as part of recent investigations by 

the U.S. Geological Survey. Preliminary models of these 

data, derived from 2D resistivity inversions, are reported 

in Rodriguez and others (2006). Since that study, revised 

and new models incorporating 13 additional MT soundings 

have been developed (Rodriguez and Sawyer, 2007; Brian 

Rodriguez, U.S. Geological Survey, written commun., 2007). 

The 2D inverse models show discrete resistivity layers that 

are interpreted as different geologic units on the basis of 

comparisons with electrical resistivity logs in boreholes 

and geologic cross sections. At the location of each MT 

sounding, the resistivity layers defined by the 2D inversion 

were assigned different depth ranges, as though the sounding 

was a borehole that penetrated layers of different electrical 

resistivities. These pseudoboreholes could then be used in this 

study to guide modeling of geologic surfaces. Two important 

results were obtained from the MT modeling: the detection 

of a conductor directly under moderately low resistivities 

associated with Santa Fe Group sediments, and resistive layers 

at the base of the electrical resistivity model that represent 

Paleozoic strata or Precambrian basement. Some ambiguities 

exist in the interpretation of the conductor, because it could 

represent Mesozoic Mancos or Chinle Formations at the top 

or middle of the pre-rift sedimentary section, respectively; 

Espinaso Formation within the package of Tertiary older 

volcanic rocks; or the clay-rich base of the Santa Fe Group 

(Brian Rodriguez and David Sawyer, U.S. Geological Survey, 

oral commun., 2008). However, consistencies between 

individual MT models and correlations with geologic maps 

provide some confidence in interpreting the resistivity layers 

in terms of trends and general depths to the base of the Santa 

Fe Group and top of the Paleozoic section.

Seismic information comes from data acquired and 

interpreted by the SAGE group. Some of the data and 

interpreted sections are published (Biehler and others, 1991; 

Baldridge and others, 1994; Ferguson and others, 1995). Only 

the interpreted model of Biehler and others (1991) is located 

within the study area, but the other data were used to constrain 

the basin-wide gravity inversion. The SAGE group has also 

had access to industry seismic reflection data from the 1970s 

in the form of paper copies of processed, common-midpoint 

stacked sections. Reflection data for several of these lines 

were first published (without locations or time scales) by 

Black (1984). The data and preliminary interpretations have 

been presented orally (Baldridge and others, 2001; Ferguson 

and others, 2007), but a final synthesis is pending. In the 

meantime, depth picks and images of some of the sections 

depicted in Ferguson and others (2007) were generously 

provided to the senior author by John Ferguson, University of 

Texas at Dallas, in 2007. (Note that Ferguson’s picks represent 

a different interpretation of the model presented in Biehler 

and others (1991), which was developed before the industry 

seismic data had been acquired.) The picks and images, which 

did not include all seismic data, were used as guides in our 

interpretation and 3D modeling but cannot be presented here. 

Methods of Data Analysis 

Data analysis, the first step in interpreting magnetic and 

gravity data, applies mathematical techniques to investigate 

the shapes of and depths to magnetization and density sources. 

The methods used for this study include edge detection, 

magnetic anomaly separation, and magnetic depth estimation. 

Edge detection is the application of techniques that locate the 

edges of causative sources that have near-vertical boundaries. 

Magnetic anomaly separation applies techniques to magnetic 

data in order to isolate various wavelength components. 

The separation helps investigators assess relations between 

regional, local, shallow, and deep features. Magnetic depth 

estimation includes techniques that estimate the depths to the 

tops of magnetic sources. For this study, edge detection methods were applied to both gridded gravity and magnetic data; 

magnetic anomaly separation techniques were applied to gridded magnetic data; and several depth estimation techniques 

were applied in different ways to both gridded and profile 

magnetic data. 

Edge Detection

Abrupt lateral contrasts in rock properties, such as those 

that occur at faults or steeply dipping contacts, correspond 

with the steepest slopes of their associated gravity or magnetic 

anomalies. Although several alternative techniques exist 

(Nabighian, Ander, and others, 2005; Nabighian, Grauch, 

and others, 2005), we use the horizontal-gradient method 

to locate edges related to faults. In this method, the steepest 

gradients can be located by computing the local maxima of 

the horizontal-gradient magnitudes (HGM) of gravity data, 

pseudogravity data, or reduced-to-pole magnetic data. The 

procedure is analogous to taking a derivative to find the value 

of the slope at the inflection point of a curve. For gridded data, 

the HGM is computed as a separate grid, and edges are located 

along HGM maxima that appear as ridges. 

Although many geophysical edges detected from 

HGM grids correspond directly with underlying faults, the 

interpretation is not always straightforward. First, steep 

gradients can be caused by steeply dipping surfaces other than 

faults, such as homoclines, dipping beds truncated by erosion, 

intrabasement or igneous lithologic contacts, and highrelief topography. Although the nature of geophysical edges 

usually becomes apparent during interpretation, the origins of 

some may remain ambiguous. Second, mapping the surface 

projections of faults from geophysical edges is affected by the 

dip of the interface and the location of the contrast in physical 
Methods of Data Analysis 21

property at depth along the interface. In both gravity and 

magnetic cases where dip is less than vertical, the maximum 

of the HGM migrates downdip farther away from the surface 

projection as depth to the magnetic contrast increases (Grauch 

and Cordell, 1987; Grauch and Hudson, 2007). The most 

serious problems are caused by basement-related contrasts 

at depths >1 km (0.6 mi), where maxima can be offset from 

the surface projection of associated faults by 500 m (1,640 ft) 

or more. Finally, in magnetic data, multiple maxima can be 

associated with one magnetic interface. These complications 

were resolved previously (Grauch and Hudson, 2007) by 

inspection of the aeromagnetic data used in this study area. 

Major gravity gradients representing regional features in the 

Española Basin were presented previously by Grauch and 

others (2006). More detailed gradients, representing individual 

faults or steep basement slopes in the vicinity of Eldorado 

at Santa Fe, are discussed in the section entitled Interpreted 

Structure.

Magnetic Anomaly Separation 

Because magnetic anomalies become broader as 

distance between the magnetometer and the source increases, 

it is common practice to apply digital filters to gridded 

aeromagnetic data to separate broad (long-wavelength) 

anomalies from narrow (short-wavelength) ones in order to 

investigate source depths. Although anomaly wavelengths 

also depend on the shapes of sources, anomaly separation 

filters allow qualitative examination of detailed (as opposed 

to regional) features that may otherwise be difficult to 

see. Several standard geophysical techniques can separate 

anomalies (Nabighian, Grauch, and others, 2005). For this 

study, we used the first vertical derivative and lowpass 

filtering. 

The first vertical derivative computes how rapidly the 

magnetic field decreases with distance from the sources 

following principles of potential-field theory (Blakely, 1995). 

The procedure is common practice because it enhances 

anomalies caused by shallow sources that are depth limited 

and generally brings out detail that is otherwise difficult to see 

(fig. 7). On the other hand, it also enhances data noise.

Lowpass filtering was accomplished by a filter designed 

using matched filtering (Syberg, 1972; Phillips, 2001), 

followed by application of an upward continuation filter 

using a continuation distance of 2 km (3.2 mi). These filters 

are designed to reduce the amplitudes of short-wavelength 

anomalies while minimizing filtering artifacts. The reader is 

referred to Blakely (1995) for more detailed discussion of this 

type of digital filter analysis. The result of the lowpass filtering 

(fig. 8) isolates the regional features of the data. Although deep 

sources generally cause regional features such as these, not 

all broad anomalies represent deep sources. For example, the 

broad region of negative magnetic anomaly values throughout 

the Cerros del Rio volcanic field is caused by shallow and 

exposed basalts within the field (Grauch and others, 2006). 

These anomalies coalesce to form a broad negative anomaly 

in the lowpass map that masquerades as a deep-source 

anomaly. Likewise, broad magnetic highs within the volcanic 

field may also be due to sources within the volcanic field. 

Comparison of anomalies between the original (fig. 5), the 

first vertical derivative (fig. 7), and the lowpass (fig. 8) maps 

shows that some anomalies have expression in both shortand long-wavelength components. Overlap of expression at 

different wavelength components was also apparent from 

more extensive matched filtering on the earlier aeromagnetic 

data compilation (Grauch and Bankey, 2003). The overlap of 

expression in areas outside of the Cerros del Rio volcanic field 

may indicate that the sources occupy a wide range of depths. 

Magnetic Depth Estimation 

Methods for estimating depth to magnetic sources from 

aeromagnetic data build on edge detection methods that locate 

magnetic edges in three dimensions. The methods rely on 

the general principle that shallow sources produce anomalies 

with steep gradients, whereas deep sources produce anomalies 

with broad gradients. However, the configuration of magnetic edges (source shape) also affects anomaly gradients 

and creates many unknown variables. To deal with unknown 

source shape, depth-estimation methods commonly consider 

idealized magnetic sources that are represented by a variable 

called the structural index (N). An infinite step (commonly 

called a “magnetic contact”) is represented by N=0; it extends 

to infinity in four directions (for example, north, south, east, 

and down). A magnetic sheet source is represented by N=1; 

it has zero thickness and extends to infinity in three directions 

(for example, north, south, and east for a horizontal sheet or 

north, south, and down for a vertical sheet, commonly called 

a “dike”). A magnetic line source (commonly called a “pipe”) 

is represented by N=2; it extends to infinity in two horizontal 

directions (for example, north and south). A magnetic point 

source, or dipole, which does not extend in any direction, is 

represented by N=3. 

Depth-estimation methods generally solve for the 3D 

coordinates of the top edge of one of these source types 

(or point location in the case of the point source) using the 

observed magnetic data and the magnetic-field equation 

appropriate for the source type. For the infinite step and sheet 

cases, many methods also solve for the dip of the step or dip 

of the sheet, respectively. The value of N has a large effect on 

the outcome of the resulting depth solutions. The shallowest 

solutions arise from N=0 and the deepest from N=3. Without 

any prior knowledge of source shape, common practice is to 

compare the solutions for different values of N. The primary 

end-member source types considered for geology are infinite 

steps (N=0) and thin sheets (N=1). The infinite step is used to 

represent a fault with large offset or the edge of a pluton. The 

thin sheet is used to represent horizontal or vertical sources, 

such as a basalt flow (observed at a distance much greater than 

its thickness) or a thin dike. In practice, shapes are likely to 
22 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 7. First vertical derivative of reduced-to-pole aeromagnetic data. The resulting map enhances detailed 

anomalies related primarily to shallow, depth-limited sources. Note linear anomaly patterns in the Precambrian 

basement along the eastern mountain front. The tiny circular features extending southwest from Santa Fe are caused 

by buildings. Image is color-shaded relief illuminated from the east.

Rio

Grande

Eldorado

at Santa Fe

Cundiyo

Seton

Village

La Cienega

Pojoaque

Española

Santa Fe

Cerros del Rio

volcanic field

Cerrillos

Hills

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

NANOTESLA

PER METER

–0.50

–0.20

–0.10

–0.05

–0.02

–0.01

0.00

0.02

0.01

0.05

0.10

0.20

0.50

–0.03

0.03

EXPLANATION

Mapped contact of Santa Fe Group 

 sediments—Generalized from 

 figure 3

Aeromagnetic data area
Figure 8. Lowpass-filtered magnetic map that enhances broad anomalies. The strong gradient extending west from 

Española may in large part be influenced by artifacts related to the poor resolution data in the northwest corner of 

the figure. Image is color-shaded relief illuminated from the east.

Methods of Data Analysis 23

Rio

Grande

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Cerrillos

Hills

Cundiyo

Seton

Village

La Cienega

Cerros del Rio

Volcanic Field

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

–300

–200

–175

–125

–150

–100

–75

–50

0

50

100

200

NANOTESLAS

EXPLANATION

Mapped contact of Santa Fe

 Group sediments—Generalized 

 from figure 3

Aeromagnetic data area
24 Geophysical Interpretations of the Southern Española Basin, New Mexico

represent a case between the end-member shapes (0<N<1), 

such as a fault with moderate offset, or have more complexity, 

such as a fold or a gradational contact. In these cases, the 

structural index N is not an integer, and it will vary as distance 

to the source changes. For example, the magnetic field due to 

a finite-width dike behaves as though N=1 at distances much 

greater than the width of the dike, but behaves as though 

N=0 at distances very close to one of the dike edges. As a 

consequence, depth solutions must be examined in context 

with other information, because the actual depths to the 

edges of sources may commonly occur somewhere between 

the shallower solutions using N=0 and the deeper solutions 

using N=1.

Magnetic depth-estimation methods can be applied either 

to grids or to profiles extracted from the aeromagnetic data 

across selected anomalies. In both cases, a window is passed 

across the data, which results in multiple depth solutions, one 

from the analysis of each window of data. The results are 

generally considered robust when solutions cluster around 

one location. For this study, both profile-based and grid-based 

methods were applied and compared. 

Profiles were analyzed by using the computer program 

PDEPTH (Phillips, 1997), which incorporates a variety 

of methods so that results can be examined together. The 

approaches giving the best results for this study are multisource Werner (Hansen and Simmonds, 1993), Euler deconvolution (Thompson, 1982), and local wavenumber or source 

parameter imaging (Thurston and Smith, 1997; Smith and 

others, 1998). These profile-based methods and their assumptions, strengths, and limitations are described in more detail in 

appendix 3. Except for the local wavenumber method, which 

computes the structural index directly, the methods require 

prior selection of N=0 or N=1. The multisource Werner 

method generally provided the most robust results for profiles. 

Solutions from this method (for both N=0 and N=1) are shown 

in subsequent figures in comparison with the geophysical 

profile models. 

Gridded data were analyzed using an adaptation of the 

local wavenumber method (Thurston and Smith, 1997; Smith 

and others, 1998) by Phillips and others (2007). Previous 

results from the application of a two-step extended Euler 

method to the southern part of the study area (Phillips and 

Grauch, 2004) were compared during construction of the 

model. Descriptions of these grid-based applications are 

also included in appendix 3. A major advantage of the local 

wavenumber method is that it solves directly for N, unlike 

other methods in which multiple values of N must be tested 

separately. A major disadvantage is that it is sensitive to 

data noise, which produces scattered or spurious results 

(Phillips, 2000). To overcome the noise problem, the method 

was applied to the magnetic data after application of the 

pseudogravity transformation. By inspection along several 

profiles, solutions using pseudogravity data had similar depths 

compared with those from the reduced-to-pole data, where 

they were focused on the same magnetic source. To further 

reduce scatter, a median filter was applied that moved a 

window across the solution points and computed the median 

for as many as 50 neighboring points. The resulting solutions 

still densely cover the entire area and represent a wide range 

of depths (more than 64,000 points). 

The Euler method is better suited to noisy data than the 

local wavenumber method, but N must be chosen before application. The two-step modification to the Euler method seeks 

to overcome the need to test different values of N by testing 

the solutions for N=0 against a user-defined minimum depth 

surface (Phillips and Grauch, 2004). When solutions are initially above the surface, N is increased until the solutions fall 

on the surface. Solutions that end up with unreasonable values 

of N are then discarded. In the previous study, the minimum 

depth surface was constructed from preliminary well data. 

For the present study, solutions from the local wavenumber 

and two-step extended Euler methods have similar results 

when compared in detail along profiles. However, the solutions from the local wavenumber method using pseudogravity 

and median filtering have less scatter, tend to provide more 

information on dipping interfaces, and are independent of any 

user-defined surfaces.

Although depth estimates related to an individual source 

may have errors as much as 20 percent of the estimated depth 

(Milsom, 1989), greater sources of error are the assumptions 

about the geologic origins of the magnetic source. Multiple 

types of magnetic sources at many depths in this study area 

make it difficult to predict which are represented by the depth 

estimates. In addition, where multiple sources are stacked 

vertically, the methods provide results only for the shallowest 

magnetic source.

Model Construction

Building on the results of our data analysis, we developed 

models to investigate various aspects of the gravity and magnetic data and integrate these results with information from 

other sources. We used a gravity inversion technique designed 

specifically to estimate the base of basin fill, joint gravity and 

magnetic modeling in 2D (profile) form to determine major 

sources of anomalies, and 3D integrated modeling to develop 

specific model surfaces across the basin. The results of the 

gravity inversion and modeling are discussed in the sections 

on interpreted subsurface geology. This section describes steps 

in the development of various models and how all information 

was integrated into a 3D model. 

Gravity Inversion 

The isostatic residual gravity data are most sensitive 

to the contrast in density between bedrock and poorly consolidated sediments, so they strongly constrain the general 

configuration of the basin floor. Deep gravity lows are closely 

associated with the thickest accumulation of sediment (fig. 6). 

However, gravity lows at the Valles caldera are instead caused 
Model Construction 25

by a thick accumulation of volcanic tuff that is less dense than 

surrounding rocks. Underlying variations in density, thickness, 

and structural relief of basin-floor rocks are expected to produce notable overprints on the overall gravity map, but they 

are difficult to predict because they are buried. 

To isolate the gravity effects of the basin fill alone, 

we used an iterative gravity inversion technique developed 

specifically for sedimentary basins by Jachens and Moring 

(1990), described by Blakely (1995), and that is very similar 

to the method of Ferguson and others (1988). The technique 

is based on separation of observed data into regional and 

residual components, where the residual component represents 

the gravity effects of the basin fill. A regional field is initially 

constructed using data only from stations located on bedrock 

not considered to be basin fill. This field is subtracted from the 

observed data to give a starting approximation of the residual 

field. Inversion techniques are then iteratively applied to vary 

the thickness of basin fill and modify the regional-residual 

separation to reach good fits to the observed data and to other 

independent constraints, such as well information. The two 

primary products are a model of basin-fill thickness (thickness 

model) and the final regional field, which represents the 

gravity effects of the bedrock (bedrock gravity) and includes 

the gravity effects of the both the basin floor and the bedrock 

of the flanking uplifts. The method is most successful where 

gravity data cover bedrock on all sides of the basin, where the 

density-depth function of basin fill compared with bedrock 

can be estimated, and where drill holes are available to help 

constrain the modeled thickness at depth. All three criteria for 

successful application of the gravity inversion technique are 

met for the Española Basin, except in the area of the Valles 

and Toledo calderas (fig. 1), where a separate approach is 

required. Because the study area skirts the caldera area, we 

elected to use the same approach throughout the basin and 

then ignore the results in that area. The steps of the gravity 

inversion are described as follows.

First, we expanded the area of analysis to include a large 

area surrounding the Española Basin. Gravity station coverage 

is fairly good at this scale except for a large region northeast of Santa Fe, which is mountainous (fig. 9). To begin the 

inversion, the area is divided into areas mapped as bedrock 

and areas mapped as Neogene and younger sedimentary and 

volcanic basin fill. Gravity stations within the areas considered 

as bedrock are separated out (blue circles on fig. 9) and used to 

develop the first regional bedrock gravity field in the iterative 

process.

Second, within the area considered as basin fill, a simple 

density-depth function must be defined. A plot of the densities 

derived from borehole density logs in the Albuquerque and 

Española Basins from table 2 provide a way to view how 

densities of different units vary with depth (fig. 10). The 

Santa Fe Group consistently possesses much lower densities 

at a given depth than any other unit. This markedly lower 

density provides the rationale for using the Santa Fe Group, 

along with the small-volume Tertiary younger volcanic 

rocks, to represent the basin fill and for all the other units to 

be considered bedrock. The density-depth function provided 

to the gravity inversion is thus the density contrast between 

the gravity reduction density of 2,670 kg/m3 and the density 

of the Santa Fe Group; this density varies in a step-wise 

fashion with depth, as shown in figure 10. Thus, the lower 

Tertiary and Mesozoic units that have densities much lower 

than 2,670 kg/m3

 at depths less than 3,000 m (9,840 ft) should 

produce low values in the bedrock gravity after inversion. 

This outcome is discussed in the section on interpretations 

regarding the pre-rift sedimentary section. Volcanic cover 

consisting primarily of basalts no more than about 250 m 

(820 ft) thick, such as in the Cerros del Rio volcanic fields 

(Rodriguez and others, 2006), are considered volumetrically 

unimportant compared with the mass of sediments contained 

in the basins (Cordell, 1976; Birch, 1982). The higher density 

volcanic rocks influence details, but not gross features, of 

the inversion results. Thus, the thickness model should be 

considered as an overall model of the combined sedimentary 

and volcanic portions of the basin fill. 

Third, lithologic and stratigraphic picks from wells, 

seismic-reflection depth picks, and model results from MT 

soundings (appendixes 1 and 4) were used to help constrain 

the inversion (fig. 9). Useful well information is particularly 

abundant in the southeast part of the basin, where many wells 

penetrate the base of the Santa Fe Group. The seismic and MT 

information, in particular, provide critical constraints in some 

of the deeper parts of the basin and extend outside the study 

area.

The isostatic residual gravity data were regridded from a 

250 m (820 ft) to a 1 km (0.6 mi) interval for the gravity inversion. The coarser grid interval minimizes excursions associated with data problems or local variations. After inversion, 

the resulting thickness model was subtracted from a regional 

terrain surface and displayed as an elevation surface (fig. 11). 

The companion product, bedrock gravity, is also shown as a 

gravity map (fig. 12). This map contains information about 

density variations and structural relief on pre-rift bedrock, discussed in a following section. Because areas where volcanic 

rocks are thick (>250 m (820 ft)) are not properly accounted 

for in the density-depth function used in the inversion process, 

the results surrounding the Valles and Toledo calderas should 

not be considered valid.

The model results should be considered in a regional 

sense owing to several problems inherent in gravity inversion. 

First, data problems and lack of coverage produce inaccurate 

results because these artifacts are incorporated as valid data. 

Second, the thickness model becomes erroneous where the 

assumed density-depth function of the basin fill misrepresents 

the actual average densities. If the assumed density contrast 

(density of basin fill minus that of bedrock) is higher than 

the actual contrast, the resulting thickness will be too great; 

if the assumed density contrast is lower, the resulting thickness will be too small. Third, the large grid-interval size and 

inherent difficulties in separating gravity effects at gravity 

stations near the basin margins result in local excursions of the 

model at the basin-bedrock margin (not apparent at regional 
26 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 9. Data coverage and types of independent constraining data used in the gravity inversion (appendixes 1 and 

2). Some gravity stations are coded as basin fill as defined by the extent of Neogene units shown in figure 1; others are 

considered to lie on pre-rift bedrock.

0 5 10 KILOMETERS

0 5 10 MILES

35°30'

36°00'

107°00' 106°30' 106°00' 105°30'

Santa Fe

Well penetrating base of Santa Fe Group—Appendix 1

EXPLANATION

Depth pick from seismic studies—Appendix 4

Gravity station on basin fill

Gravity station on pre-rift bedrock

Aeromagnetic data area

Valles and Toledo calderas—General outline

Resistivity pseudoborehole—From magnetotelluric profile

 models at sounding location, Appendix 4

Española Basin—Limit of Neogene deposits
Model Construction 27

Figure 10. Simplified density-depth functions (step functions) developed from examination of borehole 

density logs in geologic units of different ages in the Albuquerque and Española Basins (table 2). 

The function shown for the Santa Fe Group was used in the gravity inversion and two-dimensional 

geophysical models. 

scale). Finally, gridding techniques used in constructing both 

the regional gravity field and the thickness of basin fill do 

not properly account for abrupt density variations, such as 

those that may be caused by faults. Thus, faults affecting the 

basin-fill thickness may appear as smooth gradients and faults 

in the under-lying pre-rift rocks may result in unreasonable 

local excursions (either too low or too high) of the base of the 

thickness model. 

Profile Models

We developed several 2D gravity and magnetic models 

(fig. 13) to better understand the sources of aeromagnetic 

anomalies and help constrain the 3D integrated model. The 2D 

models follow profiles used as input to the 3D model; several coincide with and are modified from Grauch and Bankey 

(2003). A few partial models were also constructed to resolve 

specific uncertainties near the mountain front. The data used 

for the profile models were extracted from the isostatic residual 

gravity (fig. 6) and the reduced-to-pole aeromagnetic data 

(fig. 5). The profiles cross the central parts of aeromagnetic 

anomalies of interest and lie in the vicinity of as many gravity 

stations as possible. Gridded data were extracted rather than 

gravity-station or flight-line data to ensure that gravity gradients were represented consistently and that aeromagnetic data 

did not contain unnecessary detail. 

The 2D models employ polygonal bodies with discrete 

vertices and infinite strike in the direction perpendicular to the 

profile, following a standard 2D forward-modeling procedure 

(Talwani, 1965, as implemented by GM-SYS software of 

Geosoft®). With this approach, the vertices, magnetic properties, and densities are varied by the modeler until the computed 

magnetic and gravity fields of the model sufficiently match the 

observed data. To minimize ambiguities during this process, 

we placed fairly rigid constraints on initial model construction 

on the basis of independent information, concepts developed 

previously (Biehler and others, 1991; Baldridge and others, 

Santa Fe Group

(Basin fill)

Lower Tertiary units

Mesozoic units

Paleozoic units

Precambrian rocks

Basin Floor Elements

1,000

2,000

3,000

4,000

5,000

6,000

7,000

0

5,000

10,000

15,000

2,000 2,100 2,200 2,300 2,400 2,500 2,600 2,700

AVERAGE BULK DENSITY, IN KILOGRAMS PER CUBIC METER

DEPTH, IN METERS

DEPTH, IN FEET

20,000

0

EXPLANATION
28 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 11. Elevation of the base of basin fill derived from gravity inversion. The surface was constructed by subtracting 

the thickness model derived from the gravity inversion (not shown) from a smooth topographic surface. Artifacts related 

to topography may be introduced by this subtraction because the modeled surface tends to be smoother than the regional 

topographic surface. Note that the deepest part of the basin is just outside the northwest part of the aeromagnetic data 

area. As discussed in the text, ignore results in vicinity of Valles and Toledo calderas.

–500 0 500 1,0001,500 2,000 2,250 2,500 3,000

0 5 10 KILOMETERS

0 5 10 MILES

Model not 

valid for

caldera area

35°30'

36°00'

107°00' 106°30' 106°00' 105°30'

A

Santa Fe

Well penetrating base of Santa Fe Group—Appendix 1

EXPLANATION

Depth pick from seismic studies—Appendix 4

Gravity station

Aeromagnetic data area

Valles and Toledo calderas—General outline

A

Resistivity pseudoborehole—From magnetotelluric profile 

 models at sounding location, Appendix 4

Española Basin—Limit of Neogene deposits

Bedrock high discussed in text

METERS ABOVE SEA LEVEL
Model Construction 29

Figure 12. Bedrock gravity derived from gravity inversion. The final regional field resulting from the gravity inversion 

is shown; it is a companion of the thickness model discussed in figure 11. The map approximates the gravity effects of 

pre-rift bedrock as observed from the ground surface. Ignore results in vicinity of the Valles and Toledo calderas. Forward 

calculations of the three-dimensional model suggest that gravity values might be much lower in the northeastern part of 

the study area. If so, the thickness of pre-rift section may be greater in that area than is indicated on this map. 

–10 0 5 10 15 20 25 30 MILLIGALS

0 5 10 KILOMETERS

0 5 10 MILES

Model not 

valid for

caldera area

35°30'

36°00'

107°00' 106°30' 106°00' 105°30'

?

Relict

Laramide

uplift?

Cerrillos

Uplift

Santa Fe

Well penetrating base of Santa Fe Group—Appendix 1

EXPLANATION

Depth pick from seismic studies—Appendix 4

Aeromagnetic data area

Valles and Toledo calderas—General outline

Resistivity pseudoborehole—From magnetotelluric profile 

 models at sounding location, Appendix 4

Española Basin—Limit of Neogene deposits

Possible northeastern limit of thick pre-rift 

 sedimentary section
30 Geophysical Interpretations of the Southern Española Basin, New Mexico

MT

Alamo Creek

EW6

San Marcos

Seton Village

NSFCC

AAext

Tesuque

Pojoaque

3D Model Control Point-Source Input Profile-Based Input Miscellaneous

Section line used 

 for digitized input—

 Labeled if discussed 

 in text 

Area of detailed 

 analysis—Additional 

 closely spaced lines 

 not shown

Well that penetrates through 

 the Santa Fe Group

Well that bottoms within 

 the Santa Fe Group

Magnetotelluric sounding 

 location where resistivity 

 pseudoborehole was

 constructed

Magnetotelluric profile 

 model used to construct 

 resistivity pseudoboreholes 

Seismic-reflection time 

 section or depth picks

Geologic cross section

2D gravity and 

 magnetic model

3D model area

Study area

Aeromagnetic data area

Mapped contact of 

 Santa Fe Group 

 sediments

EXPLANATION

0

0 5 10 KILOMETERS

5 10 MILES

106°00'

36°00'

35°45'

35°30'

106°30' 106°15'
Model Construction 31

2001; Grauch and Bankey, 2003), expected physical properties 

of geophysical interpretative packages, and the results of our 

data analyses. Geologic mapping helped constrain several of 

the geophysical models near the basin margins on the eastern 

and southwestern sides of the study area (fig. 1). Geologic 

cross sections in these areas are from Sawyer and others (2006) 

and coauthor Daniel Koning, who updated cross sections from 

Read and others (2005) and developed new cross sections north 

of the Tesuque profile. Although these geologic cross sections 

span much of the basin (fig. 13), the deeper parts of the sections were based in large part on geophysical modeling, thus 

illustrating the iterative nature of the geophysical and geologic 

interpretations developed for this report. Other constraints 

on the geophysical profile models included limited well data, 

MT models, and seismic depth picks, as coded and located on 

figure 13. 

Grid points representing the base of the thickness model 

from the gravity inversion were used as a general guide to 

locate the base of the modeled Santa Fe Group. Limitations in 

the gravity inversion results were apparent near the mountain 

front where the method could not resolve detail, in areas of 

poor station coverage, and on the west side of the study area 

where large excursions in the thickness model suggest major 

density contrasts (such as faults) in the underlying pre-rift bedrock. Magnetic depth solutions from the multisource Werner 

method were used to guide the modeling in the top 2,000 m 

(6,560 ft), especially where solutions cluster. In addition, the 

grid-based local wavenumber solutions guided definition of dip 

on magnetic interfaces. 

After initial model setup, vertices of the polygonal 

bodies were varied by hand to best match computed model 

curves to both the observed gravity and the magnetic curves 

and to fit all the constraints as best as possible. Model bodies 

are based mainly on general differences in lithology. Table 

3 lists the physical properties assigned to the model bodies 

and the geologic units they are designed to represent. The 

corresponding ages and geologic unit names are given to 

help put the model into its geologic context and are subject 

to reinterpretation. The magnetic properties of the bodies 

representing Precambrian basement are not well constrained 

and yet strongly influence the computed magnetic curve. To 

minimize the ambiguities, we varied the physical properties 

of these bodies as little as possible between profiles. There are 

also discrepancies in the representation of the Santa Fe Group 

and Paleozoic limestones that are required by their physical 

properties. Where Precambrian rocks have very low magnetic 

susceptibilities, they cannot be distinguished by density or 

magnetic properties from Paleozoic limestones (fig. 4 and 

table 2) that directly overlie them. Moreover, even where 

the Precambrian basement rocks are magnetic at depth, the 

limestones are too thin to discriminate on the basis of gravity 

and magnetic modeling. Therefore, the Paleozoic limestones 

are not separated from the modeled bodies of Precambrian 

basement. Where the Santa Fe Group is thick, the section 

cannot be represented as one model layer owing to expected 

increases in density with depth caused by compaction (table 

2). Therefore, we divided the Santa Fe Group into a lower 

density shallow layer (density of 2,170 kg/m3

) and higher 

density deeper layer (density of 2,300 kg/m3

); the depth cutoff 

is 1,250 m (4,100 ft). This division of the Santa Fe Group 

sediments into two layers represents a density boundary, not a 

geologic interface. 

The geophysical models are designed to focus on the 

main features of the data and are inherently somewhat ambiguous. Grauch and Bankey (2003) describe in detail the limitations of 2D geophysical modls for the area. In this study, the 

3D model helped resolve some of these ambiguities, because 

it incorporates additional information off section and tracks 

variations in all directions. The 2D geophysical models provide general guides for the depths to basement, the configuration of magnetic units, and the thickness of the Santa Fe Group 

aquifer—a goal of this study. They also guide the construction 

of the 3D model and aid the understanding of the sources of 

aeromagnetic anomalies. Detailed discussion is reserved for 

following sections on interpreted subsurface geology. 

Integrated Three-Dimensional Model 

The primary goal in constructing a 3D model was to 

develop a surface representing the base of the basin-fill 

aquifers (Santa Fe Group) that integrated the many sources of 

information available. However, we expanded the efforts to 

incorporate also several concepts about the configuration of the 

Precambrian basement and the Tertiary older volcanic rocks 

that developed from the aeromagnetic interpretations. Unlike 

the geophysical interpretations discussed in the sections on 

interpreted subsurface geology, the 3D model ignores basalts 

of the Cerros del Rio volcanic field and intrusions related to 

the Cerrillos Hills and Espinaso Formation, and it attempts 

to distinguish Paleozoic limestones from Precambrian basement. Thus, the 3D model defines only three surfaces below 

the topography: the base of the Santa Fe Group, the base of the 

Tertiary older volcanic rocks, and the top of Precambrian basement. However, owing to the original goal of the 3D model and 

uncertainties in the lower two surfaces throughout most of the 

study area, the output for this report is limited to a digital grid 

of the base of the Santa Fe Group. The other two 3D surfaces 

are shown in subsequent figures only for illustration of key 

concepts. 

The software package 3D GeoModellerTM was used to 

develop the 3D model. Surfaces representing geologic contacts 

are interpolated between points defined in three-dimensional 

space using orientation data, then stored as equations that 

define the surfaces. The surfaces can be easily recomputed 

using new information or modified data. The primary inputs to 

Figure 13 (facing page). Locations of section lines and data 

used to construct three-dimensional integrated model. Labeled 

section lines and geophysical profile models are referred to in text 

or subsequent figures. Magnetic-depth-estimate locations not 

included because of their dense coverage. 
Table 3. Bodies used in two-dimensional geophysical profile models. [Model body codes and physical properties assigned to polygonal bodies of the two-dimensional geophysical profile models (figs. 17, 20, 22, 30), the geologic units (fig. 3), anthey are intended to represent. Assigned magnetic susceptibilities are mathematical representations that incorporate both magnetic susceptibility and a remanent component, if values represent dominant reverse-polarity magnetization. A value of 51,715 nanoteslas was assumed for the Earth’s field. ft, feet; kg/m3, kilograms per cubic meter; m, meter; 

Model body code Equivalent geologic unit Basin element Body representation As

d

(

Elements of basin fill QTv Qbt, QTv Tertiary younger volcanic rocks. Quaternary-Tertiary rift-related volcanic rocks, mainly basalts of Cerros del Rio volcanic field. The negative magnetic susceptibility is used to represent strong, reverse-polarity remanent magnetization. 

SFG QTsf, Tsf Santa Fe Group sediments Santa Fe Group sediments, shallower than 1,250 m (4,100 ft), following the density-depth function used in gravity inversion. SFGdeep Tsf Santa Fe Group sediments Santa Fe Group sediments, deeper than 1,250 m (4,100 ft), following the density-depth function used in gravity inversion. mag ? Unknown basin fill Magnetic sediments(?) within the basin fill. Magnetic susceptibility estimated on basis of magnetic portions of Santa Fe Group. 

Elements of basin floor Tov Tmov, Te Tertiary older volcanic rocks Tertiary older volcanics, mainly the Espinaso Formation. Possible reverse- polarity component not represented. 

pre-rift Tgd, cgu, hap. Pre-rift sedimentary section (incomplete). Pre-rift clastic sedimentary section, variably eroded. Includes Eocene through Paleozoic formations, except for limestones of the Paleozoic Alamitos and La Pasada Formations. pe/Pz pe, hap Precambrian basement; base of pre-rift sedimentary 

section.

Precambrian basement rocks that have negligible magnetization and overlying 

Pennsylvanian limestones. These rock types are indistinguishable from each other.

pe-weak1 pe Precambrian basement Precambrian basement rocks that have weak magnetization, less than peweak2. pe-weak2 pe Precambrian basement Precambrian basement rocks that have weak magnetization, but more than 

pe-weak1.

pe-mod1 pe Precambrian basement Precambrian basement rocks that have moderate magnetization and are somewhat denser than pe-weak. 

pe-mod2 pe Precambrian basement Precambrian basement rocks that have moderate magnetization and are moderately dense. 

pe-mag pe Precambrian basement Precambrian basement that is highly magnetic 
Model Construction 33

3D GeoModeller are points representing geologic contacts digitized along section lines and in map view, strike-and-dip points, 

and contacts defined by the well data. Where the interpreter 

places digitized points along a particular section is guided by 

georegistered images or points projected onto the plane of the 

section. For this study, images included geologic cross sections 

and 2D geophysical models. Projected points included magnetic depth solutions, thickness-model grid points, and seismic 

depth picks. Magnetic depth estimates resulting from the gridbased local wavenumber method and the two-step extended 

Euler method both represented tens of thousands of points, so 

solutions were most usefully viewed as clusters of points rather 

than as single points. Profile-based magnetic depth estimates 

were incorporated into the 2D geophysical models, which then 

could be used as an added guide when we digitized from the 

2D models. In addition, wells that do penetrate the base of the 

Santa Fe Group and the resistivity pseudoboreholes derived 

from the MT models, which both give imprecise information 

on geologic contacts, were used as guides for digitizing. The 

model sections along which points were digitized for the 3D 

model (section lines) and the types of information provided 

along or near the sections are shown in figure 13. The approach 

and data that constrain the interpretation of the three principal 

surfaces (base of the Santa Fe Group, the base of the older 

volcanic rocks, and the top of Precambrian basement) are 

described in table 1. 

Several modeling strategies developed during construction 

of the 3D model. First, we expanded the model area outside 

the immediate study area (fig. 1). This expansion allowed us 

to smoothly merge our model with subsurface models already 

published for the Santo Domingo Basin (Minor, 2006) and 

to fully characterize gravity anomalies that extend northwest 

and east of the study area (fig. 6). On the other hand, because 

many geologic complexities in the extended areas were deemed 

unimportant for understanding the study area, the model is not 

adequate for presentation in these areas.

Second, two areas (gray areas on fig. 13) required more 

detail to define the surfaces. In these areas, many closely 

spaced sections were constructed and examined one by one. 

These detail areas are located primarily in the southeastern part 

of the study area, where well information is more abundant, 

and along the eastern mountain front, where we questioned 

gravity model results. 

Third, a strategy was developed for use of the magnetic 

depth estimates, depending on the magnetic interface 

represented. A need for such a strategy became apparent when 

we realized that the magnetic depth estimates were commonly 

locating magnetic interfaces internal to the Tertiary older 

volcanic rocks and Precambrian basement rocks. Examples 

of the former situation are shown in two sections (fig. 14) 

from the southeastern detail area (fig. 13). In these examples, 

the picks in wells and the thickness model obtained from the 

gravity inversion closely constrain the contact between the base 

of the Santa Fe Group and the underlying Espinaso Formation 

(representing the package of Tertiary older volcanic rocks 

in this area). This interface is projected from the 3D model 

onto the sections (brown lines on fig. 14). Magnetic depth 

estimate solutions from both the local wavenumber and the 

two-step extended Euler methods both cluster at this interface 

in some places and in other places indicate magnetic sources 

that are much deeper. If the magnetic depth estimate solutions 

are used to represent the contact of the Santa Fe Group and 

Espinaso Formation, as was done previously (Phillips and 

Grauch, 2004), then the magnetic results suggest surface 

irregularities with more than 100 m (330 ft) of relief. Although 

irregularities in the top of the Espinaso Formation surface are 

apparent from closely spaced drill holes elsewhere (Koning and 

Hallett, 2001), relief does not appear to exceed 45 m (150 ft). 

Instead, we now consider that the magnetic depth solutions 

provide information on the internal structure of the Tertiary 

older volcanic rocks, which was used as a general guide for 

developing the shape of the base of the older volcanic rocks. 

Where magnetic rocks appear absent on the basis of lack 

of aeromagnetic anomalies, the local wavenumber depth solutions commonly showed a characteristic pattern of deepening scatter into the area with no magnetic sources (fig. 14B). 

This pattern may be due to median filtering that incorporates 

information from neighboring depth solutions as the window 

passes across the boundary between magnetic and nonmagnetic rocks. In some cases, the local wavenumber depth solutions appear more clustered within the Precambrian basement, 

generally following the dip of the basement surface but at 

greater depths, consistent with hypotheses of nonmagnetic 

Precambrian rock types overlying magnetic ones, as discussed 

in a following section. Other magnetic depth estimates show 

good clustering within the pre-rift section, but the sources are 

enigmatic.

Finally, to simplify the modeling, only three faults 

were defined in the model: (1) a west-down normal fault 

representing the La Bajada fault system and its connection 

to the Cochiti Cone fault on the north (figs. 2 and 3); (2) an 

east-down normal fault representing the Pajarito-Embudo 

fault system that bounds the west-dipping half-graben of the 

Española Basin on the northwest (fig. 2); and (3) an east-down 

normal fault representing the west limit of zones of intense 

deformation (the Agua Fria and Barrancos fault systems) 

that extend northerly across the central part of the study 

area (fig. 2). Because major gravity gradients are generally 

associated with both the La Bajada and Pajarito-Embudo fault 

systems, these gradients were used to determine the exact 

locations of the model faults. All other faults were represented 

by steep slopes on the surfaces. 

Despite the advantages of integrated multiple inputs, the 

3D model is still subject to alternative scenarios, subjective 

judgment, lack of information in certain areas, and artifacts 

of mathematical interpolation. The best-constrained surface 

throughout the area is the base of the Santa Fe Group. The 

base of Tertiary older volcanic rocks is fairly well constrained 

within most parts of the southeastern part of the area, but it 

is poorly constrained farther north owing to its greater depth. 

Because of the variability of magnetic properties of the 

Precambrian basement, this surface was constrained primarily 
34 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 14. Examples of two short east-west sections from the area of detailed analysis in the southeastern part of the 

study area. The three-dimensional model surfaces are projected onto the sections and show detail that is constrained by 

neighboring, as well as by these, sections. Digitized points used as model input not shown. Scatter and clustering is typical 

of magnetic depth estimates. The Euler solutions that closely follow a flatter surface have been forced to the surface as a 

consequence of the two-step method. A, Section follows northing 3930000 m; B, Section follows northing 3925500 m; 

Universal Transverse Mercator Zone 13, North American datum of 1927.

405000 407500 410000 412500

1,000

1,500

2,000

ELEVATION, IN METERS

1,500

1,000

2,000

2,500

ELEVATION, IN METERS

EASTING, IN METERS

EASTING, IN METERS

VERTICAL EXAGGERATION = 3

EXPLANATION

Well with model lines

Base of pre-rift sedimentary 

 section and top of 

 Precambrian basement

Representative fault—Within

 Agua Fria fault system

McMillan

X3CN X3CE R5 X11A

Magnetic sources

within pre-rift section?

Magnetic sources

internal to basement

Indication of

internal 

structure?

Source of circular

negative anomaly

Topography

A

410000 412500 415000 417500 418000

VERTICAL EXAGGERATION = 3x

ND49

ND49

G2-4

Indication of

internal 

structure?

Faults?

Magnetic sources

within Galisteo 

Formation?

Deepening

scatter of

depth solutions Magnetic

sources

within

volcanic section

Topography

B

Base of Santa Fe Group and 

 top of Tertiary older volcanic rocks

Base of Tertiary older volcanic 

 rocks and top of pre-rift 

 sedimentary section

Point extracted from the thickness model grid—

 From gravity inversion, projected from within 

 250 m (820 ft) of the plane of section

Magnetic depth estimate solution from grid-based local 

 wavenumber method—Projected from within 

 250 m (820 ft) of the plane of section

Magnetic depth estimate soution from grid-based 

 two-step extended Euler method—Projected from 

 within 100 m (330 ft) of the plane of section 
Interpreted Elements of the Basin Floor 35

by the seismic interpretation and gravity-derived thicknessmodel base, both of which have limited coverage throughout 

large areas. The model smoothes faulted surfaces and is 

insensitive to other local variations that may be recognized 

from closely spaced wells. Thus, none of the models in this 

study should be used to answer local subsurface questions.

Interpreted Elements of the Basin Floor 

The dominant features of the aeromagnetic map are 

associated with Precambrian basement rocks, intrusive rocks 

in the Cerrillos Hills, and Miocene-Pliocene basalts in the 

Cerros del Rio volcanic field (fig. 15). More subdued features 

are related to pre-rift volcanic and volcaniclastic rocks in the 

southern part of the data area, magnetic sediments contained in 

the Puye Formation of the Santa Fe Group in the northwestern 

part of the data area, and linear features related to faulting. 

Very small but prominent anomalies that look like circular or 

elongate raised areas are caused by anthropogenic structures, 

such as buildings, bridges, and landfills. 

Information about the overall structure of the basin is 

contained in the aeromagnetic data but is not readily apparent 

on the map. In contrast, the isostatic residual gravity map is 

dominated by the expression of basin geometry; the highest 

gravity values represent uplifted, relatively dense Precambrian 

basement, and the lowest values represent areas of the deepest 

parts of the basin (fig. 16). 

The following sections discuss interpretations of geologic 

features primarily from the aeromagnetic data and draw from 

the results of the data analysis and modeling. The features are 

generally discussed from oldest to youngest. 

Precambrian Basement

Some of the most prominent anomalies on the 

aeromagnetic map are caused by Precambrian basement rocks. 

These anomalies include the broad, roughly vase-shaped 

magnetic high in the center of the area; north- and northwesttrending, strong magnetic highs along the mountain front from 

Santa Fe south to Eldorado at Santa Fe; and the subtle, broad 

anomaly just northwest of Española (fig. 15). The anomalies 

are prominent in the lowpass-filtered map (fig. 8), suggesting 

that the underlying magnetic rocks are massive and extend 

to great depths. Several of these large magnetic highs are 

truncated along linear trends, such as the southern termination 

of the broad magnetic anomaly north of Santa Fe and the 

western terminations of large anomalies along the mountain 

front (bold blue dashed lines on fig. 15). The terminations 

are interpreted as a primary contrast in rock type between 

magnetic and weakly magnetic Precambrian basement that 

has only minor structural relief. This interpretation is based 

on comparisons with the gravity data, which show only minor 

gradients in association with these boundaries (fig. 16). The 

major differences in the magnetic properties are demonstrated 

by the model for the Tesuque profile (fig. 17), which requires 

within the Precambrian basement a massive, highly magnetic 

body bounded by a steep contact on the east and a gently 

dipping one on the west. The steep boundary suggests an origin 

as an intrusive contact or a major fault that formed before 

erosion of the Precambrian surface. 

Basement rocks are also associated with both magnetic 

highs and lows where they are exposed along the entire 

mountain front. The heterogeneity is not surprising, 

considering that Precambrian basement rocks in this area 

include a wide range of compositions (Read and others, 2003, 

2004). Heterogeneity of magnetic properties is evident from 

reconnaissance magnetic-susceptibility measurements at 

outcrops, which show that basement rocks range from weakly 

magnetic to strongly magnetic (fig. 4). However, a simple 

correlation between magnetic susceptibility and mapped rock 

unit is not apparent.

A characteristic pattern of linear alternating high and 

low anomalies is noticeable in the magnetic map for areas 

mapped as Precambrian rock (figs. 18 and 19). Many of these 

linear anomalies are designated as undetermined magnetic 

lineaments on plate 1. In the southeastern part of the study, 

this pattern is evident from Eldorado at Santa Fe to north of 

Seton Village (for example, A on fig. 18). The linear pattern is 

well defined in the first vertical derivative map (fig. 7). These 

patterns trend northwest to north-northwest and appear to 

terminate along a north-northeast-striking line at the mountain front just north of Seton Village. The second area is in 

the northeast part of the study area, from latitudes 35°45' to 

35°50' (for example, A on fig. 19). In contrast to patterns in 

the southeastern area, these patterns trend north-south. In both 

areas, similar linear patterns lie just west of the basin margin, 

where the source of these linear anomalies appears deeper than 

its neighboring sources because the anomalies are broader and 

more subdued (B on figs. 18 and 19). Some contribution to the 

linear anomalies northeast of Santa Fe may be due to the contact between Paleozoic limestone and Bishops Lodge member 

of the Tesuque Formation (between B and C north of Santa 

Fe on fig. 18). A significant contrast in magnetic susceptibility between these two units is suggested by reconnaissance 

measurements (fig. 4).

In the southeastern part of the study area, both 

topography and geologic trends in the Precambrian rocks 

parallel the orientation of the linear anomaly pattern. However, 

the topography does not correspond directly, suggesting that 

the sources of the linear anomalies are primarily below the 

surface. For example, five ridge-like magnetic anomalies 

parallel Interstate 25 southeast of Seton Village (between A 

and the basin margin, fig. 18). In contrast, the topography 

in the same area consists of one wide valley surrounding 

Interstate 25 and one wide zone of hills aligned northwest 

at the mountain front. There is also no definitive correlation 

between elongate trends of differing Precambrian lithology 

that are partially exposed in the same area (pl. 4 of Spiegel 

and Baldwin, 1963; Read and others, 2004). However, the 

anomalies may relate to alternating lithologic layers and 
36 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 15. Selected features of aeromagnetic map (fig. 5) and their interpreted sources, as discussed in the text. Marked 

terminations of basement-related anomalies represent large contrasts in magnetic properties (light blue dashed lines) that are 

interpreted as major contacts between different rock types within the basement. Locations of two-dimensional geophysical 

profile models (figs. 17, 20, 22, and 30) are indicated. Image is color-shaded relief with illumination from the east.

Magnetic

Precambrian

basement

Magnetic

Precambrian

basement

Magnetic

Precambrian

basement

Magnetic

Precambrian

basement

Puye

Formation

Magnetic

Precambrian

basement

Tertiary 

older

volcanic and 

volcaniclastic

rocks

Buildings

La Bajada Fault

Reversepolarity

vents

Normalpolarity

vent

Faults and

magnetic

sediments

Basalt

under

sediment

Tijeras-Cañoncito fault

Rio

Grande

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Cerrillos

Hills

Cundiyo

Seton

Village La Cienega

Cerros del Rio

volcanic field

106°00'

36°00'

35°45'

35°30'

106°15'

Magnetic

Precambrian

basement

0 5 10 KILOMETERS

0 5 10 MILES

Tesuque profile

 (fig. 17)

Alamo Creek 

profile (fig. 22)

Seton Village 

profile (fig. 30)

Pojoaque profile

 (fig. 20)

–1,200

0

–800

–400

400

800

1,200

1,600

2,000

2,400

2,800

NANOTESLAS

EXPLANATION

Mapped contact of Santa Fe Group 

 sediments—Generalized from figure 3

Aeromagnetic data area

Location of 2D geophysical model

Major intrabasement contrast 

 in magnetic properties

Major mapped fault

Geologic contact of Bandelier 

 Tuff overlying Puye Formation
Interpreted Elements of the Basin Floor 37

Figure 16. Isostatic residual gravity extracted for the study area from figure 6. Contours generally show basin form. The 

magnetic lithology boundaries of figure 15 are not apparent, suggesting that the magnetic boundaries do not correspond 

with major structural relief on the basement. The locations of two-dimensional geophysical profile models (figs. 17, 20, 22, 

and 30) are indicated.

Rio

Grande Grande

Cerrillos

Hills

Cundiyo

Seton

Village

La Cienega

Cerros del Rio

volcanic field

Tesuque profile

 (fig. 17)

Alamo Creek 

profile (fig. 22)

Seton Village 

profile (fig. 30)

Pojoaque profile

 (fig. 20)

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

MILLIGALS

0

10

20

30

40

–10

–20

–30

Mapped contact of Santa Fe Group 

 sediments—Generalized from figure 3

Aeromagnetic data area

Location of 2D geophysical model

Gravity station

EXPLANATION
38 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 17. Two-dimensional gravity and magnetic model for Tesuque profile. Model bodies are described in table 3. The profile is 

also located on the inset and on figures 13, 15, and 16. The model demonstrates variable magnetic properties of the Precambrian 

basement, both at large scale and as small-scale folding where exposed on the east. The Agua Fria fault system is manifested in 

both shallow and deep basement disruptions. The bedrock high on the west can be alternatively modeled in this location as an 

east-dipping ramp. The profile is also constrained by seismic data (not shown). Note that the horizontal division between unit SFG 

and unit SFGdeep is a density boundary, not a geologic contact.

WEST EAST

QTv 

DISTANCE, IN KILOMETERS

MT27

MT27

Tertiary younger 

 volcanic rocks

Santa Fe Group 

 sediments, divided 

 by density

Unknown basin fill

Magnetic depth estimate solutions from the profile-based multisource Werner method by using 

 several window sizes—Tail indicates estimated dip. Red and blue colors refer to infinite-step versus 

 sheet source assumptions, respectively

Base of Santa Fe Group from gravity inversion—Grid points are projected onto the profile from 

 a maximum distance of 300 m

Resistivities >500 ohm-m from magnetotelluric model, representing basalt

Resistivities of 5 to 20 ohm-m from magnetotelluric model, representing Santa Fe Group

Resistivities of 2 to 5 ohm-m from magnetotelluric model, representing fine-grained 

 rocks of undetermined age

Resistivities >50 ohm-m from magnetotelluric model, representing Paleozoic 

 section or Precambrian basement rocks

Observed 

Observed 

TESUQUE

PROFILE 

QTv LOCATION

EXPLANATION

GRAVITY, 

IN MILLIGALS 

–20 

–10 

0 

10 

20 

30 

40 

MAGNETIC FIELD, 

IN NANOTESLAS 

ELEVATION, IN FEET 

–6,000 

–4,000 

–2,000 

0 

2,000 

4,000 

6,000 

8,000 

9,000 

–500 

–250 

0 

250 

500 

ELEVATION, IN METERS 

–1,000 

0 

1,000 

2,000 

SFG mag 

pre-rift 

SFGdeep

Tov

Agua Fria fault system Santa Fe 

Mountains

Cerros del Rio volcanic field

Good fit not attempted

data problem

base not resolved

Tov alternatively modeled

as east-dipping

ramp

Calculated 

Calculated 

VERTICAL EXAGGERATION = 1.5 

0 5 10 15 20 25

SFG

SFG

deep

mag

Tov

pre-rift

pe/g

pe/g

pe-weak1

pe-weak1

pe-mod1

pe-mod1

pe-mag

pe-mag

Magnetotelluric sounding location and station number

Tertiary older 

 volcanic rocks

Pre-rift sedimentary 

 section (except Paleozoic 

 limestones)

Precambrian 

 basement and 

 Paleozoic limestones

Precambrian 

 basement of 

 differing physical 

 properties

84 285
Interpreted Elements of the Basin Floor 39

Figure 18. Closeup of the mountain front in the southeastern part of the study area. A, General geology underlain by a shaded 

digital elevation model. Geologic contacts from figure 3; additional information on the Bishops Lodge Member from Read and 

others (2004). Location and average value at one measured magnetic-susceptibility traverse is shown along the Santa Fe 

River canyon east of Santa Fe (information collected by Mark Hudson and V.J.S. Grauch (unpub. data, 2007)). B, Shaded-relief 

aeromagnetic image from figure 5. Higher gradient magnitudes indicated by thicker lines may indicate steeper basement slope 

or, more likely, faults with greater displacement than is indicated by lesser gradient magnitudes (thinner lines).

Seton Village fault

105°57'30" 105°57'30"

Buildings

Landfill

Santa Fe River fault 35°37'30"

35°45'

35°30'

Seton

Village

Eldorado

at Santa Fe

Santa

Fe

25

25

285

84

285

–1,200

0

–800

–400

400

800

1,200

1,600

2,000

2,400

2,800

NANOTESLAS

0 2 KILOMETERS

0.3 x 10–3

Ancha Formation

Tesuque Formation,

 undivided

Alluvium Paleozoic limestone

Precambrian basement rocks

EXPLANATION

A B

Bishops Lodge 

 Member of the 

 Tesuque Formation

Mapped contact of Santa Fe 

 Group sediments

Mapped fault—Dashed where 

 inferred; ball on downthrown side

Fault location interpreted from 

 gravity data—Inferred from horizontal 

 gradient magnitude (HGM) of gravity 

 data. Three different line weights, 

 from thin to thick, indicate small to 

 large HGM values

Magnetic susceptibility sample site—Value

 shown in SI units

A Feature discussed in text

A

D

D

D

C

C

C

C

H

I

B

G F

F

E

0 2 MILES
40 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 19. Closeup of the mountain front in northeastern part of study area. A, General geology underlain by a 

shaded digital elevation model. Geologic contacts are from figure 3, with additional information on Bishops Lodge 

Member from Read and others (2004). The location and average value at one measured magnetic susceptibility 

traverse is shown at Cerro Piñon (information collected by Mark Hudson and V.J.S. Grauch (unpub. data, 2007)). B, 

Shaded-relief aeromagnetic image from figure 5. Stream terrace deposits are expressed as subtle aeromagnetic 

anomalies that correlate well with topography.

Chimayo

Cundiyo

285

84

285

84

–1,200

0

–800

–400

400

800

1,200

1,600

2,000

2,400

2,800

NANOTESLAS

0.02 x 10–3

Cerro

Piñon

35°52'30"

105°57'30" 105°57'30"

35°45'

36°00'

Stream

terrace

deposits

terrace

Stream

deposits

Tesuque Formation,

 undivided

Alluvium Paleozoic limestone

Precambrian basement rocks

EXPLANATION

A B

Bishops Lodge Member 

 of the Tesuque Formation

Mapped contact of 

 Santa Fe Group sediments

Mapped fault—Dashed where 

 inferred; ball on downthrown side

Magnetic susceptibility sample 

site—Value shown in SI units

Feature discussed in text

0 2 KILOMETERS

0 2 MILES

A

D

A B

C

C

E
Interpreted Elements of the Basin Floor 41

tight folds that were later injected with magma preferentially 

along layering in Precambrian time (Spiegel and Baldwin, 

1963; Read and others, 2005; Evans and Lindline, 2004). 

Experiments with modeling the basement mountain front 

at the easternmost end of the 2D geophysical model for the 

Tesuque profile (fig. 17) confirm that buried folds rather than 

multiple dikes provide a better fit to the anomaly patterns. 

A better understanding may come from re-examination of 

Precambrian lithologic contacts, more magnetic susceptibility 

measurements, and consideration of remanent magnetization, 

all of which are beyond the scope of this study. 

Weakly magnetic rocks are evident in local areas of the 

exposed Precambrian basement at several places along the 

mountain front (C on figs. 18 and 19). In these areas, generally 

low magnetic anomaly values and little correlation between 

the anomalies and the shapes of topographic features are 

evidence that the exposed rocks have low magnetic susceptibilities. This interpretation is corroborated by measured low 

magnetic susceptibilities averaging 0.0003 (SI) for gneiss that 

crops out along the Santa Fe River (fig. 18) and averaging 

0.00002 (SI) for muscovite granite at Cerro Piñon (fig. 19). 

The character of aeromagnetic patterns associated with 

exposed Precambrian rocks changes abruptly at several 

linear trends, which may indicate terrane boundaries within 

the basement or more recent faulting. Only a few of the 

most prominent trends are annotated (D on figs. 18 and 19; 

pl. 1). East of Santa Fe, these northwesterly trends appear to 

cross the head of the Santa Fe River canyon with only minor 

disruption, suggesting that any lateral displacement along the 

Santa Fe River fault is minor. On the other hand, northerly 

trending linear anomalies are disrupted along an east-northeast 

trend that connects to the mouth of Santa Fe River Canyon 

(E on fig. 18). 

All along the basin margin, the basement rocks 

associated with detailed aeromagnetic patterns can be traced 

by their aeromagnetic expression and by the magnetic depth 

estimates no more than 1–2 km (0.6–1.3 mi) basinward 

into the subsurface, to depths of 150–300 m (500–990 ft). 

West of this zone, the aeromagnetic expression of basement 

rocks changes abruptly to very broad magnetic highs (F near 

Seton Village on fig. 18) or to broad areas of low values onto 

which more subtle anomalies are superimposed owing to the 

overlying sediments (E on fig. 19). This change in character is 

marked by bright blue dashed lines on figure 15 and plate 1. 

In some places, the detailed and broad anomalies seem to be 

superimposed, which is evident by comparing the lowpass and 

first-vertical-derivative maps, especially for the area extending 

4 km (2.5 mi) south of Cundiyo (figs. 7, 8, and 19A). The 

change from detailed to broad character is especially evident 

near the basin margin south of Seton Village (H on fig. 18). 

This difference in aeromagnetic character is not likely caused 

by a large-displacement rift fault, because modeling of the 2D 

geophysical profiles and gradients in the gravity data suggest 

mostly regional dip on the top of the basement in this area. 

Perhaps the basement terrane associated with detailed anomaly 

patterns forms a fairly thin top layer overlying the basement 

terrane represented by broader anomalies, and the top layer 

is absent just west of the basin margin. If so, the strikingly 

linear character of the western termination of this top layer 

suggests that it is related to structure developed during a prerift tectonic event. 

North of Santa Fe, the buried basement surface undulates from moderate westward dip (~20°) at the mountain 

front to shallow dip (~5°) and back to moderate dip 10–12 km 

(16–19.2 mi) west of the front; these changes are especially 

evident in the Tesuque and Pojoaque 2D geophysical profile 

models (figs. 17 and 20). South of Santa Fe, the basement 

slopes more steeply to the west and southwest, and it rises on 

the west side of the study area, as demonstrated by sections 

through the 3D model (fig. 21). In the vicinity of Eldorado 

at Santa Fe, wells indicate that the basement and overlying 

Paleozoic section are quite variable in elevation along trends 

parallel to the mountain front, but this variation cannot be 

resolved by the modeling. Moreover, the geophysical models 

do not show the individual basement faults; rather, they depict 

only the general form of the basement as a dipping surface. 

This general form shows that the greatest slope (or drop) of 

the basement surface occurs about 1.6 km (1 mi) southwest of 

the mountain front, coincident with a gravity-interpreted fault 

(I on fig. 18) rather than the change in aeromagnetic character 

(H on fig. 18).

Broad magnetic anomalies related to Precambrian basement rocks that are elongate north-south in the southeastern 

part of the study area appear to be offset across the TijerasCañoncito strike-slip fault system (fig. 15). Preliminary magnetic depth-estimate analysis indicates that the sources on both 

sides of the fault are about 1,000 m (3,280 ft) deep. If these 

depths represent similar stratigraphic levels in the Precambrian 

rocks, then the offset suggests about 5 km (3 mi) of net right 

lateral displacement on this fault.

Pre-Rift Sedimentary Section

The pre-rift sedimentary section includes strata 

deposited during Paleozoic through Eocene time. They rest 

unconformably on the Precambrian basement surface, are 

variably eroded across the Española Basin, are intruded by the 

Cerrillos Hills intrusive complex, and are overlain by Tertiary 

older volcanic rocks. The best information on the pre-rift 

sedimentary section is contained in proprietary seismicreflection data (Baldridge and others, 2001; Ferguson and 

others, 2007), which are yet to be fully analyzed. Only limited 

information from the seismic data on the total thickness of 

this package (provided by John Ferguson, University of Texas 

at Dallas, written commun., 2007) has been incorporated 

into the 3D model of this study. The seismic data show that 

the pre-rift section is thickest (almost 3,000 m (9,840 ft)) in 

the southern part of the study area, as depicted by sections 

through the 3D model (fig. 21). In this area, it forms a syncline 

with a north-south axis (Ferguson and others, 2007). To the 

north, the Paleozoic-Mesozoic sedimentary section underlies 
42 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 20. Two-dimensional gravity and magnetic model for Pojoaque profile. Model bodies are described in table 3. Profile is 

located on the inset and on figures 13, 15, and 16. This model demonstrates the faulted Barrancos monocline, which dips to the 

west. In the geophysical model, individual faults are not distinguished. Although the Agua Fria fault system is not defined in this 

area, the magnetic depth estimates and their relation to the edges of the magnetic Precambrian body (unit pe-mag) suggest 

the presence of faulting east of the Barrancos fault system. 

pe-mod1 

pe-mag 

SFG 

SFGdeep

pe-weak2 

Tov

Santa Fe

Mountains

pe-mag 

Barrancos fault system

Rio Grande 

Magnetic sediments

not modeled

GRAVITY, 

IN MILLIGALS 

–20 

–10 

0 

10 

20 

30 

40 

MAGNETIC FIELD, 

IN NANOTESLAS 

–500 

–250 

0 

250 

500 

VERTICAL EXAGGERATION = 1.5 

ELEVATION, IN FEET 

–6,000 

ELEVATION, IN METERS 

–4,000 

–2,000 

–8,000 

0 

2,000 

4,000 

6,000 

8,000 

9,000 

–1,000 

–2,000 

0 

1,000 

2,000 

0 5 10 15 20 25

WEST EAST

Calculated 

Calculated 

Observed 

Observed 

EXPLANATION

DISTANCE, IN KILOMETERS

84 285

POJOAQUE

PROFILE 

LOCATION

Santa Fe Group 

 sediments, divided 

 by density

Tertiary older 

 volcanic rocks

Precambrian basement 

 of differing physical 

 properties

SFG

SFG

deep

Tov

peweak2

pemod1

pemag

Magnetic depth estimate solutions from the profile-based multisource 

 Werner method by using several window sizes—Tail indicates estimated dip. 

 Red and blue colors refer to infinite-step versus sheet source assumptions, respectively

Base of Santa Fe Group from gravity inversion—Grid points are projected onto the 

 profile from a maximum distance of 300 m
Interpreted Elements of the Basin Floor 43

Figure 21. Sections through three-dimensional model showing general relations between the three modeled surfaces 

across the basin from south (A) to north (E). The sections, labeled in the lower left, are located on figure 13. (Only 

portions of model sections C, D, and E are shown.) Only one fault was used in the model to represent the Agua Fria and 

Barrancos fault systems. It generally follows the west sides of the zones of faulting. Fault blocks and numerous other 

faults are depicted only by change in slope of the surface. The lack of detail in these sections compared with standard 

geologic cross sections demonstrates the regional scale for which the model was intended. Universal Transverse 

Mercator Zone 13, North American Datum of 1927.

2,000

0

–2,000

ELEVATION, IN METERS

0

–5,000

5,000

ELEVATION, IN FEET

E

2,000

0

–2,000

0

–5,000

5,000

ELEVATION, IN METERS

ELEVATION, IN FEET

2,000

0

–2,000

0

–5,000

5,000

ELEVATION, IN METERS

ELEVATION, IN FEET

2,000

0

–2,000

ELEVATION, IN METERS

0

–5,000

5,000

ELEVATION, IN FEET

2,000

0

–2,000

ELEVATION, IN METERS

0

–5,000

5,000

ELEVATION, IN FEET

PROFILE 

LOCATIONS

EXPLANATION

VERTICAL EXAGGERATION = 2

0 5 KILOMETERS

Basin fill—Santa Fe Group sediments

 and Tertiary younger volcanic rocks

Tertiary older volcanic rocks and

 related intrusive rocks

Pre-rift sedimentary section and

 Cerrillos intrusion

Precambrian basement

Representative fault within

 fault system

Easting

419,000 m

Cerrillos

intrusion

(not modeled)

San Marcos

EW6

Seton Village

NSFCC

AAext

Cerrillos

intrusion

(not modeled)

La Bajada

fault

Agua Fria

fault system

Agua Fria

fault system

Agua Fria

fault system

Agua Fria

fault system

Barrancos

fault system

A

B

C

D

E

E

D

C

B

A

Cerros del Rio

volcanic field Buckman Rio Tesuque

Cerros del Rio

volcanic field

Cerros del Rio

volcanic field

SW NE

E

W

W

E

W E

SW NE

Cerrillos

Hills

paleovalley

Seton

Village

Santa Fe

River

285 84

0 5 MILES
44 Geophysical Interpretations of the Southern Española Basin, New Mexico

a north-dipping angular unconformity defining the base of 

pre-rift Tertiary units. The seismic data indicate that the 

Galisteo or Diamond Tail Formations (or both) represent 

greater than 700 m (2,300 ft) of the maximum thickness of 

the pre-rift section (Ferguson and others, 2007). A thickness 

of 2,380 m (7,810 ft) thus remains for Paleozoic-Mesozoic 

units (Baldridge and others, 2001), a value considerably larger 

than the ~1,700 m (~5,580 ft) median Paleozoic-Mesozoic 

thickness measured in the surrounding areas (Sawyer and 

Minor, 2006). 

Constraints provided by the seismic-reflection data help 

explain the strong northwest-trending gradient apparent in 

the pre-rift bedrock gravity results from the gravity inversion 

(fig. 12). The gradient implies a substantial density contrast 

within the basin floor below the Santa Fe Group; a greater volume of lower density bedrock lies on the southwest side. This 

scenario is consistent with the interpretations of the seismic 

data southwest of Santa Fe showing a lower density, pre-rift 

sedimentary section that is substantially thicker on the south 

compared with the north. On the north, the section is much 

thinner over higher density Precambrian basement. Moreover, the location and trend of this gravity gradient generally 

follow where the Paleozoic-Mesozoic units appear truncated 

in the seismic data within the Santa Fe embayment. Thus, we 

extrapolate the interpretation to suggest that the truncations 

of the Paleozoic-Mesozoic section extend the whole length of 

the remarkably linear, nearly 40-km (25-mile)-long gravity 

gradient trend. If this interpretation is correct, the trend may 

mark the southernmost limit of the Laramide uplift that has 

been hypothesized to underlie the Española Basin (Cather, 

1992; Myer and Smith, 2006). A strong gravity high associated 

with the Cerrillos uplift interrupts the gravity low associated 

with inferred thick, pre-rift sedimentary section (fig. 12). This 

scenario is consistent with previous studies that concluded 

that the Cerrillos uplift is a structurally high block formed in 

response to footwall uplift of the La Bajada fault during rifting 

(Baldridge and others, 1991; Minor, 2006). 

Synthesis of results from the thickness model obtained 

by gravity inversion, the MT profile models, and seismic 

constraints indicates the presence of a previously unreported 

bedrock high under the Cerros del Rio volcanic field. 

This high is best displayed in the thickness model as a 

north-northeast trending triangle-shaped area with about 

500–1,000 m (1,640–3,280 ft) of relief (A on fig. 11). The 

best constraints for this high come from an unpublished 

north-northwest trending 2D MT model developed by Brian 

Rodriguez and Jackie Williams (U.S. Geological Survey, 

written commun., 2007) on the basis of 12 soundings across 

the middle of the volcanic field (profile MT on fig. 13). Their 

model consistently shows a basal resistive layer (>50 ohm-m) 

overlain by a thin conductive layer (2–5 ohm-m) and a thick 

layer of less conductive crust (5–20 ohm-m), as depicted for 

station MT27 on the Tesuque (fig. 17) and station MT9 on 

the Alamo Creek (fig. 22) 2D geophysical profiles. Although 

the geologic affinity of the conductive layer is ambiguous, as 

discussed in the section “Other Data,” the base of the Santa 

Fe Group can be no deeper than the top of basal resistive 

layer, interpreted as Paleozoic rocks. Using this reasoning, the 

base of the Santa Fe Group lies no lower than 170 m (560 ft) 

below sea level (depths of 1,500–1,750 m (5,000–5,750 ft)) 

for the northern two stations on the profile, MT9 and MT27 

(appendix 4). Depth picks from a seismic line that comes 

as close as 3 km (2 mi) east of these MT stations constrain 

the base of the Santa Fe Group to elevations of about 800 m 

(2,624 ft) below sea level (a depth of 2,652 m (8,702 ft) 

for location EW_005 (appendix 4), for example). These 

constraints suggest that the bedrock high has about 600 m 

(1,970 ft) of relief in this area. Initial gravity inversion runs, 

independent of the MT data, also indicated a bedrock high in 

the area. Subsequently, MT model results at selected stations 

were used to constrain the gravity model (fig. 9), which gave 

somewhat better definition to the shape of the high in map 

view. The shape is still poorly defined owing to poor gravity 

station coverage (fig. 11). Although the shape of this gravityderived bedrock high is suspiciously coincident with the 

Cerros del Rio volcanic field, the MT results are sufficiently 

compelling to conclude that the underlying cause is in the 

pre-rift section. This hypothesis is also permissible from the 

2D geophysical models of the Tesuque and Alamo Creek 

profiles (figs. 17 and 22). The geologic configuration of this 

geophysical feature is interpreted as a west-tilted fault block 

of pre-rift sedimentary rocks in the 2D geophysical models. 

Although conjectural, it is consistent with regional geologic 

considerations. An alternative geologic model, which also 

fits the geophysical data, consists of an east-tilted block 

whose fault-bounded face coincides with the west side of the 

geophysically defined bedrock high.

Although the pre-rift sedimentary rocks are generally 

invisible to the aeromagnetic data of this study, some parts 

of the Galisteo Formation may be magnetic. North-south 

curvilinear magnetic anomalies in the southern part of the area 

near the Tijeras-Cañoncito fault (“Beds of Galisteo Fm?” on 

fig. 23A) parallel interpreted erosional contacts of the Espinaso 

Formation (discussed in a later section) and correspond with 

mapped Galisteo Formation (fig. 23B; Lisenbee, 1999). 

Similar northeast-trending anomalies parallel the EspinasoGalisteo contact just east of the fault labeled “East-down 

fault” in figure 23A. These anomalies are also evident in 

the first vertical derivative map (fig. 24A). Magnetic depth 

estimates show clustered solutions (fig. 14B) that fall within a 

depth range that has been interpreted as Galisteo Formation in 

cross-section (Lisenbee, 1999). 

Intrusions 

Both positive and negative aeromagnetic anomalies are 

associated with intrusions and volcanic vents of Tertiary age 

(fig. 23). The most prominent of these is an area of strong positive anomalies associated with mapped Eocene and Oligocene 

intrusive rocks in the Cerrillos Hills, which coalesces into one 

broad positive anomaly on the lowpass-filtered map (fig. 8). 
Interpreted Elements of the Basin Floor 45

Figure 22. Two-dimensional gravity and magnetic model of the Alamo Creek profile. Model bodies are described in table 3. Profile is 

located on the inset and on figures 13, 15, and 16. This model demonstrates well-clustered magnetic depth estimates (from profile and 

grid-based methods) and modeling that requires magnetic material at shallow depths related to the Agua Fria fault system. Using other 

geophysical constraints, the magnetic sources must be within the basin fill. The grid-based magnetic depth solutions (small red points) 

locate multiple magnetic sources within the Santa Fe Group. Faults interpreted from the aeromagnetic map correspond with abrupt level 

shifts. The bedrock high on the west can be alternatively modeled as an east-dipping ramp. The horizontal division between unit SFG 

and unit SFGdeep is a density boundary, not a geologic contact.

WEST EAST

MT9 

MT9

Magnetic depth estimate solutions from the profile-based multisource Werner method by using several window sizes— 

 Tail indicates estimated dip. Red and blue colors refer to infinite-step versus sheet source assumptions, respectively

Base of Santa Fe Group from gravity inversion—Grid points are projected onto the profile from a maximum distance of 300 m

Resistivities >500 ohm-m from magnetotell1uric model, representing basalt

Resistivities of 5 to 20 ohm-m from magnetotelluric model, representing Santa Fe Group

Resistivities of 2 to 5 ohm-m from magnetotelluric model, representing fine-grained 

 rocks of undetermined age

Resistivities >50 ohm-m from magnetotelluric model, representing Paleozoic 

 section or Precambrian basement rocks

Magnetotelluric sounding location and station number

pe-mod1 

QTv 

pe-mag 

SFG mag 

pre-rift 

SFGdeep

pe/Pz 

pe/Pz 

Tov

Cerros del Rio volcanic field

pe-weak1 

Agua Fria fault system

base not 

alternatively modeled resolved

as east-dipping

ramp

Santa Fe 

Mountains

GRAVITY, 

IN MILLIGALS 

–20 

–10 

0 

10 

20 

30 

MAGNETIC FIELD, 

IN NANOTESLAS –500 

–250 

0 

250 

500 

VERTICAL EXAGGERATION = 1.5 

ELEVATION, IN FEET 

ELEVATION, IN METERS 

–6,000 

–4,000 

–2,000 

0 

2,000 

4,000 

6,000 

8,000 

–1,000 

0 

1,000 

2,000 

0 5 10 15 20

Calculated 

Calculated 

Observed 

Observed 

EXPLANATION

DISTANCE, IN KILOMETERS

ALAMO CREEK

PROFILE 

LOCATION

Santa Fe Group 

 sediments, divided 

 by density

Tertiary younger

 volcanic rocks

Precambrian 

 basement of 

 differing physical 

 properties

Precambrian basement 

 and Paleozoic 

 limestones

Unknown basin fill

Magnetic depth estimate solutions from the grid-based local wavenumber 

 method—Solution locations are projected onto the profile from a maximum 

 distance of 250 m

Fault interpreted from magnetic data

Tertiary older 

 volcanic rocks

Pre-rift sedimentary 

 section (except Paleozoic 

 limestones)

QTv

SFG

SFG

deep

mag

Tov

pre-rift

pe/Pz

peweak1

pemod1

pemag

285 84
46 Geophysical Interpretations of the Southern Española Basin, New Mexico

Cerrillos

Intrusion

Reverse-polarity

shallow intrusions

Bonanza

Hill

Reverse-polarity

Espinaso Formation Beds of Galisteo Fm?

Basement anomaly

Basement

YLM#3

Expression

of Espinaso

Formation

Tijeras-Cañoncito fault

A1

A2

A3

Espinaso Formation or Cieneguilla volcanic

 rocks present in well below Santa Fe Group

Espinaso Formation absent in well— 

Santa Fe Group overlies pre-rift rocks (see plate 2)

Town

Isopach of modeled thickness of Tertiary 

 older volcanic rocks—Contour 

 interval = 100 feet

Area difficult to interpret because both 

 intrusive and volcanic rocks are present

Representative fault within

 Agua Fria fault system—

 Used in three-dimensional

 model

Fault mapped from surface geology—Ball 

 on downthrown side

Quaternary valley-fill alluvium and landslides

 Late Eocene-Oligocene Espinaso Formation

Mesozoic-Paleozoic clastic sedimentary 

 rocks, undivided

Precambrian basement rocks, undivided

Late Eocene and Oligocene intrusive rocks

Late Paleocene and Eocene clastic rocks of the 

 Galisteo and Diamond Tail Formations

–1,200

0

–800

–400

400

800

1,200

1,600

2,000

2,400

2,800

NANOTESLAS

1000

Magnetic unit inferred within 

 Galisteo Formation

Arrow indicating aeromagnetic ridge Plio-Pleistocene Santa Fe Group sediments 

 (Ancha Formation, Tuerto Gravel)

Plio-Pleistocene basalts and andesites of 

 the Cerros del Rio Volcanic field

Early Tertiary mafic volcanic rocks 

 (Cieneguilla volcanic complex and undivided)

Fault inferred from geophysics 

 (Speigel and Baldwin, 1963)—Ball 

 on downthrown side

Qal 

QTsf

QTv 

Tomv

Te

Ti

Tgd

pe

cgu

La Cienega

Eldorado

at Santa Fe

Los Cerrillos

Lamy

Galisteo Creek

Seton

Village

Gallina Arroyo

35°30’

A 106°07’30” 106°00’ 105°52’30”

Interpreted intrusive body associated with

 Cerrillos intrusion

EXPLANATION

A2

0

0 5 MILES

5 KILOMETERS

Interpreted volcanic vent associated with 

 Espinaso Formation

Absence of Tertiary older volcanic rocks 

 interpreted from aeromagnetic data. 

 Labels refer to discussion in text
Interpreted Elements of the Basin Floor 47

Figure 23 (facing page and above). Closeup of southern part of study area showing interpretations of 

intrusions and older volcanic rocks. A, Aeromagnetic data (from fig. 5) with interpreted intrusions and 

areas lacking Espinaso Formation superimposed. B, Geology from figure 3 underlain by shaded digital 

elevation model. Interpreted intrusions and isopach contours of Tertiary older volcanic rocks from threedimensional model are also shown. Areas of missing Espinaso Formation are indicated in three main 

areas from west to east (A1, A2, A3). Lithologic information provided after interpretation from wells drilled 

in the A1 and A3 areas correspond well with interpretations.

Geophysical modeling suggests that there are several sources 

on the basis of their densities and magnetic properties (Grauch 

and others, 2006). The physical-property differences may arise 

from the variable composition and petrology of intrusive rocks 

of various ages, which have been documented by geologic 

mapping (Maynard and others, 2002). Steep magnetic gradients on all but the western side of the area of positive anomalies suggest that the bulk of the intrusive complex is contained 

within a volume having nearly vertical sides for most of its 

perimeter. The maxima of the horizontal-gradient magnitude 

of both the reduced-to-pole magnetic data and pseudogravity 

form the basis for the interpreted outline (figs. 23 and 24). The 

roots of the intrusion extend to great depths, inferred from its 

prominent expression in both of the anomaly-separation maps 

(figs. 7 and 8). Steep sides indicated by the geophysical data 

pose an apparent conflict with the laccolithic nature of the 

intrusion indicated by geologic mapping and boreholes (Grant, 

1999; Maynard and others, 2002). However, the discrepancy 

can be explained by one or more of several factors that do not 

preclude the presence of sills: (1) sills are generally thin and 

do not have sufficient volume to produce magnetic anomalies; 

(2) sills are uniformly horizontal and thin gradually so that 

they do not produce lateral magnetic contrasts; and (3) sill 

rocks may not be magnetic enough to produce pronounced 

aeromagnetic anomalies, especially where they underlie the 

magnetic Espinaso Formation. Separation of anomalies by 

geologic unit is ambiguous in the area between La Cienega 

and the Cerrillos Hills owing to geologic complexity, the possibility that magnetic volcanic and intrusive rocks are superposed, and the presence of Espinaso Formation that is weakly 

magnetic (Phillips and Grauch, 2004). 

Despite difficulties in interpretation in this area, most 

strong positive anomalies surrounding the Cerrillos Hills 

appear to be related to the intrusive complex. An exception is 

005

1000

1000

1000

1000

2000

1500

3000

2500

2000

1500

1500

1000

500

500

500

500

Cerrillos

Hills

A1

A2

A3

YLM#3

La 

Cienega

Eldorado

at Santa Fe

Los Cerrillos

Lamy

Galisteo Creek

Seton

Village

Gallina Arroyo

San Marcos Arroyo

35°30'

B 106°07'30" 106°00' 105°52'30"
48 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 24 (above and facing page). Closeup of southern part of study area showing faults, detailed anomalies, and 

thickness of fill on the Santa Fe platform. A, First vertical derivative (a different display than in fig. 7) showing linear patterns 

and interpreted faults (from horizontal-gradient magnitude maps). Image is color shaded-relief illuminated from the west. 

B, Isopach map of thickness of Santa Fe Group from three-dimensional model, with superimposed interpreted faults and 

intrusions (fig. 23), and shaded digital elevation model in areas where the three-dimensional model is not applicable.

35°30'

A 106°07'30" 106°00' 105°52'30"

fault edges?

fault edges?

fault edges?

basement

linear

pattern

Shallow 

basement?

Cerrillos

Intrusion

Shallow 

basement

A1

A2

A3

Interpreted intrusive body associated 

 with Cerrillos intrusion

EXPLANATION

Representative fault within Agua Fria 

 fault system—Used in three-dimensional 

 model

A2

Fault mapped from surface geology—Ball 

on downthrown side

Fault location interpreted from 

 aeromagnetic data

General location of the Rancho Viejo hinge zone

–3

0

–2

–1

5

6

7

NANOTESLAS

PER METER

4

3

2

1

Interpreted volcanic vent associated 

 with Espinaso Formation

Absence of Tertiary older volcanic

 rocks interpreted from aeromagnetic 

 data—Labels refer to discussion in text

Fault location interpreted from gravity 

 data—Inferred from the horizontal gradient 

 magnitude (HGM) of gravity data. 

 Three line weights, from thin to thick, 

 indicate small to large HGM values 

Arrow indicating aeromagnetic ridge

Geologic contact of Santa Fe Group sediments

Fault inferred from geophysics (Speigel and 

 Baldwin, 1963)—Ball on downthrown side

0

0 5 MILES

5 KILOMETERS

La Cienega

Eldorado

at Santa Fe

Los Cerrillos

Lamy

Galisteo Creek

Seton

Village

Gallina Arroyo

YLM#3

Selected well location
Interpreted Elements of the Basin Floor 49

the negative anomaly that corresponds in part with Bonanza 

Hill (fig. 23A), where Oligocene hornblende monzonite is 

mapped (Koning and Hallett, 2001). This rock unit may have 

been emplaced during a reversal of Earth’s magnetic field. 

Observation of both positive and negative aeromagnetic 

anomalies associated with the Cerrillos Hills intrusive 

complex is consistent with paleomagnetic measurements, 

which record both normal and reverse polarities (Stephen 

Harlan, George Mason University, written commun., 2008). 

Smaller intrusions that served as vents for volcanoes or 

cinder cones are commonly associated with local, circular, 

high-amplitude magnetic anomalies, generally less than 

1.6 km (1 mi) in diameter, that can be can be either positive 

or negative. Examples of these circular magnetic anomalies 

are abundant in the Plio-Pleistocene Cerros del Rio volcanic 

field where vents are exposed (Grauch and others, 2006). 

The positive or negative character of the anomalies indicate 

strong normal- or reverse-polarity remanent magnetization, 

respectively, that can be tied generally to different eruption 

episodes (Thompson and others, 2006; fig. 4). Examples 

of negative anomalies associated with exposed middle- or 

late-phase vents, inferred as having reverse-polarity remanent 

magnetization, are located at the northeastern end of the 

Cerros del Rio field (fig. 15; pl. 1). An exposed vent associated 

with a positive anomaly 6.4 km (4 mi) north of La Cienega 

formed during the early eruptive phase (fig. 15; anomaly E8 

of Grauch and others, 2006). The number of anomalies in 

the Cerros del Rio that are caused by volcanic vents are too 

numerous to distinguish for this study. Therefore, they are 

indicated collectively except for several isolated ones near the 

margin of the volcanic field, such as the ones just mentioned. 

Many negative circular anomalies, similar in character to 

the ones associated with exposed volcanic vents, have sources 

that are buried under mapped Santa Fe Group in the southern 

part of the study area (fig. 23). Some of the anomalies appear 

broader and more diffuse than those observed over exposed 

vents because of their greater depth (Grauch and Bankey, 

2003). We interpret the sources of these anomalies as volcanic vents on the basis of the nature of the circular anomalies. 

We infer that they are associated with Eocene-Oligocene 

magmatic activity because magnetic depth estimation indicates that the sources of the inferred vents are located within 

the more-or-less horizontally layered Eocene-Oligocene 

Espinaso Formation, discussed in the following section. 

B

Cerrillos

Intrusion

A1

A2

A3

La Cienega

Eldorado

at Santa Fe

Los Cerrillos

Lamy

Galisteo Creek

Seton

Village

Gallina Arroyo

San Marcos Arroyo

THICKNESS,

IN FEET

5,000

2,000

1,500

1,000

750

500

400

300

200

100

YLM#3

35°30'

106°00' 106°07'30" 105°52'30"
50 Geophysical Interpretations of the Southern Española Basin, New Mexico

Part of the elliptical negative magnetic anomaly in Gallina 

Arroyo, located about 2 km east of the Yates La Mesa #3 well 

(fig. 23A), was interpreted previously as an intrusion that coincides with a ground-water high (Spiegel and Baldwin, 1963). 

Older Volcanic Rocks 

Older volcanic rocks are considered as a package; they 

contain the Espinaso Formation, overlying Cieneguilla volcanic complex, and any intervening sediments, as described in 

the section Geologic Setting. They are considered a package 

because the volcanic units closely overlie one another and are 

expected to have similar densities on the basis of their rock 

type. This similarity makes them difficult to distinguish by 

using aeromagnetic or gravity data. 

Where exposed southeast of the Cerrillos intrusion, the 

Espinaso Formation corresponds with a distinctive mottled 

pattern on the aeromagnetic map interrupted by negative 

circular anomalies, mentioned previously (“Expression of 

Espinaso Formation” on fig. 23A). The mottled pattern can be 

followed throughout an extensive area between the Cerrillos 

Hills and Eldorado at Santa Fe, implying that the Espinaso 

Formation extends into this area underneath mapped Santa Fe 

Group (fig 23B). Measurements of magnetic susceptibility on 

a few typical hand samples and outcrops of Espinaso Formation give high values (0.01–0.03 (SI), fig. 4), which result in 

induced magnetizations of about 0.4–1.2 A/m (assuming an 

inducing Earth field of 51,715 nanoteslas (nT)). In a recent 

paleomagnetic study of breccias in the Espinaso Formation 

(Stephen Harlan, George Mason University, written commun., 

2008), reverse-polarity remanent directions were established 

for a few sites, and a wide range of natural remanent magnetizations were measured overall (0.027–7.21 A/m). Comparing 

these ranges of remanent and induced magnetizations suggests 

that a reverse-polarity remanent component may dominate the 

induced component in some areas and that the opposite may 

be the case in other areas. Thus, an amalgamation of these 

sources would produce highly variable anomalies consistent 

with the observed aeromagnetic pattern, which ranges from 

strongly negative (dominant remanent magnetization) to 

moderately positive (dominant induced magnetization). One 

irregularly shaped local negative anomaly is located in the 

area of exposed Espinaso Formation on the southeast side of 

the Cerrillos intrusion (fig. 23A). This anomaly may indicate 

a local area of the Espinaso Formation where the reversepolarity remanent component exceeds the magnitude of the 

induced component. If the entire thickness of the Espinaso 

Formation is considered, detailed examination of the magnetic 

depth estimates suggests that magnetic-property differences 

may locally occur at subhorizontal interfaces (fig. 14), perhaps 

related to layering of the deposits. Recognition of this internal 

layering helped resolve problems with interpreting the magnetic depth estimates, as discussed in a previous section. 

None of the circular negative anomalies within the area 

of the mottled anomaly pattern mentioned in the previous 

section occur where the Espinaso Formation is exposed. 

However, the few sites in the paleomagnetic study yielding 

well-grouped remanent magnetizations were collected from 

small-volume, near-vent pyroclastic flows. This association 

supports the interpretation that the circular negative magnetic 

anomalies are associated with vents. 

In contrast to the Espinaso Formation, Cieneguilla volcanic rocks are associated with strong local positive anomalies where they are exposed 1 km (0.625 mi) north of La 

Cienega (Grauch and Bankey, 2003; fig. 23), which suggests 

that they will produce positive anomalies elsewhere. Similar 

strong positive anomalies extend southeast from the area of 

exposed rocks towards the Yates La Mesa #3 well as patterns of northeast-directed linear anomalies that are truncated 

along a northwest-southeast aeromagnetic ridge (fig. 23A). 

The truncation is interpreted as a concealed contact between 

Tertiary older volcanic rocks on the northeast against an 

area of mixed rock types on the southwest (pl. 2). Additional 

northeast-directed linear anomalies, parallel to each other and 

more subdued in amplitude, are apparent in an east-west swath 

including the Yates La Mesa #3 well (labeled “fault edges?” 

on fig. 24A). Grauch and Bankey (2003) interpreted these 

anomalies to be caused by paleochannels filled with basalt. 

An alternate hypothesis is that they represent upturned edges 

of fault blocks of the older volcanic rocks. Examination of the 

seismic data supports the latter interpretation. Upturned fault 

blocks of Espinaso Formation with strong, reverse-polarity 

remanent magnetization may also be the source of linear negative anomalies farther east near Eldorado at Santa Fe. 

The unique aeromagnetic patterns associated with the 

Tertiary older volcanic rocks provided guides for outlining 

their lateral extents in the integrated 3D model. In the southern part of the study area, most of the lateral limits of the 

older volcanic rocks were determined from the termination of 

aeromagnetic patterns (fig. 23). Magnetic depth solutions were 

used as far as 4 km (6.4 mi) north of Yates La Mesa #3 well 

to trace the top of the volcanic package as it plunges to the 

north. North of this, modeling of the volcanic rocks relied on 

extrapolation of widely separated information from wells and 

seismic-reflection depth picks (John Ferguson, University of 

Texas at Dallas, written commun., 2007). The volcanic surface 

forms a shallow but irregular platform in the area of mottled 

aeromagnetic pattern (fig. 23A), consistent with erosion before 

deposition of the Ancha Formation. This surface is discussed 

in more detail in the following section because it also represents the base of the Santa Fe Group in this area. 

In contrast to the top surface, the modeled base of the 

Tertiary older volcanic section is folded (fig 21A, B). The 

modeled top and base of the Tertiary older volcanic rocks were 

subtracted to construct an isopach map of modeled thickness 

(fig. 23B). The folding of the base of the older volcanic rocks 

is well reflected in the patterns of thinning and thickening of 

the isopach map, such as the two areas of >300 m (1,000 ft) 

thickness between San Marcos and Gallina Arroyos. The isopach map is consistent with maximum thicknesses in the range 

of 250–580 m (820–1,900 ft), which have been determined 
Interpreted Elements of the Basin Floor 51

in various areas from geologic mapping and the Yates La 

Mesa wells (Lisenbee, 1999; Myer and Smith, 2006; David 

Sawyer, U.S. Geological Survey, and Daniel Koning, New 

Mexico Bureau of Geology and Mineral Resources, written 

commun., 2008). 

In the southern part of the study area, where the mottled 

aeromagnetic pattern is associated with the Espinaso Formation, both the isopach map and aeromagnetic patterns indicate 

that the Espinaso Formation is notably absent in three large 

areas (A1–A3, fig. 23), first recognized by Grauch and Bankey 

(2003). In these areas, Santa Fe Group sediments likely rest 

directly on pre-rift sedimentary rocks. Interpretations of 

the geologic units that directly underlie the Santa Fe Group 

throughout the study area are depicted in plate 2 (CD in pocket 

and downloadable from the U.S. Geological Survey Web site) 

and are discussed next. 

In the western area (A1) on the northeast side of the 

Cerrillos intrusion, Santa Fe Group sediments likely overlie 

Galisteo Formation, on the basis of data from three wells 

located in the area of A1 (fig. 23B; pl. 2). Northwest of 

area A1, the underlying geologic unit is unclear. In the area 

surrounding Bonanza Hill (fig. 23A), wells intersect shallow 

Espinaso Formation but aeromagnetic anomalies lack the 

characteristic patterns of this formation. Where anomalies 

exist, the preponderance of Cerrillos intrusive rocks makes 

interpretation ambiguous. 

In the second area (A2), which is recessed into the region 

of volcanic rocks just southeast of 35°30' N., 106° W., Ancha 

Formation likely overlies west-dipping Galisteo Formation, 

on the basis of geologic map relations exposed to the south 

(Lisenbee, 1999; pl. 1). The east side of this area is defined by 

an east-down fault on the basis of its alignment with a mapped 

fault that juxtaposes Espinaso Formation against Galisteo and 

various strata within the Galisteo Formation (fig. 23; Lisenbee, 

1999). This fault and its northern extension defined by the 

aeromagnetic data form the west side of the Agua Fria fault 

system, discussed in a following section. The western side of 

the area (A2) is interpreted as an erosionally truncated contact 

between the Espinaso and underlying Galisteo Formations, 

which was buried subsequently by the Ancha Formation, 

on the basis of comparisons with geologic map relations 

of Lisenbee (1999). Thus, the area of missing Espinaso 

Formation appears to be the result of relative footwall uplift 

along the east-down fault, followed by erosion that locally 

removed the cap of Espinaso Formation volcaniclastic and 

volcanic rocks on the footwall. These relations are well 

developed in the 3D model, which are apparent in cross 

section (fig. 21A) and in the isopach map of the modeled 

thickness of the volcanic section (fig. 23B). 

The third and largest area devoid of Espinaso Formation 

on the east (A3) extends as an irregular triangle covering 

much of the community of Eldorado at Santa Fe (fig. 23). 

Grauch and Bankey (2003) recognized most of this area 

previously but were hindered on the eastern side by the limit 

of the earlier aeromagnetic coverage. Subsequent to that 

study, information compiled from local water wells confirmed 

the absence of the Espinaso Formation (fig. 23B). With the 

addition of the Santa Fe East survey in 2005 (fig. 5), the 

full area of missing Espinaso Formation is now apparent. It 

is shaped as an irregular triangle with the apex near Seton 

Village (fig. 23). Aeromagnetic interpretations combined with 

strike and dip measurements from isolated exposures of the 

Espinaso Formation (Lisenbee, 1999; pl. 1) suggest that the 

western edge of area A3 is the eastern limit of the Tertiary 

older volcanic rocks. The limit is represented primarily by the 

erosional edge of dipping Espinaso Formation, analogous to 

the western side of the triangular area A2. The western edge of 

A3 bulges to the west just north of latitude 35°32'30", which 

aligns with the prominent ridge of a strong basement-related 

anomaly (“basement anomaly” on fig. 23A). The southwestern 

side of this basement anomaly corresponds with aeromagnetic 

and gravity gradients that are interpreted as a major downto-the-west fault, on the basis of examination of seismicreflection data to the south. The northwestward continuation 

of this fault likely accounts for the southwestern side of the 

bulge. 

Near the northern apex of the triangular area (A3), linear 

anomalies characteristic of Tertiary older volcanic rocks come 

together with the linear anomalies caused by basement rocks, 

leading to some ambiguities in interpretation. North of Seton 

Village and the northern apex of area A3, these two rock types 

are nearly superposed; the older volcanic rocks rest only on a 

thin layer of Galisteo Formation that directly overlies basement rocks in Arroyo Hondo, about 2.4 km (1.5 miles) north 

of Seton Village (just off figure 23A) (Read and others, 2005). 

Despite this ambiguity, the strong linear anomaly that extends 

2 km (1.25 mi) southward from Seton Village is interpreted 

as a basement related-anomaly. Even though its north-south 

trend differs from the northwesterly trends of the linear 

basement-related anomalies in the neighboring mountain front 

to the east, its strong amplitude is more similar to amplitudes 

of basement-related anomalies than to amplitudes of older 

volcanic rocks to the west. Another isolated linear anomaly 

within area A3 is more enigmatic. It has a north-south trend, 

is located about 5 km (3.1 mi) south of Seton Village, and is 

mostly evident in the first vertical derivative map (“shallow 

basement?” on fig. 24A). Magnetic depth estimates in this 

area show sources that are clearly within the pre-rift section 

(fig. 14A). 

The older volcanic rocks thicken markedly near Yates La 

Mesa #3 well; the thickening is constrained by this and other 

wells in the area and is evident in the isopach map (fig. 23B). 

The bulk of the thickened section is due to increased thickness 

of the Espinaso Formation, which is 575 m (1,890 ft) thick 

in the Yates La Mesa #3 well and underlies 23 m (77 ft) of 

Cieneguilla volcanic rocks (David Sawyer, U.S. Geological 

Survey, and Daniel Koning, New Mexico Bureau of Geology and Mineral Resources, written commun., 2008). The 

Cieneguilla volcanic rocks thicken to the north and northwest, 

where they have a maximum thickness of 200 m (660 ft) in 

a limited area of exposure just north of La Cienega (fig. 23B; 

Sawyer and others, 2002). They are about 300 m (1,000 ft) 
52 Geophysical Interpretations of the Southern Española Basin, New Mexico

thick above a similar thickness of Espinaso Formation in the 

Yates La Mesa #2 well (Myer and Smith, 2006). Several of the 

negative circular anomalies discussed in the previous section are generally located in the thicker sections of the older 

volcanic rocks (fig. 23B). They are interpreted as volcanic 

vents synchronous with the Espinaso Formation, as argued in 

the section on intrusions. Implications of these associations are 

discussed in the Interpreted Structure section.

Interpreted Elements of Basin Fill

Santa Fe Group Sediments

Santa Fe Group sediments in the Albuquerque and 

Española Basins produce weak to moderate aeromagnetic 

anomalies where units of contrasting magnetic susceptibilities 

are juxtaposed at faults and where they directly underlie 

irregular topographic surfaces (Grauch and Bankey, 2003; 

Grauch and Hudson, 2007). Results of detailed magnetic 

property studies in the northern Albuquerque Basin (Hudson 

and others, 2008) show that the sediments have magnetic 

susceptibilities that generally fall between 0.0002 and 0.0050 

(SI), and that means are on the order of 0.001–0.003 (SI). 

The higher values generally correspond with fractions of 

coarser grain size, especially for stream-deposited sediments. 

Reconnaissance magnetic susceptibility measurements of 

Santa Fe Group sediments in the Española Basin suggest they 

have similar or even higher magnetic susceptibilities (fig. 4). 

The highest averages (about 0.010 SI) were measured from 

the Bishops Lodge member of the Tesuque Formation (fig. 

18) and the Puye Formation (fig. 15), where both contained 

considerable volcanic detritus. The moderately high values 

of lithosome E of the Tesuque Formation (fig. 4) may also 

be related to its high volcanic content, because it is derived 

primarily from erosion of the Espinaso Formation and 

Cieneguilla basanite. However, sediment containing 5–7 

percent volcanic lithic grains (basin-floor strata of lithosome B 

of Cavazza (1986)) has relatively low magnetic susceptibility 

values that average 0.001 (SI). In comparison, alluvial slope 

(piedmont slope) sediment of lithosome A of the Tesuque 

Formation (Cavazza, 1986; Kuhle and Smith, 2001) also 

averaged on the order of 0.001 (SI), even though lithosome 

B contains 10 times as much volcanic detritus as lithosome 

A (as calculated from data presented in Cavazza (1986)). 

The lack of correlation between volcanic-lithic content and 

magnetic susceptibility in lithosome B may be explained by 

volcanic source rocks that are low in magnetic susceptibility. 

The presumed source area for lithosome B volcanic detritus 

is the Latir volcanic field, about 25 km (16 mi) north of Taos 

(fig. 1) and outside the region of study. This volcanic field 

is associated with moderate- to low-amplitude aeromagnetic 

anomalies (Grauch and Keller, 2004), consistent with 

moderate to low magnetizations. 

A tentative correlation between grain size and magnetic 

susceptibility in the Tesuque Formation is suggested by the 

higher magnetic susceptibility values of the coarse-grained 

Cuarteles Member compared with those of its finer grained 

and older lithologic equivalent, lithosome A (fig. 4). However, 

reconnaissance magnetic-susceptibility measurements of sandand clay-rich fractions of lithosomes A and B in the northeastern part of the study area showed little correlation with grain 

size (V.J.S. Grauch, U.S. Geological Survey, and Daniel Koning, New Mexico Bureau of Geology and Mineral Resources, 

unpub. data, 2007). 

Regions where the Santa Fe Group tends toward higher 

magnetic susceptibilities can be interpreted from the aeromagnetic data where aeromagnetic anomalies correlate with 

topography, indicating that the material that forms the hills is 

magnetic (Grauch and others, 2006). An excellent example is 

located in the northwest part of the aeromagnetic data area, 

where the aeromagnetic expression looks very similar to 

topography (“Puye Formation” on fig. 15). The aeromagnetic 

anomalies correlate better with hills composed of Puye Formation (Kelley, 1978) than with local exposures of overlying 

Bandelier tuff, indicating that the Puye Formation is the source 

of the anomalies. Correlation of subtle aeromagnetic anomalies with topography is also evident southwest of Pojoaque 

(labeled “Faults and magnetic sediments” on fig. 15) in an area 

generally underlain by Cuarteles Member of the Tesuque Formation (Koning and Maldonado, 2003). The most prominent 

linear anomalies in this area are interpreted as faults, whereas 

the more subtle ones correlate with drainages and ridges. 

Although magnetic-susceptibility has not been measured in 

this area, analysis of the topographic-magnetic correlation 

suggests that magnetic susceptibilities may be on the order of 

0.010 (SI) in parts of the area. 

Two other areas underlain by especially magnetic sediment, as indicated by anomalies that correlate with topography, are located northeast of Highway 84 and 285 where it 

parallels the Rio Tesuque (pl. 1; “Stream terrace deposits” on 

fig. 19B), and along and north of the Santa Cruz River (pl. 1). 

The magnetic sediments northeast of the highway may be 

related to thick Quaternary-Tertiary stream-terrace gravels 

derived from granitic terrane (Read and others, 2005; Borchert 

and others, 2003). The magnetic sediment north of the Santa 

Cruz River, along the northern study area boundary, is probably due to a combined effect of the coarse-grained Cuarteles 

and Cejita Members of the upper Tesuque Formation and thick 

Quaternary stream gravels derived from granitic terrane in the 

Sangre de Cristo Mountains (Koning, 2005a; Koning, Connell, 

and others, 2005). A third area possibly underlain by magnetic 

sediments lies within the area devoid of Espinaso Formation 

near Eldorado at Santa Fe (area A3 on fig. 23A). The very 

subtle mottled pattern in the southern part of the A3 area may 

be caused by variations in the Ancha Formation, which can be 

quite magnetic (fig. 4). In contrast, the high-amplitude circular 

anomalies are caused by buildings and bridges. 

The overall thickness of Santa Fe Group sediments was 

determined by integrating information from all sources during 

development of the 3D integrated model, as described in the 

section on model construction. A map showing the elevation 
Interpreted Elements of Basin Fill 53

of the base of the Santa Fe Group from the 3D model is 

shown in figure 25 and plate 1. An isopach map of the thickness, constructed by subtracting the base elevation surface 

from a regional topographic surface, is shown in figure 26 

and plate 2. The overall form of the basin is that of a platform 

on the south and a west-dipping half-graben on the north. The 

base of the Santa Fe Group descends precipitously across a 

faulted, generally curvilinear, hinge zone, which we propose 

to call the Rancho Viejo hinge zone (fig. 24B). This hinge 

zone wraps to the southeast from 4 km (6.4 mi) north of La 

Cienega to the area around the Yates La Mesa #3 well, then 

northeast toward Seton Village. Santa Fe Group sediments 

thicken markedly northward across the hinge zone at about 

130–150 meters per kilometer (m/km) (700–800 feet per mile 

(ft/mi)). This zone is discussed in more detail in the section 

Interpreted Structure. The disruptions in the generally northsouth contours on the base of the Santa Fe Group (fig. 25), 

which continue between Eldorado at Santa Fe and Española, 

are also discussed in the Interpreted Structure section. They 

are the model depiction of the combined Agua Fria and Barrancos fault systems.

From Santa Fe to the north, the base of the Santa Fe 

Group dips relatively steeply westward (7°–20°) within 

5–10 km (3–6.3 mi) west of the present-day foot of the 

mountain front (fig. 25), following the geologic map data of 

Koning, Nyman, and others (2005), Read and others (2005), 

and Borchert and others (2003). The deepest part of the basin 

(>3,000 m (10,000 ft)) is in the west-dipping half-graben 

underneath the Pajarito Plateau west of the Rio Grande. The 

3D model depicts this area only in general form owing to 

poor constraints at depth. The bedrock high under the Cerros 

del Rio, as discussed in the section on the pre-rift sedimentary section, forms an added north-south complexity to the 

overall basin picture. Finally, the model predicts thinner Santa 

Fe Group on the footwall (northwest side) of the PajaritoEmbudo fault system (fig. 26), consistent with interpretations 

of geophysical data by Baldridge and others (1994) regarding 

thickness of basin fill near Abiquiu (fig. 1). 

The base of the Santa Fe Group on the Santa Fe platform 

generally appears to undulate from southwest to northeast 

(figs. 24B and 27B). In detail the surface is irregular, on the 

basis of exposures to the south (Maynard and others, 2002) 

and correlations between neighboring water wells (Koning 

and Hallett, 2001). The lower parts of the surface appear as 

elongated connections to the deep basin to the north, suggesting paleovalleys that drained in that direction in the 

past. Relatively small paleovalleys previously have been 

interpreted on the basis of water-well lithologic logs (Koning and Hallett, 2001). These small paleovalleys preserve 

relatively thin (<70 m (230 ft)) Tesuque Formation strata. 

However, thicker fill modeled in the vicinity of Eldorado at 

Santa Fe resembles a paleovalley containing about 90–150 m 

(300–500 ft) of Santa Fe Group sediments (fig. 24B). This 

thicker low-density material is required by the gravity inversion, which is well constrained by the gravity and well data 

coverage in this area (fig. 9). 

To assess the relative thickness of Tesuque Formation 

and overlying Ancha Formation in the areas modeled as 

thick Santa Fe Group, we used preliminary lithologic picks 

from the well data (Peggy Johnson and Daniel Koning, New 

Mexico Bureau of Geology and Mineral Resources, unpub. 

data, 2008) in the vicinity of the Rancho Viejo hinge zone 

(fig. 27). We compared elevation contours for the base of the 

Ancha Formation constructed from the well data with those of 

the modeled base of Santa Fe Group (fig. 27). The base of the 

Ancha Formation generally slopes gently to west-southwest 

(fig. 27A). The gentle slope contrasts with 3D model surface 

of the base of the Santa Fe Group (fig. 27B), which shows 

much more irregularity, greater relief, and an overall slope 

to the north. Because the interval between these surfaces is 

inferred to be Tesuque Formation, the differences suggest that 

(1) the Tesuque rather than the Ancha Formation accounts for 

much of the additional thickness of Santa Fe Group within 

paleovalleys and irregularities on the Santa Fe platform; and 

(2) considerable tectonic deformation occurred in Miocene to 

early Pliocene time, the time interval between the deposition 

of these two formations. 

A considerable thickness of Tesuque Formation is thus 

inferred for the large paleovalley modeled in the vicinity 

of Eldorado at Santa Fe (fig. 24B). This buried paleovalley, 

identified here for the first time, is located within one of the 

areas devoid of Espinaso Formation (area A3 on fig. 23A) 

and appears to be bounded by faults inferred from both the 

aeromagnetic and gravity data (fig. 24B). The faults are all 

interpreted as down-to-the-west on the basis of independent 

information (pl. 1), which is unexpected for a paleovalley 

within a fault-bounded graben. An alternative explanation 

is that the block between the west-down faults was eroded 

of its cap of Espinaso Formation after faulting and that the 

softer pre-rift sedimentary section was exposed. Further 

erosion created the paleovalley, which was then backfilled 

by the Tesuque Formation. If this scenario is correct, the 

faulting, erosion, and backfilling must have occurred 

largely in Miocene time, to account for events that postdate 

the deposition of the Espinaso Formation and predate the 

termination of aggradation of the Tesuque Formation. 

Afterwards, the Ancha Formation was deposited over the 

erosional unconformity in Pliocene to early Pleistocene time. 

The 3D modeled base of the Santa Fe Group surface was 

also used to investigate stratigraphic and structural relations 

just north of the Rancho Viejo hinge zone. Stratigraphic correlations of marker beds within the lower Tesuque Formation 

between three neighboring wells (fig. 27) give an estimated 

strike and dip of N. 95° E. and 8° N., respectively (Koning, 

Read, and others, 2006). This orientation is similar to that of 

the modeled base of Santa Fe Group, if one allows for discrepancies caused by local differences in the strike of the modeled 

surface in the vicinity of the wells (fig. 27B). Because the modeled surface in this area represents the top of the basin floor that 

is likely composed of Cieneguilla volcanic rocks, the similar 

dips imply that (1) little deformation occurred between the time 

that the Cieneguilla rocks and the lower Tesuque Formation 
54 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 25. Elevation of base of the Santa Fe Group from three-dimensional model. The surface is displayed by 

contours (fixed interval of 500 ft), colors (stretched display), and shading (illumination from the northeast) in order to 

capture details of the shallow section as well as regional trends.

6000

00 -20

0

6000

6000

6000

6000

6000

6000

2,000

4000

4000

2000

0

0

0

0

0

0

–2000

2000

–2000

2000

2000

2000

4000

–2000

–4000

–4000

–2,000

Rio

Grande

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Cerrillos

Hills

Cundiyo

Seton

Village

La Cienega

Cerros del Rio

volcanic field

Pajarito - Embudo fault system

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

ELEVATION, 

IN FEET

–2,000

–1,000

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Mapped contact of Santa Fe Group 

 sediments—Generalized from figure 3

Elevation of base of Santa Fe Group 

 sediments—From the three-dimensional 

 model. Contour interval = 500 feet; 

 datum is sea level. Hachured in areas of 

 closed lows

2000

EXPLANATION
Interpreted Elements of Basin Fill 55

Figure 26. Isopach map of thickness of Santa Fe Group from three-dimensional model. Detailed irregularities in the thickness 

map likely due to variations in the topographic surface and may not be important. The surface is displayed by contours (fixed 

interval of 500 ft), and colors (stretched display) to capture details of the shallow section as well as regional trends.

2000

2000

2000 2000

4000

4000

4000

4000

4000

6000

6000

6000

6000

6000

8000

8000

8000

8000

8000

6000

10,000

10,000

8000

Rio

Grande

4000

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Cerrillos

Hills

Cundiyo

Seton

Village

La Cienega

Cerros del Rio

volcanic field

Pajarito - Embudo fault system

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

THICKNESS, 

IN FEET

5,000

2,000

1,500

1,000

750

500

400

300

200

100

EXPLANATION

Mapped contact of Santa Fe Group 

 sediments—Generalized from figure 3

2000 Thickness of basin fill—From the 

 three-dimensional model. 

 Contour interval = 500 feet. 

 Hachured in areas of closed lows
56 Geophysical Interpretations of the Southern Española Basin, New Mexico

A

0 1 2 MILES

0 1 2 KILOMETERS

Jail Well

YLM#3

ND6

ND8

YLM#3

N

Seton

Village

Agua Fria

fault system

PALEOVALLEY

Seton

Village

Agua Fria

fault system

PALEOVALLEY

8o

8o 8o

YLM#3

Location map

Strike and dip of Tesuque Formation— 

 Estimated from triangulation of

 stratigraphic markers in ND6, ND8, 

 and Jail wells.

Selected well (Appendix 1)

Mapped contact of Santa Fe group sediments

Elevation in feet—Contour interval 100 feet

Interpreted Agua Fria fault system

EXPLANATION

Base of Ancha Formation from well data

Base of Santa Fe Group from 3D model

ND8

ND6

6100

6200

6300

6400

6500

6600

6700

6800

6900

7000

B

6000

6000

6000

6000

5000

5000

5000

4000

4000

25

25

A

B

Jail Well

8o

Rancho Viejo hinge zone

Rancho Viejo hinge zone
Interpreted Structure 57

were deposited, and (2) the basin floor to the north was tilted 

after deposition of the lower Tesuque Formation. Moreover, 

the base of the Ancha Formation dips gently to the southwest 

above the steeper northward dip of the lower Tesuque Formation in the vicinity of the three wells (fig. 27A). This contrast 

implies that northward tilting had ceased before deposition of 

the Ancha Formation in Pliocene time.

Younger Volcanic Rocks

Where exposed, the Cerros del Rio basaltic rocks are very 

magnetic, as shown by the high-amplitude magnetic anomalies that correlate with topographic contours and mapped flow 

contacts (Grauch and others, 2006). Paleomagnetic and rockproperty measurements indicate that the magnetizations of 

these rocks are generally dominated by strong remanent magnetization having both normal and reverse orientations with 

respect to the present Earth’s magnetic field, depending on the 

age of formation (Thompson and others, 2006). The normal 

and reverse orientations of the strong remanent magnetizations 

give rise to the positive and negative anomalies, respectively.

Because of the associated strong magnetic anomalies, 

the lateral extent of the Cerros del Rio volcanic field can be 

readily distinguished from Santa Fe Group sediments. This 

distinction is important for recognizing areas where PlioPleistocene Ancha Formation of the Santa Fe Group locally 

covers basalts in the southern part of the study area (fig. 3; 

Read and others, 2005). The eastern and southwestern limits 

of the volcanic field, as interpreted from aeromagnetic data, 

are shown on plate 1. The interpreted limits were determined 

from visual inspection of the aeromagnetic patterns and the 

local maxima of horizontal-gradient magnitudes to determine 

the edges (Grauch and Bankey, 2003). The interpretation is 

approximate between La Cienega and the Cerrillos Hills, 

owing to ambiguities caused by older igneous rocks that are 

also magnetic, as discussed previously. The aeromagnetically 

determined lateral extent of the Cerros del Rio volcanic field 

closely approximates geologically mapped contacts (pl. 1) 

except along the eastern margin of the field near the center of 

the study area, directly west of downtown Santa Fe. Here the 

busy aeromagnetic pattern extends about 1 km (0.625 mi) east 

of the mapped contact on the west side of the Agua Fria quadrangle (Shroba and others, 2005) at about 35°42'30" (“basalt 

under sediment” on fig. 15). The aeromagnetic pattern and 

mapped geology indicate that Ancha Formation overlies basalt 

in this area. 

The aeromagnetic data can also be used to estimate 

general areas of thin basalt (estimated at less than about 50 m 

(160 ft)). These areas (pl. 1) are derived from previous interpretations (Grauch and Bankey, 2003). These authors used two 

analytic methods to locate magnetic sources that are mathematically best represented as sheets (structural index N=1): the 

profile-based local wavenumber method (Thurston and Smith, 

1997) and the grid-based horizontal-gradient method applied 

to both reduced-to-pole magnetic data and pseudogravity 

(Phillips, 2000). A predominance of sheet sources suggests 

that the basalts are much thinner than the observation height of 

100 m (330 ft). Thin basalt determined in this way surrounds 

the intrusion north of Buckman (pl. 1), consistent with mapping by Dethier (1997). 

In contrast to the areas of thin basalt, a large area of the 

volcanic field contains many exposed vents and intrusions 

(pl. 1; Thompson and others, 2006), which are too numerous 

to distinguish within the scope of this report. MT soundings 

inside this area indicate that the total basalt thickness is typically about 200–225 m (660–740 ft; Rodriguez and others, 

2006). Thus, areas of the volcanic field on plate 1 that are not 

designated as either vent area or as thin basalt presumably 

contain basalts that are 50–200 m (160–660 ft) thick. 

The Bandelier Tuff represents exposures of Jemez 

volcanic rocks in the study area, located northwest of the 

Cerros del Rio volcanic field and west of the Rio Grande 

(fig. 3). These exposures do not produce prominent 

aeromagnetic anomalies, implying the Bandelier Tuff is only 

weakly magnetic in this area. The lack of anomalies is evident 

where the aeromagnetic pattern associated with the magnetic 

sediments of the Puye Formation appears uninterrupted 

(fig. 15), even though exposures of overlying Bandelier Tuff 

are present. However, the Bandelier Tuff is inferred to cause 

strong negative anomalies where it lies within the Valles 

caldera (Grauch and others, 2006). 

Interpreted Structure 

Faults that juxtapose Santa Fe Group units of differing magnetic properties in the Española Basin are generally 

expressed as linear aeromagnetic anomalies, with considerable 

variation in detail (Grauch and Hudson, 2007). Associated 

anomaly amplitudes typically range from 2 to 5 nT, so they 

can be difficult to distinguish from solely topographic effects 

such as the somewhat linear edges of arroyos. Faults within 

more magnetic units, such as the Tertiary older volcanic rocks, 

have much more prominent expression on the aeromagnetic 

map because they produce anomalies with much higher 

amplitudes. 

Faults interpreted from the aeromagnetic data for this 

study are from Grauch and Hudson (2007), who primarily 

used the horizontal-gradient magnitude (HGM) of the reducedto-pole aeromagnetic data to locate faults. They also discuss 

the methods used, limitations, and utility of their approach. An 

Figure 27 (facing page). Base of Ancha Formation compared 

with base of underlying Tesuque Formation in southern part of 

study area. A, Elevation contours on the base of Ancha Formation 

derived from numerous wells in the area (Peggy Johnson, unpub. 

data, 2008). B, Elevation contours on the base of the Tesuque 

Formation from three-dimensional model surface. Strike and dip 

from well data are from Koning, Read, and others (2006). 
58 Geophysical Interpretations of the Southern Española Basin, New Mexico

example of this approach from the middle of the study area 

shows the HGM of the reduced-to-pole aeromagnetic data 

first without (fig. 28A) and then with (fig. 28B) interpreted and 

mapped faults. The figure demonstrates how the HGM map 

can be used to guide fault interpretation and how the interpretation must take into account other linear sources, and the relations of the aeromagnetically interpreted faults to the mapped 

faults. The mapped faults show better detail; the aeromagnetically interpreted faults, which reflect major contrasts in bulk 

magnetic properties at depth, depict the structure at a more 

regional scale. Moreover, guides provided by Grauch and 

Hudson (2007) suggest that the faults in this area must have at 

least 30 m (100 ft) of throw to produce these anomalies. 

Interpreted faults generally strike north to northeast 

where they offset Santa Fe Group sediments in the central and 

northern parts of the study area (fig. 29). As in the example 

area (fig. 28), aeromagnetically interpreted faults generally 

correspond with mapped faults in much of the area. In particular, the dense fault patterns in the northern part of the area are 

apparent in both the geology and the aeromagnetic data. In this 

area, Kelley (1978) and Koning (2005b) recognized the Barrancos monocline and faults offsetting the monocline, called 

the Barrancos fault system. The Barrancos monocline is evident as a basement feature on the 2D geophysical model along 

the Pojoaque profile (fig. 20). In many areas, aeromagnetically 

inferred faults are corroborated by breaks in the patterns of 

magnetic depth estimates observed in profile form, as demonstrated for the Alamo Creek profile (fig. 22).

A newly defined structure in the southern part of the 

study area, called the Agua Fria fault system, is recognized by 

a combination of geologic mapping and multiple geophysical 

data sets (fig. 29). It extends from the center of the study area, 

where faults are exposed and offset Miocene Tesuque Formation, to the area near Eldorado at Santa Fe on the south, where 

normal faults terminate against the Tijeras-Cañoncito strikeslip fault system (fig. 3). In between the northern and southern 

extents, the fault system underlies a large tract of relatively 

undeformed Plio-Pleistocene Ancha Formation, which constrains the minimum age of the faulting. 

In the aeromagnetic data, the Agua Fria fault system 

is best recognized by parallel sets of inferred north-south, 

sinuous faults that cross the Santa Fe River and by a strong 

north-northwest aeromagnetic lineament on the Santa Fe platform. Some of the parallel inferred faults generally coincide 

with the mapped San Isidro Crossing fault (fig. 3), named for 

the road that crosses the river there. This fault is important 

because it coincides with a drop in the water table (Mourant, 

1980; Johnson and Frost, 2004). At shallow levels, the electrical resistivity character changes across the fault; a layer of 

high resistivity lies on the west side (Spiegel and Baldwin, 

1963). The aeromagnetic expression of the San Isidro Crossing fault is lost amid anomalies caused by buildings south of 

the river. However, the fault is imaged in seismic-reflection 

data from just north of the Santa Fe River to the southern part 

of the study area, and it corresponds with the aeromagnetic 

expression and exposures of the fault zone at the Santa Fe 

River and the north-northwest aeromagnetic lineament on the 

Santa Fe platform. As discussed previously, the aeromagnetic 

lineament also bounds the east side of the triangular area 

devoid of Espinaso Formation (area A2 on fig. 23) and connects with a mapped, east-down fault outside the area of the 

basin on the south. The seismic images show the San Isidro 

Crossing fault as a major down-to-the-east fault that bounds 

the western side of a pervasive fault zone (Ferguson and 

others, 2007). The fault zone is characterized by west-tilted 

and chaotic fault blocks across a 3–5 km (2–3 mi)-wide zone, 

as shown in the 2D geophysical model for the Seton Village 

profile (fig. 30). Faults interpreted from the gravity data 

provide additional evidence for the Agua Fria fault system, 

especially south of 35°37'30" N. (fig. 29), where its aeromagnetic expression is unclear. 

Part of the expression of the Agua Fria fault system in the 

aeromagnetic data coincides with several north-south trending, 

elongated aeromagnetic anomalies northwest of Santa Fe. 

Grauch and Bankey (2003) tentatively interpreted these 

anomalies as narrow structural highs underlain by Precambrian 

through Oligocene rocks. On the basis of 2D geophysical 

profile models and magnetic depth estimates, they concluded 

that magnetic rocks (presumably bedrock) were detected 

about 600 m (2,000 ft) below the surface. On the basis of 

integration of gravity, well, and seismic information (John 

Ferguson, University of Texas at Dallas, written commun., 

2007), this depth must be too shallow. However, the magnetic 

depth estimates and modeling results still require magnetic 

sources at this depth. These sources are depicted as layers with 

magnetic susceptibility of 0.01 (SI) (table 2) that follow the 

inferred dip of beds in fault blocks in the Tesuque and Alamo 

Creek profile models (figs. 17 and 22). These magnetic sources 

must be contained within the overlying Santa Fe Group 

sediments and have been upturned owing to tilting of faultbounded blocks within the Agua Fria fault system. Perhaps 

the sources are upturned layers of magnetic sediment, such as 

lithosome E of the Tesuque Formation (fig. 4). Alternatively, 

there could be previously unrecognized basalt interbedded 

with the Tesuque Formation, but no plausible source has been 

recognized. In any case, the 2D and 3D models and seismic 

data all indicate that the Santa Fe Group, Tertiary older 

volcanic rocks, and Precambrian basement (and pre-rift strata 

where present) are all involved in the tilted fault blocks. These 

structural highs at the modeled base of the Santa Fe Group 

surface are shown by the disrupted elevation contours on both 

sides of the modeled east-down fault (fig. 25).

Faults that can be recognized by use of aeromagnetic 

data are important because the magnetic expression indicates 

a substantial difference in bulk lithology on either side of 

the fault, such as differences in grain size or lithic type. The 

juxtaposition of such different materials identifies places 

where average aquifer properties may differ across the fault. 

Conversely, the absence of aeromagnetic expression associated with mapped faults known to offset magnetic sediments 

suggests that the sediment type, when considered in bulk, 

does not differ significantly across these faults. Such a case 
Interpreted Structure 59

Figure 28. Demonstration of horizontal-gradient magnitude of reduced-to-pole aeromagnetic data used to interpret faults 

geophysically and to see relations to faults mapped geologically. Color shaded-relief images illuminated from the west. A, 

horizontal-gradient magnitude of reduced-to-pole aeromagnetic map. Most linear ridges on the map are related to faults; 

others are related to drainages, as labeled. B, The horizontal-gradient-magnitude map overlain with interpreted faults 

following ridges (white lines) and mapped faults (pink lines) from Koning and Maldonado (2003). Relations are discussed in 

text. (Modified from Grauch and Hudson, 2007). 

EXPLANATION

Fault interpreted from magnetic data

Fault from geologic mapping—Ball on

 downthrown side, dashed where inferred

NANOTESLA/

KILOMETER

106°00'

35°52'30"

35° 45'

106°

LOCATION MAP

data

area

A B

Buildings

Drainages

N

0 2 MILES

0 2 KILOMETERS

84 84
60 Geophysical Interpretations of the Southern Española Basin, New Mexico

Figure 29. Mapped and geophysically interpreted faults. Faults interpreted from aeromagnetic data (magenta lines) are from 

Grauch and Hudson (2007). Faults interpreted from gravity data (green lines) are from the horizontal-gradient magnitude of the 

isostatic residual gravity. Mapped faults are from figure 3. Agua Fria and Barrancos fault systems are discussed in text. 

? ?

?

?

?

Aeromagnetic data area

Seton Village fault

Barrancos monocline

Cochiti Cone fault La Bajada fault

Tijeras-Cañoncito fault

Rio

Grande

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Santa Fe River fault

Cerrillos

Hills

Cundiyo

Seton

Village

La Cienega

Cerros del Rio

Volcanic Field

Cerros del Rio

volcanic field

Pajarito - Embudo Fault System

Pajarito - Embudo fault system

San Isidro Crossing fault

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

EXPLANATION

Mapped contact of Santa Fe Group 

 sediments—Generalized from figure 3

Mapped fault or fault zone from figure 3 

 —Ball on downthrown side

Fault location interpreted from magnetic 

 data—From Grauch and Hudson (2007) 

Fault location interpreted from gravity 

 data—Inferred from horizontal gradient 

 magnitude (HGM) of gravity data. 

 Four different line weights, from thin to thick, 

 indicate small to large HGM values 

General extent of Barrancos fault system

General extent of Agua Fria fault system 
Interpreted Structure 61

Figure 30. Two-dimensional gravity and magnetic model for Seton Village profile. Profile is located on inset and 

on figures 13, 15, and 16. Model demonstrates the Agua Fria fault system and a fundamental difference in the 

configuration of Precambrian basement in the southern part of study area. The disrupted surface over the Agua Fria 

fault system shows a series of east-down faults, the details of which are defined primarily by seismic information 

(John Ferguson, University of Texas at Dallas, written commun., 2007). Note the east-dipping sides of the shallow 

part of the magnetic Precambrian body. 

WEST EAST

OB-A CDEX ND36 

ND54 

Seton

Village

SFG 

pre-rift 

Tov 

ND4 

Agua Fria fault system

data problem

SETON VILLAGE

PROFILE 

LOCATION

CDEX 

Santa Fe Group sediments

Tertiary older volcanic rocks

Pre-rift sedimentary section 

 (except Paleozoic limestones)

Precambrian basement and 

 Paleozoic limestones

Precambrian basement of differing 

 physical properties

Well location, site code, and 

 total depth extent (Appendixes 1 

 and 2).

SFG

Tov

pre-rift

Observed 

EXPLANATION

GRAVITY, 

IN MILLIGALS 

–20 

–10 

10 

20 

30 

40 

MAGNETIC FIELD, 

IN NANOTESLAS ELEVATION, IN FEET 

–6,000 

–4,000 

–2,000 

0 

2,000 

4,000 

6,000 

8,000 

–500 

–250 

0 

0 

250 

500 

ELEVATION, IN METERS 

–1,000 

0 

1,000 

2,000 

Santa Fe 

Mountains

Calculated 

Observed 

Calculated 

VERTICAL EXAGGERATION = 1.5 DISTANCE, IN KILOMETERS

40 45 50 55

pe/g

pe/g

peweak1

pe-weak1

pemod1

pemod2

pe-mod1

pe-mod2

pemag

pe-mag

25

25

Base of Santa Fe Group from gravity inversion—

 Grid points are projected onto the profile from 

 a maximum distance of 300 m
62 Geophysical Interpretations of the Southern Española Basin, New Mexico

occurs south of Chimayó in the northeastern part of the study 

area, where many more mapped faults are present than are 

apparent in the aeromagnetic map (E on fig. 19B; Koning, 

Nyman, and others, 2005). Reconnaissance magnetic susceptibility measurements suggest that most sediment types in the 

area are fairly magnetic (on the order of 0.001 (SI)), so the 

lack of aeromagnetic expression implies that these magnetic 

sediments are juxtaposed against similar magnetic sediments at depth. Moreover, coarse gravels with low magnetic 

susceptibilities (<0.0005 (SI), V.J.S. Grauch, unpub. data, 

2007) measured close to the Precambrian basement contact 

about 2 km (1.25 mi) west of Cundiyo may not be extensive 

at shallow depths to the west. If they were, their juxtaposition 

with magnetic sediments at the mapped faults would produce 

linear anomalies.

A comparison of mapped and geophysically inferred 

faults with the modeled base of Santa Fe Group (fig. 31) 

shows a relation between the two. The Agua Fria fault system 

forms the faulted and irregular form of the basin floor along 

the southern mountain front. The Barrancos monocline, dissected by the Barrancos fault system, is apparent as a steep 

slope in the basin floor as it dips westward toward the deepest 

parts of the basin. 

Overall basin structure is depicted by the modeled base 

of the Santa Fe Group (fig. 25 and pl. 1) and the corresponding 

isopach map (fig. 26 and pl. 2). As an additional visualization 

tool, figure 32 shows a 3D perspective view looking east; 

the topographic surface is positioned above for comparison. 

The isopach map shows that the southern Española Basin 

transitions northward from an extensive platform structure 

in the southern Santa Fe embayment to a west-tilted half 

graben where basin-fill thickness increases to 2,000–3,000 m 

(8,000–10,000 ft). The basin deepens northward relatively 

abruptly at a north-facing, curvilinear, north-down flexure 

called the Rancho Viejo hinge zone and is superimposed on an 

asymmetric, north-plunging syncline developed in the pre-rift 

rocks. We define the hinge zone primarily by examining the 

modeled thickness of the Santa Fe Group and secondarily on 

noted differences in aeromagnetic patterns (fig. 24). It differs 

in detail from the “Santa Fe embayment hinge line” of Phillips 

and Grauch (2004), which was defined on the basis of the 

aeromagnetic data alone. The abrupt deepening is most evident 

in the southernmost extent of the hinge zone; it generally 

follows the 600-ft isopach contour on plate 2. Rate of increase 

in thickness to the north across this part of the hinge zone 

is about 130–150 m/km (700–800 ft/mi). The eastern and 

western limbs of the hinge zone generally track between the 

500 and 1,000-foot isopach contours (pl. 2). 

North of the hinge zone, the basin floor slopes fairly 

steeply westward from the mountain front, flattens slightly, 

then steepens again into the deepest parts of the basin, as discussed in the section on Precambrian basement. A subsurface 

bedrock high bounds the western side of the deep part of the 

basin on the south, generally located underneath the Cerros del 

Rio volcanic field. On the far northwestern part of the study 

area, irregular displacements along the Pajarito fault system 

are evident from large undulations of the basal surface. 

The Rancho Viejo hinge zone generally coincides with 

thickened Espinaso Formation underlying the Santa Fe Group, 

northeast-trending linear aeromagnetic anomalies (interpreted 

as upturned fault blocks of older volcanic rocks or paleochannels filled by Cieneguilla basalt flows), and circular negative 

aeromagnetic anomalies of about 1.6 km (1 mi) diameter 

interpreted as volcanic vents associated with the Espinaso 

Formation (figs. 23 and 24). These coincidences suggest that 

volcanism and possible faulting may have been associated 

with the formation of the hinge. The Rancho Viejo hinge zone 

may represent a long-lived crustal weakness that underwent a 

reversal of motion from Laramide time (north side up) to Rio 

Grande rift time (north side down). As the motion reversed, 

volcanic eruptive centers may have localized near the hinge 

zone along dilational fractures. Roughly similar dips between 

the lower Tesuque Formation and the top of the older volcanic 

rocks just north of the hinge zone suggest that pronounced 

tilting did not occur until after deposition of the Tesuque 

Formation. This tilting may have preserved Espinaso Formation to the north and facilitated erosion of Espinaso Formation 

in the uplifted area to the south. Lack of deformation in the 

overlying Ancha Formation suggests that the tilting had ceased 

before Pliocene time. 
Interpreted Structure 63

Figure 31. Geologically mapped and geophysically interpreted faults on the base of Santa Fe Group surface (shown without 

contour lines). Faults from figure 29 are overlain on the three-dimensional model surface that represents the elevation of base 

of the Santa Fe Group from figure 25. Labeled faults are from figure 3. La Bajada fault

Tijeras-Cañoncito fault

Rio

Grande

Santa Fe

Pojoaque

Eldorado

at Santa Fe

Española

Cerrillos

Hills

Cundiyo

Seton

Village

La Cienega

Cerros del Rio

volcanic field

Pajarito - Embudo fault system

106°00'

36°00'

35°45'

35°30'

106°15'

0 5 10 KILOMETERS

0 5 10 MILES

ELEVATION,

IN FEET

–2,000

–1,000

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

Mapped contact of Santa Fe Group 

 sediments—Generalized from figure 3

Mapped fault or fault zone from figure 3—

 Ball on downthrown side 

Fault location interpreted from magnetic 

 data—From Grauch and Hudson (2007) 

Fault location interpreted from gravity data—

 Inferred from horizontal gradient magnitude 

 (HGM) of gravity data. Four different line 

 weights, from thin to thick, indicate small to 

 large HGM values 

Rancho Viejo hinge zone

EXPLANATION
Figure 32. Perspective view of the modeled elevation of the base of the Santa Fe Group in relation to topography, looking east. Topographic surfaabove the base.

–1,000

0

1,000

2,000

ELEVATION,

 IN METERS 

3,990,000 3,980,000 3,970,000 3,960,000 3,950,000 3,940,000 3,930,000 3,920,00

NORTHING, IN METERS VERTICAL EXAGGE

–1,500

–1,000

–500

0

500

1,000

1,500

2,000

2,500

ELEVATION,

IN METERS

Cerrillos 

Hills

Santa Fe Platform

Santa

Fe

Cerrillos 

Hills

Santa

Fe

N

 

SANTA FE MOUNTAINS

 

SANTA FE MOUNTAINS

Rancho Viejohinge zone

Barrancos fault system

Española Agua Fria fault system

Española

25

84

68

285

285
Summary 65

Summary

The Española Basin is situated between the rightstepping, en echelon Santo Domingo and San Luis Basins of 

the Rio Grande rift in north-central New Mexico. Located 

between the Santa Fe Mountains on the east and the Jemez 

Mountains on the west, the southern Española Basin consists 

of a westward-thickening wedge of rift sediment that serves 

as an important aquifer for the city of Santa Fe and surrounding areas. Regional gravity data show that the deepest parts 

(>3,000 m (10,000 ft)) of the Española Basin are west of the 

Rio Grande under the Pajarito Plateau.

To better understand the hydrogeologic framework in 

this semiarid region, detailed aeromagnetic surveys were 

flown over the southern Española basin, including the area 

commonly known as the Santa Fe embayment. This report 

presents a synthesis of these data with gravity data and other 

constraints. The interpretations were accomplished using 

qualitative interpretation, quantitative data analysis, and 2D 

and 3D modeling. The results depict the presence of and depth 

to many geologic features that have hydrogeologic significance, such as shallow faults, various types of igneous units, 

and basement rocks. A composite of selected map interpretations is presented in plate 1. In addition, a digital surface was 

constructed that depicts the base of Santa Fe Group sediments, 

which constitutes the primary aquifer system for residents of 

Santa Fe, Española, and six Pueblo nations. The surface is 

shown as a contour map on plate 1 and is available in georegistered digital format on the CD. From this basal surface, 

an isopach of the thickness of Santa Fe Group sediments was 

computed; it estimates the maximum potential aquifer thickness across the basin (pl. 2).

The modeled base of the Santa Fe Group shows that the 

southern Española Basin transitions from a platform on the 

south to a deep, west-tilted half graben on the north. The Santa 

Fe platform is generally composed of eroded late Eocene 

and Oligocene volcanic and volcaniclastic rocks overlain by 

rift sediments of the Santa Fe Group that are typically less 

than 75 m (250 ft) thick. The sediments thicken over local 

irregularities and paleovalleys incised into the platform. 

Models that are constrained primarily by the gravity data 

imply that one paleovalley in the vicinity of Eldorado at 

Santa Fe is filled with 90–150 m (300–500 ft) of Santa Fe 

Group sediments. In this and two other areas on the platform, 

aeromagnetic analysis implies that volcanic and volcaniclastic 

rocks are absent underneath the Santa Fe Group. In these areas 

we infer a direct hydrologic connection between Santa Fe 

Group sediments and underlying early Tertiary to Paleozoic 

sedimentary rocks. 

The basin transitions abruptly from the platform to the 

deeper basin across a curvilinear zone west and southwest 

of Santa Fe, called the Rancho Viejo hinge zone. The 

Rancho Viejo hinge zone is of particular importance to the 

hydrogeologic framework as well as to rift-related and pre-rift 

structural interpretations. Santa Fe Group sediments thicken 

substantially across the hinge zone at about 130–150 m/km

(700–800 ft/mi). North of the hinge zone, the basin deepens 

less markedly but ultimately reaches depths greater than 

3,000 m (10,000 ft) under the Pajarito Plateau west of 

Española. Thus, the hinge zone defines the southern extent of a 

relatively thick Santa Fe Group aquifer. Below the area of the 

hinge zone, seismic interpretations by other workers indicate 

that most of the deeper Paleozoic and Mesozoic sedimentary 

section is unconformably truncated. The truncation results in 

a thin section of mostly basal limestones preserved below the 

deep basin on the north and a thick section of mostly clastic 

rocks preserved below the shallow basin on the south. Gravity 

modeling suggests that the erosional truncation of this prerift sedimentary section follows a northwest trend, possibly 

marking the southernmost limit of a Laramide uplift. 

The Rancho Viejo hinge zone may represent a long-lived 

crustal weakness that underwent a reversal of motion from 

Laramide time (north side up) to Rio Grande rift time (north 

side down). Magmatism may have been associated with the 

beginning of the reversal in Eocene time, on the basis of 

interpretations that thick Tertiary older volcanic rocks and 

associated vents underlie the Santa Fe Group in the vicinity 

of the hinge zone. Inferences from well data combined with 

the modeled surfaces suggest that the volcanic rocks tilted to 

the north after the deposition of the Miocene lower Tesuque 

Formation and before deposition of the Plio-Pleistocene 

Ancha Formation. 

Basin models show a west-tilted, narrow half graben 

north and west of Santa Fe; it is bounded on its west side by a 

north-northeast-elongated bedrock high underneath the Cerros 

del Rio volcanic field. This bedrock high is constrained by MT 

and gravity models considered together. Its eastern side may 

be represented by east-down faults or an east-dipping monocline. The eastern side of the half graben lies with variable 

dip parallel to the north-trending mountain front of the Santa 

Fe Mountains. The buried basement surface there undulates 

from moderate westward dip (7°–20°) at the mountain front, 

to shallow dip (~5°), and back to moderate dip 10–12 km 

(6.25–7.5 mi) west of the front. 

At several places along the front of the Santa Fe Mountains, northerly elongated, alternating narrow highs and lows 

in the aeromagnetic data suggest that Precambrian basement 

has a regional linear fabric. This fabric may influence groundwater flow within the mountain block. However, the regional 

fabric is not apparent more than 1–5 km (0.6–3 mi) west of 

the mountain front. Thus, the Precambrian basement in the 

subsurface likely has lithologic or structural characteristics 

that are much different at regional scale than the characteristics observed in exposed basement rocks. The marked difference in basement character along the western side of the 

mountain front probably originated from tectonic events that 

preceded rifting.

Using the geophysical data, we recognize numerous 

north- to northeast-striking faults throughout the basin that 

likely offset strata by at least 30 m (100 ft). In the southern 

part of the study area the faults do not penetrate the overlying 
66 Geophysical Interpretations of the Southern Española Basin, New Mexico

Plio-Pleistocene Ancha Formation of the Santa Fe Group. 

Combined with detailed geologic mapping, the geophysical 

data suggest that faults are more abundant than previously 

thought in the southern Española Basin. This abundance 

increases the potential for partial ground-water barriers or 

aquifer compartmentalization within the Santa Fe Group 

aquifer because (1) other workers have found that clay is 

common in the cores of fault zones in the basin, which reduces 

permeability at fault zones, and (2) fault-related aeromagnetic anomalies are likely caused by major contrasts in bulk 

lithologic characteristics, which may indicate permeability 

differences on either side of faults.

We categorize many of the numerous faults into two 

major fault systems, the Agua Fria and Barrancos fault 

systems. Interpretations based on several types of geophysical 

data show that the Agua Fria fault system is a zone 3–5 km 

(2–3 mi) wide consisting of generally east-down faults that 

bound west-tilted and chaotic fault blocks. It can be traced 

from the Tijeras-Cañoncito strike-slip system on the far south 

to the latitude of 35°47'30" on the north, where its northern 

termination is unclear. The western side of the Agua Fria 

fault system includes the mapped San Isidro Crossing fault, 

which is associated with a drop in water table. Near its farthest 

southern extent, the east side of the fault system marks the 

western side of an extensive paleovalley inferred to lie in 

the vicinity of Eldorado at Santa Fe. North of Santa Fe near 

latitude 35°45', magnetic modeling and analysis indicate that 

magnetic beds apparently within the basin fill are disrupted by 

the Agua Fria fault system at depths of about 610 m (2,000 ft). 

Their geologic affinity has not been determined.

The Barrancos fault system is best defined in exposures 

north of latitude 35°47'30" to the vicinity of Española. It 

is characterized by a zone ~5 km (3 mi) wide consisting of 

east- and west-down faults across a large-scale, west-dipping 

monocline. Structural relief on the monocline is 450–1,800 m 

(1,500–6,000 ft). The southern and northern extents of the 

Barrancos fault system are poorly defined.

Finally, the high-amplitude aeromagnetic anomalies associated with the volcanic rocks of the Cerros del Rio volcanic 

field and the Cerrillos intrusion can be used to determine their 

lateral extents under cover. Examination of aeromagnetic patterns suggests that basalts from the volcanic field generally do 

not extend beyond the limits of exposure except in one local 

place about 9 km (5.6 mi) west of Santa Fe, where the Ancha 

Formation of the Santa Fe Group covers the basalt. Aeromagnetic anomalies associated with the late Eocene and Oligocene 

Cerrillos intrusion show that it is a large intrusive complex 

that encompasses the Cerrillos Hills and areas to the east and 

southeast. Aeromagnetic data suggest that the bulk of the 

intrusion has fairly steep sides except on the west. This interpretation suggests that laccolithic sills known to be present 

must be volumetrically insignificant, occur only on the west 

side, or cannot be distinguished magnetically from overlying 

Tertiary older volcanic rocks.

Description of Digital Products

Table 4 presents short descriptions of digital files related 

to this study that can be downloaded from the CD in the 

pocket or from directories in the U.S. Geological Survey Web 

site. They contain grids and vector files that can be used in 

standard geographic information systems and in geophysical 

software packages.

Acknowledgments

This work was funded by the U.S. Geological Survey 

National Cooperative Geologic Mapping Program and the 

New Mexico Office of the State Engineer (OSE). Dan Koning and Peggy Johnson collaborated under the auspices of the 

New Mexico Bureau of Geology and Mineral Resources. We 

are grateful to John Ferguson (University of Texas at Dallas 

and Summer of Applied Geophysical Experience (SAGE)), 

to Shawn Biehler (University of California, Riverside and 

SAGE), and to Mark Hudson, David Sawyer, and Brian 

Rodriguez (U.S. Geological Survey) for generously sharing 

their unpublished work. We greatly appreciate reviews of 

earlier drafts by Alan Cuddy (OSE), Jack Frost (OSE), Darcy 

McPhee (U.S. Geological Survey and SAGE), Mike Sully 

(OSE), and Don Sweetkind (U.S. Geological Survey), which 

led to significant improvements. 

J.D. Phillips designed and modified magnetic depth 

estimation techniques specifically for this study (section 

“Magnetic Depth Estimation” and appendix 3). He also supplied computer programs and collaborated on general data 

analysis and magnetic interpretation. D.J. Koning provided 

extensive guidance on geologic information, much of which 

is derived from his own mapping (sections “Geologic Setting” 

and “Other Data”). P.S. Johnson provided locations, data, and 

assistance on interpreted lithology in wells that she and D.J. 

Koning compiled (section “Other Data” and appendixes 1 

and 2). Viki Bankey compiled and rectified inconsistencies in 

gravity data available as of 2006 (section “Gravity Data”) and 

helped construct two-dimensional geophysical models (section 

“Profile Models”). Additional independent constraints are in 

large part from unpublished sources (most recently from 2008) 

that are described in the text; they include physical-property 

measurements (section “Physical Properties,” table 2, and 

fig. 4) and other types of geophysical interpretations and well 

data (section “Other Data” and appendix 4). V.J.S. Grauch digitally merged high-resolution aeromagnetic data from surveys 

flown during 1997–2005 (section “Aeromagnetic Data”). She 

is also responsible for analysis, interpretation, and modeling 

that is not otherwise attributed.
References Cited 67

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no. 2, p. B31–B45, doi:10.1190/1.2347701.

Baldridge, W.S., Damon, P.E., Shafiqullah, M., and Bridwell, 

R.J., 1980, Evolution of the central Rio Grande rift, New 

Mexico—New potassium-argon ages: Earth and Planetary 

Science Letters, v. 51, p. 309–321.

Baldridge, W.S., Ferguson, J.F., Braile, L.W., Biehler, S., 

Jiracek, G.R., Gilpin, B.E., Alumbaugh, D.L., Gratwick, 

D.S., Richards, T.L., and Hasterok, D.P., 2001, Architecture 

of the southern Española Basin (Rio Grande rift) and flank 

uplift adjacent to the La Bajada fault [abs.]: Geological 

Society of America Abstracts with Programs, v. 33, no. 5, 

p. 61.

Table 4. Digital georegistered files.

[Coordinates in meters based on Universal Transverse Mercator Projection, zone 13, North American Datum of 1927. XYZ points are extracted from grids at 

intervals as indicated. Lines and polygons for import in shapefile format are represented by several files with the same prefix and different suffixes, represented below by an asterisk]

File type FileName Description

XYZ ASCII points seb_sfgbasem.xyz Base of Santa Fe Group sediments from three-dimensional model. Elevation in meters; datum is sea level. Original grid interval 100 meters (330 feet).

seb_sfgbaseft.xyz Base of Santa Fe Group sediments from three-dimensional model. Elevation in feet; 

datum is sea level. Original grid interval 100 meters (330 feet).

seb_sfgthickm.xyz Isopach of Santa Fe Group sediments from three-dimensional model. Thickness in 

meters. Original grid interval 250 meters (820 feet).

seb_sfgthickft.xyz Isopach of Santa Fe Group sediments from three-dimensional model. Thickness in 

feet. Original grid interval 250 meters (820 feet).

Shapefile format seb_mag_flts.* Lines representing faults interpreted from magnetic data. Extracted for the study area 

from digital files (crgrmag_faults1.0.*) of Grauch and Hudson (2007).

seb_no_tov.* Lines outlining areas interpreted to be devoid of Tertiary older volcanic rocks underneath Santa Fe Group sediments.

rv_hinge.* Line following general location of interpreted Rancho Viejo hinge zone.

seb_cdr_extent.* Lines representing magnetically inferred extent of Cerros del Rio volcanic field and 

related flows within study area.

seb_grv_flts.* Lines representing faults interpreted from gravity data. 

seb_intrusions.* Lines outlining interpreted extents of Tertiary older volcanic vents and intrusions. 

Includes bodies inferred to be associated with the Cerrillos intrusion and the 

Espinaso Formation.

Metadata text files sfg_model_metadata.txt Metadata describing the grid surfaces related to the three-dimensional model of 

Santa Fe Group sediments.

seb_interp_metadata.txt Metadata describing the interpretations represented by lines and polygons.

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Rodriguez, B.D., and Sawyer, D.A., 2007, Correlation of magnetotelluric resistivity soundings and regional borehole geophysical rock properties to stratigraphy of the northern Rio 

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Sawyer, D.A., 2004, Processed Landsat 7 satellite imagery of 

the Española Basin region, New Mexico: U.S. Geological 

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Appendixes

Appendix 1. Well Data Used to Constrain Three-Dimensional Integrated Model and Gravity Inversion.—Continued[Depths picked for various geologic formations in wells that penetrate the base of the Santa Fe Group. Eastings and northings are based on a Universal Transverse Mercator Procan Datum of 1927. Formation codes: Mz, Mesozoic clastic rocks; PC, Precambrian basement rocks, Pz, Paleozoic clastic and carbonate rocks; SFG, Santa Fe Group; Tgd, EocDiamond Tail Formations; Ti, Tertiary intrusive rocks; Tov, Tertiary older volcanic rocks. Tgd, Mz, and Pz constitute the pre-rift sedimentary section. Ti depth intervals may inTD, total depth of well; NMBGMR, New Mexico Bureau of Geology and Mineral Resources; USGS, U.S. Geological Survey]

Site identifier

(oil exploration well name,

if applicable)

Easting 

(meters)

Northing 

(meters)

Elevation 

(feet)

Total depth 

of well 

(feet)

Formations

in well

Depth to 

SFG base 

(feet)

Depth to top of formation 

(feet) 

Tov Tgd Mz Pz PC

96–1 419028 3931921 6,908 405 SFG/Pz/PC 200 200 2

96–4 417801 3931928 6,821 470 SFG/Pz 280 280 

castlewig1

(Castle Wigzell KellyFederal #1).

413180 3981070 6,040 2,703 SFG/Tov/Pz/ PC. 2,380 2,380 2,420 2,7

CKZ#1–G–7 

(John Gianardi #1). 

402260 3930730 6,210 7,773 SFG/Tgd/Mz/Pz. 175 175 1,659 7,109 

EB–001 398579 3935004 6,063 221 SFG/Tgd 72 72 

EB–002 399120 3935618 6,070 380 SFG/Ti 210 

EB–007 403983 3929089 6,190 146 SFG/Tov 96 96 

EB–020 414999 3943065 7,043 658 SFG/Tov 349 349 

EB–106–RSL#1 416860 3929416 6,705 1,490 SFG/Mz 110 110 

EB–118 416160 3943637 7,141 600 SFG/Tov/Tgd 80 80 240 

EB–139–RSL#2 417190 3928776 6,680 1,558 SFG/Mz 74 74 

EB–202 410066 3926479 6,318 156 SFG/Tov 111 111 

EB–209 405075 3922945 6,105 276 SFG/Tov 23 23 

EB–211 407315 3926117 6,267 200 SFG/Tov 90 90 

EB–214 401271 3932150 6,186 210 SFG/Tgd 130 130 

EB–344 407223 3930261 6,316 400 SFG/Tov 70 70 

EB–371 416308 3954596 7,209 980 SFG/PC 955 9

EB–374 403678 3928978 6,178 200 SFG/Tov 110 110 EB–378 400527 3933618 6,121 110 SFG/Tov 75 75 
Appendix 1. Well Data Used to Constrain Three-Dimensional Integrated Model and Gravity Inversion.—Continued [Depths picked for various geologic formations in wells that penetrate the base of the Santa Fe Group. Eastings and northings are based on a Universal Transverse Mercator Procan Datum of 1927. Formation codes: Mz, Mesozoic clastic rocks; PC, Precambrian basement rocks, Pz, Paleozoic clastic and carbonate rocks; SFG, Santa Fe Group; Tgd, EocDiamond Tail Formations; Ti, Tertiary intrusive rocks; Tov, Tertiary older volcanic rocks. Tgd, Mz, and Pz constitute the pre-rift sedimentary section. Ti depth intervals may inTD, total depth of well; NMBGMR, New Mexico Bureau of Geology and Mineral Resources; USGS, U.S. Geological Survey] Site identifier (oil exploration well name, if applicable) Easting (meters) Northing (meters) Elevation (feet) Total depth of well (feet) Formations in well Depth to SFG base (feet) Depth to top of formation (feet) 

Tov Tgd Mz Pz PC EB–404 403062 3927680 6,140 150 SFG/Tgd 79 79 

EB–412 404975 3931598 6,274 510 SFG/Tov 350 350 

EUI#13–95–1 417735 3932199 6,860 1,000 SFG/Pz 230 230 

EUI#14–96–2 417620 3932675 6,851 400 SFG/Pz 200 200 

EUI#15–96–3 417949 3931921 6,852 400 SFG/Pz 240 240 

G2–4 409750 3925300 6,343 2,010 SFG/Tov/Tgd 100 100 

G–8A‡ 401569 3925678 6,083 820 Tov/Tgd 0 485 

G–11A 412817 3933958 6,612 460 SFG/Tov 200 200 

G–12 405230 3923870 6,183 1,000 SFG/Tov 135 135 

G–16 405710 3924566 6,161 170 SFG/Tov 100 100 

G–25 404386 3931835 6,277 150 SFG/Tov 139 139 

G–64 405767 3929190 6,262 927 SFG/Tov/Tgd 170 170 850 

G–112 399569 3934917 6,121 160 SFG/Tov 120 120 

G–178 414743 3930998 6,624 580 SFG/Tov 120 120 

G–179/EB–581 415097 3928250 6,584 600 SFG/Tgd 80 80 

G–181 415664 3931321 6,709 500 SFG/Tov 280 280 

G–182 414060 3931538 6,625 500 SFG/Tov 170 170 

G–205 406143 3926773 6,223 600 SFG/Tov 190 190 

G–206 406949 3927579 6,264 600 SFG/Tov 105 105 

G–214 406910 3927125 6,246 500 SFG/Tov 110 110 G–215 406540 3926575 6,260 1,000 SFG/Tov 200 200 

G–218 406800 3926621 6,263 415 SFG/Tov 120 120 
Appendix 1. Well Data Used to Constrain Three-Dimensional Integrated Model and Gravity Inversion.—Continued[Depths picked for various geologic formations in wells that penetrate the base of the Santa Fe Group. Eastings and northings are based on a Universal Transverse Mercator Procan Datum of 1927. Formation codes: Mz, Mesozoic clastic rocks; PC, Precambrian basement rocks, Pz, Paleozoic clastic and carbonate rocks; SFG, Santa Fe Group; Tgd, EocDiamond Tail Formations; Ti, Tertiary intrusive rocks; Tov, Tertiary older volcanic rocks. Tgd, Mz, and Pz constitute the pre-rift sedimentary section. Ti depth intervals may inTD, total depth of well; NMBGMR, New Mexico Bureau of Geology and Mineral Resources; USGS, U.S. Geological Survey]

Site identifier

(oil exploration well name,

if applicable)

Easting 

(meters)

Northing 

(meters)

Elevation 

(feet)

Total depth 

of well 

(feet)

Formations

in well

Depth to 

SFG base 

(feet)

Depth to top of formation 

(feet) 

Tov Tgd Mz Pz PC

JAILWell 405056 3935836 6,341 1,365 SFG/Tov 1,214 1,214 

Lot12–2 415009 3927268 6,603 480 SFG/Tgd 135 135 

Lot14–3 414539 3926850 6,584 510 SFG/Tgd 125 125 

Lot7–1 416025 3927090 6,662 510 SFG/Mz 70 70 

McMillan 414140 3930160 6,491 640 SFG/Tov 160 160 

merrblk2 

(Merrion Blackshare #2). 

383255 3918401 6,010 6,820 SFG/Tov/Tgd/

Ti/Tgd/Mz.

2,150 2,150 3,534 6,407 

MonteAltoPlaza 417200 3933170 6,870 1,000 SFG/Pz 255 255 

ND1 399600 3939334 6,220 900 SFG/Tov 790 790 

ND9 409491 3932823 6,419 1,717 SFG/Tov 475 475 

ND10 412409 3932369 6,510 1,117 SFG/Tov 115 115 

ND38 401379 3937859 6,200 1,145 SFG/Tov 600 600 

ND49 406741 3925436 6,200 660 SFG/Tov 80 80 

ND85 401047 3938905 6,150 750 SFG/Tov 710 710 

OB–A 407775 3939338 6,475 1,802 SFG/Tov 1,710 1,710 

peltblk1

(Pelto Blackshare #1).

384765 3917614 6,034 7,025 SFG/Tov/Tgd/Mz. 1,480 1,480 3,230 4,300 

peltort1 

(Pelto Ortiz #1).

403901 3919011 5,804 7,450 SFG/Tgd/Mz 79 79 4,679 

R1A 420247 3931136 6,886 232 SFG/Pz/PC 42 42 2R1–EUI#8 419757 3930646 6,844 325 SFG/Pz 95 95 

R3 418445 3931260 6,860 258 SFG/Pz 164 164 

R5 419145 3930326 6,818 332 SFG/Pz 176 176 

R36 416780 3934540 6,845 405 SFG/Pz 146 146 
Appendix 1. Well Data Used to Constrain Three-Dimensional Integrated Model and Gravity Inversion.—Continued [Depths picked for various geologic formations in wells that penetrate the base of the Santa Fe Group. Eastings and northings are based on a Universal Transverse Mercator Procan Datum of 1927. Formation codes: Mz, Mesozoic clastic rocks; PC, Precambrian basement rocks, Pz, Paleozoic clastic and carbonate rocks; SFG, Santa Fe Group; Tgd, EocDiamond Tail Formations; Ti, Tertiary intrusive rocks; Tov, Tertiary older volcanic rocks. Tgd, Mz, and Pz constitute the pre-rift sedimentary section. Ti depth intervals may inTD, total depth of well; NMBGMR, New Mexico Bureau of Geology and Mineral Resources; USGS, U.S. Geological Survey] Site identifier (oil exploration well name, if applicable) Easting (meters) Northing (meters) Elevation (feet) Total depth of well (feet) Formations in well Depth to SFG base (feet) Depth to top of formation (feet) 

Tov Tgd Mz Pz PC R37 416904 3933297 6,836 540 SFG/Pz 180 180 

R49 418178 3934458 6,937 129 SFG/Pz 42 42 

R50 417744 3934277 6,900 440 SFG/Pz 79 79 

R–51 417390 3934580 6,890 311 SFG/Pz 91 91 

SVTD205 415460 3938710 6,912 205 SFG/PC 78 

TH3C 415850 3929575 6,680 800 SFG/Mz 95 95 

tomckee1‡ (Trans-Ocean McKee #1). 410103 3916593 5,936 8,128 Tgd/Mz/Pz 0 335 5,350 

ULURU 419527 3932746 6,996 770 SFG/PC 190 1

W2A 416154 3934587 6,796 330 SFG/Pz 285 285 

W3–EUI#3 417888 3934408 6,918 324 SFG/Pz/PC 62 62 3

W4–EUI#4 417829 3934581 6,931 375 SFG/Pz/PC 52 52 3

W–9/WX2E 412842 3932575 6,535 322 SFG/Tov 80 80 

WX11W 419440 3929255 6,765 260 SFG/Mz 80 80 

WX9S/X9N 414451 3928591 6,556 325 SFG/Tov 62 62 

X10N 416990 3928739 6,665 220 SFG/Mz 80 80 

X11A 419744 3929610 6,815 300 SFG/Mz 100 100 

X3C1/2S 415715 3928965 6,635 200 SFG/Mz 120 120 

X3C3/4S 416055 3928777 6,620 260 SFG/Mz 50 50 

X3CE 416690 3930015 6,741 260 SFG/Mz? 153 153 

X3CN 415970 3930380 6,700 240 SFG/Mz? 210 210 

X3CS 415872 3929550 6,680 800 SFG/Mz 97 97 
Appendix 1. Well Data Used to Constrain Three-Dimensional Integrated Model and Gravity Inversion.—Continued[Depths picked for various geologic formations in wells that penetrate the base of the Santa Fe Group. Eastings and northings are based on a Universal Transverse Mercator Procan Datum of 1927. Formation codes: Mz, Mesozoic clastic rocks; PC, Precambrian basement rocks, Pz, Paleozoic clastic and carbonate rocks; SFG, Santa Fe Group; Tgd, EocDiamond Tail Formations; Ti, Tertiary intrusive rocks; Tov, Tertiary older volcanic rocks. Tgd, Mz, and Pz constitute the pre-rift sedimentary section. Ti depth intervals may inTD, total depth of well; NMBGMR, New Mexico Bureau of Geology and Mineral Resources; USGS, U.S. Geological Survey]

Site identifier

(oil exploration well name,

if applicable)

Easting 

(meters)

Northing 

(meters)

Elevation 

(feet)

Total depth 

of well 

(feet)

Formations

in well

Depth to 

SFG base 

(feet)

Depth to top of formation 

(feet) 

Tov Tgd Mz Pz PC

X3CW 414945 3929323 6,619 250 SFG/Tov 155 155 

X3DW/WX3DE 413418 3931155 6,560 345 SFG/Tov 95 95 

YLM#2

(Yates La Mesa #2).

405534 3949690 6,610 7,710 SFG/Tov/Pz/PC. 3,966 3,966 6,018 7,5

YLM#3

(Yates La Mesa #3).

406009 3932807 6,336 4,740 SFG/Tov/Ti/Mz. 393 393 2,300 3,739 

*Data sources: [1] Database compiled by P.S. Johnson and D.J. Koning, NMBGMR, accessed May, 2008; [2] Koning, Nyman, and others (2005); [3] D.J. Koning, NMBGMR

(1999); [5] D.J. Koning, NMBGMR, unpub. data, 2006; [6] Meghan Hodgins, Glorieta Geoscience, Inc., written commun., 2007; [7] D.A. Sawyer, USGS, written commun., 20

[9] Myer and Smith (2006); [10] D.A. Sawyer, USGS, and D.J. Koning, NMBGMR, written commun., 2008.

‡G8–A and tomckee1 were not used in the gravity inversion because Santa Fe Group is not reported in those wells.
82 Geophysical Interpretations of the Southern Española Basin, New Mexico

Appendix 2. Well Data Used to Guide Three-Dimensional Integrated Model.

[Data for wells that do not penetrate below Santa Fe Group. Total depths provide minimum depths to base of Santa Fe Group. Data compiled and maintained by 

P.S. Johnson and D.J. Koning, New Mexico Bureau of Geology and Mineral Resources; accessed May 2008. Coordinates are in Universal Transverse Mercator 

Projection, zone 13, North American Datum of 1927]

Site identifier Easting

(meters)

Northing

(meters)

Elevation

(feet)

Total depth of well 

(feet)

ArchMW 413260 3953571 7,200 1,153

BuckMW 398638 3961316 5,960 2,500

EB–004 411115 3935891 6,594 560

EB–111 414112 3940815 6,909 650

EB–117 415600 3943321 7,042 465

EB–244/SF–1 412888 3946766 6,879 2,020

EB–358/CDEX 409417 3939620 6,555 1,500

EB–385 404356 3936059 6,302 802

G–55 413428 3942801 6,921 495

G–176 412172 3934258 6,579 410

G–180 412022 3934341 6,568 440

ND2 410939 3942442 6,700 1,000

ND3 409556 3941015 6,559 1,000

ND4 411129 3939205 6,660 1,490

ND5 407873 3937626 6,440 995

ND6 406018 3936207 6,385 1,600

ND7 401411 3936858 6,185 680

ND8 404028 3936442 6,305 1,000

ND11 402226 3939282 6,180 1,000

ND12 405955 3947056 6,519 980

ND17 401742 3946125 6,320 1,985

ND36 403661 3939279 6,260 1,400

ND40 400416 3941083 6,280 970

ND41 402439 3942100 6,330 2,000

ND43 401461 3944117 6,322 1,000

ND46 404520 3944262 6,400 1,940

ND52 402455 3940876 6,220 1,000

ND53 401422 3939470 6,140 1,000

ND54 402855 3939482 6,220 1,200

ND55 407108 3942283 6,450 1,450

ND59 401199 3940058 6,260 1,000

ND69 401407 3938451 6,205 1,200

ND76 401636 3939705 6,180 1,140

ND77 401021 3939656 6,240 1,000

ND80 400799 3939253 6,202 800

ND81 400412 3939492 6,190 800

ND82 400595 3939474 6,170 800

ND83 400834 3939664 6,230 1,000

ND84 400202 3939506 6,230 775

ND86 400617 3939083 6,150 810
Appendix 2 83

Site identifier Easting

(meters)

Northing

(meters)

Elevation

(feet)

Total depth of well 

(feet)

ND87 401225 3939290 6,160 840

ND88 402023 3939079 6,170 980

ND89 399902 3940244 6,250 920

ND90 400830 3940309 6,250 1,000

ND91 401631 3939089 6,160 900

ND92 401384 3938854 6,170 800

ND93 401188 3938661 6,150 670

ND94 400803 3939298 6,220 760

ND95 400899 3939494 6,220 975

ND96 400585 3938884 6,120 705

ND97 400814 3939543 6,230 795

R2B 416345 3934450 6,804 260

R11 414271 3934563 6,684 925

SVX8 414817 3938660 6,883 300

W1–EUI#1 411867 3934697 6,556 719

W2–EUI#2 414206 3934726 6,666 350

WX3BN–EUI#6 415869 3932198 6,752 280

WX3N–EUI#7 416683 3931104 6,761 270

X1E 412637 3934197 6,584 300

Appendix 2. Well Data Used to Guide Three-Dimensional Integrated Model.—Continued

[Data for wells that do not penetrate below Santa Fe Group. Total depths provide minimum depths to base of Santa Fe Group. Data compiled and maintained 

by P.S. Johnson and D.J. Koning, New Mexico Bureau of Geology and Mineral Resources; accessed May 2008. Coordinates are in Universal Transverse 

Mercator Projection, zone 13, North American Datum of 1927]
84 Geophysical Interpretations of the Southern Española Basin, New Mexico

Appendix 3. Magnetic Depth Estimation Methods Used in This Study.—Continued

Factor

Method

Multisource Werner Local wavenumber Two-step extended Euler

Input type Magnetic profile data Magnetic profile data or gridded 

magnetic data.

Gridded magnetic data.

Approach Complex linear regression within 

sliding windows using the 

profile and its derivatives.

The local wavenumber profile 

or grid is calculated from the 

derivatives of the magnetic (or 

other potential) field. Local 

curvature is used to locate the 

ridge crests of the local wavenumber and estimate the depth, 

strike (for grids), and structural 

index.

Step one—Least squares solution of Euler’s homogeneity equation within small or 

moderate-sized windows using 

the magnetic field, its first derivatives, its Hilbert transform 

components, and their first 

derivatives. 

Step two—Correct or discard 

solutions above a user-defined 

minimum depth surface, such 

as the ground surface or a surface representing the depth to a 

particular magnetic unit interpreted independently by use of 

other subsurface information.

Assumptions Magnetic anomalies within 

the analysis window are all 

produced by multiple sources 

having the same integer 

structural index (N).

Sources are two-dimensional

Strikes of sources are perpendicular to the profile azimuth.

Total magnetization is parallel to 

the inducing Earth field.

Magnetic anomalies within small 

data windows are produced by 

a single isolated twodimensional source. 

Sources strike perpendicular to 

the profile azimuth, for profile 

analysis.

Total magnetization is parallel to 

the inducing Earth field.

Magnetic anomalies within small 

or moderate-sized data windows are produced by single 

isolated sources satisfying 

Euler’s homogeneity equation. 

A minimum depth surface is 

available for testing and correcting solutions. 

All solutions below the minimum 

depth surface have a single 

known structural index (usually 

N=0). 

Solutions above the minimum 

depth surface can be corrected 

by increasing their structural 

index (N>0) until their depth 

estimates match the minimum 

depth surface or discarded if 

the required adjustment results 

in values of N considered too 

high by the interpreter.

Required input 

parameters

Window length (a multiple of 22)

Number of sources contributing to 

the anomaly within the window 

(we used two or three for this 

study).

Integer structural index of the 

sources (usually N=0 or 1).

Clustering radius (as a percentage 

of depth).

Profile azimuth

Earth field strength and direction 

(optional).

Minimum and maximum acceptable structural index (N).Window size.

Initial (minimum) structural index 

value (usually N=0).

Threshold (maximum) structural 

index value (usually N=1 for 

geologic sources or N=3 for 

anthropogenic sources).

Maximum allowable error in the 

depth (typically 15 percent). 

Minimum depth surface in grid 

form.
Appendix 3 85

Appendix 3. Magnetic Depth Estimation Methods Used in This Study.—Continued

Factor

Method

Multisource Werner Local wavenumber Two-step extended Euler

Output Source locations. Estimated dip 

and magnetic susceptibility 

contrast are provided if Earth 

field is specified. Clustering 

of sources is an indication of 

reliability, so that solutions 

are usually averaged within 

clusters.

Source locations and structural 

indices. Strikes of sources are 

provided for grids. Estimated 

dip and magnetic susceptibility 

contrast are provided for profile 

data.

Source locations, error estimates, 

estimated structural indices for 

sources on the minimum depth 

surface.

Limitations Cannot be used to determine the 

“best” structural index, so it is 

up to the interpreter to decide 

between the shallow “contact” 

solution or the deeper “sheet” 

solution typically produced by 

each source. 

3D sources will not give accurate 

results.

Depths will be overestimated if 

strike is not perpendicular to 

the profile. 

Dip and susceptibility contrast 

will be in error if remanent 

component contributes to a 

total magnetization that is 

oblique to Earth field direction. 

Deeper sources are hard to identify and locate.

Error estimates are not provided.

Interference from multiple nearby 

sources and noise can produce 

inaccurate results. 

Topographic and other 3D sources 

will produce inaccurate results. 

For profile data, depths will be 

overestimated if strike is not 

perpendicular to the profile.

For profile data, dip and susceptibility contrast will be in error if 

remanent component contributes to a total magnetization 

that is oblique to Earth field 

direction.

Deeper sources are hard to identify and locate. 

Error estimates are not provided

Anomalies resulting from interfering sources will produce 

unreliable results. 

Shallow solutions are strongly 

controlled by the minimum 

depth surface. 

Solution depths below the 

minimum depth surface depend 

strongly on the assumed minimum structural index. 

Solutions from adjacent windows 

tend to scatter and may require 

averaging within clusters.

Advantages Provides a robust multisource 

method.

Generally works better than other 

(single-source) methods on 

profile data.

Provides a robust method for 

estimating source locations and 

structural indices. 

Solutions from grid data tend to 

follow linear magnetic sources, 

for which strike estimates are 

provided.

Error estimates, which are derived 

from the least-squares regression, are provided.

3D sources are permitted.

References Hansen and Simmonds (1993) Thurston and Smith (1997)

Phillips (2000)

Phillips (2002)

Phillips and others (2007)

Reid and others (1990).

Nabighian and Hansen (2001).

Phillips (2002).

Phillips and Grauch (2004).
86 Geophysical Interpretations of the Southern Española Basin, New Mexico

Site

identifier

Easting1

(meters)

Northing1 

(meters)

Depth to 

base of SFG 

(feet)

Location

code2 Data source

Well data outside the study area used to constrain the gravity inversion

cpfed 384792 3908568 80 G Colorado Plateau Federal exploration well.

davtam1y 344677 3916345 5,600 G Davis Tamara 1-Y exploration well.

lanlaet4 350565 3962027 184 G AET-4, Goff and others (1988).

lanlfh2 348957 3971676 433 G FH-2.

lanlgt1 349289 3974409 160 G GT-1, Goff and others (1988).

lanljs1 346622 3958867 73 G JS-1, Goff and others (1988).

lanlpc1 349970 3971314 193 G PC-1, Goff and others (1988).

lanlpc2 348012 3971285 394 G PC-2, Goff and others (1988).

lanltsta 349590 3971692 155 G LANL Test Hole A.

lanltstb 350871 3978961 440 G LANL Test Hole B.

lanltstc 353558 3981670 580 G LANL Test Hole C.

lanltstd 345529 3972980 120 G LANL Test Hole D.

lanlvc1 353666 3967306 1,107 G VC-1, Goff and others (1988).

lanlvc2 354544 3974563 2,625 G VC-2, Goff and others (1988).

shsfp1 348375 3913234 2,969 G Shell Santa Fe Pacific #1.

shsfp3 332702 3910313 3,996 G Shell Santa Fe Pacific #3.

shwmf 337593 3893194 8,540 G Shell West Mesa Federal exploration well.

ugbaca12 356490 3970880 6,594 G Union Geothermal Baca 12, Goff and others (1988).

ugbaca17 357565 3973780 5,479 G Union Geothermal Baca 17.

ugbaca20 357720 3973315 5,335 G Union Geothermal Baca 22.

ugbaca22 357408 3973515 5,331 G Union Geothermal Baca 17.

ugbaca4 358240 3972680 5,971 G Union Geothermal Baca 4, Goff and others (1988).

wc23-4 352763 3974904 550 G WC 23-4, Goff and others (1988).

Point depth picks from seismic reflection data

velarde3 414759 3998974 3,150 G Ferguson and others (1995), Velarde line, using velocity=2,400 m/s.

velarde6 417121 3997224 2,440 M Ferguson and others (1995), Velarde line, using velocity=2400 m/s.

EW1_005 399779 3959097 8,702 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

EW1_025 402548 3959175 7,550 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

EW2_010 402240 3947379 4,658 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

NS1_355 404990 3958358 5,917 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

NS1_425 403056 3965269 7,297 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

NS1_445 402154 3967204 7,840 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

NS1_495 401297 3972147 7,873 S J. Ferguson, written commun., 2007, using velocity=2,526 m/s.

AB3559 381791 4008951 800 G Baldridge and others (1994).

AB3725 383607 4009167 2,200 G Baldridge and others (1994).

AB4176 387742 4009094 2,000 G Baldridge and others (1994).

Appendix 4. Other Point-Located Data.

[Depths to the base of the Santa Fe Group at single sites within or surrounding the study area from well data, representative depth picks from seismic-reflection 

and refraction sections, and 2D resistivity models from magnetotelluric soundings. Well data listed without a formal reference correspond to data available from 

the Petroleum Library at the New Mexico Bureau of Geology and Mineral Resources. m/s, meters per second; MT, magnetotelluric; SFG, Santa Fe Group] 
Appendix 4 87

Site

identifier

Easting1

(meters)

Northing1 

(meters)

Depth to 

base of SFG 

(feet)

Location

code2 Data source

Interpreted depth picks from magnetotelluric models at sounding locations

MT_EB01 406752 3978388 3,937 S B. Rodriguez, written commun., 2007.

MT4 391060 3938490 656 S Rodriguez and others (2006).

MT5 393370 3946820 3,937 S B. Rodriguez, written commun., 2007.

MT8 391127 3942655 820 S Rodriguez and others (2006).

MT9 395221 3955517 4,921 S B. Rodriguez, written commun., 2007.

MT11 392391 3945184 1,640 S Rodriguez and others (2006).

MT12 374449 3958861 2,624 M Rodriguez and others (2006).

MT14 389104 3948096 2,624 S B. Rodriguez, written commun., 2007.

MT15 388902 3956023 4,593 S B. Rodriguez, written commun., 2007.

MT17 376785 3949770 4,593 M Rodriguez and others (2006).

MT23 394527 3952588 5,741 S B. Rodriguez, written commun., 2007.

MT25 394185 3948914 5,741 S B. Rodriguez, written commun., 2007.

MT27 395460 3960730 5,250 S B. Rodriguez, written commun., 2007.

MT28 388978 3951444 4,593 S B. Rodriguez, written commun., 2007.

MT29 388978 3944059 984 S B. Rodriguez, written commun., 2007.

MT31 372461 3954777 1,640 M B. Rodriguez, written commun., 2007.

MT32 372338 3960693 2,297 M B. Rodriguez, written commun., 2007.

MT36 374863 3952577 1,640 M B. Rodriguez, written commun., 2007.

1

Sites are located by use of the Universal Transverse Mercator Projection, zone 13, North American Datum of 1927.

2

Location code: G, located in an area covered only by the gravity inversion; M, located in an area covered by both the 3D model and gravity inversion; 

S, located in the study area.

Appendix 4. Other Point-Located Data.—Continued

[Depths to the base of the Santa Fe Group at single sites within or surrounding the study area from well data, representative depth picks from seismic-reflection 

and refraction sections, and 2D resistivity models from magnetotelluric soundings. Well data listed without a formal reference correspond to data available from 

the Petroleum Library at the New Mexico Bureau of Geology and Mineral Resources. m/s, meters per second; MT, magnetotelluric; SFG, Santa Fe Group]
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Grauch and others—Geophysical Interpretations of the Southern Española Basin, New Mexico—Professional Paper 1761

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