Florissant - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 201

The stratigraphic column ( 223 ) shows
alternations of lake shales with ash-fall
tuffs, conglomerates derived from lahars,
and stream deposits of mudstone and
sandstone.
In detail, the Florissant Formation can
be subdivided into six informal units
(Evanoff et al ., 2001). The lower shale unit,
representing a lake deposit, is followed by
the lower mudstone unit, which represents
a river system. Within the lower mudstone
unit are some lenses of river channel
sandstone and at the top is the lahar
deposit with the buried tree stumps. The
succeeding middle shale unit represents a
second lake which became established in
the Florissant Valley. Following the middle
shale unit is the caprock conglomerate
unit which represents a second lahar or
debris flow; so-called because of its
hardness, it protects the softer shales
beneath from weathering away more
quickly. The upper shale unit follows
the caprock conglomerate, and was formed
when normal lake conditions became re-
established in the valley following the lahar
event. The topmost unit is the upper
pumice conglomerate, which formed as
rivers draining the volcanic hinterland
filled the lake with pebbles of pumice and
effectively ended Lake Florissant.
The succession in the Florissant
Formation can be considered as three
major cycles each starting with the
establishment of a lake which gradually
infilled with sediment; followed by the
development of a stream valley with fluvial
deposits which were later covered by a lahar
flow; and then re-establishment of a lake
again. Between the massive lahar flows,
layers of tuff resulting from volcanic ash
falls can be seen within the shale sequences.
Lahar and ash fall cycles are, of course
unpredictable in their timing. However,
within the lake shales we can also see much
finer cyclicity, possibly on an annual scale.
The shales split easily into extremely thin
sheets, so are called 'paper shales'. Under
the electron microscope, each lamination
can be seen to consist of a couplet: a layer of
diatoms (microscopic, single-
celled algae) and a layer of clay formed

from volcanic ash. Each layer is about
0.1-1.0 mm (0.004-0.04 in) in thickness.
The diatomaceous layer represents an algal
bloom, which may have been seasonal.
The bloom would have been enhanced
by the influx of siliceous ash because
diatoms make their skeletons from silica,
and an algal bloom usually ends when
overpopulation of the algal cells use up the
available nutrients and space and pollute
the water, resulting in a massive die-off.
The diatomaceous layer is the result of this
die-off. McLeroy and Anderson (1966)
counted the number of couplets in the
shales and, assuming that each couplet
represents an annual cycle, estimated that
the lake lasted for 2500-5000 years.
When an algal bloom is in full swing,
the sheer numbers of diatoms cause a
stress reaction in the cells which then
produce a mucilaginous substance, or
slime. Early ideas regarding the cause of
death of the many insects and spiders in
the Florissant lake considered it was the
volume of volcanic ash, poisonous gases,
or hot water (Licht, 1986) which caused
the demise of these creatures. More recent
studies (Harding and Chant, 2000;
O'Brien et al ., 2002), using the electron
microscope, showed that it is the microbial
slime which is more likely to have
contributed to preservation of the fauna
and flora. Soft tissues decay readily, and so
need to be protected from decaying agents
in order to stand a chance of being
preserved in sedimentary rocks. Insects
and leaves landing on the surface water of
the lake during a diatom bloom would
have become covered in microbial slime
which acts as a sealant, protecting the soft
parts from decay. Eventually, the weight of
the mat caused it to sink to the lake floor,
taking its entombed fauna and flora with
it. Successive layers of diatomaceous mat
and ash falls built up, and lahar flows
buried the shales and compressed the
rocks and fossils to the flattened state we
see when splitting the rocks today.
Any later sediments which might have
buried the Florissant Formation have been
stripped away. Later in the Tertiary, a great
uplift affected the whole southern Rocky