SOURCES OF THE SALT IN THE BIG BONE LICK SPRINGS, NORTHERN KENTUCKY

Big Bone Lick State Park, in northern Kentucky, gets its name from the large number of salt licks and springs in the area and the big animal bones found near them. The springs and bones have been of historical interest since early 1800's. Many European museums owned pieces from the site during that time, as did Thomas Jefferson, giving the area much fame in Europe as a New World fossil wonder. The source of the salt in these springs, however, is still unknown. Using a set of cores collected by the KY Geological Survey and an outcrop section, we studied the mineralogy and geochemistry of the alluvial sediments using X-ray diffraction, coulter counter grain size analysis, X-ray fluorescence, Eltra C-S analyzer, and a core gamma ray logger. The cores and outcrop were found to basically consist of two clay-rich layers, the top brown and the bottom blue-gray. Mineralogy and geochemistry were found to be strongly a function of grain size and not of stratigraphic position. Samples of the various spring and surface waters were also analyzed. The springs' water samples had Na/Cl molar ratios averaging 0.87. This is higher than the 0.62 to 0.71 range reported for Appalachian Basin oil field brines, but similar to values from the Illinois Basin, which average 0.83. Na/Br averaged 167 for the springs, which is not an exact match for any of the oil-field brines. The Illinois Basin brines from Silurian carbonates come closest at 235. Sulfur isotopes of sulfate are often used as tracers of water sources. In the Big Bone Lick springs, dissolved sulfate averaged +38.5 permil, which is quite high and suggests an evaporite source. Only sulfate deposits in rocks of Cambrian to Lower Ordovician age have values this high, suggesting these as the source of the sulfate. The co-existing sulfide, by contrast, is much lighter at -9.25 permil, which is about 50 permil lighter than the sulfate. This spread suggests that the sulfide in the springs is locally generated by bacterial reduction of the brine sulfate as it flows through the alluvium. Thus the source indicators are consistent in pointing to a Lower Paleozoic brine from the Illinois Basin. Sulfur isotopes indicate a Cambro-Ordovician source, whereas Na/Br ratios indicate a Silurian source.