Abstract

Pennsylvanian-age tidal bedforms have been generally undocumented in the Illinois Basin of the eastern U.S.A. It has been argued by some researchers that tidal currents were not strong enough to transport and deposit sediments in such a cratonic basin. Two Pennsylvanian-age tidal deposits in the Illinois Basin are described herein, both formed in strongly asymmetrical tidal regimes. One is characterized by large-scale tidal bundles (1.5 m thick); the other is characterized by planar-laminated siltstones and small-scale ripple-sized tidal bundles. Bedforms associated with these deposits were little modified by subordinate tidal currents. Analyses of the tidal bundle sequences indicate that both units were deposited in semidiurnal tidal systems, one from ebb-dominant tides, the other from flood-dominant tides. These deposits are an indication of the size range of tidal bedforms in the Illinois Basin.