Geology Reference
In-Depth Information
fetch of the Arabian Gulf. The lower wave exposure
leads to a different geomorphic character. Unlike east of
here, subtidal areas do not have oolitic deltas, but instead
include well-developed reefs offshore, perhaps refl ecting
the more limited fl ux of toxic lagoonal waters. Supratidal
islands landward of the Gulf shoreline are larger, more
equant (not âTâ shaped) (Fig.
19.16
left side), and pass
into the lagoon without a high-energy beach.
In the restricted, higher salinity, subtidal areas behind
the protection of the barrier islands all along this coast,
deposits are dominantly grey, reduced, muddy skeletal
sand with gastropods and foraminifera, sediments
broadly analogous to those found offshore in many areas
of the Bahamas. In exposed shallow subtidal to
lowermost intertidal regions, winnowing creates thin
deposits of muddy sand that are locally cemented to
crusts less than 10 cm thick, which can subsequently be
bored and fractured (Shinn
1986,
1969
) .
On the landward side of the lagoon are well-defi ned,
shore-parallel facies zones, from muddy subtidal
lagoonal sediments, to intertidal cemented gastropod-
rich sediments, to upper intertidal dark-colored lami-
nated cyanobacterial mats, intertidal to supratidal
evaporites and the sabkha (Fig.
19.16a
). As in the
Bahamas, intertidal deposits of this area have a distinct
change in character from the lower intertidal to upper
intertidal zone. Due to the larger tidal amplitude (up to
2 m in some areas, less in more restricted lagoonal
environments), however, intertidal deposits are thicker
here, with some accumulations reaching 2 m thick.
Lower intertidal deposits here are similar in many
ways to the underlying shallow subtidal deposits, and
include predominantly burrowed peloidal carbonate
mud with scattered skeletal fragments or thin sandy
layers. With the exception of laminar or irregular
fenestrae, sedimentary structures are generally not
preserved due to burrowing, and this lower intertidal
zone has few microbial mats, in large part due to the
activity of grazing cerithid gastropods.
Upper intertidal deposits differ considerably from
shallow subtidal and lower intertidal sediments (Shinn
1986
; Alsharhan and Kendall
2003
). In much of the area,
upper intertidal deposits include a distinct zone of well-
developed algal mats up to 30 cm thick. These mats pre-
serve thin, mm-scale, organic-rich laminations of
wind-blown eolian silt or fl ooding-related silt and sand.
The mats include a distinct vertical zonation (Kendall
and Skipwith
1968
), ranging from: (1) a warty black sur-
face (topographically lowest); (2) a mudcracked zone,
with cracks forming polygons that can exceed 1 m across;
(3) a crinkle zone with scattered gypsum; and (4) a zone
with a fl at, smooth algal surface (topographically high-
est). These mats may not extend into the supratidal zone
due to the intense evaporation and defl ation.
Supratidal sediments of sabkha in the United Arab
Emirates have a character transitional with upper inter-
tidal sediments. In many areas, the uppermost inter-
tidal zone is characterized by precipitation of a gypsum
mush interbedded with storm-deposited carbonates.
These deposits are fl anked by extremely fl at (slopes of
1:1,000) supratidal sabkhas characterized by anhydrite
and halite, mixed with eolian quartz-rich sand.
Trenches through this area reveal that the thin evapo-
rite layers commonly are deformed, buckled and bent
(ptygmatic-like folds). Butler (1970) suggested that
the evaporites and the deformation were the result of
evaporation of waters brought to the sabkha during
extreme fl ooding events. Between fl ooding events,
defl ation is pronounced and actively removes mud-
cracks and truncates folds in the evaporites.
As on Andros Island, the surfi cial environments in
this area have been changing in recent years
(Fig.
19.17
). Beyond the pronounced human imprint
(e.g., the canal just onshore, dredging and creation of
artifi cial islands - highlighted by white arrows in some
areas), several islands and mangrove swamps (reddish
areas, and red boxes in Fig.
19.17
) are expanding. The
aggradation refl ects a gradual infi lling of the lagoon by
progradation in the lee of the islands into the shallow
lagoon, rather than by building out from the mainland
shoreline (see below).
19.5
Facies Successions of Holocene
Carbonate Tidal Flats
The relatively few Holocene carbonate tidal flat
successions that have been cored and described in
detail collectively illustrate several consistent trends
in sedimentologic and stratigraphic features among
locations. To a large extent, these trends refl ect the
ubiquitous shallowing-upwards that occurs as a result
of aggradation and progradation outpacing rising sea
levels, and the close linkages between elevation and
depositional facies which occurs across tidal fl at sys-
tems in general. Nonetheless, bioturbation and root-
ing by mangroves in tidal fl at successions can modify
or destroy original depositional textures (Boudreau
