Applying 40ar/39ar Detrital Sanidine Geochronology to Constrain Stratigraphic Ages From proboscidean-Bearing Strata of the Plio-Pleistocene Camp Rice Formation, Southern New Mexico

Strata of the uppermost Santa Fe Group are exposed along the axis of the Rio Grande rift throughout central New Mexico and record sedimentation associated with the Plio-Pleistocene integration of the ancestral Rio Grande fluvial system. In southernmost New Mexico, these strata are referred to as the Camp Rice Formation and preserve a rich record of proboscidean megafauna (elephant relatives; i.e., mastodons, gomphotheres, and mammoths) as well as many other Plio-Pleistocene mammalian fossils. The Camp Rice Formation records sedimentation from ~5.0–0.8 Ma in southernmost New Mexico, however while fossil occurrences are abundant, radiometric age constraints are sparce for the strata. In an attempt to place new age constraint on stratigraphic horizons where known gomphothere fossils occur, N=6 samples were collected throughout Doña Ana County for 40Ar/39Ar detrital sanidine analysis. The primary goal for each sample was to analyze ca. 300 sanidine grains with the aim of using the youngest ages as a maximum depositional age (MDA) for each stratigraphic horizon. The MDAs for each sample fall between ~3–1 Ma and break out into several distinct age groups. The oldest MDAs from this study are 2.779±0.023, 2.752±0.008, and 2.745±0.010 Ma with n=2, 5, and 3 sanidine ages used to determine MDAs, respectively. The youngest three ages from the study are 1.997±0.039, 1.331±0.035, and 1.252±0.003 Ma with n= 12, 2, and 82 used to calculate MDAs for each sample, respectively. We caution that MDA determination does not preclude these samples from being younger than the ages listed above (i.e., younger sanidine grains could occur in each sample). We also note that the transport mechanism for sanidine grains into these strata could include fluvial transport and/or tephra erupted from distal sources. The youngest MDA at 1.252±0.003 Ma likely is sourced from the Upper Bandolier Tuff of the Valles Caldera in north-central New Mexico and is tentatively interpreted to represent the true age of the stratigraphic horizon of the Camp Rice Formation where it was collected. The detrital sanidine grains defining the older MDAs do not overlap with known volcanic sources from the Valles Caldera and may represent distal tephra input from other local or regional late Cenozoic volcanic fields and caldera systems. For instance, the ca. 2 Ma sanidines could be from the Huckleberry Ridge tuff in Yellowstone and the ca. 2.75 grains could be derived from Mount Taylor. The lack of ubiquitous Upper and Lower Bandelier grains in these samples could support the MDA’s as actual depositional ages. The large number of ages supporting the youngest MDA are suggestive of an eruption that could have been coincident with or led to the death of an adult but yet undated Cuvieronius gomphothere recovered from this stratum. Cuvieronius is believed to have been replaced temporally by Stegomastodon and in turn by mammoths, reflecting differences in diet imposed by aridification of the Ancient Rio Grande Valley. Nevertheless, both gomphotheres are reported from Middle Blancan to Irvingtonian NALMA in southern New Mexico.