EOLIAN FINE-GRAINED SEDIMENT IN THE NORTHERN COLORADO PIEDMONT

In the northern Colorado Piedmont, early to late Holocene eolian sediment is an important part of the Quaternary section and has been mapped in detail by the Colorado Geological Survey (CGS). It locally underlies mid- to late Holocene dune-forming eolian sand similar to the dune sand of the Colorado eastern plains, although this contact is rarely observed in exposures. The eolian sediment can be considered loess or sandy loess. It covers many upland areas and can be present over a broad range of elevations, but is not associated with particular landforms. It is widespread and occupies as much as three-quarters of some 7.5’ quadrangles. It has hardly any natural exposures but is well exposed in excavations. The unit also is identified in geotechnical boreholes, water wells, and shallow sample holes reaching the deposit’s B horizon. Eolian sediment may directly overlie bedrock, as well as sand and gravel of the Broadway Alluvium and gravel of the Slocum Alluvium. The material is sandy clayey silt or sandy silty clay, with an average sand content (very fine to fine-grained) of 19 percent, based upon 73 geotechnical samples (silt and clay analyzed together as fines) from the Windsor 7.5’ quadrangle. Observed thickness ranges from 1 to 11 m. The deposit is unstratified but locally has a Bk horizon (common), a Bt horizon (less common), and other zonation (rare). Secondary carbonate (Bk horizon) development ranges from Stage I to Stage II. CGS mapping has not discovered any fossils in the sediment. At one unusual exposure (Frederick 7.5’ quadrangle) the basal part of the deposit incorporates large, detached fragments of underlying Laramie Formation. In quadrangles mapped by the CGS, ages of the loessial eolian sediment are between ~2 ky and ~10 ky, and some of the ages coincide with the Bignell Loess of eastern Colorado, deposited from ~9 ka to ~3 ka. Sediment sources may have been glaciogenic silt (from the Front Range), bedrock, and local alluvium. It is well known that eolian sediment is important to agricultural and construction activities. Much of deposit’s surface is favorable cropland, similar to the loess of the midwestern plains. With respect to construction, eolian sediment may be locally prone to hydrocompaction under load. Because of its potential expansive clay content it may be locally classified as a swelling soil.