SEQUENCE STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS OF THE MERCER FORMATION (POTTSVILLE GROUP) AND CONTROLS ON THE DISTRIBUTION OF CRITICAL MINERALS IN UNDERCLAYS THROUGH THE NORTH APPALACHIAN BASIN

The Mercer Formation of the Pottsville Group (Pennsylvanian; Atokan) has long been recognized as a regionally important source of coal and clay. More recently, the clay facies underlying certain coal seams have also been recognized as highly enriched in critical minerals, particularly lithium (Li) and rare earth elements. However, these concentrations display considerable stratigraphic and spatial variability that remains poorly explained, and the mechanisms driving Li enrichment in underclays are not yet fully understood. This challenge is compounded by lingering uncertainty about the regional stratigraphic correlation and depositional history of the Pottsville Group.

This study combines published data with new results to develop a high-resolution sequence stratigraphic framework for the Pottsville Group and its regional equivalents. Correlations of limestone, shale, and coal reveal progressive 4th-order transgressions and marine incursions that extend into central Pennsylvania, culminating in a 3rd-order highstand recorded within the overlying Allegheny Group. Within this sequence, the Mercer clay is interpreted as a regionally variable interval formed by pedogenic alteration of argillaceous material during lowstand.

Elemental geochemistry indicates that Li enrichment is most pronounced in paleosol underclays beneath the lowest Mercer coal, particularly within the historic fireclay districts of Clarion and Clearfield counties. These areas correspond to the proximal margin of the first Pennsylvanian marine incursion into central Pennsylvania, which is marked by fossiliferous shale and limestone overlying the Mercer coal. While a complete explanation for these enrichment patterns is still in development, preliminary results suggest they likely reflect the geographically restricted interplay of local environmental conditions, depositional history, and diagenetic processes.