Beechers Trilobite Bed - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 56

P ALEOECOLOGY OF B EECHER S
T RILOBITE B ED
Cisne (1973) made a detailed
paleoecological study of Beecher's
Trilobite Bed. He made thin sections of
the bed itself and the sediments
immediately below, and studied the
sedimentary structures and fauna ( 54 ).
He showed that immediately below the
40 mm (1.5 in) trilobite bed was a 30 mm
(1.25 in) layer of mudstone with
burrows, brachiopods, trilobites, and
abundant graptolites, which itself
overlays a siltstone. The trilobite bed
consists of a thin basal layer of coarse silt
which shows a sharp lower contact, which
then grades gradually into the finer
mudrock of the rest of the trilobite bed.
The basal layer is followed by a 40 mm
(1.5 in) laminated layer with some
brachiopods and trilobites, and which
has a rippled surface. The main trilobite-
rich layer within the trilobite bed is
about 2 mm (0.08 in) thick and has no
sedimentary or biogenic structures but is
packed with brachiopod, trilobite, and

graptolite fossils. Most of the rest of the
trilobite bed has very rare trilobites,
except near the top layer, which has
more fossils as well as burrows ( 62 ).
Cisne (1973) concluded that the
trilobite bed represented a micro-
turbidite. Turbidites are packets of
sediment which flow down an underwater
slope, e.g. the front of a delta, into deeper
water by gravity, and may be triggered by
an earthquake or similar event. They are
similar to avalanches on land. The coarse
detritus in the turbidite flow settles out
first, and nearest (proximal) to the origin
of the flow, whereas finer particles can
spread for many miles as a wide apron of
thin flow consisting of silt and mud-sized
particles. As they flow, turbidites erode
and scour the sea floor, leaving behind
tell-tale sole marks such as flute casts
on the base of their bed. When they come
to a stop, coarser particles settle out
first and then finer and finer muds.
For this reason they show grading from
a sharp, erosional base up into finer
muds at the top. Beecher's Trilobite Bed

62

62 A three-branched burrow YPM. Length of
longest branch about 3 cm 1.2 in.