QUATERNARY GEOLOGIC MAP OF FLORIDA KEYS

4° x 6° QUADRANGLE, UNITED STATES

QUATERNARY GEOLOGIC ATLAS OF THE UNITED STATES

MAP I-1420 (NG-17)

State compilations by

Thomas M. Scott, Michael S. Knapp, and David L. Weide

Edited and integrated by

Gerald M. Richmond, David S. Fullerton, and David L. Weide

1986

NOTE 1: This map is the product of interorganizational collaboration. Following a regional meeting 

of state compilers with the coordinator, during which map units and related matters were established, a 

Quaternary map and map explanation of this quadrangle were prepared by T. M. Scott and M. S. Knapp 

of the Florida Bureau of Geology. Supplemental information was added by D. L. Weide, and G. M. 

Richmond, editors. Significant geologic problems requiring changes in the map were resolved in 

discussions and by correspondence. Reviewers, to whom the editors are indebted were W. A. White, 

University of North Carolina, and J. B. Cathcart, U.S. Geological Survey.

NOTE 2: The Pliocene-Pleistocene boundary defined by joint resolution of the International Union 

for Quaternary Research (INQUA) Subcommission 1–d on the Pliocene/Pleistocene Boundary (the 

International Commission on Stratigraphy (ICS) Working Group on the Pliocene/Pleistocene Boundary) 

and the Working Group on the International Geological Correlation Program (IGCP) Project No. 41 

(Neogene/Quaternary Boundary) is that at the Vrica section in southern Italy. The age of that boundary 

currently is inferred to be 1.65 Ma (Aguirre and Pasini, 1984).

Time boundaries between the early Pleistocene and middle Pleistocene and between the middle 

Pleistocene and late Pleistocene are being proposed by the INQUA Working Group on Major Subdivision 

of the Pleistocene. The boundary between the early Pleistocene and middle Pleistocene is placed at the 

Matuyama-Brunhes magnetic polarity reversal. The reversal has not been dated directly by radiometric 

controls. It is significantly older than the Bishop Tuff (revised K-Ar age 738 ka; Izett, 1982), and the 

estimated K-Ar age of 730 ka assigned to the reversal by Mankinen and Dalrymple (1979) is too young. 

In Utah, the Bishop volcanic ash bed overlies a major paleosol developed in sediments that record the 

Matuyama-Brunhes reversal (Eardley and others, 1973). The terrestrial geologic record is compatible 

with the astronomical age of 788 ka assigned to the reversal by Johnson (1982). The boundary between 

the middle Pleistocene and late Pleistocene is placed arbitrarily at the beginning of marine oxygen isotope 

substage 5e (at Termination II or the stage 6/5 transition). That boundary also is not dated directly. It 

was assigned provisional ages of 127 ka by CLIMAP project members (CLIMAP Project Members, 1984) 

and 128 ka by SPECMAP Project members (Ruddiman and McIntyre, 1984), based on uranium-series 

ages of the substage 5e high eustatic sea level stand. A sidereal age of 132 ka is derived by projection of 

the boundary onto the astronomical time scale of Johnson (1982).

The Pleistocene-Holocene boundary is being proposed by the INQUA Subcommission on the 

Holocene. Currently in the United States, it is placed arbitrarily at 10,000 B.P. (Hopkins, 1975).

Aguirre, Emiliano, and Pasini, Giancarlo, 1984, Proposal of the ICS Working Group on the 

Pliocene/Pleistocene boundary concerning the definition of the Pliocene/Pleistocene boundary 

stratotype: 6 p.

CLIMAP Project Members, 1984, The last interglacial ocean: Quaternary Research, v. 21, p. 123–224.
Eardley, A. J., Shuey, R. T., Gvosdetsky, V., Nash, W. P., Picard, M. D., Grey, D. C., and Kukla, G. J., 

1973, Lake cycles in the Bonneville Basin, Utah: Geological Society of America Bulletin, v. 84, no. 

1, p. 211–215.

Hopkins, D. M., 1975, Time stratigraphic nomenclature for the Holocene epoch: Geology, v. 3, no. 1, 

p. 10.

Izett, G. A., 1982, The Bishop ash bed and some older, compositionally similar, ash beds in California, 

Nevada, and Utah: U.S. Geological Survey Open-File Report 82–582, 47 p., 1 pl.

Johnson, R. G., 1982, Brunhes-Matuyama magnetic reversal dated at 790,000 yr B.P. by marineastronomical correlations: Quaternary Research, v. 17, no. 2, p. 135–147.

Mankinen, E. A., and Dalrymple, G. B., 1979, Revised geomagnetic polarity time scale for the interval 

0–5 m.y. B.P.: Journal of Geophysical Research, v. 84, no. B2, p. 615626.

Ruddiman, W. F., and McIntyre, A., 1984, Ice-age thermal response and climatic role of the surface 

Atlantic Ocean, 40° N. to 63° N.: Geological Society of America Bulletin, v. 95, p. 381–396.

The map contains the following illustrations:

An index map to the International Map of the World 1:100,000 topographic series showing the 

Quaternary geologic map of the Florida Keys 4°x 6° quadrangle and other published maps of the 

Miscellaneous Investigations Series (I–1420).

An illustration showing the responsibility for state compilations.

An illustration showing the correlation of map units.

An illustration showing tentative correlation of Quaternary and upper Pliocene lithostratigraphic and 

morphostratigraphic units of the Florida Keys and Jacksonville quadrangles with stratigraphic 

units in South Carolina.

LIST OF MAP SYMBOLS

CONTACT

BEACH RIDGES—Mapped along Atlantic coast and discontinuously along central carbonate ridge

DESCRIPTION OF MAP UNITS

HOLOCENE

acd ALLUVIAL CLAY AND SILT—Light- to dark-gray, yellow to brown, poorly to well-stratified, clayey 

silt and silty clay alluvium. Deposit contains varying amounts of interbedded or admixed fine 

sand, especially in channels along upper parts of regional drainages. Locally it also includes 

clay balls, lenses of kaolinitic clay, and scattered phosphatic pellets and quartz sand. Mapped 

areas include organic muck and swamp deposits (hs) on flood plains and, locally, colluvium 

along margins of valley floors. Thickness generally 5–10 m

hs SWAMP DEPOSIT—Dark-brown to black muck, mucky peat, and organic residue mixed with fine 

to very fine quartz sand, silt, and kaolinitic clay. Commonly thinly laminated; locally includes 

lenses of mottled clay. Large deposits extensively bioturbated. In topographic depressions, 

intermittently covered by 0.5 to 1 m of standing water. Where these areas have been drained 

for agriculture, oxidation of the swamp deposits has resulted in subsidence as much as 3 m. 

Small, discontinuous areas of slightly higher ground are covered with light to dark-brown, 

unconsolidated, medium to fine quartz sand. Thickness 3–10 m

hpc FRESHWATER MARSH PEAT AND CLAY—Gray to black herbaceous peat and clay, intermixed 

and interbedded. Color darkens as content of organic matter increases. Includes interbedded 

freshwater and brackish-water deposits. Mapped in part from distribution of freshwater 

vegetation. Thickness 1–3 m
hp PEAT—Dark-gray to black undecomposed to partially decomposed organic matter compacted into 

thick mats interbedded with thin layers of fine sand and silt. Mapped areas confined to deposits 

in which volume of organic matter exceeds that of mineral matter. Thickness 2–5 m 

hpd DRAINED AND COMPACTED PEAT—Light- to dark-gray fibrous peat mixed with other organic

material and interlayered with lenses of silt and fine to medium sand. Where areas are drained 

for agriculture and urban development, burning, oxidation, and compaction of the deposits has 

resulted in subsidence of as much as 4 m. Thickness 2–3 m

hcm ALGAL MUD AND CARBONATE—Blue, green, or gray, freshwater, algal matte overlying algal 

calcitic mud interbedded with thin layers of crystalline low-magnesian calcite. Deposit forms a 

rhythmite resulting from annual growth and death of calcite-secreting blue-green algae 

(Gleason, 1972, p. 34–38). Thickness approximately 1 m

hmm COASTAL MANGROVE SWAMP DEPOSIT—Dark-gray to black organic muck intermittently 

covered by as much as 1 m of sea water. Overlies wave-beveled surfaces cut on soft, porous, 

shelly limestone of late Pleistocene Anastasia Formation and Tertiary limestone. Mapped only 

in areas that support dense stands of both red and black mangrove along the Gulf of Mexico. 

Thickness 0.5–1 m 

he SWAMP DEPOSIT AND DUNE SAND—Swamp deposit is dark-gray to orange mottled, silty, 

smectitic clay with abundant organic debris. Present in parallel, linear, interdune depressions 

along Atlantic coast. Dune sand is white, unconsolidated, and fine to medium grained. It is 

composed of quartz mixed with moderate amounts of shell debris. Deposit forms linear beach 

and dune ridges parallel to shore. Dunes are chiefly stabilized by vegetation though some are 

locally active. Map unit overlies the Anastasia Formation in northeast part of the quadrangle. 

Deposit is compacted throughout most of eastern Florida, as a result of drainage for agriculture. 

Thickness 5–15 m

els DUNE SILT AND CLAY—Light-gray to light-yellow, fine to medium crossbedded sand and sandsize aggregates of silt and clay particles forming a complex of large dunes. Mapped only along 

northeast, east, and southeast shores of Lake Okeechobee where dunes have accumulated 

because of intense storm activity. Thickness 5–10 m

be BEACH AND DUNE SAND—Light-tan, angular, medium to fine sand, well-sorted, crossbedded, 

mostly quartz with traces of heavy minerals. Upper layers locally contain crushed shell that 

increases gradually in proportion from north to south. Occurs along coast at and above 

modern beach. Also present as thin deposits, too small to map, on coastal freshwater marsh 

deposits (hpc). Thickness 2–7 m 

bc BEACH SHELL-FRAGMENT AND SHELL SAND—White to light-gray shell fragment and shell 

sand including minor amounts of fine quartz sand, silt, and clay. Locally the deposit may be 

weakly cemented by calcium carbonate. Where exposed along the Florida Keys, the unit 

commonly consists of foraminiferal tests. Thickness 1–5 m

bd BEACH MUD—Gray to black silt and clay; contains shell fragments and very fine grained organic 

matter; color darkens as amount of organic matter increases. Underlies beaches bordering 

saline marsh, estuaries, and lakes from which it has been transported. Thickness 0.5–1.5 m

HOLOCENE TO MIDDLE WISCONSIN

lfm LAKE FLIRT MARL—Light- to dark-gray freshwater, calcareous fine silt and clay interbedded with 

soft, white, freshwater marl and thin layers of organic material, peat, or muck. Locally may be 

covered with swamp deposits or very thin discontinuous patches of quartz and calcareous sand. 

Thickness as much as 3 m

LATE PLEISTOCENE

ANASTASIA FORMATION—Present mostly in subsurface and shown by pattern on map. Lightgray to yellow, complexly interbedded, sandy marine limestone, calcareous sandstone and sand; 

cemented to friable coquina. Locally contains thin, discontinuous lenses of black silty muck and 

decayed organic matter. The Anastasia Formation is about 110,000 years old as determined 

by a Th230/U234 dating method (Osmond and others, 1970). Exposures of the formation, too 

small to show at scale of map, occur discontinuously along the Atlantic coastal ridge of eastern 
Florida, but most of unit is covered by thin swamp deposits (hs) and dune sand (he). Thickness 

as much as 30 m

mol MIAMI OOLITE—White to light-yellow, crystalline, oolitic limestone interbedded with thin layers of

soft, unconsolidated oolitic sand. Locally this unit is perforated with numerous solution holes 

and channels now filled with fine to medium quartz sand. Oolitic limestone portions of the 

formation are commonly brecciated and re-cemented as a result of solution. Th230/U234 ages on 

the Miami Oolite range from about 90,000 to about 145,000 years old (Broecker and Thurber, 

1965; Osmond and others, 1965). The oolite is exposed at the surface throughout much of 

southeast Florida; in places it is covered by thin, discontinuous patches of eolian oolite and 

quartz sand (indicated by pattern). Thickness as much as 20 m

LATE PLEISTOCENE TO MIDDLE PLEISTOCENE

bmk BEACH AND MARINE SAND, WEATHERED AND OXIDIZED—Light-yellow to dark-orange 

quartz sand mixed with and stained by organic matter. Underlain at depths of as much as to 

0.5 m by fine quartz sand compacted and mixed with insoluble clay; locally, weakly to strongly 

cemented with calcium carbonate. Basal part of unit consists of as much as 7 m of 

consolidated sand containing abundant whole but rotted bivalve shells. Deposit overlies sand, 

clayey sand, or shelly sand of older Pleistocene beach ridges. Much shell material has been 

leached from upper part of the deposits, resulting in reduction of thickness by as much as 1.5 

m. Locally, the dissolved shell material has been redeposited with insoluble clay to form a 

subsurface hardpan. Thickness 6–9 m

mba BEACH SAND AND LAGOONAL DEPOSITS, UNDIFFERENTIATED—Light-gray, yellowish-gray 

and brownish-gray, poorly to well-sorted sand, silt, and clay, intermixed and interbedded. 

Locally includes thin stringers of well-rounded quartz pebbles and shell hash in remnants of old 

tidal channels. In places, also contains the insoluble residues of beach sand and sandy lagoonal

deposits associated with older Pleistocene shorelines. Mapped areas include numerous small 

swamp deposits (hs), organic muck, shell debris, and thin patches of eolian sand. Thickness 

10–25 m

kls KEY LARGO LIMESTONE—Light-gray to light-yellow, coralline limestone. Highly fossiliferous 

with molds and casts of boring marine organisms. Locally composed of coral heads and algal 

mounds surrounded by amorphous limestone, coral detritus, and limestone breccia. Contains 

thin lenses of dense, hard, freshwater limestone in places. Upper surface brecciated and 

recemented. Th230/U234 ages on coral and on oolite are mostly between about 95,000 and 

145,000 years (Broecker and Thurber, 1965; Osmond and others, 1965). Maximum thickness 

about 20 m

FORT THOMPSON FORMATION—-Present only in subsurface and shown by pattern on map. 

White to very light yellow, sandy limestone and calcareous sandstone, interbedded with layers 

and pockets of fine-grained, well-sorted quartz sand, and thin beds of dense, hard, freshwater 

limestone. Exposed in numerous small outcrops but is mostly covered by thin swamp deposits 

(hs) and deposits of weathered and oxidized beach sand (bmk). Maximum thickness about 25 

m

MIDDLE PLEISTOCENE TO EARLY PLEISTOCENE

bml BEACH SAND AND SANDY CLAY—White, yellow, reddish-yellow to reddish-orange, well-sorted 

phosphatic sand and clayey sand. Locally contains lenses of red to orange-red, well-sorted, 

coarse sand stained and cemented with iron. Deposit reworked from the Pliocene Bone Valley 

Formation; in places overlies Tertiary limestone and fills many of the karst depressions along 

ridges underlain by the limestone. Locally overlain by small deposits of white, very well sorted 

quartz sand and thin deposits of black to blue-gray organic muck Thickness 1–10 m

EARLY PLEISTOCENE AND PLIOCENE

CALOOSAHATCHEE MARL—Present in subsurface only and shown by pattern on map. Grayishgreen or greenish-gray, silty, sandy marl with interbedded lenses of sand, silt, and clay. Locally 

may contain lenses of carbonaceous material that suggest former mangrove swamps. West of 
Lake Okeechobee, consists of a series of reefs composed of sandy shelly limestone and 

calcareous sandstone. Five radiometric dates on corals from the type locality range from 1.78 

Ma (million years) to 1.89 Ma and average 1.84 Ma; a single date from the St. Petersburg area 

is 2.53 Ma (Bender, 1973). All are older than the estimated age of the Pliocene-Pleistocene 

boundary (1.65 Ma). A Pleistocene age is indicated by the presence of Equus (cf. E.) Leidyi 

(DuBar, 1958a). Map unit is overlain by very thin, discontinuous patches of shelly sand. 

Maximum thickness 15 m

msa MARINE AND ALLUVIAL SAND (Facies of Citronelle Formation)—Upper part is white to lightyellow, fine to coarse, loose quartz sand, but most of unit is red, orange, yellow, or brown, 

crossbedded, pebbly, coarse to fine, kaolinitic quartz sand, casehardened by iron oxide. 

Deposit includes red- and yellow-banded lenses of generally poorly sorted, slightly kaolinitic, 

pebbly, coarse to fine sand, and thin beds of white, highly kaolinitic, well sorted, fine quartz 

sand containing numerous shrimp burrows. Locally includes pockets of white, kaolinitic clay, 

and thin stringers of discoid quartz and quartzite pebbles. Locally deposited on and collapsed 

into karst terrain developed on limestone. Coextensive with and forms an important part of 

Lake Wales Ridge (Pirkle and Yoho, 1970), a major topographic feature that forms the divide 

of the central Florida peninsula for 250 to 300 km. Considered in part to be of marine littoral 

and delta origin and in part of alluvial origin. Has been correlated with the Citronelle 

Formation (Grogan and others, 1964; Pirkle and Yoho, 1970; White, 1970) which, in its type 

locality at Citronelle in Alabama, contains a vertebrate fauna assigned a Pliocene Hemphillian 

age by F. C. Whitmore (Isphording and Lamb, 1971) and fossil leaves of Pleistocene age 

(Berry, 1916; Stringfield and Lamoureaux, 1957; Doering, 1958). Thickness 12–15 m

LAND PEBBLE PHOSPHATE DEPOSIT—Present only in subsurface and shown by pattern on 

map. Phosphatic pellets and nodules, quartz sand, and smectitic clay; chiefly massive and 

structureless but locally crossbedded or with horizontal laminations in places. Locally includes 

pockets of clay that fill sinkholes in underlying limestone. Phosphate deposit is believed by 

Cathcart and others (1952) to be a residuum derived from underlying Pliocene Bone Valley 

Formation; by Sellards (1915) to have been deposited in a shallow marine environment 

following erosion of the Miocene Hawthorn Formation. Overlain by wave-reworked beach sand 

and sandy clay (bml) as thick as 10 m. Thickness as much as 18 m

QUATERNARY AND TERTIARY

zsc SAND AND CLAY DECOMPOSITION RESIDUUM2

—White, light-yellow, grayish-orange or 

grayish-red, commonly mottled, poorly sorted, fine quartz sand and small lenses of clay, 

interbedded with light-gray, yellowish-gray, very pale orange, or light-reddish-brown, 

micaceous, medium to coarse sand; locally includes lenses of kaolinitic sandy clay that contain 

leached and partly decomposed oyster shell fragments. Mapped areas include some bedrock 

outcrops and alluvial terrace deposits too small to map separately, the latter are formed chiefly 

on the south side of the mouths of major rivers flowing into the Gulf of Mexico. Thickness 1–

2.5 m

rsi CALCAREOUS SAND SOLUTION RESIDUUM1

—Yellowish-gray to grayish-white quartz sand and 

calcareous sand residuum on soft sandy limestone and shell-hash limestone. Locally, where 

deposit mantles an underlying karst topography, it includes the clay-filling of sink holes. 

Mapped areas include some locally derived colluvium, and scattered bedrock outcrops. 

Thickness 1–3 m

1

Decomposition residuum, for purpose of this map, is defined as material derived primarily by in-place chemical decay of clastic rock with no appreciable subsequent lateral transport. 2

Solution residuum, for purposes of this map, is defined as material derived by in-place solution of carbonate 

rock or carbonate-cemented rock with no appreciable subsequent lateral transport.

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1229–1247.

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98, p. 193–202.

Broecker, W. S., and Thurber, D. L,1965, Uranium series dating of corals and oolites from Bahaman 

and Florida Key limestones: Science, v. 149, p. 58–60.

Cathcart, J. B., Blade, L V., Davidson, D. F., and Ketner, K B., 1952, The geology of the Florida Land 

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77–91. 

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121 p.

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Bulletin, v. 42, p. 1816–1862.

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River area of southern Florida: Florida Geological Survey Bulletin 40, 267 p.

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