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

摘要

In the lower Paleozoic of the Illinois Ba-sin, three illite polytypes are found: 2M_1 of detrital origin, and lAid and JM of diage-netic origin. Illite polytype quantification of detrital 2M_1 illite and diagenetic lAid and IM 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 il-lite in the lower Paleozoic of the Illinois Ba-sin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events.The Upper Ordovician Maquoketa Group contains diagenetic illite (dominant-ly lM_d with minor IM) with an extrapolat-ed age of approx 360 m.y. (356-377 m.y.) and formed from smectite at temperatures of 50-100 deg C. This age falls within the span of dates for illite/smectite (115) in K-bentonites from the Upper Mississippi Valley and is interpreted to be a combined result of illi-tization 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 Mississip-pi Valley area, or Forest City Basin.In Ordovician and Cambrian shale part-ings and sandstone older than the Maquo-keta Group, the diagenetic illite (lM_d in shale and JM in sandstone) has an age of approx 300 m.y. and formed at temperatures <140 deg C. This late Paleozoic age falls with-in 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-Guachita orogenic belt that drove hot (<140 deg C) fluids through the Illinois Basin.