This apparent anomaly is best explained if we consider the Buildex ichnofauna as produced by a terrestrial and freshwater fauna (Buatois et al., 1997). The relatively high diversity of ichnofossils probably records a secondary peak of diversity of organisms from freshwater environments that contrasts with the minimum diversity of brackish-water ecosystems (Remane and Schlieper, 1971; Ekdale, 1988; Hudson, 1990; Pickerill and Brenchley, 1991). The taxonomic composition of the arthropod-dominated Buildex ichnofauna is also suggestive of terrestrial and freshwater faunas. The assemblage actually represents a mixture of the nonmarine Scoyenia and Mermia ichnofacies, rather than the characteristically marine Cruziana and Skolithos ichnofacies. No ichnotaxa unequivocally indicative of marine settings are present. In contrast, some of the forms recorded from Buildex are typical of nonmarine environments (e.g., Undichna, Stiaria, Stiallia, Dendroidichnites, Mirandaichnium). In addition to taxonomic composition, the overall aspect of the ichnofauna suggests the activity of nonmarine organisms. Features other than ichnodiversity and taxonomic composition that suggest a terrestrial-freshwater origin include dominance of surface or shallow subsurface trace fossils, absence of infaunal burrows, and a mixture of arthropod trackways and nonspecialized grazing traces.
The presence of a mixed freshwater-terrestrial ichnofauna in these deposits seems to conflict with evidence of tidal activity. However, in estuarine systems, tidal influence commonly extends further landward than the saltwater intrusion (Dalrymple et al., 1992). For example, Allen (1991) noted that in the Gironde Estuary, the maximum limit of tidal currents is located approximately 50 km (31 mi) landward of the saltwater intrusion; therefore, the channels of the upper estuary are rarely affected by brackish water. A similar situation was documented from the Cobequid Bay-Salmon River estuary (Bay of Fundy), suggested as a modern analogue of the Tonganoxie paleovalley by Lanier et al. (1993) and Archer, Lanier et al. (1994), where the most proximal region of the inner estuary experiences essentially freshwater conditions (Dalrymple et al., 1991).
The influence of salinity on the composition of invertebrate faunas and intensity of bioturbation also was noted in these studies (Allen, 1991; Dalrymple et al., 1991). In these modern environments, no marine or brackish faunas were detected in the inner estuary, where tidal flats were essentially unbioturbated. On the other hand, infaunal brackish-water organisms, which occur in the middle estuary, thoroughly churned sediments.
Ichnologic evidence from the Buildex Quarry is consistent with deposition on tidal flats in the most proximal zone of the inner estuary (inner end of segment 2 in the terminology of Dalrymple et al., 1994 and Zaitlin et al., 1994) under essentially freshwater conditions. More specifically, this zone is situated between the maximum landward limit of tidal currents and the salinity limit further towards the sea (fig. 29). The freshwater organisms inhabiting this area do not have the adaptations necessary to survive in a brackish environment; therefore, they are restricted to the innermost zone of the paleovalley (cf., Wolff, 1983). Observations on the Buildex ichnofauna show that although lithofacies distribution within the paleovalley is, for the most part, salinity-independent, the distribution of organisms is not. Accordingly, ichnologic studies of estuarine systems can provide the high-resolution results necessary to delineate fluvio-estuarine transitions and may help to refine facies models constructed only on the basis of physical sedimentary structures.
Fig. 29. Plan view of the Tonganoxie estuarine paleovalley, showing location of the Buildex ichnofauna and relationship with salinity and tidal limits (facies model adapted from Archer, Feldman et al., 1994). Lighter shading represents sandy sediments; darker shading represents silt and mud.
Kansas Geological Survey
Web version March 19, 1998
http://www.kgs.ku.edu/Current/1998/buatois/buatois8.html
email:lbrosius@kgs.ku.edu