New Borehole at ISGS Reveals 300 Million Years of Earth History
How do geologists find out what sediments occur below land surface when there isn't any surface exposure or excavation deep enough to reveal the sequence of geologic materials? When few boreholes are available for a site or when the records of the sediments they encounter are incomplete, the only way to obtain high-quality, site-specific information is to drill the borehole yourself. And that is just what the Illinois State Geological Survey (ISGS) did, right in its own backyard. This newest borehole was drilled behind the Natural Resources Building (NRB), the Survey's home on the University of Illinois' Urbana-Champaign campus. A main reason for the drilling was to demonstrate for the public how samples of sediment are retrieved using a drill rig. The drilling and demonstration occurred before, during, and after the Institute of Natural Resource Sustainability (INRS) Naturally Illinois Expo, held March 13–14, 2009.
Although initially conceived as a demonstration of drilling equipment and technique that would penetrate about 50 to 100 feet, the effort soon evolved into a serious "push-to-bedrock, and-then-some" project to acquire a sediment record spanning the last 300 million years of earth history at this location. Only a few high-quality boreholes have been drilled to bedrock in Champaign County—fewer still within the city limits. A borehole previously drilled by the ISGS just south of campus revealed a bedrock sequence that included two seams of coal that had not previously been mapped in this part of the county, even though they are similar to seams mined in surrounding counties.
The drill crew and geologists drilled for six days total and terminated the borehole at a depth of 372 feet, having reached the target layer of bedrock. Most of the upper 304 feet of sediment is a compact mixture of gravel, sand, silt, and clay and was deposited during the last 700,000 years of earth history during three distinct glacial episodes when huge continental glaciers pushed southward out of the basins of the upper Great Lakes, notably the Lake Michigan basin. Other sediment layers of silt and very fine sand suggest periods of time when lakes, fed by water from melting glacial ice, covered this location. This sediment record is extensive and appears to contain a nearly complete record of the glacial history of this site.
Additional study by geologists will extract more detail from this part of the core. The 304 feet of glacial sediment overlies, in direct contact, bedrock that is about 300 million years old, which means that about 300 million years of earth history are not preserved as a sediment record in this area. What happened to this sediment? Erosion is the most likely culprit. The processes of deposition and erosion are constantly at work on the surface of the Earth with one or the other dominating during various periods of history.
The sediment core extracted from this borehole records a history spanning millions of years beginning about 300 million years ago. Drilling into 60+ feet of Pennsylvanian age bedrock at the bottom of the INRS Expo core hole has afforded ISGS geologists a teaching and demonstration tool for the public as well as crucial data for regional geologic maps, geology of the Pennsylvanian age, and coal resource identification. The discovery of almost 4 feet of Herrin Coal directly below the ISGS, along with evidence of thick Herrin Coal from adjacent oil records will now allow geologists to expand the known resources of that coal seam by possibly hundreds of millions of tons. The new coal information coupled with bedrock surface mapping will allow us to construct a subsurface map showing the approximate extent of the Herrin Coal in Champaign County.
The information from this new borehole will eventually be placed in the archives at the ISGS. The ISGS archives contain records for more than 500,000 boreholes that have been drilled throughout Illinois during the last century. The boreholes were drilled for a variety of reasons: exploration for oil, gas, and coal; installing residential and municipal water wells; engineering purposes related to highway, bridge, and commercial construction; and rock and aggregate resource extraction such as building stone and sand and gravel. Geologists can study the drilling records from many boreholes in an area to determine the general vertical sequence of geologic materials.