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Paleopedologic and Paleohydrologic Records of Precipitation Seasonality from Early Pennsylvanian "Underclay" Paleosols, U.S.A.

¹ Department of Geology, Baylor University, Waco, Texas 76798-7354, U.S.A.; Steven_Driese{at}baylor.edu
² 938 Norwich Avenue, Pittsburgh, Pennsylvania 15226, U.S.A.

Underclays in the Crooked Fork Group (Lower Pennsylvanian, Atokan, Langsettian) in eastern Tennessee, U.S.A., are paleosols associated with fourth-order depositional sequences that record changing paleohydrologic conditions during their formation. Climate changes (possibly Milankovitch-driven) resulted in evolution of soil landscapes from well-drained, seasonally wet floodplains and delta plains dominated by vertic (Vertisol-like) paleosols, to very poorly drained, everwet swamps dominated by sideritic gley paleosols. Coals (paleoHistosols) cap most underclays and record peat-swamp development during everwet conditions, followed by marine submergence. Pedogenic slickensides and angular blocky ped structures, in conjunction with illuviated clay pore fillings and sepic-plasmic microfabric, indicate an initial better-drained phase of paleosol development (Stage I). Gley overprinting, characterized by drab, low-chroma paleosol colors (Fe reduction) in upper portions of paleosols, sideritic rhizocretions, sphaerosiderite and pyrite nodules, extensive leaching and translocation of alkali and alkaline earth elements, and kaolinitization of smectites and hydroxy-interlayer vermiculite (HIV), indicate a later poorly drained stage of paleosol development characterized by saturated conditions, moderate to low Eh, and low pH (Stage II), as well as peat-swamp development. Lastly, burial and juxtaposition of marine facies on top of terrestrial deposits terminated paleosol and peat development (Stage III). Paleosols likely formed under conditions of marked seasonality of rainfall, judging from both modern analog systems in Sumatra and Pennsylvanian paleoclimate models. The chemical index of alteration without potash (CIA-K) was used as a proxy for Atokan paleoprecipitation, estimated as 1300 mm/yr. This estimate is comparable with mean annual precipitation (MAP) in modern tropical to subtropical rainy environments.