A seismic-reflection sequence observed at depth beneath the south flank of the Benton Uplift in Arkansas is interpreted to mark the early Paleozoic continent/ocean boundary. The sequence is similar in both external and internal geometry to wedge-shaped features commonly observed in the narrow zone separating continental from oceanic basement on modern passive margins and interpreted as layered volcanic sequences. It is suggested that slope and rise sediments now exposed in the Benton Uplift were thrust in a “thin-skinned” fashion over the edge of the North American continent during initial stages of the Carboniferous Ouachita orogeny, but that later uplift and slight northward translation of the continental margin basement occurred in a more “thick-skinned” fashion, preserving the boundary wedge intact beneath the earlier thrust sheets.

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