Neoichnology and Sedimentology of the Fluvial-Tidal Transition Zone of the Columbia River Delta, Northwest U.S.A

The Columbia River Delta, northwest U.S.A., is a complex depositional environment at the mouth of the second largest United States’ river. Through the study of tidal sand bars within the fluvial-tidal transition, neoichnological and sedimentological characteristics of the mixed-energy brackish-water setting were established. Neoichnological analysis determined trace assemblages of the area are consistent with the Teichichnus ichnofacies, with the most intense burrowing found along the bar tops and intertidal zone. Additionally, the ichnogenera burrowing depth, density and burrow diameter decrease moving up-river, and there is larval tidal recruitment of marine trace-makers into the oligohaline zone. Sedimentological analysis of the dataset led to the identification of six facies for the tidal bars of the Columbia River Delta, which were synthesized into one facies association. The more obvious sedimentological tidal indicators are not present in the representative facies and are much more subtle, encompassing changes in flow regime within a single facies.