Geology Reference
In-Depth Information
Prior to canal construction, Lake Ingraham
and the Southern Lakes (Figs 1 and 5a) were iso-
lated fresh- to brackish-water lakes, receiving
saline water only during storm surges (Simons &
Ogden, 1998). Connecting Lake Ingraham and the
Southern Lakes to the marine realm permitted
saltwater intrusion during the dry months, and a
combination of saltwater intrusion and freshwater
discharge during the rainy months. The opening
of the canals is interpreted to have triggered col-
lapse of the interior freshwater marsh and initiated
a phase of increasingly rapid marine sedimenta-
tion in areas reached by fl ood tidal waters. The
change from fresh-brackish to marine sediment
provides a visible marker horizon in the sediment
sequence, permitting calculations of thickness
and rate of historical sedimentation.
and thus the tidal volume, expands. It is expected
that with steadily rising sea level, marine water
will spill more frequently across the ridge, feeding
a progressively larger tidal volume.
Geomorphological patterns and rate of sediment
accumulation
Where connected to the ocean, the coastal bays
and lakes within Cape Sable are rapidly shallow-
ing. First to infi ll have been the Southern Lakes
adjacent to ECC (Fig. 5b). By 1953 these lakes were
fi lled to form an intertidal mud fl at dissected by
tidal creeks, features that carried sediment-laden
waters to the lakes' inner recesses. Red mangrove
forests subsequently spread across these fl ats,
capping a shallowing-upwards succession (man-
groves overlying organic-rich mud, overlying
medium grey, shelly subtidal mud, with indica-
tive brackish water mollusc species).
As the Southern Lakes fi lled and ECC and HC
widened, sediment delivery became focused
inward into Lake Ingraham (Fig. 6), illustrated by
the rapid expansion of the fl ood tidal delta and
infi lling of Lake Ingraham since 1953 (Fig. 6b
and c). Aerial photographs since 1990 indicate
that the rate of delta growth has progressively
increased over the past two decades. At present,
the southern Lake Ingraham fl ood delta forms a
sediment body over 4 km long by 1-1.5 km wide
and 50-90 cm in thickness. The delta elevation
is highest near the southeastern entrance to the
lagoon and along the margin of the axial channel.
The primary channel-margin levee remains emer-
gent even at the lower high tides and is becoming
colonized with mangrove seedlings towards its
southeastern (ECC) end.
The channel-margin levee constriction has
promoted numerous side creeks extending per-
pendicular to the main channel towards the
lake margins. These secondary channels have dis-
crete sediment lobes that broaden the delta as a
whole. One pronounced side creek connects HSC
West to the axial delta channel and is maintained
by ebb tidal discharge from HSC West (Fig. 6c).
As waters overfl ow the relict marl ridge from Lake
Ingraham into the interior marsh regularly (at
least 80 times a year), muddy overwash sediments
are also extending from the ridge into the shallow
waters of the collapsed interior marsh (Fig. 7).
The nature of these spatial changes is refi ned
by observations of
in situ
sedimentation patterns
in Lake Ingraham. Short-term sedimentation
Hurricanes
Two intense hurricanes profoundly modifi ed
Cape Sable: the Labor Day Hurricane of 1935, a
category 5 storm on the Saffi r-Simpson scale, and
Hurricane Donna in September 1960, a strong cat-
egory 4 storm. Since the 1928 aerial photograph,
the southern coast facing Florida Bay has eroded
about 180 m (Fig. 5). As documented by aerial
photographs, erosion occurred in two nearly equal
steps of approximately 75 m, one between 1928
and 1953 (Fig. 5a and b) and a second between
1953 and 1964 (Fig. 5b and c). These two steps
in erosion are interpreted to be the result of the
1935 and 1960 hurricanes. Only a very small
amount of erosion has occurred on the south shore
since 1964.
Sea-level rise
Since 1932, South Florida has experienced an
accelerated rate of sea-level rise of ~23 cm 100 yr
−1
(Wanless, 1982; Wanless
et al
., 1988; Douglas,
1991), almost six times greater than that of the
previous 2400 yr (~4 cm 100 yr
−1
) (Wanless, 1982).
This rate of sea-level rise is determined from a
continuous series of tide gauge records starting
in 1913 from Key West (Maul & Martin, 1993)
and it does not take into account the estimated
Florida subsidence rate of ~0.01 cm yr
−1
(Rona &
Clay, 1966).
At present, the slightly elevated marl ridge,
~0.5 m above MSL, is signifi cantly overtopped by
saline fl ood tide waters that have a (predicted) tidal
level of 1.28 m, which occurs during 80 high tides
a year. During these events the extent of the tide,
