Genesis of soils derived from the Kaibab Formation of the Colorado Plateau, Arizona.

Abstract

Important pedogenic processes of soils formed on dolomitic limestones are affected by the degree of in-situ weathering of the underlying bedrock. Decalcification and silicate clay illuviation of Haplustalfs and Calciustolls of the Colorado Plateau in northwestern Arizona results from the establishment of effective porosity in the parent rock. The underlying carbonate strata, the Kaibab Formation, are a complex limestone-dolomite-chert marine sediment of Permian age. Diagenetic processes have modified the initial porosity present at the time the sediment was deposited. Pleistocene meteoric waters, percolating downward, have resulted in the solutioning and partial removal of the calcium carbonate and the establishment of a porous dolomitic framework. Under these conditions, soils forming in residuum are able to decalcify and to form argillic horizons. However, in more resistant limestones, compaction during burial has resulted in a nonporous micrite which retards calcite removal by meteoric waters. Under these conditions, soil profiles maintain a high CaCO₃ content and silicate clay illuviation does not occur. Micaceous clay minerals of residual origin are converted to montmorillonite in the Alfisols. A probable sequence of events for this transformation to occur is: (1) the removal of calcite from the dolomitic limestone bedrock and the formation of a dolomitic framework, (2) the slow dissolution of dolomite and release of Mg⁺⁺ into solution, (3) the incorporation of Mg⁺⁺ into the octahedral layer of the mica and the release of K⁺ from the mica interlayer position, and (4) the illuviation of montmorillonite to form the argillic horizon. Eolian dust is also important to soil genesis on the Colorado Plateau. In particular, quartz in the coarse silt fraction (31-44 um) and quartz and biotite in the very fine and fine silt fractions (2.0-16 um). Two alluvial soils of Pleistocene age shows important morphological differences. Thick carbonate accumulations of ground water origin are present in the older soil. The effects of the Pleistocene-Holocene climatic change on this area are: (1) modification of karst topographic features, (2) aggradation of valley bottoms, and (3) the influence of Holocene slope wash Pleistocene soil properties such as calcite, dolomite and organic matter.