Sedimentary Geology

Sedimentary geology focuses on sedimentary rocks (e.g., limestone, shale, sandstone, coal) and the processes by which they were formed. It involves the description, classification, origin, and interpretation of sediments. The observational and analytical procedures used in sedimentary geology include geophysics, stratigraphy, paleontology, petrography, and chemical analyses.

Investigations of the sedimentary rocks of Illinois and adjacent regions provide important insight to the processes that led to the development of the state's abundant mineral resources including oil, gas, coal, groundwater, and industrial minerals and metals. This process-oriented approach allows for the extrapolation of information from well-understood ancient and modern analogues providing a high degree of predictability for subsurface conditions. These methods also provide fundamental information for earthquake hazards, groundwater protection, evaluation of selected construction sites, and disposal of various types of waste materials.

Event Stratigraphy

Event stratigraphy is a concept that has evolved in recent years from the recognition that, in many sedimentary successions, there are distinctive stratigraphic events, preserved either as marker beds (e.g.,i volcanic ash beds) or as discontinuities caused by regional or even global sea-level change. Study of these features can provide significant insight as to how and when sedimentary rocks were formed. Geologists are conducting research in sequence stratigraphy and K-bentonite investigations.

Ordovician K-bentonite Investigations

About 450 million years ago, near what is now the southern Appalachian Mountains, some of the largest known volcanic eruptions spewed many cubic miles of ash into the air. Southeast tradewinds blew the ash across the shallow inland sea that covered the Midcontinent, including Illinois, and carried it as far as what is now South Dakota and upper Michigan.

The ash settled and is now buried between layers of limestone and shale. Using a variety of tests, supplemented by drill cores and logs, individual ash beds (called K-bentonites) have been identified by their mineral and chemical composition. The ash fell within hours over most of the Midcontinent, and this small duration of time makes the ash layers one of the best time markers available to geologists. Like a 450-million-year-old page from a desk calendar, the ash layers are being used to date rock layers lying above and below it, as a distinct marker for matching strata from widely separated locations or to indicate where strata are absent from the geological record. Recently published papers include:

  • Kolata, D.R., W.D. Huff, and S. M. Bergstrom, 1996, Ordovician K-bentonites of eastern North America: Geological Society of America Special Paper 313, 84 p.
  • Kolata, D.R., W.D. Huff, and S.M. Bergstrom, 1998, Nature and regional significance of unconformities associated with the Middle Ordovician Hagan K-bentonite complex in the North American midcontinent: Geological Society of America Bulletins, v. 110, no. 6, p. 723-739.