Am. Stueber et Lm. Walter, GLACIAL RECHARGE AND PALEOHYDROLOGIC FLOW SYSTEMS IN THE ILLINOIS BASIN - EVIDENCE FROM CHEMISTRY OF ORDOVICIAN CARBONATE (GALENA) FORMATION WATERS, Geological Society of America bulletin, 106(11), 1994, pp. 1430-1439
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.