High-resolution Seismic Survey Helps Define Detailed Subsurface Geology of the Pesotum Bedrock Valley in Central Illinois
The Pesotum Bedrock Valley, mainly in Champaign County, is a tributary of the Teays-Mahomet Bedrock Valley System. The sediments that fill this buried valley and the topography of the underlying bedrock provide a complex record of erosion and deposition associated with repeated glacial and interglacial episodes. However, uncovering what lies beneath the land surface can only be revealed by evaluating logs from boreholes and by conducting high-resolution seismic reflection surveys. Because the majority of the over 550 boreholes did not penetrate the entire valley-fill, and only one of these had a continuous core from land surface to bedrock and only one other was a stratigraphic test hole, over 13 miles of high-resolution seismic reflection data were collected with a landstreamer to supplement the borehole data, and to demonstrate the use of this method to image the complex geology within the buried valley.
The Pesotum Bedrock Valley and its fill were classified into seven units based on the seismic surveys. By correlating these units with logs from boreholes along the seismic lines, a geologic framework was established, there was enhanced delineation of fine-and coarse-grained sediments, and the subsequent ability to visualize the succession of deposits within the valley. In addition, some small-scale features also were identified that delineated a buried paleo-landscape and small channels incised into bedrock.
These units, (shown as A-F in the lower half of the figure at the right,for the valley fill), from top to bottom are:
Wisconsin Episode — deposited ~22,000-23,000 years ago
- A. Continuous diamicton (till) of the Batestown Member, Lemont Formation, and Tiskilwa Formation. Its thickness increases to the north and east reaching a 75 ft maximum.
Illinois Episode — deposited 130,000-155,000 years ago
- B. Flat-lying interlayered diamicton, silt and clay, and sand and gravel of the Glasford Formation. Its thickness ranges from 26-75 ft.
- C. Thin, but continuous diamicton of the Vandalia Member,Glasford Formation and associated glacial lake sediment(silt, sand, and clay). A secondary unit of sand and gravel of the Pearl Formation also was identified.
Pre-Illinois episodes — deposited >425,000 years ago
- D. Clay-rich diamicton of the Hillery Member and a lower layer of bedded to massive silt, fine sand, and clay that are all reddish gray to brown, compact, hard, and have low moisture contents. The diamicton has up to 85% silt and clay.
- E. Silty diamicton of the Harmattan Member and associated glacial lake sediment (bedded silt, fine sand, and clay) that are olive brown to brown, compact, hard, have low moisture contents, and are 10-100 ft. thick.
- F. An unnamed diamicton, with silt, sand, and gravel at the bottom of the valley fill that are all typically leached or weakly calcareous and contain considerable local bedrock. Narrow channels are cut into these materials and the underlying bedrock suggesting a former land surface. One channel is >2600 ft. long and 33 ft. deep. A seismic profile shows
cross-bedded sand and gravel filling the channel.
- The bedrock is the unit most readily detected by the seismic surveys. This surface undulates as much as 26 ft. as its elevation increases towards the south.
This study illustrates the application of utilizing a high-resolution seismic survey for characterizing glacial sediments and reconstructing depositional and erosional histories. Seismic reflection profiles delineated many complexities within the valley-fill that were closely related to those described by evaluating boreholes, therefore making it possible to correlate between boreholes, and beyond boreholes to areas of otherwise sparse data.