SEQUENCE STRATIGRAPHIC FRAMEWORK OF THE SILURIAN STRATA IN NORTHWESTERN ILLINOIS

Silurian rocks in northwestern Illinois (NW IL) and contiguous areas of Wisconsin and Iowa are placed in a sequence stratigraphic framework that was established previously in the adjacent Lake Michigan region. Long the source of controversy, these rocks can now be correlated more accurately with the better-documented Silurian sections in northeastern Illinois (NE IL) and southeastern Wisconsin (SE WI). Using depositional lithostratigraphic and temporal biostratigraphic criteria, four glacioeustatically-controlled, large-scale, transgressive-regressive sequence stratigraphic units can be identified: Sequence 1) Mosalem-Tete des Morts (Hirnantian?-early Llandovery), which is equivalent to the Wilhelmi-Drummond sequence of NE IL; Sequence 2) Blanding-Marcus (mid-Llandovery), which is equivalent to the Offerman-Plaines sequence of NE IL; Sequence 3) Johns Creek-Racine (late Llandovery-early Wenlock), which is equivalent to the Brandon Bridge–lower Racine sequence of NE IL and SE WI; and Sequence 4) the Anamosa (late Wenlock-early Ludlow?), a highly truncated interval that is equivalent to much of the upper Racine of NE IL and the Waubakee of SE WI.

In general, depositional characteristics of Silurian rocks in these two areas (NW IL, E IA, SW WI) (NE IL, SE WI) are similar. Important differences are: Sequence 1) The Elwood of NE IL, with its conspicuous Platymerella brachiopod beds, is absent in NW IL; Sequence 2) The abundance of chert in the Blanding and Hopkinton of NW IL is more similar to time-equivalent rocks in NE WI and the Upper Peninsula of Michigan than to the chert free Kankakee in NE IL. Also, the Marcus increases conspicuously in biotic richness and thickens towards the N-NW. This greatly expanded sequence in NW IL may have been deposited under more normal-marine conditions than strata to the east; Sequence 3) Brandon Bridge strata, which are prominent in NE IL and SE WI, are absent or condensed in NW IL; Sequence 4) Hypersaline sediments in NW IL are present in SE WI but absent in NE IL because of the impact that sea level change had on carbonate bank, shelf and basinal environments. We have identified a prominent ∂C¹³ excursion that follows the beginning of each of these four sequences in NE IL and SE WI. An ongoing study of the NW IL section is predicted to yield the same patterns of ∂C¹³ isotope values.