SOIL GEOMORPHOLOGY OF THE CENTRAL CONEJOS RIVER VALLEY, COLORADO: FLUVIAL RESPONSE TO POST-LAST GLACIAL MAXIMUM (LGM) CLIMATES AND SEDIMENT SUPPLY

Relatively few studies have examined post-LGM alluvial deposits in the Rocky Mountains. Here we use soil geomorphic investigations on a suite of fluvial terraces and alluvial fans in the glaciated portion of the Conejos River Valley, Southern Colorado, to interpret the nature of landscape response to post-LGM climate and sediment supply variability. The surficial geology of thirty kilometers of the glacial valley was mapped in detail. Within three separate reaches (Platoro, Lake Fork and South Fork) a total of twenty-two soil pits and exposures were excavated on various surficial units.

The central Conejos River valley is characterized by a suite of four unique terrace units (Qt0-3), three alluvial fan units (Qaf1-3), a modern floodplain, colluvial slopes, as well as terminal and lateral moraine complexes. Soil profiles consistently vary with age from A/C horizonation on the floodplain to A/Bw/2C on younger surfaces (e.g. Qt3 and Qaf3) to A/Bt/2C on older units (e.g. Qt0-1 and Qaf1). A-horizon properties are similar on all units suggesting a possible influence of late-Holocene aeolian dust. Soil structure in the B-horizon notably increases with age from fine sub-angular blocks (Qt3) to medium angular blocks (Qt0-1). Qt1-3, Qaf1and Qaf3 units have been sampled for 14C dating. Comparison of soil development and stratigraphy for all units, with that of an upstream chronosequence, suggests that Qt3 and Qaf3 were deposited between 1217 and 2065 cal. yr BP. Deposition is thought to be in association with a colder, wetter climate, documented in the San Juan Mountains during this period. Stratigraphic relationships suggest that Qt0 is an outwash terrace closely associated with glacial retreat. Qt1 exhibits soil development that is somewhat similar to a unit in the headwaters thought to be around 15,000 yr BP. Clast size increases with age of terrace unit suggesting a decrease in stream competence over time. Terrace preservation diminishes downstream due to increased lateral migration of the modern channel. Ongoing development of the fluvial chronology and comparison with local paleoclimate reconstructions will help further elucidate the effect of post-LGM climate variability on landscape evolution.