Citation
Fine sediment regime of Lake Okeechobee, Florida

Material Information

Title:
Fine sediment regime of Lake Okeechobee, Florida
Series Title:
UFLCOEL
Creator:
Kirby, Robert R
Hobbs, C. H ( Carl Heywood ), 1946-
Mehta, A. J ( Ashish Jayant ), 1944-
University of Florida -- Coastal and Oceanographic Engineering Dept
Place of Publication:
Gainesville Fla
Publisher:
Coastal & Oceanographic Engineering Dept., University of Florida
Publication Date:
Language:
English
Physical Description:
ix, 68 p. : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Lake sediments -- Florida -- Okeechobee, Lake ( lcsh )
Soil mechanics -- Florida -- Okeechobee, Lake ( lcsh )
Drill cores -- Florida -- Okeechobee, Lake ( lcsh )
Coastal and Oceanographic Engineering Thesis M.S
Coastal and Oceanographic Engineering -- Dissertations, Academic -- UF
Genre:
government publication (state, provincial, terriorial, dependent) ( marcgt )
bibliography ( marcgt )
technical report ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Bibliography: p. 39.
General Note:
"November, 1989"
General Note:
Sponsored by the South Florida Water Management District.
General Note:
Final.
Funding:
This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
Statement of Responsibility:
by Robert R. Kirby, Carl H. Hobbs, Ashish J. Mehta.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
All rights reserved, Board of Trustees of the University of Florida
Resource Identifier:
22469134 ( OCLC )

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Full Text
REPORT DOCUMENTATION PAGE
1. Report No. 2. 3. Recipient a Acceslon No.
4. Title sad Subtitle :5. Report Date
FINE SEDIMENT REGIME OF LAKE OKEECHOBEE, FLORIDA November 1989
6.
7. Author(s) Robert R. Kirby 8. Performing Organization Report No.
Carl. H. Hobbs
Ashish J. Mehta UFL/COEL-89/009
9. Performing Organization Name and Address 10. Project/Task/work Unit No.
Coastal and Oceanographic Engineering Department Lake Okeechobee Phosphorus
University of Florida Tyamics Study, Task /
336 Weil Hall 11. Contract or Crant No.
Gainesville, FL 32611
13. Type of Report
12. Sponsoring Organization Name and Address
South Florida Water Management District Final
P.O. Box V, 3301 Gun Club Road
West Palm Beach, FL 33402
14.
15. Supplementary Notes
16. Abstract
The purpose of the study reported herein was to characterize the sediments of Lake
Okeechobee through field and laboratory studies, with special emphasis on the fine sediment regime. Continuous seismic profiling information, involving side-scanning and shallow reflection apparatus, was obtained. Despite the exceptionally shallow water depths in the lake, compounded by the presence of gas in the superficial muds, good quality data revealing the lake bed stratigraphy and indicating suitable sites for later sampling were derived. The sampling program was designed to establish the complete succession with special emphasis on the superficial muds. The underlying geological bedrock succession is consistent in terms of overall thickness and numbers of thin calcareous deposits with that already established
onshore.
Overlying the bedrock, around the southern and northeastern periphery at least, is a
thick in situ peat bed. The peat dates from 5,490 yr BP to 2,670 yr BP, a time when protoLake Okeechobee had a much more restricted extent than at present. Extending over much of the northern part of the lake and overlying the peat in places is a thin fan of quartz sand.
The sand is most recently of fluviatile origin and its extent and variation in thickness imply
input by streams and rivers from the north.
- Continued
17. Originator's Key Words 18. Availability Statament
Bottom characterization
Fine sediment
Lake mud
Lake Okeechobee
19. U. S. Security Classif. of the Report 20. U. S. Security Classif. of This Page 21. No. of Pages 122. Price
Unclassified Unclassified 1 77




The shallowest deposit and the one of greatest interest in respect of nutrient cycling in the lake is a black, carbonate and organic rich mud. As with the sand, the mud is restricted to the northern end of the lake, occupying about one third of the area of the lake bed. It contains 193 x 106M3 of material and is offset slightly to the northeast of the central deep of the lake. As a result its surface slopes towards the southwest at a low angle. Mud depths range from a few centimeters at the periphery to in excess of 75 cm in the deep center of the lake. The deposit has been accumulating for a long period ('- 6300 yr). Its distribution suggests input of some components from the same northern rivers which supplied the sand. Variation in deposition rate with time remains to be investigated.
Internally the deeper, central mud area shows slight lithological variations interpreted to imply that deposition commenced here. The upper and more extensive part of the mud succession looks less differentiated in cut section, but high resolution X-radiography of thin vertical slices showed that the lower horizons exhibit a microscopic interlamination of dark and light bands, thought to arise from periods of algae blooming, death and sedimentation of skeletal debris, alternating with more normal periods of deposition of organic floc material. The microscopic internal primary fabric is clear evidence that the deeper layers of the mud patch are not susceptible to frequent reworking. The X-radiographs also show that there appear to be spherical gas bubbles in the mud in some areas.
In situ density profiles of the upper part of several cores revealed a relatively thin (0-10 cm), fluid mud veneer. This material is believed susceptible to resuspension on occasions. Between the submiulimeter lower zones and the top-most fluid mud upper zone is often a broad (up to 25 cm) zone apparently with poorly developed internal primary fabric. This zone is slightly problematical because whereas shear strength profiles imply the zone to be too strong to be regularly resuspended, the presence of gas within the zone could lead to considerably enhanced susceptibility.
The present results need to be considered along with the nutrient profile data to assist the understanding of the relationship between sediment entrainment and nutrient cycling. It is concluded that the role of gas merits further close scrutiny.




UFL/COEL-89/009

FINE SEDIMENT REGIME OF LAKE OKEECHOBEE, FLORIDA
by
Robert R. Kirby Carl H. Hobbs Ashish J. Mehta

Sponsor:
South Florida Water Management District P.O. Box V, 3301 Gun Club Road West Palm Beach, FL 33402

November, 1989




UFL/COEL-89/009

FINE SEDIMENT REGIME OF LAKE OKEECHOBEE, FLORIDA
Robert R. Kirby Carl H. Hobbs Ashish J. Mehta
Coastal and Oceanographic Engineering Department
University of Florida Gainesville, FL 32611

November, 1989




ACKNOWLEDGMENT
This investigation was conducted as a part of the Lake Okeechobee Phosphorus Dynamics Study funded by the South Florida Water Management District, West Palm Beach, Florida (SFWMD). The authors wish to acknowledge Brad Jones and Dave Soballe of SFWMD for their assistance and Dr. Ramesh Reddy for coordinating the University of Florida team effort. Acknowledgement is also due to Dr. Andrew Salkield and Prof. Paul Visser for their participation in some of the field effort. Thanks are extended to the staff of the Coastal Engineering Laboratory, particularly Sidney Schofield, whose field study planning and coordination effort were critical to project execution. Graduate Assistant Xueming Shen carried out laboratory core analysis.




TABLE OF CONTENTS

ACKNOWLEDGMENT.......................................... ii
LIST OF FIGURES............................................. v
LIST OF TABLES............................................. vii
SUMMARY.................................................. vii
1 INTRODUCTION 1
2 MTHODS 3
2.1 Field Techniques.............................................3
2.2 Laboratory Techniques........................................ 7
3 RESULTS 15
3.1 Introductory Note.......................................... 15
3.2 Geophysics............................................... 15
3.3 Samples..................................................18
3.3.1 Beach rock.............................................. 18
3.3.2 Peat.................................................. 20
3.3.3 Sand.................................................. 20
3.3.4 Mud.................................................. 21
3.3.4.1 X-radiography........................................... 24
3.3.4.2 Evidence of Gas in Mud Deposits...............................26
3.3.4.3 Density and Shear Strength Profiles. .. .. .. ... ... ... .... ... ..27
3.3.5 Problematic Substrate. .. .. .. ... ... ... ... ... ... ... ......29
4 GEOLOGICAL STRUCTURE 32
5 RECOMMNDATIONS FOR FURTHER WORK 35
6 CONCLUSIONS 37
7 REFERENCES 39
APPENDICES
A REPORT ON GEOPHYSICAL FIELD OPERATION 40
A.1 Introduction.............................................. 40
A.1 Side-scan Sonorgraphy........................................50
A.1 Sub-bottom Profiles..........................................51




B REPORT ON CORING SURVEY B.1 Field Operation .............
B.2 Apparatus ................
B.3 Itinerary .................
B.4 Equipment Performance ........

C SAMPLE CORE C.1 Site: OK9 VC. C.2 Site: OK10 VC C.3 Site: OK18 VC C.4 Site: OK31 VC

DESCRIPTIONS
. . . . .. .
. . . . ..
. . . . . .
. . . . . .. .

D SEDIMENT SAMPLING IN SPRING, 1988




LIST OF FIGURES

1 Bathymetric map of Lake Okeechobee. Depths are relative to a datum which is
3.81 m above msl ...... ..................................... 4
2 Vibracorer being deployed for core collection ......................... 6
3 Core OK1 VC ....... ...................................... 8
4 Core OK2 VC ........ ...................................... 9
5 Core OK1l VC .............................................. 10
6 Vibracore sampling locations ..................................... 11
7a Core X-radiograph. X-ray (7 kv; 550 ma) exposure time was 2 seconds. Lower
part of core OK1 VC. Nail image is 5.1 cm in length ................... 12
7b Core X-radiograph. X-ray (7 kv; 550 ma) exposure time was 2 seconds. Upper
part of core OK1 VC. Nail image is 5.1 cm in length ................... 13
8 X-radiograph of core OK1l VC ................................... 14
9 Surface sediment distribution in Lake Okeechobee including coring grid pattern
(courtesy Ramesh Reddy and Don Graetz, UF Soil Science Department).
Compare with sediment distribution map produced in this study (Fig. 10). 16 10 Sediment distribution map of Lake Okeechobee .......................... 19
11 Mud thickness contour map of Lake Okeechobee ....................... 23
12 Mud vane shear strength variation with density (after Hwang, 1989) ........ ..30 13 Schematic showing velocity and concentration fields under wave action and suggested instrumented tower ....... ............................. 30
A.1 Geophysical lines with measurement time markers ...................... 41
A.2 Portion of side-scan record, line 2, October 12, 1988, west-east .............. 52
A.3 A portion of Line 9 demonstrating a shelly (?) mud layer approximately 60 cm
thick over a harder substrate. The deeper sub-bottom reflector depicts a small
paleochannel ........ ..................................... 53
A.4 A portion of Line 4 demonstrating a relatively clean mud layer over a harder
substrate. The sub-bottom reflector depicts a small paleochannel showing
signs of some internal compaction ............................... 54
A.5 A portion of Line 4 depicting a somewhat shelly (?) mud layer overlying a harder
substrate. The relatively shallow sub- bottom reflector dips toward the right. 55 A.6 A portion of Line 6 depicting both the 7 KHz and 200 KHz bottoms. The
roughness of the bottom surface is due to surface water waves approximately 0.5 m high. The strength of the multiples of the 7 KHz bottom suggests that
the bottom is relatively hard .................................. 56
C.1 Core descriptions: a) OK9 VC, b) OK10 VC, c) OK18 VC, d) OK31 VC ..... .62




D.1 Sediment/core sampling sites in Spring, 1988 . . . . . . . . . . 66
D.2 Frozen core from site 1 . . . . . . . . . . . . . . . . 67




LIST OF TABLES
3.1 Core/Clamshell Sample Description ......................... 28
4.1 Lake Okeechobee Deposit Sequence ................................ 32
A.1 Latitude and Longitude as Displayed by Micrologic 7500 LORAN- C ....... ..42
A.2 Summary of Track Lines ....... ................................ 50
D.1 Bed and Sediment Characteristics ....... .......................... 68




SUMMARY

The purpose of the study reported herein was to characterize the sediments of Lake Okeechobee through field and laboratory studies, with special emphasis on the fine sediment regime. Continuous seismic profiling information, involving side-scanning and shallow reflection apparatus, was obtained. Despite the exceptionally shallow water depths in the lake, compounded by the presence of gas in the superficial muds, good quality data revealing the lake bed stratigraphy and indicating suitable sites for later sampling were derived. The sampling program was designed to establish the complete succession with special emphasis on the superficial muds. The underlying geological bedrock succession is consistent in terms of overall thickness and numbers of thin calcareous deposits with that already established onshore.
Overlying the bedrock, around the southern and northeastern periphery at least, is a thick in situ peat bed. The peat dates from 5,490 yr BP to 2,670 yr BP, a time when protoLake Okeechobee had a much more restricted extent than at present. Extending over much of the northern part of the lake and overlying the peat in places is a thin fan of quartz sand. The sand is most recently of fluviatile origin and its extent and variation in thickness imply input by streams and rivers from the north.
The shallowest deposit and the one of greatest interest in respect of nutrient cycling in the lake is a black, carbonate and organic rich mud. As with the sand, the mud is restricted to the northern end of the lake, occupying about one third of the area of the lake bed. it contains -193 x 106M3 of material and is offset slightly to the northeast of the central deep of the lake. As a result its surface slopes towards the southwest at a low angle. Mud depths range from a few centimeters at the periphery to in excess of 75 cm in the deep center of the lake. The deposit has been accumulating for a long period ('- 6300 yr). Its distribution suggests input of some components from the same northern rivers which supplied the sand. Variation in deposition rate with time remains to be investigated.




Internally the deeper, central mud area shows slight lithological variations interpreted to imply that deposition commenced here. The upper and more extensive part of the mud succession looks less differentiated in cut section, but high resolution X-radiography of thin vertical slices showed that the lower horizons exhibit a microscopic interlamination of dark and light bands, thought to arise from periods of algae blooming, death and sedimentation of skeletal debris, alternating with more normal periods of deposition of organic floc material. The microscopic internal primary fabric is clear evidence that the deeper layers of the mud patch are not susceptible to frequent reworking. The X-radiographs also show that there appear to be spherical gas bubbles in the mud in some areas.
In situ density profiles of the upper part of several cores revealed a relatively thin (0-10 cm), fluid mud veneer. This material is believed susceptible to resuspension on occasions. Between the submillimeter lower zones and the top-most fluid mud upper zone is often a broad (up to 25 cm) zone apparently with poorly developed internal primary fabric. This zone is slightly problematical because whereas shear strength profiles imply the zone to be too strong to be regularly resuspended, the presence of gas within the zone could lead to considerably enhanced susceptibility.
The present results need to be considered along with the nutrient profile data to assist the understanding of the relationship between sediment entrainment and nutrient cycling. It is concluded that the role of gas merits further close scrutiny.




1 INTRODUCTION

Lake Okeechobee provides many functions for south-central Florida, including drainage, water supply, flood relief and recreation. The water quality of the lake has deteriorated over thirty years or more as evidenced by its chemical and biological properties. During this period farming practices and various other changes in the lake's watershed have occurred. Should water quality continue to decline, it is likely that plankton blooms will become more extensive, frequent and severe with the ultimate threat of eutrophication of the system. Steps need to be taken to reduce nutrient input to the system, but at present the major factors influencing the deteriorating water quality are not well documented or understood.
One scenario envisages that increasing input of nutrient to the inflowing waters are largely or entirely responsible for deteriorating water quality and that steps need to be taken to reduce these. Should this turn out to be the case, it requires a clearly defined course of action. A different scenario, however, envisages that, notwithstanding present nutrient loads, a large proportion of nutrients are sorbed onto fine sediment particles, which are periodically resuspended leading to partial nutrient release. According to this internal loading dependent scenario, decreasing the fresh nutrient input will have little short-term impact, because nutrient releases will continue to be dominated by fine sediment entrainment and nutrient leaching.
The study reported here is aimed at characterizing the bottom sediment regime in the lake. This has been approached by undertaking a continuous seismic profiling survey followed by a coring survey to characterize the various acoustic reflectors recognized by the geophysical instruments. The coring survey involved sampling with a small hand-held vibracorer. The vibracorer permitted the complete succession to be penetrated, except where indurated rock is exposed directly at the lake bed. The undisturbed samples were returned to the Coastal Engineering Laboratory at the University of Florida (UF) for the following sedimentological and geophysical testing to characterize the deposits. a) cutting, preparation and photography to determine the sedimentary succession, b) measurement of down-core density and shear




strength profiles to determine erosion potential, and c) cutting of thin slices from the axis of cores for X- radiography to show the primary sedimentary fabric of mud deposits.
In addition to these laboratory studies, where very loosely consolidated "fluid mud" type deposits were observed to overlay the more consolidated muds in the field, in situ density profiles of these top-most deposits were performed on board ship at the time of collection with a vibrating tube-type densimeter. Such in situ measurement was essential as the loosely consolidated fluid mud deposits would otherwise have dewatered during transport and been impossible to measure in the laboratory.
Some of the measurements carried out under this study, e.g. core density and shear strength measurements, were also useful to a companion study on lake sediment resuspension and deposition (Hwang, 1989). Those measurements therefore are reported in detail in that study and only summarized in what follows.




2 METHODS
2.1 Field Techniques
Classical geological/sedimentological investigations of the type required in this study demand application of a particular suite of techniques which must be deployed in a set order. Firstly, continuous seismic profiling techniques must be applied. These include two basic types of instruments. A side-scan sonar allows the surface topography and acoustic character of sediments exposed at the lake bed to be mapped. At the same time, penetrating acoustic devices of some kind must be deployed to map the subsurface reflectors. See Appendix A for a report on the geophysical field operation.
The maps prepared from these two types of system then provide the input and basis upon which sample localities are chosen to characterize each reflector type and the sedimentary succession. Arising from this heirachy of techniques it is clearly essential to complete the geophysical surveys before moving on to the bottom sampling program.
In this case the extremely shallow nature of the lake (maximum depths 4-5 m depending on water level, see Fig. 1), together with the suspected presence of gas in the sediments, imposed certain requirements upon the type of seismic device used. Arising from its known high resolution it was decided to use an E.G. & G side-scan sonar. The extremely shallow water depth and mud layer thickness indicated that a short pulse-length, variable frequency pinger was the best high-resolution, shallow-penetration device to chose. By the use of these devices continuous seismic profiles of a better caliber than any previously available from the lake have been obtained.
The E.G. & G SMS-960 side-scan sonar employs a towed torpedo-shaped fish with 105 kllz transducers on either side. It produces a fan- shaped pulse of sound (in the vertical plane with the transducer forming the axis or hinge of the fan). Foreward movement of the fish ensures that successive strips of the bed are scanned. By this technique two swathes of the lake bed extending from directly under the survey vessel out to a nominal range of




Fig. 1. Bathymetric map of Lake Okeechobee. Depths are relative to a
datum which is 3.81 m above msl.
4




100 m on either side are covered. The E.G. & G SMS-960 employs a signal conditioning unit which produces digital records are scale corrected to provide an undistorted image.
The Datasonics SBP-5000 pinger employs a piezo-electric crystal to produce the acoustic pulse which is directed down through the lake bed. A receiving hydrophone collects the returning acoustic signals from sub-surface reflectors. In addition to the variable frequency (3.5, 5 or 7 kHz) pinger a high frequency (200 kHz) echo sounder was operated in parallel. The high and low frequency systems are complementary, permitting precise bottom tracking and good penetration, respectively.
To complete the mapping a small mechanical vibracorer was developed and deployed from a davit on the UF research vessel Silver Bullet. The vibracorer basically has a concrete vibrator powered through a flexible drive from a gasoline motor on board the survey vessel. The concrete vibrator was clamped onto the top of the drill barrel. The drill barrel was 1.83 m in length and had an i.d. of 9.4 cm. It was fitted with a transparent liner to contain the sample. To permit core penetration and retention, a steel cutting shoe, plastic, petaltype core catcher and a non- return valve were fitted. A threaded collar on the top of the corer permitted a guide tube to be fitted. This was attached after the vessel had anchored and the corer had been hung over the side and into the water. The guide tube allowed the vertical position of the corer to be maintained during drilling operations (Fig. 2) as well as permitting visual monitoring of bed penetration. Sample sites were chosen at localities where the geophysical records indicated that particular topographic or lithological features occurred at the surface or within reach of the vibracorer, but below the mud surface. See Appendix B for a brief report on coring survey.
On recovering the vibracorer, the transparent liner was capped at its base and removed from the core barrel. The sample was then measured and described on board ship. In circumstances where the upper surface of the mud deposits was very loosely consolidated a Paar (DMA 35) densimeter was used in the field to measure the density structure of the upper, lowly consolidated horizons. The Paar densimeter is a small, battery operated device




Fig. 2. Vibracorer being deployed for core collection.
6




for accurate measurement of the density of slurries. It operates on the principle of a vibrating glass U-tube. The frequency of the vibration is directly influenced by the slurry, which is converted to density in the instrument and displayed digitally. The core liner was then capped at the top and numbered before being stored in an upright position for transport to the laboratory.
2.2 Laboratory Techniques
In the laboratory the cores were laid in a clamp and the liner only was cut down opposite sides with an electric saw. The core was then halved by drawing a cheese wire down the cuts and through the sample. The bisected core was then opened so that both halves could be described and photographed. Illustrative core photographs are included in Figs. 3, 4 and 5. Core locations are shown in Fig. 6.
No further studies were made on quartz sand, peat or beach rock, but attention was concentrated on the upper muddy zone, where this was present. Shortly after cutting and before the sample could dry to any extent, vertical profiles of density and shear strength were made.
The density profiles were made gravimetrically and the shear strength profiles were measured with a small calibrated vane (Wykeham Farrance, Model 100). Cone penetrometer tests were also carried out. Measurements were made at 5 cm increments of depth and the vane was inserted sideways into the axial (thickest) part of the halved core. This procedure disrupted the sample, rendering it inappropriate for later non-destructive testing.
The other half of the core was then tipped gently out of the liner to rest with its diameter fiat on a board. The upper, curved section was then removed with the cheese wire and a palette knife to leave behind a constant thickness (5 mm) undisturbed slice from the maximum diameter of the core. This slice was then X- radiographed using a standard X-ray machine with a low powered (75 kV, 550 mA) head. The X-ray films were then processed to show the very small scale and detailed primary fabric of the mud layers. Illustrative examples are shown in Figs. 7a, 7b and 8.




Fig. 3. Core OM VC.




Fig. 4. Core OK2 VC.
Q




OK I I vc

Fig. 5. Core OK11 VC.




27-10'
80040'

31 ,'.. 0
-

00 19 18
3
*4

7 6 29 5
16 15 14
* S o0
9 10 28
12
111
0 1 13
00
24 23 22 21
* *
26 27

1 26045' 180040'

o 5Km

Fig. 6. Vibracore sampling locations.

7 10 ",81000'1

25

2 6045' 80040'




50 mA
2 econd

Fig. 7 (a). Core X-radiograph. X-ray (7 kv; 550 ma) exposure time was 2 seconds.
Lower part of core OK1 VC. Nail image is 5.1 cm in length.




OK1V
Uppr ar
75k
55 mA

Fig. 7 (b). Core X-radiograph. X-ray (7 kv; 550 ma) exposure time was 2 seconds.
Upper part of core OK1 VC. Nail image is 5.1 cm in length.




OK1V
75 kV

Fig. 8. X-radiograph of core OK11 VC.




3 RESULTS

3.1 Introductory Note
Part of the task of characterizing the sediments is to map their areal and vertical extent. A map showing the surface sediment distribution is presented as Fig. 9; illustrative core descriptions are provided as Appendix C. For all core descriptions see Hwang (1989). These form the basis upon which the geological history is established.
3.2 Geophysics
Sub-bottom profiles were of variable quality, possibly being strongly influenced by weather conditions experienced during the survey. Some records, i.e. those from lines 2, 3 and especially Line 4, were particularly good.
The geological history revealed by the continuous seismic profiling records is of wen defined, if shallow, limestone (?) bedrock basins or swallow holes. In the north the basins are separated by a N-S orientated narrow ridge, which finds no surface expression today. Further south it appears that the bedrock rises westwards and reaches the lake bed. The extent and number of the basins is difficult to establish owing to the variable record quality.
The origin of the basins is not clear from the geophysics records alone. In addition to these series of basins the bedrock surface, at least in the west on Line 4, where record quality was excellent, is quite irregular and shows a large number of steep, often V- shaped, valleys or channels which criss-cross the margin of the basin. Similar channels are also incised into the bed of the basin. The abundance and shape of the channels gives rise to the tentative suggestion that they may be unfilled, possibly tidal, channels.
More recent calcareous deposits overlie much of this ancient topography with the result that the more accentuated, if low, relief has become muted. In the basins themselves the overlying calcareous deposits are generally of a sheet-like and extensive nature. In the more central areas of the lake up to 5 separate layers can be resolved, whereas towards the edge of the lake it is generally the case that only two layers are present. The depth of the




K L1

~006
ooSAND/SHELL
MUD
SROCK/MARL
Fig. 9. Surface sediment distribution In Lake Okeechobee including coring grid
pattern (courtesy Ramesh Reddy and Don Graetz, UF Soil Science Department).
Compare with sediment distribution map produced in this study (Fig. 10).




deposits ranges up to a maximum of 4 m, but is more generally in the range 1.5 2.5 m. The complex history of the deposits is illustrated from the fact that deeper layers of these calcareous rocks are themselves cross-cut by later channels and the channels, in turn, have been infilled. Apparently a similar environment to that under which the basins and their incised channels were formed existed during the later period when the basins were becoming infiled by the calcareous layered deposits.
The vibracorer nowhere reached down more than 30-40 cm into these indurated calcareous rocks, which are referred to here by the generic term "beach-rock." The highly cemented nature and presence of marine shell species in samples (e.g. see Fig. 7a) suggests they are likely to be in part old shallow marine sediments of possible Plio-Pleistocene age. As a result the deeper layers remain unexplored, other than by these seismic techniques.
Overlying mud deposits are difficult to isolate from the underlying beds on the geophysics records. Signals from the mud zone are often "acoustically turbid" possibly indicating the presence of dispersed gas. At some sites deep-lying strong reflectors may be from shelly layers, e.g. on Line 7 in the south. Elsewhere, for example on Line 4, in addition to the acoustic turbidity the signal from the upper horizon showed strong surface or near-surface reflectors and a phase reversal of the signal. Initial inspection might suggest the presence of coral boulders at the surface, but the 200 kHz record clearly showed a planar lake bed and the parabolic reflectors diagnostic of rock debris are absent from the record. Instead it seems more likely that these signals are due to the presence of gas accumulations in the sediment. This is also suggested by the weak reflectance of the surface seen on the side- scan records.
These zones of phase reversal are at times several tens of meters in extent on the pinger records and would undoubtedly be picked up on the side-scan records if there were patches of shell or rock at the surface. The side-scan records did, however, show a multitude of small (< 1 m) point-source strong reflectors at the lake bed (see e.g. Fig. 7a). These strong reflectors could have several origins.
To investigate the character of the unconsolidated sediments at the lake bed and interpret the geophysics, samples had to be taken.




3.3 Samples

The accessible part of the geological succession is rather straightforward. The entire lake appears to be founded on a whitish, calcareous marine "beach rock" type material. Almost all cores penetrated into this although in some cases, especially in the south, the beach rock is so indurated that the vessel could either not anchor or the corer was unable to penetrate. In a few cases the deposits overlying the beach rock are sufficiently difficult to drill and have a thickness such that they were not completely penetrated by the corer. Over part of the area the beach rock is overlain by a peat layer. Above the peat and not quite coincident with its preservation is a quartz-sand horizon. The shallowest deposit is a blackish, organic-rich clay layer, which shows both macroscopic and microscopic primary layering. The peat, sand and mud occur chiefly at the northern end and in the center of the lake, whereas the southern end is largely free from unconsolidated sediment. The extent and thickness of the various horizons are shown in the bed sediment distribution map (Fig. 10).
Much of the margin of the lake, especially on the west side, is extremely shallow and difficult to gain access to as a result of extensive beds of vegetation. This zone could not be investigated during this survey, but has been investigated by UF's Soils Science Department in a companion study. The various formations are discussed in the following sections.
3.3.1 Beach rock
A cemented calcareous white deposit forms the basement of the lake, being exposed at the lake bed for up to 50% of its area. These deposits are of variable lithology and include indurated lime muds, nodular limestones, calcareous sands and sandstones and shelly horizons. In places the upper zone shows signs of weathering and the penetration by rootlets from the overlying peat. The shelly fauna consists of gastropod and bivalve species and has not been specifically identified. These basal deposits were considered by Gleason and Stone (1975) and Brooks (1984). The calcareous deposits are probably the complete succession of the Caloosahatchee-Fort Thompson formation (Plo- Pleistocene).-




Moore Haven
OR
Paho
26-43I i26 8o40*1 Clewiston to
o SKMn
Depths in Meters Below Datum
Bathymetry-depth in feet 23 Core Station o Beach Rock in Core oR Beach Rock at Surface, not Sampled
* Peat in Core or Outcropping at Surface Conjectured Extent of Peat Deposit X Quartz Sand
---0 Margin of Sand Area
---40 SandPatch > 40cmdeep CI Black Mud 0-.0.-- Marginof MudArea
----- 30 Mud Patch > 30cm deep
----60 Mud Patch > 60cm deep ? Possible Bed Rock ? Possible Fluvial Sand
Fig. 10 Sediment Distribution Map of Lake Okeechobee.




3.3.2 Peat

One of the more unexpected discoveries of the sampling campaign is the depth and geographical extent of peat deposits at the northern end of the lake. The authors are not aware that peat deposits have been recognized at the northern end of the lake before.
The peat deposits are believed to lie in situ. In a number of cases the beach rock upon which they invariably rest is penetrated by rootlet beds from the vegetation which originally grew upon the beach rock surface. The fact that the peat is in places layered and shows a variety of textural features may also support the view that it is in situ and if the need arose could be sampled for pollen and other evidence of the past environment around Okeechobee. The relatively thick and layered peats hint at a quite prolonged period of sub-aerial exposure during which a variety of climatic or environmental changes occurred. No attempt was made in this study to carry out pollen dating, radiocarbon dating or any identification of macroscopic plant remains.
Being organic, peat layers are known to generate and hold gas. As such they present particular difficulties to seismic devices, which generally will characterize them as strong reflectors, producing a phase reversal of the acoustic signal. Peat layers are thus difficult for seismic devices to "see through" to what is underneath. In this case what is below is the beach rock and is of little direct interest to this study.
The extent of the peat suggests that at some stage the entire basin may have been a peat bog or at least *that the water may have occupied a smaller area in the deep center of the present lake. The oldest previously known peats are 5,490 yr BP and range up to 2,670 yr BP (Gleason and Stone, 1975).
3.3.3 Sand
In those few cores where the succession is complete the peat is overlain by a grayish quartz sand. The sand occupies a broader zone than the peat at the northern end of the lake, although it is generally thin (< 10.0 cm). Only around the entrance to the Kissimmee




River does its thickness increase (40-50 cm). Here a series of sand layers with differing gramnsizes and shell content overlie each other. In the west the sand layers are exposed at the lake bed and not entirely covered by the more recent black muds.
The northern distribution of the sand patch and the fact that it is thickest in the proximity of the Kissimmnee River, combined with the fact that it overlies the terrestrial peat, are all indicative of a fluviatile sand supplied by the Kissimnmee River drainage basin, as opposed to a marine beach sand.
At the southern extremity of the fan of sand thin sand layers are interbedded with the overlying black muds at two sites. This suggests that the sand sheet originally had a rather greater southerly extent and was reworked back onto the proximal mud deposits at a much later date.
The sand layer is indicative of a change in source rocks or deposits in the hinterland compared to that which is presently supplied. The Kissirmmee River watershed constitutes more than 50% of Okeechobee's drainage basin and the Plio-Pleistocene deposits are more sandy in the north. The sand layer is not directly relevant to the present investigation.
3.3.4 Mud
Black, organic-rich muds form an extensive veneer in the northeast quadrant of the lake, possibly covering a third of the entire bed. Why the mud should be absent from the southern end and western sector of the lake is unclear, especially because the western side of the lake is heavily vegetated and thus provides both more sheltered and possibly more nutrient enriched waters. Possibly the distribution is linked in some way with inputs of nutrients or inorganic fine sediment from Taylors Creek or the Kissinmmee River in the north. Equally it is not immediately apparent why the mud is absent in the south, other than to observe that the distribution of sand and mud are similar in this respect. Maps showing both bathymetric contours and the extent of the mud patch (Figs. 1 and 10) reveal that the mud patch is offset to the north-east such that its surface is inclined to the south-west. -This is presumed to reflect a hydrodynamnic control. The mud layer is generally less than 30 cm thick, although




in two areas it exceeds 30 cm and approaches 75 cm in one. The area of maximum thickness of mud is almost coincident with the area of deepest water, possibly indicating a link.
The general outline of the edge of the mud patch is closely coincident with that mapped in greater detail with more samples by UF's Soil Science Dept. (Fig. 9). Most discrepancies are comparatively minor and probably accounted for by the poor repeatability of the LORAN positioning system (see Appendix A), added to the fact that the mud area thins to a feather edge at its margins and probably is patchy. There is a greater discrepancy in the south where seven stations were attempted during this survey. At five the vessel could not anchor or the vibracorer would not penetrate, indicating the presence of rock at the surface. The anchors came up clean. At the remaining two stations the corer produced rock samples. Clearly any black mud here must be thin and very soft. The southerly limit of mud is probably about 260 541N whilst the Soil Sciences map shows a greater southerly extent down almost to 260 49'N.
Whereas the boundary of the mud area as mapped by Soil Sciences and this vibracoring survey are generally coincident the thickness of the mud patch sometimes appeared rather different. This is largely accounted for by the fact that Soil Science mapped the mud depth by measuring core length in the field. In contrast, in this study the length is measured from opened cores. The peat and sand layers omitted from this study result in a smaller mud thickness. To facilitate core retention the vibracorer used a petal-type core catcher. In the very lowly consolidated muds encountered this could have given rise to a certain amount of loss at the top of the sample. Other evidence (below) shows that the amount of disturbance caused to the sample by the vibration, core-catcher, recovery or removal and capping of the core was, however, generally slight. In Fig. 11, a mud contour map is presented to highlight the variability of mud thickness. This variability appears to be considerably greater than that suggested by Gleason and Stone (1975), although their observations regarding mud distribution in the lake are confirmed in a qualitative sense. Four sources of information were integrated in preparing this map: 1) geophysical profiling repo rted here, 2) vibracoring reported here, 3) core data derived from sampling undertaken by UF's Soil Science Depart-




27o10'1
810001
N

26045' 80040

0 5Km
Mud Thickness In cm
Mud Thickness in cm

Fig. 11. Mud thickness contour map of Lake Okeechobee.

27010,
80040'

260.45'
80040'

lk CIM
2




ment in Summer, 1988, and 4) Coastal and Oceanographic Engineering Department's data collection effort in Spring, 1988 (Appendix D). Using the values of mud thickness and area from this study, of the order of 193 x 106m' of mud lie on the bed of Lake Okeechobee.
Cutting the cores revealed the primary fabric of the black, organic-rich muds. The muds have little by way of a benthic invertebrate fauna; living organisms were largely confined to an occasional rare and small, highly ribbed, fresh water bivalve. Arising from this the samples were expected to show a well- preserved primary fabric, although there was little in the uncut cores to reveal any evidence of any significant change in lithology with depth.
Once the cores were cut the internal structure was more readily apparent and some lithological contrasts were apparent. Samples OK2, 6, 9, 10, 11 (?), 14, 15, 28 and 29 showed zones of different colored clay and thin beds of shell or sand. These lithological variations are important confirmation that the cores are largely undisturbed. These samples with a more complex stratigraphy occur in the deepest mud zones, suggesting that these are the earliest deposits, which accumulated slowly and possibly reflect major climatic perturbations in the lake or hinterland, such as hurricanes (shell and sand layers), forest fires or other short term events (clay layers). Brooks (1984) has dated these lower muds at as early as 6300 yr BP.
The upper part of the mud zone is invariably formed by a more homogeneous, black silty clay of wider extent. It appears that following the deposition of varied lithologies and beds in the deeper areas a period of more uniform, widespread and faster (?) deposition has commenced. These apparently homogeneous beds can be examined using X-radiography.
3.3.4.1 X-radiography
Unlike sand deposits, which frequently show a variety of internal primary depositional features in cut section, mud deposits generally appear massive and homogeneous in cut sections of cores. Such apparent homogeneity hides much of the evidence for how the muds were deposited and their subsequent history. In this case the apparent homogeneity could have been real and arisen from intense bioturbation, core disturbance or disruption due to




gas generation and release, or it could have been only an artefact of the small size of the sediment grains and uniformity of the sediment supply over a prolonged period.
To throw light on these issues the 5 mm thick slabs of core were X-rayed. The preparation technique employed ensured that any primary fabric was displayed with very high resolution. Arising from the fact that X-radiography was only applied to selected cores as a check on sample quality only a limited study of the internal primary sedimentary fabric could be accomplished.
Two features of the few X-radiographs (see Figs. 7a, 7b and 8 as illustrative examples) completed are worthy of note. These are that in most cases, especially towards the base of the mud layer, a distinctive alternating sequence of dark and light bands, or layers, is present on a subrnillimeter scale. A second significant feature is the apparent presence of gas. These two features are discussed in turn below.
The very delicate small scale layering is important at two levels. Firstly, it provides unequivocal evidence that the core samples obtained are largely undisturbed, despite the vibration process and the poorly consolidated nature of the deposits. This is very consistent with evidence of this kind of sample from elsewhere. Secondly, the layering shows in preserved form the history of individual sedimentary events in the lake waters stretching back in this case over several thousand years. The alternating light and dark bands clearly represent algal blooms and the detritus resulting from them, (skeletal secretions etc.) interbedded with organic deposits of more normal sedimentary processes, deposition of inorganic clays, precipitation of organic flocs etc.
The recognition of the layering has another implication relevant to nutrient cycling too, namely that any repeated resuspension and re-deposition, on whatever timescale it occurs, must only affect those sections of the bed deposits which do not show the submillimeter alternations. Any large-scale entrainment, for example during hurricanes, might be expected to give rise to single or infrequent graded units, as opposed to the alternations. Regrettably the frequency and distribution of dark and light alternations in the upper, massive




and widespread shallow mud deposits could not be ascertained owing to the absence of Xradiographs of these materials.
In this study no attempt was made to date the deposits by radiocarbon techniques, or to examine the succession to discover whether algal blooms have become more common, more prolific giving rise to thicker bed deposits, or whether the species of algae involved have remained unchanged.
In the few X-radiographs available for examination there is apparent evidence that the submillimeter intercalculations become less well defined towards the top of the cores. There are several reasons why this might be so, core disturbance, lack of consolidation to form distinctive layers and the generation and expulsion of gas being just three of the possibilities.
One X-radiograph, OKI VC (Fig. 7a), shows a series of circular or elliptical voids (light areas). This provides possible evidence of the presence of gas in the sediment. No strong smells of gas were detected at any time suggesting that H2S (hydrogen sulphide) was largely absent and that any gas was likely to be in the odorless form of CIT4 (methane). The apparently spherical nature of the voids makes it unlikely that the voids were artefacts of the cutting and preparation process.
3.3.4.2 Evidence of Gas in Mud Deposits
The 7 kHz pinger records showed phase reversals of the acoustic signal consistent with the presence of gas in the sediment. In addition, the surface of muddy deposits shown by the side-scan sonar show many point-source reflectors (Fig. A.2). The abundance of these pointsource reflectors is unusual for a mud area. The origin of the reflectors is problematical. They could arise from weed at the lake bed or could be debris and litter jettisoned from pleasure craft. An alternative possibility is that they could represent gas-seeps. This possibility has not been investigated further.
In addition to the acoustic evidence for the presence of gas, several of the cores showed signs of being gassy. Gas generation in recently collected cores can be difficult to distinguish from expulsion of air from voids created during handling. In this case recognition of the gas is




made more difficult by the fact that the change in pressure from the lake bed to atmospheric is so small. In general terms such physical evidence for gas generation and presence in the cores was limited. This seems to be borne out by the rather undisturbed nature of the cores themselves. However, further evidence for the presence of gas seems to be found in the X-radiographs of some of the muddy cores.
Furthermore, rather strong evidence for the presence of gas in sediment was first obtained during a sediment sampling cruise in Spring, 1988 (Salkield, 1988). Table 3.1 provides a brief description of the type of material found at the different sites using a small piston core (with 5 cm dia. PVC pipes varying in length from 0.6 to 1.8 m) or a clamshell grab sampler, and whether gas was present in the sediment. Site locations are shown in Fig. D.1. Gas was detected by bubbles which broke the water surface when the clamshell was dropped at the bottom. Should there be significant gas in the muddy sediment, it could have a measure of importance in terms of its effect on erosion potential of the muds. This matter is believed to merit closer scrutiny.
3.3.4.3 Density and Shear Strength Profiles
A most important aspect of characterizing the physical properties of the muddy deposits was to determine their density and shear strength characteristics with a view to calculating their erosion potential. Many of the cores had a very loosely consolidated upper zone of fluid mud in which in situ measurements of density were made. These zones range from a few to eight centimeters in depth and have densities of 1.01 to 1.03 g cm-3. No shear strength readings are available for these low strength upper zones, firstly because shear strength measurements were only made in the laboratory and secondly because the strengths were below the resolution of the instrument. Illustrative core descriptions are provided in Appendix C. For a more complete description of measurements see Hwang (1989).
The distribution of the low strength fluid mud zones showed no systematic pattern other than a slight possible tendency for the fluid mud zone to be deeper and more frequent in the south. In the firmer muds the density and shear strength measurements were generally




Table 3.1: Core/Clamnshell SampleDecito

Site Water Presence of
No. Material description depth" gas
__ ________ (in) j _

Muddy over soft marl Muddy with small shells Muddy b
Muddy, no core No core, not much mud No core, not much mud, hard bottom No core, fine sand and small shells over hard bottom
Mud over hard bottom Mud over hard bottom Mud with some sand and shell

4.6 Gas released
4.6 Gas released
4.6 Gas released
4.9 Gas released
5.2 No gas
4.3 No gas

No gas
Gas released Gas released
Gas (large quantity)
released

'Water depths were about 1.2 m above the chart datum (reported to be 3.81 mn above rnsl in NOS Chart No. 11428) at the time of measurement. bCore penetrated about 0.3 m of mud, hit a relatively hard "lens," and then broke through into the mud below.
'Not recorded.

Description




closely related. In unlayered deposits, such as 0K2 VC, the density and shear strength values showed a steady increase with depth consistent with a normally consolidated, undifferentiated substrate. Other samples with a more complex stratigraphy of interbedded weak and strong clays or clays, sands and shelly clay layers showed a general gross increase in density and strength with depth but a detailed profile which shows a series of sharp density and strength reversals. Again the strength and density peaks and troughs generally were coincident (e.g. OK10 VC). In this core, however, whilst the shear strength increased with depth the density of the weak mud layers was lower at 50 cm than at 2 cm below the surface. This type of behavior arises from the fact that density is not an unambiguous analog for strength, which, among other factors, depends strongly on mud composition.
Mud densities were in the range that might be expected, ranging up to 1.2 g cm-3 and a maximum of 1.3 g cm-3. Sand densities were higher, reaching 1.8 g CM-3. Shear strengths reached almost 6 kN m-2 at times. Even close to the surface the shear strengths were generally up to three times the critical shear stress for erosion. A heuristic explanation for this difference is provided by Hwang (1989).
A plot of shear strength versus density based on measurements from a large number of cores (Fig. 12) shows the expected scatter of data points. A best fit curve for the data intercepts the density axis at 1.065 g cm-3. At density values below 1.065 g cm-3 the shear strength becomes zero, implying that the mud essentially behaves as a fluid.
The evidence seems to indicate that the fluid mud layers could be regularly resuspended during windy weather, whilst the underlying mud is relatively resistant to erosion. The intricate and small scale lamination of the deeper mud layers supports this observation. In addition, resuspension work carried out by Hwang (1989) as well supports the same observation, indicating a depth of reworking under storm wave action on the order of 10 cm.
3.3.5 Problematic Substrate
In the west at depths less than -2 m the black mud and peat deposits are absent and at several OK localities (Fig. 6), 8, 17, 25 and 26 and possibly also at 7, 9, and 16, a




*0.

0
0 *.

1.2 1.3

1.065 g cm-3

BULK DENSITY (g cm-3)

Fig. 12. Mud vane shear strength variation with density (after Hwang, 1989).
] Data Acquisition Package

EM 0
Mobile
Lutocline EM 0-f Fluid Mud Deforming Bed AC
Stationary Bed
Hard Bottom

- ---{T
IP

Fig. 13. Schematic showing velocity and concentration fields under wave action
and suggested instrumented tower.

-n 1




variable, generally whitish or grayish sand or shelly clay occurs. The layer is believed to represent either the weathered top of the beach rock or the fluviatile sand. It is unclear whether sediments of this zone should be considered as the ancient marine foundation of the lake, as later fluviatile deposits or whether they are a mixture. The latter is considered unlikely owing to the absence of a mixing mechanism. The geophysical evidence suggests that the underlying bedrock is exposed in the west. It is unlikely that these materials play a significant role in nutrient cycling and they are not considered further here.




4 GEOLOGICAL STRUCTURE
Lake Okeechobee lies in a stable part of the earth's crust in which the overall configuration of the basin has not changed since the early Pleistocene. The Okeechobee basin has been a site of subsidence since at least the early Tertiary and a thick sequence of Miocene clay in its axis has resulted in slow differential compaction to perpetuate the feature. The other controlling influence on the geological history has been the pattern of deposition of clastic sediments during Plio-Pleistocene periods of high sea level (Brooks, 1984).
The Okeechobee Area is underlain by a sequence of Tertiary/Pleistocene Deposits as follows (Table 4.1):

Table 4.1: Lake Okeechobee Deposit Sequence

Deposit

Lake Flirt Formation (confined to the headwaters of the Caloosahatchee River)
Caloosahatchee-Fort Thompson Formation (5 or more lithological horizons one of which is the Coffee Mill Hammock Formation. Marine limestones and fresh water marls, 220,000 120,000 yrs)
Taniami Formation (white-grey sandy limestone to clayey marl and fossiliferous sands)
Hawthorn Formation (olive green and grey clays with phosphatic sandy clays > 200 m thick and controls lake position)
Tampa Limestone
Suwannee Limestone
Ocala Limestone

Age

4 -

Late Pleistocene Plio-Pleistocene Mid-late Pliocene
Miocene
Lowest Miocene
Oligocene Upper Eocene




Only the Caloosahatchee-Fort Thompson Formation is of relevance to this study, as the sub-bottom profiling data presented here shows an earlier eroded basin within the lake possibly of Tamiami limestones and marls, which has been progressively infilled by a complex sequence of calcareous deposits at least 4 in number. Brooks (1984) reported that this Caloosahatchee-Fort Thompson Formation outcrops on the bottom of Lake Okeechobee, forms the double row of "reefs" across its southern portion and underlies the peats, marls and surficial sands in areas surrounding the lake. Brooks recognized 5 typical CaloosahatcheeFort Thompson units in canals excavated in the construction of Hoover Levee on the north eastern section of the lake. Brooks traced these formations over a 24 km distance. In the north the units were predominantly sand and shelly sands of the "Pipecrest Beds." Southwards, massive but discontinuous cap rocks, usually sandy freshwater limestones with solution pipes and laminated caliche crusts occurred at the top of each marine unit. The Coffee Mill Hammock Formation does not occur around the margins of the lake and may be absent from the lake bed deposits. The thickness of the Caloosahatchee-Fort Thompson Formation on shore is generally of the order of 3 m which compares closely with continuous seismic profiling evidence for the thickness in the lake itself. These 5 units could not be penetrated during the vibracoring exercise undertaken during this study, but these descriptions from marginal locations around the coast serve to characterize the deposits. Brooks recognized a bed of Rangia cuneata overlying the Fort Thompson deposits in South Bay and extending out into the lake reaching 1 m in thickness. The Rangia beds are estuarine deposits more than 25,000 yr in age. Brooks sampled a calcitic freshwater mud in the southeastern portion of the lake. This mud overlies in part the Rangia beds as well as the cap rock of the Caloosahatchee-Fort Thompson Formation.
At Belle Glade, just southeast of the lake, peat deposits have an oldest ages of 4,400 yr BP (McDowell et al., 1969). It has long been considered that the lacustrine plain of the Florida peninsula represents in unmodified form the seabed surface at the time of its latest emergence (Heilprin, 1887; Brooks, 1984).




The northern and eastern margins of the lake are enclosed by a series of beach ridges, finding their best development between Taylor's Creek and Chauncy Bay. The lowest part of the oldest ridge has been dated by 14C on fresh water clams at 1,685 75 yr BP, presumably at this recent date these were fresh as opposed to salt water beaches formed around the lake itself.
From dates on the fresh water calcitic muds in the lake of 6,300 yr BP (Brooks, 1984) and from dates on peats determined by Gleason and Stone (1975) showing ages ranging from 5,490 90 yr BP to 2,670 80 yr BP we can imply that a lake has been present extending back many millennia. Brooks suggests to at least 12,000 years BP. At around 6,300 yrs ago the lake was small and the fresh water organic-rich muds were being deposited in the deepest, central part of the lake. Penecontemporaneously vegetation was growing which ultimately decayed to form the peats now exposed widely along the southern and north eastern margins on the exposed Caloosahatchee-Fort Thompson carbonates around the margin.
From this remnant the modern lake, with an ever increasing elevation, resulting from organic deposition along its southern rim, began to develop just over 4,000 yrs ago (Brooks, 1984). From the beach ridges, Brooks concludes a historic maximum level shortly after 265 AD. Speculative evidence dates the latest of the beach ridges at 900-1200 AD, a warm, hurricane-prone climatic interval. The organic sill rising at the southern end of the lake and blocking drainage to the south ponded the lake waters. The sill reached a maximum of 6 m above present sea level. During periods when the lake reached the 6.8 m stage, as in 1886, and again in 1878 (7.1 m) water overflowed the whole southern rim, resulting in high velocities in the rivers to the south and possibly at the southern end of the lake.
This pattern of steady and progressive ponding, accompanied by episodic overtopping, may have been typical of the last millennium and continued until the major interference by man to alter a coastline and canalize and divert the drainage during the present century.
The geophysical and sampling data obtained during this program thus support and contribute more detail on this topic of the evolution and characterization of the bed sediments.




5 RECOMMENDATIONS FOR FURTHER WORK
Three aspects of the mud deposits of Lake Okeechobee merit further study. First, both the continuous seismic profiling records and the samples show evidence for the presence of a certain amount of gas in the sediment. The continuous seismic profiling records are not adequate to map the areal distribution of gas and neither are core samples adequate to determine the vertical distribution of gas. The gas may be relevant to the erodibility of the mud in two ways, either directly through the entrainment of sediment by gas seeping from the sediment spontaneously, or indirectly through a weakening of the cohesion of the mud bed, for example during periods when large waves occur on the lake. Wave cycling at the bed leads to pressure fluctuations which will be quite large in relative terms in such shallow water. Arising from this it may be that muds of apparent strength above that normally considered stable and resistant to erosion could be entrained, should widespread gas liberation occur. A small scale project definition study is required to investigate these issues further and possibly indicate any field or laboratory tests which could throw light on the matter.
Second, the recognition of the mricrofabric of the cores and specifically the submillimeter lamination was a bonus to the investigation and indicates the value of applying Xradiographic monitoring. Its importance is that the alternating black and while bands of the elemental primary lamination are likely to be skeletal debris from algal blooms and organicrich muds, respectively. As such they represent the day by day history of the lake bed and could provide a longer time record of the evolution of the eutrophic state of the lake.
This could be evaluated by such techniques as Scanning Electron Microscopy to reveal the small scale lamination in greater detail and to identify algal species present. In addition to species the thickness and frequency of bloom deposits would also be apparent. The mineralogy could be studied at the same time using XRF and XRD methods. Such a study would be enhanced if it could be interpreted in the light of radiocarbon date profiles in the mud. At shallower depths in the samples any evidence for disruption by gas or for the presence and source of gas microbubbles would also be deduced.




Third, it is important to recognize that under wave action, the top 10 cm of the bottom mud appears to fluidize regularly but does not entrain easily into the upper water column (Hwang, 1989). Fluidization essentially implies destruction of the structural integrity of the porous solid mud matrix, which may mean new pathways for upward diffusion of soluble phosphate. A careful study of the response of mud to wave action would require a combined field/lab oratory effort addressing a number of experimental components. A key field test would involve simultaneous measurements, during periods of significant wave action, of wave properties (height, period and induced orbital velocities) in the water column, sedimentrelated turbidity and bottom mud motion.
Fig. 13 depicts the likely variation of the horizontal wave velocity amplitude (ai), the corresponding concentration (C) profile and a suggested field tower for examining the flow field and bed response. The significance of the division of the concentration profile into identifiable sublayers ranging from mobile suspension to hard bed has been discussed elsewhere (Hwang, 1989). It sufficies to note that one is interested in investigating flow conditions which lead to the fluidization of the mud bed by waves, and bed reformation by dewatering after wave action is over. This objective can be achieved by monitoring wave orbital velocities (using electromagnetic current meters, EM), mud accelerations (using accelerometer, AC), water surface variation (using pressure gage, P), and turbidity (using electro-optic meters, T).




6 CONCLUSIONS

The geophysical and vibracoring survey has permitted the sediment of Lake Okeechobee to be characterized. Where the succession is most complete a variable thickness of black mud overlies a thin veneer of fluviatile sand. The sand rests in the north-east of the lake bed on an in siupeat deposit, which is rooted into the white, calcareous beach rock forming the foundation of the lake.
Unconsolidated deposits mainly are found at the northern end of the lake, suggesting a close affinity with the Kissimmee River. Some 193 x 106 M3 (0.3 kin3) of organic-rich mud axe distributed mainly at the northern end of the lake. It is evident that the earliest mud deposits infill the deeper central portion of the lake, a fact which may indicate the control on distribution exerted by waves. Modern mud deposits are progressively spreading wider and covering more of the lake floor. The deposits are focussed in the northeast sector of the lake and form an inclined deposit extending into shallower water on the northeast margin of the lake. This eccentric distribution and sloped surface must be controlled by the hydrodynamics of the lak Ie. This could influence internal phosphorus cycling. The reason for this eccentric distribution is unclear. It could be entirely controlled by the input point in the Kissimmee River or it could be, in part, influenced by a more energetic regime at the southern end of the lake. However, there is no apparent reason why the southern end of the lake should be more energetic, either in the past or today. The depth and distribution of the mud could influence internal phosphorus cycling and the development of blooms, although the location or intensity of algal blooms is not known to the authors.
The shear strength profiles indicate that only the upper, low- strength fluid mud zone on the order of 10 cm. thickness is susceptible to resuspension, whereas the deeper sections of core, samples, which exhibit submillimeter lamination, confirm that the lower sediment layers do not participate in any sediment resuspension. An intermediate zone of apparently rather undifferentiated black mud occurs towards the tops of cores and is of widespread extent in




the lake. This is the zone which could be influenced by gas-induced resuspension and there has been inadequate opportunity in this study to investigate this important zone in detail.
If internal phosphorus cycling in the lake is linked merely with fluid mud resuspension it will be a much smaller scale process than if part of the upper, more consolidated portion of the cores could possibly be involved. For this reason investigation of the upper part of the cores and of the gas content may be instructive.
It is noted that several other lakes may provide evidence relevant to understanding and managing Lake Okeechobee. For example, "pock- marks" created by gas-seeps are known to occur in Lake Superior, whilst severe phosphorus enrichment is a problem in several lakes in Northern Ireland. In Lough Erne, County Fermanagh the phosphorus is mainly from two sources, fish farms and sewage. Great progress in improving water quality has been achieved by a phosphorus extraction plant in the local sewage works.




7 REFERENCES

Cited
BROOKS H K 1984 Lake Okeechobee. In: Patrick J Gleason (ed) Environments of South
Florida, Present and Past II. Miami Geological Society, p36-68.
GLEASON P J and STONE P A 1975 Prehistoric trophic level status and possible cultural
influences on the enrichment of Lake Okeechobee. Unpublished Report, South Florida
Water Management District, West Palm Beach, 133pp.
HEILPRIN A 1887 Explorations on the west coast of Florida and in the Okeechobee wilderness. Wagner Free Institute of Science of Philadelphia, 134pp.
HWANG K-N 1989 Erodibility of fine sediment in wave- dominated environment. M.S.
thesis, University of Florida.
McDOWELL L L, STEPHENS T C and STEWARD E H 1969 Radiocarbon chronology of
the Florida Everglades peat. Soil Sci Soc America Proc, 33, p743-745.
SALKIELD A P 1988 An evaluation of bottom sediment mapping methodologies: Task
1.6 of the Lake Okeechobee phosphorus dynamics study. Report submitted to the
University of Florida, Gainesville.
Additional
GLEASON P J, COHEN A 0, BROOKS H K, STONE P, GOODRICK R, SMITH W G
and SPACKMAN W 1984 Environmental significance of Holocene sediments from the Everglades and saline tidal plain. In: Patrick J Gleason (Ed) Environments of South
Florida, Present and Past II. Miami Geological Society, pl-67.
HOVELAND M and JUDD A G 1988 Sea bed pockmarks and seepages: Impact on Geology,
Biology, and the Marine Environment. Published by Graham and Trotman.
KAPLAN I R (Ed) 1974 Natural gases in marine sediments. Marine Science, Vol 3, Plenum
Press, NY. 324pp.
MOTHERSILL J S 1975 Lake Chad: Geochemistry and sedimentary aspects of a shallow
polymictic lake. J. Sed. Pet., vol 45, No 1, p295-309.
OTSUBO K and MURAOKA K 1987 Field studies on physical properties of sediment and
sediment resuspension in Lake Kasumigaura. Japanese Journal of Lirnmology, vol 48,
Special Issue pS131-138.
RAKOCZI L 1983 Resuspension studies in the near-shore zone of Lake Erken, p101-112.
REEBURGH W S 1972 Processes affecting gas distribution in estuarine sediments. Geol.
Soc. of America Memoir 133, p383-389.
SOMLYODY L 1983 Major features of the Lake Balaton Eutrophication problem: Approach
to the analysis pp9-44 In: Eutrophication of Shallow Lakes Modelling and Management:
The Lake Balaton Case Study. Hungarian Acad. Sci., 367pp.
TIPPETT R 1964 An investigation into the nature of the layering of deep-water sediments
in two eastern Ontario lakes. Can. J. Bot. 42, p1693-1709 (from West 1968 Pleistocene
Geology and Biology).




APPENDIX A
REPORT ON GEOPHYSICAL FIELD OPERATION A.1 Introduction
Field operations began aboard the University of Florida's R/V Silver Bullet on October 11, 1988 and continued on the 12th, 13th, and 18th. Primary equipment was a Datasonics SBP-5000 Sub-Bottom Profiling System which utilizes a Datasonics SBT-220 transceiver, the transducer set removed from a TTV-120 Transducer Vehicle and remounted in a specially fabricated catamaran surface tow vehicle, and an EPC-3202 Graphic Recorder. The transducer set consisted of four, ganged, tunable transducers for sub-bottom profiling and a single 200 kHz transducer for bottom tracking. The other major instrument systems were an EG&G SMS-960 side-scan sonar and an EG&G Model 290 side-scan sonar field access unit as well as an EPC-4800 Graphic Recorder. Both side-scan systems were operated with the same 105 kHz EG&G Model 272 tow fish. The SMS-960 provides slant-range corrected and speed adjusted, thus near planimetrically correct, records in real time. The Model 290 yields conventional, i.e. uncorrected, sonographs.
Navigation was by the ship's Micrologic 7500 LORAN-C. Locations were recorded manually in field logs each five minutes (except on Line 1, see Fig. A.1, where the interval was two minutes) and at selected other times. All logged location fixes are coincident with annotated marks on the graphic records. Table A.1 is a listing of the navigation data. The LORAN was programmed to display an internally calculated latitude and longitude. Empirically, these geographic coordinates disagree with chart data, albeit unsystematically, the error sometimes approaching 2 km. As no method with which to adjust or correct the LORAN derived coordinates could be developed, we have used the position data as recorded in the field.
During the course of the study, we collected data over nine separate lines, Line 9 being an extension of Line 5 (Table A.2). The length of the lines total 177 km (95.5 nautical miles).







Table A.1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C
Time Longitude Longitude Comment
1731 27 10.99 80 47.80 SOL 1 OCT 11, 88
1733 27 10.96 80 47.69
1735:30 27 10.96 80 47.58 1737 27 10.96 80 47.44
1739 27 10.95 80 47.29
1741:20 27 10.90 80 47.13 1743 27 10.89 80 47.00
1745 27 10.87 80 46.84
1747 27 10.88 80 46.71
1749 27 10.91 80 46.56
1751:10 27 10.85 80 46.36 1753 27 10.79 80 46.29
1755 27 10.80 80 46.09
1757 27 10.79 80 45.96 LONGITUDE 2.8 KM OFF
1759 27 10.67 80 45.86
1800 27 10.64 80 45.73 ABEAM TAYLOR CK LOCKS
1802 27 10.61 80 45.60
1804 27 10.54 80 45.45
1806 27 10.47 80 45.32
1808 27 10.48 80 45.15
1810 27 10.47 80 45.00
1812 27 10.46 80 44.85
1814 27 10.49 80 44.69
1816 27 10.46 80 44.56
1818 27 10.48 80 44.42 EOL 1 EOD OCT 11, 88
0913:30 27 07.86 80 49.89 SOL 2 OCT 12, 88
0915 27 07.94 80 49.79
0920 27 08.14 80 49.43
0925 27 08.03 80 49.02
0930 27 07.98 80 48.58
0935 27 07.92 80 48.18
0940 27 07.94 80 47.74
0945 27 07.99 80 47.34
0950 27 08.04 80 46.92
0955 27 08.13 80 46.53
1000 27 08.12 80 46.09
1005 27 08.11 80 45.68
1010 27 08.02 80 45.25
1015 27 08.00 80 44.83
1020 27 07.94 80 44.40




Table A.1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C (continued)

Time Longitude Longitude Comment

1025 1030 1035 1040 1042:30
1108 1110:30 1115 1120 1125 1130 1135 1140 1145 1150 1155 1200 1205 1210 1215 1220 1225
1240 1245 1250 1255:15 1300
1337 1340 1345 1350 1355
1400 1405 1410 1415 1420

EOL 2

07.85
07.96 08.04 08.08
08.15
04.56 04.47 04.45 04.48 04.51 04.53 04.63 04.55 04.53 04.44 04.48 04.47 04.45 04.43 04.40 04.39 04.41 04.49 04.48 04.46 04.46 04.52 04.46 04.41
59.94 59.97 59.98 00.03 59.98 59.99
00.02 00.02 59.99 59.97

43.97 43.57
43.16 42.72 42.54
40.34 40.53 40.89 41.30 41.75 42.15 42.66 43.00 43.43 43.84 44.28 44.69 45.10 45.53 45.97 46.39 46.83 47.34 47.63 48.10 48.53
48.96 49.38
49.75
56.83 56.57 56.11 55.66 55.21 54.77
54.33 53.89 53.45 53.00

SOL 3
NAV TOWER DATA TOWER
CHANGE SIDE-SCAN PAPER SIDE-SCAN ON LINE EOL 3
SOL 4




Table A. 1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C (continued)

Time
1425 1430 1435 1440 1445 1450 1455 1500 1505 1510 1515 1520 1525 1530 1535
1540 1545 1550 1555 1600 1605 1610 1615 1620 1625 1630 1635
1640 1645 1650 1655 1700 1705 1710 1715 1716:45

Longitude
26 59.98 27 00.02 27 00.05 26 59.95 27 00.07 27 00.07 27 00.01 27 00.02 27 00.00 26 59.96 27 00.04 27 00.03 27 00.01 27 00.01 27 00.00 26 59.97 26 59.98 26 59.98 26 59.96 26 59.98 27 00.01 27 00.02 27 00.01 27 00.04 27 00.05 26 59.94 26 59.98 27 00.07 27 00.01 27 00.04 27 00.01 27 00.07 27 00.10 27 00.07 27 00.06 27 00.00

Longitude
80 52.58 80 52.14 80 51.67 80 51.23 80 50.78 80 50.38 80 49.91 80 49.48 80 49.02 80 48.58 80 48.15 80 47.73 80 46.85 80 46.85 80 46.42 80 45.99 80 45.58 80 45.16 80 44.70 80 44.30 80 43.87 80 43.46 80 43.03 80 42.60 80 42.17 80 41.70 80 41.29 80 40.81 80 40.31 80 39.98 80 39.46 80 39.03 80 38.60 80 38.15 80 37.72 80 37.56

Comment
CHANGE EPC PAPER SMALL CHANNEL
SIDE-SCAN DOWN
EOL 4




Table A. 1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C (continued)
Time Longitude Longitude Comment

27

1036:02 26 1040 26
1045 26
1050 26
1055 26
1100 26
1105 26
1110 26
1115 26
1120 26
1125 26
1130 26

1754 1755 1800 1805 1810 1815 1820:30 1825 1830 1835
1840 1845 1850 1855 1900 1905:06 1910 1915 1920 1925 1930:30 1935
1940 1945 1950
1954

80

54.90 80 55.02 80 55.06 80 55.04 80 55.01 80 54.93 80 55.01 80 55.00 80 55.02 80 55.01 80 55.07 80 55.06 80

00.03 00.09
00.43 00.76
01.12 01.44 01.75 02.13
02.42 02.80 03.07 03.33 03.71
04.02 04.36 04.62 04.95 05.37 05.72 05.99
06.40 06.72 07.09
07.46 07.85 08.11

SOL 5

45.05 45.05 45.03 45.01 45.08 45.01 44.96 44.97 44.92 44.89 44.98 44.94 44.95 45.02 44.98 45.00 44.99 45.00 45.00 45.01 45.04 45.02 45.03 45.03 45.05 44.99
57.93 57.69 57.37 57.03 56.71 56.35 56.01 55.63 55.25 54.86
54.52 54.18

EOL 5 SOL 6

EOD OCT 12, 88

OCT 13, 88

SUNSET 1857




Table A.1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C (continued)

Comment

Time
1200 1136
1140 1145 1150 1155 1205
1210 1215
1220 1225 1230 1235
1240 1245 1250 1255 1300 1305 1310 1315 1320 1325 1330 1335
1340 1345 1350 1355
1400 1405 'f~410 1415 1420 1425 1430 1435 1440 1445 1450 1455

Longitude Longitude
26 54.87 80 52.12 26 54.96 80 53.74 26 54.98 80 53.47 26 54.96 80 53.11 26 55.01 80 52.78 26 54.95 80 52.45 26 54.93 80 51.79 26 54.97 80 51.50 26 55.04 80 51.19 26 55.06 80 50.85 26 55.05 80 50.52 26 55.04 80 50.15 26 54.99 80 49.79 26 54.99 80 49.39 26 55.02 80 49.04 26 55.08 80 48.67 26 55.02 80 48.27 26 55.00 80 47.90 26 54.98 80 47.63 26 54.97 80 47.13 26 55.01 80 46.77 26 54.97 80 46.37 26 54.94 80 45.98 26 54.90 80 45.61 26 55.00 80 45.25 26 54.99 80 44.86 26 54.95 80 44.47 26 54.97 80 44.11 26 54.94 80 43.68 26 55.04 80 43.35 26 55.02 80 42.96 26 55.07 80 42.62 26 54.95 80 42.25 26 54.93 80 41.85 26 55.02 80 41.49 26 55.06 80 41.11 26 54.96 80 40.73 26 55.04 80 40.36 26 55.00 80 39.96 26 55.00 80 39.58 26 55.05 80 39.24




Table A.1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C
(continued)
Time Longitude Longitude Comment
1500 26 54.99 80 38.83 1505 26 55.00 80 38.44 1510 26 55.09 80 38.08 1515 26 54.98 80 37.66
1520 26 54.96 80 37.26
1525 26 55.00 80 36.87
1530 26 55.05 80 36.72 EOL 6 MARKER 22
1602:45 26 51.02 80 38.24 SOL 7
1605 26 51.04 80 38.39 1610 26 51.02 80 38.86 1615 26 50.96 80 39.30 1620 26 51.03 80 39.75 1625 26 51.00 80 40.18 1630 26 50.99 80 40.65 1635 26 50.97 80 41.09 1640 26 51.02 80 41.53 1645 26 50.98 80 41.98 1650 26 50.96 80 42.42 1655 26 51.03 80 42.86 1700 26 51.02 80 43.34 1705 26 50.97 80 43.79 1710 26 51.03 80 44.25 1715 26 50.96 80 44.68 1720 26 51.00 80 45.16 1725 26 50.96 80 45.62 1730 26 51.00 80 46.08 1735 26 50.93 80 46.51 1740 26 51.01 80 46.98 1745 26 51.05 80 47.43 1750 26 51.03 80 47.89
1755 26 51.04 80 48.32 1800 26 51.02 80 48.76 1806 26 50.93 80 49.38 1811 26 50.96 80 49.77 1815 26 50.98 80 50.11 1820 26 50.99 80 50.58 1825 26 50.96 80 51.01




Table A.1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C (continued)
Time Longitude Longitude Comment

1830 1835 1840 1845 1846:37
0914 0920 0925 0930 0935 0940 0941:30 0945 0950 0955 1000 1005:30 1010 1015 1020 1025 1030 1035 1040 1045 1048 1050 1055 1100 1105 1110 1115 1120 1125 1130

51.04 51.04 51.04 51.03 51.00
47.98
48.03 48.11 47.97 47.97 47.95 47.93 48.00 48.00 48.03 47.97 47.99 48.05 47.86 47.99 48.03 48.00 48.15 48.14 47.97 47.98 48.07 48.36 48.78 49.17 49.58 49.89 50.30 50.67 51.07

EOL 7 SOL 8

EOD OCT 13, 88

OCT 18, 88

CHANGE STYLUS BELTS FINISH CHANGE

51.49 51.95 52.38 52.84 53.00
52.87 52.40 51.94 51.48 51.03 50.64 50.43 50.09 49.66 49.25 48.86 48.43 48.06 47.65 47.26 46.85 46.43 46.06 45.67 45.24 44.98 44.95 45.06 45.04 45.08 44.98 44.90 44.98 45.01 45.00

CONTINUOUS WITH 9

EOL 8 SOL 9 MISSED 1055




Table A.1

LATITUDE AND LONGITUDE AS DISPLAYED BY MICROLOGIC 7500 LORAN-C (continued)

Time
1135 1140 1145 1150 1155 1200 1205 1210 1215 1220 1225 1230 1235:30 1240 1245 1250 1255 1300 1306 1310 1315 1320 1325 1326:30

Comment

Longitude
26 51.48 26 51.89 26 52.29 26 52.64 26 53.15 26 53.47 26 53.87 26 54.28 26 54.69 26 55.10 26 55.42 26 55.87 26 56.30 26 56.64 26 57.03 26 57.36 26 57.74 26 58.15 26 58.60 26 58.94 26 59.35 26 59.70 27 00.05 27 00.19

Longitude
80 45.04 80 44.97 80 44.98 80 44.98 80 45.00 80 45.00 80 44.98 80 45.00 80 44.99 80 45.02 80 45.01 80 45.00 80 45.00 80 45.02 80 45.00 80 44.98 80 44.97 80 44.99 80 45.00 80 44.99 80 45.04 80 45.00 80 44.94 80 45.01

BEGIN SIDE-SCAN/EPC
CHANNEL
CHANNEL
EOL 9 EOD OCT 18, 88 END OF FIELD DEPLOYMENT




Table A.2: SUMMARY OF TRACK LINES Line Length Start and End Positions Times
No. (kin) Start End
1 5.6 27 10.99 80 47.80 27 10.48 80 44.42 1731-1818
2 12.0 27 07.86 80 49.89 27 08.15 80 42.54 0913-1042
3 15.7 27 04.56 80 40.34 27 04.41 80 49.75 1108-1300
4 31.5 26 59.94 80 56.83 27 00.00 80 37.56 1337-1716
5 14.8 27 00.03 80 45.05 27 08.11 80 44.99 1754-1954
6 37.0 26 54.90 80 57.93 26 55.05 80 36.72 1036-1530
7 25.0 26 51.02 80 38.24 26 51.00 80 53.00 1602-1846
8 13.0 26 47.98 80 52.87 26 47.98 80 44.98 0914-1043
9 22.2 126 48.07 80 44.95 127 00.19 80 45.01 11045-1326
The sub-bottom profiling system functioned at all times except for the standard, occasional few minutes to change paper and stylus belts. Due to a combination of equipment failure, shallow water, and rough conditions, the side-scan sonar was operated for only 67 kin, approximately 37 percent of the total.
A.2 Side-scan Sonography
The side-scan sonar study of the bottom of Lake Okeechobee used both the Model SMS960 (54 kin) and the Model 290 (13 kin). The side-scan systems were operated concurrently with the sub-bottom profiling system on Lines 2, 3, 4 (partial), and 9 (partial). The water depths throughout lake either are so shallow as to prohibit deployment of the tow fish or are so near the instrument's service limits as to engender less than optimum quality records. Although the SMS-960 failed near the end of Line 4, wave conditions on the following day would have precluded its use on Lines 6 and 7. The side-scan was operated again, this time using the Model 290, on the last half of Line 9; the system being deployed as the waves decreased.
The side-scan sonographs depicted a generally smooth bottom with few major perturbations. There are many "point reflectors" (pepper-flake-like depictions) perhaps indicative of very local variations in surface texture or hardness or of small (centimeters), hard high spots. If the point reflectors are the result of topography, the relief is not sufficient to cast




an acoustic shadow. They could also indicate the presence of gassy sediment, as elaborated in the text. There are a very few, isolated items protruding enough above the bottom to cast an acoustic shadow. These might be fragments of rock or anthropogenic debris. The dredged and spoil areas on Line 2 are shown very clearly on the sonographs in Fig. A.2.
A.3 Sub-bottom Profiles
The sub-bottom and bottom-tracking (200 kHz) systems functioned excellently throughout the exercise. Initial empirical experimentation demonstrated that 7 kHz provided the sharpest record; the other available frequencies being 3.5 and 5 kHz. The superior performance probably being a function of the short wave-length, approximately 21 cm. The system was operated with a trigger rate and a recorder sweep of 31.25 milliseconds, the most rapid available to the system, yielding a full scale record of 23 meters assuming an acoustic velocity of 1,500 m s-1. In practice, the data seldom exceeded 12.5 milliseconds two-way travel time (approximately 10 in). In optimum, smooth water conditions it should be possible to resolve individual, fairly widely separated layers on the order of 10 to 20 cm thick. Graphic resolution diminished with sea state. Lines 6 and 7 were run in short period, 0.6 m, white-capped seas; hence, the ability to resolve thin layers on these lines was significantly reduced.
In addition to the thickness of the mud layer, the sub-bottom profiles reveal the presence of at least two small basins in the sub-bottom underlying the general area of the mud deposit (Figs. A.3 and AA4) and the great variation in the hardness of the mud-deposit's surface. In some areas strong reflectors occur within the mud deposits perhaps indicative of a large quantity of shell on or very near the sediment surface, of partial lithification of that surface, e.g. Fig. A.5, or of gas. Signatures in Fig. A.6 are indicative of a relatively hard bottom.




West

I~ .... ....... -,-, ,-- -', "- :. -- . .... -" .
/ ;- _i- " t , "*}' '? '; ....... ......... .......&, '1,
. . ...: . . .. .; ; t : i -. -. ; -. .
j, '
!2 M '

Artifact of Shallow Water

Spoil Area

Interface Propeller/. .
Engine Noise

Dredged .
Channel "
1 k J, .W & J .1 . . !" ,..,,

Fig. A.2. Portion of side-scan record, line 2, October 12, 1988, west-east.

East




u
,-00 LAJ
INTERPRETED SECTION
6.25 ms -2
SEDIMENT SURFACE _E
SUB-BOTTOM REFLECTOR w
-70
PALEOCHANNEL
Fig. A.3. A portion of Line 9 demonstrating a shelly (?) mud layer approximately 60 cm thick over a harder substrate. The deeper sub-bottom reflector depicts a small paleochannel.




u'I 0
INTERPRETED SECTION
6.25ms
-3
SEDIMENT SURFACE
-_ _-MUD R--DL E ____" -SUB-BOTTOM REFLECTOR.-70.
a
"8
Fig. A.4. A portion of Line 4 demonstrating a relatively clean mud layer over a harder substrate. The sub-bottom reflector depicts a small paleochannel showing signs of some internal compaction.




, x\ .1
LINE 4
INTERPRETED SECTION 6. ms 1
2E
.3
SEDIMENT SURFACE--H4LLY
____.I-IFI I Y MIJO LAYER / .....-...... --

SUB-BOTTOM REFLECTOR

-6
-7
0 .8

Fig. A.5. A portion of Line 4 depicting a somewhat shelly (?) mud layer overlying a harder substrate. The
relatively shallow sub-bottom reflector dips toward the right.




M 2nd multiple
IF.
200 kHz BOTTOM
Fig. A.6. A portion of Line 6 depicting both the 7 KHz and 200 KHz bottoms. The roughness of the
bottom surface is due to surface water waves approximately 0.5 m high. The strength of the
multiples of the 7 KHz bottom suggests that the bottom is relatively hard.




APPENDIX B
REPORT ON CORING SURVEY
B.1 Field Operation
The coring survey was closely linked with the geophysical survey. The records obtained during the geophysical survey were interpreted with a view to choosing suitable core sampling sites within the lake to aid interpretation of the geophysics. Some 20 sites were initially chosen. A second suite of samples on the same grid as that previously occupied for a survey by UF's Soil Sience Department was also obtained (again approximately 20 sites of which a number were coincident with the geophysical survey lines).
For purposes of obtaining the undisturbed samples a large diameter, short barrelled vibracorer was designed and built. It was necessary to use a small vibracorer because of the intrinsic properties of the bed substrates of the lake. In many areas very soft and weak black clays overlie very hard or cemented calcareous, marine "beach-rock" type materials. To sample both substrates satisfactorily presented a logistic problem. Hand-held and driven corers were not adequate to the task because they would only penetrate soft mud, leading to doubt that the complete succession had been penetrated. Gravity corers were rejected because they would penetrate the mud but then fall over, leading to extreme disturbance of the upper, weak muddy material. Penetrating beach rock would also prove difficult for such devices. For these reasons the short barrelled vibracorer, which was hand-held from a boat moored over the sample site, proved ideal in sampling all substrates, while remaining in the vertical position and thus minimizing disturbance of the sample.
In addition to obtaining undisturbed samples, disturbed mud samples were also collected for erosion experiments in the annular flume at the University of Florida. B.2 Apparatus
The sampling platform used was the university's survey vessel "Silver Bullet." The vessel is 8 m long and has a top speed of 40 km/hr, making it efficient in steaming between survey




sites. Due to the fact that the vibracorer was small and hand-held, the vessel had to be moored fore and aft at each sample station to provide a stable coring platform. All position fixing was by the LORAN-C system.
The vibracorer was designed specifically to suite small vessel and shallow water operations. It comprised a standard concrete vibrator having a flexible drive (7.6 m) to the vibrator unit. The vibrator unit was bolted to an aluminum core barrel 8.9 cm o.d. and 7.6 cm i.d. The same assembly, mounted at the top of the core barrel was mated to an internal, thick, flexible, transparent core liner tube (cellulose-acetate-buterate, CAB). These contained each sample for return to the University of Florida.
At the top of the core barrel a nylon non-return valve expelled supernatant water during drilling operation and provided a component of the core retaining system. At the lower end of the barrel a steel cutting shoe provided a hardened edge to penetrate resistance bed deposits. This also acted as a core liner retainer and at the same time as a mounting for a petal-type core catcher. Finally, the vibrator assembly was equipped with a threaded collar to take the detachable aluminum pole used to guide and control coring operations by hand from the water surface.
In practice the corer was deployed when it was judged that the vessel had achieved a stable position at anchor. On reaching the lake bed slack was run off the lifting cable and the vibrator turned on. The vibrator was generally run for between 0.5 and 2.0 minutes and the vibracorer driven into the lake bed on all occasions to refusal.
On recovery the vessel heeled sharply to starboard and the resulting righting moment was used to break the corer free from the lake bed. On a number of occasions, the corer had penetrated material of a stiffness which necessitated judicious use of the vibrator to aid the withdrawal process. Several cases in which the core catcher had been turned inside out by the recovery forces indicated the importance of incorporating this device.
On recovery the cores were withdrawn in their liners, capped, cut to size and labelled. A Paar (model DMA 35) densimeter was used to make density profiles into the surface layers of the cores on board ship in circumstances where a poorly consolidated mud deposit was




present. Finally, the cores were sealed and retained in the vertical position for transport to the University of Florida.
For purposes of sampling the surficial mud layers for erosion tests a simple dredge (grab) samples was used.
B.3 Itinerary
The following is the travel summary for the coring survey: 10/19/88 Travel Gainesville Okeechobee. 10/20/88 Launch vessel at Okee-Tantie and rig vibracorer. Commerce drilling
operations. Steering failure of the vessel and loss of a cutting
shoe caused abandonment of operations. Three stations occupied. 10/21/88 Return to Gainesville.
10/24/88 Travel Gainesville Okeechobee; two stations occupied. 10/25/88 Nine stations occupied. 10/26/88 Eight stations occupied; 2-1/2 hours lost refueling boat. 10/27/88 Twelve stations occupied. 10/28/88 Five stations occupied; refuel boat and return to Gainesville.
B.4 Equipment Performance
In the 4-1/2 days of survey operation 39 stations were visited resulting in 30 core samples, and 4 disturbed sediment samples for erosion experiments. The lake presents a number of significant problems for coring operations including especially the combination of extremely weak clays and very hard, cemented "beach-rock" deposits. On occasion several attempts were necessary before the vessel was moored securely. Up to 3 attempts at coring were made at several stations where substrate conditions were particularly difficult..




At 5 stations it proved impossible to anchor the vessel, or the substrate proved to be sufficiently indurated that no core sample could be obtained. Particular difficulties in this respect were encountered at the southern end of the lake.
Other than the intrinsic problems of the area, which proved at time difficult and time consuming to overcome, the only other problem encountered as when grit in the threads of the cutting shoe led to a core being jammed in the barrel of the corer on 10/26/88. Forcefully unscrewing the cutting shoe led to the thread on the barrel being partly stripped. A fresh barrel and cutter were substituted.




APPENDIX C
ILLUSTRATIVE CORE SAMPLE DESCRIPTIONS C.1 Site: OK9 VC Core Sample Description (Fig. C.la)
Total 35 cm black firm mud with a sharp interface at the top made up of the following lithological units:
* 6 cm black soft mud, no shells, no smell
* Two layers 2 cm grey sand with fine shells and shell fragments separated by thin
clay layers
* 14 cm white and grey fine, small and large shells and shell fragments with a little
sand
* abc: thin layers of grey color with some very fine shells de: grey sandy layers with fine shells and shell fragments
* f: 2 cm grey sand with fine shells and shell fragments Bed Bulk Density and Shear Strength
Depth (cm) Density (g cm-3) Shear strength (Nm2)
0- 1 1.013
- 2 1.023
- 3 1.048
-4 1.035
- 9 1.138 8494
- 14 1.200 7853
- 19 1.183 5681
- 24 1.175 5833
- 29 1.128 5833
- 34 1.195 5578




6cm 6cm 6cm
9cm
5cm 10cm

]

6cm
2cm 2cm 14cm
9 cm

a
b
C 35cm
d
e f

Ta
18cm
b
C
d
36cm

T 8cm
a b
40 cm

4cm 2cm
2 c
1.5cm 3cm
T
I
10cm
18 cm

Icm
2 cm 1cm
T
5cm

1 7cm IjJcm I
2 cm i cm
Fig. C.1. Core descriptions: a) OK9 VC, b) OK10 VC, c) OK18 VC, d) OK31 VC.

1-F

e




C.2 Site: OK1O VC Core Sample Description (Fig. C.Ib)
* 6 cm soft black mud, no shells, no smell
* Visible interface at 6 cm
* 18 cm dark grey soft mud, no shells, no smell
a: thin layer (0.4 cm) of fine shells with light grey mud
b: thin light grey mud layer
* 36 cm black firm mud, a few small shells, no smell
c: thin light grey mud layer with a few fine shells
d: thin light grey mud layer
e: a dark grey thin layer of mud, small shells (Augur)
* 17 cm hard whitish clay with shells and shell fragments at the top, shells and large shell
fragment layer (4 cm) at the base
* Visible interface 4 cm above base Bed Bulk Density and Shear Strength
Depth (cm) Density (g cm3) -Shear strength(N )
0 5 1.136 1641
- 10 1.146 1158
- 15 1.216 1303
- 20 1.235 2537
- 25 1.170 2434
- 30 1.116 2586
- 35 1.269 3034
- 40 1.287 3668
- 45 1.100 3620
- 50 1.171 3572
- 55 1 1.115 14537




C.3 Site: 0K18 VC Core Sample Description (Fig. G.Ic)
* 4 cm black peat with a small amount of sand
* 2 cm light brown peat
a,b: 0.5 cm black peat
* 1.5 cm light brown peat
* 48 cm mottled firm peat with visible plant roots, no shell, no smell
40 cm mottled black and brown peat
8 cm mottled dark grey/light brown peat and sand C.4 Site: 0K31 VC Core Sample Description (Fig. C.ld)
* 1 to 3 cm dark brown coarse sand, no shell
* Fine shells and fine shell fragments with sand
* 2 cm grey medium sand, no shell
* 1 cm dark grey sand layer
* 5 cm light grey coarse sand with fine or medium shells and small shell fragments
* 5 cm brown medium sand, no shell
* 18 cm sand layer, no shell, varying color from black at the top to mottled black and light
grey at the base, varying sand grain size from fine at the top to coarse at the base




APPENDIX D
SEDIMENT SAMPLING IN SPRING 1988 A field visit was undertaken in March 1, 1988 to obtain sample bottom cores using a hand-held PVC piston corer designed at the Coastal Engineering Laboratory of the University of Florida, and to collect bottom sediment samples with a clamnshell grab sampler for sedimentary analysis. The ten sites visited are shown in Fig. D.1.
The cores were brought to the Coastal Engineering Laboratory where they were frozen in a mixture of dry ice (frozen GO2) and denatured alcohol, cut, and the bulk density of the cut pieces were measured. Fig. D.2 shows a frozen, cut core from site 1. The mud/water interface is fairly clearly defined by the observed color change. It should be pointed out that the freezing procedure leads to an uneven expansion of the core longitudinally. However, the procedure is fairly useful for examining the clarity of the low density region near the mud/water interface.
In Table D. 1 the total thickness of mud is given for different sites. The total mud thickness was found to be generally consistent with what was obtained by Gleason and Stone (1975) based on their mud thickness contours.
The fine-grained fractions of the collected mud samples were subjected to standard hydrometer test for size determination. A modification to the method was that the sediment was not dried initially, since initial drying prevents complete dispersion of the sediment. In order therefore to determine the total dry sediment mass required for calculation purposes, the sediment was dried after the hydrometer test (Hwang, 1989).
The last four columns in Table D.1 give particle size characteristics (sizes d25, d5o and d75, and sorting coefficient, S, = (din/d25) 1/2) for the fine-grained fractions at five sites. Sediment samples at these sites were first separated into coarse and fine-grained fractions by sieving through No. 200 Tyler sieve with an opening of 74 1A m. It was found that between about 75 and 90% of the material was fine-grained. This is generally consistent with the observation of Gleason and Stone (1975) who found 93% of the mud by weight to be less




Fig. D.1. Sediment/core sampling sites in Spring, 1988.
66




Fig. D.2. Frozen core from site 1.




than 74 1L m. They also found calcium carbonate to be present as a fine silt (2-5 it m), and
quartz grains to be slightly larger.
The last column in Table DA gives loss on ignition (percent by weight) of the sediment
at various sites. The percent loss is fairly uniform, ranging from 36 to 41. This, in general,
is indicative of the high fraction of organic matter present in the sediment.
Table DA: Bed and Sediment Characteristics
Site Total mud Fine particle characteristics Ignition
No. thickness d25 d5o d75 S. loss
(cm) (Am) (Am) CO M) M
I 30c 15 10 2 2.7 40
2 36 24 15 1 4.1 36
3 -a 13 7 0.6 4.5 43
4 -a 8 0.4 0.7 3.4 38
5 0 --a -a -a -a -a
5A' 0 10 ~ 3 0.6 4.2 41
6 0 --a -a -a -a -a
7 66 -a -a -a -a a
8 74 ---,a -a -a -a a
9 33 a -a ----,a -a I a
aNot measured.
b At this site the particle size analysis and ignition loss were obtained from a sample of
thin mud layer above hard bottom.
'Including soft marl.