New Mexico Geological Society Annual Spring Meeting — Abstracts

This study compares two sets of alluvial fans along the eastern margin of the Tularosa Basin. Alluvial fans of the first set are gravelly and contain both debris flow and stream flow sediment. Fan-head trenching is confined adjacent to the mountain front. Fans belonging to the second set are derived from large mountain front drainages north of Alamogordo. These alluvial fans lack debris flow sediment, have less gravel and more clay, and exhibit entrenched, main-stem arroyos throughout their length. The longer arroyos perhaps relate to their finer-grained sediment and higher discharges from their respective mountain front canyons.

Both sets of alluvial fans experienced similar erosional and depositional histories in their proximal and medial areas. Erosion was concentrated in arroyos during 18-8 ka, with pedogenesis occurring on surfaces between arroyos. Widespread aggradation occurred between 8 and 3 ka on both sets of fans. On the first set of alluvial fans, stream flow sediment commonly transitions upwards into debris flow sediment, with an inset, coarse-grained deposit found below the debris flows. Stream flow and clayey-sandy sheet flood deposits characterize the second set of alluvial fans between 8 and 3 ka. Coarse-grained channel-fill deposits interfinger with clayeysilty, very fine- to medium-grained sand. Fine-grained sediment is generally bioturbated, internally massive, and has undergone weak pedogenesis. Pebbles in the fine-grained sediment are scattered or in thin to medium lenses. These observations indicate an aggrading fan landscape during the middle Holocene. Erosion occurred between 3 and 2 ka on both sets of alluvial fans. Another episode of aggradation occurred during 0.5-2.0 ka, which on the second fan set is marked by better stratification and slightly coarser texture than earlier Holocene deposits. North of Alamogordo, there was eolian reworking of sediment and localized, periodic arroyo formation and backfilling during 0.5-2.0 ka. These observations indicate that the late Holocene experienced episodes of surface instability, perhaps due, at least in part, to sparser grass and low-lying shrubs. Pronounced erosion has occurred on the proximal and medial parts of both alluvial fan sets during the past 100-150 years, resulting in prevalent gravel lag deposits and gully dissection.