Significance of Paleosols in Alluvial Architecture: Example from Upper Jurassic Morrison Formation, Colorado Plateau: ABSTRACT

Abstract

Paleosols are rapidly becoming an important and essential aspect of alluvial architecture. Their diversity can be used extensively in detailing important subenvironments of major alluvial systems. The Upper Jurassic Morrison Formation in the Colorado Plateau exhibits some of the best examples of alluvial paleosols yet known. Three-dimensional exposures are used to illustrate the relationships between the various alluvial subenvironments. Paleosols are better developed on meandering-channel systems where the overbank areas are much broader compared to braided-channel systems where paleosol profiles are thin and less well developed. A great variation also exists between paleosols which were developed on different subenvironments. This variability ranges from degree of development (maturity) of the soil profile to the presence or absence of certain pedogenic features. Three processes are responsible for the development of the soil profiles: sedimentation, pedogenesis, and diagenesis. All three processes must be differentiated and considered separately. Sedimentation rates the paleosols to the channels, distinguishes soil profiles, and helps explain compound pedogenesis. Diagenesis reflects mineralogical alterations which may be related to climatic conditions, soil horizon coloration which does not necessarily reflect horizon boundaries, and processes such as silicification, calcification, and concretion development. Pedogenesis acts in various degrees reflecting climate, topography, vegetation, parent material,more » and time. All these aspects of paleosols can be used effectively to reconstruct ancient alluvial architecture. The Morrison Formation is a prime example because of its great regional extent and excellent outcrops.« less