|
STATE OF FLORIDA
DEPARTMENT OF ENVIRONMENTAL PROTECTION
Virginia B. Wetherell, Secretary
DIVISION OF RESOURCE MANAGEMENT
Jeremy A. Craft, Director
FLORIDA GEOLOGICAL SURVEY
Walter Schmidt, State Geologist and Chief
Open File Report 60
The geology of Warm Mineral Springs,
Sarasota County, Florida
by
Frank R. Rupert
Florida Geological Survey
Tallahassee, Florida
1994
ISSN 1058-1391
The geology of Warm Mineral Springs, Sarasota County, Florida.
Frank R. Rupert, P.G. 149
Abstract: Warm Mineral Springs is a 70 meter deep water-filled sinkhole in southern
Sarasota County, Florida. Mineral-rich anaerobic water enters the sink through a spring
vent near the bottom of the north wall of the sink. In 1991, divers explored and mapped
the spring's primary conduit to its terminus, a distance of 53 meters. Twenty-one
geological samples were collected at three meter depth intervals from the north wall of
the sink for stratigraphic analysis. The samples revealed the sinkhole is developed in
carbonates belonging to the Miocene Arcadia Formation of the Hawthorn Group. This
unit is unconformably overlain by Pleistocene Ft. Thompson Formation and
undifferentiated Pleistocene sands. A map of the spring cave and a geologic section for
the sinkhole are illustrated.
Introduction
Warm Mineral Springs is situated in
southern Sarasota County, Florida,
approximately 12 kilometers southeast of
the city of Venice (Figure 1). The area
surrounding the spring is generally flat,
sandy, terrain characteristic of the Gulf
Coastal Lowlands geomorphic zone
(White, 1970). Land surface elevation at
the spring is about three meters above
mean sea level.
Figure 1. Location Map.
The spring pool is a large, circular,
cover-collapse sink, formed in the
Miocene age carbonates which underlie
the area. It measures 72 meters in
diameter at the water surface, along its
north-south axis. In cross section
(Figure 2), the spring's profile resembles
an hour glass. At a water depth of 3 to
5 meters below the spring pool surface
(bps), the pool narrows to 48 meters in
diameter, followed by a vertical drop-off
down to 10 meters bps. Below this, the
cavity widens again, attaining a diameter
of about 55 meters at a depth of 13
meters bps. Here, the walls constrict,
forming a distinct ledge. This ledge
apparently accumulated Pleistocene
fossil material which fell into the sink; it
also served as a paleoindian burial site
during lower sealevel stands (Cockrell,
1990).
Below the ledge, the sink walls
gradually converge to a throat with
minimum diameter of 36 meters at a
depth of 30 meters bps. At 30 meters
the cavity widens again, broadening to a
width of 72 meters at the sink's
maximum depth of 70 meters bps. A
large debris cone, consisting of material
which has slumped or fallen into the
sink, rises from the spring floor to a
depth of 38 meters. This cone may offer
0
-10 c
a)
D-
r20
-30 '
O
-40 0
Q.
0
o
-50 a
-C
-60 a
-70
Explanation
Uidtmhreleted wftrdd ind
rew. que rt aldy a-ese ad pe f-sWMdte
msdale setalust ff IreeNlA
tream-pry to i e ve-ree. r e ddalu
*ring 3% quurt sad ed 5% phl hab dr.
dromnlb-lgr deoloslt day
aeetllend mUla d fstl
TIeul-iirq light dhI*e-, Ila uaully
ideoatl atdalg wIria (t 40%) qut awd.
vaibe (1 7%) phOwlodo ea, mrlla sdil.
predle adrtmbut b. En htM leeorlerl.
Figure 2. Geologic section in Warm Mineral Springs.
W..':;
I I I I I
a potentially undisturbed stratigraphic
record of the events since formation of
the sink.
Dripstone, in the form of stalactites
and stalagmites, rims the walls of the
sink at a depth of 30 meters bps. Such
dripstone formations are indicative of
subaerial exposure during Pleistocene
water table lowstands (Cockrell, 1990).
Tufa deposits, formed from the
precipitation of calcium carbonate, occur
on the ledge at 13 meters bps.
Numerous small freshwater
springs and seeps occur around the
periphery of the sink. The principal
water source for Warm Mineral Springs
is a vent in the north wall of the sink, at
a depth of 63 meters bps. An estimated
20 million gallons per day issue from this
vent (Cockrell and Murphy, 1978). This
water is warm, averaging about 30
degrees C, anaerobic, and has a high
chloride and hydrogen sulfide content.
Rosenau et al. (1977) reported sulfate
(SO,) concentrations ranging from 1,600
to 1,700 mg/I and chloride
concentrations ranging from 9,200 to
9,600 mg/I for spring water samples
taken during the period 1927 to 1972.
The warm, mineralized nature of the
water has long been an attraction to
humans, and the spring has functioned
as a bathing spa for much of the past
century.
The source of this heated,
mineralized water is uncertain. Two
primary theories have been advanced to
explain its unique nature. Stringfield
(1966) theorized that the spring is an
emergence of pre-Miocene seawater,
which was trapped beneath the Miocene
sediments over 30 million years ago.
More recently, Kohout et al. (1977)
believed the water source to be deep,
geothermally-heated seawater. These
authors hypothesized that seawater from
the Gulf of Mexico and Atlantic Ocean
seeps under the Florida Platform, to
depths approaching 1,000 meters below
mean sea level. Here it is geothermally
heated, and some rises to the near-
surface through natural fractures in the
overlying carbonates. The rising water
may be cooled and diluted in part as it
percolates through the overlying
freshwater-bearing carbonates of the
Floridan aquifer system. Warm Mineral
Springs likely represents a point where
this deep geothermal water discharges
to the surface.
The anaerobic nature of the spring
water is largely responsible for the
excellent preservation of
paleoenvironmental and archeological
remains in the sink. Well preserved
human bones and artifacts and
Pleistocene vertebrate bones have been
recovered from the 13 meter ledge in the
sink. Faunal remains from the spring
include species of Pleistocene birds, fox,
sabercat, llama, proboscideans
(mastodons and mammoths), wolf, deer,
giant ground sloth and numerous
rodents, as well as reptiles, amphibians,
and molluscs (McDonald, 1990). Leaves
and pollen from a number of tree and
plant species have also been found in
the spring (McDonald, 1990).
Mapping and Geologic Sampling
In the summer of 1991, cave divers
from the Florida State University (FSU)
Academic Diving Program, working in
conjunction with the Warm Mineral
Springs Archaeological Project
members, recovered rock specimens
from the sink walls and surveyed and
mapped the 53 meter-long primary
conduit feeding the spring. At the
request of the Florida Geological Survey
(FGS), rock specimens were collected
by divers at three meter (10 feet) water
depth intervals, from the pool surface
down to the sinkhole's maximum depth
of 70 meters (230 feet) bps.
Approximately fist-size rock samples
were removed from the north wall of the
sink, bagged, and sent to the FGS in
Tallahassee for analysis. The samples
were lithologically described and a
lithologic column for Warm Mineral
Springs constructed (Figure 2).
The divers also explored the main
conduit feeding the sink and produced
the map shown in Figure 3. This
exploration dive, at depths in excess of
62 meters, required the breathing of
special mixed gases. It is probably the
first documented use of mixed gas in
scientific diving below 62 meters (Gregg
Stanton, FSU Academic Diving Program,
personal communication, 1992).
The cave originates at a flattened,
elliptical vent, measuring about three
meters wide and one meter high. This
vent is situated 63 meters bps near the
base of the north wall of the spring sink
(Figure 2). The first segment of cave
has a clay bottom, and extends east-
northeast from the vent a distance of
approximately 8 meters. In cross
section, the cave is an elliptical tube,
typical of subaqueous phreatic caves in
Florida. About halfway along this first
segment, the floor drops from 63 to 65
meters in depth. At 8 meters into the
cave, the conduit then turns abruptly,
trending north-northeastward for 9
meters. Here, the bottom sediments
become predominantly quartz sand.
The conduit turns northwestward at a
distance of approximately 17 meters in
from the vent. From this turn, it extends
as a linear tube a distance of about 20
meters. Then, at 37 meters in from the
vent, the conduit abruptly turns east-
northeast again, and opens into a cavern
room about 10 meters long and 3.5
s64
66w
-J
METE01234
METERS
EXPLANATION
( Ceiling height in meters
65 Floor depth in meters
- Direction of flow
SPit
1 Sand
: aY
SectionA-A * '
A 63 A
r,
Figure 3. Map of main conduit feeding Warm
Mineral Springs (modified from Irving and
Wood, 1991).
meters wide. The cavern assumes a
keyhole shape in cross section, with
upper and lower portions delineated by
a horizontal constriction around the
perimeter of the cavern. The floor of the
cavern lies at about 67 meters bps. A
short, 4-meter long conduit, trending due
east, connects the east end of the
cavern with a second smaller cavern,
which forms the terminus of the cave
system, 53 meters in from the vent. This
cavern measures approximately 5 meters
long and 2.5 meters wide and has a
"keyhole" shape similar to the first
cavern. Diver Steve Irving, who explored
the last segment of cave, reported that
the water flow was barely perceptible,
and the warm, saline, anaerobic water
appears to issue from the floor of this
small cavern (S. Irving, personal
communication, 1992).
Discussion
The configuration of the primary
conduit feeding Warm Mineral Springs is
characteristic of subaqueous cave
systems in Florida (FGS unpublished
cave data). Previous workers, such as
Vernon (1951) and Windham and Spoul
(1965), have hypothesized the existence
of linear regional fracture systems in the
bedrock underlying Florida. These
fractures trend in two primary directions,
generally northwest-southeast and
northeast-southwest, with considerable
variation in the bearings of individual
fractures. Flowing water tends to exploit
fractures as paths of least flow
resistance. The linear alignment of
many sinkhole systems, some river and
stream course segments, and segments
of both coasts in Florida commonly
parallel these primary fracture directions,
and may be controlled by local fracture
systems.
The linear conduit segments in the
Warm Mineral Springs cave correspond
to the general statewide fracture
directions. If the cave was continuous
across the cavern which originally
collapsed to form the sink, the
continuation of the conduit in the
opposite (south) wall of the sink is now
plugged and obscured by the debris
cone.
The depth of the conduit, varying
between 63 and 67 meters bps, may be
largely bedding-plane controlled. Like
fractures, softer, less resistant strata in
the bedrock offer paths of least
resistance to erosion by flowing water.
Analyses of the rock specimens
reveal that the bulk of the sink is
developed in Miocene carbonates of the
Hawthorn Group (Scott, 1988). The
lithology for most of the sampled section
is comprised of phosphatic, fossiliferous,
microcrystalline dolomite. The entire
sink section is developed in the Arcadia
Formation of the Hawthorn Group, a
Late Oligocene to Early Miocene,
Chattian to Burdigalian stage unit (Scott,
1988, and T. Scott, personal
communication, 1993).
At the base of the sink (64 to 52
meters bps), this unit is a yellowish-gray
to light olive-gray, microcrystalline,
phosphatic, dolostone, containing fossil
pelecypod molds and approximately two
percent phosphate sand. Interbedded
strata of soft, clay-like dolostone,
boulders of which were observed by
divers (Steve Irving, personal
communication, 1992) in the spring
cave, may have provided a less resistant
bedding plane for development of the
spring conduit.
The Arcadia Formation in the interval
52 to 49 meters bps is a yellowish-gray
dolostone containing about seven
percent subrounded quartz sand and
five percent phosphate sand.
Recrystallization has destroyed most of
the invertebrate fossils, but small mollusc
impressions are common. Some of the
phosphatic grains resemble vertebrate
fossil fragments, possibly pieces of fish
teeth.
Above 49 meters bps, the Arcadia
Formation is comprised of yellowish-
gray, variably quartz sandy and
phosphatic microcrystalline dolomite.
The quartz sand content increases
upward from less than five percent at 49
meters bps to nearly 20 percent at 33
meters bps. Phosphate sand increases
from about one percent at 49 meters
bps to approximately seven percent at
33 meters bps. Impressions of molluscs
and vertebrate teeth fragments,
particularly shark and teleost fish teeth,
are the dominant fossils in the interval 49
to 33 meters bps.
Between 33 and 30 meters bps, the
Arcadia Formation is a very quartz
sandy, phosphatic, microfossiliferous,
microcrystalline dolostone. Quartz sand
comprises approximately 30 to 40
percent of the sample. Rounded
phosphate sand comprises about five
percent of the rock. Highly
recrystallized, unidentifiable benthic
foraminifera and other microfossil
fragments are the dominant fossils.
Perhaps due to the high sand content,
the rock in this interval is friable,
crumbling easily under pressure.
The four samples in the interval 27 to
18 meters bps are comprised of more
indurated, less sandy, recrystallized,
yellowish-gray dolostone and
microcrystalline dolostone. Quartz sand
and phosphate sand comprise five
percent or less of the samples. Mollusc
molds and recrystallized echinoids are
the predominant fossils present. These
samples, taken at 27.4, 24.4, 21.3 and
18.3 meters bps, correspond to the
position of the throat or constriction
encircling the sink and giving it the hour-
glass shape. The more resistant nature
of the strata in this interval may, at least
in part, be responsible for formation of
the ledge-like constriction at the time the
sink originally developed.
Overlying the constriction is the 13
meter ledge, upon which paleoindian
remains have been found (Cockrell,
1990). A three meter thick, greenish-
gray dolomitic clay bed occurs at 15 m,
at the edge of the burial ledge encircling
the sink. X-ray analysis of this layer
revealed it to be nearly pure, clay-size
dolomite (Meryl Enright and Dr. Ken
Osmond, F.S.U., personal
communication, 1992).
Above the clay layer, the Arcadia
Formation continues upward to the top
of the unit at three meters bps as a
greenish-gray to olive-green, very fine
dolostone, containing about 5% each of
quartz sand and phosphate sand.
The Arcadia Formation is
unconformably overlain by gray sandy
limestone and poorly consolidated
sandstone of the Pleistocene Ft.
Thompson Formation. This unit contains
what appear to be plant root casts, and
medium sand-size phosphate grains
possibly reworked from the underlying
Arcadia Formation sediments. The Ft.
Thompson is locally less than 3 meters
thick, with its top obscured by an
overlying surface veneer of
undifferentiated Pleistocene sands and
clayey sands.
Acknowledgements
The author wishes to thank a number
of people for their contributions to this
study. Dr. Wilburn Cockrell of the Warm
Mineral Springs Archeological Project
graciously provided access to the spring
for the sampling and mapping efforts,
and his cooperation is very much
appreciated. The late Mr. Parker Turner
was instrumental in coordinating the
exploration dives and sample collecting
at Warm Mineral Springs; his enthusiasm
and dedication to his work, and his
expertise in scientific cave diving were
very much appreciated, and will be
sorely missed. Special thanks are due
Mr. Steve Irving and the late Mr.
Sherwood Schile, of the Florida State
University Academic Diving Program,
and Mr. Skip Wood of the Warm Mineral
Springs Archeological Project for
collecting the geological samples,
mapping the cave system, and providing
the cave map for this report. Steve
Irving also kindly reviewed the draft text.
Drs. Tom Scott and Walt Schmidt, of the
Florida Geological Survey, reviewed
drafts of this report and provided many
useful suggestions. Dr. Ken Osmond,
F.S.U., and Meryl Enright of the F.G.S.
kindly X-rayed the dolomitic clay sample
and interpreted the results.
References
Cockrell, W.A., 1990, Archaeological
research at Warm Mineral Springs,
Florida: in: Diving for Science...'90,
Proceedings of the American
Academy of Underwater Sciences
Tenth Annual Scientific Diving
Symposium, St. Petersburg, Florida,
October 4-7, 1990, p. 69-78.
Cockreil, W.A., and Murphy, L., 1978,
Pleistocene man in Florida:
Archaeology of eastern North
America, Vol. 6, p. 1-13.
Irving, S., and Wood, S., 1991, Warm
Mineral Springs Cave, Sarasota
County, Florida: Unpublished map.
Kohout, F.A., Henry, H.R., and Banks,
J.E., 1977, Hydrogeology related
to geothermal conditions of the
Floridan Plateau, in: Smith, D.L.,
and Griffin, G.M., eds., The
geothermal nature of the Floridan
Plateau: Florida Bureau of Geology
Special Publication 21, p. 1-41.
McDonald, H.G., 1990, Understanding
the paleoecology of fossil
vertebrates: Contributions of
submerged sites: Diving for
Science...'90, Proceedings of the
American Academy of Underwater
Sciences Tenth Annual Scientific
Diving Symposium, St. Petersburg,
Florida, October 4-7, 1990, p. 273-
291.
Rosenau, J.C., Faulkner, G.L., Hendry,
C.W., and Hull, R.W., 1977, Springs
of Florida: Florida Bureau of Geology
Bulletin 31 (revised), 461 p.
Scott, T.M., 1988, The lithostratigraphy of
the Hawthorn Group (Miocene) of
Florida: Florida Geological Survey
Bulletin 59, 148 p.
Stringfield, V.T., 1966, Artesian water in
Tertiary limestone in the southeastern
states, U.S. Geological Survey
Professional Paper 517, 226 p.
Vernon, R.O., 1951, Geology of Citrus
and Levy Counties, Florida: Florida
Geological Survey Bulletin 33, 256 p.
White, W.A., 1970, Geomorphology of
the Florida peninsula: Florida
Geological Survey Bulletin 51, 164 p.
Windham, S., and Sproul, R., 1965,
Unpublished surface lineament map
derived from airphotos: Florida
Geological Survey.
~~r
|
