| In this thesis, I examine the pedogenic nature of Michigan's fragipans by: (1) characterizing four profiles with fragic or fragipan horizons having varying degrees of fragic-property expression; (2) inferring fragipan genesis based on my soil data and models from the literature; and (3) developing a model of Michigan's fragipan evolution that can also be applied to the northern Great Lakes states. The Michigan Model of Fragipan Evolution involves the self-weight collapse of a soil or parent material followed by physical ripening of the collapsed zone, which helps retain its close-packing and clay-bridging; this forms a protofragipan. Subsequently, amorphous bonding agents precipitate in the protofragipan due to its position within the solum's "weathering transition zone." The resulting fragic horizon or fragipan becomes better expressed with further pedogenic development. Fragipan degradation is initiated by an increasingly prominent perched zone of saturation that forms seasonally on the fragipan. With time, processes associated with fragipan degradation outweigh those associated with progressive development, and the fragipan is destroyed. A well-developed, bisequal solum is thought to remain with no subsequent fragipan genesis likely. |
