Neogene stratigraphy and structure of the Ojo Caliente-Rio Chama area, Espanola Basin, New Mexico
— S. Judson May

The Ojo Caliente-Rio Chama area is located in the northern Espanola basin of the Rio Grande rift, west of the basin axis, and lies between the town of Ojo Caliente in the north and the Rio Chama in the southwest. The area is characterized mainly by gently southeast-dipping Neogene sandstone, conglomerate and intercalated volcanics that fill the basin and are cut by northeast-trending Pliocene and Quaternary(?) faults. The basin is one of several north-trending, en-echelon structural depressions of late Cenozoic age that form the Rio Grande rift (fig. 1). Downfaulted "basin-fill" sediments of the Santa Fe Group and related beds are exposed throughout most of the basin. Along the western margin of the basin, these beds overlie an unconformity of locally high relief developed on Precam-brian crystalline rocks in the Ojo Caliente area and on Meso-zoic sedimentary rocks farther south and west.

 

The basic structure of the Espanola basin is a broad, gentle, northeast-trending syncline with a mainly downfaulted western margin. Within the basin, numerous north- and northeast-trending normal faults are downthrown toward the axis of the syncline. The axis extends southwest under Black Mesa and can be traced farther south where it is overlapped by the Jemez volcanic field. Typical dips on the basin limbs are 3-12° inward. Outliers of Tertiary "basin fill" rocks west of the present fault margin indicate that the original width of the depositional basin was greater than it is today. 

 

To the north of the Espanola basin, the Tertiary Brazos uplift forms an arcuate horst exposing Precambrian igneous and metamorphic rocks raised vertically along northeast- and northwest-trending faults. The Precambrian rocks are covered locally by mid- to upper Tertiary volcaniclastic rocks including the Oligocene to Pliocene Los Pihos Formation. The southern end of the range plunges gently southward beneath upper Tertiary sedimentary rocks of the basin. The Ojo Caliente-Rio Chama area is part of a gently southeast-dipping homocline forming the western limb of the basin where the uplift and the basin merge.