The Morrison Formation in the Gallup Region Alva E
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New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/18 The Morrison Formation in the Gallup region Alva E. Saucier, 1967, pp. 138-144 in: Defiance, Zuni, Mt. Taylor Region (Arizona and New Mexico), Trauger, F. D.; [ed.], New Mexico Geological Society 18th Annual Fall Field Conference Guidebook, 228 p. This is one of many related papers that were included in the 1967 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. 138 THE MORRISON FORMATION IN THE GALLUP REGION ALVA E. SAUCIER Shell Oil Co., Houston, Texas INTRODUCTION conformity is angular in the Gallup Region, for the Morrison has been progressively truncated to the south The Gallup region in northwestern New Mexico lies (Silver, 1948). completely within the western half of McKinley County, except for a strip along the Arizona side of the state line from Lupton to Fort Defiance. The area investigated ex- RECAPTURE MEMBER tends from a point six miles east of Gallup, New Mexico, The Recapture Member of the Morrison Formation west to Lupton, Arizona, and from Nutria and Zuni was named by Gregory (1938, p. 58) for exposures at the Pueblos north to Todilto Park. U.S. Highway 66 ( Inter- mouth of Recapture Creek, east of Bluff, Utah. Stokes state 40) bisects the region from east to west. (1944) amended the original description to include in the This paper discusses the areal distribution and strati- Recapture only the upper shales of the type locality. The graphic relations of the Morrison Formation in the Gal- Recapture was first recognized in northern Arizona and lup region and is based upon data given in the authors New Mexico by Harshbarger, Repenning, and Jackson masters thesis which was accepted by the University of (1951). The following year, Rapaport, Hadfield, and Ol- New Mexico in the spring of 1967. sen (1952), in a report for the Atomic Energy Commis- sion, applied the Utah terminology to rocks in the area GEOLOGIC SETTING east of Gallup. A more comprehensive stratigraphic study of the Mor- The area studied lies in the southern part of the Colo- rison by Craig and his associates (1955), extended the rado Plateau, along the southwest margin of the San Juan Utah terminology over the entire Colorado Plateau. Sub- Basin. It is bordered on the east and west by the Zuni and sequent workers, especially in the Grants-Laguna area of Defiance uplifts, respectively. Strata dipping off the up- New Mexico, have employed the Utah names (Freeman lifts into the Gallup sag (Kelley, 1957) form two long, and Hilpert, 1956; Foster, 1957; Harshbarger and others, sinuous, north-trending hogbacks which have been named 1957; Mirsky, and Treves, 1963; Hilpert, 1963). the Nutria monocline (on the east), and the Defiance Prior to the introduction of the Utah terms into New monocline (on the west ). Exposures of Jurassic rocks are Mexico, Kelley and Wood (1946) included this strati- excellent generally, but are limited to outcrops along graphic interval in their white sandstone and brown-buff these two monoclines, and to outcrops on the north flank sandstone members of the Morrison and Smith, (1954), in- of the Zuni uplift. Jurassic rocks exposed in the region troduced a new set of stratigraphic names for members of belong either to the San Rafael Group or to the Morrison the Morrison west of Grants, New Mexico. His Chaves Formation, and they consist almost entirely of sandstones Member, and Prewitt Sandstone Member are correlated of continental origin. with the Recapture and Westwater Canyon Members, respectively. GENERAL STATEMENT GENERAL DESCRIPTION West of the Rocky Mountains, strata believed to be The Recapture Member of the Morrison Formation in equivalent to the Morrison Formation of Late Jurassic age the Gallup Region is the most lithologically inconsistent have been divided, on lithologic grounds, into four mem- rock unit present. North of U.S. Highway 66 this incon- bers. In ascending order, these are: the Salt Wash Mem- sistency serves as a distinguishing characteristic of the ber, the Recapture Member, the Westwater Canyon member. Vertical and lateral changes in facies within short Member, and the Brushy Basin Member. In the Gallup distances are the rule. In gross aspect, three principal Region, all the members except the Salt Wash are present. facies can be distinguished. Essentially the same three The two lower members in this region, the Recapture and facies were defined by Craig and his associates in 1955. the Westwater Canyon are predominantly sandstone with From Zuni Pueblo, New Mexico, northwest to Oak associated conglomerate and thin mudstone beds. They Springs, Arizona, and from Nutria Pueblo, New Mexico, have been interpreted as being alluvial "fan" deposits north to Little Bear Springs, (Fig. 1) the Recapture is pre- (Craig, 1955). The Brushy Basin Member is predomi- dominantly a pale reddish-brown, fine- to medium-grained nantly shale, with minor sandstone lenses. In the more massive sandstone. The sand grains of this facies are restricted area of this study, however, only the Recap- poorly sorted, subrounded to rounded, and are composed ture and Westwater Canyon Members are present. of quartz with minor amounts of rock fragments, chert, The Morrison Formation overlies the San Rafael feldspar, and volcanic material. The sandstone is cross- Group conformably, but it is unconformable with the bedded and some units are fine-grained and well-sorted. overlying Dakota Sandstone of Cretaceous age. The un- Several beds were noted in which very coarse, angular NEW MEXICO GEOLOGICAL SOCIETY-EIGHTEENTH FIELD CONFERENCE 139 Todilto Park Tohatchi 0 0 5 10 Miles Scale N Mexican Springs Fort\ Defianceo Window Rock Navajo Resew`iro;Boundary Kit Corson Cave Hunters iPoinf x rchroc jforf ak \ oi Wingate \OSp rings Gap / • z Lupton1111)1,4 ippei\ utria \ I iak■- Cheechilgeetho Q 11,‘-N Nal3„, I Zuni Pueblo 1.1‘ I EXPLANATION • Measured section 5° lsopach, dashed where A Drill - hole Inferred, interval 50 feet. FIGURE 1 Isopachous map of the Recapture Member of the Morrison Formation. (Outcrop pattern of Jurassic rocks (J) taken from Geol. Map of New Mexico, 1965). 140 NEW MEXICO GEOLOGICAL SOCIETY-EIGHTEENTH FIELD CONFERENCE grains of feldspar and quartz were scattered in a fine- to range in size from 3 to 15 inches in diameter. They are medium-grained matrix. Small pebbles were found at some composed of radiating crystals of calcite and usually have places in these beds. Northward from Zuni Pueblo, sand- a limestone core. Most of the concretions are discoid in stones that have average grain size larger than medium be- shape, and some have a septarian development. come thin, and eventually disappear. This facies is thought Associated with the concretion-bearing mudstone are to represent the foot of an alluvial fan that extended north- several thin-bedded, very fine-grained, laminated, sand- ward from west-central New Mexico. stones. Primary current lineations, rib-and-furrow struc- The second facies is a pale-red to grayish-yellow, very tures and mudcracks are well developed in these flaggy fine to fine-grained sandstone. The grains are very well beds. The mudstone and laminated sandstones were found sorted, subrounded to well-rounded, and frosted. This associated together in approximately the same stratigraphic sandstone is cross-bedded on a large scale and it weathers position from Hunters Point, Arizona, northward. They to vertical cliffs. When well developed, as at Navajo were also seen in a section measured at Cheechilgeetho, Church and at Diamond No. 2 mine, this facies may oc- New Mexico. A similar association of units was noted in cupy almost the entire Recapture interval. approximately the same stratigraphic position in exposures The facies is distinguished by large to very large sets of along the Nutria monocline, but the limestone concretions tabular, planar, and trough cross-bedding. The scale of were absent. the trough cross-bedding sets range from less than one From a consideration of the sedimentary structures, it foot to over 12 feet in thickness, whereas the planar sets is believed that the concretion-bearing mudstone unit was may be as much as 75 feet thick.