Paleosols in clastic sedimentary rocks: their geologic applications

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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 ..
( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70 55ment for each interval. For example, the lowest 25-minterval has the highest relative degree of maturity at12.0 and occupied 9% of the total time representedby the total section Ž Fig. 7A .. Because absolute ageshad been established for the lower and upper boundariesof the 600-m-thick section, the time span foreach 25-m interval could be calculated. From thatcalculation, the sediment accumulation rate for each25-m-thick interval was estimated Ž Fig. 7B .. Krausand Bown Ž 1993.then used these short-term accumulationrates to examine up-section changes insedimentology and paleontology.Using paleosols to estimate sediment accumulationrates in a quantitative fashion suffers from severallimitations. The Leeder Ž 1975.and RetallackŽ 1983, 1984.approach assumes that times of formationof modern soil analogs can be reliably determined.The pitfalls of this assumption for calcreteshave been thoroughly discussed by Wright Ž 1990 ..He concluded that this approach is not justifiedbecause Quaternary calcretes actually show considerablevariability in their rates of formation dependingon local rainfall and the availability of airbornecarbonate dust.A second potential problem is erosion or slowsedimentation rates, which can generate compositepaleosols. With erosion, accumulation rates can beunderestimated. If the paleosols are composite, asthey are in numerous stratigraphic successions, accumulationrates can be overestimated. Finally, RetallackŽ 1998.has pointed out that studies like those ofBown and Kraus Ž 1993. and Kraus and Bown Ž 1993.assume that there are no major discontinuities in thepaleosol section and that all of the time contained ina vertical section is represented by paleosols. Henoted that this is probably an unrealistic assumptionfor stratigraphic sections that span several millionsof years.4. Paleolandscape reconstructionIn the Quaternary record, soil–landscape studiesare important for interpreting landscape evolutionŽ e.g., McFadden and Knuepfer, 1990 .. Similarly, inpre-Quaternary strata, paleosol–landscape studies canhelp in reconstructing ancient landscapes Že.g., Bownand Kraus, 1987; Besly and Fielding, 1989; Platt andKeller, 1992 .. Paleosolrlandscape studies focus onthe spatial distribution of different kinds of paleosolsand the different landscape elements and processesthat produced those paleosols. The study of JoeckelŽ 1995 ., described above, is a good example of usinglateral changes in a paleosol associated with anunconformity to analyze regional topographic variationsalong a specific degradational landscape. Furthermore,thick sedimentary sequences with multiplepaleosols provide a record of land surfaces over timeso that landscape changes and evolution can beexamined.Paleosol–landscape associations can be studied atdifferent spatial scales that range from local Že.g., achannel and associated floodplain, an eolian duneand associated interdune.to basin-wide in scopeŽ e.g., a drainage system, a sand sea .. At the localscale, lateral changes in paleosol properties arelargely the result of local variations in grain size andtopography. At the scale of the sedimentary basin,paleosols in different locations differ because ofbasinal variations in topography, grain size, climate,and subsidence rate. The following discussion providesexamples of how paleosols have been used tointerpret landscapes of the past, starting at the localscale and building to the basin-wide scale. For amore thorough discussion of paleosol–landscape associationsat different spatial and temporal scales,Ž .see Kraus and Aslan 1999 .4.1. Local landscape reconstructionPaleosol–landscape associations at this scale havebeen studied primarily in alluvial rocks Že.g., Bownand Kraus, 1987; Kraus, 1987, 1997; Besly andFielding, 1989; Platt and Keller, 1992 .. Catenas andpedofacies are common paleosol–landscape associationsthat develop at the scale of the channel andassociated floodplain. The two are not mutually exclusive,and floodplain paleosols can show a combinationof the two.In a catena, better-drained soils form on channelmarginaldeposits Ž levees, crevasse splays.becausethe deposits are elevated relative to the surroundingfloodbasin and usually consist of relatively permeablefine sands and silts Ž Fig. 8 .. The soils may showevidence of oxidized conditions, including brown Aand Bw Ž weathered or structural B.horizons and
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