Clay Mineralogy and Paleoclimatic Significance of Some Red Regoliths and Associated Rocks in Western Colorado

Abstract

ABSTRACT X-ray diffractometer analyses were made of the genetically related parent rock-regolith-sedimentary rock suites of pre-Pennsylvanian red palaeosols and the immediately overlying Pennsylvanian red beds exposed in western Colorado. For convenience the samples have been grouped into three regoliths whose limits are arbitrary, the Molas, in southwestern Colorado, the Maroon, in the Maroon trough, and the Front Range. The Moles regolith where sampled was formed on limestones in which the clay mineral is illite. The regolith contains mainly kaolinite, subordinate illite and minor montmorillonite, mixed layer illite-montmorillonite, chlorite and chlorite-vermiculite. The clay mineral assemblage in the overlying sedimentary rocks is similar to that of the regolith, but illite is dominant. The Maroon regolith formed on limestones or orthoquartzites which contain illite. In half the regolith samples mixed layer clays occur together with dominant kaolinite. Illite with subordinate kaolinite occurs in the remainder. The overlying sedimentary rocks contain mainly illite with some kaolinite but some immature clays occur where the regolith contains immature clays. The Front Range regolith was sampled where formed on three rock types, granodiorite, metamorphic rocks, and limestones in which the dominant clay mineral is illite. The regolith contains clay mineral assemblages of illite and mixed-layer illite-montmorillonite with minor kaolinite where it is formed on igneous or metamorphic rocks. Regolith formed on limestone contains dominantly illite, or illite and kaolinite in sub-equal proportions. The overlying sedimentary rocks contain kaolinite as the dominant clay mineral in most cases. The climates under which these regoliths formed have been interpreted variously as arid, or semi-arid, or humid and warm to hot, by earlier workers using conventional arguments of zonal soil types. Thus a soil type has been identified on the basis of its clay mineralogy or color, or both, or other information, and a climate of formation inferred. An analysis of soil-forming processes shows that this reasoning is inadequate, because climate is not always the controlling factor, and red color and clay mineralogy are not exclusive guides to soil type. The weathering controls may be so combined that similar net effects are achieved under a range of climates. Red color has no exclusive climatic significance. Clay mineral assemblages cannot characterize soil type adequately, and the assemblage throughout the weathering profile is variable. The duration and intensity of weathering is controlled immediately by climate and ultimately by tectonics. The regoliths studied probably formed in a sub-humid climate. This is indicated by the degree of leaching and depth of weathering, not by color or clay mineralogy. They contributed only to the basal parts of the thick red sequences overlying them, because they were buried too soon.