SEQUENCES, EXTINCTIONS AND δ13C EXCURSIONS IN THE SILURIAN THORNTON REEF OF ILLINOIS

Silurian reefs of the central U. S. show temporal changes in both depositional patterns and biotic composition, which coincide with similar changes seen in contemporary non-reef strata. In the latter, distinct large-scale sequence stratigraphic units are well defined in Wenlock and Ludlow rocks, comprising similar successions of flooding events bounded by unconformities. The early stage of each flooding event is characterized by a prominent extinction event succeeded by a significant positive δ¹³C excursion. Evidence suggests that these were global events related to glacial-eustatic sea level changes.

Large reefs and environmentally similar carbonate banks of the region appear to have had similar responses to these events, but because of their size, complexity, and limited accessibility, these structures have been more difficult to study. Long-term research at the Thornton Reef in Thornton, Cook County, Illinois, has resolved some of these issues. Depositional, biostratigraphic and δ¹³C data from the surrounding local Llandovery through early Ludlow non-reef strata exhibit a series of related sequence stratigraphic units, extinctions, and δ¹³C excursions typically found in Silurian rocks throughout the region. Similar patterns are found in adjacent reefs and carbonate banks.

As a result, important features of the Thornton Reef can be placed more accurately in a temporal and global context. Reef development at Thornton began at the peak of the Early Wenlock (Ireviken) δ¹³C excursion and ended during the Ludlow. Thornton is not a single reef, but is a composite of at least three successive generations of reefs. Historically, these individual reefs had been attributed to different environmentally-controlled growth stages within a single reef. The boundaries between these different reefs coincide with unconformable sequence boundaries recognized in surrounding non-reef strata while the sequences mark major extinction events, depositional changes, and positive excursions in δ¹³C isotope values. Identifying the δ¹³C signatures in these reef units is difficult because patterns of isotope values appear to be drawn out over significantly greater thicknesses of rock, which probably reflect a more rapid depositional rate of reef carbonates compared to that of surrounding non-reef environments.