David W. Love

Architecture of the Sierra Ladrones Formation, central New Mexico: Depositional controls on the permeability correlation structure

Statistical models of hydrogeological heterogeneity are often used in aquifer and reservoir characterization. The number of data required to estimate objectively the spatial correlation structure of permeability, however, is often prohibitive. The objective of this study was to develop a better understanding of how information about depositional processes can be used to characterize hydrogeological heterogeneity. An outcrop of the fluvial/interfluvial Sierra Ladrones Formation of New Mexico was studied for this purpose. On the basis of previous studies of paleogeography and our own field observations, deposits of the Sierra Ladrones Formation are interpreted as marginal ancestral Rio Grande flood-plain and tributary deposits. Architectural elements were mapped over a 0.16-km[sup 2] peninsular outcrop of Pliocene-Pleistocene deposits of the central Albuquerque Basin. Geostatistical analysis of the architectural-element map data indicates non-orthogonal anisotropy in the horizontal direction. The orientations of the strongest (N30[degree]W) and weakest (N90[degree]E) correlation correspond to the orientation of the tributary system and the ancestral Rio Grande flood plain, respectively. In the vertical direction, the correlation structure exhibits exponential behavior corresponding to the average-element thicknesses. The results demonstrate that information about depositional environment can be used to help to quantify statistically subsurface heterogeneity. 28 refs., 9 figs., 1 tab.

Use of radiometric (Cs-137, Pb-210), geomorphic, and stratigraphic techniques to date recent oxbow sediments in the Rio Puerco drainage Grants uranium region, New Mexico

In the absence of historic geochemical baseline data for the Grants uranium region, environmental changes resulting from uranium mine-mill activities can be determined only by indirect methods. A methodology for determining the age of recent sediments in streams draining the region has been established based on combined geomorphic, stratigraphic, and radiometric dating techniques. Because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources, sample sites which contain fine-grained deposits that both predate and postdate mine-mill activity were located in abandoned-channel segments (oxbows) of major streams draining the eastern Grants uranium region. Aerial photographs (and derivative maps) taken between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages of alluvial fills in the abandoned-channel segments. Pits were dug at these oxbow sites to determine stratigraphy and composition of the deposits. Samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by gamma ray spectrometry for the artificial radionuclide Cs-137 and the natural radionuclide Pb-210 as well as other U-238 and Th-232 daughters. Because of the dynamic nature of the system, absolute dating with Cs-137 was not possible but samples could be dated as either pre-or post-1950. The 1950 date is important because it marked the beginning of the uranium exploitation in the region. The Pb-210 dating was not possible because background Pb-210 was very high relative to fallout Pb-210.

Spatial and temporal trends in pre-caldera Jemez Mountains volcanic and fault activity

New 40 Ar/ 39 Ar dates from the Jemez Mountain volcanic field (JMVF) reveal formerly unrecognized shifts in the loci of pre-caldera volcanic centers across the northern Jemez Mountains; these shifts are interpreted to coincide with episodes of Rio Grande rift faulting. Early activity in the field includes two eruptive pulses: 10.8–9.2 Ma basaltic to dacitic volcanism on Lobato Mesa in the northeastern JMVF and 12–9 Ma mafic to silicic volcanism in the southwestern JMVF. While 9–7 Ma eruptions persisted in the southern JMVF, a new eruptive center developed on the La Grulla Plateau in the northwestern JMVF (8.7–7.2 Ma), corresponding with a period of rift widening caused by reactivation of Laramide faults in this area. The older 8.7–7.8 Ma mafic lavas emitted from Encino Point and the younger 7.7–7.2 Ma trachyandesite and dacite erupted on the La Grulla Plateau are assigned to a new unit called the La Grulla Formation. The chemical composition of a 640 m stack of lava flows exposed in the northern margin of the Valles caldera changes from dacite to andesite, then back to dacite upsection, becoming slightly more alkalic upward. The shift to more alkalic compositions occurs across a sedimentary break, marking a subtle change in magma source for the older Paliza Canyon Formation and the younger La Grulla Formation lavas. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous rhyolite deposits in the northern JMVF suggest an episode of localized, 7.6–7.8 Ma rhyolitic volcanism that occurred in the central part of the JMVF between 12–8 Ma Canovas Canyon Rhyolite and 7–6 Ma peak Bearhead Rhyolite volcanism. Younger Bearhead Rhyolite intrusions (7.1–6.5 Ma) are more widespread than previously documented, extending into the northeastern JMVF. Tschicoma Formation dacite erupted at 5 Ma in the Sierra de los Valles and then erupted throughout the northeastern JMVF 5–2 Ma. The more refined geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically and chemically distinct, relatively short-lived, episodes of activity. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10–9 Ma) to west (9–7 Ma) to east (5–2 Ma) pattern that reflects the timing of motion on faults. The new ages, coupled with detailed mapping of both volcanic rocks and the Santa Fe Group, document significant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time across the Canones fault zone in the northern JMVF.

Footprints preserve terminal Pleistocene hunt? Human-sloth interactions in North America

Contemporaneous sloth and human footprints from the terminal Pleistocene at White Sands National Monument suggest stalking. Predator-prey interactions revealed by vertebrate trace fossils are extremely rare. We present footprint evidence from White Sands National Monument in New Mexico for the association of sloth and human trackways. Geologically, the sloth and human trackways were made contemporaneously, and the sloth trackways show evidence of evasion and defensive behavior when associated with human tracks. Behavioral inferences from these trackways indicate prey selection and suggest that humans were harassing, stalking, and/or hunting the now-extinct giant ground sloth in the terminal Pleistocene.