Coastal Dune Soils in Oregon, USA, Forming Allophane and Gibbsite

by

Georg H. GrathoffCurt D. Peterson

Darren L. Beckstrand

Funded by:

Objective

• Determine the age and mechanism of dune emplacement

• Determine whether soil/paleosol mineralogy can constrain age of emplacement

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Dunesheets

Sample Locations#

Figure 1. Study area dune sheets in Oregon, SW Washington and N. California, and context map.Sample locations are where dune characteristics have already been collected by this research group, funding provided by Seagrant and USGS.Bathymetry shows 20 meter contours.

Columbia River

Oregon

Washington

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ific

Oce

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CLAT

NEHA

PACI

LINC

NEWP

FLOR

COOS

BAND

GOLD

LONG

GRAYNORT

CRESCalifornia

Study Area

Importance of the paleosols

• Influence stability

• Control groundwater chemistry and flow

• Indicate age

Dunal Paleosols

“Weeping sands”

http://nwdata.geol.pdx.edu/seagrant/oregon/newport_locations/images/P6280020_JPG.jpg

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Dunesheets

Sample Locations#

Figure 1. Study area dune sheets in Oregon, SW Washington and N. California, and context map.Sample locations are where dune characteristics have already been collected by this research group, funding provided by Seagrant and USGS.Bathymetry shows 20 meter contours.

Columbia River

Oregon

Washington

Pac

ific

Oce

an

CLAT

NEHA

PACI

LINC

NEWP

FLOR

COOS

BAND

GOLD

LONG

GRAYNORT

CRESCalifornia

Study Area

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

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

Results:Minerals in Dunal Soils

1) Gibbsite: Al(OH)3

2) Allophane/Imogolite: amorphous Al-Silicate

3) Vermiculite and/or chlorite

4) Fe oxides/hydroxides

1) Gibbsite

X-ray Diffraction results

Gibbsite002

Gibbsite110

Gibbsite

5.35 Å 200

2) Allophane replacing tree roots

Seal Rock State Park

Allophane/Imogolite replacing whole pieces of tree

Allophane replacing woodBlue: Allophane

Green: Opal-a

X-ray Diffraction

X-ray Diffraction

XRD data from Wada (1989)

Chemistry

Si:Al ratio

This study (microprobe):1: 1.3 to 1:1.8

Wada (1989):allophane 1:1 to 1:1.5imogolite 1:1.7 to 1:1.8

TGA analysis of tree replacement

Temperature (°C)50 900

Weight loss

Derivative of weight loss

TGA and DTA data forallophane and imogolite(Wada 1989)

Conclusion

• Gibbsite – 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?)