Geology Reference
In-Depth Information
Fig. 19.19
General stratigraphic patterns in the tidal fl ats south
of Crooked Island, Bahamas. (
a
) Stratigraphic cross-section.
Here, a lower layer of organic-rich intraclastic and lithoclastic
mud (transgressive deposits) overlies bedrock and is in turn
overlain by an offl apping, muddy succession. (
b
) Subtidal
deposits, including the most diverse sediments, such as
Halimeda
, foraminifera, and gastropods. (
c
) Bioturbated sub-
tidal deposits, with common mangrove roots. (
d
) Intertidal
deposits. Note the
pale yellow
/
buff
color, and the abundant
gastropods (more
orangish
color, see
white arrows
). Mangrove
roots are highlighted by the
yellow arrows
. (
e
) Intertidal-
supratidal deposits, illustrating upper
Scytonema
pincushion
(warped due to dessication during core preparation), two thin
crusts (slightly
lighter
colored, noted by the
white arrows
).
A sole mangrove root is highlighted by the
yellow arrow
.
Modifi ed from Berkeley and Rankey (submitted)
subtidal realm. Towards the platform interior, the basal
transgressive lag and thick subtidal peloid-skeletal
mud pass up into fi ne sand with conspicuous molluscs
(especially cerithid gastropods) (Fig.
19.19d
), collec-
tively interpreted as intertidal deposits. Above the sub-
tidal unit in landward areas is a ~60-cm-thick
succession of interbedded thin (~1 cm) indurated hori-
zons (Fig.
19.19e
) with angular, gravel-sized intrac-
lasts and ~40 cm of fi ne sand, refl ecting shallowing
from upper intertidal to supratidal deposition.
Generalized facies patterns (Fig.
19.19
) suggest that
this system has included marked shoreface prograda-
tion, a notion consistent with measured accumulation
rates greater in the intertidal (~0.28 cm year
−1
) than
supratidal (~0.04 cm year
−1
) environments (Berkeley
and Rankey, in review).
19.5.3 Abu Dhabi
The stratigraphy of the Abu Dhabi sabkhas indicate
extensive shoreline progradation (schematically indicated
in Fig.
19.20
), although the details of the facies patterns
vary (Alsharhan and Kendall
2003
) . For example, in
