Landform and stratigraphic influences on variability of loess thickness in northern Delaware

A detailed study of the relationships between landform, surficial geology, and variability of loess thickness on uplands was conducted at two 2.32-ha study sites in northern Delaware. The purpose of the study was to determine what factors are responsible for the short-distance variability in loess thickness observed in this region. Thickness of loess and paleotopography of the subjacent Pensauken (Columbia) Formation were measure using ground-penetrating radar at 1326 grid points within each plot. Present topography was meaured using a transit at 676 points in one plot and 1326 points in the second plot. Surface grid diagrams and isopleth maps show a lack of relationship between loess thickness and landform and relief except along ephemeral drainages. Over short distances, the thickness and even the occurrence of loess is dependent on the topography of the paleosurface. The loess was deposited upon and uneven erosional surface and conforms on a gross scale to that paleosurface but has filled the concavities and low points, thus producing a more regular present topography. Modern denudation of the present land surface occasionally exposes the coarser-textured Pensauken sediments at the surface over high points in the paleolandscape. The irregular erosional paleosurface has resulted in a high degree of short-distance variability in loess thickness that is difficult to predict except where headslope dissection has been severe enough to produce gullying and the formation of ephemeral drainages. These drainages closely conform to the occurrence of paleodrainages. The high degree of variability in loess thickness and occurrence presents mapping difficulties that need to be addressed in design, naming, and description of soil mapping units.