Geology Reference
In-Depth Information
Fig. 19.18
(
Upper
) General stratigraphic patterns in the tidal
fl ats of Three Creeks area, Andros Island, Bahamas. In this
region, the tidal fl at complex (including part of the nearshore
marine region) is underlain by a transgressive algal marsh depos-
its, suggesting a long-term onlapping pattern (Modifi ed from
Hardie
1977
) . (
Lower
) Generalized core descriptions and inter-
pretations from a suite of subenvironments, Andros tidal fl ats.
To the right of each core is a “Dunham” texture related to amount
and support of mud versus grains, but without a “-stone” suffi x.
These are generally arranged from offshore (
left
) to onshore (
right
),
but come from several areas along the shore, so no horizontal scale
is implied. Each of the cores penetrated to bedrock except the
beach ridge core (Redrafted from core descriptions graciously pro-
vided by Paul Enos, from original descriptions in 1965)
include scattered (transgressive) marsh deposits at the
base. These sediments would be overlain by subtidal
bioturbated peloid mud deposits, with gastropods and
foraminifera. Areas with continued shallowing could
include marsh or levee deposits, if near a creek
(Fig.
19.18
). Nonetheless, at a larger scale, if the creeks
and levees are not migrating markedly, aggradation and
pond fi lling (Fig.
19.7c
) may instead facilitate progra-
dation of the inland marsh over the channeled belt.
grained sediments that can exceed 2 m thickness
(Fig.
19.19
). Across the area, ~20-30 cm of organic-
rich mud with conspicuous mangrove root fragments
and gravel- to pebble-sized intraclasts and lithoclasts
lie directly atop the underlying Pleistocene bedrock,
interpreted to represent transgressive reworking and
initial mangrove colonization. Soft, white mud with
foraminifera and rare
Halimeda
plates overlie this
basal unit, suggesting continued relative rise in sea
level (Fig.
19.19b
). The homogeneity of the subtidal
mud-rich unit may partly refl ect deep burrowing or
mangrove rhizoturbation (Fig.
19.19c
), which may
obscure a clear turnaround. In more landward areas, this
subtidal unit is only 20-30 cm thick, but it thickens plat-
formward, and can exceed 2 m in the present-day
19.5.2 Crooked Island
The Holocene tidal-fl at package on Crooked Island
includes a platformward-thickening wedge of fi ne-
