Buried carbonate paleosols developed in pliocene-pleistocene deposits of the Pasco basin, south-central Washington, USA

Carbonate-rich paleosols represent a small volume of material at the Hanford Site in south-central Washington state, U.S.A., but their position below highly permeable Missoula-flood deposits and above the water table makes them an important component of contaminant remediation plans. Previous work indicated that these buried carbonate paleosols constitute a relatively continuous, low-permeability paleosurface. The distribution of the Pliocene-Pleistocene deposits containing the paleosols depends in part on erosion of the underlying Miocene-Pliocene fluvial/lacustrine Ringold Formation and post-depositional erosion by the catastrophic Missoula floods. The character of the 'caliche' layer(s), as determined from cores, varies across the study area: from 0-20 m in thickness; at depths of 20 m in the south, to 65 m in the northwest; bounded above and below by irregular surfaces with as much as 25 m relief; and in general co-varies in number of carbonate layers with the thickness of the deposits. These carbonate layers reflect the geomorphology and hydrology existing during the time they developed. I interpret these carbonate layers as paleosols but recognize morphologic features that indicate modification may have occurred after initial development at the surface. These include the absence of associated soil horizons; no apparent decrease in carbonate content with depth in some of the carbonate layers; and unusual Bk horizon fabrics (including crystalline or popcorn-like forms). These features and fabric differences influence potential flow pathways of waste fluids.