New Mexico Geological Society Annual Spring Meeting — Abstracts

In the Caballo Mountains, we studied several sections of the Atokan Red House Formation, which is the basal unit of the Pennsylvanian section locally. At these sections, the Red House Formation thickness ranges from 29 to 49 m, much thinner than reported in the literature. The Red House overlies Mississippian, Silurian, and Devonian strata. Its upper contact is at the base of the first thick, very cherty limestone unit of the overlying Gray Mesa (= “Nakaye”) Formation. The Red House Formation is composed of alternating sandstone, shale and covered shale units, and various types of limestone. A distinct facies change is recognized from the northernmost section at Apache Canyon to the southernmost section at Red House Mountain. Sandstone is most abundant in the northernmost section, decreasing towards the south, comprising only 1.7% at Nakaye Mountain and absent at Red House Mountain. Grain size also decreases from north to south, with conglomerates and pebbly sandstones present at Apache Canyon, where two sandstone horizons are present: one at the base and one in the middle of the formation overlying a thin shale-limestone unit. The basal sandstone unit is absent in all sections farther south. The upper sandstone unit is a very distinct horizon that can be traced south to Nakaye Mountain. The thickness and grain size of this sandstone decrease towards the south, and the sandstone pinches out somewhere between Nakaye Mountain and Red House Mountain. As sandstone units are decreasing in thickness, limestone units increase in thickness and abundance towards the south. In the northern part (Apache Canyon-Garfield Crest), individual limestone units are up to 2.6 m thick and limestone comprises 17-36%, whereas in the southern part (Nakaye Mountain and Red House Mountain) individual limestone units are thicker and more abundant, comprising 41-50% of the total thickness. The sandstone horizon in the middle of the Red House Formation displays trough crossbedding, contains plant remains and is interpreted as fluvial, thus marking a distinctive regressive event, probably caused by a tectonic pulse. The facies change within the Red House Formation is a continuation of the interfingering of the Red House Formation with the Sandia Formation farther north--the coarse siliciclastic “Sandia facies” thins southward within the Red House Formation.