preceded the oolite shoreline development during the
þ6 m highstand (Harrison and Coniglio, 1985). Unfortunately, more precise U-series mass spectrometric dating
has been unable to confirm the exact stratigraphic relation
between the reef and oolite units due to the impact of diagenesis (Fruijtier et al., 2000), and only one age of
128.1 ka with a strictly reliable isotopic signature has been
returned from the reef unit (Multer et al., 2002).
In the absence of reliable ages, the interpretation of reef
development in the Key Largo Limestone has focused primarily on the paleo-water depth in which the corals grew.
The problem was the lack of evidence for coral zonation or
the presence of the reef-crest coral A. palmata which
might confirm a shallow-water origin. As a result, early
interpretations suggested water depths exceeded 6 to
12 m (Stanley, 1966). This was later discounted by evidence of þ6 m shoreline development in the oolitic sand
bodies (Hoffmeister and Multer, 1968; Perkins, 1977;
Harrison and Coniglio, 1985; Multer et al., 2002). All later
interpretations therefore favored a shallow, protected,
patch-reef setting, but differed according to what afforded
that protection. Notions of a reef-crest barrier were finally
discounted by drilling data which failed to encounter reefcrest corals and indicated that shelf had a gentle and extensive ramp-style slope which dampened wave development
(Multer et al., 2002).
However, these later investigators have failed to pose
a more important question: if corals grew in shallow water,
then what prevented them from reaching the þ6 m
highstand? Why was reef development largely restricted
to elevations below þ3 m and why did it not reach the
þ6 m highstand attained by the oolitic shorelines either
side of the reef tract? Tentative evidence suggests that reef
development may have been curtailed by shifting sediment associated with enhanced ooid production during
the þ6 m highstand. But this needs to be confirmed by
precise radiometric ages of both reef and oolitic units.
Bahamas
Similar reef development during the LIG has been
reported from the adjacent Bahamian archipelago, but it
has been characterized best on the island of San Salvador
(Curran and White, 1989; Hattin and Warren, 1989; Chen
et al., 1991; White et al., 1998; Wilson et al., 1998). There,
detailed mapping shows that reefs also reached
a maximum elevation of þ3 m, and had a simple facies
association consisting of a reef-crest, dominated by clasts
and colonies of A. palmata, flanked by back-reef and
a lagoonal zone composed of large patches of
A. cervicornis and species of Montastrea (Figure 3).
Above this reefal unit, the marine sequence is dominated
by a shallow subtidal and beach-sand lithosome which
reaches an elevation of þ5 m consistent with a
þ6 m highstand (Carew and Mylroie, 1997). A similar
sequence was reported from the southern end of the island
by Hattin and Warren (1989), who documented patch reefs
up to þ3 m that were topped by a 30 cm cap of the
branching coralline algae. Subsequent development of
a separate but poorly developed patch reef above the coralline cap up to 3.5 m was taken to be evidence “. . .of
a slight additional rise of sea level. . .” (Hattin and Warren,
1989, p. 26). From this evidence, they concluded that there
was a widespread phase of reef development associated
with a þ3 m stillstand of SL, followed by a rise to
þ6 m that was associated with a higher-energy beach
and shallow subtidal sequence.
On both San Salvador and Great Inagua further south,
systematic dating of the þ3 m LIG reef unit revealed that
this simple facies association had a rather complex age
structure (Chen et al., 1991). Of the 11 samples with
strictly reliable isotopic signatures, those taken from inplace corals in the patch reefs were stratigraphically consistent and clustered around 122 ka. But ages from the
underlying coral-gravel unit gave a wider range of 122–
130 ka, with five ages in excess of 127 ka (Figure 3).
Closely juxtaposed samples from that unit also showed
large variability of $5,000 years between corals separated
by only a few centimeters (e.g., samples M1, A, and D in
Fig. 3a of Chen et al., 1991, p. 88). Although Chen et al.
(1991, p. 83) warned that “storms can mix as rubble corals
of different ages,” well-dated 3 m sections of Holocene
and late-glacial reefs show variability no larger than
1,000 years (Blanchon et al., 2002; Edinger et al., 2007).
As a consequence, it is unlikely that large magnitude age
variability in closely spaced corals from the Bahamian
reefs resulted from mixing during hurricanes, as claimed
by Chen et al. (1991), but resulted instead from mobilization of U and Th isotopes during diagenesis (Scholz and
Mangini, 2007). Coral ages from this unit are therefore
unreliable for determining the onset of the sea-level
highstand, and cast doubt on the extended duration of
the interglacial highstand claimed by Chen et al. (1991).
In addition to the duration of the LIG, these dates have
also been used to identify the timing and significance of an
internal reef-erosion surface discovered on Great Inagua
(White et al., 1998; Wilson et al., 1998). A 1,500-year
age difference between coral ages recovered from either
side of this surface was taken to indicate that the highstand
reef sequence was interrupted by a brief SL fall from
þ4 m to below the modern datum, and then a rapid rise
back to þ6 m (White et al., 1998; Wilson et al., 1998).
Unfortunately, the 2 ka analytical age uncertainty and
large true-age variation in corals dated by Chen et al.
(1991) precludes their use in determining the timing of
events of such short duration. Furthermore, although there
is extensive evidence of marine truncation and bioerosion
of corals and their clasts under this surface, there is no reliable evidence of emergence that can be separated from
emergence that occurred at the end of the highstand. As
a consequence, the evidence for a SL fall is based entirely
on marine-erosion which is a common process not specific
to that cause.
The þ3 m reef unit has also been identified on several
other Bahamaian islands (Neumann and Hearty, 1996;
Hearty and Neumann, 2001). On most of these, reef
628
LAST INTERGLACIAL AND REEF DEVELOPMENT
þ6 m highstand (Harrison and Coniglio, 1985). Unfortunately, more precise U-series mass spectrometric dating
has been unable to confirm the exact stratigraphic relation
between the reef and oolite units due to the impact of diagenesis (Fruijtier et al., 2000), and only one age of
128.1 ka with a strictly reliable isotopic signature has been
returned from the reef unit (Multer et al., 2002).
In the absence of reliable ages, the interpretation of reef
development in the Key Largo Limestone has focused primarily on the paleo-water depth in which the corals grew.
The problem was the lack of evidence for coral zonation or
the presence of the reef-crest coral A. palmata which
might confirm a shallow-water origin. As a result, early
interpretations suggested water depths exceeded 6 to
12 m (Stanley, 1966). This was later discounted by evidence of þ6 m shoreline development in the oolitic sand
bodies (Hoffmeister and Multer, 1968; Perkins, 1977;
Harrison and Coniglio, 1985; Multer et al., 2002). All later
interpretations therefore favored a shallow, protected,
patch-reef setting, but differed according to what afforded
that protection. Notions of a reef-crest barrier were finally
discounted by drilling data which failed to encounter reefcrest corals and indicated that shelf had a gentle and extensive ramp-style slope which dampened wave development
(Multer et al., 2002).
However, these later investigators have failed to pose
a more important question: if corals grew in shallow water,
then what prevented them from reaching the þ6 m
highstand? Why was reef development largely restricted
to elevations below þ3 m and why did it not reach the
þ6 m highstand attained by the oolitic shorelines either
side of the reef tract? Tentative evidence suggests that reef
development may have been curtailed by shifting sediment associated with enhanced ooid production during
the þ6 m highstand. But this needs to be confirmed by
precise radiometric ages of both reef and oolitic units.
Bahamas
Similar reef development during the LIG has been
reported from the adjacent Bahamian archipelago, but it
has been characterized best on the island of San Salvador
(Curran and White, 1989; Hattin and Warren, 1989; Chen
et al., 1991; White et al., 1998; Wilson et al., 1998). There,
detailed mapping shows that reefs also reached
a maximum elevation of þ3 m, and had a simple facies
association consisting of a reef-crest, dominated by clasts
and colonies of A. palmata, flanked by back-reef and
a lagoonal zone composed of large patches of
A. cervicornis and species of Montastrea (Figure 3).
Above this reefal unit, the marine sequence is dominated
by a shallow subtidal and beach-sand lithosome which
reaches an elevation of þ5 m consistent with a
þ6 m highstand (Carew and Mylroie, 1997). A similar
sequence was reported from the southern end of the island
by Hattin and Warren (1989), who documented patch reefs
up to þ3 m that were topped by a 30 cm cap of the
branching coralline algae. Subsequent development of
a separate but poorly developed patch reef above the coralline cap up to 3.5 m was taken to be evidence “. . .of
a slight additional rise of sea level. . .” (Hattin and Warren,
1989, p. 26). From this evidence, they concluded that there
was a widespread phase of reef development associated
with a þ3 m stillstand of SL, followed by a rise to
þ6 m that was associated with a higher-energy beach
and shallow subtidal sequence.
On both San Salvador and Great Inagua further south,
systematic dating of the þ3 m LIG reef unit revealed that
this simple facies association had a rather complex age
structure (Chen et al., 1991). Of the 11 samples with
strictly reliable isotopic signatures, those taken from inplace corals in the patch reefs were stratigraphically consistent and clustered around 122 ka. But ages from the
underlying coral-gravel unit gave a wider range of 122–
130 ka, with five ages in excess of 127 ka (Figure 3).
Closely juxtaposed samples from that unit also showed
large variability of $5,000 years between corals separated
by only a few centimeters (e.g., samples M1, A, and D in
Fig. 3a of Chen et al., 1991, p. 88). Although Chen et al.
(1991, p. 83) warned that “storms can mix as rubble corals
of different ages,” well-dated 3 m sections of Holocene
and late-glacial reefs show variability no larger than
1,000 years (Blanchon et al., 2002; Edinger et al., 2007).
As a consequence, it is unlikely that large magnitude age
variability in closely spaced corals from the Bahamian
reefs resulted from mixing during hurricanes, as claimed
by Chen et al. (1991), but resulted instead from mobilization of U and Th isotopes during diagenesis (Scholz and
Mangini, 2007). Coral ages from this unit are therefore
unreliable for determining the onset of the sea-level
highstand, and cast doubt on the extended duration of
the interglacial highstand claimed by Chen et al. (1991).
In addition to the duration of the LIG, these dates have
also been used to identify the timing and significance of an
internal reef-erosion surface discovered on Great Inagua
(White et al., 1998; Wilson et al., 1998). A 1,500-year
age difference between coral ages recovered from either
side of this surface was taken to indicate that the highstand
reef sequence was interrupted by a brief SL fall from
þ4 m to below the modern datum, and then a rapid rise
back to þ6 m (White et al., 1998; Wilson et al., 1998).
Unfortunately, the 2 ka analytical age uncertainty and
large true-age variation in corals dated by Chen et al.
(1991) precludes their use in determining the timing of
events of such short duration. Furthermore, although there
is extensive evidence of marine truncation and bioerosion
of corals and their clasts under this surface, there is no reliable evidence of emergence that can be separated from
emergence that occurred at the end of the highstand. As
a consequence, the evidence for a SL fall is based entirely
on marine-erosion which is a common process not specific
to that cause.
The þ3 m reef unit has also been identified on several
other Bahamaian islands (Neumann and Hearty, 1996;
Hearty and Neumann, 2001). On most of these, reef
628
LAST INTERGLACIAL AND REEF DEVELOPMENT