Stratigraphic and structural controls on the occurrence of saline springs within the Illinois Basin, U.S.

Abstract

Saline springs within intracratonic basins are a common, but rarely studied phenomena. Derived primarily from the mixing of basin brines (evaporated seawater) with shallow groundwater and/or freshwater recharge, there are many saline springs within the Illinois Basin. These include free-flowing springs, bedrock seeps into otherwise freshwater aquifers, and salt licks. In this study, a total of 44 saline springs, seeps, licks and wells were sampled. The chemical compositions of the saline springs of the Illinois Basin are dominated by Na and Cl- with a combined concentration as high as 60,000 mg/L. Some of the springs are enriched in H2S supporting populations of white filamentous, sulfide-oxidizing bacteria. Because the spring water contains a component of basin brines, and because basin brines of the Illinois Basin have been well characterized by others, ion mass ratios were used to identify the geologic period of the source formation(s).In general and on a basin-wide scale, saline groundwater discharge occurred along geologic structures and from strata along the margins of the Illinois Basin, originating from every period of the basin’s geologic strata (Cambrian through Pennsylvanian). The greatest number of saline springs occurred where geologic structures intersect large streams or major aquifers. Saline springs were often found in clusters that may extend over many square kilometers. The occurrence of saline springs are a reflection of the geology, geologic structure, geologic history including continental glaciation, surface water-groundwater interactions, groundwater flow regimes, and overall groundwater chemistry within the Illinois Basin. In parts of the Illinois Basin, our data suggest that brines within strata of older geologic periods have migrated upward along geologic structures, infiltrated strata of younger geologic periods, and mixed with, and in some cases, replaced brines within those strata. The source of salinity in springs within the center of the Illinois Basin is typically brines from geologic strata that outcrops and subcrops along structures associated with the LaSalle Anticlinorium. Salt licks that occur near the southeastern boundary of the Illinois Basin in Kentucky constitute points of discharge for regional groundwater flow from the basin. The Cl/Br and K/Cl mass ratios of the salt licks in Kentucky suggest that the saline groundwater is a mixture of brines from Cambrian and Ordovician strata.