Although lithofacies distribution within estuarine valleys is mostly salinity-independent, the distribution of organisms is not. Accordingly, ichnologic analysis of estuarine systems can provide the high-resolution data necessary to delineate fluvio-estuarine transitions and may help to refine facies models based only on physical evidence.
In terms of sequence stratigraphy, Buildex-type ichnofaunas characterize the lower part of the transgressive system tract, immediately overlying a coplanar surface. As transgression proceeds, tide-influenced freshwater facies are replaced by backstepping brackish-water deposits, and the mixed Scoyenia and Mermia ichnofacies may be replaced by a Skolithos and impoverished Cruziana ichnofacies. The coplanar surface (flooding surface and sequence boundary) at the base of the Tonganoxie sequence at Buildex Quarry lacks the Glossifungites ichnofacies, which is restricted to the limits of marine influence within the valley system. The coals and paleosols with upright plant remains that typify the surface represent erosional truncation and nondeposition close to the valley interfluves and may be regarded as the landward equivalent of the Glossifungites ichnofacies.
Comparisons with other trace fossil assemblages from marginal marine environments suggest that Buildex-type ichnofaunas are widespread in Pennsylvanian tidal rhythmites of the United States Midcontinent. Recognition of such ichnofaunas may be useful in the identification of freshwater inner estuarine facies and help to delineate fluvioestuarine transitions in incised valley systems.
Kansas Geological Survey
Web version March 19, 1998