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Sedimentology, Stratigraphy, and Geochronology From
Early(?)–Middle Eocene, Post-Laramide Volcanic and
Volcaniclastic Strata of the Palm Park Formation in South-Central
New Mexico
Ryan H. Creitz1 , Brian A. Hampton, Greg H. Mack and Jeffrey M. Amato
New Mexico State University, Department of Geological Sciences, P.O. Box 30001, Las Cruces, NM, 88003,
[email protected]
1
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.
pp. 22
2017 New Mexico Geological Society Annual Spring Meeting
April 7, 2017, Macey Center, New Mexico Tech campus, Socorro, NM