New Mexico Geological Society Annual Spring Meeting — Abstracts

A new compilation of recent STATEMAP projects in the Ruidoso area covers the Angus, Fort Stanton, Ruidoso, and Ruidoso Downs quadrangles, an area of New Mexico that has never previously been mapped at a scale of 1:24000. Permian through Eocene sedimentary rocks and intrusive and extrusive rocks of the Eocene-Oligocene Sierra Blanca volcanic complex are exposed in the mapped area, which covers the eastern flank of the Laramide Sierra Blanca basin.

The Permian Yeso Formation is the oldest unit in the mapped area, and is exposed along the Rio Ruidoso. Evaporite dissolution has resulted in chaotic bedding and disharmonic folding within the Yeso, and local collapse of the overlying San Andres Formation. Paleokarst within the Permian San Andres Formation is indicated by terra rossa and large thickness variations of the overlying Grayburg Formation. Triassic units are thin and are locally cut out by an unconformity beneath the Cretaceous Dakota Sandstone. Dikes, sills, stocks, and irregular igneous masses are abundant within the Cretaceous Mancos Shale and Mesa Verde Group sandstones and shales. The Eocene Cub Mountain Formation comprises the Laramide basin fill and occurs in disconnected half-grabens. Syenite of the Bonito stock underlies Monjeau Peak, the highest point in the mapped area, and intrudes andesite breccias of the Sierra Blanca volcanic complex. Mesacapping gravels are abundant in the Fort Stanton quadrangle, and are likely western equivalents of the Ogallala Formation.

The Ruidoso fault zone forms the eastern boundary of the Sierra Blanca basin and trends SW-NE through the mapped area. Maximum throw is at least several thousand feet down-to-thewest, near Moon Mountain. To the SW is a complex array of faults near Mescalero Lake. Towards Fort Stanton, the zone is composed of numerous intersecting faults, most of which are buried beneath gravel-capped mesas, suggesting topographic inversion. Evidence for dextraloblique offset along the northeast end of the fault zone includes sparse shallowly plunging slickenlines, variations in throw along strike of individual faults, and fault-related folding.

The Ruidoso region has experienced serious water supply problems for several years. Improved geologic understanding of this area should prove useful in future groundwater studies.