Abstract

Incised valleys are an important, but little studied, component of Upper Pennsylvanian cyclothems in the Mid-Continent. During the Upper Pennsylvanian, glacio-eustatic cycles resulted in alternate exposure and flooding of vast areas of the Mid-Continent. Near equatorial paleolatitudes and extensive chemical weathering of the exposed craton resulted in mud-rich siliciclastic sediment influx to drainage networks that included incised valleys. During transgressions, incised valleys flooded to form tide-dominated, muddy estuaries. The Douglas Group (Virgilian) in eastern Kansas contains several incised valley fills (IVF) and a range of tidal facies that can be best understood through comparison with modern deposits, mostly in macrotidal estuaries. Recognition of tidal influence in facies that are devoid of macrofossils is important for interpreting sea-level history, and lack of the recognition of tidal influence commonly leads to incorrect interpretations. Transitions from fluvial to estuarine facies, which were formed during marine flooding, can be recognized by the occurrence of mud-draped bedforms. Also the presence of facies that are likely the result of macrotidal conditions may have implications for paleogeographic reconstructions, because very high tidal ranges commonly occur in funnel-shaped estuaries. The IVFs occur throughout much of the Pennsylvanian in the Mid-Continent, and are probably even more common than previously has been recognized.

Several factors in addition to sea-level history influenced formation of IVFs. Climate may have had a profound influence. During wetter periods, rivers incised more deeply and developed extensive valley systems. During the drier periods, such as the Permian, valleys apparently did not develop at all. Other controls on valley morphology include bedrock lithologies exposed during incision, and basement structure.