COORDINATION OF DEPOSITIONAL SEQUENCES AND δ13C EXCURSIONS IN THE SILURIAN STRATA OF NORTHEASTERN ILLINOIS

Extensive litho- and biostratigraphic study of Silurian dolomitic strata in the Chicago region are integrated with ∂¹³C values from a core at Thornton Reef. Previously, four glacio-eustatic depositional sequences were recognized in the Silurian of this area. Sequence boundaries are developed at intensively burrowed unconformable surfaces. Each sequence begins with transgressive argillaceous strata that grade up into clean highstand carbonates. A pentameriid brachiopod epibole characterizes the upper highstand, succeeded by a series of grainstones with localized ooids.

Sequence A (early Rhuddanian?) unconformably overlies Ordovician strata. Sequence B (Aeronian) has a less argillaceous transgressive stage than other sequences. Sequence C (late Telychian-early Wenlock), distinguished by reddish argillaceous sediments and abundant diverse conodonts, is the most notable sequence in Silurian strata of the central U. S., and was previously correlated with the Snipklint Primo episode. In the youngest Sequence D (Homerian-Gorstian?), moderately argillaceous sediments overlie a pronounced unconformity. No younger Silurian rocks occur in this area.

Whole rock ∂¹³C _carbonate values derived from 250 samples from a 140-meter-long continuous drillcore through the entire Silurian section at Thornton Reef show a close relationship with depositional characteristics between and within these sequences. In three sequences, prominent positive carbon isotope excursions begin after initial transgression and reach maximum positive values as deposition of argillaceous sediments peaks and declines. Biostratigraphic evidence indicates that excursions recognized in Sequences C and D correlate with the Ireviken and Mulde excursions, respectively. Correlation of the two early Llandovery excursions is less clear, with few comparative studies available. Sequence A excursion resembles others described in North America, but Sequence B has no close analog. It succeeds a significant rise from markedly negative values, lacks transgressive argillaceous sediments, as well as a strong positive excursion. Overall, this study demonstrates that these types of depositional sequences can be used to predict the distribution of ∂¹³C excursions in the Silurian.