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The fact that the overlying Eldon
Formation passes uninterrupted across
this vertical contact shows that it is not a
tectonic fault, but instead that it was an
original feature of the Cambrian seabed,
i.e., a near-vertical submarine cliff. It is
significant that the Burgess Shale fossils
are always found at the foot of this
submarine cliff.
The conventional interpretation is that
this cliff represents the margins of an algal
reef, the top of which formed a shallow
carbonate platform with well-lit waters free
of terrigenous sediment. The Burgess
animals lived on or in the mud in the
deeper, darker water at the foot of the
cliff. The seabed sloped away from the cliff
into still deeper water which was anoxic
and hostile to life.
Various pieces of evidence suggest that
the Burgess animals were not preserved in
the area in which they were living. The
first is the lateral continuity over large
distances of the thin beds of shale, with no
evidence of bioturbation of the sediment
by crawling or burrowing organisms. The
second is the fact that the Burgess fossils
are found lying at all possible angles
within the shale, some even head-first into
the sediment. Finally, examination of the
thin shale beds in Walcott's Quarry reveals
that each shows a definite fining-upwards
sequence with coarse, orange layers at the
base and finer, dark grey layers above, such
that each bed represents a separate event,
i.e. a separate influx of sediment (
30
).
The accepted theory of the deposition
of the Burgess animals is that from time to
time storms, earth movements, or simply
instability of the wet sediment pile sent the
mud at the foot of the cliff down into the
hostile basin in a rapid cloud of sediment,
carrying with it the unsuspecting animals
which had no time to escape. Conway
Morris (1986, Figure 1) illustrated a 'pre-
slide' and 'post-slide' environment, the
former representing the area in which the
animals were living, and the latter the area
to which they were transported and
preserved (now represented by Walcott's
Quarry). Both appear to have been close
to the foot of the cliff, suggesting that the
turbidity currents flowed downslope
parallel to the cliff. The slide was followed
by quiet conditions allowing the fine
sediment to settle, giving rise to the fine
layering of the shale seen today.
How the Burgess Shale animals have
been preserved is still not entirely known.
Two of the common prerequisites for soft-
tissue preservation were undoubtedly
partly responsible, namely rapid,
catastrophic burial in a fine sediment, and
deposition on a sea floor deficient in
oxygen. Such a toxic 'post-slide'
environment would have excluded
scavengers, and when the cloud of
sediment settled the carcasses would have
been completely entombed with their
body cavities infilled by mud. However,
anaerobic microbes can break down soft
muscle tissue relatively quickly even in the
absence of oxygen, and some other factor
must have prevented such microbial
action.
Butterfield (1995) isolated tissues of
various soft-bodied Burgess animals and
showed that in many cases they were
composed of altered original organic
carbon. This was coated, however, by a
thin film of calcium aluminosilicate,
similar to mica, explaining the silvery
appearance of the Burgess fauna under
incident light. He suggested that this
coating originated from the clay minerals
in the mud in which the animals were
buried, and that these minerals inhibited
bacterial decay, perhaps by preventing
reactions of enzymes. This type of
preservation is exceptional; normally very
soft tissue such as muscle and intestine can
only be preserved if it is replaced by
another mineral during early diagenesis.
D
ESCRIPTION OF THE
B
URGESS
S
HALE BIOTA
VauxiaPhylum Porifera
Vauxia
is a bush-like, branching sponge
(
32
), which does not have discrete spicules,
but is composed of a tough spongin-like
framework, explaining why it is the most
common of the Burgess sponges.
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