Sediment characteristics at the delta of Chuckanut Creek, Mud Bay, Bellingham, Washington

Abstract

ud Bay is south of Bellingham, WA, in the northern portion of Chuckanut Bay. It is predicted to be inundated by tsunamis generated by large Cascadia earthquakes. In this report, I describe shallow sediments in the bay and at the delta where Chuckanut Creek enters the bay. I also describe shoreline changes recorded by historic charts and photographs. In these records, I do not find evidence for tsunami deposits. However, my observations may have some bearing on the sediment budget in Mud Bay. I find that historical charts show the progradation of a subaerial delta at the mouth of Chuckanut Creek from 1898 to present. I used grab samples, gouge cores, and test pits to collect samples from the banks of Chuckanut Creek, the delta, and the bay itself. There are several sandy horizons in the delta, with sand compositions similar to the glacial deposits through which the creek is incising. One distinctive horizon is traced through some additional input from the local bedrock, Chuckanut Sandstone. None of the sand horizons contain marine fossils. Two gouge cores from southeast corner of the bay show fine sand at 1 m depth abruptly coarsening at 80 cm depth and fining up toward the surface. Residents of Chuckanut Village on the shores of Mud Bay report that the bay has shallowed in the past 50 years, and I discuss my observations in the context of possible anthropogenic influences on sediment supply and retention. A companion study finds sedimentation rates up to 1 cm/year, suggesting that if tsunami deposits are preserved in Mud Bay, they are deeper than the sediments I sampled.