New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/10 Jurassic rocks of the Zuni Mountains Clay T. Smith, 1959, pp. 74-80 in: West-Central New Mexico, Weir, J. E., Jr.; Baltz, E. H.; [eds.], New Mexico Geological Society 10th Annual Fall Field Conference Guidebook, p. This is one of many related papers that were included in the 1959 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. 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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. 74 NEW MEXICO GEOLOGICAL SOCIETY O TENTH FIELD CONFERENCE JURASSIC ROCKS OF THE ZUNI MOUNTAINS Clay T. Smith The Zuni Mountains are a prominent asymmetrical anticline along the southwestern margin of the San Juan Basin. The rocks of the mountains consist of a Precam- brian core flanked by sediments that range in age from Pennsylvanian(?) to Recent. During considerable intervals of geologic time the area of the Zuni Mountains was on the margin of, or was contained within regions of, major uplift, and thus were both a source for sediments and a barrier to the spread of certain formations. In late Triassic time a positive element of considerable magnitude, extending throughout most of central New Mexico and central Arizona, (called the Navajo Highland by Smith (1951) and the Mogollon Highland by Harshbarger, Repenning, and Ir- win (1957) ) arose which affected all of the Jurassic sedi- mentation in and near the Zuni Mountains. Jurassic rocks have been recognized in the Zuni Mountains since Dutton (1885) first described the section at Fort Wingate. The almost complete lack of fossils and Figure 3.—Conglomerate of the Lukachukai member of the strikingly similar lithologies throughout the Jurassic section Wingate sandstone overlain by a narrow wedge have resulted in widely varying interpretations and cor- of Entrada sandstone showing marked cross- relations. Figure 1 shows the principal nomenclature and bedding which in turn is truncated by more the approximate equivalence of the various stratigraphic evenly-bedded Thoreau formation. South fork units which have been defined in and around the Zuni of Ruin Canyon east of Ramah, New Mexico. Mountains. Dutton (1885) originally described the Jurassic rocks mudstone unit typical of the exposures at the type locality, of the Zuni Mountains in two units, the Wingate sandstone and an upper well-sorted, cross-bedded, medium-grained below and the Zuni sandstone above. The Wingate sand- sandstone unit. Harshbarger, Repenning, and Irwin (1957) stone included all the beds from the nodular limestone beds describe, in the western part of the Navajo Reservation, a of the upper Chinle formation (Lower Trias of Dutton) to lower sandstone facies, below the silty sandstone and mud- the limestone beds overlying the massive cliffs north of stone of the typical Entrada. Thus, three members of the Fort Wingate, New Mexico; the upper limestone beds were Entrada have been recognized, the lower sandy member, later correlated with the Todilto formation by Gregory the medial silty member, and the upper sandy member. In (1917). The Zuni sandstone included all the beds from the Zuni Mountains only the medial silty member and the the Todilto limestone at the top of the massive Wingate upper sandy member are present. cliff to the capping Dakota sandstone of upper Cretaceous The medial silty member of the Entrada sandstone age. is fine-grained, massive (3- to 4-foot beds), mottled red Subsequent work by Baker, Dane, and Reeside (1936, and white, silty sandstone. The red color is chiefly a 1947), and Harshbarger, Repenning, and Irwin (1957) has surficial staining derived from the weathering and leaching resulted in considerable revision of Duttons original con- of interlayered thin red siltstone and mudstone partings. cept. Baker, Dane, and Reeside (1947) have shown that Such partings combined with prominent vertical jointing the upper cliff-forming part of Duttons Wingate sandstone yield the rectangular weathered blocks with rounded cor- actually is the considerably younger Entrada sandstone; ners to which the name "hoodoo" or "rock baby" has they recommended the abandonment of the Fort Wingate so often been applied. Along the north flank of the type locality for the Wingate sandstone and the substitu- mountains and extending eastward and westward beyond tion of the exposures in the Glen Canyon of the Colorado the range, the medial silty member is between 40 and 50 River as more typical of the Wingate sandstone recognized feet in thickness (see fig. 2 in envelope). Between Nutria over much of the Colorado Plateau. However, Harshbar- Springs and Ramah it pinches out and is absent at El Morro ger, Repenning, and Irwin (1957) show that the lower and to the southeast (see fig. 3). The medial silty mem- 355 feet of Duttons type section at Fort Wingate, New ber rests on the Lukachukai member of the Wingate sand- Mexico, are beds correlative with the Wingate sandstone stone with slight erosional disconformity and a relief of in other parts of the Colorado Plateau and thus they re- about 6 inches. The contact with the upper sandy member strict the Wingate sandstone to the lower 355 feet of Dut- is gradational; often a white bleached zone on the cliff tons section. Harshbarger, Repenning, and Irwin (1957) is selected as the top of the medial silty member. further show that these beds now are to be considered The upper sandy member of the Entrada sandstone Triassic in age, and have little relation to the overlying is massive, orange-red to pale buff or white, friable, com- Jurassic sediments. posite bedded, medium- to coarse-grained, well-sorted STRATIGRAPHY sandstone. Two grain sizes occur; the coarser grains are Entrada Sandstone concentrated along the foreset surfaces of the cross-strata The Entrada sandstone was named by Gilluly and sets. In those places where planar cros§-stratification sets Reeside (1928) from exposures in the northern part of the are replaced by horizontally bedded sets some horizon- San Rafael Swell in Utah. Eastward from the type locality tally bedded strata are ripple laminated in several direc- two facies are recognized: A lower silty sandstone and tions. The upper 35 feet of the upper sandy member is finer grained and limy, and locally, near the top of the unit, New Mexico Institute of Mining and Technology Baker, Dane, and Freeman and Harshbarger, , R epen- Dutton (1885) Reeside (1936, 1947) Smith (1957) Hilpert (1956) ning , and Irwin (1957) ;.., cd 'a a) 4, 0 0) 0 $z Dakota (?) sandstone Dakota (?) sandstone Dakota (?) sandstone Dakota (?) sandstone ]Dakota (?) sandstone H cl) P. U o g g g o Brushy Basin o Brushy Basin .9, Brushy Basin member cd member 11 member Morrison Prewitt kE Westwater Westwater 0 a. formation 44 member ,.... Canyon member 2 Canyon member GL Zuni g g g o 0 1 ,o ca to Chavez ...- Recapture ... Recapture i.■ k k sandstone $... k t4 member o member o member g Wanakah 0 o Upper Bluff sandstone Cow Springs cd -•-■ ,.§ marl ,°),, i member sandstone U .. .4 o E 10., (I Bilk Creek a. k Lower a. t, Summerville f 0 0 r, o g CO o (I sandstone 0 44 member o g Upper Sandy member (4 k 0 k k 0 1513 ml Todilto Todilto bo Ian Todilto Cg 7c1 . -4 . ti 1: °) limestone lu limestone g limestone cd it 4.. , 41 4.. cd al P4 g ,j 04) Upper Sandy z mcd cti 0 Upper Sandy Entrada g Vcd 4.+ member p Tjg .2 member g k co cdg 1-4 Cf) td Id W u) '' u) 5 ingate c4 ul 0 r4 m sandstones cd0 MedialSilty 4 g Medial Silty to member •... CO member ).1 formation- o o Glen Canyon group CO a) V undifferentiated . 4 .4.4. o 4.) 0 Lukachukai a) of Lukachukai 4. , Z 4. ..... o 4, at- member on ( d I member . Lower Chinle ran cd tO FA ..■ .5 E ! co a) 1.0 Rock Point 1-i Rock Point g la, Trias formation P" o ,.o .... c:1 44 member member ;.. Figure 1 - Nomenclature Chart of Jurassic Formations in Zuni.Mountains 76 NEW MEXICO GEOLOGICAL SOCIETY " 0' TENTH FIELD CONFERENCE lenses of sandy limestone 1 to 3 inches thick occur.