New sites of 3.1-Ma pumice beds in axial fluvial strata of the Camp Rice and Palomas formations, southern Rio Grande rift

Newly discovered pumice beds in axial-fluvial strata of the Pliocene–lower Pleistocene Camp Rice and Palomas Formations in the southern Rio Grande rift are geochemically correlated to a previously dated 3.1-Ma pumice bed at Hatch Siphon. The Lucero pumice in the Dona Ana Mountains is 1–1.5 m thick and consists of granule- and pebble-sized pumice intercalated with fluvial sand, whereas the Mud Springs pumice along the southeastern flank of the Mud Springs Mountains is 10 cm thick and is composed of sand-sized pumice. Samples from all three sites consist of vesicular, rhyolitic glass fragments and are compositionally identical, particularly with respect to Fe, Ca, and Mn, suggesting derivation from the same 3.1-Ma volcanic eruption. The composition and age of this erupted material is consistent with derivation from the Grants Ridge area, south of Mt. Taylor, implying transport to the ancestral Rio Grande via the Rio Puerco drainage system. If the correlation is correct, the Lucero pumice, along with a bed of 1.6-Ma pumice and the constructional top of the Camp Rice Formation (~0.8 Ma), provide chronologic constraints on the rate of onlap of the northwestern Dona Ana Mountains by axial-fluvial sediment of the Camp Rice Formation. From 3.1 to 1.6 Ma, the sediment accumulation rate was 46.7 m/ Ma, and the lateral rate of eastward onlap was 2 km/Ma. The corresponding values diminished to 18.8 m/Ma and 0.89 km/Ma, respectively, from 1.6 to 0.8 Ma, perhaps due to activity on the Jornada fault, which borders the northern Dona Ana Mountains. If the correlation between the Mud Springs pumice and 3.1-Ma Hatch Siphon pumice is correct, then the Mud Springs pumice provides a reliable chronologic marker within the Palomas Formation that can be compared to existing biostratigraphic data from the same region. The 3.1-Ma Mud Springs pumice is located within the stratigraphic range of the vertebrate collection of Lucas and Oakes (1986) and is consistent with their interpretation of the fauna as medial Blancan in age (~3 Ma). In contrast, the vertebrate collection of Repenning and May (1986), which has been interpreted as very early Blancan II in age (~4.5 Ma), seems anomalously old, given the fact that it is only ~20 m beneath the Mud Springs pumice.