PATTERNS OF SOIL DEVELOPMENT ON STRATH TERRACES ALONG THE COLORADO FRONT RANGE: SOIL MORPHOLOGY AND COSMOGENIC RADIONUCLIDE DATING

Large strath terraces adjacent to the Colorado Front Range record the local history of fluvial planation and incision, and soil development over time. These terraces are grouped into ~6 alluvial units, and regionally correlated largely by elevation above stream channels and soils geomorphology. We present detailed soil descriptions and preliminary soil analyses on three mapped terraces adjacent to Lefthand Canyon near Boulder, CO. Recent cosmogenic radionuclide (CRN) data indicate that the Rocky Flats and Slocum terraces in this area are ~ 95 ka (Dühnforth et al., 2012) and 91 ka (Foster, unpublished data), much younger than the correlative units to the south which are 1-2 Ma and 240 ka, respectively (Scott, 1960).

Generally, this chronosequence of soils follows an expected progression of soil development where older surfaces have more clay, thicker argillics, and redder hues. The oldest surface (Rocky Flats) has a 95 cm thick argillic horizon (2.5YR to 5YR, 22-38% clay) formed in weakly sorted, stratified sandy and gravelly alluvium. Soils on the middle terrace (Slocum) have a 40 cm thick argillic horizon (7.5YR-10YR, 25-40% clay) formed in Holocene loess over sandy and gravelly alluvium; this is the only of the three surfaces with pedogenic carbonates (stage I-III), suggestive of genesis under a more arid climate regime. The lowest terrace we investigated (Louviers correlative) has not yet been dated using CRN. This site also had loess overlying sandy, gravelly alluvium and the ~1 m thick argillic horizon (7.5YR, 20-38% clay) formed in both deposits. These soils classify as fine-loamy to loamy-skeletal Aridic Argiustolls or Calcidic Argiustolls.

Relative-age studies of soil development in this region indicate these surfaces are much older than recent CRN data indicate. This interpretation affects other regional studies based on soil development rates. Open questions include: what factors may accelerate soil development, what role do transient eolian caps play in CRN analyses and soil development in this area, and are development rates just faster than previously thought? Ultimately we will use soil development and time constraints provided by CRN dating to develop a model of soil evolution for these terraces, thereby shedding light on soil forming processes, paleoclimate, and landscape evolution in this area.