1. Coastal Dune Soils in Oregon, USA, Forming Allophane and Gibbsite by
Georg H. Grathoff Curt D. Peterson Darren L. Beckstrand
Funded by:

2. Objective Determine the age and mechanism of dune emplacement
Determine whether soil/paleosol mineralogy can constrain age of emplacement

3. Study Area

4. Importance of the paleosols
Influence stability
Control groundwater chemistry and flow
Indicate age

5. Dunal Paleosols

6. “Weeping sands”

8. Study Area

9. Map of TL and 14C dates in ka BP. Core names are in parentheses
Map of TL and 14C dates in ka BP. Core names are in parentheses. (from Beckstrand 2001)

10. Eustatic sea level fluctuations and absolute elevations
Eustatic sea level fluctuations and absolute elevations. Chappell and Shackleton (1986) (A), Shackleton (1987) (B), and Bloom and Yonekura (1985) (C). Boxes indicated probable errors of age and height related to curve C. Modified from Pirazzoli (1993). From Beckstrand; 2001

11. Results: Minerals in Dunal Soils
Gibbsite: Al(OH)3
Allophane/Imogolite: amorphous Al-Silicate
Vermiculite and/or chlorite
Fe oxides/hydroxides

12. 1) Gibbsite

14. X-ray Diffraction results
Gibbsite
002
Gibbsite
110
5.35 Å
200
Gibbsite

16. 2) Allophane replacing tree roots
Seal Rock State Park

17. Allophane/Imogolite replacing whole pieces of tree

19. Allophane replacing wood
X-ray Diffraction
Allophane replacing wood
Blue: Allophane
Green: Opal-a

20. X-ray Diffraction

21. XRD data from Wada (1989)

22. Chemistry Si:Al ratio
This study (microprobe):
1: 1.3 to 1:1.8
Wada (1989):
allophane 1:1 to 1:1.5
imogolite 1:1.7 to 1:1.8

23. TGA analysis of tree replacement
Derivative of weight loss
Weight loss 50
Temperature (°C)
900

24. TGA and DTA data for
allophane and imogolite
(Wada 1989)

25. Conclusion Gibbsite Allophane/imogolite Indication of high Al mobility
forms under highly permeable conditions (change in pH)
Fills cavities not replacing minerals
Allophane/imogolite
Replaces woody material
One phase or two phases
Indication of high Al mobility
Where does the silica go (kinetics?)