Geology Reference
In-Depth Information
a
Anhydrite
Upper
intertidal
Lower
intertidal
pond
Tidal
channel
Back barrier
tidal flat
Gypsum
Lenticular
gypsum
Wavy/planar
stromatolite
Burrowed
sediment
Ooid
tidal delta
b
Tidal
channel
Intertidal
flat
Supratidal
levee
Pond
Supratidal
beach ridge
Wavy/planar
stromatolite
Burrowed
sediment
Fig. 21.2
Diagrams showing major depositional settings of
modern arid (Persian Gulf) and humid (Bahamas) tidal systems
(Modifi ed from Shinn
1983a
) . (
a
) Major subtidal and tidal fl at
facies and sedimentary environments of the southern Persian
Gulf inner ramp setting. An ooidal barrier island separates the
open sea from the quiet back barrier subtidal lagoon. Note the
development of ebb ooid tidal delta and a narrow tidal fl at in the
back of the barrier island. Note also the presence of bioturbated
sediment in the lower intertidal, lenticular gypsum bearing pla-
nar/wavy stromatolite in the upper intertidal and gypsum/anhy-
drite deposits in the supratidal zone. (
b
) Major facies and
environments of the Andros Island tidal fl at system. Supratidal
zones are shown in
light brown
. Note the burrowed deposits in
the intertidal and planar/wavy stromatolite in the supratidal
freshwater marsh environments, respectively. Note also the pres-
ence of beach ridge at the seaward edge of intertidal zone, and
the supratidal areas on the beach ridge and tidal channel levees.
Abbreviations:
HT
high tide,
LT
low tide
or strong current energy) may destroy primary fabrics
and structures of sediments; intensity of bioturbation
and skeletal diversity decrease with increasing salinity.
In various environments of a tidal fl at system, current
energy and grain size decreases in a landward direction
and various processes, such as desiccation, cementation
and dolomitization operate. In various environments of
a tidal system, binding and trapping of sediment and
carbonate precipitation by bacteria and cyanobacteria
lead to microbial deposits (see the following section).
In the tidal fl at setting, evaporation results in higher
salinity leading to cementation and primary micritic
dolomite formation. Early cementation prevents com-
paction of soft and muddy sediments, leading to
preservation of peloids, intraclasts and primary sedi-
mentary fabrics and structures.
The high energy conditions of the platform margin
result in the development of barrier reefs and/or car-
bonate sand shoals, which separate the often restricted
back-barrier environments from the open sea. As a
result of tidal current activity, the margin is normally
dissected by tidal channels that connect the open-ocean
water with that of the quiet back-barrier subtidal
environment. Depending on tidal regime, the mouths
of these channels may develop tidal deltas. In the Abu
Dhabi region of southern Persian Gulf, for example,
