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The Abbott Formation (Atokan) conformably overlies the Caseyville Formation (Morrowan) which unconformably overlies Silurian and Mississippian units in the Rock Island area, Illinois. The two formations have a similar composition of Qm95-100, Ls0-5. More than 70% of quartz and heavy mineral grains (tourmaline and zircon; minor rutile and garnet) are rounded to well rounded. Their source is interpretated as a mature sediment on the craton, whereas equivalent sandstones in southern Illinois have an inferred metamorphic terrane component in the source.
Sediments of the Spoon Formation (Desmoinesian) conformably overlie the Abbott Formation, but their sandstones differ in composition. Spoon sandstones comprise Qm45-60, Qp7-20, F5-10, Lm5-18, Ls1-8, and heavy minerals (tourmaline and zircon; minor sphere, garnet, hornblende). More than 55% of grains are angular to subrounded. The compositional change reflects a new sediment supply to the Rock Island area during the Desmoinesian. Grains from a metamorphic terrane are inferred to be mixed with those from a mature sediment.
Paleocurrent data indicate that a change in paleoslope near Rock Island coincided with the compositional change. The early Pennsylvanian fluvial system was probably locally controlled by the Mississippi River arch and flowed to the north. The arch could then have been submerged during the Desmoinesian, and flow was toward the southwest down cratonic paleoslope (which had influenced regional flow since the Morrowan.
Paleocurrent and petrologic data indicate that lower Pennsylvanian basin fill in western Illinois was derived from the northeast rather than the northwest (Transcontinental arch) as previously suggested.
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