on antecedence inheritance. The highly permeable reef
framework and coarse rubble facies would be more resistant
to solution erosion as they would be the highest above the
water table and rainwater would pass rapidly through.
Uncemented lagoonal sand and mud facies however would
more likely be susceptible to solution. Thus, weathering
across the reef would emphasize the constructional relief
differences between reef and lagoon floor, such that resubmergence would situate reef facies above reef facies
and lagoon facies above lagoon facies. This is not at odds
with Purdy’s ideas (above) as was proposed by some, but
is a special case of it, as the karst is occurring on reef limestones (whereas Purdy’s experiments and actual examples
showed karst developing on most limestones).
Proof for these ideas requires detailed analysis of lithology immediately above and below the unconformity separating two periods of reef growth, and has been shown
particularly in Belize but also in the reefs of the southern
Great Barrier Reef where detailed mapping of facies distribution has occurred. At Ambergris Cay, at the northern end
of the Belize Barrier Reef, Tebbutt (1975) showed that the
exposed Pleistocene facies is a reef crest limestone with
the same assemblage of corals as the modern reef crest.
A priori, however this shows reef growing on reef. In the
southern Great Barrier Reef, a similar relation occurs. In
particular, the algal rim some 50–100 m wide, which acts
to protect the reef from wave destruction during the high
stand, also offers protection during the low stand against
sub-aereal weathering. It is dense, hard, and has a much
reduced porosity compared with the adjacent open framework coral dominated facies in front and behind. The reef
flat coral facies (leewards of the algal flat) which is 200–
300 m wide, has a high porosity with a likely low retention
time for percolating water during sub-aereal conditions. The
sand flat facies between the coral flat and the lagoon has
40% porosity and is unlithified; it forms a boundary of
around 30
with the lagoon and would be subject to substantial erosion and re-deposition down slope during
periods of emergence. This unlithified edge would therefore
retrograde backwards towards the inner edge of the coral
flat. The reef front facies, in spite of being well lithified is
also highly porous, and would be prone to cavern development and collapse under the influence of gravitational instability. Erosion would therefore be greatest in two areas, the
fore reef (front, spurs, and grooves) and also along the edge
of lagoon and sand flat. The effect of these specific areas of
Rain water, pH generally <7
Rain water, pH generally <7
Dissolution effects
Former sea level
Former sea level
Sea level
Sea level
Sea level
Solution rim
Shelf lagoon
Barrier platform
Sea level
Island Barrier reef
Sea level
Sea level
Conical karst
Reef
Reef
Lagoon
Maximum dissolution
Minimum dissolution
Karst marginal plain
Conical karst (slopes of 30–40Њ)
Tower karst (slopes of 60–90Њ)
Atolls
Barrier reefs
Pre-karst
Karst
Post-karst
Limestone
Non-carbonate foundation
Karst plain alluvium
Marine sediments
Min
Max
Min
Depositional slope≤45Њ
Depositional slope≤45Њ
Antecedent Platforms, Figure 4 Diagrammatic evolution of atolls and barrier reefs according to antecedent karst theory. (Purdy
Page 71)
44
ANTECEDENT PLATFORMS
framework and coarse rubble facies would be more resistant
to solution erosion as they would be the highest above the
water table and rainwater would pass rapidly through.
Uncemented lagoonal sand and mud facies however would
more likely be susceptible to solution. Thus, weathering
across the reef would emphasize the constructional relief
differences between reef and lagoon floor, such that resubmergence would situate reef facies above reef facies
and lagoon facies above lagoon facies. This is not at odds
with Purdy’s ideas (above) as was proposed by some, but
is a special case of it, as the karst is occurring on reef limestones (whereas Purdy’s experiments and actual examples
showed karst developing on most limestones).
Proof for these ideas requires detailed analysis of lithology immediately above and below the unconformity separating two periods of reef growth, and has been shown
particularly in Belize but also in the reefs of the southern
Great Barrier Reef where detailed mapping of facies distribution has occurred. At Ambergris Cay, at the northern end
of the Belize Barrier Reef, Tebbutt (1975) showed that the
exposed Pleistocene facies is a reef crest limestone with
the same assemblage of corals as the modern reef crest.
A priori, however this shows reef growing on reef. In the
southern Great Barrier Reef, a similar relation occurs. In
particular, the algal rim some 50–100 m wide, which acts
to protect the reef from wave destruction during the high
stand, also offers protection during the low stand against
sub-aereal weathering. It is dense, hard, and has a much
reduced porosity compared with the adjacent open framework coral dominated facies in front and behind. The reef
flat coral facies (leewards of the algal flat) which is 200–
300 m wide, has a high porosity with a likely low retention
time for percolating water during sub-aereal conditions. The
sand flat facies between the coral flat and the lagoon has
40% porosity and is unlithified; it forms a boundary of
around 30
with the lagoon and would be subject to substantial erosion and re-deposition down slope during
periods of emergence. This unlithified edge would therefore
retrograde backwards towards the inner edge of the coral
flat. The reef front facies, in spite of being well lithified is
also highly porous, and would be prone to cavern development and collapse under the influence of gravitational instability. Erosion would therefore be greatest in two areas, the
fore reef (front, spurs, and grooves) and also along the edge
of lagoon and sand flat. The effect of these specific areas of
Rain water, pH generally <7
Rain water, pH generally <7
Dissolution effects
Former sea level
Former sea level
Sea level
Sea level
Sea level
Solution rim
Shelf lagoon
Barrier platform
Sea level
Island Barrier reef
Sea level
Sea level
Conical karst
Reef
Reef
Lagoon
Maximum dissolution
Minimum dissolution
Karst marginal plain
Conical karst (slopes of 30–40Њ)
Tower karst (slopes of 60–90Њ)
Atolls
Barrier reefs
Pre-karst
Karst
Post-karst
Limestone
Non-carbonate foundation
Karst plain alluvium
Marine sediments
Min
Max
Min
Depositional slope≤45Њ
Depositional slope≤45Њ
Antecedent Platforms, Figure 4 Diagrammatic evolution of atolls and barrier reefs according to antecedent karst theory. (Purdy
Page 71)
44
ANTECEDENT PLATFORMS