Alluvial stratigraphy along the southern Sacramento Mountains, N.M., and inferences regarding Late Quaternary paleoclimate, soils, and sedimentation
— D. J. Koning, F. J. Pazzaglia, and R. Smartt

Geomorphic and sedimentologic studies of proximal alluvial fans along the western Sacramento Mountain front n ear A lamogordo, New Mexico, indicate four distinct stratigraphic units consisting predominately of pebbly to bouldery alluvium. The oldest unit, Qfl, is over 10m thick and extends upward into canyon mouths as a thick fill, Unit Qfl is composed of several stacked allostratigraphic subunits that are typically bounded by buried soils. Qfl is late middle to late Pleistocene in age based on two C-14 dates together with sedimentologic and pedologic similarities with the Jornada Il alluvium and older alluvial units of the Camp Rice piedmont facies of the Desert Project near Las Cruces. Unit Qfoi is a very local, buried inset gravel unit that may indicate general alluvial fan incision during the latest Pleistocene, Qf2 is 2-5 m thick, generally coarser than Qfl, and lacks buried soils. This unit commonly transitions upward from clast-supported, fluvial pebbles and cobbles to debris-flow deposits containing boulders. Three C- 14 dates constrain the age of Qf2 as early Holocene. Unit Qf3 is typically inset into Qfl and Qf2 near the mountain front and is 1-2 m thick. Soil, stratigraphic, and surface characteristics of Qf3 are consistent with a mid to late Holocene age.

Sedimentologic and soil data for these alluvial fan units, and identification of specific gastropod species in colluvium overlying Qfl, allow for generalized interpretations concerning paleoclimate and soil and sedimentation processes. The identified gastropod species in local colluvium imply that pinon-juniper-oak woodlands extended to the foot of the Sacramento Mountains during the latest Pleistocene, probably because of cooler temperatures and higher effective moisture. Abundant debris-flow deposition at 7.5-9.0 ka is interpreted to reflect a climate change from wetter to drier conditions and/or the arrival of intense summer monsoonal precipitation. Drier conditions in the middle and late Holocene were conducive to calcium carbonate and gypsum precipitation in the soil on fan surfaces, which masked Bt or Bw soils horizons and/or possibly inhibited their development.