Late Precambrian Chuar Group, Grand Canyon, Arizona
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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 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 1244 FORD AND BREED 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) 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 1. Geological map of the Chuar terrane, eastern Grand Canyon. FORD AND BREED, FIGURE 1 Geological Society of America Bulletin, v. 84, no. 4 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 c CARBON CANYON FORD AND BREED, FIGURE 2 Geological Society of America Bulletin, v. 84, no. 4 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 CHUAR GROUP, GRAND CANYON, ARIZONA 1245 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).