PEDOGENESIS AND GEOCHEMICAL ALTERATION OF COLUMBIA RIVER BASALT LAVAS

Reddened clay-rich horizons between basalt flows have historically been identified as “baked zones” or as zones of lateral groundwater movement. The reddening has been attributed to thermal or chemical alteration of the permeable portion of the basalt flow. However, many of these zones are paleosols that record significant hiatuses in basalt flow emplacement. Paleosols were identified in the field on the basis of features such as horizonation, ped morphology, and root traces. A small number of terrestrial gastropods were also found.

Plots of weathering indices such as the CIA-K and base loss are consistent with a pedogenic origin for these horizons. Down profile P₂O₅ abundance is also consistent with surficial bioaccumulation and subsurface depletion. With the well understood chemical composition of the Picture Gorge Subgroup of the Columbia River Basalt, these paleosols are also ideal for mass balance calculations that can be used to test the degree of geochemical alteration of these paleosols. Samples were collected in profiles from the overlying basalt flow, through the paleosol, down to the underlying flow. Ca, Na, Mg, Fe²⁺, Fe^*, and P have been extensively lost from the A and Bt horizons of the paleosols relative to the parental basalt, consistent with modern weathering patterns. K and Rb have been added to the upper portions of the profiles and lost in the lower portions of the profiles, with Rb added disproportionately. Historically, this pattern in K has been cited as evidence of metasomatism. However, element mobility patterns in the lower portion of the overlying basalt flows are inconsistent with fluid flow along lithological contacts. The apparent metasomatism can be explained using a new model. There was extensive contemporaneous felsic volcanism in the Cascades. K and Rb were likely added proportionately by windborne ash given their geochemically similar behaviour. The apparent discordance between K and Rb can be explained by the presence of vegetation since K is an important electrolyte and Rb does not serve any major biological role. A model of windborne addition may be a viable explanation for apparent K metasomatism in other paleosols throughout geologic time. This work suggests that interflow paleosols faithfully record pedogenesis and may be an important and largely untapped source of paleoclimatic data.