ORIGIN OF PLEISTOCENE DUNAL GRAY LAYERS: DEFLATION WETLAND DEPOSITION VERSUS UPLAND PEDOGENIC WEATHERING, LINCOLN CITY DUNE SHEET, OREGON

A gray-colored dunal paleosol from a sea-cliff exposure was examined for stratigraphy, grain texture, geotechnical properties, and alteration products to establish its origin. These tests were performed to discriminate between two mechanisms of formation, i.e., deflation wetland deposition and upland soil weathering. The 0.5 m thick layer was traced over a coast-parallel distance of 1.0 km, where it varied by less than 5 m elevation within the 15 m thick Pleistocene dune exposure. Immediately north of the 21st Street beach access stairway, the gray layer rose 1 m in elevation (from Sample B to Sample A) over a 13 m distance. Sample B included a 17 cm peat horizon with in-situ tree trunks at the top of the gray layer (38 cm total thickness). Site A contained no peat development or apparent root casts in the gray layer (36 cm total thickness). Grain size fractions in sample B are 36, 29, 30, 5 wt% and in sample A are 46, 39, 14, 0 wt% (in order of sand, silt, clay, organics). Penetrometer compressive strengths for sample A are constant (top to bottom) at about 4.5 Kgcm-2, and for sample B they drop from 4.5 Kgcm-2 (top) to 1 Kgcm-2 (bottom). Vane shear tests average about 0.15 Kgcm-2. Preliminary results of XRD analysis indicate 2:1 expanding-phyllosilicate clays. By comparison, SEM (100-2000xmag.) analysis clearly shows that both silt and clay size fractions are dominated by faceted quartz/feldspar shards. The shards are layered and/or imbricated (microfabric) indicating an origin from deposition or infiltration. This gray layer appears to have originated from loess deposition in a ephemeral wetland/bog setting. The continuity of this layer suggests the paleodunal surface was produced by deflation to near water table level. The lack of significant framework grain alteration indicates that the soil formed rapidly prior to burial. However, minor hydroxide and clay mineral cements indicate some post-burial precipitation from groundwater. Additional work is underway to luminescence-date these layers, and map their distribution. Future work should help to establish whether the dunal paleosol ‘gray-layers’ represent regional climatic-shifts or sea-level fluctuations in late-Pleistocene time.