Geology Reference
In-Depth Information
Fig. 19.6
Field photos of representative supratidal environ-
ments and sediments, Bahamian Archipelago. (
a
) Eroding
supratidal shoreline of southwest Andros Island. The shoreline
here is stabilized by grasses, which pass (northeast, to the
right
)
into cabbage palm hammocks. (
b
) Supdatidal plain of blackened
crusts, thin microbial mats, and small, stunted mangroves,
Crooked Island. (
c
) Close-up of the continuous cemented crust.
Crooked Island. The slightly darker areas are covered with a
thin, soft microbial mat; the lighter areas are cemented crust.
(
d
) Thick laminations, common in the broad supratidal inland
marsh, Andros Island. (
e
) Edge of cemented pavement, and
the adjacent cabbage palm hammock, southwest Andros Island.
(
f
) Coarse rubble of cemented clasts from a cemented pavement
southwest Andros Island
so extensive that they were used as landing strips for
small planes in more rough-and-tumble days.
neled belt, and adjacent marine. The inland marsh
(supratidal plain) comprises a more-or-less continuous
microbial mat up to 6 km wide and 25 km long that
fi lls an arcuate embayment in Pleistocene outcrops
(Fig.
19.7a
). This mat is penetrated only by a few very
shallow (<10-20 cm) and narrow (meter scale) creeks
and is extremely fl at. Some creeks represent continua-
tions of depressions in bedrock from the island to the
east, and water in these can be fresh to brackish.
The channeled belt, the 'heart' of the tidal fl at, lies
between the inland algal marsh and the offshore marine
zones (Fig.
19.7b
). This area, up to 5 km wide, includes
a complex network of tidal creeks, levees, ponds, and
algal marshes, mostly dominated by muddy sediments.
Near their emergence on the coast, tidal creeks are
fl anked by elevated levees (up to ~200 m wide, though
most are much less) that gradually slope down and out-
ward through an algal marsh zone into mangroves and
open ponds (Fig.
19.7b
). As the creeks reach head-
ward, levees are less well developed until the creeks
gradually become narrow, shallow gullies within the
mangroves and are impenetrable by boat. Although
studies of modern (Shinn et al.
1969
) and ancient
(Cloyd et al.
1990
) tidal fl ats suggest the importance of
tidal channel migration, little evidence for pronounced
19.3.2 Contrasts in Tidal Flat
Geomorphology - Bahamian
Archipelago
The tidal fl ats from the Three Creeks area are among
the best studied carbonate fl ats in the world and repre-
sent what is commonly portrayed as the 'humid chan-
neled belt' morphotype (e.g., Wright
1984
; Shinn
1986
). Although informative, they are not necessarily
representative; there are several other geomorphically
distinct types of tidal fl ats in this region.
19.3.2.1 Three Creeks Area, Andros Island,
Bahamas
The 'classic' Andros Island tidal fl ats (Black
1933
;
Shinn et al.
1969
; Gebelein
1974
; Hardie
1977,
1986
;
Shinn
1986
) occur in the Three Creeks area of
northwest Andros Island (Fig.
19.1
), where they form
an onlapping wedge of muddy subtidal, intertidal and
supratidal sediments. Shinn et al. (
1969
) recognized
three general geomorphic zones: inland marsh, chan-
