SEDIMENTOLOGY, STRATIGRAPHY AND GEOCHRONOLOGY FROM EARLY(?)–MIDDLE EOCENE ROCKS OF THE PALM PARK FORMATION IN SOUTH-CENTRAL NEW MEXICO

The final early(?)–middle Eocene phase of the Laramide orogeny is recorded in southern New Mexico by a suite of volcanic/subvolcanic, volcaniclastic, and carbonate strata that make up the Palm Park Formation and age equivalent 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 just prior to the onset of rifting. Presented here are geochronologic, sedimentologic, and volcanostratigraphic data from the Palm Park Formation near Las Cruces, New Mexico.

The base of the Palm Park is marked by a progressive (erosional) unconformity with basement rocks that range in age from Paleocene–Permian. Basal strata consist of pebble-size volcaniclastic conglomerate with outsized (boulder) limestone clasts. Conglomerate are interbedded with thin units (<5 cm) of fossiliferous limestone. Strata are interpreted to represent shallow lake sedimentation and episodic moderate- to high-energy debris flows. Adjacent to vent facies of the Palm Park, stratigraphy consist primarily of thick individual volcanic deposits (>35 m) of intermediate composition that are interbedded with massive pebble- to boulder-sized volcaniclastic conglomerate. These rocks are interpreted to represent volcanic flows and high-energy debris flows. The upper parts of the Palm Park are dominated by volcaniclastic sandstone and granule- to pebble conglomerate that were deposited in an immature braid-plain fluvial environment.

Zircons from an ash-fall tuff near the top of the Palm Park have a U-Pb age of 40.0 ± 0.49 Ma whereas an andesite flow and subvolcanic vent yield ages of 42.0 ± 0.66 Ma and 41.0 ± 0.65 Ma, respectively (errors 2σ). U-Pb detrital zircon ages from clastic intervals exhibit primary peaks between 41–44 Ma with secondary peaks between 1600–1800 Ma, 1350-1550 Ma, 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 Laramide-age stratovolcanoes. Proterozoic and Paleozoic detrital contributions are interpreted to reflect recycled zircons that were derived from the underlying Lower Permian Abo Formation.