New Mexico Geological Society Annual Spring Meeting — Abstracts

Twenty-one radiocarbon ages were obtained from charcoal and gastropod samples in alluvial fan deposits shed from the Sacramento Mountains near Alamogordo, New Mexico. The stratigraphic context of these samples and their ages (presented here as conventional radiocarbon years) give temporal and spatial constraints for sedimentation and incision events. Where sampled from the same bed, the charcoal and gastropods returned similar ages (± 200 yrs). Three charcoal samples collected from the older, late Pleistocene unit yielded ages of: 41.32 ± 1.0, 27.01 ± 0.16, and 20.32 ± 0.11 ka. Inset > 3 m into this late Pleistocene unit is a gravel that returned a C-14 age of 10.99 ± 0.06 ka from charcoal in its upper buried soil. A ~50 cm-thick, stage II+ calcic soil horizon overlain by a 10-15 cm-thick, argillic(?) soil horizon lies between strata dated at 20.32 ± 0.11 and 3.96 ± 0.4 ka.

The age constraints of this relatively common soil horizon and the inset gravel indicate widespread incision between 20 and 11 ka. Significant aggradation occurred between 6 and 3 ka, both in the proximal and distal parts of the alluvial fans. These middle Holocene deposits are relatively coarsegrained in the proximal areas of the fans. In the distal areas, the middle Holocene deposits consist of internally massive, clayey-silty sand (mostly very fine- to medium-grained) with sparse, gravelly channel-fills. Weak cumulic soils, marked by gypsum accumulation and ped development, are present in the clayey-silty sand. Widespread erosion and incision of the alluvial fans occurred between approx. 3-1.5 ka, followed by back-filling of arroyos. Over the last century, deep incision has occurred on alluvial fans of large drainages, particularly in finergrained sediment north of Alamogordo.

Having numerous radiocarbon ages reduces uncertainty and allows identification of possible reworking of older charcoal. One case of possible charcoal reworking is found at the mouth of Mule Canyon located ~7 km south of Alamogordo in a 3 m-thick deposit containing charcoal dated at 8.75 ± 0.07 ka. Overlying the aforementioned inset gravel (11 ka), this deposit may possibly correlate to the widespread 3-6 ka aggradation, considering its somewhat anomalous age and the fact that it overlies a soil with illuviated clay and a stage I+ to II calcic (+ gypsum) horizon.