Geology of the Agua Verde hills, Pima county, Arizona · GEOLOGY OF THE AGUA VERDE HILLS, PIMA COUNTY, ARIZONA by, John R. Kerns A Thesis Submitted to the Faculty of the DEPARTMENT

Geology of the Agua Verde hills, Pima county, Arizona

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GEOLOGY OF THE AGUA VERDE HILLS,PIMA COUNTY, ARIZONA

by

, John R. Kerns

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOLOGY

In Partial Fulfillment of the Requirements For the. Degree of

MASTER OF SCIENCE

In the Graduate College

UNIVERSITY OF ARIZONA

1958

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

Assistant Professor of Geology

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by

John R. Kerns

GEOLOGY OF THE AGUA VERDE HILLS,PIMA COUNTY, ARIZONA

ABSTRACT

The Agua Verde Hills are part of an imbricate sheet that has

been thrust over Pantano formation sediments of Miocene(?) age. Por

tions of the Bolsa, Horquilla, and Andrada formations comprise the

thrust sheet. Intense folding formed anticlines with steep or overturned

northern limbs and gently dipping southern limbs. The direction of

movement as interpreted from the fold axes and dips of the fold flanks

is from a southerly direction. A quartz monzonite stock of Laramide

age crops out along the northern edge of the area and is the probable

cause of the termination of thrusting movement. Nearly pure quartz

was intruded along the thrust plane and other zones of weakness during

the thrusting. After the interval of siliceous intrusion, basic intrusives

came in along zones of weakness during the thrusting and show some

shearing effects from the faulting. Later tension faulting has offset

some of the fold axes and uplifted the Pantano formation in the southern

part of the area.ii

CONCLUSIONS

1. . Major thrusting is post-Pantano formation.............. .

2. Major thrusting did not include the Pantano formation.

3. Igneous activity was simultaneous with the thrusting. .

4. Thrust block came from a southerly direction.

RECOMMENDED SCOPE OF FUTURE PROBLEMS

1. Accurately date quartz monzonite.

2. Consider tectonic gliding in addition to, or in place of, thrusting

in the area.

3. Sedimentary fabric study of the Pantano formation to possibly

determine the direction of the source.

TABLE OF CONTENTS

INTRODUCTION

Page

1

Location 1

Extent and Access ..............Climate, Drainage, Ecology

Purpose and Method of Study ..Previous Work .........................Acknowledgments......................

DESCRIPTIVE GEOLOGY----- PART I

STRATIGRAPHY ...................................

G eneral...............................................

Sedimentary R o ck s ......................

Cambrian Rocks

Balsa Quartzite . Measured Section

12

6

6

6

7

7

711

Pennsylvanian R ocks......................................................... 13

Horquilla Formation ........................... 13Measured Sections---- . . . . ............ 16

Pennsylvanian-Permian R o ck s....................................... 26

Andrada Formation ...................................... 26Lower Andrada M em ber............................................. 26Upper Andrada Member ............................................. 28Measured Section ......................................................... 30

Cenozoic Rocks 36

iv

cn ►£

«. eo

Page

Pantano Form ation........................................ 36Alluvium ................................................ 37

Igneous Rocks ....................................................................... 39

Quartz Monzonite.......... .............. 39Basic Intrusions ................................................ 41Quartz Intrusions................................................................ 43

STRUCTURE ...... ....... .................................. 48

General ............................. 48Folding .......................................................................................... 48Faulting ................................................................................. 50Jointing .............................................. 53

ECONOMIC GEOLOGY..................................................................... 58, ' ; . < . . : ' . v , . -

GEOLOGIC HISTORY - ---- PART H .......... ......................... 61

STRUCTURAL AND IGNEOUS HISTORY ....................................... 61

PALEOGEOGRAPHY.......................................................... 64

REFERENCES CITED ......... 68

LIST OF FIGURES

FIGURE Page

1. Index map showing location of the Agua VerdeHills .................................................................................. 3

2. Geologic cross-sections of the Agua Verde. Hills . . . . . . . 51

v

LIST OF PLATES

PLATE , , Page* -V V ' V 1 ^ • V 1 ' ' *6

I Geologic map of the Agua Verde Hills, PimaCounty, Arizona........ ..................— . . . — . . . . . . . in pocket

H Photomicrograph of oolitic limestone from the . . .Horquilla form ation........................................... ................ 25

m Chert pebble conglomerate marker bed of the LowerAndrada m em ber------------------------- -- . . . . . . . . 34

IV White crystalline dolomite stringers that are .abundant in the Upper Andrada m em ber............ .......... . 35

V Angular unconformity between the Pantano formationand alluvium ......................................... . 38

VI Dark inclusions in quartz monzonite at an outcropnear the Day R anch ....................... ............ ................... 45

VII Diabase intruding Bolsa quartzite approximately l/2 ,.mile northwest of Red Hill .................... ......... . 46

VIII Intrusive quartz surrounding limestone . . . . . . . . . . . . . . . 47

IX View of the west side of Agua Verde Hill . . . . . . . . . . . . . . 55

X Brecciation of Bolsa quartzite along a thrust planenorth of Horse Hill ......................................................... 56

XI Thrust contact of Pantano formation and overridingPaleozoic limestone ........................................................ 57

XII Barite vein above Agua Verde Creek west of RedH il l .............. ...................................................................... 60

XIII Asymmetrical ripple marks from a sandstone unit inLower Andrada member ............ . 67

vi

INTRODUCTION

Location

The Agua Verde Hills, located 30 miles southeast of Tucson,

are a portion of the southwestern foothills of the Rincon Mountains.

The mapped area lies within sections 7, 8, 9, 16, 17, 18, 19, 20, and

21 of T. 16 S«, R. 17 E ., and sections 13 and 24 of T. 16 S., R„ 16 E«

Colossal Cave, a Pima County Park, is located one mile north of the

Agua Verde Hills. The Agua Verde Hills have a maximum elevation of

3,981 feet. The maximum relief within the area is 660 feet. In gen

eral the higher elevations are in the northern portion and lower hills

are in the southwestern portion of the area. With the exception of Agua

Verde Hill, arbitrary names have been given to the more prominent top

ographic features of the area to facilitate use of geographic locations

(Fig. 1).

Extent and Access

Agua Verde Creek is used as both the northern and western

limits of the mapped area. The southern and eastern boundaries have

no single geographic feature as a boundary. Rather the contact of the

Pantano formation with the Paleozoic rocks is used to delimit the mapped

area along the eastern and southern boundaries.

2

Access to the area from Tucson is via U. S. Highway 80 (Benson

Highway) to Vail Junction, then, northeastward approximately seven

miles on the Colossal Cave Road to the Day Ranch, A private, dirt

road is located parallel to Agua Verde Creek along the northern bound

ary of the area.

Climate, Drainage and Ecology

The climate of the area is nearly the same as that of the re st of

Pima County. Rainfall slightly exceeds that of the Tucson area because

of the higher elevation of the Rincon Mountains, The region is drained

by a dendritically arranged system of intermittent streams that even

tually discharge into Pantano Wash, most of them flowing first into Agua

Verde Creek. Geologic structures provide controls for determining the

location of the drainage pattern. Many abrupt changes in streamflow

direction are caused by fault traces, along which the valleys have been

downcut.

The flora and fauna of the area are typical of the 2, 000 to 5,000

foot elevations of the Lower Sonoran Life Zone. Bryan (1925) described

this life zone in detail.

Purpose and Method of Study

The purpose of this study is to determine the structural and

stratigraphic relationships of the rocks in the region of the Agua Verde

3

INDEX MAP SHOWING LOCATION OF THE AGUA VERDE HILLS

RINCONM IN S

TUCSOIPIMA COUNTY

M X

5 0 MILES

__ ,/D DAY RANCHCOLOSSALCAVE ROAD

; RED

HORSEHILL

RODHILL

SANDWICH

- h i l l AGUA VERDE HILL

TRACKS

0 .5 MILE

FIGURE I

4

Hills.

The problem included making a detailed geologic map to a scale

of 1 inch equal to 500 feet and measuring columnar sections of the

Paleozoic formations. The faunal assemblages of each formation are

listed. Petrographic thin sections were studied of the igneous and sedi

mentary rocks that occur within the mapped area.

The topographic map used as a base was made by the floating dot

method of stereo-photo contouring. A United States Geological Survey

map, prepared in 1957, of the southeastern quarter of the Tucson 30

minute quadrangle was used for control. Field mapping was done on

aerial photographs and transposed to the base map.

Previous Work

The first geologic reconnaissance of the area was made by

Barton (1925). He grouped all the carbonate rocks as Devonian-

Carboniferous and the granitic rocks as Precambrian.

Moore and Tolman (undated) designated the greater portion of

the Agua Verde Hills as rocks of Carboniferous and Permian age on

their map of the Tucson 30 minute quadrangle. They also recognized

the presence of the Pantano formation to the south and the presence of

some Cretaceous rocks to the southeast.

Brennan (1957) in his reconnaissance of Cienega Gap called the

entire area of the Agua Verde Hills undifferentiated Carboniferous rocks

5

and recognized that the Pantano formation to the south was in fault con

tact.

Acknowledgments

-

Thanks are expressed to Mr. Charles Day for permission to

enter his patented and leased land.

I am also grateful to F. W. Galbraith, chairman of the Geology

Department, and to the rest of the faculty of the department for their

help in preparing this thesis, and especially to H. W. Miller under

whose direction this thesis was prepared.

PARTIr DESCRIPTIVE GEOLOGY

STRATIGRAPHY

v General

The.Paleozoic sedimentary rocks of the area represent a thrust

block resting on Laramide quartz monzonite and Miocene(?) Pantano

formation. The Bolsa quartzite of Cambrian age is the oldest known

rock unit in the area and shows evidence of imbricate thrusting. A1-

though both the top and bottom of the formation are believed to be ex

posed in the area, no complete section could be measured due to the

imbrication.

Pennsylvanian carbonates form the topographic highs represent

ed by Agua Verde Hill, Horse Hill and prominent ridges. These rocks

generally are massive, thick bedded limestones that form cliffs up to

20 feet high. Limestones and clastic rocks of Pennsylvanian-Per mi an

age {Lower Andrada member) form the topographic lows in the central

portion of the area. Black dolomites of the Upper Andrada member cap

numerous hills in the southwest.

The Pantano formation outcrops to the south and east of the area

and forms the base of the thrust plane on which the Paleozoic rocks now

6

7

rest. This formation is predominately a poorly consolidated conglom

erate with clasts from boulder to pebble size. Alluvium rests uncon-

formably on the Pantano formation.

A quartz monzonite stock outcrops in.the northwestern corner

of the area. This stock probably was exposed prior to the thrusting

and acted as a terminating resistance to the thrusting. Siliceous and

basic intrusives accompanied the thrusting and outcrop in many thrust

zones and other zones of faulting.

Sedimentary Rocks

Cambrian Rocks

Bolsa quartzite. —The Bolsa quartzite is a purple and maroon,

highly indurated, orthoquartzite that crops out several places along the

south side of Agua Verde Creek. The most complete section of Bolsa

quartzite in the area forms Red Hill. At this locality 290 feet were

measured (Section A). This section is predominately composed of the

middle member of the formation. The lower member which contains

a basal quartz pebble conglomerate is best observed in contact with the

quartz monzonite south of the Day Ranch.

The Bolsa quartzite was first defined by Ransome (1904) where

it crops out at Bolsa Canyon on the southwest side of Escabrosa Ridge

of the Bisbee quadrangle. At the type locality Ransome measured a

total thickness of 430 feet. The formation is unfossiliferous and was

8

dated as Middle Cambrian on the basis of stratigraphic position. Ransome

concluded that the Bolsa quartzite is the stratigraphic equivalent of the

Tonto sandstone of the Grand Canyon area.

Weidner (1957) made a study of the Bolsa quartzite at Quartzite

Ridge and Beacon Hill (Pistol Hill) which are immediately northwest of

the Apia Verde Hills. He measured 779 feet of Bolsa quartzite on

Quartzite Ridge and was able to distinguish three members of the forma

tion. The lower member has a basal conglomerate consisting of white

quartz pebbles up to 3 inches diameter. The middle member is the

most resistant to erosion and, therefore, forms the topographic highs.

The upper member is lithologically similar to the middle member ex

cept for some shaly beds that are present.

The same three members are recognized in the Agua Verde

Hills. The lower member contains a basal conglomerate with white

vein-quartz pebbles of up to 3 inches diameter. These pebbles are

sub-angular to sub-rounded and are poorly sorted. Cross-bedding

and thin laminations are present at several outcrops. Black circular

spots about 1/2 inch diameter are present on the rock at outcrops near

the top of the lower member, giving it a black speckled appearance,

Weidner (1957) called these spots manganese stains. Study of petro

graphic thin sections and micro-chemical tests indicate that no manga

nese is present. A barite vein approximately 10 feet long and 2 feet

wide occurs near the top of the member just west of Red Hill (PI. I).

9

The middle member forms the summit of Red Hill. This mem

ber is highly indurated, and purple and white layering approximately

1/4 inch wide is common. The typical Bolsa quartzite coloration of

bright purples and maroons makes the lithology distinctive. Study of

petrographic thin sections indicates that the rock is composed of more

than 90% quartz and about 5% iron minerals. The iron minerals cause

the rock's distinctive colors. The following is a mineral assemblage

of the middle member as identified in microscopic thin section:

Quartz 90%+Iron oxides and hydrous silicates 5%+Others: ' , 1%+

Rutile■iv- ■' Sphene -

CalciteApatiteZircon

The rock is equigranular, microcrystalline, and some of the quartz

crystals have sutured margins.

Only a small portion of the upper member of the Bolsa quartzite

is exposed in the Agua Verde Hills. Along the east side of Red Hill,

within a few feet of the Bolsa-Horquilla contact, there are small outcrops of

red siliceous siltstone that weather dull green. This siltstone may be

near the base of the upper member of the Bolsa quartzite.

In the Agua Verde Hills the Bolsa quartzite is a thrust block

that rests on quartz monzonite. The general dip to the east was caused

by a period of tension faulting after the initial thrust, Weidner (1957)

believed that the thrust was from the southwest In the Agua Verde

10

Hills there is little evidence within the formation to either confirm or

deny Weidner's interpretation of the direction of the thrusting. The

repetition of the lower member of the Bolsa quartzite at three locations

suggests an imbricate structure. The lower member overlies quartz

monzonite south of the Day Ranch and west of Red Hill. The portion of

the Bolsa quartzite that comprises the most southwestern part of Red

Hill is believed to be part of the lower member. The absence of the

middle and upper members at the two previously mentioned locations

where the Bolsa quartzite rests on quartz monzonite was probably

caused by the telescopic effect of the thrusting and not erosion. The

middle member is the most resistant of the three members, therefore,

it is concluded that this competent member has been missing from these

two localities since the thrusting.

Folding within the Bolsa quartzite is not prevalent. Broad

flexures are the only evidence that compressive forces have been exert

ed on the formation.

11

Measured Section A

Partial section of the Bolsa quartzite measured at Red Hill,

Cambrian: Bolsa quartzite, 290 feet.

Horquilla limestone

Thrust faultThickness

Unit (feet)

13, Shale; red, weathers greenish brown, micaceous, 2moderate fissility. Locally becomes a siltstone.Poorly indurated, forms slope.

Approximate middle-upper member contact

12. Quartzite; white and purple. Weathers to orange. 62Very resistant and massive, highly indurated, fine grained, typical layering is absent. Strike S30E dip 55E.

Possible fault

11. Same as No. 12 above. Forms summit of Red Hill. 23

10. Quartzite; dark purple with very thin (l/8") white 4layers, fine grained, forms slope.

9. Covered interval 12

8. Quartzite; maroon with up to one inch white layers, 55massive. Some two inch layers of quartzite conglomerate with quartz and rock fragments up to one inch diameter. Strike SIDE dip 35E.


6. Quartzite; dark purple with white, sub-rounded quartz 12pebbles up to 1/2 inch diameter. Forms slope.

5. Quartzite; massive, forms cliff, maroon to purple 37with alternating purple and white layers about 1/2 inch thick, fine grained. Strike S5E dip 40E.

12

Unit. ■ ‘ * Thickness

(feet)

4. Covered interval; approximate lower-middle member contact.

7

3. Quartzite; maroon with purple and white layering (1/4"), medium grained, some circular black spots about l/2 inch diameter.

18


1 . Quartzite; maroon to purple, medium grained, white vein quartz pebbles up to 1 inch diameter. Locally purple layering.

7

Diabase

13

P ennsylvanian Rocks

Horquilla formation. ^-The Horquilla formation comprises the

greatest area of total outcrop in the Agua Verde Hills. Agua Verde

Hill, the highest hiU in the area, Horse Hill, and topographically

prominent ridges represent Horquilla rock outcrops.

The HorquiUa formation was originally defined by Gilluly,

Cooper, and Williams (1954) in the Tombstone Hills. Here the forma

tion is represented largely by a series of thin bedded, gray limestones.

The lithology of the Horquilla formation in the Agua Verde Hills is most

ly thick bedded, finely crystalline, gray and pink limestones, although

some thin bedded limestones occur infrequently (see measured sections

B, C, and D). Some limestones are dolomitic but no true dolomites

were recognized in the formation. Near the top of the formation clastic

beds become more numerous. The contact between the Horquilla for

mation and the overlying Andrada formation is arbitrarily picked where

the clastic rocks become dominant over the limestones. This corresponds

to the Horquilla-Earp contact to the east in Cochise County (Gilluly,

1956). The contact between the HorquiUa formation and the underlying

Escabrosa formation is dubious. There is no marked lithologic bound

ary, and there was no erosion during the latest part of the Mississippian

(Gilluly, 1956). The presently accepted Escabrosa formation lithology

in the vicinity of Colossal Cave is a massive gray to buff limestone that

forms huge cUffs. This cliff former can be traced over considerable

14

distance north and northeast of the Agua Verde Hills. For this reason

and lack of faxmal evidence, the Escabrosa limestone is not believed to

be present in the Agua Verde Hills.

, Chert is common as layers or lenses within the limestones.

Layton (1958) showed by thin section analysis that at least some of the

chert is of secondary origin and not syngenetic with the limestone. The

secondary origin is further indicated by the fact that all fossils found

have been replaced by chert,

A three foot thick, black, oolitic limestone bed was found at

four different localities and was of some assistance in determining

stratigraphic position within the area. Two outcrop locations are north

of the summit of Agua Verde Hill. A third is northwest of the summit,

topographically about 300 feet lower than the summit (measured section

C). The fourth locality is southwest of Horse Hill (measured section D).

At each outcrop the bed is stratigraphicaUy near the top of exposed

Horquilla rocks in the area. The rock has a slightly conglomeratic ap

pearance in hand specimen, but in thin section it is oolitic. The crys

tals of calcite within the oolites are cryptocrystalline. The oolites

show evidence of some deformation by their ellipsoidal outline and

fracturing (PI. H). Calcite outside the oolites shows a preferred

twinning parallel to the oolite border. Other minerals in the rock in

clude plagioclase (range of andesine), quartz, and muscovite. These

minerals total 5 to 10 percent of the rock.

15

Folding within the Horquilla formation consists mostly of broad

flexures due to the relative high competency of the rocks. The upper

fifty feet of Agua Verde Hill shows the most intense folding observed

involving Horquilla rocks. Here the north limb of an anticline is ver

tical. Faults involving the formation are abundant. Thrust faults are

especially common along the northern outcrop limits of the formation.

These thrust zones are usually small in lateral extent (rarely exceed

50 feet) and are bedding plane faults. Basic intrusives often mark these

small thrust zones.

. The Horquilla formation in the Agua Verde Hills has numerous

localities where fossils are common. The fossils are concentrated in

single beds and over short lateral distances, rather than being distrib

uted throughout the entire outcrop. Generally, the fossils are badly

broken and recrystallized, so although fossils are common, those in

identifiable condition are sparse. Two coral genera date rock as

Pennsylvanian: Syringopora and Lophophyllidium. Both of these coral

genera were recognized south of Horse Hill and east of Agua Verde Hill.

Other identifiable fauna include Composita, Dictyoclostus, Caninia(?),

zaphrentid corals, spiriferids, bryozoans, crinoid stems, and echinoid

spines. The tooth of a fish was found north of Rod Hill in a one foot

thick bed of limestone.

16

Measured Section B

Pennsylvanian: Horquilla formation, 197 feet.

P artia l section of the Horquilla formation m easured southwest of RedHill.

Horquilla formation .

Thrust faultThickness

Unit (feet)

21. Limestone; dark gray, weathers smooth, thin 8bedded, solution cavities up to 6" diameter and 6" deep, N-S joint pattern has calcite making up 25% of the rock.

20. Limestone; dark gray, massive, some shell frag- 8ments, no vein calcite, forms cliff.

19. Limestone; dark gray, thick bedded, Syringopora 7and Lophophyllidium at bottom, smalEcalcite veins (l/4"), very fetid odor upon fracture, forms steep slope.


17. Limestone; dark gray, thick bedded, weathers 8rough except on dip slope, no vein calcite.

16. Limestone; light gray, thick bedded weathers 4rough, Composita, forms slope.

15. Shale; red and green, very fissile, 6” dark gray 8limestone bed near top has some fossil fragments.

14. Limestone; dark gray, medium bedded, badly frac- 3tur ed with fr actur es filled with calcite and caliche, forms slope.

13. Limestone; grayish pink, thick bedded, many l/2" 14calcite veins, weathers smooth where very pink, small fossil fragments up to 1/2", forms cliff.

17

ThicknessUnit (feet)

12. Limestone; pinkish gray, thin bedded, chert nodes 7common, calcite veins up to 1/4”, forms slope.

11. Limestone; grayish pink, very thick bedded, weath- 17 ers rough, 2” chert lenses about every 2r, horn corals and other fossil fragments near top, calcite and caliche present where limestone is very pink, forms cliff.

10. Shale; green, very fissile, vitreous luster on fresh 9surface, forms slope.


8. Limestone; dark gray, massive, weathers rough, 15no vein calcite, forms steep cliff.

7. Shale; green, weathers red in some places, forms 2slope.

6. Limestone; gray, massive, weathers rough. Com- 13posita and some productid remains, horn corals and crinoid stems, calcite stringers up to 1” wide, chert in vugs, forms steep cliff.

5. Limestone; dark gray, abundant chert as lenses, 8weathers rough, some shell fragments, forms cliff.

4. Shale; reddish gray, weathers dark red, shows some 5 fissility, forms slope,

3. Limestone; dark gray, thin bedded, chert layers 19to l/2 " thick, weathers smooth where chert is not present, forms slope.

2. Siltstone; reddish gray, calcareous, 6M caliche 4layer at top, shows some fissility, forms slope.

1. Quartzite; white and pink, thrust zone, some light 13gray limestone as breccia present, calcite stringers up to l/2" wide, chert is sparse.

Alluvium and diabase

18

Measured Section C


P artia l section of the Horquilla formation m easured on the northwestslope of Agua Verde Hill.

Lower Andrada member

Alluvium= * = Thickness

Unit (feet)

30. Limestone; dark gray, thick bedded, shell frag- 4ments near top, Composita, no vein calcite.

29. Covered interval, some brick red and buff shale 18float. : :

28. Limestone; gray, arenaceous, thick bedded, 2" 8calcite veins, weathers off in slabs, forms cliff.

27. Limestone; gray, thick bedded, weathers very 4rough, chert lenses parallel to bedding, becomes arenaceous and pink near top.


25. Limestone; dark gray, chert knots perpendicular to 5bedding, massive, weathers rough, becomes pink towards the top, forms ledge.


23. Limestone; pink, thick bedded, weathers rough, 6forms ledge.


21. Limestone; dark gray, arenaceous, oolitic, weath- 3ers rough (thin section made of this unit, see text).

20. Limestone; pinkish gray, pinkest near fractures, 46thin bedded, small amount of caliche in fractures, weathers rough, forms slope.

19



18. Limestone; gray, thin bedded, weathers smooth, 7forms slope.

17. Limestone; pinkish gray, medium bedded, pinkest 7at the bottom, weathers very smooth with solution pits up to 1 cubic foot, numerous shell fragments, forms slope.

16. Limestone; dark gray, massive, chert near bottom 38as knots but absent near top, no vein calcite, weathers smooth on dip slope, forms ledges.

15. Limestone; pink, thin bedded, some caliche in the 8bedding planes, forms slope.

14. Limestone; pink, medium bedded, weathers rough, 3forms slope.

13. Covered interval; 27

12. Shale; green, shows some red streaks, contains 6some quartz, very fissile, forms slope.

11. Limestone; pink, weathers rough, medium bedded, 3forms slope.


9. Limestone; gray, productid fragments, 10% of rock 16is chert as knots, calcite stringers up to 1/2” wide, forms cliff..

8. Limestone; pink, medium bedded, weathers gray, 5some chert knots and calcite stringers, forms slope.

7. Limestone; pink, thick bedded, weathers very rough 3and to a buff color, forms slope.

20


6. , Limestone; light gray, thick bedded, weathers 6rough, large knots of chert up to 12” diameter, no vein calcite, forms cliff.

5. Limestone; gray, medium bedded, weathers 15raspy, locally arenaceous, l /2 ” chert lenses

.... cap the top.

4. Limestone; gray, massive, dip slope weathers 16smooth with large solution pits, locally arenaceous, very pink at top, l /2 ” .chert lenses cap the top.

3. Limestone; pinkish gray, 1” beds of chert makes 12up over 25% of rock, calcite veins perpendicular to bedding, large crinoid stems, forms cliff.

2. Limestone; dark gray, thick bedded, weathers 10rough and pink, chert knots to 6” . diameter, no vein calcite, forms cliff.

1. Limestone; light gray, weathers rough, horn 13corals and crinoid stems at bottom, up to 1/2” calcite veins and some chert lenses parallel to bedding.

Fault unknown displacement

Horquilla limestone

21

Measured Section D


P artia l section of the Horquilla formation m easured south of HorseHill.

Lower Andrada member

FaultThickness

Unit (feet)

44. Limestone; gray, weathers pink and very rough, 3medium bedded, small chert nodules at bottom, forms slope. .

43. Limestone; dark gray, weathers pink and rough, 10massive, caliche in fractures near bottom, forms ledge.

42. Limestone; gray, weathers orange«=brown at top, 11" calcite veins, forms slope.

41. Limestone; gray, weathers rough, chert lenses 2'* 4thick, massive, forms ledge. Strike N45W dip 308.

40. Limestone; black, arenaceous, oolitic, contains 3some red chert, weathers rough, solution cavities 12” deep, forms slope.

39. Limestone; gray, weathers pink and rough, medium 8bedded, forms cliff.

38. Limestone; gray, weathers rough, medium bedded, 3forms ledge.

37. Siltstone; red, calcareous, may conceal fault. 10

36. Limestone; black, weathers gray and rough, 7medium bedded, small amounts of chert and calcite, arenaceous at bottom, forms long dip slope.

22


35. Limestone; black, arenaceous, weathers rough, 6numerous calcite stringers, forms slope. Strike N20W dip 40S.

34. Covered interval; possible fault. 18

33. Limestone; gray, arenaceous, weathers rough, 6small calcite vehis common, forms slope.


31. Limestone; dark gray, weathers pink and smooth on 11dip slope, solution cavities 6” deep on dip slope, l /2 n chert and limestone, clasts at bottom, crinoid stems, forms cliff.

30. Sandstone; greenish black, weathers red, 1M calcite 3layers, some slickensides found, forms slope.


28. Limestone; dark gray, weathers black and rough, 5chert lenses common up to 2,‘ thick, forms long dip slope.


26. Limestone, gray, weathers pink and rough, medium 2bedded, some productid and coral fragments, forms slope.

25. Limestone; black arenaceous and dolomitic, numerous 9small calcite stringers, forms slope.

24. Limestone; gray, weathers pink and rough, chert 6as lenses constitutes over 30% of rock, horn coral and productid fragments, forms slope.

23. Same as unit No. 25. . 2


23

21. Limestone; black, weathers smooth, chert knots 66’' diameter common, forms slope. Strike N25W dip 35S.

.. . .

20. Limestone; red and gray, weathers orange and red, 3silty, forms slope.

19. Limestone; same as unit No. 21. 3


17. Limestone; same as unit No. 21, forms cliff. 8

16. Limestone; gray, weathers smooth, forms ledge. 2

15. Limestone; pink, weathers smooth, forms ledge. 2

14. Limestone; pink, thin bedded, limestone clasts up 3to 3n, forms slope.

13. Limestone; black, weathers rough, some calcite 5veins, forms slope.

12. Limestone; black, weathers smooth, medium bedded, 3crinoid stems and spiriferid fragments, forms slope.


10. Same as unit No. 12. 2

9. Limestone; black, arenaceous, weathers rough, 14medium bedded, chert knots up to 2", some red stains near chert, forms ledges. Strike N-S dip SOW.

8. Limestone; gray, weathers smooth, l/2" calcite 2stringers, crinoid stems, forms slope.

7. Limestone; black, arenaceous, weathers rough, 2chert at top.



24


5. Limestone; black, weathers rough, some chert 8lenses, crinoid stems abundant.


3. Limestone; gray, arenaceous, weathers smooth, 31/2" calcite veins, forms ledge.

2. Limestone; gray, weathers smooth, some chert as 2nodules, forms slope.


Bottom of wash; probable fault, unknown displacement

PLATE H

Photomicrograph of oolitic limestone from the Horquilla forma

tion (x 11 diameters).

PLATE II

26

Pennsylvanian-Permian Rocks

Andrada formation. —The history of the numenclature applied to

Pennsylvanian and Permian rocks of Southern Arizona is discussed by

Bryant (1955). Since the individual formations of the Andrada group

cannot be separated as far west as the Agua Verde Hills, the group is

lowered to formation status and divided into two members.

Wilson (1951) first gave the name Andrada to a sequence of rocks

in the Empire Mountains, but he did not designate a type section. Bryant

(1955) formally described the Andrada formation near the Andrada Ranch

in the Patagonia quadrangle, and he measured a thickness of about 1,300

feet at this locality.

In the Agua Verde Hills the Andrada formation can be divided into

a clastic (lower) member and carbonate (upper) member. The lower

member is composed of thin bedded sandstones, limestones, shales

and siltstones that form slopes and topographic lows except where capped

by the more resistant upper member. The upper member is predominate

ly black dolomite with numerous white crystalline dolomite stringers.

The upper member caps many MUs in the southern portion of the area.

Lower member. —The lower member of the Andrada formation

is characterized by a great diversity of rock types of various colors

and is roughly equivalent to the Earp formation described by Gilluly,

Cooper, and Williams (1954). One of the best criteria for recognition

27

of this member in the Agua Verde Hills is a very distinctive chert

pebble conglomerate. This unit is found listed in many measured sec

tions of the Earp formation in Southern Arizona. Barton (1925) called

it a jasperiod conglomerate. The thickness of this marker bed in the

Agua Verde Hills varies from two to fifteen feet. The clasts are round

ed to sub-rounded chert pebbles and granules. The matrix is generally

a pink to tan calcareous siltstone but at some localities becomes a pink

limestone.

Thin section analysis of a specimen taken from northeast of

Sandwich Hill shows that the carbonate in the matrix is mostly siderite.

The chert composes about 50% of the rock and is partly recrystallized.

Sericite and hematite are also present in minor amounts. The percent

age of chert clasts will vary from about 10% to 80% of the rock within

a short lateral outcrop distance. Often the marker bed is separated by

a layer (one foot or less) of tan sandstone that is free of any chert clasts

(PI. m).

Localities in the Agua Verde Hills where the marker bed crops

out are east of Sandwich Hill, north of Rod Hill, and east of Red Hill.

North of Rod Hill where the bed was measured (Section E), it is 5 feet

thick. This thickness remains fairly constant tracing the bed eastward

until east of Sandwich Hill, the thickness increases to 15 feet at some

outcrops.

The other clastic beds of the lower member are mostly thin

28

bedded siltstones that alternate with thin bedded limestones. Ripple

marks and cross bedding indicate a probable near shore depositional

environment. Red and brown siltstones are the most common clastic

rock type, although green and red shales are not rare . A petrographic

thin section of a maroon siltstone shows a composition of over 60% an

gular quartz grains. Other minerals include calcite, sphene, zircon,

hematite, clay, and hydrous iron silicates.

Fossils are relatively ra re and those that are found are usually

poorly preserved. The faunal assemblage found in the Agua Verde Hills

include Composita, productids, bryozoans, bellerophontids, echinoid

spines, and crinoid stems.

The measured thickness of the member on the north side of Rod

Hill is 330 feet (Section E). This thickness represents approximately

only the lower two-thirds of the total lower Andrada member composing

Rod Hill since the upper portion is covered by the upper member.

Folding within this clastic member is intense and at some lo

calities strata are overturned. Where this member is not covered by

alluvium, the limits of the outcrop pattern can be seen from aerial

photographs by observing the high degree of folding.

Upper member. —The Upper Andrada member is predominately

a black dolomite that locally rests with apparently unconformable re

lations on the Lower Andrada member. The reason for the apparent

unconformity is not depositional but structural. Due to the high

29

competency of the upper member relative to that of the lower member,

detachment has taken place and folding of the two members is not equal.

The lower member shows intense folding while the upper member shows

only shallow undulations or remains horizontal.

The dominant lithology of the Upper Andrada member is gray and

black, fine grained, carbonates. The carbonates are mostly dolomite.

White crystalline dolomite stringers up to one inch wide (PI. IV) and

white to tan chert lenses are abundant.

The upper member caps Sandwich Hill, Rod Hill, and numerous

small hills in the south central portion of the area. The relative dark

colored lithology of the member compared to the lower member makes

the outcrop limits very easy to distinguish on aerial photographs.

Fossils in the upper member are abundant, but poorly preserved.

Crinoid stems are numerous and there are some echinoid spines.

Brachiopod and pelecypod valves are present but they are badly broken

and re-crystallized. Composita, Cleiothyridina(?), Dictyoclostus, and

bellerophontids could be identified. Gastropods are abundant in the

member but are too poorly preserved for identification. One gastropod

specimen (probably euomphalid) collected has a cross-section normal

to the axis of coiling of two inches.

30

Measured Section E

Partial section of the Andrada formation measured at Rod Hill.

P ennsylvanian-Per mi an: Andrada formation, 330 feet.

Erosion surface .

UnitThickness

(feet)

39. Dolomite; gray and black, numerous small dolomite 20+ veins that are usually white but sometimes pink.Beds vary from 1* to 6r thick, weather very rough, fetid odor upon fracture, fossil fragments common.Forms summit of Rod Hill, represents lower part of Upper Andrada member. Strike N40W dip 20S.

Contact of upper and lower members of Andrada formation


37. Limestone; gray, weathers tan and rough, forms 4slope, some orange chert near top.


35. Limestone; gray, weathers tan at top and pink at 3bottom, medium bedded, some clasts to 1/2" at bottom, weathers smooth, forms slope.

34. Limestone; pink, arenaceous weathers smooth and 3red-brown to black, laminations l / l6 " and crossbedding visible, forms slope.

33. Limestone; gray, massive, weathers pink and rough, 4 horn coral fragments, 1 inch calcite veins, forms cliff. Strike N40W dip 3 58.

32. Limestone; pink, weathers gray and smooth, l/2" 4sand layers cause a differential erosion, sand weathers orange, forms ledge.

31. Limestone; pink, weathers orange-brown and smooth, 2 small amount of chert as nodes, forms slope.

Unit

31

Thickness(feet)

30. Limestone; gray, arenaceous thick bedded, weathers pink at top, forms ledge.

6

29. Limestone; gray, weathers reddish brown and smooth, finely laminated, forms slope.

2

28. Limestone; gray, weathers pink and rough, thick bedded, some limestone clasts up to 4" near bottom, solution cavities up to 2” diameter, forms ledge.

4

27. Limestone; black, weathers rough, medium bedded, calcite stringers l/4 n. Strike N35W dip 40S.

2

26. Limestone; black, dolomitic, weathers smooth, medium bedded, 1/4" calcite stringers, forms ledge. Strike N35W dip 40S.

6

25. Limestone; pinkish gray, weathers green and gray, thin bedded, forms slope.

1


23. Limestone; gray, weathers green and brown, l/2 " sand layers cause a differential erosion, fine grained, forms slope.

2


21. Limestone; gray, weathers pink at bottom and tan at top, no visible bedding, weathers rough, forms slope.

1

20. Limestone; pink, weathers red-brown, thin bedded, silty near bottom, weathers smooth, forms slope. Strike N30W dip 55S.

2


18. Siltstone; red, weathers brown, caliche in fractures, medium bedded, forms slope.

11


32


16. Limestone; gray, arenaceous, weathers pink, . < 2forms slope.

15. Limestone; gray, weathers orange-brown and 3rough, no visible bedding, 1/4" calcite stringer s, forms ledge.

14. Limestone; gray, massive, weathers pink and rough, 2 some productid fragments.

13. Conglomerate; chert pebbles, matrix silty limestone, 5 forms ledge, strike N40W dip 308 (marker bed, see text). ----

12. Limestone; pink, arenaceous, weathers orange-brown 4and rough, massive, chert lenses up to 6" thick near top, forms ledge.

11, Covered interval 12

10. Limestone; gray, arenaceous, fine bedded, weathers 3pink and rough, small chert nodules, forms slope.


8. Siltstone; maroon, weathers smooth and brown, 8caliche in fractures, calcareous near bottom, forms slope.


6. Siltstone; gray, calcareous, weathers green and 6brown, shows some fissility, forms slope.

5. Limestone; gray, weathers pink and rough, some red 2 shale inclusions, forms slope.

4. Shale; brick red, weathers green, poor fissility, no 8calcite content, forms slope.

3, Siltstone; gray, weathers orange-brown, calcareous, 13forms slope.

33

Unit

2. Shale; maroon and green (maroon most fissile), some thin siltstone beds, forms topographic saddle.

1. Siltstone; gray, weathers orange-brown, calcareous, fine bedded, forms ledge. Strike N45W dip 40S.

Horquilla fault contact

Thickness(feet)

5

24

PLATE m

Chert pebble conglomerate marker bed of the Lower Andrada

member.

PLATE III

PLATE IV

White crystalline dolomite stringers that are abundant in the

Upper Andrada member.

PLATE IV

36

Cenozoic Rocks

Pantano formation. —The name Pantano has been in general

geologic use throughout Southern Arizona for many years, but it was

not formally defined as a formation until Brennan (1957) made his re -

connaissance of the Cienega Gap area. Brennan measured 13, 762

feet of clastic and volcanic beds of the Pantano formation and dated

them as Miocene(?).

The Pantano formation occurs to the south and east of the Agua

Verde Hills. Everywhere, except in nearly vertical walls of stream

cut valleys, the Pantano formation is covered unconformably by alluvium

(PI. V). The best exposures were observed at two locations in Agua

Verde Creek: to the south of Rod Hill and one-half mile east of Red

Hill. The formation is interpreted as being present under the south

ern portion of the Paleozoic thrust sheet.

The predominant lithology of the Pantano formation around the

Agua Verde Hills is a maroon, polymictic conglomerate. The matrix

is red, quartz sandstone with a high percentage of carbonate cement.

The consolidation is generally poor and the beds are easily eroded.

Clasts of limestone, dolomite, arkose, sandstone, granitic and volcanic

rocks range from 3 feet to about 3 inches diameter. The largest clasts

are granitic and are most abundant to the east of the area, and the

smaller carbonate clasts predominate to the south. The poor stratifica

tion, lack of sorting, and sub-angular shape of the limestone clasts

37

indicate a dump type deposit. The sources of these various clasts

must have been relatively nearby. Locally an imbrication of the small

er clasts can be observed. Southwest of Sandwich Hill the direction of

imbrication was measured at 220 degrees and the average dip about 30

degrees NE.

South of the Agua Verde Hills there are a few localities where

the maroon conglomerate lithology is not present. Here the lithology

is a medium grained, green, volcanic sandstone with some thin beds of

coarse, gray sandstone.

Alluvium. —Alluvial deposits of the area are characterized by

elevation above stream level. In the southwestern part of the area al

luvial deposits occur up to 100 feet above the stream bed of Agua Verde

Creek. Streams in the southern portion of the area are presently cutting

down into the alluvium leaving valley walls of exposed alluvium more

than 30 feet high. Two river terraces may be observed that approximate

ly parallel Pantano Wash near the confluence of Agua Verde Creek with

Pantano Wash.

The greatest extent of aUuvium in the area is to the south of the

Agua Verde HiUs. .These deposits rest unconformably on the Pantano

formation and contain igneous, metamorphic, and sedimentary rock

fragments. Where this conglomeratic debris has become consolidated,

the cement is caliche or calcarenite.

PLATE V

Angular unconformity between the Pantano formation and alluvi

um. The Pantano rocks dip approximately 40 SW.

PLATE V

39

Igneous Rocks

The igneous rocks of the area consist of a quartz monzonite

(Williams, 1955) stock, basic intrusives, and highly siliceous intru-

sives of almost pure quartz. The quartz monzonite crops out along

the northwestern part of the area and is part of the same rock desig

nated by Moore and Tolman (undated) as a stock intruding Cretaceous

sediments. The basic and siliceous intrusives appear locally along

the principal thrust plane and less frequently in zones of weakness in

the Late Paleozoic sedimentary rocks.

Quartz Monzonite

P art of a quartz monzonite stock is exposed in topographic lows

along the northwestern boundary of the Agua Verde Hills. This rock

was named the Rincon Granite by Moore and Tolman (undated) and dated

as Late Cretaceous or Early Tertiary.

In hand specimen the rock is deeply weathered, phaneritic, and

contains quartz, feldspar, biotite, epidote and hornblende. Dark in

clusions up to one cubic foot are not ra re (PI. VI). The border between

the inclusions and host rock is transitional within about 1/2 inch. There

is a definite concentration of quartz phenocrysts around the inclusions.

From study of the boundary between the inclusions and the host rock,

the basic inclusions probably were assimilated before the quartz had

crystallized.

40

Composition of a typical specimen of the quartz monzonite as

determined from thin section analysis is as follows:

Quartz 30%Plagioclase 25%Orthoclase 25%Hornblende 5%Biotite & clinochlore 5%Others: 10%

Sericite & clayClinozoisiteEpidote

. SpheneHematiteApatite

The texture of the rock is holocrystalline, hypidiomorphic, seriate

granular. A large percentage of the feldspars are in various stages of

being altered to sericite and clay. Clinozoisite present is also an altera

tion mineral from the plagioclase. The composition of the plagioclase

is in the range from sodic andesine to calcic oligoclase. Fine granula

tion along micro shears and undulatory extinction of the quartz is evi

dence that the rock has been subjected to deformation. Locally the

quartz is sagenitic. Biotite also shows evidence of deformation by

curved or bent crystals. Most of the biotite is altering to.clinochlore.

Some of the biotite is a result of the alteration of hornblende.

The megascopic inclusions in the quartz monzonite are very

dark green, medium grained, phaneritic and contain phenocrysts of

quartz. In addition to the quartz, hornblende and biotite are discernible

in hand specimen. The composition of an inclusion as resolved by thin

41

section study follows:

Hornblende")Biotite lClinochlore j

30%

Quartz 25%Plagioclase") Sausserite j 35%Others:

MagnetiteHematiteApatite

5%

The plagioclase is subhedral and of oligoclase composition. Biotite is

altering to clinochlore and shows some bending but not to the extent

that it does in the host rock. The quartz is seriate and has many small

inclusions of apatite.

Basic Intrusions

Outcrops of basic intrusives in the area are found in zones of

thrust faulting and usually have small areal extent. The largest out

crop is west of Red Hill. The rock at this outcrop is a green, phaneritic,

deeply weathered diabase. The diabase is definitely post Bolsa (PI. VII)

and probably post major thrusting. Petrographic thin section analysis

shows the following composition:

Plagioclase altering to sericite and clinozoisite 65%

Uralite altering to •clinochlore 15%

Hypersthene (relict) 5%Magnetite and hydrous

iron silicates 10%

42

Others: 5%Sphene and leucoxeneZirconApatiteEpidote

The subhedral plagioclase laths tough each other and show progressive

zoning with a more calcic center. The composition of the plagioclase

ranges from labradorite to calcic andesine.

Layton (1958) suggested that the basic intrusives in his area

may be the result of magmatic differentiation from one parent source.

This reasoning also can be projected to the Agua Verde Hills area. The

diabase occupies the topographic low areas. Topographically higher

above the diabase, the basic intrusives become finer grained and have a

slightly more acidic composition. A typical specimen taken from a

small thrust zone in Horquilla limestone about 150 feet topographically

above the diabase shows the following composition:

Quartz and calcite(amygdaloidal) 10%

Plagioclase (altered) 20%Pennine (uralite

psuedomorphs) 40%Clinozoisite 5%Others: 20%

Hydrous iron silicatesHematiteMagnetiteApatiteSphene and leucoxene

. Clay :• / • :

Although the plagioclase has been altered some original andesine re

mains. The composition of this rock is essentially the same as Layton*s

43

meladiorite (Williams, 1954).

Quartz Intrusions

Layton (1958) mapped three large outcrops of white quartz which

he called quartz pods in the western part of his area. He concluded that

the quartz was the result of recrystallization after brecciation.

The intrusive quartz bodies in the Agua Verde Hills form large

ovate outcrops that occur along the thrust plane to the east and south.

Other outcrops occur locaUy in zones of weakness throughout the

Horquilla formation and Lower Andrada member (PI. vm). The silica

replaced the elastics of the Lower Andrada member and carbonates of

the Horquilla formation and left many relict structures. Because of the

presence of the relict structures, it seems probable that the silica was

intruded post major thrusting. However, since a basic intrusive that

shows shearing effects from the thrusting intruded the quartz, the dating

of both the basic rock and quartz is interpreted as during thrusting.

This relationship can be observed in an exploration pit located near the

center of the largest quartz outcrop in the area (southeast corner).

From this evidence the deduced sequence of events was 1) thrusting

started, 2) quartz intruded, 3) basic intrusives, 4) thrusting ended.

A stockwerk structure is present in many places where the

silica replaces limestone. Where the silica replaces Lower Andrada

member elastics, there are vugs of up to 4 inches diameter with large

secondary euhedral quartz crystals growing toward the center of the vug.

Study of petrographic thin section of the quartz shows most of

the quartz to be anhedral or subhedral. The sutured margins of the

grains is clean and no lattice layer minerals surround the grains. The

size of the grains varies from cryptocrystalline to megascopic. Granu

lation and re-crystallization are evident along zones of micro shearing.

Hornblende and crystalline hematite are present but constitute less than

1% of the rock.

PLATE VI

Dark inclusions in quartz monzonite at an outcrop near the Day

Ranch.

PLATE VI

PLATE VII

Diabase intruding Bolsa quartzite approximately 1/2 mile north

west of Red Hill.

PLATE VII

PLATE Vin

Intrusive quartz surrounding limestone (limestone is the dark

rock at the head of the hammer).

PLATE VIII

STRUCTURE

General

Basically, the geologic structure of the Agua Verde Hills is a

thrust block of Paleozoic rocks overriding Cenozoic sedimentary rocks

and Laramide igneous rocks. Locally the structure becomes very com

plicated by the inversion of beds, the imbricate nature of the thrust

faulting, and later super-imposed faulting. Tension faulting that follow

ed the thrust faulting offset some fold axes in the Lower Andrada mem

ber, and elevated the Pantano formation in the southwestern part of the

area. Folding in the area is a result of the compression during the

thrusting.

Folding

The intensity of folding in the Agua Verde Hills varies with the

lithology. The less competent Lower Andrada member is tightly folded,

and the axial planes of some of the folds show rupture. The intensity of

folding at most locations has slightly surpassed concentric or parallel

type folding and progressed to similar folding. Slaty cleavage has

started to form at the crest of some anticlines, but no apparent thinning

of the limbs could be detected. The large syncline southwest of Agua

48

49

Verde Hill shows an outcrop pattern of an accordion fold (de Sitter,

1956). Of all the folds in the Lower Andrada member where a plunge

could be determined, the plunges were toward the west 10 to 30 degrees.

The reason for this continuity of plunge direction is not apparent but is

probably due to either differential forces during thrusting or the slope

of the thrust plane was toward the west. The northern flanks of the

anticlines dip steeply or are overturned, while the dip of the southern

flanks is usually at least 10 degrees less than the northern flank. The

best illustration in the area of an overturned anticline is the one located

southwest of Agua Verde Hill near the Lower Andrada member-Pantano

contact. Here, one limestone bed on the northern flank of this anticline

can be traced for a distance of over 25 feet. In this interval the bed dips

45N at the west end and at the east end is overturned to the south 60

degrees.

The massive, thick bedded, limestone lithology of the Pennsyl

vanian Horquilla formation makes it less susceptible to the extreme

degree of folding that is common to the Lower Andrada member. Prob

ably the most severe folding in the Horquilla formation can be observed

on the western side of Agua Verde Hill (pi. IX). At this location the

northern flank of the anticline is nearly vertical, and the southern flank

dips only 25 degrees. Rupture at the crest of this fold is apparent.

50

Faulting

Faulting in the area has resulted in complex, confused strati

graphic sections which make fault displacement determinations virtually

impossible. Thrust faults, shear faults, and tension faults are all ap

parent in the area.

The major structural feature of the area is thrust faulting that

put Paleozoic rocks on top of the quartz monzonite and Pantano forma

tion (Fig. 2). As this thrusting progressed an imbrication took place

making many small thrust zones and taking many beds out of their cor

rect stratigraphic sequence. All the thrust faults observed are bedding

plane faults or very nearly so. All Paleozoic rocks mapped in the area

were involved in the thrusting. Brennan (1957) showed that quartz mon

zonite was involved in a thrust block several miles south of the Agua

Verde Hills. If this is also true in the Colossal Cave area, there should

be a major basal thrust zone under the quartz monzonite. The possibility

of such an observation in the region seems feasible on a local scale, but

not to include all quartz monzonite adjacent to the area.

Compressive forces have further complicated the area by invert

ing some beds. Those beds definitely inverted are so indicated on Plate

I. However, due to the lack of diagnostic primary features in most of

the rocks, many facings may be down that are assumed to be up. This

possible situation is further exemplified by the structurally controlled

position of the Lower Andrada member. This member more often is

B' BEAST SLOPE OF

AQUA VERDE HILL

SKETCH OF SECTION B-B

A' ANW <-

SKETCH OF SECTION A-A

FIGURE 2

52

below the Horquilla formation than it is above; its correct stratigraphic

position. This may be suggestive of major overturning.

Thrust faults in the area can be recognized by zones of breccia-

tion that follow a near horizontal attitude. Siliceous or basic intrusives

are often emplaced in these zones. Shearing and recrystallization is

common within the intrusives. North of Horse Hill the Bolsa quartzite

has been thinned by imbrication and a zone of brecciation can be observed

(Pi. X) where the Bolsa quartzite overrides the quartz monzonite. South

of Agua Verde Hill at the east end of the large quartz outcrop, Paleozoic

limestone beds can be observed on top of the Pantano formation (PI. XI).

Here, Pantano rocks show the effect of some squeezing but the limestone

shows more severe grinding and shearing.

Shear faults and tension faults assume an expected pattern from

a general north-south compression. Tension faults assume north-south

and east-west trends. Shear faults which are less conspicuous than the

tension faults assume northeast-southwest and northwest-southeast

trends.

North-south trending tension faults can be traced for considerable

lateral distance compared to other fault trends in the area. East-west

tension faults are best expressed at the crests of folds in the Lower

Andrada member. Many of these folds have caliche deposits along

their crests in addition to crumpling and twisting of the rocks. All of

the observed tension faults in the area are steeply dipping and often

53

have caliche or intrusives associated with the fault zone. The faults

probably formed during a relaxation of the compressive,forces that

caused the thrusting. Although the direction of movement and displace

ment is obscure, both strike slip and dip slip are believed to be present

on most of the tension faults. Displacement along east-west faults south

of Rod Hill uplifted the Pantano formation in the southwestern portion of

the area. Within the Pantano formation there are numerous steep dipping

faults with small displacements. Determining the relative movement of

the sides is not possible at most localities. Where the relative move

ment could be determined by associated drag folding, the faults are

normal.

Shear faults are much less distinct than the tension or thrust

faults and have a greater deviation in their trends. Lateral extent of

these faults can be traced much less distance than the tension faults.

The shear faults are steeply dipping (usually over 60 degrees), apparent

ly have small displacements, and are mostly normal.

Jointing

Jointing within the quartz monzonite is common and at least two

directions seem to be consistent on the major outcrop in the northwestern

portion of the area. One direction of jointing has an average strike of

N70E and dips 65 to 80 degrees to the northwest. At one location this

joint set looks like it might be a foliation plane. The other joint set

54

strikes N50W and dips 65NE.

The diabase west of Red Hill has one major direction of jointing

and one minor direction. The major direction is N10E and dips 70E.

This joint set is continuous over the entire outcrop, and probably rep

resents tension joints. The other direction is N80E and dips 45N and

is not found as extensively or pronounced over the outcrop as the first

joint set.

Jointing within the sedimentary rocks of the area is not well

developed, however, the chert pebble conglomerate bed of the Lower

Andrada member shows some jointing. East of Sandwich Hill the prom

inent joint set in this bed has an average strike of N10W. A less develop

ed set strikes N80E. This is very similar to the joint pattern within the

diabase and is further evidence that the diabase was intruded before the

thrusting ceased.

PLATE IX

View of the west side of Agua Verde Hill. Note the steep dip of

the northern flank of the anticline (arrow) compared to the dip

of the southern flank.

PLATE IX

PLATE X

Brecciation of Bolsa quartzite along a thrust plane north of

Horse Hill. (Dark rocks in the background are limestones

of the HorquiUa formation.)

PLATE X

PLATE XI

Thrust contact of Pantano formation and overriding Paleozoic

limestone.

PLATE XI

ECONOMIC GEOLOGY

Several prospect pits have been dug in the area, but apparently

no ore has been found. The combination of the quartz-monzonite and

carbonate rocks has, no doubt, led to some optimism. However, the

carbonate rocks have been thrust over the quartz-monzonite rather

than intruded by it, and, therefore, the possibility of ever finding a

large ore body in the Agua Verde Hills is remote.

Mineralization that does occur is associated with the quartz

intrusives. Malachite, chrysocolla, hematite, and limonite were the

only minerals recognized in hand specimen. No primary minerals

were found. Layton (1958), however, found galena and chalcopyrite in

his area immediately west of the Agua Verde Hills.

A barite vein west of Red Hill occurs within the Bolsa quartzite

(PI. I). The outcrop is on a cliff about 30 feet above the bed of Agua

Verde Creek (PI. XII). On the face of the cliff the vein is approximately

10 feet long and 2 feet wide. The barite is white with some orange tints.

Previous prospecting activity near the vein is obvious by small pits and

claim corners.

Magnetite sands can be observed at many locations in the stream

bed of Agua Verde Creek. The source of the magnetite was not ascertained.

58

59

The possibility of oil beneath the area of the Agua Verde Hills

is left to conjecture. No traps survived the thrusting action since most

folding has passed beyond parallel folding into similar folding. Any oil

would have escaped once similar folding is attained (de Sitter, 1956).

Beneath the thrust sheet is the Pantano formation which would be a poor

potential reservoir rock. If the Pantano formation is over 13, 000 feet

thick as Brennan (1957) postulates, there is little hope that reservoir

rocks may be projected beneath it.

PLATE XH

Barite vein (arrow) above Agua Verde Creek west of Red Hill.

PLATE XII

PARTEGEOLOGIC HISTORY

STRUCTURAL AND IGNEOUS HISTORY

Evidence of any structural activity previous to the thrusting of

the Paleozoic rocks has been destroyed by the intense deformation of

the thrusting. Brennan (1957) believes that there was some regional

tilting prior to the thrusting as evidenced by the present dip (up to 50

degrees) of the Pantano formation. Proof of this is lacking in the Agua

Verde Hills. The dip of the Pantano formation is definitely too great

to be completely attributed to initial dip. The Pantano formation was

tilted either prior to the thrusting or as result of the tension faulting

that followed the thrusting.

On the basis of observations by Moore and Tolman (undated)

the quartz monzonite in the northwestern portion of the area is dated

as Laramide. This igneous body is tentatively considered a stock.

Uplift accompanied the intrusion of the stock, and erosion later exposed

the igneous body. Clasts of quartz monzonite are found in the Miocene(?)

Pantano formation that formed on the flanks of the uplift.

During late Cenozoic time (post-Pantano) forces from the south

thrust a block of Paleozoic sediments over the Pantano formation and

61

62

the quartz monzonite. Layton (personal communication, Jan ., 1958)

included the Pantano formation within the thrust block on the basis of

the similar fracture pattern of both the Pantano formation and late

Paleozoic rocks. However, the Pantano formation is a very incom

petent formation and should show a more direct participation in the

thrusting than it does if it were involved. F irst, noticeable folding

within the Pantano formation is lacking. Folding from the compressive

forces of the thrusting should be intense since the Pantano formation is

even a less competent formation than the Lower member of the Andrada

formation. Second, the contact pattern is regular and no Pantano rocks

outcrop within the thrust block. Third, where the thrust plane can be

seen (PL XI) the Pantano rocks are relatively undisturbed while the

carbonates above show evidence of shear and grinding.

During the thrusting siliceous intrusives came in along zones of

weakness. Some mineralization was emplaced by these intrusions. The

quartz intrusions were followed by basic intrusives of various composi

tion as discussed under Basic Intrusions. Since the basic intrusives

show the effects of shear, it is interpreted that the thrusting was still

proceeding during the emplacement of the intrusions.

After the thrusting, tension faulting offset some fold axes in the

Lower Andrada member. Tension faulting is also interpreted to have

uplifted the Pantano formation in the area southwest of Sandwich Hill.

At localities in the Pantano formation where relative movement of the

63

faulting can be ascertained the faulting is normal with steep dip. Accord

ing to de Sitter (1956) normal faulting within the Basin and Range province

ranged from Late Middle Miocene to Late Pleistocene.

The present topographic relief is the result of erosion during late

Cenozoic time. The two river terraces that roughly parallel Pantano

Wash indicate renewed incision by drainage. Gilluly (1956) dated renewed

incision along the San Pedro River (twenty miles southeast of the Agua

Verde Hills) at about 1880.

PALEOGEOGRAPHY

Deriving seaways and shorelines from a study of rocks in the

Agua Verde Hills is impaired for two reasons: First, the area is too

small, and second, the Paleozoic rocks present are not at their original

site of deposition. A depositional environment, however, may be inferred

for several of the rock units from study of their primary features.

The sequence of the late Paleozoic rocks in the Agua Verde Hills

shows a regressive trend of the seas and shallow water depositional

environment. The dense, autochthonous limestones of the Pennsylvanian

Horquilla formation represent maximum transgression. Alternating

limestones and thin clastic beds near the top of the Horquilla formation

grade into the clastic Lower Andrada member near the close of Penn

sylvanian time. Layton (1958) further proves the regression on evidence

of fusilinid location. The alternating thin carbonate beds and clastic beds

of the Lower Andrada member indicate a series of short oscillations of

the shoreline. Current ripple marks were found in some sandstone beds

of the Lower Andrada member southeast of Sandwich Hill (PI. XHI).

Bucher (1919) points out that current ripples may form in limestones at

a depth of 250 meters. More recent studies of current ripple marks

indicate that ocean currents are active on the bottom beyond the conti

nental slope to at least 9,000 feet deep (Dietz, 1958). However, since

64

65

these current ripples are in a medium grained sandstone rather than

limestone and the beds above and below the ripple marks alternate from

coarse, bright colored elastics to carbonates, it seems reasonable to

postulate a shallow water origin for these current ripple marks. There

is also some cross-bedding within the lower member. The broken con

dition of the fossils may be indicative of a littoral or a shallow neritic

zone environment or may be the result of similar folding. The source

of the elastics according to McKee (1951) was the Uncompahgre-San

Luis Highland. Elsewhere in Southern Arizona thick beds of gypsum

were being deposited simultaneously with the dolomites of the Upper

Andrada member, indicating intervals of restricted circulation (Bryant,

1955).

Conditions of deposition during Bolsa time were also near shore.

The base of the Bolsa quartzite is a poorly sorted conglomerate of sub

rounded quartz pebbles up to 3 inches diameter. Cross-bedding also

occurs in the lower member of the Bolsa quartzite. Ransome (1904)

concluded that the Bolsa quartzite was deposited as the sea advanced

over subsiding Precambrian crystalline rocks that were nearly a level

plain.

The Pantano formation is the only sedimentary unit in the Agua

Verde Hills that outcrops at its site of deposition. This formation has

all of the salient features of an orogenic conglomerate (Pettijohn, 1949).

An imbricate structure of the clasts may be observed at several localities.

66Although insufficient readings were taken to ascertain a definite trend,

the few that were taken indicate a current flow to the southwest.

PLATE Xm

Asymmetrical ripple marks from a sandstone unit in the Lower

Andrada member. Arrow indicates N65E (strike of the bed).

Current direction was toward the camera (approximately N40W).

Average wave length of the ripple marks is 1.7 cm.

PLATE XIII

REFERENCES CITED

Brennan, D. J . , 1957, ”Geological Reconnaissance of Cienega Gap,Pima County, Arizona, ” Arizona Univ., Ph. D. thesis.

Bryan, K ., 1925, "The Papago Country, Arizona," U. J5. Geol. Survey Water- Supply Paper 499.

Bryant, D. L ., 1955, "Stratigraphy of the Permian System in Southern Arizona," Arizona Univ., Ph. D. thesis.

Bucher, W. H., 1919, "On Ripples and Related Sedimentary SurfaceForms and Their Paleogeographic Interpretation," Amer. Journ. of Science, vol. XLVI, no. 280, p 254-256.

Darton, N. H., 1925, "A Resume of Arizona Geology," Arizona Bur. Mines Bull. 119.

Dietz, R. S ., Lewis R. V ., and Rechnitzer, A. B ., 1958, "TheBathyscaph," Scientific American, vol. 198, no. 4, p 27-33.

Gilluly, J . , Cooper, J. R ., and Williams, J. S ., 1954, "Late Paleozoic Stratigraphy of Central Cochise County, Arizona," U. J3. Geol. Survey Prof. Paper 266.

Gilluly, J . , 1956, "General Geology of Central Cochise County, Arizona," U. S. Geol. Survey Prof. Paper 281.

Layton, D. W., 1958, "Stratigraphy and Structure of the Southwestern Foothills of the Rincon Mountain, Pima County, Arizona,"Arizona Univ., M. S. thesis.

McKee, E. D ., 1951, "Sedimentary Basins of Arizona and Adjoining A reas," Bull. Geol. Soc. America, vol. 62, p 481-506.

Moore, B. N ., Tolman, C. F . , and others, undated, "Geology of theTucson Quadrangle," Unpublished map and text on file at Arizona State Bureau of Mines, Tucson, Arizona.

Pettijohn, F. J . , 1949, Sedimentary Rocks, Harper B ros., P 208, 209.

68

69

Ransome, F. L ., 1904, "The Geology and Ore Deposits of the Bisbee Quadrangle, Arizona, M U. Js Geol. Survey Prof. Paper 21.

de Sitter, L. U ., 1956, Structural Geology, McGraw Hill, p 156, 213, 216.

Weidner, M. I., 1957, "Geology of the Beacon Hill-Colossal Cave Area, Pima County, Arizona, " Arizona Univ. M. S. thesis.

Williams, H., Turner, F. J . , Gilbert, C. M., 1955, Petrography,An Introduction to the Study of Rocks in Thin Section, W. H. Freeman and Co., p l2 l, 131.

Wilson, E. D., 1951, "Arizona Zinc and Lead Deposits, Part II," Arizona Bur. Mines Bull. 158.

Univ. of Arizona Library

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