EPISODES OF INCREASED FROST SHATTERING FACILITATE NON-STEADY STATE ACCRETIONARY SOIL DEVELOPMENT ON HILLSLOPES IN THE SANDIA MOUNTAINS, NEW MEXICO

In the Sandia Mountains foothills, differences in the weathering properties of aplite and granite produce vastly different slope deposits and forms. Granite decomposes to erodible grus, producing transient pockets of thin regolith with weak soil development and irregular slope profiles with corestones. Outcrops of aplite produce a thick regolith layer, well-developed soils, and curvilinear slopes. Aplite is less susceptible to chemical weathering than granite and serves as a local source for large, blocky clasts in downslope deposits. Aplite clast competence, clast diameter, and outcrop fracture spacing show that physical weathering is the dominant process generating blocky clasts and that weathering of aplite during downslope transport is minimal. Aplite clast-rich colluvium favors entrapment and accumulation of dust, resulting in cumulic soils with well-developed Bt and Bk horizons that reflect both the antiquity and stability of slope materials.

Currently, physical weathering of aplite is minimal, but previous environmental conditions must have been favorable to this process given the large concentration of coarse aplitic clasts in the soil. Colder temperatures during glacial periods may have enhanced frost shattering of aplite. Current minimum monthly winter temperatures (-0.5 to -3.5°C) are too warm to sustain significant frost action; a minimum temperature depression of ~5°C likely is necessary to generate aplite colluviation by this process. In support, reconstructed temperatures in the Sangre de Cristo Mountains for the last glacial maximum indicate temperature depressions of 3.5-10.0°C (Leonard, 2007). Elsewhere in the Sandias, extensive quartzite exposures on steep upper slopes have produced blocky colluvium up to 5 m thick, where a silt-rich matrix also implies abundant dust trapping. Soils within these deposits contain multiple buried Bk and Bt horizons representing numerous cycles of slope stability and erosion. These features demonstrate that in the Sandia foothills, colluvial thickness and slope form are not primarily the outcome of a balance between colluvial production and downslope transport. Instead, they are the product of variable weathering and transport of materials intimately linked to antecedent environmental conditions, rock type variability, and dust input.