New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/9 Tertiary stratigraphy of the Navajo country Charles A. Repenning, J. F. Lance, and J. H. Irwin, 1958, pp. 123-129 in: Black Mesa Basin (Northeastern Arizona), Anderson, R. Y.; Harshbarger, J. W.; [eds.], New Mexico Geological Society 9th Annual Fall Field Conference Guidebook, 205 p. This is one of many related papers that were included in the 1958 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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IRWIN4 INTRODUCTION of the Zuni Mountains, named the erosion surface on which The Tertiary rocks of northeastern Arizona include two the Bidahochi formation was deposited the Zuni surface. units of late Tertiary age that will be seen during the field Hack (1942, p. 345) called the same surface in the Hopi conference: the Chuska sandstone of Pliocene(?) age and Buttes area the Hopi Buttes surface in belief that he was the Bidahochi formation of Pliocene age. Other Tertiary applying the name as defined by Gregory (1917, p. 121- units border northeastern Arizona, but they are located in 122) . However, Gregory defined the Hopi Buttes "pene- areas quite distant from the route of the conference and, plain" as the erosion surface that underlies the lava of the as such, are not of particular importance to the present Hopi Buttes (the volcanic member of the Bidahochi forma- discussion. These units include rocks of possible early tion) and described it as overlying beds now called the Tertiary age, southeast of St. Johns in both Arizona and lower member of the Bidahochi, which he referred to the New Mexico, that may be correlative with the Baca forma- McElmo formation (idem, p. 89), a name since abandoned tion of Eocene(?) age; gravel deposits near Fence Lake, by the Geological Survey. Thus Gregorys Hopi Buttes sur- New Mexico, of presumed Miocene age; the Datil formation face is in the middle of the formation and not at its base. in western New Mexico and adjacent Arizona of possible Because of these facts, the pre-Bidahochi surface is here Miocene age; and gravel deposits along the Mogollon Rim referred to as the Zuni surface, as was first done by McCann; of late Pliocene and/or early Pleistocene age. the name Hopi Buttes surface is here used for the erosion Of the two units discussed, the Bidahochi formation surface beneath the volcanic member of the Bidahochi in can be observed more closely along the route of the field the Hopi Buttes area, as was first done by Gregory; and conference; the Chuska sandstone will be seen only from Hacks term "Hopi Buttes surface" is considered both sy- a distance of several miles to the north of the route to nonymous to the Zuni surface and preoccupied by Gregorys Window Rock on the first day of the conference. Because term for the younger surface beneath the lava. of this, the description of the Chuska sandstone is inten- The Zuni surface can be seen at various points along tionally brief. the route of the conference between Lupton and the Pet- Information summarized in this report was largely rified Forest National Monument. Over this route it is cut obtained with the cooperation of the Bureau of Indian on the Chinle formation of Triassic age. The Zuni surface Affairs, U. S. Department of the Interior. is also visible at many points between Holbrook and Keams BASAL UNCONFORMITIES Canyon. Along this route it is cut on units ranging from The Chuska sandstone and the Bidahochi formation the Chinle to the Cretaceous Toreva formation. About 2 were deposited on erosion surfaces that are of distinctly dif- miles north of Bidahochi Trading Post, excellent exposures ferent origin. The surface beneath the Chuska sandstone can be seen to the east of the road in which the lower is essentially planar and it truncates all Laramide structures. member of the Bidahochi formation is separated from the As a result, the contact of the Chuska with underlying units underlying Triassic Wingate sandstone by the Zuni surface is conspicuously angular. The pre-Chuska surface now and from the overlying volcanic member of the Bidahochi lies at an elevation of about 8,000 feet. by the Hopi Buttes surface of Gregory ( not Hack). In contrast with the pre-Chuska surface, the surface CHUSKA SANDSTONE beneath the Bidahochi formation is, in part, the dip slope The Chuska sandstone occurs almost exclusively in of resistant beds of Mesozoic and Paleozoic age. Thus the the Chuska Mountains of Arizona and New Mexico (fig. topography in the area of the Black Mesa basin at the 2). Gregory (1917, p. 80) briefly described and named start of the Pliocene epoch generally reflected Laramide the unit. structural relief, and the Bidahochi formation was deposited Wright (1954, p. 1831-1833) named the lower, pre- in an exhumed tectonic basin. Modern elevations on the dominantly fluvial, 250 feet of Gregorys type Chuska sand- preserved parts of the surface range from slightly below stone the Dezas formation. Although the Dezas formation 6,000 feet to 7,200 feet (fig. 1 and fig. 2). can be recognized in some exposures throughout the Chus- There are exceptions to the structural control of the ka Mountains, good exposures are not plentiful and for the pre-Bidahochi surface. Some of these resulted from local most part the Deza-Chuska contact, which is gradational, is truncation of Laramide features by streams conforming to very difficult to map. Because of this the Deza formation the regional gradient as they carved the pre-Bidahochi has not been accepted as a map unit by the authors and surface. Others resulted from the fact that the pre-Chuska Gregorys original definition of the Chuska is followed. surface completely truncated the Defiance upwarp, expos- Lithology and Distribution ing rocks that range from the DeChelly sandstone of Per- Composition mian age to the Mesaverde group of Late Cretaceous age. The Chuska sandstone consists of pinkish-gray and With Miocene uplift, erosion began downcutting from the yellowish-gray sandstone that weathers yellowish gray pre-Chuska surface along these several different resistant (color terminology from the National Research Council units to form the pre-Bidahochi surface and thus the sur- Rock-Color chart). The lower part of the formation, which face is regionally non-conformable. was called the Deza formation by Wright, contains a con- McCann (1938, p. 171), working on the west side siderable amount of silt and clay. Clay beds are most common near the base, although they occur locally through- 1- Publication authorized by the Director, U. S. Geological out the formation. Survey. Two types of sandstone alternate throughout the Chus- 2 U. S. Geological Survey, Menlo Park, California. ka. One sandstone type is composed of coarse- to fine- 3 University of Arizona, Department of Geology, Tucson. grained, rounded to subrounded, clear and stained quartz 4 U. S. Geological Survey, Denver, Colorado. with prominent black, white, and red accessory grains that 124 NEW MEXICO GEOLOGICAL SOCIETY • NINTH FIELD CONFERENCE SCALE io .0 A A DEFIANCE UPWARP CHUSKA MTS bor. Feet Fluted Rock Black Creek ? _ _To Valley sz2_ -a HOPI BUTTES 5 Pueblo Colorado Tbu Tbv Tbv Tbv Tbl T bl Volley Arosyduggymegyozwymmayaam.„,srus -5 11111111111.111- FIGURE 1. Diagrammatic cross section A-A-A" from the sive rocks (Ti); and the pre-Chuska (S-1) and Zuni (S-2) Chuska Mountains to the Hopi Buttes showing: the Chuska surfaces and their relation to generalized Laramide struc- sandstone (Tc); the lower (Tbl ), volcanic (Tbv), and upper ture. Jurassic (J), Triassic (T), and Permian (P) beds (Tbu) members of the Bidahochi formation; Tertiary intru- are generalized. Vertical exaggeration X 10. average, respectively, about 4, 3, and 2 percent of the Sandstone units of the fluvial type, as well as silt total volume by megascopic estimate.