Bulletin 92—Geology of the Cebolla Quadrangle, Rio Arriba County, New Mexico

By H. H. Doney, 1968, 114 pp., 1 table, 21 figs., 1 plate, 1 appendix, 1 index.

Studies the geology and structure of the two physiographic provinces contained in the quadrangle, the San Juan Basin and the Tusas Mountains. Discussions include the Kiawa Mountain Formation, Dakota Formation, Mancos and Mesaverde Groups, Ritito Formation, Brazos Basalt, Chama Basin, Brazos uplift, mass-wasting, economic geology for coal, gold, and petroleum, and geologic history. The appendix lists measured sections.

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 ft 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 near shore molluscan fauna in the upper 200 ft of the Mancos Group.

The eastern half of the Cebolla quadrangle, within the Tusas Mountains physiographic province, contains Precambrian and Mesozoic rocks unconformably overlain by eastward-dipping Cenozoic units. The Precambrian rocks are assigned to the Moppin Formation, Kiawa Mountain Formation, Burned Mountain metarhyolite, and Maquinita granodiorite. The Cenozoic rocks consist of 1,300 ft 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 NM.

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 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.