Regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west-central Florida

Regional stratigraphic framework linking continentalshelf and coastal sedimentary deposits

of west-central Florida

Stanley D. Locker a;�, Albert C. Hine a, Gregg R. Brooks b

a College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USAb Department of Marine Science, Eckerd College, St. Petersburg, FL, USA

Accepted 1 June 2003

Abstract

A regional study of the Holocene sequence onlapping the west-central Florida Platform was undertaken to mergeour understanding of the barrier-island system with that of the depositional history of the adjacent inner continentalshelf. Key objectives were to better understand the sedimentary processes, sediment accumulation patterns, and thehistory of coastal evolution during the post-glacial sea-level rise. In the subsurface, deformed limestone bedrock isattributed to mid-Cenozoic karstic processes. This stratigraphic interval is truncated by an erosional surface,commonly exposed, that regionally forms the base of the Holocene section. The Holocene section is thin anddiscontinuous and, north or south of the Tampa Bay area, is dominated by low-relief sand-ridge morphologies.Depositional geometries tend to be more sheet-like nearshore, and mounded or ridge-like offshore. Sand ridges exhibit0.5^4 m of relief, with ridge widths on the order of 1 km and ridge spacing of a few kilometers. The central portion ofthe study area is dominated nearshore by a contiguous sand sheet associated with the Tampa Bay ebb-tidal delta.Sedimentary facies in this system consist mostly of redistributed siliciclastics, local carbonate production, and residualsediments derived from erosion of older strata. Hardground exposures are common throughout the study area.Regional trends in Holocene sediment thickness patterns are strongly correlated to antecedent topographic control.Both the present barrier-island system and thicker sediment accumulations offshore correlate with steeper slopegradients of the basal Holocene transgressive surface. Proposed models for coastal evolution during the Holocenetransgression suggest a spatial and temporal combination of back-stepping barrier-island systems combined withopen-marine, low-energy coastal environments. The present distribution of sand resources reflects the reworking ofthese earlier deposits by the late Holocene inner-shelf hydraulic regime.4 2003 Elsevier B.V. All rights reserved.

Keywords: west Florida shelf; Holocene stratigraphy; barrier island; sand ridge

1. Introduction

An investigation of the inner continental shelfo¡ west-central Florida was conducted for the

0025-3227 / 03 / $ ^ see front matter 4 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0025-3227(03)00191-9

* Corresponding author. Tel. : +1-727-553-1502;Fax: +1-727-553-1189.

E-mail address: [email protected] (S.D. Locker).

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Available online at www.sciencedirect.com

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purpose of de¢ning the geologic framework anddepositional history of nearshore sediments, andestablishing their relationship with the modernbarrier-island system. Much of the coastline isheavily developed, presenting considerable man-agement concerns related to shoreline stabilityand o¡shore sand resources needed for beachnourishment. The widespread occurrence of hard-bottom and rock outcrops is well-known to rec-reational and commercial industries, and a reviewof bathymetric maps suggested the presence ofsand ridges, which are often targeted as potentialsand resources for beach nourishment (Finkl etal., 1997; Gelfenbaum et al., 1995; see reviewby Balsillie and Clark, 2001). The potential forexchange of sediment between the nearshore andbarrier island was largely unknown, hence a fun-damental question concerned determining thequantity, characteristics, and distribution of thesediment which exists seaward of the barrier-is-land system. To understand the origin of thepresent shelf and coastal sedimentary frameworksit is important to look at the region as a whole.Key questions include those concerning the tran-sition and succession of environments during thelate Holocene transgression. How much of theshelf sedimentary sequence is reworked? Towhat extent do present shelf sedimentary faciesand distribution patterns re£ect patterns that areinherited, or were controlled by earlier deposition-al environments such as £uvial or barrier islandsand lagoon systems?We describe a regional stratigraphic framework

for the inner continental shelf and barrier/lagooncomplex of west-central Florida. Achieving thisinvolved merging o¡shore data with barrier-islandand lagoon stratigraphy to de¢ne the full onlapsequence. Moreover, we look at the broad region-al trends in depositional patterns, and considerthe large-scale processes and controls that havein£uenced the highstand systems tract develop-ment on this sediment-starved, mixed carbonate/siliciclastic, low-energy, mid-platform ramp set-ting. Companion papers by Brooks et al.(2003) and Davis et al. (2003) focus in greaterdetail on the facies architecture of the continentalshelf and the barrier-island stratigraphy, respec-tively.

2. Background

The study area is a 175 km stretch of coastlinecentered on Tampa Bay, extending from AncloteKey in the north, south to Venice, Florida, andextending approximately 40 km o¡shore (Fig. 1).The low-gradient continental shelf is consideredto be a moderate- to low-energy setting, subjectto higher-energy winter frontal passages. Tidalranges are less than 1 m, with mean annualwave heights of 10^30 cm (Tanner, 1960).The study location is situated in the middle of

the Mesozoic^Cenozoic Florida carbonate plat-form that received signi¢cant terrigenous sedi-ment from the southeastern United States duringintermittent highstands of sea level in the Neo-gene (McKinney, 1984; Warzeski et al., 1996;Guertin et al., 1999; Cunningham et al., 2001,2003; Hine et al., 2001, 2003).Along the west coast of Florida, a broad low-

relief ramp is developed upon the early Cenozoiccarbonate platform deposits. This carbonate sec-tion is a¡ected by signi¢cant karst developmentthat continues to the present day, as evidencedby the many springs and sinkholes in the region.Evidence for subsurface dissolution is widespreadthroughout the central portion of the FloridaPlatform, and has caused widespread collapseand deformation of Miocene depositional sequen-ces (Missimer, 1999; Duncan et al., 2003). Post-Miocene deformation appears to be limited to lo-calized vertical structures such as sinkholes andsolution shafts (Lane, 1986). The modern Holo-cene sediment cover discontinuously overlies a re-gional erosional unconformity that truncates thesevaried depositional units.Earlier studies on the shelf in this area indicated

a thin, patchy sediment cover typi¢ed by quartzsand, carbonate gravel, and live hardbottoms(Gould and Stewart, 1955; Cherry et al., 1970;Ginsburg and James, 1974; Riggs and O’Conner,1974; Doyle and Sparks, 1980). However, the lowsampling density was inadequate to predict spatialtrends of sediment types. Even less was knownabout sediment thickness or depositional geome-tries.Considerable prior e¡ort has been focused on

understanding the stratigraphy and morphody-

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namics of the barrier-island system (Davis, 1989;Davis and Hine, 1989; Davis et al., 1992; Davis,1994). The present coastal zone is characterizedby a barrier-island/lagoon system described asone of the most complex barrier-island systemsin the world, with evidence for both tide- andwave-dominated processes. The adjacent innershelf contains a variety of sedimentary featuresincluding sediment-starved sand-ridge systemswhich display a wide variation in morphologyand orientation (Gelfenbaum and Guy, 1999),and the Tampa Bay ebb-tidal delta, ranked asthe largest in the Gulf of Mexico (Hine et al.,1986). Other investigations have emphasized theimportance of antecedent topographic control oncoastal evolution, and the importance of near-shore sediment supply on barrier-island evolution(Evans et al., 1985; Davis and Kuhn, 1985; Hineet al., 1987; Davis et al., 1992).

3. Methods

The primary data sets used in this study werecollected from 1993 to 1998. Geophysical surveysincluded high-resolution single-channel ‘boomer’seismic data and 100-kHz side-scan sonar imagery(Fig. 1; Locker et al., 2002c). Most of the recon-naissance seismic and side-scan sonar data wereacquired during two o¡shore cruises in 1994. Bot-tom samples were collected during the 1994cruises using an underway grab sampler at 4-kmintervals along track.Seismic data were acquired using low power

levels ranging between 100 and 200 J in order tomaximize vertical resolution of the thin Holocenesection. Most data were collected using a HUN-TEC boomer, 10-element Innovative Transducerstreamer and Elics Delph2 digital seismic acquisi-tion and processing software. Older data sets in-cluded analog paper records acquired using anORE Geopulse boomer system. Processing ofthe seismic data shown here includes 1000^3500Hz bandpass ¢lter, automatic gain control, andtrace summing. Vertical resolution of the digitalseismic data is very good, on the order of 10^20cm at the sea£oor.Side-scan sonar imagery was collected coinci-

dent with all o¡shore seismic lines. This pairingof data types was vital for interpreting sedimentthickness in this region of discontinuous sedimentcover. In most areas the Holocene sediment accu-mulations tend to be quartz sand of low acousticbackscatter. In contrast, bedrock and a coarse-grained veneer produces high-backscatter imagerythat can be used to constrain the border of sandbodies. Three continuous-coverage mosaics wereacquired within the study region in support ofcomprehensive investigations of local settings(Twichell and Paskevich, 1999; Donahue et al.,2000; Harrison et al., 2000; Twichell et al.,2000). In addition, a fourth survey conducted pri-or to this program in 1988 o¡ Anclote Key alsoincluded a towed underwater video camera at thesame time as both side-scan sonar and seismicdata recording. The video provided direct visualveri¢cation and classi¢cation of the relationshipbetween backscatter and bottom type. Althoughnot fully overlapping in its coverage, the closelyspaced Anclote Key survey demonstrated the im-portance of integrating methods (seismic, back-scatter, video) for habitat and sediment thicknessmapping (Locker et al., 2000).Over the duration of this study the side-scan

sonar methodology which we used evolved signif-icantly. Initial reconnaissance-style single-track-line surveys used an EGpG 272-TD analog tow-¢sh operated at 100 kHz and an EGpG Model260 recorder for slant-range correction and out-put to paper records. The continuous-coveragemosaics involved using 100 kHz analog tow¢sh(EGEGpGG or Klein) with digital acquisitionand processing systems (Donahue et al., 2003;Harrison et al., 2003; Twichell et al., 2003).O¡shore vibracore locations were selected

based upon seismic data and were focused inareas likely to contain su⁄cient sediment thick-ness for core retrieval (Brooks et al., 1999). Seis-mic interpretations of Holocene sediment thick-ness were ground-truthed by these vibracores.Additional sediment thickness evidence includedexposed hardbottoms (pre-Holocene bedrock)and probe-rod measurements of sediment thick-ness in the nearshore zone. Vibracores and probedata provided stratigraphic control in the barrier-island and bay areas (Davis et al., 2003).

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Fig. 1. Study location showing data coverages and ¢gure locations.

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4. Results

4.1. Side-scan sonar imagery

Throughout the study areas the backscatter im-agery is associated with four main bottom types(Fig. 2). Low-backscatter areas (Type-I) are mostcommonly related to siliciclastic facies of the Ho-locene sand sheet. Typical associations for thelow-backscatter bottoms are sand ridges andebb-tidal delta deposits. Two types of high-back-scatter bottoms are: Type-IIA, hardbottoms ex-hibiting a blocky pattern in imagery, or Type-IIB, a more uniform high backscatter, at somelocations including sub-meter scale ripples, whichcorresponds to coarse-grained sediments with ahigh carbonate content (Vs 30%). A fourth bot-tom, Type-III, includes di¡use textures of moder-ate to high backscatter that are related to sea-grass. Seagrass distribution patterns range fromsparse and widely dispersed patches (on Type-Ibottoms) to dense and continuous cover. Seagrassis largely limited to the back-barrier bays, exceptin the northern area o¡ Anclote Key where athinning seagrass cover extends seaward of thebarrier (Locker et al., 2000). Hine et al. (1987)document more extensive seagrass beds o¡shorefrom Anclote which underwent a decline in thelate 1950s^1960s. The presently sparse seagrasscover o¡shore may therefore represent the linger-ing remnants of a once-abundant seagrass cover.Backscatter imagery combined with bathymetry

is a useful proxy for sediment thickness through-out the study area. Because the Holocene silici-clastic sand accumulations commonly form posi-tive relief features such as sand ridges, the lowbackscatter tends to correspond to sand-ridgedepositional morphology. The high-backscatterareas correspond to inter-ridge lows where ex-posed bedrock or a winnowed veneer of coarsecarbonate molluscan-rich sediment predominates(Brooks et al., 2003). Exceptions to this patterndo occur, as shown by Twichell et al. (2000, 2003)in the Sarasota area (Fig. 1) where low-relief sandridges developed signi¢cant textural variationacross the ridge, with a distinct boundary betweenhigh and low backscatter controlled by this tex-tural change occurring at the ridge crest.

4.2. Stratigraphic framework

The regional stratigraphic framework is pre-sented using a selection of 18 dip-oriented cross-sections that combine o¡shore seismic data withbarrier-island stratigraphy (Fig. 3). Wherever pos-sible these cross-sections extend from the o¡shorelimit of seismic lines in approximately 26 m waterdepth, landward to the bayline, or mainland on-lap point. Nine of these cross-sections are mergedwith barrier-island cross-sections that were devel-oped by researchers at the University of SouthFlorida Department of Geology (Davis et al.,2003). These sections are presented in greater de-tail in the 5-km-wide swath-transect map seriespublished as U.S. Geological Survey Open-FileReports on CD-ROM, and accessible via theworld-wide-web (Locker et al., 2002a,b,d^j ;http://coastal.er.usgs.gov/w£a/). The remainingcross-sections include the estimated landward pro-jections of Holocene sediment thickness beneaththe barrier islands and lagoon.The alignment of cross-sections in Fig. 3 is cen-

tered at 10 m bedrock depth, in order to facilitatethe comparison of regional bedrock structuraltrends with barrier-island and o¡shore sedimentaccumulation patterns. North of Tampa Bay,the inner shelf is broader, and the 10 m bedrockdepth is located 12^16 km o¡shore from the bar-rier-island beaches. North of the Indian RocksBeach headland, the barrier islands are developedalong the seaward side of a shallow bedrock ter-race that underlies the broader back-barrier la-goon (Evans et al., 1985; Davis and Kuhn,1985). South of Tampa Bay, the inner shelf typi-cally lacks the shallow bedrock terrace and, moreimportantly, is comparatively deep, with the 10 mbedrock depth located within 3^8 km of thebeach.Three key features shown by these cross-sec-

tions are: (1) a thin and discontinuous Holocenesequence that unconformably overlies (2) a highlydeformed Cenozoic section containing solutioncollapse structures, across (3) a relatively £at ero-sional surface that truncates the pre-Holocenestrata. While the focus of this paper is the Holo-cene section, a brief review of the pre-Holocenestratigraphy is provided because of the important

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Fig. 2. Classi¢cation of typical 100 kHz side-scan sonar backscatter imagery from the study area.

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Fig. 3. Line drawing interpretations of dip-oriented seismic lines merged with coastal stratigraphy. The Holocene sequence is thin and discontinuous. Deformationof the pre-Holocene strata appears more signi¢cant to the south. Sections are aligned at the 10 m isobath (excepting J) to illustrate narrowing of the nearshorezone from north to south.

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Fig.3(Continued).

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Fig.3(Continued).

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in£uence of antecedent topographic controls andrelict sediment sources on the development of Ho-locene depositional environments.

4.3. Pre-Holocene

Wavy parallel and chaotic re£ections character-

ize the regional bedrock section, which is identi-¢ed as the Arcadia Formation of Miocene age(Figs. 3 and 4; Scott, 1988; Duncan et al.,2003). These stratal patterns are characteristic ofvertical deformation and lithostratigraphic inho-mogeneities caused by karst processes. The bed-rock deformation and associated subsidence

Fig. 4. Example of shelf valley showing lack of acoustic contrast between valley ¢ll sediment and the Holocene section. In thisexample, some truncation of earlier deformed strata, perhaps Miocene-age Arcadia Formation, contributes to the valley forma-tion. The in¢lling clinoforms (late Neogene or possibly Pleistocene) have not been deformed. The basal Holocene unconformitydeveloped during high-amplitude Quaternary sea-level £uctuations and long-term subaerial exposure of the Florida Platform.

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forms km-scale shelf-valley depressions (Fig. 4).From Tampa Bay south, such shelf valleys arewell-developed (Fig. 5). In contrast, shelf valleysare absent north of John’s Pass at approximately27‡47PN. The origin of the shelf-valley systems istentatively attributed to collapse deformation dueto deep-seated dissolution of the mid-Cenozoicplatform carbonates. The relief of these depres-sions is of the order of several tens of meters. Insome local areas, erosional truncation plays a rolein valley formation (Fig. 4). However, the westFlorida valley systems lose de¢nition toward theseaward limit of this study.Sigmoidal prograding clinoforms or transparent

sequences in¢ll the shelf-valley depressions (Fig.4). Shelf-valley in¢ll is assumed to be late Mio-cene to Pliocene in age (Duncan et al., this vol-ume; Ferguson and Davis, 2003), although wehave little age control on a regional basis to dis-tinguish the time of valley formation from thein¢lling sequences. The in¢lling clinoforms aresimilar in occurrence to the Pliocene-age UpperPeace River Formation sequences which havebeen identi¢ed further to the south in centralFlorida (Cunningham et al., 2001, 2003). The in-¢ll section of one shelf valley investigated beneatha barrier island just north of Tampa Bay containsQuaternary siliciclastic in¢ll cycles (Ferguson andDavis, 2003). Valley-forming deformation of thebedrock section would therefore be pre-late Mio-cene in age, as the in¢lling clinoforms are notdeformed. No evidence has been found for defor-mation of Holocene strata by karst processes.

4.4. Basal Holocene unconformity

A major erosional unconformity marks the baseof the Holocene section (Fig. 3). This unconform-ity re£ects extensive periods of exposure and ero-sional processes accompanying numerous Quater-nary sea-level £uctuations. During the Holocenetransgression, evidence for a ravinement on thissurface varies, as not all areas were subject toshoreface erosion (Brooks et al., 2003).Pre-Holocene bedrock is exposed as hardbot-

toms throughout the study area, sometimes sup-porting ‘live bottom’ benthic communities includ-ing sponges, and soft and hard corals (Harrison et

al., 2003). Some exposures include meter-scaleledges that support benthic communities andbio-eroders that constitute an important modernsediment source (Obrochta et al., 2003).Due to lengthy periods of exposure during sea-

level lowstands, this surface also includes a varietyof weathered and residual products ranging fromblue-green clays and carbonate lithoclasts tocoarse-grained phosphatic grains (Brooks et al.,2003). These varied lithofacies and the weatherednature of the exposure surface result in generallypoor acoustic properties that limit penetration,and often yield little impedance contrast betweenmodern shelf sands and mid-Cenozoic strata.Vibracores in o¡shore areas commonly recov-

ered pre-Holocene weathered limestone and resid-ual clay facies (Brooks et al., 2003). Beneath thebarrier-island system, Miocene limestone, residualclay facies, and Pleistocene sand are commonlypresent below the Holocene onlap (Davis et al.,2003; Locker et al., 2002a,d, 2001a).

4.5. The Holocene

The inner continental shelf can be divided intothree main areas or provinces, based on a combi-nation of bedrock geology, sedimentary facies as-sociations and sediment thickness-distributionpatterns. The Northern Province (north of TampaBay) is characterized by somewhat shallow depthsacross the inner shelf (10 m isobath located 10^16km seaward of beach) and NW^SE trending sandridges. A central region, Tampa Bay Province, isdominated by the Tampa Bay ebb-tidal delta. TheSouthern Province is characterized by a deepershelf (10 m isobath located 3^8 km seaward ofbeach) and a sand-ridge system of variable trend.In general, stratal re£ections within the Holocenesection are absent, although weak re£ections as-sociated with a ravinement surface are observedat some locations (see Edwards et al., 2003), andsome low-angle re£ections within the Tampa Bayebb-tidal delta suggest late-Holocene prograda-tion or phased buildup of this depositional fea-ture.

4.5.1. Sand-ridge trendsO¡shore sand-ridge morphology predominates

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on the continental shelf away from the TampaBay ebb-tidal delta, beginning just seaward ofthe barrier islands shoreface. Gelfenbaum andGuy (1999) produced a bathymetric surface modelusing NOAA and NOS hydrographic survey data

collected in the 1950s and 1970s. A modi¢ed ver-sion of this database is shown in Fig. 6. The moststriking features are sand-ridge lineations, roughly1 km in width and several km in spacing.Although the geometries of sand ridges in depths

Fig. 6. (A) Bathymetry, modi¢ed from Gelfenbaum and Guy (1999), is shown by white-to-black transitions that repeat for each5 m depth interval out to the 25 m isobath. This is done to enhance the meter-scale topographic features dominated by sand-ridge morphology. (B) Line drawing of sand-ridge and large sand-dune orientations. Regional trends indicate both onshore^o¡-shore and north^south variation. The spacing of features increases in an o¡shore direction. In an alongslope direction, threezones are evident. The northern zone is dominated by NW^SE trending sand ridges. The Tampa Bay zone displays a rotationalpattern with SW^NE trending ridges becoming more common. The southern zone displays a mixed pattern of ridge orientationsin the nearshore zone and generally SW^NE trending ridges o¡shore.

Fig. 5. Generalized map showing location of main deformation styles of the pre-Holocene strata. From Tampa Bay south, largevalley systems contain prograding clinoforms or transparent ¢ll. Channel structures located at the base of the Holocene sectionare very rare.

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Fig. 7. Holocene sediment thickness map for the Northern Province. Continuous contouring is not possible without closelyspaced survey lines. Note that the thickest areas of sediment accumulation correspond to the barrier islands, or to sand ridgeso¡shore in approximately 20 m water depth. The sand-ridge morphology is easily seen in the background bathymetry map (fromGelfenbaum and Guy, 1999).

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greater than 12 m are not well mapped, the gen-eral trends indicate an increase in ridge spacingwith depth.In the Northern Province, ridges trend NW^SE

at oblique angles of 30‡^65‡ to the shoreline. Thesand-ridge trends are most variable as they sweeparound the Indian Rocks Beach headland. Sea-ward of 10 m water depth, the ridge trends aremore consistently oriented at approximately 50‡from the shoreline, the oblique angle opening tothe north. O¡ Tampa Bay, ridges are not clearlydeveloped and the Tampa Bay ebb-tidal deltadominates this coastal sector (Donahue et al.,2003). The sand-ridge systems south of TampaBay are oriented at oblique angles with the coast-

line but display mixed NW^SE and NE^SWtrends. NE^SW trends at 45‡^55‡ relative to thecoastline are common in the nearshore (out toV10 m water depth). In 10^15 m water depths,mixed NW^SE and NE^SW trends with 30‡^50‡angles to the coastline are inferred from bathym-etry. In greater than 15 m water depth, NE^SWtrends of 50‡^60‡ appear most common (Fig. 6).

4.5.2. Sediment thicknessRegional data on o¡shore sediment-thickness

patterns are presented in Figs. 7^9. Because thetransgressive surface is relatively £at, bathymetryis a good proxy for sediment thickness; hencemost sediment accumulation patterns correspond

Fig. 8. Holocene sediment thickness map for the Tampa Bay Province. Thicker sediments associated with the Tampa Bay ebb-tidal delta dominate this coastal sector. The area contoured is estimated to be a contiguous sand sheet.

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to the low-relief sand-ridge morphologies (0.5^2m of relief). Holocene sediment accumulationsreaching 3^4 m are associated with small ebb-tidaldeltas in the nearshore, or localized sand-ridgecrests o¡shore (Fig. 10).In the Northern Province (Fig. 7), an area of

continuous isopach contouring o¡ Indian RocksBeach reveals a high variability in thickness con-trolled primarily by sand-ridge trends, and secon-darily by large sand dunes (100-m-scale bedforms)that comprise the sand ridges in this area (Fig. 11;also see Harrison et al., 2003). Sediment thicknessover 2 m in this area is rare, while more than 50%of the sea£oor in 10 m water depth is interpretedas hardbottom or a coarse sediment veneer belowthe resolution of seismic data (in this sub-studyabout 0.5 m). However, spot accumulations of 4 mor greater sediment thickness were found 20^25km o¡shore (Fig. 3G; Locker et al., 2002h).The Holocene barrier-island chain is commonly5^7 m thick (Fig. 3A^R; Davis et al., 1992; Evanset al., 1985; Locker et al., 2002d,e,g,h, 2001a).The Tampa Bay Province is dominated by the

Tampa Bay ebb-tidal delta and is distinguished bya thicker Holocene sequence beneath the barrierislands in this region (Figs. 3J^L and 8). A largeebb-tidal delta extends approximately 10 km o¡-shore, which, combined with some smaller inletslinked to Tampa Bay, has resulted in late-Holo-cene sediment accumulation of 5 m or greater in anearshore zone 20^25 km in width centered on themouth of Tampa Bay. However, here also there islimited sediment cover o¡shore between 10 and15 m water depth, and then in 15^20 m depthsthere are some substantial local accumulations of4 m or more (Fig. 3K).Sediment thickness in the Southern Province is

shown in Fig. 9. Most sediment is less than 2 mthick, with only a few isolated areas in the 3^4-mrange located in nearshore ebb-tidal deltas or fur-ther o¡shore in 15^20 m water depths (Figs. 3M^P and 11). As in the Northern Province, the thick-ness patterns are primarily controlled by sand-ridge morphology. An area of continuous isopachcontours in the nearshore o¡ Sarasota, Floridawas constructed by Twichell et al. (2000, 2003)using a tight grid of 3.5 kHz pro¢le data alongwith a continuous side-scan mosaic and bottom

sampling. The NE^SW aligned sand ridges thereexhibit a £atter morphology in comparison tosand ridges o¡ Indian Rocks Beach. Again,much of the area has little-to-no Holocene sedi-ment cover.

4.5.3. Sediment distribution patternsOne of the objectives for this regional study

was to assess the inventory of o¡shore sedimentresources in relation to the barrier-island system ^how is the available Holocene sediment distrib-uted within this onlapping coastal/shelf deposi-tional sequence? To address the sediment inven-tory question, and perhaps to gain insight intocontrols on sediment accumulation patterns, sedi-ment-thickness distributions as a function ofdepth to the underlying transgressive surfacewere analyzed. Previous studies have cited ante-cedent topographic control as a factor in barrier-island development (Evans et al., 1985; Davis,1994). For each cross-section in Fig. 3, the aver-age sediment thickness was determined for 2-mdepth intervals (measured from sea level to thebasal Holocene boundary) along the section.Due to di¡erences in slope, the horizontal distan-ces for each 2-m depth interval varied. The result-ing data set re£ects the relationship of sedimentaccumulation to antecedent topography (Fig. 12).These results were further grouped according tothe three provinces previously identi¢ed to facili-tate comparisons.The relationship between sediment thickness

and depth of the transgressive surface providessome quantitative assessment of the previouslydescribed north-to-south, and onshore/o¡shore,trends in sediment thickness (Fig. 12). The bar-rier-island accumulations are at a maximum in the2^6 m bedrock depth interval, averaging 3^4 mthick overall. In both the north and south prov-inces, this coastal sediment wedge thins dramati-cally to less than 1 m average sediment thicknessin the 10^15-m depth interval. O¡ Tampa Bay,the ebb-tidal delta extends this pro¢le further sea-ward. However, in all three provinces minima arereached o¡shore by 10^15 m depths. From Tam-pa Bay to the north, increases in average sedimentthickness are found farther seaward at depthsgreater than 15 m, while in the Southern Province,

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Fig. 9. Holocene sediment thickness map for the Southern Province. Sand ridges of mixed orientations (noticeable in the back-ground bathymetry) are also indicated by detailed sediment thickness mapping in the nearshore o¡ Sarasota (modi¢ed fromTwichell et al., 2000).

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Fig. 10. Example of seismic data south of Tampa Bay showing £atter sand bodies nearshore (A) and thicker sand bodies o¡shore(B). The seismic data used a 100 J boomer source, and 1000^3000 Hz bandpass ¢lter. Location in Fig. 1.

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Fig. 11. Typical con¢guration of sand ridges seaward of Sand Key (Indian Rocks Beach headland). (A) 100 kHz side-scan sonarimagery provides information on bedform orientations. Coarser-grained sedimentary facies characterized by increased shell mate-rial are common in troughs. (B) Uninterpreted seismic pro¢le. The sand dunes commonly have meter-scale relief, 60^100 m wave-length, and comprise the broader sand ridge (V1 km wide). (C) Interpretation showing the relief associated with these bedformscan account for signi¢cant variability in sediment thickness. This variability in thickness is only partially represented in the iso-pach contours shown in Fig. 7. See Fig. 1 for location.

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the average thickness remains low, although themaximum thickness of sand bodies consistentlytends to be greater o¡shore, as previously dis-cussed.To consider whether the slope of the transgres-

sive surface may have been in£uential in sedi-ment-distribution patterns, the slope gradient ofthe basal surface is plotted versus average sedi-ment thickness for each of the three provinces(Fig. 13). In the Northern and Tampa Bay prov-inces, the slope gradients increase above 1:1500seaward of the 16 m bedrock depth contour, butfurther south the gradient remains at 1:1500.Overall, the regional sediment thickness and

slope gradient data show a positive correlationbetween increased gradients and increased sedi-ment thickness (Fig. 14). Thicker sediment accu-mulations are positioned over the more steeplyinclined basal surface. Conversely, £attening ofgradients correlates with a general thinning ofthe Holocene sediment cover. This suggests a¢rst-order control by bedrock topography on

the sediment distribution patterns in the westernFlorida coastal and inner-shelf system.

5. Discussion

5.1. Depositional environments

Holocene sediments mainly re£ect the redistri-bution and sorting of quartz sand during the Ho-

Fig. 13. Plots showing relationship of Holocene sedimentthickness to slope gradients for the three provinces. Slopegradients greater than 1:1500 correlate to thicker accumula-tions known, or inferred, to be barrier-island systems.

Fig. 12. Plot of average Holocene sediment thickness versusbedrock (transgressive surface) depth. These data are derivedfor the cross-sections presented in Fig. 3 and grouped by thethree provinces. On a regional basis, the Tampa Bay ebb-tidal delta and the barrier-island chain contain most of theexisting sediment volume within the onlapping Holocene se-quence. However, increases in sediment volume are indicatedfor the Northern and Tampa Bay Provinces in o¡shore areasdeeper than 15 m water depth.

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locene transgression and the response to the open-shelf hydrodynamic regime during the past fewthousand years. Additional sediment sources in-clude locally produced carbonate material that isprimarily molluscan in origin, and reworkingfrom pre-Holocene sequences exposed at theseabed. Here, we summarize regional sedimenta-ry-facies characteristics. More detailed discussionsare presented in accompanying papers by Davis etal. (2003) for the barrier islands, and by Brooks etal. (2003) for the o¡shore sediment distribution. Athorough presentation of these core and surfacesediment data, with interpretations, is also avail-able on CD-ROM (Locker et al., 2002a,b,d,2001b http://coastal.er.usgs.gov/w£a/), and onthe internet (http://coastal.er.usgs.gov/w£a/).Vibracoring throughout the barrier islands re-

cords a succession of Pleistocene sand or Miocenelimestone bedrock overlain by vegetated paralicfacies which grade upward to back-barrier/over-wash facies. In turn, this is overlain by inlet chan-nel facies (typically incised to the bedrock), andbeach ridge and dune facies. The vertical com-pleteness of these facies successions and lateralrelationships varies, and some back-barrier faciesoccur seaward of the present beach (Davis et al.,

2003). Thus, while coastal retreat has occurred incertain areas during the late Holocene, other areashave prograded seaward and/or laterally in re-sponse to alongshore sediment transport or thelandward transport and shoaling of nearshoresediments (Hine et al., 1987; Davis and Kuhn,1985). The oldest of the barriers have been datedat 3000 years (Stapor et al., 1988).Brooks et al. (2003) focused on the o¡shore

facies architecture recovered by vibracoring andfound distinct north^south di¡erences in sedimen-tary facies associations. Brooks identi¢ed six Ho-locene facies types and associated depositional en-vironments, consisting of three back-barrier facies(organic muddy sand, olive-gray mud, and muddysand facies) and three open-marine facies (well-sorted quartz sand, shelly sand, and black sandfacies).Vibracore sampling in the Northern Province

typically recovered a medium-grained quartz-sand facies with subordinate amounts of coarse-grained carbonate shell material, which comprisesmost of the 1^3 m thick Holocene sand-ridge sec-tion. This quartz-sand facies is interpreted as de-posited under open-marine conditions. Movingsouth, closer to Tampa Bay, core recovery ap-

Fig. 14. Summary of all sediment thickness and slope gradient data derived from the Fig. 3 cross-sections illustrating the positivecorrelation between slope gradient and sediment thickness.

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proached 5 m. Back-barrier facies, including mudand mud-rich sand containing burrows and la-goonal foraminiferal assemblages, was recoveredin the further o¡shore core locations in this north-ern locale. The o¡shore back-barrier facies wereradiocarbon-dated at 8300T 90 yr BP, with atransition to open-marine facies dated from 5900to 7790 yr PB.O¡shore in the Tampa Bay Province, vibra-

cores penetrated similar facies to those describedfarther north. However, there is a trend for lesssediment cover and an increase of blackenedgrains mixed with the open-shelf, well-sortedquartz sand. Additionally, the base of the Holo-cene contains some muddy facies that are typicalof back-barrier/lagoonal deposits.O¡shore core recovery in the Southern Prov-

ince was usually less than 2 m. Carbonate sandand gravel with subordinate amounts of quartzsand is common, with some increase of quartzsand in the inner 3-km portion of the shelf. Aquartz sand with blackened grains facies com-monly represents the full Holocene section, andrarely reaches a thickness of 1 m (Brooks et al.,2003; Locker et al., 2001a).Sediment grab samples recovered coarse-

grained and gravel-sized carbonate skeletal mate-rial from the high-backscatter, low-elevation, in-ter-ridge trough areas. In contrast, the elevatedlow-backscatter sand ridges contain medium-grained sand (Brooks et al., 1998; Edwards etal., 2003; Harrison et al., 2003). Such a segrega-tion of grain populations is common in sediment-starved shelf environments (e.g. Riggs et al.,1998). However, exceptions to this topographyand texture association can be found in the south-ern portion of the study area.A comparison of nearshore sand ridges in the

Northern (Indian Rocks Beach) and Southern(Sarasota) Provinces reveals signi¢cant di¡erencesin sedimentary facies relative to ridge morphol-ogy. The northern sand ridges display low side-scan backscatter, associated with the full bathy-metric expression of the sand ridges and alsoacross the ridge’s crest, which re£ects a more uni-form texture for these sand bodies as seen by 100kHz acoustic imaging (Harrison et al., 2003). Incontrast, o¡ Sarasota, a signi¢cant variation in

sediment texture occurs within the bathymetricexpression of a sand ridge. Twichell et al. (2003)found an abrupt change in side-scan backscatterand sediment texture at the ridge crest, character-ized by the presence of coarser-grained carbonate-rich sediment on the northern £ank and ¢ner-grained quartz sand on the southern £anks.Twichell et al. (2003) suggest that current win-nowing of the northern £anks of the sand ridgemay control the sharp textural transition in theSarasota area, whereas o¡ the Indian RocksBeach headland, the net e¡ect of tides, waveshoaling, and seasonal storms appears to be amore balanced, bidirectional sediment transport.This is partly supported by regional circulationstudies that show a seasonal reversal of north^south shelf currents centered o¡ Tampa Bay(Yang and Weisberg, 1999; Yang et al., 1999).

5.2. Controls on sediment distribution patterns

The primary late-Holocene depositional sys-tems represented by this study of the west-centralFlorida shelf/coastal system include the beach/barrier-island/back-barrier complex, inlets and as-sociated tidal deltas of widely varying scale, ando¡shore isolated sand bodies commonly shapedinto broad, low-relief sand ridges. Perhaps themost important di¡erence that distinguishes thewest Florida shelf from other coastal settings isthe location on a carbonate platform interior hav-ing a very low slope gradient, and a limited, pri-marily relict, mixed carbonate and siliciclasticsediment supply with little-to-no £uvial input.Thus, the evolution of depositional environmentsappears to have been more sensitive to subtletopographic controls.

5.2.1. Barrier islandsSediment supply and rates of sea-level rise are

certainly fundamental controls on barrier-islandformation (Morton, 1994; Oertel and Kraft,1994). However, the results of this study empha-size the role that antecedent topography, and spe-ci¢cally slope gradient, can play in the initiationof barrier islands during sea-level transgression.Moreover, we suggest that the slope gradient con-trol can operate independently of the rate of sea-

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level rise or sediment supply, especially in a low-gradient ramp setting hosting a thin and discon-tinuous sediment source.Evidence for a slope gradient control is shown

by this study in Fig. 13, and the present-daynorthern transition from the west-central Floridabarrier-island chain to the Big Bend open shelfand marsh coastline. The plot of sediment thick-ness versus slope gradient in Fig. 13 shows that inall three provinces, where the slope gradient risesabove a 1:1500 limit, the sediment volume in-creases. Where the slope remains £at in the South-ern Province, the sediment thickness trend re-mains relatively constant, but represents asediment source that could have been concen-trated into barriers given the necessary antecedenttopography. Coincident with the thicker o¡shoreaccumulations is the presence of back-barrier fa-cies beneath the sand-ridge facies in vibracores.The barrier-free inner shelf north of the study

area may re£ect a setting similar to the o¡shoreSouthern Province. Decreasing slope gradientsjust north of Anclote Key, which comprises thenorthernmost barrier island of the west-centralFlorida barrier-island chain, may control thenorthern limit of the barrier island system. Thebedrock slope below the northern end of AncloteKey is approximately 1:1500. This slope dimin-ishes to approximately 1:3000 a few kilometersnorth of Anclote Key (estimated from bathymet-ric charts), and is calculated to be 1:5000 at 43km to the north (Hine et al., 1988). This is con-sistent with exceeding the slope gradient thresholdfor barrier-island formation ^ in an area whereadequate sediment resources are available for bar-rier development. Thus, the Holocene sedimentdistribution patterns and the modern extent ofthe west Florida barrier-island tract, together sug-gest a critical threshold value for barrier-islandformation occurs at gradients near 1:1500, suchthat barrier islands will not develop on slopes lessthan 1:1500.

5.2.2. Sand ridgesThe observed o¡shore sediment distribution

patterns raise a key question ^ do these featuresoriginate from deposition associated with theshoreline transgression (such as drowned barrier

islands), the modern hydrodynamic regime, or acombination of these primary and secondary pro-cesses? The results of the study indicate bothmechanisms are important, but that the reworkingby open shelf hydrodynamics appears dominant.Paleo barrier-island deposits have not been iden-ti¢ed in vibracores and appear unlikely to be pre-served as original depositional units. Only back-barrier sedimentary facies have been identi¢edfrom o¡shore areas in the northern portion ofthe study area. However, the legacy of these prob-able barrier^lagoon systems in the north is thegreater volume of o¡shore sediment shown inFig. 13.Perhaps the most similar coastal setting hosting

barrier-island and sand-ridge depositional systemsis the well-studied eastern US Atlantic continentalshelf (Duane et al., 1972; Stubble¢eld et al.,1984). Many east-coast studies have emphasizedbarrier-island retreat and associated models forthe origin of sand ridges that are attributed toshoreline retreat mechanisms ^ including shore-face-attached ridges and ebb-tidal delta retreatpaths (Duane et al., 1972; Swift et al., 1972;McBride and Moslow, 1991; Snedden et al.,1994, 1999). Fewer studies consider open shelfformation unrelated to shoreline retreat (Stubble-¢eld et al., 1984; Swift and Rice, 1984; Tillmanand Martinsen, 1984). Regardless of the initiationmechanisms, the open shelf hydraulic regime isconsidered most important for continuing tobuild, rework, and shape the evolving sand-ridgesystems (Huthnance, 1982; Trowbridge, 1995).Two papers in this volume (Edwards et al.,

2003; Twichell et al., 2003) address in detail pos-sible origins for local sand-ridge development and¢nd evidence both for and against shoreface re-lated origins. From a regional perspective, the in-itial development phase only seems to be re£ectedin the sediment volume now contained within thesand ridges. The distribution of sand ridges on thewest-central Florida shelf suggests that they canform directly on an open, shallow-marine innershelf, without having experienced barrier-islandmigration or erosional shoreface retreat. We envi-sion that the very low-gradient areas of the westFlorida shelf (slopes less than 1:1500) transitionedfrom marsh and seagrass to open-marine environ-

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ments during sea-level transgression without anyintervening high-energy beach transgression. Thisis supported by the vibracore results showing atransition from low- to higher-energy sedimentaryfacies without evidence for a ravinement surface(Edwards et al., 2003). The sand ridges which ex-tend north of the west-central Florida barrier-is-land system, and are located seaward of the open-marine marsh and seagrass-dominated coastline,appear to be developing directly from these earlierlow-energy environments, suggesting a sand-ridgeorigin independent of barrier-island migrationand erosional shoreface retreat. Additionally,many areas within this study area, such as muchof the Southern Province shelf and portions of theNorthern Province in 10^15 m water depth, couldhave evolved similarly.

5.3. Depositional history models

A three-phase model is suggested here for theevolution of the west-central Florida shelf andcoastal system, based on sediment-thickness pat-terns and sedimentary facies patterns summarizedherein and presented in greater detail in compan-ion papers in this volume. The existence of earlierHolocene barrier islands o¡ west-central Floridais not con¢rmed because of reworking by open-marine processes ^ it is inferred based on thepresence of back-barrier facies in cores. As dis-cussed elsewhere, back-barrier and open, low-en-ergy, marsh coastlines can produce similar faciespatterns (Hine et al., 1988; Brooks et al., 2003). Itis most likely that the Florida region experienceda combination of barrier and open shelf/marshcoastlines, both spatially and temporally.Phase I: In the early Holocene (8.3^5.3 ka; see

Brooks et al., 2003), barrier-island development at315^20 m elevation occurred at least in theNorthern province and perhaps extended o¡ thearea of Tampa Bay. A greater abundance ofquartz sand and back-barrier facies across thenorthern shelf regions appears to be consistentwith this pattern. Barrier islands would not haveformed until a few meters of accommodationspace had been created over the areas of steepergradient. These early barriers probably did notmigrate across the shelf but were drowned as

the system became a low-gradient, low-energy,open-marine shelf similar to the present-day BigBend region north of Anclote Key.In the south, lower gradients did not favor bar-

rier-island formation, and back-barrier facies areabsent from the low-gradient southern province.Rather we envision an open rocky or marshcoastal system occurred that supported activebio-erosion of the Miocene bedrock facies to pro-duce the black sand facies now found in the area.The estuarine/bay system may not have existedthen, or was only a broad open embayment.Phase II: During the mid-Holocene (V5^3 ka),

the low-gradient inner shelf experienced gradualdrowning of an open-marine, low-energy, sea-grass-dominated environment, and barrier islandsare less likely to have occurred. During this time,the low-energy seagrass-stabilized open shelf expe-rienced a switch to more active sediment trans-port, with sand ridges forming when the waterdepth exceeded some undetermined depth limit.Due to deeper bedrock elevations in the SouthernProvince, the sand ridges probably began to de-velop earlier by scavenging from limited relictsediment supplies. The barriers o¡shore in thenorth were drowned and underwent reworkingand dispersal. The Tampa Bay embayment devel-oped, and with increasing tidal exchange began tobuild ebb-tidal deposits on the inner shelf at themouth of Tampa Bay.Phase III: Finally, in the late Holocene (V3 ka

to present) the present barrier-island system alongwest-central Florida developed in concert with de-clining rate of sea-level rise combined with ante-cedent topographic control. At this time, an ex-pansion of the Tampa Bay estuarine systemoccurred in combination with shoaling at themouth of Tampa Bay caused by ebb-tidal deposi-tion. The extensive development of sand barriersaround the mouth of Tampa Bay re£ects a redis-tribution of sediments which were trapped withinthe expanding ebb-tidal system as the estuarinetidal prism increased in size.

6. Summary

This regional study of the west-central Florida

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shelf and coastal environments identi¢es thestratigraphic relationships of an onlapping Holo-cene sequence on a low-relief carbonate platformin a mixed carbonate/siliciclastic depositional re-gime. The Holocene sequence is thin and discon-tinuous and is signi¢cantly in£uenced by the ante-cedent topography and composition of the pre-Holocene strata. The present development of thebarrier-island system is probably the most exten-sive it has been during the Holocene. A positivecorrelation between the slope gradient and sedi-ment volume suggests that barrier islands wereprobably absent from low-gradient sectors ofmuch of the southern portion of the study area,and across the middle portion of the northernstudy area. However, thicker sediment accumula-tions o¡shore from Tampa Bay, and to the northo¡shore, probably re£ect accumulation associatedwith early development of barrier islands ^ a sce-nario supported by sediment facies associationsrecovered by vibracoring. Such slope gradientcontrol appears to operate independently ofchanges in rate of sea-level rise or sediment sup-ply.O¡shore, sediment accumulation is commonly

limited to 1^2 m in thickness, while the most sig-ni¢cant sediment depocenter, which reaches 5^8 mthick, is the Tampa Bay ebb-tidal delta. Ba-thymetry and backscatter imagery are an impor-tant guide, in e¡ect a proxy, for predicting near-shore sediment thickness. The o¡shore Holocenesediment is dominated by a quartz-sand facies de-posited under open-marine conditions. Proposedmodels for coastal evolution during the Holocenetransgression suggest a spatial and temporal com-bination of back-stepping barrier-island systemsaccompanied by open-marine low-energy coastalenvironments. The present distribution of sandresources and hard-bottom habitats re£ects thereworking of these earlier deposits by the late-Ho-locene inner-shelf hydraulic regime.

Acknowledgements

The study was funded by the U.S. GeologicalSurvey Center for Coastal Geology and RegionalMarine Studies as part of the West-Central Flor-

ida Coastal Studies Project under project managerGuy Gelfenbaum. We thank the crews and sup-port sta¡ of the research vessels R/V Bellows, R/VSuncoaster (Florida Institute of Oceanography)and R/V Gilbert (U.S. Geological Survey) fortheir assistance. Many ideas and data in this pa-per draw heavily on contributions and discussionswith Richard Davis, David Twichell, Dave Dun-can, Scott Harrison, Jim Edwards, and countlessgraduate students working in this area over theyears. Constructive reviews by Robert Carterand Louis Bartek are greatly appreciated.

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