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Depositional Environment

Since the Yakima Ridge deposit is in the same basalt flow complex as the well-known fossil wood assemblage at Vantage, it is interesting to contrast the two localities. As stated earlier, the Yakima Ridge deposit is a low diversity assemblage of trees preserved in life position. This indicates that the deposit represents a forest assemblage of local trees that were entombed at or near the place where they grew. In contrast, the Vantage deposits represent a highly diverse assemblage of trees preserved as prostrate logs and jumbled log rafts. While the genera that make up the Yakima Ridge deposit all have modern representatives that live in similar habitats (i.e. riparian/floodplain), the genera found at the Vantage deposit represent a diversity of habitats, from riparian to upland. Professor G.F. Beck of Central Washington University postulated that the deposit at Vantage represented a river drainage that carried the logs from various upriver areas into a large lake where they were waterlogged and subsequently buried in the basalt (Beck, 1945). Later workers have examined the sediments associated with the Vantage deposit and suggest that large accumulations of logs were carried into the Vantage area by a volcanic lahar that stripped trees from the surrounding countryside (Tolan, et al. 1991).

One interesting aspect of the Yakima Ridge deposit is the lack of root traces in the underlying clay layer. The presence of standing trees in the basalt invites speculation that clay represents a dried lakebed or a paleosol that the trees grew into. This is difficult to prove, since the tree trunks terminate abruptly at the clay/basalt interface with no indication of any roots penetrating into the clay. It may have been that the clay was waterlogged and nutrient poor, thus the trees were shallow rooted and one would not expect visible root traces in the clay. This might explain the narrow, tightly spaced growth rings in the wood indicating significant growing stress. At this point, it is not possible to be certain that the trees grew in the exact positions in which they were fossilized. They may have been transported after being entombed in the basalt, although it is unlikely that there was any significant transport due to the fact that all but two of the trees are in a vertical orientation, they are relatively unbroken, and many small diameter boles are preserved intact. Harold Coffin of Loma Linda University studied trees in Spirit Lake after the 1980 eruption of Mount St. Helens in Washington State, and observed a high proportion of logs floating vertically in the lake. He advanced this as an explanation to explain supposedly in situ fossil forests in Yellowstone Park and other places (Coffin, 1987). It is unlikely that his observations would account for the preservation at the Yakima Ridge site, since he noted that approximately 40% of the logs in Spirit Lake were horizontal, and many of the upright logs were held in that position by a balance between the weight of the root mass and the buoyancy of the wood. Since there are no root masses associated with the Yakima Ridge trees and only two of the logs were horizontal, the site does not fit his model.

Log mold in basalt
View of log mold in basalt. Note basal swelling of trunk and lack of root traces in clay layer.

Hickory (Carya sp.) log encased in basalt flow.

Next read about the Conclusion and References or return to the Miocene Forest main page.

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