GLACIAL RECHARGE AND PALEOHYDROLOGIC FLOW SYSTEMS IN THE ILLINOIS BASIN - EVIDENCE FROM CHEMISTRY OF ORDOVICIAN CARBONATE (GALENA) FORMATION WATERS

The Illinois basin provides an opportune setting for elucidating the r oles of remnant evaporite brines and meteoric waters in the evolution of formation waters in an intracratonic sedimentary basin. Formation w aters from carbonate reservoirs in the Upper Ordovician Galena Group h ave been analyzed geochemically to study the origin of their salinity, their chemical and isotopic evolution, and their relationship to pale ohydrologic flow systems. Chloride/bromide ratios and CI/Br-Na/Br rela tions indicate that initial brine salinity resulted from subaerial eva poration of seawater rather than from halite dissolution. Subsequent s ubsurface dilution of the brines by meteoric waters is disclosed by de lta D-delta(18)O covariance; however, the remnant evaporite brine has not been completely expelled from these Ordovician strata. Galena form ation waters have Sr-87/Sr-86 ratios that range from 0.708 17 (a value nearly equal to that of coeval seawater) to 0.710 43. This is the gre atest range of Sr isotopic ratios found in waters from any stratigraph ic unit in the basin. Two fluid mixing events are revealed in plots of Sr-87/Sr-86 vs. 1/Sr: introduction of Sr-87-enriched fluids from a si liciclastic source, probably overlying Maquoketa shale, and a later ev ent that only affected reservoir waters in the western shelf of the ba sin. General covariance between Sr and H-O isotopes suggests that the later event is related to meteoric water recharge. The point of inters ection of the delta D-delta(18)O trend with the meteoric water line im plies that this mixing event involved Pleistocene glacial meltwater th at recharged Galena reservoirs near outcrops along the western margin of the basin. Ordovician Galena formation waters are geochemically dis tinct from those in both Silurian-Devonian and Mississippian-Pennsylva nian strata, a distinction that has evidently been maintained by the o verlying Maquoketa regional aquitard.