Paleosols below the Ames Marine Unit (Upper Pennsylvanian, Conemaugh Group) in the Appalachian Basin, U.S.A.; variability on an ancient depositional landscape

A Vertisol-like paleosol complex, ranging from 3 to > 10 m thick, is developed below the Ames marine interval (Conemaugh Group, Upper Pennsylvanian) in the Appalachian Basin (eastern Ohio, western Pennsylvania, West Virginia, and adjacent parts of Maryland and Kentucky). Sub-Ames mudstones contain the following pedogenic features: very large slickensides, microsparitic calcite nodules, nodules or coatings of radial calcite spar, preserved soil microstructure, soil-compatible birefringence fabrics, and prominent mottling (commonly restricted to the lower part of the paleosolum). Soil formation comprised: (1) long-term pedogenesis in alluvial (and probably lacustrine) sediments, and (2) rapid development of Histosols (Harlem Coal) as much as 8 m thick encouraged by the approaching Ames transgression. Pedogenic carbonates in the sub-Ames interval at three localities yielded delta ¹⁸ O values ranging from -6.18 per thousand to -1.21 per thousand PDB (average = - 4.02 per thousand PDB; s.d. = 1.42 per thousand ) and delta ¹³ C values ranging -9.17 per thousand to -6.72 per thousand PDB (average = -8.12 per thousand PDB; s.d. = 0.59 per thousand ). A binary plot of these values suggests mixing (probably seasonal) of evaporative and meteoric effects on isotope partitioning. The stratigraphy of the Ames-Harlem Coal interval, the regional distribution and thickness of the Harlem Coal, and features of the sub-Ames paleosol show that the pre-Ames landscape had significant local relief (in the form of shallow paleovalleys with broad interfluves) along the western to northern margin of the Appalachian Basin (Ohio, southwestern Pennsylvania). The stratigraphic relationships of sub-Ames paleosols, the Harlem Coal-Ames marine unit interval, and the Ames marine unit itself are compatible with a significant effect of eustatic sea-level rise in this area. Greater regional tectonic subsidence was probably the strongest control on sub-Ames sedimentation and pedogenesis along the eastern to southern margin (south-central Pennsylvania, West Virginia, northeasternmost Kentucky), where there appears to have been very little relief. The morphology and stable-isotope geochemistry of sub-Ames paleosols are compatible with seasonally wet-dry climates, probably with moderate ( nearly equal 500-1000 mm) annual rainfall.