CLAY MINERALOGY OF ORTSTEIN HORIZONS FROM LAKE SUPERIOR BEACH RIDGES: IMPLICATIONS FOR THE HYDROLOGY OF RIDGE AND SWALE ENVIRONMENTS

Well-defined ortstein horizons were discovered at depth upon excavation of individual beach ridges at three strandplains studied parallel the shoreline of Lake Superior. The ortstein horizons consist of spodic materials that are cemented, typically redder than 7.5 YR, and darker in appearance than the overall soil profile. In low relief ridges the ortstein layers were found to be associated with the water table. Penetration of the ortstein layer resulted in upward movement of groundwater, suggesting that this horizon serves as an aquiclude, possibly influencing the hydrology of wetlands that commonly form between individual beach ridges in the ridge and swale depositional environment.

Ortstein samples were studied using powder X-ray diffraction and transmission electron microscopy (TEM) techniques. Powder X-ray diffraction data from the < 2 micrometer size fraction indicate that the clay material is dominantly X-ray amorphous. TEM investigation shows that the clay material is composed of amorphous Fe-Al oxides of variable chemical composition. Four types of textures are recognized: capsule-shaped particles, sponge-like particles, platy particles and spherical particles. Capsule-shaped particles are interpreted as being the result of accumulation of oxides on bacterial surfaces. Other particles appear to be inorganic in origin. Examples of transformations between amorphous and more crystalline textures were observed. Crystalline oxides and clay-sized tectosilicates were observed in some samples. The wide range in size and shape of amorphous Fe-Al oxide particles within and between samples may explain why ortsteins can develop low vertical permeability even when they are relatively young. This study shows that ortstein formation involves complex mineralogic and biologic processes with profound implications for local hydrology.