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. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico.
Free Downloads
NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks.
Copyright Information
Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. 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 author s New Mexico by Harshbarger, Repenning, and Jackson master s 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 Hunter s 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. deposited in a broad, wind-swept playa lake, and that this The best development of the planar type cross-bedding bed may serve as a useful stratigraphic marker over a very was observed at Pyramid Peak, just east of Gallup, where large area along the Arizona-New Mexico border. one set of cross-strata exposed in a vertical face can be FIELD RELATIONS distinguished from U.S. Highway 66, a distance of 1.5 miles. Harshbarger, and others (1957) believed that this The Recapture Member overlies the Cow Springs Sand- facies, north of Fort Wingate, was the upper tongue of stone conformably and gradationally throughout the re- the Cow Springs Sandstone. A detailed examination of gion. North of U.S. Highway 66 the contact is easily this distinctive facies near Gallup revealed it to be a local distinguished as it is placed at the base of the lowest accumulation of sand which is most probably eolian in persistent red-brown siltstone in the Recapture. This origin. A similar development of large-scale cross-bedding siltstone is three to nine feet thick and commonly marks occurs in the Recapture between Fort Defiance and Todil- a break in slope that separates the massive cliff-forming to Park. Tongues of this facies were found in most of the Cow Springs Sandstone below from the overlying less sections measured. resistant sandstones and siltstonc beds of the Recapture. The third facies consists of interstratified very fine- Elsewhere the basal siltstone unit makes a prominent grained sandstone and lenticular siltstone and mudstone. dark-brown stripe across the face of outcrops. The sandstone is usually a pale greenish-yellow, medium South of U.S. Highway 66, the lower contact is not so to poorly-sorted, and angular to sub-rounded. The thin easily defined. The dark color band and break in slope dis- siltstone and mudstone lenses are a dark red-brown, and appear rapidly to the south as the lower Recapture be- are somewhat fissile. The best development of this facies comes coarser-grained, and more resistant. The Cow is at Window Rock and at the northwest apex of the Zuni Springs Sandstone, however, remains fine-grained and well- uplift. Southward, the facies intertongues with the other sorted in the southern part of the area; therefore the con- two facies, and eventually disappears completely in the tact is placed arbitrarily at the base of the lowest sand- vicinity of Nutria Pueblo. This facies weathers to colorful stone composed of grains that average larger than fine. cones or "hoodoos" near Kit Carson Cave, but, more fre- The Recapture Member is overlain in the northern half quently, it simply castellates upon weathering. of the region by the Westwater Canyon Member. Where The third facies, together with the eolian facies, sug- the Westwatcr Canyon is absent in the southern one third gests a dry climate. Cross-bedding indicates that the pre- of the region, the Dakota Sandstone rests on the Recap- vailing wind blew almost due east. It is postulated that ture unconformably. In almost all instances where it is at the foot of the alluvial fan, from Nutria Pueblo north- overlain by the Westwater Canyon, the Recapture exceeds ward, there may have been a broad expanse of playa. Many 300 feet in thickness, the average being 330 feet. Ap- dunal mounds and ridges separating interdunal basins proaching Zuni Pueblo from the north, the Recapture is probably existed on the playa. The small basins were the progressively truncated by the Dakota Sandstone; and sites of ephemeral lakes where the red-brown, calcareous only 80 feet was measured at Zuni River Gap. silts were deposited. The ultimate disappearance of the Recapture a short A possible regional stratigraphic marker occurs in the distance south of Zuni Pueblo is inferred (Fig. 1). Smith third facies, near the top of the member. A gray to green (1957, p. 58) and Edmonds (1961, p. 36) concur in their mudstone unit, 2 to 10 feet thick, contains calcium car- belief that the Morrison is lost between Nutria Springs bonate nodules and concretions that were found in no and Ramah, New Mexico. The termination of the Recap- other part of the section. The concretions are gray, and ture is due principally to pre-Dakota beveling, although NEW MEXICO GEOLOGICAL SOCIETY-EIGHTEENTH FIELD CONFERENCE 141 gradation into the Zuni Sandstone may also contribute to localities revealed that almost 90 per cent of the conglom- its disappearance. erate consists of sedimentary rocks. The most common pebble lithology is a yellowish-brown, very fine grained WESTWATER CANYON MEMBER sandstone. Next in abundance are greenish-gray to yellow- ish-brown pitted cherts. The third most common lithology The Westwater Canyon Member of the Morrison represented is a violet, fine- to very coarse-grained quartz- Formation was named by Gregory (1938, p. 59) for ex- ite. Approximately five to ten per cent of the pebbles are a posures in Westwater Canyon, 15 miles southwest of light-weight, off-white to pink welded tuff or rhyolite. The Blanding, Utah. This member subsequently has been great bulk of the sandstone and pebbles in the member recognized over much of northeastern Arizona and were derived from pre-existing sedimentary rocks. No northwestern New Mexico (Harshbarger, Repenning, and granite pebbles were observed anywhere in the Westwater Jackson, 1951, p. 98), and may be correlated approximate- Canyon Member. ly with part of the white sandstone member of Kelley The Westwater Canyon is everywhere trough or festoon and Wood (1946), and with the Prewitt Sandstone Mem- cross-bedded. Concentrations of very coarse sand and ber of Smith (1954). Some of the more important refer- pebbles commonly occur along cross-bedding foresets. ences to this area are Smith (1954, 1957, 1959); Allen and Clay galls may be locally abundant above the scoured Balk (1954); Craig and others (1955); Freeman and Hil- contacts which separate many cross-bedding cosets. The pert (1956); Foster (1957); and Harshbarger and others small- to medium-scale of the cross-bedding and the medi- (1957). um- to very coarse-grained, unsorted sand composing the member all strongly suggest a fluvial environment of de- GENERAL DESCRIPTION position. This is in complete agreement with the conclu- The Westwater Canyon Member, unlike the Recapture sions reached by previous workers. The average of a num- Member, is more consistent lithologically. Throughout ber of paleo-current indicator readings taken on cross- the Gallup Region it is a yellow-gray to pale-red, fine- to bedding, primary lineations, and elongate concretionary coarse-grained, poorly sorted to unsorted, locally con- structures is N. 13° E. glomeratic sandstone. Color was found to be the least The maximum thickness of the member was measured consistent physical character of the member. The average in Black Creek Valley where it is 287 feet thick. From grain size is 0.38 mm, or medium-grained. The grains are this point southward, the member thins rapidly, and angular to subrounded and are composed predominantly wedges out completely at Oak Springs, Arizona. This is of quartz, chert, and quartzite rock fragments. contrary to the stratigraphic interpretations of Craig and Examination of ten disaggregated specimens under a Freeman who measured 94.4 feet of Westwater Canyon binocular microscope revealed the presence of volcanic at Lupton, Arizona (in Cooley, and others, 1964, p. 150). material such as sanidine, and small doubly-terminated, They placed a question mark on this part of the section to bipyramidal quartz crystals called quartzoids. In addition, emphasize its doubtful status; however, the measurement tuffaceous galls, rhyolitic pebbles and montmorillonite was accepted without reservation by subsequent workers. clay found in the member strongly suggest that a consider- The interpretation presented here is illustrated in Fig. 2. able portion of the Westwater Canyon was derived from a The zero isopach passes from Oak Springs across the volcanic source. The presence of volcanic material in the Gallup sag, slightly south of east to Stinking Springs, Morrison has been previously noted by Kelley (1955), New Mexico. The disappearance of the Westwater Can- Waters, and Granger (1953), and Allen and Balk (1954). yon Member to the south is believed to be entirely the In places, enough feldspar is present to label the sand- result of post-Jurassic, north to northeastward tilting, and stone feldspathic. The sandstone generally has a distinctly subsequent erosion prior to the deposition of the Dakota "gritty" texture. Sandstone. Exposures of the Westwater Canyon generally are mas- sive and cliff forming. FIELD RELATIONS A somewhat distinctive feature of the Westwater Can- In the area of this study, the Westwater Canyon Mem- yon Member is the relatively widespread occurence of ber is bounded by the Dakota Sandstone above, and the fossil woods within the unit. Much of the wood is in Recapture Member below. The contact between the Da- local concentrations but specimens can be found generally kota and the Westwater Canyon is uncomformable. This throughout the member. unconformity could be identified throughout the region, In the northern part of the region, the upper half of and it served as a valuable stratigraphic marker. the Westwater Canyon Member is somewhat coarser- At Church Rock, New Mexico, one or more thin, len- grained and more conglomeratic than the lower half. This ticular, maroon and green mudstone beds appear in the characteristic was noted also by Smith (1954, p. 17) in member. These minor interstratified mudstones become the Thoreau Quadrangle. Pebbles and cobbles in the more persistent and thicken in an easterly direction. In conglomeratic units were seldom in contact, but rather some cases, mudstone beds in the upper half of the mem- were scattered in a matrix of medium- to coarse-grained, ber can be traced directly into the Brushy Basin Member sandstone. of the Morrison which is exposed east of Kit Carson Cave. A close examination of the pebbles and cobbles at 13 The Westwater Canyon displays a sharp, erosional, 142 NEW MEXICO GEOLOGICAL SOCIETY—EIGHTEENTH FIELD CONFERENCE
Red Tod ilto Lake Park Tohatchi o 5 0 5 10 I 11,,,t, , i i 1 Miles Scale
\ \ Mexican Springs N\ Fort ko \Defianceo 0
Window Rock Reservation oundory
Corson Cave
Hunters Point x ort Oak 0 Wingate Springs Gap ManuelitcX
Lupton
per utria 0 Cheechilgeetho
Zuni Pueblo
EXPLANATION • Measured section 5c) Isopach, dashed where • Drill-hole inferred, interval 50 feet. See Fig. 3 for Section on Li ne A–A
FIGURE 2 Isopachous map of the Westwater Canyon Member of the Morrison Formation. (Outcrop pat- tern of Jurassic rocks(J)—taken from Geol. Map of New Mexico, 1965).