A FERRICRETE LAYER FROM A LATE HOLOCENE BEACH RIDGE OF LAKE SUPERIOR, AU TRAIN, MICHIGAN

Excavation of strand plain beach ridges at three sites along the southeastern shore of Lake Superior commonly revealed a ferricrete layer located within the B horizon of the soil profile. The iron content of beach ridge pedons exceeds what would be expected by translocation processes alone. In multiple beach ridges this ferricrete was observed to function as an aquiclude, forming an upper limit to fluctuation of the water table. The presence of this ferricrete layer potentially influences infiltration, podzolization, and local hydrology within beach ridge complexes. A sample of this ferricrete was isolated for mineralogical analysis from the B horizon of a late Holocene beach ridge of Lake Superior located in Au Train Michigan.

Bulk XRD of the < 2 micron fraction indicates that the oxide material is amorphous. TEM investigation reveals several textures oxide materials. Capsule-shaped particles composed approximately 3 % of the sample. Platy particles are common in the sample material and compose approximately 20% of the total sample material. Two types of spongy particles were observed. Type1 spongy particles compose approximately 10% of the total sample while type 2 spongy particles compose approximately 65% of the sample volume. The least common texture observed is that of crystalline Fe oxide minerals which comprises 2 % of the sample. Selected area electron diffraction shows sharp diffraction spots in concentric rings consistent with an aggregate of randomly oriented micro-crystals. These minerals are always enveloped in type 2 type particles. EDS analyses show that type 2 particles have Cr2O3 content commonly on the order of 5 wt% and crystalline oxides enveloped in type 2 particles have Cr2O3 content varying between approximately 10 and 25 wt. %. Both texture and chemistry argue for a transformation of type 2 particles to micro-crystals.