Seismic imaging: Geologic Background
Near-surface Quaternary Geology
Quaternary sediments in this area are found both at the top of the river bluffs and at the ground surface, at the base of the bluffs. The bluffs, which stand about 200 ft high, are capped by up to 5 ft of Peoria Silt, a late Wisconsin Episode, wind blown loess (Hansel and Johnson, 1996). Beneath the loess are about 40 ft of sandy diamicton and sand lenses of the Tiskilwa Formation, Wedron Group (Hansel and Johnson, 1996). These glacial sediments were deposited during the Wisconsin Glacial Episode. Beneath the roadway, at the margin of the Illinois River Valley, Quaternary and Recent sediments range from less than 10 to over 30 ft thick. Most of these materials are alluvial and colluvial sediments of the Cahokia Formation. Thicker sediments of the Henry and Equality Formations, which were deposited during the Wisconsin Glacial Episode (Hansel and Johnson, 1996), may be present at the north and south ends of the study area where the roadway leaves the margin of the valley.
Paleozoic Bedrock Geology
Most of the bluff material consists of sedimentary rocks of the Modesto and Carbondale Formations of Pennsylvanian age (Smith and Berggren, 1963). Undercutting of the Lonsdale Limestone Member of the Modesto Formation, which occurs near the top of the bluff, has resulted in large blocks of limestone in the sediments near the base of the bluffs. The Danville (No. 7) Coal Member, the uppermost member of the Carbondale Fm. occurs near the base of the bluff. In places this coal bed is 3.5 ft thick and was the target of many small surface and drift mines. Just above the Danville Coal is the Farmington Shale Member which was mined for brick production at a large facility in the southern part of the study area (White and Lamar, 1960).
The cyclic sedimentary rocks of the Carbondale, Spoon, and Abbot Formations are up to 300 ft thick beneath the study area. Most of these sediments are shale, but thin limestone and variable sandstone layers provide important stratigraphic markers and seismic reflectors. The coal members in this area are, for the most part, thin and poorly developed. Only one subsurface coal mine is known from the study area that is not associated with bluff-side drifts. This mine used the long-wall method to extract the relatively thin (30 inches) Colchester (No. 2) Coal at a depth of about 180 ft beneath the town of Sparland (Smith and Berggren, 1963). One of our seismic lines skirts the edge of this mined area.
When the brick company had exhausted the Farmington Shale pits, it began to mine clay with a room and pillar operation at a depth of about 250 ft. This mine extracted an 8 to 10 ft thick clay layer near the contact between the Spoon and Abbot Fms. Mine maps indicate that this operation extended beneath the study area for about 0.5 miles. Our Borehole #3 was located near now abandoned surface structures from this facility. Drilling in this borehole ceased when a void was encountered at a depth of 251 ft, presumably in the abandoned mine works.
An unconformity separates the Abbot Fm from the distinctive dark and variegated shales of the New Albany Fm (Devonian) (Willman et al., 1975). These shales are 200 to 300 ft thick in this area. The unconformity is significant because thick carbonate rocks of Mississippian age usually intervene between the Abbot and the New Albany. This accounts for the absence of the pronounced seismic reflection which would be expected from the carbonate rocks in our study area.