Sedimentology of a fine-grained aggrading floodplain

Abstract

Historical land use changes accelerated both soil erosion and subsequent floodplain alluviation within the Galena River basin, Wisconsin. A cross-valley transect in a low-gradient, downstream section of this watershed permitted documentation of the lateral and vertical sedimentological properties occurring during this aggradational phase. Mechanical and statistical analyses of 97 sediment samples reveal two dominant controls determining the resulting floodplain sedimentology. Firstly, the initial fine-grained loessial texture dominated the particle size of the early post-settlement alluvium, generally yielding fine-textured sediments across the valley bottom. Further landscape de-stabilization promoted upland gullying into coarser-textured residuum below the loess, with a concomitant particle size increase in the foodplain alluvium. Thus, the resulting foodplain stratigraphy exhibits a coarsening upward sequence. Secondly, distance from the stream channel exerts a strong control on overall sedimentological properties. Distal settings possess massive, high clay-content sediments usually with less than 1% sand. Samples become coarser and more stratified near the modern stream channel, with channel fill facies containing approximately 20% sand in the upper sections. The fine-grained loessial sediment source affected numerous sedimentological properties of the deposited alluvium. Sorting correlates negatively with particle size because all particle sizes were entrained during large overbank floods. Dominance of silt in this loess-mantled basin maintains the maintains the median of the particle size distribution within a fairly narrow range. Only the tails of the distribution vary, and they are primarily controlled by the distance to the stream channel. In distal settings the fine tail emerges, whereas the coarse tail appears in the channel fill and proximal settings. Although all sediments were deposited in an overbank floodplain environment, Passega's CM diagrams erroneously indicate a pelagic depositional environment.