Paleosols in clastic sedimentary rocks: their geologic applications

( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70 47These profiles reflect the deposition of successive,thin increments of sediment accompanied by pedogenesis.Using fluvial systems as an example, channeldeposits may show little evidence of pedogenesis ormay contain compound soils because sedimentationis so rapid Že.g., Marriott and Wright, 1993; Krausand Aslan, 1999 .. As the channel migrates laterallyover time, composite or well-expressed soils with Bthorizons may form in the upper deposits. Overbankdeposition is usually steady but slow, commonly onthe order of 1–10 mmryear Že.g., Walling et al.,1992 ., and overbank deposits tend to thin and decreasein grain size away from the active channelŽ Guccione, 1993; Marriott, 1996 .. As a result, compoundsoils with weakly expressed profiles tend toform on natural levees Ž Fig. 3A ., whereas cumulativeprofiles develop in floodbasin areas distal to theactive channel Ž Fig. 3B .. Compound soils can alsoform in crevasse deposits because of rapid and unsteadydeposition Ž Fig. 3C ..Autogenic and allogenic processes that operateŽ3 4over intermediate time-scales 10 to 10 years.canproduce relatively lengthy pauses in deposition aswell as local erosion, both of which can influencesoil development. An example of the effects of anautogenic mechanism is provided by avulsion in afluvial system. By causing channels to shift theirpositions on the floodplain, avulsion can terminatesedimentation on a particular part of the floodplainfor periods of time that are probably on the order of310 years, which is the periodicity of avulsion Že.g.,Bridge and Leeder, 1979 .. A well-developed paleosolcan form in floodplain areas cut off from sedimentationby this autogenic process. In the PaleogeneWillwood Formation, episodic avulsions haveproduced stratigraphic intervals that are meters thickand that have two kinds of paleosols: Ž. 1 relativelywell developed cumulative paleosols, which formedon floodbasin deposits and Ž. 2 very weakly developedcompound or cumulative paleosols, whichformed on the sediment deposited by the avulsion ofthe trunk channel ŽKraus and Aslan, 1993; Kraus,1996; Kraus and Gwinn, 1997.Ž Fig. 3B .. Verticalpaleosol sequences show alternations of the weaklydeveloped and more strongly developed paleosols,and no allogenic controls need be invoked to generatesuch an alternation.Turning to extrinsic mechanisms, climatic changescan also influence soil development at an intermediatetime scale by initiating and terminating sedimentation.Many Quaternary loess successions show avertical alternation between pedogenically-unmodifiedsediment and relatively well developed paleosolsŽe.g., Kukla and An, 1989; Frakes and Sun,1994; Pecsi, 1995 .. In fact, these alternations providea loess-paleosol stratigraphy Ž e.g., Pecsi, 1995 .. Thepaleosols are believed to have formed during periodsof reduced sediment input associated with more humidclimates Ž Fig. 4 .. During drier times, sedimentinput overwhelmed pedogenesis, and no soil formed.Although few pre-Quaternary examples of loessiteand loess-paleosols have been described, an exceptionis provided by Soreghan et al. Ž 1997.whoobserved alternations between paleosols and unmodifiedloessite horizons in the Late Paleozoic MaroonFormation. In this example, the paleosols show significantlyhigher magnetic susceptibilities than dothe loessite horizons, which aids in their recognition.As with the Quaternary cases, the paleosols werelinked to more humid climatic conditions and theunaltered loessite layers to drier climates Ž Fig. 4 ..Although some processes merely halt sedimentation,other intermediate-scale mechanisms cause incision.Mechanisms like climate change can producetruncated soil profiles due to erosion of the upperpart of a developing soil Že.g., Marriott and Wright,1993.Ž Fig. 2C .. In the sedimentary record, episodesof truncation are eventually followed by renewedsedimentation. If that sedimentation is slow andsteady, a cumulative-truncated profile can result;however, if sedimentation is so rapid that the truncatedsoil becomes buried, a compound-truncated orcomposite-truncated set can develop ŽMarriott andWright, 1993 .. In the former, the truncated paleosolis separated by unweathered sediment from an overlyingpaleosol; in the latter, the older, truncatedpaleosol and the younger paleosol partially overlap.In a study of upper Silurian and lower Devonianpaleosols, Marriott and Wright showed how thesedifferent kinds of paleosols can be used to understandthe complex processes that operate in ancientfluvial systems. They compared and contrasted twostratigraphic sections, one dominated by cumulativepaleosols and one with numerous truncated paleosols.They inferred that the first section had a stable