Geology Reference
In-Depth Information
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shrimp burrows, red mangrove rooting).
These deposits can be indistinguishable from under-
lying subtidal deposits, especially with gradational
contacts due to aggradation from subtidal up into
lower intertidal elevations, and downward penetra-
tion of mangrove roots.
Marked changes in depositional conditions occur
between lower intertidal and upper intertidal environ-
ments, causing pronounced differences in biota and
sediment facies. In particular, halophyte grasses (espe-
cially
Spartina
), shrubs, and black mangroves
(
Avicennia sp
.
nitida?
) appear in the upper intertidal
zone, red mangroves are less abundant, and the sedi-
ment surface is covered by microbial mats (
Scytonema
pincushions at lower elevations Fig.
19.4d
; mixed with
Schizothryx
mats at higher levels, Fig.
19.4e
). Short
cores from upper intertidal zone reveal that, due to the
decreased density of vegetation and burrowing, sedi-
mentary structures are more commonly preserved.
Sediments typically are a light buff color, and include
fenestrae (multigranular-roofed pores; Shinn
1983a
) ,
desiccation cracks and resultant intraclasts. These
occur in lenticular layers with mm- to cm-scale lami-
nations. Gastropods, foraminifera, and polychaete
worms are quite uncommon relative to their high abun-
dance in the mangrove and open ponds. Cemented
crusts are locally abundant in the intertidal zone, asso-
ciated with
Scytonema
(Fig.
19.4f
).
Supratidal
regions lie above mean high tide, and are
fl ooded only during spring tides, storms, or prolonged
onshore winds. Supratidal areas from across the archi-
pelago include creek-adjacent levees, beach ridges,
and supratidal plains. Some tidal fl ats in the Bahamas
(parts of Andros, Crooked-Acklins), include hammocks,
supratidal areas which support cabbage palm trees and
grasses, and are essentially terrestrial.
Supratidal deposits include the most diagnostic
suite of sedimentary structures on the tidal fl at. On
parts of the Andros tidal fl at, supratidal levees fl ank the
larger creeks near their outlet to the open ocean,
and are bounded by an abrupt channel margin with
abundant
Uca
crab burrows (Fig.
19.5a
). These levees
typically have a sparse vegetation cover (black
mangrove, scattered grasses and shrubs) (Fig.
19.5b
),
and are covered with a distinct continuous, fi rm mat of
Schizothryx
(Fig.
19.5c-e
). Although “air domes”
(incipient fenestrae; Fig.
19.5d
) and small, shallow
mud cracks occur on the levee backslope, on the levee
crest, this mat is tightly bound and mud cracks are
absent. Short cores reveal that the sediments include
mm-scale discontinuous normally graded laminations
of peloidal silt and fi ne sand (Fig.
19.5e
).
Beach ridges with grainy deposits occur on the
shorelines of some tidal fl ats, most notably those of
the northwest-facing parts of Andros tidal fl ats and the
east-facing appendages of Crooked-Acklins and Caicos
tidal fl ats. These ridges commonly have an erosional
oceanward face with small, scalloped embayments,
irregular at a decameter scale (Fig.
19.5f
). This face
cuts into fi ne, laminated peloid-skeletal sand
(Fig.
19.5g, h
), and locally is covered by thin coarse
skeletal debris in the shoreline recesses (Fig.
19.5h
). In
numerous locations on the northwest Andros tidal fl ats
near Three Creeks, thin, platy cemented crusts, similar
to those found in the marsh on the beach-ridge back-
slope, occur within the erosional faces. Beach ridges
parallel the shoreline and are generally less than 100 m
wide, but reach up to ~250 m in a few exceptional
cases; in scattered locations, discontinuous wedges of
storm-derived coarse skeletal sand and gravel extend
landward from the shoreline (Fig.
19.5i
). The beach
ridges pass landward and downdip into marshes and
ponds either abruptly (in the presence of sandy skeletal
wedges) or gradationally (mimicking the lateral
changes from levees down to ponds).
In contrast to the beaches on the northwest-facing
tidal fl ats of Andros Island, the shorelines of the
southwest-facing Andros tidal fl ats are oblique to both
normal trade winds and the brisk winds from south-
eastward-moving cold fronts. These areas include
supratidal beach ridges with a different character.
Along much of this coast, the shoreface is essentially a
low scarp with up to 1 m relief (Fig.
19.6a
) cutting into
gastropod-bearing muddy carbonates. The scarp is
capped with grasses, bushes, and small trees, indicat-
ing that it is rarely fl ooded, and gravels are
uncommon.
Inboard supratidal plains form some of the most are-
ally extensive regions on the tidal fl ats, perhaps best
developed on parts of Andros complexes and on the
Crooked Island system. These features are extremely
fl at; gentle gradients of 10 cm/km (1:10,000) (Hardie
1977
), with only subtle undulations, are not uncommon
(Fig.
19.6b
). These areas are covered by a vast expanse
of dark
Scytonema
pincushions (Andros) or scattered
pincushions with a hardened crust (Crooked Island,
Fig.
19.6c
), with only rare stunted black mangroves and
scattered grass. Flooding is limited to major storms.
