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Pedogenesis and geochemical alteration of the Picture Gorge subgroup, Columbia River basalt, Oregon

Nathan D. Sheldon^,1

¹ Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA

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. Many of these red horizons are instead 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. Terrestrial gastropods, fossil plants, and peats also occur locally.

Plots of weathering indices, such as base loss, are consistent with a pedogenic origin for these horizons. Ca, Na, Mg, Fe²⁺, and P have been extensively lost from the A and Bt horizons of the paleosols, 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 relative to the parent material, with Rb added disproportionately. Historically, this pattern of enrichment has been cited as evidence of K metasomatism. The apparent metasomatism can be explained using a new model in which there was extensive felsic volcanism contemporaneous with pedogenesis, and K and Rb were likely added by windborne ash. The apparent discordance between K and Rb contents of the paleosols can be explained by the presence of land plants because K is an important electrolyte and Rb does not serve any major biological role. This model could account for apparent K metasomatism in other paleosols following the advent of land plants and may be applicable to older paleosols as well.

Key Words: Picture Gorge • basalt • paleosols • Oregon • paleoclimate

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