Geology Reference
In-Depth Information
The shallowest parts of the crests of these ridges
can be exposed at low tide. Many ridge crests are
ornamented with small simple dunes up to 1 m tall,
and include the best sorted and most oolitic sedi-
ments on these bars. Crests can have early marine
cementation, which can facilitate the growth of corals
or other organisms that require a hard substrate to
grow. Ridges have steep fl anks, locally at the angle
of repose, although a systematic asymmetry is
absent in many tidal sand ridge fi elds, unlike that
which occurs in some siliciclastic examples (e.g.
Houbolt
1968
) .
Transverse shoulder bars
are asymmetric, straight
to slightly sinuous crested bars, with crests oriented
roughly normal to the dominant tidal fl ow, which in
many cases is roughly normal to the shelf margin
(Fig.
20.8b
). These bars can reach up to several meters
in height (generally less than tidal sand ridges, how-
ever), and have a distinct steep side (up to the angle
of repose) and a more gently sloping fl ank (<2°).
Nonetheless, because they are normal to the shelf
margin, shoulder bars generally are at high angles to
either ebb or fl ood tidal currents, and one tide will be
dominant across much of the barform (Hine
1977
;
Rankey et al.
2006
) , enhancing their asymmetry.
These features can be quite large; on Lily Bank, indi-
vidual shoulder bars extend along strike for 5 km and
reach up to 2 km in breadth, and in the western parts
of the Schooner Cays shoal complex, shoulder bars
can exceed 30 km long and 5 km in breadth. These
features can have up to 5 m relief.
The crests of many shoulder barforms are not
exposed at low tide. Elevated tidal fl ow velocities lead
to active sediment transport over the crest. As with
tidal sand ridges, the crests of shoulder bars are orna-
mented with symmetrical small sandy dunes up to 1 m
tall, and include some of the best-sorted oolitic sands
in the system. These dunes can be oriented at a range
of orientations, from normal to fl ow (Rankey et al.
2006
), to oblique to fl ow, to sinuous, depending on the
details of the patterns of ebb and fl ood tides across the
crest. Although scattered cemented zones occur there,
the crests of shoulder bars of Lily Bank shoal complex
do not have widespread hardgrounds, probably also
related to the active sand transport. Sedimentologically,
shoulder bar crests have the best-sorted and most
oolitic sediments, and they pass laterally into channels
with less-well sorted sands and silts that can include
up to 20% mud (Rankey et al.
2006
).
Parabolic bars
are complex, lobate features with
variable crest orientations that change systematically
by more than 90°. Broadly speaking, and as used here,
parabolic bars include both tidal deltas (Fig.
20.8c
),
frequently associated with bedrock highs in the
Bahamas, and open-marine forms not associated with
any local
a priori
restriction, because the morphology
and sedimentology of both are strongly infl uenced by
mutually exclusive fl ood- and ebb-dominated conduits
(Fig.
20.8d
; van Veen
1936
; Rankey et al.
2006
; Reeder
and Rankey
2008
). Although perhaps broadly analo-
gous, we avoid the use of “spillover lobes” because of
the implicit (or explicit) implication that these are
created by storms (Ball
1967
; Hine
1977
; Wanless
and Tedesco
1993
) (see discussion below).
With the exception of tidal deltas anchored by bed-
rock highs, parabolic bars do not occur as isolated fea-
tures; in many cases, the fl ank of fl ood parabolic bars
form the margin of adjacent ebb parabolic bars and vice
versa, forming a sinuous-crested barform (Fig.
20.8d
;
cf. Caston
1972
). Parabolic bars have crests with dis-
tance from fl ood-oriented apex to ebb-oriented apex
(amplitude, in map-view) ranging from 0.3 to 3.0 km
and aperture (width) between 0.5 and 3.0 km; mean
amplitude: aperture ratios for sets of parabolic bars
range from 1:2 (in tidal deltas of the Abacos) to almost
1:1 (in the southern TOTO area) (Rankey et al.
2006
) .
Parabolic bars can include up to ~5 m bathymetric
relief, although most are between 2 and 4 m. In most
cases, bar crests remain submerged by 0.5 m or more
at low tide (e.g., Lily Bank; Abaco tidal deltas, Ocean
Cay tidal deltas), but in some, bars have aggraded and
now form stabilized islands (e.g., Schooner Cays).
Linked parabolic bars pair fl ood-dominant bars with
ebb-dominated bars; commonly, this pair is orna-
mented by subaqueous dunes up to 1 m tall with sys-
tematically variable orientations that refl ect the
reversing ebb- and fl ood-tides that fl ow over their
crests (Rankey et al.
2006
) .
As with the other barforms, the best-sorted and most
ooid-rich sediments occur on the crests of parabolic
barforms. In many open-marine parabolic bars (such as
those on Lily Bank or Schooner Cays), the crests pass
laterally into poorly sorted muddy, skeletal-peloidal
sands in grass-stabilized lows between the bars.
Although bar crests pass laterally into less well-sorted,
less oolitic sediments in Bahamian tidal deltas, they
differ markedly in that the deepest parts of the inlets
at the center of the parabolic tidal deltas have hard
