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Stratigraphy and geologic history, Bunces Key, Pinellas County, Florida

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Title:
Stratigraphy and geologic history, Bunces Key, Pinellas County, Florida
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Book
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English
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Crowe, Douglas E
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University of South Florida
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Tampa, Fla.
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Subjects / Keywords:
Geology, Stratigraphic -- Holocene   ( lcsh )
Barrier islands -- Florida -- Pinellas County   ( lcsh )
Bunces Key (Fla.)   ( lcsh )
Bunces Key
Florida
lithostratigraphy
Pinellas County Florida
stratigraphy
Dissertations, Academic -- Geology -- Masters -- USF   ( lcsh )
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government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
ABSTRACT: Bunces Key, a narrow, linear, barrier island on the west-central coast of Florida, was formed in 1961. Its growth and development since that time is well documented by aerial photography. Cores taken from the Key and surrounding areas reveal a stratigraphic succession of facies reflecting rapid vertical aggradation. Sedimentation began on a gently sloping platform through the landward migration of large scale bedforms (sand waves) during fair weather periods. Migration of these bedforms ceased when emergence and lack of continued overwash precluded further movement.Vertical accretion to supratidal levels resulted from the continued onshore transport of sediment and subsequent welding to the previously formed bars. Stratigraphically, the barrier exhibits a "layer-cake" type of stratigraphy, with nearshore sediments overlain by foreshore, backbeach, and dune deposits. The backbarrier generally exhibits muddy lagoon sediments intercalated with washover and channel margin sediments.Fining upward washover sequences reflect the unstable nature of the island.Low pressure systems commonly cause overtopping of the barrier, with the subsequent formation of tidal inlets and washover fans. Aerial photographs document the formation of an initial barrier that was breached twice prior to 1973. A second barrier formed in late 1973 just seaward of the initial island and subsequently grew through littoral drift to a length of 1.8 km. A narrow inlet (30 m) formed through the northern end of the island in 1982.
Thesis:
Thesis (M.S.)--University of South Florida, 1983.
Bibliography:
Includes bibliographical references.
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by Douglas E. Crowe.
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Title from PDF of title page.
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Document formatted into pages; contains 113 pages.

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oclc - 10653880
notis - AJJ0744
usfldc doi - E14-SFE0000022
usfldc handle - e14.22
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STRATIGRAPHY AND GEOLOGIC HISTORY. 8UNCESKEY,PINElLASCOUNTY,FLORIDA by DouglasE. A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Department of Geology fn the Un1versityof South Florida 1983 Major Professor: RichardA.Dav1s.Jr.

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Graduate Council IJniversity of South Florida Tampa,Florida CERTIFICATE OF APPROVAL MASTER'S THESIS This is to certify that the Master's Thesis of Douglas E. Crowe with a major in Geology has been approved by the Examining Corrrnittee on October 25, 1983 as satisfactory for the thesis requirement for the Master of Science degree. Major Professor: RichardA.Davis,Jr. Member: MarkT.Stewart Menber: 11mB. Upchurch

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To my parents

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ACKNOWLEDGEMENTS I would like to thank Dr. Richard A. Davis, Jr. for serving as principal thesis advisor, suggesting the problem, providing invaluable insight,andcriticallyreviewingthemanuscript. Thanks also to Drs. Sam Upchurch and Mark Stewart for their time spent answering many questions. and reviewing the manuscript. Thanks to Dave Schrader, Heidi Knudsen,Al Stodghill,BobbyFierr 0, Paul Sterkenberg, Debbie Robinson, Skip Davis, and Dave Bagget tfor assistinginthealwayspleasurablevibracoringprocedure. Thanksto Heidi Knudsen for aiding in sample collecting and beach profiling. Special thanks to Sitka for making sure that no seabird or stray beach comber was left undisturbed. Funding was provided by Sigma Xi, the Scientific Research Society. Finally, special thanks to Heidi for providing support and encouragE ment, and enduring many frustrating moments.

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lISTQFTABlES. LIST OF FIGURES ABSTRACT. TABLE OF CONTENTS INTRODUCTION ......... Rationale and Objectives location and General Description General Geologic Setting ... Climate and Coastal Conditions PreviousWorks FtElOMETHODS Coring ..... Surface Samples . LAB METHODS .... Core Preparation and Analysis. Sample Preparation Page vii 11 11 13 15 15 15 SURFACE SEDIMENTARY ENVIRONMENTS. .... . 18 Dune..... ..... 18 Foreshore... 21 Backbeach... 21 Nearshore. ..... . 21 P1ungeStep.............. 21 lagoon............. . 23 Channel... . 23 __ M SUBSURFACE FACIES 26 Dune . ... . 26 Backbeach ..... 26 Foreshore . 28 Nearshore. . 28 lagoon .. 29 Washover 29 Channel Margin 30 iv

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Page 31 32 32 36 36 37 37 37 BARRIER BREACHING 42 Critical Factors .... .... 42 Occurrence of Breaches at Bunces Key 43 Morphologic Changes. . . . . 46 % STRATIGRAPHY . Traverse El-E3 Traverse Dl-D4 TraverseCl-C5 Traverse Bl-B4 Traverse Al-M TraverseA2-E2 BARRIER ISLAND FORMATION: EVIDENCEFORVERTICALAGGRADATION. Critical Factors Model for Barrier Island Genesis Stagel .. Stage2-3 . Stage 4 SUMMARYANDCONCLUSIONS REFERENCES ...... 50 61 64 65 66 APPENDICES..... .... .... 76 Appendix A -Flow Chart Showing Lab Methods for Sediment 77 Samples. AppendixB-CoreLogs . 78 AppendixC Faunal List.... 101

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LIST OF TABLES Table Page Textural parameters of surface environment samples. Textural parameters, sedimentary structures, and common 27 fauna of facies units.

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Figure location of Bundes Key. west-central Florida. Bathymetry and delta positions near study area. location of cores and cross-section lines. Beach profiles, Bunces Key. Ternary diagram. surface environment samples. Quartz-carbonate histograms showing minimal overlap of two fractions. Top: shelly sample. Bottom: non-shelly sample. Trench on Gulf side of Bunces Key. Note gently seaward dipping foreshore beds. Trench on lagoon side of Bunces Key. Notethick washoverdepositoverlyingcleanforeshoresands. Morphologic changes. Bunces Key. 1957-1983. Aerial photograph. Bunces Key area, 3-21-57. BP= BuncesPass. SC=SouthChannel. Page Aerial photograph. Bunces Key. 12-4-62. BP;; 35 Bunces Pass. SC '" South Channel. BK'" Bunces Key. Aerial photograph. Bl.Inces Key. 2-17-73. BP;; Bunces Pass. SC '" South Channel. BK '" Bunces Key. Aerial photograph, Bunces Key. 2-26-75. BP;; BuncesPass. SC"'SouthChannel. BK"'BuncesKey. 14 Aerial photograph. Bunces Key. 10-26-80. BP'" 40 Bunces Pass. SC '" South Channel. BK'" Bunces Key. Aerial photographs, Bunces Key. before and after 44 latest breach event. Top: Pre-breach, 3-81. Arrow points to future breach location. Bottom: Post-breach. 3-83, arrow points to existing breach. Stratigraphic cross section -, Bunces Key. Refer 49 to Figure 3 for core and cross section locations.

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Figure Stratigraphic cross section 01-04, BuncesKey. Refer to Figure 3 for core and cross section locations. Stratigraphic cross sectionCl-C5. Bunces Key. Refer to F1gure 3 for core and cross section locations. Stratigraphic cross section Bl-B4, Bunces Key. Refer to Figure 3 for core and cross section locations. Stratigraphic cross section Al-A4, BuncesKey. Refer to Figure 3 for core and cross section locations. Stratigraphic cross sectionA2-E2,Bunees Key. Refer toFigure3forcoreandcrossseetionlocations. 22 Graph plotting mean wave height (em) vs. mean tidal range (m), with line showing approximate limit of barrier island formation (after Davis and Hayes). Aerial photograph. Bunees Key, 1-9-76. BP: Bunees Pass. SC:o South Channel. BK'" Bunces Key. SZ=shoalingzone. Sample core log with key to core symbols. Page

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STRATIGRAPHY AND GEOLOGIC HISTORY. BUNCEsKEY,PINELLASCOUNTY,FLORIDA by DouglasE. An Abstract Ofa thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the DepartrnentofGeology in the University of South Florida December. 19B3 Major Professor: RichardA.Davis,Jr.

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ABSTRACT Bunces Key. a narrow. linear, barr1er island on the west-central coast of Florida. was fonned in 1961. Its growth and development since that time is well documented by aerial photography. Cores taken from the Key and surrounding areas reveal a stratigraphic succession of facies reflecting rapid vertical aggradation. Sedimentationbeganonagently s10ping platform through the landward migration of large scale b edforms (sand waves) during fair weather periods. Migration of these bedforms ceased when emergence and lack of continued overwash precluded fur ther IOOvement. Vertical accretion to supratidal levels resulted from the continued onshore transport of sediment and subsequent welding tot he previously formed bars. Stratigraphically, the barrier exhibits a type of stratigraphy, with nearshore sediments overlain by foreshore. backbeach. and dune deposits. Thebackbarriergenerally exhibits muddy lagoon sediments intercalated with washover and channel margin sediments. Fining upwardwashoversequences reflect the unstable nature ofth e island. Low pressure systems cOlJIII)nly cause overtopping of the barrier. with the subsequent fonnation of tidal inlets and washover fans. Aerial photographs document the fornation of an initial barrier that was breached twice prior to 1973. A second barr1er fonned in late 1973 just seaward of the initial island and subsequently grew through littoral

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drift toa length of 1.8 km. Anarrow;nlet (30m) formed through the northern end of the island in 1982. Abstract approved: xl R;chardA.Davis,Jr. Professor Geology Date of Approval

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INTRODUCTION RationaleandObiectives Vertical aggradation has been cited as a model for barrier-island However, later models for barrier-island genesis. such as cut-off spits (Gilbert.1885)anddrownedcoastalridges(McGee,1890),castdoubts as to the validity of vertical aggradation. Furthermore. wave-tank experiments by leontyev and Nikiforov (1965) failed to produce vertically-aggraded. barrier-type structures. Recent wave-tank data (Davis, pers. COI1ll1.) as well as strati graph ic evidence from the northern Gulf of Mexico (Otvos, 1970; 1981) have revived interest in the vertical aggradation theorY as a valid working model for barrier-island genesis. The fonnation and growth of Bunces Key, a small barrier-island on the west-central Florida coast, since 1960, suggests thatbarri erislandfonnation through vertical aggradation is a valid mechanismf or barrier-island genesis. This project was designed to examine factors critical to this process by investigating in detail the stratigraphy and sedimentology of Bunces Key and surrounding areas. Washoverfansandbarrierbreacheshavecharacterizedthedevelopment of Bunces Key since its fonnation. This study defines factors critical to the fonnatfon of washovers and breaches. and attempts to show where they will occur in the future, as well as predict future IOOrphologicchangesofthekey.

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Additionally, barrier-island sands represent excellent hydrocarbon traps because of their high porosity and permeability, and their close spatial relation to downdip mar'ine shales that are hydrocarbon so urces (Weidie, 1968). Therefore, studies such as this will add to the growing body of literature examining barrier-island systems and make clearer the understanding of analogs found in the rock record. Location and General Description Bunces Key;s located on the west-central coast of Florida in Pinellas County. It is 7.5 km north of Egmont Key and 0.7 km south west of SUll1lTer Resort Key in the Gulf of Mexico (Fig. 1). BuncesKey is 1.85 km long and its southern end extends 0.8 km in an east-west directionparalTeltoBuncesPass. to colonize the back barrier and the eastern side of the key, and several Casuarina1l!!!!.on the initial recurved spit now located in the lagoon half-way between the northern and southern ends of the island have attained heignts of 7-8 meters since 1961. Thenorthern portion of Bunces Key is covered solely by low scrub and grasses, such In early 1981 a breach cut the key into two sections and has remained open to date, leaving Bunces Key divided roughly in half. General GeoloqicSett1nq Bunces Key is located at the mouth of Bunces Pass on the north side of an ebb delta that is superimposed on a much larger ebb delta at the mouth of Egmont Channel {Fig. 2). Romans (1779) noted in

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Figure 1. location of Bunces west-central Florida.

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Fi9ure2. Bathymetry and delta positions near study area.

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1775 the existence of these sand shoals west of Mullet Key. Today, the Egmont Channel ebb delta extends roughly 15 km west into the Gulf of Mexico. The Bunces Pass ebb delta extends approximately 2 km west into the Gulf before its outline (in map view) is indistinguishable fro mthe form of the EgmontChannel ebb delta. BuncesKeyislocatednearthesouthernterminusofthechainof barrier islands that extends northward from the mouth of Tampa Bay to Anclote Key in northern Pinellas County, To the south of Bunces Key lie Mullet and Egmont Keys. A series of keys extends to the north, terminating with Anclote Key. which lies at the mouth of the Anclote River. Further north. in what Tanner (l960) described as the "zeroenergy coast". no barrier islands are present. Stratigraphically. the Holocene sands of the Bunces Key area are underlain by undifferentiated Plio-Pleistocene sediments which. in turn. are underlain by the Miocene Hawthorn Formation. Heath and Smith (l954) describe this unit as a clayey quartz sandstone to a sandy clay which is locally carbonate cemented. The unit dips gently to the south. Well log data from Fort DeSoto, 0.5 km south of Bunces Key. place the upper surface of the Hawthorn at -45 m and the top of the undifferentiated Plio-Pleistocenesedimentsat-9m(Applin.1907). This represents a significant difference in surface sediment thickness when comp aredto the barriers along the central and northern Pinellas County coast, where a thin veneer of sediment overlies bedrock. Somenorthernbarriers appear to be structurally controlled by the underlying bedrock surface . 1982; Davis and Kuhn. in press), while Bunces Key is not. The source of the sediments that comprise the sand sheets and barrier islands along this portion of the Florida coast is uncert ain.

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Clearly, the ultimate sources for the silica fraction were the Appalachian and Piedmont Provinces to the north. Erosion of shallow. quartz-bearing subsurface strata. such as the Tampa and Hawthorn Formations, combined with the remobilizing of various latercoasta 1 plain sediments seems to be the most likely explanation. Modern terrigenous input is too low to account for the sediments in the nearshore zone off west-central Florida (Davis 1982). Offshore, in approximately 6-9 m of water. the sediment sheet pinches out, precluding this area as the primary sediment source as well. Climate and Coastal Conditions Florida and the eastern Gulf of Mex;co lie;n a subtropical climatic belt that exhibits distinct seasonal changesinweathe r. During the spring and surmner months. atmospheric circulation patte rns are controlled by the Ber!l1Jda high, with prevailing wind directions from the southeast (Jordan, 1973). locally, thermal convection cells are established daily. resulting in severe thunderstorms during the 1 ate afternoonanrlearlyevening. These storms do not generate large waves, however, due to their limited extend and short duration. During the fall and winter. c1n::ulation patterns are controlled by an anticyclonic system generating winds from the northwest to north (Jordan. 1973). Locally. the west coast of Florida is subjected to frontal systems originating in canada that track across the Gulf of Mexico from west to east. These fronts produce winds from the southwest as they approach. and strong w1nds from the north as they pass. Depending upon dlJration and wind speed. these fronts can generate waves 9reaterthanlminheight(Rosen.1976).

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Wave energy along the Pinellas County coast is low. Rosen (1976) observed wave heights of 6-30 cm with periods of 2-4 sec. during calm weather and average wave heights of 50-60 cm with periods of 5 sec. during the passage of frontal systems. These low wave energy values are the result of a broad continental shelf and a limited fetch. Additionally, Bunces Key is located on the inner margin of the extremely shallow m) platform that extends seaward 1-2 km from Bunces Pass. This ebb-delta platform serves to greatly reduce wave energy through frictional losses. The tidal system is mixed with semi-diurnal cycles of unequal height during most of the lunarmonth,anddiurnal tides the remain der of the time (Dept. of Convnerce, NOAA. 1981). Bunces Key is located in a hydraulically active area. Boca Ciega Bay, located to the north. empties into the Gulf via South Channel. which runs directly into Bunces Key. Bunces Pass, which empties a portion of Tampa Bay, passes Bunces Key on its southern boundary. Although the back-barrier area of Bunces Key is properly termed a lagoon. it is not a tidally restricted area but rather a tidally active area exhibiting good cirtulation. The origin and genesis of barrier islands has been debated since DeBeaullDnt(1845)firstproposedamodelforbarrierformation. Presently, there are no less than a half dozen models currently in use. Zenkovitch (1969) and Schwartz (1973) offer good reviews of some of OeBeaUloont (1845) initially proposed that barriers fonned through theupwardgrowthofsubt1dalshoals. This model has since been

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supported by Johnson (1919) and Otvos (1970), who studied Atlantic and Gulf of Mexico barriers. respectively. Leontyev (1969) proposed a similar model, but felt that some sort of sea-level lowering was necessary to allow the subtidal shoals to emerge. He proposed both a eustatic sea-level change. such as theFlandriantransgression, aTlda post -stormtida11oweringaspossibilities. Fisher (1968), following the ideas of Gilbert (1885), claimed that barrier roorphology and dating of beach ridges along the Atlantic a TId Texas coasts indicated that barriers formed throughspitdevelopm ent and subsequent cut-off and segmentation. McGee (1890),Hoyt (l967),FieldandDuane (l976),andHalsey (1979) suggested an offshore barr1er formation during lower sea-l evel, fo1lowed by a marine transgression, and ultimate welding of the barrier to a landward topographic high. RampinoandSanders(198l) proposed an alternative to this landward migration during sea-level transgression. They propose that, if sea-level rise is rapid enough, barriers will "drown" in place, and the shoreline will jump landward. making the fonnerlagoonthenearshorezone. Schwartz (1971) offers the idea of "llUltiplecausalit,Y". suggesting that, in fact, most of the aforementioned theories (as well as others not mentioned) are valid for a particular set of circumstances. Washovers during stonns are the primary method whereby barriers migrate landward. Barrier tidal inlets are related to washovers 1n that both may fom due to overtopping of the barrier during storm surges. Kahn and Roherts (1982) offer a good overview of the pro cesses involved in inlet breaching. Pierce (1970) and Boyd and Penland (1981) detail what factors are critical in determining where a washover

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or breach will occur. (l980) suggest that previous areas of over wash and infil1ed tidal inlets are prime targets for future overwashing and breaching. A significant amount of literature has been written concerning the economic importance of facies as hydrocarbon reservoirs. Weidie (1968) offers a general view of why, stratigraphically, barrierislands are good potential oil traps. (1971), and Hobday and Horne (1977) all detail various aspects of the problems associated with identifying facies in the rock record and in subsurface exploration. Tanner (1960) offers a coastal classification scheme for the west coast of Florida based on wave energy. Inparticular,heidentifies the"zeroenergycoast"locatedonthenorthwestFloridacoastbetween St. Marks and Tarpon Springs. Oavis g1.e.i (1982) examined the barrier system of northern Pinellas County. The study reveals that these barrierislands appear to be structurally controlled by a subsurface high in the limestone bedrock surface beneath the barriers. TheHolocenemarinetransgression is also examined and documented with geophysical and vibracore methods. Sediment thickness in northern Pinellas County is 3-6 m and in many areasbedrockisexposed,suggestingthatthesedimentcoverisbut a tnin patchy veneer in this area. Brame (1976) and Kuhn (l983) investigatedCaladesi Island and Anclote Key, respectively. Both took an extensive number of cores in order to detail the stratigraphy and geologic history of these areas. Both document the eustatic. Holocene, sea-level rise, as evidenced by

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basal mangrove peats overlain by muddy lagoonal sediments and, then, clean open-marine and barrier sands. Rosen (T976) examined beach and nearshore sedimentation on Caladesi Island. Short term variation in morphology due to storms is compared to long term variation due to sea-level rise and normal marine energy conditions.

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FIELD METHODS Twenty-three vibracores were taken during the spring and fall of 1982 using a system similar to that described by lanesky (1979). Five east-west traverses, perpendicular to the long axis of the island, were cored {Fig. 3}. A vibermite vibracore machine was used to drive 7.63 cm (3 in.) aluminum irrigation tubing into the sediment. Extraction of the pipe was accomplished with a heavy-duty tripod and comealong. To prevent core loss due to slippage during extraction a plug was inserted in the top of the pipe. Penetration generally ceased between 2-3 m due to compaction of unconsolidated. very fine sand around and inside the tube. Compaction of the core sample was measured by subtracting the distance from the tube top to the sediment surface on the outside from that inside the tube. Upon extraction. excess pipe was cut off to prevent slumping inside the tube. and the ends sealed with duct tape for transport to the lab. Previous studies using thevibracoremethod (Kuhn, 1983; Knowle s. 1983; Evans, 1983} along the Florida Gulf coast document relatively insignificant (generally <10%) compaction. Cores taken on Bunces Key, however, generally compacted approximately 20% with SOIRe instances of 40-50% compaction occurring when coring through dune sediments. Possibly. rapid sediment accumulation resulted in very loosely packed grains such that when vibrated by the coring apparatus, they became organized 1n a more compact configuration. This compaction was

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Figure 3. Location of cores and cross section lines.

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sufficient to prevent further penetration of the dune and beach Core locations were surveyed with a sextant. Core elevations above mean sea-level were determined using the Emery method (1961) for beach profiles. while core elevations below mean sea-level were detennined using water depth and tide tables. Surface Samples Thirty surface samples were collected for analysis. sampling was deemed impractical as certain environments to be sampled were spatially too narrow to guarantee being sampled. Instead.4or 5 samples were collected from each environment at predetennined points within that particular environment. as selected from recent aerial photographs. Exact sample locations were detennined using sextant readings taken from reference points in the field. Care was taken to relOOve no more than the top 1-2 em of sediment to prevent mixing with potentially different underlying sediments. Transects were surveyed across the Key during the spring of 1983 in order to obtain a general knowledge of the cross-sectional topography of Bunees Key (Fig. 4). The Emery method (1961) was employed. using 1.5 m rods marked with 1 cm divisions.

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& J ;;: J!

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LAB METHODS Core Preparation and Analysis Cores were stored in a freezer prior to analysis in order to pre vent organic decomposition. dessication. and sediment disturbance. Twelve hours prior to logging and sampling. individual cores were allowed to defrost. Core tubes were cut on opposite sides with a circular saw equipped with an abrasive blade and an arc brace. The core was gently separated into halves with a sharp knife for lab analysis. Individual cores were logged using a standard logging form. Structure.approximategrainsize.color.flora/fauna,bioturbation. and unit thicknesses were noted. Samples were stored in plastic bags with fresh water sufficient to prevent dessication. Sample Preparation Core and surface samples were all run through the procedure outlined in Appendix A. Methods outlined in Folk (1968) were followed for sand and gravel fractions. Clay and silt were not differentiated. Percent gravel. sand,mud. graphic mean. inclusive graphic stand ard deviation and inclusive graphic skewness were calculated for all samples. All data were entered on computer cards and run through the SPSS subroutine SCATTERGRAM. which crossplots each variable against every other variable. The overwhelming preponderance of very fine quartz sand resulted incross-plots clustering too close together to discern

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different environments. Ternary plots of percent gravel. sand, and mud, however do allow lagoon. plunge step. channel, overwash and dune deposits to be categorized while allowing for some overlap (Fig. 5).

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EXPLANATION o Lagoon. Washoverlstorm x Dune Plunge Step Channel. Foreshore Nearshora /\ Figure 5. Ternary diagram. surface environment samples.

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SURFACE SEDIMENTARY ENVIRONMENTS Eight surface environments were identified and sampled. These include nearshore. plunge step. foreshore. dune, washover, backbeach. lagoon and channel margin. Ridge and runnel systems were not present during sample collection. All sediments are composed of various admixtures of skeletal carbonate shells, grains of very fine qua rtz sand. and a minor mud-sized carbonate/organic fraction. Overlap of the carbonate and quartz fractions occurs in the 14> to 34> interval {Fig.6}. Lackofacoarsergrainedsourcerestrictsthegrain-sizedistr;bution of the quartz fraction. Sediment textures and constituents are described below and surrmarizedinTablel. Different surface environments are distinguished by grain-size distribution, color, and biota. Dune sediments are white. very well sorted. slightly positively skewed fine sands. Disseminated and laminated layers of phosphate (fluorapatite?) and heavy minerals are sparse but ubiquitous. plants are found throughout. Whole shells are sparse and occur as pavements deposited during oventash. Clay-sized sediments are lacking .10%). The dune crests are generally 0.5 m higher than proximal backbeachse
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WT .. Figure 6. QUARTZ FRACTION r2LI CARBONATE FRACTION OVERLAP 2 PHI OIAMETER QQUARTZ FRACTION rza CARBONATE FRACTION OVERLAP 2 PHI DIAMeTER Quartz-carbonate histograms showing minimal overlap of two fractions. Top: shelly sample: Bottom: non-shelly sample.

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Tablel. Textural parameters of surface environment samples. SEDIMENTARY MEAN GRAIN SORTING SKEWNESS ENVIRONMENT SIZE (P) (q) (SkI) DUNE (n=4) BACKBEACH (n .. 4) FORESHORE (n"'S) PLUNGE STEP (n=S) NEARSHORE (n",4) LAGOON (u .. 4) WASHOVER (u .. 3)

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Foreshore sediments are light gray to white, well sorted. negatively-skewed fine sands from along the western margin of Bunces Key between the berm and thesubtidaT zone. Shellsaresparse overall but commonly are concentrated along the strand line and in small Tag deposits left during higher than normal tides. Mud is nearly absent .20%) and flora are very sparse. These sediments are parallel laminated and dip gently ,-2_3) seaward (Fig. 7). Backbeach Backbeachsedimentsarewhite,wellsorted.negativelyskewedfine sands. Shell pavements are common. Mud is nearly absent. Roots commonly penetrate from the overlying dunes into this unit. Nearshore located seaward of the foreshore surface environments, nearshore sediments are light gray, well-sorted, non-skewed fine sand. Molluscs terized by relatively high wave energy conditions that result in a neartotal absence of mud .20%) and benthicrnacroflora. Disarticulated shells are broken and abraded by wave action within this zone. This enVironment, which is a narrow. linear zone beneath the near-shore and foreshore zones, is usually grouped with the foreshore (Davis. 1978). It 15 markedly different texturally than both, so it receives separate treatment here. Characteristically, it is a poorly-sorted,

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Figure 7. Trench on Gulf side of Bunces Key. Note gently seaward dipping foreshore beds.

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negatively-skewed, gravelly. medium sand that contains up to 40% skeletal carbonate fragments with whole and broken shells thatar e coarser than As in the nearshore and foreshore environments, mud is nearly absent .25%) and the high energy conditions preclude the growth of benthic macroflora. 1!5IQQ.!l Lagoon sediments are composed of dark olive gray, well sorted. slightly positively skewed, pellet-bearing fine sand. Areally. these sediments are accumulating east of Bunces Key, except where ther eopened SouthChanne11snowlocated. Conmonfaunal elements include shells are not significantly abraded when compared to shell material from Gulf environments, thus allowing for differentiation between ..!.!l situ and transported faunal assemblages. Low wave and current energy conditions allow for some mud to accumulate. Mud content ranges from 0.8% to 9.4%,withanaverageof 1.6%. The mud fraction is comprised of 1) a carbonate silt fraction, 2) anorganic, silt-clay fraction (pellets), and 3) a minor clay mineral fraction. identified by X-ray diffraction as smectite. Flora are abundant. and consist of algal mats and seagrasses that occur in patches throughout the ba ck-Channel sediments are light gray, IlI)derately sorted. strongly coarse-skewed, slightly gravelly fine sand. Channel samples were taken from South Channel proximal to Bunces Key. Mud 1s negligible .20%), probably due to winnowing by currents. Mollusc assemblages

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are admixtures of abraded gulf and lagoonal species. (1978) document similar channel facies characterized by mixtures of abraded open marine fauna and nonabraded lagoonal fauna. Washover This environment isa fining-upward sequence that consists ofa coarse.poorlysortedbasalshellpavementgradingupwardintopr0-gressively finer and better sorted sands (Fig. 8). Washover fans are readily identifiable on the lagoonal side of Bunces Key as lobate bodies extending outward into the back-barrier, protected lag oonzone. Size data for the overall unit are meaningless because the unit is heterogeneous. Identification of this unit is based upon the presence or absence of abraded shell material and the fining upward texture. As the low energy conditions of the lagoon preclude shell abrasion. the presence of abraded shells. especially at the base ofa fining upward sequence, is indicative of washover from the Gulf side, where shell abrasion is conmon in the surf zone.

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Figure 8. Trench on lagoon side of Bunces Key. Note thick washover deposit overlying clean foreshore sediments.

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SUBSURFACE FACIES Seven subsurface facies have been defined on the basis of biota. texture and composition. and elevation relative to mean sea lev el. These facies are shown in core logs {Appendix B) and.instratigra phic cross sections (Figs. 16-21). Table 2 sunmarizes textural. structural. and faunal differences between the units. Dune The dune facies is identical to the dune surface unit. The sediment is white. well to very well sorted. coarse skewed. fine sand. Shell material is unconmon (average 2%) where present. is col11l1only abraded. Mud is rare (average 0.2%). Heavy mineral grains (fluorapatite?) are cOll111on both in laminae and as disseminated grains. grasses. is ubiquitous throughout this zone and cOllIlIOnlypenetra testa the backbeach unit below. Unfortunately. this unit receives significant input from humans in the form of disgarded cans, bottles, and other effluvia. The dunes are stabilized by the scrubby vegetation Florida coast, and are 1ndicativeofa supratidal environment. Backbeach This facies occurs just below the dune unH and ;s between 0.05 and 0.2 m in thickness. The sediment is white to gray

PAGE 39

1 11 all 11 I it it it 11 [i 6 150 1 i I ]c Ii h i :3 S 0 I b L 0 N Ii Ix h n o .s

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moderately well-sorted, strongly coarse-skewed. fine sand. Shell materialcomprisesanaverageof3%ofthetotalsample,whilemud averages 0.15%. Root material is sparsebutpersistant, as are heavy mineral grains. The unit is often characterized by a thin shelly pavement at the top of the unit. The presence of this pavement, plus the presence of roots and heavy minerals is indicative of deposition in a high intertidal to supratidal zone. Foreshore The foreshore facies occurs stratigraphically below the backbeach and extends down to the mean low tide level. The unit dips gently (20 _3) seaward and exhibits plane laminations (Fig. 7). The sediment is light gray to gray, moderately well sorted, coarse skewed, fine sand. Shell material averages between 6-7% and 1s characterized by a Gulf type of faunal assemblage, including Diplodonta punctata, Do nax of the total sample and is between 90-95% calcium carbonate, which is produced by shell abrasion and subsequent filtering into the porou s andpenneableforeshoresediments. Thelackoforganic-rich,lagoonal mud,thepresenceofGulforopenmar1netypeoffaunalassemblage,and the stratigraphic position directly below backbeach sediments indicate an intertidal depositional environment. Nearshore The nearshore unit incorporates a number of texturally different sediments. ranging from clean. well-sorted, fine sand to very shelly, poorly sorted, medium sand. The clean. well sorted sand reflects periods of normal wave ene.rgy. The shelly, poorly sorted, medium sand

PAGE 41

29 reflects high energy events, such as storms, that result in thick sh ell pavements, as seen in the upper O.5mofcore El (Appendix B). Typical nearshore sediments are gray, moderately sorted, strongly coarse-skewed. fine sands, containing 7-8% gravel-sized shell material and 0.35% mud. Muds from the nearshore zone average 70-95% calcium carbonate, reflecting a greater amount of infaunal activity than in the higher energy, fore-shore zone. Molluscs CORlllon to this unit are typical of an open marine The lagoon facies is similar to the lagoon surface unit but contains significantly more mUd. The sediment is dark olive gray to olive gray, moderately well-sorted. slightly positively skewed. pellet-bearing. fine sand. The gravel fraction accounts for an average of 2.5% of the total sample and is cOllIlIOnly composed of articulated Chionecancel1ata. Mud ranges from 0.8% to 12% and is composed of an average of 50% pelleted material and 50% calcium carbonate silt and clay formed by the breakdown of shell material. X-ray diffraction yields very weak patterns indicating a minimal amount of clay minerals identified as smectite. The lagoon facies is deposited inbackbarrier areas where wave and current energy is low as evidenced by significant (average 3.5%) mud accumulations. This facies occurs in juxtaposition with the protected lagoon and channel margin units. Each unit generally fines upward and has a sharp

PAGE 42

or scoured basal contact with underlying units. Thesedimentisa poorly sorted, strongly coarse-skewed, shelly, medium sand. Shell material comprises an average of 24% of the sample, most of which shows some abrastondue to agitation in the surf zone prior to trans -port to the lower energy backbarrier area. Brokenshellsarecolmlon in this and other units, but are not used as indicators of trans po rt, as various organisms will break shells during feeding. Mud comprises 0.45% of the sediment and represents a mixture of carbonate silt and pellets. The carbonate sflt forms due to shell abrasion on the open marine side of the barrier and is subsequently transported to the lagoon during washover. Pellets, which comprise the remainder of the mud fraction, are deposited in the lagoon by infaunal filter feeders. The pellets become suspended during washover events and mix with the carbonate silt from the Gulf. Channel Margin Sediments from the channel margin facies are light gray. well sorted. coarse-skewed. fine sands. Shell material averages 2.5% of the total sediment and mud accounts for 0.45%. Molluscs found within this facies include species characteristic of the open marine nearshore,theforeshore.andtheprotectedlagoonfacies. This facies exhibits the best degree of sorting among all facies except the dun e unit. It represents a depositional environment in very close proximity M) to an act1ve tidal channel. Identification of the channel margin facies in cores A4. 84. C5, and D4 (Appendix 8) was accomplished by examining aerial photographs and sorting values using Folks (1974) sorting equation.

PAGE 43

SEQUENCE OF EVENTS Examination of aerial photographs. local climatological data. tide data. and historical charts and records indicates that Bun cesKey has undergone numerous periods of aggradation and erosion since i ts initial formation in 1961. Periods of aggradation are characterized by an increase in the size of the island through the welding of ridge and runnel systems and spit progradation. Some landward growth may also occur due to overwashing of the barrier. During an aggradational period. major low pressure systems (barometric pressure < 996mb) are rare or do not occur Periodsoferosionarecharacterizedbyareductioninthesize of the key through barrier breachin9,washover. and wave 1nduced beach erosion. Breaching results in the formation ofa tidal inlet through the barrier andsubsequentmobllization and redeposition of sediment s formerly comprising the barrier. Was hover results in the landward migrationofthebarrierbyremovingforeshore,backbeach,anddune sediments and depositing them on the landward margin of the island. During these periods, major low pressure systems occur more frequently, suggesting that the frequency of major law pressure systems controls, to some extent. the rate of erosion and deposition. Tide data were examined for all periods during which a major low pressure system occurred to determine whether the effects of the storm were amplffied or lessened by the lunar and tide stage at the time of the storm.

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Historical data were examined for the period prior to 1961, when the present barrier formed, in order to determine if there was any indicationthatbarriershadformedatthislocationpreviollsly. Sevenperiodsaredefined;apre-1961phase,threepost-1961 aggradationalperiods,whichalternatewiththreeerosionalperiods. The tlme boundaries between periods are not precise but rather approxi-mationsbecausetheabso1utedatesarenotknown. Morphologicchanges aresunmarlzedinFig.9. Pre-Barrier Period (T779-l960) Romans (1779) cites evidence that the Bunces Pass ebb-delta existed in 1775 when he was charting the west coast of Florida. Aerial photographs from 1957 show a complex sand body located between Bunces Pass and South Channel (Fig. 10). The sand body is a combination of channel margin deposits located just north of Bunces Pass and an ebb delta at the mouth of South Channel. The ebb delta is dissected by several subtidal channels that have formed due to currents generated by Bunces Pass. Sand waves, predominantly oriented N-S, are superimposed upon most of the structure. There is no indication of any subtidal topography that would later control the fonnation of the During the period 1949-1960. eight major low pressure systems, culminated by Hurricane Donna on September9-1l,l969. passed over the area. Aggradational Period I (l 960-1 962) Aerial photographs from mid-1961 show a 0.5 km long linear barrier island (Fig. 11) situated just north of Bunces Pass and due south of

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Figure 9. Morphologic changes, Bunces Key,

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Figure 10. Aer1alphotograph,8unceSKey area, 3-21-57. 8P-SuncesPass. sc .. South Channel.

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Figure 11. Aerial photograph,Bunces Key, 12-4-62. BP=BuncesPass. SC= South Channel. BK= Bunces Key.

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South Channel. By late 1962 the key had become more arcuate in shape. with a prograding spit developed at its northern end. This spit pro gradation extended the island to 0.7 kminlength. Sparse vegetation had begun to stabilize both ends of the island. South Channel defined the northern limit of the island. During the initial aggradational period. only one major low pressure system moved through the area; it occurred during spring Erosional Period I 0963-1970) Aerial photographs taken between 1963 and 1970 are of poor quality and for this reason an accurate outline of the islandcountnotbed rawn for inclusion here. It is apparent. however. that during this phase, Bunces Key was breached at its midpoint. Climate data indicate that there were two possible events that could havecausecl the barrier to be breached. The first was a series of five major low-pressure systems associated with neap tide conditions that occurred from late 1962 through late 1964. Secondly, and more likely, was the passage of Hurricane Alma. which struck on June 9-10, 1966, coincident with spring tide conditions. During this period ten major low-pressure systems moved over the southern Pinellas County coast. Eight occurred during neap tide conditions and two during spring tide conditions. A9gradational Period II (1970-1972) Aerial photographs from early 1971 show Bunces Key as one con tinuous barrier, O.B kID long. South Channel remained unchanged, however the island has moved landward approximately 0.2 km since 1962 (aggrada-

PAGE 49

tional period I position}. During this period, no major low-pressure systems were recorded. Erosional Period II (late 1972-1ate 1973} Aerial photographs from early 1973 show Bunces Key as almost entirely subtidal save for two small islands located at the former northern and southern extent of the key (Fig. 12). Approximately 0.1-0.2 km seaward of the aggradational period II barrier was a linear, shore-parallel,subtidaltolowintertidalsandbar. Itextendedfrom north of South Channel south to a point between the two remnant islands of Bunces Key. South Channel was still open to the Gulf of Mexico. Weather data show one major low-pressure system affected the Pinellas County area in late 1972. Thissystemoccurredduringspring tide conditions and may have been responsible for breaching the b arrier. Aggradational Period III pate 1973-late 198]) Aerial photographs from early 1975 show Bunces Key as a continuous barrier 1.4 km long (Fig. 13). Remnants of aggradational period II lay just landward of the present barrier. By early 1975 South Channel no longer opened into the Gulf due to the rapid spit progradation of BuncesKey. From1975unti11ate1981,thebarriergrewnorthwardby spit progradation toalength ofl.8 km (Fig. 14). During this period, four major low-pressure systems were recorded, two corresponding to neap tides, and two with spring tides. These systems occurred at regular intervals throughout this period. Eros;onalPeriodIII(1ate1981-present) Aerial photographs from early 1982 show a narrow (20 m) breach located approximately 1.2 km north of Bunces Pass. At present, the

PAGE 50

Figure 12. Aerialphotograph,BuncesKey. 2-17-73. BP=BuncesPass. SC'" South Channel. BK'" Bunces Key.

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Figure 13. Aerial photograph. Bunces Key, 2-26-75. BP=BuncesPass. SC .. South Channel. BK=Buncesl
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Figure 14. Aerial photograph. Bunces Key. 10-26-80. BP=BuncesPass. SC= South Channel. BK= Bunces Key.

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breach has expanded to 0.3 km and a tidal inlet has been established connecting South Channel with the Gulf of Mexico. The exact date of the breach event remains uncertain. but most probably lies within a three month period from early Novemher. 1981 to late January. 1982. Weather data from this period show ten low-pressure systems during this period. eight of which coincided with neap tides and two with spring tides. A significant low-pressure system during June". 1981 caused extensive damage to Mullet Key and the surrounding area. but did not breach Bunces Key. No major lows passed the west coast of Florida during the per10dthatBunces Key is believed to have been hreached. Greenwood and Keay (1979) suggest that significant low pressure systems are not necessary to hreach a barrier. Rather. a threshold pOint is reached due to previous weather conditions that makes it possible tooverto pthe barrier during the passage of "average" low-pressure systems.

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BARRIER BREACHING Barrier breaches have characterized BuncesKeythroughout its history. Since 1961, the barrier has been breached on three separate occasions, with an unstable tidal inlet forming asa result each tim e. The first, opened in approximately 1966. remained open until mid-1970. The second. breached in roughly 1972. closed by late 1973. Currently, the barrier is divided into northern and southern segments by a tidal inlet due to a breach that formed in early 1982. Critical Factors Greenwood and Keay (1979), in a study of a breached barrier in the microtidal Kouchibouguac Bay in the Gulf of St. Lawrence, Canada, determined that tidal regime, wind and wave climate, and sediment availability were critical factors in detennining if barrier breaching would occur. as well as whether or not the resultant inlet formed would be stable. Pierce (1970) and Boyd and Penland (1981) further showed that the la900nand nearshore structure, and the height and width of the barrier are also critical factors indetennining if a breachwil 1 occur. These studies were done on both microtidal (Boyd and Penland, Louisiana Coast) andmesotidal (Pierce, U.S. Atlantic Coast) coastlines. Breach events generally occur during the passage of high-magnitude. low-frequency storms. Overtopping of the barrier occurs and eroding waters may flow either into the lagoon or into open waters on the seaward side. Water flows seaward when a low-pressure system piles water

PAGE 55

up in backbarrier areas, effectively increasing the tidal prism. As the system passes, this prism is driven seaward through existing inlets. If the flow is greater than the capacity of the inlets, overtoppin gof the barrier from the lagoon side may occur. Overtopping waters flow from the GuTfdue toa combination of 1) exceptional tides, 2) concentration of wave energy through re fraction, 3) extreme wind and barometric conditions (Greenwood and Keay. 197 9), as well as, 4) frontal wave attack (Pierce. 1970). Occurrence of Breaches at Bunces Key Aerialphotographcoverageofthefirsttwobreacheventsispoor, as is any first-hand knowledge. Photo coverage and first-hand knowledge of the 1981 breach are substantially better and therefore this event will be considered in some detail. Overtopping of a narrow structure. such as Bunces Key, usually resultsintheformationofatidalinlet,whereasovertoppingofa wide barrier usually results in washover fan formation (Pierce, 1970). Bunces Key is no more than 75 m wide and it narrows to 30 m in several areas. Examination of Fig. 15 shows Bunces Key just prior to and just after the 1981 breach. Two important factors are obvious. First, the initial breach point was one of the narrowest paints along the entire barrier. Second,thelocationofSouthChannelnearthenarrowpoint has some control over the breach in that it allowed tidal currents to be illll'lediately established, rather than opening into a flatter lagoonal area with subsequently reduced flow. The open nature of the backbarrierareaprecludesbreach1ngfromthelagoonsideasthet1dal prism could not be "trapped" behind the barrier and forced to overtop

PAGE 56

Figure 15. Aerial photographs, Bunces Key, before and after latest breach event. Top: 3l. Arrow points to future breach location. Bottom: Post-breach. 3-83. Arrow points to existing breach.

PAGE 57

the island to excape. Thus, the assumption w;l1 be made that the barrier was overtopped from the seaward side. Waves approaching the Bunces Key area from the south or southwest areattenuatedbytheshoal,which1s1.8mdeep.thatextends7km due west into the Gulf of Mexfco parallel to the northern margin of Egrront Channel (Fig. 2). Waves approaching from the west, northwest, or north, however, are not attenuated until they contact the Bunces Pass delta approximately 1 km seaward of the barrier. Most frontal systems will generate waves in the following manner as they pass over the west central Florida coast. As the front passes, the winds shift to the west, northwest, and finally north, and increase in velocity, generating waves that approach from the west and northwest. where no significant attenuation occurs. Early to mid-19Bl saw the passage of three low-pressure systems. Two (March 5 and May 7) approached from the west, and one (March 18) from the north-northwest. These closely spaced fronts may have pro duced a threshold state that permitted the key to be overtopped and breached a short time later during a low-intensity front. A probable sequence of events for the breaching of Bunces Key follows. A IOOderate. low-pressure event, coincident with a spring tide, generates waves approaching from the west-northwest as it passes. The island is overtopped at the narrowest and possibly lowest point by frontal wave attack. The surge carries into the lagoon. dune and back beach sediments and erodes/steepens the shoreface. Overwasheventually reduces the height of the barr1er below water level and connects the seaward side of the island with the prev10usly cut-off South Channel. Continued wave attack combined with the now re-established tidal inlet

PAGE 58

system, erodes a narrow tidal inlet to below mean low water. Following the passage of the front, the tidal scouring of the inlet has been sufficient to keep the inlet open through the present. Morphologic Changes Examination of aerial photographs from late 1980 (prebreach), early 1982. and early 1983, reveals a number of distinct changes in morphology. 1) An initial narrow (20 m) channel and intertidal zone connecting South Channel and the Gulf of Mexico progressively widened tothepresentO.3km. Occurring concurrentwHh the widening of the breach was the destruction of the nearshore bar system in the nearby vicinity. 2)Oevelopmentofa subsidarychanneloccurred south of the main (original) channel during mid-1982, creating a roughly triangular, high-subtidal spit platform between the two channels. This channel presently empties into a runnel that parallels the southern segment of Bunces Key. 3) Development of a small, intertidal ebb delta on the northern margin of the initial channel occurred during mid-to late-l982. FutureChanaes In light of historical data (twopreviousbreachesandsubseque nt closure). the abundant sediment supply of Bunces and Egmont ebb deltas, and the inherent unstable nature of breach inlets (Greenwood and Keay. 1979), the tidal inlet currently dividing Bunces Key can be expected to close within the near future years). Previous breaches, including the breach event of 1973 that almost entirely destroyed the barrier, haverespondedbyl)inletclosing,and2)sp1tprogradationresultin9 in overall barrier lengthening. This may indicate that although sediment is abundant. the availab1lity to the barrier system proper is

PAGE 59

sporadic, or at best cyclic. At present, although the breach is over 300 m wide, roost of the area is in the form of a broad spit platform which is barely subtidal; the channel is only 35 m wide. Sediment sufficient to close the breach. on the order of roughly 20.000 cubic meters, is readily available from nearshore sediment sources. Near shore bars generally form during the winter months and migrate shoreward to be welded onto the barrierduringsuFI111ermonths (Coastal Engineering Research Center, 1973). The introduction of sediment in this fashion may ultimately result in the closure of the present breach.

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STRATIGRAPHY Stratigraphic cross sections have been constructed from data derived from cores and aerial photographs. Six cross sections of Bunces Key are shown in Figs. 16 to 21. and crosS section locations are shown in Fig. 3. Approximate time lines are based primarily on a time series of photographs. Cross sectionEl-E3 (Fig. 16) along the south end of the barrier. displays the longitudinal facies relationships of a typical prograding spit. In this instance the spit is the southern portion of the island that is building eastward in response to the flood tidal current of Bunces Pass in combination with waves that impinge on that shore. Channel-margin facies deposits underlie the southern end of the barrier. Sediments are very well-sorted. fine sand, deposited in a relatively high energy environment. Somewhat unexpected was a muddy saAd lense which occurs at -2.5 m 1n core E1. Dating of this sediment by l37cs methods indicates a post-1954 age. Although this is a high energy area. shielding may have been provided by a series of sand waves located along the margin of the channel, permitting an areallyrestricted, muddy sand to be deposited. Foreshore sediments overlie the channel margin sediments in cores E2andE3. The foreshore deposits toward the seaward end of the cross sectionweredepositedthroughtheweldingofsubtfdalbarstointer-

PAGE 62

tidal levels following erosional period II. Foreshore deposits toward the eastern end of the island are younger and reflect deposition through spit progradation eastward along Bunces Pass. Theseaward,forshore sediments were deposited rapidly, as ev;denced by aerial photo graphs taken during aggradational period III, while the foreshore deposits to the east accumulated more slowly. Core E2 displays a thin (20 cm), shelly, backbeach deposit 20 cm above present high tide. As the barrier continued to accrete vertically, vegetation began to stabilize the supratidal areas. The entrapment of sediment by this vegetation established the dunes and furthersta bilized the barrier. Cross section 01-04 (Fig. 17) shows the ideal stratigraphic succession of facies fora vertically accreted barrier island. Nearshore sediments underlie the barrier as well as the lagoon landward of the barrier. CoresD2andD4bothpenetratetounderlyingnearshoresediments, as evidences by the low percent mud. light gray color. and never completely penetrated during the study as the contact with under lying Plio-Pleistocene material lies at approximately -9 m (Applfn, 1907). As in all other cross sections except El-E3. the nearshore facies exhibits 1) a gently seaward dipping surface from the barrier westward and 2) a more steeply landward dipping surface from the present day barrier position eastward. underlying sediments of tne lagoon facies. ihis may indicate that the area underlying the barrier has been a shoaling area allowing for deposition of the muddy sediments landward of the present day barrier position.

PAGE 63

] ,g !iI! j a: i

PAGE 64

Overlying the nearshore sediments in thebackbarrierzone is the slightly muddy (_2%) lagoon facies. These sediments were deposited landward of the subtidal shoaling sandbodies prior to the develo pment of the supratidal present day barrier. Interfingeredwith the lagoon facies deposits are two washover unit. The upper deposit (at -1.5 m) probably represents one of the recent breach events (erosional period I and/or II). This unit extends approximately 300 m into the backbarrier zone. The lowerwashover (-2 to-3m} overlies lagoon facies sediments whose age is uncertain. The facies relationships of the intertidal to supratidal porti on of the barrier are identical in all traverses. The foreshore overlies the nearshore and is capped by the backbeach and dune sediments. The foreshore sediments were deposited through the welding of subtida 1 bars to high intertidal levels. The shelly. backbeach pavement is located 20 cm above present mean high tide level and is capped by almost 1 m of dune sediment. TraverseCl-C5 Cross section Cl-C5 (Fig. 18) transects the middle barrier and the northern end of the initial (aggradational period I) barrier located landward of the present barrier. The northern end of the initial barrier formed through spit progradation of an originally smaller key. while the present day barrier developed by welding of subtidal bars to intertidal and supratidal elevat1ons. The foreshore sediments underlying the barrier exhibit a configurat1onidenticaltotheothercrosssections. From the barrier seaward. the unit dips gently to the west. From the aggradational

PAGE 65

53

PAGE 66

period I barrier landward. the unit dips more steeply to the east. CoreC5(AppendixB),whichpenetratedto-3.75m,didnotpenetrate to the nearshore facies. In the baekbarrierzone. lagoon facies sediments overlie nearshore deposits. These sediments are muddy, fine sands containing a lagoon-type faunal assemblage. including Noetia ponderosa a nd Braehidontes exustus. indicating a quiet. low-energy depositional environment. A l37Cs "date",takenatthetopofthisunit, indieates a pre-l954 age, implying the presence of protective subtidal sh oals prior to this data. Overlying the lagoon facies in the backbarrier is a channel margin unit with awashover unit contained within. Theehannelmargin sediments are cleaner than the under Tying lagoon facies sediments and are slightly better sorted. These two facies lap onto the supratidal barrier. Between the two supratidal barriers (aggradational I and III period barriers) lies a small, laO m wide. shallow. mud flat. which was penetrated by core C3. This unit is 50 em thick and represents deposition of sediments since roughly 1975. Sedimentation rates since 1975 have been roughly 7.5 em/yr. The sediment is highly bioturbated, Intertidal and supratidal stratigraphy of the barriers is identical to other cross sections. The aggradational period I barrier has a dune system overlying backbeach sediments. The and.!.EmJJ9.!. The aggradational period III barrier has not accreted

PAGE 67

vertically to the elevation of the aggradational period 1 barrier as it is substantially younger and less vegetated. Traverse81-B4 Stratigraphically, traverse 81-B4 (Fig. 19) strongly resembles traverseCI-C4(Fig. IB),exceptthatonlyonesupratfdal barrie ris present in traverse 81-84. The supratidal barrier here has apparently accreted vertically through a combination of spit progradation and subtidal bar welding. Aerial photographs from197S show subtidal bars migrating shoreward, toward the island, presumably to be eventually welded to the beach. Well developed beach ridges at the northern end of the island, indicate spit progradation through littoral drift transport. Nearshore sediments resemble those seen in traverses 01-04 and Cl-C5(Figs.l7andlS}. In thebackbarrierarea, a thick (>2.Sm) channel-margin deposit over11es the nearshore sediments. The channel sediments thin to the west and pinch out against foreshore sediments of the barriers. Numerous flasers are present throughout the upper two-thirds of core 84, possibly suggesting deposition in close proximitytoSouthChannel. The intertidal and supratidal stratigraphy is identical to previously discussed cross sections 01-04 and Cl-CS. Traverse Al-A4 This cross section (Fig. 20) across the northern end of the barrier displays stratigraphy typical of a prograding spit. The positions oftfle time lines reflect the series of welded offshore bars that formthedowndrift end of the key.

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N -i M .;: J i M s j =: ill i -\ =.. \ \ t

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As in all the cross sections. the northern traverse is underlain by nearshore sediments. Thenearshoresedimentsaresubtidal.representingdepositioninashallow,shoaling-upwardenvironmentcharacterized by relatively high wave energies. Contained within the near shore zone sediments are severaT coarse shell deposits, represent ing deposition on the shallow Bunces Pass ebb delta during high energy events. These deposits are texturally very similar to washover 58 deposits found in the backbarrier areas, but do not contain mixed mollusc assemblages from the Gulf and lagoon. Overlying the nearshore sediments is a 2.5 m thick channel margin unit identical to that seen in core B4 on traverse BlS4 (Fig. 19). AO.5mthickwashoveris located in the channel margin sequence an d pinches out to the east. Supratidal stratigraphy along section Al-A4 is identical to supratidal stratigraphy in all other sections. TraverseA2-E2 This longitudinal cross section (Fig. 2l) displays the "layercake" stratigraphy that seems to characterize vertically-accreted barriers. Nearshore and foreshore sediments basically represent the early accretionary stages of barrier formation, while the backbeach and dune sediments characterize later stages of development and stabilizat ion. Channel margin sediments penetrated by core E2 reflect deposition proximal to Bunces Pass. As is reflected in Fig. 21, Bunces Key is currently divided into northern and southern portions by a deep (5 m) tidal channel that appears

PAGE 72

to have fonned during the passage of a late 1981 frontal system that caused severe flooding on nearby Mullet Key.

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BARRIER ISLAND FORMATION: EVIDENCE FOR VERTICAL AGGRADATION Critical Factors There are six factors critical to the formation of barrier islands through vertical aggradation. Sea-floor slope, nearshore topography. sediment availability. wave/tide climate. and littoral drift exert direct control on harrier formation. Climate directly influences waves, littoral drift, and tides. thus indirectly controlling barrier formation. Sea-level variation will not be considered, because the process of vertical aggradation in this example represents a response on a time scale much shorter than any glacioeustatic rises. Sea-level variations certainly are capable of drowning or migrating barriers. but have no appreciable control over formational processes as discussed here Sea-floor slope controls barrier formation in that a barrier can form only when the slope gradient is lower than the equilibrium gradient (Johnson, 1919). Essentially. a sea floor slope with a 9radient lower than the equilibrium profile indicates that wave energies and currents are insufficient to move sediments offshore. thus creating a system with excess sediment to construct coastal landforms. Kearshoretopographyismostcriticalpriortoanddurin9the initial formation of the barrier. Wave bore currents are the primary sediment transport mechanisms responsible for vertical aggradation. These currents are established when waves encounter shoaling waters, steepen. then break. entraining sediment and moving it landward.

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Therefore. in the nearshore zone, proximal to an aggrading barr; er, the topography must be such that a breaker zone exists, creat;ngt hese essential currents. Sediment availability is a factor that, intuitively. ;s critical to the formation of barrier islands, as well as many other coastal landforms. Sediment-starved areas are not as likely to exhibit constructional landforms as areas with an abundant sediment supply. Sediment availability does not reflect the amount of terrigenous input but rather the amount of sediment within the coastal area that is available for entrainment and subsequent deposition. The energy produced by waves and tides is an important factor in determining whether barrier can form or not. Davis and Hayes (in press) plot mean wave height (i.e. wave energy) versus mean tidal range and define a field representing energy conditions that permit the fonnati on of barrier islands (Fig. 22). If the combined wave and tide energies plotabovethelimit-of-barrier-formationline,barrierswillnotform. Low tidal range m) and wave height em), however, may result in exceptions to this because the delicate balance between tide and wave elimatesmakesthesystemverysusceptibletominorexternalfluctuations, sueh as storms and sediment flux (Davis and Hayes, in press). littoral drift is a critical factor during the initial fonnational period of barrier islands and. more importantly, is a key factor contributing to island elongation by spit progradation through time. During the initial formation ofa barrfer. Ifttoral driftinfluen ces sediment transport directions such that sediment entrained by bore currents will not be transported directly shoreward but rat her will have a certain shore-parallel component. During later growth stages.

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5 o 100 200 MEAN WAVE HEIG HT (em) Figure 22. Graph plotting mean wave height (em) vs. mean tidal range (m).withlineshow1ngapprox1matelirnitofbal"rierisland formation (after Dav1s and Hayes).

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littoral drift will result in the downdrift elongation of the barrier through spit progradation. Climate indirectly controls barrier formation by controlling waves, longshore drift, and influencing tides. Large frontal systems have been used to explain barrier island formation through vertic al aggradation (Leontyev, 1969). During passage of these strong fronts storm surges occur and nearshore bars respond by building up toa new, elevated wave base. When the storm subsides, these bars are left as barrier above normal sea-level. No evidence from this study, however, indicates that this mechanism was responsible for the formation of Bunces Key. Davis (1978) points out that storm surge periods tend to be characterized by foreshore and upper nearshore erosio n,and not deposition as suggested above. Model for Barrier Island Genesis Otvos (l98l) proposes the followingroodel for barrier-island genesis: 1. Formationofanearshore,subtidal,shoalarea. 2. Subtidalbarbuilduptointertidal1evels. 3. Accretiontohighintertidal1evels. 4. Ridge integration and island stabilization. Stage 1 This initial phase involves "laying the foundation" for future barriers. This can be accomplished in a number of ways, including littoral drift aggradation and the formation of ebb deltas. These ;hoaling areas can vary 1n size from a Chandeleur or Isles Dernieres lrc{large)toaBuncesDelta(small).

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The foundation of Bunces Key i-s the ebb-oriented Bunces delta (Fig. 2). Critical factors most important during stage 1 in the Bunces Key area appear to have been sediment. availability, littoral drift, and a tide-dominated coast. Although the west coast of Florida is considered to be sediment starved (Oavisetal,l982),theEgmont-Bunces tid aldeltacomplexrepresentsalocalsedimentsink,wheresedimentisavailable for the construction of barriers. Sediment is transported by longshore currents from a point in the vicinity of Indian Rocks Beach (located 30 km north of Bunces Key on Sand Key) southward, down the coasttotheBunces-Egmontarea(Tedrick,l972). Upon reaching Egmont Channel, it appears: that most sediment being carried southward by littoral currents: is stored in the deltaic complex. This appears to be the result of the nature of the coast in this area, as evidenced by the ebb-delta morphology characteristic of such a system (Oertel, 1975). Stages 2&3 These stages "fnvolve developing an upper nearshore breaker zone upon the stage 1 shoal. Wave-bore currents established by breakers transport sediment landward to form shoaling barrier bars. Stage 2 is characterized by the initial development of barrier bars andthei r growth to intertidal levels. Stage 3 is characterized by the welding of these bars and continued vertical aggradation to highest int ertidal levels. Davis (pers. COIl1l'l.) andOtvos (l98l) envision this process as occurring during fair weather periods, avoiding the necessity for a higher sea-level stand of either short (stonn) or long (eustatic ) term, as suggested by Leontyev (1969).

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DUring these stages, sea-floor slope and nearshore topography appear to have been the critical factors of greatest importance i nthe Bunces Key area. Slope measurements of the shoreface perpendicular to Bunces Key westward across Bunces Key indicate an average slope of 0.65 m/km. Slope measurements across the westF10rida shelf to the 50 fathom (90 m) depth contour indicate an average shelf gradient of 0.96 m/km. Clearly, the nearshore slope gradient in the Bunces Key area is substantially lower than the gradient for the west Florida she lf, which in itself is relatively low when compared to other continent al shelves. This very low shoreface gradient, coupled with the abundant sediment supply of the Bunces-Egmont deltaic complex, has allowed for the periodic development of breaker zones in the nearshore zone. Wave bore currents generated by these breaker zones move sediment lan dward and develop barrier bars (Fig. 23)thatthroughtimeweldtogeth erand accrete to highest intertidal levels. Stage 4 Stage 4 is characterized by continued shoreward transportofse diment. elongation through littoral drift,bennwidening, andeventu a1 dune formation and stabilization. Ouring this stage, barrier bars generated during stage 3 widen due to continued deposition on the seaward side by onshore transport of sediment and/or landward migration throughoverwash. linkage of nearby barrier bars and the continued downdriftelongation due to littoral drift result in the fonnation of large,stablebarrierislands. Bunces Key. during stage 4. is the result of the sum of all the previously mentioned critical factors. Figure 23 is a good example of these factors. Nearshore topography is such that a breaker zone is

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figure 23. Aerial photograph. Bunces Key. 1-9-76. BP"'SuncesPass. SC= South Channel. BK=BuncesKey. SZ=shoalingzone.

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evident offshore of Bunces Key. Sediment is being transported to the key by wave-bore currents, as evidenced by the presence of a ridge and runnel system in the process of welding to the barrier. Longshore currents are forming recurved spits to the north. thus elongating the island. Overal1,theseprocessesservetostabilizeandenlargethe key. At this pOint in time. vegetation has colonized approximately 50% of the island and will serve to trap sediment, further stabilizing the key.

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SUMMARY AND CDNClUSIDNS Bunces Key was chosen for;nvestigation because it is anexcellen t exampleofa harrier island formed by vertical aggradation. A general description of the stratigraphy and origin of the barrier has hee n documented. Breaches formed by barr;erovertopping and sUDsequent tidal inlet formation are documented as well. Stratigraphically, Bunces Key exhibits a "layer-cake" stratigraphy characteristic of vertically-aggraded barriers. Typically, the vertical sequence of facies is, from bottom to toP. nearshore, foreshore. back-beach,anddune,thusdisplayingaWalther-typestratigraphicsuccession of facies units. In the lagoon east of Bunces Key. washover facies are intercalated with protected and channel margin units. reflecting episodic storm deposition of sediment. A four stage model for the formation of barrier islands. following theideasofOtvos (1970i1981). is proposed. Stage 1 involves the formation of nearshore, shallow shoals upon which barriers can form ;in this study the ebb tidal Buncesdelta is such a zone. Stages2and3 involvetransportingsedimentlandwardbywave-borecurrentsduring fair weather periods and fanning nearshore bars that, :through time, accrete and coalesce to high intertidal levels. Stage 4 is the pen ultimate formational stage whereby the island is stabilized by accretion to supratidal levels and where it cOlllllOnly elongates and migrates. as

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Factors critical to one or more of the above stages include nearshore slope. nearshore topography. sediment availability, wave/tide climate,littoraldrift.andclimate. Sea-level variation is not critical as the process of vertical aggradation is considered to be almost instantaneouswhencomparedtoglacio-eustaticsea-levelchanges. Barrier breaching and washover is COll1llon and controlled by facto rs such as barrier width and height, nearshore and lagoon topography. tidal regime, wind and wave climate. and sediment availability. Breach events are important sediment contributors to the lagoon. carrying sediment from the nearshore and barrier proper landward to fonn fining-upward stann deposits. Washoverscontribute significant amounts of sediment to the backbarrier. widening the barrier and migrating it landward. Future trends for the area can be predicted with some degree of certainty. Growth of the barrier will continue northward due to spit progradation and it will eventually link with the small arcuate sand body 0.2 km to the north. Sediment supply is adequate. as evidenced by the continuous growth and migration of subtidal nearshore bars due west of the island. To the south. Bunces Pass represents the limiting boundary of southern growth of the island. Tidal currents produced by Bunces Pass and waves will continue to elongate the southern end of the island to the east. Historically, breaches that have fanned through the island have eventuallY been closed within the span of 2 to 7 years. Sediment needed to close the breach is readily available in the proximal nearshore. and wave-bore currents needed to bring the sedfment landward are operative in this area. However. South Channel. which was cut off from the Gulf by the northward growth of Bunces Key during constructional phase III. has

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beenre-connectedbythelatestbreachviaanarrowtidalchanne1 that may prevent the healing of the breach. Furthermore, as most tidal inlets of this nature are inherently unstable (Greenwood and Keay, 1979), migration north or south is highly probable. If Bunces Key is not ultimately destroyed by storms, it can be expected to migrate landward bywashoveras sealevel rises, pro ducing a stratigraphic sequence similar to Honeymoon and Caladesi Islands to the north. These barriers exhibit lagoonal facies overlain by barrier deposits characteristic of a marine transgressive sequence. Finally,itisimportanttostressthattheevidenceandmodetfor barrierisTandgenesisthroughverticalaggradationpresentedhere is Rather, as Schwartz (1972)proposed,itisbutoneofanumberofapparentlyvalidmodels, but one that has recently come under fire as invalid. The intention of this study has been to show that the model is indeed valid and worth considerationalongwithothermethodsofbarrierislandfonnation.

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REFERENCES Applin, E. R., 1907, "Well log report to the U.S.G.S."; U.S.G.S., 5pg. Beaumont. l. E. de. 1845. leconsdegeologiepractique, Paris, F rance, p.221-252. Boyd,R . andPenlancl.S . 1981.Washoverofdeltaicbarriersonthe louisiana coast; Trans. Gulf Coast Assn. Geol. Soc., 31: 243-248. Brame, J. W . 1976, The stratigraphy and geologic history of Caladesi Island,PinellasCounty,Florida;unpb1.M.S.thesis,Univ.South Florida.l09pg. CampbelT,C.B.,1971,Oepositionalmodel-UpperCretaceousGallup beach shoreline, Ship Rock Area, New Mexico; Jour. Sed. Pet., 41:2:395-409. Coastal Engineering Research Center. 1973,ShoreProtectionM anual,v.l, Washington. C.O . U. S. Army Corps of Engineers. Davis, R. A., Jr., 1978, Coastal Sedimentary Fnvironments SpringerVerlag. New York. 420 pg. Davis. R. A., Jr., Hine,A. C., IIr,andBelknap.D. F., 1982. Coast al zone atlas: northern Pinellas County, Florida; Fla. Sea Grant Project RjOE-l7. 37pg. Davis, R. A . Jr., and Kuhn. 8 . in press, Origin and development of Anc10te Key, West-Peninsular Florida; Marine Geology. Dept. ofConrnerce, NOAA, 1981,TheFloridaCoastal ManagementProgr am, Draft Environmental Impact Statement, Office of Coastal Zone Management, Tallahassee. Flo Dickinson, K. A . Berryhill. H. l., and Holmes, C. W., 1972, Criteria forrecognizingancientbarriercoastlines.inR1gby.J.K.,and Hamblin. W. K . (eds.). Recognition of Ancient Sedimentary Environ ments, S..P.M., Spec. Pub1.no.16. Dil1on.W.P.,1970.SubmergenceeffectsofaRhode!slandbarrierand lagoon and inferences on migration of barriers; Jour. Geology, 78:94-106.

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Emery, K. 0., 1971, A simple method of measuring beach profiles; Limnology and Oceanography. 6: 90-93. Field, M. E., and Duane, D. B., 1976, Post-Pleistocene history of the U.S. inner continental shelf; significance to origin of barrier islands; Geol. Soc. Amer. BUll., 87: 69l-702. Fisher,J. J., 1968, Barrier island formation: discussion; Geol. J!.mer. Bull., 79: 1421-1426. FOlk,R. L., 1968,PetrologYofsedimentaryRocksi Hemphill Pu b.Ca., Austin,Tx,l82pg. Gilbert,G.K.,1885,ThetopographicfeaturesofLakeShores:U.S.G.S. 5th Ann. Rept., 87-88. Greenwood, B., and Keay, P. A 1979, Morphology and dynamics of a barrier breach: a study in stability; Can. Jour. of Earth Sci., 16:1533-1546. Halsey,S. 0., 1979,Nexus: newmodelofbarrierfslanddevelopment ,in Heath, R. C.,andSmith,P. C., 1954, Groundwater reSources of Pinellas County, Florida; Fla. Geol. Survey. Rept. lnv. 12, 139 p. Hobday, D. K., and Horne, J. C 1977. Tidally influenced barrier island and estuarine sedimentation in the Upper Carboniferous of Souther n WestVirg1nia;sed.Geol.,18:97-122. Hoyt,J. D., 1967. Barrier island formation, Geol. Soc.Amer. Bull.. 78:1125-1136. Johnson, D. W 1919. Shore Processes and Shore Line Development: John Wiley and Sons, Inc., New York. of Fla., Inst. of OceanographY, p. IIA-l to 22. Kahn, J. H., and Roberts. H. H . 1982,Variationsinstonnresponse alongamicrotidaltransgressivebarrier-islandarc.,Sed.Geo1., 33:2:129-146. Lanesky,D.E.,et.a1.,1979,Anewapproachtoportablevibracoring underwater and on land; Jour. Sed. Pet., 49: Leontyev. O. K 1969, Flandrean transgression and the genesis of barrier barS, in Wright, H. E. (ed.), Quaternary Geology and Climate, Nat1.Acad. Sci. Publ. 1701.

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leontyev, O. K., and Nikiforov, L. G . 1965. On the causes of the global occurrence of barrier bars, Oceanology, 4: 653-661. McGee, W. D., 1890, Encroachments of the sea, The Forum, 9: 437-449. Morris,P.A.,1973,AFieldGuidetoShellsoftheAtlanticand Gulf coasts and the West Indies: HougritenMlftilnCo., Boston. Moslow, T. F., and Heron, S. D., Jr., 1978, Relict inlets: preservation and occurrence in the Holocene stratigraptly of southern core ban kso North Carolina; Jour. Sed. Pet., 48:4: 1275-1286. Nurrrnedal, D., et. al., 1980, Geologic response of hurricane impact on low-profiTe Gulf Coast barriers; Trans. Gulf Coast Assn. of Geol. Soc., 30: 183-195. Oertel,G.F.,1975,Ebb-tidaldeltasofGeorgiaestuaries,in Cronin, L. E. (ed.), Estuarine Research, v. 2, Geology and Engineering, Academic Press, New York, p. 267-276. Otvos, E.G.,Jr., 1970a, Development and migration of barrier is lands, northern Gulf of Mexico; Geol. Soc. Amer. Bull., 81: 241-246. Otvos. E. G., Jr . 1970b, Development and migration of barrier islands, northern Gulf of Mexico: reply; Geo1. Soc. Amer. Bu'l., 81: 37833788. Otvos,E.G.,Jr.,198l,Barrierislandformationthroughnearshore aggradation; stratigraphic and field evidence; Marine Geology 43:3-4:195-243. Pierce, J. W . 1970. Tfdal inlets and washover fans; Jour. Geol., 78:230-234. Rampino, M. R . and Sanders. J. E., 1981, Evolution of the barrier is1ands of southern long Is1and,N.Y.; Sedimentology, 28: 37-47. Romans. 8., 1775, Concise Natural HistorY of East and West Florida; PelicanPublisningco., New Orleans. LA. Rosen,D.S . 1976.BeachandnearshoresedimentationonCaladesiIsland State Park, Pinellas County. Florida; unpbl. M.S. thesis, Univ. South Fla . 89pg. Schwartz. M. lo. 1971, The multiple causality of barrier islands; Jour. Geology, 79: 91-94. Schwartz,M.l . Inc.,Stroudsberg.PA. Tanner,W. F., 1960,Flor1dacoastal classification; Trans. Gulf Coast Assn. Geo1. Soc 10:259-266.

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Weidie.A. E., 1968. Bar and barrier island sands; Trans. Gulf COg st Assn. of Geo1. Soc . 18: 405-415. Zenkovich, V. P . 1962. Some new explorgtion results gbout sand shores development during the sea transgression; De Ingenieur, 9: 113-1 21.

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APPENDICES

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Split core/surface ,amplar 40-80g Disperse with NaPo4 O',(h'" .. ) Lat I Decant excess BZO, dryrmpla Weigh sample Dry 4' ("ction "aiOh lamPla Sieve to remove Run sand fraction Weigh gravel thru settling tube I j Calculate sand, ouval, mud Redraw cumulative % curve to lncorporate clay/gravel fractions I Calculate graphic parameters Appendix A. Flow chart showing lab methods for sediment samples.

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AppendixB-CoreLogs Core logs are marked in 0.1 m increments relative to mean sea level, and mean high and low tides are indicated as well. Appropriate symbols are used to indicate general grain size, heavy minerals. flasers, rootmaterial,pellets.andmolluscs. In the first column to the right of the visual log. gravel. sand, and mud are shown as weight percents. Munsell color notation ;s recorded in the second column. Fauna, as identified in Morris (1975), is listed in column three. Only genus names are listed; species names can be found in Appendix C. Fauna are listed in order of abundance. Bioturbation is indicated by a solid vertical line for the appropriate interval. Shellabrasion1sdividedintothreecategories. Common indicates that the majority of the shells examined were abraded. Moderate indicates that roughly 50% of the shells examined were abraded, white rare indicates that the majority of the shells were unabraded. An asterisk in this column indicates that either not enough shelT material was present or the shell fraction was too finely broken up to make a valid interpretation. Facies are shown in the right hand column, and facies changes are indicated in the visual log by a solid line. Intrastratal changes are shown on the v;sual log by a dashed line. Faciesdivisionsweredetermined by changes in the above categories and on !\tratigraphic relationships noted in the field. A key to all the core logs appears on the following page (Fig. 24).

PAGE 91

FAUNA .'OTU.S. -.} intr&st:tatume.hange '"--_---in lithology Figure 24. Sample core log with key to core samples. I 1

PAGE 92

COREE3 COl.OR FAUNA. BIOTURB. 1

PAGE 93

COREE2 FAUNA BIOTURB. I f 1

PAGE 94

COREEl FAUNA BIOTUAB. 1 I

PAGE 95

CORE 04 -

PAGE 96

CORED4(cont.) .. ;.;.:z;:._S_-t:.: . i$ .4/98.4/.2 COLO. = .,OT1U.0. Diplodonta Mode:eat ..

PAGE 97

85 CORE 03 m"t'ON T Diplodonta I Dfplodonta t Brachidontea IN Tereii"ra 1

PAGE 98

CQRED2 I 1 1

PAGE 99

87 COREDl Di-elodonta Chi.
PAGE 100

COREC5 D1ploclol'lu l T 1" 1

PAGE 101

COREC4 FAUItA BIOTURB. iUAXION I ..

PAGE 102

COREC3 FAUNA BIOTURB. T =::onta =tw..

PAGE 103

COREC2 T

PAGE 104

CORECl I

PAGE 105

CORES4 D1pl"odouta 1-

PAGE 106

COREB3 ,.s. -[ T

PAGE 107

COREB! FAUNA B'O,TU.B. ChionO:!

PAGE 108

CORES! DiplodODta Diplodonta :::;!Ontes ptp"lodonta :dOnta 5Y1/2 Diplodouta

PAGE 109

COREA4 I l' T T

PAGE 110

COREA3 FAUHA 810TU.8. 1 Diplodonta :ar:ra :;:vta Br4C:hidontes

PAGE 111

COREA2 COLOR FAUNA BIOTURB. I -.. T T T T

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:i;:outa

PAGE 113

AppendixC-FaunalList Anadaralienosa Anomalocan:liacuneimeris Brachidontesexustus Calvptraeacentralis Cerithiummuscarum Chionecancellata Conussozon; Crepidulafornicata Dentaliumeboreum Dinocardiumrobustum Donaxvariabilis Oosineadiscus Glycymerispectinata Macrocallistra nimbosa Mercenariamercenaria Murexrecurvirostris Nassariusalbus Noetiaponderosa Olivel1amutica Prunumlabiatum Pyramidellacrenulata Terebradislocata Trachycardium eomontium Turbocastaneus


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Stratigraphy and geologic history, Bunces Key, Pinellas County, Florida /
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ABSTRACT: Bunces Key, a narrow, linear, barrier island on the west-central coast of Florida, was formed in 1961. Its growth and development since that time is well documented by aerial photography. Cores taken from the Key and surrounding areas reveal a stratigraphic succession of facies reflecting rapid vertical aggradation. Sedimentation began on a gently sloping platform through the landward migration of large scale bedforms (sand waves) during fair weather periods. Migration of these bedforms ceased when emergence and lack of continued overwash precluded further movement.Vertical accretion to supratidal levels resulted from the continued onshore transport of sediment and subsequent welding to the previously formed bars. Stratigraphically, the barrier exhibits a "layer-cake" type of stratigraphy, with nearshore sediments overlain by foreshore, backbeach, and dune deposits. The backbarrier generally exhibits muddy lagoon sediments intercalated with washover and channel margin sediments.Fining upward washover sequences reflect the unstable nature of the island.Low pressure systems commonly cause overtopping of the barrier, with the subsequent formation of tidal inlets and washover fans. Aerial photographs document the formation of an initial barrier that was breached twice prior to 1973. A second barrier formed in late 1973 just seaward of the initial island and subsequently grew through littoral drift to a length of 1.8 km. A narrow inlet (30 m) formed through the northern end of the island in 1982.
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