--> Abstract: Neoichnological Trends at the Fluvial-Tidal Transition of the Columbia River Delta, Northwest USA, by Robynn Dicks, Shahin E. Dashtgard, John R. Suter, and Murray Gingras; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Neoichnological Trends at the Fluvial-Tidal Transition of the Columbia River Delta, Northwest USA

Robynn Dicks1; Shahin E. Dashtgard2; John R. Suter3; Murray Gingras1

(1) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.

(2) Earth Sciences, Simon Fraser University, Burnaby, BC, Canada.

(3) ConocoPhillips, Calgary, AB, Canada.

The Columbia River Delta, northwest U.S.A. is a complex depositional environment, wherein the neoichnological and sedimentological character of fluvially dominated, tide-influenced brackish-water subenvironments can be established. Although several neoichnological studies have considered bioturbation in brackish-water, the neoichnological characterization of very-low-salinity fluvial-tidal settings has not been presented in the literature.

The aim of this study is to identify and interpret neoichnological trends of tide-influenced bars along a longitudinal transect of the Columbia River Delta (i.e., from the fluvial-dominated through to the tide-dominated regions of the estuary). The collected dataset includes: ichnological observations from x-rays and sediment peels, grain-size analyses, total organic carbon (TOC), and salinity concentrations. The first-order control on ichnological distributions appears to be salinity, which decreases very gradually landwards. Trace assemblages throughout the Columbia River Delta are typical of the Teichichnus Ichnofacies (c.f. Pemberton et al., 2009), and the diversity of traces decreases landwards. Near the mouth of the delta (i.e., 5 km inland from the Pacific Ocean), at salinities of 12 ppt, trace diversity is comparably high, and includes modern burrows akin to Skolithos, Arenicolites, Polykladichnus, Palaeophycus, and Siphonichnus. Twenty to 40 km landward, salinity reaches only 2 ppt, but the river still experiences notable tides. The trace assemblage here is lower in diversity, and includes Skolithos, Arenicolites, and Planolites; although in some sheltered locales, the trace assemblage also includes Thalassinoides and Polykladichnus. Locally, very high bioturbation intensities are observed. Fifty km inland, fluvial processes progressively dominate with maximum salinities of approximately 0.5 ppt. At this landward position, the trace assemblage includes Skolithos, Arenicolites, Planolites, and locally Siphonichnus.

The longitudinal mapping of neoichnological distributions of the Columbia River distributary reveals a decrease in trace diversity with decreasing salinity. At near-freshwater salinities, a brackish-water trace assemblage is still prominent, providing evidence for marine larval recruitment into fluvial-dominated regions of the delta.