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Bright Angel and Eminence Faults, Eastern Grand Canyon, Arizona

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Bright Angel and Eminence Faults, Eastern Grand Canyon, Arizona Bright Angel and Eminence Faults, Eastern Grand Canyon, Arizona 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 schist against younger Pre- sition of the basal Shinumo Quartzite. Additional reverse move- cambrian sedimentary rocks along a reverse fault, whereas the ment occurred following intrusion of the Unkar Group by diabase 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 Chuar Group 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 Colorado River, 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 granite gneiss 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 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 112° 00' lll°45' 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. 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 BRIGHT ANGEL AND EMINENCE FAULTS, EASTERN GRAND CANYON, ARIZONA 467 Figure 2. Idealized stratigraphic section in the eastern Grand Canyon. Pk, Kaibab Formation; Pt, Toroweap Formation; Pc, Coconino Sandstone; Ph, Hermit Shale; IPPs, Supai Formation; Mr, Redwall Limestone; Dtb, Temple Butte Formation; Cm, Muav Limestone; Cba, Bright Angel Shale; Ct, Tapeats Sandstone; pCsm, Sixty Mile Formation; pCk, Kwagunt Formation; pCg, Galeros Formation; pCn, Nankoweap Formation; pCc, Cardenas Lavas; pCd, Dox Formation; pCs, Shinumo Quartzite; pCh, Hakatai Shale; 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 Phantom Ranch. 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. 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 468 HUNTOON AND SEARS sill 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.
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