Ichnology of Micro-Tidal Incised-Valley Fill Deposits: A Comparison of the Ichnology of a Cretaceous Ferron Sandstone Incised Valley System, East-Central Utah and the Neoichnology of the Pleistocene-Holocene Nueces River Incised Valley, Texas Gulf Coast

Low tidal ranges and tidal current energy along micro-tidal coastlines result in barrier islands that contain few perennial tidal inlets which conduit normal salinity marine waters into back barrier lagoons and estuaries. Therefore, both infaunal and epifaunal organisms are subject to a high degree of environmental stress. An analysis of the ichnology of an incised valley within the Cretaceous Ferron Sandstone of the Western Interior with the neoichnology of the Holocene Nueces Incised Valley estuary, Texas Gulf Coast provides insight into the effects of substrate, water energy, salinity variability and climate on the behavior, abundance, and diversity of burrowing and trace-making benthic invertebrates and implications for sedimentological and paleoecological interpretations.

This study concludes that substrate composition, water salinity (and gradient), and hydrologic energy are the dominant controls on neoichnofaunal abundance and diversity. Medium to low energy, sand-prone facies, such as bayhead deltas, cross-valley spits and bay marine tidal flats, are dominated by a low diversity opportunistic fauna that would produce burrows and traces that could be assigned to either Skolithos or a mixed Skolithos/Cruziana ichnofacies. The low energy, muddy sand facies of the abandoned portions of the bayhead delta and the muddy facies of the shallow central basin contain a more diverse equilibrium fauna that produces burrows and traces that would be assigned to the Cruziana ichnofacies.

Since micro-tidal estuarine and barrier island/lagoon systems are stressed ecosystems, it is essential when working in ancient micro-tidal facies, to continually evaluate the trophic and environmental factors controlling the ecology of benthic organisms and condition ichnological and sedimentological interpretations with that information.