PRESERVED PEDOGENIC MINERAL MAGNETIC SIGNATURE, PEDOGENESIS, AND PALEOCLIMATE CHANGE - PENNSYLVANIAN ROCA SHALE (VIRGILIAN, ASSELIAN), CENTRAL KANSAS, USA

This study focuses on the field characteristics and mineral magnetism of four stacked paleosols in the Upper Pennsylvanian (Virgilian, Assel ian) Roca Shale exposed near Manhattan, Kansas, USA. The Roca Shale pa leosols (pi at base, P4 at top) include a protosol (Pi), argillic calc isol (P2), calcic argillisol (P3), and calcic vertisol (P4). Soil clim atic changes during the development of each soil profile are suggested by cross-cutting relationships and superposition observations, includ ing: (1) clasts of a lower, vertic part are incorporated in an upper, calcic horizon, and (2) carbonate glaebules have argillans covered by (hematitic) ferrans. Decreasing carbonate content, more strongly devel oped of vertic properties and argillans, and more mature profiles in s uccessive paleosols are consistent with an interpretation of a longer- term soil climate shift from more arid to more seasonal or humid durin g development of the four paleosols. Several observations are consiste nt with a preserved pedogenic mineral magnetic signature. In particula r: (1) each type of soil horizon (B, Bt, Bk, BE, and C), within and am ong paleosols, contains consistent magnetic susceptibility (chi), natu ral remanent magnetization (NRM), anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) signatures, differen t from other horizons; (2) similar redox trends are indicated by magne tic mineralogy and uranium concentrations; (3) observed magnetic chara cter and uranium concentrations are compatible with those expected in gleyed and oxidized intervals, as defined by field observations; (4) p edogenic carbonate glaebules and adjacent pedogenically-altered silici clastic strata preserve the same chi trends; and (5) paleosols contain single domain (SD) magnetite and some maghemite, similar to many mode m soils. Comparison with other Paleozoic vertisols indicates that the Roca Shale paleosols are characterized by more complex pedogenic and m ineral magnetic attributes. This variability in the Roca Shale paleoso ls is probably related to glacial-interglacial climatic variability du ring pedogenesis. The results of this study show that coupled field ob servation and mineral magnetic characterization of other pre-Quaternar y paleosols potentially will yield significant paleoclimatic and pedog enic information.