The geology of the Washington margin

Accreted sediments along the Cascadia

subduction zone contain high

concentrations of organic matter, which is converted to methane.

Where does the organic material come from?

Organic material formed from photosynthesis and other biological activity settles on the sea floor. This material trapped within the thrust complexes within the formed accretionary wedge.

faults

McNeill, L.C., K.A. Piper, C. Goldfinger, L.D. Kulm, and R.S. Yeats (1997). Listric normal faulting on the Cascadia continental margin. J. Geophys. Res. 102 (B6): 12,123-12,138.

The image above is from McNeill et al (1997) and right image is EM300 swath bathymetry from the WA margin in 2007; showing faults that appear to be associated with methane emission sites on the WA shelf.

Faulting possibly associated with methane emissions

48° 00' N

rock outcrop

0 - 5

5 - 10

10 - 20

20 - 30

30 - 40

Thickness of Mid-Shelf Holocene Mud Layer (m)

10 0 10 Miles

1 inch represents 7.89 milesSCALE 1:500,000

Projection - UTM 10 Datum - NAD83

10 0 10 Kilometers

125° 00' W 124° 00' W 123° 00' W

46° 00' N

47° 00' N

48° 00' N

125° 00' W 124° 00' W 123° 00' W

Willapa Bay

GraysHarbor

Washington

Columbia River

Queets River

Oregon

Quinault River

Willapa Canyon

Grays Canyon

Quinault Canyon

GuideCanyon

Astoria Canyon

Vancouver Island

Strait of Juan de Fuca

MoclipsRiver

PointGrenville

RaftRiver

HohRiver

HohHead

TillamookHead

TillamookBay

CopalisRiver

Juan de Fuca Canyon

Bathymetric contour (interval = 10 m)

50100

150

50

150

50100

150200

10

9 Nittrouer estimate of sedimentthickness in meters

Figure 1. Isopach map of the Mid-Shelf Silt Deposit compiled from pre-existing USGS seismic survey data. Bathymetry contours generated by Michael R. Hamer from NOS hydrographic soundings obtained from NOAA

.

3

PACIFIC

OCEAN

Flattening and truncation of the margin sediments during low sea stands during glacial periods has exposed the folded sediment layers.

Mid-shelf

mud deposit

Grays Canyon sponge reef and methane plume site

Olympic Marine Sanctuary

Site of Interest in relation to the Olympic

Marine Sanctuary