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Paleoecology of shallow-marine carbonate environments, middle Eocene of Peninsular Florida
Sedimentary Geology, 66 (1990) 1-11 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
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Paleoecology of shallow-marine carbonate environments, middle Eocene of Peninsular Florida ANTHONY
F . R A N D A Z Z O 1, M I E K E K O S T E R S 2, D O U G L A S and ROGER W. PORTELL 3
S. J O N E S
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J Department of Geology, University of Florida, Gainesville, FL 32611 (U.S.A.) 2 Comparative Sedimentology Division, Instituut voor Aardwetenschappen, Rijksuniversiteit Utrecht, Budapestlaan 4, 3508 TA Utrecht (The Netherlands) 3 Florida Museum of Natural History, University of Florida, Gainesville, FL 32611 (U.S.A.)
Received March 22, 1989; revised version accepted November 23, 1989
Abstract Randazzo, A.F., Kosters, M., Jones, D.S. and Portell, R.W., 1990. Paleoecology of shallow-marine carbonate environments, middle Eocene of Peninsular Florida. Sediment. Geol., 66: 1-11. The middle Eocene Avon Park Formation of west central Florida contains a carbonate sequence that accumulated under supratidal to shallow subtidal conditions. Vertically repetitive sedimentary sequences document transgressive regressive, open marine to shoreline cyclicity. Newly discovered cross-bedding and abundant fossils support these paleoenvironmental interpretations. The rocks are dolomitic and bioclastic packstones and grainstones. Allochems are chiefly benthic foraminifers, bivalves, echinoids, and algae. Planar cross-bedding occurs within megaripple sets. Bed thicknesses, lateral extent of bed types, and paleocurrent directions are nonuniform, indicating migrating or shifting hydrodynamic regimes. Trace fossils, including Ophiomorpha, are well preserved and suggest maximum water depths of three to four meters in a shoreline transition zone. Invertebrate, vertebrate, trace, and plant fossils represent more than 70 species. Abundant irregular echinoids (clypeasteroids, spatangoids, cassiduloids, and a holectypoid) also suggest sandy shoreline conditions. Erosional truncations of burrows indicate periodic storm action along the sboreface. Modern environmental and ecological analogues to this study area include the south Florida shelf and the Bahamian Platform. Mud-free, carbonate sands displaying megaripp!ed cross-beds and extensive burrowing occur adjacent to sea grass beds with finer-grained sediments and greater faunal diversity/density. Shifting conditions often result in redistribution of facies.
Introduction Sedimentological episodes which p r o d u c e d the m i d d l e Eocene c a r b o n a t e sequences in west central p e n i n s u l a r Florida, as well as the diagenetic changes to these rocks, are well defined ( R a n d a z z o a n d Saroop, 1976). T h e discovery of large scale cross-bedding a n d associated paleontological features i n these sequences has provided new insight c o n c e r n i n g their n a t u r e a n d significance, The q u a r r y in which exposures of these sequences oc0037-0738/90/$03.50
© 1990 Elsevier Science Publishers B.V.
cur has o n l y recently b e e n m a d e accessible for study (Fig. 1). Such p r e s e r v a t i o n of c r o s s - b e d d i n g has n o t b e e n previously reported for Paleogene c a r b o n a t e rocks of F l o r i d a a n d is relatively rare in the rock record generally. This p a p e r emphasizes lateral a n d vertical variations in s e d i m e n t a r y structures a n d rock fabrics, as well as the paleoecological significance of f a u n a a n d flora a n d b i o t u r b a t i o n . R e c o g n i t i o n of shallow m a r i n e depositional e n v i r o n m e n t s , t h r o u g h c o m p a r i s o n with m o d e r n analogues a n d with other
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Fig. 1. Index map of study area (crossed picks). The Dolime Minerals Quarry is located in Citrus County, Florida, Yankeetown Quadrangle, SE 1/4, Section 11, T17S, R16E.
Eocene sequences, provides important insights into the evolution of the Florida Carbonate Platform and similar regions of the world.
Stratigraphic and depositional setting The sedimentary sequences studied represent the top of the Avon Park Formation and are directly overlain by the Ocala Limestone (Miller, 1986). The Avon Park Formation is generally characterized by complex cyclical sequences of peritidal carbonate sediments which have been extensively dolomitized (Randazzo and Zachos, 1984). Vertically repetitive deposits representing supratidal to shallow subtidal environments have been recognized. Subtidal facies in the lower part of this unit exhibit a low faunal diversity, dominated by species of the benthic foraminifer Dictyoconus and large miliolid foraminifers. The unit becomes highly fossiliferous and more diverse faunaUy near the top. Lenses of peat and marine grass beds occur locally and can be found at the base of the sequence described in this paper. In contrast to the complex cyclical sequences expressed by the Avon Park strata, the rocks of the Ocala Limestone represent a more constant
ET AL.
and stable depositional environment. The presence of a more abundant and diverse marine fauna in the Ocala reflects this transition. Deposition of the Ocala Limestone is interpreted to have taken place relatively close to shore and the deposits represent shallow subtidal to intertidal, comparatively high-energy, open marine conditions (Fenk, 1979). Randazzo (1980) identified three commonly occurring subfacies in the rocks of the lower Ocala: subtidal sand facies, detrital shoal facies, and intertidal shoal facies. These subfacies delineated distinctive depositional subdivisions of the open shelf environment. Chen (1965) concluded that sediments of the Ocala Limestone were deposited under warm, shallow-water marine conditions on a relatively broad carbonate shelf or platform, similar to the present-day Bahama Bank. In broad general terms, the change from the tidal flat, restricted subtidal deposits of the Avon Park Formation to the offshore, high- and low-energy marine deposits of the Ocala Limestone is indicative of an overall transgressive sequence of sedimentation. This sea level change mimics the general trend for the middle to late Eocene depicted in the Vail et al. (1977) and Haq et al. (1987) global sea level curves. Studies by Randazzo and Saroop (1976) and Zachos (1978) support this contention. Within these stratigraphic units are recognizable cyclical patterns of marine transgressions and shoreline progradations (Randazzo, 1987). This study illustrates the importance of fluctuating sedimentary regimes and associated environments within these broader sea level changes. Well developed cross-bedding and associated sedimentologic and paleontologic features reflect a near-shore transitional environment. This environment of deposition has not been previously documented in Paleogene rocks from Florida. Fig. 2 depicts the general distribution of depositional environments of the Avon Park and Ocala units.
Petrography The quarry faces consist of tan colored, very fine- to fine-grained dolomite with a high percentage of visible moldic porosity (average 27%). Preservation of original depositional fabrics is un-
PALEOECOLOGY OF SHALLOW-MARINE CARBONATE ENVIRONMENTS
CYCLIC S ,EDIMENTATION
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NEARSHORE SHELF
I Intertidal \ Shallow _ t" Subtidal to and ~ R e s rlctedlntertidalt~/Shallow / | ISupratidal ~__~ Lagoon i S h o a l s ' " ) / S u b t i d a l ~ l I Mudflats/=lP~~/Sandflats/ ~ ~ ~ f f s h o r e l I
I
I
I
Opeo I
Fig. 2. Representativeenvironmentsof deposition for the carbonate sequences of the Avon Park Formation and Ocala Limestone.
even but is relatively high for much of the sequences. The beds appear to have been predominantly bioclastic packstones and grainstones (Dunham, 1962) originally. Allochems are chiefly whole and fragmented large foraminifers, bivalves, echinoids, and algae. A few peloidal wackestones also occur. Pore-filling dolomite cement is c o m m o n and up to 5% quartz cement also occurs. Only four samples contained calcite and these were collected from the very top of the section. A non-dolomitic, bioclastic packstone caps the sequences. Bed geometry The quarry walls studied are 590, 20, and 500 m long respectively. They reach a maximum height of 10 m above the quarry floor. The general types of bedding present include cross-beds (megarippies) and sheet to lenticular bedding (Fig. 3). The beds range from 2 cm up to 1 m thick. Cross-bedding is planar but occurs within megaripple sets. These sets show reactivation surfaces and truncations. Wavelengths of the megaripples are not continuous. Sheet to lenticular bedding occurs above, below, and lateral to the megaripple crossbedding. Paleocurrent directions, measured on the four
sets of the megaripple cross-beds, are not uniform but tend to dip 10-15 ° to the south (Fig. 4). The differences in thicknesses and lateral extent of the bed types along the parallel east and west walls of the quarry suggest migrating or shifting hydrodynamic regimes in response to systematic changes in shoreline and shoal channel geometries, sediment budgets, and more turbulent local, meteorological events. The absence of bipolar cross-bedding generally indicates a lack of tidal current transport reversals. Megaripple development represents an important but isolated episode of sedimentation. The absence of trough cross-beds or changes in megaripple geometry from undulating to straight crested or lunate (Reineck and Singh, 1975), as well as the inability to recognize changes from lower to upper flow regime conditions, suggest peritidal deposition attributable to several possible environments. Fauna and flora
Trace fossils Trace fossils are abundant in several parts of the exposed sections. In general the cross-bedded zones have the greatest concentration of burrows. The most distinctive burrows are Ophiomorpha sp.
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Fig. 3. Planar cross-beds within a megaripple set (west wall of the Dolime Minerals Quarry). Bar is 1 m.
and occur as shafts and tunnels which are smooth on the interior surface and somewhat irregularly pelleted or mamillated on the exterior surface. Burrow walls and lithified burrow fillings are both present (Fig. 5). Two types of Ophiomorpha are
NUMBER pERCENT FREQUENCY
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N
IO
20
31%
X Fig. 4. Rose diagram illustrating principal directions of dip among the four sets of megaripple cross-beds.
present. The first is more robust and occurs near the base of the west wall of the quarry with a predominantly horizontal orientation. It is generally less abundant than the second type which is principally vertical and commonly branching (including " Y " shapes) in configuration. The latter type exhibits smaller diameters and thinner walls. Minimum and maximum tube diameters of all the Ophiomorpha specimens are 0.4 and 1.7 cm, respectively. The two types of Ophiomorpha burrows are interpreted as components of an integrated burrow system. Such systems are characteristic of the best known modern analogues for the Ophiomorpha organism, thalassinidean shrimp (Callianassa major Say and other callianassids as well as certain species of Upogebia and Axius) that routinely construct knobby walled burrows (Weimer and Hoyt, 1964; Howard, 1972; Frey et al., 1978). Burrow systems include shafts, mazes, and boxworks. Branching is common and tends to occur
PALEOECOLOGY OF SHALLOW-MARINECARBONATE ENVIRONMENTS
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Fig. 5. Ophiomorpha sp. displaying irregular pelleted and mammillated exterior surfaces, and branching configuration.
at acute angles. Typically, shafts predominate in higher-energy environments while boxworks and mazes are more characteristic of lower-energy regimes (Frey et al., 1978). Occurring in all of the Ophiomorpha zones throughout the quarry are fossil remains of the decapod crustacean, Callianassa inglisestris Roberts. The fossils consist mainly of internal casts of the propodus and various other elements of the claw. In light of the observation that Callianassa major represents the best modern analogue for an Ophiomorpha-producing organism, C. inglisestris appears responsible for the Ophiomorpha burrows in this quarry. This association was postulated earlier by Randazzo and Saroop (1976) and Saroop (1977) who reported Ophiomorpha from cores which penetrated the lower Ocala and the Avon Park in this vicinity.Ophiomorpha is locally common in the Pleistocene Miami Oolite of Florida and in Mesozoic Cenozoic oolitic shoal facies generally (Kennedy, 1975; Sellwood, 1986). Other
modern analogues of subtidal carbonate sand facies with well developed Ophiomorpha have been described by White and Curran (1987) in the Bahamas. Shinn (1968) and Braithwaite and Talbot (1972) report knobby burrows of Cailianassa spp. from modern carbonate terrains in South Florida Bahamas and the Seychelles, respectively. Ophiomorpha sp. is also known from San Salvador Island (Bahamas) where it is abundant in skeletal calcarenites which interfinger with the coralline facies of the fossil reef complex and in tabular cross-stratified, Halimeda-rich, coarse-grained calcarenites interpreted to have been a Pleistocene tidal delta (Curran, 1984). Pemberton and Jones (1988) described an extremely well preserved suite of ichnofossils (including two species of Ophiomorpha) from a Pleistocene carbonate sequence on Grand Cayman Island. The ichnofossils were created in response to hydrodynamic conditions rather than the composition of the substrate, emphasizing that ichnofossils of some carbonate sys-
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tems should be treated in a similar manner to their siliciclastic counterparts. The Eocene ichnofossils reported here offer important, additional support for this concept. Less common, and less distinctive, are burrows attributed to worms, probably polychaetes. These burrows are also found near the base of the studied sections and range from vertical to horizontal in orientation. In the limestone unit at the top of the sequence, several specimens of a third trace fossil were recovered. This enigmatic structure of conical to subcylindrical shape, possessing an internal, central tube, is well known from the Ocala Limestone and is interpreted as the burrow of a soft-bodied organism (Diblin et al., 1987).
Macrofossils Diagenetic processes, including dolomitization, have resulted in selective preservation of what was once a fairly diverse fauna. No original shell material is present, except perhaps in some calcitic fossils from the thin limestone bed at the top of the sequence. Most fossils are preserved as molds and casts. Nevertheless, it is possible to identify many taxa (particularly irregular echinoids) and to offer a paleoecological assessment of the environment of deposition. Much of the molluscan fauna is represented by unidentifiable molds and casts; yet, many species are recognizable. Gastropods comprise at least 10 taxa, including Velates floridanus Richards, an archaeogastropod with Tethyan, Old World affinities (Palmer and Richards, 1953). Some 17 genera of bivalves are present, with the large lucinid, Pseudomiltha megameris Dall, being particularly distinctive. In general, the shallow marine molluscan fauna is composed of species representing a diversity of life habits and habitats so that more precise paleoenvironmental assessments are difficult. Only one bryozoan specimen, a large, branching cheilostome which has eluded identification, has been recovered. The remainder of the macroinvertebrate fauna, excluding the above mentioned ichnofossils, an unidentifiable balanomorph barnacle, a solitary coral, and the decapod crustacean, Callianassa inglisestris, consists of echinoderms. Two partial
A.F. RANDAZZO
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impressions and numerous ossicles from an unidentified sea star and large spines from one species of regular echinoid [cf. Phyllacanthus mortoni (Conrad)] indicate that irregular echinoids were not the only echinoderms to inhabit this environment. Irregular echinoids are represented by at least 12 species. Few, if any, specimens were found in life position and no preferred orientations were recorded. Evidently currents were strong enough to transport and overturn echinoid tests over short distances but did not damage fragile forms (such as sand dollars) in the process. Crushing of some of the more robust species, however, indicates that compaction and diagenesis have played significant roles. Irregular echinoids are vagrant, benthic organisms which are predominantly infaunal~ though epifaunal forms are widespread. Most modern species are very sensitive to the type of substrate in which they burrow and this is reflected in the morphology of the test (Nichols, 1959). By comparison with modern analogues, substrate preferences of fossil forms can be inferred. As with almost all echinoderms, echinoids are dependent upon fairly constant, normal marine salinity (Boolootian, 1966). Additional factors influencing their spatial distribution include temperature and water depth; however, the comparatively wide range of depths and temperatures inhabited by extant representatives of genera found in the lower Ocala limits our ability to interpret Eocene paleodepths or paleotemperatures. Irregular echinioids from four orders are represented: clypeasteroids, spatangoids, cassiduloids, and a holectypoid. Oligopygus phelani Kier, a guide fossil for the lower Ocala (Hunter, 1976; Zachos and Shaak, 1978; McKinney and Jones, 1983) is the only holectypoid present. The distribution of Oligopygus in the Ocala seems to be influenced little by substrate types (Croft and Shaak, 1985) and since the extinct oligopygoids are only known from middle and upper Eocene strata, it is difficult to make paleoecological inferences beyond the observation that their geographic distribution suggests they preferred warm water (Kier, 1967). Three species of cassiduloids, Rhyncholampas (= Cassidulus ) ericsoni (Fischer), R. globosus (Fischer), and an unidentified species of Rhyncho-
PALEOECOLOGY
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CARBONATE
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ENVIRONMENTS
lampas, were also recovered. Though the ecology of m o d e m cassiduloids deserves more study, Holocene species tend to live on sandy substrates, perhaps buried up to their petals (Kier, 1962; Smith, 1980). It seems likely the species from the Ocala also lived on sandy bottoms (Croft and Shaak, 1985). Dominating the echinoid assemblage are the spatangoids (heart urchins) and the clypeaster0ids (including the sand dollars). Eupatagus antillarum (Cotteau), and particularly the large individuals of E. clevei (Cotteau) (= E. ingens) Kier (1984), represent the most striking spatangoids in the quarry. The former is found only in the limestone bed at the top of the section whereas the latter occurs throughout the dolomitized zone. Kier (1984) stated that it is doubtful Eupatagus could burrow in fine sediment. From functional morphological arguments he suggested Eupatagus lived buried in sand. A similar habitat is also postulated for the much smaller Agassizia clevei Cotteau ( = A . floridana) Kier (1984). A single specimen of a brissid heretofore unreported from the lower Ocala, Plagiobrissus curvus (Cooke), adds a fourth spatangoid species to the list. Plagiobrissus, like Eupatagus, lives buried in sand and builds no tunnel to the sediment-water interface (Kier, 1984). The most distinctive clypeasteroid in this sequence is the large, circular, stratigraphically diagnostic sand dollar, Periarchus lyelli floridanus Fischer. This species is characteristic of the lowest faunizone (Inglis Formation) in the Ocala (Puri, 1957). Like the smaller sand dollars from this quarry [Neolaganum durharni (Cooke) and Durhamella ocalanum (Twitchell)] and their m o d e m counterparts, Periarchus probably lived in a sandy substrate, buried a few millimeters deep in the sand (Croft and Shaak, 1985). The conditions preferred by the tiny, more spheroidal, clypeasteroid, Fibularia vaughani (Twitchell), are uncertain. Near the bottom of the section, plant fossils of marine grasses occur. These sea grasses are well known from the Avon Park Formation of Citrus and LevY counties (Dixon, 1972; Randazzo and Saroop, 1976; Lumbert et al., 1984) where they occur, not only as impressions of leaf blades, but also as entire rhizome mats in growth position.
The sea grass assemblage closely resembles the Thalassia syringodium communities typical of south Florida today and probably served as a principal food source of sea cows such as Protosiren (Domning et al., 1982), whose remains are also found at this site. Other vertebrate fossils include a marine crocodile, a sea turtle, a barracuda, a porcupine fish, three types of shark (tiger, sand, and lemon), and a primitive whale (Basilosaurus sp.). The plant and vertebrate fossils are all indicative of shallow-marine, tropical to subtropical conditions.
Environment of deposition Sedimentologic and paleontologic features of these rocks indicate a shallow, normal marine, near-shore environment. These sequences show evidence of recurring high-energy events in a subtidal setting. Poor preservation limits paleoenvironmental interpretations based on most invertebrate groups except the irregular echinoids. From the information presented above, it appears that the echinoid assemblage is uniformly composed of sand-dwelling species to the exclusion of mud or gravel substrates. While the salinity of the water must have been near normal, the depth range suggested by the echinoids could have extended from the shallow subtidal to much deeper. In general, the clypeasteroids (especially sand dollars) seem to be adapted to near-shore, high-energy, sandy environments (Seilacher, 1979) but the spatangoids, being less mobile and liable to suffocate if stranded at low tide, suggest a slightly deeper habitat. Using independent approaches, Croft and Shaak (1985) and McKinney and Zachos (1986) investigated the paleoenvironmental and stratigraphic relationships of the echinoid fauna of the Ocala Limestone. Their conclusions regarding the lower Ocala, based on functional morphological and statistical (cluster) analyses, respectively, generally agree with the inferences drawn here from an integrated sedimentologic and paleontologic approach. Both investigations concluded that the lower Ocala was deposited under shallow subtidal conditions which deepened higher in the section before eventually shoaling in the uppermost
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Ocala. Water depths were probably less than 10 m (McKinney and Zachos, 1986) and the energy levels were high. Total echinoid diversity and abundance were found in both studies to be very high in the lower Ocala with large and small sand dwellers predominating. Trace fossil evidence supports the paleoenvironmental assessments based on the macrofossils. The Ophiomorpha association is interpreted as indicating a shoreface transition zone. Frey et al. (1978, p. 215) cautioned that both the ecology and ethology of the best known Ophiomorpha organism, Callianassa major, are variable and it should not be assumed without qualification that Callianassa burrows and Ophiomorpha are necessarily indicative of beaches or the shallow sublittoral sands seaward of them. Yet, the weight of the paleontological and sedimentological evidence in this instance strongly supports this conventional interpretation. Ophiomorpha generally increases in density from the offshore to the shoreface environment (Howard, 1972). In this Florida sequence predominantly horizontal Ophiomorpha burrows, characteristic of the shoreface, become more common as both forms occur contemporaneously. The presence of both forms in these sequences suggests water depths were probably no more than 3 - 4 m (H.A. Curran, personal communication, 1988). Probable polychaete burrows at the base of the sections, however, indicate that the substrate was stable, with normal shallow shelf water movement. Above this zone Ophiomorpha burrows are truncated by erosional surfaces indicating the action of periodic storm action along the shoreface. The distribution of burrows is uneven, suggesting burrowers responded to hydrodynamic conditions by constructing new burrows rather than repairing old ones. Carbonate sand facies are generally a part of higher-energy environments in which modern carbonate sediments form. Sand belts occur with sandwaves and sandbars of various types (Ball, 1967). Mobile sand belts commonly have impoverished benthic faunas because of the instability of the substrate. These sand bodies undergo rapid migration during storms. Spatial changes in the cross-bedded megaripples and lenticular beds,
A.F. R A N D A Z Z O ET AL.
accompanied by differences in wavelength, suggest complicated variations in flow regimes. Lenticular beds were formed by stronger currents which truncated and modified megaripple geometry. Megaripple cross-beds reflect an environment of shifting sediments. The complex environments of the shorezone are well documented by the exposures studied. Peloidal and bioclastic facies, displaying several bed geometries and accompanied by extensive bioturbation, suggest a shifting environmental setting typical of the shorezone. Periodic higher-energy conditions, probably resulting from storm surges, have superimposed sedimentary characteristics upon the deposits. The presence of thin, planar, well sorted, finegrained dolomite beds (Randazzo and Cook, 1987), with significant concentrations of sea grasses at the base of the section, and the occurrence of calcitic packstones at the top of the section, reflect major changes in sedimentological conditions. A more normal, open shelf marine environment replaced the shallower, near-shore ecologic habitat.
Modern analogues Carbonate environments in which carbonate sand sedimentation is occurring are c o m m o n in the modern Bahamian Platform and south Florida shelf. High current velocities have produced mudfree carbonate sands with complex geometric configurations (Sellwood, 1986). These sand bodies are frequently associated with actively growing coral reefs, shoal areas, and grass covered flats. The island of San Salvador, Bahamas is just one example of the interactive carbonate facies related to energy conditions and faunal distribution. Megarippled, cross-bedded sands, with extensive burrowing (Curran and White, 1984) but sparse faunal population, can be found adjacent to sea grass beds with finer carbonate sand and greater faunal diversity and density. More open marine conditions occur just a few tens of meters from the shoreline. Shifting conditions associated with storms and shoreline and substrate configuration changes could bring about a redistribution of these associated facies. Likewise, variations in sea level could also bring about such facies distributions. The sedimentary
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PALEOECOLOGY OF SHALLOW-MARINE CARBONATE ENVIRONMENTS
and faunal characteristics observed in the study area closely resemble those of some of the nearshore carbonate environments of San Salvador. The principal difference between this modem analogue and that of the study area is the absence of a reefal facies in the Eocene sequence. This difference is poorly understood (Randazzo, 1987) and poses an interesting challenge for future study.
sediments and greater faunal diversity and population. The sedimentological and paleontological features of these carbonate sequences are clearly indicative of a shallow, normal marine, near-shore environment. They represent recurring high-energy conditions in a setting of subtidal deposition.
Acknowledgements Summary Middle Eocene carbonate sequences of peninsular Florida represent an important component of the development of the Florida Platform. Vertically repetitive, dolomitic and bioclastic packstones and grainstones suggest alternating open marine to shoreline conditions within broader ranging changes in sea level. Well developed, planar cross-bedding (reported for the first time for these Eocene sequences), occurring within megaripple sets, expresses non-uniform paleocurrent directions. The carbonate sequences indicate migrating or shifting hydrodynamic regimes in response to systematic changes in shoreline and shoal channel geometries, sediment budgets, and local meteorological events. Abundant burrows of the trace fossil Ophiomorpha sp. have horizontal and vertically branching systems including shafts, mazes, and boxworks. Comparison with modem burrow forms from the Bahamas suggest maximum water depths of 3-4 m in a shoreline transition zone. More than 70 species of organisms were identified in the carbonate sequences. Abundant specimens of four orders of echinoids (clypeasteroids, spatangoids, cassiduloids, and a holectypoid) reflect their preference for sandy substrates within a shoreline transition zone. Other invertebrate, vertebrate, and plant fossils within the sequences are indicative of shallow-marine, tropical and subtropical conditions. Sedimentological and ecological conditions of the Florida Platform during the middle Eocene appear to be remarkably similar to modern carbonate analogues of the south Florida shelf and the Bahamian Platform. Cross-bedded, extensively burrowed, mud-free sands are commonly found adjacent to sea grass beds with finer-grained
We gratefully acknowledge the cooperation of the Dolime Minerals Corporation, and in particular, Tom Jones, the quarry manager. David Kendrick assisted in the collection of fossils and Gary Morgan identified the vertebrates. Dr. H. Allen Curran, Smith College, visited the site, and offered significant insights concerning the ichnofauna. We are particularly appreciative of his input and thorough review of this manuscript. Heather Quinn provided valuable help in collecting field data and through group discussions. This paper represents University of Florida Contribution to Paleobiology No. 342.
References Ball, M.M., 1967. Carbonate sand bodies of Florida and the Bahamas. J. Sediment. Petrol., 37: 556-591. Boolootian, R.A. (Editor), 1966. Physiology of Echinodermata. Wiley Interscience Publishers, New York, 822 pp. Braithwaite, C.J.R. and Talbot, M.R., 1972. Crustacean burrows in the Seychelles, Indian Ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 11: 265-285. Chayes, F., 1956. Petrographic Modal Analysis--An Elementary Statistical Appraisal. John Wiley and Sons, New York, N.Y., 113 pp. Chen, C.S., 1965. The regional lithostratigraphic analysis of Paleocene and Eocene rocks of Florida. Fla. Geol. Surv. Bull., 45:105 pp. Croft, M. and Shaak, G.D., 1985. Ecology and stratigraphy of the echinoids of the Ocala Limestone (Late Eocene). Tulane Stud. Geol. Paleontol., 18: 127-143. Curran, H.A., 1984. Ichnology of Pleistocene carbonates on San Salvador, Bahamas. J. Paleontol., 58: 312-321. Curran, H.A. and White, B., 1984. Field guide to the Cockburn Town fossil coral reef, San Salvador, Bahamas. In: J.W. Teeter (Editor), Proceedings of the 2nd Symposium on the Geology of the Bahamas. College Center of the Finger Lakes Bahamian Field Station, pp. 71-95. Diblin, M.C., Randazzo, A.F. and Jones, D.S., 1987. Description, occurrence, and origin of an enigmatic sedimentary structure from the Ocala Limestone, Florida. Soc. Econ.
10 Paleontol. Mineral. Annu. Midyear Meet., Austin, Texas, IV: 20. Dixon, F.S., 1972. Paleoecology of an Eocene mud flat deposit (Avon Park Formation, Claibornian) in Florida. M.Sc. Thesis, University Florida, Gainesville, Fla. 44 pp. (unpublished). Domning, D.P., Morgan, G.S. and Ray, C.E., 1982. North American Eocene sea cows (Mammalia: Sirenia). Smithsonian Contrib. Paleobiol., 52:69 pp. Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E. Ham (Editor), Classification of Carbonate Rocks. Am. Assoc. Petrol. Geol., Mem., 1: 108-121. Fenk, E.M., 1979. Sedimentology and stratigraphy of middle and upper Eocene carbonate rocks, Lake, Hernando, and Levy Counties, Florida. M.Sc. Thesis, University Florida, Gainesville, Fla., 133 pp. (unpublished). Frey, R.W., Howard, J.D. and Pryor, W.A., 1978. Ophiomorpha: its morphologic, taxonomic, and environmental significance. Palaeogeogr., Palaeoclimatol., Palaeoecol., 23: 199-229. Friedman, G.M., 1959. Identification of carbonate minerals by staining methods. J. Sediment. Petrol., 29: 87-97. Haq, B.U., Hardenbol, J. and Vail, P.R., 1987. Chronology of fluctuating sea levels since the Triassic. Science, 235: 1156-1167. Howard, J.D., 1972. Trace fossils as criteria for recognizing shorelines in the stratigraphic record. In: J.K. Rigby and W.K. Hamblin (Editors), Recognition of Ancient Sedimentary Environments. Soc. Econ. Paleontol. Mineral., Spec. Publ. 16: 215-225. Hunter, M., 1976. Mid Tertiary carbonates, Citrus, Levy, and Marion Counties, west central Florida: Southeast. Geol. Soc. (Tallahassee, Fla.), Field Trip Guideb., 18: 2-14. Kennedy, W.J., 1975. Trace fossils in carbonate rocks. In: R.W. Frey (Editor), The Study of Trace Fossils. SpringerVerlag, New York, N.Y., pp. 377 398. Kier, P.M., 1962. Revision of the cassiduloid echinoids. Smithson. Misc. Collect., 144:262 pp. Kier, P.M., 1967. Revision of the oligopygoid echinoids. Smithson. Misc. Collect., 152:149 pp. Kier, P.M., 1984. Fossil spatangoid echinoids of Cuba. Smithson. Contrib. Paleobiol., 55:336 pp. Lumbert, S.H., den Hartog, C., Phillips, R.C. and Olsen, F.S., 1984. The occurrence of fossil seagrasses in the Avon Park Formation (late middle Eocene), Levy County, Florida (U.S.A.). Aquat. Bot., 20: 121-129. McKinney, M . L and Jones, D.S., 1983. Oligopygoid echinoids and the biostratigraphy of the Ocala Limestone of peninsular Florida. Southeast. Geol., 24: 21-30. McKinney, M.L. and Zachos, L.G., 1986. Echinoids in biostratigraphy and paleoenvironmental reconstruction: a cluster analysis from the Eocene Gulf Coast (Ocala Limestone). Palaois, 1: 420-423. Miller, J.A., 1986. Hydrogeologic framework of the Floridan aquifer system in Florida and parts of Georgia, Alabama,
A.F. RANDAZZO ET AL. and South Carolina. U.S. Geol. Surv. Prof. Pap., 1403B: 91 pp. Nichols, D.. 1959. Mode of life and taxonomy in irregular sea-urchins. Spec. Publ. System. Assoc., 3: 61-80. Palmer, K.V.W. and Richards, H.G., 1953. Part 4: Discussion. In: H.G. Richards and K.V.W. Palmer (Editors), Eocene Mollusks from Citrus and Levy Counties, Florida. Fla. Geol. Surv. Bull., 35: 56-59. Pemberton, S.G. and Jones, B., 1988. lchnology of the Pleistocene lronshore Formation, Grand Cayman Island, British West Indies. J. Paleontol., 62: 495-505. Puri, H.S., 1957. Stratigraphy and zonation of the Ocala Group. Fla. Geol. Surv. Bull., 38:248 pp. Randazzo, A.F., 1980. Geohydrological model of the Floridan aquifer in the southwest Florida Water Management District. Water Resour. Res. Cent., Univ. Fla, Publ. 46:79 pp. Randazzo, A.F., 1987. Tertiary sedimentology and evolution of the Florida Platform. Geol. Soc. Am., Abstr. Progr., 19: 812. Randazzo, A.F. and Cook, D.J., 1987. Characterization of dolomitic rocks from the coastal mixing zone of the Floridan Aquifer, U.S.A.. Sediment. Geol., 54: 169-192. Randazzo, A.F. and Saroop, H.C., 1976. Sedimentology and paleoecology of middle and upper Eocene carbonate shoreline sequences, Crystal River, Florida, U.S.A. Sediment. Geol., 15: 259-291. Randazzo, A.F. and Zachos, L.G., 1984. Classification and description of dolomitic fabrics of rocks from the Floridan aquifer, U.S.A. Sediment. Geol., 37: 151-162. Reineck, H.E. and Singh, I.B., 1975. Depositional Sedimentary Environments. Springer-Verlag, New York, N.Y., 439 pp. Saroop, H.C., 1977. Callianassid burrows in the Avon Park Formation of Florida. Fla. Sci., 40: 160-166. Seilacher, A., 1979. Constructional morphology of sand dollars. Paleobiology, 5: 191-221. Sellwood, B.W., 1986. Shallow marine carbonate environments. In: H.G. Reading (Editor), Sedimentary Environments and Facies. Elsevier, Amsterdam, 2nd ed., pp. 283-342. Shinn, E.A., 1968. Burrowing in recent lime sediments of Florida and the Bahamas. J. Paleontol., 42: 879-894. Smith, A.B., 1980. The structure, function, and evolution of tube feet and ambulacral pores in irregular echinoids. Paleontology, 23: 39-83. Vail, P.R., Mitchum, R.M. and Thompson, S., 1977. Seismic stratigraphy and global changes of sea level, 4. Global cycles of sea level changes. In: C.E. Payton (Editor), Seismic Stratigraphy--Applications to Hydrocarbon Exploration. Am. Assoc. Petrol. Geol., Mere., 26: 83-98. Vernon, R.O., 1951. Geology of Citrus and Levy Counties, Florida. Fla. Geol. Surv. Bull., 33:365 pp. White, B. and Curran, H.A., 1987. Coral reef to eolianite transition in the Pleistocene rocks of Great Inagua Island, Bahama. In: H.A. Curran (Editor), Proceedings of the 3rd Symposium on the Geology of the Bahamas. College Center of the Finger Lakes Bahamian Field Station, pp. 165-179. Weimer, R.J. and Hoyt, J.H., 1964. Burrows of Callianassa
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major Say: geologic indicators of littoral and shallow neritic environments. J. Paleontoi., 38: 761-767. Zachos, L.G., 1978. Stratigraphy and petrology of two shallow wells Citrus and Levy Counties, Florida. M.Sc. Thesis, University Florida, Gainesville, Fla., 105 pp. (unpublished).
11 Zachos, L.G. and Shaak, G.D., 1978. Stratigraphic significance of the Tertiary echinoid Eupatagus ingens Zachos. J. Paleontol., 52: 921-927.
1
Paleoecology of shallow-marine carbonate environments, middle Eocene of Peninsular Florida ANTHONY
F . R A N D A Z Z O 1, M I E K E K O S T E R S 2, D O U G L A S and ROGER W. PORTELL 3
S. J O N E S
3
J Department of Geology, University of Florida, Gainesville, FL 32611 (U.S.A.) 2 Comparative Sedimentology Division, Instituut voor Aardwetenschappen, Rijksuniversiteit Utrecht, Budapestlaan 4, 3508 TA Utrecht (The Netherlands) 3 Florida Museum of Natural History, University of Florida, Gainesville, FL 32611 (U.S.A.)
Received March 22, 1989; revised version accepted November 23, 1989
Abstract Randazzo, A.F., Kosters, M., Jones, D.S. and Portell, R.W., 1990. Paleoecology of shallow-marine carbonate environments, middle Eocene of Peninsular Florida. Sediment. Geol., 66: 1-11. The middle Eocene Avon Park Formation of west central Florida contains a carbonate sequence that accumulated under supratidal to shallow subtidal conditions. Vertically repetitive sedimentary sequences document transgressive regressive, open marine to shoreline cyclicity. Newly discovered cross-bedding and abundant fossils support these paleoenvironmental interpretations. The rocks are dolomitic and bioclastic packstones and grainstones. Allochems are chiefly benthic foraminifers, bivalves, echinoids, and algae. Planar cross-bedding occurs within megaripple sets. Bed thicknesses, lateral extent of bed types, and paleocurrent directions are nonuniform, indicating migrating or shifting hydrodynamic regimes. Trace fossils, including Ophiomorpha, are well preserved and suggest maximum water depths of three to four meters in a shoreline transition zone. Invertebrate, vertebrate, trace, and plant fossils represent more than 70 species. Abundant irregular echinoids (clypeasteroids, spatangoids, cassiduloids, and a holectypoid) also suggest sandy shoreline conditions. Erosional truncations of burrows indicate periodic storm action along the sboreface. Modern environmental and ecological analogues to this study area include the south Florida shelf and the Bahamian Platform. Mud-free, carbonate sands displaying megaripp!ed cross-beds and extensive burrowing occur adjacent to sea grass beds with finer-grained sediments and greater faunal diversity/density. Shifting conditions often result in redistribution of facies.
Introduction Sedimentological episodes which p r o d u c e d the m i d d l e Eocene c a r b o n a t e sequences in west central p e n i n s u l a r Florida, as well as the diagenetic changes to these rocks, are well defined ( R a n d a z z o a n d Saroop, 1976). T h e discovery of large scale cross-bedding a n d associated paleontological features i n these sequences has provided new insight c o n c e r n i n g their n a t u r e a n d significance, The q u a r r y in which exposures of these sequences oc0037-0738/90/$03.50
© 1990 Elsevier Science Publishers B.V.
cur has o n l y recently b e e n m a d e accessible for study (Fig. 1). Such p r e s e r v a t i o n of c r o s s - b e d d i n g has n o t b e e n previously reported for Paleogene c a r b o n a t e rocks of F l o r i d a a n d is relatively rare in the rock record generally. This p a p e r emphasizes lateral a n d vertical variations in s e d i m e n t a r y structures a n d rock fabrics, as well as the paleoecological significance of f a u n a a n d flora a n d b i o t u r b a t i o n . R e c o g n i t i o n of shallow m a r i n e depositional e n v i r o n m e n t s , t h r o u g h c o m p a r i s o n with m o d e r n analogues a n d with other
2
A.F, R A N D A Z Z O
7'
Fig. 1. Index map of study area (crossed picks). The Dolime Minerals Quarry is located in Citrus County, Florida, Yankeetown Quadrangle, SE 1/4, Section 11, T17S, R16E.
Eocene sequences, provides important insights into the evolution of the Florida Carbonate Platform and similar regions of the world.
Stratigraphic and depositional setting The sedimentary sequences studied represent the top of the Avon Park Formation and are directly overlain by the Ocala Limestone (Miller, 1986). The Avon Park Formation is generally characterized by complex cyclical sequences of peritidal carbonate sediments which have been extensively dolomitized (Randazzo and Zachos, 1984). Vertically repetitive deposits representing supratidal to shallow subtidal environments have been recognized. Subtidal facies in the lower part of this unit exhibit a low faunal diversity, dominated by species of the benthic foraminifer Dictyoconus and large miliolid foraminifers. The unit becomes highly fossiliferous and more diverse faunaUy near the top. Lenses of peat and marine grass beds occur locally and can be found at the base of the sequence described in this paper. In contrast to the complex cyclical sequences expressed by the Avon Park strata, the rocks of the Ocala Limestone represent a more constant
ET AL.
and stable depositional environment. The presence of a more abundant and diverse marine fauna in the Ocala reflects this transition. Deposition of the Ocala Limestone is interpreted to have taken place relatively close to shore and the deposits represent shallow subtidal to intertidal, comparatively high-energy, open marine conditions (Fenk, 1979). Randazzo (1980) identified three commonly occurring subfacies in the rocks of the lower Ocala: subtidal sand facies, detrital shoal facies, and intertidal shoal facies. These subfacies delineated distinctive depositional subdivisions of the open shelf environment. Chen (1965) concluded that sediments of the Ocala Limestone were deposited under warm, shallow-water marine conditions on a relatively broad carbonate shelf or platform, similar to the present-day Bahama Bank. In broad general terms, the change from the tidal flat, restricted subtidal deposits of the Avon Park Formation to the offshore, high- and low-energy marine deposits of the Ocala Limestone is indicative of an overall transgressive sequence of sedimentation. This sea level change mimics the general trend for the middle to late Eocene depicted in the Vail et al. (1977) and Haq et al. (1987) global sea level curves. Studies by Randazzo and Saroop (1976) and Zachos (1978) support this contention. Within these stratigraphic units are recognizable cyclical patterns of marine transgressions and shoreline progradations (Randazzo, 1987). This study illustrates the importance of fluctuating sedimentary regimes and associated environments within these broader sea level changes. Well developed cross-bedding and associated sedimentologic and paleontologic features reflect a near-shore transitional environment. This environment of deposition has not been previously documented in Paleogene rocks from Florida. Fig. 2 depicts the general distribution of depositional environments of the Avon Park and Ocala units.
Petrography The quarry faces consist of tan colored, very fine- to fine-grained dolomite with a high percentage of visible moldic porosity (average 27%). Preservation of original depositional fabrics is un-
PALEOECOLOGY OF SHALLOW-MARINE CARBONATE ENVIRONMENTS
CYCLIC S ,EDIMENTATION
3
NEARSHORE SHELF
I Intertidal \ Shallow _ t" Subtidal to and ~ R e s rlctedlntertidalt~/Shallow / | ISupratidal ~__~ Lagoon i S h o a l s ' " ) / S u b t i d a l ~ l I Mudflats/=lP~~/Sandflats/ ~ ~ ~ f f s h o r e l I
I
I
I
Opeo I
Fig. 2. Representativeenvironmentsof deposition for the carbonate sequences of the Avon Park Formation and Ocala Limestone.
even but is relatively high for much of the sequences. The beds appear to have been predominantly bioclastic packstones and grainstones (Dunham, 1962) originally. Allochems are chiefly whole and fragmented large foraminifers, bivalves, echinoids, and algae. A few peloidal wackestones also occur. Pore-filling dolomite cement is c o m m o n and up to 5% quartz cement also occurs. Only four samples contained calcite and these were collected from the very top of the section. A non-dolomitic, bioclastic packstone caps the sequences. Bed geometry The quarry walls studied are 590, 20, and 500 m long respectively. They reach a maximum height of 10 m above the quarry floor. The general types of bedding present include cross-beds (megarippies) and sheet to lenticular bedding (Fig. 3). The beds range from 2 cm up to 1 m thick. Cross-bedding is planar but occurs within megaripple sets. These sets show reactivation surfaces and truncations. Wavelengths of the megaripples are not continuous. Sheet to lenticular bedding occurs above, below, and lateral to the megaripple crossbedding. Paleocurrent directions, measured on the four
sets of the megaripple cross-beds, are not uniform but tend to dip 10-15 ° to the south (Fig. 4). The differences in thicknesses and lateral extent of the bed types along the parallel east and west walls of the quarry suggest migrating or shifting hydrodynamic regimes in response to systematic changes in shoreline and shoal channel geometries, sediment budgets, and more turbulent local, meteorological events. The absence of bipolar cross-bedding generally indicates a lack of tidal current transport reversals. Megaripple development represents an important but isolated episode of sedimentation. The absence of trough cross-beds or changes in megaripple geometry from undulating to straight crested or lunate (Reineck and Singh, 1975), as well as the inability to recognize changes from lower to upper flow regime conditions, suggest peritidal deposition attributable to several possible environments. Fauna and flora
Trace fossils Trace fossils are abundant in several parts of the exposed sections. In general the cross-bedded zones have the greatest concentration of burrows. The most distinctive burrows are Ophiomorpha sp.
4
A.F. R A N D A Z Z O E T Air
Fig. 3. Planar cross-beds within a megaripple set (west wall of the Dolime Minerals Quarry). Bar is 1 m.
and occur as shafts and tunnels which are smooth on the interior surface and somewhat irregularly pelleted or mamillated on the exterior surface. Burrow walls and lithified burrow fillings are both present (Fig. 5). Two types of Ophiomorpha are
NUMBER pERCENT FREQUENCY
6
N
IO
20
31%
X Fig. 4. Rose diagram illustrating principal directions of dip among the four sets of megaripple cross-beds.
present. The first is more robust and occurs near the base of the west wall of the quarry with a predominantly horizontal orientation. It is generally less abundant than the second type which is principally vertical and commonly branching (including " Y " shapes) in configuration. The latter type exhibits smaller diameters and thinner walls. Minimum and maximum tube diameters of all the Ophiomorpha specimens are 0.4 and 1.7 cm, respectively. The two types of Ophiomorpha burrows are interpreted as components of an integrated burrow system. Such systems are characteristic of the best known modern analogues for the Ophiomorpha organism, thalassinidean shrimp (Callianassa major Say and other callianassids as well as certain species of Upogebia and Axius) that routinely construct knobby walled burrows (Weimer and Hoyt, 1964; Howard, 1972; Frey et al., 1978). Burrow systems include shafts, mazes, and boxworks. Branching is common and tends to occur
PALEOECOLOGY OF SHALLOW-MARINECARBONATE ENVIRONMENTS
5
Fig. 5. Ophiomorpha sp. displaying irregular pelleted and mammillated exterior surfaces, and branching configuration.
at acute angles. Typically, shafts predominate in higher-energy environments while boxworks and mazes are more characteristic of lower-energy regimes (Frey et al., 1978). Occurring in all of the Ophiomorpha zones throughout the quarry are fossil remains of the decapod crustacean, Callianassa inglisestris Roberts. The fossils consist mainly of internal casts of the propodus and various other elements of the claw. In light of the observation that Callianassa major represents the best modern analogue for an Ophiomorpha-producing organism, C. inglisestris appears responsible for the Ophiomorpha burrows in this quarry. This association was postulated earlier by Randazzo and Saroop (1976) and Saroop (1977) who reported Ophiomorpha from cores which penetrated the lower Ocala and the Avon Park in this vicinity.Ophiomorpha is locally common in the Pleistocene Miami Oolite of Florida and in Mesozoic Cenozoic oolitic shoal facies generally (Kennedy, 1975; Sellwood, 1986). Other
modern analogues of subtidal carbonate sand facies with well developed Ophiomorpha have been described by White and Curran (1987) in the Bahamas. Shinn (1968) and Braithwaite and Talbot (1972) report knobby burrows of Cailianassa spp. from modern carbonate terrains in South Florida Bahamas and the Seychelles, respectively. Ophiomorpha sp. is also known from San Salvador Island (Bahamas) where it is abundant in skeletal calcarenites which interfinger with the coralline facies of the fossil reef complex and in tabular cross-stratified, Halimeda-rich, coarse-grained calcarenites interpreted to have been a Pleistocene tidal delta (Curran, 1984). Pemberton and Jones (1988) described an extremely well preserved suite of ichnofossils (including two species of Ophiomorpha) from a Pleistocene carbonate sequence on Grand Cayman Island. The ichnofossils were created in response to hydrodynamic conditions rather than the composition of the substrate, emphasizing that ichnofossils of some carbonate sys-
6
tems should be treated in a similar manner to their siliciclastic counterparts. The Eocene ichnofossils reported here offer important, additional support for this concept. Less common, and less distinctive, are burrows attributed to worms, probably polychaetes. These burrows are also found near the base of the studied sections and range from vertical to horizontal in orientation. In the limestone unit at the top of the sequence, several specimens of a third trace fossil were recovered. This enigmatic structure of conical to subcylindrical shape, possessing an internal, central tube, is well known from the Ocala Limestone and is interpreted as the burrow of a soft-bodied organism (Diblin et al., 1987).
Macrofossils Diagenetic processes, including dolomitization, have resulted in selective preservation of what was once a fairly diverse fauna. No original shell material is present, except perhaps in some calcitic fossils from the thin limestone bed at the top of the sequence. Most fossils are preserved as molds and casts. Nevertheless, it is possible to identify many taxa (particularly irregular echinoids) and to offer a paleoecological assessment of the environment of deposition. Much of the molluscan fauna is represented by unidentifiable molds and casts; yet, many species are recognizable. Gastropods comprise at least 10 taxa, including Velates floridanus Richards, an archaeogastropod with Tethyan, Old World affinities (Palmer and Richards, 1953). Some 17 genera of bivalves are present, with the large lucinid, Pseudomiltha megameris Dall, being particularly distinctive. In general, the shallow marine molluscan fauna is composed of species representing a diversity of life habits and habitats so that more precise paleoenvironmental assessments are difficult. Only one bryozoan specimen, a large, branching cheilostome which has eluded identification, has been recovered. The remainder of the macroinvertebrate fauna, excluding the above mentioned ichnofossils, an unidentifiable balanomorph barnacle, a solitary coral, and the decapod crustacean, Callianassa inglisestris, consists of echinoderms. Two partial
A.F. RANDAZZO
E T AL.
impressions and numerous ossicles from an unidentified sea star and large spines from one species of regular echinoid [cf. Phyllacanthus mortoni (Conrad)] indicate that irregular echinoids were not the only echinoderms to inhabit this environment. Irregular echinoids are represented by at least 12 species. Few, if any, specimens were found in life position and no preferred orientations were recorded. Evidently currents were strong enough to transport and overturn echinoid tests over short distances but did not damage fragile forms (such as sand dollars) in the process. Crushing of some of the more robust species, however, indicates that compaction and diagenesis have played significant roles. Irregular echinoids are vagrant, benthic organisms which are predominantly infaunal~ though epifaunal forms are widespread. Most modern species are very sensitive to the type of substrate in which they burrow and this is reflected in the morphology of the test (Nichols, 1959). By comparison with modern analogues, substrate preferences of fossil forms can be inferred. As with almost all echinoderms, echinoids are dependent upon fairly constant, normal marine salinity (Boolootian, 1966). Additional factors influencing their spatial distribution include temperature and water depth; however, the comparatively wide range of depths and temperatures inhabited by extant representatives of genera found in the lower Ocala limits our ability to interpret Eocene paleodepths or paleotemperatures. Irregular echinioids from four orders are represented: clypeasteroids, spatangoids, cassiduloids, and a holectypoid. Oligopygus phelani Kier, a guide fossil for the lower Ocala (Hunter, 1976; Zachos and Shaak, 1978; McKinney and Jones, 1983) is the only holectypoid present. The distribution of Oligopygus in the Ocala seems to be influenced little by substrate types (Croft and Shaak, 1985) and since the extinct oligopygoids are only known from middle and upper Eocene strata, it is difficult to make paleoecological inferences beyond the observation that their geographic distribution suggests they preferred warm water (Kier, 1967). Three species of cassiduloids, Rhyncholampas (= Cassidulus ) ericsoni (Fischer), R. globosus (Fischer), and an unidentified species of Rhyncho-
PALEOECOLOGY
OF SHALLOW-MARINE
CARBONATE
7
ENVIRONMENTS
lampas, were also recovered. Though the ecology of m o d e m cassiduloids deserves more study, Holocene species tend to live on sandy substrates, perhaps buried up to their petals (Kier, 1962; Smith, 1980). It seems likely the species from the Ocala also lived on sandy bottoms (Croft and Shaak, 1985). Dominating the echinoid assemblage are the spatangoids (heart urchins) and the clypeaster0ids (including the sand dollars). Eupatagus antillarum (Cotteau), and particularly the large individuals of E. clevei (Cotteau) (= E. ingens) Kier (1984), represent the most striking spatangoids in the quarry. The former is found only in the limestone bed at the top of the section whereas the latter occurs throughout the dolomitized zone. Kier (1984) stated that it is doubtful Eupatagus could burrow in fine sediment. From functional morphological arguments he suggested Eupatagus lived buried in sand. A similar habitat is also postulated for the much smaller Agassizia clevei Cotteau ( = A . floridana) Kier (1984). A single specimen of a brissid heretofore unreported from the lower Ocala, Plagiobrissus curvus (Cooke), adds a fourth spatangoid species to the list. Plagiobrissus, like Eupatagus, lives buried in sand and builds no tunnel to the sediment-water interface (Kier, 1984). The most distinctive clypeasteroid in this sequence is the large, circular, stratigraphically diagnostic sand dollar, Periarchus lyelli floridanus Fischer. This species is characteristic of the lowest faunizone (Inglis Formation) in the Ocala (Puri, 1957). Like the smaller sand dollars from this quarry [Neolaganum durharni (Cooke) and Durhamella ocalanum (Twitchell)] and their m o d e m counterparts, Periarchus probably lived in a sandy substrate, buried a few millimeters deep in the sand (Croft and Shaak, 1985). The conditions preferred by the tiny, more spheroidal, clypeasteroid, Fibularia vaughani (Twitchell), are uncertain. Near the bottom of the section, plant fossils of marine grasses occur. These sea grasses are well known from the Avon Park Formation of Citrus and LevY counties (Dixon, 1972; Randazzo and Saroop, 1976; Lumbert et al., 1984) where they occur, not only as impressions of leaf blades, but also as entire rhizome mats in growth position.
The sea grass assemblage closely resembles the Thalassia syringodium communities typical of south Florida today and probably served as a principal food source of sea cows such as Protosiren (Domning et al., 1982), whose remains are also found at this site. Other vertebrate fossils include a marine crocodile, a sea turtle, a barracuda, a porcupine fish, three types of shark (tiger, sand, and lemon), and a primitive whale (Basilosaurus sp.). The plant and vertebrate fossils are all indicative of shallow-marine, tropical to subtropical conditions.
Environment of deposition Sedimentologic and paleontologic features of these rocks indicate a shallow, normal marine, near-shore environment. These sequences show evidence of recurring high-energy events in a subtidal setting. Poor preservation limits paleoenvironmental interpretations based on most invertebrate groups except the irregular echinoids. From the information presented above, it appears that the echinoid assemblage is uniformly composed of sand-dwelling species to the exclusion of mud or gravel substrates. While the salinity of the water must have been near normal, the depth range suggested by the echinoids could have extended from the shallow subtidal to much deeper. In general, the clypeasteroids (especially sand dollars) seem to be adapted to near-shore, high-energy, sandy environments (Seilacher, 1979) but the spatangoids, being less mobile and liable to suffocate if stranded at low tide, suggest a slightly deeper habitat. Using independent approaches, Croft and Shaak (1985) and McKinney and Zachos (1986) investigated the paleoenvironmental and stratigraphic relationships of the echinoid fauna of the Ocala Limestone. Their conclusions regarding the lower Ocala, based on functional morphological and statistical (cluster) analyses, respectively, generally agree with the inferences drawn here from an integrated sedimentologic and paleontologic approach. Both investigations concluded that the lower Ocala was deposited under shallow subtidal conditions which deepened higher in the section before eventually shoaling in the uppermost
8
Ocala. Water depths were probably less than 10 m (McKinney and Zachos, 1986) and the energy levels were high. Total echinoid diversity and abundance were found in both studies to be very high in the lower Ocala with large and small sand dwellers predominating. Trace fossil evidence supports the paleoenvironmental assessments based on the macrofossils. The Ophiomorpha association is interpreted as indicating a shoreface transition zone. Frey et al. (1978, p. 215) cautioned that both the ecology and ethology of the best known Ophiomorpha organism, Callianassa major, are variable and it should not be assumed without qualification that Callianassa burrows and Ophiomorpha are necessarily indicative of beaches or the shallow sublittoral sands seaward of them. Yet, the weight of the paleontological and sedimentological evidence in this instance strongly supports this conventional interpretation. Ophiomorpha generally increases in density from the offshore to the shoreface environment (Howard, 1972). In this Florida sequence predominantly horizontal Ophiomorpha burrows, characteristic of the shoreface, become more common as both forms occur contemporaneously. The presence of both forms in these sequences suggests water depths were probably no more than 3 - 4 m (H.A. Curran, personal communication, 1988). Probable polychaete burrows at the base of the sections, however, indicate that the substrate was stable, with normal shallow shelf water movement. Above this zone Ophiomorpha burrows are truncated by erosional surfaces indicating the action of periodic storm action along the shoreface. The distribution of burrows is uneven, suggesting burrowers responded to hydrodynamic conditions by constructing new burrows rather than repairing old ones. Carbonate sand facies are generally a part of higher-energy environments in which modern carbonate sediments form. Sand belts occur with sandwaves and sandbars of various types (Ball, 1967). Mobile sand belts commonly have impoverished benthic faunas because of the instability of the substrate. These sand bodies undergo rapid migration during storms. Spatial changes in the cross-bedded megaripples and lenticular beds,
A.F. R A N D A Z Z O ET AL.
accompanied by differences in wavelength, suggest complicated variations in flow regimes. Lenticular beds were formed by stronger currents which truncated and modified megaripple geometry. Megaripple cross-beds reflect an environment of shifting sediments. The complex environments of the shorezone are well documented by the exposures studied. Peloidal and bioclastic facies, displaying several bed geometries and accompanied by extensive bioturbation, suggest a shifting environmental setting typical of the shorezone. Periodic higher-energy conditions, probably resulting from storm surges, have superimposed sedimentary characteristics upon the deposits. The presence of thin, planar, well sorted, finegrained dolomite beds (Randazzo and Cook, 1987), with significant concentrations of sea grasses at the base of the section, and the occurrence of calcitic packstones at the top of the section, reflect major changes in sedimentological conditions. A more normal, open shelf marine environment replaced the shallower, near-shore ecologic habitat.
Modern analogues Carbonate environments in which carbonate sand sedimentation is occurring are c o m m o n in the modern Bahamian Platform and south Florida shelf. High current velocities have produced mudfree carbonate sands with complex geometric configurations (Sellwood, 1986). These sand bodies are frequently associated with actively growing coral reefs, shoal areas, and grass covered flats. The island of San Salvador, Bahamas is just one example of the interactive carbonate facies related to energy conditions and faunal distribution. Megarippled, cross-bedded sands, with extensive burrowing (Curran and White, 1984) but sparse faunal population, can be found adjacent to sea grass beds with finer carbonate sand and greater faunal diversity and density. More open marine conditions occur just a few tens of meters from the shoreline. Shifting conditions associated with storms and shoreline and substrate configuration changes could bring about a redistribution of these associated facies. Likewise, variations in sea level could also bring about such facies distributions. The sedimentary
9
PALEOECOLOGY OF SHALLOW-MARINE CARBONATE ENVIRONMENTS
and faunal characteristics observed in the study area closely resemble those of some of the nearshore carbonate environments of San Salvador. The principal difference between this modem analogue and that of the study area is the absence of a reefal facies in the Eocene sequence. This difference is poorly understood (Randazzo, 1987) and poses an interesting challenge for future study.
sediments and greater faunal diversity and population. The sedimentological and paleontological features of these carbonate sequences are clearly indicative of a shallow, normal marine, near-shore environment. They represent recurring high-energy conditions in a setting of subtidal deposition.
Acknowledgements Summary Middle Eocene carbonate sequences of peninsular Florida represent an important component of the development of the Florida Platform. Vertically repetitive, dolomitic and bioclastic packstones and grainstones suggest alternating open marine to shoreline conditions within broader ranging changes in sea level. Well developed, planar cross-bedding (reported for the first time for these Eocene sequences), occurring within megaripple sets, expresses non-uniform paleocurrent directions. The carbonate sequences indicate migrating or shifting hydrodynamic regimes in response to systematic changes in shoreline and shoal channel geometries, sediment budgets, and local meteorological events. Abundant burrows of the trace fossil Ophiomorpha sp. have horizontal and vertically branching systems including shafts, mazes, and boxworks. Comparison with modem burrow forms from the Bahamas suggest maximum water depths of 3-4 m in a shoreline transition zone. More than 70 species of organisms were identified in the carbonate sequences. Abundant specimens of four orders of echinoids (clypeasteroids, spatangoids, cassiduloids, and a holectypoid) reflect their preference for sandy substrates within a shoreline transition zone. Other invertebrate, vertebrate, and plant fossils within the sequences are indicative of shallow-marine, tropical and subtropical conditions. Sedimentological and ecological conditions of the Florida Platform during the middle Eocene appear to be remarkably similar to modern carbonate analogues of the south Florida shelf and the Bahamian Platform. Cross-bedded, extensively burrowed, mud-free sands are commonly found adjacent to sea grass beds with finer-grained
We gratefully acknowledge the cooperation of the Dolime Minerals Corporation, and in particular, Tom Jones, the quarry manager. David Kendrick assisted in the collection of fossils and Gary Morgan identified the vertebrates. Dr. H. Allen Curran, Smith College, visited the site, and offered significant insights concerning the ichnofauna. We are particularly appreciative of his input and thorough review of this manuscript. Heather Quinn provided valuable help in collecting field data and through group discussions. This paper represents University of Florida Contribution to Paleobiology No. 342.
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