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Conclusions and References

The current working hypothesis is that the clay was deposited in a lake that formed on top of the Grande Ronde basalt flow during a hiatus in volcanic activity. At some point, the lake dried up and a low diversity assemblage of trees colonized the exposed lakebed. The site was subsequently reflooded, and the drowned trees were entombed when the Ginkgo Flow of the Frenchman Springs basalt inundated the lake. Evidence for this includes the following:

  • Presence of a fine, silty clay layer underlying the wood-bearing basalt indicates deposition in a low energy water environment.
  • Low diversity forest with closely spaced, small diameter trees and narrow growth rings indicate colonization of disturbed/new habitat and growth in stressed conditions.
  • Modern relatives of the tree genera found in the fossil deposit are commonly found in wet soils, riparian or floodplain deposits.
  • The presence of palagonite and basalt pillows indicates that the basalt was emplaced in a water environment.

Further work
Additional samples will be obtained to confirm the presence of Liquidambar and Gleditsia. Work is in progress to further understand the depositional environment of the clay layer, including a palynological analysis. If pollen can be successfully extracted, a study will be conducted to determine the constituents of the pollen rain. This will give a more comprehensive picture of the regional forest assemblage.

Acknowledgements
The author gratefully acknowledges the support of those who have assisted in this investigation, most notably Clyde Friend of Yakima, Washington for access to his property and the donation of wood specimens for thin section analysis. Pete Wald of the University of Washington assisted with the geological study; George Mustoe and Terry Tolan provided valuable help with data analysis and interpretation.

References

Beck, G.F. 1945. Ancient Forest Trees of the Sagebrush Area in Central Washington. Journal of Forestry, vol. 43, no. 5. pp 334-338

Beeson, Marvin H., K.R. Fecht, S.P. Reidel, T.L. Tolan. 1985. Regional correlations within the Frenchman Springs Member of the Columbia River Basalt Group: New insights into the middle Miocene tectonics of northwestern Oregon. Oregon Geology, Vol 74, No. 8. pp 87-96.

Brown, Claud L. and L. K. Kirkman. 1990. Trees of Georgia and Adjacent States. Timber Press, Portland, OR. 292 pp.

Camp, V.E. 1995. Mid-Miocene propagation of the Yellowstone mantle plume head beneath the Columbia River basalt source region. Geology, vol. 23, no. 5. pp 435-438

Camp, V.E. and M.E. Ross. 2004. Mantle dynamics and genesis of mafic magmatism in the intermontane Pacific Northwest. Journal of Geophysical Research, vol. 109, B08204. pp 1- 14.

Campbell, Newell P. 1989. Structural and stratigraphic interpretation of rocks under the Yakima fold belt, Columbia Basin, based on recent surface mapping and well data. in Reidel, S.P. and Hooper, P.R., eds., Volcanism and tectonism in the Columbia River flood-basalt province: Boulder, Colorado, Geological Society of America, Special Paper 239, pp 209-222.

Coffin, Harold G. 1987. Sonar and scuba survey of a submerged allochthonous "forest" in Spirit Lake, Washington. Palaios, vol. 2. pp 178-180.

Hooper, Peter R. 2000. Chemical discrimination of Columbia River basalt flows. Geochemisty, Geophysics, Geosystems, Vol. 1. Paper number 2000GC000040.

Reidel, S.P., Tolan, T.L., Hooper, P.R., Beeson, M.H., Fecht, K.R., Bentley, R.D., and Anderson, J.L. 1989. The Grande Ronde Basalt, Columbia River Basalt Group; Stratigraphic descriptions and correlations in Washington, Oregon, and Idaho. in Reidel, S.P., and Hooper, P.R., eds, Volcanism and Tectonism in the Columbia River flood-basalt province: Boulder, CO, Geological Society of America, Special Paper 239.

Schweingruber, Fritz H. 1996. Tree Rings and Environment. Dendroecology. Birmensdorf, Swiss Federal Institute for Forest, Snow and Landscape Research. Berne, Stuttgart, Vienna, Haupt. 609pp.

Tolan, T.L.; Reidel, S.P, et al. 1989. Revisions to the estimates of the areal extent and volume of the Columbia River Basalt Group, in Reidel, S.P. and Hooper, P.R., eds., Volcanism and tectonism in the Columbia River flood-basalt province: Boulder, Colorado, Geological Society of America, Special Paper 239, pp 1-20

Tolan, T.L., Reidel, S.P., and Fecht, K.R., 1991, The unusual occurrence of fossil logs within middle Miocene flood-basalt pillow lava complex - an examination of geological events and processes that created the "Vantage Forest" of central Washington State: EOS (Transactions of the American Geophysical Union), v. 72, no. 44, p. 602.

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