New Mexico Geological Society Annual Spring Meeting — Abstracts

New LA-ICP-MS U-Pb ages on zircon crystals from airfall tuff beds within the Skunk Ranch Formation, a Laramide synorogenic unit exposed in the Little Hatchet Mountains, southwestern New Mexico, indicate a chronostratigraphic age of 71-70 Ma. Ranging from 374 m thick in its southern exposures to 738 m in northern exposures near Playas Peak, the Skunk Ranch Formation is divided into three informal members including a lower conglomerate member, a middle member of lacustrine shale and basaltic-andesite flows, and an upper member of conglomerate and sandstone. Near Playas Peak, the Skunk Ranch Formation overlies a thick section of the Campanian Ringbone Formation, but south of the Mojado thrust fault, the lower member unconformably overlies Albian carbonate strata. The Mojado thrust cuts the lower member and terminates in a fault-tip anticline in the middle member, which thus provides a minimum age on fault movement. Three tuff beds from the middle member were dated and yielded stratigraphically consistent ages. The stratigraphically lowest sample yielded a weighted mean age of 71.4 ± 0.5 Ma (n=81; MSWD=0.89). The middle tuff bed has a weighted age of 70.6 ± 0.7 Ma (n=32; MSWD=0.39). The stratigraphically highest tuff bed yielded a weighted mean age of 70.4 ± 0.5 Ma (n=60; MSWD=1.04). The new ages indicate that the Skunk Ranch is Campanian-Maastrichtian in age, rather than Eocene as previously interpreted on the basis of ostracodes and that it is correlative with the Hidalgo Formation, a thick section of andesitic flows and flow breccias. The Skunk Ranch Formation was formerly interpreted to record the second phase of a two-stage history of Laramide deformation in southwestern New Mexico. In addition to significantly improving stratigraphic correlation of Laramide strata, the new radiometric ages indicate that Laramide deformation in the Little Hatchet Mountains likely was restricted to the latest Cretaceous.