Pleistocene forest preserved in Oregon coast sediments

W. D. Smyth, R.A. Hart, Oregon State University
P. J. Reimer, Lawrence Livermore National Laboratory

This note documents the discovery of a paleosol located near Cape Perpetua, Oregon, containing well-preserved tree trunks dated at prior to 50Ka. The forest appears to have been buried during a debris flow, preserved due to anoxia, and finally exposed by wave erosion of the surrounding sediments.

The exposure is located at 124.108W, 44.298N, about one km south of the town of Yachats. A 10m-high sediment bank occupies the north slope of a creek mouth that empties into a small embayment of the Pacific Ocean. The bank is composed of a homogeneous mixture of sand and angular clasts with diameters up to ~10cm. Underlying the bank is the Yachats Basalt bedrock. Enclosed within the bank, about 1m above the bedrock, is a layer of horizontally oriented tree trunks. The largest trunks have diameter near 40cm, and the maximum thickness of the wood layer is about 1m. The layer extends for about 20m along the bank. Thick, ruddy bark suggests that the trees are spruce, a common species on the modern Oregon coast. Above the paleosol is an 8-10m thick layer of sediment atop which runs the Coast Highway (figs. 1, 2). Both the horizontal orientation of the largest trunks and the homogeneity of the overlying sediments suggest that the forest was buried in a catastrophic landslide. Such events may occur, for example, in association with the subduction zone earthquakes that occur every few hundred years on the Oregon coast (e.g. Darienzo & Peterson, 1995). The other likely source of horizontally oriented logs is driftwood (e.g. Komar 1998, fig 2.5), but the presence of thick bark would appear to rule out this possibility.

The sediments containing the paleosol are near the bottom of a wave-cut platform formed around 80Ka ago during a previous interglacial high stand  (Lund 1972).Two wood samples have been radiocarbon dated at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. Each sample was dated twice. The first sample gave an initial date of 44690 ± 1100 RCYBP. After dissection to reduce possible contamination by modern material, a second run yielded 49900 RCYBP.  As this is within two standard deviations of the detection limit, it is considered a lower bound (Stuiver & Pollack 1977). The second sample was taken from a higher elevation in the wood layer, but gave an earlier date, suggesting that some modern contamination remained in the first sample. Two measurements of the second sample yielded 55500 and 54750 RCYBP, which are again considered a lower bound due to their proximity to the detection limit.

These results suggest that the forest lived at least 50K years ago, and possibly much earlier. Further work is underway to compare this paleosol with other, similar features on the south Oregon coast.

 

Figure 1: Paleosol and overlying sediments. Coast highway 101 runs atop the bank.

Figure 2: Close view of paleosol and surrounding sediments. The yellow handle of the increment borer is visible.

References

Darienzo, M. E., and Peterson, C. D., 1995, Magnitude and frequency of subduction-zone earthquakes along the northern Oregon coast in the past 3,000 years, Oregon Geology 57, 3-12.
Komar, P., 1998: The Pacific Northwest Coast, Duke University Press, Durham, NC.
Lund, E.H., 1972, Coastal landforms between Yachats and Newport, Oregon: The Ore Bin, 34 (5), 73‑91.
Stuiver, M. and H. A. Pollack, 1977, Discussion: reporting of 14C data, Radiocarbon 19(3), 355-363.