BULLETIN 92 Geology of the Cebolla Quadrangle Rio Arriba County, New Mexico by HUGH H. DONEY 1 9 6 8 STATE BUREAU OF MINES AND MINERAL RESOURCES NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY CAMPUS STATION SOCORRO, NEW MEXICO NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY STIRLING A. COLGATE, President STATE BUREAU OF MINES AND MINERAL RESOURCES FRANK E. KOTTLOWSKI, Acting Director THE REGENTS MEMBERS Ex OFFICIO The Honorable David F. Cargo ...................................... Governor of New Mexico Leonard DeLayo ................................................. Superintendent of Public Instruction APPOINTED MEMBERS William G. Abbott .........................................................................................Hobbs Henry S. Birdseye ............................................................................... Albuquerque Thomas M. Cramer ................................................................................... Carlsbad Steve S. Torres, Jr. ....................................................................................... Socorro Richard M. Zimmerly .................................................................................... Socorro For sale by the New Mexico Bureau of Mines and Mineral Resources Campus Station, Socorro, N. Mex. 87801—Price $3.00 Abstract The Cebolla quadrangle overlaps two physiographic provinces, the San Juan Basin and the Tusas Mountains. Westward-dipping Mesozoic rocks, Quaternary cinder cones and flow rock, and Quaternary gravel terraces occur in the Chama Basin of the San Juan Basin, which occupies the western half of the quadrangle. The Mesozoic rocks are assigned to the Triassic Chinle Formation, the Jurassic Entrada and Morrison Formations, and the Cretaceous Dakota Formation, Mancos and Mesaverde groups, and the Lewis Formation. In the upper 600 feet of shale of the Mancos Group and in the basal sandstone of the overlying Mesaverde Group, faunal and lithologic changes occur from normal marine to coal swamp environment, with a distinctive nearshore molluscan fauna in the upper 200 feet of the Mancos Group. The eastern half of the Cebolla quadrangle, within the Tusas Mountains physiographic province, contains Precambrian and Mesozoic rocks unconform- ably overlain by eastward-dipping Cenozoic units. The Precambrian rocks are assigned to the Moppin Formation, Kiawa Mountain Formation, Burned Moun- tain Metarhyolite, and Maquinita Granodiorite. The Cenozoic rocks consist of 1300 feet of gravel and tuff assigned to the El Rito, Ritito, and Los Pinos Formations. Local sources for much of the gravel incorporated in these Cenozoic units have created considerable lithic variation in the Tusas Mountains. The Ritito Formation interfingers with the Conejos Quartz Latite of the San Juan Mountains to the north and with the lower part of the Abiquiu Tuff to the south. The Los Pinos Formation is correlated with the upper part of the Abiquiu Tuff and the basal part of the Santa Fe Formation, units recognizable in the Rio Grande valley of northern New Mexico. Major folding and faulting occurred after deposition of the Precambrian Kiawa Mountain Formation, Cretaceous Lewis Formation, and Tertiary Los Pinos Formation. Folding of the entire Precambrian sequence created a series of anticlines and synclines, accompanied by rhyolite and granodiorite intrusions and regional metamorphism. Iron, gold, and copper have been mined from these rocks intermittently since 1881. Post-Lewis deformation, probably related to the Laramide orogeny, uplifted and folded the Mesozoic and older rocks. Post-Los Pinos block-faulting and tilting have imparted a regional east-northeast dip to the Cenozoic units. Outwash material and periglacial features associated with three periods of Pleistocene glaciation occur within the quadrangle Recent movement along the western edge of the Tusas Mountains has been accompanied by volcanic eruptions and associated flow rock. Slump and earthflow mass-wasting is typically associated with the Mancos and Mesaverde groups and 2 NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES the El Rito and Ritito Formations. These slides cover wide areas and have disrupted the drainage in the headwaters of the westward-flowing, gravel-terraced streams draining into the Rio Chama. Introduction The Cebolla quadrangle overlaps the boundary of two physiographic prov- inces in north-central New Mexico. These are the Tusas Mountains, part of the southern Rocky Mountain province, and the San Juan Basin, part of the Colorado Plateau province. Cebolla is a fifteen-minute quadrangle about seven- teen miles south of the Colorado—New Mexico state line (fig. 1). The western edge of the area approximately coincides with U.S. Highway 84, along both of which are the towns of Cebolla, Park View, and Tierra Amarilla, county seat of Rio Arriba County. The northern boundary of the quadrangle, at latitude 36 ° 45' N., is nearly coincident with the lower part of the Rio Brazos, a tributary of the Rio Chama. Its southern boundary, at latitude 36° 30' N., lies close to the Rito de Canjilon. The eastern boundary is longitude 106° 15' W. The stratigraphy and geology of this area are of interest both from an economic and an academic viewpoint, because it is here that several economic provinces meet. In the Tusas Mountains, the western edge of the old Hopewell—Bromide gold, silver, and copper mining district abuts the eastern edge of the quadrangle. The easternmost lignite and bituminous Upper Cretaceous coal beds of the San Juan Basin extend into the western part of the quadrangle. The arm of the San Juan Basin that forms part of the quadrangle is called the Chama Basin, and it lies between the Tusas Mountains on the east and a monoclinal flexure on the west, stretching from the Nacimiento and San Pedro mountains north through the Archuleta anticlinorium (fig. 1). Immediately west of the map area, this flexure is reflected in the El Vado domes. Within the Chama Basin, the Mesozoic rocks do not dip consistently basinward (west) but show some reversals from local flexures (p1. 1). This report describes various rock units that outcrop in the Cebolla quad- rangle, as well as the geomorphology, structural geology, and economic geology. Different names for the Cenozoic units that crop out in areas north, east, and south of the Cebolla quadrangle were correlated with those of the quadrangle, lithologic and biostratigraphic units in the 1700-foot-thick Mancos Group were established, and the Mancos—Mesaverde contact was studied. PREVIOUS WORK Some of the earliest geologic work in this area was done by members of the Wheeler survey in 1873 and 1874. Professor J. J. Stevenson of the Wheeler survey described the Cretaceous rock units near Tierra Amarilla and the Brazos Box in 1873 (Wheeler, 1889). In 1874, both paleontologist Edwin D. Cope and GEOLOGY OF THE CEBOLLA QUADRANGLE 5 G. M. Wheeler of the survey investigated the rocks cropping out around Tierra Amarilla (Wheeler). In the same year, Lieutenant G. S. Anderson made a study concerning the feasibility of constructing a wagon road from Fort Garland, Colorado, to Fort Wingate, New Mexico (Anderson, 1874). One of his survey lines cut east-west through the Cebolla quadrangle. He described in general terms the rocks and topographic features of this area. In 1905, Lindgren and Graton made a reconnaissance study of the Hopewell—Bromide mining district and briefly discussed the Precambrian rocks (Lindgren and Graton, 1906). They followed this survey with a rather detailed account of the Hopewell and Bromide districts in 1910 (Lindgren, Graton, and Gordon, 1910). Nelson H. Darton described the rocks in the Cebolla area (Darton, 1928), and Atwood and Mather referred to those in the Brazos Canyon and briefly described the glacial features of the area (Atwood and Mather, 1932). A considerable amount of geologic work has been done in and around the Cebolla quadrangle since the early 1930's. Major mapping projects have been completed by Smith (1935,1936, and 1938), Butler (1946), Barker (1958), and Smith, Budding, and Pitrat (1961) in areas to the south and east. Dane (1948), Landis and Dane (1967), and Muehlberger (1960, 1967) mapped areas north and west. A number of theses and dissertations for the New Mexico Institute of Mining and Technology and the University of Texas also cover parts of some of these major areas (Adams, 1957; Trice, 1957 ; Longgood, 1960; St. John, 1960; and Davis, 1960). Bingler (1968) discussed the geology and mineral resources of Rio Arriba County. ACKNOWLEDGMENTS Many people have assisted the writer in the preparation of this report and given guidance and direction in the field. William R. Muehlberger of the University of Texas has immeasurably aided him through suggestions and criticisms, both in the field and in preparing the report. Professors R. K. DeFord, K. P. Young, and S. E. Clabaugh, also of the University of Texas, have offered many constructive suggestions and spent much time editing and improving this manuscript. The New Mexico Bureau of Mines and Mineral Resources furnished financial assistance and supplied equipment, and its staff members also offered many constructive suggestions. Eugene Callaghan, former director of the Bureau, and A. J. Thompson, director, and Frank Kottlowski, assistant director, gave the writer much help. Thanks are also due William E. Arnold, for drafting of 6 NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES illustrations, Joyce Aguilar, Cheryl LePlatt, and Juarine Wooldridge, for typing the manuscript, and Teri Ray for