Paleosols in clastic sedimentary rocks: their geologic applications

54( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70Ž .rivers e.g., Blum, 1994; Blum and Valastro, 1994 ,and paleosol development reflects those effects ratherthan sealevel fluctuations.3.4. Sediment accumulation ratesSediment accumulation rates are generally calculatedby dividing the thickness of a particular stratigraphicsection by its known or estimated time span.Sediment accumulation rates estimated for continentalstratigraphic successions are usually severelytime-averaged because radiometric dates are notavailable or widely spaced in time and because thetemporal resolution of paleomagnetic dating or continentalbiostratigraphy is generally relatively coarseŽ e.g., Kraus and Bown, 1993 .. Because the developmentalhistory of a sedimentary paleosol reflects therate of pedogenesis relative to the total time of soildevelopment and how steady or unsteady that depositionwas Ž Fig. 2 ., the kind of paleosol that developsis a good indicator of sediment accumulation ratesfor thin stratigraphic intervals. The relative degree ofpedogenic development is used as a proxy for therelative rate of sediment accumulation, and, as discussedin a later section, paleosols have been used tocompare and contrast sediment accumulation rates,in a qualitative sense, in different parts of a depositionalbasin Že.g., Atkinson, 1986; Platt and Keller,1992. and through time Že.g., Kraus and Aslan,1993; Kraus, 1997; Soreghan et al., 1997; examplesin Kraus and Aslan, 1999 ..In addition to this qualitative approach, severalworkers have attempted to use paleosols to quantitativelyestimate short-term sediment accumulationrates, using somewhat different approaches. In aseminal paper on this topic, Leeder Ž 1975.compiledages of Quaternary calcretes to establish ages offormation for particular stages of calcrete development.The ages were then incorporated into a modelfor estimating floodplain accretion rates. RetallackŽ 1983, 1984.also took a quantitative approach toaccumulation rates by estimating development timesfor different kinds of paleosols through analogy tomodern soils.Bown and Kraus Ž 1993.and Kraus and BownŽ 1993.took a different approach. On the basis ofmorphologic criteria including the combined thicknessof the A and B horizons, geochemical trendswithin a paleosol profile, and degree of clay translocation,those workers assigned floodplain paleosolsin the Paleogene Willwood Formation to seven stagesof pedogenic development. Bown and Kraus Ž 1993.constructed a composite stratigraphic section extendingfrom the base to the 600-m level of the formationfrom numerous measured sections and then subdividedthe composite section into 25-m-thick intervals.The paleosols found within a particular 25-minterval were assigned to a stage of development andthen averaged to yield a relative degree of pedogenicmaturity for each interval Ž Fig. 7A .. The relativeamount of time represented by each interval wasthen estimated from the relative degree of develop-Fig. 7. Ž A.Plot of relative degree of pedogenesis vs. meter levelin composite section of the lower Tertiary Willwood Formation.Relative degree of pedogenic development was calculated for25-m-thick intervals by assigning paleosols in each interval to astage of pedogenic development outlined by Bown and KrausŽ 1993 .. Ž B.That information was combined with the absolute ageof the 600-m-thick section to estimate sediment accumulationrates for each 25-m-thick interval. See text for more detailsŽ modified from Bown and Kraus, 1993; Kraus and Bown, 1993 ..