Coordination of depositional sequences and Delta (super 13) C excursions in the Silurian strata of northeastern Illinois; Geological Society of America, 2010 annual meeting

Extensive litho- and biostratigraphic study of Silurian dolomitic strata in the Chicago region are integrated with ∂> 13) C 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 ∂> 13) C 13) C excursions in the Silurian.