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Bulletin 75—Geology of the southeastern part of the Chama Basin

By C. T. Smith, A. J. Budding, and C. W. Pitrat, 1961, 57 pp, 2 tables, 11 figs., 8 plates, 2 appendices, 1 index.

The Chama Basin is a shallow basinal structure closely related to the San Juan Basin to the northwest and lying between the Rio Grande trough to the east and the northward extension of the Nacimiento uplift to the west. Late Paleozoic and Mesozoic sediments fill the south-eastern part of the basin and Tertiary and Quaternary volcanic rocks and sediments overlap the older beds from the northeast and south. The southeastern part of the basin is characterized by broad open folds, and gentle regional dips to the north and west; steep normal faults with a north to northeast trend are prominent, and exhibit stratigraphic throws of 200 feet or more. Some of the faults in the extreme southeastern portion of the area have been filled by basalt. The basin was filled with marine Pennsylvanian rocks and nonmarine Permian rocks with scarcely any hiatus; the alternating marine and nonmarine environment continued throughout the Mesozoic, culminating in the extensive marine invasion of the upper Cretaceous Mancos sea. During Tertiary time widespread volcanism to the north, east, and south covered much of the area with tuffaceous debris, gravel fans, and lava. Glaciating was extensive during the Pleistocene in the northern part of the Chama Basin, but no glacial deposits can be identified in the southeastern part. However, solifluction and landsliding probably became pronounced under the periglacial conditions of the Pleistocene, and continued post-glacial erosion has developed the numerous terraces and alluvial fills found in the larger valleys.

Ranching and farming are the principal economic resources of the southeastern part of the Chama Basin. A paucity of water has hampered development. Sparse showings of uranium were staked, but no production has been developed. Two wells have been drilled in the southeastern part of the basin, but no shows of oil or gas were encountered.

The Chama Basin is a shallow basinal structure merging to the northwest with much larger San Juan Basin. The Chama Basin is bounded on the west by Gallina Mountain and Capulin Mesa, northward extensions of the Nacimiento-San Pedro uplift, on the south by the Jemez plateau, on the southeast by the Arroyo del Cobre anticline, and on the northeast by the Brazos uplift.

The southern portion of the Chama Basin is an area of high mesas and low plains, intricately dissected by the Rio Chama and its ephemeral tributaries. Exposed bedrock ranges from a small outcrop of Pennsylvanian rocks in the northeastern part of Arroyo del Cobre in the Canjilon SE quadrangle to the Tertiary Santa Fe Formation. This formation conceals the older rocks along the southeast margin of the basin in the southeastern part of the Canjilon SE quadrangle. Quaternary surficial deposits mask bedrock in areas of low relief on the high mesas as well as in many of the lower canyons and gullies.

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