New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/11 Geology of the southeastern part of the Chama basin Antonius J. Budding, C. W. Pitrat, and C. T. Smith, 1960, pp. 78-92 in: Rio Chama Country, Beaumont, E. C.; Read, C. B.; [eds.], New Mexico Geological Society 11th Annual Fall Field Conference Guidebook, 129 p. This is one of many related papers that were included in the 1960 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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In the north-central part of the Canjilon SE quad- It is bounded on the west by Gallina Mountain, Capulin rangle in Arroyo Del Cobre, thin-bedded carbonaceous, Mesa, and northward extensions of the Nacimiento-San gypsiferous, and micaceous siltstone is exposed along a Pedro uplift. On the south it is bounded by the Jemez small fault. A well-preserved flora, consisting predom- plateau, on the southeast by the Abiquiu-El Rito embay- inantly of Alethopteris serlii (identified by C. B. Read, ment of the Rio Grande trough, and on the northeast by the of the U. S. Geological Survey in a personal communication ) Brazos uplift. is found in the upper part of this unit. Read states that The southern portion of the Chama basin is an area these beds are either Des Moines or Missouri in age; prob- of high mesas and low plains, intricately dissected by the ably Des Moines. About 35 feet of beds are exposed, Rio Chama and its ephemeral tributaries. Exposed bed- and the upper contact is partly gradational. rock ranges from nonmarine Pennsylvanian siltstone of Des A middle Des Moinesian fossil assemblage, apparent- Moinesian age found in Arroyo del Cobre to the Los Pinos ly of marine origin, has been described by Muehlberger and Santa Fe formations of Tertiary age that conceal the (1957) from Chavez Canyon, about 35 miles north of the older rocks along the southeast margin of the basin on Arroyo del Cobre exposures. Approximately 250 feet of the eastern edges of the Magote Peak and Canjilon SE grayish-red and brown siltstones and sandstones, calcareous quadrangles. Quaternary surficial deposits mask the bed- in their upper part, here rest on Precambrian quartzite. rock in the low-relief areas of the high mesas as well as These limited exposures suggest a rather irregular in many of the lower canyons and gullies. coastline and relatively low-lying masses during Des Moines The mapping on which this paper is based was done time along the eastern margin of the Chama Basin. Thick- in 1956 and 1957 as part of the joint summer field camp ening of these Pennsylvanian rocks westward has been operated by the Geology Department of the New Mexico demonstrated by drilling in the San Juan Basin ( see Institute of Mining and Technology and the Louisiana State Bradish and Mills, 1950, and Wengerd and Matheny, 1958), University. Six 7 1/2 quadrangles, the Echo Amphitheater, although the irregular topography suggests by the differing Ghost Ranch, Canjilon SE, Alire, Canjilon, and Magote environments of the two exposures extends to the north- Peak, were mapped, and the work will be issued as a western part of the Chama Basin, where younger rocks bulletin of the New Mexico Bureau of Mines and Mineral rest directly on Precambrian, as in wells drilled on El Resources during the 1960-61 fiscal year. Sincere thanks Vado dome. A few miles east of these localities Pre- are extended to the Presbyterian Church and to the staff cambrian rocks are exposed on which only Tertiary sedi- of Ghost Ranch for permission to camp on Ghost Ranch ments are found. property and for their generally helpful attitude. The The outcrop exposes insufficient section to determine U. S. Forest Service was most cooperative, providing w,:,!er whether these beds are more closely related to the Hermosa- for the camps from their installations at Coyote and El Rico beds of the Colorado Pennsylvanian section or to Rito; rangers at Canjilon, Coyote, and El Rito provided the Magdalena group of the central New Mexico ex- much information regarding access roads and property posures. ownership. All ranchers in this region were most gracious Permian Rocks in allowing access over their property. The entire project South of 36° N. latitude in New Mexico the Permian was made possible through the cooperation of the College beds are divided into the Abo, Yeso, Glorieta, and San Division of the New Mexico Institute of Mining and Tech- Andres formations. North of that line the formations nology, the Louisiana State University, and the New Mexico lose their distinguishing characteristics, and the strata are Bureau of Mines and Mineral Resources. all assigned to the Cutler formation, a term borrowed from GEOLOGY southwestern Colorado (Northrop and Wood, 1946). The Chama Basin lies in a reentrant between the Rio Cutler Formation Grande trough to the east and the northward extension The Cutler formation was described by Cross and of the Nacimiento uplift to the west. The Jemez volcanic Howe (1905) from exposures along Cutler Creek near plateau to the south conceals the relations between the Ouray, Colorado, where the formation comprised the "bright southeastern part of the basin and the junction of the red sandstones, and lighter red or pinkish grits and conglo- bounding structures. The basin is filled with late Paleozoic merates alternating with sandy shales and earthy lime- and Mesozoic sediments, over whose upturned edges to stones of varying shades of red" between the underlying the east and south are spread Tertiary and Recent volcanic Rico formation and the overlying Dolores formation. rocks and sediments. The southeastern part of the basin Outcrops of the Cutler formation are limited to the exhibits a gentle northwestward plunge; the southeastern Chama Canyon and its deeper tributaries to the south as margin is marked by several northeast- and north-trending far as Arroyo del Cobre, and to the canyon of El Rito faults, some filled with basaltic dikes, whose irregular Creek and some of its tributaries. In the southeastern displacements suggest late Tertiary adjustments to the corner of the Ghost Ranch quadrangle the formation is sinking of the Rio Grande trough. brought up on the southeast sides of a series of southwest- trending faults. The outcrops in the Chama Canyon are Geology Department, NMIMT. terminated on the east by faulting, where as elsewhere 2 Geology Department, Univ. of Kansas. the exposures disappear westward and northward as a NEW MEXICO GEOLOGICAL SOCIETY <> ELEVENTH FIELD CONFERENCE 79 result of a gradual rise of the stream valleys and overlap exists, the basal Chinle consists of pebble conglomerate. by younger rocks. The overlying beds consist of white, gray or buff quartzose The Cutler consists of a seemingly cyclic alternation of to micaceous sandstone which weathers buff to brown. cross-bedded, purple, arkosic sandstones that are locally Bedding ranges from massive to slabby, and cross-bedding conglomeratic, and purple to orange mudstones. The ar- is exceedingly common. For the most part, the massive kosic sandstones, which average 10-15 feet in thickness, beds are coarse grained, whereas, the slabby beds range are resistant to erosion and form small cliffs; whereas from medium to fine grained. Conglomerate lenses occur the thicker, less competent mudstone units form slopes. at various levels. Fossils are rare, but a few pieces of All units exhibit rapid lateral changes in lithology. At carbonized wood were noted. Thickness of the lower least 1,500 feet of Cutler beds are present in the south- sandstone member does not exceed 250 feet, and the unit eastern Chama Basin. The basal contact is exposed in thins eastward and westward from the Ghost Ranch quad- Arroyo del Cobre, where a massive channel sandstone rangle.