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