SEQUENCE, BIO-, AND CHEMO- STRATIGRAPHY OF SILURIAN ROCKS IN WEST-CENTRAL ILLINOIS

Silurian rocks are exposed extensively in bluffs along the Illinois and Mississippi Rivers in west-central Illinois (Jersey, Calhoun, Greene, Pike Counties). Although first described in the 1850s, they are poorly known compared with Silurian elsewhere in Illinois. Recent sequence stratigraphic study of new subsurface samples, plus biostratigraphic and chemostratigraphic data, has clarified the nature of these rocks.

The Silurian is divided into three unconformity-bound depositional sequences. The oldest, Bowling Green Dolomite unconformably overlies Ordovician Noix Oolite or Maquoketa Shale. Historically, the Bowling Green had been assigned to the Silurian. Although chitinozoans at its top are late Ordovician to early Silurian in age, δ¹³C data from the Bowling Green do not show the Hirnantian excursion, suggesting a Silurian age.

 The second sequence, mostly Kankakee Dolomite, overlies the Bowling Green. The conspicuous high relief contact between these units has been described as a dolomitization front, but our evidence shows an unconformity. The age of the Kankakee (Aeronian) is defined by brachiopods, with basal beds containing Platymarella and upper beds, Pentamerus oblongus and Stricklandia protriplesiana. Elsewhere, Platymarella are confined to an older sequence and are not part of the Kankakee; a series of glauconitic surfaces may indicate a subtle depositional break above its occurrence here. The prominent late Aeronian unconformity at the top of the Kankakee marks a major depositional change during the Silurian.

 The third sequence includes all younger Silurian strata (late Telychian-Sheinwoodian). Well dated by conodonts, the basal meter of this sequence is the Brandon Bridge Member of the Joliet Dolomite. The overlying Romeo Member of the Joliet contains δ¹³C data that depict the early Sheinwoodian excursion. Around Grafton, the Romeo is overlain by a thick section of Sugar Run Dolomite building-stone beds, characterized by the trilobite Gravicalymene celebra. No carbon isotopic evidence for the younger Homerian excursion has been found, although a few fossils from older collections suggest it might occur locally. The Silurian is truncated and overlain by an unusually thin sequence of Middle Devonian rocks; these sediments also fill karst features through most of the Silurian section.