New Mexico Geological Society Annual Spring Meeting — Abstracts

A middle Eocene phase of intermediate volcanism marks the transition from deformation associated with the Laramide orogeny to the late Eocene initiation of the Rio Grande Rift in south-central New Mexico. This tectonic transition is recorded by a suite of volcanic/subvolcanic, volcaniclastic, gypsiferous, and carbonate strata that make up the Palm Park Formation and age-equivalent Orejon Andesite, Cleofas Andesite, and Rubio Peak Formation. A number of studies have focused on constraining the timing and geochemistry of the late Eocene initiation of the Rio Grande Rift in southern New Mexico, yet little is known about the eruptive and depositional history following cessation of Laramide deformation and preceding the onset of rifting. Presented here are new geochronologic, sedimentologic, and stratigraphic data from the Palm Park Formation and Orejon Andesite in south-central New Mexico. The base of the Palm Park is marked by a progressive (erosional) unconformity with underlying basement rocks that range in age from Permian–Paleocene (Abo/Hueco and Love Ranch Formations). These basal strata consist of matrix-supported, pebble-cobble conglomerate units that contain Permian marine fossiliferous limestone clasts suspended in a granule-sandy, volcaniclastic matrix. Conglomeratic units are overlain by nonmarine, fossiliferous limestone and gypsum-bearing strata. This lower stratigraphic interval is interpreted to represent shallow lake sedimentation and episodic moderate- to high-energy lahar debris flows that contained a mix of intermediate volcanic debris and clasts of Permian bedrock. Lake and lahar deposits are overlain by a succession of interbedded volcanic lava flows, pyroclastic flows, and volcaniclastic conglomerate and sandstone, all of which have intermediate compositions. These rocks are interpreted to represent lava flows, basal surge deposits, and high-energy lahar debris flows. Volcanic tuffs are very rare throughout the Palm Park and have only been observed near the base and top of the formation. The youngest stratigraphic intervals in the Palm Park are dominated by volcaniclastic mudstone, sandstone, and granule–pebble conglomerate that range in thickness from 0.01–0.5 meters. These strata are interpreted to represent low-energy lahar hyperconcentrated flows and water-laden sheet flows. Igneous zircons from an ash-fall tuff near the top of the Palm Park yield a U-Pb age of 39.6 ± 0.5 Ma whereas volcanic flows near the base yield a U-Pb age of 44.0 ± 1.5 Ma. Volcanic flows and porphyry deposits in the middle parts of the Palm Park yield ages ranging from 41.6 ± 0.7–41.0 ± 0.6 Ma, respectively (errors 2σ). U-Pb detrital zircon ages from volcaniclastic intervals exhibit primary peaks between 41–44 Ma with secondary peaks between 1600–1800, 1350-1550, and 1000-1250 Ma. Paleozoic–Mesozoic age zircons are present in nearly all samples but do not make up statistically-relevant peaks. Middle Eocene peaks are interpreted to represent detrital contributions from stratovolcano sources. Rare Proterozoic and Paleozoic detrital contributions are interpreted to reflect recycled zircons that were derived from relict, inactive Laramide uplifts and support previous models that call for the termination of Laramide deformation by the middle Eocene in south-central New Mexico.