UNITED STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY

Fracture history of the Redwall Limestone, lower

Supai Group, and Tertiary units on the

Hualapai Indian Reservation,

northwestern Arizona:

Additional data to

Open-File 

Report 87-359

by 

Julie A. Roller 1

Open-File Report 89-463 

1989

This study was funded by the Bureau of Indian Affairs in cooperation with the 

Hualapai Tribe. This report is preliminary and has not been reviewed for 

conformity with U.S. Geological Survey editorial standards and stratigraphic 

nomenclature.

1 10765 Nighthawk Lane 

Flagstaff, Arizona 86004
Table of Contents

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

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

Methods.........................................................4

Fracture pattern................................................ 9

Fracture set 1 (F1) - Redwall Limestone.................... 13

Fracture set 2 (F2) - Redwall Limestone.................... 13

Fracture set 2.5 (F2.5) - Redwall Limestone................ 16

Fracture set 2.5 (F2.5) - Supai Group...................... 16

Fracture set 2.5 (F2.5) - Tertiary units................... 16

Fracture set 3 (F3) - Redwall Limestone.................... 19

Fracture set 3 (F3) - Supai Group.......................... 19

Fracture set 3 (F3) - Tertiary units....................... 19

Fracture set 4 (F4) - Redwall Limestone....................22

Fracture set 4 (F4) - Supai Group.......................... 22

Fracture set 4 (F4) - Tertiary units.......................22

Fracture set 5 (F5) - Redwall Limestone.................... 22

Fracture set 5 (F5) - Supai Group.......................... 25

Fracture set 5 (F5) - Tertiary units.......................25

Fracture set 6 (F6) - Redwall Limestone....................25

Fracture set 6 (F6) - Supai Group.......................... 25

Fracture set 6 (F6) - Tertiary units.......................27

Fracture set 7 (F7) - Redwall Limestone....................27

Fracture set 7 (F7) - Supai Group..........................27

Fracture set 7 (F7) - Tertiary units.......................27

Fracture set 8 (F8) - Redwall Limestone, Supai Group,

and Tertiary units..................................... 27

Mineralization along joints.................................... 30

Fractures in collapse breccias within the Redwall Limestone....30

Summary ....................................................... 31

Tectonic implications of the Hualapai and Coconino Plateau

fracture pattern....................................... 36

Possible controls of breccia pipe locations by fractures ......36

Acknowledgements............................................... 40

References..................................................... 41
Fracture History Of The Redwall Limestone, Lower Supai Group, And Tertiary

Units On The Hualapai Indian Reservation, Northwestern Arizona:

Additional Data To Open File Report 87-359

ABSTRACT

Paleozoic strata and Tertiary units on the Hualapai Plateau and western 

Coconino Plateau are cut by a complex fracture network that record at least 

eight different fracture events. The Mississippian Redwall Limestone was 

sliced by two early fracture sets prior to deposition of the overlying Supai 

Group. The Supai Group and Tertiary volcanic rocks and conglomerates appear 

to share the same fracture history. Breccia within pipes seems to have been 

fractured by the same events common to the Supai Group and Tertiary units. 

Ring fractures found in Supai rock directly adjacent to breccia pipes have not 

been observed in the underlying Redwall Limestone, which suggests that the 

limestone was fractured prior to formation of the pipes. Abundance and 

prominence of the various fractures sets varies spatially from the Hualapai 

Plateau to the western Coconino Plateau, although the relative-age 

relationships between the various sets remains constant.

INTRODUCTION

The Hualapai Indian Reservation is located in northwestern Arizona in the 

southwestern part of the Colorado Plateau physiographic province. The 

reservation is north of U.S. Highway 66 and south of the Lower Granite Gorge 

of the Colorado River (fig. 1). The main portion of this study (31 of 47 

stations) focused on the eastern Hualapai Reservation, known as the Coconino 

Plateau. To augment a similar fracture study (Roller, 1987) conducted on the 

Hualapai Reservation, sixteen additional stations in the Redwall Limestone, 

Supai Group, and Tertiary units were located on the western half of the 

reservation known as the Hualapai Plateau.

The Permian Kaibab Limestone caps the Coconino Plateau (fig. 2), whereas 

the Mississippian Redwall Limestone caps the Hualapai Plateau, an erosional 

surface that formed during Paleocene time (Young, 1979). The Devonian Temple 

Butte Formation and the Cambrian Muav Limestone also crop out in the 

southwestern part of the reservation around Meriwhitica Canyon. Several 

isolated outcrops of the Mississippian Surprise Canyon Formation fill canyons 

as deep as 400 ft (122 m) that were eroded into the upper Redwall Limestone in 

late Mississippian time (Billingsley and Beus, 1985). Remnants of the 

Pennsylvanian/Permian Supai Group crops out sparsely in the northwestern part 

of the Hualapai Plateau and north of Hindu Canyon, and more extensively on the 

Coconino Plateau. Early and middle Tertiary basalt flows lie directly on the 

Redwall Limestone, Temple Butte Formation, and the Muav Limestone on the 

Hualapai Plateau. In only a few areas does basalt cap erosional remnants of 

the Supai Group. Tertiary gravels that filled ancient stream valleys crop out 

across the reservation.

Several large, high-angle, north- to north-northeast-trending fault zones 

cut the region, including the Aubrey, Toroweap, Hurricane, and Separation 

Canyon faults. A complex and lengthy history of both normal and reverse 

movement characterizes most of these faults. Deformation commenced in
Ridenour 

Mine

Aubrey Cliffs

Colorado River

*\ Blue Mountain 

Canyon v

Peach Springs

to\Williams

Explanation \ q 

East dipping monocline

Fault, U and 0 indicate 

upthrown and dawnthrawn 

sides

Figure 1 - Map of the Hualapai Indian Reservation and major structural

features, northwestern Arizona. (Modified from Huntoon, 1981, and 

Billingsley and Huntoon, 1983)
Average or range

in thickness

(in feet)

Harrisburg Gypsiferous 

Member

Fossil Mountain Member

Woods Ranch Member 

Toroweap

Formation Brady Canyon Member 

Seligman MemberWescogame Formation

Watahomigi Formation

Surprise Canyon

Formation 

(Mississippian/Pennsylvanian?j

Temple Butte Limestone

Figure 2 - Stratigraphic column of Paleozoic units that are exposed on the 

Coconino Plateau and Hualapai Plateau. (Adapted from Verbeek and 

others, 1988)

Huolopol 

Ploteou
Proterozoic time for many of the faults, and some, such as the Hurricane 

fault, offset Holocene alluvial deposits (Huntoon, 1974; Shoemaker 1978). 

Huntoon (1974) noticed that some normal faults offset the Redwall Limestone 

and underlying units but not the overlying units.

Several large monoclines, believed by Young (1979) to have formed during 

two phases of deformation during the Late Cretaceous-to-Eocene Laramide 

Orogeny, have locally folded the Paleozoic sediments of the Hualapai and 

Coconino Plateaus. The Aubrey, Toroweap, Hurricane, Meriwhitica, and Peach 

Springs monoclines are thought by Huntoon (1974) to overlie high-angle reverse 

faults, which may be reactivated Proterozoic faults. The monoclines and 

inferred underlying faults trend north and north-northeast.

Recent mapping of the Hualapai Reservation has revealed the presence of 

over 900 collapse features, including about 100 that have been identified as 

breccia pipes (Billingsley and others, in prep.; Wenrich and others, 1986; 

Billingsley and others, 1986; Wenrich and others, 1986; Wenrich, 1985; 

Billingsley and Huntoon, 1983). Breccia pipes are generally cylindrical, 

vertical structures that wholly or partly contain angular to rounded rock 

fragments with or without a matrix (Bryner, 1961). Exposures of pipes along 

canyon walls show that the pipes are steep-sided features, which many workers 

believe originated as caverns in the Redwall Limestone (Wenrich and Sutphin, 

1983; Sutphin and others, 1983; O'Neil and others, 1981; Baillieul and 

Zollinger, 1980; Wenrich-Verbeek and Verbeek, 1980; Bowles, 1977). Continued 

enlargement of caverns within the Redwall prompted the collapse of overlying 

formations into the caves filling the caverns with collapse breccia.

Solution caverns and passages subparallel to bedding are associated with 

areas of breccia pipes, especially in Blue Mountain Canyon. In Blue Mountain 

Canyon, the Redwall Limestone has been dissected by caverns and passages that 

are now clogged with numerous layers of white to gray, massive, and coarse- to 

very coarse-grained calcite crystals, some as large as 15 to 20cm across. The 

different layers of calcite suggest several episodes of fluid flow through the 

caverns. Age relations between breccia pipes and the caverns are unknown, but 

the two features could be related in time. Calcite-filled passageways might 

be the link between subhorizontal passages and the caverns that stoped upward 

to eventually form pipes.

Sutphin and Wenrich (1983) and Wenrich (1985) suggest that the location 

of breccia pipes in the Redwall Limestone farther east on the Marble Plateau 

was controlled by northwest- and northeast-trending fracture zones that 

propagated upward from Proterozoic basement structures. They propose that 

areas in which to focus exploration can be identified from the hypothesis that 

pipes are concentrated along these trends.

This study of the fracture pattern on the Hualapai Plateau is a 

continuation of a preliminary analysis (Roller 1987) of the fracture history 

of the Redwall Limestone, lower Supai Group, and Tertiary units. In addition 

to more fracture stations in the previously studied units, this study (1988) 

focused on six breccia pipes and adjacent Redwall Limestone to determine the 

controls, if any, of the Redwall Limestone fracture network on the location of 

breccia pipes.

METHODS

Forty seven stations were studied across the Hualapai Reservation (figs. 

3 and 4). The majority of stations (26) are in the Redwall Limestone. Of
refer to 

figure 3b

Figure 3 ~ Location map of stations, which start at #61 for continuity with 

Roller (1987). Open circles are stations in the Redwall Limestone; 

black circles are in the lower Supai Group, and asterisks designate 

Tertiary basalts and conglomerates.
V

61

66

^ s

- 66

WOO 2000 FEET

Figure ^4 - Blow-up of station localities in the Ridenour mine area (upper map) 

and Blue Mountain Canyon (lower map). The Ridenour mine is located 

on the Vulcan f s Throne SW 7/£ minute topographic map, and Blue 

Mountain Canyon is found on Robbers Roost Canyon and Peach Springs NE 

7*4 minute topographic maps. Symbols are the same as in figure 3.
these 26 stations, 18 stations are in both breccia pipes and the adjacent 

country rock. Less intensive examination of fracture systems within the Supai 

Group (9 stations), and Tertiary basalts and gravels (10 stations) was also 

conducted to determine if the fracture history of the Redwall Limestone 

differs from that of these overlying formations as proposed in Roller 

(1987). Station numbers in this report start at 161, which is where the 

stations in the 1987 report left off.

At each station, fractures were separated into sets based on such 

characteristics as orientation, size and shape of the fracture plane, surface 

features, mineral fillings, and relative age. Terminating relationships 

between fracture planes establish relative ages of different sets at each 

station. A detailed and thorough discussion of these methods can be found in 

Grout and Verbeek (1983).

Designation of different fracture sets is based not only on joint 

orientation, but more importantly, the evaluation of relative-age 

relationships among joints of both similar and different orientations. Thus 

in this terminology, F1 refers to a discrete fracture episode. For example, a 

fracture that is parallel to F1 fractures, but is determined to be younger, is 

not an F1 fracture because it did not originate during the F1 event (fig. 

5). In addition, a segment of an F1 joint that formed later by reactivation 

of the early joint by later stress is also not an F1 fracture, because it is 

younger than the F1 event. Therefore, although orientations of some of the 

sets in the Redwall Limestone and Supai Group are similar, the relative ages 

of the sets are different in the two units, so not all sets of similar 

orientations are correlative. Instead, in the Redwall Limestone, fractures of 

similar orientation developed at widely separated times, and at least nine 

episodes of fracturing took place. This conclusion will be developed further 

in the following sections.

Techniques used by Grout and Verbeek (1983) were developed in clastic 

rocks and appear to be applicable to limestones as well. However, weathering 

of joint surfaces in the limestone commonly destroys all vestige of surface 

features, such as plumose structures, twist hackle, or arrest lines that 

indicate an extensional origin for joints. Nevertheless, if both sides of 

open, curved fracture surfaces match across the opening, then no shear offset 

has occurred, and the fractures can be inferred to have formed from 

extensional processes. This is the case for the majority of fracture surfaces 

in the Redwall Limestone.

Understanding the fracture history of limestone is further complicated by 

the commonly extensive solutioning and consequent widening of either large, 

single fractures or zones of closely spaced fractures in the Redwall 

Limestone. Terminating relationships among solution-widened fractures of 

different sets are commonly obscure when two parallel joints that once 

terminated on opposite sides of an older joint now coalesce due to solution. 

The result is one apparently large joint that crosscuts the older joint, thus 

erasing the original terminating relationships between the two sets. 

Nevertheless, enough consistent relative-age information was found between 

fractures across the study area to formulate a working hypothesis for the 

history of fractures in the Redwall Limestone, the lower Supai Group, and 

Tertiary units.

In addition, undulating, irregular joint surfaces are common in limestone 

and cause changes in orientation along both the height (measured perpendicular 

to bedding) and the length (measured parallel to bedding of single 

fractures). It is commonly difficult to discern whether variable orientations 

actually reflect 1) an original wide spread in orientation of the same set,
Figure 5 - Simplistic drawing shows a young fracture set (F3 in the drawing) 

that formed parallel to an earlier set (F1). Early F1 fractures 

truncate later F2 fractures, while the younger F3 fractures terminate 

against F2 fractures. These relationships can lead to apparent 

conflicting age relations if it is not understood that F1 and F3 

fractures, although parallel in orientation, formed at distinctly 

separate times in the fracture history of the rock.
2) undulatory joint surfaces due to intense irregular weathering, or 3) 

several closely spaced but short joints of one set that are connected along 

steps whose orientation coincides with that of a different set. Widening of 

the fractures due to solution has destroyed the details. This widening of 

fractures sometimes created difficulties in determining the exposed height and 

length of individual fractures, so measurements were taken of "composite" 

heights and lengths. Composite measurements include the heights and lengths 

of several smaller joints that have coalesced into one large joint. Dips in 

the limestone are commonly difficult to measure precisely, as they often vary 

as much as 20 degrees on either side of vertical along an undulatory joint 

surface.

A continuous spread of orientations between fracture sets (commonly 30 to 

40 degrees) is common amongst all of the sets at one time or another so that 

in areas of complex fracture networks, it is almost inevitable that some 

overlap in orientation occurs. As an example, the relationship between joints 

whose orientations cluster around N25W with the north and N50W sets is not 

always clear. Joints of the N25W set commonly are large and prominent, but at 

some stations, a continuous range in orientations between north and N50W 

occurs. A distinct clustering does exist at numerous stations, however, so 

the N20-30W joints are given set status. It is also sometimes difficult to 

discern whether N70-79E belongs to the northeast set or the east set when the 

entire spectrum of orientations are found at a station. This is why 

orientation alone is an insufficient guide to sets and why relative-age 

determinations are essential to understanding the number and sequence of 

fracture events. The combined use of orientation and relative ages serves to 

distinguish the various sets.

The 1987 study included joints with orientations of N16-39E in either the 

north set or the northeast sets. However, this grouping appears to be a set 

of its own because at 62% of the stations, both low northeast (N16 -39E) and 

higher northeast (N40-70E) joints exist. Age relations indiate that joints 

with low northeast orientations formed during two events, one after F2 

fracturing and the other post F7 (N50W) fracturing. An insight into this 

situation might be gleaned from observations at several stations where joints 

with approximately N30E orientations curve along strike into larger joints 

with N60E orientations. Given that F1 (N50E) joints formed early in the 

Redwall Limestone, then at some later time, if the least principal stress 

direction was within 20 to 30 degrees of the F1 sigma 3, then a new set with 

low northeasterly orientations might use the terminations of the F1 joints as 

points of propagation, in addition to new fractures of the low northeasterly 

orientation forming in unbroken rock. The result would be twofold: a wide 

range of low northeast and northeast (F1) strikes could occur along strike of 

the same joint, and low northeast joints might also occur as shorter 

individual joints, not obviously connected to the earlier formed northeast 

joints. In the lower Supai and Tertiary units, the low northeast set (F2.5) 

is older than the N50E (F6) set, although at three stations, age relationships 

indicated that N35E joints are younger than F7 joints. These young N35E 

joints are designated F3 fractures.

FRACTURE PATTERN

Roller (1987) indicated that the fracture pattern of the Redwall 

Limestone differs significantly from that of the overlying Supai Group and 

Tertiary units on the Hualapai Plateau. The 1987 study stated that the
Redwall Limestone contains seven recognizable fracture sets. This report 

agrees with the findings of 1987, but it also shows that the fracture history 

of units on the Hualapai Plateau is more complex than previously shown. It 

appears, based on relative-age relations, that there have been several 

episodes of reactivation or fracturing parallel to earlier sets in the Redwall 

Limestone. The number of sets as determined by this study (1988) has 

increased to nine: F2.5 (N28E) is now separated out from orientations included 

in F1 based on relative age determinations, and F8 (N38E) is a set that 

parallels F2.5 orientations but was determined to be younger than F1 - F7 

fractures. Reactivation of joints has also occurred within Supai Group and 

Tertiary units. The Supai Group and Tertiary units now contain a minimum of 

seven differentiable sets, plus an uncertain number of reactivated sets.

In the Redwall, joints of the two oldest sets (designated F1 and F2 

respectively) strike N50E and N50W (Roller, 1987). Four younger sets (F2.5, 

F3, F4 and F5) strike approximately N28E, east, north, and N25W. In addition, 

there are three still younger sets (F6, F7, and F8) that have orientations 

similar to F1, F2, and F2.5 respectively, but are demonstrably younger than F3 

through F5 joints. In contrast, the two oldest sets of the Redwall Limestone 

(F1 and F2) are missing in the lower Supai Group: F2.5 - F8 are common to both 

formations. Joints of the two oldest sets in the lower Supai Group strike 

N30E (F2.5), east (F3) and north (F4), and joints of the younger sets strike 

N25W, N50E, N50W, and N30E (F5, F6, F7, and F8 respectively). Although N50E 

and N50W joints are found in both the Redwall Limestone and the lower Supai 

Group, it can be demonstrated by relative age determinations that N50E and 

N50W joints in the Supai are younger than F2.5 - F5 joints. Ten stations 

studied in Tertiary basalts and gravels that cap the Redwall Limestone also 

contain seven sets (F2.5 - F8) common to both the Redwall Limestone and the 

Supai Group, but their fracture history is similar to that of the Supai Group.

Another interesting outcome of this years study is that the most 

prominent sets in the Redwall Limestone on the Hualapai Plateau are not, for 

the most part, the dominant sets in the Redwall on the Coconino Plateau. It 

appears that the relative ages of the sets have remained the same from one 

side of the plateau to the other, but due to a differential stress gradient 

across the plateaus, different fracture sets are of varying prominence from 

place to place. On the Hualapai Plateau, northeast (F1) and northwest (F2) 

fractures are consistently dominant, but on the Coconino Plateau, the younger 

N80E (F3) and N10W (F4) are commonly the most prominent sets in the Redwall 

Limestone. These field observations are substantiated by aerial photographs 

of the Redwall surface. Fracture orientations observed on photographs in the 

Hindu Canyon area of the Hualapai Plateau include N45W, N45E, and N20W. N45E 

fractures are the longest and most prominemt on the photographs. The N20W 

fractures are locally abundant and large. In constrast, north of Blue 

Mountain Canyon, observed orientations include N10W, east, N40-55E, and 

N45W. The N10W and east fractures are the most strongly displayed in the 

aerial photographs on the Coconino Plateau.

Solution along subhorizontal fractures is pervasive in the Redwall 

Limestone. Numerous open caves several meters across and high occur at the 

junction between large joints of the N50E, N50W, N10W and N25W sets and 

horizontal partings.

Joints are well developed in all members of the Supai Group. Unlike the 

weathered surfaces of joints in the Redwall Limestone, delicate plumose 

structures, twist hackle, and arrest lines, characteristic of extension 

joints, are beautifully displayed on joint surfaces in the clastic units of 

the Supai Group. The lower Supai, however, contains several thick limestone

10
units whose joint surfaces have not retained surface features. All sets 

contain coarse-grained pink to white calcite cement along joint surfaces.

The fracture set nomenclature is based on both orientation and age 

relations, so the earliest Supai Group fracture set is designated F2.5 (N30E) 

to match the F2.5 (N28E) fracture episode in the Redwall Limestone. This is a 

new set designation for the Supai Group and Tertiary units as well as the 

Redwall Limestone. In Verbeek and others (1988), they observed in the 

Ridenour Mine area that the north set (their F1) is older than the N70E set, 

which is their F2 set. This F2 fracture set ranges from N60E to N79E, and 

averages N70E (Verbeek and others, 1988). Orientations of their F2 fracture 

set overlap with both my F3 set (N75E to N75W) and F6 (N40E to N75E). The 

often prominent N86E set (F3) and newly designated F2.5 (N30E) that I measured 

in the lower Supai Group do not have an exact correlatives in the upper Supai 

Group in the Ridenour Mine area.

Fracture measurements were taken at eight stations in both cliff 

exposures and stream beds in Tertiary basalts that cap the Redwall Limestone 

and Muav Limestone. At many of the stations, a clear distinction exists 

between fractures formed from extensional forces and those formed during 

cooling of the basalt. Extensional fractures cut several meters of section 

and have fairly planar, smooth surfaces in cross section, and are laterally 

continuous for several meters along strike in plan view (fig. 6). Fracture 

zones, which contain numerous closely-spaced joints, were observed with 

orientations of northwest, northeast, and N25W joints. These criteria suggest 

that these are not cooling joints. At some stations, it was not certain if 

relatively short (less than a meter long), sinuous fractures were extension 

joints or cooling joints. It is possible that some fractures measured include 

a mixed population of both types of joints, although an attempt was made to 

measure only those that appeared to be extension joints.

F2.5 through F8 fracture sets that cut the Redwall Limestone and the 

Supai Group also cut Tertiary basalt and conglomerate. Fractures within the 

N50W and N50E sets are not F1 and F2 sets, but are of the F6 and F7 fracture 

episodes. East fractures are commonly the most prominent, planar, and largest 

of fractures (see fig. 6). At stations #94 and #106, however, the east set 

was not prominent at all, and terminated against north, northeast, and low 

northeast fractures. The fracture history of the Redwall Limestone and Supai 

Group both appear to be complex and suggest multiple episodes of reactivation 

along most orientations, and this is substantiated by age relationships in 

Tertiary basalts.

Two thick (6-25 m), early-middle Tertiary conglomerate outcrops were 

studied in Peach Springs Canyon. The six fracture orientations that cut the 

basalt also cut the entire section of conglomerate. Fracture lengths and 

heights are up to several meters, and spacing ranges from 0.5 to several 

meters. At station #90, fractures were apparent, but they were not as 

abundant or closely spaced as at station #89, possibly because the gravels at

#90 are massive and the thickness of the unit is great, approximately 25 

meters thick. In constrast, layering ranged from 0.5 to 2m thick at station

#89, which agrees with the observation of Grout and Verbeek (1983) that units 

with thin layering are more highly fractured than more massive units.

Below is a description of the fracture sets in terms of orientation, 

relative age, and geometry for the Redwall Limestone, Surprise Canyon 

Formation, Supai Group, and Tertiary basalts and gravels.

11
Figure 6 - Long (up to 5m), relatively planar N80W (F3) joint of extensional 

origin cuts Tertiary basalt at station #101 near Milkweed Spring.

12
Fracture set 1 (F1) Redwall Limestone

The average orientation for F1 is N57E, with a range between N46E-N68E 

(fig. 7). On the western Hualapai Plateau, F1 joints were present at all but 

one station, and they were always the largest and most planar joints. The 

situation is different on the Coconino Plateau, specifically in the Blue 

Mountain Canyon area. N57E fractures remain the oldest set, but spatially 

they vary in relative prominence. They are either poorly developed or are 

totally absent at several stations in the Redwall Limestone on the eastern 

part of the reservation. Instead, N80W (F3) and/or N10W (F4) joints are 

commonly the most prominent joints in the Redwall as well as in the overlying 

lower Supai rocks. However, in Redwall Limestone that bordered breccia pipes 

#369 (adjacent to station #96) and #876 (adjacent to station #78), N57E joints 

were prominent and closely spaced. This was also true with F2 (N55W) joints 

near the pipes. This will be described further in the section on breccia 

pipes and possible explanations will be discussed in the conclusion section.

Exposed lengths of subvertical F1 joints range from 10cm to 1m, with 

exposed heights of 15cm to 1.2m. Composite lengths and heights are larger, up 

to 10m and 15m respectively. It is commonly difficult to tell if the large 

measurements belong to an individual fracture due to solution and the 

resultant coalescence of two or more very closely spaced joints, which is so 

common in the Redwall limestone. Average spacing for F1 joints is 

approximately 10-40cm locally, with spacing up to several meters across some 

outcrops where F1 is poorly developed.

Crosscutting of mineralized F1 joints by F2 joints is relatively common, 

indicating that F1 was mineralized prior to F2 formation, which allowed the 

younger F2 fractures to propagate across the filled F1 fractures.

Fracture zones comprising several closely-spaced joints that cut tens of 

meters of section are widespread on the western Hualapai Plateau, but are not 

as common on the Coconino Plateau. Because F1 is not well developed on the 

Coconino Plateau, fewer fracture zones have formed from the coalescence of 

numerous closely spaced joints. However, solution along the few fracture 

zones that were observed has sometimes created zones up to 1 meter wide, which 

are either open or partially filled by iron-oxide-stained calcite cement.

Fracture set 2 (F2) Redwall Limestone

Subvertical F2 joints strike an average of N55W (fig. 8). Analogous to 

the F1 fractures, the prominence of F2 fractures varies across the 

reservation. F2 joints are found at a majority (37 of 44) of stations, and on 

the Hualapai Plateau they are planar and prominent. However, many of the 

outcrops studied on the Coconino Plateau contain only a few N55W joints, and 

these are commonly not prominent, which is similar to the F1 fracture 

network. For example, at stations #61-#66 north of the Ridenour Mine, N55W 

joints are planar, closely-spaced, the dominant set, and are older than east 

(F3), north (F4), and northeast (F6) joints at three stations, but at two 

stations N55W joints are undulatory, short and weakly developed, and they 

terminate against the east (F3) north (F4) sets. These weakly developed 

joints belong to the F7 (F50W) fracture set as defined by relative age 

relations.

In the Blue Mountain canyon area, early N55W (F2) joints were observed at 

8 out of 14 stations. At the remainder of the stations, they either were not 

observed or were judged younger than F2 joints by relative-age

13
'\ \^~^>\ < 

* \^^ "ff cu\

\ °* *^

N X V \

\ \

t 1 ° ' ' i4 ' \ x\ X\

Kilometers N ^^__^ 

\ to Kingmon |^^ *

/

>( /

\ /

)v ®-^/

L^^ /^^-^^ to Willioms 

^.WPeoch Springs / ^"~~~^^ v

/ ^^

Figure 7 - Map of mean strikes for the F1 fracture set in the Redwall 

Limestone. Overall average for F1 is N50E.
Figure 8 - Map of mean strikes for the F2 fracture set in the Redwall 

Limestone. Overall average for F2 is N51W.

15
relationships. Figure 9 shows a photograph of a planar N55W joint (parallel 

to the pencil) that is filled with Redwall Limestone breccia, which shows that 

the joint formed prior to brecciation.

Exposed heights of 0.2 to 1.3m and lengths of 0.15 to 0.7m are common. 

Composite heights and lengths of 2.5 to 2 meters respectively were measured. 

Average spacing of individual joints outside fracture zones is from 5-20cm 

locally, averaging 40-70cm, with spacing of up to 1-5m across the outcrop 

where F2 joints are not well developed. Fracture zones as wide as 20 to 50cm 

wide were observed.

Fracture set 2.5 (F2.5) Redwall Limestone

Fracture set 2.5 (F2.5) was included in the F1 set in the 1987 report, 

but it is given set status in this report because it occurs at 62% of the 

stations studied, along with the F1 (N57E) set. It is designated set 2.5 so 

that sets F3 through F7 of this report refer to the same sets described in 

Roller (1987). Fifteen relative age relations show that these fractures are 

younger than F1 and F2 fractures and older than F4 through F8. F2.5 fractures 

range from N20E to N37E and average N28E (fig. 10).

At several outcrops, it was observed that N20-30E (F2.5) joints curved 

into N55E joints along strike. One possible explanation for this observation 

is that the least principle stress direction (sigma 3) during F2.5 time was 

close but not quite perpendicular to sigma 3 of the F1 event, so new fractures 

formed using the tips of F1 fractures as points of propagation. The new set 

has orientations close to those of the F1 set, but formed at a later time.

Fracture set 2.5 (F2.5) Supai Group

Age relations indicate that joints with orientations averaging N30E (fig. 

10) occurred both early (F2.5) in the fracture history of the lower Supai 

Group and late (F8). The 1987 fracture report did not distinguish this N30E 

set from the higher northeast set, but with more age relation observations, it 

is now evident that an early N30E fracture set does exist. Subvertical F2.5 

fractures were observed at five of the nine lower Supai stations.

Along the base of a 25m high sandstone cliff west of the Ridenour Mine, 

N30E fractures have exposed heights of 6m and exposed lengths of 2.5m 

average. Layering in most Supai outcrops measured is much thinner, from 30cm 

to several meters. F2.5 fractures terminate against bedding planes, so 

exposed heights average 40cm and composite heights average 2.5m. Exposed 

lengths average 30-60cm. Average spacing of joints is 0.4-1m. F2.5 is also a 

prominent set at the two Supai stations measured in the Quartermaster Canyon 

area.

Fracture set 2.5 (F2.5) Tertiary units

F2.5 fractures were measured in both Tertiary gravels, the Peach Springs 

tuff (I8my) and olivine basalts (fig. 10). Subvertical F2.5 fractures were 

observed at two out of ten stations; at three other stations, the age 

relations are ambiguous.

16
Figure 9 - Photograph of N55W joint (parallel to pencil) that defines zone 

filled with breccia. Breccia consists of Redwall Limestone with 

cemented, comminuted, red Surprise Canyon Formation or Supai Group 

sediment as matrix. Wall of N55W joint is lined with 2mm thick white 

crystalline calcite and altered Redwall Limestone with large calcite 

rhombs bordering the breccia zone. N10W joints (middle right to 

lower left of photograph) cut the breccia.

17
Figure 10 - Map of mean strikes for F2.5 fracture sets in the Redwall

Limestone (solid lines), Supai Group (dashed lines), and Tertiary 

units (lines of dashes and dots). Overall average strike of F2.5 for 

the Redwall Limestone is N28E; Supai Group is N31E; Tertiary units 

are N32E.

18
F2.5 fractures have composite heights of 0.6-3m and composite lengths of 

0.4-2.5m. Average spacings across the outcrop are 30-50cm, and can be several 

meters where they are not well developed. Where they are well developed, F2.5 

joints are relatively planar (fig. 11).

Fracture set 3 (F3) Redwall Limestone

Subvertical joints of F3 range in strike from N81E to N87W with the mean 

centering around N87E (fig. 12). They are not abundant at many of the 

stations in the Redwall Limestone on the western Hualapai Plateau, although 

they are well expressed at some stations (29 out of 44). In contrast, F3 

joints are often very well developed and prominent on the Coconino Plateau, 

especially in the Blue Mountain Canyon area. Exposed heights range from 0.2 

to 1m and exposed lengths range from 0.1 to 1m. Where F1 and F2 are well 

developed, F3 fractures tend to be short segments that terminate against the 

earlier formed joints. Composite heights and lengths range from 1.0 to 2.5m 

and 0.7 to 4.5m respectively. Spacings of individual F3 fractures average 

between 20 and 50cm where they are best developed to several meters where they 

are least abundant.

Fracture set 3 (F3) Supai Group

Strikes of F3 joints range from N84E to N89E; average strike is about 

N86E in this study (fig. 12) and was N86W in the 1987 study. Dips are 

subvertical. F3 joints range from planar in clastic units to undulatory in 

limestone layers. Exposed heights range from 0.3-5m and exposed lengths from 

0.2-2m. Similar to the Redwall Limestone, the larger lengths and heights may 

be composite dimensions resulting from solution along fractures. Spacings 

average 20-50cm where F3 is well expressed to several meters where only a few 

F3 fractures are present. Fracture zones up to 30cm wide are locally 

abundant.

Joints of the east (F3) and the north (F4) sets are commonly the largest 

and most prominent joints in the Supai Group, especially in the Blue Mountain 

Canyon area. At some outcrops, F3 joints were not abundant but nevertheless 

were very large and widely spaced. This is often the case with the oldest 

fractures.

Fracture set 3 (F3) Tertiary units

F3 fractures range from N87W-N87E and average 90 degrees (fig. 12). They 

were observed at seven out of ten stations. At many of the stations in 

olivine basalt, F3 fractures are the most prominent joints and are planar. 

Average lengths range from 0.7-1.5m and average heights range from 0.4-1.5m. 

Composite lengths are up to 7.0m and composite heights are up to 3.5m. 

Average spacing is 20-50cm where F3 is the prominent and abundant fracture 

set, and spacing increases to several meters where they are not well 

developed.

19
Figure 11 - Set of joints directly behind person in Tertiary basalt strike

N20E (F2.5). N50E (F6) joints are well expressed to the right of the 

person. Station

20
Figure 12 - Map of mean strikes for F3 fractures sets in the Redwall Limestone 

(solid lines); Supai Group (dashed lines); and Tertiary units (lines 

of dashes and dots). Overall average strike for F3 fractures in the 

Redwall Limestone is N87E: Supai Group fractures average W86E; 

average for Tertiary units is 90 degrees.

21
Fracture set 4 (F4) Redwall Limestone

F4 joints range in strike between N02E and N14W and average N7W (fig. 

13). They usually dip subvertically. In the Blue Mountain Canyon area they 

are often the most prominent set along with F3 joints. F4 joints have exposed 

heights of 0.2 to 0.9m and exposed lengths of 30 to 40cm. Composite lengths 

and heights are up to 2m. Similar to F3 joints, individual spacings range 

from close, averaging 20-30cm, to several meters apart where poorly 

developed. Abundant fracture zones with composite lengths of 50m and up to 1m 

wide were observed at several stations. Joints in these zones are filled with 

several generations of calcite, with some calcite crystals up to 2cm long, and 

locally with Redwall breccia and red silty filling.

At one locality between station #69 and #70, fractures trending N10W are 

partially filled with finely laminated, red, quartz- and calcite-rich 

siltstone layers cemented by calcite. The siltstone consists of resedimented 

Surprise Canyon or Supai sediments that were deposited into the irregular 

shaped N10W fracture zones.

Fracture set 4 (F4) Supai Group

F4 joints strike from N03E to N13W, averaging N6W (fig. 13). In many 

places, these joints comprise the most prominent set. Joint surfaces are 

planar to broadly curving to undulatory. Exposed heights range from 0.30-1.4m 

and exposed lengths range from 0.5-2.7m. Composite lengths are up to 6m and 

composite heights range from 5-7m. Average spacing where F5 is prominent is 

30-80cm. Calcite fills joints with apertures around 1-2mm.

Fracture set 4 (F4) Tertiary units

F4 joints range from 04E-03W, and average north (fig. 13). They were 

observed at eight out of ten stations. Average heights are 0.2-0.3m and 

average lengths are 0.2-1m. Composite heights and lengths are up to 5m. 

Spacing averages 25-60cm and was commonly several meters. These joints are 

usually planar and are a prominent set in the Blue Mountain Canyon area.

Fracture set 5 (F5) Redwall Limestone

F5 joints have average strikes of N27W (fig. 14) and range from N25W to 

N30W. This N20W-N30W fracture set is not developed everywhere, although 

locally the joints can be quite large and prominent.

Composite heights are up to 2.5m and lengths as long as 3m. Many 

straight, steep drainages and clefts in the Redwall Limestone trend N25W and 

cut hundreds of meters of stratigraphic section, especially on the Hualapai 

Plateau.

This set was observed at only one of the six Redwall stations north of 

the Ridenour Mine (station #63) and it was not well developed. The set is 

also not well developed at the northern-most stations in the Diamond Creek 

area, but becomes more prevalent around Blue Mountain Canyon and on the 

western Hualapai Plateau.

22
Figure 13 - Map of mean strikes for F4 joints in Redwall Limestone (solid 

lines), Supai Group (dashed lines), and Tertiary units (lines of 

dashes and dots). Overall average for F4 in the Redwall Limestone is 

N7W; Supai Group is N6W; and Tertiary units is north.

23
Figure 14 - Map of mean strikes for F5 fracture sets in the Redwall Limestone 

(solid lines); Supai Group (dashed lines); and Tertiary units (lines 

of dashes and dots). Overall average for the F5 set in the Redwall 

Limestone is N27W; average for the Supai Group is N26W; average for 

Tertiary units is N26W.

24
Fracture set 5 (F5) Supai Group

F5 joints range in strike from W20W to W31W, and average W26W (fig.

14). They have exposed heights and lengths of 0.2-1m and 0.2-3m respectively, 

and composite heights and lengths of 1.5-2.5m and 2-7m. In a study of the 

Esplanade Sandstone of the upper Supai Group in the Ridenour Mine area, 

Verbeek and others (1988) did not observe this set at the numerous stations 

studied. They did, however, note its presence approximately 1 mile southwest 

of the mine during photogeologic fracture mapping of the Esplanade platform 

(Verbeek and others, 1988). This is another example of fracture sets that 

vary spatially in prominence across the region.

Calcite-filled joints with apertures less than 5mm are common. Average 

spacing between F5 joints is 10-50cm, increasing to several meters where they 

are less well developed. At station #93, F5 joints are very planar but 

conflicting age relations were observed at this outcrop between W26W (F5) and 

N86E (F3) joints. The W26W joints have the longest composite lengths at this 

station and are closely-spaced in fracture zones.

Fracture set 5 (F5) Tertiary units

F5 fractures occur at seven out of ten stations (fig. 14). They average 

W25W and have exposed heights and lengths of 0.4-1.5m and 0.4-0.7m, 

respectively. Composite heights and lengths are up to 3.0m and 5.0m, 

respectively. Average spacing ranges from 0.2-1.5m. These fractures are 

prominent sets at a few stations where they are also very planar. Fracture 

zones are 20cm wide and contain numerous closely-spaced joints.

Fracture set 6 (F6) Redwall Limestone

Joints of fracture set 6 strike an average of N50E, subparallel to the 

older F1 joints. F6 joints terminate against older fractures and are commonly 

short (less than 20cm), which is typical of young joints. These joints appear 

to be more common on the Coconino Plateau than on the Hualapai Plateau (fig.

15), which consistently exhibits well-developed, large early F1 fractures. F6 

joints may represent a combination of reactivation along F1 trends, and/or 

fractures that formed parallel to F1 trends but at a later time than F1. More 

work is necessary to clearly sort out the F6 history. They were not observed 

at all stations.

At stations #61-#66, northwest of the Ridenour mine, both F1 and F6 

joints were observed based on relative age relations. It is sometimes 

difficult to separate the younger W50E joints from the older ones in limestone 

where few age relations exist and seemingly conflicting age relationships are 

common.

Fracture set 6 (F6) Supai Group

The average strike of F6 joints is W58E (fig. 15). These joints are

commonly sparse but are locally well developed in lower Supai units, such as

at stations #80, #88, and #99 where they are one of the dominant joints

25
Figure 15 - Map of overall strikes for the F6 fracture set in the Supai Group 

(dashed lines) and Tertiary units (lines of dashes and dots). 

Overall average for F6 fractures in the Supai Group is N58E and 

average for Tertiary units is N56E. Too few measurements for F6 were 

taken in the Redwall Limestone to warrant representation on the map.

26
sets. In zones with very long composite lengths, the individual segments are 

very undulatory. Although this set is one of the younger sets, it is commonly 

well developed locally whereever the older sets have wide spacings. This was 

also observed in the 1987 study. At station #95, F6 joints terminated against 

subhorizontal parting joints.

Fracture surfaces are planar to undulatory. Exposed heights range from 

0.2-2m and exposed lengths from 0.2-2.5m. Composite heights and lengths range 

from 0.6-3.5m to 0.2-10m. Average spacing of F6 joints is 20-30cm where well 

developed to several meters in outcrops where they are sparse. Fracture zones 

are relatively common with numerous closely spaced joints within a 15-30cm 

wide zone.

Fracture set 6 (F6) Tertiary units

F6 joints average N50E and were observed at all ten Tertiary stations 

(fig. 15). They have exposed heights and lengths that average 0.3-1.5m and 

0.4-1.5m, respectively. Composite heights and lengths are up to 8m and 6m, 

respectively. Average spacing of F6 joints is 0.2-1m. At station #106, F6 

joints are prominent and planar, although they often exhibit irregular 

surfaces. Fracture zones are common.

Fracture set 7 (F7) Redwall Limestone

Joints of fracture set 7 have orientations similar to those of the F2 

set. F7 joints tend to be short and terminate against fractures F1 through 

F6.

Fracture set 7 (F7) Supai Group

The average strike of F7 joints is N54W (fig. 16), and ranges from N47W 

to N62W. Other characteristics are similar to those of F6 joints in terms of 

prominence, height and length, and spacing.

Fracture set 7 (F7) Tertiary units

Fracture 7 joints occur at seven of the ten stations. The average strike 

of F7 joints is N50W (fig. 16). Exposed heights average 0.4-1m and exposed 

lengths average 0.4-1m. Composite heights and lengths average 0.3-2m and 0.5- 

2.5m respectively. Average spacing of F7 joints is 0.2-1m and is often 

several meters spacing where F7 joints are not well expressed. These young 

joints are usually undulatory (fig. 17).

Fracture set 8 (F8) Redwall Limestone, Supai Group, and Tertiary units

Fracture set 8 has similar orientations of F2.5, but has been determined 

to be younger than all previous sets based on relative age relationships. 

These joints have irregular surfaces and are small both in terms of exposed 

height and length. They are bound by subhorizontal uplift joints. This 

fracture set was also observed by Verbeek and others (1988) in the upper Supai

27
Figure 16 - Map of mean strikes for the F7 fracture set in the Supai Group 

(dashed lines) and Tertiary units (lines of dashes and dots). 

Overall average strike for the Supai Group is N54W and average for 

Tertiary units is N59W.

28
Figure 17 - Caliche-filled N50W (F7) joints parallel to the magic marker are 

terminated by a planar N50E (F6) joint that parallels the pencil. 

Station #94 in Tertiary basalt.

29
Group near the Ridenour Mine, and this F8 set occupies the same relative age 

position in their fracture history of the Ridenour Mine area.

Mineralization along joints

All joint sets in the Redwall Limestone contain calcite cement, usually 

consisting of a lustrous, white to gray coarse-grained calcite. Two types of 

calcite cement can also occur in the same fracture, and typically, the gray or 

white variety is found with an iron-stained one. The iron-stained calcite was 

observed both in the middle of the opening and lining the fracture. In 

addition to terminations of younger joints by earlier joints, crosscutting of 

joints is prevalent in the Redwall Limestone and shows that many early joints 

were closed by cementation prior to formation of later joints.

No cement unique to the two early sets in the Redwall was observed, nor 

was there any cement that was common in proximity to or within breccia 

pipes. The only exception is within pipe #874 (near station #84): limonite 

and goethite nodules were locally ubiquitous and a thin (2mm) black 

(goethite?) filling coated a very small area of a subhorizontal fracture in a 

solution cavern. This was the only case of cement other than calcite observed 

to fill joints in the Redwall Limestone.

Several F2 joints at station #61 contained angular limestone pieces (1- 

4cm long) that were cemented by a caliche matrix. This feature could have 

formed either by very closely spaced anastomosing fractures that coalesced and 

branched apart isolating pieces of limestone or could be brecciated limestone 

that was later cemented by caliche.

Coarsely crystalline white calcite was observed to fill fractures of all 

seven sets in the lower Supai Group and Tertiary units. Aperatures ranged 

from 0-3mm wide.

FRACTURES IN COLLAPSE BRECCIAS WITHIN THE REDWALL LIMESTONE

Stations adjacent to and within six breccia pipes were studied to 

determine if early F1 and F2 joints in the Redwall Limestone controlled the 

formation of the pipes, as hypothesized by Sutphin and Wenrich (1983), Wenrich 

and Sutphin (1988), and suggested by the results of Roller (1987). The 

results of studying these six pipes are not conclusive with regards to 

controlling pipe formation, but some interesting observations were made that, 

with more detailed work, might aid in answering this question. It was 

possible to examine only the upper exposed sections of most of the breccia 

pipes because much of the breccia is exposed on sheer cliff faces that are not 

readily accessible. An exception is pipe #369 (from Billingsley and others, 

1986) in the lower Supai (station #96), which is well exposed and easily 

accessible.

Stations #78 and #79 are not in a pipe proper but are in the Redwall 

Limestone on opposite sides of pipe #876 south of Blue Mountain Canyon. Very 

little of the actual breccia pipe was accessible. In this area, east (F3) and 

north (F4) fractures are generally the most prominent sets in both the Redwall 

Limestone and lower Supai Group. F1 and F2 joints are either weakly 

developed, nonexistent, or prominent only very locally. However, at stations 

#78 and #79, F1 and F2 joints are prominent, planar, and closely-spaced, on 

the order of 10-20cm versus several tens of centimeters to meters at 

surrounding stations. Redwall outcrops at these two stations look like rubble 

due to very close density of fracture spacing of the northeast (F1) and 

northwest (F2) joints (fig. 18).

30
Densely-spaced F1 and F2 joints also occur at station #97 next to pipe 

#369. Figure 19 shows a photograph of prominent, densely-spaced joints in 

proximity to the breccia pipe. The fracture density in the cliff wall several 

hundred meters away from the breccia pipe is reduced. North (F4) fractures 

are most obvious in the photo, but F1 and F2 fractures are also well developed 

and closely-spaced.

Breccia visible at pipe #369 is composed of lower Supai limestone blocks 

(Watahomigi Formation). Most of the joint surfaces in the breccia are very 

irregular and the few that are planar have both subvertical dips or relatively 

shallow (42 to 65 degrees) dips. The most prominent sets in the breccia are 

N20-40E (F2.5) and N20-30W (F5). Less prominent north (F4), northeast (F6) 

and northwest (F7) joints cut the breccia. Immediately adjacent to the pipe, 

however, and approximately 5 meters below station #98, northwest (F2) joints 

(based on relative age determinations) in the Redwall Limestone are very 

densely spaced (1-15 cm) and prominent (fig. 20). Spacing of F2 joints 

increases to several meters away from the pipe.

One possible scenerio is that a densely-spaced early F1 and F2 fracture 

system in the Redwall Limestone localized and concentrated fluid flow, which 

initiated cavern formation along these early fracture networks. Some of these 

caverns later developed into pipes. A densely-spaced subvertical, nearly 

perpendicular fracture network would encourage formation of rectangular blocks 

of Redwall Limestone like the blocks of rubble observed adjacent to several 

pipes. This bears on the apparent absence of ring fractures in the Redwall 

Limestone and the existence of ring fractures as the first fracture network to 

be imposed in the overlying Supai Group as is well-documented by Verbeek and 

others (1988). Verbeek and others (1988) have documented in detail that 

mineralized ring fractures associated with the Ridenour Mine pipe formed in 

unjointed upper Supai rock (Esplanade Sandstone) immediately adjacent to the 

breccia pipe. Ring fractures are unlikely to form or be well-defined in rock 

that is already jointed. Several large ring fractures were observed in Supai 

rock just outside of breccia pipe #369 approximately 100m above the denselyspaced northwest fractures in the Redwall Limestone. No ring fractures were 

observed in the underlying Redwall Limestone.

Summary

Table 1 summarizes the orientations of the different fracture events 

within the Redwall Limestone, Supai Group, and Tertiary units on the Hualapai 

and Coconino Plateaus. The two oldest fracture sets in the Redwall Limestone, 

have no correlatives in terms of both orientation and relative age among joint 

sets in the Supai Group. Thus, the fracture histories of the Redwall 

Limestone and Supai Group overlap only in part. Fracture sets 2.5 through 8 

of the Redwall Limestone appear to correlate with fracture sets 2.5 through 8 

of the Supai Group: the orientations agree and the relative ages are in 

identical sequence. Therefore, these suggested correlations are almost 

certainly correct.

All six orientations of fractures cut breccia within collapse structures 

in the Redwall Limestone and Supai Group. These fractures also cut the 

Tertiary basalts and gravels. No definitive age relations of the fracture 

sets, however, were observed for the breccia, so the suggested correlations of 

sets and inferred similarities of fracture histories are based only on 

fracture orientations.

31
Figure 18 - View of stations #78 and #79 and pipe #876 south of Blue Mountain 

Canyon. Closely-spaced northeast (F1) and northwest (F2) joints give 

the outcrop a rubbly appearance.

32
Figure 19 - Prominent north-striking joints (F4) and closely-spaced northweststriking joints (F2) at station #97 near pipe #369.

33
Figure 20 - Closely-spaced northwest-striking joints (F2) parallel to the 

hammer immediately adjacent to pipe //369. Outcrop is Redwall Limestone.
Table 1 - Summary of average strikes and suggested correlations for fracture 

sets F1 through F8 for the Redwall Limestone, lower Supai Group and 

Tertiary units.

GEOLOGIC UNIT FRACTURE SET

F1 F2 

(oldest 

F2.5 F3 F5 F6 F7 F8 

. youngest)

Redwall Limestone N50E N51W N28E N87E N7W N25W N50E N50W N28E 

Lower Supai Group n.f n.f N30E N86E N6W N26W N58E N54W N30E

collapse breccia n.f n.f N30E N86E N6W N25W N58E N54W N30E 

in Supai Group

Tertiary units n.f n.f N35E east north N25W N50E N50W N35E

Upper Supai Group n.f n.f n.f n.f N2W ap* N68E N60W N35E 

(Verbeek and 

others, 1988)

n.f. 

*ap

= not found

; this set was not observed in the immediate vicinity of the Ridenour 

Mine, but is a relatively prominent set on aerial photographs 

approximately one mile southwest of the mine. Age relations were not 

observed between this set and the other sets, so the placement of 

this set in the fracture history scheme is based on conjecture.

35
TECTONIC IMPLICATIONS OF THE HUALAPAI AND COCONINO PLATEAU FRACTURE PATTERN

Paleozoic strata and Tertiary units on the Hualapai and Coconino Plateaus 

have been subjected to numerous stresses that initiated joint development. 

Although I do not attempt to determine the event responsible for each fracture 

set, it is reasonable to propose that the early northeast- and northwesttrending fracture sets (F1 and F2) in the Redwall Limestone, which do not have 

correlatives in the Supai Group, formed prior to deposition of the Supai 

Group. As mentioned in the 1987 report, residual stress from the Antler 

Orogeny, that caused gentle upwarping during regional uplift at the end of 

Mississippian time (Skipp, 1979), might have imposed the early northwest and 

northeast fracture network upon the Redwall Limestone. Karst features, 

including sinkholes, caverns, and solution breccia were also widespread in the 

Redwall Limestone during the late Mississippian (McKee, 1979; Billingsley, 

1986), and it is possible that karst development was promoted by an extensive 

fracture network.

Fracture sets 2.5-8 cut the lower Supai Group, breccia, and Tertiary 

units, which shows that a great deal of fracturing has occurred in relatively 

recent time, at least post I8my (age of Peach Springs Tuff). Thus a large 

part of the fracture history of the Hualapai region does not overlap in time 

with deformation events that produced the large monoclines that fold 

Paloeozoic strata on the Hualapai Indian Reservation during the late 

Cretaceous to early Tertiary.

The southwest margin of the Colorado Plateau has been uplifted two to 

three miles since the Cretaceous, and more than 3000 feet of uplift has 

occurred along the Grand Wash Fault in the last 5 million years (Wenrich and 

others, 1986). It would be reasonable to expect that this large amount of 

uplift has affected the rocks of the Hualapai Plateau either directly as 

McGill and others (in press) suggest (see below) or possibly millions of years 

later as Ramsay and Huber (1987) suggest. Ramsay and Huber (1987) state that 

it is doubtful that joints form during major tectonic events but most likely 

are related to the last phases of erogenic activity or to a "release of stored 

erogenic elastic strains during periods of uplift, perhaps millions of years 

after the main tectonic event." However, McGill and others (in press) propose 

that normal faults in the Grand Canyon area affect regional stress fields and 

cause fracturing of rocks over wide regions that are not faulted. Locally, 

the faults cause densely-spaced jointing close to the fault traces. If 

faulting does cause some fracture formation, the extensive record of recurrent 

movement along major faults on the Hualapai and Coconino Plateaus (Billingsley 

and others, 1986) might have produced a very complex fracture system composed 

of many reactivated joints or segments parallel to earlier-formed fractures 

during the late Tertiary.

POSSIBLE CONTROLS OF BRECCIA PIPE LOCATIONS BY FRACTURES

It remains premature to state definitively that early F1 and F2 fractures 

in the Redwall Limestone controlled pipe formation, although the following 

observations support the suggestion: 1) Northeast (F1) and northwest (F2) 

fracture sets were imposed upon the Redwall Limestone prior to fracturing 

within the Supai Group.' At what time the fracturing of the Supai Group rocks 

occurred is not known from the available data, but the fact that all of the 

Tertiary units studied also contain the same sets as the Supai Group 

introduces the possibility that the Supai was not fractured until tens of

36
millions of years after it was deposited, perhaps during uplift of the 

Colorado Plateau in the late Tertiary. 2) The close association of pipes and 

the Surprise Canyon Formation (Billingsley, 1986; Wenrich and others, 1986) 

supports the notion that cavern development within the Redwall Limestone 

occurred shortly after deposition and lithification, prior to and/or 

synchronous with deposition of Surprise Canyon sediments in river valleys and 

caverns. Rivers and karst features are both dependent on large water 

discharge, which is facilitated by a fractured and porous rock unit. 3) A map 

of underground passageways and caverns in the Redwall Limestone west of Peach 

Springs Canyon shows five pipes that are all located in northwest (F2)- and 

northeast (F1) - striking passages (Wenrich and Sutphin, 1989). Numerous 

passageways with N05E, N15W, and N30E trends were also mapped, but no pipes 

were found along these trends. 4) Ring fractures appear to be well developed 

in Supai rocks adjacent to pipes, but none have been observed in the Redwall 

Limestone. If the Redwall already contained a network of fractures then the 

ring fractures, which form as a sort of pressure release in unjointed rock, 

would not form in the Redwall as it was already fractured. 5) Four stations 

studied in the Redwall adjacent to pipes exhibited anomolously closely-spaced 

early formed fractures . The closely-spaced joints may have concentrated 

fluid flow, thus localizing groundwater movement and promoting solution and : 

cavern development.

Wenrich and others (1986) state that with a few exceptions, breccia pipes 

on the Hualapai Plateau appear to be distributed irregularly (fig. 21) 

compared to the Marble Plateau where a strong northeast and northwest 

distribution has been mapped (Sutphin and Wenrich, 1983). Alignments that 

were noted on the Hualapai Plateau include N40E, N50W, and N10-20W trends. 

The apparent irregular distribution of pipes on the Hualapai Plateau does not 

necessarily preclude the possiblity of early fracture control of cavern 

formation. Figure 22 is a simplistic illustration showing a network of 

northeast and northwest fractures in the Redwall Limestone. In this diagram, 

it is assumed that caverns occur at both the junction of two major fractures 

and along single fractures, as observed in the cavern map in Wenrich and 

Sutphin (1989). Pipe locations can either appear totally irregular if the 

pipes are spaced far apart or in areas where several pipes occur, one could 

connect pipes in virtually all orientations described in this report - 

northeast, northwest, N25E, northerly, N25W, and easterly. The impression is 

of a seemingly irregular distribution when in fact pipe locations in this 

illustration were originally controlled by early fractures. If one looks at 

just the mineralized pipes (squares with circles) on the Hualapai Plateau (see 

fig. 21), one sees two distinct northerly trends with at least four pipes (Bat 

Tower area and north of Plain Tank Flat) in addition to several easterly, 

northwesterly, and northeasterly trends. Thus it is possible that the early 

northeast and northwest fractures did control cavern location even though the 

distribution pattern appears irregular.

37
Mapped by G.H. Billingsley 

 Mapped during present study 

H Mineralized pipe

^j^m^m^g^^-^

Figure 21 - Map of collapse features, including known and suspected breccia 

pipes, on the Hualapai Indian Reservation. The tree on the eastern 

side of the reservation represents heavily forested areas within 

which collapse features are difficult to detect. Some of these 

collapse features were mapped by G. H. Billingsley prior to 1982; the 

majority were mapped by K. J. Wenrich, G. H. Billingsley, and H. B. 

Sutphin during 1982-1988. Taken from Wenrich, Billingsley, and Van 

Gosen (1986). I

38
A/25£

Figure 22 - Simplistic illustration showing how early northeast (F1) and 

northwest (F2) fractures in the Redwall Limestone could control breccia pipe 

locations at depth in the Redwall Limestone, although the surface expression 

of the pipes, when connected by lines, shows numerous orientations that imply 

that an irregular distribution of pipes exists.

39
ACKNOWLEDGEMENTS

This research was supported by the U.S. Geological Survey and the Bureau 

of Indian Affairs in conjunction with the Hualapai Indian Tribe. The efforts 

of Karen Wenrich made this study possible. Earl Verbeek provided important 

discussions concerning the behavior of fractures. Brad Van Gosen and Anna 

Wilson provided valuable assistance and support in the study area. Thorough 

reviews by Earl Verbeek and Karen Wenrich greatly improved the manuscript.

40
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