In the lower Paleozoic of the Illinois Basin, three illite polytypes are found: 2M1 of detrital origin, and 1Md and 1M of diagenetic origin. Illite polytype quantification of detrital 2M1 illite and diagenetic 1Md and 1M illite, combined with K-Ar age dating, allows extrapolation to apparent detrital and diagenetic illite ages. Kinetic modeling of smectite illitization, combined with the calculated age of illitization, can evaluate different origins of illite. The diagenetic illite in the lower Paleozoic of the Illinois Basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events.

The Upper Ordovician Maquoketa Group contains diagenetic illite (dominantly 1Md with minor 1M) with an extrapolated age of ∼360 m.y. (356–377 m.y.) and formed from smectite at temperatures of 50–100 °C. This age falls within the span of dates for illite/smectite (I/S) in K-bentonites from the Upper Mississippi Valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a K-rich brine from the Michigan Basin, Upper Mississippi Valley area, or Forest City Basin.

In Ordovician and Cambrian shale partings and sandstone older than the Maquoketa Group, the diagenetic illite (1Md in shale and 1M in sandstone) has an age of ∼300 m.y. and formed at temperatures <140 °C. This late Paleozoic age falls within the range of illites from sandstone in the Upper Mississippi Valley and K-bentonites of the Appalachian Basin; it coincides with the Alleghany orogeny and is interpreted as having formed by gravity-driven flow from the uplifted Alleghanian-Ouachita orogenic belt that drove hot (<140 °C) fluids through the Illinois Basin.

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