TREVOR D. FORD Department of Geology, University of Leicester, Leicester, England WILLIAM J. BREED Museum of Northern Arizona, Flagstaff, Arizona 86001

Late Precambrian Chuar Group, Grand Canyon, Arizona

ABSTRACT Chuar only for the topographic features Chuar Butte and Chuar Creek. The Lexicon of The Chuar Group is exposed in tributary Geologic Names attributes the name Chuar to canyons to the Colorado River over an area Walcott (1883). some 15 mi (24 km) long and 4 mi (6 km) wide. Walcott (1894, 1895) gave a measured The rocks are faulted against Paleozoic rocks section of the Grand Canyon Series totaling ap- by the Butte fault on the east, and uncon- proximately 12,000 ft (3,657 m), but this was formably overlain by Paleozoic rocks to the assembled from several different sections, some west. The group is 6,610 ft (2,013 m) thick and unspecified, and with little or no indication of has been divided into three formations and where he had moved from one section to an- seven members. The lower two formations, other. We found Walcott's estimate of 5,120 ft Galeros (below) and Kwagunt (above) are pre- (1,554 m) for Chuar terrane to be quite low. dominantly argillaceous with subordinate thin Following his survey of 1882-1883, Walcott limestone beds, while the highest, Sixty Mile (1899) described primitive fossils in the Chuar Formation, is mostly coarse breccia. Stromato- Group and presented a generalized section of lites are present at three horizons, one of them the Chuar Group. Other published works on biohermal. The form-genera Inzeria, Baicalia, the Chuar are those by Hinds (1935) and Ford and Boxonia indicate an upper Riphean age. and Breed (1969). The mega-planktonic fossil Chuaria occurs near The Unkar terrane was studied by Noble the top of the Kwagunt Formation. The Chuar (1914), White (1928), Van Gundy (1934. rocks are probably younger than any other 1937a, 1937b, 1946, 1951), and Maxson (1961). Precambrian rocks in Arizona. They may be Noble was the first to consider the Unkar and contemporary with rocks below the Cambrian Chuar as Groups, and he subdivided the Unkar in eastern California, and with the Windermere into Dox Sandstone, Shinumo Quartzite, Formation of the northern Cordillera. Hakatai Shale, Bass Limestone, and Hotauta Conglomerate. The last two are not always INTRODUCTION separable in the eastern Grand Canyon. These The Precambrian rocks of the Grand Canyon formations are in general more accessible than were observed by Powell during his voyage is the Chuar, as they crop out in Bright Angel down the Colorado River in 1869 (Powell, Canyon, near Phantom Ranch. Van Gundy 1874, p. 18; 1875, p. 81). He recognized the (1946, 1951) and Maxson (1961, 1967) mapped presence of two groups of rocks, both now the Unkar formations, although they provided known to be Precambrian; of these, he referred no detailed descriptions. Apart from Hinds' the lower complex of gneiss, schist, and granite summary (1935), no subdivision of the Chuar to the Eozoic and the upper group to the Group has been attempted prior to our work. Silurian. Walcott (1883) recognized that both Van Gundy reassigned the topmost division groups were Precambrian; he divided the of Walcott's Unkar succession to the Chuar, younger Grand Canyon Series into the Unkar and elevated the next division below (Walcott, (below) and Chuar (above), both of which he 1899, p. 216, Unkar divisions 1 b-e) to the regarded as being Algonkian in age. Walcott status of a new Nankoweap Group on the attributed the naming of the Chuar rocks to basis of lithological distinction and uncon- Powell (1883, p. 440), but he used the name formable contacts. Van Gundy (1951) also

Geological Society of America Bulletin, v. 84, p. 1243-1260, 12 figs., April 1973 1243

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figured and described a jellyfish, Broo\sella The outcrops lie to the west of the river in the canyonensis Bassler, from the Nankoweap heads of a series of tributary canyons (Fig. 1), Group. This has been noted briefly by Van in an area some 15 mi king and as much as 4 mi Gundy (1937a), Hinds (1935), Bassler (19«), wide (24 km by 6 km). and Seilacher (1956), although Cloud (1968, p. 27) has since expressed the opinion that it is an CHUAR GROUP inorganic structure formed by "compaction of fine sands deposited . . . over possibly a small The Chuar Group is herein subdivided into gas blister." three formations, two of which are further The topmost bed of the Chuar terrare as divided into seven members. Both formations defined by Walcott has also caused some and members are named and defined for the difficulty, as it crops out on the isolated summit first time. of Nankoweap Butte. Walcott regarded it as The three formations are named for the belonging to the Chuar, but both Van Gundy Galeros promontory that overlooks the south- (1951) and Maxson (1967) showed it on ':heir ern part of the Chuar outcrops in Chuar and maps as an outlier of the Cambrian Tapeats Carbon Canyons, for Kwagunt Canyon in the Sandstone. We regard it as being Precambrian northern slopes of which the formation is fully in age, and have named it the "Sixty Mile exposed, and for Sixty Mile Canyon between Formation." the previous two types areas. Between the Unkar and Nankoweap Groups, The total thickness of the group is 6,610 ft there is about 980 ft (294 m) of basaltic lava (2,013 m) in contrast: to Walcott's figure of flows, first noted by Walcott (1894). They 5,120 ft (1,554 m) and Hinds' estimate of 5,135 were named the "Cardenas Lavas" by Keyes to 5,323 ft. As neither of these authors defined (1938, p. 110), but subsequently Maxson called their boundaries clearly, except for the sand- them the "Rama Formation" (1961, 1967). It stone at the base of the: upper Chuar (Kwagunt has been recommended that the name Rama Formation), the members herein defined can- be dropped and the name Cardenas restored to not be compared directly with the older esti- use (Ford and others, 1972). mates. The figures given in Table 1 are those The Chuar outcrops are relatively inacces- taken from the type sections. Some variation sible as they are entirely in the eastern Grand does take place in some members, but there are Canyon, away from public trails, and cut off too few measurable sections to detect any from the Colorado River by the Butte fault. systematic regional change.

TABLE 1. SUBDIVISIONS OF THE CHUAR GROUP

Thickness Type section Other sections Formation Member Meters (feet) (lettered on Fig. 2) measured

Sixty Mile 36 120 Sixty Mile Canyon(H) Top of Nankoweap Butte (120 ft) Awatubi Canyon (36 m) Walcott 255 838 Head of Walcott Glen and upper part of Kwagunt Nankoweap Butte(G) (2,218 ft) Awatubi 344 1,128 Awatubi Canyon(F) Southeast slope of (676 m) Nankoweap Butte

Carbon Butte 76 252 Carbon Butte(E) South fork of Nankoweap Canyon Duppa 174 570 Below Duppa Butte in Kwagunt Canyon(D) Carbon Canyon 471 1,546 Carbon Canyon west Galeros fork and mid-Chuar (4,272 ft) Canyon(C) (1,302 m) Jupiter 462 1,516 Below Jupiter Temple in lower part of Chuar Canyon(B) ^Tanner 195 640 Overlooking Tanner Lower end of Chuar Raoids in cliffs Canyon of 3asalt Canyon(A)

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FORD AND BREED, FIGURE 1 Geological Society of America Bulletin, v. 84, no. 4

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FORD AND BREED, FIGURE 2 Geological Society of America Bulletin, v. 84, no. 4

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Figure 3. Basalt Canyon, looking north-northwest. center is the dolomite at the base of the Tanner Member The lowest Chuar strata are seen beneath the uncon- (pCt); the middle part of the canyon is in the shale in formable Cambrian Tapeats Sandstone (Ct). The top of the upper part of the Tanner. The inclined light bed the Cardenas Lavas is just visible in the bottom right, toward the top of the canyon is the stromatolite horizon overlain by the Nankoweap Group sandstone (pCn). at the base of the Jupiter Member (pCj) (photo The ledge forming a prominent platform in the lower courtesy of Parker Hamilton). GALEROS FORMATION conformity claimed by Van Gundy (1934, 1951) has been seen. In the eastern branch of The Tanner Member is named from the Basalt Canyon, the dolomite appears to be Tanner Rapids on the Colorado River and con- only some 20 ft (6 m) thick, but the section is sists of some 60 ft (18 m) of massive coarsely disturbed by faulting. Some variation in thick- crystalline dolomite at the base and 580 ft ness from 40 to 80 ft (12 to 24 m) is, however, (177 m) almost exclusively shale above (sec- visible near the top of the basalt cliffs west of tion A on Figs. 1 and 2; Fig. 3). The dolomite Basalt Canyon, and the dolomite lies in wide forms the topmost ledge of the cliffs overlook- shallow channels cut in the Nankoweap Forma- ing the mouth of Basalt Canyon and Tanner tion there. The inaccessibility of these sections Rapids. It also forms a wide bench between the has prevented a closer examination of the two branches of Basalt Canyon. The contact contact. with the underlying Nankoweap Group is not The dolomite of the Tanner member is also easily accessible, but little indication of the un- present as an inlier west of Lava Butte on the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1243/3418135/i0016-7606-84-4-1243.pdf by guest on 02 October 2021 Figure 4. Lower Carbon Canyon (foreground) and (pCcc). The darker {¡round beyond and to the left is middle Chuar Canyon looking southwest. The hills formed of the shale of the Jupiter Member (pCj) between the two canyons are formed of alternating (photo courtesy of Packer Hamilton). limestone and shale of the Carbon Canyon Member

Figure 5. Lower Chuar Canyon looking north. colored shale of the Jupiter Member (pCj) (photo by Shale of the Tanner Member (pCt) in the foreground T. D. Ford), overlooked by crags of stromatolitic limestone and

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southern side of Chuar Canyon, a patch incor- part of the Tanner Member appears to cor- rectly shown in Maxson's map (1967) as Rama respond to Walcott's Lower Chuar Division 9. Basalts. The massive dolomite also crops out The Tanner Member appears to correspond to between two branches of the Butte fault, to Hinds' Division 1 and part of Division 2. form a low hill north of the camp site in The Jupiter Member is named from Jupiter Nankoweap Canyon. No primary sedimentary Temple which overlooks the main outcrops in structures have been seen in the dolomite, and the upper part of Basalt Canyon and the mid- no variations in lithology have been deter- dle reaches of Chuar Canyon. Like the Tanner mined. It is cream-colored coarsely crystalline Member below, the Jupiter Member consists of dolomite, weathering to a light brown. carbonate below and thick shale above, totaling Numerous small faults cut it in both the Chuar 1,516 ft (462 m) (section B on Figs. 1 and 2; Canyon and Nankoweap Canyon outcrops, and Figs. 4, 5, and 6). The carbonate beds reach a these carry secondary minerals in the form of thickness of 32 to 42 ft (10 to 13 m) and are a barite and hematite. complex of stromatolitic limestones associated Immediately overlying the massive dolomite with fragmental, laminated, and argillaceous is a thin, fine-grained limestone bed from 1 to limestone, apparently corresponding to Wal- 2 ft thick, which occasionally passes laterally cott's Division 8. The limestone is dolomitized into a series of lenses in the base of the shale. to varying degrees, and one thin bed near the The upper part of the Tanner Member con- top shows carbonate casts of gypsum crystals. sists of fissile blue-gray micaceous shale, some The stromatolites are mostly wide low domes, 580 ft (177 m) thick, with a few scattered silt- more or less confluent and commonly 2 to 3 stone beds, rarely more than an inch or two ft in diameter. Many of them have a nucleus of thick, which bear ripple marks and occasional flakes of laminated carbonate mudstone. mud cracks. The top of the Tanner Member is Lenses of flat-pebble conglomerate are fre- taken at the base of a prominent stromatolitic quent. The top of the stromatolite beds shows limestone bed. The dark blue-black shale beds sporadic low columns, 2 or 3 in. (5 to 7.5 cm) immediately below this have yielded a few high and wide, in a matrix of fragmental lime- poorly preserved Chuaria. The measured sec- stone with clasts of laminated limestone, tion (A) was taken up the north slopes of cemented by a tufa-like calcite deposit. The Basalt Canyon. Another measured section in stromatolitic limestone beds form a prominent the lower part of Chuar Canyon totaled 697 ft feature along the middle reaches of Basalt (212 m), the shale being 637 ft (194 m) thick, Canyon (Fig. 3), particularly below the though not so well exposed. The lower part of promontory of Tapeats Sandstone overlooking the Tanner Member appears to correspond to Tanner Rapids, and along the north side of the Walcott's Unkar Division la, while the upper middle reaches of Chuar Canyon (Fig. 5). They

Figurc6. Chuar Canyon. Alternating limestone-shale succession of the Carbon Canyon Member (pCcc) (photo by T. D. Ford).

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reappear briefly in the fault complex north of Jupiter Formation, which was taken from the the campsite in Nankoweap Canyon, but their point where the strornatolitic limestone crosses character has there changed somewhat. The Chuar Creek due north to the lowest limestone stromatolites are very fine-grained dc'omite of th: overlying Carbon Canyon Member in partly silicified, with lamination shown by the interfluve betvveen Chuar and Carbon changes in grain size and carbonaceous matter. Canyons (Figs. 4 and 5). Here the section has a Some stromatolite structures show a columnar few prominent sandstone ledges 1 or 2 ft thick, development of the form Inzeria (Fig. 7). but r.o limestone was seen. About a mile farther The overlying shale of the Jupiter Member, upstream, two thin limestone beds appear which corresponds broadly to Walcott's -150 and 300 ft (45 and 90 m) from the top Division 7, shows a greater variation in of the member. lithology than those of the Tanner Msmber. The Carbon Canyon Member is named from The lowest few feet contain gray calcareous the west fork of Carbon Canyon which has siltstone. Above these is shale with colors vary- most of its course cut in these beds (section C ing from light gray-green, through red-brown on Figs. 1 and 2; Figs. 4, 6, and 8). The to dark blue-gray. The blue-gray beds are member forms the interfluve between Chuar micaceous, and contain many quartzcse silt- and Carbon Canyons, and its outcrops extend stone beds which are usually not more t aan 2 to into the heads of both canyons, although there 4 in. thick. The siltstone beds show ripple they are largely concealcd by landslips and marks and mud cracks on the top of almost debris of Tapeats Sandstone. The member also every surface as well as rain prints and oc- crops out in the heads of Nankoweap and casional salt pseudomorphs. Within this argil- Kwagunt Canyons, although outcrops are dis- laceous division of the Jupiter Member, groups continuous owing 1:3 talus and terraces. of beds tend to have one color dominant, and as The member con sists of a rapid alternation of weathering has intensified the colors to various limestone beds a few feet thick and similar shades of red and ocherous browns and yellows, thicknesses of shak, with some thin sandstone they form a striking multi-colored scenic area beds. A measured section (C) was taken from overlooking Chuar Canyon. They aopear to close to the junction of Chuar South and North correspond to Walcott's Lower Chuar Divisions Forks, northeastward over the interfluve and 5, 6, and 7, and to Hinds' Division 3 ard upper across Carbon Canyon toward Carbon Butte. part of 2. The shale has a thickness of 1,484 ft Part of this section was badly obscured by (452 m) in the measured section (E) of the talus, and detailed measurements were not

Figure 7. Basal Jupiter Member limestone with stromatolites (XO.ii) (photo by T. D. Ford).

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Figure 8. Carbon Canyon, looking south. Promi- To the right a ridge of the Carbon Canyon Member nent cuesta of Carbon Butte Member (pCcb) dips to the (pCcc) overlooks the north fork of Chuar Canyon left (east). Duppa Member (pCd) is largely in shadow. (photo courtesy of Parker Hamilton). taken. The base is taken at the lowest limestone lamination. Occasional limestone flake breccia bed of this sequence, and the top is taken at at base, ~1 in. thick. Commonly 10 ft thick (3 the highest limestone bed. Between these, the thickness is 1,546 ft (471 m), and the member 4. Limestone: weathering light buff color, appears to correspond broadly to Walcott's medium gray on fresh surface. Coarsely lam- Lower Chuar Divisions 2b-2e, 3, and 4, and to inated, with laminae commonly fractured by part of Hinds' Divisions 4 and 5. pull-apart structures. Top surface almost al- There is a poorly developed cyclicity in the ways ripple-marked, and usually mud-cracked Carbon Canyon Member, with an ideal cycle also. Salt pseudomorphs are not uncommon. approximating to the following description. There may be several such laminae at the top. 1. Sandstone: green, argillaceous, with well- Other laminae occasionally show broad ir- rounded quartz grains probably of aeolian regular undulations probably of algal origin. origin; some are cross-bedded, some are Small chert nodules are scattered in some beds. laminated. Deeply mud-cracked on the top In thin section, the beds are micritic carbonate and with truncated mud cracks simulating with scattered dolomite rhombs and streaks of "worm burrows" on some laminae. Rarely sutured quartz. The limestone locally replaced more than 3 ft (1 m) thick. by calcareous siltstone. Most limestone beds 2. Green mudstone: poorly laminated, oc- are about 2 ft (60 cm) thick, but beds as much casionally interlayered with red mudstone. as 6 ft (2 m) thick or more are present, though Usually only 1 to 2 ft thick. with shale partings. 3. Red mudstone: light red at top, darker A thin mudstone bed may intervene between red below. Blocky fracture. Occasional poor the sandstone top and the base of the overlying

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limestone. Most contacts are gradational, and section. The member appears to correspond to no channeling has been seen. Incomplete cycles Walcott's Lower Chuar Divisions 1 and 2a, and are frequent, usually with sandstone missing. At in part to Hinds' Division 5. times, only an alternation oi limestone and red Both Walcott and Hinds noted an oolitic iron mudstone is present. ore near the top of the Duppa Member, but the The interpretation of this cyclic sedimenta- only horizon which seemed to fit their brief tion is of shallow quiet waters, with repeated description was a 1 -in. bed of well-rounded emergence and dessication. Horizons indicative sand grains, with small angular quartz grains in of emergence are at the top of the sandstone the interstices, set in a hematite cement. and at the top of the limestone, the former representing the washing in of clastic sedi- KWAGUNT FORMATION ment while the latter represents the filling in The Carbon Butte Member is named from situ by calcium carbonate, perhaps biogenically the prominent shelf of red sandstone surround- precipitated. ing Carbon Butte; it includes the overlying A single horizon, 230 ft (70 m) from the top shale and some thinner sandstone beds (section of the member, shows good stromatoliie de- C on Figs. 1 and 2; Fig. 8) (corresponding to velopment. It is a bed of limestone about 2 ft Walcott's Upper Chuar Divisions 12 and 13, thick, with heads of rapidly widening, ir- and to Hinds' Division 6 and part of Division regularly branching columnar stromatolites 7). The member thu<; contains the only thick dispersed throughout its outcrop. It has been sandstone in the Chuar Group, and as it forms recognized in all the sections where this pirt of the most obvious topographic feature, it is a the member can be expected. The columns are useful horizon at which to subdivide the Chuar more dolomitic than the matrix, and they stand into the upper Kwagunt and lower Galeros out on weathered joint faces. They appear to be Formations. Walcott similarly divided the close to the form Baicalia aff rara Semikhatov. Chuar at this horizon (below Bed 13). The The Duppa Member is named from Duppa sandstone forms strong scarp features in Butte, which dominates a northwesterly Nankoweap and Kwagunt Canyons, and is also branch (Shale Wash) from the middle reaches exposed in Sixty Mile and Carbon Canyons. of Kwagunt Creek (section D on Figs. 1 a id 2; The sandstone division of the Carbon Butte see also Fig. 8). The member is much more Member totals 94 ft in thickness. It is formed of argillaceous than the Carbon Canyon Member two massive beds, with a few feet of purple below, but it still carries many thin beds of micaceous shale between them. Micaceous limestone and calcareous siltstone, few of laminae are present bcth above and below this which are more than a few inches thick. Toward parting, and at the base and top of the sand- the top, there are red or purple micaceous silty stone, but otherwise it is an even-grained shale and a few thin sandstone beds, but the quartz sandstone with hematite pellicles giving junction with the overlying Carbon Butte it color. Current bedding is poorly developed, sandstone is sharp. The shale is generally purple and only seen to any extent at the top of the to gray and micaceous. The Duppa Member sandstone on Carbon Butte, where currents thus marks a partial return to the argillaceous from the northwest are indicated. Barite veins character of much of the Tanner and Jupiter and small limonitic concretions are present in Members with a partial transition to the places, particularly near faults. Mud cracks are arenaceous Carbon Butte Member above. The present in the shale patting, and ripple-marked thickness was measured as 570 ft (170 m) in the surfaces are frequent on fallen slabs, particularly type section "Shale Wash," 532 ft (162 m) in from the topmost beds. The red sandstone is the western slopes of Carbon Butte, and about overlain by 78 ft (24 m) of purple-gray mud- 460 ft (140 m) south of the campsite in stone, and then by several feet of soft white Nankoweap Canyon. The base of the Duopa and red mottled quartz sandstone, with con- has been taken at the highest limestone more spicuous carious weathering surfaces. Purplish- than 3 ft (1 m) thick in each of these sections, gray mudstone with thin siltstone lenses follows but as the detailed sequences of beds are dis- to the top of the member. similar, there may be some diachronic variation The member totals 251 ft (76 m) in the type in the unit taken as base. No complete section section on the escarpment west of Carbon Butte of the whole member could be measurec in (Fig. 8), 198 ft (60 m) in Nankoweap South detail, owing to a cover of talus on parts of the Fork, and 188 ft (57 m) in Kwagunt Creek.

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The Awatubi Member is named from the tervals of 6 in. (15 cm) or so. Both columns and upper reaches of Awatubi Canyon, which is laminae consist of fine-grained dolomitized largely cut into this member (section F on limestone; carbonaceous wisps preserve the Figs. 1 and 2). The member is also exposed in laminae in the columns, but the matrix is Sixty Mile Canyon and at the head of both coarsely crystalline dolomite. Coarse-grained forks of Carbon Canyon below the Galcros dolomitic sediment lies between the columns. promontory. It appears to correspond to A few bioherms can be seen to have a nucleus of Walcott's Lower Chuar Divisions 10 and 11 and limestone flakes at the base. Many bioherms to part of Hinds' Division 7. The member is have weathered out as discrete units and rolled mainly argillaceous, with considerable varia- down the hill slopes into the nearest stream tion in color, not unlike the Jupiter Member. bed. Siltstone beds up to about 4 in. (10 cm) thick Some 6 ft (2 m) above the stromatolite commonly mark the boundaries between dif- horizon, there is a greenish shale unit with ferent-colored shale. Such siltstone beds are numerous spheroidal marcasite nodules as much usually ripple marked or mud cracked, and as 3 in. (7.5 cm) in diameter. Overlying this some carry salt pseudomorphs. are the varicolored shale beds of the main part At the base of the member, there is a of the member. At several levels, there are conspicuous stromatolitic limestone bed, 12 ft blue-black shale beds, and all of them have (3.5 m) thick (Fig. 9). This forms a shelf yielded specimens of Chuaria. They are, how- feature about 200 to 250 ft (60 to 75 m) above ever, poorly preserved and few in number the red sandstone of the Carbon Butte Mem- except near the top of the member. Some 30 ber. The bed consists of closely packed bio- ft (9 m) below the Flaky Dolomite basal bed herms more or less in contact. They are about of the Walcott Member, blue-black, slightly 8 to 12 ft (2.5 to 3,5 m) in height and diameter, micaceous shale has yielded abundant Chuaria, and they result in the upper surface of the bed and this is believed to be the type horizon having a marked undulose character. Each defined ambiguously by Walcott (1899, p. 234) bioherm consists of radiating to subparallel as "730 feet beneath the summit of the Chuar columns of the form Boxonia 2 or 3 in. (5 or terrane." The Chuaria shale is best exposed in 7.5 cm) in diameter, sometimes interrupted by the saddle east of Nankoweap Butte, but it can confluent domal or undulating laminae at in- also be found on the western ridge and in the

Figure 9. A stromatolite bioherm from the base of the Awatubi Member seen from above. Chuar Canyon (photo by T. D. Ford).

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northwestern slopes of Nankoweap Butte. The arusy quartz linings. The rop surface of the bed horizon is probably present in Awatubi, Sixtv sometimes shows a curious rippled effect, with Mile, and Carbon Canyons below the Flaky ridges 3 or 4 in. (7.5 or 10 cm) wide and 1 in. Dolomite, but it was not located during the (2.5 cm) high, separated by flat troughs 1 ft limited time available. The horizon wit a wide (30 cm). Occasionally, intersecting sets of abundant Chuaria is only a few inches thicx, such ripples can be seen. but they have been found scattered through Above the Flaky Dolomite, there is ~66 ft some 80 ft (24 m) of shale below, and in black (20 m) of blue-gray shale, with scattered shale well up into the overlying Walcott Chuaria and a 6-in. silicified oolite bed. This is Member. followed by a very distinctive pisolitic chert The Awatubi Member has been measured on bed about 2 ft (60 cm) thick, containing algal the southern slope of Nankoweap Butte as filaments. Most of the pisoliths are about kji 1,128 ft thick (344 m). inch in diameter, formed of black chert, in a The Walcott Member is named from a matrix of white granular chert and sideritic newly named Walcott Glen, which branches carbonate. Patches of carbonate rhombs occur northwest from the lower parts of Kwagunt within the pisoliths. Weathering bleaches both Canyon to head on the southeastern slopes of to white or yellow. A few lenses of unsilicified Nankoweap Butte (section G of Figs. 1 and 2; pisolite are present, formed of buff-weathering Fig. 10). The lower part of this glen contains dolomite. The upper surface of the pisolitic spectacular exposures of the Carbon Butte and chert is nodular with patches of pisolite 2 in. Awatubi Members, and a complete section of (5 cm) across separated by narrow troughs. the Kwagunt Formation can be followed north- The pisolitic chert forms a prominent ledge westward up this glen from Kwagunt Canyon around the foot of the upper part of Nankoweap to the summit of Nankoweap Butte. The mem- Butte, and it extends along the ridge to the ber is also exposed in Awatubi and Sixty Mile west with the Flaky Dolomite below, in a Canyons, and the lower part of it is exposed in down-faulted block. These beds may be the faulted block at the head of the west fork present on the south side of Kwagunt Canyon, of Carbon Canyon. but talus completely obscures the section. The sediments of the Walcott Member are These beds are well exposed in Awatubi and more diverse than those below and include Sixty Mile Canyons and are the highest beds some distinctive horizons. At the base is the seen at the head of Carbon Canyon below Flaky Dolomite, buff-weathering dolomite 8 Galeros oromontory. ft thick, containing relics of what was probably The pisolitic chert is followed by shale, algal lamination (Fig. 11). These relics are in mainly blue-black, with another bed of cherty the form of silicified flakes, commonly an inch oolite 6 in. (15 cm) thick, and with black or so in length and a quarter inch thick, wildly pyritous shale forming a spur on the southwest disoriented, and apparently heaped irregularly flank of Nankoweap Butte. About 420 ft (128 with a matrix of smaller fragments of the same m) above the pisolitic chert, there are two material all set in a matrix of dolomite mic.rite. layers of dolomitic limestone 25 and 40 ft thick Some subparallel strings offtakes merge into an (7.5 and 12 m), separated by 40 ft (12 m) of ill-defined mass of laminated siliceous dolomite. shale. These are the highest dolomite beds in Rarely these have undulations of stromatolitic the Chuar sequence. The top of the lower leaf type. Some sheets of silicified laminae arc- of limestone shows undulating laminae, prob- folded back on themselves. The whole aspect ably of stromatolitic origin, while the top 3 ft of this rock is of a disrupted laminar stromatoli- of the upper leaf is a coarse oolite. tic limestone which has been dolomitized and A further 228 ft (70 m) of blue-gray shale subsequently selectively silicified. The cause of the disruption is thought to be contemporary completes the Walcott Member, making a total sea-floor slumping. The beds immediately thickness of 838 ft (255 m) measured up the above and below are unaffected, and the bed is east face of Nankoweap Butte. It appears to present over an area at least 7 mi long and L mi correspond broadly to Walcott's Divisions 2 to wide (11 X 1.6 km) without obvious variation. 9 and to the upper part of Hinds' Division 7. The Flaky Dolomite has a few lenses of oclite, SIXTY MILE FORMATION particularly toward the top. Irregular nodules This newly described formation is named of chert are scattered throughout, some with from outcrops of breccia and sandstone in the

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Figure 10. Nankoweap Butte looking northeast, Dolomite and Cherty Pisolite of Walcott Member lower Nankoweap Canyon in far left; Walcott Glen in (pCw). Prominent ledges toward the top of the Butte shadow to right. Slopes in lower foreground and lower are massive dolomite in Walcott Member. Summit cap right are in shale of Awatubi Member (pCa). Lowest is breccia and sandstone of the Sixty Mile Formation two ledges on the Butte in lower left center are Flaky (pCs) (photo courtesy of Parker Hamilton).

cliffs on the north side of the upper part of varying degrees of silicification resting on an Sixty Mile Canyon (section H on Figs. 1 and 2; eroded surface of shale of the Walcott Member. see also Fig. 12). This canyon and its neighbor The uppermost 50 ft or so of the Walcott Awatubi Canyon are not easily accessible from Member are silicified, with frequent chert the Colorado River, and so they have rarely nodules and with mottling in purple and been visited, and the existence of this formation yellow. Near the contact, they are often creamy with its very distinctive lithology has hitherto white. The clasts in the breccia are of the same been unknown. The formation makes up the material and obviously derived from former topmost 120 ft (36 m) of Nankoweap Butte continuations of the Walcott Member. The (Fig. 10). Walcott (1894) recorded some 200 base of the breccia is generally parallel with the ft (60 m) of sandstone, shale, and fragmental bedding of the shale of the Walcott Member, beds, and placed them as the topmost division although channels and scarps as much as 20 ft of his Chuar terrane, bed 1; Hinds recorded high are sporadically present. The basal breccia only sandstone as his Division 8, ignoring the bed varies from about 50 ft (15 m) thick in breccia. Both Van Gundy (1946, 1951) and parts of the Sixty Mile and Awatubi Canyons Maxson (1967) regarded these beds as an to only about 20 ft (6 m) in Nankoweap Butte. outlier of Tapeats Sandstone of Cambrian age. Above this is as much as 100 ft (30 m) of fine- The base of the formation in all three grained red sandstone, generally flat-bedded, sections is sharp, with breccia beds showing with frequent lenses of breccia ~1 ft in thick-

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Figure 11. Flaky dolomite from the base of the Walcott Member (pCw) on Nankownap Butte (photo by T. D. Ford).

Figure 12. Breccia of the Sixty Mile Formation (pCs). Sixty Mile Canyon (photo by T. D. Ford).

ness. Both in Nankoweap Butte and in the the massive sandstone in Sixty Mile Canyon, south side of Awatubi Canyon, the sandstone is and large irregular lenses of breccia are present sometimes massive and unbedded with slump among the slumps on the south side of the structures, sometimes as much as 100 ft (30 rn) Awatubi Canyon. high, usually indicating movement toward the The relation of the Sixty Mile Formation to axis of the Chuar syncline. A second layer of the overlying Tapeats of the Cambrian presents breccia up to 20 ft (6 m) thick is present above some problems of inte rpretation. While the

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lower contact of the Sixty Mile Formation is has been found. Such an argument strengthens clearly erosional, the formation occupies the the recognition of the Sixty Mile Formation as core of the main synclinal fold of the Chuar a distinct unit, as it is thus earlier than the Group. The overlying Tapeats is horizontal and Butte fault, which is in turn earlier than the clearly crosscuts the Chuar. In sections in the Tapeats. cliffs of Awatubi and Sixty Mile Canyons The nature of the Sixty Mile Formation toward the extremities of the wedge-shaped invites comparison with the early Tapeats outcrops of the Sixty Mile Formation, the slide breccia masses some 30 mi to the west Tapeats is again clearly in cross-cutting relation, described by Sharp (1940), and it is clear that but in the centers of the wedges, lenses of in the light of the discovery of the Sixty Mile breccia also occur interbedded with the Tap- Formation, Sharp's findings should be re- eats Sandstone, and both Tapeats and Sixty examined. Mile Formations are horizontal. Traced laterally, these lenses of breccia appear to have STRUCTURE been derived from the main beds of breccia in As Walcott (1890) recognized, the Chuar the Sixty Mile Formation toward the flanks of terrane is synclinal in structure with a north the fold. In these circumstances, we feel justi- trend, cut off by the Butte fault on the east fied in regarding the Sixty Mile Formation as a and unconformably overlain by the Paleozoic distinct unit and not simply as a local basal on the west. The syncline is somewhat asym- facies of the Tapeats. Thus we regard the Sixty metrical, with gentle dips in the heads of the Mile Formation as a part of the Chuar Group, canyons to the west, and steep dips on the east as did Walcott. The summit of Nankoweap close to the Butte fault. In places, beds in the Butte can no longer be regarded as an outlier eastern limb are vertical. There may well be of the Tapeats. In contrast, an outlier of some folding, more or less along the lines sug- Tapeats is present close to the Butte fault, east gested by Maxson (1967), but we hesitate to of Nankoweap Butte. It is possible to confuse define the axes so clearly owing to the cover of the sediments here with those of the Sixty talus. Some minor folding is present in Chuar Mile Formation, as they too are of sandstone Canyon and is well seen close to the mouth of with lenses of clasts derived from the Chuar. Chuar North Fork. A more westerly anticline However, in this outlier, the sandstone matrix shown by Maxson in the upper reaches of commonly has the amber-colored quartz Chuar Canyon could not be found by us. pebbles typical of the Tapeats. Blocks of Tapeats Sandstone on the slopes of Nankoweap The main synclinal axis passes through Butte suggest that it has been eroded off the Nankoweap Butte in a northwest trend. It can top in very recent times. The Sixty Mile be traced across Nankoweap Canyon toward Formation is about 120 ft (36 m) thick on Marion Point with little difficulty. To the Nankoweap Butte and slightly thicker in Sixty southeast of the trend, the axis crosses Kwagunt Mile and Awatubi Canyons. The inaccessibility Canyon, but is totally obscured by talus in of the highest beds there prevented complete Malgosa Canyon. Farther southeast, the axial measurement. trace swings to a more southerly trend. The two episodes of movement, pre-Paleo- The environment of deposition of the Sixty zoic and post-Paleozoic, of the Butte fault Mile Formation was clearly in marked contrast were described by Walcott (1890), who esti- to that of the rest of the Chuar Group. Some mated a westward downthrow of 400 to 4,000 uplift and warping of the Chuar syncline is ft for the pre-Paleozoic episode. The former indicated, with short-distance transport and figures relates only to the branches of the Butte minimal abrasion of clasts derived from the fault around the mouth of the Chuar Canyon, immediately underlying strata, probably in a but the latter can be revised in the light of the fluvial regime. It is probable that the whole greater stratigraphic knowledge discussed process of warping, erosion, transport, and above. deposition took place during the early stages of By plotting the altitude of the Tapeats Sand- folding of the Chuar syncline, before the pre- stone of each side of the fault, it can be Paleozoic movement of the Butte fault. The shown that the eastward downthrow in post- latter would have produced a high fault scarp Paleozoic times, probably during the Laramide only one-half mile away, with all the Chuar movements (Huntoon, 1971), was about 2,700 and some older Precambrian rocks exposed to ft (810 m). By noting which Precambrian erosion. No evidence of clasts from the latter members of formations are now in contact

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across the fault, where possible, a present-day local faults close to the Butte fault to the net displacement during the Precambrian southeast of Nankoweap Butte that displace episode of about 2,300 to 2,500 ft (690 to 750 the steeply dipping Awatubi stromatolite m) may be deduced. To this must be added horizon against the Carbon Butte Member. the post-Paleozoic movement in the opposite In the heads of the Carbon Creek East and direction, so that it follows that the Precam- West Forks, another northwesterly trending brian fault movement amounted to 5,000 to fault drops the same beds down to the north- 5,200 ft (1,500 to 1,560 m) down to the west. east by about 100 ft (30 m), and a branch This major fault scarp was planed off before affects the Carbon Butte Member east of deposition of the Tapeats. The reversal of tne Carbon Butte itself. direction of throw and partial cancellation of Maxson (1967) showed a strong fault in the displacement by post-Paleozoic movements Basalt Canyon along the length of the canyon has been noted on other faults in the Grand throwing the Chuar and Nankoweap Groups Canyon and has been discussed by Huntoon down to the east, and this is clearly seen in the (1971). eastern branch of Basalt Canyon. The displace- The Butte fault is only exposed as a clean ment estimated is about 400 ft (120 m). break in Kwagunt Canyon, where a horizon Three other faults, trending northeast, were near the top of the Carbon Canyon Member is also shown by Maxson (11567), but none of these in contact, base to base, with the Bright Angel has been seen by the present authors. On Shale (Cambrian) with both rocks being Maxson's map, they have little displacement in younger away from the fault. To the north In the Paleozoic strata, and none is indicated in Nankoweap Canyon, the Butte fault splits the Chuar; there may be some displacement in northwestward into several branches, delimit- the Unkar Group of Unkar Creek. ing blocks made up as follows: (a) unmoved A small branch of the Butte fault trends block east of Butte fault complex-Nankoweap northwest across the south flank of Chuar Group on Cardenas Lavas on Dox Sandstone. Canyon and bounds the Tanner Dolomite Moderate dip toward the east, (b) Dox Sard- inlier there. The displacement is probably no stone, dipping gently east, (c) Tanner Member more than 20 ft (6 m) down to the northeast, of the Lower Chuar, on Nankoweap Group, on but there is considerable slickensiding and Cardenas Lavas. Nearly horizontal, mineralized some barite and copper mineralization. in places with hematite and barite. (d) Tanner A minor fault cutting; the Carbon Canyon Member, dipping steeply west, with a few blocks of basal Jupiter Member stromatolite Member in the North Fcrk of Chuar Creek has limestone, (e) Duppa and Carbon Canyon an unusual development of radiate masses of Members of Lower Chuar, almost horizontal, gypsum. close to the axis of the syncline. PALEONTOLOGY OF THE Faulting within the Chuar outcrop is mainly CHUAR GROUP along a northwesterly trend, in disagreement The fossils found in the Chuar Group in- with the pattern shown on Maxson's map clude the primitive orga nism Chuaria and algal (1967). In Nankoweap Canyon, we could find stromatolites. no trace of the northeast-trending Nankoweap Chuaria circularis was first described by Wal- fault along the creek as deduced by Maxson cott (1899, p. 233-234, ?1. 27, figs. 12 and 13) from aerial photographs. Instead two faults as a discinoid brachiopod. Another "species" were found with a northwest trend across C. wimani, which is probably conspecific, was Nankoweap Canyon, and across the head of two recorded from Sweden (Brotzen, 1941, p. 258). tributaries below Duppa Butte. They were also Ford and Breed (1969, 1973) discussed these seen to displace the Carbon Butte sandstone in and the apparently similar fossil from India and Kwagunt Creek. Each has a downthrow to the Iran, Fermoria, and gave reasons for regarding southwest of about 100 ft (30 m). To the south- that also as synonymous with C. circularis. east, these faults appeared to converge but Timofeev (1970) recently referred Chuaria were lost beneath the talus in Malgosa Canyon. wimani (and hence the cithers) to a new group Smaller faults, which may have been little of giant microplankton, the Megasphaeromor- more than landslide fractures, displace the phidae, without discussing reasons, although Carbon Butte sandstone on either side of the noting a further occurrence in eastern Siberia. mouth of Nankoweap South Fork. There are Leaving aside dubious assignment of Chuaria

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to the Brachiopoda, tryblidiacean gastropods, are more common, taller and at times close chitinous Foraminifera, megaspores, or to "in- enough for laminae to pass from one column to organic objects," we support Timofeev's in- the next. They appear to fall into the category ferred reference of these obscure carbonaceous, named Inzeria. spheroidal corpuscles with irregular shrinkage The second stromatolite horizon is near the folds and furrows to the planktonic algae. top of the Carbon Canyon Member. Although Timofeev did not assign his group Megasphae- dolomitized, the internal structure is partly romorphidae to any particular type of alga, but preserved and shows irregularly branching, the inference seemed to relate them to early rapidly widening columns. The laminae over- acritarchs. They are the largest known algae of hang the sides of the columns toward the top, this type and seem to be restricted to late giving a distinct "mushroom" appearance. Precambrian rocks. They appear to be fairly close to Baicalia aff The various claims of medusoid markings in rara Semikhatov. the Precambrian rocks of the Grand Canyon The third stromatolite horizon is at the base all refer to structures in the Nankoweap and of the Awatubi Member (Fig. 9), which forms Unkar Groups, older than the Chuar. That the spectacular biohermal horizon around which was named Brooksella canyonensis was Nankoweap and Carbon Buttes. This horizon dismissed by Cloud (1968, p. 27) as being a gas is believed to be that which supplied Walcott's blister, although Glaessner (1969, p. 374-375) specimens named Cryptozoon cf. occidentale by argued that it might be an annelid burrow. The Dawson in 1897. Walcott was somewhat con- circular structures from the Hakatai Shale of fused concerning the position of this bed in the the Unkar, described as medusae by Alf (1959) sequence (see discussion in Ford and Breed, were also dismissed as being raindrop prints by 1969); he also called it Stromatopora at a time Cloud (1968), but Glaessner (1969, p. 375- when the nature of this genus was not under- 376) regarded them as casts of algal colonies. stood. Walcott later (1914, p. 111) referred it to The present authors find it difficult to support Collenia occidentale, but Rezak (1957, p. 132) Glaessner's interpretations, and no such fossils referred it back to Cryptozoon occidentale on have been found in the Chuar. the basis that the colonies originate from a Stromatolitic limestone is now known at point. three definite horizons in the Chuar Group, at The bioherms consist essentially of alterna- the base of the Jupiter Member (Fig. 7), in the tions of domal and columnar stromatolites. upper part of the Carbon Canyon Member, and Domes are commonly 20 to 50 cm wide and as at the base of the Awatubi Member (Fig. 9). much as 20 cm high, while columns are usually Minor occurrences of algal lamination have about 5 to 10 cm wide, and they may be up to been noted elsewhere in the Carbon Canyon 2 m high without branching. More often, in- Member and in the Walcott Member. The dividual columns are 20 cm high, and then three main horizons are each quite distinct in covered by a dome, bearing more columns. character from the others, but all three are pre- Branching of columns is uncommon, but con- dominantly undulating dome-shaped, and the fluent laminae join columns frequently. In the digitate columnar forms used in recent cumbersome literal terminology of Logan and stratigraphical subdivision of the Younger Pre- others (1964) the nearest code would be SH- cambrian are rare (Cloud and Semikhatov, C—LLH-C—SH-C—LLH-C. The columns 1969; Raaben, 1969; Glaessner and others, do not fit the diagnostic nomenclature of Cloud 1969). and Semikhatov (1969) very well, but the well- The lowest horizon, at the base of the Jupiter developed enveloping walls suggest Boxonia. Member, consists mainly of wide low domes, Stromatolites are generally regarded as in- commonly 15 cm high and 50 cm wide, with dications of a littoral or even intertidal en- much porous tufa-like material between them. vironment, but those in the Chuar show little Only on the uppermost surface are there direct evidence of intertidal conditions. While scattered low confluent domes of the form the nuclei of some bioherms were flakes of Stratifera. In the block between branches of the laminae piled up, a few bioherms had mud- Butte fault north of Nankoweap Creek, what is cracked tops. The other sediments above and thought to be the same bed is present, but below the stromatolite horizons were pre- mineralization has largely destroyed the in- dominantly shale. However, the thin siltstone ternal structure of the stromatolites. Columns beds within the shale commonly showed ripple

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marks and mud cracks, and so an inference of the same phase of igneous, activity as that which quiet shallow waters with some temporary produced the sills in the Bass Formation of the emergence can be drawn from the stromatolite Unkar, as suggested by chemical and petro- growth. No variation in stromatolite structures graphic data (Hendricks, 1972), and probably across the Chuar terrane was found which the numerous diabase sills of the Apache could be taken to indicate the direction Group. This intrusive phase of igneous activity toward either a shore or deeper water. In- is clearly pre-Chuar and has been dated radio- dividual bioherms in single exposures, however, metricallv at 1.2 b.y. (Silver, 1960; Damon showed variation and might be entirely and others, 1962). A minimum K/Ar age of columnar, but the next might be largely con- 845 + 15 m.y. has recently been obtained for fluent domes. No reason for this variation can the Cardenas Lavas (Ford and others, 1972). be put forward. Allowing for some argon loss, this provides some The presence of the stromatolite forms support for the hypothesis that these lava flows Inzeria, Baicalia aff rara, and Boxonia suggests are the extrusive phase of the Apache and an upper Riphean age. Rama diabases. Studies by J. W. Schopf (in litt.) have re- No other sedimentary group of late Pre- vealed algal filaments in the pisolitic chert of cambrian age is known in Arizona which could the Awatubi Member on Nankoweap Butte. be correlated with the Chuar; but in the Death Further work on these is in progress. Valley region of California, late Precambrian rocks thousands of feet thick lie conformably AGE AND CORRELATION OF below the presumed equivalent of the Tapeats THE CHUAR GROUP (Wright and Troxel, 1966; Stewart, 1970). The The Chuar Group is unconformably covered Pahrump Group, suggested by Cloud (1971) to by Tapeats Sandstone of Cambrian age. A be 1.2 to 1.4 b.y. old, is at the base of these structural episode involving folding, faulting, strata. This would seem to be similar to the and the erosion of the 6,610 ft (2,013 m) of younger age limit of the Apache Group, into Chuar and varying parts of the underlying which diabase is intruded, for which Silver Unkar from areas east and west of the present (1960) gave a minimum age of 1,075 ± 50 outcrops intervened before the accumulation m.y. and a probable age of 1.2 b.y. As the Chuar of the Tapeats. The Tapeats is probably of is clearly younger than the Cardenas Lavas early Middle Cambrian age in the eastern with probable consanguinity to the diabase, it Grand Canyon (McKee, 1969), but it is must be equivalent to some part of the late diachronous; in the western Grand Canyon, the Precambrian of eastern California. No single Tapeats is Early Cambrian in age. Thus it is horizon can be correlated between the Chuar conceivable that part of the Chuar could have and the late Precambrian rocks of California, been deposited in Early Cambrian times in the but both constitute a predominantly littoral eastern Grand Canyon, contemporarily with sequence of sediments with stromatolitic the Tapeats in the west. But no evidence to limestone. support such a concept has been found, and the Farther north in the Cordillera, parts of the Chuar is here regarded as being clearly ?re- Beltian appear to be of similar age to the cambrian in age. Apache Group, and the overlying Windermere Shride (1967) suggested a correlation of the Group was apparently deposited during the underlying Unkar Group with the Apache period from 850 m.y. ago to the start of the Group of central and southern Arizona. The Cambrian at 570 m.y. (Harrison and Peter- correlation of the Chuar with the Troy man, 1971). The Windermere Group thus falls Quartzite of the Apache Group suggested by into the same period :>f geologic time as the Wilson (1962) seems unlikely in view of the Chuar, although the chronological limits of predominantly argillaceous character of the both are uncertain in the absence of radio- Chuar in contrast to the arenaceous Troy. metric dates. Further reliable radiometric Unlike the Chuar, the Troy was intruded by dates are clearly needed both in the Arizona numerous diabase sills. Furthermore, the Precambrian and elsewhere. stromatolites in the Mescal Limestone of the The Chuar Group thus is older than the Apache Group are unlike those of the Chuar. Cambrian, younger than the Cardenas Lavas Thus it seems to us that the Chuar Group is (at least 845 ± 15 m.y. and possibly 1.2 b.y.), post-Apache but older than the Cambrian. The unique in Arizona, possibly to be correlated pre-Chuar Cardenas Lavas appear to represent with some part of the Late Precambrian of

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eastern California and with the Windermere Ford, T. D., Breed, W. J., and Mitchell, J. G., Group of the northern Cordillera. 1972, The name and age of the late Pre- cambrian basalts in the eastern Grand Canyon: ACKNOWLEDGMENTS Geol. Soc. America Bull., v. 83, p. 223-226. Glaessner, M. F., 1969, Trace fossils from the Pre- This work was the outcome of a preliminary cambrian and basal Cambrian: Lethaia, v. 2, expedition undertaken in 1966 at the request p. 369-393. of the U.S. National Museum to relocate Wal- Glaessner, M. F., Preiss, W. V., and Walter, M. R., cott's type locality for the primitive fossil 1969, Precambrian columnar stromatolites in Chuaria circularis. Ellis Yochelson of the Australia: Morphological and stratigraphical Museum and E. D. McKee of the U.S. analysis: Science, v. 164, p. 1056-1058. Geological Survey encouraged the work. Both Harrison, J. E., and Peterman, Z. E., 1971, the preliminary work and field work of 1969- Windermere rocks and their correlatives in the 1970 could not have been undertaken without western United States: Geol. Soc. America, the co-operation of Hatch River Expeditions, Abs. with Programs (Ann. Mtg.), v. 2, no. 7, p. Arizona Helicopters, Inc., the Grand Canyon 592-593. National Park authorities, and a grant pro- Hendricks, J. D., 1972, Precambrian diabase sills and dikes of the Grand Canyon National vided by the Grand Canyon Natural History Park: Geol. Soc. America, Abs. with Programs Association. The Museum of Northern Arizona (Rocky Mtn. Sec.), v. 3, no. 5, p. 381. provided field support and full use of its Hinds, N.E.A., 1935, Researches on Algonkian facilities. formations at Grand Canyon National Park: One of us (Ford) acknowledges the receipt Carnegie Inst. Washington Year Book, no. 34 of a Fulbright Travel Grant and assistance (for 1934-1935), p. 326-329. from the Research Board of the University of Huntoon, P., 1971, The deep structure on the monoclines in eastern Grand Canyon, Arizona: Leicester. Plateau, v. 43, no. 4, p. 148-158. Parker Hamilton of Flagstaff provided aerial Keyes, C. R., 1938, Basement complex of the photographs. Grand Canyon: Pan-American Geologist, v. 70, p. 71-116. Logan, B. W., Rezak, R., and Ginsburg, R. N., REFERENCES CITED 1964, Classification and environmental signifi- Alf, R. M., 1959, Possible fossils from the early cance of stromatolites: Jour. Geology, v. 72, Proterozoic Bass Formation, Grand Canyon, p. 68-83. Arizona: Plateau, v. 31, p. 60-63. Maxson, J. H., 1961, Geological map of the Bright Bassler, R. S., 1941, A supposed jellyfish from the Angel quadrangle (revised 1966): Grand Precambrian of the Grand Canyon: U.S. Canyon Nat. History Assoc. Pub. Natl. Mus. Proc., v. 89, no. 3104, p. 519-522. 1967, Geologic map of the Grand Canyon and Brotzen, F., 1941, Nagra bedrog till Visingsoforma- vicinity, Arizona—eastern section: Grand tionens stratigrafi och tektonik: Geol. Foren. Canyon Nat. History Assoc. Pub. Stockholm Fiirh., v. 63, no. 426, p. 245-261. McKee, E. D., 1969, Paleozoic rocks of Grand Cloud, P. E., 1968, Pre-Metazoan evolution and Canyon: Four Corners Geol. Soc. Grand the origin of the Metazoa, in Drake, E. T., ed., Canyon Guidebook, p. 78-90. Evolution and environment: New Haven and Noble, L. F., 1914, The Shinumo quadrangle, London, Yale Univ. Press, p. 1-72. Grand Canyon district: U.S. Geol. Survey 1971, Precambrian of North America: Geo- Bull. 549, 96 p. times, v. 16, no. 3, p. 13-18. Powell, J. W., 1874, Report of explorations in 1873 Cloud, P. E„ and Semikhatov, M. A., 1969, of the Colorado River of the West: Smithso- Proterozoic stromatolite zonation: Am. Jour. nian Inst. Pub., 36 p. Sci., v. 267, p. 1017-1061. 1875, Exploration of the Colorado River of the Damon, P. E., Livingston, D. E., and Erikson, West: Smithsonian Inst. Washington, 291 p. R. C., 1962. New K/Ar dates for the Pre- Raaben, M. E., 1969, Columnar stromatolites and cambrian of Pinal, Gila, Yavapai and Coconino late Precambrian stratigraphy: Am. Jour. counties, Arizona: New Mexico Geol. Soc. Sci., v. 267, p. 1-18. 15th. Ann. Conf., Mogollon Rim Guidebook, Rezak, R., 1957, Stromatolites of the Belt Series of p. 56-57. Glacier National Park and vicinity, Montana: Ford, T. D., and Breed, W. J., 1969, Preliminary U.S. Geol. Survey Prof. Paper 294-D, p. report of the Chuar Group, Grand Canyon, 127-153. Arizona: Four Corners Geol. Soc., Grand Seilacher, A., 1956, Der Beginn des Kambrium: Canyon Guidebook, p. 114-122. Neues Jahrb. Geologie u. Paläontologie Abh., 1973, The problematical Precambrian fossil Bd. 103 (Hft 1/2), p. 155-180. Chuaria: Palaeontology, v. 16, pt. 3. Sharp, R. P., 1940, A Cambrian slide breccia,

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