Origin and controls on the occurrence of saline springs of the Illinois Basin

Saline springs within intracratonic basins of the Midwestern U.S. are common phenomena that are rarely studied on a basin-wide scale. Saline springs, seeps and licks within and along the margins of the Illinois Basin were identified using historic records and published literature, and many are of historic significance. All occurrences were investigated, and spring water samples were collected and analyzed for field parameters, cations and anions, and stable isotopes. The chemical compositions of the saline springs are dominated by Na and Cl with combined concentrations as high as 60,000 mg/L. While most of the springs were saline, many, often described as "mineral springs", were found to contain background levels of Na and Cl. Some of the springs (both saline and fresh) are enriched in iron and H (sub 2) S and are home to white filamentous, sulfur-loving bacteria. Chloride/Br and K/Cl ratios were used in the identification of the geologic age of the source formation(s) of the springs. On a basin-wide scale, saline groundwater discharges were found to occur along geologic structures and basin margins of the Illinois Basin, especially where major geologic structures intersect large streams and major aquifers. Saline springs and licks were often found in clusters that extend over many square kilometers, and referred to herein as saline anomalies. The occurrence of saline springs and anomalies are a reflection of the geology, geologic structure, groundwater composition and flow regime of basin brines within the Illinois Basin. For example, saline groundwater discharging within the Starved Rock State Park area (northern Illinois) has created saline springs, salt marshes and sporadically-occurring saline water wells that are all part of a structurally-controlled saline anomaly that covers over 4000 km (super 2) . The Cl/Br and K/Cl ratios of that anomaly suggests that the saline groundwater is a mixture of Cambrian and Ordovician brines. Within the center of the Illinois Basin, the source of salinity of springs and seeps are typically from Pennsylvanian strata subcrop beneath glacial sediments. In addition, Panno et al. (2017) recently found evidence suggesting that saline springs and salt licks that occur near the southeastern boundary of the Illinois Basin in Kentucky constitute an area of basin brine discharge from Cambrian and Ordovician strata.