New Mexico Geology — Back-issues

Cover Image: A late Miocene Rio del Oso dike cross-cutting the light colored Miocene Ojo Caliente Sandstone of the Tesuque Formation

A late Miocene Rio del Oso dike cross-cutting the light colored Miocene Ojo Caliente Sandstone of the Tesuque Formation (Santa Fe Group). The Rio del Oso dike swarm is located in the Chili 7.5’ quadrangle, western Española Basin, and crops out as backbone-like dikes cutting across the landscape in stark contrast to the Ojo Caliente sandstone. The western margin of the Rio Grande rift at the latitude of Española, NM, is characterized by a zone >17 km wide of oblique-slip faults. The dikes are tabular, fine grained, and mafic in composition (basalt). The dikes range from 0.5–3.5 m wide, form en echelon segments, and strike generally north-south. The minerals in hand samples are 1 mm- to 2 mm long, often with elongated geometries, and include olivine (vitreous luster and olive-green color), plagioclase (white to gray tabular crystals), and minute augite (stubby prisms) phenocrysts. Many of the dikes have vesicles that are often filled with secondary calcite. At a few locations elongated vesicles and lineations occur on the faces of the dikes. The dike mineralogy in thin section includes major Ca-plagioclase, augite, olivine, and magnetite. Swallowtail morphologies of the plagioclase laths as well as dendritic habit of the Fe-Ti oxide phases indicate undercooling of the host magmas consistent with shallow emplacement and rapid cooling.

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