Bright Angel and Eminence Faults, Eastern ,

PETER W. HUNTOON | JAMES W SEARS ( Department of Geology, University of Wyoming, Laramie, Wyoming 82071

ABSTRACT PREVIOUS INVESTIGATIONS

The Bright Angel and Eminence faults trend northeastward for Precambrian faults in the vicinity of Bright Angel Canyon are approximately 60 mi (100 km) through the eastern Grand Canyon shown on Figure 3. Ransome (1908)was the first to identify recur- region. The Bright Angel fault parallels basement foliation. Activity rent structural deformation along the Bright Angel fault. He ob- along the fault dates from Precambrian time. The first record of served that the southeastern block was uplifted during late Pre- movement indicates that it was reverse; it coincided with the depo- cambrian time, displacing basement against younger Pre- sition of the basal . Additional reverse move- cambrian sedimentary rocks along a reverse fault, whereas the ment occurred following intrusion of the Unkar Group by southeastern block was downthrown in the usual sense in post- sills and dikes resulting in a total of as much as 1,300 ft (400 m) of Paleozoic time. Noble and Hunter (1916) noted that the fault coin- displacement, east side up. The Precambrian was cides with a contact between distinctive basement rock types; Max- later broken by a series of northwest-trending normal faults that son and Campbell (1933) observed that the fault was parallel to tilted the section toward the northeast, causing minor adjustments basement foliation. The evidence for Precambrian and post- along the Bright Angel fault. Paleozoic movement was reviewed by Van Gundy (1946). Maxson Possible reverse movement along the Bright Angel fault is re- (1961) published an interpretation of the structural history of the corded in a local angular unconformity between the Paleozoic Bright Angel fault and intersecting structures which involved the Redwall and Supai Formations. East-dipping Laramide (?) mono- following periods of deformation: clines developed as reverse movement occurred along selected pre- (1) Pre-Unkar faulting of the basement rocks along northeast- existing northwest-trending Precambrian faults. Tensional faulting and northwest-trending faults. (2) Pre-diabase, post—Dox Sand- beginning in Miocene (?) or Pliocene (?) time caused downfaulting, stone faulting along northeast- and northwest-trending faults. (3) east side down, along the Bright Angel fault. The Eminence fault, Post-Chuar Group block faulting along northwest-trending faults. west side down, is part of a graben complex. Key word: structural (4) Reverse faulting, east side up, along the Alpha fault (Fig. 3), ap- geology. proximately along the line of a major synclinal axis in the basement rocks. (5) Renewed normal faulting and possible left-lateral strike- INTRODUCTION slip faulting along existing northwest-trending faults that offset the Alpha fault. (6) Renewed reverse faulting, east side up, along the The objective of this paper is to document recurrent movements Bright Angel (Beta) fault (Fig. 3) and possible right-lateral strike- along the Bright Angel fault of the Grand Canyon and to examine slip movement along the Beta fault. Laramide (?) movements in- the relationship between the Bright Angel and Eminence faults. clude the following: (7) Reverse faulting, west side up, along The Bright Angel fault (Fig. 1) is the most prominent and well- selected northwest-trending Precambrian faults that produced the known member of a system of northeast-trending faults in the east-dipping Grandview-Phantom monocline. (8) Normal faulting, Grand Canyon region. The fault is conspicuously exposed in the west side down, along the Grandview-Phantom monocline (Fig. 1). Grand Canyon and dips steeply toward the northeast. It can be (9) Normal faulting, east side down, and 1,500 ft (473 m) of right- traced in Permian rocks for more than 20 mi (32 km) to the south- lateral strike-slip movement along the Bright Angel (Gamma) fault west of the Grand Canyon; it terminates against a northwest- that offset the northwest-trending faults. The Gamma fault de- trending monocline near Cataract Creek on the Coconino Plateau. veloped in the Paleozoic rocks above the underlying Precambrian The fault controls the strike of Bright Angel Canyon and terminates Beta fault. to the northeast near the East Kaibab monocline on the Kaibab Shoemaker and others (1974) placed the Bright Angel and Emi- Plateau. nence faults in regional perspective as components of a still active The Eminence fault, which dips steeply toward the northwest, is northeast-trending lineament consisting of Cenozoic normal faults marked by a prominent northeast-trending fault-line scarp on controlled by a Precambrian fault system. Marble Platform northeast of the junction of the Grand and Mar- We have not found evidence for strike-slip movement along the ble Canyons. It terminates against the Echo Cliffs monocline to the Bright Angel fault or along any of the northwest-trending faults in north and the East Kaibab monocline to the south. the area. Consequently, the interpretations that follow are substan- As shown in Figure 1, the Eminence and Bright Angel faults are tially simpler than those proposed by Maxson. closely aligned. The two faults form a structural lineament more than 60 mi (100 km) long. The post-Paleozoic displacement across PRECAMBRIAN DEFORMATION the Eminence fault is west down, whereas the east side is downthrown along the Bright Angel fault. Subsidiary grabens The orientation of the Bright Angel fault plane in the Precam- parallel the Eminence fault. brian rocks was strongly influenced by pre-existing structural The stratigraphy of the eastern Grand Canyon is summarized in weaknesses in the basement complex. At the , the Figure 2. The reader is referred to Noble (1914), McKee (1969), Bright Angel fault and the Alpha fault (Fig. 3) of Maxson (1961) and Ford and Breed (1974) for detailed information on the local are localized at the contacts of a large body enclosed stratigraphy. in amphibolite schist. Northward along Bright Angel Canyon, the

Geological Society of America Bulletin, v. 86, p. 465-472, 10 figs., April 1975, Doc. no. 50404.

465

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36 30 — 36° 30

36° 15 — 36° 15'

36° 00' — 36° 00'

112° 00' III0 45' Figure 1. Principal post-Paleozoic structures in the eastern Grand Canyon region.

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Figure 2. Idealized stratigraphic section in the eastern Grand Canyon. Pk, Kaibab Formation; Pt, Toroweap Formation; Pc, Coconino ; Ph, Hermit ; IPPs, Supai Formation; Mr, Redwall ; Dtb, Formation; Cm, Muav Limestone; Cba, ; Ct, ; pCsm, Sixty Mile Formation; pCk, Kwagunt Formation; pCg, Galeros Formation; pCn, Nankoweap Formation; pCc, Cardenas Lavas; pCd, ; pCs, Shinumo Quartzite; pCh, ; pCi, diabase intrusive rocks; pCb, Bass Limestone; pCv, Vishnu Schist.

Bright Angel fault is subparallel to northeast-frending, dominantly II2°5' vertical foliation in the basement rocks. The fault diverges from the foliation in strike by an average of 8 degrees west and 19 degrees east in exposures respectively north and south of . The dips of the foliation vary as much as 25 degrees from the dip of the fault. No conclusive evidence indicating pre-Unkar movement along the Bright Angel fault was observed by the writers, although Shoemaker and others (1974) used magnetic data to infer that faulting of the basement rocks along northeast trends probably has occurred in the vicinity. Because the contact between the gneiss and schist diverges from the fault plane north of Phantom Ranch, we do not consider the contact to be an ancient expression of the fault. Rather, the contact offered a locally convenient zone of basement weakness for subsequent deformation. Variation in the attitude of foliation by as much as thirty degrees on opposite sides of the fault may be the result of later faulting. The earliest demonstrable movement along the Bright Angel fault occurred during deposition of the basal Shinumo Quartzite (Fig. 4A). At Ribbon Falls (Fig. 3), the 200-ft-thick (61-m) basal member of the Shinumo Quartzite (Division H of Noble, 1914) pinches out from west to east across the Bright Angel fault over a distance of about 1,500 ft (457 m) by successive thinning of indi- vidual strata. In the immediate vicinity of the fault, the maximum dips of the upper Hakatai Shale are 66° northwest, whereas max- imum dips of the superjacent Shinumo Quartzite are 55° north- west. The absence of an erosional unconformity, the general con- formity of the Hakatai-Shinumo contact, and the nature of the thinning of the Shinumo Quartzite suggest that deformation ac- companied deposition of the basal Shinumo Quartzite. The result- ing monocline occurs in the underlying Hakatai Shale and was em- placed in response to reverse faulting, east side up, in the Vishnu Schist along the Bright Angel fault. No other movements along the Figure 3. Principal Precambrian faults in the Bright Angel Canyon area. Bright Angel fault contemporaneous with deposition of the Unkar Traces of faults are projected to the base of the Paleozoic section. The Group have been identified. Bright Angel monocline, which trends along the Bright Angel fault is not Minor deformation has occurred along portions of the Bright shown. Dips of fault planes shown.

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next crossed the monocline to an offset stratigraphic level in the Hakatai Shale and inflated an additional 400 ft (122 m) as shown in Figure 4C. Because the diabase intrusions are confined to the Unkar Group, Ford and Breed (1972) concluded that they predate deposition of the Nankoweap Formation and Chuar Group. Following the intrusion of the diabase, the southeastern block was uplifted an additional 600 ft (183 m) along the Bright Angel fault and overlying monocline. This resulted in a cumulative dis- placement of 800 ft (244 m) to 900 ft (274 m) at the base of the Unkar Group, 600 ft (183 m) at the top of the diabase sill, and 560 w ft (171 m) in the Shinumo and Dox Formations at Wall Canyon (Fig. 3). During this episode, the limbs of the monocline overlying p€d the Bright Angel fault steepened in the Hakatai, Shinumo, and Dox P€U Formations. The diabase sill is displaced along the structure in con- formity with the enclosing sediments and is sheared by joints which 1ft dip 35° west and trend along the axis of the monocline. p€h This period of reverse faulting was responsible for the emplace- J ment of both the Alpha and Beta faults (Fig. 3) of Maxson (1961). p€b As will be shown, the Alpha fault is subsidiary to the Bright Angel p€v (Beta) fault. Bedding in the lower part of the Unkar Group in the footwalls of both faults is severely deformed. In places, bedding in the Bass Limestone and Hakatai Shale is vertical. Bedding of rocks B in the hanging wall is less deformed, although areas of folding coincide with discontinuous, antithetic faults of small displace- ment. The next period of faulting that left a record in the Phantom Ranch area in Precambrian time consisted of movement along large-scale, northwest-trending normal faults and grabens. Many of these faults, including the Precambrian Pipe and Phantom faults, terminate against the Bright Angel fault (Fig. 3). Other faults in the system cross the Bright Angel fault and offset it. The system of northwest-trending faults was responsible for dif- ferential northeastward tilting of the section. As the blocks rotated, smaller adjustments occurred along the Bright Angel fault. The block to the west of the Bright Angel fault was tilted more steeply than that to the east between Ribbon Falls and Transept Canyon (Fig. 3). This caused the section to rupture along a scissors fault coplanar with the underlying Bright Angel fault. The scissors fault had a maximum displacement of 170 ft (52 m) west down at Wall Canyon (Fig. 3) in Precambrian time. A few northwest-trending faults offset both the Bright Angel fault and Alpha fault at Phantom Ranch. The Tipoff graben (Fig. 3) was downfaulted about 500 ft (150 m) at Phantom Ranch and dis- 500 1000 METERS placed the trace of the Bright Angel and Alpha faults in the graben Figure 4. Early development of the Bright Angel fault near Ribbon Falls. block about 250 ft (75 m) toward the northwest. A. Monocline develops in the Hakatai Shale above a reverse fault in the From this geometry, it is clear that no episode of normal faulting basement causing the basal Shinumo Quartzite (pCs) to pinch out to the along the Phantom and Cremation faults occurred between the east. B. Diabase (pCi) intrudes from the west, causing an expansion fault to movement of the Bright Angel (Beta) fault and Alpha fault, as develop along the axis of the monocline. Diabase then intrudes the fault and Maxson (1961) proposed. Maxson also inferred on the basis of feeds a sill to the east. C. Diabase sill crosses the monocline and inflates the structural similarity that the Cremation and Phantom faults were section on both sides of the fault. Explanation of symbols on Figure 2. once continuous and concluded that they were offset by 1,550 ft (473 m) of right-lateral movement along the Bright Angel fault. However, the continuity of the Tipoff graben across the Bright Angel fault contemporaneous with intrusion of the Unkar Group Angel fault precludes the possibility of such horizontal movement. by diabase sills and dikes. At Ribbon Falls, a branches from a Stratigraphic relations along the northwest-trending Butte fault thick underlying sill and intrudes a northwest-dipping, high-angle (Fig. 1) suggest that movement on the northwest-trending faults reverse fault in Hakatai and Shinumo Formations. The reverse fault near Phantom Ranch occurred after deposition of the Chuar has a displacement of about 50 ft (15 m), terminates downward in Group. The Chuar Group is displaced at least 5,000 ft (1,525 m) the sill, and generally coincides in trend with the synclinal axial along the Precambrian Butte fault, which is the primary component plane of the fold overlying the Bright Angel fault (Fig. 4C). The sill of the northwest-trending system of faults in the eastern Grand Can- thins from 450 ft (137 m) on the west side of the Bright Angel fault yon (Walcott, 1890; Ford and Breed, 1973). A post-Chuar date is to 400 ft (122 m) on the east side and steps down 150 ft (46 m) in further supported by the fact that the Chuar Group lies conform- stratigraphic position. It is our interpretation that the sill intruded ably on the Unkar Group where the contact is exposed in outcrops from the west along a favorable zone in the Hakatai Shale and was between the Bright Angel and Butte faults, whereas if the large- halted by the limb of the monocline. It then expanded the section scale block faulting and tilting of the strata dated from pre-Chuar on the west by 50 ft (15 m), producing the reverse fault in the time, an angular unconformity would occur between the Unkar superjacent strata which was injected with diabase (Fig. 4B). The and Chuar Groups.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/4/465/3443756/i0016-7606-86-4-465.pdf by guest on 27 September 2021 A B Figure 5. The relationship between the Bright Angel (Beta) fault and the Alpha fault. A. Block diagram of the detached sliver bounded by the Bright Angel (Beta) and Alpha faults. B. Block diagram of the detached sliver fol- lowing tensional faulting of the sliver.

C Figure 7. Squeezing of the wedge of Vishnu Schist and Bass Limestone between beds of the Bass Limestone. A. Monocline developed in response to reverse faulting along the Alpha fault. B. Wedge of Bass Limestone and Vishnu Schist squeezed between beds of the Bass Limestone. C. Wedge Figure 6. Wedge of Vishnu Schist and Bass Limestone between beds of sheared off as movement continued along the reverse fault. Explanation of the Bass Limestone. symbols on Figure 2; pCbl, lower Bass Limestone.

ALPHA FAULT faults, its radius of curvature was less than that of the space into which it moved. Consequently, the sliver deformed in the center Maxson (1961) identified the Alpha and Beta faults (Fig. 3) and along six tensional faults oriented perpendicular to the Alpha and proposed two episodes of Precambrian reverse movement to ac- Beta faults (Fig. 5). These faults are restricted to the sliver and have count for them. His map shows that these faults join near Pipe displacements that range from 50 ft (15 m) to 200 ft (61 m). The Canyon but parallel each other for 4 mi (6.4 km) to the north be- cumulative displacement across the tensional faults is about 500 ft fore they merge in upper Bright Angel Canyon. (150 m), but the net displacement across the zone is minimal be- The writers were unable to find the Alpha fault north of Phan- cause the faults occur in an interlocking horst and graben sequence. tom Canyon, leaving only a 1.5-mi (2.4-km) segment of the double Assuming an average dip of 70°, the tensional faults account for structure between Pipe and Phantom Canyons. The displacements approximately 180 ft (55 m) of extension in the sliver parallel to across the Alpha and Beta faults are, respectively, 330 ft (100 m) the Alpha and Beta faults. This mechanism not only explains the and about 1,000 ft (300 m), west down, near Phantom Ranch. The simultaneous development of both the Alpha and Beta faults but dip of the Alpha "fault ranges between 55° and 65° southeast. In also accounts for the presence of the cross faults that are restricted plan, the Beta fault is straight, but the Alpha fault scribes an arc of to the sliver. about 73° between its intersections with the Beta fault. Figure 6 shows a curious structural feature found along the A down-dip projection of the Alpha fault (Fig. 5) forms a cone Alpha fault near the Bright Angel Trail south of the Colorado that intersects the Beta fault. This geometry suggests that the block River. The wedge composed of tightly folded Bass Limestone and between the Alpha and Beta faults is a sliver that detached from the Vishnu Schist was squeezed between beds of the Bass Limestone as west wall of the Beta fault as reverse movement occurred along it. reverse movement occurred along the Alpha fault. Figure 7 recon- As the sliver was dragged upward between the Alpha and Beta structs the development of the injected wedge.

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SUMMARY OF PRECAMBRIAN TECTONISM LATE CENOZOIC FAULTING

At the close of Precambrian time, the net reverse displacement A system of normal faults developed in the Grand Canyon area across the Bright Angel fault near Phantom Ranch was 1,300 ft during late Cenozoic time as normal movements occurred along (400 m), east side up. The displacement diminished to 560 ft (170 many of the northwest-trending Precambrian faults. Prominent m) near Wall Canyon 5 mi (8 km) to the northeast. The Precam- faults in this system include the West Kaibab fault zone and a sys- brian faulting that left a record in the vicinity of the Bright Angel tem of faults that trend along the axis of the Kaibab upwarp fault in the Grand Canyon is summarized as follows: (Strahler, 1948; Maxson, 1967; Huntoon, 1969). The Cremation, 1. Approximately 200 ft (61 m) of reverse movement, east side Phantom, Roaring Springs, and Uncle Jim faults are included in up, along the Bright Angel fault during deposition of the basal this system. In general, movement on these faults was west Shinumo Quartzite. The fault yields upward to an unbroken, gently side down. west-dipping monocline at the level of the Hakatai Shale. Normal displacement of the Paleozoic rocks along the Cenozoic 2. Minor expansion faulting along portions of the Bright Angel Phantom and Cremation faults was restricted to the upper structure associated with intrusion of the Unkar Group by diabase. Paleozoic section along the synclinal axis of the Laramide (?) 3. As much as 1,100 ft (335 m) of reverse faulting along the Grandview-Phantom monocline (Fig. 1). Deformation of the lower Bright Angel fault, probably prior to deposition of the Chuar Paleozoic rocks was accomplished through a combination of Group. The beds in the monocline in the upper Unkar Group were flowage and intense jointing. In essence, the monocline in the lower steeply folded, and the sliver between the Alpha and Beta faults de- Paleozoic rocks unfolded slightly to accommodate the tensional veloped. stress. 4. Normal faulting throughout the region along northwest- This period of faulting cannot be dated on the basis of strati- trending faults following deposition of the Chuar Group. During graphic evidence in the eastern Grand Canyon. Initiation of fault- this episode, the rocks broke into northeastward-tilting blocks. ing is tentatively assigned a Miocene (?) or possibly Pliocene (?) age Minor adjustments to this deformation occurred along portions of to be consistent with radiometric dates obtained for similar parallel the Bright Angel fault. faults in the western Grand Canyon (see Hamblin, 1970). Movement along the Bright Angel structure in Cenozoic time re- PALEOZOIC DEFORMATION sulted in 200 ft (61 m) of normal displacement in Bright Angel Canyon (Fig. 1), which produced a high-angle, normal fault with a The only evidence for Paleozoic deformation along the Bright dip of about 85° toward the east in the Paleozoic rocks. The beds Angel fault is a local angular unconformity that occurs between on the east side of the fault dip toward the fault plane at a slight west-dipping beds of the and the overlying angle. This antithetic dip is particularly noticeable in the outcrops Supai Formation at the head of Garden Canyon (Fig. 3). The upper of the Kaibab Formation in Grand Canyon Village and is known as 30 ft (9.1 m) of the'Horseshoe Mesa Member of the Redwall reverse drag. Reverse drag has been documented along other nor- Limestone is truncated over a distance of about 0.25 mi (0.4 km) mal faults in the Colorado Plateau (Hamblin, 1965). across the fault. The underlying rocks also have an anomalous dip Reverse drag along the Bright Angel fault appears to be related to of 10° toward the west, which suggests that minor reverse move- the change in dip of the fault plane between the Paleozoic and Pre- ment occurred along the Bright Angel fault during the interval be- cambrian rocks. The fault dips between 76° and 87° southeast in tween Redwall and Supai deposition. McKee and Gutschick (1969) the Paleozoic rocks, compared to about 45° to 80° southeast in the noted a similar relationship along the Butte fault to the east. Precambrian rocks. Figure 8 illustrates that as the adjacent blocks displaced under tensional stress, the theoretical separation that LARAMIDE (?) DEFORMATION NEAR would have developed along the fault plane above the Precambrian PHANTOM RANCH units was prevented by sag of the Paleozoic strata in the downthrown block. The onset of the Laramide orogeny of late Mesozoic and early The Cenozoic displacement followed the trace of the Precam- Cenozoic time was responsible for reverse faulting in the Colorado brian Bright Angel (Beta) fault near Phantom Ranch and was Plateau and much of the uplift of the region (Hunt, 1956; Damon, named the Gamma fault by Maxson (1961). The Alpha fault was 1971). On the basis of structural similarities with datable Laramide not involved in the Cenozoic movement. structures on the Colorado Plateau, Huntoon (1974) has provi- sionally assigned reactivation of the ancient northwest-trending RELATIONSHIP BETWEEN THE faults of the Grand Canyon area, including the Butte, Cremation, NORTHWEST-TRENDING FAULTS AND THE and Phantom faults, to the Laramide (?) period. Reverse faulting, BRIGHT ANGEL FAULT west side up, along the Phantom and Cremation faults was sufficient to displace the overlying Cambrian Tapeats Sandstone The geometric relationship between the Bright Angel, Roaring and Bright Angel Shale (Maxson, 1961; Huntoon, 1971). These re- Springs, and Uncle Jim faults suggests that initiation of Cenozoic verse faults yield upward to a northeast-dipping monocline above faulting along the Bright Angel fault postdates movement of the the level of the Tapeats Sandstone in the vicinity of Phantom northwest-trending faults. Figure 1 illustrates that: (1) the Bright Ranch. Angel fault turns sharply northward in Bright Angel Canyon and Displacement on the Cremation and Phantom faults during the then terminates near the East Kaibab monocline, and (2) the Roar- Laramide (?) orogeny was as great as 250 ft (76 m), west side up, ing Springs fault and southern part of the Uncle Jim fault are re- and was opposite to the Precambrian movement. The Laramide (?) verse and downthrown to the east where they terminate against the component of displacement shifts from the Phantom to the Crema- Bright Angel fault. The structure of the Roaring Springs and south- tion faults across a zone of northeasterly dipping rocks in the im- ern part of the Uncle Jim faults is incompatible with the normal, mediate vicinity of the Bright Angel fault. In general, the Precam- west-down movement that occurred along the other northwest- brian and Laramide fault planes do not coincide; for example, un- trending faults in the vicinity. faulted Tapeats Sandstone overlaps the Precambrian Phantom fault The fault planes along the reverse portions of the Uncle Jim and near the mouth of Phantom Canyon. Roaring Springs faults dip toward the west. The Uncle Jim fault is

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scissored 6 mi (9.7 km) north of Bright Angel Canyon, and the west block is downthrown along a normal fault with a west-dipping fault plane. Consequently, the northern part of the Uncle Jim fault has the identical sense of displacement, dip, and trend as other components in the northwest-trending system. This suggests that the Uncle Jim and Roaring Springs faults were originally normal with the west sides down. Their locations indicate that they de- veloped along Precambrian faults that terminated to the south against the Precambrian Bright Angel (Beta) fault. As the Cenozoic Bright Angel (Gamma) fault developed, the east block was down- faulted, and the strike of the fault in upper Bright Angel Canyon assumed a position subparallel to the Uncle Jim and Roaring Springs faults. We propose that part of the movement was accom- modated along the pre-existing Uncle Jim and Roaring Springs faults, implying that the northwest-trending Cenozoic faults pre- date the Cenozoic Bright Angel (Gamma) fault.

SUMMARY OF PALEOZOIC AND CENOZOIC TECTONISM

The numbering below is continued from the section titled "Summary of Precambrian Tectonism" above. 5. Possible minor reverse faulting, east side up, along the Bright Angel fault in the time interval between deposition of the Redwall and Supai Formations. 6. Approximately 250 ft (76 m) of Laramide reverse faulting along the Phantom and Cremation faults, which resulted in the de- velopment of the overlying east-dipping Grandview-Phantom monocline. 7. Normal faulting, west side down, along the Grandview- Phantom monocline and development of the Roaring Springs and Figure 9. Photograph toward the east across the Marble Platform from Uncle Jim faults. This episode is tentatively assigned an age of a point over the Kaibab Plateau. Eminence fault is the prominent left-facing Miocene (?) or Pliocene (?). fault-line scarp right of center. Note that the fault dies out eastward toward the Echo Cliffs monocline, which is the shaded cliff facing the viewer. U.S. 8. Approximately 200 ft (61 m) of normal faulting, east side Geological Survey photograph. down, along the Bright Angel fault that caused local reversals in displacement along the Roaring Springs fault and the southern part placement on the order of 150 ft (46 m), southeast side down. The of the Uncle Jim fault. This deformation is tentatively assigned an intervening rocks are rifted by numerous minor, discontinuous, age of Miocene (?) or Pliocene (?). normal faults. The net displacement across the tensional zone is west down a few tens of meters. The Eminence fault is also directly EMINENCE FAULT associated with a local graben 0.5 to 1 mi (0.8 to 1.6 km) wide. The Eminence and Bright Angel faults are very closely aligned, The Eminence fault is topographically expressed as a 100- to and the strike of each turns away from the East Kaibab monocline 250-ft (30- to 76-m) fault-line scarp in the Kaibab Formation on where they terminate (Fig. 1). They are both high-angle normal the Marble Platform (Fig. 9). No Precambrian rocks are exposed faults with 100 to 250 ft (30 to 76 m) of displacement, but they along the fault, and so information at depth is unavailable, and no have opposite displacements and opposite dips. The Bright Angel Tertiary rocks are in association with it to indicate when it formed. fault is not associated with a large graben structure as is the Emi- The Eminence fault is the southeastern boundary fault in a large nence fault. graben complex that is well exposed in the walls of Marble Canyon The alignment of the Bright Angel and Eminence faults may be (Fig. 10). The northwestern boundary of the tensional zone is the fortuitous; however, it is more likely that they both reflect the Fence fault which lies 5 to 8 mi (8 to 13 km) away and has a dis- northeasterly strike of basement foliation or that they share a common northeast-trending early Precambrian fault zone. If the latter is the case, the late Precambrian Butte fault should offset the controlling northeast-trending early Precambrian fault. The pri- mary argument against a shared Precambrian fault is that the Emi- nence fault dips westward, whereas the Bright Angel fault dips eastward. If the Bright Angel and Eminence faults share the same early Pre- cambrian fault zone, their change in strike in the vicinity of the Butte fault and overlying East Kaibab monocline must be ex- « E Figure 8. Development of re- plained. Possibly these segments are unrelated to Precambrian verse drag along the Bright Angel structure and resulted directly from the complex Cenozoic stresses fault near Grand Canyon Village. A. Geometry before Cenozoic that existed in the vicinity of the pre-existing Butte fault and East faulting. B. Faulting without sag of Kaibab monocline. It is also possible that the Vishnu fault is the the hanging wall. C. Faulting with southern extension of the Eminence fault and that the Fence fault is sag of the hanging wall. the northern extension of the Bright Angel fault (Fig. 1).

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O I 2 3 4MIL.ES

0 I 2 3 4 KILOMETERS

SCALE Figure 10. Geologic section across the Eminence and Fence faults. 1. Kaibab Formation; 2. Toroweap Formation; 3. Coconino Sandstone; 4. Hermit Shale; 5. Supai Formation; 6. Redwall Limestone; 7. Temple Butte Formation; 8. Muav Limestone; 9. Bright Angel Shale; 10. Tapeats Sandstone; 11. Precambrian rocks. Section location on Figure 1.

CONCLUSIONS Soc. America Bull., v. 84, p. 1243-1260. 1974, The younger Precambrian rocks of the Grand Canyon, in Breed, The northeast-trending Bright Angel fault became active during W. J., and Roat, E. C., Geology of the Grand Canyon: Mus. Northern deposition of the basal Shinumo Quartzite. The fault trends sub- Arizona and Grand Canyon Nat. History Assn., p. 21—33. Hamblin, W. K., 1965, Origin of reverse drag on the downthrown side of parallel to basement foliation and had reverse movement with the normal faults: Geol. Soc. America Bull., v. 76, p. 1145-1164. east side upthrown. Minor expansion faulting occurred along the 1970, Structure of the western Grand Canyon region, in Hamblin, W. fault as the Unkar section was intruded by diabase sills and dikes. K., and Best, M. G., eds., The western Grand Canyon district: This episode was followed by renewed reverse faulting which, Guidebook to the geology of Utah, No. 23, Utah Geol. Soc., p. 3-37. when combined with earlier deformation, resulted in a displace- Hunt, C. B., 1956, Cenozoic geology of the Colorado Plateau: U.S. Geol. ment of as much as 1,300 ft (400 m) across the fault. This move- Survey Prof. Paper 279, 99 p. ment appears to have occurred prior to Chuar deposition. A series Huntoon, P. W., 1969, Recurrent movements and contrary bending along of northwest-trending normal faults developed following deposi- the west Kaibab fault zone: Plateau, v. 42, p. 66-74. tion of the Chuar Group and broke the region into northeastward- -1971, The deep structure of the monoclines in eastern Grand Canyon, tilted blocks. Minor adjustments occurred along the Bright Angel Arizona: Plateau, v. 43, p. 148-158. fault in response to this deformation. 1974, The post-Paleozoic structural geology of the eastern Grand Canyon, Arizona, in Breed, W. J., and Roat, E. C., Geology of the Additional minor reverse motion appears to have occurred along Grand Canyon: Mus. Northern Arizona and Grand Canyon Nat. His- the Bright Angel fault in the interval between deposition of the tory Assoc., p. 82-115. Paleozoic Redwall and Supai Formations. Maxson, J. H., 1961, Geologic map of the Bright Angel quadrangle, Grand The Cenozoic displacements in the region were opposite in direc- Canyon National Park, Arizona: Grand Canyon Nat. History Assoc. tion to those of the Precambrian. Several northeast-trending Pre- 1967, Preliminary geologic map of the Grand Canyon and vicinity, cambrian normal faults were reactivated in Laramide (?) time. Arizona, eastern section: Grand Canyon Nat. History Assoc. Movement along the faults was reverse at depth with the west sides Maxson, J. H., and Campbell, I., 1933, Faulting in Bright Angel quad- rangle, Arizona [abs.]: Geol. Soc. America, Proc., p. 301. upthrown and is expressed in the overlying Paleozoic rocks as McKee, E. D., 1969, Stratified rocks of the Grand Canyon: U.S. Geol. Sur- east-dipping monoclines. Extension commencing in Miocene (?) or vey Prof. Paper 669-B, p. 23-58. Pliocene (?) time was responsible for downfaulting of the mono- McKee, E. D., and Gutschick, R. C., 1969, History of the Redwall Lime- clines to the west. Subsequently, the east block was downthrown in stone of northern Arizona: Geol. Soc. America Mem. 114, 612 p. normal fashion along the Bright Angel fault. Noble, L. F., 1914, The Shinumo quadrangle, Grand Canyon district, The record of recurrent movement along the Bright Angel fault Arizona: U.S. Geol. Survey Bull. 549, 100 p. supports the tenet that deformation attempts to take place along Noble, L. F., and Hunter, J. F., 1916, A reconnaissance of the Archean pre-existing zones of structural weaknesses. complex of the Granite Gorge, Grand Canyon, Arizona: U.S. Geol. Survey Prof. Paper 98, p. 95-113. Ransome, F. L., 1908, Pre-Cambrian sediments and faults in the Grand ACKNOWLEDGMENTS Canyon of the Colorado: Science, v. 27, p. 667—669. Shoemaker, E. M., Squires, R. L., and Abrams, M. J., 1974, The Bright This study was supported by grants from the Grand Canyon Angel and Mesa Butte fault systems of northern Arizona, in Natural History Association. Portions of the data used in the Pre- Karlstrom, T.N.V., Swann, G. A., and Eastwood, R. L., eds., Geology cambrian sections were developed by James Sears for his Master of of northern Arizona, Part 1 — Regional studies: Flagstaff, Arizona, Science thesis at the University of Wyoming. Geol. Soc. America (Rocky Mountain Sec.), p. 355—391. Strahler, A. N., 1948, Geomorphology and structure of the west Kaibab fault zone and Kaibab Plateau, Arizona: Geol. Soc. America Bull., v. REFERENCES CITED 59, p. 513-540. Van Gundy, C. E., 1946, Faulting in the east part of the Grand Canyon of Damon, P. E,, 1971, The relationship between late Cenozoic volcanism and Arizona: Am. Assoc. Petroleum Bull., v. 30, p. 1899-1909. tectonism and orogenic-epeirogenic periodicity, in Turekian, K. K., ed., The late cenozoic glacial ages: New York, John Wiley & Sons, Walcott, C. D., 1890, Study of line of displacement in the Grand Cañón of Inc., p. 15-35. the Colorado in northern Arizona: Geol. Soc. America Bull., v. 1, p. 49-64. Ford, T. D., and Breed, W. J., 1972, The Chuar Group of the Proterozoic, Grand Canyon, Arizona: 24th Internat. Geol. Cong., Proc., Montreal, sec. 1, p. 3-10. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 24, 1974 1973, Late Precambrian Chuar Group, Grand Canyon, Arizona: Geol. REVISED MANUSCRIPT RECEIVED SEPTEMBER 25, 1974 Printed in U.S.A.

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