T. D. FORD Geology Department, University of Leicester, Leicester, England W. J. BREED Museum of Northern Arizona, Flagstaff, Arizona 86001 J. S. MITCHELL School of Physics, University of Newcastle, Newcastle, England

Name and Age of the Upper Precambrian

Basalts in the Eastern

ABSTRACT Owing to the doubtful validity of many of Keyes' names, the clearly defined Cardenas The nomenclature of the thick basaltic lavas Lavas Series was subsequently overlooked as a at the top of the Unkar division of the Grand stratigraphic name. The basalts were not named Canyon upper Precambrian is discussed. It is on Van Gundy's map (1951), but on Maxson's recommended that Keyes' (1938) name of map (1967), they were named the Rama Forma- Cardenas Lavas be retained for these flows in tion (Maxson, 1961). This was an unexplained preference to Maxson's term of Rama Forma- extension of the name Rama from its applica- tion. A new K-Ar date of at least 845 + 15 tion to the diabase sills which intrude the Bass m.y. has been determined for these lavas. Limestone at the mouths of Mineral and Asbestos Canyons some 6 mi to the west. NOMENCLATURE Maxson in 1961 applied the name Rama from A thick series of basaltic rocks is clearly the somewhat tenuous projection that the visible in the midst of the upper Precambrian diabase sills occur "near" the base of the butte Grand Canyon Series below Desert View look- known as Rama Shrine. On the back of his out tower. First recognized by Powell in his Bright Angel quadrangle map (1961) Maxson famous voyage of 1869 (Powell, 1874, p. 18; denned the Rama Formation as "largely 1875, p. 81), the succession of these basalts was diabases and basalts, . . . intruded as plugs, described in some detail by Walcott (1894, p. dikes and sills following deposition of the Dox 503-519); the general petrography was covered Formation." This latter clause cannot be by Iddings in the same report (1894, p. 520- proved as regards the sills and dikes cutting the 524). In spite of their clear description of these Bass Limestone, as no intrusion in this area is rocks as basalt flows, some subsequent writers seen to cut rocks younger than the Hakatai have called them sills, though without giving Shale. The dikes and sills in the Bass and reasons. No detailed petrological account of Hakatai have no direct connection with the these basalts has been published since 1894. dikes in the upper Dox Sandstone, which in Walcott noted ten separate flows separated turn are not connected with the Cardenas by thin layers of sandstone, the whole totaling Lavas. In spite of this, Pewe (1968) showed a about 1,000 ft in thickness. He regarded them feeder dike connecting the Rama sill with the as forming a part of the lower division of the Cardenas Lavas in a diagrammatic columnar Grand Canyon Series, the . section. However, Maxson added a sentence Noble (1914) named the other sedimentary indicating that he included the flows above the formations within the Unkar but the basalts Dox Sandstone within the Rama Formation. remained unnamed until Keyes (1938, p. 110) Molenaar (1969) included the Cardenas Lavas proposed the name of Cardenas Lavas as they within the Rama Formation, although admit- outcropped at the foot of Cardenas Butte, and ting the priority of Keyes' term (Fig. 1). in the head of Cardenas Creek, immediately It is here proposed that the name of Cardenas below Desert View. These names in turn honor Lavas be maintained for the basalt flows above the first white man known to have visited the the Dox Sandstone on the grounds that (a) Grand Canyon, Garcia Lopez de Cardenas, the name has priority, (b) the name has a more who in 1540 probably saw the canyon from logical definition tied to a locality where the somewhere close to this locality. formation outcrops, (c) the possibility of

Geological Society of America Bulletin, v. 83, p. 223-226, 1 fig., January 1972 223

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verbal confusion of Rama with Brahma Schists map of the eastern part of the Grand Canyon in the underlying lower Precambrian, and (d) (1967). The Cardenas basalts are conformable no field evidence has yet been found which on the Dox Sandstones in Unkar, Basalt, and proves a connection in time and space between Chuar Canyons to the north of the Colorado, the Cardenas lavas and the Rama sills in the and below the Palisades of the Desert and in Bass Limestone. No detailed petrographic or Cardenas Creek to the south. The unconformity geochemical studies of these rocks have yet indicated on Molenaar's stratigraphic chart been made so that any evidence of consanguin- (1969) between the basalts and the Dox Sand- ity between the two formations is not available. stone could not be found in the field. A faulted Under this argument the name Rama is applied patch of Cardenas Lava occurs in Nankoweap only to intrusions into the Bass and Hakatai, Canyon. and to add the designation of formation seems inappropriate. AGE OF THE CARDENAS LAVAS The distribution of the Cardenas Lavas has While there is no doubt that the Cardenas been depicted on Van Gundy's map (1951) of basalts are Precambrian in age,, aas they lie the eastern Grand Canyon, and on Maxson's within the Grand Canyon Series below the

N

Et Cambrian Tapcats p£c p£n Nankoweap Group pCd Cardenas Lavas vv'vV p£d Dox Sandstone p£s Shmumo Quartzite 3 Miles p£h p£b Bass Limestone Figure 1. Sketch map of the eastern Grand Canyon showing p£v Vishnu Schist the position of the Cardenas Lavas and the Rama intrusions. p£r Rama Diabase «*^

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TABLE 1. RESULTS OF DUPLICATE ANALYSIS

Atmospheric K20 Radiogenic argon contamination Mean age (percent) (mm3 • gm~') (percent) (m.y.) 2.39 (8.46 ± 0.08) X 10-" 2.9 2.41 (8.42 ±0.10) X 10~2 4.0 Means 2.40 (8.44 ± 0.13) X 10-* 845 ± 15

Great Unconformity at the base of the Cam- significant argon loss from Precambrian whole- brian, their age within the Precambrian has so rock samples, this result does not preclude the far been regarded only as younger Precambrian possibility that the Cardenas lavas may have or "Proterozoic." The Grand Canyon Series been part of the same phase of igneous activity of sediments lies unconformably on an eroded as the Rama sills. surface of lower Precambrian or "Archean" If such dating is accepted for the Unkar gneisses and schists intruded by granite, which Group as a whole, as the Cardenas Lavas are has been dated at 1.4 b.y. Placement of the conformable to the Dox Sandstone, it provides Grand Canyon Series within some part of the a shorter time span for the formation of the time span from 1.4 b.y. to the beginning of the unconformably overlying Nankoweap and Cambrian at 600 m.y. would help considerably Chuar Groups, which form the upper part of in regional correlation and analysis of the the younger Precambrian in the Grand Canyon. Precambrian history. Diabase from an intrusion The Rama sills have been correlated by of the Rama sills has been dated by Donald Shride (1967) with the Precambrian units of E. Livingston, Laboratory of Isotope Geochem- central Arizona. If this correlation is correct the istry, University of Arizona. K-Ar determina- 1.2 b.y. date on the Rama may be more ac- tions on pyroxene from this sample gave a date curate for radiometric dates. Radiometric of 934 m.y. However, Livingston states that dates provided by Damon and others (1962) the potassium content was very low and his indicate that the sills cutting the Troy Quart- confidence in the date is not very great. A date zite and the Apache Group are about 1.1 b.y. of 1.2 b.y. for similar rocks has been reported in age. Pre-Apache Group granites in the Pinal by Silver (Cloud, 1971). Schists have been dated at 1.4 to 1.6 b.y., sug- Random specimens were collected from the gesting that it was part of the same event which Cardenas basalts in Basalt Canyon during 1969 produced the granite sheets in the "Archean" and one of these has now been dated by the Vishnu Schists of the Grand Canyon. K-Ar method. The potassium determinations The above arguments regarding dating sup- were by flame photometer using the lithium port the conclusions of Ford and Breed (1969 internal standard. Argon analyses were carried and in press) that the Chuar Group is younger out by isotope dilution on an Omegatron than any other Precambrian rocks exposed in mass-spectrometer (Grasty and Miller, 1965) Arizona, probably falling within a time span using the techniques described by Miller and from little more than 845 m.y. to the beginning Brown (1964). The results of duplicate of the Cambrian at 600 m.y. ago. analysis are shown in Table 1 and indicate a The age relation to the various Precam- minimum age of 845 + 15 m.y. for the basalt brian formations may be summarized thus sample analyses. Because of the likelihood of (radiometric dates are given in brackets): Grand Canyon Central Arizona Cambrian Devonian Upper Precambrian Chuar Grand Canyon Nankoweap Cardenas Lavas [845 m.y.] Series Dox Sandstone Unkar Hakatai Shale Troy Quartzite and Bass Limestone Apache Group [1.1 b.y.] (with Rama sills) [1.2 b.y.] with diabase sills Lower Precambrian Zoroaster Granite [1.4 b.y.] Granites [1.4-1.6 b.y.] Vishnu and Brahma Schists Pinal Schists

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REFERENCES CITED Washington, D. C., Grand Canyon Nat. His- tory Assoc. Cloud, Preston, 1971, Precambrian of North Miller, }. A., and Brown, P. E., 1964, How old is America: Geotimes, v, 16, no. 3, p. 13-18. Scotland?: Adv. Sci., v. 20, p. 527-539. Damon, P. E., Livingston, D. E., and Erikson, Molenaar, C. M., 1969, Lexicon of stratigraphic R. C., 1962, New K-Ar dates for the Pre- names used in northern Arizona and southern cambrian of Final, Gila, Yavapai and Coconino Utah east of the Paleozoic hinge line: Four Counties, Arizona: New Mexico Geol. Soc. Corners Geol. Soc. Grand Canyon Guidebook, 13th Ann. Conf. Mogollon Rim Guidebook, p. p. 68-77. 56-57. Noble, L. F., 1914, The Shinumo quadrangle, Ford, T. D., and Breed, W. J., 1969, Preliminary geologic report of the Chuar Group, Grand Grand Canyon district: U.S. Geol. Survey Canyon, Arizona: Four Corners Geol. Soc. Bull. 549, 100 p. Grand Canyon Guidebook, p. 114-122. Pewe, T. L., 1968, guidebook, Lees 1972, The Chuar Group of the Grand Canyon Ferry to : Tempe, Arizona, upper Precambrian: Montreal International T. L. Pewe, 78 p. Geologic Conference (in press). Powell, J. W., 1874, Report of explorations in 1873 Grasty, R. L. and Miller, J. A., 1965, The Omeg- of the Colorado River of the West: Wash- atron: a useful tool for argon isotope investiga- ington, D.C., Smithsonian Inst. Pub., 36 p. tions: Nature, v. 207, p. 1146-1148. 1875, Exploration of the Colorado River of Iddings, J. P., 1894, Petrographic character of the the West: Washington, D.C., Smithsonian lavas (Grand Canyon): U.S. Geol. Survey, Inst. Pub., 291 p. 14th Ann. Rept., pt. 2, p. 520-524. Shride, A. F., 1967, Younger Precambrian geology Keyes, C. R., 1938, Basement complex of the in southern Arizona: U.S. Geol. Survey Prof. Grand Canyon: Pan-American Geol., v. 70, Paper No. 566, 89 p. no. 2, p. 91-116. Van Gundy, C. E., 1951, Nankoweap Group of the 1940, Physiographic significance of Proterozoic Grand Canyon Algonkian of Arizona: Geol. Cardenas Lava flows of the Grand Canyon: Soc. America Bull., v. 62, p. 953-959. Pan-American Geol., v. 74, no. 3, p. 235-236. Walcott, C. D., 1894, Precambrian igneous rocks Maxson, J. H., 1961, Geological map of the Bright of the Unkar terrane, Grand Canyon of the Angel quadrangle, Grand Canyon National Colorado, Arizona. U.S. Geol. Survey, 14th Park: Washington, D. C., Grand Canyon Nat. Ann. Rept., pt. 2., p. 497-519. History Assoc. 1967, Preliminary geologic map of the Grand MANUSCRIPT RECEIVED BY THE SOCIETY MAY 28, Canyon and vicinity, Arizona, eastern section: 1971

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