Holocene Pedogenesis in Fluvial Deposits of the Conejos River Valley, Southern Colorado

Abstract

Relatively few geomorphic studies have examined Holocene-aged soils developed in alluvial deposits in the Rocky Mountains. Here, we present a soil morphological investigation from a suite of fluvial terraces in the glaciated portion of the Conejos River Valley, southern Colorado. The surficial geology of 25 km of the glacial valley was mapped in detail. Within three separate sub-reaches (Platoro, Lake Fork and South Fork) a total of thirteen soil pits and exposures were excavated and described on alluvial deposits. Soil samples were analyzed for particle size and extractable iron. Soil horizonation (A/C to A/B/2C), structure (fine sub angular to medium angular blocks), clay content of the B horizon (8.0% to 22.8%) and Feo/Fed (0.39 to 0.80) illustrate trends with relative terrace deposit age in individual sub-reaches. However, only Feo/Fed ratios displayed similar rates of development between all sub-reaches highlighting the usefulness of this metric for determining accurate rates of pedogenesis and relative age for Holocene-aged deposits in sub-alpine environments. Results indicate that clay content and structure developed in alluvial deposits of similar ages vary between sub-reaches. Clay contents were found to be lower in the Platoro sub-reach (e.g. 13.1% at Platoro and 20.0% at South Fork on Qt1 deposits). This variation is attributed to heterogeneity in the nature of the inherited parent material and potential variability in aeolian dust contributions throughout the Holocene.