Paleosols in clastic sedimentary rocks: their geologic applications

( )M.J. KrausrEarth-Science ReÕiews 47 1999 41–70 51The Bozeman Group is a thick continental successionin MT, USA, which spans the Cenozoic. Hannemanand Wideman Ž 1991.and Hanneman et al.Ž 1994.showed that problems with the stratigraphicrelations of this group could be clarified using calcretesto delimit basin-wide unconformities and thusto subdivide the succession into five sequences. Ofparticular interest is that Hanneman et al. Ž 1994.integrated surface geologic, seismic, and well logdata and showed that the calcretes produced brightseismic reflections that could be traced in the subsurface.This study not only highlights the potential ofpaleosols for solving stratigraphic problems, but alsoshows that, in some cases, paleosols can be recognizedin the subsurface using data other than coredata and the paleosols can then be used for regionalcorrelations. Similarly, Ye Ž 1995.used paleosols tocorrelate Miocene strata in the subsurface and toconstrain the position of a petroleum reservoir. Thepaleosols were identified in cores primarily on thebasis of color, the presence of roots and calcretenodules, and mineralogy. Although Ye was also ableto use log characteristics to distinguish paleosolsfrom rock that was not pedogenically modified, incontrast to Hanneman et al., he found it very difficultto identify, and thus correlate, individual paleosolsfrom log characteristics alone.Going beyond local or regional correlation usingpaleosols, a study by Koch et al. Ž 1992.showed thatsome paleosols have the potential to make globalcorrelations. A sharp decrease in the 13 Cr 12 C ratiosof biogenic marine carbonates has been well documentedat approximately the PaleocenerEoceneboundary and has been attributed to global marinewarming Že.g., Shackleton, 1986; Zachos et al.,1993 .. Koch et al. were able to use pedogenic carbonatesto identify this sharp isotopic change inPaleogene alluvial rocks in the Bighorn Basin ofWyoming and, thus, to correlate the continentalrecord to the marine isotopic record. Through thiscorrelation, they showed that the PrE boundary inthe marine record corresponds to a particular biostratigraphicboundary in the Bighorn Basin and werethen able to relate changes in mammalian faunas atthis boundary to global climate change.As anyone working in the stratigraphic record isaware, direct absolute dating of the sedimentaryrecord is difficult. Yet, recent work indicates thatUrPb dating of pedogenic carbonates offers a newapproach to obtaining absolute dates for continentalrocks Ž Rasbury et al., 1998 .. Those authors estimatedthe U–Pb ages of pedogenic carbonates from Pennsylvanian–Permianstrata. Because they argued thatthose strata were deposited rapidly and show noevidence of later diagenetic modification, they concludedthat the time of formation of the calcretes is aproxy for the time of deposition of the strata thatenclose them. This technique is potentially powerfulbecause pedogenic carbonates are found in numerousstratigraphic successions throughout the Phanerozoic.Finally, various authors have emphasized the sequencestratigraphic importance of paleosols, bothfrom a modeling approach Že.g., Wright and Marriott,1993.and, more commonly, through field studiesŽe.g., Aitken and Flint, 1996; McCarthy andPlint, 1998 .. Wright and Marriott Ž 1993.developed amodel that predicts how the degree of pedogenicmaturity and soil drainage of coastal plain paleosolsvaries at different times in the sea level cycle. Theirmodel suggests that, during sea-level lowstands, matureand well-drained soils should form on terracesthat are produced by channel incision because thewater table drops and the floodplains receive nosediment Ž Fig. 5 .. When sea level first begins to rise,hydromorphic soils develop because baselevel is rising.As sea level continues to rise, accommodationspace is created and floodplain sedimentation is rapid,leading to weakly developed paleosols. Later, duringearly highstand, the rate of aggradation begins todecrease and more strongly developed soils shouldform. Finally, when highstand is fully achieved,accommodation space is low. Mature soils form;however, their preservation potential is probably lowbecause aggradation rates are so low that sedimentreworking by channel combing is intense.A key feature of the Wright and Marriott model isthat, contrary to other models of sequence stratigraphy,it predicts that the highest rates of aggradationare characteristic of the transgressive systems tractŽ TST ., and that, because of low accommodationspace, the highstand systems tract Ž HST.had lowrates of aggradation. Consequently, abundant overbankdeposits with weakly developed paleosolsshould characterize the TST, whereas lower amountsof overbank deposits with well-developed paleosolsare typical of the HST. In contrast some field studies