Regional-scale assessment of a sequence-bounding paleosol on fluvial fans using ground-penetrating radar, eastern San Joaquin Valley, California

Abstract

Recently developed sequence stratigraphic models for fl uvial fans suggest that sequence boundaries in these deposits are marked by laterally extensive paleosols; however, these models were based on paleosol correlations inferred between wells. To test this, we collected ~190 km of ground-penetrating radar (GPR) profi les on three fl uvial fans from the eastern San Joaquin Valley, California, to determine the lateral extent and character of a buried near-surface sequence-bounding paleosol. This paleosol, recognized on GPR by rapid shallow signal attenuation, extends across large areas on all three fl uvial fans. Limited areas of signifi cantly increased signal penetration were also identifi ed, and these zones are interpreted to indicate the absence of the paleosol. The zones where the paleosol is missing likely correspond to paleooutwash channel activity on the fan surfaces that, when active, was able to partially or fully scour through the paleosol and deposit coarse-grained channel sediments in place of the sequence boundary. Erosional breaks are most common on the Kings River fan, while few breaks on the Tuolumne and Merced River fans may indicate less paleochannel activity on these fan surfaces during the last outwash event. Differences in channel activity between fans indicate that the Kings River migrated across its fan during the last outwash event, as evidenced by the large number of areas with increased GPR signal penetration and the presence of numerous channel deposits recorded on the soil surveys, while the Tuolumne and Merced Rivers only deposited fl oodplain fi nes, with the channels remaining inside a shallow incised valley, as evidenced by the relatively low number of areas with increased GPR signal penetration and the presence of primarily fi ne-grained material recorded on the soil surveys. Factors controlling these differences may include variable valley subsidence rates and differences in the San Joaquin Basin overall width at each fan location.