Page 248 - Sedimentology and Stratigraphy

Shallow Marine Carbonate Environments 235

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Fig. 15.12 Facies distribution in a reef complex.

remove mass from the reef structure, a process of
bioerosion carried out by some types of fish and
molluscs that bore into the reef. The voids between
the framework structures may be filled with the
remains of organisms, debris formed by the mechan-
ical breakdown and bioerosion of the framework and !
by carbonate mud. If burial occurs before the voids
have been filled with sediment, crystalline calcite
cement may subsequently be precipitated during dia-
genesis (18.2).
Break-up of the reef core material by wave and
storm action leads to the formation of a talus slope
of reefal debris. This forereef setting is a region of
accumulation of carbonate breccia to form bioclastic
rudstone and grainstone facies. As these are gravity
deposits formed by material falling down from the reef
crest they build out as steeply sloping depositional
units inclined at 10˚ to 30˚ to the horizontal. Behind Fig. 15.13 Reefs can be recognised as occurring in three
the reef crest the back reef is sheltered from the high- settings: (a) barrier reefs form offshore on the shelf and
est energy conditions and is the site of deposition of protect a lagoon behind them, (b) fringing reefs build at the
debris removed from the reef core and washed coastline and (c) patch reefs or atolls are found isolated
towards the lagoon. A gradation from rudstone to offshore, for instance on a seamount.
grainstone deposits of broken reef material, shells
and occasionally ooids forms a fringe along the mar- (Fig. 15.13). Fringing reefs are built out directly
gin of the lagoon. from the shoreline and lack an extensive back-reef
lagoonal area. Barrier reefs, of which the Great
Barrier Reef of eastern Australia is a distinctive exam-
Reef settings
ple, are linear reef forms that parallel the shore-
Three main forms of reef have been recognised in line, but lie at a distance of kilometres to tens of
modern oceans, and in fact were recognised by kilometres offshore: they create a back-reef lagoon
Charles Darwin in the middle of the 19th century area which is a large area of shallow, low-energy