FLOATSTONE
Peter Flood
University of New England, Armidale, NSW, Australia
Floatstone is a type of limestone which displays 10% or
more of the grains larger than 2 mm in diameter and there
is mud (matrix) between the grains (like packstone – sand
sized).
Bibliography
Embry, A. F., and Klovan, J. E., 1971. A Late Devonian reef tract on
Northeastern Banks Island. NWT: Canadian Petroleum Geology
Bulletin, 19, 730–781.
FLORIDA KEYS
Barbara H. Lidz
U.S. Geological Survey, St. Petersburg, FL, USA
Synonyms
Asymmetric reef-tract evolution; Calcified organisms;
Cemented oolitic tidal bars; Cumulative coral reefs; Emergent reefal and oolitic limestone; Florida Keys National
Marine Sanctuary; Holocene Acropora palmata;
Inhabited island chain off south Florida; Key Largo Limestone; Miami Limestone; Outlier reefs; Pleistocene
Montastraea annularis; Replicated antecedent facies and
geomorphic landforms; Shallow south Florida shelf;
Windward platform-margin rim
Definitions
Quaternary: The Quaternary Period consists of the
Pleistocene and Holocene Epochs, the most recent ages
of geologic time. The Pleistocene ranged from $1.8 Ma
to 10 ka. The Holocene began at 10 ka and includes the
present. On the basis of glacial/interglacial intervals as
determined by oxygen-isotope analyses of fossil planktic
foraminifera, the Quaternary has been divided into numbered Marine Isotope Stages (MISs). Even numbers represent glacial intervals: relative to the Florida Keys reef
record, MIS 10, 8, 6, 4, and 2 are pertinent. Interglacial
intervals are referred to as MIS 11, 9, 7, 5, 3, and 1, with
MIS 1 being the present warm interlude. MIS 5 is
subdivided into substages 5e through 5a. Coral ages show
that Florida Keys reefs flourished during transgressive and
regressive intervals immediately surrounding late Quaternary sea-level maxima at $230 (MIS 7), 125 (MIS 5e),
112 (MIS 5d), 106 (MIS 5c), 92 (MIS 5b), and 80 ka
(MIS 5a) (Muhs, 2002; Lidz et al., 2007). The highest
stand among those maxima occurred at $125 ka when
the sea level was $10.6 m higher than at present. Conditions then produced the now emergent fossil coral reef
(Key Largo Limestone; Multer et al., 2002) and tidal-bar
oolite (Miami Limestone; Hoffmeister et al., 1967), which
comprise islands of the Florida Keys.
Q Units: Quaternary marine sequences in the Florida Key
Largo Limestone have been divided into Q1 (oldest)
through Q5 (youngest) Units (Perkins, 1977). The first
flourishing reef occurs in the Q3 Unit. The oldest Q3
corals (Montastraea annularis), cored in the upper Keys,
have been dated to $370 and 366 ka (Multer et al.,
2002). Those authors and others (e.g., Muhs et al., 2003)
correlate those ages to the highstand of MIS 9. However,
some authors (e.g., Droxler et al., 2003a) argue that the
Q3 reef could belong to MIS 11. The MIS 11 highstand
was the highest ($20–25 m higher than at present) and
longest (30–40 kyr duration) in the Quaternary (see papers
in Droxler et al., 2003b).
Mangrove peat: Modern mangrove forests and calm water
rim the Florida Keys. The forests form peat, a compacted
accumulation of partially decomposed organic debris.
Dates on and depths of Holocene peat recovered from
beneath offshore coral reefs provide times of peat formation and positions of paleoshorelines relative to the present
sea level. Peat presence implies quiescent conditions at
those sites during formation. Higher elevation outer-shelf
bedrock topography protected those sites from waves as
sea level rose. The deepest Keys mangrove peat cored
was recovered from $15 m below the present sea level
(Lidz et al., 1985).
Bedrock: Researchers regard the uneven Pleistocene limestone surface on the south Florida shelf as “bedrock.”
Wherever bedrock in contact with overlying Holocene
coral reefs and sands has been dated, the dates point to
the highstand of 80 ka (Lidz, 2006).
Ooids/oolite: Ooids are concentric grains of calcium
carbonate that precipitate in warm, highly agitated seawater. During the 125-ka highstand when the Key Largo
Limestone reef was growing, seaward-flowing platforminterior waters piled up behind the reef, causing
high-energy convergence zones around its northeastern
and southwestern ends. Ooids formed in these zones and
amassed as unstable tidal bars. With sea withdrawal, the
exposed bars became cemented into (Miami Limestone)
oolite.
Geographic and geomorphic setting
The Florida Peninsula, located at the southeast corner of
the continental United States, is the emergent part
of a broad, shallow, southward-extending carbonate platform. Seawater (Gulf of Mexico, Straits of Florida, Atlantic Ocean) surrounds the peninsula to the west, south, and
east (Figure 1). The Florida Keys form a discontinuous
island chain trending west–southwest from the southern
peninsular tip. The submerged bedrock $5–7 km seaward
of the Keys is Pleistocene, generally <12 m deep (Enos
and Perkins, 1977), and harbors the largest coral reef ecosystem in the contiguous United States. Several meters
deeper to the southwest than northeast, the shelf curves
westward into the Gulf of Mexico where its shallow
406
FLOATSTONE
Peter Flood
University of New England, Armidale, NSW, Australia
Floatstone is a type of limestone which displays 10% or
more of the grains larger than 2 mm in diameter and there
is mud (matrix) between the grains (like packstone – sand
sized).
Bibliography
Embry, A. F., and Klovan, J. E., 1971. A Late Devonian reef tract on
Northeastern Banks Island. NWT: Canadian Petroleum Geology
Bulletin, 19, 730–781.
FLORIDA KEYS
Barbara H. Lidz
U.S. Geological Survey, St. Petersburg, FL, USA
Synonyms
Asymmetric reef-tract evolution; Calcified organisms;
Cemented oolitic tidal bars; Cumulative coral reefs; Emergent reefal and oolitic limestone; Florida Keys National
Marine Sanctuary; Holocene Acropora palmata;
Inhabited island chain off south Florida; Key Largo Limestone; Miami Limestone; Outlier reefs; Pleistocene
Montastraea annularis; Replicated antecedent facies and
geomorphic landforms; Shallow south Florida shelf;
Windward platform-margin rim
Definitions
Quaternary: The Quaternary Period consists of the
Pleistocene and Holocene Epochs, the most recent ages
of geologic time. The Pleistocene ranged from $1.8 Ma
to 10 ka. The Holocene began at 10 ka and includes the
present. On the basis of glacial/interglacial intervals as
determined by oxygen-isotope analyses of fossil planktic
foraminifera, the Quaternary has been divided into numbered Marine Isotope Stages (MISs). Even numbers represent glacial intervals: relative to the Florida Keys reef
record, MIS 10, 8, 6, 4, and 2 are pertinent. Interglacial
intervals are referred to as MIS 11, 9, 7, 5, 3, and 1, with
MIS 1 being the present warm interlude. MIS 5 is
subdivided into substages 5e through 5a. Coral ages show
that Florida Keys reefs flourished during transgressive and
regressive intervals immediately surrounding late Quaternary sea-level maxima at $230 (MIS 7), 125 (MIS 5e),
112 (MIS 5d), 106 (MIS 5c), 92 (MIS 5b), and 80 ka
(MIS 5a) (Muhs, 2002; Lidz et al., 2007). The highest
stand among those maxima occurred at $125 ka when
the sea level was $10.6 m higher than at present. Conditions then produced the now emergent fossil coral reef
(Key Largo Limestone; Multer et al., 2002) and tidal-bar
oolite (Miami Limestone; Hoffmeister et al., 1967), which
comprise islands of the Florida Keys.
Q Units: Quaternary marine sequences in the Florida Key
Largo Limestone have been divided into Q1 (oldest)
through Q5 (youngest) Units (Perkins, 1977). The first
flourishing reef occurs in the Q3 Unit. The oldest Q3
corals (Montastraea annularis), cored in the upper Keys,
have been dated to $370 and 366 ka (Multer et al.,
2002). Those authors and others (e.g., Muhs et al., 2003)
correlate those ages to the highstand of MIS 9. However,
some authors (e.g., Droxler et al., 2003a) argue that the
Q3 reef could belong to MIS 11. The MIS 11 highstand
was the highest ($20–25 m higher than at present) and
longest (30–40 kyr duration) in the Quaternary (see papers
in Droxler et al., 2003b).
Mangrove peat: Modern mangrove forests and calm water
rim the Florida Keys. The forests form peat, a compacted
accumulation of partially decomposed organic debris.
Dates on and depths of Holocene peat recovered from
beneath offshore coral reefs provide times of peat formation and positions of paleoshorelines relative to the present
sea level. Peat presence implies quiescent conditions at
those sites during formation. Higher elevation outer-shelf
bedrock topography protected those sites from waves as
sea level rose. The deepest Keys mangrove peat cored
was recovered from $15 m below the present sea level
(Lidz et al., 1985).
Bedrock: Researchers regard the uneven Pleistocene limestone surface on the south Florida shelf as “bedrock.”
Wherever bedrock in contact with overlying Holocene
coral reefs and sands has been dated, the dates point to
the highstand of 80 ka (Lidz, 2006).
Ooids/oolite: Ooids are concentric grains of calcium
carbonate that precipitate in warm, highly agitated seawater. During the 125-ka highstand when the Key Largo
Limestone reef was growing, seaward-flowing platforminterior waters piled up behind the reef, causing
high-energy convergence zones around its northeastern
and southwestern ends. Ooids formed in these zones and
amassed as unstable tidal bars. With sea withdrawal, the
exposed bars became cemented into (Miami Limestone)
oolite.
Geographic and geomorphic setting
The Florida Peninsula, located at the southeast corner of
the continental United States, is the emergent part
of a broad, shallow, southward-extending carbonate platform. Seawater (Gulf of Mexico, Straits of Florida, Atlantic Ocean) surrounds the peninsula to the west, south, and
east (Figure 1). The Florida Keys form a discontinuous
island chain trending west–southwest from the southern
peninsular tip. The submerged bedrock $5–7 km seaward
of the Keys is Pleistocene, generally <12 m deep (Enos
and Perkins, 1977), and harbors the largest coral reef ecosystem in the contiguous United States. Several meters
deeper to the southwest than northeast, the shelf curves
westward into the Gulf of Mexico where its shallow
406
FLOATSTONE