Recent continental slope sediments and sedimentary processes bordering a non-rimmed carbonate platform : southwest Florida continental margin

Recent continental slope sediments and sedimentary processes bordering a non-rimmed carbonate platform : southwest Florida continental margin

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Recent continental slope sediments and sedimentary processes bordering a non-rimmed carbonate platform : southwest Florida continental margin
Brooks, Gregg R.
Place of Publication:
Tampa, Florida
University of South Florida
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x, 153 leaves : ill. ; 29 cm


Subjects / Keywords:
Marine sediments -- Analysis -- Mexico, Gulf of ( lcsh )
Dissertations, Academic -- Marine science -- Doctoral -- USF ( FTS )


General Note:
Thesis (Ph. D.)--Univesity of South Florida, 1986. Bibliography: leaves 146-153.

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University of South Florida
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University of South Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
020836979 ( ALEPH )
16633513 ( OCLC )
F51-00167 ( USFLDC DOI )
f51.167 ( USFLDC Handle )

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RECENT CONTINENTAL SLOPE SEDIMENTS AND SEDIMENTARY PROCESSES BORDERING A NON-RIMMED CARBONATE PLATFORM: SOUTHWEST FLORIDA CONTINENTAL MARGIN by Gregg R. Brooks A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Marine Science in the University of South Florida May, 1986 Major Professor: Larry J. Doyle, Ph.D.


Graduate Council University of South Florida Tampa, Florida CERTIFICATE OF APPROVAL Ph.D. Dissertation This is to certify that the Ph.D. Dissertation of Gregg R. Brooks with a major in Marine Science has been approved by the Examining Committee on April 10, 1986 as satisfactory for the dissertation requirement for the Ph.D. degree. Examining Committee: Member: Robert M. Garrels, Ph.D. Member: A. Dennis Kirwan, Ph.D. Member: Pamela Hallock-Muller, Ph.D.


ACKNOWLEDGEMENTS I wish to extend my deep appreciation to the following people for their continued support throughout the project. Dr. Larry J. Doyle for his guidance and support; Dr. Charles W. Holmes for his guidance and continued encouragement in seeing the project through; and, Drs. Robert M. Garrels, A.D. Kirwan, and Pamela Hallock-Muller for their many valuable comments and contributions. I would also like to thank Dr. Charlotte Brunner, Lenore Tedesco and Jim Bannon for their help in performing some analyses; Tracy Logue for help in preparing the manuscript; Ginger Shuert in helping to cut through the red tape; Steve Snyder for the many hours of stimulating discussions; Steve Walker, Jennifer McNeillie and Walt Bowles for their help in collecting data; and, Doug Parker, Rick Wall, Paul Schroeder, Dennis Latta, Gail McGarry, and Bruce Barber for rounding out the program. Finally, I would like to thank my mother, Margaret Brooks, for her never ending support and my wife, Becky, for her encouragement and confidence for all of these years. ii




LIST OF TABLES Table 1. Core numbers and depth (down-core) of each sample 23 Table 2. Major characteristics of each seismic sequence 41 Table 3. Depths of core sample sites and sequences penetrated 63 Table 4. Texture and calcium carbonate content of sediment samples 68 Table 5. Carbonate mineralogies of some biogenic carbonates 71 Table 6. Carbonate mineralogy and strontium concentration of sediment samples Table 7. Sediment constituents-sand fraction Table 8. Oxygen and carbon isotope values for selected sediment samples Table 9. Radiocarbon age dates for all cores Table 10. Biostratigraphic data from all cores Table 11. Carbonate mineralogy of sediments from different 73 78 88 91 92 provinces of the west Florida continental margin 96 Table 12. Sediment constituents of some carbonate environments iv 101


LIST OF FIGURES Figure 1. Location map showing study area. 3 Figure 2 Study area. 7 Figure 3. Physical processes influencing southwest Florida margin. 10 Figure 4. Surface currents influencing study area. 12 Figure 5 Vertical current structure in Straits of Florida. 14 Figure 6 Seismic track lines and core sites. 21 Figure 7. Seismic profile showing basal reflector. 28 Figure 8. Structural contour map of the base of sequence 9. 30 Figure 9. Seismic profile showing all 9 sequences. 32 Figure 10. Seismic profile showing erosional unconformity. 34 Figure 11. Seismic profile showing oblique prograding clinoforms at the base of the sequence grading upward into sigmoidal prograding clinoforms. 37 Figure 12. Seismic profile showing lower slope gullies. 39 Figure 13. Isopach map of sequence 9. 44 Figure 14. Location map showing outcrop of geologic units. 46 Figure 15. Isopach map of sequence 8. 48 Figure 16. Isopach map of sequence 7. 50 Figure 17. Isopach map of sequence 6. 52 Figure 18. Isopach map of sequence 5. 54 v


Figure 19. Isopach map of sequence 4. Figure 20. Isopach map of sequence 3. Figure 21. Isopach map of sequence 2. Figure 22. Isopach map of sequence 1. 56 58 60 62 Figure 23. Core X-radiographs showing current structure at the base (a) and homogeneous sediments at the top, a Zoophycos burrow is shown by (b). 65 Figure 24. Sr concentrations of some major carbonate sediment components. 77 Figure 25. Sediment constituents in the sand fraction from the top (a) and base (b) of core 10, and the top (c) and base (d) of core 7. Figure 26. Scanning Electron Micrographs of the mud fraction from the middle of core 9 (a) and middle of core 5 (b). Figure 27. Scanning Electron Micrograph of a clay mineral (kaolinite ?) from the middle of core 5. Figure 28. Oxygen and carbon isotopes. Figure 29. Sr concentrations in sediments on the southwest Florida continental margin. Figure 30. Conceptual depositional model. Figure 31. Vertical profile of Florida Straits showing thickest sediment accumulations. Figure 32. Late Quaternary sea level curve for the Gulf of Mexico. Figure 33. Map of southwest Florida continental margin vi 81 83 85 90 99 115 117 123


showing potential provenance area (arrow). 132 vii


RECENT CONTINENTAL SLOPE SEDIMENTS AND SEDIMENTARY PROCESSES BORDERING A NON-RIMMED CARBONATE PLATFORM: SOUTHWEST FLORIDA CONTINENTAL MARGIN by Gregg R. Brooks An Abstract Of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the department of Marine Science in the University of South Florida May 1986 Major Professor: Larry J. Doyle, Ph.D. viii


The southwest Florida continental slope bordering the Straits of Florida contains a thick sequence of seaward prograding sediments. Over 1,500 km of high resolution seismic reflection data and 8 sediment cores were collected in order to determine: (1) provenance; (2) depositional mechanisms; (3) their relationship to high frequency sea-level fluctuations;(4) how they compare with other carbonate slope deposits in both the modern and ancient rock record. Nine seismic sequences have been identified, each bounded by an erosional unconformity. Sediments consist of a mixture of shallow-water (principally biogenic carbonates) and pelagic material deposited rapidly on the upper slope. Sedimentary patterns are interpreted to be a function of high frequency sealevel fluctuations. Most vigorous off-shelf transport and highest sedimentation rates (possibly exceeding 2.5 m/1,000 yrs) occur during early flooding of the shelf. During sea-level highstands, off-shelf transport is less vigorous and sedimentation rates decrease. During sea-level lowstands, no off-shelf transport takes place and erosion of the previously deposited sequence occurs. Based upon conservative estimates for rates of shallow-water biogenic carbonate sediment production, it is shown that sufficient quantities of sediments can be produced in the provenance area during a 6th order (1,000-10,000 yrs) sea-level cycle to account for the thickest sequence identified in the study area. High rates of sedimentation coupled with the sensitivity of ix


the Florida Current to climatic fluctuations permits the identification of such high frequency events where they may otherwise remain undetectable. Comparisons with s lopes borderin g other carbonate platforms show the southwest Florida slope may represent a transition zone between rimmed and non-rimmed carbonate platforms. The southwest Florida slope provides a valuable modern analog for identifying similar transitional environments in the geologic past. Abstract approved: r {; 9;pf Jl' , ( .L Major ro : rry )?"D ):;ptr.D. Professor, Marine Science c X


INTRODUCTION Scope Carbonate platforms have been common throughout most of the geologic record, occurring in every epoch of the Phanerozoic (Schlager, 1981). Recent sedimentologic research pertaining to carbonate margins has concentrated on shallow-water environments because they are more accessible and of interest as petroleum reservoirs. Research on continental slopes bordering these platforms has lagged behind to such an extent that the common approach to the study of modern carbonate slope sedimentation has become "the past is the key to the present" (Cook and Enos, 1977). The most extensively studied modern carbonate slopes are those of the northern Bahamas (Cook and Mullins, 1983), classic rimmed platforms, in the sense of Ginsburg and James (1975). In contrast, slopes bordering the west Florida margin, a non-rimmed platform, have received comparatively little attention. The southwest Florida continental slope, which borders the southern Straits of Florida (Fig. 1), has been the site of extensive sedimentation for much of the recent geologic past. Holmes (1985) identified a large lobe of sediment projecting southward into the Florida Straits. Using seismic reflection characteristics and external form he interpreted this lobe as two sedimentary fans. The existence of these sediment bodies in l


Figure 1. Location map showing study area.


840 30' 83 30' 820 DRY TORTUGAS ... ... OJ; MARQUESAS KEYS 30' t+ STUDY AREA o 5 I 10 15 20 w


this environment is puzzling, as strong currents flow through the straits from the Gulf of Mexico to the Atlantic Ocean. A detailed investigation of these deposits was initiated in 1983. The objectives of the study were: 1) To determine the depositional processes operating on the southwest Florida slope. 2) To determine sediment types and provenance. 3) To determine how the slope responds to glacially induced sealevel fluctuations that have been common throughout the Quaternary. Specific questions to be addressed include: 1) How are sediments deposited in such a high energy environment? 2) How do processes compare with those operating on slopes bordering the better known "rimmed platforms"? 3) Is deposition continuing today? By answering these questions, this research will provide a much needed modern analog for ancient non-rimmed carbonate platforms, which have been common throughout the geologic past (Wilson, 1975). Geologic Setting The southwest Florida continental slope is part of the vast Florida platform, a massive, southward-thickening wedge of Mesozoic and Cenozoic carbonates and evaporites reaching at least 6,000 min thickness beneath Cay Sal Bank (Antoine, et al., 1974). The west Florida margin is an "open shelf" (Ginsburg and James, 1975) or "drowned platform" (in the sense of Read, 1982). 4


The margin is a result of pre-Cretaceous rifting in the Gulf of Mexico. Since then subsidence and sea-level fluctuations have been the dominant influences. The study area (Fig. 2) is that portion of the margin which trends east-west and is bordered by the deep Florida Straits to the south, the Pourtales Terrace to the east, the Florida Canyon system to the west, and the west Florida shelf to the north. Along the shelf edge, just up-slope from the study area, a series of small isolated carbonate banks occur (Shinn, et al, 1982). Depths range from approximately 100 m at the shelf-slope break (a rather arbitrary division), to over 1,000 m on the floor of the Straits of Florida. The gradient is gentle, averaging less than 1 for the entire slope, but reaching 2.8 between the 250 m and 500 m isobaths. Along the lower slope, in depths of 500 m to 1,000 m, are a series of gullies collectively known as the Agassiz Valleys with relief ranging from less than 10 m to over 50 m. The gullies appear to be erosional in origin (Minter, et al., 1975). The southern Straits of Florida is a 100 km wide trough, approximately 1,000 m deep, which separates the Florida platform from the island of Cuba. Its origin remains uncertain, but some early workers suggest it may represent a half-graben downfaulted against Cuba and rising gently to the north (Hurley, 1964; Malloy and Hurley, 1970). The Pourtales Terrace, on the eastern margin of the study area is a Miocene phosphatic limestone cropping out between the 200 m and 300 m isobaths. Scouring by the strong bottom currents associated with the eastward flowing Florida Current has 5


Figure 2. Study area.


\ \ \ '\ 87' N FLORIDA CANYON 8 5. ; Q Q \ \ \ I 7 25' / -...... --/1 00o ______., .:..--POURTALES FLORIDA sTRAits t TERRACE _..J 2 3' 83 81 19'


restricted sedimentation (Gomberg, 1976). The junction between the east-west trending slope and the north-south trending slope is an area dissected by canyons. These canyons, known as the "Florida Canyon System" have acted as conduits funneling shallow water carbonate sediments to the basin floor (Holmes, 1985). Modern headward erosion of these canyons is exposing older sediments on the outer shelf and upper slope in the western portion of the study area. Phys ical Oceanography The physical environment is dominated by the Florida Current an eastwardly flowing current linking the Loop Current of the Gulf of Mexico with the Gulf Stream (Figs. 3 and 4). Surface velocities average 100 em/sec but reach over 250 em/sec in the core of the current (Richardson, et al., 1969). It encounters the bottom to depths of 200 m (Gomberg, 1976) where velocities of 45 em/sec have been recorded. A westward flowing countercurrent (Fig. 5) with velocities of 20-25 em/sec have been observed between 430 m and at least 600 m water depths (Brooks and Niiler,1975; Stewart, 1962). Between 200 m and 430 m exists a zone of reversal over which the flow gradually changes direction (Brooks and Niiler, 1975; Stewart, 1962). Flow velocities are sluggish throughout the entire zone. Below 600 m, few data have been gathered. A westward flowing surface countercurrent occurs intermittently on the outer shelf just l andward of the Florida Current. Its origin is uncertain, but it may represent a cyclonic recirculation of Florida Current water (Brooks and Niiler, 1975). 8


Figure 3. Physical processes influencing southwest Florida Margin.




Figure 4. Surface currents influencing study area.




Figure 5. Vertical current structure in Straits of Florida.




Superimposed on the dominantly east-west current pattern are north-south currents originating on the continental shelf to the north (Fig. 3). Strong reversing north-south tidal currents have been observed as they are funneled between the shallow carbonate banks dotting the outer shelf (Shinn, et al., 1982; Barron, Pers. Comm.). Additionally, strong southerly currents occur periodically across the entire shelf in response to Gulf Loop Current intrusion up onto the shelf (Neurauter, 1980), and the passage of storm frontal systems averaging every 5 to 10 days during the winter (Niiler, 1976). Large, southward trending sand waves, which are found over large portions of the shelf surface, indicate these currents have the ability to transport relatively large quantities of sediments for great distances (Neurauter, 1980; Holmes, 1984). Background and Previous Works To date, most research on modern carbonate slopes has concentrated on classic rimmed platforms such as the northern Bahamas (Mullins, et al., 1978; Mullins and Neumann, 1979; Crevello and Schlager,1980; Mullins et al., 1980; Boardman and Neumann, 1984; Mullins, et al., 1984). In general, modern carbonate slopes bordering the northern Bahamas can be divided into three morphological parts: (1) A marginal escarpment that drops precipitously from the shelf edge (30 m-50 m) to depths of 200 m or more; (2) an upper gullied slope dipping seaward at slopes of 3 to 15 and dissected by numerous small canyons 20 m-150 m in relief; (3) a more gentle (1-5), smooth to gently undulating lower slope or rise consisting of thick gravity flow deposits 15


(Schlager and Chermak, 1979). Mullins and Neumann (1979) described seven deep carbonate bank margin types and suggest the processes most responsible for this configuration include basement faulting, off-bank transport, gravity and pelagic sedimentation, physical circulation patterns, submarine cementation, and deep water biogenic buildups. The slope bordering the west Florida margin, a non-rimmed carbonate platform, i s considerably different from that of the northern Bahamas. Research has been mainly confined to the northern segment (north of 26 N) where it is a relatively smooth, continuous feature. The shelf-slope transition is a complex series of steps starting as shallow as 60 m with gradients between the steps averaging less than 2 (Uchupi and Emery, 1968). No continuous energy-absorbing rim of reefs and banks is present. 16 Below this complicated transition zone lies the steep West Florida Escarpment. Beginning at 1,000 m-2,000 m depths, it dips seaward at 15-55 with total relief ranging from 1,000 m-1,500 m (Bryant, et al. 1969). Recent sediments are dominated by coccoliths and planktonic foraminifera, with minor amounts of clay minerals (dominantly smectite) originating from the Mississippi River system (Doyle and Sparks, 1980; Doyle and Feldhausen, 1979). Mass sediment movements occur periodically indicated by huge slump blocks (Doyle, 1983), slide scars (Mullins, et al. 1986), and turbidites (Walker, 1984; Doyle and Holmes, 1985). Mitchum (1976) found the post mid-Cretaceous history of the northern slope to be one characterized by continued seaward progradation of sediments


into deep water interrupted occasionally by sea-level fluctuations. On the southwest Florida slope, bordering the southern Straits of Florida, Milligan (1962) identified sediments on the slope as consisting of shallow-water carbonates similar to sediments in the adjacent Florida Bay. He suggested that the material was transported around the western end of Key West by storms and deposited down slope by gravity flow processes. Hurley (1964) looked at deeper sediments in the Straits and noticed a graded axial slope indicating a westward transport of material. He suggested the most likely depositional mechanism is some sort of deep westwardly flowing turbidity current. Similarly, Malloy and Hurley (1970) noted that patterns of sediment dispersal and erosion indicate that currents act along the lower slope and bottom of the Florida Straits. The erosional gullies were first described by Minter, et al. (1975). They noticed these gullies, termed Agassiz valleys, are being infilled from the top by sediments rich in sponge, mollusc and coral debris. They also noticed a complex current pattern consisting of a reversing eastwest flow and periodic, sluggish, down-slope flow within the gullies. Gomberg (1976) reported that the Florida Current reaches the bottom to depths of at least 200 m and is probably responsible for limiting the accumulation of modern sediments on the Pourtales Terrace. Biostratigraphy of sediments from the deeper Straits of Florida has been intensively studied by Brunner (1975, 1983, in press). She found that these deep sediments have recorded a history of fluctuating bottom current intensity that can be 17


18 related to glacial episodes.


METHODS All data were gathered on a 13-day research cruise to the southern Straits of Florida aboard the R/V Lynch during November, 1983. Over 1,500 km of high resolution seismic reflection data and eight gravity cores (1-3 m in length) were collected (Fig. 6). The seismic system consisted of a Teledyne Mini-sparker powered by a Del Norte 800J power supply, a 100 element hydrophone, and an analog recorder. Resolution is ideally less than 1 m (based on the dominant frequency of 500 Hz and an estimated velocity of 1,650 m/sec through unconsolidated carbonate sediments; Sherrif, 1977; Gregory, 1977). Positions were determined by Loran-e with fixes plotted at 5-minute intervals. Satellite navigation was used to correct for any variances in the Loran-e data. In the region of study, the navigational error is less than 200 m. Analysis of seismic reflection data consisted of a detailed sequence and facies analysis. Initially, unconformities and their correlative conformities were determined. Horizontal and vertical facies relationships were then identified. Isopach maps for all sequences were constructed as well as a structural contour map for the base of the lowest sequence. Time-depth conversions were made based on the velocity estimates mentioned above. Sediment cores were initially split vertically, and visually described. Smear slide samples from the top, middle, and bottom of 19


Figure 6. Seismic track lines and core sites.


840 30' 83 30' 820 25 DRY TORTUGAS MARQUESAS KEYS 10m 30' 4-KILOMETERS II 0 5 10 15 20 8 e CORE LOCA TION N .....


each core were examined microscopically. Each core was slabbed and X-rayed for sedimentary structures according to the method described by Roberts (1972). Each core was then sampled at the topt middle and bottom for further analyses. The decision for sampling locationswas based on the lack of sharp breaks in texture or color within the cores as well as their short lengths. Three cores (St 7t and 9) were sampled an additional two times based on aget length of caret or trends determined by previous analyses. Sample numbers and depths (down-core) are given in Table 1. Textural analysis was performed on all samples. Percentages each of sandt siltt and clay were determined by the sieve and pipette method described in Folk (1965). Calcium carbonate content was determined for all samples by the acid leaching method (Millimant 1974). Relative percentages of calcitet high Mg calcitet and aragonite were determined for all bulk samples and the mud-sized fraction of selected samples by X-ray diffraction (Millimant 1974). The ratio of peak areas was used to determine percentage aragonite vs calcite (both high and low Mg). The ratio of peak heights was used to determine percentage low Mg vs high Mg calcite. The relative percentages of each was read from a standard curve (Millimant 1974; Boardmant 1978) verified for this study using pure biogenic carbonates (all species characteristic of the west Florida margin) as standards. Sediment constituent composition for the sand-sized fraction 22


23 Table 1. Core numbers and depth (down-core) of each sample. Sample No. Depth of Sample (em) 1-t 10-12 1-m 116-118 1-b 221-223 4-t 10-12 4-m 49-51 4-b 88-90 5-t 10-12 5-tm 37-39 5-m 64-66 5-mb 91-93 5-b 118-120 6-t 10-12 6-m 49-51 6-b 88-90 7-t 10-12 7-tm 62-64 7-m 110-112 7-mb 160-162 7-b 210-212 8-t 10-12 8-m 49-51 8-b 88-90 9-t 10-12


24 Table 1. (cont.) 9-tm 54-56 9-m 99-101 9-mb 142-144 9-b 188-190 10-t 10-12 10-m 92-94 10-b 173-175


of all samples was determined by visual estimation under the li&ht microscope (Carver, 1971). The mud-sized fractions of selected samples were analyzed using the Scanning Electron Microscope (SEM). SEM analysis was also used to check for evidence of dissolution or precipitation in the fine (mud-sized) fraction. Strontium composition was determined for all samples by atomic absorbtion. Strontium analyses were conducted by Dr. C. W. Holmes, USGS. Biostratigraphic analysis was performed on top, middle, and bottom samples from each core. Fossil assemblages were used in order to determine the approximate age and environment of deposition for each sample. Analyses, concentrating mainly on planktonic foraminifera, were performed by Dr. C. Brunner, University of California, Berkeley. Stable isotope analyses were made under the supervision of Dr W. M. Sackett, University of South Florida. Values for o18o and 13 o C (PDB) were determined for selected bulk samples. Bulk samples were used to gain information on water temperatures and may be useful in determining sediment provenance. Bottom samples from each core were dated by radiocarbon methods by Beta Analytic, Inc., Coral Gables, Florida. 25


RESULTS AND DISCUSSION Seismic Data The basal reflector identified in the study area is a heavily eroded surface (Fig. 7) that eventually rises and crops out in the easternmost portion of the study area to form the Pourtales Terrace, a Miocene phosphatic limestone (Gomberg, 1976). This reflector cannot be traced westward for any great distance (probably because of signal loss), but previous studies indicate that it rises and crops out once again on the upper slope northwest of the study area (Holmes, 1981), thereby forming a broad reentrant. This reentrant is reflected in a structural contour map of the base of sequence 9 (Fig. 8), the lowermost sequence identified in the study area. The easternmost segment rises abruptly from greater than 900 m to outcrop between 200 m and 300 m below sea level. The outcropping terrace to the northwest lies between 475 m and 500 m depths (Holmes, 1981). Overlying the basal reflector is a thick sequence of post Miocene sediments. Thickest deposits have accumulated between the 250 m and 500 m isobaths. Nine seismic sequences have been identified (Fig. 9). Each sequence is bounded by an erosional unconformity (Fig. 10), some reaching as deep as 400 m below present sea level. Below thi s they become conformable. These erosional surfaces are similar to the intraformational truncation 26


Figure 7. Seismic profile showing basal reflector.


E 0 0 E 0 0 E 0 0 co E 0 0 co ........ :-, : 28


Figure 8 Structural contour map of the base of sequence 9 (contours in meters below present sea level).


840 30' 83 30' 820 25 30' t-CONTOURS IN METERS KILOMETERS 0 5 10 15 20 DRY TORTUGAS ,_t';:f. ; c!)/,J' STRUCTURAL CONTOUR MAP BASE OF SEQUENCE 9 w 0


Figure 9. Seismic profile showing all 9 sequences.


' " I I 'I I I ---.qw (/) 1 .>:_J E 0 0 C\1 E 0 0 .qE 0 0 co 32


Figure 10. Seismic profile showing erosional unconformity.


------1 CJ) 0 z ----'-----.. L E 0 0 C\J ,., : I r p .. .'/ 1 I }I I . l '" , . 34 6 / 0 l j f /. .. .


surfaces seen in ancient carbonate slope environments (Cook and Enos, 1977; Davies, 1977). Each sequence consists of southward trending (off-shelf) prograding clinoforms, indicating transport of material from the shallow shelf to the north. The clinoform pattern is one of oblique prograding clinoforms at the base of the sequence indicating relatively rapid sedimentation in a high energy environment, grading upward into sigmoidal prograding clinoforms (Fig. 11) indicative of relatively slow sedimentation in a low energy environment (Mitchum, et al., 1977). Several local erosional unconformities exist but are not mappable for any significant distance. These features may have been caused by local mass wasting or current scour. All nine sequences have similar characteristics. All are lobate in shape and appear to be radiating from the shallow shelf to the north. Internal reflectors are dominantly continuous and parallel to sub-parallel. Hummocky to chaotic and reflection free configurations occur locally. Erosional gullies are prevalent on the lower slope between approximately 500 m and 900 m below sea level (Fig. 12). Gullies range from 5 m to 20 m in relief and are generally less than 1 km across. They are presently being infilled by prograding sediments on the upper slope suggesting they have been inactive throughout the recent geologic past. Gullies were probably formed by erosion as shallow water-material was funneled down-slope as gravity flows. Each sequence is eroded with greatest erosion concentrated on 35


Figure 11. Seismic profile showing oblique prograding clinoforms at the base of the sequence grading upward into sigmoidal prograding clinoforms.


Cf) E 0 0 C\J Cf) E 0 0 37 E 0 0 co . ;. .... .. : .....


Figure 12. Seismic profile showing lower slope gullies.


E 0 0 co w \\00 '-"'-I 0 t..-11 ( II) . r.. .... \ \ ) '\ t\. '1"\ \-.: ,-::r-.:59 -... I _J 0 E 0 0 1'E 0 0 co ,.. ...... '1:. J ..... 4 0 39


the westernmost portion. Erosion may be a result of scour associated with the strong eastward flowing Florida Current. The depositional center of each sequence is displaced just eastward, or down-current from that of the preceding sequence as the reentrant was being infilled in progressive fashion from west to east, also a possible result of the Florida Current. Individual characteristics of each sequence are given in Table 2. Locations of the depositional centers and thicknesses of each sequence, as well as a geologic outcrop map, are shown in Figures 13 through 22. Sediments Sediments in the study area were sampled by gravity cores. A total of ten cores were attempted and eight were retrieved. Core targets were areas where each of the nine sequences outcrop. Logistical problems prevented sampling of all sequences. Core numbers, locations, and sequences penetrated are given in Table 3. All cores look alike having an olive-gray color similar to carbonate sediments collected from the Florida-Hatteras slope and Blake Plateau (Stanley, 1969). Sediments are generally homogeneous with some mottling. Mottles are principally concentrated near the core tops and are lighter in color than surrounding sediments. Concentrations of shells, generally pteropods, are abundant locally. Shell concentrations are lens shaped and approximately 5 em wide and 2 em thick. Shell concentrations can be caused by current winnowing or grazing by carnivores (Bouma, 1972; Reineck and Singh, 1975). Current 40


Table 2. Major characteristics of eac h seismic sequence. Sotntc: Soquonc:o I 4 HalltThlc:ltaou so 120a 100. 60. Loc:attoa of O.poettlonol Centar Duo Iouth of Horquooao hyo overlytoa Pourtaloo Terrace South-oouthwoot of H.orquooao ltoyo Soath-oouthwoot of tt.rqueeaa leya South-oouth .. at of Dry Tortus Roflec:tlon ConflKUratlono Contlnuouo ond porollol to oubporollol; Htah anal parallel-ob llque proar.dtng occur locally Parallel and conttnuouo otronaly roflocthe in davnelope aes-ente; Reflection froo h.....,clty ond chaotic locally obundont in ohallow eection of quence Conttnuouo and porollel to oub-porollel; Htahly rafluttve c:llnofo"'' vtth top11ets truncated occur locally Sub-parallel ond dlocontlnuouo, vlth locally h.,_.,clty areoo; Obllqua progrodtna cltnoforao Dovn lap onto Pourtalee Terrace OUtcrop Loc:atlono Contlnuouo alon11 tho 150 a-200 a ioobatho Lower elope Patchy, throuahout etudy area Broad &one aoutheaat of Dry Tortuaoo Othu P'ooturoo 70 Ita Ions ocorp followtns 100 a ioobath; Co.proootonal Coldo on upper olopa, .,..ndo (reofo?) along tho 60 .-70 a loobatho Eroefoo In veatem portion of aequence: Oblique cltnoforao oDd c:hootlc untto tn Pourtoloo lerrac:e area lCNer elope; Gulltee preeent but appear Ieee developed thaa thoaa of underlytntt eaquencee Heevt ly eroc!ed on veatana portion; Chaotic unite Ia Pourtale Terrace reaton; lovn elope sullleo preeent but appear leaa developed than undarlytna eequeneee Heovlly erodecl on .. oton portion; Houndo (r .. fo1) on upper elope; Culltao pr .. aat but head further clovnelopa aad r leal concentrated then thoae of underlytna oaquanc:ao lataqorouttono Rtall uta of depeltioa to a rolotiYaly htlh onorl}' ..... tr ..... nt; Bartel of Pourtalea Terraea c:onttautna Roptd, htall oneriJ dopoetttoa tn Pourtoloo Tarraee area Reptd. htah eaar., dapoaltlon to clepoaltloaol ceatar; .... wi.atl"' Currant eroatoa doalaant oroeloaal proc:ooo loptd, htah anarl}' depooition tn dapooittoeal center'; tceaa waatiDI tn ohollow, c:antrol portloo nf otudy area, tho lower olopo aulley field oDd tha Florida Caayoo realoo; laatantq of burial of Pourtaleo Terrace .......


Table 2 (cont.). Sdntc Sequence s 6 a 9 HaatThlckoo .. 120 90. ss so 160 Locatioo of Depoolttonal Center Duo aoutb of Dry Tortu1 South-oouth, of Dry Tortua South-oouthveot o f D r y Tortua South-oouthveot of Dry Tortua P lorlda Caoyon erea Rflectto n Conftauratlona Contlnuouo and parallel to eub-parallel; Chaotic locally ln thtck .. t portion Contl nuouo and parallel to aub-paretlal; Jaflectto n free, hUIIIIOcky and chaoti c in Florida Canyon and auJ lied lover a lope areaa Cont t nuoua and parallel to eubparallol; Cheotlc locally on lower elope eut of 1ully flold Cont tuuoua and P "rallal to auh-parallal; Hu..oclty o n lovor elope ond rofloctlon free t n rlorida Canyon a rea Ohcont1noua and porallel to oub-parelld; Reflection froo unite In Plortdo Caoyoo rt:Rion Outcrop Locatlone Narrow (S lta-10 Klo) band froa tho Florida Caoyo n ro1l on to tho sullied lover elope Other raaturae Haa v tly eroded on waatara portion aod gullied lover elope ; Hound (reof7} oo upper elop e ; aulUoo coaaon on lover elope Narrov ( S b-I 0 X a) b a nd HeaY 11 y eroded o n vee tern elon& lover olope portion and gullled lover olope Florida Canyon rea ton florida Conyoo end aullled elope Florida Canyon area Haavlly eroded oa vaetan portion and aullled lover a lope !roded o o veatem porttoa aDCI aullied lover elope !roded oo veatern portion; l.cNor elope aulllee -u developed I oterprotatlona Rapid, hl&h ooer17 depoaittoa to dopoaitt.,...l c.aatar; Maaa tla a oo lover alopa and to thlckoot portloo of aaquanca laptd hl&h aoer17 depoettton t o dopoelttoool canter; troaloo by currant a n d .... w .. ttna -o ln Plortda Caoyoo aod lover elope aaptd ht&h oOOr J J depoaitioa t o depoottiooal cooter; !roeton by bottoa currant aD4 .... waattaa c-o to Plortda Canyaftd lover elope hptd, ht&h anoriJ depoeltioo la depoo1tloaal cooter; Eroaloa by bottcurcenta aacl Mea waattaa coaaoa ia Plorlda Caayoo and 1-r elope laptd, hl&h anoriJ dopoaltloa 1a Florida C.ayoo ro&loa N


Figure 13. Isopach map of sequence 9.




Figure 14. Location map showing outcrop of geologic units.


840 30' 83 30' 820 25 30' KILOMETERS 0 5 10 15 20 tDRY TORTUGAS (} GEOLOGIC OUTCROP MAP MARQUESAS KEYS + 2 -------? 0\


Figure 15. Isopach map of sequence 8


840 30' 83 30' 820 25 30' t" CONTOURS IN METERS KILOMETERS 0 5 10 15 20 DRY TORTUGAS ISOPACH MAP SEQUENCE 8 .p. <


Figure 16. Isopach map of sequence 7.




Figure 17. Isopach map of sequence 6.




Figure 18. Isopach map of sequence 5.




Figure 19. Isopach map of sequence 4.


84 30' 25 I J R E EF TRE N D CONTOURS I N METE R S KILOMETERS 0 5 10 1 5 20 83 30' DRY TOR T UG A S fl'' I + 820 ISOPACH MAP SEQUENCE 4 MARQUE SAS KEYS ........-..... .....--_ I VI 0\


Figure 20. Isopach map of sequence 3.


840 30' 83 30' 820 25 30' -t-CONTOURS IN METERS KILOMETERS 0 5 10 15 2 0 DRY TORTUGAS II) ft:: : + ISOPACH MAP SEQUENCE 3 MARQUESAS KEYS 10m VI 00


Figure 21. Isopach map of sequence 2.




Figure 22. Isopach map of sequence 1.


840 30' 83 30' 820 25 30' CONTOURS IN METERS KILOMETERS 0 5 1 0 15 20 DRY TORTUGAS _.,is';! c!Y; .' (jj + ISOPACH MAP SEQUENCE 1 MARQUESAS KEYS 10m 0\ N


63 Table 3 Depths of core sample sites and sequences penetrated. Core No. Water Depth (m) Sequence Penetrated 1 725 sediment drift 2 N 0 RECOVERY 3 N 0 RECOVERY 4 450 sediment drift 5 975 5 6 450 3(?) 7 960 1 8 930 1 9 340 2(?) 10 200 1


Figure 23. Core X-radiographs showing current structure at the base (a) and homogeneous sediments at the top, a Zoophycos burrow is shown by (b).


..... N 0 ,... >-1 A. e 65 E E .


winnowing is probably the major process here as shells are of planktonic organisms and concentration of planktonic organisms by carnivores is unlikely. Additionally, currents competent to concentrate shells have been identified in the study area. X-ray radiography can show features not recognizable to the naked eye. All cores have similar structures, although not necessarily in the same order of occurrence. First-order structures consist of wispy, cross-bedded and wavy parallel-bedded units. Second-order features are homogeneous, mottled and burrowed structures. Most cores consist of wispy cross-bedded, and wavy parallel-bedded sediments at the base and grade upward into homogenous, mottled and bioturbated sediments (Fig. 23) Wispy cross-bedded, and wavy parallel-bedded sediments have coarse (sand/silt) layers intercalated with finer material. These layers are typically 2-5 em thick. Homogeneous and mottled sediments are a result of intense bioturbation. Individual burrows occur in all directions (Fig. 23) Wispy cross-bedded, and wavy parallel-bedded sediments preserved in the core bottoms are indicative of current winnowing after deposition (Hill, 1984; Walker, 1984) or possibly deposition from nepheloid layers or low density turbidity currents (Bouma, 1972; Reineck and Singh, 1975). Hill (1984) found a very close association between wispy, laminated sediments and homogeneous muds collected from the Nova Scotian upper continental slope. He interpreted these structures to be the result of hemipelagic deposition with periods of winnowing under a higher flow regime. 66


Heavily burrowed, homogeneous, and mottled sediments reflect secondary reworking by organisms that act to destroy primary structures. The presence of these features generally indicate a decrease in sedimentation rate that allows more complete reworking of the sediments by burrowing organisms. Bioturbated sediments are common in late Holocene sediments of the Gulf of Mexico, reflecting the decrease in rate of sea-level rise (Bouma, 1972). The general down-core pattern of sedimentary structures from cores that penetrated the modern sequence indicates that sediments at the base of the core were deposited rapidly and under a relatively high energy flow regime. Both energy and sedimentation rate decreased as sediments in the upper portion of the cores were deposited. Intense bioturbation has destroyed any primary structures present. Results of textural analysis (Table 4) show sand-sized material ranges from 7.9% to 73.9% of the sample, with an average of 26.5%. Most is in the fine to very fine sand size range. Silt-sized material comprises 20.4% to 71.5% of the sample, with an average of 54.3%. In most samples, the silt-sized fraction makes up the bulk of the sediment. Clay-sized material comprises 5.8% to 39.9% of the sample, with an average of 19.2%. Clay-sized material shows less of a range indicating little variation in input of clay-sized sediment. Lateral distribution shows a decrease in sand-sized material and an increase in silt-sized material with depth (water) and distance from the shelf. Amount of clay-sized sediment remains 67


68 Table 4. Texture and calcium carbonate content of sediment samples. Sample No. /.Sand /.Silt /.Clay 7.CaC03 1-t 15.3 67.3 17.3 81.0 1-m 17.1 63.0 19.9 74.3 1-b 8.3 71.5 20.2 88.2 4-t 42.7 47.1 10.2 92.8 4-m 44.0 44.2 11.8 93.2 4-b 48.6 41.3 10.1 93.5 5-t 39.8 45.4 14.8 91.8 5-tm 31.9 47.7 20.4 90.6 5-m 27.5 51.8 20.6 91.8 5-mb 15.0 60.5 24.5 93.9 5-b 13.0 63.5 23.5 78.8 6-t 34 4 54.7 10.9 93.3 6-m 14.4 74.9 10.8 93.1 6-b 33.7 53.5 12.8 92.5 7-t 11.7 62.6 25.7 90.0 7-tm 7.2 61.1 31.7 90.0 7-m 8.6 57.8 33.7 57.9 7-mb 6.6 53. 7 39.7 62.2 7-b 7.6 62.6 29.8 73.3 8-t 7.9 63.7 28.4 91.5 8-m 9.6 59.9 30.5 87.6 8-b 12.2 53.6 34.2 87.4


69 .Table 4 (cont.). Sample No. 7.Sand 7.Silt 7.Clay 7.CaC03 9-t 56.4 32.8 10.8 83.2 9-tm 55.8 32.5 11.7 77.3 9-m 25.2 60.8 14.0 88.0 9-mb 24.0 61.5 14.5 89.0 9-b 22.0 64.2 13.8 74.4 10-t 24.7 62 5 12.8 82.2 10-m 54.5 33.4 12.1 86.4 10-b 73.9 20.4 5.7 99.4


virtually unchanged (Table 4). Vertical (down-core) distributions show virtually no trends (Table 4). Exceptions are cores 5 and 10. Core 10, which penetrated the center of the modern sequence, shows strong grading, containing over 70% sand-sized material at the base. The bulk consists of relatively coarse, shallow-water, carbonate sands. Core 5 penetrated sequence 5, the oldest sequence sampled in the study area. Sediments of core 5 are inversely graded with coarser grained material at the core top. Calcium carbonate content ranges from 57.9% to 99.4% and averages 85.6% of the sample (table 4). The non-carbonate fraction consists dominantly of quartz with minor amounts of kaolinite and smectite. Unlike textural data, there are no trends of %Caco3 of samples from core tops with depth and distance from the shelf. Values for all lie between 80% and 95% Caco 3 Downcore values also show no trend (Table 4). A more quantitative determination of shelf versus pelagic input can be made using sediment mineralogies. In most carbonate environments shallow-water carbonate sediments are rich in aragonite and high Mg calcite, whereas deep-water carbonates from pelagic sources are dominated by calcite (Milliman, 1974; Bathurst, 1975). Table 5 shows carbonate mineralogies of the dominant sediment constituents of the west Florida continental shelf and slope. All shallow-water forms consist of either aragonite or high Mg calcite. Deep-water forms, with the exception of pteropods, are dominantly calcitic. Pteropods, a pelagic form of gastropod, are aragonitic. Relative percentages 70


71 Table 5. Carbonate mineralogies of some biogenic carbonates. Aragonite Halimeda Pteropods Benthic Molluscs High Mg Calcite Coralline Algae Benthic Forams (most) Echinoderms -------------Bryozoa--------------Corals (most) Soft Corals Calcareous Sponge Spicules Calcite Planktonic Forams Coccoliths


of dominant carbonate minerals are given in Table 6. Aragonite constitutes 34% to 61% of the samples with an average of 45%. High Mg calcite (greater than 10 mole % Mgco3 ) comprises 10% to 36% and averages 23.4%. Calcite (less than 4 mole % MgC03 ) ranges from 13% to 49% and averages 31.6% of all samples. Lateral distribution (Table 6) shows an increase of calcite with depth and, therefore, distance from the shelf. Conversely, aragonite and high Mg calcite decrease strongly with depth. No down-core trends exist, with the exception of core 10, which penetrated the center of the modern unit. Core 10 shows a slight increase in aragonite and high Mg calcite and subsequent decrease in calcite down core (Table 6). Carbonate mineralogy of the mud-only fraction shows no significant differences from bulk samples. Strontium content can also aid in determining sediment provenance. Strontium concentrations may be used in distinguishing between shallow and deep-water derived aragonite. Figure 24 shows strontium concentrations in typical carbonate sediment constituents. Pelagic forms (i.e., pteropods) typically found in deep-sea sediments have consistently low (less than 2,000 ppm) values. Shallow-water forms common to the tops of carbonate platforms have higher values that range from approximately 3,000 ppm to over 10,000 ppm. Values from southwest Florida slope bulk samples range from 1,920 ppm to 4,260 ppm and average 2,370 ppm (Table 6). No down-core trends were identified but strontium concentrations of samples from core tops decrease with increasing water depths (Table 6). Values, intermediate between typical shallow water and deep water sources, indicate that sediments are 72


73 Table 6. Carbonate mineralogy and strontium concentration for sediment samples. Sample No. 7oArag. /.MgCC 7.CC [Sr] (ppm) 1-t 47 17 36 2300 1-m 41 24 35 2390 1-b 41 34 25 2300 4-t 47 24 29 2580 4-m 43 27 30 2500 4-b 41 36 23 2720 5-t 40 19 41 2040 5-tm 41 19 40 1960 5-m 36 29 35 2200 5-mb 41 10 49 1970 5-b 34 24 42 1920 6-t 58 18 24 2520 6-m 48 25 27 2670 6-b 51 24 25 2 630 7-t 40 17 43 2320 7-tm 42 18 40 2350 7-m 43 21 36 2400 7-mb 47 22 31 2370 7-b 38 27 35 2170 8-t 43 17 40 2330 8-m 41 18 41 2260 8 b 43 20 37 2290


74 Table 6 (cont.). Sample No. 7.Arag. 7.MgCC 7.CC [Sr] (ppm) 9-t 48 27 25 2770 9-tm 52 23 25 2620 9-m 49 24 27 2290 9-mb 48 30 22 2310 9-b 48 25 27 2240 10-t 48 27 25 2550 10-m 51 29 20 2890 10-b 61 26 13 4260


a mixture of relatively high strontium aragonite derived from the shelf and low strontium aragonite from pela&ic sources (Fig. 24). Results of sediment constituent analysis for the sand-sized fraction are shown in table 7. Dominant constituents include planktonic foraminifera, pteropods, sponge spicules, benthic molluscs, and benthic foraminifera. Echinoid fragments make a variable contribution and ostracods, bryozoa, octacorals, coralline algae, Halimeda, and pelletoids contribute minor quantities. Barnacle and rock fragments, blackened grains, and annelid tubes are present in trace amounts. Typical sediment assemblages of the sand fraction are shown in figure 25. Sponge spicules, benthic molluscs, and benthic foraminifera, all present in appreciable quantities, are contributed from shallow-water sources. Molluscs are typically fragmented indicating Benthic foraminifera consist of a typical shallow-water assemblage dominated by Peneroplis sp.t Archaias sp., Amphistigina sp., Elphidium sp., and Nonionella sp . The mud-sized fraction consists of aragonite needles, coccoliths, clay minerals, and unidentifiable debris (Figs. 26 and 27). Although no quantitative analysis was attempted, the mudsized fraction appears to be dominated by aragonite needles and coccoliths, the relative amounts varying directly with depth and distance of the sample site from the shelf margin. Clay minerals are identifiable in only a few samples. The presence of aragonite needles is indicative of input from shallow-water sources because pteropods do not degrade into needles. Aragonite needles are the 75


Figure 24. Sr concentrations (ppm) of some major carbonate sediment components (after Bathurst, 1975 and Boardman, 1978).




Table 7. Sediment constituents sand fraction. -Ill () M ..c: C1) .... ... <11 0 00 C1) <11 4! '"0 .tl .... C1) () "'-./ 0 Ill Ill Ill oM M Ill Ill ...-i Ill Ill C1) 00 '"0 '"0 Ill '"0 <11 "0 C1) <11 <11 oM oM 0 0 () () 'M .... <11 0 ...-i M '"0 .... 0 ... ... p. C1) Ill 'M 0 0 0 () () ...-i C1) ... 0 M ::3 ..c: () N <11 <11 ...-i C1) C1) .... ...-i ... oM 0 0 .... <11 oM ...-i '"0 Sample <11 C1) 0 ...-i ..c: ... >. ... .... .... ...-i () ...-i 'M ...-i ... p. C1) () () .... Ill <11 0 <11 0 C1) p.. p.. C/) ft:l rzl 0 ft:l 0 ft:l u ::c ,:X: p.. ::::> Ly-1-T A A c A c Tr-C Tr -Tr -Tr --Tr-r c Ly-1-M A A A c c Tr --r ----Tr-r c Ly-1-B c c A c r -Tr ------Tr c Ly-4-T c c C c r-C r Tr -r Ly-4-M c c c c c r Tr Tr r -Tr --Tr Ly-4-B c c r-C c c r-C r Tr r-C Ly-5-T A A A c c r-C --------A Ly-5-TM A A c A c r Tr -Tr --Tr --A Ly-5-M A A A c c r-C -Tr Tr-r ----A Ly-5-MB A A c A c r-C --Tr -----A Ly-5-B C-A c A A c r-C --------A Ly-6-T A C-A C-A C-A r-C r Tr Tr Tr-r Ly-6-M C-A r-C r-C C-A r-C r Tr -r Ly-6-B C-A c c C-A c r r Tr r-C --Tr Ly-7-T A A A c r-C r --c ----c Ly-7-TM A A A A c Tr-r Tr -r Ly-7-M A A C-A A r r --r ----r Ly-7-MB A A c A c Tr --r Ly-7-B A A A A c r-C --Tr ----r ........ CXl


Table 7. (cont.) ....._ (/) u 13 r-l CIS ..c: al 1-1 4-J (/) CIS 0 13 00 al CIS .-j 'Q ..c 1-1 < al u '--' 0 (/) (/) (/) r-l -r-l (/) (/) .-j (/) (/) al 00 'Q 'Q (/) 'Q CIS 'Q al CIS CIS -r-l -r-l 0 0 u u r-l 1-1 CIS 0 .-j r-l 'Q 1-1 0 4-J 4-J 0.. al (/) r-l 0 0 0 u u .-j Q) 4-J 0 00 ::l ..c: u N CIS CIS .-j 13 al Q) 1-1 .-j 4-J r-l 0 0 1-1 CIS r-l .-j 'Q Sample 'CIS al 0 .-j ..c: 4-J >-4-J 1-1 1-1 .-j u .-j -r-l .-j 4-J 0.. 0 Q) u u 1-1 (/) rn 0 rn 0 al (/.) 0 0 u :I:: ll:O P. ::> Ly-8-T C-A C-A r-C c c r C Tr -Trr ----Tr-r Ly-8-M A A c A r-C Tr --Tr ---Tr Ly-8-B A A c A c Tr Tr-r -Tr r ---Tr Ly-9-T A C-A c A c r Tr-r Tr r-C Ly-9-TM A A c C-A c r-C r -r-C ----Tr Ly-9-M A A c A A r C Tr Tr r-C -Tr Ly-9 -MB A A A A c r --r Ly-9-B A A A C-A C-A ---r-C Ly-10-T A A A C-A C-A r-C -Tr c -Tr --c Ly-10-M C A C-A c A C-A c -c Tr r Tr r -r-C c Ly-10-B R R c C-A c c -r --c Tr c c D = Dominant ( >30%) A Abundant (20%-30%) c Common (10 %-20% ) r rare (1%-10%) T R Trace ( <1%) ., none detected "'-1 \D


Figure 25. Sediment constituents in the sand fraction from the top (a) and base (b) of core 10, and the top (c) and base (d) of core 7. Photographs show sediments dominated by planktonic foraminifera, pteropods, benthic foraminifera and benthic molluscs.




Figure 26. Scanning electron micrographs of the mud fraction from the middle of core 9 (a) and middle of core 5 (b). Micrographs show sediments dominated by coccoliths and aragonite needles.




Figure 27. Scanning electron micrograph of a clay m ineral (kaolinite ?) from the middle of core 5.




product of degradation of calcareous green algae and ooids. Neither is a significant contributor to the sand fraction of slope samples, but large Halimeda sources exist in nearby Florida Bay (Gebelein, 1977), the Dry Tortugas (Ginsburg, 1956) and the Quicksands region due west of the Marquesas Keys (Hudson, 1984; Shinn, et al., 1977). No present ooid sources exist nearby but 86 the Marquesas Key Bank and possibly other shelf-edge banks rest on Pleistocene oolitic rock (Shinn, et al., 1982). Needles could be derived from modern Halimeda sources, ancient oolitic sources, or a combination of the two. Coccoliths are dominated by Emiliani huxleyi and Geophyrocapsa sp. (Fig. 26). Clay minerals are difficult to identify by SEM but X -ray diffraction shows minor amounts of both kaolinite and smectite. An example of a clay crystal (kaolinite?) is shown in Figure 27. Quartz was not identified by light microscope or SEM in either the sand or mud-sized fractions. This is puzzling because X-ray diffraction indicates quartz to be the dominant mineral of the non-carbonate fraction. Sponie spicules consist of opaline silica which gives no appreciable X -ray signal. Quartz therefore, must account for a large portion of unidentifiable debris in the mud-sized fraction. Lateral distributi on of sediment constituents (core-top samples versus depth) shows a general increase in shallow-water forms toward the shelf (Table 7). This is especially noticeable in the minor contributors such as echinoids and ostracods, as well as aragonite needles in the mud-sized fraction. Pelagic forms such as planktonic foraminifera and pteropods consistently make a


major contribution. Down-core, virtually no trends exist with the exception of core 10. Core 10 shows a dramatic down-core decrease in pelagic material. 18 13 Isotope values for 6 0 and 6 C are given in Table 8. 13 Values for 6 C range from 0.718 to 2 199 (PDB) and values for o18o range from -1.325 to -0.503 (PDB). All are typical values for oceanic sediments (Milliman, 1974). Figure 28 shows a plot of o13c versus o18o including values for typical sediment constituents. Values from southwest Florida slope samples fall within the shallow water mollusc and benthic foraminifera zone. Core 9 shows a general down core decrease in both 0 18o and 513c (Table 8). Radiocarbon dates from the bases of all cores are shown in Table 9. With the exception of core 5, all show radiocarbon ages of late Pleistocene-Holocene to mid-Holocene. Core 5 is beyond the limits for radiocarbon age dating techniques. Comparing data from Table 9 with those of Table 2 shows some cores that penetrated (presumably) older (underlying) sequences have younger radiocarbon age dates. These samples may have been contaminated, redistributed by intense bioturbation or cores did not penetrate through the overlying sediment veneer into the appropriate sequence. Biostratigraphic analysis (Table 10) shows cores 1, 4, 8, 9, and 10 all to be younger than 11,000 ybp. Co-occurrences of the planktonic foraminifera Globorotalia fimbriata and Globorotalia cultrada indicate sediments from these cores were deposited during Ericson zone Z (Ericson and Wollin, 1968), which 87


88 Table 8. Oxygen and carbon isotope values for selected sediment samples. Sample No. 5-t 5-m 5-b 7-t 7-m 7-b 9-t 9-m 9-b a18o (PDB) -0.967 -0.503 -0.522 -0.858 -1.223 -0.968 -0.768 -0.879 -1.325 a13C (PDB) 1.512 1. 718 1. 331 1.415 1.146 1.204 2.199 1.151 0.718

PAGE 100

Figure 28. Oxygen and carbon isotopes of some common carbonate constituents (after Milliman, 1974). Boxes indicate values from this study.

PAGE 101


PAGE 102

91 Table 9. Radiocarbon age dates for all cores. Sample No. Core Length (m) 14c Age (ybp ) 1-b 2.33 5240+70 4-b 1.00 8880+1 0 0 5-b 1.3 0 > 38180 6-b 1.00 7 660+90 7-b 2.2 0 10150+130 8-b 1.00 7 820+100 9-b 2.00 10440 + 100 10-b 1.85 12690+130

PAGE 103

Table 10. Sample No. 1-t 1-m 1-b 4-t 4-m 4-b 5-t 5-m 5-b 6-t 6-m 6-b 7-t 7-m 7-b 8-t 8-m 8-b 9-t 9-m 10-t 10-m 10-b Biostratigraphic data from all cores. Ericson Zone z z z z z z z,x X X z(x?) z(x?) z(x?) z z z,x z z z z z z Ericson and Wollin, 1968 z z/y=llOOO ybp z y/x=90000 ybp x/w=128000 ybp 92

PAGE 104

is restricted to the Holocene. Core 5 is considerably older. Co occurrences of Globorotalia tumida flexuosa, Globorotalia ungulata and Globigerina calida calida indicate that sequence 5 (which was penetrated by core 5) was deposited between 84,000 and 127,000 ybp during Ericson zone X (Ericson and Wollin, 1968). Core 6 and the basal sample of core 7 contained a few specimens of Globorotalia tumida flexuosa and, therefore, may have been deposited somewhere between 11,000 and 84,000 ybp. 93 Both dating techniques indicate very high rates of sedimentation. Radiocarbon dates indicate rates for Holocene sedimentation ranging from 11.0 to 41.5 cm/1,000 years. This is an order of magnitude greater than rates for pelagic sedimentation and significantly greater than the rates (6.9 to 14. 3 cm/1,000 years) for bank-derived input to slopes bordering the northern Bahamas (Boardman and Neumann, 1984) Sedimentation rates may even be higher considering the age of the shallow-water sediments derived from the shelf. A mixed assemblage of surface sediments collected from the west Florida shelf surrounding the Florida Middle Ground, a mid-shelf reef, ranges from 3,200 to 7,500 years in age (Brooks, 1981). If these relict sediments were to be transported off-shelf and deposited on the upper slope, an underestimate in sedimentation rate would result; therefore, sedimentation rates calculated from radiocarbon ages may be an underestimation. Biostratigraphic analysis gives even higher sedimentation rates. As mentioned previously, core 5 (which probably penetrated sequence 5) is dated at 84,000 to 127,000 years in age. Sequences 1-4, which overlie sequence 5,

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94 collectively attain a maximum thickness of 330 m, indicating a sedimentation rate (for approximately the last 100,000 years) exceeding 2.5 m/1,000 years. High sedimentation rates are continuing today. The Spanish galleon "Atoche" which was lost in 1622 was recently discovered due south of the Marquesas Keys just up-slope from the northeast portion of the study area. The wreck was covered by sediment ranging in thickness from 5 em to 2 m, but averaged approximately 60 em (Barron, Pers. Comm.). Sedimentation rates from 1622 to the present, therefore, have ranged from 13.8 em to 5.5 m/1,000 years, with an average of 1.65 m/1,000 years. Although the presence of the wreck could cause accelerated sedimentation, rates of such magnitude are consistent with this study, which are 1 to 2 orders of magnitude higher than can be accounted for by normal pelagic sedimentation.

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PROVENANCE Sediments on the southwest Florida slope are a mixture of pelagic organisms derived from the overlying water column and shallow-water debris consisting of biogenic calcium carbonate detritus, quartz and clay minerals. Southward trending (offshelf) prograding clinoforms indicate the west Florida continental shelf as the source for shallow-water material. Average carbonate mineralogies for surface samples from the southwest Florida slope are shown in Table 11 along with values from the west Florida shelf, west Florida Bay (Enos and Perkins, 1977) and the west Florida slope to the north (Bannon, Pers. Comm). Values for shelf and slope samples show many similarities. The slightly higher low Mg calcite and slighty lower high Mg calcite values for slope samples reflect the greater input of pelagic organisms. Both planktonic foraminifera and coccoliths, which are common in slope sediments, consist of low Mg calcite and their input effectively dilutes sediment derived from shallow-water sources. The higher aragonite concentration for slope samples can be accounted for by the addition of aragonite from pteropods, or possibly input from the aragonite-rich sediments of Florida Bay. Florida Bay may contribute a large quantity of sediments to the slope. Milligan (1962) suggested the bulk of sediment on the southwest slope is derived from Florida Bay. He maintains Florida Bay muds are 95

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96 Table 11. Carbonate mineralogy of sediments from different provinces of the west Florida continental margin. s.w. Slope s.w. Shelf W. Fla. Bay N.W. Slope Aragonite 447. 417. 607. 117. Mg Calcite 237. 307. 257. 67. Calcite 337. 297. 157. 837.

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transported west by a nearshore countercurrent and eventually deposited down-slope by gravity flows. Stockman, et al., (1967) have shown that the rate of production of biogenic lime mud is more than enough to account for the total accumulation in Florida Bay. The excess therefore, must be transported elsewhere and the physical processes operating in the area are consistent with transport to the southwest slope. Carbonate mineralogy of surface sediments from the slope northwest of the study area is considerably different. Sediments are dominated by low Mg calcite indicating input primarily from pelagic sources. Significant input from shallow-water sources is not indicated. The west Florida slope therefore, can be virtually eliminated as a potential major provenance area for southwest slope sediments. Strontium data show sediments on the southwest slope are a mixture of high Sr and low Sr varieties. High Sr aragonite is indicative of shallow-water carbonates. Hi g nest Sr values arise from sediments rich in ooids, Halimeda and coral fragments which dominate many modern carbonate platforms, such as the Bahamas. Sediments of the southwest Florida shelf however, contain lesser amounts of these constituents and are dominated by the lower Sr varieties such as molluscs and benthonic foraminifera. Sediments of the shelf therefore, contain Sr concentrations higher than pelagic sources but substantially lower than platform sediments rich in ooids, Halimeda and coral debris. Figure 29 shows the distribution of Sr on the southwest Florida margin (Holmes, 1973). 97

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Figure 29. Sr concentrations (ppm) in sediments on the southwest Florida continental margin (after Holmes, 1973).

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100 Slope values, averaging 2,370 ppm indicate a mixture of shelfderived and pelagic-derived strontium. Highest Sr values on the shelf are concentrated along the outer margin in the vicinity of the shallow carbonate banks, reflecting input from ooids and Halimeda. Strontium concentrations decrease northward reflecting a higher input of lower Sr varieties such as molluscs and benthonic foraminifera. Values on the southwest slope may arise from low input of high Sr (shelf-edg e banks) carbonates, a high input of low Sr (shelf interior) varieties, or a combination of the two. Other analyses indicate a high input of shelf (interior) derived material rich in benthonic foraminifera and mollusc fragments. Ooids and Halimeda are rare. Thus, the bulk of shallow-water material transported to the slope is derived from the shelf interior behind the shelf edge banks. Sediment constituents support the contention of significant input of shallow water material. Table 12 shows the general sediment compositions of the west Florida shelf, west Florida slope, southwest slope and Florida Bay. Shallow-water molluscs and benthonic foraminifera, the dominant shallow-water components of southwest slope samples, are the principal constituents of sediments from both the west Florida shelf (Brooks, 1981; Martin, 1984; Clayton, Pers. Comm.) and Florida Bay (Ginsburg, 1956; Enos and Perkins, 1977). Sponge spicules, also a major constituent of southwest slope samples, may be derived from Florida Bay sediments. Gebelein (1977) discovered very dense sponge communities and found spicules account for up to 107. of the sediment composition in Florida Bay. Quartz and clay minerals,

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Table 1 2 Molluscs B. Forams Halimeda Coral Sponge P. Formas Pteropods Quartz Sediment constituents of some carbonate environments (sand fraction). S.W. Fla. Slope* W. Fla. Shelf0 Fla. Bay X C-A+ A R-D C-A c R-C Tr -c Tr --C-A R c A R -A ---R-A R This study 0 Martin, 1984; Clayton, pers. comm.; Brooks, 1981 x Ginsburg, 1956; Enos and Perkins, 1956; Gebelein, 1977 e Ginsburg, 1956 Walker, 1984 + D c Dominant ( >40%) A Abundant (20%-40%) C Common (10%-20%) R Rare (1%-10%) Tr Trace ( <1%) none identified Outer Shelf Banks R Tr D A-D --N.W. Fla. Slope R-C Tr-C Tr A-D Tr-R Tr-A ...... 0 ......

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102 which make up the non-carbonate fraction in slope samples, may also originate on the west Florida shelf. A nearshore quartz band presently extends seaward from the coast with an average width of 32 km (Martin, 1984). Quartz is no longer being added but the band is believed to migrate laterally across the shelf with fluctuating sea level (Doyle and Sparks, 1980; Doyle and Feldhausen, 1981). Minor amounts of fine quartz (silt-sized?) are probably transported south with the shallow water carbonate debris, and deposited on the slope. Similarly, the clay minerals kaolinite and smectite, found in minor quantities along the southwest slope, are input from the shelf. Although they co-exist on the shelf they originate from different sources. Smectite is introduced into the Gulf of Mexico by the Mississippi River system. Kaolinite enters via the rivers of northwest Florida (Doyle and Sparks, 1980). As both exist on the west Florida margin, input to the slope must be from this source. No other single source for both exist in the i mmediate vicinity. Bethonic foraminifera in southwest slope samples are similar to both west Florida shelf and Florida Bay forms. As previously stated, Peneroplis, Archaias, Amphistigina, E lphidium and Nonienella are the dominant forms in southwest slope samples. All are common shallow-water foraminifera. Peneroplis and Archaias are the dominant forms in Florida Bay sediments (Bathurst, 1975). Shallow-water material, both carbonate and terrigenous clastic, in southwest Florida slope sediments appear to be derived from both the west Florida shelf and Florida Bay. It is

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103 impossible to determine quantitatively which dominates but both appear to contribute significant quantities. Paradoxically, the carbonate bank-dominated outer shelf, located just up-slope from the study area, contributes little sediment. Sand-sized material from the Dry Tortugas is dominated by Halimeda (417.) and coral fragments (397.) (Ginsburg, 1956). Very little of either is found in the sand-sized fraction of slope samples. Aragonite needles, common in the mud-sized fraction, may represent some input from this source or from Florida Bay muds also rich in Halimeda. However, as aragonite needles occupy the clay-sized fraction (Bathurst, 1975), which averages less than 207. of the total sediment, a contribution from either source is not significant. Shelf and Florida Bay sediments are winnowed by physical processes and transported through the channels between the carbonate banks (incorporating little bank material) and deposited on the slope where they are mixed with the remains of pelagic organisms which have settled out of the overlying water column The relative input of shelf versus pelagic material varies both with time and distance from the shelf. Down-core variabilities in texture, mineralogy, sediment constituents and radioisotopes indicate variations throughout time. A general down-core increase in shelf characteristics in the modern sequence indicates greater shallow-water sediment contribution during the early Holocene when sea level was lower. The decrease in shelf-derived material with distance from the shelf not only indicates the shelf as a source, but that most deposition of shelf material occurs on the upper slope. The

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104 perennial rain of pelagic material probably varies little, therefore differences in relative amounts of pelagic versus shelf derived material is related to input of shelf-derived sediments.

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105 DEPOSITIONAL MECHANISMS As shown in the preceding section, sediments in the study area originate from multiple sources. It can be assumed therefore, that deposition is a result of multiple and probably complex processes. To determine these processes the following requirements must be satisfied: 1) the processes must be capable of contributing large quantities of material at relatively high rates; 2) the processes must be capable of transporting material for relatively long distances from the shelf interior and Florida Bay to the depositional site on the slope; and, 3) the processes must be able to account for rapid deposition in a relatively high energy environment. Shallow water sand and mud-sized material is eroded and transported southward to the southwest slope by various processes. Strong southward flowing currents exist across the entire width of the southern shelf as well as Florida Bay. These currents, resulting from tides, intermittent storms and Gulf Loop Current intrusion up onto the shelf, are presently strong enough to erode and transport fine sand and mud-sized material during normal climatic conditions. Extreme high energy events, such as tropical storms and hurricanes can move significant quantities of coarse sand and even granule sized material. Large fields of sand waves 0 have been identified on the mid to outer shelf south of 26 N

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106 (Neurauter, 1980; Holmes, 1985) indicating southward sediment transport has been active for much of the Holocene (Holmes, 1985). Processes most responsible for erosion and transport of shelf material are probably storms and Gulf Loop Current intrusion. Florida Bay, which is more protected, is more influenced by tides Strong, reversing north-south tidal currents exceeding 50 em/sec have been observed on the outer shelf during tidal exchange between Florida Bay and the Straits of Florida (Shinn, eta!., 1982; Barron, Pers. Comm.). Once the sediment is transported to the slope, it is deposited and mixed with pelagic sediments originating from the overlying water column. Possible depositional processes include deposition from gravity flows, settling of grains through the water column, deposition from nepheloid layers and deposition from contour currents. Gravity flows have been considered the dominant mechanism for deposition of large quantities of sediment at relatively high rates for carbonate slope environments i n both the modern and ancient (Cook and Mullins, 1983). Turbidity currents and debris f l ows appear to be the dominant mechanisms in both cases. Data from both seismic and core analyses of sediments from the southwest Florida slope however, show a conspicuous lack of gravity-flow characteristics. Internal reflection configurations are dominantly continuous and parallel to sub-parallel. Hummocky, chaotic and reflection-free units indicative of gravity flow processes, are found only locally at the base of the slope and the

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107 Florida Canyon region. Additionally, as thickest sediments have accumulated on the upper slope, it is doubtful they were deposited by large scale gravity flows. Since gravity is the driving force, large scale deposition is not expected until that force is no longer active. This would normally occur at the base of slope. Turbidites may not be identifiable using conventional seismic techniques (Cook, 1979), but core X-rays and sediment characteristics show no evidence of turbidite deposition. Low density/low velocity turbidity currents have been suggested as a possible mechanism for rapid deposition in both modern and ancient slope environments (Bouma, 1972; Reineck and Singh, 1975; Cook, 1979; Hill, 1984). In practice however, a continuum exists between low density/low velocity turbidity currents, low velocity bottom currents and rapid settling of particles out of the water column (Stow and Lovell, 1979). Therefore, there exists a gradation between deposits formed by these mechanisms. Pure deposits of each are only end members of the continuum. As most deposits in nature are not pure end members, it is difficult at best, to distinguish between the dominant mechanisms. In fact, there probably is no real difference between deposition from low density/low velocity turbidity currents, low velocity bottom currents and rapid settling through the water column. The common characteristics of deposits interpreted to result from low density/low velocity turbidity currents are a large input of sediment and rapid deposition. It is suggested that deposits on the southwest Florida upper slope are formed, in part, by a frequent input of

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shelf-derived sediments which are deposited rapidly by settling out of the water column. 108 Deposits resulting from the more common higher density gravity flows are found only locally at the base of slope and Florida Canyon regions. No sedimentological data are available, but these units have the characteristic chaotic, hummocky and/or reflection free seismic configurations. Gravity flows may have been initiated by upper slope processes such as sediment overloading or erosion by the Florida Current. Gravity flows most likely redistribute material previously deposited on the upper slope and therefore are secondary rather than primary depositional mechanisms. Slumps have also been identified. Slumps generally occur on the mid to upper slope in the zone of highest accumulation and steepest gradient. Probably due to instability resulting from sediment overloading, slumps are also secondary processes redistributing previously deposited material. Pelagic sediments, principally pteropods, planktonic foraminifera, and coccoliths, constitute an appreciable amount of sediments on the southwest slope. None are found in significant quantities on the shelf; therefore, they must have settled through the overlying water column. They are found throughout the entire sedimentary section. A continuous rain of pelagic material is indicated. This is common on most slopes, both modern and ancient, but surprising here as currents in the overlying water column average 100 em/sec. Low density (i.e., pteropods) or clay-

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109 sized (i.e., coccoliths) sediments would normally be swept away by currents of this magnitude. Pelletoids have been identified (albeit in small quantities) in the sand-sized fraction of many samples (Table 7). Pelletoids may in fact be fecal pellets consisting of remains of planktonic organisms living in the overlying water column. Fecal pellets are relatively large (sand-sized) and dense, allowing rapid settling through the water column even under high energy conditions. Their scarcity in bottom sediments may result from destruction during bioturbation or simple decomposition of the organic coating (Kennett, 1982). As mentioned previously, olive-gray mud such as that found on the southwest slope often signifies a large input of fecal pellets (Reineck and Singh, 1975). Similar processes have been identified for fine-grained material in Northwest Providence Channel, Bahamas (Boardman and Neumann, 1984). Mud-sized sediment deposited beneath the strong surface currents have been attributed to deposition by fecal pellets. Deposition of fine-grained pelagic material on the southwest Florida slope may be explained by rapid settling of fecal pellets through the relatively high energy environment of the overlying water column. Deposition from nepheloidlayers, a dense layer of suspended sediments, have been proposed as a process for deposition of sediments under low flow regimes (Bouma, 1972). Keith and Friedman (1977) suggest this process may have been responsible for depositing lime muds on the slope of the Cambrian Taconic sequence of New York and Vermont. They further state that the exact origin, composition and areal extent of nepheloid layers are still

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not known but they may account for deposition of significant quantities of hemi-pelagic sediments in deeper waters of slopes and ocean basins. It is doubtful that shallow-water derived sediments in the study area have been deposited from nepheloid layers because no such layers have been observed. 110 Deposition from contour currents is another possible mechanism. Contour currents, generally found in the deep sea, follow bathymetric contours and are capable of transporting and depositing large quantities of material (Heezen and Hollister, 1971). Large carbonate sediment drifts discovered off the northwest corners of Little Bahama and Grand Bahama banks have been attributed to this process (Mullins, et al., 1980). While the strong Florida Current easily qualifies as a possible transporting agent, it is doubtful sediments in the study area have been deposited by contour currents for several reasons. First, sediments upcurrent are unlike those found in the study area. The logical origin of sediments entrained by the current would be terrigenous clastics introduced by the Mississippi River and slope carbonates from the west Florida upper slope where the current interacts with the margin (Doyle, 1983; Freeman-Lynde, 1983). As discussed in the preceding section, the west Florida slope is unlikely as a provenance area. Similarly, as terrigenous clastic material makes up such a small fraction of sediments in the study area, its deposition from contour currents would be only of minor importance. Seismic reflection data offer more evidence against contour

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111 current origin. Primary stratal surfaces are oriented off-shelf in a southerly direction indicating sediments are being transported from the shelf to the north and not from the west as would be expected if the Florida Current were a major transporting agent. Additionally, no contourite characteristics have been found, such as thinning of stratal units down current or wedge shaped deposits, both of which are common in Bahama sediment drifts (Mullins, et al., 1980). Internal reflectors such as wavy upper surfaces and reflection-free units, also contourite characteristics (Stow and Lovell, 1979), are found only locally. Sedimentation rates of sediments deposited by contour currents are consistently less than 10 cm/1000 yrs (Stow and Lovell, 1979). Sedimentation rates of deposits on the southwest Florida slope have been shown to be considerably greater, therefore it is doubtful contour currents are a major depositional mechanism. Erosional unconformities bordering each sequence truncate underlying strata and merge tangentially down-slope with underlying beds. These surfaces are analogous with intraformational truncation surfaces so common in the ancient (Cook and Enos, 1977; Reinhardt, 1977; Yurewicz, 1977; Davies, 1977) except they are "convex up" instead of "concave up". Most examples in the ancient rock record are believed to represent slide scars (Cook and Mullins, 1983) originating from mass wasting processes. Few examples have been attributed to current erosion such as those described here. This basic difference in morphology may prove valuable in distinguishing between different erosional agents.

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Influence of the Florida Current is also recognized in the sedimentary structures at the base of the cores. Wispy laminae, also known as current structure (Bouma, 1972; Hill, 1984; Walker, 1984) are formed as material is winnowed and reworked by the current. The concentration of these features down-core indicates an increase in current influence during lower sea level. 112 The continuing influence of the Florida Current throughout the Quaternary is also illustrated in the depositional pattern of each sequence. The depositional center of each sequence is Jisplaced just eastward, or down current, from that of the preceding sequence. The reentrant is being infilled in progressive fashion from west to east. Although influenced by the current, the primary stratigraphic pattern of off-shelf prograding clinoforms remains unaltered. In summary, the dominant depositional processes operating on the west Florida slope are settling of pelagic detritus (probably in the form of fecal pellets) from the overlying water column, and rapid deposition of shelf material, which is frequently supplied to the depositional site. Gravity flows are important as secondary processes redistributing previously deposited material to the base of the slope. Contour currents are not an important depositional process but may modify previously deposited sediments by winnowing and reworking.

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113 DEPOSITIONAL MODEL Sediments on the southwest Florida slope consist of a mixture of shallow-water shelf-derived material and pelagic material. Vertical seismic and down-core trends show that the relative contribution from each provenance area has varied throughout geologic time. A conceptual depositional model is shown in Figure 30. High frequency fluctuations in sea level are the ultimate control. During sea-level highstands the shelf surface is flooded. Shallow water carbonate sediments are generated in significant quantities (Wilson, 1975). As shallow-water sediments are produced they are swept southward o f f the platform and deposited on the slope by the processes mentioned previously. During early flooding when the shore was near the present shelf-slope break, long shore transport may also have been an important depositional mechanism. Thickest deposits have accumulated in the form of offshelf prograding clinoforms on the upper slope between the 250 m and 500 m isobaths, within the low energy "zone of reversal" (Fig. 31). Higher energy zones, both shallower and deeper, prohibit significant deposition. The relative contribution by off-shelf transport is a function of sea level. During those periods in the sea level cycle when water depths over the shelf surface are shallow, as may

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Figure 30. Conceptual depositional model.

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Figure 31. Vertical profile of Florida Straits showing thickest sediment accumulations.

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118 occur during early transgressions and late regressions, off-shelf transport is vigorous. Large quantities of shallow-water sediments are transported southward as physical processes operating on the shelf have more influence on the shallow bottom. It is during this period that sand as well as mud-sized sediment is transported off-shelf. This material is deposited rapidly on the slope mixing with pelagic sediments settling through the overlying water column Sediments are deposited rapidly under relatively high flow conditions as shown by wispy laminated sedimentary structures and oblique prograding clinoforms at the base of the sequences. High flow conditions may arise from vigorous off-shelf transport and depositional processes, strong current activity or most probably a combination of the two. During sea-level highstands, like the present, off-shelf transport is less vigorous as much of the shelf surface lies below wave base (approximately 10 m to 20 m under normal energy conditions; from Seibold and Berger, 1982). During normal energy events only fine grained material is transported off shelf. Pelagic deposition makes an increasingly important contribution. Material is deposited at significantly lower rates under low flow regimes as indicated by sigmoidal prograding clinoforms and heavily bioturbated muds overlying the high flow regime deposits (Fig. 30). During sea-level lowstands when the shelf surface is exposed, no off-shelf transport takes place. Unlike terrigenous clastic margins, which have accelerated off-shelf transport during

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119 these periods, carbonate margins quickly become cemented upon exposure to fresh water (Bathurst, 1975). Those carbonate sediments that are eroded by fresh water runoff, go into solution so no significant quantities of particulate carbonate material is transported off-shelf. Some shallow-water carbonate sediments are produced during this time as the carbonate producing zone shifts seaward down the low angle slope, with regressing sea level. Deposition of these sediments on the southwest slope however, is prohibited by strengthened currents. Erosion of the previous depositional sequence takes place in response to an increase in erosive capacity of the eastward flowing Florida Current, which is a product of three factors: 1) There may be an increase in current velocity associated with an increase in wind velocity as climate changes during glacially induced sea-level lowstands (Kennett, 1982; Brunner, 1985). 2) There is an increase in current velocity as the cross-sectional area of the relatively narrow (approximately 100 km) Florida Straits decreases as sea level lowers (Brunner, 1985). 3) There is a downward shift of the Florida Current with lowering sea level, into the previous "zone of reversal" thereby bringing the strengthened current into contact with the previously deposited sequence. A lowering of sea level to roughly -130 m (as that postulated for the previous lowstand) would drop the bottom margin of the Florida Current to approximately 330 m (assuming all other factors remained constant). This is within a reasonable range to produce the erosional unconformities identified at approximately 400 m

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below present sea level. Adding the effect of current strengthening b y the factors mentioned above, erosive capacity to depths greater than 330 m and possibly down to 400 m is likely. Heaviest erosion is concentrated on the western, or up current, portion of each sequence. Eroded material is either transported out of the study area or carried to the heads of the erosional gullies where it is eventually funneled down-slope in the form of gravity flows. 120

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SLOPE DEVELOPMENT Late Quaternary Sedimentary History. The presence of at least 9 seismic sequences, all with similar characteristics, indicates the depositional processes described in the previous section have been operating for a relatively long period of time. Age dates show these processes have been occurring since at least the late Pleistocene. Sedimentary cycles from Pliocene to mid-Pleistocene are not unlikely. The erosional unconformity forming the reentrant is bounded below by material of Miocene age, therefore, no evidence exists for similar depositional episodes prior to the Miocene Biostratigraphic and radiometric age dates indicate cycles 121 may be a result of high frequency sea level fluctuations. Fluctuations of 5th (10,000 to 100,000 year frequencies) or possibly even 6th (1,000 to 10,000 year frequencies) order are indicated. Figure 32 shows a sea level curve for the Gulf of Mexico for the last 125,000 years (Beard, et al., 1982). Sequence 5, dated at 84,000 to 127,000 ybp, was probably deposited during the sea-level highstand associated with the end of the last interglacial stage, the Sangamon. The sensitivity of the Florida Current to climatic changes coupled with high sedimentation rates allow us to identify such high frequency events where they may otherwise remain undetected.

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Figure 32. Late Quaternary sea-level curve for the Gulf of Mexico (after Beard, et al., 1982).

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123 0 FALL-RISE QB HOLOCENE WOODFORDIAN z 25,000 FARMDALIAN < z LATE ALTON IAN en a. z ca 0 MID ALTON IAN () (f) en a: < UJ EARLY AL TONIAN >-.._, 75,000 UJ < 100,000 Q5 SANGAMONIAN 12 5, _____ _j

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124 Although all nine sequences exhibit similar characteristics, subtle changes may indicate a shift in depositional processes. This shift is most apparent between sequences 4 and 5 (Table 2). Mass wasting deposits which are found only locally in previous sequences, become more prevalent with sequence 4. Gullies become less prevalent and more spread out. Additionally, gullies gradually begin to become buried from the top by prograding upper slope sediments. Mound shaped deposits, generally concentrated on the upper slope, also begin to increase with sequence 4. This shift in processes is probably more gradual than abrupt, it may begin to be noticeable with sequence 4 only because the quality in data increases nearer the surface. The reason for a shift in processes is not known. The decrease in gully activity and increase in upper slope gravity flow deposition may signify a change from funneling of material down-slope in gullies, to random redistribution of sediment further up-slope by mass wasting processes. This switch in processes may reflect a more basic change in overall margin development, as similar features have been found in the ancient. Exhumed Jurassic carbonate platforms from the Umbria-Marches Apennines of Italy show a well developed gullied slope that suddenly began to infill and eventually became buried. The ultimate control was drowning of the platform, which led to a change from a bypass margin to a depositional margin (Bice and Stewart, 1985). Slope morphology may influence both gravity flow distribution and reef growth. Greatest accumulation of each sequence is in the

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125 up-slope region between 250 m and 500 m water depths. Down-slope, little or no sedimentation takes place. As this slope-front facies progrades seaward it steepens. Oversteepening can be a primary factor in initiating gravity flows. Since this would occur during the depositional phase, material would not necessarily be funneled down-slope via the gullies. Gullies may still be active during the ensuing sea-level lowstand. Reefal growth, creating mounded configurations, may be encouraged by steepening during progradation. Steepening would result in an increase in wave action at the sharp break in slope. Corals thrive in areas of high wave action as food supply and oxygen increase (Milliman, 1974). Additionally, both reefal growth and mass wasting may begin to increase with sequence 4 as the slope front fill begins to become a more positive feature. Sequences prior to sequence 4 have been infilling the large reentrant and the depositional environment has been somewhat sheltered. As the reentrant filled, a more positive and less sheltered feature was developed. As this feature became more exposed to high energy physical processes such as waves and currents, both reefal growth and mass wasting were encouraged. Modern Configuration The present sedimentological configuration on the west Florida slope is the product of the modern sedimentologic regime, typical of patterns operating during sea-level highstands throughout the late Quaternary. Three major facies are present: 1) the upper slope progradational facies; 2) the lower slope gullied or 11sediment starved11 facies; and, 3) the "base of slope" facies

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consisting largely of material supplied by gravity flows. Other sub-facies which will be discussed include the Florida Canyon/Agassiz Valley erosional region and the scarp/11accordian morphology11 of the mid to upper slope. The upper slope progradational facies consisting of a thick accumulation of sediments originating from the shelf and overlying water column has already been described. Prograding sediments represent a southward extension of the west Florida carbonate platform. Erosional scarps are scars resulting from down-slope movement of sediments in response to high rates of sediment input. The lower gullied slope, existing between 500 m and 900 m, contains very little modern sediment cover. Sediments consist dominantly of pelagic detritus. Gullies are presently inactive, as shown by their progressive infilling by prograding upper slope sediments. Further down-slope they contain little sediment fill. Sediment starvation is probably the result of the deep westward flowing countercurrent which effectively prohibits settling of particles. Velocities of 20-25 em/sec have been recorded (Stewart, 1962; Brooks and Niiler, 1975), which are sufficient to prevent deposition of fine sand and clay-sized particles. Base of slope deposits, existing between approximately 900 m and the floor of the Florida Straits at approximately 1,000 m, consist dominantly of onlapping sediments deposited by gravity flows. These deposits were not sampled by cores and are identified on only a few seismic profiles. Previous studies discuss these 126

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127 deposites in more detail (Hurley, 1964; Brunner, 1985). Onlapping sediments may originate on the southwest Florida upper slope, funneled down-slope during sea-level lowstands; or the up-slope portion of the Florida Straits to the east (Hurley, 1964). Based upon sedimentary structures and thickness, Brunner (1985) suggests some sediments may have been deposited by westward flowing contour currents. The western margin of the study area is dominated by the Florida Canyon system and Agassiz Valleys (Fig. 2). Both are large features presently being eroded headward exposing portions of the underlying sequences to the sediment-water interface. As headward erosion occurs, unconsolidated sediment collapses into the chasm, forming large sediment drifts characterized by reflection free or chaotic to hummocky internal reflectors. Reflection surfaces are extremely irregular. Cores 1 and 4 have penetrated this sediment drift. No sedimentary structures are present. The erosional mechanism is unknown, but it is the controlling factor in exposing underlying sedimentary sequences in the pattern illustrated in figure 14. The outcrop pattern as well as the entire modern configuration reflects a combination of several processes operating both simultaneously and in series. These include: 1) The seaward progradation of the upper slope by rapid input of shelf-derived and pelagic sediments. 2) Sediment starvation of the lower slope by the present sedimentologic regime and bypassing of this area during lower sea levels.

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1 2 8 3) The input of sediments to the base of slope by gravity flows originating on the upper slope during lower sea levels, gravity flows originating in the eastern Florida Straits, westward flowing contour currents, or a combination of the three. 4) The currently active, headward erosion of the Florida Canyon/Agassiz Valley system into the western regions of the study area.

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SEDIMENT PRODUCTION AND SEA-LEVEL CYCLICITY: A SEDIMENT BUDGET It has been shown that numerous sea-level fluctuations have occurred throughout the Phanerozoic and that these fluctuations, or cycles, have occurred with different frequencies and amplitudes (Vail, et al., 1977). High frequency sea-level fluctuations (4th through 6th orders) have a most profound effect on shallow-water carbonate deposition and may actually dominate the longer, low frequency events (Kendall and Schlager, 1981; Hine and Steinmetz, 1984). High frequency cycles are rarely identifiable, however, as they are commonly beyond the resolving power of most state of the art seismic systems (Kendall and Schlager, 1981). Sediments produced during high frequency sea-level cycles can rarely accumulate rapidly enough to produce deposits of sufficient thickness to be resolved by modern seismic techniques. High frequency events can be identified on the southwest Florida slope because extremely high sedimentation rates are coupled with the sensitivity of the Florida Current to climatic fluctuations. It will be shown that frequencies as high as 6th order, with durations of 1,000-10,000 years, can produce sufficient quantities of sediments to account for even the thickest sequence identified in the study area. With the limited data available it is difficult to be quantitative, but by making a few assumptions it 129

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is possible to show these sequences could be deposited during high frequency events. In the following calculations, the most conservative estimates for shallow-water carbonate production will be used to account for the maximum thickness of sediment accumulations and highest sedimentation rates observed in the study area. In addition, the following assumptions are made: 1) All sediment found on the southwest Florida slope was produced on the shelf. There is no input from pelagic sources. 2) The mass of sediment in each sequence is estimated by multiplying the areal extent by the maximum thickness. Therefore, the unit is assumed to be the same thickness throughout. 3) There has been no loss of sediment from each unit. What has been estimated by assumption 2 is the original amount deposited Provenance area and average production rates were determined in order to estimate the input of shallow-water carbonate 130 sediments to the slope. Provenance area is estimated as that area where southward trending sand waves have been identified indicating southward and probably off-shelf transport of shallow-water material (Fig 33; Neurauter, 1980; Holmes, 1981; Brooks, 1982). The carbonate production rate on the shelf is estimated at 0.23 m/1,000 years based on an average of Moore's (1972) values of 0.33 m/1,000 years for littoral macrobenthos and 0.13 m/1,000 years for sublittoral macrobenthos on the Florida shelf. This is an extremely conservative estimate as rates ranging from 1 m to 10 m/1,000 years for carbonate sediments on healthy platforms have

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Figure 33. Map of southwest Florida continental margin showing potential provenance area (arrow).

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/ :::. ..... \ ), < !;/ I l ( \ \ \ I \ \ I I 132

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been reported (Kendall and Schlager, 1981). Multiplying the provenance area (4.2 x 10 10 m 2 ) by the average carbonate production rate (0.23 m/1,000 yrs) yields 9.7 x 10 9 m 3 /1,000 yrs of shallow water carbonate sediments being produced in the potential provenance area. Comparing this volume with the volume of sediments in the thickest sequence; sequence 9 (5.6 x 10 10 m 3 following the assumptions discussed above) indicates it would take 5.8 x 10 3 years to produce the sediment volume required. This is assuming all sediments produced in the potential provenance area are transported to the southwest slope, which of course is unlikely. However, over a 10,000 year period off-shelf transport of only 57.77. of sediments produced would sufficiently account for the required volume. Taking into account all of the transport mechanisms in effect, off-shelf transport of this magnitude is very reasonable. Examples with similar values have been reported from the northern Bahamas. Off-shelf transport of carbonate muds from the Bight of Abaco has been estimated at 507. to 677. of those produced on the bank top (Neumann and Land, 1975; Boardman and Neuman, 1984). An additional assumption is that the entire shelf surface is flooded and therefore can produce shallow water carbonate sediments for the entire 10,000 year period. This as we know, has not been the case throughout the Quaternary. However, even during lower sea levels when most of the shelf surface is exposed, shallow-water carbonate sediments can be produced in significant quantities on the southwest Florida margin. The last sea-level cycle will be used as a model as that is the one in which the most 133

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accurate data are available. Additional assumptions include: 1) Sea level 18,000 years ago was 130 m below present. 2) Sea level rose at an average rate of 8 m/1,000 years between 15,000 and 7,000 ybp. 3) Transgression slowed to average 1.4 m/1,000 years from 7,000 to 4,000 ybp, since which time it has remained relatively stable. 4) The average seaward gradient of the shelf and upper slope (to 200 m) at approximately 26 30'N is 0.46 m/km (Martin, 1984). During early transgression, sea level was rising at 8 m/1,000 years and the shoreline was progressing landward at a rate of 17 km/1,000 yrs (Martin, 1984). Assuming that 14,000 ybp sea level was 98 m below present and the shoreline lay at the present shelfslope break (a rather arbitrary division on the west Florida slope). It is also assumed that between 18,000 and 14,000 ybp no shallow-water carbonate sediments were contributed to the southwest slope and that erosion of the previous sequence was occurring. Between 14,000 and 13,000 ybp sea level rose from -98 m to -90 m and the shoreline transgressed 17 km. The newly formed shallow water environment (0-8 m deep) from 26N to Key West (following the 100 m bathymetric contour) could produce an average of 2.3 x 10 9 m 3 of shallow water carbonate material (based on Moore's production rate of 0.33 m/1,000 years for sublittoral macrobenthos). This is 4.1% of the total calculated for sequence 9 not including any shallow-water sediment produced in depths greater than 8 m which certainly occurs and may make a significant contribution. Therefore, even during lower sea level when the area 134

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of carbonate production is greatly restricted, a significant portion of sediment in the most massive unit can be produced in a 1,000 year period in the potential provenance area. For every 1,000 years of sea-level rise (at the rate of 8 m/1,000 yrs) an additional 6.9 x 10 9 m 2 is added to the provenance area further increasing potential production. Lag effects, because of the time it takes for a carbonate platform to respond to rising sea level has been cited as a potential cause for platform drowning (Kendall and Schlager, 1981; Hine and Steinmetz, 1984). Kendall and Schlager (1981) estimate lag times to be on the order of 500 to 1,000 years. Lag times probably have negligible effects on shallow-water carbonate production and subsequent off-bank transport on the southwest Florida margin. The response consists of "starting up" and "catching up'1 of carbonate production to sea level as it begins to flood the banktop. This, of course, is based on the assumption that carbonate production has been 11turned off11 during the preceding lowstand. This may be a viable assumption for classic "Bahama type'1 platforms with their flat tops and steep marginal escarpments. For the west Florida margin however, carbonate production has probably never been turned off. Because the gradient of the upper slope is relatively low, the carbonate producing zone, though narrowing, migrates seaward during regressions and back landward during transgressions. Kendall and Schlager (1981) discuss examples of such migrations on low gradient margins in the ancient and show that shallow-water carbonate production during sea-level lowstands may not be 135

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entirely terminated. In summary, it has been shown that it is possible for shallowwater carbonates to be produced in sufficient quantities in the potential provenance area to account for the entire mass of sediments in each of the sequences on the southwest Florida upper slope, even during 6th order sea level fluctuations. The southwest Florida upper slope therefore acts as a sink for shallow-water carbonate sediments produced on a large portion of the west Florida margin. The controlling factors are the transport mechanisms which act to export material off-shelf to the southwest slope. In addition to promoting large scale deposition on the slope, these processes may contribute to drowning of the platform. Hine and Steinmetz (1984) have suggested that efficient export of sediments off-shelf, as described here, can prevent vertical accumulation and the ability to keep up with rising sea level. 136

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GEOLOGIC SIGNIFICANCE Continental slopes bordering the northern Bahama platform are by far the best documented of modern carbonate slopes ( Neumann, 1977; Mullins, 1978; Mullins and Neumann, 1979; Schlager ar.d Chermak, 1979; Crevello and Schlager, 1980; Mullins, et al., 1980; Sc hlager and Ginsburg, 1981; Cook and Mullins, 1983; Boardman and Neumann, 1984; Droxler, 1984; Mullins, et al., 1984; Droxler and Schlager, 1985; ODP Leg 101, 1985; Boardman, et al., 1986). Comparing the southwest Florida slope with northern Bahama slopes show both differences and similarities. The most obvious and possibly most basic difference is in slope morphology. Specifically, the 11concave up11 configuration of the no::-thern Bahamas and the "convex up11 configuration of southwest Florida. Differences in slope morphology reflect the basic difference in geologic processes operating on both slopes. The northern Bahamas rep_resent, for the most part, a "healthy11 carbonate platform cabable of maintaining consistent upward, vertical growth. 137 Vertical growth may be augmented by submarine cementation which acts to stabilize sediments on the upper part of the slope thereby maintaining the near vertical escarpment (Mullins, et al., 1984). Sediments movi ng as gravity flows bypass the upper slope and accumulate at the base of slope. Vertical growth, cementation of the escarpment, sediment bypassing, and base of slope accretion are

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the processes responsible for the present configuration. The southwest Florida slope borders a drowned platform with limited vertical growth, no observed cementation, and, therefore, no steep marginal escarpment. Large quantities of sediments prograding seaward from the upper slope, the lack of lower slope sediments and minor base of slope accumulations give the slope its ''convex up" configuration. Prograding upper slope sediments and the comparative lack of major down-slope transport mechanisms are the depositional processes responsible for the present configuration. Compared to the Bahamas, other modern carbonate slopes have received little attention. Belize and South Florida slopes both border rimmed platforms and are similar in many ways to Bahamas slopes. (Enos, eta!., 1979; Mc!lreath and James; 1979; Read, 1985). As with the Bahamas, the difference between these slopes and the southwest Florida slope can be attributed to the "health" of the platform. South F lorida and Belize, like the Bahamas, are "healthy", rimmed margins. Slopes bordering the Yucatan Peninsula and the Sahul shelf of northwestern Australia both border non-rimmed platforms and may be similar to the southwest Florida slope. Unfortunately, neither have been studied in detail. Slopes are relatively gentle and facies patterns resemble those of terrigenous clastic slopes in that there is a decrease in texture and shelf components down slope (Van Andel and Veevers, 1967; Read, 1985). In comparing the southwest Florida slope with the slope 138

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139 bordering the remainder of the margin (especially north of 26 N), many differences are noticed. West Florida slopes receive little shelf-derived material and large scale mass wasting as well as current scour is common. This difference reflects the strong depositional nature of the southwest slope as it acts as a major sink for shallow-water material. The northern slope appears to be more influenced by erosional processes. Like slopes bordering the Yucatan and Sahul shelves, sedimentary processes on the southwest Florida slope resemble those of a terrigenous clastic slope. The decrease in shallowwater components down-slope is similar to intercanyon regions of terrigenous clastic slopes as gravity flows are typically confined to canyons. Reflection characteristics on a prograding terrigenous clastic slope consist of sigmoidal and oblique prograding clinoforms, often grading into one another. Sedimentary processes occurring on the southwest Florida slope may be more like processes on terrigenous clastic slopes (Sangree and Widmier, 1977) than those on slopes bordering healthy, rimmed carbonate platforms. Ancient carbonate slopes with characteristics similar to those previously described for the southwest Florida slope are common throughout much of the rock record. The Cretaceous Talme Yafe Formation of northern Israel (Bein and Weiler, 1976) shows many similar characteristics. The mid to lower slope and rise i s characterized by a large sedimentary prism consisting of shelf carbonates mixed with pelagic sediments. Shallow-water sediments are transported off-shelf by storms, tides and seasonal currents,

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and deposited down-slope from nepheloid layers or low density/low velocity turbidity currents. There is a noticeable lack of identifiable turbidites. An alternation of homogeneous bioturbated units with thinly laminated (some cross-laminated) units indicates a regime where weak and strong currents alternate periodically. Morphologically, the deposit has an elongated, asymmetric prismatic shape, much like the depositional sequences on the southwest Florida slope. Thickness reaches 3,000 m in the center and sedimentation rates as high as 20 cm/1,000 years are indicated. The depositional site is believed to be a very low angle, prograding slope. The Lime Kiln Member of the Cambrian Fredrick Limestone of the central Appalachians (Reinhardt, 1977) also shows similar characteristics. The Lime Kiln member consists of fine-grained, . mottled sediments. Deposition occurred by settling of finegrained material winnowed from the nearby high energy shelf. Gravity flow deposits have not been identified. Fining upward cycles 10 m-20 m in thickness, with coarse, poorly sorted material at the base is thought to result from long term migration of channels below wave base. The depositional environment is interpreted to be the outer shelf to upper slope. The Mississippian Rancheria Formation of west Texas and New Mexico (Yurewisz, 1977) may contain some analogous units. The deep water (100 m-250 m) spiculitic wackestone facies contains 57.357. sponge spicules. Deposition occurred by a combination of pelagic fallout, low density turbidity currents and from bottom 140

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currents. Two major groups of sediments are identified: nonlaminated beds and laminated beds. Non-laminated beds are heavily bioturbated and were probably deposited slowly by pelagic fallout. Laminated beds contain cross-laminations and were probably deposited rapidly from low density/low velocity turbidity currents periodically influenced by bottom currents. Gravity flows were rare and intraformational truncation surfaces are attributed to corrasion by bottom currents. The Cambrian Taconic sequence of New York and Vermont (Keith and Friedman, 1977) contains two facies that resemble southwest Florida slope sediments. Structureless micrite consists of shallow-water derived lime mud transported to the slope by low density/low velocity turbidity currents, nepheloid layers or bottom currents. This facies may be analogous to the finegrained, homogeneous sediments deposited during sea-level high-stands on the southwest Florida slope. Laminated sediments, closely associated with the structureless micrite mentioned above, consists of parallel or cross-laminated beds of pelleted limestone or fine grained quartz. This unit is attributed to current reworking of shelf-derived sediments and is analogous to the coarser-grained sediments deposited rapidly under high energy conditions during periods of lower sea level on the southwest Florida shelf. Late Cambrian to early Ordovician upper continental slope sediments of central Nevada (Cook and Egbert, 1981) consist of hemi-pelagic periplatform lime mudstones and packestones derived from the shelf. Off-shelf transport was provided by tides, 141

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storms and seasonal currents. Depositional mechanisms on the upper slope include turbidity currents and contour currents. Resulting deposits include medium-bedded, burrowed or horizontallaminated units, thin-bedded turbidites and well rippled contourites. Off-shelf transport mechanisms and some sedimentary characteristics are similar to southwest Florida slope sediments. Bottom current and turbidity-current processes appear to have played much larger roles in deposition and redistribution in the ancient unit, however. Continental slopes bordering rimmed carbonate platforms with steep marginal escarpments and sedimentation patterns dominated by gravity flow deposits are more abundant in the ancient as well as modern geologic record. This may be because the southwest Florida slope is unique in that it represents a transition zone between an area characterized by high rates of shallow-water carbonate production and accumulation (ie: rimmed platforms such as the Florida Keys and Bahamas) and areas where shallow-water carbonate production is suppressed (ie: the non-rimmed remainder of the west Florida margin). The modern configuration of the southwest Florida slope is a manifestation of these transitional conditions. Carbonate production and accumulation rates are relatively high but lack significant contributors (such as corals) that build shelf edge reef frameworks. If these organisms were present, the southwest Florida margin would be a rimmed platform. As the organisms present produce sand and mud-sized material as opposed to reef framework, transport and depositional processes can be 142

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expected to be more like those on siliciclastic margins. The southwest Florida slope may provide a valuable modern analog for the identification of such transitional environments in the geologic record both temporally as well as spatially. James (1983) has suggested that non-rimmed platforms may develop during those geologic periods when reef-forming organisms are lacking. The ancient analogs to the southwest Florida slope (mentioned above) are all characteristic of times when reef-building organisms were removed by extinction events or before they were fully developed (i.e.: Cambrian, Mississippian and Cretaceous; James, 1983). The southwest Florida slope therefore, may provide an analog for identification of transition zones between rimmed and non-rimmed platforms both in the modern and ancient rock record. 143

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CONCLUSIONS Thick sediment accumulations on the southwest Florida upper continental slope bordering the Straits of Florida consist of a mixture of shallow-water derived material (principally biogenic carbonate) and pelagic detritus derived from the overlying water column. Shallow-water sediments are frequently transported offshelf by tides, storms and oceanic currents and deposited rapidly on the upper slope. Pelagic detritus, probably in the form of fecal pellets, is deposited by settling through the overlying 144 water column. Fecal pellets, because of their relatively large size can settle rapidly through the Florida Current, which averages 100 cm/s in the Florida Straits. Sedimentation pat :erns are controlled by high frequency sealevel fluctuations. During early flooding of the margin, offshelf transport is vigorous as physical processes have more influence o n the shallow shelf surface. Sedimentation on the upper slope is rapid, possibly exceeding 2.5 m/1,000 yrs. During sea-level highstands, off-shelf transport is less vigorous because much of the shelf surface lies below wave base. Sedimentation rates decline. During sea-level lowstands, no off-shelf transport takes place as the shelf surface is exposed. The strengthened erosional capacity of the Florida Current erodes and reworks the previously deposited sequence.

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145 The presence of nine seismic sequences, all with similar characteristics, indicate that these processes, which contribute to southward accretion of the margin, have been occurring since at least the late Pleistocene. Each sequence is the result of 5th or 6th order sea-level fluctuations. The unusually high sedimentation rates and sensitivity of the Florida Current to climatic fluctuations allows the identification of such high frequency events where they may otherwise remain undetected. It has been shown that sufficient quantities of shallow-water biogenic carbonate sediments can be produced during a 6th order sea-level cycle (even considering lower sea levels) to account for the thickest unit identified in the study area. Rapid accumulation is continuing in the present as southward prograding sediments are burying the Pourtales Terrace. Comparisons with slopes bordering other carbonate platforms indicate that the southwest Florida slope represents a transition zone between rimmed and non-rimmed platforms. Similar deposits in the ancient rock record have been identified in those times when reef-forming organisms were lacking. The southwest Florida slope provides a valuable analog for the identification of similar transitional environments in both the modern and ancient rock records.

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146 LIST OF REFERENCES Antoine, J.W., Bryant, W.R. and Pyle, T .E., 1974, Structural framework of the west Florida continental shelf and recommendations for further research, in Smith, R.E., Editor, Procedures in marine environment implications of offshore drilling, eastern Gulf of Mexico: State Univ Syst. Fla. Inst. Oceanogr., St. Petersburg, p. 295-300. Bathurst, R.G.C., 1975, Carbonate sediments and their diagenesis: New York, Elsevier, 658pp Beard, J.H., Sangree, J.S. and Smith, L.A., 1982, Quaternary chronology, paleoclimate, depositional sequences and eustatic cycles: Amer. Assoc. Pet. Geol., v. 66, p. 158-169. Bein, A. and Weiler, Y., 1976, The Cretaceous Talme Yafe Formation; a contour current shaped sedimentary prism of calcareous detritus at the continental margin of the Arabian craton: Sedimentology, v. 23, p. 511-532. Bice, D .M. and Steward, K.G., 1985, Exhumed by-pass margins of carbonate platforms from the Umbria-Marches Apennines of Italy: GSA Abstracts with Programs, p. 46. Boardman, M.R., 1978, Holocene deposition in Northwest Providence Channel, Bahamas; a geochemical approach (Ph.D. Dissertation): Chapel Hill, Univ. N Carolina, 155 pp Boardman, M.R. and Neumann, A.C., 1984, Sources of peri-platform carbonates: Northwest Providence Channel, Bahamas: Journ. Sed. Pet., v. 54, p. 1110-1123. Boardman, M .R., Neumann, A.C., Baker, P.A Dulin, L.A., Kenter, R.J., Hunter, G.E. and Kiefer, K.B., 1986, Banktop responses to Quaternary fluctuations in sea level recorded in peri-platform sediments: Geology, v. 14, p. 28-31. Bouma, A.H., 1972, Distribution of sediments and sedimentary structures in the Gulf of Mexico, in Rezak, R. and Henry, V.J. Eds., Contributions on the geological and geophysical oceanography of the Gulf of Mexico: Texas A&M University Oceanographic Studies, v. 3, p. 35-65.

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147 Brooks, G.R., 1981, Recent carbonate sediments of the Florida Middle Ground reef system; northeastern Gulf of Mexico (M.S. Thesis): St. Petersburg, Univ. So. Fla., 137 pp. Brooks, G.R., 1982, West Florida continental margin as interpreted by seismic reflection methods: Unpublished Manuscript, 41 pp. Brooks, I.H. and Niiler, P.P., 1975, The Florida Current at Key West: summer, 1972: Journ. Mar. Res., v. 33, p. 83-92. Brunner, C.A., 1975, Evidence for intensified bottom current activity in the Straits of Florida during the last glaciation: GSA Abstracts with Programs, p. 49. Brunner, C.A., 1983, Evidence for increased volume transport of the Florida Current in the Pliocene and Pleistocene: Marine Geology, v. 54, p. 223-235. Brunner, C.A., In Press, Deposition of a muddy sediment drift in the southern Straits of Florida during the late Quaternary: Marine Geology. Bryant, W.R., Meyerhoff, A.A., Brown, N.K., Furrer, M.A., Pyle, T.E. and Antoine, J.W., 1969, Escarpments, reef trends and diapiric structures, eastern Gulf of Mexico: Amer. Assoc. Petrol. Geol., v. 53, p. 2506-3542. Carver, R.E., 1971 Processes in sedimentary petrology: New York, Wiley-Interscience, 653 pp. Chin, H., 1983, Seasonal variability on the southwest Florid a shelf, in Southwest Florida ecosystem study: Final Report to MMS, p .13-71. Cook, H.E., 1979, Ancient continental slope sequences and their value in understanding modern slope development in Doyle L.J. and Pilkey, O.H.,Eds., Geology of continental slopes: SEPM Sp. Pub 27, p. 287-305. Cook, H.E. and Egbert, R.M., 1981, Late Cambrian-early Ordovician continental margin sedimentation, central Nevada, in Taylor, M.E., Ed., 2nd International Symposium on the Cambrian System Proceedings: USGS Open File Rept. 81-743, p. 50-56. Cook, H.E. and Enos, P., 1977, Introduction, in Cook, H.E. and Enos, P., Eds., Deep-water carbonate environments: SEPM Sp. Pub. 25, p. 1-3. Cook, H.E. and Mullins, H T., 1983, Basin margin environment, in Scholle, P.A., Bebout, D.G. and Moore, C.H.,Eds., Carbonate depositional environments: AAPG Memoir 33, p. 540-617. P.D. and Schlager, W 1980, Carbonate debris sheets and

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turbidites, Exuma Sound, Bahamas: Jour. Sed. Pet., v. 50, p. 1121-1147 148 Davies, G.R., 1977, Turbidites, debris sheets and truncation structures in upper Paleozoic deep-water carbonates of the Sverdrup Bas in, Arctic Archipelago, in Cook, H.E. and Enos, P Eds., Deep-water carbonate environments: SEPM Sp. Pub. 25, p. 221-247. Doyle, L.J., 1983, Shallow structure and stratigraphy of the carbonate west Florida continental slope and their implications to sedimentation and geohazards: USGS Open File Report 83-425, 19 PP Doyle, L.J. and Feldhausen, P.H., 1979, Bottom sediments of the eastern Gulf of Mexico exam ined with traditional and multivariate statistical methods: Mathematical Geology, v. 13, p. 93-117. Doyle, L.J. and Holmes, C.W., 1985, Shallow structure, stratigraphy and carbonate sedimentary processes of the west Florida upper continental slope: Amer. Assoc. Petrol. Geol. v 69, p. 11331144. Doyle, L.J. and Sparks, T.N., 1980, Sediments of the Mississippi, Alabama and Florida (MAFLA) continental shelf: Journ. Sed. Pet. v. 50, p. 905-916. Droxler, A.W., 1984, Late Quaternary glacial cycles i n the Bahamian deep basins and in the adjacent Atlantic Ocean (Ph.D. Dissert.): Miami, Univ. Miami, 165 pp. Droxler, A.W. and Schlager, W., 1985, Sources of peri-platform carbonates: Northwest Providence Channel, Bahamas-Discussion: Journ. Sed. Pet. v. 55, p. 928-929. Enos, P., Koch, J W and James, N.P 1979, The geophysical anatomy of the southern Belize continental margin and adjacent basins, in James, N.P. and Ginsburg, R .N., Eds., The seaward margin of Belize barrier and atoll reefs: Intern. Assoc. Sed. Sp. Pub. 3, p. 15-23. Enos, P. and Perkins, R.D., 1977, Quaternary sedimentation in south Florida: GSA Memoir 147, 198 pp. Ericson, D.B. and Wollin, G., 1968, Pleistocene climates and chronology in deep-sea sediments: Science, v. 162, p. 1227-1234. Folk, R .L., 1965, Petrology of sedimentary rocks: Austin, Hemphills, 159pp. Freeman-Lynde, R.P 1983, Cretaceous and Tertiary samples dredged from the Florida Escarpment, eastern Gulf of Mexico: Transactions, Gulf Coast Assoc Geol. Soc v. 33, p. 91-99.

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149 Gebelein, C.D., 1977, Dynamics of recent carbonate sedimentation and ecology, Cape Sable, Florida: Leiden, E.J. Brill, 120 pp. Ginsburg, R.N., 1956, Environmental relationships of grain size and constituent particles in some south Florida carbonate sediments: Amer. Assoc. Petrol. Geol., v. 40, p. 2384-2427. Ginsburg, R.N. and James, N.P., 1975, Holocene carbonate sediments of continental shelves, in Burk, C.A. and Drake, C.L., Eds., The geology of continental margins: New York, Springer-Verlag, p. 137-155. Gregory, A.R., 1977, Aspects of rock physics from laboratory and log data that are important to seismic interpretation: AAPG Memoir 26, p. 15-46. Gomberg, D.N., 1976, Geology of the Pourtales Terrace, Straits of Florida (Ph.D. Dissert.): Miami, Univ. of Miami, 330 pp. Gould, H.R. and Stewart, R.H., 1955, Continental terrace sediments in the northeastern Gulf of Mexico: SEPM Sp. Pub. 3, p. 2-19. Heezen, B.C. and Hollister, C.D., 1971, The face of the deep: Oxford, Oxford Univ Press, 659 pp. Hill, P.R., 1984, Facies and sequence analysis of Nova Scotian slope muds: turbidite versus hemi-pelagic deposition, in Stow, D.A.V. and Piper, D.J.W., Eds., Fine-grained sediments: deep water processes and facies: Boston, Blackwell Sci. Pub., p. 311-318. Hine, A.C. and Steinmetz, J.C., 1984, Cay Sal Bank, Bahamas-a partially drowned carbonate platform: Marine Geology, v. 59, p. 135-164. Holmes, C.W., 1973, Distribution of selected elements in surficial marine sediments of the northern Gulf of Mexico shelf and slope: USGS Prof. Paper 814, 7pp. Holmes, C.W., 1981, Late Neogene and Quaternary geology of the southwestern Florida shelf and slope: USGS Open File Report 81-79, 29pp. Holmes, C.W., 1984, Carbonate fans in the Florida Straits: SEPM Abstracts with Programs. Holmes, C.W., 1985, Accretion of the south Florida platform, late Quaternary development: Amer. Assoc. Petrol. Geol., v. 69, p. 149-160. Hudson, J.H., 1984, Seasonal growth rates and carbonate production in Halimeda opuntia at Marquesas Keys, Florida: AAPG Abstracts With Programs.

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150 Hurley, R.J., 1964, Bathymetry of the Straits of Florida and the Bahama Islands: part 3, southern Straits of Florida: Bull. Marine Science of the Gulf and Caribbean, v. 14, p. 373-380. James, N.P., 1983, Reef environment: AAPG Memoir 33, p. 346-462. Keith, B.D. and Friedman, G.M. 1977, A slope-fan-basin-plain model, Taconic sequence, New York and Vermont: Journ. Sed. Pet., v 47, p 1220-1241. Kendall, C.G. and Schlager, W., 1981, Carbonates and relative changes in sea level: Marine Geology, v. 44, p. 181-212. Kennett, J., 1982, Marine Geology: Englewood Cliffs, N.J., Prentice Hall, 813 pp. Malloy, R.J. and Hurley, R.J., 1970, Geomorphology and geologic structure: Straits of Florida: Geol. Soc. Amer., v. 81, p. 19471972. Martin, D.W., 1984, Clastic to carbonate transitions-a modern example: the west Florida continental shelf (M.S. Thesis): St. Petersburg, Univ. So. Florida, 89 pp. Mc!lreath, I.A. and James, N.P., 1979 Facies models 13; carbonate slopes: Geoscience Canada, v. 5, p. 189-199. Milligan, D.B., 1962, Marine geology of the Florida Straits (M.S. Thesis): Tallahassee, Florida State Univ., 130 pp. Milliman, J.D., 1974, Marine carbonates: New York, Springer-Verlag, 375 pp. Minter, L.L., Keller, G.H. and Pyle, T E 1975, Morphology and sedimentary processes in and around Tortugas and Agassiz sea valleys, southern Straits of Florida: Marine Geology, v. 18, p. 47-69. Mitchum, R.M., 1976, Seismic stratigraphic investigation of west Florida slope, Gulf of Mexico: AAPG Studies in Geology No. 7, p 193-223 Mitchum, R.M., Vail, P.R. and Sangree, J.B., 1977, Stratigraphic interpretation of seismic reflection patterns in depositional sequences: AAPG Memoir 26, p. 117-133. Moore, H.B. 1972, An estimate of carbonate production of macrobenthos in some tropical soft-bottom communities: Marine Biology, v. 17, p. 145-148. Mullins, H .T., 1978, Deep carbonate bank margin structure and sedimentation in the northern Bahamas (Ph.D. Dissert.): Chapel

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