Geology and history of the Burgess Shale

In Cambrian times, the Burgess Shale was at the base of a tall undersea cliff called the Cathedral Escarpment which at that time (around 510 million years ago) was on the edge of the Laurentia continental plate (or craton) which was located at the equator.

Canada today	Burgess Shale formation 510 mya

Mud and creatures from the shallow sea floors at the top of the cliff was periodically swept over the edge, falling to deeper water where the creatures were buried and preserved as fossils. Unusually some of the fossils are preserved as carbon films which means that the fossils are formed from the original creatures themselves, rather than the more usual replacement by minerals (Ref 1.)

The exact cause of this unusual preservation is unknown, but the deeper water would have been colder and less oxygenated than the shallow waters above and being covered with fine mud would have both protected the creatures and further reduced oxygen levels. The particular chemistry of the site would have played a role in the preservation process and studies of similar sites in China have indicated that this process is linked to the sediment being silicate, rather than carbonate based (Ref 2.) Surprisingly, it is also possible that bacteria may be the cause of the unusually good preservation (Ref 3.)

Note that deep sea fossils from the Cambrian period are rare because plate tectonics creates new ocean crust at oceanic ridges and destroys it by subduction, mostly at continental boundaries. Oceanic crust sinks below continental crust because it is denser. This process means that most of the ocean floor is replaced roughly every 200 million years. Since the period we are interested in is around 540 to 490 mya and life was restricted to only the oceans at that time, it means that most of the potentially fossil bearing ocean floor has been recycled several times since then.

The oldest deep ocean floor is currently only about 180 million years old as the ocean crust age maps show. The ancient ocean floor locations which escaped destruction by subduction are those which were actually the underwater edges of continental plates.

More information about the Burgess Shale geology and history, both ancient and modern, is available from the The Burgess Shale Geoscience Foundation web pages and the process by which geologists date sites like this is explained by NewScientist.