Paleosols in clastic sedimentary rocks: their geologic applications

Interest in pre-quaternary paleosols has increased over the past decade, in large part, because they have proved to be beneficial in solving diverse g eological problems. The majority of paleosols are described from continenta l deposits, most commonly from alluvial strata. Criteria for recognizing th ese paleosols have been extensively described; however, classifying them ha s proved more complicated. Pre-Quaternary paleosols are generally classifie d according to one or more modern soil classification systems, although one new classification has been proposed exclusively for paleosols to avoid pr oblems using the modern soil classifications. In addition to taxonomic clas sification, paleosols can be categorized according to the interplay among d eposition, erosion, and the rate of pedogenesis when they formed. Paleosols can be solitary if they formed during a period of landscape stability foll owing the development of an unconformity. Such paleosols are commonly thick and extremely well developed. More commonly, paleosols are vertically stac ked or multistory because they formed in sedimentary systems undergoing net aggradation. If erosion was insignificant and sedimentation was rapid and unsteady, compound paleosols generally formed. If the rate of pedogenesis e xceeded the rate of deposition, composite paleosols developed. Thick, cumul ative paleosols indicate that erosion was insignificant and that sedimentat ion was relatively steady. Both autogenic and allogenic processes can influ ence depositional and erosion patterns and, thus, affect the kinds of soils that form. Consequently, paleosols can help to interpret the history of se diment deposition and the autogenic and allogenic processes that influenced a sedimentary basin. Paleosols are also helpful in stratigraphic studies, including sequence stratigraphic analyses. They are used for stratigraphic correlations at the local and basinal scale, and some workers have calculat ed sediment accumulation rates based on the degree of paleosol development. In addition to their stratigraphic applications, paleosols can be used to interpret landscapes of the past by analyzing paleosol-landscape associatio ns at different spatial scales, ranging from local to basin-wide in scope. At the local scale, lateral changes in paleosol properties are largely the result of variations in grain size and topography. At the scale of the sedi mentary basin, paleosols in different locations differ because of basinal v ariations in topography, grain size, climate, and subsidence rate. Paleosol s are used to reconstruct ancient climates, even to estimate ancient mean a nnual precipitation (MAP) and mean annual temperature (MAT). Ancient climat ic conditions can be interpreted from modern soil analogs or by identifying particular pedogenic properties that modem studies show to have climatic s ignificance. Stable carbon and oxygen isotopes are also used to interpret a ncient climate, and some effort has been made to estimate MAT from isotopic composition. On the basis of modern soil analogs, paleo-precipitation has been estimated from the depth at which calcic horizons originally formed. F inally, paleosol carbonates have been used to estimate ancient atmospheric CO2 values. (C) 1999 Elsevier Science B.V. AU rights reserved.