PRELIMINARY GEOLOGY OF THE BULL GAP QUADRANGLE, LINCOLN COUNTY, NEW MEXICO

The Bull Gap Quadrangle is located in the northern Tularosa Basin, south of Carrizozo and north of Oscura, New Mexico. Exposures of upper Paleozoic and Mesozoic strata are present in the center of the map area and large tracts of modern alluvium. Older fan deposits derived from the Sacramento Mountains to the east and the Oscura Mountains to the north and west cover bedrock geology in many areas. The western one third of the quadrangle lies to the north, south, and east of bedrock uplands; surficial clastic sediments in this area are classified as piedmont deposits with relative age designations based on inset relationships. The ~5 ka Carrizozo lava flow trends northeast-southwest across the quadrangle. An unnamed Tertiary(?) age conglomerate is locally incised into underlying Cretaceous strata and its base is an angular unconformity with older strata. Tertiary igneous intrusives occur as both sills and dikes and are presumably related to the Oligocene Sierra Blanca volcanic field to the east. Sills observed in the field area range from aphanitic to porphyritic to phaneritic and are felsic to intermediate in composition. Dikes in the map area are oriented east-west to southeast-northwest and exposures tend to be short in length (less than 0.75 km). Permian strata in the Bull Gap area include the San Andres Formation and the overlying Grayburg Formation of the Artesia Group, which are in turn overlain by the Lower to Middle Triassic Moenkopi Formation. Cretaceous strata include the Dakota Sandstone that forms the distinctive hogback in the center of the map area, the lower tongue of the Mancos Shale, Tres Hermanos Formation, D-Cross Tongue of Mancos Shale, Gallup Sandstone and Crevasse Canyon Formation. The limestone in the lower tongue of the Mancos Formation are the Bridge Creek Limestone Beds. Faults in the Bull Gap quadrangle are primarily oblique-slip faults are oriented northwest-southeast to east-west. Both right lateral and left lateral strike-slip motion is suggested by field relations in this group of faults. A few faults trend north-south to northeast-southwest and appear to be normal faults. These faults are primarily down to the west and are presumably part of the Neogene extensional basin-forming structural fabric.