Estimation of Alluvial-Fill Thickness

in the Mimbres Ground-Water Basin,

New Mexico, from Interpretation of

Isostatic Residual Gravity Anomalies

Prepared in cooperation with the

NEW MEXICO OFFICE OF THE STATE ENGINEER

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4007
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02-4007

Estimation of Alluvial-Fill Thickness

in the Mimbres Ground-Water Basin,

New Mexico, from Interpretation of

Isostatic Residual Gravity Anomalies

Prepared in cooperation with the

NEW MEXICO OFFICE OF THE STATE ENGINEER

By Charles E. Heywood

Albuquerque, New Mexico

2002
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Denver, CO 80225-0286

District Chief

U.S. Geological Survey

Water Resources Division

5338 Montgomery Blvd. NE, Suite 400

Albuquerque, NM 87109-1311

from:

Information regarding research and data-collection programs of the U.S. Geological Survey is available

on the Internet via the World Wide Web. You may connect to the home page for the New Mexico District

Office using the URL http://nm.water.usgs.gov.
iii

CONTENTS

Page

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

Introduction........................................................................................................................................................................... 1

Purpose and scope....................................................................................................................................................... 1

Previous work.............................................................................................................................................................. 2

Physiographic setting .................................................................................................................................................. 2

Geologic setting .......................................................................................................................................................... 2

Acknowledgments....................................................................................................................................................... 4

Isostatic residual gravity field in the Mimbres Basin............................................................................................................ 4

Separation of anomalies .............................................................................................................................................. 9

Density profiles ........................................................................................................................................................... 10

Alluvial-fill thickness in structural subbasins ....................................................................................................................... 12

Mangas Trench............................................................................................................................................................ 12

Seventysix Subbasin.................................................................................................................................................... 12

Mimbres Trench .......................................................................................................................................................... 13

Cross Graben............................................................................................................................................................... 13

Florida Subbasin ......................................................................................................................................................... 13

Tres Hermanas Graben................................................................................................................................................ 13

Columbus Subbasin..................................................................................................................................................... 13

Boundary areas............................................................................................................................................................ 13

Summary ............................................................................................................................................................................... 14

Selected references................................................................................................................................................................ 14

FIGURES

1-4. Maps showing:

1. Location of physiographic features and subbasins of the Mimbres Basin ..................................................... 3

2. Surficial geology of the Mimbres Basin area and locations of seismic-refraction

lines and deep exploratory drill holes....................................................................................................... 7

3. Measurement points and isostatic residual gravity in the Mimbres Basin .................................................... 8

4. Estimated thickness of alluvial fill in the Mimbres Basin............................................................................. 11

TABLES

1. Geologic units in the Mimbres Basin area............................................................................................................... 5

2. Depth intervals and densities for lithofacies groups 1, 2, and 3 in the Mimbres Basin........................................... 6

3. Location and depth to bedrock of deep exploration drill holes in the Mimbres Basin............................................ 10
iv

CONVERSION FACTORS AND VERTICAL DATUM

Altitude, as used in this report, refers to distance above sea level.

Multiply By To obtain

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

square kilometer (km2) 0.3861 square mile (mi2)

square kilometer (km2) 2.471 acre

kilogram per cubic meter (kg/m3) 0.001 gram per cubic centimeter (g/cm3)

1 milligal (Mgal) = 0.001centimeter/second/second 
1

ABSTRACT

The geologic structure of the Mimbres

ground-water basin in southwest New Mexico is

characterized by north- and northwest-trending

structural subbasins. Sedimentation of Miocene

and Pliocene age has filled and obscured the

boundaries of these subbasins and formed potentially productive aquifers of varied thickness. The

location and depth of the subbasins can be estimated from analysis of isostatic residual gravity

anomalies. Density contrasts of various basement

lithologies generate complex regional gravity

trends, which are convolved with the gravity signal

from the Miocene and Pliocene alluvial fill. An

iterative scheme was used to separate these

regional gravity trends from the alluvial-fill gravity signal, which was inverted with estimated

depth-density relations to compute the thickness

of the alluvial fill at 1-kilometer spacing. The

thickness estimates were constrained by exploratory drill-hole information, interpreted seismicrefraction profiles, and location of bedrock lithology from surficial geologic mapping. The resulting map of alluvial-fill thickness suggests large

areas of thin alluvium that separate deep structural

subbasins.

INTRODUCTION

Ground water is a principal supply for the domestic, industrial, and agricultural water demands in the

Mimbres Basin in southwest New Mexico. In his report

on the ground-water supply of the Mimbres Valley,

White (1930) stated “It is recognized that overdevelopment would be fatal to the best interests of the community and that future expansion in pumping activities

should be guided by as adequate information as it is

possible to obtain concerning the extent of this

resource.” Substantial ground-water withdrawals since

the 1930’s resulted in ground-water drawdowns of 30

m south of Deming and 40 m east of Columbus by 1970

(Hanson and others, 1994). Land subsidence and associated fissuring south of Deming resulted in abandonment of some agricultural land (Contaldo and Mueller,

1991). To better manage the ground-water resources,

the New Mexico Office of the State Engineer needs an

improved understanding of the shallow geologic structure of the Mimbres Basin. Thickness variations of the

Tertiary and Quaternary alluvium, which composes the

principal aquifer system in the basin, influence the

three-dimensional flow of ground water in the Mimbres

Basin. Ground-water flow simulations of the basin

should consider aquifer-thickness variability in geohydrologic conceptualization and numerical discretization.

Purpose and Scope

In 1989, the U.S. Geological Survey, in cooperation with the New Mexico Office of the State Engineer,

began a two-part investigation using gravity measurements to study alluvial-basin aquifer systems. In the

first part, a gravity database was developed to enable

interpretation of shallow crustal geologic structure

throughout New Mexico (Heywood, 1992). Because

the unconsolidated sedimentary alluvium that fills

many structural basins in New Mexico typically has a

Estimation of Alluvial-Fill Thickness in the Mimbres

Ground-Water Basin, New Mexico, from Interpretation of

Isostatic Residual Gravity Anomalies

By Charles E. Heywood
2

much lower density than surrounding consolidated sedimentary or igneous rocks, the isostatic residual gravity

anomaly is particularly useful for estimating the alluvial thickness in such basins. This report provides an

estimation of the thickness of the alluvial-fill aquifer,

the principal water supply, in various areas within the

Mimbres Basin in the United States. The Mimbres

ground-water basin extends into Mexico, but areas

south of the international border are not considered in

this report.

Within the Mimbres Basin, areas of greater alluvial thickness may contain more ground water, but this

water may not be recoverable. Aquifer-system permeability and water-quality characteristics also need to be

evaluated but are beyond the scope of this report.

Previous Work

Birch (1980) constructed several two-dimensional

regional gravity profile models for New Mexico, one of

which crossed the Mimbres Basin. Hanson and others

(1994) used well logs, gravity maps, and seismic profiles to estimate the alluvial thickness in different parts

of the Mimbres Basin. Their ground-water flow model

was two dimensional, and initial estimates of aquifer

transmissivity and storativity were not affected by their

definition of alluvial thickness. The thickness estimates

were used to estimate ground-water flux across several

vertical cross sections of the ground-water system for

comparison with net recharge estimates. Heywood

(1992) documented the isostatic-residual gravity database used for the analysis in this report. Ackerman and

others (1994) reprocessed oil industry seismic-refraction lines, which were interpreted by Klein (1995).

Several of these seismic lines cross the Mimbres Basin

and were used to help constrain this gravity interpretation.

Physiographic Setting

Subbasins and the geographical extent and physiographic features of the Mimbres Basin in southwest

New Mexico are depicted in figure 1. Basin extent is

defined by the topographically defined surface-water

drainage. The limits of the Mimbres ground-water

basin within the United States are assumed to correspond with this topographically defined surface-water

drainage basin. The northern and western extents of the

basin correspond to the Continental Divide; from

Reeds Peak in the Black Range, the divide extends

through the Pinos Altos Range, the Big Burro Mountains, and the Cedar Mountain Range to the Carizalillo

Hills just north of the United States–Mexico border.

From the United States–Mexico border, the eastern

side of the basin extends northward through the West

Potrillo Mountains, Aden Hills, Sleeping Lady Hills,

Sierra de las Uvas, and Mimbres Mountains to the

Black Range. In the United States, this area encompasses about 11,283 km2

 or 4,356 square miles.

Between the mountain ranges, the basin boundary follows the topographic high on the intervening plains.

Because the location of the ground-water divide does

not correspond exactly to a topographic high, minor

differences between the Mimbres ground-water basin

and the Mimbres surface-water drainage area may

exist.

Ground water in the basin generally flows southward and discharges to a playa south of the Mexican

border (Hanson and others, 1994). The climate is typical of the arid desert Southwest: average annual precipitation varies from less than 0.25 m (about 10 in.) on

the basin floor to about 0.75 m (about 30 in.) in the

mountainous northern areas of the drainage basin.

Geologic Setting

The geologic structure of the Mimbres groundwater basin in southwest New Mexico is characterized

by north- and northwest-trending structural subbasins.

Sedimentation of Miocene and Pliocene age has filled

and obscured the boundaries of these subbasins, which

have variable thickness.

The Mimbres Basin is located at the convergence

of three tectonic provinces: the Basin and Range, Colorado Plateau, and Rio Grande Rift. The northern section of the basin lies within a “transition zone” to the

Colorado Plateau, which is relatively undeformed. The

eastern flank of the basin is transitional to the Rio

Grande Rift, a region of substantial crustal extension.

The remaining southern and western parts of the basin

lie within the extensional tectonic environment of the

Basin and Range Province. These extensional tectonic

environments have generated structural subbasins, or

grabens, bounded by normal faults within the Mimbres
3

Figure 1. Location of physiographic features and subbasins of the Mimbres Basin.

Mangas Trench

Tres Hermanas

Graben

Seventysix

Subbasin

Columbus

Subbasin

Mimbres Trench

Florida Subbasin

0 10 20 30

0 10 20 30 40 MILES

40 KILOMETERS

Cross

Graben

33°

32°

45 ́

32°

30 ́

32°

15 ́

32°

108°15 ́ 108° 107°45 ́

NEW MEXICO RIO

GRANDE

Mimbres

Basin

West Potrillo

Mountains

Lad

Sleeping

y Hills

Mimbres Mountains

Big

Burro

Mountains

Pinos

Altos

Range

Cedar Mtn Range

Reeds Peak

Black

Range

Sierra de

las Uvas

Carrizalillo

Hills

Burdick Hills

Aden

Hills

Cookes Range

Florida

Mountains

Tres

Hermanas

Mts

Continental

Divide

U.S. Geological Survey digital elevation data

Scale 1:1,000,000

Lambert projection: central meridian = 106°,

standard parallels = 33°, 45°

Basin

boundary

Deming

107°30 ́ 107°15 ́
4

Basin. Many of these faults have a northwest strike,

similar to compressional faults of the Late Cretaceous

and early Tertiary Laramide Orogeny. This coincidence of structural trends suggests that some relict

structures may have been reactivated in the extensional

tectonic environment. Although the tops of intervening horsts (elongate blocks bounded on both sides by

normal faults) are occasionally exposed in ranges,

such as the Florida Mountains, the locations of graben-bounding faults are generally overlain by alluvium filling the intermontane basins.

The major surficial geologic units that have been

mapped in and near the Mimbres Basin area are summarized in table 1. These units were compiled from

Anderson and others (1997) and Kennedy and others

(2000). For further lithologic summaries and general

geologic history of the area, the reader is referred to

Hawley (1975), Cather and Johnson (1986), Ross and

Ross (1986), Seager and Mack (1986), Clemons and

Mack (1988), Mack and Clemons (1988), and Kennedy

and others (2000). For this report, these units have been

categorized into four lithofacies groups, which are

listed in table 1 and depicted in figure 2.

 Indurated clastic and carbonate sedimentary,

metamorphic, plutonic, and silicic volcanic rocks of

Precambrian through Tertiary age (group 4 of table 1

and fig. 2) are considered “bedrock,” and the locations

of their land-surface outcrop are shown in black in figure 2. These rock types underlie groups 1, 2, and 3

(table 1 and fig. 2), which partially fill the structural

grabens. The density contrasts among the various rock

types considered bedrock in group 4 are accounted for

in the regional isostatic residual gravity field; therefore,

explicitly defining them is not necessary.

Basalts of Pliocene age and younger (group 3 in

table 1 and fig. 2) occur in large flow fields in the southern Mimbres Basin, as shown in yellow in figure 2.

These basalts may overlie the basin-filling alluvial

deposits (groups 1 and 2) and may be as much as 100

m thick. The density of homogenous basalt is typically

in the range of 2,700 to 3,100 kg/m3

 (Milsom, 1989).

Because these basalts are probably interbedded with

lower density alluvial sediments, the bulk density used

to represent these basalts in the gravity model varies

with depth from 2,120 to 2,420 kg/m3

 (table 2).

The basin-fill alluvium was subdivided into two

groups on the basis of age and depositional environment. Group 1 lithofacies, shown in magenta in

figure 2, are typically older and more lithified than

lithofacies in group 2 (shown in blue) and have slightly

higher average density. Typical depositional environments of group 1 lithofacies are piedmonts or basin

floors. The Gila Group and the lower and middle units

of the Santa Fe Group are representative of group 1

lithofacies. Densities of these deposits increase with

depth of burial from 2,000 to 2,300 kg/m3 (table 2).

Group 1 deposits probably are the most voluminous of

the three basin-fill lithofacies groups, though this is not

apparent in figure 2 because these deposits may be

overlain by the younger basin-fill alluvium of group 2.

Group 2 lithofacies are generally younger and less

dense than those of group 1 and include late Quaternary

deposits of major streams as well as eolian, basin-floor,

and piedmont deposits. The upper Santa Fe Group typifies deposits of this lithofacies group. Densities

increase with depth of burial from 1,800 to 2,100 kg/m3

(table 2).

Acknowledgments

Dr. John Hawley provided invaluable insight for

the author’s understanding of the geologic setting and

sedimentation of Quaternary age.

ISOSTATIC RESIDUAL GRAVITY FIELD IN

THE MIMBRES BASIN

For this study, existing gravity measurements and

corresponding values of isostatic residual gravity in

and near the Mimbres Basin were obtained from the

New Mexico database developed by Heywood (1992).

These data were interpolated with MINC (Webring,

1981), a program that minimized the second horizontal

derivative of the interpolated potential field. The portion of the isostatic residual gravity surface within the

lateral boundaries of the Mimbres Basin, horizontal

location of measurement points, and surrounding

topography (depicted in shaded relief) are shown in figure 3. As explained by Heywood (1992), the colors on

the isostatic residual gravity map (fig. 3) represent variations in the Earth’s gravity field caused by density
5

Table 1. Geologic units in the Mimbres Basin area

Symbol Description

Lithofacies

group (fig. 2)

Quaternary and Tertiary

Qe Eolian deposits 2

Qa Alluvium 2

Qoa Older alluvial deposits 1

Qp Piedmont alluvial deposits 2

Qb Basalt and andesite flows and local vent deposits 3

Qv Basaltic volcanics; tuff rings, cinders, and proximal lavas 3

Qbo Basalt or basaltic andesite 3

QTg Gila Group: includes Mimbres Formation and several informal units 1

QTp Older piedmont alluvial deposits and shallow basin fill 1

QTs Upper Santa Fe Group 2

QTsf Upper Santa Fe Group, undivided 2

 Tertiary

Tnv Neogene volcanic rocks 4

Tnb Basalt and andesite flows; Neogene 3

Tpb Basalt and andesite flows; Pliocene 3

Tus Upper Tertiary sedimentary units 2

Tuau Lower Miocene and uppermost Oligocene basaltic andesites 4

Tui Miocene to Oligocene silicic to intermediate intrusive rocks; dikes, stocks, plugs, and diatremes 4

Turp Upper Oligocene rhyolitic pyroclastic rocks (ash-flow tuffs) 4

Turf Upper Oligocene silicic (or felsic) flows and masses and associated pyroclastic rocks 4

Tual Upper Oligocene andesites and basaltic andesites 4

Tlrp Lower Oligocene silicic pyroclastic rocks (ash-flow tuffs) 4

Tlrf Lower Oligocene silicic (or felsic) flows, domes, and associated pyroclastic rocks and intrusions 4

Tos Oligocene and upper Eocene sedimentary and volcaniclastic rocks with local andesitic to

intermediate volcanics

1

Tlv Lower Oligocene and Eocene volcanic rocks, undifferentiated 4

Tli Quartz monzonites in the Silver City Range, intermediate intrusives of the Cookes Range 4

Tv Middle Tertiary volcanic rocks, undifferentiated 4

Tps Paleogene sedimentary units 1

Tuv Volcanic and some volcaniclastic rocks, undifferentiated 4

Tla Lower Tertiary andesite and basaltic andesite flows and associated volcaniclastic units 4

Ti Tertiary intrusive rocks, undifferentiated 4

Tsf Lower and middle Santa Fe Group 1

Tertiary and Cretaceous

TKi Paleogene and Upper Cretaceous intrusive rocks 4

TKav Andesitic volcanics 4
6

Cretaceous

Kbm Mancos Shale or Beartooth Quartzite 4

Ku Upper Cretaceous, undivided 4

Kl Lower Cretaceous, undivided 4

Paleozoic

PP Permian and Pennsylvanian rocks, undivided 4

Pa Abo Formation, Permian 4

Pz Paleozoic rocks, undivided 4

Pys Yeso Formation, Permian 4

Ph Hueco Formation, Permian 4

P Pennsylvanian rocks, undivided 4

M Mississippian rocks, undivided 4

MD Mississippian and Devonian rocks, undivided 4

D Percha Shale 4

SO Silurian and Ordovician rocks, undivided 4

SOC Silurian through Cambrian rocks, undivided 4

OCp Ordovician-Cambrian plutonic rocks of Florida Mountains 4

 Precambrian

Yp Middle Proterozoic plutonic rocks 4

Table 2. Depth intervals and densities for lithofacies groups 1, 2, and 3 in the Mimbres Basin

[>, greater than]

Depth interval

(meters below land

surface)

Density (kilograms per cubic meter)

 Group 1 Group 2 Group 3

0 - 100 2,000 1,800 2,420

101 - 200 2,100 1,900 2,120

201 - 1,200 2,200 2,000 2,320

 > 1,200 2,300 2,100 2,420

Table 1. Geologic units in the Mimbres Basin area--Concluded

Symbol Description

Lithofacies

group (fig. 2)
7

0 10 20 30

0 10 20 30 40 MILES

40 KILOMETERS

Figure 2. Surficial geology of the Mimbres Basin area and locations of

seismic-refraction lines and deep exploratory drill holes.

EXPLANATION

Bedrock (group 4 lithofacies)

Basalt (group 3 lithofacies)

Older Quaternary age (group 1 lithofacies)

Younger Quaternary age (group 2 lithofacies)

Exploration drill-hole location and number (table 3)

Trace of seismic-refraction lines and locations of shot points

alluvium of

alluvium of

6

33°

32°

45 ́

32°

30 ́

32°

15 ́

32°

108°15 ́ 108° 107°45 ́ 107°30 ́ 107°15 ́

Basin

boundary

Burdick - Tres Hermanas fault zone

Seventysix fault zone

Snake Hills fault zone

Columbus fault zone

Camel

Mountain

fault

zone

Malpais fault

Good S gi ht af ult zone

Mimbres fault

Treasure Mountain fault zone

1 2

3 4

5

6

7 8 9

10

Silver City fault zone

Seismic line 5

Seismic line 4

Seismic line 7 a10

a22

b4

U.S. Geological Survey digital elevation data

Scale 1:1,000,000

Lambert projection: central meridian = 106°,

standard parallels = 33°, 45°

Geology and faults from Anderson and others (1997)

and Kennedy and others (2000)
8

Figure 3. Measurement points and isostatic residual gravity in the Mimbres Basin.

0

0

10

10

20

20

30

30 40 MILES

40 KILOMETERS

Isostatic residual

gravity, in milligals

-27

-24

-21

-18

-15

-12

-9

-6

-3

0

3

6

9

12

15

18

21

24

27

EXPLANATION

Measurement point

33°

32°

45 ́

32°

30 ́

32°

15 ́

32°

108°15 ́ 108° 107°45 ́ 107°30 ́ 107°15 ́

Basin

boundary

U.S. Geological Survey digital elevation data

Scale 1:1,000,000

Lambert projection: central meridian = 106°,

standard parallels = 33°, 45°
9

variations in rocks composing the Earth’s crust.1 Positive anomalies (shades of green to red) result from

rocks denser than the Bouguer reduction density of

2,670 kg/m3 (Heywood, 1992), whereas negative

anomalies (shades of blue to magenta) result from

rocks of lower density. The patterns and magnitudes of

these anomalies may be used to interpret crustal structure, such as the location of Quaternary alluvial subbasins bounded by buried faults.

Potential fields, such as the isostatic residual gravity field discussed in this report, are mathematically

continuous surfaces. Readers unfamiliar with potential

field theory may consult texts such as those by Garland

(1979), Dobrin and Savit (1988), Milsom (1989), and

Telford and others (1990). The calculations described

in this report were performed on a computer that

required discrete representation of the surfaces as

grids. The grids used in this study are two-dimensional

arrays of values at a constant horizontal spacing of

1,000 m. Grids generated with MINC (Webring, 1981)

and subsequently calculated or derivative grids were

identically georeferenced in a Lambert Conformal

Conic Projection. The magnitude of the isostatic residual gravity field in areas with a paucity of measurement

points is poorly constrained and thus precluded geologic interpretation in some areas.

Separation of Anomalies

Estimation of basin-fill alluvial thickness from

gravity anomalies in the Mimbres Basin is complicated

by the presence of substantial quantities of volcanic

rocks of Tertiary age that also have the low densities

that generate negative gravity anomalies. To quantitatively estimate the thickness of the basin-fill alluvium,

it was necessary to separate the gravitational effect of

the basin-fill alluvium from that of Tertiary volcanic

rocks as well as consolidated sedimentary rocks of

Mesozoic and Paleozoic age. This separation was

accomplished through an iterative process similar to

that described by Jachens and Moring (1990), Saltus

and Jachens (1995), and Langenheim and Jachens

(1996). The process is summarized in the steps below

to obtain a first approximation of basin-fill thickness at

each grid location. The basin-fill thickness is the sum

of the thicknesses h for each successive “slab” or depth

interval defined in table 2.

(1) Mapped geologic units were separated into four

lithofacies groups (table 1), as previously discussed, according to the stratigraphic classifications of Anderson and others (1997) and

Hawley (in Kennedy and others, 2000).

(2) A subset of the gravity measurement data set consisting of measurements collected above group

3 lithofacies was extracted from the main gravity data set. These data were gridded using the

minimum curvature algorithm MINC

(Webring, 1981) to generate a potential surface, B0. This surface was a first approximation

of the anomalous isostatic residual gravity

from the "basement" rocks.

(3) The anomalous gravitational potential of the lowdensity basin fill, F0, was first approximated by

subtracting the basement field, B0, from the

total isostatic residual gravity field I (fig. 3).

The value of F0 at each grid location was then

inverted by the appropriate depth-density relation (table 2) and the infinite slab formula from

Garland (1979)

(1)

where is the anomalous gravitational potential of

the infinite slab, in meters per second

squared; is the constant 3.14; is the

difference in density of the slab interval

from the Bouguer reduction density, in

kilograms per cubic meter; G is the universal gravitational constant, in cubic meters

per kilogram second squared (6.67 x 10-11

m3kg-1 s-2); and h is the slab thickness, in

meters.

(4) A revised estimate of the anomalous gravitational

potential, F1, of the grid of basin-fill thickness,

T, was calculated using the depth-density relations (table 2) by a Fast-Fourier Transform

technique (Parker, 1972). The anomalous gravity values of the basement rocks data subset

used in step 2 were adjusted to account for the

1The isostatic residual gravity field was derived from Bouguer gravity anomaly data by removing the gravitational effect of

the compensating mass that supports topographic loads. The thickness of this compensating mass was calculated using averaged digital topography by assuming a crustal thickness for sea-level

topography of 20 km, crustal density of 2,670 kg/m3, and a density

contrast between the crust and upper mantle of 0.3 kg/m3. The

reader is referred to Heywood (1992) for more information.

∆g = 2πρGh

∆g

π ρ
10

influence of the low-density basement fill on

those gravity measurements. This adjusted set

was then regridded to generate B1, a revised

estimate of the basement gravity field.

(5) As in step 3, the basement gravity field, Bn, was

subtracted from the total isostatic residual

gravity field, I, to obtain an improved estimate

of the anomalous gravity field, Fn, from the

basin fill. (The subscript n is an iteration identifier.) This field was inverted as in step 3 to

obtain an updated estimate of basin-fill thickness, Tn.

Steps 4 and 5 were repeated for six iterations,

which was sufficient to separate the “basement” and

“basin-fill” isostatic residual gravity fields. The calculated basin-fill thickness was then imported into a geographic information system (GIS) to facilitate

visualization and comparison with constraining seismic-refraction data (Ackerman and others, 1994;

Klein, 1995) and drill-hole data (Clemons, 1986;

Broadhead, 1991). The location and depth to bedrock

in the deep exploratory drill holes (fig. 2) used to constrain the depth calculation are presented in table 3.

Locations of drill holes and traces of seismic-refraction

profiles are shown in figure 2.

Following comparison of calculated basin-fill

thickness to the constraining information, density profile modification and other adjustments were made and

steps 1 through 5 were repeated. The depth-density

relations for lithofacies groups used for the final thickness calculation (fig. 4) are presented in table 2. For the

size of geologic structures in the Mimbres Basin, locations more than 3 km from a known gravity measurement do not warrant estimation of alluvial-fill thickness

from gravity data alone. To discourage such interpretation, these areas are depicted in black in figure 4. Calculated thickness near these areas should likewise be

interpreted with caution. Readers may obtain digital

gravity or magnetic data (Kucks and others, 2001) for

New Mexico through the Internet at

http://greenwood.cr.usgs.gov/pub/open-file-reports/

ofr-01-0061/data.

Density Profiles

Information regarding the density of alluvial-fill

deposits is typically obtained by analyzing drill cuttings, borehole density logs, or borehole gravity measurements. However, no such data were available for

the alluvial-fill sediments in the Mimbres Basin.

Although borehole density logs are available for some

deep exploratory drill holes listed in table 3, these measurements were collected in deeper “bedrock” lithologies and therefore provide no information on density

variations within the basin fill. The relation of rock

Table 3. Location and depth to bedrock of deep exploration drill holes in the Mimbres Basin

Well number (fig. 2) Well name (fig. 2)

Well location Bedrock depth

(meters below

North latitude West longitude land surface)

1 City of Deming #2 32°14.65’ -107°42.05’ 1,250

2 McSherry #1 32°14.65’ -107°40.28’ 1,244

3 City of Deming #1 32°14.43’ -107°41.80’ 1,288

4 Hurt Ranch #1 32°14.15’ -107°53.63’ 610

5 Skelly #1-A 31°51.87’ -107°23.27’ 1,417

6 Sunray Mid-Continent #1 31°50.35’ -107°19.98’ 152

7 Cockrell #1 32°18.93’ -107°54.57’ 70

8 Guest & Wolfson #1 Diana 32°18.05’ -107°58.63’ 27

9 Sycor Newton #1 32°19.37’ -107°57.37’ 61

10 Seville Trident 32°18.40’ -107°58.28’ 0
11
12

density to depth for the alluvial-fill deposits was therefore estimated by analogy with similar deposits elsewhere in New Mexico and the Western United States.

Birch (1980, 1982) correlated density measurements

using numerous wells to their presumed lithologic age.

In his two-dimensional gravity-profile models, Birch

used a value of 2,200 kg/m3

 for the Miocene and

Pliocene deposits that correspond to the alluvial-fill

deposits of this study.

Density-depth relations were assigned to model

grid points on the basis of geology as mapped by Anderson and others (1997) and Kennedy and others (2000).

Basin-fill rock types were divided into three groups, as

described previously. Although density varies among

the rocks in the fourth “basement” group, explicitly

defining these densities was not necessary because density variations are accounted for in the regional gravity

field.

ALLUVIAL-FILL THICKNESS IN

STRUCTURAL SUBBASINS

The estimates of alluvial-fill thickness in the Mimbres Basin based on gravity analysis are presented in

figure 4. Areas of bedrock outcrop within the basin are

depicted in grey shaded relief. Areas devoid of gravity

measurements within 3 km are shown in black. A graduated rainbow color scale indicates increasing (red to

purple) estimated thickness of basin-fill alluvium.

Large areas in the basin, shown in red in figure 4,

probably consist of bedrock overlain by a thin veneer of

alluvium. Unless the water table is close to land surface

in these areas, they probably do not overlie significant

alluvial aquifers. Many of these areas overlie “horsts”

that separate the “grabens,” or subbasins, which are

described separately in subsequent sections of this

report. These subbasins attain depths up to 2,200 m

(7,200 ft).

Mangas Trench

 The Mangas Trench, also termed the San Vicente

Subbasin (Kennedy and others, 2000) and the San

Vicente half graben (Seager and others, 1982), is

bounded on the north and east by the Silver City and

Treasure Mountain fault zones (fig. 2) and by northwest-trending normal faults to the south. The southeastern portion of the Mangas Trench near Deming is also

known as the Deming Basin (Kennedy and others,

2000). The Mangas Trench produces a prominent negative gravity anomaly (fig. 3) extending northwest from

the Deming area to the Continental Divide north of the

Big Burro Mountains. Hanson and others (1994) estimated an average thickness of 430 m for the basin-fill

aquifer in the Mangas Trench. This gravity analysis suggests as much as 2,200 m of basin fill in the deepest

parts of the trench near Deming (fig. 4).

Seventysix Subbasin

The Seventysix Subbasin, also referred to as the

Iona Subbasin, is a northwest-trending graben separated

from the Tres Hermanas Mountains and Burdick Hills

(fig. 1) to the southwest by the Seventysix fault zone. It

is bounded on the north by the Snake Hills fault zone

(fig. 2). Hanson and others (1994) estimated the average

thickness to be 1,070 m for the basin-fill aquifer in the

Seventysix Subbasin. Seager (1995) interpreted approximately 200 m of alluvium of Tertiary and Quaternary

age to overlie approximately 800 m of Tertiary basaltic

andesite, undifferentiated volcanic rocks, and the Lobo

Formation, which in turn unconformably overlie a Paleozoic sedimentary sequence. The deep exploratory drill

hole “M.R. Young Oil Co. #1 Bisbee Hills” was drilled

to a total depth of 2,184 m and penetrated 30 m of alluvium on top of Tertiary volcanic and older sedimentary

rocks.

 The Seventysix Subbasin is crossed by seismicrefraction line 7 (fig. 2) (Ackerman and others, 1994;

Klein, 1995). A northwest-trending portion of this line

(between shots 7-a10 and 7-a22) traverses the southwestern margin of the graben near the Seventysix fault

zone and flanking pediment of the Tres Hermanas

Mountains. A north-trending portion of the seismic line

(between shots 7-a22 and 7-b4) crosses the middle of

the graben to the flanking horst to the north. A refracting

horizon with an average depth of approximately 200 m

on both parts of this line probably represents the interface of unconsolidated basin-fill alluvium with underlying consolidated volcanic rocks. This geophysical

interpretation agrees with the geologic interpretation of

Seager (1995). Because of the paucity of other wellcontrol data, this refracting horizon is the principal constraint on the thickness of basin-fill alluvium in the Seventysix Subbasin. If a layer of basaltic andesite, which
13

may have a relatively high velocity, is present (as suggested by Seager), it may mask deeper, lower velocity

layers, preventing reliable interpretation of deeper basin

structure from the refraction data. This gravity analysis

suggests a maximum basin-fill thickness of 874 m in the

Seventysix Subbasin (fig. 4).

Mimbres Trench

The Mimbres Trench is a north-northwest-trending

structure that extends from the Cookes Range to the

Pinos Altos Range in the northern Mimbres Basin. Hanson and others (1994) estimated an average thickness of

300 m for most of the Mimbres Trench west of the

Black Range. The thickness estimated in this study generally agrees with the earlier estimates. The thickest

estimates (500-600 m; fig. 4) west of the Black Range

are probably artifacts of the gravity interpolation process and are not constrained by drill-hole data.

Cross Graben

The Cross Graben, which connects the Mimbres

Trench with the Mangas Trench, is also known as the

Dwyer Subbasin (Kennedy and others, 2000). Hanson

and others (1994) estimated an average thickness of 150

m for the basin-fill aquifer in this area. This analysis

suggests that the basin fill thickens to the southeast up

to about 400 m (fig. 4).

Florida Subbasin

The Florida Subbasin is composed of two northtrending structures located northeast and southeast of

the Florida Mountains near the central part of the Mimbres Basin. These structures are distinctly separated into

northern and southern grabens by an intervening horst

just east of the Florida Mountains. The grabens correspond to the Akela and Mesquite Lake Subbasins of

Seager (1995). Kennedy and others (2000) suggested

that these structures be referred to as the Akela and

Mesquite Lake Sections of the Florida Subbasin. Hanson and others (1994) estimated an average thickness of

150 m for the basin-fill aquifer in this subbasin, which

is less than that estimated in this analysis. A paucity of

gravity measurements to the east of these grabens precludes interpretation in that area.

Tres Hermanas Graben

Located northeast of the Cedar Mountain Range

(fig. 1), the Tres Hermanas Graben (also referred to as

the Hermanas Basin (Seager, 1995) and the Hermanas

Subbasin (Kennedy and others, 2000)) trends northwest

from the Mexican border to the Continental Divide. It is

separated from the Seventysix Subbasin to the northwest by an intervening horst structure. Hanson and others (1994) estimated an average thickness of 760 m.

This gravity analysis suggests that much of the basin is

about 200 m thick, with a slightly deeper area to the

southeast and a significantly deeper area (in excess of

500 m) to the northwest (fig. 4). A structural high, possibly a horst, appears to separate these deeper regions.

Columbus Subbasin

Hanson and others (1994) estimated the thickness

of the alluvial-fill aquifer in the Columbus Subbasin,

which is in the southern part of the Mimbres Basin, to

vary from 170 to 760 m. Blandford (1987) constructed

three hydrostratigraphic cross sections from water-well

drill logs in the Columbus Subbasin. Four of these

water-well logs were used to constrain the gravity

model of basin-fill thickness. Two of the four were used

to constrain the depth to the Malpais Basalt underlying

the basin fill in the northwest part of the subbasin. This

gravity analysis suggests that much of the basin is about

50 m thick, with a maximum basin-fill thickness of

about 200 m (fig. 4).

Boundary Areas

This gravity analysis has revealed two additional

areas, previously assumed to have bolson-fill thicknesses of about 15 to 30 m (Hanson and others, 1994),

that appear to overlie thick alluvial fill. In the southeast

corner of the Mimbres Basin on the west flank of the

West Potrillo Mountains, Quaternary basalts presumably overlie basin-fill alluvium, which may be 1,000 m

thick in some areas (fig. 4). On the western side of the

Mimbres Basin, just east of the Continental Divide at

N.32°20’, W.108°15’, basin-fill thickness is estimated

to be as much as 500 m.
14

SUMMARY

Isostatic residual gravity anomalies in the Mimbres Basin contain a signal from low-density alluvial

fill convolved with regional trends resulting in density

variations in basement rocks. When separated from the

regional trend, the signal from the alluvial fill may be

inverted, assuming depth-density relations, to estimate

the thickness of alluvial fill. Seismic-refraction, drillhole, and surficial geologic data constrain resulting calculations of the alluvial thickness in the Mimbres Basin

in southwest New Mexico. Estimates suggest that alluvial thickness beneath much of the Mimbres Basin is

less than 100 m. The thickness of alluvial fill in structural grabens is much greater, typically as much as 300

m and locally as much as 2,200 m. Because Tertiary

volcanic rocks that may be in proximity to alluvial

basin fill have similar densities, differentiating these

lithologies in the subsurface solely with gravity data is

generally not possible. These estimates were obtained

using the known surface distribution of the lithologies

with limited subsurface drill-hole and seismic-refraction data.

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