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.