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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