APLITE COLLUVIUM AND DUST INFLUX PRODUCE ACCRETIONARY HILLSLOPE SOILS IN THE SANDIA MOUNTAINS, NEW MEXICO

To determine the control of bedrock on hillslope soil formation in the Sandia Mountains of New Mexico, soil catenas were documented on two slopes with similar aspect and relief but associated with different rock types. One slope is underlain entirely by granite; the weak A/C soil profiles are less than 0.5 m in thickness. The other slope is capped by an aplitic dike; soils on this slope progressively increase to thicknesses exceeding 2 m in lower slope positions. Maximum soil development with both Bt and Bk horizons occurs on the lower backslope where illuvial clay exists as both unoriented interstitial plasma and oriented on grain, ped, and root pore surfaces.

Five lines of evidence indicate that the source of much of the fine matrix in the aplite slope soils reflects atmospheric dust additions rather than local bedrock weathering: (1) Aplite clasts exhibit no appreciable chemical weathering in outcrop, along the slope, or even in the well-developed Bt horizons (2) Grusified granite under the aplite slope colluvium shows neither alteration of feldspars to clay or B horizon development. (3) Major increases in conservative elements (i.e., Fe₂O_3, MnO and TiO₂) and major losses of the highly mobile cations CaO and MgO are observed in the soil profile relative to the granite and aplite parent material. (4) Fine quartz sand in the Bt horizons yielded strong response OSL shine-down curves indicative of eolian material. (5) Significant pedogenic calcium carbonate exists in the Bk horizon, despite limited chemical weathering of bedrock.

The contrast in soils between aplite and granite slopes reflects the different weathering properties of aplite and granite. Alteration of biotite in granite facilitates the production of erodible grus and thus slopes are dominated by detachment limited corestones. In contrast, the lack of easily weatherable minerals and fine-grained texture of aplite results in the production of large angular colluvium relative to grus. The aplite colluvium produces a relatively rough, erosionally resistant surface that enhances dust entrapment and b horizon development and therefore allows for the generation of soils that are not solely the product of weatherable bedrock. Soil thickness on these steep, vegetated curvilinear dryland slopes is neither a function of hillslope curvature or soil thickness.