REGOLITH EVOLUTION IN THE OUACHITA MOUNTAINS AS INDICATED BY ROCK FRAGMENT DISTRIBUTIONS

Conventional models of weathering and regolith evolution involve the progressive downward migration of a bedrock weathering front. This implies that if surface erosion and mixing are relatively minor, then at the profile scale there should be a systematic decrease in the degree of weathering from the surface downward. The vertical distribution of rock fragments in soils and regoliths of the Ouachita Mountains, Arkansas is used in this context to shed light on landscape and regolith evolution. A model is developed which assumes that the concentration of rock fragments in the lowermost soil horizon (immediately above bedrock or a paralithic contact) represents the initial concentration of rock fragments produced as the weathering front migrates. The concentration of rock in any overlying horizon is therefore a function of this initial content; depletions due to further clast weathering, removal by mixing, or erosional removal; and additions due to concentration via erosional winnowing, surface deposition, or accumulations by mixing. In the study area surface erosion and deposition of fragments is negligible, and the majority of mixing is due to bioturbation. Detailed profile descriptions from 39 soil pits in the Winona Lake area show irregular vertical trends in rock fragment concentrations. In 27 cases the concentration of rock fragments is highest in the lowermost regolith horizon, as expected. However, the concentration of rock fragments was at a minimum in the surface (A) horizon in only six pits, and only four of these showed an uninterrupted trend of constant or increasing rock fragments with depth. Rock fragment minima occurred in the second (E or Bt1) horizon in 25 (64%) of the pits, and an additional seven showed a percentage of clasts at least 10 percent higher in the surface horizon than in the second layer. Results suggest a combination of upward transport of fragments via bioturbation (particularly tree throw) and erosional winnowing at the surface, and indicate that the regolith does not represent a gradient of the age or degree of weathering from the surface to the weathering front.