Geology Reference
In-Depth Information
Coastal Stratigraphy: Cascadia
Coast
Inland
Saltwater
marsh site
Spruce
swamp site
Lowland
river site
tideflat mud
roots of saltwater
tideflat plants
spruce snag
tsunami sand layer
thinning and fining
tufted grasses sticking
into sand layer
well
developed soil
~300 yr BP
river silt
tideflat mud
weak soil
tsunami sand?
~1700 yr BP
well developed soil
A
Seismic Cycle Depositional Envrionments
rapid
coseismic
subsidence
well developed soil
well developed soil
weak soil
Forest
Marsh
Tideflat
earthquake
possible earthquake
earthquake
B
2400
2000
1600
1200
800
400
0
Years Before Present
Fig. 6.15 Stratigraphic record of subduction-related submergence and emergence.
A. Schematic stratigraphy of buried soils and associated plants in saltwater tideflats, spruce swamps, and riverine settings
along the Washington coast above the Cascadian subduction zone. Abrupt drowning of trees and burial of grasses in
growth position result from coseismic submergence and an accompanying tsunami. Buried soils can indicate coseismic
submergence, but also could result from slow, interseismic subsidence. B. Schematic chronology of depositional
environments created by subduction-zone deformation across several seismic cycles. Modified after Atwater (1992).
address what had been a major unsolved ques-
tion: Is there a major seismic hazard associated
with this rather slow subduction zone? In the
mid-1980s, Thatcher (1986a) wrote: “No great
earthquake has been recorded off the Washington
coast and the hypothesis that subduction occurs
seismically will not be proven until one does.”
Today, as a result of stratigraphic studies by
Atwater and co-workers (Atwater, 1992; Atwater
et al. , 1991), we now know that Cascadia produced
very large subduction-zone earthquakes as recently
as 300 years ago.
In estuaries along the coastal areas of
Washington and Oregon, several superposed,
buried lowland soils crop out at low tide
(Fig. 6.15). The estuaries are typically within
Search WWH ::




Custom Search