Paleosols in clastic sedimentary rocks: their geologic applications

58( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70Fig. 10. Paleosol–landscape associations in alluvial rocks at both the local and basinal scales. Pedofacies developed over a distance of -2km, with weakly developed paleosols Ž Entisols or Inceptisols. grading into better-developed paleosols Ž Alfisols and Vertisols .. Over adistance of ;75 km, paleosols became even more strongly developed Ž Stage 4.in response to basinal changes in the rate of aggradation.See text for more details Ž modified from Platt and Keller, 1992 ..lateral changes in parent material produce lateralchanges in the cumulative soils that form on theloess Ž e.g., Smith, 1942; Ruhe, 1983 .. For exampleMcDonald and Busacca Ž 1990.found that, with increasingproximity to the sediment source, a singlesoil bifurcated into two well-developed soils. HollidayŽ 1990.also documented lateral variations insoils developed on eolian deposits and attributedthese primarily to variations in the grain size of thesediment, although local thickness variations alsoplayed a role.Catenas and pedofacies are produced by variationsin topography and grain size related to landscapeposition. Modern soil studies show thatcompositional differences are linked to grain sizedifferences and suggest that those compositional differencesalso influence soil development. In fluvialsystems, for example, sands and coarse silts thataccumulate in channel-marginal environmentsŽ levees, splays.tend to be dominated by quartz,feldspar, and lithic fragments Že.g., Schumacher etal., 1988 .. In contrast, clay and fine silt, whichtypically accumulate in distal floodbasins, consistprimarily of clay minerals such as smectite, illite,kaolinite, and chlorite. Aslan and Autin Ž 1998.concludedthat, in Mississippi River floodplains, thesedepositionally controlled compositional differenceshad a greater impact on the chemistry of alluvialsoils than did weathering processes. Similarly, withQuaternary loess deposits, compositional changes,including an increase in clay minerals, are foundwith increasing distance from source areas, Že.g.,