U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3354

Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, 

Santa Cruz County, Arizona
Cover. Photograph of southwest view along U.S.-Mexico border fence, east of the Nogales urban area; Nogales, Sonora, is in distance 

on left, and Nogales, Arizona, is in upper right. Hilly terrain is mostly volcaniclastic sandstone and conglomerate of the Miocene Nogales 

Formation. Atascosa Mountains are on the skyline. Photograph by W.R. Page.
Geologic Map of the Rio Rico and Nogales 

7.5’ Quadrangles, Santa Cruz County, 

Arizona

By William R. Page, Christopher M. Menges, Floyd Gray, Margaret E. Berry, 

Mark W. Bultman, Michael A. Cosca, and D. Paco VanSistine

Scientific Investigations Map 3354

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Suggested citation:

Page, W.R., Menges, C.M., Gray, Floyd, Berry, M.E., Bultman, M.W., Cosca, M.A., and VanSistine, D.P., 2016, Geologic 

map of the Rio Rico and Nogales 7.5 ́ quadrangles, Santa Cruz County, Arizona: U.S. Geological Survey Scientific 

Investigations Map 3354, 32 p. pamphlet, 2 sheets, 1:24,000-scale, http://dx.doi.org/10.3133/sim3354.

ISSN 2329-132X (online)
iii

Contents

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

Methods...........................................................................................................................................................1

Description of Map Units..............................................................................................................................6

Alluvial and Basin-Fill Deposits..........................................................................................................6

Bedrock Units......................................................................................................................................10

Stratigraphy ..................................................................................................................................................12

Jurassic................................................................................................................................................12

Upper Cretaceous...............................................................................................................................12

Tertiary..................................................................................................................................................12

Grosvenor Hills Volcanics ........................................................................................................12

Nogales Formation ....................................................................................................................12

Revised Stratigraphy for Nogales Formation...............................................................14

Nogales Formation Geochronology...............................................................................15

Proto Canyon Member..............................................................................................................17

Nogales Wash Member............................................................................................................19

Mariposa Member.....................................................................................................................20

Late Tertiary to Early Pleistocene Basin-Fill Deposits..................................................................22

Quaternary Alluvium...........................................................................................................................23

Structure........................................................................................................................................................24

Summary........................................................................................................................................................26

Acknowledgments.......................................................................................................................................27

References Cited..........................................................................................................................................27

Figures

1. Location map of Rio Rico and Nogales (Arizona) 1:24,000-scale quadrangles, 

showing major physiographic, geologic, and hydrologic features in the map area.............2

2. Location map of upper Santa Cruz basin area, showing major physiographic, 

geologic, and hydrologic features in the region......................................................................3

3. Stratigraphic chart of sediments and rocks in map area ......................................................4

4. Plots of new 40Ar/39Ar geochronology incremental heating data for amphibole 

and biotite analyses from the Grosvenor Hills Volcanics ....................................................13

5. Photograph of northeast view of ledge-forming, resistant volcaniclastic 

sandstone beds in the Nogales Wash member of the Nogales Formation (Tnn) 

overlying less-resistant arkose sandstone and conglomerate beds in the lower 

part of the Proto Canyon member (Tnp), east of Nogales, Arizona, along State 

Highway 82...................................................................................................................................14

6. Plots of new 40Ar/39Ar incremental-heating analyses of whole-rock samples 

from basalts in the Nogales Formation........................................................................................... 15

7. New 40Ar/39Ar ideogram plots for laser-fusion analyses of A, sanidine from 

rhyolite tuff in Grosvenor Hills Volcanics, B, detrital sanidine and plagioclase 

from volcaniclastic sediments in the Nogales Formation, and C, sanidine 

and plagioclase from rhyolite tuff in the Nogales Formation from Agua Fria 

Canyon area.................................................................................................................................16
iv

8–26. Photographs of:

 8. Basal debris-flow conglomerate in the Proto Canyon member of the 

Nogales Formation south of Mount Benedict, near contact with Jurassic 

Quartz Monzonite of Mount Benedict..............................................................................17

 9. Sequence of alternating beds of arkosic sandstone, conglomeratic 

sandstone, and debris-flow conglomerate in lower parts of Proto Canyon 

member, above basal debris-flow conglomerate; hammer in middle left 

for scale ................................................................................................................................17

 10. Cliff-forming, cyclic, fining-upward, fluvial sequences in lower part of the 

Proto Canyon member exposed in Proto Canyon ..........................................................18

 11. Red and light-gray gravelly claystone (below black line) and overlying gray 

volcaniclastic sandstone (above black line) in the middle part of the Proto 

Canyon member exposed in Yerba Buena Canyon........................................................18

 12. Fluvial cross-bedded volcaniclastic sandstone and conglomerate with lag 

monzonite gravel in channels, in upper part of the Proto Canyon member 

exposed west of Grand Avenue, and north of Mariposa Road....................................18

 13. Pinkish-gray volcaniclastic sandstone in the uppermost part of the Proto 

Canyon member, exposed north of Mariposa Road, between Interstate 19 

and Grand Avenue; pick in middle for scale ...................................................................19

 14. Boulder of Proto Canyon member exposed in Sonoita Creek area where it 

laps onto the Grosvenor Hills Volcanics; unit in lower part of member and 

consists of volcaniclastic conglomerate with clasts mostly of white and red

porphyritic volcanic rocks, and brownish-gray rhyodacite derived from the 

Grosvenor Hills Volcanics; boulder is about 0.5 m in diameter....................................19

 15. Massive, cliff-forming volcaniclastic sandstone and conglomeratic sandstone 

in the basal part of the Nogales Wash member along Grand Avenue, south of

Mariposa Road...........................................................................................................................19

 16. Alternating thin- to thick-bedded, volcaniclastic sandstone and conglomeratic

sandstone of the Nogales Wash member south of Mariposa Road and west of 

Grand Avenue; small-displacement fault diagonally cuts beds in center of 

photograph..................................................................................................................................20

 17. Faulted and highly rotated beds (dipping 48° northwest) of Mariposa member

along Interstate 19, north of Mariposa Road .......................................................................20

 18. Massive claystone unit in Mariposa member exposed south of Mariposa 

Road, between Interstate 19 and Grand Avenue ...........................................................20

 19. Mariposa member in basin south of Sonoita Creek and east of the Santa 

Cruz River, composed of alternating beds of volcaniclastic conglomeratic 

sandstone, sandstone, and few thin silt and clay partings; hammer and 

field notebook for scale......................................................................................................21

 20. Fine-grained facies of the Mariposa member in basin south of Sonoita 

Creek and east of the Santa Cruz River, composed of alternating beds of 

fine-grained volcaniclastic sandstone, siltstone, and claystone; hammer 

in lower left for scale ..........................................................................................................21

 21. Sandy and gravelly claystone beds in the Mariposa member south of Sonoita

Creek and east of the Santa Cruz River; height of outcrop about 3 m............................21
v

 22. View to west of basal part of basin-fill deposits (QTa) in Potrero Canyon area .........22

 23. A, View to east of reddish brown basal basin-fill deposits (QTa) in trough 

cut into underlying Nogales Wash member of the Nogales Formation (Tnn) 

in a road cut at Crawford Hill; height of outcrop about 20 m. B, West view 

directly across Interstate 19 from road cut at Crawford Hill, showing 

subvertical fault (black line with arrow on downthrown side) juxtaposing 

Nogales Wash member (Tnn) in hanging wall, against basin-fill deposits 

(QTa) in footwall; height of outcrop about 18 m............................................................22

 24. Northwest view along Mount Benedict fault (black line with arrow on 

downthrown side) east of Santa Cruz River; gray and dark brown beds 

(on left) of the Salero Formation (Ks) in the footwall, and yellowish to 

pinkish gray beds (on right, on which geologists are standing) are the 

Mariposa member of Nogales Formation (Tnm) in the hanging wall; fault 

plane strikes N. 40° W. and dips 65° northeast ..............................................................24

 25. Southwest view of Grand Avenue fault (black line with arrow on downthrown

side) just south of Mariposa Road and west of Grand Avenue; gray sandstone 

of the Nogales Wash member (Tnn) on left in footwall, and pink claystone 

in Mariposa member (Tnm) on right in hanging wall; height of large shrub in 

center of photograph is about 1 m............................................................................................... 25

 26. North view along southern end of the east-dipping San Cayetano fault just 

south of Sonoita Creek .......................................................................................................26

Tables

1. 40Ar/39Ar geochronology for Grosvenor Hills Volcanics and Nogales Formation 

in the Nogales and Rio Rico 1:24,000-scale quadrangles, Arizona ....................................13

2. 40Ar/39Ar geochronology for Nogales Formation in the Agua Fria Canyon area 

(fig. 2), Peña Blanca Lake 1:24,000-scale quadrangle, adjacent and to the west 

of the Rio Rico quadrangle........................................................................................................16

Appendix Table

1–1. Combined 40Ar/39Ar data for Grosvenor Hills Volcanics and Nogales Formation.

(Click here to access digital file)..............................................................................................29

Conversion Factors

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, 

Santa Cruz County, Arizona

William R. Page, Christopher M. Menges, Floyd Gray, Margaret E. Berry, Mark W. Bultman, Michael A. Cosca, 

and D. Paco VanSistine

Previous regional-scale geologic mapping in the area 

was completed by Simons (1974), Drewes (1972, 1980), 

and Peterson and others (1990). Gettings and Houser (1997) 

and Gettings (2002) described the geometry and structure of 

the upper Santa Cruz basin based mostly on aeromagnetic, 

gravity and other geophysical data, and well data. Youberg 

and Helmick (2001) mapped the adjacent Tubac and Amado 

1:24,000-scale quadrangles (fig. 2), with emphasis on defining 

the surficial geology in terms of assessing geologic hazards 

and geomorphology.

Methods

Bedrock geology was digitally mapped in ArcMap 

(version 9.3.1), with the aid of 2010 National Agriculture 

Imagery Program (NAIP) orthophotography, imagery viewed 

with Google Earth, 1996 National Aerial Photography Program 

(NAPP) 1:40,000-scale color infrared aerial photographs, and 

1:24,000 and 1:100,000-scale topographic maps. Mesozoic 

bedrock units were compiled from existing published geologic 

maps in the area, including those of Simons (1974), and Drewes 

(1972, 1980); most bedrock unit descriptions were from these 

reports with minor modifications, and the location of some contacts and faults were slightly adjusted based on interpretation 

of orthophotography and imagery viewed with Google Earth. 

Simons (1974) mapped numerous northwest-trending dikes in 

the Jurassic Quartz Monzonite of Mount Benedict and in the 

Salero Formation in the San Cayetano Mountains. These dikes 

were not shown on this geologic map because of difficulty georeferencing and reproducing their precise locations; see Simons 

(1974) for location and description of these dikes.

Due to the scarcity of deep drill holes with subsurface 

data in the map area, the location, geometry and thickness of 

the Nogales Formation, and estimated depth to Cretaceous 

and Jurassic bedrock shown in cross sections were based 

primarily on geophysical models, combined with data from 

surface outcrops. Contacts between members of the Nogales 

Formation are approximately located in cross sections, where 

not constrained by nearby outcrops.

Introduction

The Rio Rico and Nogales (Arizona) 1:24,000-scale 

quadrangles are located in the Basin and Range Province of 

southern Arizona, and the southern edge of the map is the 

international border with Sonora, Mexico (figs. 1 and 2). The 

major urban area is Nogales, a binational city known as “the 

gateway to Mexico.” Rocks exposed in the map area range in 

age from Jurassic through Quaternary (fig. 3). Major physiographic, geologic, and hydrologic features in the map area 

include the southern San Cayetano Mountains, Grosvenor 

Hills, and Sonoita Creek in the northern part, and Mount 

Benedict and the Mount Benedict horst block in the southcentral part (figs. 1 and 2). The horst block is bounded by the 

Santa Cruz River on the east and Nogales Wash (tributary to 

the Santa Cruz River) on the west.

The objectives of our mapping were to define the geologic framework for the Nogales area and the upper Santa 

Cruz basin (fig. 2) (Callegary and others, 2011; Page and 

others, 2011), to support ongoing multidisciplinary projects. 

These projects include the U.S. Geological Survey (USGS) 

Tucson Water Science Center Transboundary Aquifer Act 

Project (hydrologic and climatic modeling and water quality and quantity studies of the upper Santa Cruz basin); the 

United States-Mexico Border Environmental Health Initiative 

(evaluating contaminants in the basin and impacts to human 

and wildlife health); and the assessment of concealed porphyry 

copper deposits in the region. This work will also provide fundamental data used for flood hazard and water storage investigations in the Nogales Wash and Santa Cruz River areas, and 

in efforts to detect and predict the location of illegal tunnels 

beneath the international border. We significantly revised 

the Miocene Nogales Formation based on geologic mapping 

combined with new geochronologic, geophysical, and hydrogeologic studies. The Nogales Formation is poorly understood 

because it has not been mapped in detail, and in turn, its 

groundwater resource potential is poorly known. This new 

work will improve understanding of the Nogales Formation to 

more fully assess its groundwater resource potential (Gray and 

others, 2014).
2 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Figure 1. Location map of Rio Rico and Nogales (Arizona) 1:24,000-scale quadrangles, showing major physiographic, geologic, and hydrologic 

features in the map area. Black squares are town locations and black ball and bar symbols indicate the downthrown side of normal faults.

19

19

19

82

UNITED STATES

MEXICO

NOGALES

RIO RICO

GRAND

AVENUE

MA

RIPOSA ROAD

Rio Rico

Nogales

Rio Rico

Basin

San Cayetano

Mts.

Mount Benedict 

horst block

Mount Benedict

Grosvenor Hills Fresno Canyon

Guevavi

Canyon

Yerba Buena Canyon

Canyon

Proto

Ephraim

 Canyon

Mariposa Canyon

Potrero Canyon

SAN CAYETANO FAULT

MOUNT BENEDICT FAULT

GOLD HILL FAULT

PROTO CA

NYON FAULT

YERBA BUENA CANYON FAULT

GRAND AVENUE FAULT

RIO RICO FAULT

Sonoita Creek

Santa Cruz River

Nogales Wash

31°30'

31°22'30"

111°00' 110°52'30"

0 0.5 1

0 0.5 1

1.5 MILES

1.5 KILOMETERS

Shaded-relief base from U.S. Geological 

Survey National Elevation Dataset (NED) 

(accessed in 2015 at http://ned.usgs.gov/), 

10-meter resolution elevation data

Late Miocene−Holocene alluvium

Miocene Nogales Formation

Bedrock

EXPLANATION
Methods 3

Figure 2. Location map of upper Santa Cruz basin area, showing major physiographic, geologic, and hydrologic features in the region. 

Black squares are town locations. Inset map in upper right shows location of upper Santa Cruz River basin with boundary in gray, Santa 

Cruz River (SCR) in blue, and boundaries of Rio Rico and Nogales quadrangles in red.

289

82

19

19

UNITED STATES

MEXICO

RIO RICO

NOGALES KINO SPRINGS

Tubac

Nogales

Amado

Rio Rico

Patagonia

Tucson

Basin

Patagonia

Mountains

Santa Rita Mountains

Grosvenor

Hills

San Cayetano

Mts.

Atascosa 

Mountains

 Tumacacori Mountains

Pajarito Mountains

Sierrita Mountains

Cumero Canyon

Agua Fria Canyon

Josephine Canyon

Walker Canyon

Peña Blanca

Canyon

Cottonwood Canyon

Kino Springs

Creek

Santa Cruz River

Nogales Wash

Sopori Wash

Sonoita

Sa nt a Cruz River

AMADO

PEÑA BLANCA LAKE

TUBAC

31°45'

31°30'

111°00' 110°45'

0 1 2 3 4

0 1 2 3 4

5 MILES

5 KILOMETERS

Late Miocene−Holocene alluvium

Miocene Nogales Formation

Bedrock

EXPLANATION

111°00' 110°30'

31°15'

31°45'

31°30'

Patagonia Tubac

Amado

USA

MEXICO

0

0

5

5

10 MILES

10 KILOMETERS

SONORA 

ARIZONA

Rio

Rico

Nogales

Santa Cruz River

PIMA COUNTY

SANTA CRUZ COUNTY
4 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Figure 3. Stratigraphic chart of sediments and rocks in map area. Unconformities and 

disconformities are indicated by the wavy line.

Mariposa member—Moderately to weakly consolidated calcareous, volcaniclastic sandstone 

and conglomerate, siltstone, claystone, basalt flows dated at 11.67 Ma; 300–350 m thick

Alluvium—Poorly to moderately bedded, unconsolidated to weakly consolidated, poorly sorted, 

interstratified sandy gravel and gravelly sand, deposited in major and tributary drainages and 

in piedmont alluvial fans; generally 3 to 4 m thick

Basin-fill deposits—Moderately to poorly bedded, weakly consolidated, interstratified gravelly 

sand to sandy gravel; generally ≤100 m thick

Nogales Wash member—Moderately consolidated volcaniclastic sandstone and conglomerate, 

abundant pumice fragments, pervasively fractured; basal 20–30 m of member forms massive 

cliffs; 50–150 m thick

Proto Canyon member—Moderately to weakly consolidated volcaniclastic conglomerate and 

sandstone, arkosic sandy and clayey conglomerate, and gravelly claystone; includes basal 

debris-flow conglomerate which grades upward into arkose conglomerate and sandstone; 

maximum thickness about 450 m

Upper rhyodacite member—Rhyodacite and rhyolite tuff, agglomerate, flow breccia, and latite; 

rhyolite tuff samples dated at 24.8–23.5 Ma, and another at 26.9–26.3 Ma; 75 m thick

Middle rhyolite member—Vitric-lithic tuff, rhyolite lava, tuffaceous sandstone and conglomerate; 

rhyodacite plugs dated at 27.8 Ma; 100 m thick

Andesite lava and flow breccia, latitic to rhyolitic tuff and lava, tuffaceous conglomerate; 

10–20 m thick

Diorite stock—Augite, hypersthene, and biotite, altered

Arkosic conglomerate and conglomeratic sandstone, argillite, and conglomeratic quartzite; 

50–400 m thick

Porphyritic biotite-hornblende quartz monzonite; lamprophyre, microdiorite, or diabase dikes

QUATERNARY TERTIARY

Miocene

JURASSIC CRETACEOUS

Middle

Quartz 

Monzonite of

Mount Benedict

Upper Paleocene Oligocene Pleistocene Pliocene

Quaternary alluvium Basin-fill deposits Nogales Formation Salero Formation Diorite Gringo Gulch Volcanics Grosvenor Hills Volcanics Proto Canyon member Nogales Wash member Mariposa member

Holocene

Period Epoch Units Description
Methods 5

Most previous geologic mapping within the Rio 

Rico–Nogales area could not be used in the compilation of 

Quaternary surficial deposits, because these published maps do 

not define Quaternary units in sufficient levels of detail or coverage. Thus, the surficial geology in the map area was compiled from interpretation of stereoscopic pairs of 1996 NAPP 

color-infrared aerial photography (1:40,000-scale), 2010 

NAIP orthophotography, imagery viewed with Google Earth, 

topographic data, and digital soils data (U.S. Department of 

Agriculture Natural Resources Conservation Service, 2008), 

using previous mapping in adjacent areas by Youberg and 

Helmick (2001) and Helmick (1986) as a guide. The surficial 

geology was digitally mapped on the NAIP imagery using 

ArcMap GIS software. Interpretive mapping was constrained 

by, and unit description and age estimates are based on data 

collected at GPS-located field stations along a series of 

traverses. Terrace heights were estimated from 1:24,000-scale 

topographic maps, Digital Elevation Model (DEM) data, and 

for lower (younger) units, local field measurements.

Surficial units are classified and mapped on the basis of 

both their genesis and relative age. These units are primarily 

alluvial in origin and were deposited either by axial streams 

and their major tributaries or by distributed, shallow, channel 

systems on piedmont alluvial fans, which also include sediment deposited by debris flows. Age estimates for the deposits 

are based primarily on age-related physical characteristics 

that change progressively with time. Chief among these are 

preservation of original depositional form or microtopography 

(for example, bar-and-swale morphology), development of 

desert pavement, degree of varnish development and amount 

of physical weathering of clasts (Bull, 1991), and the degree 

of soil development. The latter includes depth and degree of 

alteration of surficial deposits (parent materials), accumulation of secondary materials (such as eolian silt, illuvial clay, 

and (or) carbonate), and soil horizon morphology (Birkeland, 

1999). The relative heights of stream-terrace and tributary-fan 

deposits provide an additional constraint on age estimates, 

because progressive and persistent dissection of major and 

tributary drainage systems, punctuated by periodic depositional events, produces a series of deposits where progressively younger deposits are inset within and topographically 

lower than older deposits. No radiometric ages have been 

obtained for Pliocene or Quaternary deposits in the map area. 

Thus general age designations for these units are based on correlations with chronosequences developed in adjacent mapped 

areas (Helmick, 1986; Pearthree and Biggs, 1999; Pearthree 

and Youberg, 2000; Youberg and Helmick, 2001; Lindsey and 

Van Gosen, 2010).

(Division of Pliocene-Quaternary time used in this report 

is from Cohen and others [2014]: Pliocene, 5.33–2.58 Ma; early 

Pleistocene [Calabrian and Gelasian], 2.58 Ma–781,000 yr; 

middle Pleistocene 781,000–126,000 yr; late Pleistocene, 

126,000–11,700 yr; Holocene, 11,700–0 yr. Map unit symbols 

for Quaternary deposits follow Youberg and Helmick (2001) to 

retain consistency with previous mapping in adjacent areas).
6 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

DESCRIPTION OF MAP UNITS

ALLUVIAL AND BASIN-FILL DEPOSITS

Qycr Active river-channel deposits (latest Holocene)—Modern river channel deposits of Youberg and Helmick 

(2001). Alluvium transported and deposited by discrete channel systems during the past several years to 

decades. Unit mapped mostly along sections of the Santa Cruz River, Nogales Wash, and Sonoita Creek 

(fig. 1). Channels are anastomosing to braided systems of small subchannels and low bars (local relief as 

much as 1−2 m) within a larger discrete meandering channel belt up to tens of meters wide. In urbanized 

areas of Nogales, from the diversion tunnel north of the international border to the Mariposa Canyon area, 

channel of Nogales Wash is artificially restrained within concrete abutments. Deposits are composed of 

unconsolidated, poorly to moderately sorted, weakly to moderately bedded, and interstratified silt, fine to 

coarse sand, and pebble- cobble-, and locally small-boulder gravel. Clasts rounded to subangular. No discernible modification of original depositional surfaces. No soil development other than very thin (≤1−2 cm) 

surface crusts composed of silt and very fine sand in channel reaches that have not had recent flows. Most 

reaches currently experience discontinuous, intermittent to ephemeral flow, with the exception of Nogales 

Wash and nearby downstream reaches of the Santa Cruz River that receive limited perennial artificial 

recharge from south of the international border at Nogales. Many channels locally contain significant sparse 

to dense growths of riparian vegetation. Most large channels are prone to moderate to extreme flow discharges during prolonged and (or) intense precipitation events. Larger flooding episodes commonly are 

associated with major sediment transport, deposition, and local erosion within channels and adjacent lowland 

areas. Main channel system generally incised 1−5 m below lowest river terraces of units Qyr2 and Qyr1. 

Much of the most recent valley incision is related to pronounced arroyo incision that initiated in the late 

1880s along all major drainages in the region (Betancourt, 1990; Wood and others, 1999). Estimated thickness 1−3 m

Qyr2 Young river floodplain and terrace deposits (late Holocene)—Part of Holocene floodplain and terrace deposits 

of Youberg and Helmick (2001). Alluvial deposits associated with floodplain and lowest-level river terraces 

flanking main channels on valley floors of major drainages. Surfaces occasionally flooded. Mapped mostly 

along sections of the Santa Cruz River, Nogales Wash, and Sonoita Creek. Locally includes active channel 

deposits too narrow to map separately. Unit associated with flat to gently undulating (local relief generally 

<50 cm), low floodplain surfaces, many abandoned after post-1880s arroyo cutting, and one or more higher 

terraces within heights of 1 to 2 m above adjacent active channels. Deposits are composed of unconsolidated, 

poorly to moderately sorted, poorly to moderately bedded, and interstratified sand, silt, and pebble-, cobble-, 

and locally small-boulder gravel, with a larger percentage of fine-grained components in the upper part of 

the unit. Clasts rounded to subangular. Surfaces typically extensive and undissected. Original depositional 

surfaces artificially altered to varying degrees by agricultural development or urbanization, but display little 

to no natural surface modification. Soil development weak and restricted to the accumulation of fine silt and 

sand at shallow depths. Estimated thickness 1−3 m

Qyr1 Young river terrace deposits (Holocene)—Part of Holocene floodplain and terrace deposits of Youberg and 

Helmick (2001). Alluvial deposits associated with low-level river terraces on valley floors of major drainages. Surfaces have not received discharge or alluvial sediment in the past several decades to millennium. 

Mapped primarily along the Santa Cruz River, Nogales Wash, and Sonoita Creek. Locally includes active 

channel deposits too narrow to map separately. Unit associated with terrace surfaces about 2–5 m above 

adjacent active channels and 1–3 m above lower Qyr2 terraces. Deposits are composed of weakly consolidated, poorly to moderately sorted, poorly bedded, and interstratified silt, sand, and pebble to cobble gravel. 

Clasts rounded to subangular. Original depositional surfaces artificially altered by agricultural development 

or urbanization. Natural surface modification limited to some smoothing of original depositional surfaces 

and development of light varnish on surface clasts. Incipient to weak soil development to depths of 1−1.5 m. 

Soils on the highest (oldest) terraces characterized by A or Av, Bw, and Bk horizons with stage I carbonate 

morphology. Estimated thickness 1−3 m

Qy Young alluvial deposits, undivided (Holocene)—Undifferentiated Holocene alluvium of Youberg and Helmick 

(2001). Deposits associated with channels, adjacent floodplains, low terraces, and small alluvial fans along 

tributaries to major drainages. Locally includes deposits forming narrow alluvial plains at the toe of piedmont slopes along the Santa Cruz River valley margin. Most surfaces are 0.5 to 3 m above active local 

channels. Lower surfaces rarely to occasionally flooded within decadal to millennial time scales by high-
DESCRIPTION OF MAP UNITS 7

discharge runoff during extreme precipitation events, whereas higher surfaces are rarely if ever inundated. 

Deposits are composed of unconsolidated to very weakly consolidated, poorly to moderately sorted, poorly 

bedded, and interstratified sand, silt, and pebble to cobble gravel, with an increase in fine-grained sediment 

near the top of the unit. Locally contains a few scattered small boulders. Clasts rounded to subangular. Clast 

size and relative abundance typically decrease downstream from mostly sandy pebble-cobble gravel in 

upland reaches to sand and pebbly gravel in distal reaches. Unit subdivided into Qy2 and Qy1 where more 

than one set of Holocene alluvial deposits are discernible. Estimated thickness 1−3 m

Qy2 Younger of the young alluvial deposits (late Holocene)—Late Holocene alluvium of Youberg and Helmick 

(2001). Deposits associated with channels, adjacent floodplains, lowest terraces, and small alluvial fans 

along tributary drainages. Forms surfaces mostly 0.5 to 2 m above active local channels in tributary drainages and at the toe of piedmont slopes along the Santa Cruz River valley margin. Surfaces generally flat to 

gently undulating, and undissected with minimal or no modification of original depositional surface. No to 

very weak soil development, limited to thin fine-grained A horizons developed on some terrace surfaces. 

Estimated thickness 1−3 m

Qy1 Older of the young alluvial deposits (Holocene)—Holocene alluvium of Youberg and Helmick (2001). 

Deposits associated with low terraces locally preserved along margins of incised tributary valleys at heights 

of 2 to 3 m above active channels; also may include some small fan remnants near canyon mouths of the 

tributary drainages. Surfaces generally flat to gently undulating with minor dissection by widely spaced shallow rills. Some smoothing of original depositional surfaces, poorly preserved bar-and-swale morphology, and 

incipient pavement development. Some surface clasts are physically weathered and (or) lightly varnished. 

Very weak to weak soil development, about 1–1.5 m deep, characterized by Av, Bw, and Bk horizons with 

thin stage I carbonate morphology. Estimated thickness 1−3 m

Qlr Young intermediate river-terrace deposits, undivided (late Pleistocene)—Late Pleistocene river terraces of 

Youberg and Helmick (2001). Alluvial deposits associated with intermediate-height terraces preserved along 

valley margins of major drainages. Unit mapped along sections of Santa Cruz River, Nogales Wash, and 

Sonoita Creek. Terrace surfaces are generally 8 to 25 m above active channels, and flat to gently sloping 

with mostly minor low-relief dissection. Deposits are composed of unconsolidated, poorly sorted, poorly 

bedded, and interstratified sand, gravelly sand, and sandy pebble-, cobble-, and minor boulder-gravel. Clasts 

subangular to rounded. Varnish on surface clasts imparts orange to reddish-brown coloration to terrace 

surfaces in outcrop and on color photographic imagery. Unit subdivided into Qlr2 and Qlr1 where more than 

one set of late Pleistocene alluvial deposits are discernible. Thickness typically 1 to 2 m, but locally up to 

3 to 4 m

Qlr2 Younger of the young intermediate river-terrace deposits (late Pleistocene)—Part of late Pleistocene river 

terraces of Youberg and Helmick (2001). Generally forms the more prominent, wider and less dissected set 

of terraces, at heights of 8 to 15 m above adjacent channels. Surfaces have weakly to locally moderately 

developed pavement, light-toned orange to pale red varnish, and weak to locally moderate physical clast 

weathering. Generally moderate soil development, to depths of 1–1.5 m, characterized by distinct Av horizons, weak to moderately developed Bt horizons, and Bk horizons with stage I–II carbonate morphology. 

Typically 1 to 2 m thick

Qlr1 Older of the young intermediate river-terrace deposits (late Pleistocene)—Part of late Pleistocene river terraces of Youberg and Helmick (2001). Generally forms terraces with minor to moderate dissection, at heights 

of 10 to 25 m above adjacent channels. Surfaces have weakly to moderately developed pavements, bright 

orange to reddish-brown varnish, and weak to moderate physical clast weathering. Moderate soil development, to depths of 1–1.5 m, characterized by distinct Av horizons, weak to moderately developed Bt horizons, and Bk horizons with stage II carbonate morphology. Typically 1 to 2 m thick

Ql Young intermediate alluvial deposits, undivided (late Pleistocene)—Late Pleistocene alluvium of Youberg and 

Helmick (2001). Deposits associated with intermediate-height terraces and alluvial fans along tributaries to 

major drainages. Top of unit commonly projects to, or grades to top of unit Qlr. Deposits of unit Ql typically overlie older sediments and (or) bedrock. Surfaces flat to gently undulating benches at heights of 5 to 

25 m above adjacent channels. Deposits are composed of unconsolidated, poorly sorted, poorly bedded, and 

interstratified sand, gravelly sand, and sandy gravel. Clasts mainly subrounded to subangular and range in 

size from pebbles to cobbles, with local scattered small boulders. Varnish on surface clasts imparts an overall 

orange to reddish brown coloration to surfaces in outcrop and color photographic imagery. Unit subdivided 

into Ql2 and Ql1 where more than one set of late Pleistocene alluvial deposits are discernible. Typically 1 to 

2 m thick
8 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Ql2 Younger of the young intermediate alluvial deposits (late Pleistocene)—Latest Pleistocene member of 

Youberg and Helmick (2001). Forms benches 5 to 10 m above adjacent channels in valley floors of tributary 

drainages. Surfaces slightly dissected by widely spaced gullies and rills 1 to 2 m deep. Original surface morphology slightly to moderately modified by weak to locally moderate pavement, light-toned orange to palered varnish, and weak to locally moderate physical clast weathering. Moderate soil development, to depths of 

1–1.5 m, characterized by distinct Av horizons, weakly to moderate developed Bt horizons, and Bk horizons 

with stage I–II carbonate morphology. Typically 1 to 2 m thick

Ql1 Older of the young intermediate alluvial deposits (late Pleistocene)—Late Pleistocene member of Youberg 

and Helmick (2001). Forms benches 12 to 25 m above adjacent channels in valley floors of tributary drainages. Surfaces characterized by broad, gently rounded to locally flat interfluves interspersed with widely 

spaced gullies and rills 2 to 5 m deep. Original depositional surface modified by moderate to weak pavement 

development, moderate to locally strong reddish brown to orange varnish, and moderate to weak physical 

clast weathering. Generally moderate soil development, to depths of 1–1.5 m, characterized by distinct Av 

horizons, weakly to moderately developed Bt horizons, and Bk horizons with stage I–II+ carbonate morphology. Typically 1 to 2 m thick

Qmr Older intermediate river-terrace deposits (middle Pleistocene)—Middle Pleistocene river terrace deposits 

of Youberg and Helmick (2001). Alluvial deposits associated with scattered remnants of high-level terraces 

along Sonoita Creek. Terrace deposits form accordant series of broad, rounded, isolated benches, 5 to 10 m 

wide and about 30 to 40 m above active channels of adjacent drainage. Deposits are composed of unconsolidated, poorly sorted, poorly bedded, interstratified sand and gravel. Clasts mainly subangular to rounded 

and range in size from pebbles to cobbles, with scattered small boulders. Surfaces typically have moderate 

to strong modification of original morphology, with moderate to locally strong pavements that subsequently 

have been partially stripped, moderate to strong physical clast weathering, and moderate to strong varnish on 

surface clasts. Varnish on surface clasts imparts dark-reddish-orange to brown coloration to terrace surfaces 

in outcrop and on color photographic imagery. Locally eroded, moderately to strongly developed soils, about 

1–2 m thick, characterized by well-developed, clay-rich Bt horizons, and Bk and cemented K horizons with 

stage II+ and III+ carbonate morphology, respectively. Typically 1 to 3 m thick

Qm Older intermediate alluvial deposits (middle Pleistocene)—Middle Pleistocene alluvium of Youberg and 

Helmick (2001). Deposits preserved mainly as isolated and scattered erosional remnants of high-level 

terraces and alluvial fans related to tributary drainages. Surfaces typically strongly dissected, with broad, 

rounded interfluves, and benches at relative heights of 10 to 30 m above incised tributary channels. Deposits 

are composed of unconsolidated, poorly sorted, poorly bedded, and interstratified sand and gravel. Clasts 

mainly subangular to rounded, and range in size from pebbles to cobbles with scattered small boulders. 

Moderate to strong modification of original surface morphology, with moderate to locally strong pavements 

that subsequently have been partially stripped, moderate to strong physical clast weathering, and moderate 

to strong varnish on surface clasts. Varnish on surface clasts imparts dark reddish-brown or orange-brown 

coloration to surfaces in outcrop and on color photographic imagery. Locally eroded, moderately to strongly 

developed soils, about 1–2 m thick, characterized by well-developed, clay-rich Bt horizons, and Bk and 

cemented K horizons with II to III+ carbonate morphology, respectively. Typically 0.5 to 3 m thick

Qo Old alluvial deposits (early Pleistocene)—Early Pleistocene alluvium of Youberg and Helmick (2001). Alluvialfan deposits preserved on a ridge crest about 170 m above the channel of the Santa Cruz River near the 

northwestern corner of the map area, where the deposits overlie unit QTa. North of the map area, unit is 

more extensively preserved east and west of the Santa Cruz River (Youberg and Helmick, 2001; Lindsey and 

VanGosen, 2010). Deposits are composed of moderately to thickly bedded, moderately to weakly consolidated sandy gravel to gravelly sand. Clasts typically subrounded to angular, and range in size from pebbles 

to cobbles, with a few scattered small boulders. Clasts are derived locally from adjacent highlands. Original surface is strongly modified and only partially preserved, with some erosional remnants of strongly to 

moderately varnished surface clasts on partially stripped former pavements. Locally preserved surface soil is 

characterized by a strongly developed Bt horizon above a well cemented K horizon with stage IV–V carbonate morphology, which includes laminar carbonate at the top of the K horizon. Deposits range in thickness 

from 1 to 2 m above exposures of gently westward sloping basal contact

QTa Alluvial basin fill (early Pleistocene? to Miocene)—Early Pleistocene to Pliocene alluvium of Youberg and 

Helmick (2001). Deposits of basin-fill sediment overlying Nogales Formation and bedrock units. Unit 

consists of stratified sequences of mostly alluvial-fan deposits that are deeply eroded and dissected into a 
DESCRIPTION OF MAP UNITS 9

series of ridges and ravines (ballena topography), with local relief of 10 to 70 m. No original depositional 

surfaces are preserved. Deposits are composed of light-reddish-brown, and pinkish-gray to light-gray, weakly 

consolidated, moderately to poorly bedded, and interstratified sandy gravel and gravelly sand. Locally 

includes finer-grained sections of mostly sand and pebbly sand. Bedding contacts diffuse to gradational in 

character. Clasts commonly subangular to subrounded, and range in size from pebbles to cobbles. Clasts 

are composed of various rock types exposed along basin margins. Volcanic rock types, including rhyolite, 

rhyodacite, and dacitic flows and tuffs, are most common, but some sections, especially near the base of unit, 

contain significant granitic clasts in areas near exposed granitic bedrock. Vertical exposures of unit erode to 

form characteristic shallow vertical fluting that crosscuts bedding. Bedding dips are typically shallow and 

oriented in a basinward direction. Generally few fractures, faults or bedding partings are present in most 

exposures. Unit differentiated from sediments of the underlying Nogales Formation based on weak induration, diffused bedding, vertical fluting in outcrop, and general absence of deformation. The exception is the 

basal part of the unit, which exhibits local internal faulting, and is slightly more indurated and has greater 

stratal tilt relative to sediments higher in the unit. Basal part of unit QTa is faulted against the informally 

named Mariposa member of Nogales Formation along Interstate 19 south of Potrero Creek; along the 

Rio Rico fault on the west flank of the San Cayetano Mountains (fig. 2), where unit QTa in the hanging wall 

is faulted against Salero Formation in the footwall; and at Crawford Hill where unit QTa is faulted against 

the Nogales Wash member (informal name) of the Nogales Formation. Thickness of unit poorly constrained, 

but exposed sections, combined with subsurface estimates based on geophysical and borehole data, suggest 

thicknesses of 50–100 m; possibly as much as 150 m thick in deeper subbasins

QTu Alluvium and basin-fill deposits, undivided (Holocene to Miocene)—Shown in cross sections only; maximum 

thickness about 150 m

Nogales Formation (Miocene)—Includes informally named Mariposa, Nogales Wash, and Proto Canyon members; maximum age of formation is about 20 to 18 Ma, and minimum age is about 11 Ma (Cosca and others, 

2013); combined maximum thickness estimated at 950 m

Tnm Mariposa member—Exposed in vicinity of Mariposa Road and Interstate 19, in basin south of Sonoita Creek 

and east of Santa Cruz River, and in southeastern part of map near international border. In Mariposa Road 

and Interstate 19 areas, consists mostly of grayish-orange-pink to pinkish-gray, moderately to weakly consolidated, alternating beds of volcaniclastic, calcareous, conglomerate and sandstone. Conglomerate and sandstone are coarse- to fine-grained and contain subangular to subrounded grains of quartz (some polymictic), 

plagioclase, biotite, hornblende, porphyritic volcanic rocks, pumice fragments, and altered volcanic glass. 

Member locally contains grayish-orange-pink claystone unit in Mariposa Road area (between Grand Avenue 

and Interstate 19); about 30 to 40 m of unit is exposed, but total thickness may be 60 m or more. Claystone 

is weakly consolidated and has some sandstone and conglomeratic sandstone interbeds. Claystone is calcareous, and some beds contain white, rounded pumice fragments, and angular quartz and biotite grains. Claystone unit gradational with overlying sandstone and conglomerate. In basin south of Sonoita Creek and east 

of Santa Cruz River, member contains units like those in urban Nogales, but conglomerate and sandstone is 

less coarse, and units represent a finer-grained facies, consisting of alternating beds of fine-grained, volcaniclastic sandstone, siltstone, and claystone. Exposed claystone beds are grayish-orange-pink, poorly sorted, 

calcareous, and sandy to gravelly like those in urban Nogales, but are generally thinner, and range from 0.5 

to 3 m thick. Along the international border in the southeastern part of the map, only lower part of Mariposa 

member is exposed and consists of grayish-orange, volcaniclastic, pebble conglomerate with mostly subangular, light-brownish-gray porphyritic volcanic clasts, and some fine to medium grained, subangular to 

subrounded, volcaniclastic sandstone containing mostly porphyritic biotite- and hornblende-bearing rocks. In 

this area, member contains thin discontinuous, medium dark gray, olivine basalt flows (indicated by stipple 

pattern). Basalt flows are 3 to 5 m thick, and have a 40Ar/39Ar age of 11.67±0.09 Ma (Cosca and others, 2013; 

table 1). Member is generally disconformable with overlying unit QTa, and is exposed in scattered isolated 

outcrops where it is about 80–100 m thick, but maximum thickness is estimated to be 350 m in deeper basins

Tnn Nogales Wash member—Only exposed in southern part of map area in urban Nogales, Arizona area, south 

of Mariposa Road, and along the international border from Ephraim Canyon to about 4 km east of Grand 

Avenue. Member is mostly moderately consolidated, pinkish-gray, yellowish-gray, and grayish-orange-pink, 

alternating thin- to thick-bedded, fine- to coarse-grained volcaniclastic sandstone, conglomeratic sandstone, 

and minor siltstone and claystone; grains are poorly to moderately sorted, subangular to subrounded, and are 

composed of pumice fragments, quartz, plagioclase, biotite, hornblende, biotite- and hornblende-bearing por-
10 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

phyritic volcanic grains, and minor chert, microcline, spherulite fragments, and altered volcanic glass. Beds 

typically contain tabular planar crossbeds, and few show mudcracks, bioturbation, and leaf impressions. Unit 

locally contains fining-upward sequences of conglomeratic sandstone at the base, medium-grained sandstone 

in the middle, and fine-grained sandstone and siltstone at top. The basal 20 to 30 m forms distinctive massive 

cliffs composed of pinkish-gray, moderately well consolidated volcaniclastic sandstone and conglomeratic 

sandstone, with abundant pumice fragments. Member is pervasively fractured in most places in map area, 

and is disconformable with overlying Mariposa member. Thickness estimated at about 50 to 150 m

Tnp Proto Canyon member—Upper part consists mostly of thin-bedded, moderately consolidated, poorly sorted, 

fine- to coarse-grained, yellowish-gray to pinkish-gray volcaniclastic sandstone and conglomerate, with 

subangular to subrounded grains of quartz (some coarser grains polymictic), plagioclase, biotite, hornblende, 

pumice, porphyritic biotite- and plagioclase-bearing volcanic grains, and altered volcanic glass. Some beds 

show trough crossbedding, and contain monzonite cobble to pebble lag gravel in channels. Middle parts of 

the member consist of alternating beds of yellowish-gray, poorly sorted, volcaniclastic sandstone and conglomerate; pale-red sandy and gravelly claystone; and pale-red clayey and sandy arkosic conglomerate that 

contains poorly sorted, subangular clasts of monzonite. Below middle part of unit and above the basal conglomerate are cyclic, fining-upward fluvial sequences; cycles generally consist of (from base to top) pinkishgray monzonite clast conglomerate, massive gray-weathering volcaniclastic conglomeratic sandstone, and 

white horizontally bedded pumice-rich siltstone; sequences also contain some debris-flow conglomerate 

beds. The cyclic sequences form massive cliffs in the Proto Canyon area south of State Highway 82. Basal 

conglomerate consists mostly of grayish-red, clast-supported monzonite boulder, cobble and pebble debrisflow conglomerate which grades upward into pale-reddish-brown, weakly consolidated arkose sandstone and 

conglomerate with monzonite boulders, cobbles, and pebbles. North of State Highway 82, member includes 

dacite dikes (Tnpd) intruding unnamed north-northeast striking fault cutting arkose conglomerate units in 

lower part of the Proto Canyon member. Dacite is greenish-gray to light-olive-gray, and consists mostly of 

phenocrysts and granular clots of plagioclase, and lesser amounts of amphibole, biotite, pyroxene, and some 

quartz and feldspar inclusions; dikes 1 to 2 m thick. In the Sonoita Creek area, the Proto Canyon member 

consists of upper and lower parts; upper part is mostly thin-bedded, fine-grained, moderately sorted, volcaniclastic sandstone and conglomeratic sandstone which weathers to form distinctive rounded ledges. Lower 

part is volcaniclastic conglomerate and sandstone with cobbles, pebbles, and grains primarily of subangular 

white and red porphyritic volcanic rocks, rhyodacitic to rhyolitic tuff, pumice, and some basal debris-flow 

conglomerate eroded mostly from the upper rhyodacite member of the Grosvenor Hills Volcanics. Proto 

Canyon member is unconformably overlain by the Nogales Wash member, and by the Mariposa member 

where the Nogales Wash member is absent. Member thickness variable, but maximum thickness about 450 m

Bedrock Units

Grosvenor Hills Volcanics (Oligocene)—Descriptions from Simons (1974) and Drewes (1972). Consists of 

rhyolite member, rhyodacite and rhyolite intrusive rocks (Tr), and rhyodacite members of Drewes (1972); 

basal silt and clay member of Drewes (1972) not exposed in map area; combined thickness for the unit about 

175 m thick in map area (Drewes, 1972)

Tghu Upper rhyodacite member—Dark gray to dark brown, rhyodacite agglomerate, grayish-red, brownish-gray, 

and light-gray, coarse-grained porphyritic biotite-hornblende rhyodacitic tuff, rhyolite tuff, flow breccia, 

tuff breccia, and biotite latite tuff. In western part of southern Grosvenor Hills, upper part is brownish-gray 

coarse-grained porphyritic biotite rhyodacitic agglomerate with blocks up to 1 m across. In Fresno Canyon 

area (fig. 1 and sheet 1), unit consists of tuff breccia, flow breccia, and some gray to grayish red hornblendehypersthene-biotite vitrophyre. 40Ar/39Ar geochronology on a rhyolite tuff (sample 12SCB03, table 1) yielded 

ages ranging from 24.4 to 23.5 Ma on sanidine single-grain laser fusion analyses, and a plateau age of age 

of 24.1±0.5 Ma (Cosca and others, 2013; table 1). Another rhyolite tuff collected from a lower stratigraphic 

level in the unit (sample 12SCB04, table 1) produced 40Ar/39Ar plateau ages of 26.3±0.7 Ma for amphibole 

(one grain) and 26.9±0.2 Ma for biotite (Cosca and others, 2013; table 1)

Tr Rhyodacite and rhyolite intrusive rocks—Mostly brownish- to pinkish-gray porphyritic rhyodacite and 

rhyolite dikes and plugs north of Sonoita Creek. 40Ar/39Ar geochronology analyses of one amphibole grain 

from this unit (sample 12SCB05, table 1) yielded a plateau age of 27.8±0.3 Ma and a near-plateau age from a 

single biotite grain of 27.6±0.4 Ma (fig. 4) (Cosca and others, 2013; table 1)
DESCRIPTION OF MAP UNITS 11

Tghm Middle rhyolite member—Mostly orange-pink and light-gray biotite rhyolitic, vitric, lithic tuff, and some 

rhyolite lava, agglomerate, tuff breccia and volcaniclastic conglomerate and sandstone. In Fresno Canyon 

area, lower part of member includes (1) pale-red spherulitic rhyolite with flow layering, (2) red, brown, and 

dark green and gray well-bedded volcaniclastic sandstone and tuff, and (3) grayish-red fined grained porphyritic biotite andesite and andesitic agglomerate

Tv Tertiary volcanic rocks undivided (Oligocene)—Cross section only; consists of Grosvenor Hills Volcanics and 

equivalent rocks; about 100 to 300 m thick

Tgg Gringo Gulch Volcanics (Paleocene)—Description from Simons (1974). Mostly green, greenish-gray, and 

reddish-purple pyroxene andesite lava and flow breccia; andesitic tuff and breccia; light-gray to pink latitic 

to rhyolitic tuff and lava; and coarse polymictic volcanic conglomerate. Only exposed on northeast edge of 

map, and about 60 Ma, based on geologic relations and radiometric ages for rocks above and below the unit 

northeast of map area (Drewes, 1972). Most of unit exposed to the northeast of the map area, along upper 

Sonoita Creek, southwest of Patagonia; about 10 to 20 m thick

Kd Diorite (Upper Cretaceous)—Stock composed of highly weathered and altered grayish-red, fine-grained diorite 

on southern border of map area, in downtown Nogales; about 90 percent (or greater) altered plagioclase, 

10 percent (or less) augite, hypersthene, and biotite. Simons (1974) also reported some porphyritic biotite 

latite and rhyolite in easternmost exposures of unit in the map area. Similar stocks in the southern Santa Rita 

Mountains were dated at 67 Ma (Drewes, 1980)

Ks Salero Formation (Upper Cretaceous)—Exposed along Santa Cruz River southeast of Sonoita Creek and 

northwest of Guevavi Canyon, and on northern edge of map area in southern San Cayetano Mountains (fig. 1 

and sheet 1). Along Santa Cruz River, lower part of formation unconformably overlies the Quartz Monzonite 

of Mount Benedict, and consists of well consolidated pale-red to light-brown conglomerate and conglomeratic sandstone; pebble, cobble, and some boulder-size clasts. Clasts and grains are subrounded and consist 

mostly of white porphyritic volcanic rocks, gray sandstone and siltstone, and granite; generally clast and 

grain supported, but contains some sand, silt, and clay matrix. In southern San Cayetano Mountains, unit 

consists mostly of pale-red to grayish-red conglomeratic quartzite, and greenish-gray thin-bedded argillite; 

intruded by numerous pale-red latite dikes and grayish-red rhyolitic to rhyodacite dikes and plugs (unit Tr) 

of Oligocene age (Simons, 1974). Drewes (1968) reported K-Ar age of 72.5±2.2 Ma (biotite) from a welded 

tuff in the Salero Formation in Santa Rita Mountains, 10 km north of the map area. Formation is at least 400 

m thick in San Cayetano Mountains, and 50 to 150 m thick in the Santa Cruz River area

Quartz Monzonite of Mount Benedict (Jurassic)—Descriptions from Simons (1974)

Jb Biotite-hornblende quartz monzonite—Light- to light-brownish-gray, coarse-grained, porphyritic quartz 

monzonite; 20 percent quartz, 36 percent plagioclase, 36 percent perthitic potassium feldspar, 4 percent biotite, 3 percent pale green hornblende, and 1 percent accessories; also contains some granodiorite and quartz 

diorite. K-Ar age on biotite 160±7 Ma, and on hornblende, 164±19 Ma (Simons, 1974). Contains abundant 

northwest-trending lamprophyre, microdiorite, or diabase dikes of Jurassic age (Simons, 1974)

Jbm Quartz monzonite—Light brown and light gray, fine to coarse grained; abundant fine-grained or aplitic quartz 

monzonite at Mount Benedict, and epidote-quartz-chlorite rock (epidosite) north and east of Mount Benedict. 

Probably older than unit Jb

KJu Cretaceous and Jurassic rocks undivided—Cross section only; mostly includes Quartz Monzonite of Mount 

Benedict and equivalent units, but may also include some Cretaceous igneous and sedimentary rocks
12 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Stratigraphy

Jurassic

The oldest rocks exposed in the quadrangle include the 

Quartz Monzonite of Mount Benedict of Late to Middle Jurassic 

age (160±19 Ma and 164±19 Ma; Simons, 1974). These rocks 

are highly deformed and are intruded by numerous diorite and 

diabase dikes (Simons, 1974). The dikes are mostly northwest striking and southwest dipping, and although most are a 

few meters thick, some are as much as 60 m thick. We follow 

Simons (1974), and subdivided the formation into two units; 

biotite hornblende quartz monzonite (Jb), the younger unit, 

and quartz monzonite (Jbm), the older unit. These rocks form 

a confining unit below the Miocene Nogales Formation and 

Quaternary to Tertiary basin-fill surficial deposits.

Upper Cretaceous

The lower part of the Upper Cretaceous Salero Formation 

unconformably overlies the Quartz Monzonite of Mount 

Benedict along the Santa Cruz River southeast of Sonoita 

Creek and northwest of Guevavi Canyon (sheet 1 and fig. 1). 

Most of the unit in that area consists of thick-bedded, wellconsolidated conglomerate and sandstone, with clasts of 

silicic volcanic rocks and some granite. The Salero Formation 

forms the high ridge of the San Cayetano Mountains on the 

north edge of the map, where it is greater than 400 m thick, 

and consists mainly of light-brown conglomerate and sandstone, green and gray argillite and sandstone, and pale red 

quartzite and conglomerate. These rocks are intruded by 

abundant west- to northwest-trending, reddish-gray latite 

dikes and plugs of Tertiary age. Drewes (1968) reported a 

K-Ar age of 72.5±2.2 Ma (biotite) from a welded tuff in the 

Santa Rita Mountains, about 10 km north of the map area, 

and Hayes (1970) reported that this K-Ar date combined with 

stratigraphic position indicates a late Campanian to earliest 

Maastrichtian age (about 72 to 69 Ma) for the formation.

Tertiary

Grosvenor Hills Volcanics

The Grosvenor Hills Volcanics (Drewes, 1968) are 

exposed north of Sonoita Creek and east of the San Cayetano 

Mountains, and consist mostly of pale red to pink rhyolite and 

rhyodacitic tuff, vitric lithic tuff, agglomerate, flow breccia, 

and latite. These rocks are well exposed north of the map area 

in the Grosvenor Hills, and age-equivalent rocks are exposed 

to the west in the Atascosa and Tumacacori Mountains, and to 

the south in the Nogales, Sonora area. Drewes (1968, 1972) 

subdivided the Grosvenor Hills into a basal gravel and silt, 

a moderately thick middle rhyolite member, and an upper 

rhyodacite member. The basal member is not exposed in the 

map area. Drewes (1972, 1980) reported ages from 28 to 27 Ma 

for the Grosvenor Hills Volcanics, north of the map area in 

the Grosvenor Hills. Two samples collected from laccoliths 

intruding the rhyolite member had K-Ar ages (hornblende) of 

27.8±2.8 Ma and 27.6 Ma. The Grosvenor Hills were interpreted to be underlain by a thick, subsurface intrusion, based on 

a corresponding 10 km-in-diameter, circular, positive magnetic 

anomaly (Gettings, 2002).

Drewes (1972) described the rhyolite member in the 

Grosvenor Hills to consist mostly of tuff and tuff breccia, 

with lesser amounts (about 5 percent each) of tuffaceous 

sandstone, agglomerate, lava flows, and welded tuff. Mapping 

by Drewes (1972) suggests these rocks were erupted from 

multiple vents. We collected samples from the rhyolite member of the Grosvenor Hills Volcanics in the map area north 

of Sonoita Creek (sheet 1). One sample from a rhyolitic to 

dacitic intrusive rock (sample 12SCB05, table 1) in the middle 

rhyolite member yielded ages of 27.8±0.3 Ma (amphibole) 

and 27.6±0.4 Ma (biotite) (Cosca and others, 2013; table 1 and 

fig. 4). These ages are similar to those reported for the member 

by Drewes (1972).

Drewes (1972) reported the rhyodacite member to consist 

mostly of agglomerate and lava flows, with some welded tuff 

and minor non-welded tuff. Our new 40Ar/39Ar geochronology 

(table 1) indicates units of the upper rhyodacite member are 

significantly younger than originally interpreted by Drewes 

(28–27 Ma), and are about 27 to 23 Ma. These conclusions are 

based primarily on new 40Ar/39Ar geochronology ages determined for sanidine and biotite from rhyolite tuff samples, one 

ranging from 24.8±0.1 Ma to 23.5±0.1 Ma (sample 12SCB03, 

table 1), and another (sample 12SCB04, table 1) ranging from 

26.9±0.2 Ma to 26.3±0.7 Ma (Cosca and others, 2013; table 1 

and fig. 4). Additional evidence supporting a younger age 

for the upper rhyodacite member includes numerous detrital 

sanidine and plagioclase grains with ages ranging from about 

26 to 23 Ma in the overlying Proto Canyon and Nogales Wash 

members of the Nogales Formation (fig. 4; samples 11SCB01, 

11SCB02, 11SCB03, 11SCB08, and 11SCB09 from the 

Nogales Wash member, and sample 12SCB01 from the Proto 

Canyon member). Additional studies are needed to better 

determine the geochronologic, stratigraphic, genetic, and 

tectonic relations between the middle and upper members, 

and with the onlapping members of the Nogales Formation.

Nogales Formation

The Nogales Formation was named by Simons (1974) 

for exposures in Nogales, Arizona, and areas to the east and 

north. In the Nogales area, the formation overlies the Jurassic Quartz Monzonite of Mount Benedict, which was a local 

source area for sediments in the formation. Simons (1974) 

recognized correlative sediments in the Sonoita Creek area 

that overlie the Grosvenor Hills Volcanics, which were a 

local source for detrital material in sediments of the Nogales 

Formation in that area. Because of the different source terrains for the formation, many of the clastic units commonly 

contain mixtures of Oligocene volcanic rocks and Jurassic 
Stratigraphy 13

monzonite. Simons estimated a thickness of about 2,290 m for 

the entire Nogales Formation, but our new mapping indicates 

a maximum thickness of about 950 m for the formation in the 

Nogales, Arizona area.

Simons (1974) originally subdivided the Nogales Formation 

in the Nogales area into lower, middle, and upper members, 

which are generally equivalent with the Mariposa, Nogales Wash, 

and Proto Canyon members in this report. Simons (1974) mapped 

his lower member mostly south of Mount Benedict, and reported 

that it contained conglomerate, fanglomerate, volcaniclastic 

conglomerate, sandstone, and tuff; thickness of the lower member 

was estimated to be about 1,500 m. Simons (1974) mapped his 

middle and upper members east of Nogales, Arizona, south of 

State Highway 82, and west of the Santa Cruz River; he described 

the middle member to consist of about 150 m of volcaniclastic 

conglomerate and sandstone and white biotite-hornblende tuff, 

and his upper member to consist of about 610 m of volcanic 

conglomerate with clasts of plutonic and metasedimentary rocks. 

The upper member contains basalt flows dated at 11.67±0.09 Ma 

(Cosca and others, 2013). In the basin south of Sonoita Creek 

and east of the Mount Benedict fault, Simons (1974) mapped 

undivided Nogales Formation, that mostly contained volcanic 

conglomerate and sandstone which locally overlies the Grosvenor 

Hills Volcanics.

Table 1. 40Ar/39Ar geochronology for Grosvenor Hills Volcanics and Nogales Formation in the Nogales and Rio Rico 1:24,000-scale 

quadrangles, Arizona. Data analyzed by Cosca and others (2013) at USGS Argon geochronology laboratory, Denver, Colorado. See 

combined 40Ar/39Ar data in appendix.

[Ma, age reported in mega-annums. UTM listed in northing and easting coordinates, zone 12]

Sample Geologic unit Argon age Material analyzed UTM coordinates

11SCB01 Nogales Fm. 25.09 to 23.6 Ma Sanidine, plagioclase 3466868 505543

11SCB02 Nogales Fm. 25.93 to 24.7±0.6 Ma Plagioclase 3467200 506142

11SCB03 Nogales Fm. 25.57 to 23.7 Ma Sanidine, plagioclase 3467850 506764

11SCB07 Nogales Fm. 11.67±0.09 Ma Whole rock 3467793 509993

11SCB08 Nogales Fm. 24.9±0.2 Ma Sanidine, plagioclase 3466710 508185

11SCB09 Nogales Fm. 25.2±0.7 Ma Sanidine, plagioclase 3466814 508250

12SCB01 Nogales Fm. 24.76 Ma Sanidine 3470259 509952

12SCB03 Grosvenor Hills Volc. 24.8±0.1 to 23.5±0.1 Ma Biotite 3483451 505870

12SCB04 Grosvenor Hills Volc. 26.9±0.2 to 26.3±0.7 Ma Amphibole 3483522 506075

12SCB05 Grosvenor Hills Volc. 27.8±0.3 to 27.6±0.4 Ma Biotite, amphibole 3483676 505884

Figure 4. Plots of new 40Ar/39Ar geochronology incremental heating data for amphibole and biotite analyses 

from the Grosvenor Hills Volcanics. Incremental analysis indicated in red for sample 12SCB03 as omitted from 

calculation of plateau; number of grains analyzed for each sample indicated on plots. Apparent age reported 

in mega-annums (Ma); %, percent.

26.3±0.7 Ma*

12SCB04 (single grain)

27.8±0.3 Ma

12SCB05 (3 grains) 

Amphibole

Apparent Age (Ma) Apparent Age (Ma)

10

20

30

40

10

20

30

40

Cumulative % 39Ar Released

0 10 20 30 40 50 60 70 80 90 100

Grosvenor Hills Volcanics Ca/K Apparent Age (Ma) Apparent Age (Ma) Apparent Age (Ma) Ca/K

10

30

5

15

10

20

30

40

20

30

40

20

30

40

Biotite

24.1±0.5 Ma

12SCB03 (3 grains)

26.9±0.2 Ma

12SCB04 (single grain)

27.6±0.4Ma*

12SCB05 (single grain)

Cumulative % 39Ar Released

0 10 20 30 40 50 60 70 80 90 100
14 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Rocks of the Nogales Formation were mapped in other 

areas of the upper Santa Cruz basin. Drewes (1971) mapped the 

Nogales Formation near the mouth of Cottonwood Canyon in 

the northern Santa Rita Mountains (fig. 2), which he described 

as consisting mostly of gravel, conglomerate, and sand with 

abundant volcanic detritus. He described the unit as Miocene to 

Pliocene in age, and estimated its thickness to be slightly more 

than 300 m. Lindsey and Van Gosen (2005, 2010) mapped the 

Nogales Formation in the Josephine Canyon area (fig. 2), south 

of Cottonwood Canyon. Cooper (1973) mapped the Nogales 

Formation in the southern Sierrita Mountains (fig. 2), where 

he noted volcaniclastic units in the lower 30 m. Drewes (1980) 

mapped the Nogales Formation on the east flank of the Atascosa 

Mountains, in Agua Fria Canyon (fig. 2), where it was estimated 

to be about 800 m thick. In this area, the unit consists mostly 

of sandstone and conglomerate, with an ash-flow tuff near the 

base, and basalt flows in the middle and upper parts. Nelson 

(1963) mapped units of the Nogales Formation in the Walker 

and Peña Blanca Canyon areas (fig. 2), east of the Atascosa 

Mountains; he referred to these rocks as the “Peña Blanca 

Formation,” which is about 110 m thick.

Revised Stratigraphy for Nogales Formation

New mapping for this report combined with geochronologic, geophysical, and hydrogeologic data, resulted in significant stratigraphic revision of the Nogales Formation as defined 

by Simons (1974), and recognizes three informal members, 

including (from base to top) the Proto Canyon, Nogales Wash, 

and Mariposa members. Sediments in the Nogales Formation 

were deposited mostly by streams and debris flows on alluvial 

fans on the flanks of the upper Santa Cruz basin, and in playas 

in axial parts of the basin. The depositional basin extends from 

west of the Patagonia Mountains, south of the Grosvenor Hills, 

west of the San Cayetano and Santa Rita Mountains, and east 

of the Atascosa and Tumacacori Mountains (fig. 2), and the 

basin extends southward into Sonora, Mexico. Basin geometry 

in the Nogales and Rio Rico quadrangles during the Miocene 

was complex due to a highly irregular pre-Nogales Formation 

erosion surface developed on Jurassic and Cretaceous intrusive and sedimentary rocks which were deformed by episodes 

of Miocene basin and range extension, and older pre-Tertiary 

deformational events, most notably Cretaceous plutonism and 

faulting related to the Laramide orogeny (Simons, 1974).

Within the map area, rocks of the Proto Canyon member 

represent initial deposits that accumulated in the southern 

part of the upper Santa Cruz basin, and were laid down across 

an area coincident with that of the modern day basin. During deposition of the Proto Canyon member, Mount Benedict 

(fig. 1, sheet 1) was likely a paleotopographic high near the 

center of the basin, based on coarse debris-flow conglomerate 

at the base of the Proto Canyon member shed off the flanks 

of Mount Benedict. The Mount Benedict horst block (fig. 1) 

likely stood topographically high in the basin owing to faulting during earlier stages of extension, or from some older 

pre-Tertiary deformational events, or both. The horst block 

likely experienced multiple stages of uplift during deposition 

of the Nogales Formation based on unconformities bounding 

each member of the formation, and onlapping relations 

between the Proto Canyon member and the Nogales Wash 

and Mariposa members.

Uplift of the Mount Benedict horst block occurred during 

and/or following deposition of the Proto Canyon member to 

expose basal debris-flow conglomerate of the formation within 

the horst block, and further compartmentalized the southern 

part of the Santa Cruz basin into two main subbasins; one west 

of the horst block and east of the Atascosa and Tumacacori 

Mountains, and another east of the horst block and Santa Cruz 

River, south of the Grosvenor Hills, and west of the Patagonia 

Mountains. The Proto Canyon member was deeply eroded 

following this uplift, based partly on (1) the high degree of 

weathering of abundant Jurassic monzonite clasts contained 

within thick, arkose units in lower and middle parts of the 

member, and (2) the onlapping relations of the Nogales Wash 

and Mariposa members onto Proto Canyon member units in 

the southeastern part of the map area (fig. 5 and sheet 1). Further faulting and uplift of the horst block continued throughout 

and following deposition of the Nogales Formation, based 

on the observation that all members are highly deformed, 

and major faults cut the very oldest part of post-Nogales 

Formation alluvial basin-fill deposits (unit QTa) overlying the 

formation. Strata in all members of the Nogales Formation are 

moderately to highly rotated, with dips ranging from 10° to 

40°, and locally as much as 70° along major faults (sheet 1). 

The formation is locally folded in the southeast part of the 

map near the international border, and in the central part of the 

map area west of the Gold Hill fault (fig. 1 and sheet 1).

Figure 5. Northeast view of ledge-forming, resistant volcaniclastic 

sandstone beds in the Nogales Wash member of the Nogales 

Formation (Tnn) overlying less-resistant arkose sandstone and 

conglomerate beds in the lower part of the Proto Canyon member 

(Tnp), east of Nogales, Arizona, along State Highway 82.

Tnn

Tnp

1 METER
Stratigraphy 15

Nogales Formation Geochronology

Previous geochronology for the Nogales Formation was 

reported by Simons (1974), Marvin and others (1973), and 

Houser and others (2004). Simons (1974) reported a K-Ar age 

of 12.6±0.8 Ma (Marvin and others, 1973) on groundmass from 

a basalt in the Nogales Formation from Agua Fria Canyon, in 

the adjacent Peña Blanca Lake quadrangle (fig. 2). Houser and 

others (2004) indicated the presence of multiple generations 

of volcanic-sourced sanidine and plagioclase using 40Ar/39Ar 

geochronology, with the youngest grains providing an accurate 

determination of the age of Nogales Formation deposition. 

Houser and others (2004) dated two samples of the Nogales 

Formation in the upper Santa Cruz basin by single-grain, laserfusion 40Ar/39Ar geochronology—one from Agua Fria Canyon, 

and one in Cottonwood Canyon (fig. 2). In Cottonwood Canyon, 

a tuff 30 m above the base of the formation yielded two possible 

ages of 15.53±0.38 Ma (sanidine) and 17.38±0.77 Ma (whole 

rock) (Houser and others, 2004). In the Agua Fria Canyon area, 

an ash-flow tuff near the basal contact of the formation yielded 

a weighted mean age of 13.23±0.10 Ma determined from 14 

sanidine grains (Houser and others, 2004).

New 40Ar/39Ar geochronology of the Nogales Formation 

(table 1) determined for this study generally supports these 

earlier geochronologic studies, and indicates the formation may 

be as old as early Miocene (about 20 to 18 Ma) and as young as 

early late Miocene (about 11 Ma). The minimum age limit of 

11 Ma is based on a whole-rock age of 11.67±0.09 Ma (fig. 6; 

table 1) (Cosca and others, 2013), determined for a basalt in the 

Mariposa member, the upper member of the formation. Because 

the stratigraphic position of these basalts is in the lower part of 

the Mariposa member in the Nogales quadrangle, and to account 

for deposition of sediments in the upper part of the member, 

we estimated an overall minimum age of about 11 Ma for the 

formation. Although our new 40Ar/39Ar age is the youngest age 

determined for the Nogales Formation, an older K-Ar age of 

12.6±0.8 Ma (Marvin and other, 1973) was obtained from a 

basalt flow (whole rock) in the upper part of the formation in 

Aqua Fria Canyon (fig. 2). We dated two basalt samples from 

the Nogales Formation from Aqua Fria Canyon, possibly the 

same unit dated by Marvin and others (1973), and the basalts 

yielded 40Ar/39Ar plateau whole-rock ages of 13.64±0.11 Ma 

and 13.65±0.11 Ma (Cosca and others, 2013; table 2 and fig. 6). 

These basalt ages are similar in age to the weighted mean age of 

13.23±0.10 Ma age of sanidine 40Ar/39Ar fusion ages from the 

same area (Houser and others, 2004).

The maximum age of about 20 to 18 Ma for the formation is based on the oldest age determinations for the formation in the upper Santa Cruz basin, in the Cottonwood 

Canyon area, where a sample collected in the basal 30 m of 

the formation yielded an age range between 17.38±0.77 Ma 

(whole rock) and 15.53±0.38 Ma (sanidine) (Houser and 

others, 2004). Further, new single-grain laser-fusion 40Ar/39Ar 

geochronology on four plagioclase grains from a rhyolite 

tuff in Agua Fria Canyon yielded a mean age of 18.4±1.5 Ma 

(11SCB05; fig. 7; table 2) (Cosca and others, 2013). The large 

Figure 6. Plots of new 40Ar/39Ar incremental-heating analyses 

of whole-rock samples from basalts in the Nogales Formation. 

Apparent age reported in mega-annums (Ma); %, percent.

0 10 20 30 40 50 60 70 80 90 100

Cumulative % 39Ar Released

0 10 20 30 40 50 60 70 80 90 100

Cumulative % 39Ar released

0 10 20 30 40 50 60 70 80 90 100

Cumulative % 39Ar released

0.2

0.6

1.0

6

10

14

18

Apparent age (Ma)

K/Ca

0.1

0.3

0.5

6

10

14

18

Apparent age (Ma)

K/Ca

0.1

0.3

0.5

6

10

14

18

Apparent age (Ma)

K/Ca

11SCB07

11.67±0.09 Ma

11SCB06

13.65±0.11 Ma

11SCB04

 13.64±0.11 Ma

Basalt in Mariposa member near U.S.–Mexico border

Basalts of Agua Fria Canyon
16 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Figure 7. New 40Ar/39Ar ideogram plots for laser-fusion analyses of A, 

sanidine from rhyolite tuff in Grosvenor Hills Volcanics, B, detrital sanidine 

and plagioclase from volcaniclastic sediments in the Nogales Formation, 

and C, sanidine and plagioclase from rhyolite tuff in the Nogales Formation 

from Agua Fria Canyon area. Age reported in mega-annums (Ma).

20 22 24 26 28 30

Age (Ma)

16 18 20 22 24 26 28 30

Age (Ma)

A Sanidine from rhyolite tuff in Grosvenor Hills Volcanics

Detrital sanidine and plagioclase in fluvially reworked 

volcaniclastic sediments of the Nogales Formation

B

Sanidine and plagioclase from rhyolite tuff 

in Nogales Formation in Agua Fria Canyon

C

Grosvenor Hills volcanism 12SCB03 

12SCB01

11SCB01

11SCB02 

11SCB03

11SCB08

11SCB09

11SCB05

Sanidine

Plagioclase

EXPLANATION

Table 2. 40Ar/39Ar geochronology for Nogales Formation in the Agua Fria Canyon area (fig. 2), Peña Blanca Lake 1:24,000-scale quadrangle, 

adjacent and to the west of the Rio Rico quadrangle. Data analyzed by Cosca and others (2013) at USGS Argon geochronology laboratory, 

Denver, Colorado; see combined 40Ar/39Ar data in appendix.

[UTM listed in northing and easting coordinates, zone 12]

Sample Geologic unit Argon age Material analyzed UTM coordinates

11SCB04 Nogales Fm. 13.64±0.11 Ma whole rock 3480096 495337

11SCB05 Nogales Fm. 18.4±1.5 Ma sanidine, plagioclase 3480019 495020

11SCB06 Nogales Fm. 13.65±0.11 Ma whole rock 3480001 494884
Stratigraphy 17

sorted debris-flow conglomerate with subangular, monzonite 

boulders, cobbles, and pebbles, in a mostly sandy matrix 

(fig. 9). Unlike most all other units in the Nogales Formation, 

the basal debris flow conglomerate and arkose units characteristically lack pumice and altered volcanic glass fragments, and 

consist almost entirely of monzonite detritus eroded from the 

Quartz Monzonite of Mount Benedict. The basal debris-flow 

conglomerate and arkose units are well exposed in the basin 

north of Proto Canyon and State Highway 82, and south of 

Mount Benedict. The thickness of the basal debris-flow conglomerate and arkose units combined is variable, but estimated 

at about 100 to 250 m thick.

The lower units of the Proto Canyon member above 

the basal conglomerate and arkose units are exposed in 

Proto Canyon south of State Highway 82, where they consist of cliff-forming, fining-upward, cyclic fluvial sequences 

of volcaniclastic conglomerate at the base, volcaniclastic 

conglomeratic sandstone in the middle, and laminated white, 

water-lain pumice siltstone at the top (fig. 10); the sequences 

also contain some debris flow conglomerate beds. The middle 

part of the Proto Canyon Member contains alternating beds 

analytical uncertainties associated with all of these analyses 

support any age between 20 and 16 Ma for the maximum age 

of the Nogales Formation, thus their usefulness for precise 

geochronology is limited.

Single-grain, laser-fusion 40Ar/39Ar geochronology on 

detrital sanidine and plagioclase from the Proto Canyon and 

Nogales Wash members of the Nogales Formation for this 

study yielded ages ranging from 26 to 23 Ma (table 1), which 

likely reflect the ages of the Grosvenor Hills Volcanics (fig. 7) 

or equivalent rocks from an unknown source in Sonora. Detrital sanidine grains from the Proto Canyon member yielded a 

40Ar/39Ar age of 24.76 Ma (table 1, sample 12SCB01). Several 

samples from the Nogales Wash member (table 1; samples 

11SCB01, 02, 03, 08, and 09) yielded 40Ar/39Ar ages from 

26 to 23 Ma. The restricted age range of volcanic feldspars 

closely matches the age of volcanism in the Grosvenor Hills, 

the presumed source area for these grains (fig. 7).

Proto Canyon Member

The lower member of the Nogales Wash Formation is 

herein informally named for exposures in Proto Canyon, in 

the southeastern part of the map area (fig. 1, sheet 1). South 

of Mount Benedict (fig. 1), the basal part of the Proto Canyon 

member consists of distinctive debris-flow conglomerate 

(fig. 8) which grades upward into arkose conglomerate and 

sandstone (fig. 9). The basal coarse debris-flow conglomerate consists entirely of monzonite clasts, is primarily clastsupported (fig. 8), and is well exposed near the contact with 

the Jurassic monzonite. These beds grade upward into distinctive weakly consolidated sequences of pale-reddish-brown 

arkose conglomerate and sandstone, with some beds of poorly 

Figure 8. Basal debris-flow conglomerate in the Proto Canyon 

member of the Nogales Formation south of Mount Benedict, 

near contact with Jurassic Quartz Monzonite of Mount Benedict. 

Conglomerate is mostly clast-supported and well consolidated; 

notebook for scale.

Figure 9. Sequence of alternating beds of arkosic sandstone, 

conglomeratic sandstone, and debris-flow conglomerate in 

lower parts of Proto Canyon member, above basal debris-flow 

conglomerate; hammer in middle left for scale.
18 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Figure 10. Cliff-forming, cyclic, fining-upward, fluvial sequences in

lower part of the Proto Canyon member exposed in Proto Canyon. 

Depositional cycles (one complete cycle, about 1.0 m thick, outlined 

by black lines) generally consist of (from base to top) volcaniclastic 

conglomerate (vc); volcaniclastic conglomeratic sandstone (vcs); 

and overlying white horizontally bedded pumice siltstone (ps) 

(center of photograph). Sequences also contain some debris-flow 

conglomerate (dfc) beds.

of thin-bedded yellowish-gray volcaniclastic sandstone and 

conglomeratic sandstone, interbedded red and gray sandy 

claystone (fig. 11), and red arkosic, clayey and sandy conglomerate, best exposed in the Yerba Buena Canyon area 

(fig. 1). Claystone samples and clay-fraction samples from 

arkosic conglomerate in the area contain abundant kaolinite, 

formed primarily by the weathering of feldspar sourced from 

the Quartz Monzonite of Mount Benedict. This clay mineral 

assemblage is in contrast to those of most other claystones 

and clay-fraction samples from volcaniclastic sandstone in the 

formation, especially those in the Mariposa member, which 

contained greater amounts of smectite and illite, the zeolite 

clinoptilolite, and minor amounts to no kaolinite.

The upper part of the Proto Canyon member consists 

mainly of pinkish-gray to yellowish-gray, mostly poorly 

sorted, volcaniclastic sandstone and conglomerate (figs. 12 

and 13) exposed in the Grand Avenue area, north of Mariposa 

Road. The sandstone and conglomerate beds in the upper part 

of the member are similar in appearance to clastic units in 

the Nogales Wash member of the Nogales Formation, but the 

Proto Canyon member units are generally thinner bedded and 

have abundant monzonite clasts, reflecting locally derived 

detritus from the Quartz Monzonite of Mount Benedict in the 

Mount Benedict horst block.

The thickness of the Proto Canyon member is highly 

variable, but maximum thickness is about 450 m. The variability in thickness is due to structural deformation combined 

with deep erosion of the member prior to deposition of the 

overlying Nogales Wash and Mariposa members. The Proto 

Canyon member is mostly exposed in the topographically high 

hills of the Mount Benedict horst block east of the Nogales 

urban area, and west of the Santa Cruz River, and from Proto 

Canyon southeastward to across the international border 

(sheet 1). Rocks of the Proto Canyon member are also exposed 

in northeast Nogales, Sonora; the northern extent of outcrop 

crosses into the U.S. just east of downtown Nogales, where 

it flanks Cretaceous diorite (map unit Kd) (sheet 1). Other 

exposures of the Proto Canyon member are in the Sonoita 

Creek area, where the member laps onto the Grosvenor Hills 

Volcanics. In this area, the lower part of the member is pinkish-gray, cliff- to ledge-forming, poorly sorted volcaniclastic 

conglomerate and sandstone containing clasts and grains 

composed of mostly white and red porphyritic volcanic rocks, 

brownish-gray rhyodacite (fig. 14), and some pumice and latite 

Figure 11. Red and light-gray gravelly claystone (below black line)

and overlying gray volcaniclastic sandstone (above black line) in the 

middle part of the Proto Canyon member exposed in Yerba Buena 

Canyon. Exposed part of gray sandstone bed above black line at far 

right is about 0.5 m thick.

Figure 12. Fluvial cross-bedded volcaniclastic sandstone and 

conglomerate with lag monzonite gravel in channels, in upper part 

of the Proto Canyon member exposed west of Grand Avenue, and 

north of Mariposa Road.

0.5 METER

dfc

ps

vcs

vc

ps
Stratigraphy 19

contain alternating beds of thin- to thick-bedded, volcaniclastic 

sandstone and conglomeratic sandstone (fig. 16), which locally 

display tabular-planar cross-bedding. Limitation of the member 

to the southern part of the map area, combined with onlapping relations with the lower Proto Canyon member, suggest 

the Nogales Wash sediments were deposited in a paleo-valley 

eroded into the Proto Canyon member, coincident with modernday Nogales Wash.

Figure 13. Pinkish-gray volcaniclastic sandstone in the uppermost 

part of the Proto Canyon member, exposed north of Mariposa Road, 

between Interstate 19 and Grand Avenue; pick in middle for scale.

derived from the Grosvenor Hills Volcanics. The upper part of 

the member is a sequence of mostly thin-bedded, fine-grained, 

moderately sorted, volcaniclastic sandstone and conglomeratic 

sandstone, which weathers to form distinctive rounded ledges.

Nogales Wash Member

The middle member of the Nogales Formation is herein 

informally named the Nogales Wash member, for exposures in 

the Nogales Wash area, south of Mariposa Road (sheet 1). The 

member is only present in the southern part of the map area, and 

outcrops extend from Ephraim Canyon on the west, to about 

4 km east of Grand Avenue (sheet 1 and fig. 1). The member 

typically contains ledgy to massive cliff-forming, yellowishgray to pinkish-gray, volcaniclastic conglomerate and sandstone with abundant pumice fragments, and it generally lacks 

claystone beds unlike the other two members of the formation. 

Outcrops of the member are pervasively fractured in the map 

area. The Nogales Wash member is unconformable with the 

Proto Canyon member, and laps onto the latter member along 

State Highway 82, about 1.5 km northeast of Grand Avenue 

(fig. 5; sheet 1). The Nogales Wash member is about 30 to 80 m 

thick in scattered outcrops in the urban Nogales area, but it is 

estimated to be as much as 150 m thick, based on the subsurface 

projection of surface unit exposures shown in cross sections 

E–E′ and F–F′ (sheet 2). The basal 20 to 30 m of the member 

contains distinctive massive cliffs of yellowish-gray to pinkishgray, moderately consolidated volcaniclastic sandstone and conglomeratic sandstone (fig. 15). Most other parts of the member 

Figure 14. Boulder of Proto Canyon member exposed in Sonoita 

Creek area where it laps onto the Grosvenor Hills Volcanics; unit in 

lower part of member and consists of volcaniclastic conglomerate 

with clasts mostly of white and red porphyritic volcanic rocks, and 

brownish-gray rhyodacite derived from the Grosvenor Hills Volcanics; 

boulder is about 0.5 m in diameter.

Figure 15. Massive, cliff-forming volcaniclastic sandstone and 

conglomeratic sandstone in the basal part of the Nogales Wash 

member along Grand Avenue, south of Mariposa Road. Note internal 

deformation including abundant fractures, and breccia zone (center 

of photograph), indicating syndepositional extensional tectonism. 

Pen in circle for scale.
20 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

It is unclear whether units equivalent to the Nogales 

Wash member are present in the subsurface in the northern 

part of the map area, either in (1) the basin west of Nogales 

Wash or (2) the basin south of Sonoita Creek and east of the 

Santa Cruz River. However, several map relations suggest 

the member may be absent in those basins. In the basin west 

of Nogales Wash, the northernmost outcrops of the Nogales 

Wash member are just south of Mariposa Road and east of 

Grand Avenue (sheet 1). These relations indicate the member 

onlaps the Proto Canyon member, and pinches out north of 

Mariposa Road, as it does to the east along Highway 82. In 

addition, south of Mariposa Road, the Mariposa member is 

faulted against the Nogales Wash member along the Grand 

Avenue fault, whereas north of Mariposa Road, the Mariposa 

member is faulted against the Proto Canyon member, indicating the Mariposa member directly overlies the Proto Canyon 

member in the subsurface, where the Nogales Wash member 

has pinched out. Map evidence indicating the Nogales Wash 

member is absent in the basin south of Sonoita Creek and east 

of the Santa Cruz River includes an angular unconformity 

between the Mariposa member and the Proto Canyon member 

just south of Sonoita Creek, and the Nogales Wash member 

is apparently absent. More subsurface information (drill hole 

data) and surface stratigraphic data are needed to further 

evaluate these preliminary conclusions.

Mariposa Member

The upper member of the Nogales Formation is herein 

informally named the Mariposa member, for outcrops north 

and south of Mariposa Road, between Interstate 19 and Grand 

Avenue (sheet 1). The member is also exposed in southeastern 

parts of the map area, and in the basin south of Sonoita Creek 

and east of the Santa Cruz River (sheet 1). The Mariposa 

member generally has more pinkish-red color tones compared 

to dominant yellowish-gray tones in the Nogales Wash member, is less consolidated than the Nogales Wash member, and 

is more calcareous than the other two members of the formation. North of Mariposa Road along Interstate 19, the member 

consists of alternating beds of pale-red to pinkish-gray, coarseto fine-grained, volcaniclastic conglomeratic sandstone, 

Figure 16. Alternating thin- to thick-bedded, volcaniclastic 

sandstone and conglomeratic sandstone of the Nogales Wash 

member south of Mariposa Road and west of Grand Avenue; smalldisplacement fault diagonally cuts beds in center of photograph. 

Height of shrub in upper left is about 0.6 m.

Figure 17. Faulted and highly rotated beds (dipping 48° northwest) 

of Mariposa member along Interstate 19, north of Mariposa Road. 

Beds consist of alternating volcaniclastic conglomeratic sandstone, 

sandstone, and minor siltstone. Height of concrete curb in lower 

right is about 0.15 m.

Figure 18. Massive claystone unit in Mariposa member exposed 

south of Mariposa Road, between Interstate 19 and Grand Avenue. 

Height of walnut shrub in center of photograph is about 1.5 m.
Stratigraphy 21

sandstone, and minor siltstone (fig. 17). South of Mariposa 

Road between Interstate 19 and Grand Avenue, a massive 

claystone unit (fig. 18) likely represents playa deposits in an 

axial-basin facies of the member, based on its fine-grained 

character. X-ray diffraction analyses were conducted on 

several samples from this unit, and results indicated the claystone contained abundant mixed-layer smectite and illite, and 

varying amounts of the zeolite clinoptilolite. About 30 to 40 m 

of the claystone unit is exposed, but total thickness may be 

60 m or more, indicating the unit may locally be a significant 

confining unit. More subsurface data are needed to determine 

the claystone’s lateral extent and thickness in the basin. No 

evaporite deposits were noted in the claystone, which may 

indicate the Miocene Nogales basin was not a closed basin; 

however, evaporites may be preserved at lower stratigraphic 

levels in axial parts of the basin, like those noted in drill holes 

in the Tucson basin (fig. 2), north of the map area (Houser and 

others, 2004).

The Mariposa member is also exposed in the southeastern part of the map area (sheet 1), southeast of highway 82 

and north of the international border, where lower parts of the 

member consist mostly of volcaniclastic, pebble conglomerate and sandstone, and some fine-grained, moderately sorted 

sandstone. In this area, clasts are mostly subangular, lightbrownish-gray rhyolite that may be derived from an unidentified volcanic source likely in northeastern Nogales, Sonora. 

The member here also contains small discontinuous basalt 

flows which are about 2 to 5 m thick, and a sample from one 

flow dated near the international border yielded a 40Ar/39Ar 

geochronology plateau age of 11.67±0.09 Ma (whole rock) 

(fig. 6; table 1; Cosca and others, 2013); this date provides a 

minimum age for the Nogales Formation.

Figure 19. Mariposa member in basin south of Sonoita Creek 

and east of the Santa Cruz River, composed of alternating beds of 

volcaniclastic conglomeratic sandstone, sandstone, and few thin 

silt and clay partings; hammer and field notebook for scale.

Figure 20. Fine-grained facies of the Mariposa member in basin 

south of Sonoita Creek and east of the Santa Cruz River, composed 

of alternating beds of fine-grained volcaniclastic sandstone, siltstone, 

and claystone; hammer in lower left for scale.

Figure 21. Sandy and gravelly claystone beds in the Mariposa 

member south of Sonoita Creek and east of the Santa Cruz River; 

height of outcrop about 3 m.

In the basin south of Sonoita Creek and east of the Santa 

Cruz River, the Mariposa member contains pinkish-gray, alternating beds of volcaniclastic conglomeratic sandstone, sandstone, 

and some siltstone and claystone (fig. 19), like in the Mariposa 

Road type area, but units are generally thinner bedded and 

finer-grained than those in the type area, and consist mostly of 

alternating beds of fine-grained volcaniclastic sandstone, siltstone, 

and claystone (fig. 20). The Mariposa member is this area also 

contains claystone units similar to those in the Mariposa Road 

type area, (fig. 21), but these units are thinner than those in the 

type area, range from about 0.5 to 3 m thick, and are generally 

interbedded with volcaniclastic sandstone beds.
22 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Late Tertiary to Early Pleistocene 

Basin-Fill Deposits

Basin fill refers to a relatively thin (generally ≤100 m) 

sequence of alluvial deposits that unconformably overlies a 

variety of Tertiary strata (including Nogales Formation sediments and the Grosvenor Hills Volcanics), as well as many 

older bedrock units that are exposed in dissected basins in the 

western, north-central, and east-central parts of the map area. 

Basin-fill deposits typically represent the youngest set of aerially extensive sediments that are in turn locally overlain by 

thin and spatially restricted veneers of ridge-capping and inset 

Quaternary surficial deposits described elsewhere in this report. 

Basin-fill deposits consist of a reddish-brown to pinkish-gray, 

moderately to poorly bedded, and weakly consolidated sequence 

of interstratified gravelly sand and sandy gravel (figs. 22 and 

23). The sediments typically display provenance relations and 

stratal dips consistent with fan deposition sourced from nearby 

highland bedrock source terrains. The absence of strongly 

rotated bedding, faults, or fractures in most exposures of basinfill deposits indicates no significant post-depositional deformation in most of these deposits. The primary exceptions are local 

outcrops of basal sections of basin fill that exhibit low to moderate structural rotation and widely spaced internal faults and 

fractures near the traces of exposed basin-bounding faults. Basin 

fill can generally be distinguished from older sediments of the 

Nogales Formation by a combination of weaker consolidation, 

more diffuse and poorly differentiated interbedded sequences 

of sand and gravel, pinkish gray color, low stratal dips, lack of 

internal faulting or fracturing, and commonplace development 

of a subvertical erosional fluting pattern that crosscuts bedding 

in outcrops (figs. 22 and 23).

The age of basin-fill deposits in the map area is poorly 

constrained due to the lack of geochronologic age control 

for the unit. A maximum age constraint is provided by the 

11.67 Ma age for basalt flows within the underlying Mariposa 

member of the Nogales Formation. A minimum age constraint 

cannot be derived in the map area, but basin-fill deposits that 

underlie a prominent high, very old, capping gravel deposit 

immediately north of this map area (Youberg and Helmick, 

2001) may correlate with a similar high capping gravel fan 

deposit in upper Sonoita Creek, east of the map area. Menges 

(1981) and Menges and McFadden (1981) constrained the 

age of the latter deposit to early Pleistocene. These relations 

provide a general late Miocene to early Pleistocene age for the 

Figure 22. View to west of basal part of basin-fill deposits (QTa) 

in Potrero Canyon area. Unit is light reddish-brown, pinkish-gray, 

to light gray, weakly consolidated, moderately to poorly bedded, 

sandy gravel to gravelly sand. In comparison with Nogales 

Formation, unit has shallow dips, and has prominent verticalfluting due to erosion across bedding.

Figure 23. A, View to east of reddish brown basal basin-fill 

deposits (QTa) in trough cut into underlying Nogales Wash 

member of the Nogales Formation (Tnn) in a road cut at Crawford 

Hill; height of outcrop about 20 m. B, West view directly across 

Interstate 19 from road cut at Crawford Hill, showing subvertical 

fault (black line with arrow on downthrown side) juxtaposing 

Nogales Wash member (Tnn) in hanging wall, against basin-fill 

deposits (QTa) in footwall; height of outcrop about 18 m.

QTa

Tnn

QTa

Tnn

A

B
Stratigraphy 23

basin-fill deposits, with much of the deposition most likely 

during late Miocene to Pliocene time. This age assignment 

is consistent with the estimated age ranges of the majority of 

basin-fill deposits observed in southeastern Arizona (Menges 

and Pearthree, 1989).

Basin-fill deposits in the region represent the uppermost 

sections of alluvial-fan deposits that filled the stratigraphically 

highest portions of the youngest basins, which correspond to 

the present day system of basins and ranges in the map area. 

These deposits typically form a relatively thin sedimentary 

veneer (thicknesses estimated at 50 to 150 m) that unconformably overlies older rocks in the depositional basins. There is 

little to no geophysical or borehole data to constrain subsurface accumulations of buried depositional subbasins that exist 

along and west of the Santa Cruz River in the continuation of 

the primary basin to the north (Gettings and Houser, 1997). 

These relations suggest that the youngest basin-forming episode commonly associated with regional basin-range deformation from 13 to 10 Ma (Menges and Pearthree, 1989), did not 

form to any significant extent in the map area. Instead, basin 

fill in this area primarily comprises the final thin and variably 

distributed depositional-fill sequence in the upper parts of a 

series of exhumed older middle to early Miocene basins filled 

primarily with members of the Nogales Formation. Apparently, this early basin-forming interval, recorded by members 

of the Nogales Formation, did not extend significantly into the 

late Miocene deformational episode that is regionally associated with basin-fill deposits.

Quaternary Alluvium

Quaternary deposits in the map area comprise a relatively 

thin series of alluvial deposits associated with both piedmont 

alluvial fans and axial streams and their major tributaries that 

initially developed during the early Pleistocene, and have 

continued to form into modern time. The primary drainage is 

the southern reach of the Santa Cruz River, which forms an 

axial drainage that traverses two physiographic and depositional 

basins, and an intervening bedrock narrows. This axial drainage 

is joined by two major tributary drainages, Sonoita Creek and 

Nogales Wash (figs. 1 and 2), which themselves drain significant areas outside of the two local basins, and numerous smaller 

intrabasin tributaries of varying sizes that are spatially restricted 

to a given basin side-slope and, for the larger tributaries, the 

adjoining bedrock highlands.

Each of the drainages in this system contains a discrete 

meandering channel belt made up of multiple, braided to anastomosing subchannels and bars, and flanked by undissected floodplains and low terraces. Some smaller tributaries locally contain 

small alluvial fans between the dominant channelized reaches. 

The central lowlands are flanked by dissected valley side-slopes 

that are underlain by mostly pre-Quaternary basin-fill deposits, 

and (or) older sedimentary and bedrock units. One or more 

variously eroded and dissected topographic benches covered 

by thin deposits of terrace or fan gravels are commonly inset 

into the valley side-slopes, and record successively older and 

higher former levels, ranging in age from middle Pleistocene to 

Holocene. These terrace and fan gravels were deposited during 

local depositional episodes that periodically interrupted longterm stream downcutting. The interfluve areas between incised 

tributary valleys within the basin piedmonts are characterized 

by narrowly rounded ridgecrests, developed on the mostly preQuaternary substrate, but locally are capped by flat to gently 

rounded alluvial-fan surfaces and associated thin fan deposits of 

middle to early Pleistocene age.

Quaternary alluvium in the map area typically consists 

of unconsolidated, poorly to moderately bedded, poorly sorted 

sandy gravel to gravelly sand. The clasts consist of rounded to 

subangular pebbles, cobbles, and local small boulders, have 

rock types that generally reflect adjacent bedrock highlands on 

local tributaries, and more heterogeneous subregional source 

terrains on the major axial drainages.

The Quaternary deposits in the map area are associated 

with and controlled by the upper Santa Cruz River system, 

which flows northwest out of the map area, and ultimately 

is integrated with the Gila River and lower Colorado River 

regional drainages. The final phases of closed-basin aggradation, recorded by the highest stratigraphic and topographic 

levels of basin-fill deposits (QTa), probably ended sometime 

prior to the deposition of the oldest (highest) alluvial fan 

deposits of probable early Quaternary age (Qo) that locally 

cap the basin-fill deposits. This inference is based on regional 

correlations with similar deposits in surrounding areas (Menges 

and McFadden, 1981; Menges and Pearthree, 1989). Whether 

these early Pleistocene fan deposits were graded to an externally 

drained ancestral Santa Cruz River system cannot be determined 

from geologic relations in the map area. Regardless, the suite 

of progressively younger (lower) inset terrace and fan deposits indicates a mostly continually downcutting stream system, 

which, in turn, indicates that the Santa Cruz River system was 

externally draining at the time of their formation. Each major 

map unit at a given height range typically displays diagnostic, 

age-related geomorphic-surface and pedogenic characteristics 

that can be correlated with deposits and surfaces with similar 

characteristics throughout the region. This similarity in diagnostic, age-related characteristics suggests regional-scale, climatecontrolled deposition that periodically interrupted the long term, 

system-wide pattern of drainage incision. The ultimate source 

and control for this persistent downcutting in the map area 

does not appear to be related to any specific local Quaternary 

tectonic events, given the general absence of deformation in 

Quaternary deposits, other than a few short isolated Quaternary 

faults and associated scarps in the eastern Santa Cruz River 

basin to the north of the map area (Helmick, 1986; Pearthree 

and Calvo, 1987; Youberg and Helmick, 2001). Long-term 

dissection is most likely related to local base levels controlled 

by progressive integration of formerly isolated local basin 

drainages that evolved into the present-day externally linked, 

regional Santa Cruz–Gila River drainage system (Menges and 

Pearthree, 1989).
24 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Structure

Structural features in the map area primarily formed during 

early to late Miocene extensional deformation, which included a 

phase of early Miocene basin extension from 20 to 13 Ma, and 

main episodes of basin-range extension that occurred in southern 

Arizona mostly from 13 to 10 Ma (Menges, 1981; Menges and 

McFadden, 1981; Menges and Pearthree, 1989). Extensional 

deformation was broadly contemporaneous with deposition of 

the Nogales Formation, based on observations that all members 

of the formation are moderately to highly faulted, with abundant 

intra-formational faults and fractures, and moderate to steeply 

inclined bedding dips. The Nogales Wash member is pervasively 

fractured compared to moderate fracturing in the other two members, because the member is more consolidated and contains 

more brittle, volcaniclastic sandstone units and less claystone 

than the other two members. Extensional faulting lasted until late 

Miocene time in the area based on faults which cut only the very 

oldest post-Nogales Formation late Miocene basin-fill deposits, 

and Pliocene through Holocene deposits are undeformed.

The main structural feature in the map area is the Mount 

Benedict horst block (fig. 1), a zone of uplifted Quartz 

Monzonite of Mount Benedict (Jurassic) and overlying members of the Nogales Formation. The major fault bounding the 

east side of the horst block is the Mount Benedict fault, and 

the west side is bound by several north- to northwest-striking 

faults that progressively downdrop Jurassic monzonite and 

Miocene Nogales Formation rocks westward into the subsurface. These faults include the Grand Avenue fault, and several 

concealed faults to the west (sheet 1) which were mapped 

based on geophysical data.

The main fault in the map area is the northwest-striking 

Mount Benedict fault (sheets 1 and 2 A–A′, B–B′, and C–C′; 

fig. 24), originally named and described by Gettings and 

Houser (1997). The fault forms the northeast side of the Mount 

Benedict horst block, and is mostly concealed by surficial 

deposits along the Santa Cruz River (sheet 1); the fault generally controls the course of the river from the Rio Rico area, 

southward across the international border and into Sonora, 

Mexico. The only known exposures of the fault in the map area 

are northeast of the Santa Cruz River, northwest of Guevavi 

Canyon, and southeast of Sonoita Creek (sheet 1), where the 

fault plane strikes about N 40° W and dips about 65° northeast. 

In this area, the fault downdrops rocks of the Mariposa member of the Nogales Formation against the Quartz Monzonite 

of Mount Benedict and also some Salero Formation, which 

unconformably overlies the monzonite locally. Dark-gray dogtooth calcite fills fractures in the fault plane in places, indicating seepage, or discharge from a carbonate source at depth. A 

concealed strand of the fault is located east of the exposed fault 

(sheets 1 and 2 B–B′), along the course of the Santa Cruz River. 

Northward in the Rio Rico area, the fault is concealed and forms 

the western margin of the Rio Rico basin (fig. 1), and it projects 

northwestward into the adjacent Peña Blanca Lake and Tubac 

quadrangles (fig. 2). Our reconnaissance mapping indicates the 

fault is also exposed in the adjacent Cumero Canyon quadrangle, southeast of the intersection of State Highway 82 and 

the Santa Cruz River (fig. 2). In this area, the fault juxtaposes 

red arkosic conglomerate of the Proto Canyon member in the 

footwall against pinkish- to yellowish-gray volcaniclastic sandstone of the Mariposa member in the hanging wall. Like many 

other extensional faults in the Basin and Range Province, the 

northwest-striking Mount Benedict fault is likely a reactivated 

older structure that had a long complex history; northweststriking faults formed in the region during the Mesoproterozoic, 

Late Triassic, and during late Miocene basin and range extension (Drewes, 1980).

The Gold Hill fault (sheets 1 and 2 C–C′ and D–D′) was 

originally mapped by Simons (1974). It strikes north, and dips 

65° west, and terminates against the Mount Benedict fault at 

its northern end. Along its northern segment, the fault cuts the 

Quartz Monzonite of Mount Benedict, but southward, the fault 

downdrops rocks of the Proto Canyon member of the Nogales 

Formation against Jurassic monzonite. At its southernmost 

end, the Gold Hill fault loses throw where it offsets units of 

the Proto Canyon member, and then it becomes concealed and 

likely terminates beneath an east-west-trending wash, east of 

Mariposa Road and Grand Avenue (sheet 1).

The Grand Avenue fault (named here) is a newly 

mapped fault (sheet 1; fig. 25) concealed along much of its 

trace. At its southern end, the fault strikes northeast and is 

Figure 24. Northwest view along Mount Benedict fault (black line 

with arrow on downthrown side) east of Santa Cruz River; gray and 

dark brown beds (on left) of the Salero Formation (Ks) in the footwall, 

and yellowish to pinkish gray beds (on right, on which geologists are 

standing) are the Mariposa member of Nogales Formation (Tnm) in 

the hanging wall; fault plane strikes N. 40° W. and dips 65° northeast. 

Peaks in the San Cayetano Mountains are on skyline.

Tnm

Ks
Structure 25

concealed in Ephraim Canyon (sheets 1 and 2 F–F′); north 

of the canyon, the fault is concealed, but it separates isolated 

outcrops of the Mariposa member on the downthrown side 

from rocks of the Nogales Wash member on the upthrown 

side (sheet 1). The only presently known exposures of the 

fault are just south of Mariposa Road and west of Grand 

Avenue (sheet 1 and fig. 25), where it strikes about N. 30° E. 

and dips 60° northwest, and juxtaposes massive claystone 

of the Mariposa member in the hanging wall against volcaniclastic sandstone of the Nogales Wash member in the 

footwall (sheet 2, E–E′). Slickenlines on the fault plane trend 

N. 70° W. and plunge 56° NW, indicating latest movement 

on the fault was mostly dip-slip. North of Mariposa Road, 

the fault bends northwestward (sheet 1), and juxtaposes 

volcaniclastic sandstone and conglomerate of the Mariposa 

member in the hanging wall, against volcaniclastic sandstone 

of the Proto Canyon member in the footwall (sheet 2, D–D′). 

Farther north, the Grand Avenue fault is concealed (sheet 1), 

and hanging wall rocks of the Mariposa and Proto Canyon 

members in the subsurface are interpreted (based on geophysical modeling) to be progressively downdropped northward 

against the Quartz Monzonite of Mount Benedict in the 

footwall (sheet 2, B–B′ and C–C′), indicating significant westside down offset. At its northern end, the Grand Avenue fault 

terminates at the Mount Benedict fault (sheet 1), similar to the 

Gold Hill fault. Several other faults are mapped west of the 

Grand Avenue fault, and these faults are concealed and were 

interpreted based on airborne transient electromagnetic (TEM) 

data and other geophysical data. The Grand Avenue fault and 

faults to the west together form a complex graben system in 

the subsurface of the western part of the map area (sheets 1 

and 2, C–C′).

The Proto Canyon fault (sheets 1 and 2, D–D′, E–E′, and 

F–F′) parallels Proto Canyon along State Highway 82, where 

it strikes about N. 45°E. and dips southeast, and offsets rocks 

in the lower part of the Proto Canyon member. In the Proto 

Canyon area, the fault downdrops cliff-forming, cyclic units 

in the lower part of the member (fig. 10) in the hanging wall, 

against arkose conglomerate and sandstone (fig. 9) in the basal 

parts of the Proto Canyon member in the footwall. From Proto 

Canyon northeastward, the fault eventually cuts out the arkose 

units, and downdrops the cyclic, cliff-forming units against the 

Quartz Monzonite of Mount Benedict near the eastern edge 

of the map area (sheet 1). The southern segment of the Proto 

Canyon fault offsets rocks mostly of the Nogales Wash and 

Mariposa members (sheet 1), and it extends southward into 

Mexico. The Yerba Buena Canyon fault (sheet 1) is subparallel 

to the Proto Canyon fault, and is also down to the southeast. 

Like the Proto Canyon fault, it offsets units mostly of the 

Proto Canyon member along its northern segment, and offsets 

the Mariposa and Nogales Wash members along its southern 

segment, where it extends southward into Mexico.

In the northwestern part of the map area, faults bound the 

west and east flanks of the San Cayetano Mountains to form 

a horst block (sheets 1 and 2, A–A′). The Rio Rico fault 

(named here) is the range-front fault on the west flank of the 

San Cayetano Mountains, and is down-to-the-west where it 

forms the eastern margin of the Rio Rico basin (fig. 1). The 

fault is exposed at several locations; at the southernmost 

location, the fault juxtaposes post-Nogales Formation basin 

fill (unit QTa) in the hanging wall against rocks of the Salero 

Formation in the footwall, and strikes about N. 35° W. and 

dips 70° southwest (sheet 1). Northward along the Rio Rico 

fault, Tertiary rhyolitic intrusive rocks in the hanging wall are 

juxtaposed against mostly sedimentary rocks of the Salero 

Formation in the footwall; these latter rocks compose most of 

the high ridge of the San Cayetano Mountains.

The San Cayetano fault of Drewes (1972) bounds the 

east flank of the San Cayetano Mountains, is down-to-the-east, 

and offsets rocks mostly of the Proto Canyon member of the 

Nogales Formation and Grosvenor Hills Volcanics in the hanging wall against the Salero Formation in the footwall (sheet 1). 

North of the map area, the fault offsets Grosvenor Hills 

Volcanics in the hanging wall against the Salero Formation in 

the footwall, where Drewes estimated about 300 to 760 m of 

vertical displacement along the fault. Drewes (1972) reported 

this displacement was likely cumulative over a period of time 

following late Oligocene volcanism and continuing after 

deposition of the Miocene Nogales Formation. South of the 

San Cayetano Mountains and Sonoita Creek (sheet 1), the 

San Cayetano fault offsets rocks mostly of the Mariposa member of the Nogales Formation (sheet 1, and fig. 26), and then it 

terminates against the Mount Benedict fault.

Figure 25. Southwest view of Grand Avenue fault (black line with 

arrow on downthrown side) just south of Mariposa Road and west of 

Grand Avenue; gray sandstone of the Nogales Wash member (Tnn) 

on left in footwall, and pink claystone in Mariposa member (Tnm) on 

right in hanging wall; height of large shrub in center of photograph is 

about 1 m.

Tnn Tnm
26 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Summary

The objectives of mapping the Rio Rico and Nogales 

1:24,000-scale quadrangles were to define the geologic framework to support ongoing multidisciplinary USGS projects in 

the Nogales area, including the Tucson Water Science Center 

Transboundary Aquifer Act Project, U.S.-Mexico Border 

Environmental Health Initiative, and the assessment of concealed porphyry copper deposits in the area. This work will 

also provide important information for other agencies investigating flood hazards and water storage in the Nogales Wash 

and Santa Cruz River areas, and for studies detecting and 

predicting the location of illegal smuggling tunnels beneath 

the international border.

The oldest rocks in the quadrangle include the Quartz 

Monzonite of Mount Benedict of Late to Middle Jurassic age. 

These rocks are highly deformed and are intruded by numerous 

mostly northwest-trending diorite and diabase dikes, and they 

form a confining unit below the Miocene Nogales Formation 

and Tertiary through Quaternary deposits in the map area.

The Upper Cretaceous Salero Formation (about 72 to 

69 Ma) unconformably overlies the Quartz Monzonite of Mount 

Benedict along the Santa Cruz River, southeast of Rio Rico. 

Most of the unit in that area consists of thick-bedded, wellconsolidated conglomerate and sandstone, with clasts of silicic 

volcanic rocks and some granite. The Salero Formation forms 

the high ridge of the San Cayetano Mountains on the north 

edge of the map area, where it consists chiefly of conglomerate, 

sandstone, argillite and quartzite, intruded by abundant west- to 

northwest-trending latite dikes and plugs of Tertiary age.

The Grosvenor Hills Volcanics (Drewes, 1968) are 

exposed north of Sonoita Creek and east of the San Cayetano 

Mountains, and consist mostly of rhyolite and rhyodacitic 

tuff, vitric lithic tuff, agglomerate, flow breccia, and latite tuff. 

We follow Drewes (1972) and subdivide the Grosvenor Hills 

Volcanics into middle rhyolite and upper rhyodacite members; 

the basal silt and gravel member is not exposed in the map 

area. Our new argon geochronology reveals some units in the 

upper rhyodacite member are significantly younger than previously interpreted (28–27 Ma) and are from about 27 to 23 Ma.

Our new mapping and supporting geochronologic, 

geophysical, and hydrogeologic studies resulted in significant stratigraphic revision of the Nogales Formation, and 

we defined three new informal members, including (from 

base to top) the Proto Canyon, Nogales Wash, and Mariposa 

members; all three members are separated by unconformities. 

New 40Ar/39Ar geochronology indicates the maximum age of 

the formation is about 20 to 18 Ma, and the minimum age is 

about 11 Ma; the minimum age is based on whole-rock dates 

from basalt flows in the Mariposa member near the international border (11.67 Ma; Cosca and others, 2013). 40Ar/39Ar 

geochronology analyses of detrital grains from the Nogales 

Formation range in age from 28 to 23 Ma, and correspond to 

the provenance of detrital material from the Grosvenor Hills 

Volcanics. The Proto Canyon member consists of basal debrisflow conglomerate and arkose sandstone and conglomerate; 

a lower cliff-forming part consisting mostly of fluvial, cyclic, 

fining-upward sequences; a middle part consisting of alternating beds of volcaniclastic conglomerate, arkosic conglomerate, and sandstone and claystone; and an upper part containing 

volcaniclastic conglomeratic sandstone and sandstone. The 

Nogales Wash member is exposed in the southern part of the 

map area, where it consists of massive, cliff-forming volcaniclastic sandstone and conglomeratic sandstone in the basal 

part, and ledge-forming, alternating beds of thin- to thickbedded, volcaniclastic sandstone and conglomeratic sandstone 

in the upper part. Units of the Nogales Wash member are 

pervasively fractured and faulted, contain abundant pumice 

fragments, and generally lack claystone units unlike the other 

two members of the formation. The Mariposa member consists 

of alternating beds of calcareous, volcaniclastic conglomeratic 

sandstone, sandstone, siltstone, and claystone. Basalt flows 

dated at 11.67 Ma are in the lower parts of the member near 

the international border, and this age constrains the minimum 

age of the Nogales Formation at about 11 Ma. Massive claystone units of the member in the Mariposa Road area represent 

playa deposits in an axial basin facies of the member and may 

be significant confining units.

Major episodes of extensional deformation were contemporaneous with deposition of the Nogales Formation as 

indicated by the observation that all units of the formation are 

moderately to highly faulted, with abundant intraformational 

faults and fractures, and moderate to steeply inclined bedding 

dips. Extensional faulting lasted until latest Miocene time 

based on faults which cut only the very oldest post-Nogales 

Formation, late Miocene basin-fill deposits; Pliocene through 

Figure 26. North view along southern end of the east-dipping 

San Cayetano fault just south of Sonoita Creek. Main fault (thick 

black line with arrow on downthrown side) offsets rocks of the 

Mariposa member of the Nogales Formation, which dip about 

15° west in the hanging wall (right of fault), and are gently eastdipping in the footwall (left of fault); bedding dips indicated by thin 

black dashed lines on both sides of fault.
References Cited 27

Holocene basin-fill deposits exposed in the map area are 

undeformed. The main structural feature in the map area is 

the Mount Benedict horst block, a zone consisting of uplifted 

Jurassic Quartz Monzonite of Mount Benedict and overlying 

units of the Nogales Formation. The major fault in the map 

area is the Mount Benedict fault bounding the east side of 

the horst block, and it controls the location of the course of 

the Santa Cruz River from the Rio Rico area, southward into 

Mexico. The fault is exposed northwest of Guevavi Canyon 

and southeast of Sonoita Creek where it dips northeast and 

offsets rocks of the Jurassic monzonite and overlying Salero 

Formation in the footwall, against the Mariposa member of the 

Nogales Formation in the hanging wall.

The Grand Avenue fault is a newly mapped fault that 

forms the west side of the Mount Benedict horst block. The 

fault is exposed just south of Mariposa Road and west of 

Grand Avenue where it strikes northeast and dips northwest, 

and slickenlines on the fault plane indicate the sense of movement was mostly dip-slip. The southern part of the fault juxtaposes rocks of the Nogales Wash member against those of the 

Mariposa member of the Nogales Formation, but northward, 

rocks of the Mariposa member are faulted against those of the 

Proto Canyon member and the Quartz Monzonite of Mount 

Benedict. Several other concealed faults are mapped west of 

the Grand Avenue fault, and these faults were interpreted from 

airborne transient electromagnetic and other geophysical data. 

The Grand Avenue fault and concealed faults to the west form 

a complex graben system in the subsurface in the western part 

of the map area.

The newly mapped Proto Canyon fault parallels Proto 

Canyon along State Highway 82, where it strikes northeast, 

is down to the southeast, and mainly offsets rocks of the Proto 

Canyon member. Farther south, it offsets the Nogales Wash 

and Mariposa members and extends southward into Mexico. 

The Yerba Buena Canyon fault is subparallel to the Proto 

Canyon fault, and like the Proto Canyon fault, it is down to 

the southeast and offsets rocks of the Proto Canyon member to 

the north and the Nogales Wash and Mariposa members to the 

south where it extends into Mexico.

Major faults in the northern part of the map area bound the 

west and east flanks of the San Cayetano Mountains to form a 

horst block. The Rio Rico fault is the range-front fault on the 

west flank of the mountains; it is down to the west and forms 

the east margin of the Rio Rico basin. The fault juxtaposes 

lower basin-fill units (late Miocene) in the hanging wall against 

rocks of the Salero Formation in the footwall. The San Cayetano 

fault on the east side of the San Cayetano Mountains downdrops 

rocks—mostly of the Nogales Formation and Grosvenor Hills 

Volcanics in the hanging wall—against Salero Formation in 

the footwall. At its southern end, the San Cayetano fault offsets 

rocks of the Mariposa member of the Nogales Formation, and 

then terminates at the Mount Benedict fault.

Acknowledgments

We greatly appreciate the technical reviews of the report 

completed by Ralph Shroba and Kenzie Turner; their comments 

and suggestions vastly improve the report. Jeremy Havens 

was responsible for digitally drafting all of the report illustrations, and the cross sections on sheets 2. Thanks to Gary Skipp 

who conducted X-ray diffraction analyses on samples from the 

Nogales Formation. Ted Brandt digitally compiled, edited, and 

mosaicked the topographic base map for sheet 1.

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Appendix 29

Appendix Table 1–1. Combined 40Ar/39Ar data for Grosvenor Hills Volcanics and Nogales Formation. (Click here to access digital file.)

—Continued

Laser

power

watts

s,p

Relative isotopic abundances Derived results 40Ar

±1σ

39Ar

±1σ

38Ar

±1σ

37Ar

±1σ

36Ar

±1σ

39Ar Mol

∞ 10–14

39Ar %

of total

Ca/K

±1σ

%40Ar Age (Ma)

±1σ

11SCB01; Sanidine (s), Plagioclase (p); J=0.0004191±0.0000009

5 s 44.6479 0.0514 1.3646 0.0038 0.0164 0.0006 0.0065 0.0011 0.0015 0.0004 0.0301 100.0 0.01 0.00 99.0 24.3 0.1

5 s 42.9186 0.0474 1.3319 0.0052 0.0168 0.0006 0.0138 0.0016 0.0026 0.0004 0.0294 100.0 0.02 0.00 98.2 23.8 0.1

5 s 26.3185 0.0454 0.8204 0.0036 0.0096 0.0006 0.0038 0.0011 0.0004 0.0006 0.0181 100.0 0.01 0.00 99.5 24.0 0.2

5 s 27.8223 0.0364 0.8698 0.0037 0.0106 0.0005 0.0000 0.0016 0.0002 0.0004 0.0192 100.0 0.00 0.00 99.8 24.0 0.1

5 s 31.6243 0.0912 0.9819 0.0033 0.0113 0.0005 0.0008 0.0011 0.0028 0.0004 0.0217 100.0 0.00 0.00 97.4 23.6 0.1

5 s 33.6432 0.0493 1.0148 0.0033 0.0121 0.0006 0.0042 0.0013 0.0002 0.0004 0.0224 100.0 0.01 0.00 99.8 24.9 0.1

5 s 29.4658 0.0394 0.8803 0.0044 0.0116 0.0006 0.0062 0.0011 0.0022 0.0004 0.0194 100.0 0.01 0.00 97.8 24.6 0.2

5 s 44.9224 0.0523 1.3821 0.0036 0.0182 0.0006 0.0081 0.0011 0.0001 0.0004 0.0305 100.0 0.01 0.00 99.9 24.4 0.1

5 s 43.4177 0.0484 1.2774 0.0037 0.0185 0.0006 0.0081 0.0010 0.0085 0.0004 0.0282 100.0 0.01 0.00 94.2 24.1 0.1

5 s 24.7183 0.0398 0.7618 0.0030 0.0085 0.0006 0.0000 0.0028 0.0010 0.0004 0.0168 100.0 0.00 0.01 98.8 24.1 0.2

5 s 20.3850 0.0378 0.5907 0.0027 0.0071 0.0006 0.0121 0.0029 0.0040 0.0004 0.0130 100.0 0.04 0.01 94.3 24.4 0.2

5 s 22.5934 0.0443 0.7078 0.0028 0.0079 0.0007 0.0061 0.0038 0.0004 0.0004 0.0156 100.0 0.02 0.01 99.4 23.8 0.2

5 s 27.3487 0.1001 0.8587 0.0030 0.0114 0.0007 0.0092 0.0045 0.0009 0.0004 0.0190 100.0 0.02 0.01 99.1 23.7 0.2

5 s 22.8081 0.0364 0.7108 0.0028 0.0083 0.0006 0.0050 0.0026 0.0002 0.0004 0.0157 100.0 0.01 0.01 99.8 24.0 0.2

5 s 16.3697 0.0150 0.5234 0.0024 0.0069 0.0006 0.0004 0.0033 0.0002 0.0004 0.0116 100.0 0.00 0.01 99.6 23.4 0.2

5 s 52.0239 0.0612 1.5625 0.0049 0.0221 0.0006 0.0036 0.0025 0.0060 0.0004 0.0345 100.0 0.00 0.00 96.6 24.2 0.1

5 s 48.0324 0.0523 1.4561 0.0045 0.0213 0.0006 0.0097 0.0042 0.0036 0.0004 0.0322 100.0 0.01 0.01 97.8 24.2 0.1

5 s 57.0990 0.0573 1.7545 0.0048 0.0230 0.0007 0.0088 0.0026 0.0056 0.0004 0.0387 100.0 0.01 0.00 97.1 23.7 0.1

5 s 10.8098 0.0179 0.3290 0.0022 0.0032 0.0005 0.0000 0.0025 0.0002 0.0004 0.0073 100.0 0.00 0.01 99.5 24.6 0.3

5 s 12.7103 0.0129 0.3947 0.0026 0.0050 0.0006 0.0000 0.0035 0.0003 0.0004 0.0087 100.0 0.00 0.02 99.3 24.0 0.3

5 s 15.3680 0.0452 0.4669 0.0025 0.0074 0.0006 0.0000 0.0030 0.0019 0.0004 0.0103 100.0 0.00 0.01 96.3 23.8 0.2

5 s 17.2742 0.0225 0.5098 0.0037 0.0068 0.0006 0.0001 0.0028 0.0012 0.0004 0.0113 100.0 0.00 0.01 98.0 24.9 0.2

11SCB02; Plagioclase (p); J=0.0004191±0000009

5 p 11.6764 0.0170 0.1543 0.0017 0.0049 0.0006 0.3576 0.0038 0.0185 0.0005 0.0034 100.0 4.54 0.07 53.4 30.3 0.8

5 p 9.8184 0.0142 0.2197 0.0017 0.0024 0.0005 0.2190 0.0020 0.0059 0.0004 0.0049 100.0 1.95 0.02 82.3 27.6 0.5

5 p 5.9828 0.0315 0.1586 0.0016 0.0017 0.0005 0.3330 0.0024 0.0029 0.0004 0.0035 100.0 4.12 0.05 86.2 24.5 0.6

5 p 5.4747 0.0153 0.1120 0.0015 0.0021 0.0005 0.0111 0.0011 0.0008 0.0004 0.0025 100.0 0.19 0.02 95.9 35.1 0.8

5 p 4.4089 0.0126 0.0980 0.0013 0.0023 0.0005 0.1994 0.0020 0.0016 0.0004 0.0022 100.0 3.99 0.07 89.6 30.3 0.9

5 p 3.3776 0.0105 0.0846 0.0011 0.0008 0.0004 0.1588 0.0018 0.0002 0.0004 0.0019 100.0 3.68 0.06 98.5 29.5 1.0

5 p 6.9398 0.0127 0.1374 0.0015 0.0039 0.0005 0.2359 0.0018 0.0076 0.0004 0.0030 100.0 3.37 0.05 67.9 25.8 0.7

5 p 5.8069 0.0125 0.1465 0.0016 0.0026 0.0005 0.2619 0.0019 0.0015 0.0004 0.0032 100.0 3.50 0.05 92.7 27.6 0.7

5 p 9.7674 0.0227 0.1861 0.0018 0.0040 0.0005 0.2974 0.0036 0.0115 0.0004 0.0041 100.0 3.13 0.05 65.5 25.8 0.5

11SCB03; Sanidine (s), Plagioclase (p); J=0.0004191±0.0000009

5 s 64.6944 0.0653 1.8814 0.0051 0.0248 0.0007 0.0194 0.0013 0.0023 0.0004 0.0415 100.0 0.02 0.00 98.9 25.5 0.1

5 s 20.0327 0.0315 0.5794 0.0026 0.0051 0.0005 0.0000 0.0016 0.0006 0.0004 0.0128 100.0 0.00 0.01 99.1 25.7 0.2

5 s 27.5668 0.0384 0.8335 0.0027 0.0096 0.0005 0.0020 0.0011 0.0009 0.0004 0.0184 100.0 0.00 0.00 99.0 24.6 0.1

5 p 6.6170 0.0144 0.1832 0.0018 0.0016 0.0005 0.1220 0.0024 0.0005 0.0004 0.0040 100.0 1.31 0.03 97.9 26.6 0.5

5 s 35.6346 0.0743 1.0353 0.0039 0.0135 0.0005 0.0105 0.0011 0.0002 0.0004 0.0229 100.0 0.02 0.00 99.8 25.8 0.1

5 s 31.6513 0.0524 0.9175 0.0034 0.0113 0.0006 0.0080 0.0015 0.0002 0.0004 0.0203 100.0 0.02 0.00 99.8 25.8 0.1

5 p 12.4019 0.0179 0.3610 0.0023 0.0049 0.0005 0.1787 0.0018 0.0035 0.0004 0.0080 100.0 0.97 0.01 91.8 23.7 0.3

5 s 42.4009 0.0443 1.2507 0.0044 0.0154 0.0005 0.0126 0.0016 0.0017 0.0003 0.0276 100.0 0.02 0.00 98.8 25.2 0.1

5 s 15.3042 0.0153 0.4176 0.0023 0.0060 0.0006 0.0230 0.0028 0.0027 0.0004 0.0092 100.0 0.11 0.01 94.9 26.1 0.3

5 p 2.9837 0.0101 0.0851 0.0013 0.0023 0.0006 0.0636 0.0031 0.0008 0.0004 0.0019 100.0 1.46 0.07 92.4 24.3 1.1

5 s 27.8767 0.0483 0.8069 0.0031 0.0108 0.0007 0.0158 0.0036 0.0015 0.0004 0.0178 100.0 0.04 0.01 98.4 25.5 0.2

5 s 19.4074 0.0178 0.5668 0.0027 0.0083 0.0007 0.0124 0.0035 0.0018 0.0004 0.0125 100.0 0.04 0.01 97.2 25.0 0.2

5 s 13.8490 0.0216 0.4141 0.0023 0.0062 0.0006 0.0151 0.0035 0.0008 0.0004 0.0091 100.0 0.07 0.02 98.4 24.7 0.3

5 s 20.7926 0.0345 0.5957 0.0027 0.0092 0.0006 0.0111 0.0027 0.0027 0.0004 0.0132 100.0 0.04 0.01 96.1 25.2 0.2

5 s 35.3112 0.0454 1.0316 0.0035 0.0150 0.0007 0.0067 0.0026 0.0029 0.0004 0.0228 100.0 0.01 0.01 97.6 25.1 0.1

5 p 6.9948 0.0118 0.1806 0.0015 0.0022 0.0006 0.2054 0.0032 0.0021 0.0004 0.0040 100.0 2.23 0.04 91.4 26.6 0.5

5 s 28.0028 0.0412 0.8233 0.0032 0.0078 0.0006 0.0000 0.0036 0.0002 0.0005 0.0182 100.0 0.00 0.01 99.8 25.5 0.2

5 s 9.6542 0.0157 0.2770 0.0019 0.0047 0.0005 0.0015 0.0028 0.0019 0.0004 0.0061 100.0 0.01 0.02 94.2 24.6 0.4

5 s 12.6190 0.0304 0.3656 0.0021 0.0046 0.0006 0.0008 0.0029 0.0005 0.0004 0.0081 100.0 0.00 0.02 98.8 25.6 0.3

5 p 4.3064 0.0099 0.1157 0.0014 0.0010 0.0005 0.0334 0.0030 0.0013 0.0004 0.0026 100.0 0.57 0.05 91.1 25.4 0.8

11SCB05; Sanidine (s) and Plagioclase (p); J=0.0006756±0.0000008

5 p 2.1868 0.0088 0.1087 0.0015 0.0020 0.0008 0.1237 0.0027 0.0019 0.0005 0.0024 100.0 2.23 0.06 75.3 18.4 1.8

5 s 10.9427 0.0196 0.5672 0.0024 0.0076 0.0008 0.0088 0.0020 0.0001 0.0005 0.0125 100.0 0.03 0.01 99.8 23.3 0.4

5 s 15.1624 0.0357 0.7540 0.0031 0.0099 0.0007 0.0026 0.0016 0.0008 0.0005 0.0167 100.0 0.01 0.00 98.5 24.0 0.3

5 p 1.5676 0.0135 0.0776 0.0013 0.0000 0.0006 0.0688 0.0020 0.0001 0.0005 0.0017 100.0 1.74 0.06 99.3 24.3 2.3

5 p 1.6418 0.0121 0.0767 0.0013 0.0013 0.0006 0.0708 0.0031 0.0001 0.0006 0.0017 100.0 1.81 0.08 97.8 25.3 2.7

5 p 2.8145 0.0126 0.1544 0.0016 0.0021 0.0007 0.0782 0.0021 0.0013 0.0005 0.0034 100.0 0.99 0.03 86.8 19.2 1.2

5 p 2.3321 0.0132 0.1245 0.0015 0.0026 0.0007 0.0604 0.0018 0.0014 0.0005 0.0028 100.0 0.95 0.03 82.2 18.7 1.5

5 s 16.5349 0.0157 0.8367 0.0029 0.0131 0.0007 0.0001 0.0028 0.0011 0.0004 0.0185 100.0 0.00 0.01 98.0 23.5 0.2

5 p 1.2944 0.0066 0.0542 0.0011 0.0020 0.0006 0.0379 0.0027 0.0016 0.0004 0.0012 100.0 1.37 0.10 63.4 18.4 3.0

5 p 1.9556 0.0069 0.0928 0.0013 0.0036 0.0006 0.1947 0.0031 0.0019 0.0004 0.0021 100.0 4.11 0.09 71.5 18.3 1.7

5 s 11.6035 0.0131 0.5536 0.0033 0.0091 0.0007 0.0000 0.0026 0.0014 0.0004 0.0122 100.0 0.00 0.01 96.5 24.5 0.3

5 p 1.1124 0.0075 0.0494 0.0016 0.0001 0.0007 0.0273 0.0028 0.0001 0.0004 0.0011 100.0 1.08 0.12 98.5 26.8 3.2

5 p 0.9592 0.0071 0.0517 0.0013 0.0005 0.0006 0.0275 0.0025 0.0003 0.0004 0.0011 100.0 1.04 0.10 90.5 20.4 2.9

5 s 4.5498 0.0109 0.2158 0.0018 0.0043 0.0007 0.0122 0.0039 0.0005 0.0004 0.0048 100.0 0.11 0.04 96.9 24.7 0.7
30 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Appendix Table 1–1. Combined 40Ar/39Ar data for Grosvenor Hills Volcanics and Nogales Formation. (Click here to access digital file.)

—Continued

Laser

power

watts

s,p

Relative isotopic abundances Derived results 40Ar

±1σ

39Ar

±1σ

38Ar

±1σ

37Ar

±1σ

36Ar

±1σ

39Ar Mol

∞ 10–14

39Ar %

of total

Ca/K

±1σ

%40Ar Age (Ma)

±1σ

11SCB08; Sanidine (s), and Plagioclase (p); J=0.0006756±0.0000008

5 p 1.6587 0.0118 0.0736 0.0014 0.0003 0.0009 0.1315 0.0026 0.0004 0.0005 0.0016 100.0 3.50 0.10 94.2 25.7 2.6

5 p 3.6946 0.0137 0.1603 0.0017 0.0027 0.0008 0.1275 0.0026 0.0001 0.0005 0.0035 100.0 1.56 0.04 99.8 27.8 1.2

5 s 34.1908 0.0453 1.6512 0.0034 0.0219 0.0007 0.0108 0.0017 0.0003 0.0005 0.0365 100.0 0.01 0.00 99.8 25.0 0.1

5 p 2.3765 0.0152 0.1015 0.0012 0.0025 0.0006 0.1663 0.0022 0.0013 0.0005 0.0022 100.0 3.21 0.06 84.8 24.1 1.9

5 s 17.7992 0.0190 0.8509 0.0031 0.0105 0.0006 0.0000 0.0019 0.0015 0.0005 0.0188 100.0 0.00 0.00 97.5 24.7 0.2

5 s 316.3722 0.1703 3.0455 0.0060 0.0424 0.0009 0.0119 0.0017 0.0168 0.0006 0.0673 100.0 0.01 0.00 98.4 120.5 0.3

5 p 18.7777 0.0280 0.1832 0.0016 0.0005 0.0006 0.3060 0.0029 0.0063 0.0005 0.0040 100.0 3.27 0.04 90.2 109.4 1.4

5 p 1.6165 0.0068 0.0646 0.0013 0.0021 0.0007 0.0916 0.0030 0.0005 0.0004 0.0014 100.0 2.78 0.11 90.6 27.4 2.2

5 s 25.6162 0.0521 1.2029 0.0042 0.0160 0.0008 0.0008 0.0028 0.0021 0.0004 0.0266 100.0 0.00 0.00 97.6 25.2 0.2

5 p 2.4172 0.0095 0.0949 0.0015 0.0022 0.0006 0.3163 0.0064 0.0011 0.0004 0.0021 100.0 6.53 0.17 87.1 26.9 1.6

5 p 1.9853 0.0072 0.0729 0.0013 0.0014 0.0007 0.4224 0.0064 0.0009 0.0004 0.0016 100.0 11.36 0.27 88.7 29.3 2.1

5 p 3.2972 0.0085 0.1275 0.0014 0.0036 0.0007 0.4345 0.0042 0.0031 0.0004 0.0028 100.0 6.68 0.10 73.6 23.1 1.2

5 s 27.5266 0.0352 1.2888 0.0041 0.0166 0.0007 0.0183 0.0037 0.0014 0.0004 0.0285 100.0 0.03 0.01 98.5 25.5 0.1

5 p 20.1559 0.0263 0.2346 0.0019 0.0036 0.0007 0.0142 0.0035 0.0094 0.0004 0.0052 100.0 0.12 0.03 86.2 88.1 1.0

11SCB09; Sanidine (s) and Plagioclase (p); J=0.0006756±0.0000008

5 s 89.1287 0.0660 4.0878 0.0079 0.0519 0.0008 0.1423 0.0018 0.0127 0.0005 0.0903 100.0 0.07 0.00 95.8 25.3 0.1

5 p 9.5609 0.0164 0.4202 0.0024 0.0019 0.0006 1.2398 0.0050 0.0014 0.0005 0.0093 100.0 5.78 0.04 96.6 26.7 0.5

5 s 24.9659 0.0318 1.1854 0.0043 0.0124 0.0007 0.0038 0.0017 0.0007 0.0006 0.0262 100.0 0.01 0.00 99.2 25.3 0.2

5 s 73.1961 0.0550 2.8586 0.0052 0.0444 0.0006 0.0472 0.0017 0.0414 0.0006 0.0631 100.0 0.03 0.00 83.3 25.8 0.1

5 s 58.6795 0.0629 2.4541 0.0045 0.0355 0.0007 0.0097 0.0018 0.0273 0.0007 0.0542 100.0 0.01 0.00 86.3 25.0 0.1

5 s 90.2385 0.0668 4.1222 0.0062 0.0554 0.0009 0.0422 0.0021 0.0167 0.0006 0.0911 100.0 0.02 0.00 94.6 25.1 0.1

5 s 67.1304 0.0472 3.1146 0.0064 0.0412 0.0008 0.0214 0.0018 0.0064 0.0005 0.0688 100.0 0.01 0.00 97.2 25.4 0.1

5 p 5.9784 0.0174 0.2591 0.0020 0.0036 0.0007 0.4106 0.0044 0.0031 0.0004 0.0057 100.0 3.11 0.04 85.4 23.9 0.6

5 s 31.3132 0.0373 1.2393 0.0042 0.0163 0.0007 0.0321 0.0029 0.0191 0.0004 0.0274 100.0 0.05 0.00 82.0 25.1 0.2

5 s 14.7834 0.0147 0.6616 0.0028 0.0076 0.0007 0.0017 0.0028 0.0001 0.0004 0.0146 100.0 0.00 0.01 99.8 27.0 0.3

5 p 1.9624 0.0090 0.0834 0.0019 0.0000 0.0006 0.1631 0.0048 0.0003 0.0004 0.0018 100.0 3.83 0.14 96.3 27.4 1.9

5 s 26.2695 0.0373 1.2118 0.0041 0.0148 0.0007 0.0023 0.0030 0.0036 0.0004 0.0268 100.0 0.00 0.00 96.0 25.2 0.2

5 p 17.3680 0.0215 0.7859 0.0028 0.0104 0.0007 0.1259 0.0032 0.0037 0.0004 0.0174 100.0 0.31 0.01 93.8 25.1 0.2

12SCB01; Sanidine (s); J=0.001085±0.000001

5 s 76.9575 0.0240 5.5703 0.0157 0.0719 0.0060 0.0719 0.0057 0.0215 0.0002 0.2731 100 0.09 0.01 91.8 24.6 0.1

5 s 102.7870 0.0277 6.7626 0.0172 0.1472 0.0058 0.3874 0.0054 0.0594 0.0004 0.3315 100 0.39 0.01 83.0 24.5 0.1

5 s 109.2773 0.0296 7.8127 0.0169 0.0942 0.0060 0.3144 0.0056 0.0336 0.0003 0.3830 100 0.27 0.00 91.0 24.7 0.1

5 s 94.2278 0.0240 6.7018 0.0157 0.0621 0.0060 0.2219 0.0054 0.0318 0.0003 0.3285 100 0.22 0.01 90.1 24.6 0.1

5 s 46.8246 0.0187 2.8536 0.0156 0.0246 0.0060 0.2753 0.0055 0.0361 0.0003 0.1399 100 0.65 0.01 77.3 24.7 0.2

12SCB03; Sanidine; J=0.001085±0.000001

6 s 52.5713 0.0204 4.0369 0.0160 0.0425 0.0059 0.0000 0.0058 0.0125 0.0002 0.1979 100 0.00 0.01 93.0 23.5 0.1

6 s 49.4673 0.0204 3.7959 0.0160 0.0445 0.0058 0.0000 0.0058 0.0112 0.0002 0.1861 100 0.00 0.01 93.3 23.6 0.1

6 s 39.3389 0.0178 2.8536 0.0159 0.0163 0.0061 0.0025 0.0061 0.0118 0.0002 0.1399 100 0.01 0.01 91.2 24.4 0.1

6 s 85.3881 0.0239 6.2819 0.0178 0.0654 0.0061 0.0147 0.0058 0.0312 0.0002 0.3079 100 0.02 0.01 89.2 23.6 0.1

6 s 75.7498 0.0248 5.5455 0.0158 0.0126 0.0061 0.0135 0.0059 0.0171 0.0002 0.2718 100 0.02 0.01 93.3 24.8 0.1

6 s 71.0326 0.0240 4.9960 0.0156 0.0392 0.0059 0.0165 0.0057 0.0290 0.0003 0.2449 100 0.02 0.01 87.9 24.3 0.1

6 s 90.7391 0.0268 6.8580 0.0166 0.0899 0.0055 0.0255 0.0053 0.0229 0.0002 0.3362 100 0.02 0.01 92.5 23.8 0.1

6 s 167.2104 0.0402 11.7222 0.0176 0.1494 0.0060 0.0253 0.0058 0.0796 0.0004 0.5746 100 0.01 0.00 85.9 23.8 0.1

12SCB03; biotite; J=0.001085±0.000001

0.7 173.2674 0.0398 2.2625 0.0165 0.1289 0.0065 0.0000 0.0057 0.5447 0.0009 0.1109 9.2 0.00 0.02 7.1 10.6 0.9

0.9 308.4867 0.0715 5.0949 0.0175 0.2417 0.0062 0.0295 0.0054 0.8764 0.0013 0.2498 20.6 0.04 0.01 16.1 18.9 0.7

1 279.5708 0.0605 4.4620 0.0168 0.1898 0.0065 0.0426 0.0057 0.7647 0.0013 0.2187 18.1 0.07 0.01 19.2 23.4 0.7

1.1 114.2230 0.0301 1.8093 0.0159 0.0816 0.0066 0.0006 0.0053 0.3114 0.0007 0.0887 7.3 0.00 0.02 19.4 23.9 0.7

1.2 133.3972 0.0369 1.9821 0.0165 0.1041 0.0065 0.0034 0.0057 0.3663 0.0008 0.0972 8.0 0.01 0.02 18.9 24.7 0.7

1.3 94.4497 0.0244 1.4511 0.0158 0.0887 0.0061 0.0142 0.0057 0.2595 0.0007 0.0711 5.9 0.07 0.03 18.8 23.8 0.8

1.4 78.1376 0.0234 1.2199 0.0171 0.0611 0.0061 0.0116 0.0057 0.2133 0.0006 0.0598 4.9 0.07 0.03 19.3 24.1 0.8

1.5 105.5837 0.0301 1.6503 0.0153 0.0776 0.0063 0.0082 0.0057 0.2883 0.0007 0.0809 6.7 0.04 0.03 19.3 24.0 0.7

1.6 44.7953 0.0188 0.7359 0.0158 0.0267 0.0065 0.0116 0.0056 0.1197 0.0005 0.0361 3.0 0.12 0.06 21.1 24.9 0.9

1.7 61.9211 0.0197 1.0225 0.0164 0.0429 0.0065 0.0245 0.0057 0.1657 0.0005 0.0501 4.1 0.18 0.04 20.9 24.6 0.8

2 51.5161 0.0207 0.7963 0.0158 0.0428 0.0065 0.0156 0.0057 0.1391 0.0005 0.0390 3.2 0.15 0.05 20.2 25.4 0.9

2.2 12.8154 0.0129 0.2692 0.0152 0.0172 0.0063 0.0054 0.0056 0.0335 0.0003 0.0132 1.1 0.15 0.15 22.7 21.0 1.3

2.4 12.8109 0.0145 0.2764 0.0150 0.0155 0.0066 0.0197 0.0055 0.0315 0.0002 0.0135 1.1 0.53 0.15 27.5 24.8 1.4

2.6 12.9515 0.0129 0.3034 0.0152 0.0068 0.0062 0.0000 0.0058 0.0313 0.0002 0.0149 1.2 0.00 0.14 28.6 23.8 1.3

2.8 11.4236 0.0137 0.3154 0.0158 0.0141 0.0065 0.0073 0.0059 0.0251 0.0002 0.0155 1.3 0.17 0.14 35.2 24.8 1.3

4 25.3472 0.0153 1.0381 0.0158 0.0070 0.0067 0.0174 0.0056 0.0396 0.0003 0.0509 4.2 0.13 0.04 53.9 25.6 0.4

12SCB04; amphibole; J=0.001085±0.000001

1 86.2416 0.0256 0.5031 0.0150 0.0449 0.0062 0.1436 0.0061 0.2688 0.0007 0.0247 2.3 1.61 0.08 7.9 26.5 2.2

1.5 60.2113 0.0219 3.1963 0.0152 0.0372 0.0067 0.5590 0.0061 0.0542 0.0003 0.1567 14.5 0.99 0.01 73.6 26.9 0.2

1.7 42.8214 0.0175 2.6841 0.0159 0.0276 0.0067 0.4091 0.0058 0.0186 0.0002 0.1316 12.2 0.86 0.01 87.4 27.1 0.2

1.9 33.3828 0.0166 2.1051 0.0144 0.0307 0.0065 0.7895 0.0059 0.0157 0.0002 0.1032 9.6 2.11 0.02 86.6 26.7 0.2

2.1 41.4255 0.0192 2.6822 0.0152 0.0315 0.0064 3.5903 0.0063 0.0212 0.0002 0.1315 12.2 7.55 0.04 86.6 26.1 0.2

2.3 33.7893 0.0175 2.1994 0.0144 0.0291 0.0063 3.1730 0.0064 0.0172 0.0002 0.1078 10.0 8.14 0.05 86.9 26.0 0.2

2.5 27.4661 0.0175 1.8168 0.0144 0.0156 0.0066 2.7507 0.0061 0.0129 0.0002 0.0891 8.3 8.54 0.07 88.2 26.0 0.2

2.7 34.6100 0.0158 2.1294 0.0144 0.0245 0.0063 4.1121 0.0065 0.0228 0.0002 0.1044 9.7 10.90 0.07 82.9 26.3 0.2

2.9 16.3491 0.0130 1.1093 0.0151 0.0108 0.0060 2.0389 0.0060 0.0087 0.0001 0.0544 5.0 10.38 0.14 86.8 24.9 0.3
Appendix 31

Appendix Table 1–1. Combined 40Ar/39Ar data for Grosvenor Hills Volcanics and Nogales Formation. (Click here to access digital file.)

—Continued

Laser

power

watts

s,p

Relative isotopic abundances Derived results 40Ar

±1σ

39Ar

±1σ

38Ar

±1σ

37Ar

±1σ

36Ar

±1σ

39Ar Mol

∞ 10–14

39Ar %

of total

Ca/K

±1σ

%40Ar Age (Ma)

±1σ

12SCB04; amphibole; J=0.001085±0.000001—Copntinued

3.2 7.8060 0.0127 0.5065 0.0150 0.0000 0.0064 0.8796 0.0061 0.0046 0.0001 0.0248 2.3 9.81 0.28 85.0 25.5 0.7

3.5 10.0979 0.0142 0.5996 0.0144 0.0274 0.0062 1.2151 0.0059 0.0071 0.0001 0.0294 2.7 11.45 0.27 81.6 26.8 0.6

4 15.3325 0.0142 0.9512 0.0144 0.0100 0.0066 2.1173 0.0060 0.0105 0.0002 0.0466 4.3 12.58 0.18 82.7 26.0 0.4

5 22.6825 0.0142 1.5034 0.0137 0.0287 0.0062 3.4686 0.0060 0.0127 0.0002 0.0737 6.8 13.04 0.12 86.6 25.5 0.2

12SCB04; biotite; J=0.001085±0.000001

0.9 59.3366 0.0209 0.6274 0.0151 0.0123 0.0062 0.0014 0.0059 0.1819 0.0006 0.0308 0.5 0.02 0.07 9.4 17.3 1.2

1 35.4551 0.0165 0.7092 0.0151 0.0105 0.0062 0.0126 0.0061 0.0937 0.0004 0.0348 0.6 0.13 0.06 21.9 21.3 0.7

1.1 54.7002 0.0200 2.2340 0.0152 0.0407 0.0062 0.0079 0.0055 0.0919 0.0004 0.1095 1.8 0.03 0.02 50.4 24.0 0.3

1.2 42.9335 0.0183 2.2300 0.0158 0.0322 0.0061 0.0270 0.0058 0.0496 0.0003 0.1093 1.8 0.09 0.02 65.9 24.6 0.2

1.3 61.8963 0.0219 3.4004 0.0147 0.0377 0.0065 0.0241 0.0056 0.0564 0.0004 0.1667 2.7 0.05 0.01 73.1 25.8 0.2

1.4 72.4410 0.0228 4.2850 0.0161 0.0551 0.0060 0.0222 0.0057 0.0493 0.0003 0.2101 3.4 0.04 0.01 79.9 26.2 0.1

1.5 86.5317 0.0246 5.3938 0.0156 0.0654 0.0061 0.0157 0.0055 0.0441 0.0003 0.2644 4.3 0.02 0.01 85.0 26.5 0.1

1.6 91.8639 0.0284 5.8085 0.0157 0.0892 0.0061 0.0299 0.0055 0.0435 0.0003 0.2848 4.6 0.04 0.01 86.0 26.4 0.1

1.7 79.4038 0.0246 5.0697 0.0162 0.0621 0.0060 0.0158 0.0057 0.0358 0.0003 0.2485 4.0 0.02 0.01 86.7 26.4 0.1

2 211.4131 0.0498 13.3078 0.0200 0.2053 0.0058 0.0824 0.0059 0.0960 0.0005 0.6524 10.5 0.05 0.00 86.6 26.7 0.1

2.2 193.2880 0.0459 12.4140 0.0185 0.1687 0.0060 0.0643 0.0057 0.0708 0.0004 0.6086 9.8 0.04 0.00 89.2 27.0 0.1

2.4 185.1089 0.0439 11.8338 0.0188 0.1799 0.0062 0.0566 0.0057 0.0698 0.0004 0.5801 9.4 0.04 0.00 88.9 27.0 0.1

2.6 207.2727 0.0459 13.3878 0.0189 0.1901 0.0062 0.0820 0.0058 0.0754 0.0004 0.6563 10.6 0.05 0.00 89.3 26.8 0.1

2.8 172.4781 0.0410 11.0814 0.0185 0.1371 0.0060 0.0822 0.0058 0.0619 0.0004 0.5432 8.8 0.05 0.00 89.4 27.0 0.1

3 160.5932 0.0390 10.5928 0.0178 0.1311 0.0064 0.1908 0.0058 0.0485 0.0004 0.5193 8.4 0.13 0.00 91.1 26.8 0.1

4 356.3883 0.0735 24.0213 0.0259 0.3536 0.0060 0.5104 0.0056 0.0845 0.0004 1.1776 19.0 0.16 0.00 93.0 26.8 0.1

12SCB05; amphibole; J=0.001085+0.000001

0.5 0.0000 0.0124 0.0000 0.0164 0.0000 0.0060 0.0000 0.0057 0.0000 0.0001 0.0000 0.0 1.52 0.56 113.9 2.2 0.9

1 15.3132 0.0130 0.2385 0.0152 0.0035 0.0060 0.0139 0.0059 0.0383 0.0003 0.0117 3.0 0.33 0.14 26.1 32.5 2.1

1.5 10.9507 0.0138 0.4523 0.0152 0.0000 0.0064 0.6795 0.0059 0.0139 0.0002 0.0222 5.6 8.50 0.28 63.8 30.1 1.0

1.7 10.4537 0.0125 0.5446 0.0158 0.0000 0.0059 1.4965 0.0061 0.0080 0.0001 0.0267 6.8 15.55 0.43 80.3 30.1 0.8

1.9 15.2732 0.0146 0.9349 0.0164 0.0000 0.0059 2.7365 0.0064 0.0097 0.0001 0.0458 11.6 16.57 0.28 84.9 27.1 0.5

2.1 20.6066 0.0138 1.1153 0.0171 0.0040 0.0063 3.2045 0.0064 0.0170 0.0002 0.0547 13.9 16.27 0.24 78.8 28.4 0.4

2.3 35.3458 0.0163 2.1400 0.0165 0.0322 0.0060 6.0048 0.0075 0.0211 0.0002 0.1049 26.6 15.89 0.12 85.8 27.7 0.2

2.5 25.2461 0.0146 1.6229 0.0148 0.0056 0.0061 4.9459 0.0071 0.0102 0.0001 0.0796 20.2 17.26 0.15 92.0 28.0 0.3

2.7 3.2705 0.0127 0.1874 0.0158 0.0000 0.0061 0.6486 0.0062 0.0013 0.0001 0.0092 2.3 19.61 1.58 92.4 31.5 2.5

3.2 7.7477 0.0120 0.4721 0.0158 0.0000 0.0060 1.2732 0.0060 0.0046 0.0001 0.0231 5.9 15.29 0.49 86.0 27.5 0.9

3.5 1.8850 0.0116 0.1048 0.0150 0.0000 0.0063 0.3157 0.0061 0.0009 0.0001 0.0051 1.3 17.08 2.35 89.6 31.4 4.3

4 1.5128 0.0121 0.0846 0.0146 0.0000 0.0058 0.3222 0.0059 0.0009 0.0001 0.0041 1.1 21.61 3.56 86.5 30.2 5.0

5 2.8037 0.0122 0.1446 0.0158 0.0000 0.0062 0.6218 0.0058 0.0013 0.0001 0.0071 1.8 24.39 2.54 91.1 34.5 3.6

12SCB05; biotite; J=0.001085+0.000001

0.7 136.6897 0.0350 1.4417 0.0155 0.1020 0.0059 0.0000 0.0059 0.4340 0.0009 0.0707 2.0 0.00 0.03 6.2 11.4 1.1

0.9 45.4422 0.0189 2.4909 0.0142 0.0403 0.0062 0.0027 0.0057 0.0877 0.0004 0.1221 3.4 0.01 0.02 43.0 15.3 0.2

1 38.9876 0.0163 2.4554 0.0156 0.0472 0.0061 0.0000 0.0061 0.0228 0.0002 0.1204 3.4 0.00 0.02 82.7 25.5 0.2

1.1 43.6494 0.0180 2.8324 0.0156 0.0415 0.0062 0.0034 0.0063 0.0160 0.0002 0.1389 3.9 0.01 0.02 89.2 26.7 0.2

1.2 52.1783 0.0207 3.4286 0.0150 0.0179 0.0064 0.0000 0.0059 0.0140 0.0002 0.1681 4.7 0.00 0.01 92.1 27.2 0.1

1.3 63.7147 0.0207 4.1999 0.0144 0.0495 0.0060 0.0000 0.0058 0.0145 0.0002 0.2059 5.8 0.00 0.01 93.3 27.5 0.1

1.4 71.2201 0.0217 4.7265 0.0145 0.0523 0.0062 0.0116 0.0059 0.0125 0.0002 0.2317 6.5 0.02 0.01 94.8 27.7 0.1

1.5 84.6582 0.0254 5.5686 0.0161 0.0527 0.0062 0.0000 0.0061 0.0135 0.0002 0.2730 7.7 0.00 0.01 95.3 28.1 0.1

1.6 82.2364 0.0245 5.4601 0.0154 0.0953 0.0062 0.0000 0.0057 0.0132 0.0002 0.2677 7.5 0.00 0.01 95.2 27.9 0.1

1.7 75.1684 0.0235 4.9952 0.0138 0.0496 0.0061 0.0000 0.0062 0.0120 0.0002 0.2449 6.9 0.00 0.01 95.3 27.8 0.1

2 169.2794 0.0438 11.2702 0.0185 0.1507 0.0067 0.0232 0.0058 0.0280 0.0003 0.5525 15.6 0.02 0.00 95.1 27.7 0.1

2.2 123.5714 0.0302 8.4225 0.0162 0.1155 0.0064 0.0670 0.0061 0.0157 0.0002 0.4129 11.6 0.06 0.01 96.3 27.4 0.1

2.4 115.6774 0.0292 8.0203 0.0174 0.0967 0.0063 0.1364 0.0062 0.0097 0.0002 0.3932 11.1 0.13 0.01 97.5 27.3 0.1

2.6 52.1966 0.0189 3.6188 0.0150 0.0372 0.0063 0.0977 0.0063 0.0052 0.0001 0.1774 5.0 0.20 0.01 97.1 27.2 0.1

2.8 34.9834 0.0163 2.3867 0.0149 0.0378 0.0066 0.0755 0.0061 0.0034 0.0001 0.1170 3.3 0.24 0.02 97.2 27.7 0.2

3 16.7162 0.0138 1.1314 0.0141 0.0009 0.0064 0.0063 0.0059 0.0020 0.0001 0.0555 1.6 0.04 0.04 96.4 27.7 0.3

11SCB04; whole rock chip; J=0.006560±0.000012

0.1 26.7409 0.0403 2.5337 0.0079 0.0463 0.0010 3.3390 0.0157 0.0754 0.0008 0.0561 0.7 0.01 0.02 17.5 21.8 1.1

0.3 33.0594 0.0582 9.9924 0.0131 0.1398 0.0016 14.0168 0.0293 0.0747 0.0008 0.2211 2.9 0.01 0.01 36.2 14.1 0.3

0.5 39.1656 0.0514 22.0672 0.0360 0.2954 0.0023 36.4758 0.0572 0.0497 0.0008 0.4881 6.4 0.01 0.01 69.1 14.5 0.1

0.7 38.8603 0.0464 27.2483 0.0580 0.3326 0.0022 34.6457 0.0825 0.0322 0.0007 0.6030 7.9 0.01 0.01 81.8 13.8 0.1

0.9 37.4687 0.0504 27.4174 0.0350 0.3378 0.0020 37.7136 0.0643 0.0275 0.0007 0.6067 7.9 0.01 0.01 85.5 13.8 0.1

1.1 55.9389 0.0484 43.3737 0.0500 0.5479 0.0025 74.1922 0.0827 0.0374 0.0008 0.9598 12.5 0.01 0.01 89.7 13.6 0.1

1.3 50.7195 0.0583 40.9589 0.0620 0.5181 0.0021 66.1568 0.1195 0.0271 0.0008 0.9066 11.8 0.01 0.01 93.5 13.7 0.1

1.5 44.1940 0.0445 35.8754 0.0530 0.4515 0.0022 54.6852 0.0736 0.0243 0.0007 0.7939 10.4 0.01 0.01 92.5 13.5 0.1

1.7 39.7320 0.0405 32.4070 0.0671 0.4116 0.0027 48.6040 0.0599 0.0193 0.0007 0.7171 9.4 0.01 0.01 94.3 13.6 0.1

1.9 32.6682 0.0395 25.6023 0.0630 0.3316 0.0024 42.6266 0.0876 0.0151 0.0007 0.5665 7.4 0.01 0.01 95.6 14.4 0.1

2.2 24.4054 0.0375 19.5279 0.0420 0.2594 0.0023 35.0510 0.0923 0.0143 0.0007 0.4321 5.6 0.01 0.01 92.8 13.7 0.1

2.5 21.6011 0.0425 17.1491 0.0250 0.2257 0.0018 43.1225 0.0924 0.0160 0.0007 0.3794 5.0 0.01 0.01 92.3 13.7 0.2

2.8 15.7632 0.0145 12.2418 0.0220 0.1721 0.0020 38.1060 0.0694 0.0150 0.0007 0.2708 3.5 0.02 0.01 89.0 13.5 0.2

3.1 11.1034 0.0229 8.4761 0.0094 0.1226 0.0016 31.0009 0.0833 0.0125 0.0007 0.1875 2.4 0.02 0.01 86.6 13.4 0.3

3.5 9.4148 0.0145 7.0967 0.0099 0.1014 0.0016 35.2859 0.1111 0.0132 0.0007 0.1570 2.1 0.03 0.01 85.3 13.4 0.3

4 7.4505 0.0336 5.5201 0.0082 0.0772 0.0012 31.0930 0.0927 0.0102 0.0007 0.1221 1.6 0.04 0.01 89.2 14.2 0.5

4.5 5.7175 0.0118 4.3620 0.0088 0.0622 0.0013 26.9858 0.0465 0.0088 0.0007 0.0965 1.3 0.03 0.01 88.3 13.7 0.5
32 Geologic Map of the Rio Rico and Nogales 7.5’ Quadrangles, Santa Cruz County, Arizona

Appendix Table 1–1. Combined 40Ar/39Ar data for Grosvenor Hills Volcanics and Nogales Formation. (Click here to access digital file.)

—Continued

Laser

power

watts

s,p

Relative isotopic abundances Derived results 40Ar

±1σ

39Ar

±1σ

38Ar

±1σ

37Ar

±1σ

36Ar

±1σ

39Ar Mol

∞ 10–14

39Ar %

of total

Ca/K

±1σ

%40Ar Age (Ma)

±1σ

11SCB06; whole rock chip; J=0.006560±0.000012

5 6.0280 0.0219 4.3238 0.0075 0.0621 0.0012 35.7661 0.0558 0.0063 0.0007 0.0957 1.2 0.04 0.01 111.4 18.4 0.6

0.5 33.8749 0.0713 15.2345 0.0191 0.1993 0.0016 21.1116 0.0524 0.0521 0.0008 0.3370 3.3 0.01 0.01 59.0 15.5 0.2

0.7 49.3235 0.0465 33.2874 0.0661 0.4038 0.0025 41.7231 0.0762 0.0449 0.0008 0.7364 7.2 0.01 0.01 79.1 13.8 0.1

0.9 55.7515 0.0594 43.0273 0.0661 0.5356 0.0023 57.8770 0.1334 0.0324 0.0008 0.9519 9.3 0.01 0.01 90.2 13.8 0.1

1.1 69.8337 0.0663 55.6585 0.0961 0.6951 0.0025 82.4812 0.1526 0.0392 0.0008 1.2318 12.1 0.01 0.01 91.8 13.6 0.1

1.3 77.4193 0.0574 62.9580 0.0771 0.7959 0.0031 109.2696 0.1957 0.0426 0.0009 1.3934 13.7 0.01 0.01 93.7 13.6 0.1

1.5 63.7337 0.0683 51.3540 0.0761 0.6444 0.0031 110.8370 0.2484 0.0431 0.0011 1.1365 11.2 0.01 0.01 92.4 13.5 0.1

1.7 50.3109 0.0793 40.2054 0.0560 0.5016 0.0024 94.0554 0.1435 0.0382 0.0008 0.8899 8.7 0.01 0.01 90.8 13.4 0.1

1.9 44.1077 0.0484 35.5962 0.0671 0.4457 0.0025 73.7883 0.0910 0.0340 0.0008 0.7878 7.7 0.01 0.01 89.1 13.0 0.1

2.2 49.5110 0.0633 39.2241 0.2401 0.5239 0.0027 78.1136 0.0959 0.0378 0.0008 0.8680 8.5 0.02 0.01 88.6 13.2 0.1

2.5 37.9920 0.0425 29.9134 0.0901 0.4016 0.0028 86.5946 0.1103 0.0343 0.0008 0.6619 6.5 0.02 0.01 89.5 13.4 0.1

2.8 27.3964 0.0336 21.1050 0.0370 0.3082 0.0022 106.6702 0.2352 0.0423 0.0008 0.4669 4.6 0.03 0.01 82.0 12.6 0.2

3.1 15.0828 0.0336 11.7838 0.0300 0.1690 0.0015 66.1091 0.1682 0.0237 0.0007 0.2607 2.6 0.04 0.01 84.7 12.8 0.2

3.5 7.9149 0.0229 6.1416 0.0111 0.0887 0.0014 30.8003 0.0482 0.0125 0.0007 0.1358 1.3 0.02 0.01 81.0 12.3 0.4

4 6.3710 0.0201 4.8789 0.0070 0.0780 0.0013 28.1854 0.0531 0.0130 0.0007 0.1079 1.1 0.03 0.02 71.0 11.0 0.5

4.5 6.6559 0.0201 4.9501 0.0111 0.0735 0.0013 40.7765 0.1975 0.0167 0.0007 0.1095 1.1 0.09 0.01 69.2 11.0 0.5

5 7.4509 0.0201 4.9121 0.0085 0.0790 0.0012 57.6757 0.0772 0.0233 0.0008 0.1087 1.1 0.05 0.02 62.7 11.3 0.6

11SCB07; whole rock; J=0.006560±0.000012

0.1 9.7125 0.0220 2.1379 0.0080 0.0425 0.0011 1.4352 0.0130 0.0245 0.0007 0.0473 0.4 0.01 0.02 26.6 14.2 1.2

0.3 31.1760 0.0703 13.5355 0.0290 0.2389 0.0019 12.9154 0.0364 0.0628 0.0008 0.2995 2.5 0.01 0.02 43.4 11.8 0.2

0.5 53.9568 0.2301 38.3681 0.0390 0.6317 0.0030 22.8107 0.0451 0.0645 0.0008 0.8491 7.1 0.00 0.02 67.7 11.2 0.1

0.7 89.4397 0.0813 71.1962 0.0711 1.1088 0.0035 41.4883 0.0872 0.0793 0.0008 1.5757 13.2 0.00 0.02 77.1 11.4 0.1

0.9 101.5562 0.0922 77.2487 0.0721 1.1618 0.0048 48.7935 0.1019 0.0973 0.0009 1.7095 14.3 0.00 0.02 75.1 11.7 0.1

1.1 97.8601 0.0992 69.5568 0.0651 1.0311 0.0041 62.9879 0.1262 0.1145 0.0009 1.5393 12.9 0.00 0.01 70.0 11.6 0.1

1.3 88.3604 0.0932 59.7004 0.0931 0.8630 0.0037 73.8273 0.1117 0.1149 0.0011 1.3210 11.0 0.01 0.01 67.5 11.8 0.1

1.5 63.3343 0.0673 42.1354 0.0711 0.5908 0.0027 64.6049 0.1459 0.0897 0.0009 0.9324 7.8 0.01 0.01 65.4 11.6 0.1

1.7 45.0243 0.0524 31.0115 0.0540 0.4352 0.0025 55.3136 0.0731 0.0629 0.0008 0.6861 5.7 0.01 0.01 67.5 11.6 0.1

1.9 32.0722 0.0614 22.9950 0.0400 0.3273 0.0024 46.5467 0.0780 0.0409 0.0007 0.5088 4.3 0.01 0.01 72.6 12.0 0.1

2.2 23.6732 0.0415 17.9105 0.0250 0.2600 0.0022 42.9144 0.0732 0.0315 0.0007 0.3963 3.3 0.01 0.01 73.5 11.5 0.1

2.5 19.1620 0.0220 13.8255 0.0230 0.1996 0.0015 43.7427 0.0781 0.0274 0.0007 0.3059 2.6 0.02 0.01 74.0 12.1 0.2

2.8 23.4549 0.0435 16.9311 0.0220 0.2393 0.0024 72.9093 0.1319 0.0440 0.0008 0.3746 3.1 0.02 0.01 66.6 10.9 0.2

3.5 40.0035 0.0390 25.6920 0.0581 0.3708 0.0023 141.0033 0.2670 0.0889 0.0009 0.5685 4.8 0.03 0.01 59.3 10.9 0.2

4 41.6872 0.0448 25.7361 0.0380 0.3730 0.0023 151.1846 0.3959 0.0927 0.0009 0.5694 4.8 0.04 0.01 60.0 11.5 0.2

4.5 13.1816 0.0303 7.3429 0.0121 0.1056 0.0014 53.0925 0.0895 0.0349 0.0007 0.1625 1.4 0.03 0.01 50.3 10.7 0.4

5 8.9796 0.0227 5.2498 0.0083 0.0802 0.0016 40.4447 0.0747 0.0227 0.0008 0.1162 1.0 0.04 0.02 57.4 11.6 0.6

Note: For this study ages were calculated with decay constants in Steiger and Jaeger (1977), a 40Ar/36Ar value of atmosphere = 298.56±0.31 (Lee and others, 

2006) and an age of 28.02±0.16 for Fish Canyon sanidine (Renne and others, 1998). Argon isotope data acquired by simultaneous collection of all masses on 

Faraday collectors (masses 40–37) and ion counter (mass 36) with a Thermo Scientific ARGUSVI mass spectrometer (samples with a 12SCB-prefix), and by 

peak hopping with a single electron multiplier in analog mode using a Mass Analyser Products 215-50 mass spectrometer (samples with 11SCB prefix). All data 

corrected for blanks, radioactive decay, nucleogenic interferences, mass discrimination, and detector intercalibration.

Lee, J.Y., Marti, K., Severinghaus, J.P., Kawamura, K., Yoo, H.S., Lee, J.B., and Kim, J.S., 2006, A redetermination of the isotopic abundances of atmospheric 

Ar: Geochim. Cosmochim, Acta70, p. 4507–4512.

Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998, Intercalibration of standards, absolute ages and uncertainties in 

Ar40/Ar39 dating: Chemical Geology 145(1-2), p. 117–152. doi: 10.1016/S0009-2541(97)00159-9.

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology—Convention on the use of decay constants in geo- and cosmochronology: Earth and 

Planetary Science Letters 36(3), p. 359–362.
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