New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/25 Stratigraphy and uranium potential of the Burro Canyon Formation in the southern Chama Basin, New Mexico A. E. Saucier, 1974, pp. 211-217 in: Ghost Ranch, Siemers, C. T.; Woodward, L. A.; Callender, J. F.; [eds.], New Mexico Geological Society 25th Annual Fall Field Conference Guidebook, 404 p. This is one of many related papers that were included in the 1974 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. 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No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. 211 STRATIGRAPHY AND URANIUM POTENTIAL OF THE BURRO CANYON FORMATION IN THE SOUTHERN CHAMA BASIN, NEW MEXICO by A. E. SAUCIER Continental Oil Company Albuquerque, New Mexico INTRODUCTION northeast. As pointed out by Silver (1948), and Craig and others (1955), the Dakota Formation overlies progressively The Chama basin is both a structural and a topographic younger formations from south to north across the state. basin situated in the center of Rio Arriba County, New Rocks of Lower Cretaceous age have been identified below Mexico. Highway 84 runs north-south through the Chama the Dakota in southeast Utah, southwestern Colorado, the basin and divides it into almost equal portions. The southern extreme northeastern corner of Arizona, and in northwestern part of the basin is one of the few areas in New Mexico outside and northeastern New Mexico (Craig and others, 1955). In of the Grants Mineral Belt in which potentially economic north-central New Mexico, rocks of Lower Cretaceous age uranium mineralization has been discovered in recent years. have not been distinguished, but should be present as indicated The objective of this article is to attempt evaluation of the in Figure 1. Silver (1951) reported: uranium potential of this area. In order to do so it will be necessary to discuss the stratigraphic relations of the principal "Rocks of early Cretaceous age have been tentatively identi- uranium-bearing unit, which is a whitish, massive-appearing, fied by Reeside (1944) only in the northern part of the (San Juan) Basin near the town of Dolores in Montezuma county, conglomeratic sandstone lying between the Upper Cretaceous Colorado, where they have a thickness of approximately 100 Dakota Formation and the Late Jurassic Morrison Formation. feet. They consist of white, medium to fine grained sand- Its relationship to the productive members of the Morrison in stones and green and gray shales. Similar rocks of like thick- the Grants Mineral Belt is important in economically evalu- ness appear to be locally present in northern New Mexico, ating the exploration potential of this area. 150 miles to the east at the head of Arroyo Canjilon in Portions of this paper, in particular the isopachous map, could not have been included without the consent and helpful cooperation of the following companies: Atlantic Richfield Company, Bokum Resources Corporation, Continental Oil Company, Earth Resources Company, Kerr-McGee Corpora- tion, and United Nuclear Corporation. These companies, how- ever, are in no way responsible for the interpretations or con- clusions that are presented. STRATIGRAPHY The geology of the Chama basin is fairly well known, mostly through publications of the New Mexico State Bureau of Mines and Mineral Resources, and from a number of Univer- sity of New Mexico Master's theses. In reviewing the earlier work one finds that the prospective unit has been placed in the Morrison Formation by some workers (Lookingbill, 1953; and Sears, 1953), and in the Dakota Formation by others (Smith, and others, 1961: McPeek, 1965; Muehlberger, 1967; Bingler, 1968; and Doney, 1968). The stratigraphic position of this unit is important not only for establishing the boundary be- tween the Jurassic and Cretaceous Systems, which is rather academic, but also for economic reasons. With the discovery of uranium, the tendency has been to correlate this unit with the "Jackpile Sandstone" of the Laguna area. The "Jackpile Sand- stone" is the host for two of the largest sandstone uranium deposits known. The correlation with the "Jackpile Sandstone" has been on the basis of stratigraphic position and similar lithologic character. Ordinarily these criteria would be un- assailable; however, the major regional unconformity at the base of the Dakota Formation must be taken into considera- tion. Over most of the state this unconformity is angular with strata beneath it dipping at a very low angle to the north and 212 SAUCIER T25N, R4E. Subsurface information from the few wells pulverulent and vuggy chalcedony, varicolored quartzite and presently drilled to that horizon indicates that these rocks are chert pebbles. The pebbles range from 'A inch to 1 inch in generally present in the subsurface through the north half of diameter, and are well rounded when whole, but are com- the basin." monly broken. Lookingbill (1953) describes the conglomerate All subsequent workers in the Chama basin have acknowl- as having a "popcorn" appearance which results from the great edged the probability of Lower Cretaceous Burro Canyon in number of white, tripolitized chert pebbles. This white altered the Chama basin, but have arbitrarily included these rocks in conglomerate is one of the most distinctive lithologic features either the Dakota or Morrison Formations. The one exception of this formation in north-central New Mexico. The mudstone is Swift (1956) who believed that this persistent and easily lenses in the Burro Canyon are similar to the mudstones of the recognized unit in the Chama basin was at least partly underlying Brushy Basin Member of the Morrison Formation; equivalent to the Burro Canyon, and he proposed the name however, the green and pink colors appear to be more "washed Deadman's Peak Formation. Contrary to Swift, McPeek out," or of a pastel shade in the Burro Canyon. (1965) affirmed that the Burro Canyon is present in the The Burro Canyon Formation forms massive, vertical cliffs Chama basin, but he places it in the Dakota Formation on the below the more resistant Dakota Sandstone. It is universally basis that it is not a mappable unit. According to McPeek trough cross-bedded, and commonly displays intraformational (1965), E. H. East of Union Oil Company of California has scoured contacts with associated clay galls. The sedimentary traced the Burro Canyon by a series of measured sections, structures, along with occasional silicified logs, indicate that from southwestern Colorado to North El Vado Dome in the the unit was deposited by streams flowing in a generally north- middle of the Chama basin. McPeek also presents two strati- east direction. graphic cross-sections which tie the Burro Canyon of the San Juan Basin to his measured section south of El Vado Reservoir. Contacts There appears to be sufficient evidence, therefore, to correlate this unit with the Burro Canyon Formation of southwest Although the Burro Canyon is widely exposed around the Colorado. While acknowledging that a new formation name southern periphery of the Chama basin, the base of the forma- may be appropriate for this unit in the Chama basin as pro- tion is seldom well exposed due to talus which accumulates at posed by Swift (1956), it will be referred to as the Burro the bottom of the steep cliffs. The Burro Canyon overlies the Canyon Formation in this paper. Emphasis will be on the massive, green to varicolored mudstones of the Brushy Basin relationship of this unit to the Jackpile Sandstone. Member of the Morrison Formation. Elsewhere in the northern San Juan Basin, the contact has been described as conformable Burro Canyon Formation and gradational (Craig and others, 1961). It is possible that along the southern margins of the Burro Canyon in New The name Burro Canyon was proposed by Stokes and Mexico, the basal contact may be a minor disconformity. The Phoenix (1948) for a relatively thin sequence of rocks lying subsurface information available for this study was inadequate between the Morrison Formation and the Dakota Formation. to determine the character of this contact, but it appears to be The type locality is in Burro Canyon (Sec. 29, T44N, R18W) fairly sharp. in San Miguel County, Colorado.