Case Studies of Ancient Soil Systems
A basin-scale pedostratigraphic model that focuses on paleosols and their pedostratigraphic relationships has been established for the Cenomanian Dunvegan Formation, a unit that represents a large delta complex. A detailed sequence stratigraphic and paleogeographic framework permits analysis of paleosol development with respect to distance from marine shorelines and coeval valleys. Paleosols that bracket sequence boundaries vary depending upon their paleo-landscape position. The sequence-bounding package of paleosols can be partitioned into three spatial zones based upon both the degree of development and the architecture of the paleosols. Zone 1 occurs in seaward localities near the maximum regressive shoreline and is characterized by hydromorphic, weakly developed paleosols typical of a poorly drained, progradational, and aggradational coastal plain. Zone 2 occurs in an intermediate location and is characterized by well-developed Alfisol-like welded paleosols that record a complex architecture indicating (i) an aggradational phase; (ii) a subsequent static and/or degradational phase related to valley incision, nondeposition, and soil thickening; and (iii) a final aggradational phase related to valley filling and renewed sedimentation across the coastal plain. Zone 3 occurs in more up-dip settings and is characterized by compound and complex Inceptisol-like paleosols that developed as the result of a reduced aggradation rate when valleys were being incised further down-dip. Because accommodation, sediment supply, and hydrological conditions vary in both dip and strike directions, the three zones represent lateral soil facies equivalents. The soil-forming interval bracketing the sequence boundary comprises a geosol composed of welded paleosols that subdivide both up-dip and down-dip into more weakly developed aggradational paleosol complexes. Above the sequence boundary, a high accommodation phase (equivalent to the Transgressive Systems Tract) is represented by widespread lacustrine and poorly drained floodplain facies and weakly developed hydromorphic paleosols. As accommodation rate decreases (late Highstand Systems Tract time) the alluvial succession becomes paleosol dominated, comprising floodplain pedocomplexes that record a regional decrease in the accommodation/sediment supply ratio. Up-dip variability along the sequence boundary and within sequences is controlled primarily by variations in the accommodation/sediment supply ratio, by hydrological variations associated with floodplain incision during valley formation, and by tectonic subsidence rates that vary in space and in time.
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New Frontiers in Paleopedology and Terrestrial Paleoclimatology: Paleosols and Soil Surface Analog Systems
After initial breakthroughs in the discovery of fossil soils, or paleosols in the 1970s and early 1980s, the last several decades of intensified research have revealed the much greater role that these deposits can play in reconstructing ancient Earth surface systems. Research currently focuses on terrestrial paleoclimatology, in which climates of the past are reconstructed at temporal scales ranging from hundreds to millions of years, using paleosols as archives of that information. Such research requires interdisciplinary study of soils conducted in both modern and ancient environments. These issues and many others were discussed at the joint SEPM-NSF Workshop “Paleosols and Soil Surface Analog Systems”, held at Petrified Forest National Park.