Geology Reference
In-Depth Information
Fig. 19.20
Schematic conceptual models of shoreline progradation and resultant facies patterns from different areas in the United
Arab Emirates. See text for discussion (
upper
- Modifi ed from Kenig et al.
1990
; the
lower
- Modifi ed from Shinn
1986
)
some areas (Kenig et al.
1990
) at the base of the
Holocene succession, reworked bioclastic and oolitic
deposits include clasts of Pleistocene eolianite in a unit
up to 1 m thick (Fig.
19.20a
). This unit is overlain by
transgressive intertidal microbial mat and mangrove
deposits up to 35 cm in thickness. The lowermost
15 cm of these contain well-preserved sedimentary
structures such as polygonal desiccation cracks and
mangrove root casts, but the upper parts are biotur-
bated, resulting in the loss of primary sedimentary
structures and nearly half of the original organic con-
tent. Above these microbial and mangrove deposits,
1-2 m of subtidal bioturbated mud contains seagrass
roots and leaves. Above these lagoonal deposits, a
40 cm thick interval of sand, silt and mud originally
deposited in non-vegetated, lower intertidal environ-
ments. This interval is overlain by 25 cm of organic-rich,
mangrove deposits with dense accumulations of root
and leaf material; roots deeply penetrate and disrupt
the underlying sand, silt and mud. The intertidal
package is capped by microbial mat deposits, contain-
ing fi ning-upwards laminae. This unit is typically
~35 cm thick, although the upper ~15 cm contain
gypsum precipitates which cause the destruction of
fi ne laminations and the organic matter. In contrast
with the more humid examples of the Bahamian
Archipelago, the regressive mangrove and microbial
mat facies show no evidence of post-depositional
reworking through bioturbation. Up to 1 m of
supratidal eolianite and evaporite deposits ('chicken-
wire' anhydrite, gypsum), typically containing negli-
gible organic matter, cap the succession.
In contrast, other regions that lack mangroves
exhibit a slightly different facies pattern (e.g. Shinn
1986
; Warren
1991
; Alsharhan and Kendall
2003
) ,
with a general shoaling upward trend (Fig.
19.20b
). In
these areas, basal Holocene brown eolian quartzose
carbonate sand (which can exceed 6 m in thickness) is
