Origin of illite in the lower Paleozoic of the Illinois basin: Evidence for brine migrations

In the lower Paleozoic of the Illinois Basin, three illite polytypes are fo und: 2M(1) of detrital origin, and 1M(d) and 1M of diagenetic origin. Illit e polytype quantification of detrital 2M1 illite and diagenetic 1M(d) and I M illite, combined with K-Ar age dating, allows extrapolation to apparent d etrital and diagenetic illite ages. Kinetic modeling of smectite illitizati on, combined with the calculated age of illitization, can evaluate differen t origins of illite. The diagenetic illite in the lower Paleozoic of the Il linois 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 1M(d) with minor 1M) with an extrapolated age of similar to 360 m.y. (356- 377 m.y.) and formed from smectite at temperatures of 50-100 degreesC. This age falls within the span of dates for illite/smectite (I/S) in K-bentonit es from the Upper Mississippi Valley and is interpreted to be a combined re sult of illitization by burial diagenesis and either a hydrothermal brine f rom the southern and deeper part of the basin or a K-rich brine from the Mi chigan Basin, Upper Mississippi Valley area, or Forest City Basin. In Ordovician and Cambrian shale partings and sandstone older than the Maqu oketa Group, the diagenetic illite (1M(d) in shale and IM in sandstone) has an age of similar to 300 m.y. and formed at temperatures < 140 degreesC. T his 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 Allegheny orogeny and is interpreted as having formed by gravity-driven flow from the uplifted Alleghanian-Ouachita orogenic belt t hat drove hot (< 140 degreesC) fluids through the Illinois Basin.