Chronosequence development and soil variability from a variety of sub-alpine, post-glacial landforms and deposits in the southeastern San Juan Mountains of Colorado

Abstract

Surficial processes acting on post-glacial alpine and sub-alpine landscapes vary at small temporal and spatial scales and are thus often difficult to conceptualize in the context of large-scale landscape evolution models. Soils developing in this setting can thus provide valuable information about landform genesis, sedimentology and age. Relatively few post-glacial chronosequences have been examined in these settings however, particularly for the variety of landforms and parent materials that exist within alpine and sub-alpine environments. Here, we examine a chronosequence of relatively young, post-glacial landforms with varying parent materials and climate histories. We dug and described 39 soil pits in the upper Conejos River Valley of Colorado on a variety of deposits and landforms, including alluvial fans, terraces, colluvium, glacial till, and terminal moraines, and compared soil properties with radiocarbon ages from the area. Our results suggest that some typical chronosequence soil properties (e.g., pH, structure, color) do not correlate with time over short time scales. However, extractable iron ratios (Feo/Fed) show a relatively strong correlation with age across late-Pleistocene and Holocene time scales and maximum profile clay content shows a weak but statistically significant relationship with age. Both of these trends are stronger when examined across a single parent material. Differences in initial parent material texture and dust inputs seem to be the most significant complicating factors over post-glacial time scales. Soil property development through time is most inconsistent in cumulic alluvial fan soils. This observation may indicate that alluvial fans are more responsive to sub-basin scale processes as opposed to fluvial terraces that are more likely respond to processes active across the entire basin. These differences would explain why stratigraphically similar alluvial fans are mantled by soils with varying development. Nonetheless, horizonation, clay content, and extractable iron ratios provide a useful tool for correlating young deposits, assigning ages, and interpreting the geomorphic history of complex post-glacial environments.