GLACIO-EUSTATIC CONTROLS ON EARLY SILURIAN MARINE DEPOSITION IN ILLINOIS: IMPLICATIONS FOR GLOBAL CORRELATION AND BIOTIC EXTINCTION

Early Silurian strata in Illinois and adjacent Wisconsin and Indiana are characterized by conspicuous depositional cycles, which appear to be controlled by glacio-eustatic sea level fluctuations. Beginning at the Ordovician/Silurian boundary, each of the three recognized cycles displays a similar succession of distinctive lithologies, and is bounded by prominent depositional breaks. These cycles, in combination, temporally represent approximately ten million years, although whether the cycles are each of equal duration is unknown.

Each lithologic cycle begins with a flooding event and culminates with emergence, representing a parasequence. More precisely, initial transgression deposited argillaceous carbonate sediments on mineralized hardgrounds, some being exposure surfaces. As transgression continued, increasingly carbonate rich sediments, representing more normal-marine subtidal environments, were deposited. As accommodation space filled and relative sea level likely fell, a pentameriid brachiopod epibole developed, followed by deposition of coarser carbonate sediments, sometimes containing ooids, in shallow marine settings. Eventually, sea level dropped so that nondeposition, and even exposure, ensued.

The widespread distribution of these parasequences throughout the region and their depositional characteristics suggest a relationship to early Silurian glacial events known from the southern hemisphere. Unfortunately, limited precision in dating these glaciations currently precludes exact correlation. However, it is not surprising that the epieric seas in this intracratonic region would have been readily impacted by glacio-eustatic sea level fluctuations. If they are glacio-eustatic, the timing and character of these parasequences may provide a precise means of correlating early Silurian rocks globally.

Each of these parasequences exhibits an important extinction event at or near its base, where a complete change in the composition of specific taxonomic groups has been observed. Carbon isotope excursions may coincide with these extinction events, but do not necessarily correlate directly with transgression.