| Abstract/Notes | The 15-minute Fort Defiance and Tohatchi quadrangles lie south of latitude 36 degrees and include a part of San Juan Basin, New Mexico, as well as the bordering region in Arizona. The area contains a part of the synclinal "Gallup embayment," a part of the Defiance uplift and monodine, the southern end of the high Chuska Mountain plateau, and the north end of the lower Manuelito Plateau. Smaller features are the Todilto Park dome and Buell Park diatreme with its central kimberlite - tuff plug. Several dikes and smaller central intrusives are aligned in an east-northeast direction.Altitudes range from 6,200 feet in Chuska Valley along the eastern border of the area to more than 9,000 feet on Chuska Mountain. The climate is semiarid; life-zones range from upper Sonoran to Hudsonian.No mineral resources are being exploited currently in the area. Coal and nonmetals have commercial possibilities, but metallic minerals found in the course of this survey consist solely of a few traces of manganese oxides in sandstone concretions. In spite of the fact that elsewhere the formations exposed in this area are uraniferous, so far only one locality has been found sufficiently radioactive to affect the Geiger counter, and no uranium mineral was seen. Large deposits of bentonitic shales which occur in the Tohatchi formation on the slopes of Chuska Mountain may be of value as well-drilling muds, though tests thus far have not been favorable. Sands which possibly could be beneficiated and used for a local green glass industry occur in the Chuska sandstone. Semiprecious stones, garnet and peridot, which have been used to a slight extent for native jewelry, are associated with some of the igneous rocks, particularly at Buell Park. Coal beds of commercial thickness and extent occuring in the Crevasse Canyon formation, are adequate for any local industry. No oil or gas has yet been discovered in these quadrangles, but formations which elsewhere are petroliferous underlie most of Tohatchi quadrangle. Water occurs in the Chuska sandstone, which supplies the perennial springs along the west side of Chuska Mountain, and in Cretaceous sandstones, which can be tapped as underground water sources in Tohatchi quadrangle.The sedimentary sequence is 11,000 to 12,000 feet thick. It has been divided into 19 major and 16 subordinate units. Four igneous rock-types are described. Pre-Cretaceous rocks include Precambrian (?) quartzite, exposed only in two canyons northwest of Fort Defiance; Permian Cutler formation and De Chelly sandstone (1,060-1,300'); Upper Triassic Shinarump conglomerate and Chinle formation (1,0001,350'); Lower Jurassic Wingate sandstone (80-265'); Upper Jurassic Entrada sandstone, including Carmel silty facies (220-293';); Todilto limestone (0-14'); Summerville formation (340-367'); and Recapture shale and Westwater Canyon sandstone members of the Morrison formation (464-857'). Tongues of Cow Springs sandstone come into the area from the south within the Recapture shale and at the top of the Summerville formation. A few thin lenses of maroon shale appearing above the Westwater Canyon sandstone possibly are equivalent to the Brushy Basin shale to the north of the area.Upper Cretaceous rocks include 4,800 to more than 6,100 feet of marine and nonmarine sediments which have been mapped as the Dakota (?) sandstone (218-253'), Mancos shale (630-750'), and Mesaverde group (4,000-4,825'). The Mesaverde group has been divided into five formations and several members, as follows: Gallup sandstone (100 - 400'); Crevasse Canyon formation (252-410'), which includes the Dilco, Dalton sandstone, and lower Gibson members; Point Lookout sandstone (0-365'), which includes the Satan tongue of Mancos shale and Hosta sandstone tongue; Menefee formation (2,300'); and Tohatchi formation (1,350' plus), subdivided into lower carbonaceous and upper bentonitic members. The sandstone members of the Mesaverde group thicken and thin rapidly; they split and pinch out or are replaced by siltstone within short distances.Tertiary sedimentary rocks consist of the Pliocene (?) Chuska sand - stone (1,100' plus), which was deposited unconformably upon the beveled edges of rocks ranging from the Summerville formation to the Tohatchi formation.Nine igneous plugs, with associated discontinuous dikes, penetrated the sedimentary rocks in late Tertiary time. Buell Park is a diatreme characterized by a ring dike, composite plugs, and kimberlite and lapilli tuff. North of Tohatchi quadrangle, at Washington Pass, a similar igneous mass cuts through the Chuska sandstone, and plugs in the mapped area may have been emplaced after the erosion of much of the Chuska sandstone. However, the igneous activity was terminated before the present major valleys were incised into the post-Chuska rejuvenated "Zuni erosion surface." The earliest igneous rocks are kimberlite tuff and lapilli tuff, composed of partly altered olivine (fa8-11), enstatite, chrome diopside, pyrope garnet, actinolite, ilmenite, magnetite, titanclinohumite, and a pale-green, aphanitic groundmass of antigorite and indeterminable, minute mineral fragments. Xenoliths in the tuff include various siliceous crystalline rocks, and cognate fragments of peridotites. Fragments of the surrounding sedimentary rocks are rare. Closely associated are plugs and dikes of minette and minette-breccia, composed for the most part of diopside, biotite, sanidine, and variable amounts of brown glass, possibly with small amounts of olivine and leucite in places. Leucocratic phases in Buell Park approach trachyte in composition. No contact effects were seen at kimberlite tuff contacts, but the minette has vitrified the sandstones in several places. The trachytic phases of Buell Mounta in, Fluted Rock, and the minette plug of Zilditloi Mountain may have approached the surface closely, but true surface flows are lacking.Quaternary sediments consist of Pleistocene and Recent terrace and pediment gravels, which occur at several elevations, and of landslide debris which is widespread around the south and east sides of Chuska Mountain and below the Gallup sandstone cuesta along the southern junction of the two quadrangles. These units rarely reach 50 feet in thickness. Recent alluvium, called the Nakaibito formation (0-50'), occupies the wider valley bottoms.The sedimentary sequence has been folded to produce the broad, northplunging Defiance uplift, whose axis passes through the center of Fort Defiance quadrangle, and the north-plunging Nakaibito syncline, whose axis lies just east of the center of Tohatchi quadrangle. The Todilto Park dome is a subordinate fold whose long axis strikes N. 15 deg E. Dips as steep as 35 degrees occur on the eastern flank, but to the west the beds dip gently into the shallow Zilditloi syncline, which swings to the southeast to form a minor trough in the Nakaibito syndine terminating near Mexican Springs. Another minor crenulation near the center of the southern edge of the area represents the northern termination of the syndine which has brought down the coals mined at the Window Rock coal mine in the quadrangle adjoining the Tohatchi on the south. On the east flank of the Nakaibito syncline, a wide structural nose may reflect the northwesternmost extension of the Zuni uplift.Fracture systems are prominent in the Permian formations. Vertical northeasterly-trending joints transect the southwestern and central parts of the Defiance uplift within the map area. Northwest-trending fractures extend from the vicinity of Sawmill to the northwest corner of Fort Defiance quadrangle. Conceivably these fracture systems are radial tension joints related to the Buell Park diatreme. Steeply west-dipping joints are a feature of the east slope of the Defiance uplift. They are probably tension joints related to tangential stretch in the uparched zone. The Mesozoic sandstones on the east side of Black Creek Valley locally display slip zones with small displacements, and east-west striking striae. Some eastwest striking joints accompany the zone of igneous plugs extending from Fluted Rock to Beelzebub.The area lies wholly within the Colorado Plateau Province, and its physiographic divisions reflect the structures and composition of the underlying rocks. On the west, the Defiance Plateau is essentially a broad anticlinal arch in the Permian and Triassic rocks pierced by the Buell Park diatreme near its axis. It is bounded on the east by the monoclinal Black Creek Valley, incised in the soft Triassic Chinle shales. Todilto Park is formed by a local anticlinal arch which exposes the Chinle along its axis. The Manuelito Plateau, bounded on the west by cliffs of east-dipping Jurassic sandstones, and on the east by badlands and eastdipping cuestas of Point Lookout sandstone and sandstones of Menefee formation, is essentially a structural terrace imposed on the Defiance monoclinal structure. Chuska Valley is carved in the gently-dipping Menefee rocks, and is divided into two parts, the western badlands, terraces and pediments, and the eastern alluviated portions. Chuska Mountain consists of the nearly flat high plateau, underlain by Chuska sandstone, with the subdivisions of lower-lying structural terraces on the west and landslide areas on the south and east. Subsequent to the long erosional epoch which extended throughout nearly all of Paleozoic time, the depositional history of the area was initiated by Permian continental flood-plain and sand-dune deposition forming the Cutler and De Chelly shales and sandstones. Lower Triassic (Moenkopi) beds are missing in the area, and Upper Triassic deposition commenced with a widespread but thin sheet of coarse conglomerate (Shinarump) followed by thick shale and minor sandstone sequence (Chinle) representing river flood-plains and swamps. Above these rocks, with only an inconspicuous disconformity, lie Jurassic units consisting of thick, massive-weathering, crosslaminated and predominantly red-colored sandstones and subordinate shales. These rocks represent a widespread and thick accumulation of river flood-plain and sand-dune sediments interrupted by one period of lacustrine or marine deposition (Todilto) of sandy limestone.Epeirogenic subsidence, following a period of nondeposition or erosion which lasted throughout early and middle Cretaceous time, permitted the first definitely recorded advance of the sea, from the northeast, over the area. The lowermost Upper Cretaceous rocks consist of siltstone, coal, and thin sandstone of the Dakota (?) formation, representing the lagoons. The thicker cuesta-making sandstone represents the offshore bar and beach, the marine (Mancos) shale the deeper marine deposits. Repeated transgression and regression of this shoreline across the area from the northeast to the southwest, with influx of abundant silt and sand from highlands to the southwest, led to a complicated series of intertonguing sandstones, siltstones, and coals which make up the Mesaverde group of rocks. Within this area marine deposits occur twice during Mancos and Satan transgressions; apparently the shoreline did not quite reach the area during the Mulatto or Lewis transgressions.Post-Upper Cretaceous (Laramide) folding produced the structures previously listed. Subsequent extensive Tertiary erosion formed a widespread surface of low relief (the "Zuni erosion surface") upon which was deposited the dominantly eolian Chuska sandstone of Pliocene (?) age. Later, the Chuska sandstone was variably eroded and the Zuni erosion surface was exhumed over large areas. Volcanic activity penetrated this surface and formed the igneous features previously discussed. Quaternary uplift and gentle westward tilting superposed established drainages across the folds, incised numerous subsequent valleys, and produced several series of sloping pediments and terraces. This postChuska degradation may have removed over 1,000 feet of Chuska sandstone and up to an additional 2,000 feet of Cretaceous rocks from much of the west half of San Juan Basin.During glacial pluvial epochs (Wisconsin) and before present canyons and badlands were completely formed, landslides developed around the south and east escarpments of Chuska Mountain, especially where the sandstone was underlain by the upper Tohatchi bentonitic shales. Following the late Pleistocene and Recent cutting of the present canyons and badlands, alluviation filled the lower stream-courses with Nakaibito sediments, and, in historical time, gullying of this fill has begun again.The petrology of a section of sedimentary rocks that range in age from Precambrian through part of Cretaceous was studied in detail. Chip samples from nearly all of the stratigraphic units were disaggregated; the silt-clay size fraction was removed by elutriation, and the remaining sediment was mechanically analyzed.The median diameter, angularity, solubility, and coefficient of sorting were determined from each sample. The percent of feldspar and heavy minerals in the .125 or .062 mm size fractions also was determined. The frequency percent of the heavy minerals was determined by grain counts. Differential thermal analyses were made of all the silt-clay concentrates, and many of the samples were tested by X-ray diffraction and staining.Differential thermal analyses suggest that the De Chelly and Chinle formations are the product of uninterrupted sedimentation, partially in a marine environment. The heavy-mineral assemblage and the abundance of montmorillonite in the Chinle indicate that in this area the Chinle is a mixture of volcanic and nonvolcanic material.The Jurassic sediments present many of the features of the rocks above and below them. No major change in the mineralogy occurs across the contact between the Jurassic and Cretaceous formations. The minerals of the rocks from the upper part of the Westwater Canyon sandstone through the lower part of the Mancos shale appear to be the remains of preexisting sediments. Above the Mancos shale they appear to have been derived from a crystalline source.The observed changes in the mineralogy and physical state of the rocks in the studied section, may prove to have a restricted range in time, and hence may aid materially in subsurface correlations.Two measured sections in the Menefee formation, located about 2 miles apart, were described in detail in order to determine lateral variation of the beds within relatively short distances. Comparison of the descriptions of these sections (pl 11, nos 5 and 6) and of an additional section nearly 4 miles farther south (pl 11, n 9) bring out two features: first, that the sandstone units thicken and thin and disappear within short distances, and hence cannot with assurance be correlated over even these short distances without walking them out along the strike; and, second, that the ratio between the sandstone and the shale and siltstone changes southward from about 1:4 to more than 1:3. Two sections of the Crevasse Canyon formation were also described.A seismic reconnaissance in Buell Park was made in an attempt to determine whether the caldera has vertical walls, or whether it is saucer-shaped and has a bottom, possibly approximating the Precambrian surface. Results were not conclusive.Climatic data from 2 stations in the area and from 3 other nearby stations indicate that the climate, although variable by reason of elevation differences, consists of temperature average from 25 to 30 degrees in January to 65 to 75 degrees in July. The average annual precipitation is 10 to 13 inches, more than half of which falls between April and September.A complete list of plants in the area, a glossary of technical terms employed, a list of Navajo words used for geologic terms, and a bibliography and index complete the report. |
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