New Mexico Geological Society Annual Spring Meeting — Abstracts

The U-Bar Formation of southwestern New Mexioo contains five informal members. In ascending order they are the brown limestone, oyster limestone, limestone-shale, reef, and suprareef members. TheU-Bar with the Mural Limestone of southeastern ,Arizona. The Mural Limestone contains coral-rudist patch reefs analogous to subsurface buildups in Albian rocks of Texas. Recently we found four small (10-15 m high, 10-20 m long) coral-rudist patch reefs at Sierra Rica in southwestern New Mexico. These small reefs are in the limestone-shale member of the U-Bar Formation. About 130 m separate the patch reefs from the overlying reef member. Branching coraIs (Ovalastrea?, Dermosmilia? and Diploasytea?) dominate the lower part of the patch reefs and appear to be in growth position. The upper parts of the patch reefs are dominated by caprinid rudists with minor amounts of Toucasia. The limestone beds above and beneath the patch reef horizon are interbedded with medium-gray shale and consists of bioclastic mudstones, wackestones and packstones that contain trace amounts of echinoderms, gastropods, globigerinids, miliolids and other shell fragments. Bioclasts increase in abundance near the base of the patch reef horizon and in the upper part of the Iimestone-shale member. The overall environment during Aptian-Albian deposition was a marine shelf. The presence of patch reefs and their fossil assemblage indicate a period of shallower water depth (10-20m).