U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004–5076

Stratigraphy and Tectonic History of the Tucson Basin, Pima 

County, Arizona, Based on the Exxon State (32)-1 Well
A detailed stratigraphy is presented, based on well cuttings and geophysical logs, for the 3,658 m of Upper 

Jurassic through upper Tertiary sedimentary and volcanic rocks overlying Precambrian(?) granitoid basement in the Tucson Basin. This is a revision of previous interpretations of Tucson Basin stratigraphy and is 

based in part on four new 40Ar/ 39Ar dates. The stratigraphic units are interpreted in terms of depositional 

environments and tectonic setting.

Stratigraphy and Tectonic History of the Tucson 

Basin, Pima County, Arizona, Based on the Exxon 

State (32)-1 Well

By Brenda B. Houser, Lisa Peters, Richard P. Esser, and Mark E. Gettings

Scientific Investigations Report 2004–5076

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2005

This publication is available only online at:

http://pubs.usgs.gov/sir/2004/5076/

Text edited by Peter H. Stauffer

Production and design by Stephen L. Scott

Manuscript approved for publication, April 30, 2004

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

FRONT COVER

Landsat satellite image of the Tucson Basin. See figure 2 (page 6) for explanation.
Contents

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

Introduction -------------------------------------------------------------------------------------------------- 2

Acknowledgments ------------------------------------------------------------------------------------ 3

Tucson basin ------------------------------------------------------------------------------------------------- 3

Geologic setting --------------------------------------------------------------------------------------- 3

Age of the Basin --------------------------------------------------------------------------------------- 5

Tectonic Setting --------------------------------------------------------------------------------------- 7

Stratigraphy of the Exxon (32)-1 well --------------------------------------------------------------------- 7

Methods and Data used ------------------------------------------------------------------------------ 7

Stratigraphic nomenclature of basin-fill units ----------------------------------------------------- 8

Fort Lowell Formation -------------------------------------------------------------------------- 11

Tinaja Beds ------------------------------------------------------------------------------------- 11

Descriptions of stratigraphic units ----------------------------------------------------------------------- 12

Upper Basin-Fill Deposits --------------------------------------------------------------------------- 12

Stratigraphic Unit A ---------------------------------------------------------------------------- 15

Stratigraphic Unit B ---------------------------------------------------------------------------- 17

Lower Basin-Fill Deposits --------------------------------------------------------------------------- 17

Stratigraphic Unit C ---------------------------------------------------------------------------- 17

Stratigraphic Unit D ---------------------------------------------------------------------------- 18

Pantano Formation ----------------------------------------------------------------------------------- 18

Conglomerate, mudstone, and andesite(?) flow -------------------------------------------- 19

Rock-avalanche deposit ---------------------------------------------------------------------- 20

Middle Tertiary volcanic and sedimentary rocks ------------------------------------------------- 21

High-potassium (lamprophyre ?) dike or sill ------------------------------------------------ 22 

Andesite(?) lava flow -------------------------------------------------------------------------- 22 

Conglomerate ----------------------------------------------------------------------------------- 23 

Silicic tuff --------------------------------------------------------------------------------------- 23 

Limestone conglomerate ---------------------------------------------------------------------- 23 

Pyroxene trachyte ----------------------------------------------------------------------------- 24 

Bisbee Group ----------------------------------------------------------------------------------------- 24 

Turney Ranch Formation ---------------------------------------------------------------------- 25 

Shellenberger Canyon Formation ------------------------------------------------------------ 26 

Apache Canyon Formation -------------------------------------------------------------------- 26 

Willow Canyon Formation --------------------------------------------------------------------- 27

Glance Conglomerate ------------------------------------------------------------------------- 27

Granitoid rock ---------------------------------------------------------------------------------------- 28

Discussion and Summary --------------------------------------------------------------------------------- 29

References Cited ------------------------------------------------------------------------------------------- 31

Appendix: Age Spectra and Probability Distribution Diagrams for 40Ar/ 39 Ar

 Determinations ------------------------------------------------------------------------------------ 34

III
Figures

 1. Map of part of southeastern Arizona showing basins, ranges, location of 

 the Exxon State (32)-1 well in the Tucson Basin, location of the Catalina 

 detachment fault, ash sample locations, and locations of other features 

 mentioned in the text -------------------------------------------------------------------------2

 2. Satellite thematic mapper image of the Tucson Basin showing the geomor-

 phic expression of the late Pliocene to early Pleistocene Cienega Creek 

 alluvial fan --------------------------------------------------------------------------------------6

 3. Map of simplified residual gravity anomalies in the Tucson Basin ----------------------8 

 4. Stratigraphic column for the Exxon State (32)-1 well -------------------------------------9

 5. Graph showing bulk density of the sedimentary units in the Exxon State 

 (32)-1 well ------------------------------------------------------------------------------------ 10

 6-13. Geophysical logs for units in the Exxon State (32)-1 well:

 6. Unit A, Upper basin-fill deposits ------------------------------------------------------ 12

 7. Unit B, Upper basin-fill deposits ------------------------------------------------------ 13

 8. Unit C, Lower basin-fill deposits ------------------------------------------------------ 14

 9. Unit D, Lower basin-fill deposits ----------------------------------------------------- 15

 10. Pantano Formation --------------------------------------------------------------------- 19

 11. Middle Tertiary volcanic and sedimentary rocks ----------------------------------- 22

 12. Bisbee Group (Turney Ranch, Shellenberger Canyon, and Apache Canyon 

 Formations and Glance Conglomerate ------------------------------------------ 25

 13. Granitoid plutonic rock ---------------------------------------------------------------- 28

 14-17. Age probability distribution diagrams or ideograms for dated mineral 

 separates:

 14. Feldspar separate from 613PEN97 (Nogales Formation) -------------------------- 34

 15. Feldspar separate from 615BAT99 (formation of Tinaja Peak) -------------------- 35

 16. Sanidine separate from 616MTHOP00 (Nogales Formation) ---------------------- 36

 17. Age spectra of biotite separates from a silicic tuff penetrated in the 

 Exxon State (32)-1 well ------------------------------------------------------------ 37

 Tables 

 1. Summary of geophysical logs and borehole data for the Exxon State (32)-1 

 well ----------------------------------------------------------------------------------------------3

 2. Summary of 40Ar/ 39Ar data and analytical methods for new radiometric 

 ages obtained in this study -------------------------------------------------------------------4

 3. Names and ages (where known) of paired upper and lower basin-fill units 

 in basins of southeastern Arizona and southwestern New Mexico ------------------ 11

IV
1

Abstract

The Tucson Basin is a relatively large late Cenozoic 

extensional basin developed in the upper plate of the Catalina 

detachment fault in the southern Basin and Range Province, 

southeastern Arizona. In 1972, Exxon Company, U.S.A., 

drilled an exploration well (Exxon State (32)-1) near the 

center of the Tucson Basin that penetrated 3,658 m (12,001 

ft) of sedimentary and volcanic rocks above granitoid basement. Detailed study of cuttings and geophysical logs of the 

Exxon State well has led to revision of the previously reported 

subsurface stratigraphy for the basin and provided new insight 

into its depositional and tectonic history.

There is evidence that detachment faulting and uplift of 

the adjacent Catalina core complex on the north have affected 

the subsurface geometry of the basin. The gravity anomaly 

map of the Tucson Basin indicates that the locations of subbasins along the north-trending axis of the main basin coincide with the intersection of this axis with west-southwest 

projections of synforms in the adjacent core complex. In other 

words, the subbasins overlie synforms and the ridges between 

subbasins overlie antiforms. The Exxon State well was drilled 

near the center of one of the subbasins.

The Exxon well was drilled to a total depth of 3,827 m 

(12,556 ft), and penetrated the following stratigraphic section:

Pleistocene(?) to middle(?) Miocene

 upper basin-fill sedimentary rocks (0-908 m [0-2,980 ft])

 lower basin-fill sedimentary rocks (908-1,880 m 

 [2,980-6,170 ft])

 lower Miocene and upper Oligocene 

 Pantano Formation (1,880-2,516 m [6,170-8,256 ft])

 upper Oligocene to Paleocene(?)

 volcanic and sedimentary rocks (2,516-3,056 m 

 [8,256-10,026 ft])

 Lower Cretaceous to Upper Jurassic 

 Bisbee Group (3,056-3,658 m [10,026-12,001 ft]) 

 pre-Late Jurassic 

 granitoid plutonic rock (3,658-3,827 m [12,001- 

 12,556 ft]). 

Stratigraphy and Tectonic History of the Tucson 

Basin, Pima County, Arizona, Based on the Exxon 

State (32)-1 Well

The 1,880 m (6,170 ft) of basin-fill sedimentary rocks 

consist of alluvial-fan, alluvial-plain, and playa facies. The 

uppermost unit, a 341-m-thick (1,120-ft) lower Pleistocene 

and upper Pliocene alluvial-fan deposit (named the Cienega 

Creek fan in this study), is an important aquifer in the Tucson 

basin. The facies change at the base of the alluvial fan may 

prove to be recognizable in well data throughout much of the 

basin.

The well data show that a sharp boundary at 908 m (2,980 

ft) separates relatively unconsolidated and undeformed upper 

basin fill from denser, significantly faulted lower basin fill, 

indicating that there were two stages of basin filling in the 

Tucson basin as in other basins of the region. The two stages 

apparently occurred during times of differing tectonic style in 

the region.

In the Tucson area the Pantano Formation, which contains 

an andesite flow dated at about 25 Ma, fills a syntectonic basin 

in the hanging wall of the Catalina detachment fault, reflecting middle Tertiary extension on the fault. The formation in 

the well is 636 m thick (2,086 ft) and consists of alluvial-fan, 

playa, and lacustrine sedimentary facies, a lava flow, and rockavalanche deposits. Analysis of the geophysical logs indicates 

that a K-Ar date of 23.4 Ma reported previously for the Pantano interval of the well was obtained on selected cuttings collected from a rock-avalanche deposit near the base of the unit 

and, thus, does not date the Pantano Formation. 

The middle Tertiary volcanic and sedimentary rocks have 

an aggregate thickness of 540 m (1,770 ft). We obtained a 

new 40Ar/ 39Ar age of 26.91+0.18 Ma on biotite sampled at a 

depth of 2,584-2,609 m (8,478-8,560 ft) from a 169-m-thick 

(554-ft) silicic tuff in this interval. The volcanic rocks probably correlate with other middle Tertiary volcanic rocks of the 

area, and the sedimentary rocks may correlate with the Cloudburst and Mineta Formations exposed on the flanks of the San 

Pedro Basin to the northeast.

The Bisbee Group in the Exxon well is 602 m (1,975 

ft) thick and includes the five formations of the northeastern 

facies described in the Empire and Whetstone Mountains to 

the east. The lithologies of the five formations in the well and, 

hence, their inferred depositional environments are very similar to those in the type sections to the east (shale, lacustrine 

limestone, and conglomerates of alluvial fans); however, the 

section in the well is only about 5 to 30 percent as thick as in 

the outcrop belt, which may indicate that, at the drill site, the 

Bisbee subbasin did not subside as rapidly as to the east. The 

By Brenda B. Houser1, Lisa Peters2, Richard P. Esser2, and Mark E. Gettings1

1

U.S. Geological Survey, 520 North Park Avenue, Tucson, Arizona 85719 

bhouser@usgs.gov

2

New Mexico Bureau of Mines, 801 Leroy Place, Socorro, New Mexico 

87801
2 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

Bisbee Group was not recognized in the initial report on the 

stratigraphy of the Exxon well. Instead, the rocks in this interval were correlated with the Pantano Formation. 

The well bottomed in 169 m (555 ft) of equigranular, 

slightly chloritic granitoid crystalline rock at the total depth of 

3,827 m (12,556 ft). Because this granitoid rock is overlain by 

the Upper Jurassic(?) or Lower Cretaceous Glance Conglomerate of the Bisbee Group, it must at least be older than 138 

Ma. Granitoid rocks beneath the Bisbee Group in the Cienega 

Gap area to the east of the Tucson Basin are Precambrian.

INTRODUCTION

In 1972, as part of an exploration program in the Basin 

and Range Province of southwestern Arizona, Exxon Company, U.S.A., drilled a 3,827-m-deep (12,556-ft) test well near 

the center of the Tucson Basin (Exxon State (32)-1, T. 16 S., 

R. 15 E., sec. 5, Pima County, Arizona) (fig. 1). Granitoid 

rock was penetrated beneath 3,658 m (12,001 ft) of Mesozoic 

and Cenozoic sedimentary and volcanic rocks, and the well 

bottomed in the granitoid rock at the total depth of 3,827 m 

(12,556 ft). This is an important well for the Tucson Basin 

because, in addition to being the only one to reach granitoid 

crystalline rock, it is the only well that has penetrated more 

than about the first 1,000 meters of the 3,658-m-thick (12,001-

ft) overlying sedimentary and volcanic section. Unfortunately, 

only cuttings (no core) are available for study. However, a 

standard suite of geophysical logs was run, which provides 

coverage for nearly the entire depth of the well with the exception of the upper 61 m (200 ft) of surface casing and a 13-mthick (42-ft) interval between 899 and 912 m (2,950-2,992 ft) 

at the base of the second casing (table 1). The combination 

of geophysical log data and drill cuttings analysis provides 

insight into lithologic identification, sedimentologic trends, 

and structural interpretations that would not be given by either 

analysis alone.

The stratigraphy and radiometric ages obtained for this 

well were summarized by Eberly and Stanley (1978) and were 

correlated by them with surface data and with data from other 

deep wells in the basins of southwestern Arizona. The surface 

and well data, together with seismic data, provided the basis 

for their interpretation of the regional Cenozoic stratigraphy 

of southwestern Arizona. Eberly and Stanley recognized that 

some of the K-Ar whole-rock ages obtained on selected cuttings of volcanic rocks from Exxon State (32)-1 were out of 

chronologic order. To resolve this discrepancy, they inferred 

that rocks lower in the well (giving younger dates than rocks 

higher in the well) were intrusive volcanic rocks.

The present study has three objectives, which will ultimately provide part of the basis for a broader study of the 

Tucson Basin: (1) to interpret as much of the tectonic history 

of the basin as possible from the sedimentary and volcanic 

stratigraphy of the well, (2) to resolve some of the uncertainties associated with radiometric dates from well cuttings given 

in Eberly and Stanley’s (1978) paper, and (3) to develop a 

revised stratigraphy for the Tucson Basin. To these ends, we 

obtained four new 40Ar/ 39Ar ages. Three of these are on surface exposures of ash-flow and ash-fall tuffs from lower basinfill rocks and from Pantano-age conglomerates located in the 

upper Santa Cruz Basin south of the Tucson Basin, and one is 

from a 169-m-thick (554-ft) silicic tuff in the middle Tertiary 

volcanic rocks in the well (table 2). In addition, we present a 

detailed revised stratigraphy that includes Lower Cretaceous 

and Upper Jurassic Bisbee Group sedimentary rocks overlying 

crystalline granitoid rock at the bottom of the well.

Eberly and Stanley (1978) mistakenly identified 1,142 

m (3,745 ft) of Tertiary volcanic and sedimentary rocks and 

the Cretaceous and Jurassic Bisbee Group in the Exxon well 

as part of the upper Oligocene and lower Miocene Pantano 

Formation. This is understandable in that the focus of their 

Figure 1. Part of southeastern Arizona showing basins, ranges 

(stippled), location of the Exxon State (32)-1 well in the Tucson Basin, 

location of the Catalina detachment fault (CDF, ticks on upper plate), 

ash sample localities (dots with sample numbers), and locations 

of other features mentioned in the text. Crosshatched area shows 

the extent of the Cienega Creek fan in the Tucson Basin. Inset map 

shows location of study area.

0

0
3

study was the Cenozoic stratigraphy of southwestern Arizona, 

rather than the middle Tertiary and Mesozoic stratigraphy of 

southeastern Arizona. Also, as noted by Dickinson (1999), 

superficial similarities of some of the formations of the Bisbee 

Group to the Pantano Formation have resulted in exposures of 

the Bisbee Group being mapped as Pantano in the northeastern 

part of the Tucson Basin.

We also discuss the correlation of the geophysical logs 

with the physical properties of various rocks penetrated by 

the Exxon State (32)-1 well. This analysis contributes to the 

methodology of maximizing drill hole data and thus will be 

useful to other researchers where similar data are available.

Acknowledgments

We thank J.C. Matti, F.N. Houser, P.G. Vikre, and Floyd 

Gray for stimulating discussions and critical reviews of the 

manuscript. Thanks are also extended to W.R. Dickinson 

for his encouragement in starting this project. The late H.W. 

Peirce showed one of us (Houser) important conglomerate 

exposures in the northeastern part of the Tucson Basin. T.G. 

McGarvin provided assistance in accessing the well cuttings 

at the Arizona Geological Survey core and cuttings repository. 

We thank Walter A. Harrison of Caterpillar Inc. for allowing 

us to conduct field work on the Tucson Proving Ground southeast of the Sierrita Mountains where we studied the formation 

of Tinaja Peak and collected an ash sample.

TUCSON BASIN

Geologic Setting

The Tucson Basin is a large structural basin in the southern Basin and Range Province of the southwestern United 

States, a region of northwest-trending sediment-filled basins 

separated by fault-bounded mountain ranges, which form the 

characteristic basin-and-range topographic expression of late 

Tertiary extension (fig. 1). The Tucson Basin is 15 to 40 km 

wide (10 to 25 mi), about 80 km long (50 mi), generally crescent shaped (concave to the west), and forms part of the valley 

of the north-flowing Santa Cruz River. The elevation of the 

floor of the basin near central Tucson is about 727 m (2,385 ft) 

above sea level.

Mountain ranges on the north and east side of the Tucson 

Basin are the Santa Catalina and Rincon Mountains, respectively, with elevations up to 2,791 m (9,157 ft). These two 

ranges make up the Catalina core complex, which is bounded 

on the southwest by the Catalina detachment fault (Dickinson, 1991). Cienega Gap is a broad bedrock saddle between 

the Rincons on the north and the Santa Rita Mountains on 

the south. The Santa Rita Mountains are as high as 2,881 m 

(9,453 ft) and border the Tucson Basin on the east and south. 

The Sierrita and Tucson Mountains on the west side of the 

basin are about 1,800 m (5,900 ft) and 1,430 m (4,700 ft) 

high, respectively, more than 1,000 m (3,300 ft) lower than the 

mountains on the north and east sides.

A wide range of lithologies is present in the mountains 

bordering the Tucson Basin, including Precambrian and 

Laramide-age (latest Cretaceous and Paleocene) granitoid 

plutonic rocks, Paleozoic sedimentary rocks, and volcanic and 

sedimentary rocks of Mesozoic, Laramide, and middle Tertiary age (Drewes, 1971a,b, 1977; Cooper, 1973; Banks, 1974; 

Reynolds, 1988; Lipman, 1993; Force, 1997). At the present 

erosion level, granitoid gneiss is the predominant lithology in 

the Catalina core complex, but before and during late Tertiary 

uplift and unroofing of the core complex, other lithologies had 

to be present, particularly Tertiary and Cretaceous volcanic 

rocks and Paleozoic limestones of the hanging wall of the 

Log types Date logged Depth interval Borehole diameter1

Compensated gammagamma (density), natural

gamma, caliper, induction,

spontaneous potential,

compensated sonic (transit

time)

9-21-72 61 – 899 m

(199 – 2,950 ft)

Bit = 34.9 cm

(13.75 in)

Compensated neutron

formation density, natural

gamma, caliper, dual

induction laterolog,

spontaneous potential,

compensated sonic (transit

time)

12-14-72 912 – 3,827 m

(2,992 – 12,556 ft)

Bit = 24.5 cm (9.6 in) from

 912 to 2,925 m

(2,992 – 9,598 ft)

Bit = 19.4 cm (7.6 in) from

 2,925 to 3,825 m

(9,598 – 12,556 ft)

�

��������������������������������������������

Table 1. Summary of geophysical logs and borehole data for the Exxon State (32)-1 well.

[Casing: Surface to 61 m (199 ft)—40.6 cm (16 in) diameter; surface to 912 m (2,992 ft)—27.3 cm (10.75 in) diameter. Location data: 

lat 32°04’07”N., long 110°50’02”E.; NE1/4NE1/4SW1/4, sec. 5, T. 16 S., R. 15 E. Surface elevation: 876 m (2,873 ft). Total depth: 

3,827 m (12,556 ft).] 

Tucson Basin
4 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

detachment fault. These rocks were shed into the northern 

half of the basin in large quantity, both as rock avalanches 

and as fluvial sediments. The erosional sequence is clearly 

demonstrated in the clast composition of lower basin-fill conglomerate units exposed in the area termed the Catalina Foothills along the north side of the Tucson basin (fig. 1) (Voelger, 

1953; Pashley, 1966; Dickinson, 1999). The oldest unit in the 

Foothills area was named the lower Rillito Formation by Voelger (1953) and the type I Rillito beds by Pashley (1966). It 

is probably equivalent to the middle Tertiary Pantano Formation described in Cienega Gap by Finnell (1970b) and Balcer 

(1984) and contains clasts of volcanic rock, limestone, quartzite, and schist but no gneiss. The progressively younger lower 

basin-fill rocks were named the middle and upper Rillito Formation by Voelger (1953), the type II and III Rillito beds by 

Pashley (1966), and the Swan-Craycroft gravels by Dickinson 

(1999). These rocks contain increasing amounts of gneissic 

clasts and proportionally decreasing amounts of other clast 

lithologies upward in the section until, finally, the Pleistocene 

fans are composed dominantly of granitoid gneiss clasts. 

Except for discontinuous exposures of more consolidated 

lower basin-fill rocks next to the mountains, exposures in the 

Tucson Basin consist of slightly dissected upper basin-fill 

sediments, probably late Pliocene to early Pleistocene in age, 

variously covered by middle Pleistocene through Holocene 

surficial materials (Pearthree and others, 1988). Holocene 

alluvial deposits of the Santa Cruz River occupy the western 

edge of the basin. No vertebrate fossils and only one tephra 

bed (which contains no datable minerals) have been found in 

the Tucson basin fill, partly because of a paucity of exposures, 

but chiefly because the rocks that are exposed were deposited 

in fairly high-energy fluvial environments where vertebrate 

remains and tephra beds are unlikely to be preserved.

Most of the basin, from the mouth of Cienega Creek west 

to the Santa Cruz River and northwest to Tucson, is occupied 

by a large upper Pliocene to lower Pleistocene alluvial fan 

Sample

Number

Stratigraphic

unit

Latitude

Longitude

Irradiation1 Mineral Analysis2,3 Number

of steps

Age ( Ma)

4

(MSWD)

 613PEN97 Nogales

Formation

 31o

26’02”

111o

02’50”

NM-82 Sanidine Laser

fusion

14 13.23+0.10

(1.6)

615BAT99 formation of

Tinaja Peak

 31o

50’10”

111o

08’27”

NM-119 Sanidine Laser

fusion

3 24.70+0.19

(0.5)

616MTHOP00 Nogales

Formation

 31o

38’29”

110o

58’38”

NM-129 Sanidine Laser

fusion

2 or 12 15.53+0.38

or

17.38+0.77

(18)

2,584 – 2,609

m Exxon State

(32)-1

Middle Tertiary

volcanic

rocks

 32o

04’07”

110o

50’02”

NM-110 Biotite Furnace

step heat

7 26.91+0.19

(1.5)

Table 2. Summary of 40Ar/ 39Ar data and analytical methods for new radiometric ages obtained in this study.

1

Sample preparation and irradiation: Mineral separates were prepared using standard crushing, heavy liquid, and hand-picking techniques. The separates were loaded into a machined Al disc and irradiated for either 7 or 14 hours in D-3 position at the Nuclear Science Center, College Station, Texas. Neutron flux monitor is Fish Canyon Tuff sanidine (FC-1) with an assigned age of 27.84 Ma (Deino and Potts, 1990) relative to Mmhb-1 at 520.4 Ma (Samson 

and Alexander, 1987). 2

Instrumentation: Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system. Biotite samples step-heated 

in Mo double-vacuum resistance furnace; heating duration 8 minutes. Reactive gases removed by reaction with three SAES GP-50 getters, two operated at 

about 450°C and one at 20°C. Gas also exposed to a W filament operated at about 2,000°C. Single sanidine crystals were fused by a 50-watt Synrad CO2 laser. 

Reactive gases removed during a 1-minute reaction with two SAES GP-50 getters, one operated at about 450°C and one at 20°C. Gas also exposed to a W 

filament operated at about 2,000°C and a cold finger operated at –140°C. 3

Analytical parameters: Electron multiplier sensitivity averaged 2.31x10–16 moles/pA for the furnace and 1.25x10–16 for the laser. Total system blank and 

background for the furnace averaged 5,030, 44.1, 0.9, 5.6, 18.2x10–18 moles and 700, 15, 1.8, 3.5, 4.2x10–18 moles for the laser at masses 40, 39, 38, 37, and 36, 

respectively. J-factors determined to a precision of ±0.1% by CO2 laser-fusion of four single crystals from each of four or six radial positions around the 

irradiation tray. Correction factors for interfering nuclear reactions were determined using K-glass and CaF2, and are as follows: (40Ar/ 39Ar) K = 

0.00020±0.0003; (36Ar/ 37Ar) Ca = 0.00026±0.00002; and (39Ar/ 37Ar) Ca = 0.00070±0.00005. 4

Age calculations: Total gas ages and errors calculated by weighting individual steps by the fraction of 39Ar released. Plateau ages calculated for the 

indicated steps by weighting each step by the inverse of the variance. Plateau age errors calculated using the method of Taylor (1982). Mean sum weighted 

deviate (MSWD) values are calculated forn-1 degrees of freedom for plateau ages. If the MSWD is outside the 95% confidence window (Mahon, 1996, table 

1), the error is multiplied by the square root of the MSWD. Decay constants and isotopic abundances after Steiger and Jager (1977). All final errors reported 

at ±2σ.
5

(herein termed the Cienega Creek fan) composed of detritus 

eroded from Davidson Canyon, Cienega Gap, and the lower 

part of Cienega Creek Valley (fig. 2). Dickinson (1999, p. 4) 

noted that the “floor of the northeastern corner of the Tucson 

basin is a sediment apron in the form of a half fan * * * with 

its apex where Pantano Wash exits from Cienega Gap.” However, lacking subsurface data, he apparently did not recognize 

that the near-surface sediment is actually part of a thick upper 

basin-fill alluvial fan and not just a sediment apron with the 

form of a fan. The surface morphology of the alluvial fan 

shows clearly on figure 2, which is a thematic mapper image 

of the Tucson basin and of areas to the southeast that were the 

source of the fan sediment. Analysis of cuttings and logs of 

the Exxon State (32)-1 well indicate that at the well site the 

fan is about 340 m (1,120 ft) thick, and the gamma ray log 

indicates that tephra beds are present in a finer-grained facies 

of basin-fill deposits just 10 m below the base of the alluvial 

fan.

Age of the Basin

Basin-range taphrogeny and the formation and filling 

of the Tucson Basin resulted from crustal extension directed 

dominantly east-northeast/west-southwest on a regional scale 

during the late Cenozoic. Normal faulting, basin subsidence, 

and basaltic and bimodal basalt-rhyolite volcanism began in 

southeastern Arizona as early as 17 Ma and continued into the 

late Pliocene and early Pleistocene.

 Basaltic or bimodal basalt-rhyolite magmatism is commonly taken to be an indicator of crustal extension and, thus, 

the oldest basaltic lavas or high-silica ash-flow tuffs associated with a basin and its basin fill will give a minimum age 

for basin formation. For example, in the Safford Basin and 

adjacent Gila, Whitlock, and Peloncillo Mountains, about 150 

km northeast of the Tucson Basin, numerous radiometric dates 

show that the change from intermediate composition volcanism, characteristic of the mid-Tertiary volcanic period in 

the southern Basin and Range (Shafiqullah and others, 1978), 

to basaltic and bimodal volcanism occurred at about 17 Ma 

(Richter and others, 1981; Richter and others, 1983; Houser 

and others, 1985; Wrucke and others, 2004). The oldest dated 

true basalt flows in southeastern Arizona are in the Whitlock 

Mountains, where K-Ar ages were obtained of 16.6 to 16.2 Ma 

(Richter and others, 1981).

The maximum age of the beginning of basin-range extension and formation of the Tucson Basin is similar to that of the 

Safford Basin, but it is not as tightly constrained. The youngest dates reported for middle Tertiary intermediate composition volcanic rocks in the ranges surrounding the Tucson Basin 

are 20.3+3.0 Ma in the Tucson Mountains (Marvin and others, 

1978; Lipman, 1993) and 23.7 Ma in the Sierrita Mountains 

(Cooper, 1973), whereas the oldest dated tuff associated with 

lower basin-fill sedimentary deposits of the Nogales Formation in the upper Santa Cruz Basin (south of the Tucson Basin) 

is 15.53+0.38 Ma (see below).

The Nogales Formation is a fluvial middle to upper Miocene unit in the upper Santa Cruz Basin named by Simons 

(1974) for exposures near Nogales. Drewes (1971b) extended 

the geographic range of the unit to include well indurated 

faulted alluvial-fan deposits exposed at the edge of the upper 

Santa Cruz Basin on the west side of the Santa Rita Mountains. In this study, 40Ar/39Ar ages were obtained on sanidine 

from two tuffs interbedded near the base of the Nogales Formation on either side of the upper Santa Cruz Basin (fig. 1; 

table 2; figs. 14 and 16 in appendix). 

A sample (MTHOP00) for dating was collected from a 

relatively unreworked ash-fall tuff bed 0.5 m thick in a highly 

tuffaceous conglomerate facies, probably within 100 m of 

the base of the Nogales Formation, in Cottonwood Canyon 

on the western side of the Santa Rita Mountains (fig. 1). The 

stratigraphic position of the tuff bed relative to the base of the 

Nogales Formation is estimated from well data. In nearby 

surface exposures, the Nogales is in fault contact chiefly with 

Precambrian gneiss and with minor Late Cretaceous diorite 

(Drewes, 1971b). Unfortunately, small crystal size of this 

sample led to analytical problems. In addition, the age probability distribution diagram (fig. 16) shows that the sample 

contained probable xenocrystic material. The most likely 

age of the ash bed we sampled is 15.53+0.38 Ma, based on 

the weighted mean of the two youngest sanidine crystals in 

the sample. The older date (17.38+0.77 Ma), based on the 

weighted mean of the whole sample (fig. 16), may represent 

the age of the dominant tuff of the reworked tuffaceous matrix 

of the conglomerate. It is possible that this older tuff could 

be part of a bimodal eruption sequence and, thus, could date 

more closely the beginning of Basin and Range extension in 

the upper Santa Cruz Basin, but in the absence of unreworked 

exposures, this relationship is conjectural.

The second date was obtained on sanidine from an ashflow tuff bed about 5 m thick interbedded with conglomerate 

of the Nogales Formation on the west side of the upper Santa 

Cruz Valley in Agua Fria Canyon, 70 km south of the Exxon 

well site (613PEN97 in fig. 1). The base of the conglomerate 

is not exposed at the sample locality, although field relations 

indicate that the tuff is fairly near the bottom of the Nogales 

Formation. About 3 km to the northwest, the conglomerate 

and tuff are in fault contact with middle Tertiary rhyolite of the 

Atascosa Mountains. The 40Ar/ 39Ar date obtained in this study, 

on sanidine from pumice in the ash-flow tuff, is 13.23+0.10 Ma 

(table 2; fig. 14 ). This agrees well with the whole-rock K-Ar 

age reported by Simons (1974) of 12.6+0.8 Ma from basalt 

interbedded near the base of the conglomerate about 1 km to the 

northeast. The ash-flow tuff and basalt samples, closely positioned in age and space, suggest the possible presence of a small 

bimodal eruptive center buried within or beneath the lower 

basin-fill deposits of the upper Santa Cruz Basin to the east.

Scarborough and Peirce (1978) quoted a K-Ar date of 

11.6 Ma for the beginning of extension in the Tucson Basin. 

The date was obtained on drill cuttings from the Exxon well 

that (based on Eberly and Stanley’s (1978) stratigraphic interpretation) were collected from the top of the volcanic section 

Tucson Basin
6 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

Figure 2. Landsat satellite thematic mapper (TM) image of the Tucson Basin showing the geomorphic expression of the late Pliocene to 

early Pleistocene Cienega Creek alluvial fan. The fan head is at the eastern side of the basin near the mouth of Cienega Creek, and the toe 

is within the developed southeastern outskirts of Tucson. The fan is composed of sediments derived from basin-fill deposits and bedrock 

eroded from the Davidson Canyon and Cienega Creek drainage basins. The Cienega Creek fan is the youngest unit penetrated by the Exxon 

well, and is about 340 m (1,120 ft) thick at the well site (locality marked by an X).
7

just beneath the basin fill at a depth of 2,192 m (7,191 ft) (fig. 

4) (Damon and others, 1996). However, in our revised interpretation of the stratigraphic section, this depth corresponds 

to playa-facies sediments in the Pantano Formation, so the 

selected cuttings that were dated probably were conglomerate 

clasts from uphole contamination.

Soil studies and maps of surficial deposits in the Tucson 

Basin and upper Santa Cruz Valley (Pearthree and others, 

1988; Helmick, 1986) show that there are no basin-filling 

deposits younger than about earliest Pleistocene, suggesting 

that crustal extension and basin subsidence in southeastern 

Arizona were waning by this time as noted by Machette and 

others (1986) and Menges and Pearthree (1989). Although 

Pleistocene fault scarps of the Santa Rita Fault Zone on the 

eastern piedmont of the Tucson and upper Santa Cruz Basins 

(Drewes, 1971a, b) are evidence of continued extension, 

Pearthree and Calvo (1987) have estimated a fault recurrence 

interval of about 200 k.y. on the fault zone, indicating relative quiescence. Through-flowing drainage by the Santa Cruz 

River was probably initiated near the beginning of the Pleistocene as basin subsidence slowed, but, as in the case of the fault 

scarps, the modern river also provides evidence of continuing minor tectonic subsidence. Gettings and Houser (1997) 

noted that the gradient of the upper Santa Cruz River flood 

plain increases where the river crosses buried bedrock highs 

between subbasins. They suggested the changes in gradient 

are caused by differential movement of subbasins relative to 

intervening bedrock highs during the Holocene.

Tectonic Setting

The Tucson Basin lies in the hanging wall of the early 

Miocene Catalina detachment fault (Dickinson, 1991). In fact, 

the detachment fault more or less coincides with the topographic boundary of the basin adjacent to the Santa Catalina 

and Rincon Mountains (fig. 1), although aeromagnetic data 

(Sweeney, 2000) indicate that the middle Miocene basin-range 

fault along the northern side of the basin in the Catalina Foothills area lies about 1.5 km (1 mi) southwest of the surface 

trace of the detachment fault.

Like mountain ranges, which are marked by visible 

saddles and peaks, basins have deeper parts termed subbasins 

separated by buried bedrock highs. However, whereas the 

topography of ranges is shaped by both erosion and tectonics, 

the topographic shape of the basins is chiefly a function of 

tectonic forces. The location and shape of buried subbasins 

and intervening bedrock highs are delineated by drilling and 

by gravity, magnetic, and electrical data. The location of subbasins within extensional basins in the southern Basin and 

Range Province commonly appears to have been controlled 

by activation of pre-existing structures. For example, in the 

upper Santa Cruz Valley, the northwestern extension of the 

Mount Benedict Fault crosses the valley obliquely and separates the Rio Rico and Tubac subbasins (Gettings and Houser, 

1997). In the Tucson Basin, there is evidence that detachment 

faulting and uplift of the core complex, in addition to creating 

the Santa Catalina and Rincon Mountains, have had an effect 

on the subsurface geometry of the basin. Figure 3 is a simplified residual gravity anomaly map of the basin (modified 

from Davis, 1971). The schematic gravity anomaly contours 

indicate that, although the axis of the basin is generally north 

trending, the locations of the subbasins along the axis coincide 

with the intersection of the basin axis with west-southwest 

projections of the synforms of the core complex. Projections 

of the antiforms, or mullions, of the core complex correspond 

to ridges between the subbasins. This geometry is analogous 

to the sediment-filled basins present in the synforms between 

mullions of the Harcuvar, Rawhide, and Buckskin Mountains 

core complexes in west-central Arizona (Spencer and Reynolds, 1989) and demonstrates that the tectonic influence of the 

Catalina core complex may extend into the subsurface at least 

as far west as the axis of the Tucson Basin.

This correlation of the location of late Tertiary subbasins 

of the Tucson Basin with underlying synforms of the Catalina 

core complex implies that the southwestern flank of the core 

complex was tectonically active when the subbasins were being 

filled, at least during the early stages in middle to late Miocene 

time. The magnitude of differential displacement between the 

subbasins and intervening ridges is not trivial. On the basis of 

density analysis of gravity anomaly data, Litinsky (1989) estimated the topographic relief between subbasins and ridges in 

the Tucson Basin to range from 1 to 3 km (0.6 to 1.8 mi).

STRATIGRAPHY OF THE EXXON 

STATE (32)-1 WELL

Methods and Data Used

This study involved a detailed microscopic examination of drill cuttings from the Exxon State (32)-1 well. The 

sampled interval begins at 70 m (230 ft) and extends through 

the total depth of the well at 3,827 m (12,556 ft). Names and 

numerical designations of colors of the cuttings are based on 

the Munsell system as estimated from the rock color chart 

(Goddard and others, 1975). Geophysical logs were invaluable in the interpretation of the stratigraphy and sedimentology 

of the rock units represented by the cuttings and provide 

much of the basis for tectonic interpretations. The logs were 

obtained by a commercial well-logging company, Schlumberger Well Services. Table 1 summarizes the depth intervals 

for which the logs were run and the borehole and casing diameters. Discussions of the interpretation of geophysical logs 

can be found in Keys and MacCary (1971), Telford and others 

(1976), Hilchie (1978), and Merkel (1979).

Radiometric ages of stratigraphic units related to the 

Tucson Basin have been determined by both the K/Ar method 

(Cooper, 1973; Eberly and Stanley, 1978; and Shafiqullah and 

others, 1978) and the 40Ar/ 39Ar method (reported in this paper). 

The 40Ar/ 39Ar dates (one from interval 2,584-2,609m [8,478-

Stratigraphy of the Exxon (32)-1 well
8 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

8,560 ft] of the Exxon State (32)-1 well and three from surface 

rocks correlated with units present in the well) were obtained 

by the New Mexico Geochronology Research Laboratory. A 

summary of the 40Ar/ 39Ar results can be found in table 2. The 

complete set of 40Ar/ 39Ar analytical data can be found in the 

appendix.

Two problems with the cuttings were recognized that bear 

on the anomalously young K-Ar ages obtained from selected 

cuttings in Eberly and Stanley’s (1978) study: (1) In some cases, 

the cuttings in the sample vials do not match the lithologies indicated by the geophysical logs, suggesting that in these cases the 

cuttings in the vials are from some other interval and may have 

been improperly bagged at the drill site or later. (2) The biggest 

problem with the cuttings is contamination from washing out 

of sediments farther up the well. The amount of contamination 

is variable and could be estimated with a moderate degree of 

certainty in monolithologic intervals. For example, the pyroxene trachyte between 2,897 m and 3,056 m (9,504-10,026 ft) 

is a monolithologic interval directly below poorly consolidated 

limestone conglomerate (2,753-2,897 m [9,032-9,504 ft]). 

Contamination in the trachyte interval from the conglomerate is 

estimated to be as much as 50 percent. Selected cuttings from 

the trachyte interval yielded two anomalously young K-Ar dates 

(Eberly and Stanley, 1978); these anomalously young dates are 

presumed to have been due to contamination.

In contrast, contamination was not a significant problem 

in the monolithologic silicic tuff interval dated in this study 

(2,584-2,753 m [8,478-9,032 ft]) (table 2). It was found 

to contain only an estimated 10 percent contaminant chips 

consisting of mudstone, volcanic lithics, and rounded quartz 

grains. Contamination in this interval is minimal because the 

overlying lower basin-fill units and Pantano Formation are 

relatively well consolidated. In addition, the second casing 

was set at 912 m (2,992 ft), just below the base of the loosely 

consolidated upper basin-fill units (figs. 4 and 5), which effectively eliminated contamination from these materials. Biotite 

was the mineral dated in the silicic tuff interval and, although 

biotite is present in the uphole sedimentary rock cuttings 

above about 2,200 m (7,200 ft), the dated sample consisted 

only of euhedral biotite books with adhering tuff.

Thickness estimates for fault zones and dikes interpreted 

from the geophysical logs are maximum values because the 

attitudes of the faults and dikes relative to the logging tools 

and well bore are not known. In general, the steeper the attitude, the thicker the feature will appear to be. Down section 

from the upper basin-fill units (which presumably are flat 

lying), thickness estimates of bedding and of stratigraphic 

units also are maximum values if the units have been tilted.

Stratigraphic Nomenclature of Basin-Fill Units

The most comprehensive record of the stratigraphy of the 

Tucson Basin is provided by the Exxon State (32)-1 well (fig. 

4). The record of the upper Cenozoic sedimentary basin-fill 

units is particularly good because the well was drilled near the 

deep part of one of the subbasins as indicated by the residualgravity anomaly map (fig. 3). This location provides a thick, 

relatively complete sedimentary section having few hiatuses 

or complicating influxes of locally derived sediment. In the 

upper 1,880 m (6,170 ft) of sediments in the Tucson Basin, 

four units are here recognized and informally designated as 

Units A and B of the upper basin fill and Units C and D of the 

lower basin fill.

The separation into upper and lower basin fill is based 

on age, degree of consolidation, and amount of deformation 

(figs. 4, 5; table 3). Where suitable materials are available for 

dating elsewhere in the southern Basin and Range Province of 

southeastern Arizona and southwestern New Mexico, upper 

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Figure 3. Simplified residual gravity anomalies in the Tucson Basin 

(modified from Davis, 1971). The schematic gravity contours correlate 

with depth of basin fill. Note that, although the axis of the basin is 

generally north trending, the locations of the subbasins along the axis 

correspond to the intersection of the basin axis with west-southwest 

projections of the synforms of the Catalina core complex. The antiforms, or mullions, correspond to ridges separating the subbasins.
9

Figure 4. Stratigraphic column for the Exxon State (32)-1 well. Dashed lines under clast-lithology 

heading indicate intervals where the lithology is rare. Queried radiometric dates are from Eberly and 

Stanley (1978); the 26.91 Ma age date on the ash-flow tuff was obtained in this study. The stratigraphic correlation of Eberley and Stanley (1978) for southwestern Arizona is shown at the left. In 

the Exxon well, they correlated Unit II with basin fill, middle Unit I with middle Tertiary volcanic and 

sedimentary rocks, and lower Unit I with the Pantano Formation.

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Stratigraphy of the Exxon (32)-1 well
10 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

basin-fill deposits generally are found to be Pleistocene and 

Pliocene in age, poorly to moderately consolidated, flat lying 

or nearly so, and broken by only a few faults, most of which 

have relatively small displacement. Lower basin-fill deposits generally are late to middle Miocene in age, moderately 

consolidated, slightly to moderately deformed with dips of as 

much as 15o (higher adjacent to faults), and broken by numerous faults having small displacement and by some faults with 

very large displacement. Examples of paired upper and lower 

basin-fill units in nearby basins of Arizona and New Mexico 

are given in table 3.

The distribution of facies of both upper and lower basinfill sedimentary rocks in the region indicates that they were 

deposited in basins having more or less their modern configurations. Clasts in upper basin-fill conglomeratic beds 

were derived from adjacent ranges, whereas lower basin-fill 

conglomeratic deposits (particularly near the base of the units) 

commonly contain lithologies that are not present in the adjacent ranges and(or) do not contain lithologies that are locally 

abundant. This is a consequence of erosional stripping and 

reflects the greater age of the lower basin fill relative to the 

upper fill. In outcrops near basin margins, the contact between 

upper and lower basin-fill deposits commonly is sharply gradational or paraconformable. 

Although Davidson (1973) named two stratigraphic units 

in the Tucson Basin (Fort Lowell Formation and Tinaja beds) 

that correlate, in part, with the upper and lower basin-fill 

deposits described above, we prefer to use the more general 

and informal terminology of upper and lower basin fill in this 

report for reasons discussed below.

Figure 5. Bulk density of the sedimentary units in the Exxon State (32)-1 well. Boxes represent average high and low bulk density within each sedimentary unit interval. Average bulk densities were 

estimated visually from the formation density logs (figs. 6-13). Volcanic units are labeled, but their bulk 

densities are not plotted because they are not age and depth dependent. .
11

Fort Lowell Formation

Davidson (1973, p. E25-E30) defined the lower to middle 

Pleistocene Fort Lowell Formation on the basis of cuttings and 

core from a well (the type section) located in the northern part 

of the Tucson Basin, just south of exposures of lower basinfill conglomerates in the Catalina Foothills. Because the well 

is so close to the contact between upper basin-fill and lower 

basin-fill rocks, and to the edge of the basin, it probably is not 

representative of basin-wide sedimentation. In fact, Davidson 

stated (1973, p. E27) that the base of the formation is difficult 

or impossible to identify in the subsurface data of wells farther 

out in the basin.

Davidson (1973, plate 2, cross section E-E’) estimated 

the thickness of the Fort Lowell Formation to be 90 to 120 m 

(about 300 to 400 ft) and showed the base of the formation 

at a depth of 107 m (350 ft) in a well near Exxon State (32)-

1; later, Anderson (1987, plate 1, cross section F-F’) showed 

the basal contact of the Fort Lowell at the same depth (107 m 

[350 ft]) in the Exxon well. However, study of the cuttings 

and geophysical logs of the Exxon well does not indicate any 

significant change in the sedimentary rocks near the depth of 

107 m (350 ft) or, in fact, at any depth above a major facies 

change at 341 m (1,120 ft) in stratigraphic Unit A (figs. 4, 

6). That facies change, which marks the base of the Cienega 

Creek alluvial-fan deposit (see sections on “Geologic Setting” above and “Stratigraphic Unit A” below), may prove to 

be recognizable in well data throughout much of the basin. 

If so, the Cienega Creek alluvial-fan deposit will be a more 

useful and identifiable stratigraphic unit than the Fort Lowell 

Formation.

Tinaja Beds

Davidson (1973, p. E20-E25) applied the informal name 

Tinaja beds (separated into upper and lower) to basin-fill 

sedimentary rocks in the Tucson Basin underlying the Pleistocene(?) Fort Lowell Formation and overlying the 24.9-

Ma Pantano Formation (Shafiqullah and others, 1978). In 

Davidson’s usage, the Tinaja beds span all of Pliocene and 

Miocene time. We consider this to be too long a time period 

Table 3. Names and ages (where known) of paired upper and lower basin-fill units in basins of 

southeastern Arizona and southwestern New Mexico.

Stratigraphy of the Exxon (32)-1 well
12 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

Figure 6. Geophysical logs of Unit A, Upper basin-fill deposits, in 

the Exxon State (32)-1 well.

for the term to be very useful as a stratigraphic designation. 

In particular, Davidson included both upper and lower basinfill units in the upper Tinaja beds and thus made a single 

unit of sedimentary rock packages that are separated by a 

hiatus and were deposited under different tectonic conditions 

(table 3). Anderson (1987, p. 10-12) modified Davidson’s 

terminology by separating the Tinaja beds into three subunits 

(upper, middle, and lower) and correlated the middle Tinaja 

with the Nogales Formation and with the lower basin fill of 

Menges and McFadden (1981) as shown in table 3. However, Anderson (1987) assigned gypsiferous clayey silt in the 

central part of the Tucson Basin to the middle Tinaja beds. 

In our interpretation of the stratigraphy of the Exxon well, 

the gypsiferous clayey silt is not lower basin fill; it is a playa 

facies at the top of Unit B of the upper basin fill. Thus, in 

Davidson’s usage the Tinaja beds span too long a period of 

time, and in Anderson’s usage the part of the unit correlated 

with lower basin fill (middle Tinaja beds) was defined as 

containing sediments that, on close inspection, turn out to be 

upper basin fill.

Perhaps the chief problem with use of the term Tinaja 

beds is the age of the unit at its type locality. Cooper (1973) 

originally used the name “formation of Tinaja Peak” for 

interbedded andesitic to dacitic volcanic and volcaniclastic 

rocks of a small eruptive center on the southeast side of the 

Sierrita Mountains. On the basis of Cooper’s early unpublished mapping, Davidson apparently thought the volcaniclastic conglomerate was younger than the Pantano and could 

be traced northward into the subsurface of the Tucson Basin. 

However, Cooper’s published map (1973) gives an age of 

23.7 Ma for andesite at the base of the formation of Tinaja 

Peak, and we have obtained an 40Ar/39Ar age of 24.70+0.19 

Ma on sanidine crystals from an ash-fall tuff bed in the thick 

tuffaceous conglomerate unit at the top of the formation of 

Tinaja Peak (table 2; fig. 15 ). These dates demonstrate that 

the overlying conglomerate is similar in age to the volcanic 

rocks of the eruptive center (late Oligocene to early Miocene) and that both are about the same age as the Pantano 

Formation (24.9 Ma). Therefore, it is inappropriate to apply 

the term Tinaja beds to basin-fill units that probably are no 

older than 17 Ma.

DESCRIPTIONS OF STRATIGRAPHIC 

UNITS

Upper Basin-Fill Deposits, 0-908 m

 (0-2,980 ft)

The sedimentary rocks here termed upper basin fill 

in Exxon State (32)-1 can be separated into two units that 

reflect, from younger to older, (A) medial alluvial-fan, distal 

alluvial-fan, and alluvial-plain facies, and (B) playa, playa 

margin, and distal alluvial-fan facies (figs. 4, 6, 7). We des-
13

Figure 7. Geophysical logs of Unit B, Upper basin-fill deposits, in the Exxon State (32)-1 well.

Descriptions of Stratigraphic units 
14 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

Figure 8. Geophysical logs of Unit C, Lower basin-fill deposits, in the Exxon State (32)-1 well.
15

ignate these as units A and B to avoid confusion with the 

numerical terminology used by Pashley (1966) and Eberly 

and Stanley (1978) to designate stratigraphic units in the 

basin. Similarly, we subdivide the lower basin-fill sequence 

into units C and D (figs. 8 and 9).

Stratigraphic Unit A, 0-566 m (0-1,856 ft)—

Lower Pleistocene(?) and Upper Pliocene(?)

There are no cuttings for the upper 70 m (230 ft) of Exxon 

State (32)-1, and no geophysical logs were run in the upper 61 

m (200 ft). However, drill cuttings and logs of nearby water 

wells provide adequate coverage of this interval. In addition, the Quaternary and upper Tertiary geologic map of the 

Tucson 1° x 2° quadrangle (Pearthree and others, 1988) shows 

that basin-fill deposits are exposed at the well site. Satellite 

images (fig. 2) indicate that these deposits are part of a slightly 

dissected low-gradient alluvial fan derived from eastern source 

areas in Davidson Canyon, Cienega Gap, and the Cienega 

Creek drainage basins (see section on “Geologic Setting” 

above). The fan extended from the mouth of Cienega Creek 

westward 25 km to the Santa Cruz River and northward 20 

km to the present outskirts of Tucson. The location of Exxon 

State (32)-1 near the middle of the fan surface suggests that 

alluvial-fan deposits are likely to be encountered in the well, 

and are likely to be of medial alluvial-fan facies. The age of 

the uppermost basin-fill deposits at the well site probably is 

early Pleistocene or late Pliocene (Pearthree and others, 1988).

The upper half of Unit A consists of these fan deposits. 

Unit A is a fluvial unit more than 566 m (1,856 ft) thick that 

represents (1) medial alluvial-fan facies (0-341 m [0-1,120 ft]) 

overlying and grading to (2) distal alluvial-fan and alluvialplain facies (341-566 m [1,120-1,856 ft]). Unit A consists 

chiefly of interbedded sandy conglomerate, pebbly sandstone, 

sandy siltstone, and unconsolidated calcareous sandy mud (fig. 

4).

The cuttings indicate that conglomerate clasts in the upper 

part of Unit A (70-341 m [230-1,120 ft]) consist of limestone, 

quartz, and both unaltered and chloritized granitoid lithics. 

Absence of gneissic granitoid chips in the cuttings indicates 

that the source area did not include the mylonitic granitoid terrane of the Rincon and Santa Catalina Mountains in the foot 

wall of the Catalina detachment fault. Chips of quartzite and 

intermediate composition volcanics are also present, but only 

down to 165 m (540 ft). The cuttings contain fragments of 

slightly indurated, calcareous micaceous siltstone and sandstone that presumably are interbedded with the conglomerate. 

Sand-size mineral fragments in the cuttings include quartz, 

muscovite, chlorite, epidote, and magnetite. The average dry 

color of the cuttings is pale yellowish-brown (10 YR 6/2).

Figure 5 and the density log (fig. 6) show that the bulk 

density of sediment in the upper part of Unit A varies considerably. The maximum range is from 1.85 to 2.45 g/cm3

, 

and the average is about 2.1 g/cm3. The higher density peaks 

probably correspond to calcite-cemented gravel beds as much 

as 1.5 m (5 ft) thick; the areas of intermediate density probFigure 9. Geophysical logs of Unit D, Lower basin-fill 

deposits, in the Exxon State (32)-1 well.

Descriptions of Stratigraphic units 
16 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

ably correspond to slightly indurated, calcareous micaceous 

siltstone and sandstone in beds 1.5-3 m (5-10 ft) thick interbedded with the gravel; and the low density areas correspond 

to unconsolidated, very calcareous sandy mud. The sonic log, 

induction electrical log, gamma ray log, and caliper log (fig. 

6) also demonstrate the relatively thin bedded, chiefly poorly 

consolidated, but highly variable nature of the upper part of 

Unit A.

The contact of the upper and lower parts of Unit A near 

341 m (1,120 ft) is gradational and coincides with a downward 

decrease in thin, relatively dense, moderately well cemented 

conglomerate beds accompanied by an increase in sandy lime 

mud. This change in lithology results in an overall decrease 

in the variability of the bulk density for the lower part of Unit 

A as shown on figures 5 and 6. The contact between the upper 

and lower parts of Unit A is further marked by the absence 

of limestone lithic chips below about 317 m (1,040 ft), and a 

color change from pale yellowish-brown (10 YR 6/2) to light 

brown (5 YR 6/4) at 341 m (1,120 ft). The contact was chosen 

as 341 m (1,120 ft) because the color change of the cuttings is 

fairly abrupt at this depth. The next color change is at 503 m 

(1,650 ft) from light brown to pale red (5 R 6/2), following a 

trend to more reddish color with increasing age that is typical 

of upper Cenozoic continental sediments in the southern Basin 

and Range Province.

The color change at 341 m (1,120 ft) and the influx of 

limestone clasts into the basin fill at 317 m (1,040 ft) probably 

record the beginning of deposition of the alluvial fan that now 

occupies much of the surface of the Tucson Basin. The fan 

head is at Cienega Gap (fig. 1), and the first Paleozoic limestone clasts to be deposited may have come from exposures in 

the hanging wall of the Catalina detachment fault on the north 

side of Cienega Gap. With continued erosion, limestone clasts 

could have been transported northward by Cienega Creek and 

Davidson Creek from the Whetstone and Empire Mountains. 

The influx of quartzite and volcanic clasts at 165 m (540 ft) 

suggests that Upper Cretaceous sedimentary and volcanic 

rocks were being eroded from Davidson Canyon and that 

Bisbee Group conglomerates containing quartzite clasts were 

being eroded from both Davidson Canyon and Cienega Creek 

Valley.

Field relations indicate that Cienega Gap, Davidson 

Canyon, and Cienega Creek Valley were sediment-filled 

basins when deposition of the alluvial fan began. Nearly all 

of Cienega Gap and Davidson Canyon have been exhumed, 

and basin-fill alluvium has been eroded from the lower part 

of Cienega Creek Valley. The event, whether climatic or tectonic, that triggered the erosion of basin fill from these three 

areas has not been identified and is the subject of ongoing 

studies.

The lower part of Unit A extends from 341 to 565 m 

(1,120 ft to 1,856 ft) and has a thickness of 224 m (736 ft). 

It consists chiefly of light brown (5 YR 6/4) unconsolidated, 

sandy lime mud and somewhat better indurated, very finegrained micaceous sandstone. Soft, light greenish-gray (5 

GY 8/1) lime mudstone is present as a minor constituent. The 

only lithic fragments present are granitoid lithics and quartz; 

the only minerals are quartz, feldspar, muscovite, biotite, and 

chlorite. The granitoid lithics are actually abraded quartz fragments that contain bits of mica or other small mafic mineral 

inclusions, implying that the source area for the granitoid 

clasts was either remote from the depositional area or was a 

mature weathered terrane. Clusters of 1-mm-long calcite crystals are common. The caliper log of figure 6 shows numerous 

washouts and the sediment is termed “very soft gummy” on 

the mud log.

Two intervals near the top of the lower part of Unit A 

(350-358 m and 392-411 m [1,150-1,175 ft and 1,285-1,350 

ft]) show significant increases in radiation on the gamma 

ray log, from a background 80 to 100 API units for sediment 

above and below the intervals to as much as 160 API units 

for sediment within the intervals. The increased radiation 

can be attributed to volcanic ash beds (tephra) deposited with 

the sediment (fig. 6). The shape of the curves on the gamma 

ray log suggests that a few individual beds of relatively 

clean ash as much as 1.5 m (5 ft) thick occur within zones 

of reworked ash and sediment. White grains consisting of 

aggregates of clay and glass shards are present in cuttings 

from this interval, lending credence to our interpretation 

of the gamma ray log. The presence of interbedded tephra 

indicates a low-energy depositional environment and supports our interpretation that the lower part of Unit A consists 

of sediments of distal alluvial-fan and alluvial-plain facies. 

Davidson (1973, p. E23) reported a 1-m-thick (3-ft) bed of 

silty tuff that was cored in a well located about 4 km (2.5 

mi) northwest of the Exxon well. The elevation of the cored 

tuff is 361 m (1,185 ft), 104 m and 165 m (340 ft and 540 ft) 

lower than the two tuffaceous intervals in the Exxon well. 

This relationship suggests that the tephra beds are probably 

discontinuous and lenticular.

The contact of the lower part of Unit A with Unit B is 

sharp and is evidenced by a downward increase in bulk density and decreases in interval transist time and in conductivity (figs. 5, 6, 7). The higher bulk density and lower interval 

transit time of Unit B are important because they indicate that 

a significant amount of time must have elapsed between the 

deposition of Units B and A, long enough for diagenesis and 

compaction to increase the bulk density from 2.05-2.28 g/cm3

to 2.15-2.30 g/cm3.

The reason for the difference in conductivity of the two 

units is more difficult to interpret, but may be related to the 

presence of saline connate water and to variation in porosity associated with diagenesis. Figures 6 and 7 show that the 

conductivity of the alluvial-plain sediments in the lower part 

of Unit A increases from about 120 millimhos/m to 350 millimhos/m in the 60-m-thick (200-ft) interval above the contact 

with the gypsiferous playa deposit of Unit B. At the contact, 

the conductivity decreases abruptly to about 160 millimhos/

m, then gradually increases downhole throughout the playa 

deposit to about 400 millimhos/m in the underlying playamargin deposit. The conductivity again begins to decrease 

near the bottom of the playa-margin deposit and is about 100-
17

200 millimhos/m in the underlying distal alluvial-fan deposit. 

We suggest that the observed increases in conductivity in the 

alluvial-plain and playa-margin deposits are caused by the 

presence of connate saline water associated with the playa, 

which has had limited circulation through the more permeable 

sediments above and below the playa deposits.

Stratigraphic Unit B, 566-908 m (1,856-2,980 ft)—

Lower(?) Pliocene to Upper Miocene(?)

Unit B extends from 566 to 908 m (1,856 to 2,980 ft) 

and has a thickness of 343 m (1,124 ft). The unit can be 

divided into three parts (figs. 4, 7): (1) gypsiferous sandy 

lime mud between 566 and 686 m (1,856-2,250 ft), (2) 

sandy lime mud with sparse gypsum from 686 to 805 m 

(2,250-2,640 ft), and (3) from 805 to 908 m (2,640-2,980 ft), 

sandy lime mud and poorly indurated muddy sandstone with 

minor pebble conglomerate beds. The three-part sequence 

is inferred to represent a 120-m-thick (394-ft) gypsiferous 

muddy playa deposit overlying 119 m (390 ft) of playamargin sediments that in turn overlie 104 m (340 ft) of distal 

alluvial-fan sediments.

The logs and cuttings indicate that the playa and playamargin deposits constitute fairly uniform, poorly consolidated sediment sequences containing lime mud and very 

fine- to medium-grained sand (figs. 4, 7). The distal alluvialfan deposits consist of sandy lime mud, increasing amounts 

(downward) of sandstone and slightly indurated pale-red 

calcareous micaceous siltstone, and a pebble conglomerate 

interval from about 841 to 856 m (2,760-2,810 ft). As in the 

fine-grained sediments in the lower part of Unit A, the only 

lithic fragments in Unit B are quartz and abraded granitoid 

lithics, indicative of a distant or deeply weathered source terrane.

We were not able to place the contact between Unit 

B (upper basin fill) and Unit C (lower basin fill) precisely 

because the bottom of the second casing was set at 912 m 

(2,992 ft), very close to the contact, and because geophysical logs were not run between 899 and 912 m (2,950 and 

2,992 ft). Based on cuttings from this interval, we place the 

contact at 908 m (2,980 ft), which corresponds to the base 

of the muddy gypsiferous sediment of Unit B. Overall, Unit 

C of the lower basin fill is slightly coarser grained than Unit 

B of the upper basin fill and is better consolidated and less 

calcareous. The better consolidation is shown by comparison 

of the bulk density on figure 5 and on the density logs (figs. 

7, 8). The bulk density of the lower 213 m (700 ft) of Unit 

B is 2.10 to 2.25 g/cm3, whereas the bulk density of lower 

basin-fill sediments (Units C and D) is significantly higher, 

typically 2.25 to 2.45 g/cm3. The difference in consolidation 

apparently was immediately obvious to the drillers because 

they set the bottom of the casing about 3 m (10 ft) below the 

contact of the two units, thus sealing off the less consolidated 

upper basin-fill rocks so that they would not be continually 

washing out as drilling progressed.

Lower Basin-Fill Deposits, 908-1,881 m (2,980-

6,170 ft)

 The lower basin-fill deposits, in addition to being 

better consolidated than the upper basin fill, are more 

deformed, as indicated by numerous faults identified on the 

geophysical logs, particularly on the density and caliper logs 

(figs. 7, 8). The lower basin fill consists of two members, both 

fluvial, designated Units C and D.

Stratigraphic Unit C, 908-1,170 m (2,980-3,840 

ft)—Upper(?) Miocene(?)

Unit C is a relatively fine-grained fluvial deposit 

about 262 m (860 ft) thick that consists of the following downhole sediment sequence: (1) interbedded sandstone and siltstone 86 m thick from 908 to 994 m (280 ft thick from 2,980 

to 3,260 ft), (2) mostly sandstone with minor siltstone 146 m 

thick from 994 to 1,140 m (480 ft thick from 3,260 to 3,740 

ft), (3) interbedded sandstone and siltstone 12 m thick from 

1,140 to 1,152 m (40 ft thick from 3,740 to 3,780 ft), and (4) 

conglomerate 18 m thick from 1,152 to 1,170 m (60 ft thick 

from 3,780 to 3,840 ft).

Cuttings from the two sandstone and siltstone intervals 

contain abundant chips of pale red (5 R 6/2) to grayish-orangepink (5 YR 7/2) moderately well indurated, slightly calcareous, 

micaceous sandy siltstone and very calcareous, micaceous 

muddy sandstone. The sandy siltstone lithology is the same as 

in Unit A except that it is better consolidated and less calcareous. The cuttings consist of abundant fragments of granitoid 

and quartz clasts, and rare fragments of volcanic and limestone 

clasts. The mineral grains are quartz, feldspar, muscovite, biotite, chlorite, and magnetite. Some of the granitoid lithics are 

rounded quartz with included micas or mafic minerals, as in 

Units A and B, implying a source area that was distant or was a 

weathered terrane of low relief. However, the granitoid lithics 

also include subangular polycrystalline fragments composed of 

quartz, feldspar, micas, and mafics, implying a closer source. 

Rare malachite crusts and pyrite crystals suggest the presence 

of mineralized rocks in the source area. Small amounts of 

gypsum and anhydrite are present throughout Unit C.

The depositional environment of Unit C is interpreted for 

the most part to be a sandy braidplain. This is indicated by the 

relatively good separation of sandstone and siltstone into well 

defined interbeds rather than the muddy, poorly sorted mix of 

the alluvial-plain and playa facies of Units A and B and by the 

occurrence of only minor amounts of gypsum and anhydrite. 

The presence of a conglomerate zone 18 m (60 ft) thick at the 

base of Unit C suggests that a significant unconformity exists 

between Unit C and the underlying Unit D. Lithic fragments 

and mineral grains in the conglomerate are the same as elsewhere in Unit C, except for uncommon fragments of quartzite 

that resemble Precambrian or Paleozoic lithologies of the 

region.

Descriptions of Stratigraphic units 
18 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

Faults are numerous throughout Unit C, as shown by 

thin, sharply defined intervals of decreased bulk density on 

the density log of figure 8. Ten individual faults and zones of 

faulting are recognized, in contrast to Units A and B, which 

show no identifiable evidence of faulting. The density log 

indicates that the intervals of crushed rock marking individual 

faults range from 0.6 m (2 ft) wide to as much as 2.5 m (8 ft) 

wide and that zones of faulting are as wide as 8.5 m (28 ft). It 

is important to realize that these widths of crushed rock are 

maximum values and may be more a function of the angle of 

the fault than of the amount of deformation associated with it. 

In outcrop most faults observed in lower basin-fill rocks are 

high-angle faults, which means that a vertical drill hole will 

penetrate a considerable thickness of brecciated rock bordering the fault plane itself.

The caliper and sonic logs show that most faults correspond to washed-out intervals and increased interval transit 

time, but a few do not show these characteritics, suggesting 

that some fault breccias might be recemented. The gamma ray 

log shows no pronounced increases in gamma radiation corresponding to faults, suggesting that the brecciated fault gouge 

contains little or no clay.

The thickness of Unit C penetrated in the well (262 m 

[860 ft]) may not be an accurate measure of its true thickness 

for several reasons: (1) zones of faulting at both the top and 

bottom of the unit could have cut out some of the section; 

(2) faults within the unit may have repeated or cut out part of 

the section; and (3) outcrops of faulted lower basin-fill units 

are commonly tilted 10° to 15°; such dips would increase the 

apparent thickness in a vertical drill hole.

Stratigraphic Unit D, 1,170-1,880 m (3,840-6,170 

ft) — Upper(?) and Middle Miocene

Unit D of the lower basin fill is 710 m thick (2,330 ft), 

pale red in color (5 R 6/2), and consists of calcareous mud, 

siltstone, sandstone, and conglomerate that vary from poorly 

to moderately indurated over short intervals. The depositional setting is inferred to be medial to distal alluvial fan, 

similar to the upper part of Unit A. Lithics in the cuttings 

consist of abundant granite and quartz and rare quartzite, 

limestone, and volcanics. Granitoid lithics are of two types—

(1) abraded quartz fragments containing bits of mica and 

mafic inclusions and (2) angular polycrystalline fragments of 

quartz and feldspar with micas and mafic minerals—indicating both distant and nearby sources for the granitoid clasts. 

The mineral grains are quartz, feldspar, muscovite, biotite, 

and chlorite. Chips of pale red (5 R 6/2) well indurated, noncalcareous, micaceous sandy siltstone and calcareous, micaceous muddy sandstone are abundant. Gypsum is present, but 

uncommon, between 1,170 and 1,344 m (3,840 and 4,410 ft).

The geophysical logs (fig. 9) indicate that the sequence 

can be separated into three parts on the basis of degree of consolidation. The upper 271 m (890 ft) of Unit D, from 1,170 to 

1,441 m (3,840 to 4,730 ft), and the lower 128 m (420 ft) from 

1,753 to 1,880 m (from 5,750 to 6,170 ft), show extreme variations in density, interval transit time, and resistivity, and the 

caliper log shows that the well diameter was greatly enlarged 

by washouts in these two intervals. The highly variable degree 

of consolidation is probably caused by the presence of numerous thin beds of unconsolidated calcareous mud interbedded 

throughout the more typical, moderately well consolidated 

siltstone, sandstone, and conglomerate beds.

The drill cuttings and the mud log show that the 311-

m-thick (1,020-ft) middle interval (1,442 to 1,753 m [4,730-

5,750 ft]) contains siltstone, sandstone, and conglomerate 

similar to the upper and lower intervals, but contains little 

unconsolidated mud. The paucity of unconsolidated mud is 

evidenced on the geophysical logs (fig. 9) which show that 

the middle interval has a much more uniform bulk density 

(2.3-2.4 g/cm3), interval transit time (85-100 microseconds/

ft), and resistivity (3-5 ohm-m), and, thus, gives a more realistic indication of the typical degree of consolidation to be 

expected of lower basin-fill deposits at this depth in the basin.

The geophysical logs (fig. 9) show that Unit D is highly 

faulted, perhaps more so than Unit C. There are four zones 

of faulting and associated brecciation from 15 m (50 ft) to 

more than 30 m (100 ft) wide, but individual faults are more 

difficult to identify because of the variable consolidation of 

Unit D. As with Unit C, the faulting and probable tilting 

(by analogy to similar age units in outcrop) of Unit D cause 

the thickness of the unit penetrated in the well to be only an 

approximation of its true thickness.

The contact between Unit D and the upper conglomerate 

of the underlying Pantano Formation is sharp, and on the geophysical logs (figs. 9, 10) is characterized by several distinct 

signatures: (1) an increase in bulk density from about 2.35 to 

2.55 g/cm3 (fig. 5); (2) an increase in resistivity from about 2 

to 10 ohm-m; and (3) a decrease in interval transit time from 

90 to 80 microseconds/ft. There is, however, only a small 

change in gamma radiation, which remains at about 100 API 

units. The washouts and muddy sediment typical of Unit D of 

the lower basin fill cease abruptly at 1,880 m (6,170 ft). The 

presence of abundant volcanic fragments in the Pantano Formation (fig. 4) further serves to differentiate the Pantano from 

the overlying lower basin-fill, which contains only rare volcanic fragments. The sharpness of the contact, the markedly 

dissimilar characteristics on the geophysical logs of Unit D 

and the upper Pantano Formation, and the absence on the logs 

of sharp peaks that might indicate a fault at the contact, all 

suggest that the contact is probably an erosional unconformity 

representing a considerable hiatus.

Pantano Formation, 1,880 -2,516 m 

(6,170-8,256 ft)—Lower Miocene and 

Upper Oligocene(?)

In roadcuts, clay quarries, and outcrops in Cienega 

Gap (fig. 1)—the saddle between the Rincon and Santa Rita 
19

Mountains about 17 to 25 km (10 to 15 mi) east of the site 

of the Exxon State (32)-1 well—the Pantano Formation is 

well exposed and has been described by Brennan (1957), 

Finnell (1970b), and Balcer (1984). An andesite flow near the 

middle of the Pantano, exposed where the Southern Pacific 

Railroad bridge and Pantano Road cross Cienega Creek, has 

been dated at 24.93+2.6 Ma (K-Ar on plagioclase; Shafiqullah and others, 1978). Because the andesite flow has been 

pervasively chloritized and is cut by numerous jasperoid 

veins, this date probably should be used with caution. Furthermore, potassium metasomatism associated with the 

uplift of the Catalina core complex may have affected rocks 

in the Cienega Gap area, which lies in the hanging wall of 

the Catalina detachment fault just south of the trace of the 

fault. Thus, it may be more realistic to bracket the age of the 

Pantano Formation between about 17 Ma (the earliest date 

for beginning of basin formation) and somewhat less than 

26.91+0.18 Ma, which is the date obtained in this study for 

the silicic tuff 68 m (222 ft) downsection from the base of the 

Pantano (see section on “Silicic Tuff” below).

The Pantano Formation in Exxon State (32)-1 is broadly 

similar to the formation in surface exposures, in that both 

(as inferred by our correlation) are syntectonic deposits of 

alluvial fans, playas, rock avalanches, and volcanic flows in 

a region of middle Tertiary extension. However, a significant 

difference that may reflect the local tectonic setting of the 

two sites at the time of deposition is that in the Exxon well, 

rock-avalanche deposits occur at the bottom of the Pantano, 

whereas at Cienega Gap, there is a rock-avalanche deposit at 

the top. 

In Exxon State (32)-1, the 703-m-thick (2,308-ft) interval 

assigned to the Pantano Formation consists of two parts: (1) a 

449-m-thick (1,472-ft) well-consolidated gypsiferous, muddy 

conglomerate (1,880-2,329m [6,170-7,642 ft]) containing 

an andesite(?) flow in the lower half; and (2) a 187-m-thick 

(614-ft) sequence we infer to be a composite rock-avalanche 

deposit composed chiefly of intermediate-composition volcanic rocks (2,329-2,516 m [7,642-8,256 ft]). The overall color 

of the cuttings darkens gradually from pale red (5 R 6/2) at 

1,880 m (6,170 ft) to grayish-red (5 R 4/2) by 2,225 m (7,300 

ft).

Conglomerate, Mudstone, and Andesite (?) Flow, 

1,880-2,329 m (6,170-7,642 ft)

The geophysical logs (fig. 10) show the Pantano conglomerate and mudstone to be uniformly well consolidated 

except for a few broad zones of minor washouts and a poorly 

consolidated interval between 2,133 and 2,194 m (7,000-7,197 

ft), which contains gypsum and a 3.7-m-thick (12-ft) bed of 

anhydrite. Few faults are evident on the logs for this part of 

the Pantano compared to the number interpreted for the overlying lower basin fill. A possible explanation for this may be 

that fault breccia and gouge has been recemented in the better 

consolidated Pantano conglomerate so the faults are not obviFigure 10. Geophysical logs of the Pantano Formation, in 

the Exxon State (32)-1 well. Asterisk indicates location of 

interval of sampled cuttings that gave a whole-rock K-Ar 

age of 23.4+0.6 Ma (Eberly and Stanley, 1978).

Descriptions of Stratigraphic units 
20 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

ous on the logs. The geophysical logs and the composition of 

lithics and mineral grains in the cuttings allow the conglomerate, mudstone, and andesite of the Pantano Formation to be 

described in four parts:

(1) 1,880 to about 2,133 m (6,170-7,000 ft).—The conglomerate in this interval is inferred to be a medial alluvial-fan 

deposit. It contains abundant volcanic clasts and common 

granitoid clasts interbedded with well-consolidated pale-red 

(5 R 6/2) slightly calcareous to noncalcareous, slightly gypsiferous, micaceous shale and muddy calcareous micaceous 

sandstone. The bulk density ranges from 2.35 to 2.65 g/cm3, 

with an average of about 2.50 g/cm3 (figs. 5, 10). Lithic clasts 

consist of angular quartz fragments, quartz with mica and 

mafic inclusions, granitoid chips of various kinds (unaltered; 

with red feldspar; pink chloritized; epidotized), abundant gray 

volcanic chips of probable intermediate composition, and 

rare limestone chips. Mineral grains in the cuttings consist of 

muscovite, biotite, chlorite, epidote, and two types of quartz 

(rounded with frosted or polished surfaces and angular).

(2) About 2,133 to 2,194 m (7,000-7,197 ft).— This is 

a poorly consolidated muddy evaporite interval that contains 

a 3.7-m-thick (12-ft) anhydrite bed between 2,163 and 2,167 

m (7,098-7,110 ft). The depositional environment may have 

been a transgressive/regressive sequence of distal alluvial fan, 

playa margin, and short-lived lake. Cuttings in the interval are 

coated with mud, and the caliper log (fig. 10) shows that the 

diameter of the well was washed out from 28 cm to as much as 

38 cm (from 11 to 15 in). The bulk density is variable, ranging from 2.25 to 2.60 g/cm3

 (the bulk density of the anhydrite 

bed is 2.90 g/cm3). Lithic and mineral fragments in the cuttings are the same composition as in the overlying better consolidated conglomerate. Fragments of interbedded shale and 

sandstone include greenish-gray (5 GY 6/1) to light-olive-gray 

(5 Y 6/1) calcareous micaceous shale in addition to pale-red 

shale and sandstone.

The cuttings, density log, and sonic log indicate that the 

grain size, bedding, and consolidation of the sediment vary 

considerably over short distances in the evaporite interval. 

The electrical log, however, shows that the resistivity of the 

part of the interval between 2,158 and 2,188 m (7,080-7,180 

ft) is fairly constant at about 3 ohm-m (except for the anhydrite bed, where it is 100 ohm-m). This may indicate that the 

pore space of both fine-grained and coarse-grained lithologies 

in this interval is filled with saline connate water.

(3) 2,194 to 2,222 m (7,197-7,290 ft).— This interval 

contains the densest rock encountered to this level in the well, 

nearly 2.70 g/cm3, and is inferred to be a lava flow of intermediate composition. The geophysical logs (fig. 10) show that 

rock properties in the interval (bulk density, interval transit 

time, and resistivity) are similar to those of the intermediate 

volcanic rock in the inferred rock-avalanche deposit below 

(2,329-2,516 m [7,642-8,256 ft]) and of the intermediate-composition volcanic flow between 2,524 and 2,553 m (8,282-

8,376 ft). However, cuttings from this interval indicate that it 

should be a conglomerate of virtually the same composition as 

the overlying intervals 1 and 2. Because the geophysical logs 

are not likely to be in error, it is more likely that the cuttings 

were not collected from this 28-m-thick (93-ft) interval or that 

the cuttings contain a very large amount of contamination. 

Thus, the 28-m-thick (93-ft) interval is inferred to consist of 

one or more intermediate-composition lava flows.

The contact between the volcanic flow of interval 3 and 

the overlying evaporite interval may be a fault. This is suggested by the density and sonic logs, which both show a sharp 

change at 2,194 m (7,197 ft). There is probably also a fault 

or a flow breccia near the middle of the interval at 2,207 m 

(7,242 ft), as shown by a sharp increase in interval transit time 

and decrease in resistivity at this depth.

(4) 2,222 to 2,329 m (7,290-7,642 ft).—Conglomerate 

clasts in this interval are mostly volcanic, as in interval 1 of 

the conglomerate. However, the volcanic clasts are more 

varied in composition and many are propylitically altered. 

Granitoid clasts are sparse to rare, as are rounded detrital 

quartz grains. The interval is still slightly gypsiferous. The 

overall color of the cuttings has darkened to grayish red (10 R 

4/2). The bulk density is fairly constant at 2.50 g/cm3, about 

the same as in interval 1 but lower than in the overlying intermediate-composition lava flow (interval 3).

Rock-Avalanche Deposit, 2,329-2,516 m (7,642-

8,256 ft)

Cuttings from this 187-m-thick (614-ft) interval consist 

chiefly of intermediate-composition volcanic rocks. Propylitic 

alteration of the volcanics is uncommonly present and there 

are rare fragments of copper-bearing minerals. The color of 

the cuttings varies from grayish red (5 R 4/2) and blackish 

red (5 R 2/2), through medium dark gray (N 4), medium light 

gray (N 6), and brownish gray (5 YR 4/1). Micaceous shale, 

sandstone, gypsum, and quartz grains are present in amounts 

of 10 to 50 percent. Because the variation in quantity of these 

sediment-derived grains does not correlate with variations in 

rock properties recorded by the geophysical logs (fig. 10), the 

grains are attributed to contamination from uphole washouts. 

This implies that a similar amount of the volcanic fragments 

in the cuttings (10 to 50 percent) are contamination also, probably from volcanic clasts in conglomerates higher in the Pantano Formation.

This interval is inferred to be a rock-avalanche deposit 

probably composed of intermediate-composition volcanic 

rocks. This inferrence is based on unique patterns shown 

by the geophysical logs in this interval (fig. 10) and their 

likely correspondence with physical properties that might be 

expected of rock-avalanche bodies. Rock-avalanche bodies 

have been described by many workers ( for example, Shreve, 

1968; Kreiger, 1977; Yarnold and Lombard, 1989; Yarnold, 

1993; Beratan, 1998) and are relatively common in upper Oligocene and lower Miocene syntectonic sedimentary rocks in 

southeastern Arizona (Creasey, 1965; Kreiger, 1977). One of 

the occurrences described by Yarnold and Lombard (1989) is 

the Cross Hill rock-avalanche deposit at the top of the Pantano 
21

Formation in Cienega Gap (fig. 1). Cooper (1973) mapped 

rock-avalanche deposits (described as monolithologic breccia of landslide origin) within the Helmet Fanglomerate in the 

northeastern Sierrita Mountains. 

Rock-avalanche deposits are very large volume, tabular 

or lensoid megabreccia bodies, commonly monolithologic, 

that begin as giant rockfalls and traverse down several kilometers of relatively gentle slopes at high speed. Other than the 

opinion that water probably is not involved, there is little consensus as to the medium of support that allows for high-speed 

nonturbulent transport of the megabreccia bodies, while preserving relict stratigraphy within the megabreccia and disturbing the substrate only minimally. Various mechanisms have 

been proposed by Kent (1966), Shreve (1968), Hsü (1975), 

and Melosh (1983).

Yarnold and Lombard (1989) compiled a table of sedimentary characteristics used in distinguishing rock-avalanche 

deposits from four other types of mass-movement deposits in 

the field. Most of these field characteristics are not useful for 

this study because well data are one-dimensional. However, 

the following five characteristics of rock-avalanche megabreccia bodies are important in the context of this report: (1) they 

are tens to hundreds of meters thick; (2) they are commonly 

monolithologic and consist of identifiable lithologic units 

having relict stratigraphy preserved; (3) stratigraphic repitition 

occurs along shear planes; (4) they are made up of pervasively 

brecciated fresh rock and, for the most part, the breccia is 

dense, being composed of a tight mosaic of unrotated angular fragments (crackle breccia) or fragments separated from 

each other by thin bands of comminuted rock (jigsaw breccia) 

(Krieger, 1977); and (5) the unconfined tops of some megabreccia bodies consist of rotated clasts (Kreiger, 1977, fig. 13), 

which would change the character of the brecciation and have 

the effect of reducing the bulk density of that part of the avalanche body.

The inferred rock-avalanche deposit in Exxon State (32)-1 

appears on the geophysical logs (fig. 10) as a sequence of 13 

asymmetrical humps with an aggregate thickness of as much 

as 187 m (614 ft). However, because the Pantano Formation 

is a syntectonic deposit, it is very likely tilted and, choosing 

an abitrary dip of 30o, the actual thickness of the rock-avalanche deposit may be closer to 162 m (531 ft). The thickness 

of individual humps ranges from 10 to 23 m (32 to 76 ft) and 

averages about 15 m (50 ft). Individual humps are densest 

in their bottom two-thirds (2.65 to 2.75 g/cm3) and tail off 

upward to bulk densities of 2.25 to 2.50 g/cm3. It is possible 

that the 13 humps are the result of stratigraphic repetition of 

13 landslide megabreccia sheets, each composed of dense 

crackle and jigsaw breccia in their bottom part and grading 

upward to unconfined tops characterized by less dense breccia 

having rotated clasts. The bulk density of the denser parts of 

the humps (2.65 to 2.75 g/cm3) and the presence of abundant 

volcanic fragments in the cuttings indicates that the rock is 

probably volcanic. However, the possibility that the cuttings 

in this interval represent contamination means that other dense 

rock types cannot be ruled out. 

Repetition of stratigraphic sequences along shear planes 

in rock-avalanche deposits is common and is discussed in 

the literature (for example, Kreiger, 1977), but we found no 

descriptions similar to the aggregate of thirteen deposits that 

we infer here. It is tempting to construct hypothetical conditions which might result in stacked stratigraphic sequences, for 

example, ramping of the avalanche body over a topographic 

high. However, the available one-dimensional well-log data 

are inadequate to resolve hypothetical models.

The rock-avalanche event probably was associated with 

extension on the Catalina detachment fault (Dickinson, 1991) 

to the north and east of the present well site. However, the 

location of the volcanic rocks that were the source of the 

deposit is not known. Both middle Tertiary and Laramide-age 

intermediate-composition volcanic rocks were widely distributed in the region in late Oligocene and early Miocene time. 

The highland that supplied the volcanic rock of the avalanche 

deposit was probably located to the east along a splay of the 

Catalina detachment fault. The splay very likely was buried 

beneath younger conglomerates of the Pantano Formation that 

later filled the basin created by displacement on the fault.

One of the whole-rock K-Ar ages (23.4+0.6 Ma) reported 

by Eberly and Stanley (1978) was obtained on selected cuttings taken within the inferred rock-avalanche deposit. The 

location of the dated interval (2,420-2,426 m [7,940-7,960 

ft]) is shown on figure 10, where it appears, on the basis of 

geophysical characteristics, to be within the upper part of one 

of the megabreccia sheets. The age of the rocks in the rockavalanche deposit has no bearing on the age of the Pantano, 

except that they must be older than the part of the Pantano 

Formation in which they were emplaced. Additionally, the 

date is queried because of the presence in the rock-avalanche 

interval of a significant amount of contaminant fragments of 

Tertiary sedimentary rocks and probably, therefore, of contaminant fragments of volcanic conglomerate clasts as well.

Middle Tertiary Volcanic and Sedimentary 

Rocks, 2,516-3,056 m (8,256-10,026 ft)—Miocene(?)-Oligocene (?)

This 540-m-thick (1,770-ft) interval consists of a diverse 

group of rocks with uncertain correlation to surface units. It 

contains the following rock units (figs. 4, 11): (1) lamprophyre(?) dike or sill, (2) intermediate-composition lava flow, 

(3) conglomerate, (4) crystal-lithic ash-flow tuff, (5) limestone conglomerate, and (6) pyroxene trachyte flow. Similar 

rocks are present south of Cienega Gap where Finnell (1971) 

mapped Tertiary rhyolite tuff and andesite. He assigned a 

Paleocene(?) age to quartz latite porphyry and to dikes and 

sills of lamprophyre, andesite, and diabase. In the Exxon well 

the crystal-lithic ash-flow tuff (our unit 4) has been determined to be upper Oligocene (see discussion below), so units 

1 through 3 must be younger. However, the ages of the limestone conglomerate and pyroxene trachyte flow (units 5 and 6) 

are uncertain. They could be Oligocene(?), or Laramide-age 

Descriptions of Stratigraphic units 
22 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

(Paleocene and Upper Cretaceous), or they could be Upper 

Cretaceous. An Oligocene age is favored for the limestone 

because of the presence of distinctive micaceous shale and 

sandstone interbeds and the absence of nonmicaceous shale—

shale beds in the Cretaceous sedimentary rocks of the region 

are not micaceous. An Oligocene age also is favored for the 

pyroxene trachyte, rather than a Laramide age, because no 

propyllitic alteration was seen in the cuttings.

High-Potassium (Lamprophyre?) Dike or Sill, 

2,516-2,524 m (8,256-8,280 ft)—Early Miocene or 

Late Oligocene

The cuttings in this 8-m-thick (24-ft) interval consist of 

about 50 percent pale-red and greenish-gray shale and sandstone and 50 percent intermediate-composition volcanic fragments similar to the conglomerate in the Pantano Formation. 

The geophysical logs (fig. 11) show that the interval has a 

relatively high gamma radiation of 160 API units, and resistivity of 60 ohm-m. The bulk density and interval transit time 

are 2.55 g/cm3 and 65 microseconds/ft, respectively. Although 

the cuttings are similar to those of a conglomerate or intermediate-composition volcanic rock, both the gamma radiation 

and resistivity are higher than Pantano conglomerate above 

the interval (fig. 10) and higher than the middle Tertiary conglomerate and intermediate-composition volcanic rocks below 

the interval (fig. 11). The characteristics of this interval more 

closely resemble those of the pyroxene trachyte at the base of 

the middle Tertiary part of the section (fig. 11).

We interpret this interval to be a dike or sill of an igneous 

rock with a moderately high potassium content, such as trachyte, dacite, or lamprophyre. The interval of high resistivity 

is only about 4 m (12 ft) thick and is centered within the high 

gamma radiation interval. This geometry may indicate the 

presence of chilled margins enclosing the dike or sill. The lack 

of an obvious distinctive lithology in the cuttings may be attributed to the thinness of the interval and to uphole contamination.

Andesite(?) Lava Flow, 2,524-2,553 m (8,280-8,376 

ft)—Upper Oligocene

The rocks in this 29-m-thick (96-ft) interval are inferred 

to be intermediate-composition volcanic rocks, chiefly on the 

basis of the bulk density of about 2.70 g/cm3 (fig. 11). The 

cuttings contain mostly fragments of volcanic rocks with 

only 10 to 20 percent shale and sandstone (probably as contaminants). A 2.5-m-thick (8-ft) zone of low density, high 

interval transit time, and low resistivity at 2,533 m (8,310 ft) 

is interpreted to be a fault. The caliper log shows that the well 

diameter was enlarged considerably in this zone, which may 

indicate that the volcanic rocks are highly fractured. Alternatively, the low-density zone could be a flow breccia or a deuterically alterated interflow lens.

Figure 11. Geophysical logs of the middle Tertiary volcanic and sedimentary rocks, in the Exxon State (32)-1 well. Asterisks indicate location of intervals where cuttings were sampled for radiometric dates. 

Age of 26.91+0.18 Ma was obtained in this study by 40Ar/ 39Ar analysis 

of biotite. Ages of 16.1+0.6 Ma and 18.0+2.0 Ma are whole-rock K-Ar 

ages (Eberly and Stanley, 1978).
23

Conglomerate, 2,553-2,584 m (8,376-8,478 ft)—

Upper Oligocene

This conglomerate interval is 31 m (102 ft) thick and 

contains intermediate-composition volcanic fragments and 

more than 50 percent pale-red and greenish-gray shale and 

sandstone fragments. The shale fragments are still micaceous 

and calcareous, but are noticeably harder than the shale higher 

in the well. This is verified by comparison of the bulk density of the conglomerate overlying the rock-avalanche deposit 

in the Pantano Formation (about 2.50 g/cm3, fig. 8) with the 

bulk density of this conglomerate, which is about 2.55 to 2.60 

g/cm3 (figs. 5, 11). This higher bulk density indicates that this 

conglomerate is probably significantly older than the conglomerate in the Pantano Formation

The contact of the conglomerate with the underlying tuff 

is very sharp on the geophysical logs and could be either a 

fault or a depositional contact on an erosional surface. The 

contact probably is depositional, because the bottom 6 m (20 

ft) of the conglomerate contains 10 to 20 percent chips of the 

underlying tuff, and the gamma ray log shows that the radiation level of the conglomerate gradually increases in the lower 

9 m (30 ft) toward the contact with the tuff (fig. 11).

Silicic Tuff, 2,584-2,753 m (8,478-9,032 ft)—Upper 

Oligocene

This interval is a 169-m-thick (554-ft) crystal-lithic tuff 

containing crystals of biotite and quartz in the upper 25 m (82 

ft) and chiefly quartz and opaque oxide in the bulk of the unit. 

Biotite books (with adhering tuff) collected between 2,584 and 

2,609 m (8,478-8,560 ft) yielded an 40Ar/ 39Ar age of 26.91+0.18 

Ma (table 2, appendix ). Between 2,721 m (8,930 ft) and the 

base of the unit at 2,753 m (9,032 ft), the tuff contains andesitic 

lithic fragments in addition to crystals of quartz and opaque 

oxide. The color of the tuff varies from pale red (5 R 6/2) to 

light brownish-gray (5 YR 6/1). The basal vitrophyre (2,746-

2,753 m [9,008-9,032 ft]) is 7 m (24 ft) thick and consists of 

moderate orange-pink (10 R 7/4) waxy-appearing altered glass. 

The presence of a basal vitrophyre indicates that the tuff probably was emplaced as an ash flow rather than an ash fall.

Both the geophysical logs (fig. 11) and examination of the 

cuttings indicate that the tuff is relatively homogeneous and 

shows no sharp discontinuities, except at the top of the vitrophyre. The characteristics that define the tuff are high gamma 

radiation (as much as 240 API units) and lack of washouts. 

The caliper log indicates that the well diameter in the tuff 

interval was fairly constant at 25 to 28 cm (10-11 in). Minor 

variation in the bulk density suggests that the tuff may be a 

compound cooling unit: it is less dense below 2,682 m (8,800 

ft) (about 2.40 g/cm3), more dense above 2,682 m (8,800 ft) 

(about 2.45 g/cm3), and capped by a nonwelded zone at the top 

about 15 m (50 ft) thick. The tuff’s considerable thickness and 

our interpretation that it was emplaced as a single compound 

cooling unit suggest that the tuff was derived from a nearby 

caldera-forming eruption.

Although ash-flow tuffs this thick commonly are welded, 

no traces of fiamme were seen in the cuttings. However, the 

apparent absence of fiamme could be a function of the small 

size of the cutting fragments. The regular sinusoidal pattern 

seen on the sonic log (fig. 11) in three intervals (2,658-2,670 

m; 2,694-2,716 m; 2,723-2,737 m [8,720-8,760 ft; 8,840-8,910 

ft; 8,935-8,980 ft]) may correspond to the sonic properties of 

densely welded tuff. This pattern is also seen on the sonic log 

in the trachyte interval deeper in the well, between 2,897 and 

3,056 m (9,504 and 10,026 ft), and in the granitoid rock near 

the bottom of the well.

Limestone Conglomerate, 2,753-2,897 m (9,032-

9,504 ft)—Oligocene(?)

This interval is a 144-m-thick (472-ft) moderately well 

indurated conglomerate that is composed chiefly of clasts of 

Paleozoic(?) limestones, with subordinate clasts of reddish 

andesite or dacite (some propylitically altered), grayish-red 

quartzite, and granitoid rocks. The overall color of the cuttings is speckled pale red (10 R 6/2) and grayish red (10 R 

4/2). Fragments of pale-red (5 R 6/2) and greenish-gray 

(5 GY 6/1) micaceous, calcareous shale and sandstone are 

common. The dominance of limestone clasts is demonstrated 

by the density log (fig. 11), which shows that the bulk density 

of the conglomerate is about 2.60 to 2.75 g/cm3. Figure 5 

shows that the limestone conglomerate is denser in part than 

the underlying Lower Cretaceous Turney Ranch and Shellenberger Canyon Formations of the Bisbee Group, although 

the presence of micaceous shale in the cuttings (representing 

either matrix material or shaly interbeds) indicates that the 

limestone conglomerate is Tertiary rather than Cretaceous. 

Shale beds in the Cretaceous sedimentary rocks of the region 

are not micaceous.

The caliper and gamma ray logs suggest that the conglomerate may consist of two slightly different facies having 

a gradational contact between them at about 2,818 m (9,245 

ft). The caliper log indicates that the well diameter was considerably enlarged, from 25 cm to 36 cm (10 in to 14 in), by 

washouts in the upper facies and that the conglomerate washed 

out uniformly, not preferentially (as in shaly interbeds, for 

example). The gamma radiation of the upper facies ranges 

from about 60 to 90 API units, whereas the lower facies shows 

a more uniform gamma radiation of about 60 API units. Small 

differences in the density, sonic, and induction logs for the two 

facies show that the lower facies is slightly better indurated.

There are two well defined faults at 2,804 m (9,199 ft) 

and at 2,882 m (9,459 ft); they are characterized by sharp 

decreases in bulk density and resistivity and by increases in 

interval transit time. Although the contact of the limestone 

conglomerate with the overlying tuff is very sharp, the presence of the basal vitrophyre of the tuff suggests that it is not a 

fault contact.

Descriptions of Stratigraphic units 
24 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

Pyroxene trachyte, 2,897-3,056 m (9,504-10,026 

ft)—Oligocene(?)

This 159-m-thick (522-ft) unit is identified as a pyroxenebearing trachyte on the basis of the combination of high K2O 

content as indicated by the gamma ray log (fig. 11) and the 

presence in cuttings of pyroxene phenocrysts and large euhedral tabular feldspar crystals. Assuming the gamma ray log 

is measuring mostly K2O content, then the K2O content of the 

trachyte (140 to 160 API units) is about midway between that 

of the intermediate-composition volcanic rocks (80 to 120 API 

units) in the Pantano Formation (fig. 10), which are probably 

high-K calc-alkaline rocks (2.0 to 5.0 weight percent K2O), 

and that of the silicic tuff (200 to 240 API units) (fig. 11).

Eberly and Stanley (1978) reported two whole-rock KAr dates from the trachyte of 16.1 Ma and 18.0 Ma. Because 

both dates are younger than the 23.4 Ma age obtained by them 

from rocks higher in the well, they interpreted the younger 

dates to be from an intrusive dike or sill. This was a reasonable interpretation, considering that magmas of trachyte 

composition are highly viscous and are commonly emplaced 

as dikes, plugs, or short, thick flows. The cuttings provide no 

clues as to whether the trachyte is an intrusion or a flow, and 

evidence from the geophysical logs is equivocal. The balance 

of the evidence leans toward the trachyte being a thick extrusive flow, however, which suggests the 16.1 and 18.0 Ma dates 

reported by Eberly and Stanley (1978) are anomalously young 

(discussed below).

Evidence for a flow origin of the trachyte is shown by the 

geophysical logs (fig. 11). Zones at the top and bottom of the 

trachyte interval have lower bulk density and resistivity and 

higher interval transit time than the main body of trachyte. 

The zone at the top is about 21 m (70 ft) thick and the one at 

the bottom is about 8 m (25 ft) thick; both are much thicker 

than would be expected from chilled intrusive contacts. The 

differences in thickness of the two zones are consistent with 

the zone at the top being a subaerial, brecciated flow carapace 

and the bottom zone being an annealed basal flow breccia. A 

sharp decrease in bulk density and increase in interval travel 

time near the middle of the top zone (fig. 11) may represent a 

boundary between carapace slabs. 

Assuming the trachyte body is a flow, the K-Ar dates 

obtained by Eberly and Stanley (1978) must be in error, 

because the new 40Ar/ 39Ar age reported in this paper for biotite 

from the overlying silicic tuff (which is about 152 m (500 ft) 

higher in the well than the trachyte) is 26.91+0.18 Ma (table 

2). The most likely cause for errors in the K-Ar whole-rock 

dates is contamination of the selected cuttings by similarappearing lithologies from weathered volcanic clasts in the 

limestone conglomerate above.

Eberly and Stanley’s (1978) age of 16.1+0.6 Ma was 

obtained on selected cuttings from 2,895 to 2,898 m (9,498-

9,508 ft). However, the geophysical logs (fig. 11) show that 

the upper 2 m (6 ft) of that sampled interval are in the overlying limestone conglomerate, and inspection of the cuttings 

show that the sampled interval as a whole contains about 50 

percent limestone fragments as contamination from the limestone conglomerate. Therefore, there is a strong possibility 

that the selected cuttings may have contained fragments of 

volcanic clasts from the overlying limestone conglomerate 

unit, similar in appearance to the trachyte. The second interval 

sampled by Eberly and Stanley (1978), from 2,972 to 3,002 m 

(9,751-9,850 ft) in the central part of the trachyte, yielded an 

age of 18.0+2.0 Ma. While the cuttings in this interval are less 

contaminated than those higher in the trachyte, the contamination is still significant, and the relatively large standard deviation makes this age also suspect.

An additional indication that contamination of the selected 

cuttings by overlying clasts probably was the cause of the 

anomalously young dates for the trachyte interval is given by 

Eberley and Stanley’s (1978) description of the rock unit. They 

called it a varicolored, porphyritic andesitic basalt. Our inspection of the cuttings showed that the trachyte is distinctive, 

uniformly light gray, fine grained, and contains large tabular 

feldspar phenocrysts. Numerous varicolored volcanic fragments are also present and have been washed in from uphole.

Bisbee Group, 3,056-3,658 m (10,026-12,001 

ft)—Lower Cretaceous and Upper Jurassic

The Bisbee Group was described and named by Ransome (1904) for exposures in the Mule Mountains in Cochise 

County, Arizona (fig. 1). The formations recognized in the 

Mule Mountains (southeastern facies) are the basal Glance 

Conglomerate and overlying Morita Formation, Mural Limestone, and Cintura Formation. In the Empire and Whetstone 

Mountains (northwestern facies), 80 km (50 mi) northwest 

of the Mule Mountains and 25 to 50 km (15-30 mi) southeast 

of the well site, the Bisbee Group comprises five formations 

that are partly correlative with strata in the Mule Mountains 

as time equivalent facies (Tyrrell, 1957; Schafroth, 1965; 

Finnell, 1970a). They are the basal Glance Conglomerate and 

overlying Willow Canyon Formation, Apache Canyon Formation, Shellenberger Canyon Formation, and Turney Ranch 

Formation. The Bisbee Group ranges in age from uppermost 

Jurassic to lowermost Cretaceous for the Glance Conglomerate (Bilodeau and others, 1987) through Lower Cretaceous for 

the Turney Ranch Formation (Archibald, 1987). The Bisbee 

Group was deposited in the Bisbee Basin, which consisted 

of a series of northwest-trending en-echelon extensional subbasins at the northwestern end of the Chihuahua Trough (a 

northwest-trending rift basin related to the opening of the Gulf 

of Mexico) (Bilodeau, 1982; Dickinson and others, 1986). In 

southeastern Arizona, exposures of Bisbee Group rocks indicate that the Chihuahua Trough was as wide as 130 km (80 mi) 

and may have extended as far to the northwest as the Tucson 

Mountains (Risley, 1987)(fig. 1).

In the Exxon well, we correlate the 602-m-thick (1,975-

ft) interval of shale, sandstone, thin-bedded limestone, and 

conglomerate from 3,056 to 3,658 m (10,026-12,001 ft) with 
25

sedimentary rocks of the Bisbee Group. This is the interval 

beneath the middle Tertiary volcanic and sedimentary rocks 

and above granitoid rock in the bottom of the well. Except for 

the fact that each formation in the section is much thinner than 

in sections of the Bisbee Group in the Empire and Whetstone 

Mountains, the lithologies interpreted from the cuttings and 

geophysical logs correlate reasonably well with the Bisbee 

Group in those ranges. A comparison of the thickness of 

each formation in the well to its thickness in exposures in the 

Empire Mountains shows that the Glance Conglomerate and 

Willow Canyon Formation are very much thinner in the well 

(only 2 and 4 percent, respectively, as thick as in the Empire 

Mountains) but the other three formations average 30 percent 

as thick.

In the Empire and Whetstone Mountains, the Glance and 

Willow Canyon are coeval deposits of proximal and of medial 

to distal alluvial fans, respectively. They contain clastic debris 

shed southward into a subsiding half graben from the main 

bounding fault on the north side of this segment of the Bisbee 

Basin (Archibald, 1987; Soreghan, 1999). The thickness of 

this alluvial fan is a function of the rate of subsidence of the 

half graben and the rate of uplift and erosion along the fault on 

the north side of the basin. Although the two formations vary 

greatly in thickness, the Glance Conglomerate is generally 

about 1,525 m (5,000 ft) thick on the north end of the Empire 

Mountains near the paleofault and is absent on the south side 

of the paleobasin, whereas the Willow Canyon Formation is 

about 915 m (3,000 ft) thick at the south end of the Empire 

Mountains, on the south side of the paleobasin, and is absent 

on the north side near the paleofault.

The fact that the Glance and Willow Canyon intervals 

in the Exxon well are both thin suggests that the part of the 

Bisbee basin they were deposited in was less tectonically 

active than the half graben to the southeast in the Empire and 

Whetstone Mountains, where the alluvial fan deposits are 

thicker. A less active tectonic setting in the depositional basin 

also fits with the more moderate thinning of the remaining 

three units of the Bisbee Group evidenced in the Exxon well. 

The lithologic similarity of the Apache Canyon, Shellenberger 

Canyon, and Turney Ranch Formations in the well to their 

counterparts in the Empire and Whetstone Mountains suggests 

that the rate of subsidence in the basin slowed while the depositional environments along the basin axis remained the same.

Turney Ranch Formation, 3,056-3,165 m (10,026-

10,384 ft)—Lower Cretaceous

In exposures in the Empire and Whetstone Mountains, 

the Turney Ranch Formation has been described as a thick- to 

thin-bedded repetitive sequence of pale-red calcareous shale 

and siltstone and light pinkish-gray and pale yellowish-orange 

sandstone deposited as fluvial and estuarine facies in a coastal 

plain environment (Shafroth, 1965; Finnell, 1970a; Archibald, 

1987). Lenses of arkosic pebble conglomerate contain clasts 

of chert, quartzite, and light-colored volcanic rocks. The 

Figure 12. Geophysical logs of the Bisbee Group (Turney Ranch, 

Shellenberger Canyon, and Apache Canyon Formations and Glance 

Conglomerate), in the Exxon State (32)-1 well. Inferred depositional 

environments are from field studies of Tyrrell ( 1957), Schafroth 

(1965), Finnell (1970a), Archibald (1987), and Soreghan (1999). 

Descriptions of Stratigraphic units 
26 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

thickness ranges from more than 975 m (3,200 ft) in the 

Whetstones to more than 305 m (1,000 ft) in the Empire 

Mountains.

Geophysical logs (fig. 12) indicate that the 109-m-thick 

(358-ft) interval interpreted to be Turney Ranch Formation 

in Exxon State (32)-1 consists of uniformly well consolidated rock with an average bulk density of about 2.65 g/cm3

, 

gamma radiation of about 80 API units, average interval transit time of about 60 to 65 microseconds/ft, and average resistivity of about 20 to 40 ohm-m. There are no sharp spikes in 

any of the logs to indicate faulting, and the caliper log shows 

no washed-out intervals. Although these characteristics could 

be representative of an intermediate-composition volcanic 

rock instead of a consolidated sedimentary rock, there are not 

very many volcanic fragments in the cuttings.

The lithology of the Turney Ranch Formation in the well 

is difficult to determine from the cuttings, which contain 

considerable uphole contamination. Limestone fragments 

that are common to abundant indicate most of the contamination is from washouts in the limestone conglomerate interval 

higher in the well (2,753-2,987 m [9,032-9,504 ft]). Pale-red 

nonmicaceous mudstone and sandstone fragments are distinctive and probably are representative of the Turney Ranch. 

Other mineral grains and lithic fragments (granitoid, volcanic, 

limestone) could be from either conglomerate lenses in the 

Turney Ranch Formation or contamination from the higher 

limestone conglomerate.

Two main criteria were used for correlating the inferred 

Turney Ranch interval in Exxon State (32)-1 with sections in 

the Empire and Whetstone Mountains: (1) the greater bulk 

density of the sedimentary rocks in the interval compared to 

those in the Pantano Formation (fig. 5), indicating that the 

unit is significantly older, and (2) the presence in the cuttings 

of pale-red, nonmicaceous mudstone and sandstone chips 

similar to the lithologic descriptions of the Turney Ranch 

Formation in outcrop. However, because the well site is 

40 to 80 km (25-50 mi) west of Turney Ranch exposures in 

the Empire and Whetstone Mountains, it is possible that the 

Turney Ranch Formation interval penetrated in the well had a 

different source area and may contain conglomerate clasts of 

lithology different from that at the type locality. Thus, some 

of the limestone fragments may come from conglomerate 

clasts within the Turney Ranch Formation rather than from 

uphole contamination. 

Shellenberger Canyon Formation, 3,165-3,447 m 

(10,384-11,310 ft)—Lower Cretaceous

In the Empire and Whetstone Mountains, the Shellenberger Canyon Formation consists of about 1,250 m (4,100 

ft) of shale, siltstone, and sandstone (Tyrrell, 1957; Schafroth, 

1965; Finnell, 1970a; Archibald, 1987). About two-thirds of 

the section is sandstone, and some of the sandstone beds in the 

upper half contain thin lenses of pebble conglomerate. Most 

of the shale and siltstone beds and a few limestone beds are in 

the lower half. The predominant colors of the strata are shades 

of red, gray, olive, brown, and green. Archibald (1987) interpreted the depositional facies to be lacustrine, marginal lacustrine, paludal, and fluvial in the upper and middle part of the 

formation and marine, fluviodeltaic, and marginal lacustrine in 

the bottom part.

In Exxon State (32)-1, we correlate the 282-m-thick (926-

ft) interval from 3,165 to 3,447 m (10,384-11,310 ft) with the 

Shellenberger Canyon Formation. As in the inferred Turney 

Ranch Formation interval, the cuttings contain considerable 

contamination from washouts higher in the well; however, the 

interval contains semi-indurated to indurated chips of nonmicaceous sandstone, mudstone, and waxy shale in distinctive 

colors of dusky red (5 R 3/4), grayish red (5 R 4/2), dark gray 

(N3), brownish gray (5 YR 4/1), and dusky brown (5 YR 2/2). 

These colors are similar to those described in the literature 

for the Shellenberger Canyon, and are differerent from those 

of the overlying Turney Ranch Formation and the underlying 

Apache Canyon Formation.

The geophysical logs (fig. 12) show that the characteristics of the Shellenberger Canyon Formation generally are 

similar to those of the Turney Ranch Formation, but the Shellenberger Canyon contains many thin clay-rich(?) zones of 

low bulk density, more numerous in the lower part of the unit. 

The average bulk density is about 2.65 g/cm3, with spikes as 

low as 2.05 g/cm3. The average gamma radiation is 80 to 100 

API units, with spikes as high as 140 API units corresponding, in part, to the low bulk density spikes. The interval transit time is 65 to 75 microseconds/ft, slightly slower than the 

Turney Ranch Formation, with low sonic velocity spikes corresponding to low bulk density spikes. The resistivity of the 

Shellenberger Canyon Formation is also lower than that of the 

Turney Ranch Formation, averaging about 8 to 20 ohm-m.

The thin zones of low bulk density probably are caused 

by faults, some of which may be bedding-plane faults in shaly 

intervals. This is suggested by the presence of lens-shaped 

slickensided waxy shale fragments in the cuttings. The slickensided shale fragments are inferred not to be an artifact of 

drilling, because they are not found in any other intervals 

containing fine-grained sedimentary rocks.

Apache Canyon Formation, 3,447-3,609 m 

(11,310-11,840 ft)—Lower Cretaceous

Studies of the Apache Canyon Formation in the Whetstone Mountains (Tyrrell, 1957; Archibald, 1987) and in 

the Empire Mountains (Schafroth, 1965; Finnell, 1970a) 

indicate that it is primarily a lacustrine deposit composed of 

thin-bedded to laminated limestone, shale, and minor sandstone, which grades laterally and vertically (downsection) 

to coarser-grained alluvial-fan facies of the Willow Canyon 

Formation and Glance Conglomerate. The lithology of the 

Apache Canyon Formation interpreted from the well cuttings 
27

is very similar to that in exposures of the Apache Canyon in 

the Empire Mountains as described by Schafroth (1965) and 

Finnell (1970a).

Finnell estimated the Apache Canyon to be more than 488 

m (1,600 ft) thick, and Schafroth measured a thickness of 265 

m (870 ft). Schafroth and Finnell both placed the upper contact above the last significant limestone sequence. In the well, 

the contact was placed at 3,447 m (11,310 ft), which is just 

above the highest occurrence of black limestone fragments 

and corresponds to the top of the interval of highly variable 

density as shown on the density log (fig. 12). This interval 

contains the highest significant limestone beds of the Apache 

Formation penetrated in the well and, thus, correlates with 

Schaforth’s and Finnell’s stratigraphic interpretation.

The interval inferred to be the Apache Canyon Formation 

is 162 m (530 ft) thick, and the cuttings show that it consists 

of dark-gray to medium-dark-gray (N 3 to N 4) shale, brownish-gray (5 YR 4/1) calcareous sandstone, and grayish-black 

(N 5) limestone. Minor amounts of gypsum are present from 

3,529 to 3,609 m (11,580-11,840 ft). An unidentifiable carbonaceous plant fossil was seen on a brownish-gray shale 

fragment; the fragment may have washed down from the Shellenberger Canyon Formation, which contains abundant woody 

plant fossils (Finnell, 1970a; Archibald, 1987)

The density log (fig. 12) shows that the Apache Canyon 

varies greatly in bulk density (from 2.0 to 2.8 g/cm3) across 

intervals less than a meter to as much as 3 m (10 ft) wide. 

Fissile shale is probably responsible for the lowest density 

and limestone for the highest density. The caliper log shows 

numerous washouts, particularly in the vicinity of the gypsiferous interval between 3,529 and 3,609 m (11,580-11,840 ft), 

where the well diameter washed out from 20 cm to more than 

40 cm (8 in to more than 16 in). These characteristics indicate 

that the unit is thin-bedded, lithologically variable, and not 

uniformly indurated.

The gypsiferous interval in the bottom part of the unit 

may be correlative with a gypsum bed and overlying gypsiferous sequence exposed in the Empire Mountains. In the well, 

the interval begins at the base of the Apache Canyon and is 

about 80 m (260 ft) thick (fig. 12). In outcrop, the gypsum 

zone begins about 60 m (200 ft) above the base of the Apache 

Canyon and is about 30 m (100 ft) thick (Schafroth, 1965; 

Finnell, 1970a).

Willow Canyon Formation(?), 3,609-3,642 m 

(11,840-11,948 ft)—Lower Cretaceous

In the Empire and Whetstone Mountains, the Willow 

Canyon Formation consists of an alternating sequence of 

brown and gray siltstone and locally conglomeratic lightcolored sandstone (Finnell, 1970a; Archibald, 1987). Thin 

limestone beds are present near the top of the formation in 

a marginal lacustrine facies that is gradational with lacustrine facies of the overlying Apache Canyon Formation. The 

bulk of the Willow Canyon Formation consists of distal and 

medial facies of alluvial-fan systems that grade laterally and 

downward to the Glance Conglomerate, which is the timeequivalent proximal alluvial-fan facies of the lower part of the 

Willow Canyon. Because it both overlies and intertongues 

with the Glance, the thickness of the Willow Canyon is highly 

variable in outcrops in the Empire and Whetstone Mountains, 

ranging from 0 to more than 1,000 m (about 3,000 ft).

It is not possible to identify the Willow Canyon Formation 

in the Exxon well on the basis of cuttings, because of the large 

amount of uphole contamination in the cuttings and because 

the lithology of the Willow Canyon (as described from outcrops) is gradational with the overlying and underlying units. 

In general, the cuttings in the interval that may be the Willow 

Canyon Formation are dark gray (N3) and brownish-gray 

(5YR 4/1) shale and sandstone. Dark gray shale chips in the 

bottom 3 m (10 ft) of the interval are pyritic.

A 33-m-thick (108-ft) interval is tentatively assigned to 

the Willow Canyon Formation on the basis of the following 

characteristics shown on the geophysical logs (fig. 12): (1) 

The density and sonic logs show considerably less variation in 

this interval than in the overlying Apache Canyon Formation. 

(2) The caliper log shows that between 3,612 and 3,627 m 

(11,850-11,900 ft) the well diameter was not enlarged but was 

constant at about 20 cm (8 in). (3) There is a steplike decrease 

in the gamma ray log at 3,642 m (11,948 ft) from about 150 

API units to about 120. (4) At about 3,609 m (11,840 ft) there 

is a small but sustained increase in resistivity from slightly 

less than 10 ohm-m to slightly more than 10 ohm-m.

Glance Conglomerate, 3,642-3,658 m

 (11,948-12,001 ft)—Lower Cretaceous 

and Upper Jurassic

A 16-m-thick (53-ft) interval consisting of interbedded 

medium-dark-gray (N4) calcareous shale, brownish-gray (5 

YR 4/1) sandstone, and conglomerate containing unaltered 

to slightly chloritized granitoid clasts and medium-gray (N 

5) limestone is interpreted to be the Glance Conglomerate. 

The granitoid fragments increase in abundance downward 

toward the top of the underlying granitoid basement(?) rock 

at 3,658 m (12,001 ft), from sparse at 3,642 m (11,950 ft) to 

about 20 percent of the sample at 3,654 m (11,990 ft). Some 

granitoid fragments in the interval directly overlying the 

granitoid rock (3,654-3,658 m [11,990-12,001 ft]) contain 

chalky weathered-looking feldspar. Limestone fragments are 

rare to sparse, indicating that there are relatively few limestone clasts in the conglomerate. This is suggested also by 

the density log (fig. 12), which shows that the density of the 

conglomerate assigned to the Glance is about the same as that 

of the underlying granitoid rock (2.60-2.65 g/cm3). Thus, the 

Glance penetrated in the well is probably a granitoid-clast 

conglomerate with a matrix of gray shale and sandstone. It 

can be correlated with the upper granitoid-clast conglomerate member of the Glance described in the northern part of 

Descriptions of Stratigraphic units 
28 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

the Empire Mountains by Finnell (1970a) and Bilodeau and 

others (1987).

The granitoid-clast conglomerate member of the Glance 

Conglomerate in the northern Empire Mountains is as much 

as 800 m (2,625 ft) thick, whereas the interval interpreted 

to be the Glance in Exxon State (32)-1 is only 16 m (53 ft) 

thick. This difference can be attributed to normal variation 

in the thickness of the Glance. The Glance Conglomerate 

varies greatly in thickness throughout its outcrop area, and 

in the northern Empire Mountains it thins toward the south 

to as little as 1 m (3 ft) thick as it interfingers laterally with 

the Willow Canyon and Apache Canyon Formations (Finnell, 

1970a; Bilodeau and others, 1987).

Zones of low-density rock (2.1 g/cm3) about 3 m (10 ft) 

thick at both the top and bottom of the Glance Conglomerate 

interval are inferred to be shear zones associated with faults 

(fig. 12). Resistivity peaks on the induction log suggest the 

presence of cemented brecciated rock about 0.5-1.0 m (2-3 ft) 

wide within these shear zones. It is not possible to estimate 

how much of the Glance section may have been cut out by the 

faults, but the thicknesses of the inferred shear zones implies 

that they are major structures. The basal fault may account 

for the relatively low percentage of granitoid clasts directly 

above the contact with granitoid rock. 

Granitoid Rock, 3,658-3,827 m (12,001-12,556 

ft)—pre-Upper Jurassic

Between 3,658 m (12,001 ft) and the total depth of 3,827 

m (12,556 ft), Exxon State (32)-1 penetrated 169 m (555 ft) 

of equigranular granitoid crystalline rock, termed quartz monzonite by Eberly and Stanley (1978). In the present report, 

this rock is merely termed granitoid because of the difficulty 

of differentiating granitoid compositions from examination of 

cuttings. On the geophysical logs (fig. 13) and the mud log, 

the top of the granitoid rock appears to be at 3,658 m (12,001 

ft), although Eberly and Stanley placed it a little lower, at 

3,660 m (12,008 ft).

Eberly and Stanley (1978) reported two ages for the granitoid rock, a K-Ar whole-rock age of 61 Ma, which they said 

was a reduced age, and a Rb-Sr whole-rock age of 120+60 

Ma. Although the contact of the overlying Upper Jurassic(?) 

or Lower Cretaceous Glance Conglomerate with the granitoid 

rock may be faulted, it was probably depositional originally as 

evidenced by the weathered granitoid fragments at the base 

of the Glance. Thus, the granitoid rock must be older than 

the Glance. As the Cretaceous-Jurassic boundary is about 

138 Ma, it follows that the granitoid rock is very likely older 

than 138 Ma. It may correlate with the Triassic or Jurassic 

granitoid rocks of the Sierrita Mountains (Cooper, 1973); or 

it may correlate with either the Middle Proterozoic Oracle 

Granite of the Santa Catalina and Rincon Mountains (1,351 to 

1,430 Ma; Reynolds and others, 1986) or the similar-age Continental Granodiorite of the Santa Rita Mountains (Drewes, 

1971b). In a cross section on the Rincon Valley geologic map, 

Drewes (1977) inferred the crystalline rocks in the bottom of 

the Exxon well to be a thrust-faulted sequence of Precambrian 

diabase, Pioneer Shale, Rincon Valley Granodiorite, and Cambrian Bolsa Quartzite.

The cuttings consist of about 50 percent granitoid rock 

and 50 percent shale, sandstone, and limestone presumably 

washed in from above during drilling, mainly from poorly 

consolidated intervals of the Apache Canyon Formation. The 

granitoid fragments are white, gray, pink, red, or pale green 

and are composed of quartz and feldspar along with muscovite, biotite, or chlorite. Mafic minerals are rare. All the granitoid fragments show mild chloritic alteration, which gives the 

cuttings a pale greenish cast; however, chloritic alteration is 

less conspicuous in the interval from about 3,776 to 3,795 m 

(12,390-12,450 ft). Feldspar in the lithic chips in this interval 

is pink to red. 

The geophysical logs (fig. 13) show that, although the 

characteristics of the granitoid rock are fairly uniform, a moderate amount of variation exists, and there may actually be two 

different igneous bodies separated by a fault. Over all, the 

bulk density varies from about 2.55 to 2.70 g/cm3; the average 

gamma radiation is about 120 API units; the interval transit 

time on the sonic log averages about 60 microseconds/ft; and 

the resistivity is high, between 30 and 300 ohm-m in the upper 

part of the granite and rising to more than 1,000 ohm-m in the 

part of the granite below the inferred fault.

Figure 13. Geophysical logs of granitoid plutonic rock, in the Exxon 

State (32)-1 well. Asterisk indicates location of interval of sampled 

cuttings that gave a whole-rock Rb-Sr age of 120+60 Ma (Eberly and 

Stanley, 1978).
29

There is evidence in the geophysical logs (fig. 13) for 

several dikes and a fault. Two small dikes about 1 m (4 ft) 

wide are tentatively identified at 3,676 and 3,698 m (12,060 ft 

and 12,132 ft). They correspond to increases in bulk density 

of 0.1 g/cm3, lower resistivity, and peaks on the gamma ray 

log to about 160 API units. A feature at 3,704 m (12,154 ft), 

interpreted to be a pegmatite dike about 2.5 m (8 ft) wide, corresponds to a gamma ray peak of 320 API units. The sonic, 

density, and induction logs show no corresponding peaks for 

this interval in the well, which implies that, except for its high 

gamma-radiation level, the dike is very similar in other physical characteristics to the granitoid rock surrounding it. There 

are no lithologic changes in the cuttings to indicate the presence of these three dikes.

A 10-ft-wide interval of low-bulk-density rock between 

3,788 and 3,792 m (12,430-12,440 ft) may be caused by a 

fault zone. The caliper log shows that considerable rock was 

washed out over a vertical distance of 15 m (50 ft) centered on 

this inferred fault zone. The cuttings in this washed-out interval change from an average of about 50 percent granitoid rock 

to about 20 percent granitoid rock and 80 percent dark-gray 

shale, black pyritic limestone, and grayish-red sandstone. It 

is possible that the abundant cuttings of shale, limestone, and 

sandstone in the interval are derived from a roughly 6-m-thick 

(20-ft) tectonic slice of sediment inferred to be the Apache 

Canyon Formation that was caught in the fault zone.

The bulk density of the rock in the faulted interval is as 

low as 2.30 g/cm3, whereas the bulk density of the granitoid 

rock above the interval is about 2.58 to 2.63 g/cm3 and that 

below the interval is about 2.63 to 2.68 g/cm3. The difference 

in bulk density of the rock on either side of the inferred fault 

suggests the juxtaposition of two slightly different granitoid 

bodies. In addition, the rock below the faulted interval has a 

lower interval transit time and higher resistivity than the rock 

above. The sinusoidal pattern of the sonic log is slightly more 

pronounced and regular in the granitoid rock below the fault 

than in that above, which may indicate a difference in the crystalline homogeneity of the two igneous bodies.

Discussion and Summary

This section summarizes the geologic events that have 

shaped the Tucson basin, with particular reference to the interpretations of the cuttings and geophysical logs of the Exxon 

State (32)-1 well in this study. The principal events in this 

section are given from oldest to youngest rather than youngest to oldest as in the body of the paper (the way well data are 

typically presented). A comprehensive analysis of the depositional history and tectonic framework of southeastern Arizona 

has been presented by Dickinson (1991) in his study of the 

Catalina core complex. 

The Proterozoic and Paleozoic geologic events that 

shaped the area of the present Tucson Basin were regional in 

extent. The granitoid pluton penetrated at the bottom of the 

Exxon well probably was emplaced in Middle Proterozoic 

time (1.4 Ga). This is the age of similar granite to granodiorite 

basement rock, overlain by Paleozoic and Mesozoic sedimentary rocks, on the flanks of the core complex that makes up 

the Santa Catalina Mountains and Rincon Mountains and in 

Cienega Gap and the Empire Mountains to the east of the well 

site (Reynolds and others, 1986; Reynolds, 1988; Dickinson, 

1991; Drewes, 1996). The granitoid rock in the well is not 

mylonitic, thus indicating that it is in the hanging wall of the 

Catalina detachment fault.

Paleozoic (Middle Cambrian to mid-Permian) marine 

platform sedimentary rocks were deposited on deeply eroded 

Precambrian granitoid basement throughout southeastern Arizona. Although deposition was intermittent and the section 

records many hiatuses, a total of about 1,750 m (5,750 ft) of 

sediment was deposited, much of it carbonate (Peirce, 1976). 

Paleozoic rocks are absent at the well site, indicating they 

were removed by erosion prior to the latest Jurassic, when the 

Glance Conglomerate was deposited. However, on the basis 

of exposures in the surrounding mountains, remnants of the 

Paleozoic rocks probably are present in scattered fault blocks 

beneath the Tucson Basin. The 144-m-thick (472-ft) limestone conglomerate interval in the middle Tertiary volcanic 

and sedimentary rocks of the well demonstrates that, perhaps 

as late as the Oligocene, a sizeable exposure of carbonate rock 

was relatively close by.

Beginning in latest Jurassic time and continuing through 

the Early Cretaceous, northeast-southwest directed extension in southeastern Arizona and northern Mexico created 

the Bisbee Basin. The extension was associated with rifting 

of the northwest-trending Chihuahua Trough and formation of the Gulf of Mexico. The Bisbee Basin was a series 

of block-faulted, asymmetrical subbasins in which fluvial, 

lacustrine, and marine sedimentary rocks of the Bisbee Group 

were deposited. Paleozoic rocks may have been eroded from 

horst blocks during basin formation and filling. The section 

of Bisbee Group rocks in Exxon (32)-1 is complete, and the 

depositional environments appear to be the same as those of 

the Bisbee in the Empire and Whetstone Mountains (however, 

considerable uphole contamination of the cuttings makes 

this interpretation tenuous). A significant difference between 

Bisbee Group sedimentary rocks in the well and those exposed 

in the mountains to the southeast is that the formations in the 

well are only 10 to 30 percent as thick as their counterparts in 

outcrop. This suggests that the subsidence rate was slower in 

this part of the Bisbee subbasin.

Upper Cretaceous sedimentary and volcanic rocks (Fort 

Crittenden Formation) and Laramide-age (Late Cretaceous and 

Paleocene) hypabyssal intrusive granodiorite and quartz monzonite and associated volcanic rocks are widespread but scattered in the region, and were not penetrated in the Exxon well. 

If Upper Cretaceous and Paleocene sedimentary and volcanic 

rocks were deposited at the well site, they were removed by 

erosion, probably during the Eocene.

Products of mid-Tertiary volcanism and extension are 

well represented in the Exxon well (see Dickinson (1991) for a 

Discussion and Summary
30 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

discussion of this period of tectonism). Interbedded volcanic 

and sedimentary rocks emplaced or deposited during the initial 

phase of extension consist of 168 m (522 ft) of pyroxene trachyte, 142 m (472 ft) of limestone conglomerate, 169 m (554 

ft) of ash-flow tuff, 31 m (102 ft) of conglomerate, and 29 m 

(96 ft) of intermediate-composition volcanic rocks. There is 

also a contemporaneous 8-m-thick (24-ft) dike or sill. The 

pyroxene trachyte is assumed to be Oligocene because of the 

lack of alteration of its cuttings and the late Oligocene age of 

the stratigraphically higher ash-flow tuff. Although the possibility that the trachyte is Late Cretaceous or Laramide in 

age cannot be ruled out, volcanic rocks of these ages in the 

area commonly are propylitically altered. K-Ar age dates of 

18.0 and 16.1 Ma reported by Eberly and Stanley (1978) for 

the pyroxene trachyte probably reflect the presence of uphole 

contamination.

Although volcaniclastic conglomerate units are quite 

common in exposed middle Tertiary volcanic terranes in 

southeastern Arizona, where many appear to be medial to 

distal lahar deposits, the presence of nearly 150 m (500 ft) of 

limestone conglomerate underlain and overlain by volcanic 

rocks is unusual. It implies that crustal extension or volcanism 

had created a local fault-bounded basin in the vicinity of the 

well and that the basin was being filled by alluvial-fan facies 

sediments principally derived from a nearby upland of Paleozoic carbonate rock. The depositional setting may have been 

similar to that of the mid-Tertiary Cloudburst and Mineta Formations exposed on the flanks of the San Pedro Basin to the 

northeast (Dickinson, 1991).

The 169-m-thick (554-ft) ash-flow tuff overlying the 

limestone conglomerate records an instant in geologic time 

when a caldera-forming eruption occurred within a few tens 

of kilometers of the well site and terminated deposition of the 

limestone conglomerate. The only radiometric age in the well 

that we consider to be reliable is the 40Ar/ 39Ar date of 26.91 

Ma obtained in this study on biotite from this ash-flow tuff 

(table 2; appendix). The basin that contained the limestone 

conglomerate probably continued to subside after it was filled 

with ash-flow tuff. Thus, the 31 m (102 ft) of volcaniclastic 

conglomerate that overlies the ash-flow tuff may indicate 

continued deposition in the same basin, but with a source area 

modified (no exposed limestone) by the blanket of ash-flow 

tuff. 

The 636-m-thick (2,086-ft) Pantano Formation is inferred 

to represent syntectonic upper-plate response to the second 

phase of mid-Tertiary volcanism and extension of the region, 

which was characterized by extensional shear zones at midcrustal levels beneath subhorizontal detachment faults. This 

inference is drawn chiefly by analogy to exposures of the 

Pantano Formation in Cienega Gap and in the Catalina Foothills, where the formation is complexly tilted and faulted, is 

in direct contact with the detachment fault in places, and is 

overlain by a rock-avalanche deposit. Displacement on the 

Catalina detachment fault, which included doming and the 

beginning of denudation of the core complex (Dickinson, 

1991), probably began at about 25 Ma, near the end of the Oligocene, and continued to at least 20 Ma and perhaps longer. 

This timing is in agreement with the age of about 25 Ma (K-Ar 

on plagioclase from andesite) for the Pantano in Cienega Gap, 

although, as discussed above in the section on the Pantano 

Formation, we suggest caution in using this date because of 

alteration of the andesite.

The Pantano Formation in Exxon (32)-1 is thinner, but 

quite similar lithologically, to the Pantano in outcrop in 

Cienega Gap and, apparently, similar to the Pantano in the 

Catalina Foothills, although the exposures there are not as 

good. Both in outcrop and in the Exxon well, the formation consists chiefly of moderately well consolidated muddy 

conglomerate, gypsiferous in part; in Cienega Gap it also 

contains an andesite flow and is capped by a rock-avalanche 

deposit. If the deposition of the Pantano was approximately 

coeval in the Catalina Foothills, at the Exxon well site 

(middle of the present Tucson Basin), and in Cienega Gap, 

the depositional basin was a large one, at least 60 km by 

25 km (about 40 mi by 15 mi) (fig. 1). Reconstruction of 

the late Oligocene to early Miocene geography of the area, 

based on Dickinson’s (1991) estimate that the upper plate 

of the Catalina detachment fault was displaced 20-30 km to 

the southwest relative to the lower plate, places the Pantano 

depositional basin more or less squarely on top of the present Catalina core complex. This perhaps is not surprising, 

because this may be where midcrustal extension was greatest 

and, therefore, where an extensional basin was most likely 

to form. The relative stratigraphic position of the rock-avalanche deposits (at the bottom of the Pantano in the Exxon 

well and at the top of the formation at Cienega Gap) may 

reflect eastward migration (as a function of time) of splay 

faults propagating upward from the Catalina detachment fault 

during deposition of the Pantano.

The Tucson Basin contains more than 1,880 m (6,000 

ft) of basin-fill sedimentary rocks, about equally divided 

between lower basin-fill and upper basin-fill deposits. The 

age of the basin fill is not constrained for the Tucson basin. 

However, an approximate age for the base of the lower basinfill deposits is given by an 40Ar/39Ar age for an ash-fall tuff 

interbedded near the base of the Nogales Formation in the 

upper Santa Cruz Valley (fig. 1). Sanidine crystals in that tuff 

were dated at 15.53 Ma or 17.38 Ma (table 2; see also fig. 

16, in the appendix). A minumum age for the upper basin-fill 

deposits of late Pliocene to early Pleistocene is based on the 

ages of deposits in nearby basins (table 3).

The basin-fill units are deposits of alluvial fans and 

playas. The chief differences between the lower and upper 

basin fill are that the lower fill is better consolidated and is 

significantly faulted whereas the upper fill does not appear, 

on the basis of the geophysical logs, to be faulted at all. The 

boundary separating the upper and lower basin-fill units has 

regional tectonic significance, because a similar boundary can 

be recognized in all the basins in Arizona and New Mexico 

where the basin fill has been mapped in detail (table 3). It 

represents a recognizable hiatus of perhaps a few million 

years and signals a change in tectonic activity.
31

The uppermost 341 m (1,120 ft) of upper basin-fill sediments at the well site are deposits of a low-gradient alluvial 

fan, herein named the Cienega Creek Fan. The fan head is 

at the mouth of Cienega Creek at the east side of the Tucson 

Basin. The source of sediment for the fan was older basin fill 

and bedrock in the area of Cienega Gap, Davidson Canyon, 

and the lower part of Cienega Creek. Field relations indicate 

that these drainages were sediment-filled basins when deposition of the alluvial fan began. Nearly all of Cienega Gap and 

Davidson Canyon have been exhumed and basin-fill alluvium 

has been eroded from the lower part of Cienega Creek Valley. 

The event, whether climatic or tectonic, that triggered the erosion of basin fill from these three areas has not been identified, 

but is currently being investigated.

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32 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

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34 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

APPENDIX: AGE SPECTRA AND PROBABILITY DISTRIBUTION DIAGRAMS FOR 

40AR/ 39AR DETERMINATIONS

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Figure 14. Age probability distribution diagram or ideogram for the 613PEN97 feldspar separate (Nogales Formation). The weighted 

mean of the fourteen single-crystal sanidine ages (13.23+0.10 Ma) is the preferred age for this sample. The plagioclase crystal that 

yielded an anomalously young age was probably altered (as supported by its lower K/Ca ratio and lower radiogenic yield) and was not 

used in calculating the mean. Error bars are two sigma.
35

Figure 15. Age probability distribution diagram or ideogram for the 615BAT99 feldspar separate (formation of Tinaja 

Peak). The weighted mean of the three single-crystal sanidine ages (24.70+0.19 Ma) is the preferred age for this 

sample. The sanidine crystals yield a population interpreted to record the age of eruption of the ash-fall tuff. Although 

the plagioclase crystals (K/Ca<10) yield a weighted mean age (24.9+1.8 Ma) indistinguishable from that of the sanidine 

crystals, the higher uncertainty of the individual plagioclase ages warrants excluding these data from the weighted 

mean age of the sample. Error bars are two sigma.

Appendix
36 Stratigraphy and Tectonic History of the Tucson Basin, Pima County, Arizona, Based on the Exxon State (32)-1 Well

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Figure 16. Age probability distribution diagram or ideogram for the sanidine separate from 616MTHOP00 (Nogales Formation). 

The analyzed crystals were very small (60-80 mesh), which resulted in signal sizes barely above the normal blank level. When 

these data were reduced in the usual way, we noted that eight of the eighteen analyzed crystals had radiogenic yields over 

100 percent. Negative 36Ar signals (caused by overestimation of the blank) resulted in the 40Ar/39Ar ratio being overcorrected 

for atmospheric argon. The precision of these analyses was improved by making the assumption that the crystals are all 100 

percent radiogenic and using the measured 40Ar/39Ar ratio rather than the corrected one. When calculated in this way, the crystals reveal a spread in ages from 15.51+0.42 Ma to 118+20 Ma. After eliminating the oldest six crystals (indicated by the dashed 

curve) as probable xenocrysts, the remaining crystals yield a weighted mean age of 17.38+0.77 Ma. The age spread even in the 

data not corrected for atmospheric argon is much greater than expected for a normal population, as evidenced by an unacceptable mean sum weighted deviation (MSWD) of 18 (Mahon, 1996; see discussion of MSWD in table 2 footnote). If the spread 

to older ages is due not only to analytical problems because of the small signal size, but to xenocrystic material, a weighted 

mean of the two youngest crystals (15.53+0.38 Ma) would be assigned as the best age of this sample. Error bars are two sigma.

Figure 17. Age spectra of biotite separates from a silicic tuff penetrated in the Exxon State (32)-1 well (middle Tertiary volcanic and sedimentary rocks). The biotite was collected from 2,584-2,600 m (8,478-8,530 ft) and 2,600-2,609 m (8,530-8,560 ft). The two contiguous intervals 

were sampled separately because there is a slight color change in the tuff at 2,600 m (8,530 ft). Both samples yield age spectra with high 

K/Ca ratios, increasing radiogenic yields, and young apparent ages in the first 15-40 percent of the 39Ar released during step heating (steps 

labelled by letters and temperature in  ̊C). Integrated ages include the data of the initial heating steps. Weighted mean ages are calculated 

from the remaining parts of the age spectra. Biotite from 2,584-2,600 m (8,478-8,530 ft) yields a weighted mean age of 26.91+0.18 Ma that 

includes 94 percent of the 39Ar released and has an acceptable MSWD (Mahon, 1996). The biotite from the deeper interval (2,600-2,609 m 

(8,530-8,560 ft)) yields a weighted mean age of 26.80+0.44 Ma that includes 61 percent of the 39Ar released and also has an acceptable MSWD. 

The dates from both samples are analytically indistinguishable, indicating the tuff in both intervals is the same. The disturbed early parts 

of both spectra are probably due to alteration of the biotite. The age spectrum of the upper interval suggests less alteration than the more 

disturbed age spectrum of the lower interval. The weighted mean of the sample from the upper interval also yields a more precise weighted 

mean age (26.91+0.18 Ma) and is, therefore, the preferred age for the eruption of the tuff.
37

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Appendix