DEVONIAN PALEOSOLS AS INDICATORS OF ACADIAN VOLCANIC ACTIVITY

Bentonites deposited in distal marine strata of the Appalachian foreland basin are the most commonly recognized evidence for volcanic activity associated with the Acadian Orogeny. Terrestrial sections may offer an alternative record of volcanic activity in the form of distinct paleosol morphologies. The majority of paleosols within the terrestrial portion of the Acadian clastic wedge are represented as four primary types: entisols, inceptisols, alfisols, and vertisols. Entisols are weakly developed soils representing the onset of pedogenesis, inceptisols represent the initiation of B horizons, and alfisols represent the subsurface accumulation of silicate clay in the B horizon. These three soil orders generally represent increasing periods of pedogenesis before burial and exclusion from the pedogenic environment. Vertisols are characterized by the enrichment of smectitic clays which shrink and swell with individual wetting events and pedoturbate soil materials, producing distinctive slickensided curvilinear planes and wedge shaped soil aggregates. These four soil types commonly alternate vertically in any given exposure, suggesting punctuated fluxes of volcanic materials into the terrestrial ecosystem that were hydrolyzed to produce smectitic clays, and in turn vertisols. The expression of vertic properties is proportional to the amount of smectite in the soil profile. Additionally, since vertisols require repetitive episodes of wetting and drying to generate vertic features, seasonality of precipitation can be inferred, especially in vertisols with subsurface accumulations of pedogenic carbonate. Soils return to non-vertic varieties with continued deposition and burial by fluvial siliciclastics derived from the orogen, or by relative sea level rise depending upon the proximity to marine environments, until the next episode of explosive volcanism. In the absence of distinct bentonite beds, vertisols can serve as an indicator of volcanic activity in convergent orogenic belts.