Geology Reference
In-Depth Information
factors controlling reef development: in this case
the mixed coral-microbial-coralline algae-coral
systems of Highborne Cay and Stocking Island.
In the rock record, we are limited to outcrop
or core data from which to infer paleoenvir-
onmental information (Feldmann & McKenzie,
1997; Kahle, 1994; Riding
et al.
, 1991). For instance,
in the Upper Jurassic Smackover formation,
stromatolites were interpreted to form in low-
energy peritidal settings characterized by waters
with elevated salinity, whereas thrombolites
are associated with higher energy conditions in
subtidal setting of normal sea water (Mancini
et al.
, 2004). In contrast (Leinfelder
et al.
, 1993)
concluded that microbialites are not restricted
by water depth, salinity, temperature, light pen-
etration, oxygen content or nutrient supply - but
occur where other reef organisms are excluded
due to some factor.
How would the Highborne Cay or Stocking
Island reef system be interpreted in an outcrop?
The high spatial variability might intrigue geolo-
gists to interpret the different facies as non time-
equivalent. Moreover, the basal coral surface
below the stromatolites could be interpreted as
an erosional surface formed during sea-level low-
stand or representing a non-depositional surface.
Alternatively, the demise of the coral reef could
be attributed to drowning after a rapid sea-level
rise and the microbialite attributed to the next
sea-level cycle. From a sequence stratigraphic
perspective, stromatolite ridges overlying colum-
nar structures could be interpreted within a shal-
lowing upward sequence - which is correct on a
small scale. However, as seen within the Highborne
Cay reef system, the high spatial variability is due
to the co-habitation and reef development within
the same sea-level cycle.
Modern analogue studies offer the unique
opportunity of directly characterizing and
quantifying the physical environment in three
dimensions and time. More importantly, they
provide an opportunity to study and understand
processes sparking new ideas and alternate sce-
narios with which to revisit the rock record.
material and dating itself. Microbial build-ups are
surprisingly young (less than 1000 years) and a
recent addition to the fringing reef systems along
the Exuma margin.
The present distribution of microbial versus
metazoan reef builders in the Stocking Island and
Highborne cay reef is dominantly controlled by
sediment and sediment dynamics, more specifi c-
ally by the intensity and frequency of sediment
stress, sand abrasion and burial of reef biota. Late
Holocene reef history appears to refl ect variable
sedimentation patterns, antecedent topography
and late Holocene sea-level transgression. Initial
reef development was relatively unaffected by
sedimentation, due to a lag in sediment produc-
tion with respect to sea-level transgression, and
more importantly deposition of sediment below
the infl uence of average wave base, and below the
infl uence of current suspension.
Slowing of sea-level rise combined with increas-
ing sediment production decreased accommo-
dation space. Mobile sediment progressively
impinged upon the reef environment and, due
to an emerging algal ridge, sediment was now
trapped in the back reef area. Although corals
can tolerate some sediment stress, they cannot
cope with extended periods of burial. Increasing
sediment stress and more importantly increasing
amplitude and frequency of sediment burial are
interpreted as dominant factors leading to the
demise of metazoan reef builders and the prolifi c
growth of microbialites in the back reef lagoon.
The study of modern analogues is important for
a process-orientated understanding, which in
turn provides concepts and hypothesis to apply
and validate in the rock record.
ACKNOWLEDGEMENTS
This paper is a tribute to RNG, a walking ency-
clopaedia of carbonate sedimentology - whom
we admire for his breadth of knowledge, wit and
humour. This paper, to us, represents a classic
'Ginsburgian' drama featuring some of RNG's
most favourite and long standing research actors:
stromatolites, reefs and ooids on one of his favou-
rite geologic stages, the shallow, warm waters of
the Bahamas.
We thank reviewers R. Riding and L. Montaggioni
for useful suggestions and comments. Funding was
provided by the US National Science Foundation.
This is RIBS contribution No. 36.
CONCLUSIONS
Based on outcrop stratigraphy and dating, the
evolution of the Highborne Cay and Stocking
Island reef systems is comparable and events
can be correlated within the limits of the dated
