Geology Reference
In-Depth Information
Beach
Ocean
Back reef
Reef flat
Reef crest
Fore reef
1.0
0.5
1.4a
1.2 1.1
Sawed
head
1.3
Thrombolite
Stromatolite
Microbial Mat
Coralline Algae
A
A
N
bsc-1.2
bsc-1.1
A
A'
bsc-1.3 Sawed Head
bsc-1.4a
Fig. 4. Transect and aerial image of
Site 1. Transect across Site 1 with
dated coral and stromatolite sam-
ples projected onto transect line.
Reef zones (back reef, reef fl at, reef
crest, and fore reef) characterized
by different reef facies (thromb-
olites, stromatolites, microbial mat
and coralline algae). Aerial image
overlain by facies to illustrate spa-
tial distribution thereof.
5m
below in a seaward to shoreward direction:
fore-reef pinnacle zone, reef crest, reef fl at and
back-reef lagoon (Fig. 4).
Prominent pinnacle structures, not previously
documented in the Highborne Cay reef com-
plex, occur in the fore-reef area, ~25 m east of the
coralline algal reef platform (Fig. 3e). Located
in ~3 m of water depth, the pinnacles tower up
to 2 m in height if their bases are exposed from
sand. They are composed of dense coralline algal
limestone and sand-sized sediment fi lling the
voids between the branching framework. Living
macroalgae and scattered encrusting corals form
bulging caps at the top of these pinnacles.
Interestingly, a healthy patch reef grows
adjacent to the above-mentioned 'relict' reef
(Fig. 3a; arrow indicating patch reef). Its shape
is reminiscent of a mushroom: round tabular top
with undercut base.
The maximum relief observed at the seaward
edge of the reef crest zone is ~2 m; due to a lack
of cores, the absolute thickness of the Holocene
reef is unknown. Based on visual outcrop observa-
tions and samples chiselled from the surface, the
wall of the outer reef, like the fore-reef pinnacles,
consists of well-indurated, hard, dense algal lime-
stone. Smooth surfaces are the result of constant
sand abrasion. Hand samples and observations
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