A Generalized Genetic Framework for the Development of Sinkholes and Karst in Florida, U.S.A

A Generalized Genetic Framework for the Development of Sinkholes and Karst in Florida, U.S.A.

BARRY F. BECK limestone and subsidence of the overlying unconsolidated Florida Sinkhole Research Institute sediments causes surface collapse a subsidence doline or University of Central Florida sinkhole This process may penetrate up to 60 m of the Florida 32816 semi-consolidated Hawthorn cover, as occurred when the Winter Park sinkhole developed Dense clusters of solution ABSTRACT / Karst topography in Florida is developed on the pipes may have formed cenotes which are now found on the Tertiary limestones of the Floridan aquifer Post-depositional exposed limestone terrain diagenesis and solution have made these limestones highly permeable, T = ca. 50,000 m2/d. Zones of megaporosity have formed at unconformities, and dissolution has enlarged Groundwater moves laterally as diffuse flow except where joints and fractures Erosion of the overlying clastic Miocene input or outflow is concentrated. At sinking streams, vertical Hawthorn group strata on one flank of a structural arch has shafts, and springs, karst caves have formed, but only the exposed the limestone The elevated edge of the Hawthorn major sinkng streams form through-flowing conduit systems cover forms the Cody scarp Ubiquitous solution pipes have Shaft recharge dissipates diffusely. Spring discharge is con- previously formed at joint intersections and are now filled centrated from diffuse flow In both cases, conduits taper Downwashing of the fill deeper into solution cavities in the and merge into a zone of megaporosity

Geologic Setting sediments. In the northern peninsula and the coastal plains, the clastic, Miocene Hawthorn group caps this sequence of limestones. However, in many areas of the Florida, as well as Georgia and South Carolina, is a Florida peninsula, the majority of the lower Hawthorn part of the Southeastern Coastal Plain. During Meso- is also calcareous or dolomitic. This thick sequence of zoic and Cenozoic times, shallow marine sediments carbonate rocks constitutes one of the world's most pro- were deposited in an off-lapping sequence on the coasts lific aquifers. In Florida it is called the Floridan aquifer of both the Atlantic Ocean and the Gulf of Mexico. At and elsewhere it is termed the principal artesian the juncture of these two provinces, the Floridan pla- aquifer. teau extends southerly from the North American main- The central portion of the northern two-thirds of land. Since Cretaceous time, the Floridan plateau has peninsular Florida is occupied by a distinct subsurface been a dominantly carbonate platform on which have structural axis or elevated ridge commonly referred to accumulated thousands of feet of limestones and do- as the Peninsular Arch (Fig. 1), or "backbone." This lomites with comparatively minor amounts of evapo- area was the first to rise above sea level and become an rites and clastic sediments. The Floridan peninsula is the exposed land mass. Later, the arch itself was covered exposed portion of this plateau. Most of the peninsular by shallow transgressive and regressive seas, as evi- area is mantled by thin deposits of recent and residual denced by those rocks that now constitute the Floridan sands and clayey sands, peat, or wetlands and lakes, so aquifer in central and north Florida. The Ocala uplift that the majority of the geologic history must be inter- is a younger axis of gentle folding (Fig. 2) parallel to, preted from cores. and 30-35 miles west of, the Peninsular Arch. This The Floridan aquifer comprises a continuous se- rather broad, tension-faulted anticlinal structure is quence of limestones deposited from the Paleocene to mappable (Faulkner 1973) through exposures of the Miocene epochs (Fig. 1). Correlative limestones ex- Middle and Upper Eocene limestone and dolomite tend into the Florida panhandle (the portion extending rocks. Relationships between the Peninsular Arch, westward along the Gulf Coast), Georgia, and South Ocala uplift, tension faults, surface and subsurface Carolina. However, away from the peninsula, limestone units, and their respective general thicknesses are may be only the marine facies, and the updip (inland), shown in cross-section in Figure 1. The vertical scale in continental/littoral equivalents are frequently clastic Figure 1 is exaggerated approximately 160 times; ac-

Environ Geol Water Sci Vol 8, Nos 1/2, 5-18 1986 Springer-Verlag New York Inc 6 Bar~ F Beck

SOUTHWEST b, NORTHEAST

X C~, e..,. q.1 q~' X IO00

~ I TM, L I TMu.. I TQu.~Cb ~ u~ <~ kc(~ ~,,~ [ "- To l TMu I I TQu ] TQu..,~_.l '~ - Mean Sea Level

To p~__..~-T~ ~ ~ ',j, ~ To

T~c " '"~, r ~" I000

Tck 2000 -

_J Upper Cretaceous ~ -r (sedimentary) u~Zcr

LU 3000' CL -7

Lower Cretaceous 4000-

5000

EXPLANATION

TQu - Tertiary - Quarternary, undifferentiated TMu- Mio- Pliocene (?), undifferentiated

To- Upper Eocene, Ocolo Limestone Tap-Middle Eocene, Avon Park Limestone Tic- Middle Eocene, Lake City Limestone Tol- Lower Eocene, Oldsmar Limestone 1 Tck- Paleocene, Cedar Keys Limestone

0 I0 20 30 40 50 MILES I I I I I I LOCATION

Figure 1. E-W cross-section through peninsular Florida showing structure and its relationship to surficial geology (from Faulkner 1973). Framework for Sinkhole Development 7

(1 / \l I " ;-'-'- "- / )---/-- ~ .... I-- <'--~ / <'~ >%; ,. ,,~,o,v,~,

~,4 ,K~ '~_r '''~ .,, ~ \~.

r-,--Xr~,Ck

YANKEETOWN r,,---. ,~ \'q/_ ) 0 2 .f/ 4, -1 x, \ --/- ) \*r % o <- #-,

Approximate edge /~ of Floridon Plateau5 \ "~ , 6, (datum: mean sea level) ( L__~ ~---

% _i ~ -

o 50 Ioo MILES I 1 I

Figure 2. The Floridan plateau and peninsular Florida showing the main geologic structures (from Faulkner 1973).

tually the flanks of the Peninsular Arch and Ocala over 4,600 m thick have been encountered. The Cedar uplift dip one-half degree or less. Keys limestone, Oldsmar limestone, and Lake City The subsurface units thicken outward to the east limestone are not exposed and are known only through and west and particularly to the south were sediments cuttings and cores. The Lake City limestone, the Avon 8 Barry F Beck

Park limestone, and the Ocala group limestones gen- dish-brown, river-type water. However, there is also a erally constitute the Flor.idan aquifer. To the south and substantial groundwater addition to this flow (Ceryak north in Figure i, younger units including the Su- 1977). wannee limestone, the Tampa limestone (or its equiv- alent), and limestones within the Hawthorn group also Hydrogeology are included in the Floridan aquifer. In the central and northern peninsula, the clastic The limestones that compose the Floridan aquifer sediments of the Miocene Hawthorn group overlie and have been variously subjected to repeated cycles of sea confine the limestones of the Floridan aquifer. Along level fluctuation. During some regressions, they were the crest and western flank of the Ocala uplift, erosion subaerially exposed, producing unconformities marked has stripped off this cover leaving an exposed limestone by paleokarst surfaces (Randazzo 1982; Schmidt and surface, or one thinly mantled by younger unconsoli- Scott 1984; Popenoe and others 1984). During the dated sands and clayey sands (Figs. 1 and 3). This area transgressions and regressions, the saltwater/freshwater is broadly referred to as the Gulf coastal lowlands mixing zone also oscillated. This zone has been related (Head and Marcus 1984). It contains broad areas of to various diagenetic changes. "A major environment low, level karst plains marked by cenotes, small subsi- of regional dolomitization is in the mixing zone (zone dence sinkholes, lakes, and scattered springs (Abbott of dispersion) where profound changes in mineralogy 1972). Surface drainage is absent, except for major and redistribution of porosity and permeability occur streams or limited areas in which the cover contains from the time of early emergence and continuing impermeable strata. Groundwater flows toward major through the time when the rocks are well-developed rivers or coastal springs and swamps, or to a few major aquifers. The reactions and processes, in resonse to inland springs. Within the Gulf coastal lowlands are mixing waters of differing chemical composition, in- several extensive, high "sand" ridges and several broad, clude dissolution and precipitation of carbonate min- alluvial valleys which irregularly interrupt the karst erals in addition to dolomitization" (Hanshaw and Back plains. 1979, p. 287). It has been noted that the more perme- Because the axis of the Ocala uplift is far to the able zones of the Floridan aquifer are dolomitic (e.g., western side of the peninsula (Fig. 2), coastal erosion Lichder and others 1968). On the other hand, Ran- has not stripped the clastic cover from all of the eastern dazzo (1982, p. 9) identifies the same processes, "dis- flank of the uplift (Fig. 1). This elevated region of solution and the creation of cavernous porosity, ctastic sediments is divided into the northern and cen- infilling of pore spaces by sparry calcite cements, cal- tral highlands. The landscape in this zone is not gen- citization and dolomitization," but ascribes their erally karst, but along the western margin there is a occurrence to "interaction with a phreatic meteoric variable region (shown in Figure 3) where subsidence water environment." sinkholes (or ravelling sinks) disrupt the surface. These Whatever the exact mechanisms, the limestones and sinkholes mark areas where unconsolidated surface dolostones of the Floridan aquifer now have a high sediments, in some areas up to 60 m thick, have sub- overall permeability. For instance, Hunn and Slack sided into preexisting voids in the underlying lime- (1983) estimate a T of 45,000 m2/day in the Santa Fe stone. River Basin of north Florida. However, many authors The boundary between the highlands and the low- have also noted that certain zones, or levels, within the lands is the irregularly dissected Cody escarpment. aquifer are more cavernous (megaporosity) than others Above the escarpment the thin, clastic cover is com- (Lichder and others 1968; Robertson 1973; Krause monly breached by erosion, both from above by 1979). Zones of unusually high macroporosity are de- streams and from below by subsidence sinkholes. This veloped at unconformities at the contact of the Ocala erosional dissection has broadened the escarpment into limestone and the Suwannee limestone and at the con- a transition zone. Where the cover is breached, virtually tact of the Suwannee limestone and the Hawthorn all surface drainage flowing off the highlands disap- group (Burnson 1981). The Ocala:Suwannee contact pears underground; a short distance below the escarp- has been shown to be a locus of karst development and ment numerous springs occur. Several of these, such groundwater circulation at Peacock Springs Cave in as the Alapaha River Rise or the unnamed resurgence Florida (Fisk and Exley 1977). Within the Floridan of the Santa Fe River, constitute discrete resurgences aquifer, then, lateral flow of groundwater is somewhat apparently connected to the sinking point by contin- diffuse and homogeneous, although the greatest uous conduits, although this has not been absolutely volume of flow occurs within these zones of megapo- documented. The outflow is frequently turbid, red- rosity. Framework for Sinkhole Development 9

N. C.

-4OO %

EXPLANATION

Approximate outcrop of Selma Chalk

Tertiary limestone at or near land surface 2001

Quaternary limestone at or near land surface ~'~

Principal area in which s~nks breach the Hawthorn Formation

Line north and west of which some thin patches of Tertiary limestone may occur near land surface

Line beyond which limestone thickens and is more deeply buried

25 0 25 100 200 Miles I=~l,L I I I I I I I I Top of Tertiary limestone, m feet below sea level

Figure 3. Map showing outcrop areas of Quaternary, Tertiary, and Cretaceous limestone. Contours show top of Tertiary limestone in the subsurface (from Stringfield and LeGrand 1966).

Despite the high overall permeability, solutional en- ida and southwest Georgia in spring flow from enlarged largement of joints, fractures, and bedding planes, and joints (Rosenau and others 1977; Hunn and Slack preferential flow along these enlarged features, does 1983), joint control of cave systems (Beck and Arden still occur. This can be seen in central and north Flor- 1983), and the effects of major joint trends on surface 10 Barry F Beck

topography (Vernon 1951). Limestone dissolution is an large source of aggressive water. Beck and Arden exceedingly slow process in human terms, even though (1983) hypothesize that the caves localized beneath the it is considered rapid on a geologic time scale. The Pelham escarpment in Georgia are formed by this limestones of the Floridan aquifer have been dissolved mechanism. These caves have a spongework character over millions of years. The effects of this dissolution by and lack a distinct flow path and spring outlet. They groundwater have been combined with, and controlled are centered around the entrance shafts, and passages by, the older defects in the limestone, such as paleokarst become smaller with increasing distance from the input surfaces, zones of megaporosity, or structural disrup- points. tions. The enlargement of these joints is not more At the same geomorphic position, along the edge of prominent because the high overall permeability does the clastic cover overlying the limestone (the Cody allow diffuse flow. scarp), larger karst conduits may form where a major Recharge into the Floridan aquifer occurs by three stream sinks into the limestone and a very large volume principal mechanisms: of aggressive flow is concentrated. Along the Cody 1. Where the Floridan aquifer is confined, diffuse scarp, virtually all streams go underground. Both the recharge occurs through the confining layers Alapaha and Santa Fe rivers emerge again after a pe- whenever and wherever the shallow water table riod of subterranean flow. level is higher than the potentiometric surface; About 60% of the time, the Alapaha flows the entire the rate of recharge varies with the leakage of length of its bed; the remainder of the time, a group the confining layer, the head difference, and the of sinkholes.., captures the entire river flow It is integrity of the confining layer. suggested that once underground, the river travels 2. Point recharge occurs where the confining, or through solution channels in the limestone for ap- overlying, strata are breached by surface ero- proximately nineteen miles and emerges at two sion, structural defects, or sinkholes. In partic- springs (resurgences) that flow into the Suwannee ular, near the thinning edge of the confining River. These resurgences (Alapaha Rise and layers, surface streams may erode downward to Holton Creek) usually pump turbid, reddish-brown, the limestone and be pirated underground by river-type water; but they become clearer during sinkholes (ponors or swallets). extended periods of low flow. Holton Creek be- 3. Direct recharge occurs where the limestone is comes entirely clear and will dry up during extreme exposed, or only thinly mantled by permeable low flows when only small amounts of surface wa- sediment, such as in the coastal lowlands. This ters disappear underground in the upstream is most effective through sinkholes that open di- area... During low-flow periods, it is assumed that rectly to the aquifer. These may be solution the water that enters the sinkholes is confined to a sinks, solution pipes, or cenotes. narrow underground corridor and emerges, mixed As noted earlier, the Floridan aquifer has a high with groundwater, at the Alapaha Rise (Ceryak overall permeability and thus groundwater flow is dif- 1977, p 10) fuse. However, where the geomorphic setting has produced a concentration of groundwater flow at one Groundwater flow also converges at springs. Many point, or from one input to one outlet, a karstic con- of Florida's springs upwell from large, steeply sloping duit--a cave--has developed. In Florida this has oc- conduits that become horizontal at some greater depth curred in three settings: beneath individual, or clusters (Rosenau and others 1977). The horizontal develop- of, sinkholes where water recharges the aquifer (as ment is probably concentrated at a level at which pre- in Thrailkill 1968); where streams flow off insoluble viously developed megaporosity facilitated ground- cover and sink at one site to resurge later as predom- water flow. Such is the case at Peacock Springs in Su- inantly conduit flow (as in Shuster and White 1971); wannee County where Fisk and Exley (1977) have or at springs where the convergence of groundwater found that a three-mile-long, horizontal, underwater, flow lines create a concentration of flow (as first pop- cavern system is localized along the unconformable Su- ularized by Rhoades and Sinacori 1941). wannee:Ocala contact. Most of Florida's springs, except Apparently, the formation of caves beneath indi- those previously described along the Cody escarpment, vidual sinkholes is minimal, except in the case of ver- arise from a convergence of diffuse flow. Springs like tical shafts. Vertical shafts generally form along the Blue Springs (Volusia County), Alexander Springs, edge of an overlying, noncarbonate layer where ag- Juniper Springs, Silver Springs, Rock Springs, and gressive perched drainage first reaches the limestone. Wekiva Springs (all major springs) are marked by cir- They are basically solution pipes with a moderately cular depressions in the potentiometric surface indi- Framework for Sinkhole Development 11

I (~ I l

LOCATION OF AREA I

SILVER SPRINGSCATCHMENT ~ ~%~~~~/__

INDICATES DIRECTIONOF GROUNO WATER FLOW

0 ::' 4 MILES MOOIFiEO FROM A I I = FAULKNER, 1973 I l 1

Figure 4A. Potentiometric contours in the Silver Springs catchment area showing converging groundwater flow (from Rosenau and others 1977). B. Potentiometric contours in north-central Orange County showing cones-of-depression and flow convergence at two major springs, Wekiwa Springs (to the northwest) and Sanlando Springs (from Schiner and German 1983). cating converging radial flow. The catchment area of the converging discharge at the spring and do not con- Silver Springs has been delineated in detail by Faulkner stitute actual conduit flow as described by Schuster and (1973), as shown in Figure 4A; the relationship of two White (1977). other central Florida springs to the potentiometric surface is shown in Figure 4B. Further evidence of the Sinkhole Formation Today diffuse nature of the spring flow can be seen in the flow duration curves for Wekiva and Rock springs (Fig. Sinkholes are more properly called dolines (Jen- 5). Note the unvarying discharge as compared to other nings 1971). However, in Florida the term sinkhole or local streams. Even though these springs discharge simply sinh has become thoroughly entrenched in both from discrete, diveable conduits, they are formed by the common and professional English language, and it 12 Barry F Beck

O.Oq 00..50I 02 Q5 I 2 5 I0 20 30 40 50

5,000 '~ Econlockhatchee River near Bith~o.

3,000 \ oo oooL'--I I " \ 2 Ajay-EQst Tohopekaliga Canalnear Narcoossee

5 1,000 "~-. I ~.3("~ , ' ~'~ % 3 Jim Creek .... Christ .... a- 700 ~ .~. %. ,"-. soo ix u. SO0 ~ "~ ~ 4 Reedy Creek near Vineland. ZOO ~' " 200

=- ,oo -~--~-v> 2_~ \ \ ,oo "" -- 70 ,o ' .,&.\ ~ 50

.~ :, \ '~

I0 "~ h I0 "i\\ 5 I Z

I 6 80nnet Creek near Vinelond ~. 07 OS

7 Wekivo Springs near Apopka. '~/(~)~ .. "'-.., O03 2 Figure 5. Flow duration curves for Orange County surface streams (1-6) 8 ROCk Sl~rings near Apopka I and two springs (7 and 8) (from Licht- I 40 50 60 70 80 90 95 98 99 995 ler and others 1968). PERCENT OF TtME DISCHARGE EQUALED OR EXCEEOED THAT SHOWN will be retained herein. Sinkholes are "enclosed hollows karst landform produced by areally diffuse recharge of moderate dimensions" (Williams 1964), originating on an exposed limestone surface. They may be more because of the solution of the underlying bedrock. common here than elsewhere owing to the very low However, the mechanism by which the solution of the surface gradient favoring infiltration rather than bedrock, usually limestone, produces enclosed hollows runoff. In the Mendip Hills of England, Ford (1963) varies as does the form of the depression and the cir- found that the number of dolines was inversely pro- cumstances of development. portional to the surface gradient, thus supporting this While sinkholes in Florida are, of course, forming hypothesis. today as they have in the past, it is misleading to think The sinkholes that occur on the land surface today of the catastrophic collapses that make newspaper are generally of three types: ponors (also called swallets headlines as originating recently. The sudden subsi- or stream-sinks), subsidence sinkholes, and cenotes. dence which disrupts the present land surface is only Most of the sinking streams, as mentioned previously, a late stage expression of processes that have been oc- are confined to the transition zone along the Cody es- curring beneath the surface over thousands of years. It carpment. At the time of formation of these sinkholes, is quite clear that solution pipes are features of some the subterranean piracy frequently created a semi-blind antiquity. Solution pipes are vertical, generally cylin- valley on the surface, a surface drainage channel that drical, frequently tapering downard, voids extending received flow only when the capacity of the stream-sink from the limestone surface downward. They are gen- was exceeded by flood flow. Near Lake City in north erally located at the intersection of vertical joints. It is Florida, the former course of the Little River can be the author's observation that they frequently terminate traced to its confluence with the Suwannee River as a downward at local zones of megaporosity which facili- dry valley marked by a series of abandoned stream tate lateral flow. Infilling clays have been found to con- sinks which retreated as the Cody escarpment re- tain fossils ranging from Miocene to Pleistocene age treated. These ponors probably formed initially as the (Jones 1982). Solution pipes are quite common on the reactivation of paleokarst solution pipes when the re- limestone surface in Florida and are probably the basic treat of the cover exposed them to concentrated inflow. Framework for Sinkhole Development 13

The high volume of aggressive water was then capable karst terrain very similar to Florida. Further research of additional enlargement of the pipe and underlying is necessary to refine or refute this hypothesis. permeable zone. While these stream-sinks are very im- However, in Florida, as elsewhere (Newton 1984), portant hydrologically, particularly in relation to the vast majority of the sinkholes that collapse today groundwater pollution, their development and collapse are subsidence sinkholes or subsidence dolines. Jen- is rarely witnessed and does not significantly impact nings (1971) gives the following description: "Where human use of the land. superficial deposits or thick residual soils overlie karst In the coastal lowlands, where the limestone is near rocks, dolines can develop through spasmodic subsi- the surface, cenotes are common. Cenotes are vertically dence and more continuous piping of these materials walled or overhanging shafts in a bare, or thinly man- into widened joints and solution pipes in the bedrock fled, limestone terrain, generally with width/depth ra- beneath. They vary very much in size and shape. A tios approaching unity, and usually containing water in quick movement of subsidence may temporarily pro- the bottom. They have generally been considered col- duce a cylindrical hole which rapidly weathers into a lapse structures (Jennings 1971) where the limestone gentler, conical or bowl-shaped depression." Although roof of a preexisting horizontal conduit system col- Sweeting (1973) would call these alluvial dolines or pos- lapses. Certainly, sinkhole collapse into cave systems sibly solution subsidence dolines, none of her descrip- does occur, although rarely (Beck 1984). Fisk and Exley tions so aptly match the Floridan examples as does Jen- (1977) have noted that the many sinkhole entrances nings' quote above. Sowers (1975) has termed these rav- into the Peacock Springs Cave System are roof col- elling sinks and this terminology is prominent among lapses. Similarly, Beck and Arden (1984) have noted Florida's professional engineers. However, Jennings' cave roof collapse sinkholes in the water-filled cave (1971) term not only predates Sowers (1975), but the feeding Radium Springs in the Georgia outcrop of the study of karst has historically been the realm of geol- Ocala limestone. On the other hand, of 650 recent sink- ogists and geographers. Therefore, the term subsidence hole collapses tabulated by Beck (1984), none were due doline or subsidence sinkhole seems to have priority. As to the collapse of the roof of a limestone cave. a practical matter, the two terms should be considered In discussing the origin of cenotes, Sweeting (1973, synonyms. p. 216) states, "IT]heir distribution and frequency are In the coastal lowlands the limestone is commonly not entirely characteristic of collapse phenomena." She overlain by unconsolidated sediment, varying in thick- speculates that these may be former artesian springs ness from less than 3 m to more than 30 m. Only rarely that no longer overflow. However, the dry spring hy- is the bare limestone exposed at the land surface. pothesis is easily refuted since the cenotes in central Above the Cody escarpment, the limestone is overlain Florida do not physically resemble the presently ex- by partially indurated Hawthron group sediments and isting artesian springs that occur in the same setting, younger, less consolidated, overlying strata. Subsidence and none of the numerous cenotes show any evidence sinkholes develop throughout the lowlands and even of a former surface drainage channel leading away above the escarpment in a zone within which the cover from the site. It is possible that cenotes are an integral is generally less than 60 m thick (Fig. 3). site-specific phenomenon. Florida cenotes are fre- A simplified example of the subsidence mechanism quently marked by numerous solution pipes exposed was clearly revealed in a recent (1983) sinkhole north in their walls. Since lineaments are usually trends of of Ocala, Florida. At the site there is only approxi- closely spaced, parallel joints, where two lineaments mately three meters of sandy sediment overlying the cross, many individual joints will intersect. This could limestone. This sinkhole, which formed rapidly, con- create a dense concentration of solution pipes and di- sisted of a 3-4-meter wide, 3-meter deep, funnel- rect recharge to the aquifer. Over time the concen- shaped depression in the sand leading to a 5-6-meter trated recharge beneath the pipe cluster would dissolve deep, 1-1.5-meter diameter, almost perfectly cylin- the limestone, forming a localized cavity or void drical shaft in solid limestone. A vertical fracture was (Thrailkill 1968). The roof of this void would eventu- visible in one side of the limestone pipe. The bottom ally collapse as continued solution along joints, and of the pipe was floored with loose sand and recent within and below 'the solution pipes, would eventually vegetation debris. Numerous similar solution pipes riddle and weaken the overlying limestone. By this hy- filled with sediment are visible in the walls of the many pothesis a large, through-flowing karst cave system is local quarries. The obvious hypothesis is that this sink- not a prerequisite for cenote formation. Southworth hole formed when the sediment formerly filling the (t 984) has noted the alignment of cenotes along a frac- pipe settled, or was washed downward. Surface sedi- ture system in one area of eastern Yucatan. Mexico, a ment immediately fell into the open pipe and the sides 14 Barry F Beck

of the surface opening quickly eroded to a semi-stable the clay. Any mechanism that increases the head dif- angle of repose. Examples such as this are common in ference between the artesian water in the limestone and the thinly mantled karst area. In Citrus County, near the perched water in the surface sands will increase the the west coast, small sinkholes (less than 3 m in diam- downward flow and the downwashing of sediment and eter and 3 m deep) form at a rate approaching one per may trigger sinkhole development. The head differ- week. They are also quite common in Marion County, ence may be increased in two ways: the piezometric on the axis of the Ocala uplift, where the cover is quite surface may be lowered, or the water table may be thin. raised. The relationship of water levels and hydrostatic The major subsidence sinkholes, however, have a forces to collapse is complicated by the buoyant support more complicated history. In the covered karst area, that water may provide to the overlying sediment and the clayey Hawthorn group sediments overlying the the shrinking and cracking that may occur as sediments limestone provide a durable, cohesive bridge or roof dessicate. over vertical channels in the limestone. When the fill During the intense freeze of late December 1983, in these channels subsides, the clay bridges the void. agricultural interests used irrigation constantly to pro- Over a long period of time, the clayey sediment may tect delicate crops from freezing. In these areas, pie- gradually crumble into the channel only to wash into zometric levels dropped rapidly leaving some of the deeper conduits in the limestone. As this cavity in the shallower domestic walls without water for days. clay grows upward, its sides also crumble and it grows During this period a new sinkhole, 25 m wide, 11 m wider. Thus, over an extended period of time, a size- deep, and steep-walled, developed in Pierson, Florida, able void may develop in the clay, much larger than where cover thickness is approximately 30 m. An ad- the diameter of the underlying limestone conduits. jacent pond, approximately 90 m in diameter and 3 m When the void enlarges so much that it nears the top deep, drained into the new sink which accepted the of the Hawthorn group sediment, the thin clay roof water without filling. Witnesses reported seepage remaining may suddenly collapse, allowing the over- emerging on the sides of the sink approximately 3 m lying sandy sediments to rapidly subside into the large below land surface. This indicates that the shallow void in the clay. water table was depressed in the sinkhole area during If the limestone conduit remains open, unconsoli- or prior to its formation, probably by initial recharge dated surface sediment may continue to erode into the downward through the developing openings. Local opening and disappear to greater depths as the sink- depression of the shallow water table has been noted hole widens to stable side slopes. If there is a perched in association with other sinkhole collapses (Jammal water table in the sand, then the sand/water slurry may and Associates 1982). Over the next week, the sinkhole flow in toward and down into the conduit and cause gradually filled with water to this level, but nearby lake rapid widening of the sinkhole. Such was the case when levels indicated that the local water table around the the Winter Park Sinkhole developed in May 1981 (Fig. sinkhole was still somewhat depressed. Other examples 6). A vertical "throat" some 22 m in diameter pene- of sinkhole formation due to irrigation for freeze pro- trated the Hawthorn group and the overlying water- tection have previously been documented in various saturated sands rapidly eroded into the shaft until a areas of Florida (Rutledge 1982; Metcalfe and Hall stable slope angle was achieved. The final diameter at 1984). the land surface was approximatley 100 m and the vis- The development of subsidence sinkholes is a spas- ible depth was on the order of 31 m. Approximately modic process. Episodes of rapid subsidence, such as 147,000 m s of sediment disappeared along with several occurred at Winter Park, alternate with long periods trees, a house, and three Porsche cars. The sinkhole when the sinkhole outlet to the subsurface is plugged occurred in recreational property adjacent to a business by debris and the surface depression holds water. Over district and completely disrupted two city streets as well time surface erosion will cut back the original steep as damaging several businesses. The final damage total sides to a stable slope broadening and infilling the is estimated to have exceeded four million dollars. basin. This is the origin of many of central Florida's In all these subsidence processes, downward sedi- small, circular lakes. Eventually, several sinkhole basins ment movement is a~ded by infiltrating groundwater. may merge to form large, more irregular composite However, where it is present, the clayey, low perme- lakes. Many of these sinkhole lakes still periodically ability Hawthorn group effectively caps the limestone drain into the subsurface when the plug is disrupted. and perches water in the sands above. Water recharges Even large lakes, such as Alligator Lake or Mills Pond the limestone slowly through the thick clay, or more (in the transition zone in north Florida), periodically rapidly through cracks or sand-filled pipes breaching drain through small sinkholes in the lake bottom Framework for Sinkhole Development 15

Figure 6. Hydrogeologic conditions leading to the collapse of the Winter Park sinkhole as interpreted byJammal and Associates (1982, Fig: 10-3). 16 Barry F Beck

(Copeland 1982). Lake Jackson, located near Talla- 1. The Tertiary limestones of the Floridan aquifer hassee, has a surface area on the order of 25 km2; in are generally highly permeable. May 1982, it drained completely through a small sink- 2. Within the Floridan aquifer, there are laterally hole in the lake bottom. persistent zones of megaporosity due to diagen- Before completing this section, it is also important esis and paleokarst. to emphasize that the location of catastrophic collapse 3. Over millions of years,joints, fractures, and bed- sinkholes in Florida (and elsewhere) is not generally ding planes in the limestone have been enlarged due to increased limestone solution occurring presently by karstic dissolution. However, dissolution does (i.e., arising from groundwater pollution by industrial not generally cause perceptible changes within wastes or concentrated pumping). This has been ably the human time frame. demonstrated by Sinclair (1982, p. 13), who showed 4. Although dissolution may have created aniso- that the increase in limestone dissolution due to tropy in the aquifer, groundwater flow is gen- pumping 3,900 m3/day from four well fields would erally diffuse. amount to only 0.0027 m3/m 2 of limestone surface in a 5. Where limestone has been exposed at the land 15-year-period. However, it is still errantly stated that surface, or only thinly mantled by permeable "sinkholes are most likely to occur in areas of active cover, the ubiquitous karren features that form groundwater recharge because the dissolving action of are solution pipes (also called geological organs). the water is greatest when it first enters the limestone 6. Where the limestone aquifer is confined, re- aquifer" (Rutledge 1982, p. 96). It is more probable charge is diffuse, except where sinkholes or that the coincidence of sinkholes and recharge areas is structural or erosional defects breach the con- caused by the fact that in recharge areas downward fining layer; where the limestone is exposed at infiltration is most rapid and thus downward transpor- the land surface, sinkholes may still facilitate tation of unconsolidated sediment into preexisting rapid recharge. voids is most effective. 7. Modern sinkhole collapse is generally caused by subsidence of the unconsolidated (or semicon- solidated) overburden into preexisting solution A Generalized Genetic Framework for the pipes or other karstic voids. Development of the Present Karst Surface 8. Karstic conduits or caves have formed where in Florida groundwater flow is concentrated: (a) beneath The present karst surface topography in Florida is individual sinkholes or clusters of sinkholes; (b) complex and it generally varies from bare to thickly beneath vertical shafts or ponors (sinking mantled by overlying consolidated and unconsolidated streams) along the edge of the confining layer sediments. It differs from most other karst areas in the (the Cody escarpment) where greater volumes United States because the young (Tertiary) limestones of aggressive water enter the limestone; conduit have a high overall permeability, as compared to older enlargment is greatest where flow is greatest limestones, and because it has an extremely low surface and the resurgences of major rivers may show gradient and is very close to sea level. It shares many true conduit flow; and (c) at artesian springs characteristics in common with the karst topography in where a large volume of diffuse flow converges Yucatan (Back and Hanshaw 1970) and Puerto Rico, radially to a relatively small outlet. despite the fact that tectonic uplift has elevated the 9. Natural discharge from the aquifer occurs: (a) limestones on the latter island. as conduit flow from sinking streams as de- Karst in Florida has developed by all the same prin- scribed in 8(b); (b) as diffuse flow from artesian ciples and processes as elsewhere and has produced springs [8(c) above]; (c) into broad lowland analogous landforms (contrary to the assertions of swamps where the limestone is exposed below Thornbury 1969, p. 305), although much muted by the the potentiometric surface level; and (d) lack of gradient. The present complex karst surface in through submarine karst springs which formed Florida can be related to a ubiquitous genetic frame- in response to past lowered sea levels. work which applies equally well to the various geologic areas producing variations in resulting landforms. Most of the details of Florida's karst hydrogeologic cycle have References Cited been discussed previously. The unifying framework is Abbott, E. M. F., 1972, Karst topography as an influence on summarized below (an expansion on Beck and others land use in west central Florida; unpub. Ph.D. thesis, Dept. 1984): of Geography, University of Florida, 199 p. Framework for Sinkhole Development 17

Back, W., and B. B. Hanshaw, 1970, Comparison of chemical faunas of Northwest Peninsular Florida, /n Cenozoic ver- hydrogeotogy of the carbonate peninsulas of Florida and tebrate and invertebrate paleontology of North America: Yucatan: J. Hydrol. v. 10, p. 330-368. Southeastern Geological Society Guidebook no. 24, p. 14- Beck, B. F., 1984, A computer-based inventory of recorded 32. recent sinkholes in Florida: Sinkhole Research Inst., Uni- Krause, R. E., 1979, Geohydrology of Brooks, Lowndes, and versity of Central Florida, report 84-85-1, 12 p. western Echols Counties, Georgia: U.S. Geological Survey, Beck, B. F., and D. D. Arden, 1983, Hydrogeology and geo- Water-Resources Investigations Open-File Report 78-117, morphology of the Dougherty Plain, Southwest Georgia-- 48 p. Guidebook for Field Trip, Southeastern Geological Society Lichtler, W. F., A. Warren, and B. F. Joyner, 1968, Water of America, Tallahassee, Florida: Americus, GA, Georgia resources of Orange County, Florida: Florida Geological Southwestern College. Survey, Report of Investigation, no. 50, 150 p. Beck, B. F., and D. D. Arden, 1984, Karst hydrogeology and Metcalfe, S. F., and L.E. Hall, 1984, Sinkhole collapse in- geomorphology of the Doughtery Plain, Southwest duced by groundwater pumpage for freeze protection ir- Georgia: Southwestern Geological Society Guidebook no. rigation near Dover, Florida, January, 1977, in B. F. Beck, 26, 59 p. ed., Sinkholes: Their geology, engineering, and environ- Beck, B. F., R. Ceryak, D. T. Jenkins, T. M. Scott, and D. P. mental impact: Rotterdam, A. A. Balkema, p. 29-34. Spangler, 1984, Field guide to some illustrative examples Newton, J. G., 1984, Review of induced sinkhole develop- of karst hydrogeology in central and northern Florida: ment, in B. F. Beck, ed., Sinkholes: Their geology, engi- Sinkhole Research Inst., University of Central Florida, re- neering, and environmental impact: Rotterdam, A. A. Bal- port 84-85-2, 43 p. kema, p. 3-10. Burnson, T., 1981, Hydrogeology of the Suwannee River Popenoe, P., F.A. Kohout, and F.T. Manheim, 1984, Water Management District:/n G. Fisher, ed., Groundwater Seismic-reflection studies of sinkholes and limestone dis- in Florida. Proceedings of the First Annual Symposium on solution features on the Northeastern Florida Shelf, in B. F. Florida Hydrogeology: NW Fla., Water Mgt. Dist., Public Beck, ed., Sinkholes: Their geology, engineering, and en- !nfo. Bull. 82-2, p. 51-64. vironmental impact: Rotterdam, A. A. Balkema, p. 43-58. Ceryak, R., 1977, Hydrogeology of a river basin in a karst Randazzo, A. F., 1982, Comments on the geology of the field- terrain, Alapaha River, Hamilton County, Florida: Su- trip area, in Cenozoic vertebrate and invertebrate paleon- wannee River Water Management District Information Cir- tology of North Florida: Southeastern Geological Society cular Series IC-5, 20 p. Guidebook no. 24, p. 5-13. Copeland, R. W., 1981, Mature karst features in north Cen- tral Florida, in Karst hydrogeology and Miocene geology of Rhoades, R. G., and B. W. Sinacori, 1941, Patterns of ground- the Upper Suwannee River Basin, Hamilton County, water flow and solution: J. Geol., v. 49, p. 785-794. Florida: Southeast Geological Society Guidebook no. 23, Robertson, A.F., 1973, Hydrologic conditions in the Lake- 36 p. land Ridge area of Polk County, Florida: Florida Bureau Faulkner, G.L., 1973, Geohydrology of the cross-Florida of Geology Report of Investigation no. 64, 54 p. barge canal area with special reference to the Ocala vicinity: Rosenau, G. C., G. L. Faulkner, C. W. Hendry, Jr., and R. W. U.S. Geological Survey WRI 1-73, 117 p. Hull, 1977, Springs of Florida: Dept. of Natural Resources, Fisk, D. W., and I. S. Exley, 1977, Exploration and environ- Bureau of Geology Bull. no. 31,461 p. mental investigation of the Peacock Springs Cave system, Rutledge, A. T., 1982, Hydrology of the Floridan Aquifer in in R. R. Dilamarter, and S. C. Csallany, eds., Hydrologic Northwest Volusia County, Florida: U.S. Geological Survey problems in karst regions: Bowling Green, KY, Western Water-Resources Investigations Open-File Report 82-108, Kentucky University, p. 297-307. l16p. Ford, D.C., 1963, Aspects of the geomorphology of the Schiner, G. R., and E. R. German, 1982, Effects of recharge Mendip Hills: unpub. Ph.D. thesis, Oxford University, Bod- leian Library. from drainage wells on quality of water in the Floridan Aquifer in the Orlando area, Central Florida: U.S. Geolog- Hanshaw, B. B., and W. Back, 1979, Major geochemical pro- ical Survey Water-Resources Investigation Report no. 82- cesses in the evolution of carbonate-aquifer systems: J. Hy- 4094, 124 p. drol., v. 43, p. 287-312. Schmidt, W., and T. M. Scott, 1984, Florida karst--its rela- Head, C. M., and R. B. Marcus, 1984, The face of Florida: tionship to geologic structure and stratigraphy, in B.F. Dubuque, Iowa, Kendall/Hall, 209 p. Beck, ed., Sinkholes: Their geology, engineering, and en- Hunn, J. D., and L.J. Slack, 1983, Water resources of the vironmental impact: Rotterdam, A. A. Balkema, p. 11-16. Santa Fe River basin, Florida: U.S. Geological Survey Shuster, E. T., and W. B. White, 1971, Seasonal fluctuations Water-Resources Investigations report, no. 83-4075, 105 p. in the chemistry of limestone springs: A possible means for Jammal and Associates, 1982, The Winter Park Sinkhole: Or- characterizing carbonate aquifers: J. Hydrol., v. 14, p. 93- lando, FL, Jammal and Associates, Consulting Engineers, 128. 274 p. Sinclair, W.C., 1982, Sinkhole development resuhing from Jennings, J. N., 1971, Karst: Cambridge, MA. M.I.T. Press, ground-water withdrawal in the Tampa area, Florida: U.S. 252 p. Geological Survey Water-Resources Investigations 81-50, Jones, D.S., 1982, Some considerations of the late Eocene 19p. 18 Barry F Beck

Southworth, C. S., 1984, Structural and hydrogeologic appli- Sweeting, M.M., 1973, Karst landforms: New York, Co- cations of remote sensing data, eastern Yucatan Peninsula, lumbia University Press, 362 p. Mexico, in B. F. Beck, ed., Sinkholes: Their geology, engi- Thornbury, W. P., 1969, Principles of geomorphology: New neering, and environmental impact: Rotterdam, A. A. Bal- York, John Wiley & Sons, Inc., 305 p. kema, p. 59-64. Thrailkill, J., t968, Chemical and hydrologic factors in the Sowers, G.F., 1975, Failures in limestones in humid sub- excavation of limestone caves: Geological Society of tropics: J. Geotech. Eng. Div., Proc. A.S.C.E., GT8, p. 771- America Bull., v. 79, p. 19-46. 787. Vernon, R. O., 1951, Geology of Citrus and Levy Counties: Stringfield, V. T., and H. E. LeGrand, 1966, Hydrology of Florida Geological Survey Bull. 33, 265 p. limestone terrains in the coastal plain of the southeastern Williams, P. W, 1964, Aspects of the limestone physiography United States: Geological Society of America Special Paper of parts of counties Claire and Galway, West Ireland: no. 93, 46 p. unpub. Ph.D. thesis, Cambridge University.