SEDIMENTOLOGY OF SAND UNITS WITHIN THE HENRY FORMATION (QUATERNARY) NEAR THE MOUTH OF SANDY CREEK, PUTNAM COUNTY, ILLINOIS

More than 30 m of the Henry Formation is exposed along the west bluff of the Illinois River near the mouth of Sandy Creek in Putnam County, Illinois. This exposure was studied in detail in preparation for a field trip administered by the Education Extension office of the Illinois State Geological Survey. The Henry Formation is interpreted to be outwash that was deposited during the Wisconsin Episode.

The Henry Formation here consists of interbedded (in decreasing abundance) sand, gravel, and silt. A detailed textural and compositional analysis was conducted on four sand units present in the exposure. The lowest two sand units are each about 1 m in thickness and consist of medium-coarse sand, are crudely stratified, and contain pebbly interbeds. They are very heterogeneous in composition, and have wide variety of accessory minerals. The heavy mineral suite is dominated by magnetite, but also includes high concentrations of ilmentite, garnet, brown tourmaline, and zircon. The middle sand unit is about 10 m in thickness, and consists of medium-fine sand. Flaser bedding is pervasive in the lowest 2 m, and climbing ripples are present in the middle and upper parts of the unit. The upper sand unit is medium grained and contains crude planar cross stratification and gravelly interbeds. This unit has the highest percentage of quartz grains, many of which are very well rounded and nearly spherical. It also contains clasts of coal.

The sand units studied here are all glaciofluvial in origin, and formed as melt waters were released down the Illinois River valley. Based on structural and textural evidence, the lower and upper sands were likely associated with larger melt water release events. The preponderance of coal and mature quartz sand grains in the upper sand unit may signal valley incision in the LaSalle area where the St. Peter Sandstone and Colchester Coal are present.