| Basis for focus area |
The focus area includes underclays, fireclays, tonsteins, Bolivar clays and other clays in stratigraphic units associated with coals in Pennsylvanian cyclothem strata. All states listed here have Pennsylvanian black shales, but the nature of the rocks are different across this large focus area for states listed here. Some parts of the Pennsylvanian are more clay-rich than other parts, thus the prospectivity for critical minerals from this mineral system is variable this mineral system, the focus area encompasses the extent of Pennsylvanian and Springfield paleosols in the Illinois Basin, the Upper Cherokee Group underclays in Iowa, Pennsylvanian underclays in Kentucky and Tennessee, and the mapped extent of Pennsylvanian Pottsville and Allegheny Formation underclays in Maryland, Ohio, Pennsylvania, and West Virginia. Areas of interest are the subcrop and outcrop of the Pennsylvanian. |
| Identified resources |
Historical production of clay in Maryland for refractory purposes. |
| Production |
Unknown. |
| Status |
Past mining for refractory clays. |
| Estimated resources |
Unknown. |
| Geologic maps |
Horton and others (2017), scale 1:500,000; Brezinski (2018, 2019), scale 1:24,000. |
| Geophysical data |
Inadequate coverage; majority of the area is Rank 4 and 5, whereas central Illinois, southwest Indiana, and eastern Ohio are Rank 3. Some rank 3 in southwestern Missouri. |
| Favorable rocks and structures |
Pennsylvanian paleosols, underclays. Key units/formations include: Pottsville and Allegheny (Mercer, Mt. Savage/Clarion, Ellerslie and Bolivar of Maryland, Pennsylvania, West Virginia, and Ohio), Cheltenham clay (Missouri), Olive Hill Clay (Kentucky), and Brazil and Staunton Formation underclays (Indiana), Upper Cherokee Group (Iowa). |
| Deposits |
Unknown. |
| Evidence from mineral occurrences |
None. |
| Geochemical evidence |
Geochemical analyses of underclays in West Virginia suggest ion exchangeable REE sometimes in excess of 300 ppm in clay layers, and several deposits of associated coal-waste have potentially economic REE concentrations (Montross and others, 2017; Tetra Tech, Inc., 2018).Similar clays in Pennsylvania have elevated Li concentrations. Elevated Li contents have been correlated with enriched alumina in Pennsylvania and Kentucky (Patterson and Hosterman, 1962; Tourtelot and Brenner-Tourtelot, 1977; Tourtelot and Meier, 1976). |
| Geophysical evidence |
Unknown. |
| Evidence from other sources |
Unknown. |
| Cover thickness and description |
Variable; polygon shows the extent of subcrop and outcrop. |
| Authors |
William Andrews, David Brezinski, Sarah Brown, Ryan Clark, Kristen Hand, Will Junkin, Gina Lukoczki, Maria Mastalerz, Patrick McLaughlin, Jessica Moore, Steve Shank, Stephanie Tassier-Surine. |
| New data needs |
Geochemical data to determine thickness and extent of underclays. |
| Geologic mapping and modeling needs |
Geologic mapping of specific clay beds. |
| Geophysical survey and modeling needs |
High resolution aeromagnetic and aeroradiometric coverage needed. |
| Digital elevation data needs |
Variable lidar quality; some complete, some planned/funded, some inadequate. |
