Paleosols in clastic sedimentary rocks: their geologic applications

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52( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70Fig. 5. Schematic diagram showing Wright and Marriott Ž 1993. model of pedogenic development related to the sea-level cycle. Ž A.Thelowstand systems tract Ž LST.is characterized by channel incision and strongly developed and well-drained paleosols that form on terraces.Ž B.Early in the TST, hydromorphic paleosols may form because base level is rising; channel sandstones may overlap becauseaccommodation space is still low. Ž C.Increased accommodation space in the TST produces rapid sediment accumulation and weaklydeveloped soils. Ž D.The HST is characterized by low accommodation space, thus, well-developed paleosols Žmodified from Wright andMarriott, 1993 ..Že.g., figure supplied by M. Uliana and L. Legarreta,cited in Shanley and McCabe, 1994.have shown thatthe HST, rather than the TST, is dominated byoverbank deposits and that paleosols are most commonin the HST. Shanley and McCabe Ž1994, p.562 ., suggested that this discrepancy may have arisenbecause the Wright and Marriott model ‘‘takes noaccount of the basinal position of systems tracts northe importance of recognition of the landward equivalentof marine condensed intervals’’. Nonetheless,the model shows that paleosols are an important areaof future research in sequence stratigraphy.Sequence stratigraphic field studies generallystress the importance of paleosols for identifyinginterfluvial sequence boundaries and for distinguishingincised valley fills from major channel sandstones.For example, Aitken and Flint Ž 1996.foundpaleosols to be critical for identifying interfluvialsequence boundaries in Pennsylvanian rocks, because,beyond the incised valley fills, the boundariesare subtle features. The paleosols, which developedon delta plain deposits, are typically hydromorphicor gleyed paleosols; however, the presence of compositepaleosols indicates that many of the soils wereinitially well drained when water tables were low,and then they underwent a secondary gleyed stagewhen the water table rose in response to transgression.Despite their potential value in identifying sequenceboundaries, paleosols like these probably havelow preservation potential because the ensuing transgressioncan obliterate them Ž Aitken and Flint, 1996 ..Interfluvial sequence boundaries with associated paleosolshave also been described from entirely fluvialsuccessions Ž e.g., Leckie et al., 1989 .. The presenceof numerous, closely spaced paleosols in the Cretaceousstrata suggested to Leckie et al. that sediment
( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70 53supply to the floodplain was cut off and they attributedthis to channel incision caused by loweredbase level.Despite the progress using paleosols to recognizeinterfluvial sequence boundaries, McCarthy and PlintŽ 1998.noted that more attention should be paid todetails of the paleosols. In particular, those authorsshowed how examining the micromorphologic attributesof the paleosols in conjunction with fieldmorphology and stratigraphic relationships leads to amore complete understanding of the sequence ofevents that produced paleosols at a sequence boundary.In a study of Cretaceous paleosols that formedon coastal plain deposits, they found that grey soilcolors and the presence of siderite and preservedorganic matter indicated that the soils first formedunder poorly drained conditions Ž Fig. 6A .. The presenceof clay coatings and various ferruginous featuressuggest that drainage then improved, andpapules Ž fragmented clay coatings.and pedorelictsindicate that the interfluves underwent an episode oferosion Ž Fig. 6B .. They attributed these features tolowered base level, which caused the channels toincise and water tables to drop. Silty layers directlyabove the sequence boundary within the paleosolsreflect renewed sedimentation on the interfluves becausebase level rose again. Finally, wet soil indicators,present above the sequence boundary, show thatwater tables also rose, although multiple organic-richbeds indicate that water tables fluctuated somewhatŽ Fig. 6C ..Although sea level fluctuations are an importantcontrol on coastal plain rivers and the developmentand morphology of paleosols associated with thoserivers, the current opinion is that sea-level effectsprobably do not extend inland more than about 100–150 km from the shoreline Že.g., Shanley and Mc-Cabe, 1994; North, 1996 .. In alluvial systems moredistant from the sea or in closed basins, climate andregional tectonic activity are the major controls onFig. 6. Schematic diagram showing sequential development of an interfluve paleosol that marks a sequence boundary. Ž A.Siderite and greysoil colors show that initial soil drainage was poor. Ž B.Base level dropped, the floodplain was incised, and the water table dropped. Claycoatings and ferruginous nodules formed in the better-drained soil. Ž C.Base level rose and new sediment was deposited. The water tablealso rose, producing hydromorphic features including multiple organic horizons. See text for more details Žmodified from McCarthy andPlint, 1998 ..
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Paleosols in clastic sedimentary rocks: their geologic applications

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