New Mexico Geological Society Annual Spring Meeting — Abstracts

The Big Hatchet Mountains in southwestern New Mexico contain excellent outcrop exposures of the Pennsylvanian to Permian (Morrowan to Wolfcampian) Horquilla Formation. At New Well Peak in the Big Hatchets, the Horquilla section is about 1 km thick and consists of interbedded limestones and marine shales, all of which generally dip ~25º to the southeast. This section is one of the few in the western USA where sedimentation was apparently continuous across the Pennsylvanian-Permian boundary. We sampled each limestone bed in the ~ 180-m-thick part of the Horquilla section that spans the Pennsylvanian-Permian boundary, resulting in 42 sites (4-6 samples/site). To date, half of the samples (about 100 specimens) show well-defined magnetizations in both progressive thermal and alternating field demagnetization. Some 90 percent of the NRM is unblocked by about 420ºC and in AF demagnetization, median destructive fields are about 40-60 mT. Two magnetization vectors are typically isolated in progressive demagnetization, although the second-removed (and presumably older) vector is not yet clearly defined. The in situ grand mean direction of the first-removed vector, which is exclusively of normal polarity, is D = 008.9º, I = 43.3º, α₉₅ = 4.8º and k = 28.5, and a corrected grand mean direction is D = 30.7º, I = 63.0º. A subset of the samples showing the secondremoved vector yields a preliminary in situ grand mean direction of D = 306.6º, I = 32.6º, α₉₅ = 12.8 and k = 11.5 (corrected grand mean direction of D = 291.5º, I = 51.8º). These initial data indicate that these upper Paleozoic carbonate rocks have clearly been remagnetized (probably during the Cenozoic); the direction of the first-removed, well-defined secondary magnetization implies that the Big Hatchet Mountains have experienced a modest clockwise rotation after remagnetization, in the Cenozoic.