Geology Reference
In-Depth Information
1
(a)
Suspension
transport
Bed load
transport
0.1
No movement
0.01
0.1
Grain diameter (mm)
1
10
Fig. 2.
Entrainment diagram for quartz sands. When there is
suffi cient wave or current energy to move grains fi ner than
200
(b)
m, then there is also enough turbulence to transport
them in suspension (
τ
= shear stress;
ρ
s
= density of grain;
ρ
= density of fl uid; and
D
= grain diameter). Modifi ed from
McCave (1970).
at the bottom of the North Sea, showed that grains
fi ner than 200 μm tend to move in suspension
(Fig. 2). Specifi cally, when there is enough energy
to move particles fi ner than about 200 μm, there
is enough turbulence to put them in suspension.
So even in hurricanes surges, except within about
100 m of the shore, the tidal fl at layers are nearly
entirely composed of less than 200 μm, suspen-
sion-transported sand. Risi
et al
. (1995) found that
this was true for the 1-10 cm thick layer swept
10 km inland through the broad mangrove swamp
of southwest Florida during Hurricane Andrew,
a Category 5 storm in 1992.
Beaches display this principle well. Sand fi ner
than 200 μm will not stay on a beach because
whenever the sand moves (by water or wind),
it will move in suspension and be carried from
the beach. The minimum grain size of nearly all
beaches is around 200 μm. In areas with higher
persistent energy, the bedload boundary (what
remains on the beach) will shift to the coarser
grain size as indicated in Fig. 2. The rapid ero-
sion of many beach renourishment projects is
the result of ignoring this fundamental behaviour
of sands.
Grain size explains a lot about the origin of
stratifi cation in the modern and ancient environ-
ment, whether one is looking at the centimetre-
thick sand and shale beds in the Cambrian Bright
Angel Shale in the Grand Canyon, water-density-
driven canal sedimentation in Marco Island,
Florida, or the sediments washing off to the fl anks
of Great Bahama Bank during storms. Rapid sedi-
ment dispersal of suspension-transported sands
can, with suffi cient sediment supply, totally
(c)
Fig. 1.
With less frequent sedimentation events on
carbonate tidal fl ats, the tufted cyanobacteria,
Scytonema
has an increasing infl uence on layering form. (a) Frequent
sedimentation events permit only the beginning of tufts
to form and the layering takes on a crinkly appearance.
(b) With more time between sedimentation events tufts
become more pronounced and disrupt layer continuity.
(c) In areas removed from signifi cant sedimentation
events for years, a pronounced tufted cushion mat forms
and adsorbs minor sedimentation events. Millimetre or
thicker sediment layers (light) smother and kill the mat
which will take 0.5-1 year to recolonize. The decayed
tufted mat and its contained sediment form the thicker,
darker layers in the sequence. Andros Island tidal fl ats,
Bahamas.
