Geology Reference
In-Depth Information
Fig. 19.21
Schematic of evolution of sabkhas on Trucial Coast
(Modifi ed from Purser and Evans
1973
) . (
a
) Offshore Pleistocene
islands (
black
), separated from the mainland (also
black
) by a
trough, the Khor al Bazm lagoon. (
b
) Formation of beaches
(
yellow
) of bioclastic sand on windward side of islands, tidal
fl ats (
gray
) on leeward side. Sand beaches form on the mainland
shoreline (see Fig.
19.16a
). (
c
) Beaches expand laterally due to
longshore currents, tidal fl ats (
grey
) fronted by microbial mats
(
red
) nucleate on mainland once wave energy is suffi ciently
restricted. (
d
) Suffi cient restriction occurs between islands leads
to development of oolitic tidal deltas (
fuzzy dots
). Coral reefs
grow oceanward of islands, protected from toxic lagoon waters.
Tails of islands continue to accrete landward and lagoons gradu-
ally infi l l
overlain by grey, subtidal burrowed, sparsely fossilif-
erous mu that in turn grades upwards into slightly win-
nowed muddy skeletal sand, silt and mud (Evans et al.
1969
; Shinn
1986
). In the absence of the disrupting
infl uence of mangroves, the upper parts of these sandy,
shallow subtidal deposits can be well cemented, with
syndepositional polygonal fractures (teepees) and bor-
ings. This succession is overlain by 1-2 m of peloidal
mud and organic-rich, laminated mud with prism
cracks and fenestrae, corresponding to the well-
developed microbial mat zones (e.g. Kendall and
Skipwith
1968
). Although many surface mats include
mudcracks, Shinn (
1986
) noted that mudcracks are
rare in core from this area, likely due to erosion by
wind. This succession is in turn overlain by a complex
succession of evaporite and carbonate that can include
nodules, “chicken-wire” texture, or contorted beds
(“gyspsum mush zone,” Fig.
19.20
) (Warren
1991
) ,
locally truncated by defl ation (e.g., Kirkham
1997
) .
Cross-bedded eolian quartz or carbonate sand or supratidal
mud, the sabkha deposits, locally caps the succession.
At a larger scale, the geologic history of this coast-
line is quite complex (Fig.
19.21
) (Kirkham
1998
;
Walkden and Williams
1998
) . Purser and Evans (
1973
)
suggested that the general geomorphic confi guration
was established by a pre-Holocene, westerly plunging
structural depression (presently manifest as the Khor
al Bazm lagoon to the west of the area of Fig.
19.16
),
fl anked to the south by the mainland, and to the north
by a subtle offshore high (Fig.
19.21a
). These subtle
highs provided enough relief for establishment of
patch reefs, which then produced abundant sediment,
and led to expansion of the islands by wind and current
action (Fig.
19.21b
). With continued leeward expansion,
