The Burgess Shale, discovered by
Charles Doolittle, contains a diverse range of animals. It is located
in British Columbia's, Yoho National Park, in the Canadian Rocky
Mountains. The fossil bearing shales are found in a 150 m high and 2 m
thick unit, called the Stephen Formation which consists of Walcott and
Raymond's quarry. These strata were formed 520 million years ago during
the Cambrian Period.
The preservation of the soft bodied fauna is extraordinary. Usually the
soft parts decay away leaving no fossil evidence of the animal's
existence. The Burgess strata, and their constituent fauna, were
deposited at the base of an algal reef in a deep-water basin. Unstable
mud and turbidity currents caused periodic mud slumps. These
transported the organisms down the sloping banks, to be deposited into
anoxic basin waters. These conditions allowed the soft body parts to be
preserved as the decay rate was slow and there were no scavengers or
predators. Compacted by overlying mud, the fauna is preserved as thin
carbon films in graded units consisting of fine mud layers.
During the Mesozoic era the
Stephen Formation, including the Burgess Shale was transported along a
thrust fault to the present location. This contributed to the
extraordinary preservation by allowing protection from heat and
pressures of nearby tectonic activities.
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