Sedimentary Features
In west coast Holocene stratigraphy sections show a rapid drop in sea level
associated with particular sedimentary features created by tsnami events
and not with storm surges.
The marsh sequence starts at the bottom with the Muddy/Sandy Layer.
This represents a low energy beach area which was probably in close proximity
to the coastline.
After the Muddy/Sandy Layer the strata grades into a Organic Mud which represents
the low energy accumulation of mud and organic debris over time. These layers
are usually quite thick and represent accumulations over lengthy periods
of time. Some where between 200 to 500 hundred years.
After the Mud/Sand facies, an abrupt change in the lithologies is marked
by a coarse sandy layer with larger clasts within it and sometimes a plant
debris on the top of the bed. These layers are typically not very thick
(from 2 to 5 centimetres) and represent a drastic and rapid advance of the
ocean over the land. This is direct evidence that a Tsnami swept over the
area depositing sand and followed by plant remains of which were most probably
torn up by the wave action. This can be also seen in present day earthquakes
along coastlines all over the planet.
After the Tsnami deposit the facies again abruptly change back to the Muddy
Organic which again are quite thick and represent extended periods of deposition.
As one sees in the stratagraphic profile the Tsnami deposits repeat over
time. This is evidence that this phenomenon has occured many times in the
past.
Scientists can then correlated the relative rate of deposition of the organic
mud layers between the Tsnami deposits and find the frequency of which the
Tsnamis' (earthquakes) occur.
Potential Risk
Cascadia Subduction Zone
How microfossils are used.
How big and how long before the next one?
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