The Silurian Salina Group is an extensive evaporite series consisting of interbedded halite, anhydrite, dolomites, and shale that extends from Michigan into Ohio and through New York. It was deposited during restriction of the Michigan and Appalachian basins, combined with the prevailing arid conditions during the Silurian, leading to deposition of what previous workers have deemed as “layer-cake” stratigraphy. Historically, the Salina has been mined for salt and gypsum in the shallow subsurface; it is currently mined for its 99.9% pure rock salt for roads and consumption. Current interests in extensive evaporitic bodies have shifted towards the search for “critical minerals” including Rare Earth Elements (REEs). Even at low concentrations (e.g., 7 ppm of Li), these elements could have commercial value, and current hard-rock mining of REEs is economical for concentrations that are a few hundred ppm. Furthermore, REEs could also be useful indicators of palaeoceanographic conditions and lead to insights in deposition, diagenesis, and halokinesis of salt bodies.

Here, we present bulk and trace geochemical data on core and detailed subsurface mapping of the Silurian Salina salt horizons to determine depositional history and palaeoceanographic conditions across eastern Ohio. Bulk mineralogy was determined with qXRD, while trace elements were measured with a Thermo Niton Xl3t GOLDD+ handheld XRF. Crystal structure was identified using SEM. Well logs, Petra, and ArcGIS were used to map the thickness and structure of the salt horizons across Ohio. Results show that the rock salt horizons are primarily halite, although there is an enrichment compared to the shale horizons in the core in Nd (avg. 404 ppm vs. 92.3 ppm, respectively), Pr (avg. 258 ppm vs. 52.3 ppm), La (avg. 169 ppm vs. 46.1 ppm), Cr (avg. 26.5 ppm vs. 2.64 ppm), Ni (avg. 112 ppm vs. 42.4 ppm), and Ce (avg. 151 ppm vs. 38.5 ppm). However, concentrations vary throughout the salt horizons, with some locations void of REEs and other trace elements. In contrast, the anhydrite layers are further enriched compared to the halite, reaching ~1400 ppm. Core descriptions show a host of shallow water sedimentary features, including mud cracks, burrows, and inclusions of organic matter in halite and anhydrite beds. Crystal structures consist of both chevron halite crystals and mosaic crystals, suggesting that some horizons have been recrystallized, whereas others still have the primary depositional crystal structure. Isopach maps of salt horizons show highly variable thicknesses across the region; notable are the inversion of thicknesses of the lowermost salt horizon (Salina B salt) and the underlying Lockport Dolomite in the north, the inversion of thicknesses of the F1 and F2 salt horizons, and the abrupt thickness changes in Harrison County in the Salina E salt horizon—a highly structured area. These indicate changes in depocenters over time, with bathymetric lows being locally restricted and allowing ocean water to pool and concentrate in these areas, rather than continuous salt being deposited throughout the 1.5 my as previously thought.

Combined with core data, the Salina Group was most likely deposited in a sabkha to salina setting, rather than a deepwater environment, with fluctuations in anoxia and organic productivity within the water column. Subsequent basin flushing events caused dissolution and reprecipitation of several salt horizons, resulting in variable concentrations of REE and other trace elements within the halite horizons.