83-8 Weathering-Resistant Aplite Dikes and Climatic Variability Control Non-Steady Hillslope Evolution, Sandia Mountains, New Mexico

Abstract:

In the foothills of the Sandia Mountains, exposures of porphyritic granite result in widespread corestone topography with patchy colluvium, minimal soil development, and irregular slope profiles. There are also prominent curvilinear slope forms associated with thick colluvium and well-developed soils. Along these smooth slopes, the shoulder and summit positions are composed of aplite dikes that are up to 5 m in thickness. The weathering-resistant aplite produces angular clasts (predominantly 10-30 mm in diameter) weathered from summits and transported downslope where they mantle granitic bedrock. The aplite clasts are larger than the granite-derived grus and favor the entrapment and accumulation of dust in pore spaces between clasts. Aplite clast and dust accumulation creates cumulic soils with thick Bk and Bt horizons. The lower B horizons are stable, resulting in accretionary soil development that thickens the overall hillslope deposit. Aplite clasts on the surface and in the shallow subsurface, however, are transported downslope by creep-related processes at a faster rate relative to clasts in Bk and Bt horizons. In the Holocene, ¹⁰Be-derived rates of aplite clast production are low, and there has been net erosion of hillslope soils. Higher rates of aplite clast production occurred in the late Pleistocene, prior to the LGM, at ~35 ka. Temperature depressions of 4-10°C and increased moisture during this time likely favored higher rates of frost shattering. During this period, clast production far exceeded erosion rates, and hillslope soils thickened. Soil morphologic features, ¹⁰Be abundances in aplite clasts, and optically stimulated luminescence dates of eolian material indicate an earlier late-Pleistocene period of aplite accumulation (~65 ka). This period of accumulation was followed by stabilization, incorporation of eolian material at depth, and erosion of surface soil horizons. These aplite slopes highlight how volumetrically minor occurrences of rock with different weathering properties, combined with climatic variability, can profoundly impact the complex geomorphic evolution of hillslope landforms. Colluvial thickness and slope form are not the result of a long-term balance between colluviation and erosion. Instead, these slopes and their forms are the product of non-steady rates of weathering and erosion that are intimately linked to prior environmental conditions, rock type, and eolian accumulation.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-21435

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