Paleopedology of a residual clay associated with the Mississippian-Pennsylvanian (Mid-Carboniferous) unconformity, southwestern Pennsylvania

The Mississippian-Pennsylvanian boundary near Johnstown, Pennsylvania is marked by an iron-rich mudstone ("boundary mudstone") that grades downward into a quartzose sandstone ("subjacent sandstone") in the Mauch Chunk Formation (Mississippian). Field, micromorphological, geochemical, and mineralogical characteristics indicate that these strata constitute a well-developed paleosol (herein referred to as the "Route 56 paleosol") with a complex pedogenic history. Specifically, the boundary mudstone comprises ferruginous glaebules that bear evidence of both ferralitic/oxilic weathering (abundant kaolinite, ferric iron, and runiquartz) and gleization (sphaerosiderite). Such mineralogical and micromorphological disparities suggest that the Route 56 paleosol has a polygenetic origin involving contrasting soil-forming regimes. Although the Route 56 paleosol is lithologically similar to a gleyed underclay, micromorphological and geochemical analyses suggest that it formed under well-drained conditions on an interfluve of the Mississippian-Pennsylvanian unconformity. Basic paleosol characteristics are consistent with pedogenesis under a tropical rainy climate, as existed during early Pennsylvanian time. This study demonstrates how paleopedology, particularly micromorphology, can be used to provide sedimentological evidence for an otherwise cryptic unconformity surface.