Geology Reference
In-Depth Information
Sand bodies stabilize and become cemented in
the deepening of the fore-reef area due to sea-level
rise - a viable process explaining the fore-reef
pinnacle structures. The relict structures in the
fore reef at Highborne Cay (Fig. 3e) are of further
note: the description, and in particular the photo-
graph of the fore-reef pinnacles at Stocking Island
could be the spitting image of each other (see
Fig. 3e, Macintyre et al. , 1996). According to
Macintyre et al., around 2220 yr BP , the offshore
sandbars lithifi ed, forming the bases for subse-
quent coralline algae growth at Stocking Island.
No explanation on the processes is given by
Macintyre et al. (1996). Here it is proposed that as
sea level rose, these formerly very agitated bottoms
fell below average wave base; sands were shifted
around less and less frequently allowing for their
stabilization and subsequent cementation.
The slow-down of sea-level rise further enabled
sediment production to fi ll accommodation space
and impact adjacent reefs. For approximately
2000 years, reefs began catching up with sea level
(Boardman et al. , 1989; Kindler, 1992) as evid-
enced by the formation of an intertidal coralline
algal lip at Stocking Island (Macintyre et al. , 1996).
A slower rate of sea-level rise forced the reef into
lateral expansion, but allowed the sand to catch
up and fi ll accommodation space. The differential
speed at which sand relative to the reef aggrada-
tions fi lled accommodation space explains most
of the variance in the recent reef history as eluded
to in the following.
the reef. From 1000 to 500 yr BP the stromatolite
and algal ridge at Stocking Island are interpreted to
be at the apex of their development. Stromatolites
thrived in the back-reef lagoon and once reach-
ing sea level coalesced to form a broad fl at reef
(Fig. 2). A mature algal ridge characterizes the
outer reef crest. In contrast, at Highborne Cay, the
coral-dominated basal surfaces indicate that cor-
als were the dominant reef-building organisms
~1000 years ago. Corals of the species Diploria
and Siderastraea are tolerant with respect to sedi-
ment, shallow water and elevated salinities sug-
gesting that conditions increasingly shifted to
the present-day conditions. Also noteworthy
is that although coral ages span roughly 400 years,
there is a lack of any signifi cant vertical growth.
This could be attributed to limited accommodation
space, i.e. the reef had reached sea level within a
metre. Alternatively, environmental conditions
were not optimal for strong vertical growth, i.e.
high energy and/or sediment stress.
Sediment also leaves the reef system. Today,
accommodation space at Highborne Cay fi lls up,
at least temporarily. Sand bars, which frequently
migrate over the seaward reef edge, cover the
reef fl at and fi ll the back reef lagoon; these sand
waves bury the reef and obliterate relief (Fig. 5b).
Moreover, sand is exported from the reef system
today. Emergent sand partially dries during low
tides and midday sun. Easterly winds pick up sand
grains and blow them onto the beach up against
the Pleistocene eolianites, forming modern dune
deposits which become cemented (Fig. 3a).
These observations and interpretations high-
light the major differences between the two sys-
tems: the present day Highborne Cay reef is
considered the modern analogue to the Stocking
Island reef ~1000 years ago, based on the promin-
ent and intact algal ridge, as well as prolifi c stro-
matolite formation (Macintyre et al. , 1996). These
observations are confi rmed with the dating of
the underlying coral surface and initial stromato-
lite growth, yet in turn, beg the question of why
stromatolite development at Highborne Cay lags
that of Stocking Island by 500-1000 years and
further why are there no corals reported from the
Stocking Island cores?
With all physical boundary conditions such as
rate of sea-level rise, regional climate and oceano-
graphic conditions being equal, the authors spec-
ulate that the antecedent topography might have
been different at the two island sites. Assuming
a one metre thicker Pleistocene buildup at
Highborne Cay and the proposed sea-level rise
Reef-sediment interactions
Macintyre et al. (1996) attributed the start of
stromatolite growth behind the algal ridge to the
wave-reducing effect of the ridge. We agree that
the emerging algal lip dissipated wave energy
but suggest that, more importantly, sediment was
increasingly trapped behind this barrier, and that
the sediment accumulation over increasingly lon-
ger time periods negatively affected metazoan
reef builders and progressively favoured stro-
matolite development at Stocking Island. Here,
the formation of an emergent algal ridge ~1500
years ago coincided with stromatolite growth in
the back-reef lagoon. Similarly, ooids trapped
within the vertically cut and dated stromatolite
from Highborne Cay yielded ages of ~1500 years.
This average age of stromatolite-bound grains
suggests that by ~1500 yr BP hydrodynamic con-
ditions were suitable for ooid and sand produc-
tion; their presence demonstrating the effect on
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