Sedimentology of a Tidally-Influenced Fluvial Zone (TIFZ): Columbia River, USA

Abstract:

This paper quantifies, for the first time, the surface morphology and subsurface structure of deposits in the tidally-influenced fluvial zone (TIFZ) of the Columbia River, NW USA. Channel and bed morphology are quantified using multi-beam echosounding, with the subsurface sedimentology revealed using Ground Penetrating Radar and a Parametric Echosounder, combined with shallow (< 5m) cores.

Data from the landward, fluvially-dominated, tidally-altered region is characterised by medium-scale dune deposits that are indistinguishable from those of purely fluvial origin. The central minimum “energy flux divergence” (EFD) region displays a highly complex structure. This comprises classic signatures associated with the turbidity maximum that occur away from the main channel in distributaries, backwater bays, tidal creeks and bar remnants (e.g. increased proportion of silt, and prevalence of long, low-angle lateral accretion surfaces associated with fine deposition on point bars; Inclined Heterolithic Stratification). However, closer to the main channel in the landward section of the minimum EFD region an anabranching planform dominates and bar vertical grain size trends display a coarse-fine-coarse succession. These bars are characterised by coarser-grained dune deposits at their base, pervasive sand-silt in the central part of the bar with smaller sand dunes on the surface or steep bar margin sets in the bartails. The finer-grained part of this succession is interpreted as a subtle tidal influence on relatively stable bars with vertical accretion that undergoes reworking by wave action.

In the seaward part of the minimum EFD region, close to the main channel, long, low-angle surfaces predominate, although some steep bar margin and dune deposits are still present. Silts are found as laminae within sand deposits, and dunes in this zone are small, more symmetrical, or absent entirely.

Overall, the sedimentology of the Columbia River TIFZ does not correspond well to other published models because of its relatively coarse grain size, anabranching planform, greater main channel flow velocities and entrenched fluvial/deltaic environment. More typical TIFZ facies described in the literature are largely restricted to embayments and in the very seaward part of the minimum EFD region.