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Late Holocene estuarine–inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida?
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Marine Geology 200 (2003) 219^241 www.elsevier.com/locate/margeo
Late Holocene estuarine^inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida? B.T. Donahue a , A.C. Hine a; , S. Tebbens a , S.D. Locker a , D.C. Twichell b b
a College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA U.S. Geological Survey, 384 Woods Hole Road, Quissett Campus, Woods Hole, MA 02543-1598, USA
Accepted 1 June 2003
Abstract The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sealevel rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped ^ one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebbtidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf. 1 2003 Elsevier B.V. All rights reserved. Keywords: shelf valley; ebb-tidal delta; shelf evolution; estuarine retreat path
* Corresponding author. Fax: +1-727-553-1189. E-mail addresses: [email protected] (B.T. Donahue), [email protected] (A.C. Hine), [email protected] (S. Tebbens), [email protected] (S.D. Locker), [email protected] (D.C. Twichell).
0025-3227 / 03 / $ ^ see front matter 1 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0025-3227(03)00184-1
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1. Introduction An important component in the development of continental shelves during the sea-level rise since the last glacial maximum, particularly along passive margins, has been the landward retreat of £uvial^deltaic^estuarine systems (e.g. Dalrymple et al., 1994). Although laterally restricted in places, the sur¢cial features generated by the landward tracking of such large paleo-drainage systems such as the Hudson, Delaware, Savannah, Altamaha, and Albemarle Rivers and the suite of rivers associated with the Chesapeake Bay estuary are impressive in bathymetric complexity and cross-shelf continuity. Indeed, features such as estuarine retreat shoals/blankets, shelf valleys and levees inherited from earlier estuary mouth environments may dominate certain shelf sectors, such as the Georgia Embayment (South Atlantic Bight of Swift, 1976). Equally as important as the distribution of the sur¢cial features is the subsurface lithosomes resulting from the landward translation of £uvial^ deltaic^estuarine systems during sea-level rise. Incised channels, in¢lled with a complex sequence architecture beneath the modern shelf, have been shown clearly to exist (e.g. Kraft et al., 1974; Belknap and Kraft, 1985; Hine and Snyder, 1985; Niedoroda et al., 1985; Penland et al., 1985; Nummedal et al., 1987; Locker and Doyle, 1992; Dalrymple et al., 1994; Riggs et al., 1995). Such areas have been fertile ground for the detection and measurement of ¢ne-scale (millennia) sea-level £uctuations and, conversely, have been used to link the sedimentary response to these £uctuations (e.g. Thomas and Anderson, 1994). However, all of these studies have occurred on the seaward extension of coastal plains, which have supported sizeable, well-developed £uvial drainage systems during sea-level lowstands. Our study area along the west-central coast of peninsular Florida (Fig. 1) is situated in a signi¢cantly di¡erent setting even though it is still classi¢ed as an autochthonous shelf (Swift, 1976) as is the more intensely studied US east coast shelf. Autochthonous shelves are those characterized by rapid transgression and sediment bypassing via shoreface erosion (Swift, 1976, p. 314). Alloch-
thonous shelves experience slow transgression, have signi¢cant river-mouth bypassing, support ¢ne-grained (large mud component), mobile sediment sheets, and have shorefaces that merge imperceptibly with the shallow inner shelf (Swift, 1976, p. 316). Tampa Bay has 1032 km2 of surface area, making it the largest estuary in Florida and one of the largest estuaries in the US. In spite of its areal extent, its average water depth is V 6 4 m (http://www.tbep.org/portrait/fastfacts.html). It has only several small streams feeding directly into it. However, it sits upon an impressive in¢lled valley system, which becomes a shelf valley o¡shore having as much as 40 m of subsurface relief (Herbert, 1985; Duncan, 1993; Ferguson and Davis, 2003; Duncan et al., 2003) and has been mapped approximately 40 km to the west (Fig. 2). This shelf-valley system lies in the middle of an ancient carbonate platform whose western portion is a gently sloping, former carbonate ramp. Peninsular Florida is entirely underlain by limestones, which crop out extensively or lie in the shallow subsurface beneath a thin quartz sanddominated veneer (see Randazzo and Jones, 1997; Hine, 1997; Hine et al., 2003). So the regional setting is unusual and contrasts sharply with the regional setting of other, better-studied shelf-valley, estuarine systems. Furthermore, antecedent topography, developed on this carbonate platform through karstrelated processes, has exerted strong controls on regional sedimentation (Locker et al., 2003). First and foremost is that £uvial-dominated coastal plains did not form on peninsular Florida in spite of its humid climate and relatively high rainfall (V120 cm/yr). The drainage basins on Florida are not large compared to those surrounding the rest of the Gulf of Mexico and those along the US east coast. This is because dominantly carbonate terranes do not support large rivers due to the highly permeable nature of karst features, which may redirect sur¢cial water £ow into the subsurface, particularly during periods of sea-level lowstand (Trudgill, 1985). Thus, along the west-central Florida coast we have the paradox of a large estuarine system situated on top of a large subsurface valley system
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Fig. 1. Location map of this and other survey areas on the West Florida shelf. Gray-scale background is the bathymetric map by Gelfenbaum and Guy (1999).
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MARGO 3352 25-8-03 Fig. 2. Seismic data collected west of Tampa Bay, indicating a shelf-valley system which decreases in relief to the west. Inset is a depth to seismic basement map from Duncan (1993), which indicates bedrock rising to the west.
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with no signi¢cant, modern, connected £uvial drainage system. In fact, the Charlotte Harbor estuary to the south along this same coastal system presents the same paradox (Evans et al., 1989; Evans and Hine, 1991). This apparent contradiction allows us to pose some fundamental questions, which form the basis of this manuscript. First, in the absence of a large modern £uvial drainage system, was there ever a paleo-£uvial drainage system that extended across the west Florida shelf during sea-level lowstands forming an extensive shelf valley carved into the interior of this ancient carbonate platform? Do seismic data reveal the existence of such a system? Second, as sea level rose since the last glacial maximum (Termination 2 deglaciation event) approximately 20 ka, did a paleo-Tampa Bay estuarine system track back up across the shelf leaving behind the tell-tale bathymetry and subsurface lithosomes of an estuarine retreat path? Third, conversely, are Tampa Bay and its associated barrier-island system relatively recent features, having formed at their present position only in the past one thousand years? If so, has the enormous ebb-tidal delta (1.4U108 m3 ; Hine et al., 1986), one of the largest shelf sand bodies in the Gulf of Mexico, recently prograded out onto the shelf? Or, has this sand sheet transgressed across the shelf as part of the estuarine retreat path? Does the modern estuarine system a¡ect sediment transport patterns on the inner shelf beyond the distal ebb-tidal delta? Fourth, how does this shelf sector, located seaward of a major estuary, compare to and contrast with the other well-studied shelf sectors located seaward of barrier islands on the west-central Florida shelf (Locker et al., 2003; Harrison et al., 2003; Twichell et al., 2003)?
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2. Methods The data used for this study consist primarily of remotely sensed geoacoustical data. These data sets include: bathymetric surveys, side-scan sonar surveys, and high-resolution seismic re£ection pro¢ling. In addition, sediment samples were collected using a bottom grab sampler deployed from a small boat and during underwater SCUBA transects. The sediment samples were analyzed with the primary goal of ground-truthing the sidescan sonar imaging. The geoacoustic data sets used are listed in Table 1. 2.1. Bathymetry Historical bathymetric data were previously collected in 1950, 1958, and 1975 in the study area by the National Ocean Service (NOS) to construct nautical charts. The 1950 and 1958 data sets were combined to yield a more complete coverage of the study area. A regional bathymetric map derived from NOS charts is presented in Gelfenbaum and Guy (1999) (Fig. 1). Bathymetric data were directly measured in sub-study areas with a Furuno model 667 echo sounder, which operated simultaneously with the side-scan sonar. Once all data sets were in X, Y, Z format, they were imported into MapInfo software. In MapInfo the areas between data points were interpolated by a nearest-neighbor analysis and coverages were created. The coverages were thematically mapped to generate color bathymetric maps (see Donahue, 1999 for details) for the purpose of detecting estuarine retreat path features. Finally, comparisons were made between maps of di¡erent time slices to see if there was measurable bathymetric change.
Table 1 Geoacoustic data sets used Data set
Date collected
Coverage
Resolution
Seismic survey Bathymetry 100 kHz side scan 500 kHz side scan
1982, 1994 1950, 1958, 1975, 1997 07/18^08/01/97 04/04/98
28.3 km variable 80 km2 2.4 km2
V0.5 m vert. res. variable 1 m/pixel 0.1 m/pixel
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Fig. 3. Main 100 kHz side-scan mosaic with rectangles showing the smaller 500 kHz survey locations. Westernmost rectangle is presented in Fig. 5, easternmost rectangle is presented in Fig. 4.
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2.2. Side-scan sonar data acquisition and processing Elics Delph sonar software was used to acquire and process all side-scan data in the three surveys conducted from a 7.5 m survey vessel equipped with a Trimble Navtrac XL global positioning system (GPS). The GPS was used in conjunction with a Trimble Nav Beacon to receive di¡erential corrections (DGPS). Typically this system has an accuracy of 2^7 m (Valldeperas, personal communication, 1998) in the selective-availability mode. Fig. 3 shows the ¢rst and largest side-scan survey of this study, which covered approximately 80 km2 . The survey was conducted over 10 days, from July 18 to August 1, 1997, and utilized an EGpG model 272-TD single-frequency (100 kHz) tow¢sh. The tow¢sh cable was attached to a ¢xed position on the vessel to limit the variation in position corrections required during processing. The ¢sh was towed approximately 1 m below the sea surface, with only slight variations in depth caused by changes in speed, heave, pitch and roll of the boat. The 100 kHz survey consists of 29 east^west lines of side-scan data, each approximately 12 km long with a full swath width of 300 m, and line spacing of 240 m. The line spacing and swath range allow for 60 m of overlap of adjacent swaths. The overlap provides full coverage for the study area to remedy any navigation errors or corrections for the outer areas of the side-scan record. The second and third surveys were conducted on April 4, 1998, with each covering approximately 1.2 km2 (see box inserts in Fig. 3). These areas were chosen to closely examine some of the major bedforms and other bottom features in the 100 kHz study area. Side-scan data were collected with an EGpG model 272-TD dual-frequency (100/500 kHz) tow¢sh set at 500 kHz. Each area was comprised of 13 north^south lines. The sidescan was con¢gured to collect a full swath of 100 m. Line spacing was 75 m for swath overlap of 25 m. The settings provided high-resolution data that were processed so that each pixel represented 0.1 m. As in the larger survey, the gains and tow¢sh depth were set at the beginning of the
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survey and were not changed. All of the mosaic images have the same shading scale, i.e. the darker the gray scale, the greater the amount of backscatter and the coarser the sedimentary material. This relationship was shown to be true in Harrison et al. (2003). 2.3. Sediment sampling and analysis Surface sediment samples were taken by deploying a clam-shell grab from the sea surface and by SCUBA divers deployed along 10 selected transects. A total of 63 samples were analyzed. Locations were determined by DGPS. Every attempt was made to document the dive areas with still photographs or video. All samples were treated with the same analysis method based on Folk (1980). Grain-size statistics and percent carbonate were generated. 2.4. Seismic data All seismic data utilized were acquired during 1982 and 1994 (Fig. 2). The earlier data, SC8202, were acquired with an ORE electromagnetic ‘boomer’ system using an eight-element EGpG streamer and recorded in analog format. The 1994 (cruise B94N) data were digitally acquired and processed using the Elics Delph2 system. The Elics software was used in conjunction with a Huntec Sea Otter ‘boomer’ sled powered at 135 J and a 10-element ITI hydrophone streamer.
3. Results 3.1. Bathymetry The bathymetry shown in Fig. 1 and the bathymetric changes shown in Donahue (1999) reveal that the dominant bathymetric feature is the distal, arcuate portion of the modern ebb-tidal delta cut by a dredged channel for shipping. This sand deposit grades seaward from 33 to 37 m water depth along a slope of 1.3 m/km (1:770) to the inner shelf along its distal edge over a distance of V3 km. Seaward, the inner shelf grades westward to 313 m water depth in the western study area
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Fig. 4. Detail of side scan with distinct NW^SE oriented sand bodies adjacent to the modern ebb-tidal delta (see Fig. 4). This is a spatially restricted ¢eld of these features in this study area. They do not occur elsewhere and are smaller and much straighter in appearance than the features described by Harrison et al. (2003) and Edwards et al. (2003). Their relationship to the main ebb-tidal delta is enigmatic (500 kHz side scan processed at 0.1 m/pixel).
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Fig. 5. Detail of side scan of lobate sand features located toward the western portion of the study area (see Fig. 3). These are aligned along a NE^SW trend. The areas of lower backscatter (light gray) are elevated and composed of ¢ne sand. These are interpreted as sediment-starved 3D dunes. The darker areas are bathymetrically lower and composed of coarse, small 2D dunes consisting of molluscan shells (500 kHz side scan processed at 0.1 m/pixel).
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Fig. 6. Detail of side-scan imagery showing ubiquitous small 2D dunes surrounding sediment-starved 3D dunes. Note the very distinct boundary between the ¢ne sand of the larger bedforms and the coarse molluscan gravel. The persistence of this sharp boundary indicates that physical processes maintain these features regularly, otherwise this boundary would become di¡used with bioturbation (side scan processed at 0.1 m/pixel).
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Fig. 7. Grain-size data and sample locations from the modern ebb-tidal delta. Side-scan data to the right are from the easternmost portion of the 100 kHz mosaic shown in Fig. 3.
along a £atter slope of 0.8 m/km (1:1250). This sand body measures approximately 8 km alongshore and extends about 6 km seaward. The bathymetric changes of the distal portion of the ebb-tidal delta suggest a slight contraction of this feature as the 38 m isobath is displaced to the east or toward the estuary nearly 1 km in some areas, but appears to be stable in others. There does not appear to be any systematic change in bathymetry on the inner shelf seaward of the ebb-tidal delta over the 39 year time frame. The appearance of the dredged channel and the corresponding dredge spoil deposits are clearly evident in the bathymetric change analysis, indicating that vertical changes on the order of several meters and horizontal changes on the order of 100^200 m can be detected using this methodology. If natural changes of this magnitude occurred they would have been detected by our bathymetric map comparison. However, smaller,
but undetected, bathymetric changes over this time frame probably did occur. Sand ridges, similar to those described by Harrison et al. (2003) and Edwards et al. (2003), are not shown in the bathymetric data. 3.2. Side-scan sonar mosaic The centerpiece of the data in this study is the side-scan mosaic presented in Fig. 3, which reveals complex patterns of light and dark gray shades. Acoustically low backscatter (light gray) represents topographically elevated areas consisting of ¢ne sands (see Section 3.3) and the acoustically dark areas represent topographically lower areas dominated by coarse shells or exposed bedrock similar to that shown in Harrison et al. (2003). From east to west, the mosaic reveals the following features: (1) an acoustically light and featureless modern ebb-tidal delta (Fig. 3),
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Fig. 8. Grain-size data from the deeper sand plain interpreted to be a relict ebb-tidal delta. Note that it is coarser, which is indicative of a larger carbonate content. Side-scan data to the right are from the center of the 100 kHz mosaic located in Fig. 3.
which transitions into (2) a restricted zone of distinct, but small NW^SE oriented sand ridges (Fig. 4), which are unlike the ones to the north; (3) a roughly circular, mostly featureless, 4 km wide zone having acoustic gray tones darker than the active ebb-tidal delta located just to the west of the ebb-tidal delta (Fig. 3); (4) at the west-central portion as well as the south-central portion of the mosaic, complex ¢elds exist of lobate-circular features (10^300 m diameter ; 50 cm relief) of low acoustic scatter surrounded by areas of high acoustic backscatter. The transition between these varying backscatter areas is very sharp, abrupt, and distinct (Figs. 5 and 6). These are interpreted to be sediment-starved 3D dunes (Ashley, 1990; Harrison et al., 2003). The acoustically darker areas are dominated by distinct small 2D dunes having 50 cm spacings and heights of 10 cm and whose crests trend N^S in very straight lines for many 10s to 100s of meters (Fig. 6). These features are identical to those small 2D dunes seen in
the sand-ridge ¢eld described by Harrison et al. (2003). 3.3. Sediments and benthic community Surface-grab samples from the modern ebb-tidal delta are ¢ne sands (mostly within 2^3 P interval) and 100% fall within the well to very well sorted category (Fig. 7). Percent CaCO3 was very low, varying from 1.0% to 7.7%, averaging 3.17%. This invariant nature of the surface sediment texture and composition also reveals itself in the evenness of the acoustic backscatter intensity shown in the side-scan data (Figs. 3 and 7). The roughly circular, mostly featureless 4 km wide zone just west of the active ebb-tidal delta is also a broad sand plain with similar grain-size textural and compositional characteristics (Fig. 8) as most of the sediments are ¢ne sands falling within the 2^3 P interval. However, due to a higher carbonate content (3.9^72%, averaging 12.5%),
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Fig. 9. Grain-size data from the sediment-starved sand waves and the surrounding coarser area. Side-scan data are a portion from the side scan shown in Fig. 5, where the dive transect was completed.
the sediment was less well sorted, with 92% of the samples being well sorted to moderately well sorted and 8% being poorly sorted. Within this group of samples, there is a trend of increasing CaCO3 content with increasing distance o¡shore. The increased CaCO3 content in this area could explain that, although acoustically featureless in side-scan data, the area does yield a darker pattern indicative of slightly higher acoustic backscatter intensity. A bottom-sampling strategy whereby divers obtained surface sediments along speci¢c transects to cross zones of highly contrasting side-scan backscatter intensity was conducted to determine the sedimentological cause of closely juxtaposed areas of high backscatter contrast (Fig. 9). As expected, samples from the high-backscatter (dark) area were distinctly coarser and consisted of high concentrations of molluscan shells and a hash of shell fragments. These shells were mostly Chione cancellata and Anadara transversa with some Atrina serrata, which were commonly arranged in the distinct, small 2D dunes. Also within the high acoustic backscatter area were hard-
ground areas ranging in size from 150 m2 to 3.4 km2 . The hard-grounds typically have 40 cm relief within pock-marked karstic limestone composed of friable, coquina-like rock that possibly had been phosphatized. These hard-bottoms attract ¢sh populations and support small corals, sponges (Xetospongia muta), gorgonians (Telesto riisei), nudibranchs (Hypselodoris edenticulata), and sea whips (Leptogorgia virgulata). To illustrate the correlation between side-scan acoustic intensity and sediment data, Fig. 10 was constructed to summarize the observation that low acoustic intensity was generally associated with the elevated, broad, ¢ne quartz sand plains having little carbonate content. The high acoustic backscatter areas were associated with the bathymetrically lower patches of higher carbonate content of coarser particles (molluscan shells). 3.4. Seismic data Previous seismic data have delineated the geometry and size of the shelf valley that lies beneath the Tampa Bay estuary extending to approxi-
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Fig. 10. Overall map of grain-size distribution showing broad spatial trends controlled by the presence of the modern and relict ebb-tidal delta.
mately 40 km seaward (Fig. 2 ; Duncan et al., 2003). Further west on the middle shelf, no data have been collected to map the full seaward extent of this shelf valley. However, our most seaward seismic line suggests that this basin disappears. Data from Duncan et al. (2003), also show that basin relief is reduced toward the west. Seismic lines cross the side-scan mosaic and reveal that the modern ebb-tidal delta and the sand plain to the west as well as the remaining sand cover rest upon a prominent re£ector which we interpret to be a limestone unit (Pleistocene?) capping the shelf valley (Fig. 11A,B). To the north, opposite Indian Rocks headland, the underlying limestones are most likely the Arcadia Formation of the Hawthorn Group (Green et al., 1995). However, this limestone cap is undoubtedly younger, probably Quaternary in age, and forms the scarps and ledges described by Obrochta et al. (1998, 2003).
Seismic Line 10 (Fig. 11A) is a W^E line, which traverses the inner-shelf, distal ebb-tidal boundary and runs across this sand body at the eastern end of this line. Here, the modern ebb-tidal delta is approximately 5 m thick and reveals some internal seismic re£ectors. The topography of the restricted zone of NW^SW trending sand ridges (Fig. 4) seen in the side-scan mosaic at the transition between the distal ebb-tidal delta and inner shelf is revealed in this seismic line and has about 1 m relief. At the extreme western end of this line, exposed hard-bottom is shown. Line SC84 (Fig. 11B) runs N^S, crosses Line 10, and illustrates the northward prograding clinoforms that have in¢lled the shelf valley. This line also shows several sinkhole deformational features, the dredged main shipping channel leading into Tampa Bay, a buried channel thalweg (3^ 4 m relief, 250 m wide) located just to the north of the dredged channel, and ¢nally, the £at, thin
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MARGO 3352 25-8-03 Fig. 11. (A) Seismic re£ection pro¢le of Line 10 oriented W^E (see Fig. 2 for location) illustrates the thin sediment cover in the most distal portion of the study area, the sand plain interpreted to be a relict ebb-tidal delta, some small sand ridges, and ¢nally, to the east, the distal portion of the modern ebb-tidal delta. Note the overall thin nature of the modern sedimentary cover with the modern ebb-tidal delta being the thickest. (B) Seismic re£ection pro¢le of Line SC84 oriented N^S (see Fig. 2 for location) illustrates the subsurface basin or shelf valley lying beneath the study area. One expanded view shows prograding clinoforms, indicating extensive lateral in¢lling that occurred in the Neogene. The other expanded view shows a buried channel that might represent paleo-£uvial drainage occurring during the late Pleistocene lowstand.
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(V2 m thick) featureless sand plain. The northern end of seismic line SC84 also reveals a marked rise in a re£ector to the north, which crops out on the sea£oor (not shown in Fig. 11B). This re£ector is limestone, probably of the Tampa Member of the Arcadia Formation (Hawthorn Group; Green et al., 1995), and its abrupt rise to the sea£oor marks the northern boundary of the shelf valley (Fig. 2).
4. Discussion 4.1. Interpretation of seabed features As mentioned above, the elevated sand plain on the easternmost and shallowest portion of the study area is clearly the modern ebb-tidal delta. Large ebb-tidal deltas are integral components of large inlet systems with large tidal prisms (Hayes, 1975; Nummedal et al., 1977). The bathymetric data show that a distinct boundary exists between the distal portion of this sand body and the inner shelf, except where the side-scan imagery reveals the restricted area of NW^SE trending sand ridges (Fig. 11A). These sand ridges appear quite di¡erent from the ones described in Harrison et al. (2003) in that they are straighter, more precisely aligned, more restricted in spatial extent, smaller, and not covered with multiple-scale bedforms. Nevertheless, they could represent ridges forming from ebb-tidal deltas ^ a process described by McBride and Moslow (1991). However, if this were a dominant process by which shoreline-oblique sand ridges are generated, why is not the entire distal margin of this huge ebbtidal delta marked by these sand bodies? In fact, there are no other modern sand ridges o¡ ebbtidal deltas along the entire west-central barrier island coast of Florida that transition into the inner shelf through a series of linear sand ridges. Conversely, there were probably very few inlets associated with the shoreline o¡ Indian Rocks headland, yet the inner shelf there is covered with sand ridges. The deeper, more carbonate-rich, featureless sand plain seaward of the modern ebb-tidal delta could be an earlier ebb-tidal delta that became
stranded on the inner shelf and is now relict. It is much thinner (2 m vs. s 5 m) than the active delta and smaller in area (by about 75%), suggesting that it was not active long enough to accumulate the volume of sand equal to the modern system. Its increased percentage in molluscan shell hash (Fig. 8) could be due to its older age, whereby this carbonate-producing infauna had more time to contribute its skeletal material to the overall mass of the sand body. If this sand plain is an older ebb-tidal delta, it formed earlier in the late Holocene marine transgression as a proto-Tampa Bay estuary was forming. The establishment of the modern ebb-tidal delta might represent a distinct back-step in the estuarine system and not a smooth, eastward horizontal translation. Possible causes of such an estuarine back-step include a sudden sea-level jump (Blanchon and Shaw, 1995) or changes in sand supply recon¢guring barrier islands at or near the mouth of the estuary forced by wave climate changes. The lobate sand features (Figs. 5 and 6) found in the western part of the side-scan mosaic are interpreted to be sediment-starved 3D dunes, which probably respond to £ows from a variety of directions, although their apparent alignment suggests dominant £ows from the NW. These features are similar to the sediment-starved 3D dunes described by Harrison et al. (2003). The distinct boundary between the ¢ne sand composing these bedforms and the coarse, shelly, much smaller 2D dunes surrounding them, formed by wave oscillatory motions on the seabed, suggests that sediment movement is active at least on a seasonal basis (Fig. 6). Otherwise, the small 2D dunes and the distinct textural boundary would be blurred by active bioturbation. Both of these bedform types respond to shelf processes and are not associated with ebb or £ood tidal currents emanating from or £owing towards Tampa Bay. 4.2. E¡ect of modern estuarine system on inner shelf What is striking about the inventory of features described above is the overall sediment-starved nature of this shelf sector seaward of Tampa Bay and the lack of tide-dominated, linear, sinu-
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Fig. 12.
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Fig. 12 (Continued).
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ous, and paired sand bodies commonly associated with the mouths of other large estuarine systems on the immediately adjacent inner shelf. The modern ebb-tidal jet seems not to have any in£uence on the inner shelf beyond the distal margins of the ebb-tidal delta. In fact, the overall lack of seaward-oriented bedforms, except in the dredged channel at the more proximal locations of this sand body, suggests that the ebb-tidal delta is no longer building seaward. The bathymetric change data (Donahue, 1999) indicate that it, in fact, might be contracting at least as far as its distal boundary is concerned. However, volume measurements (Hine et al., 1986) based on past bathymetric charts indicate that this sand body continues to accumulate sediments through aggradation and not progradation. So the western portion of the side-scan survey mosaic, dominated by these sediment-starved 3D dunes, probably responds to inner-shelf dynamics and is devoid of any in£uence from the modern estuary. 4.3. Estuarine retreat path? The distribution of sedimentary features shown in the bathymetry and side-scan sonar and the overall lack of sedimentary cover suggests that there is no paleo-estuarine retreat path. The subsurface data also indicate that one probably did not exist during the last rise of sea level. A strong seismic re£ector beneath the thin, modern sedimentary cover and the presence of limestone hard-bottom (Fig. 12B) both indicate that the shelf valley, so prominently seen in seismic data, was capped. As a result, very little accommodation space (V2^5 m) was available to create a deepwater paleo-estuarine embayment out on the
237
present inner shelf as sea level rose. This is consistent with the present average depth of the modern Tampa Bay, which is about 5 m. Duncan et al. (2003) also indicate that most of the in¢lling of the subsurface shelf valley occurred well before the last glacial maximum. Their data and those of Hebert (1985) also indicate that the shelf valley decreases in relief to the west. Although we have few seismic data farther seaward of these datasets, it appears that the shelf valley essentially disappears or becomes very broad, having subsurface relief, perhaps, of no more than a few meters. From these observations, we suggest that the shelf valley is cross-shelf-restricted, does not extend westward to some sealevel lowstand point, but is contained to a regional depression in the middle portion of the carbonate platform. We further speculate that beneath the Tampa Bay area is a restricted zone of deepseated limestone dissolution resulting in stratigraphic collapse of the overburden, creating a spatially con¢ned, shallow basin at the ground surface. Although this basin has as much as 40 m of subsurface relief, the dimension of the Tampa Bay basin prior to Holocene £ooding was approximately 60 km in its longest direction with an average relief (now depth) of V5 m ^ a very broad, and a very shallow depression. Regardless of origin, the Tampa Bay area, including the inner shelf immediately to the west, was a lowrelief, sur¢cial depression before the last rise in sea level. Such a depression would have formed a wide, but shallow, embayment in the coastline as sea level was rising. It was this sur¢cial basin that caused local streams to £ow into it and out onto the shelf during sea-level lowstand. As has been mentioned,
Fig. 12. (A^D) The theoretical development of the inner shelf, Tampa Bay estuary, and ebb-tidal delta system. Panel A, around 10^11 ka, shows a shoreline consisting of low, overwash-dominated, transgressing barrier islands with sand ridges forming seaward. The mangrove/marsh mainland lagoon shoreline has intercepted the slight topographic depression (Tampa Bay Basin) lying on top of the buried shelf valley forming a slight embayment. The area of the modern Tampa Bay and area seaward is a freshwater swamp surrounded by uplands vegetated by trees. Panel B, around 8^9 ka, shows that the coastline has transgressed towards the east, with subtidal/intertidal swash bars and very low barrier islands forming the coastline. The embayment and headland morphology has become more prominent. Panel C, around 5^8 ka, shows further transgression. However, a small barrier island or swash-bar system has formed or migrated into the embayment, focusing an ebb-jet. This ebb-jet may create the lower sand plain, which we interpret to be a relict ebb-tidal delta. In panel D, around 3 ka, sea level has slowed down, the modern barrier system is becoming established. The modern ebb-tidal delta starts to prograde seaward.
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present-day streams are relatively small and probably were even smaller during the last sea-level transgression, as Florida was much more arid during the very early Holocene than today (Grimm et al., 1993; Wright, 1995). We suggest that the channel thalweg seen in Fig. 11B might have been the full extent of seaward-£owing stream activity when this portion of the inner shelf was subaerially exposed. Its cross-sectional dimension of V4 m of relief and V250 m in width is much larger than any single stream now £owing into the modern Tampa Bay. Thus, we speculate that Tampa Bay and the area just to the west was probably no more than a broad, very shallow depression that contained freshwater wetlands through which very small rivers passed, draining the central interior of peninsular Florida. This depression did not extend westward very far across the shelf and the rivers dissipated into smaller streams, creating thalwegs below seismic resolution, or simply disappeared down sinkholes ^ a common fate of streams in carbonate karsti¢ed terrain (Trudgill, 1985). 4.4. Late Holocene sea-level £ooding model Using a local, relative sea-level curve assembled by Wright (1995) and the Barbados curve (Fairbanks, 1989) as well as water depth and shelf gradient, we present a paleogeography/paleoenvironmental scenario depicting how this inner shelf developed during the last deglacial rise in sea level (Fig. 12A^C). Fig. 12A occurs at approximately 10^11 ka, when sea level was about 330 m below present and the shoreline was located some 50 km seaward (west) of its present position. Given an average sea-level rate of rise at that time (V10 m/1000 yr; Fairbanks, 1989) and the average gradient (1:2000), the eastward shoreline translation was approximately 20 km/1000 yr, transgressing a subaerial surface having a thin, quartz-sand veneer covering limestone bedrock. The coastline was probably a mixture of ephemeral washover-dominated barrier islands and/or subtidal swash bars. Perhaps a small river emanating from the upland Tampa Bay area discharged along this coast similar to the modern Suwannee River (Wright,
1995). More importantly, this coastline was oriented N^S and was approximately straight, not having signi¢cant headlands or embayments. A slight embayment seaward of the modern Tampa Bay might have started to form. The mid-platform basin (Tampa Bay Basin) occupying the Tampa Bay area had not yet been occupied or £ooded by the rising marine waters and was probably a freshwater wetland as mentioned above. By V8^9 ka (Fig. 12B), the marine transgression had reached the mid-platform basin, £ooding it and creating a broad embayment in the regional shoreline trend. The broad, elevated high forming Indian Rocks headland de¢ned the northern boundary of this embayment. As sea level rose, the embayment extended eastward very quickly as marine waters £ooded the basin. This was the proto-Tampa Bay, which probably did not have strong tidal £ow as the mouth was not constricted as it is today, and probably experienced very low wave energy as it was, essentially, a broad, shallow shelf embayment. Seagrass meadows probably dominated the embayment’s £oor and mangroves dominated the embayment’s shoreline. For this reason, no sand ridges formed, thus making it distinctive from the evolving shelves to the north and south. The development of the lower sand plain as an early precursor to the modern ebb-tidal delta is somewhat of an enigma. It lies at about 38 m water depth, whereas the top of the modern ebb-tidal delta lies at about 4^5 m depth. If subaqueous, subtidal ebb-tidal deltas build to an equilibrium depth and the modern system seaward of Tampa Bay is currently at this depth, then the sand plain must have been an active ebb-tidal delta approximately 5 ka. This was a period of deceleration in the rate of sea-level rise. Hence, the mouth of the estuary could have become partially closed o¡ by barrier islands, thus focusing the ebb-jet (Fig. 12C). This incipient barrier-island system could have stepped back to its modern position as a result of local changes in sand supply. Or, perhaps the relict ebb-tidal is older and was abandoned around 7.6 ka as a result of a sudden increase in the rate of sea-level rise (Blanchon and Shaw, 1995). This would have drowned the local coastal system and forced the
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estuarine mouth to retreat landward soon after the ¢rst ebb-tidal delta had formed. Either scenario, regardless of timing, would have left this sand body stranded and inactive on the inner shelf, never to obtain the size and thickness of its modern counterpart. It has become relatively enriched in molluscan shelly gravel through time by continuing to support a benthic infaunal molluscan community. By V3 ka (Fig. 12D), the rate of sea-level rise had slowed and sea level had nearly reached its current elevation. In response, the present-day barrier-island coastline was becoming established. The barriers changed from being less transgressive to more progradational/aggradational (Stapor and Mathews, 1980). The longshore transport system began to build spits into the broad mouth of Tampa Bay, restricting it laterally. Barrier islands became established on these spits. With this restriction came the focusing of the ebb-tidal jet, which started to develop a modern ebb-tidal delta. We speculate that this sand body prograded seaward and aggraded to its present depth as the barrier islands were forming around the mouth of Tampa Bay. Today, there is no obvious evidence of a sediment transport pathway (i.e. no persistent seaward-oriented bedforms) to continue to extend the ebb-tidal delta seaward, so it remains a huge subtidal sand body modi¢ed by the ambient wave climate, and acts as a signi¢cant wave-refracting agent controlling wave-energy £ux to the adjacent beaches. However, the net longshore sand transport continues today to introduce sand to the £anks of the ebb-tidal delta, probably making this an area of continued sand-body growth (Hine et al., 1986). 4.5. Comparison to other shelf sectors The inner shelf seaward of Tampa Bay is fundamentally di¡erent from the inner shelf o¡ the Indian Rocks headland (Harrison et al., this volume; Edwards et al., this volume) to the north. It is also fundamentally di¡erent from the inner shelf o¡ Sarasota (Twichell et al., this volume) to the south, indicating that distinct shelf sectors or provinces can be de¢ned. Seaward of Tampa
239
Bay are no closely spaced, narrow (V500 m), bedform-dominated, well-oriented NW^SE sand ridges as seen to the north. The boundary between these shelf provinces is de¢ned by the northern margin of the shelf valley (Fig. 2). As mentioned earlier, these well-de¢ned sand ridges could not form in the broad, shallow, low-energy embayment that occupied the area seaward of Tampa Bay during the mid-Holocene. See Brooks et al. (2003) and Locker et al. (2003) for a broader discussion on shelf sedimentology and geology variability.
5. Conclusions There was not a large paleo-£uvial system, nor a high-relief valley system that extended across the exposed west Florida shelf during the last sea-level lowstand. Instead, a broad mid-platform depression existed in the proto-Tampa Bay area with its western boundary located probably within 40 km west of the present-day estuary. As sea level started to rise after the last glacial maximum, the shoreline, starting at the 3120 m isobath, transgressed rapidly across the shelf but did not track up a shelf-valley system until it reached the western, most seaward limit of the mid-platform depression. Hence, there is no estuarine retreat path on the west Florida shelf well seaward of Tampa Bay similar to those described on the shelf o¡ the US east coast. The inner shelf seaward of Tampa Bay is dominated by two sand bodies, the present-day modern ebb-tidal delta and a more distal and deeper sand plain, which we interpret to be an older, but short-lived ebb-tidal delta. The modern ebb-tidal delta has progaded out onto the inner shelf during the past few thousand years as the rate of sealevel rise decelerated, the modern barrier island system formed and narrowed the mouth of Tampa Bay. This, in turn, focused the ebb-jet, which exported sands seaward to form the ebb-tidal delta. A model of sea-level rise, coastal retreat, and shelf evolution is presented in the text and not summarized here. The most seaward part of the study area consists of sediment-starved 3D dunes surrounded by coarse, molluscan shelly material
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and/or exposed hard-bottom. These bedforms respond to modern shelf hydraulics and do not appear to be in£uenced by the modern estuarine system. The inner shelf seaward of Tampa Bay did not form sand ridges as seen in the other two provinces, located to the north and to the south. Instead, a broad, shallow, low-energy embayment, forming proto-Tampa Bay evolved into the modern Tampa Bay and was not conducive to creating such sand bodies.
Acknowledgements This work was funded by the US Geological Survey’s Center for Coastal and Regional Marine Studies in St. Petersburg, FL, as part of the WestCentral Florida Coastal Studies Project. This work was also partially funded by the O⁄ce of Naval Research Contract #N00014-96-1-5032. We thank Drs. Guy Gelfenbaum and Terry Edgar, both of the USGS, for their program management skills in running this project. We thank Mark Hafen for enormous assistance in the ¢eld as well as in the computer lab. We also thank Dr. David Naar and Dr. David Duncan for thoughtful advice and technical assistance. We appreciate the graphics assistance provided by Chad Edmisten. Finally, we thank Drs. Charles (Chip) Fletcher and Steven Goodbred for providing key advice. References Ashley, G.M., 1990. Classi¢cation of large-scale subaqueous bedforms: a new look at an old problem. J. Sediment. Petrol. 60, 160^172. Belknap, D.F., Kraft, J.C., 1985. In£uence of antecedent geology on stratigraphic preservation potential and evolution of Delaware’s barrier systems. Mar. Geol. 63, 235^262. Blanchon, P., Shaw, J., 1995. Reef drowning during the last deglaciation: evidence for catastrophic sea-level rise and icesheet collapse. Geology 23, 4^8. Brooks, G.R., Doyle, L.J., Suthard, B.C., Locker, S.D., Hine, A.C., 2003. Facies architecture of a mixed carbonate/siliciclastic inner continental shelf of west-central Florida: Implications for Holocene barrier development. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00190-7, this issue.
Dalrymple, R.W., Boyd, R., Zaitlin, B.A., 1994. Incised-valley systems: origin and sedimentary sequences. SEPM Spec. Publ. No. 51. Soc. Sediment. Geol., Tulsa, OK. Donahue, B.T., 1999. Modern morphodynamics and Holocene evolution of an inner shelf/estuarine transition zone on the west Florida shelf. M.S. Thesis, Department of Marine Science, University of South Florida, St. Petersburg, FL. Duncan, D.S., 1993. Neogene to Recent seismic stratigraphy of the lower Tampa Bay estuary: West-Central Florida. M.S. Thesis, Department of Marine Science, University of South Florida, St. Petersburg, FL. Duncan, D.S., Locker, S.D., Brooks, G.R., Hine, A.C., Doyle, L.J., 2003. Mixed carbonate-siliciclastic in¢lling of a Neogene carbonate shelf valley system: Tampa Bay, west-central Florida. Mar. Geol. 200, doi:10.1016/S00253227(03)00180-4, this issue. Edwards, J.H., Harrison, S.E., Locker, S.D., Hine, A.C., Twichell, D.C., 2003. Stratigraphic framework of sedimentstarved sand ridges on a mixed siliciclastic/carbonate inner shelf o¡ west-central Florida. Mar. Geol. 200, doi:10.1016/ S0025-3227(03)00183-X, this issue. Evans, M.W., Hine, A.C., Belknap, D.F., 1989. Quaternary stratigraphy of the Charlotte Harbor estuarine-lagoon system, southwest Florida: Implications of the carbonate-siliciclastic transition. Mar. Geol. 88, 319^348. Evans, M.W., Hine, A.C., 1991. Late Neogene sequence stratigraphy of a carbonate-siliciclastic transition: southwest Florida. Geol. Soc. Am. Bull. 103, 679^699. Fairbanks, R.G., 1989. A 17,000 year glacio-eustatic sea-level record: in£uence of glacial melting rates on the Younger Dryas Event and deep ocean circulation. Nature 342, 637^ 642. Ferguson, T.W., Davis, R.A. Jr., 2003. Post-Miocene stratigraphy and depositional environments of valley-¢ll sequences at the mouth of Tampa Bay, Florida. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00181-6, this issue. Folk, R.L., 1980. Petrology of Sedimentary Rocks. Hemphill Publ. Co., Austin, TX. Gelfenbaum, G., Guy, K., 1999. Bathymetry of west-central Florida. USGS Open File Report 99-417. Green, R., Arthur, J.D., DeWitt, D., 1995. Lithographic and hydrostratigraphic cross section through Pinellas and Hillsborough Counties, southwest Florida. Open File Report 61, Florida Geol. Surv., Tallahassee, FL. Grimm, E.C., Jacobson, G.L., Watts, W.A., Hansen, B.C.S., Maasch, K.A., 1993. A 50,000 year record of climate oscillations from Florida and its temporal correlation with the Heinrich Events. Science 261, 198^200. Harrison, S.E., Locker, S.D., Hine, A.C., Edwards, J.H., Naar, D.F., Twichell, D.C., 2003. Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf o¡ west-central Florida. Mar. Geol. 200, doi:10.1016/ S0025-3227(03)00182-8, this issue. Hayes, M.O., 1975. Morphology of sand accumulations in estuaries. In: Cronin, L.E. (Ed.), Estuarine Research, vol. 2, Geology and Engineering. Academic Press, New York, pp. 3^22.
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B.T. Donahue et al. / Marine Geology 200 (2003) 219^241 Hebert, J.A., 1985. High-resolution seismic stratigraphy of the inner west Florida shelf, west of Tampa Bay. M.S. Thesis, Department of Marine Science, University of South Florida, St. Petersburg, FL, 52 pp. Hine, A.C., 1997. Structural and paleoceanographic evolution of the margins of the Florida Platform. In: Randazzo, A.F., Jones, D.S. (Eds.), The Geology of Florida. University Press of Florida, Gainesville, FL, pp. 169^194. Hine, A.C., Brooks, G.R., Davis, R.A., Jr., Duncan, D.S., Locker, S.D., Twichell, D.C., Gelfenbaum, G., 2003. The west-central Florida inner shelf and coastal system: A geologic conceptual overview. Mar. Geol. 200, doi:10.1016/ S0025-3227(03)00161-0, this issue. Hine, A.C., Snyder, S.W., 1985. Coastal lithosome preservation: Evidence from the shoreface and inner continental shelf o¡ Bogue Banks, North Carolina. Mar. Geol. 63, 307^330. Hine, A.C., Mearns, D.L., Davis, R.A., Jr., Bland, M., 1986. Impact of Florida’s Gulf coast inlets on the coastal sand budget. Final Report to The Division of Beaches and Shores, Florida Dept. of Natural Resources, Tallahassee, FL. Kraft, J.C., Sheridan, R.E., Moose, R.D., Strom, R.N., Weil, C.B., 1974. Middle-late Holocene evolution of the morphology of a drowned estuary system - the Delaware Bay. In: International Symposium on Interrelationships of Estuarine and Continental Shelf Sedimentation, Institute Geol. Du Bassin d’Aquitaine, July 9^14, 1973, pp. 297^ 306. Locker, S.D., Doyle, L.J., 1992. Neogene to recent stratigraphy and depositional regimes of the northwest Florida inner continental shelf. Mar. Geol. 104, 123^138. Locker, S.D., Hine, A.C., Brooks, G.R., 2003. Regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west-central Florida. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00191-9, this issue. McBride, R.A., Moslow, T.F., 1991. Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A. Mar. Geol. 97, 57^85. Niedoroda, A.W., Swift, D.J.P., Figueredo, A.G., Jr., Freeland, G.L., 1985. Barrier island evolution, middle Atlantic shelf, USA part II: Evidence from the shelf £oor. Mar. Geol. 63, 363^396. Nummedal, D., Oertel, G.F., Hubbard, D.K., Hine, A.C., 1977. Tidal inlet variability - Cape Hatteras to Cape Ca-
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naveral. Coastal Sediments ’77 Proceedings, 5th Symposium. Am. Soc. Civ. Eng., Charleston, SC, pp. 543^562. Nummedal, D. Pilkey, O.H., Howard, J.D., 1987. Sea-level £uctuation and coastal evolution. SEPM Spec. Publ. No. 41. Society for Sedimentary Geology, Tulsa, OK. Obrochta, S.P., Duncan, D.S., Brooks, G.R., 1998. Hardbottoms of the inner West-Central Florida continental shelf. Trans. Gulf Coast Assoc. Geol. Soc. 48, pp. 312^319. Obrochta, S.P., Duncan, D.S., Brooks, G.R., 2003. Hardbottom development and signi¢cance to the sedimentstarved, west-Florida inner continental shelf. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00188-9, this issue. Penland, S., Suter, J.R., Boyd, R., 1985. Barrier island arcs along abandoned Mississippi River deltas. Mar. Geol. 63, 197^233. Randazzo, A.F., Jones, D.S., 1997. The Geology of Florida. University Press of Florida, Gainesville, FL, 327 pp. Riggs, S.R., Cleary, W.J., Snyder, S.W., 1995. In£uence of inherited geologic framework on barrier shoreface morphology and dynamics. Mar. Geol. 126, 213^234. Stapor, F.W., Mathews, T.D., 1980. C-14 chronology of Holocene barrier islands, Lee County, Florida: a preliminary report. In: Tanner, W.F. (Ed.), Shorelines Past and Present. Department of Geology, Florida State University, Tallahassee, FL, pp. 47^67. Swift, D.J.P., 1976. Continental shelf sedimentation. In: Stanley, D.J., Swift, D.J.P. (Eds.), Marine Sediment Transport and Environmental Management. Wiley, New York, pp. 311^350. Thomas, M.A., Anderson, J.B., 1994. Sea-level controls on the facies architecture of the Trinity/Sabine incised-valley system, Texas continental shelf, 1994. In: Dalrymple, R.W., Boyd, R., Zaitlin, B.A. (Eds.), Incised-Valley Systems: Origin and Sedimentary Sequences. SEPM Spec. Publ. No. 51, pp. 63^82. Trudgill, S., 1985. Limestone Geomorphology. Longman Group, Essex, 196 pp. Twichell, D., Brooks, G.R., Gelfenbaum, G., Paskevich, P., Donahue, B., 2003. Sand ridges o¡ Sarasota Florida: a complex facies boundary on a low energy, inner shelf environment. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00185-3, this issue. Wright, E.A., 1995. Sedimentation and stratigraphy of the Suwannee River marsh coastline. Ph.D. Thesis, University of South Florida, St. Petersburg, FL.
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Late Holocene estuarine^inner shelf interactions; is there evidence of an estuarine retreat path for Tampa Bay, Florida? B.T. Donahue a , A.C. Hine a; , S. Tebbens a , S.D. Locker a , D.C. Twichell b b
a College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA U.S. Geological Survey, 384 Woods Hole Road, Quissett Campus, Woods Hole, MA 02543-1598, USA
Accepted 1 June 2003
Abstract The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sealevel rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped ^ one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebbtidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf. 1 2003 Elsevier B.V. All rights reserved. Keywords: shelf valley; ebb-tidal delta; shelf evolution; estuarine retreat path
* Corresponding author. Fax: +1-727-553-1189. E-mail addresses: [email protected] (B.T. Donahue), [email protected] (A.C. Hine), [email protected] (S. Tebbens), [email protected] (S.D. Locker), [email protected] (D.C. Twichell).
0025-3227 / 03 / $ ^ see front matter 1 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0025-3227(03)00184-1
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1. Introduction An important component in the development of continental shelves during the sea-level rise since the last glacial maximum, particularly along passive margins, has been the landward retreat of £uvial^deltaic^estuarine systems (e.g. Dalrymple et al., 1994). Although laterally restricted in places, the sur¢cial features generated by the landward tracking of such large paleo-drainage systems such as the Hudson, Delaware, Savannah, Altamaha, and Albemarle Rivers and the suite of rivers associated with the Chesapeake Bay estuary are impressive in bathymetric complexity and cross-shelf continuity. Indeed, features such as estuarine retreat shoals/blankets, shelf valleys and levees inherited from earlier estuary mouth environments may dominate certain shelf sectors, such as the Georgia Embayment (South Atlantic Bight of Swift, 1976). Equally as important as the distribution of the sur¢cial features is the subsurface lithosomes resulting from the landward translation of £uvial^ deltaic^estuarine systems during sea-level rise. Incised channels, in¢lled with a complex sequence architecture beneath the modern shelf, have been shown clearly to exist (e.g. Kraft et al., 1974; Belknap and Kraft, 1985; Hine and Snyder, 1985; Niedoroda et al., 1985; Penland et al., 1985; Nummedal et al., 1987; Locker and Doyle, 1992; Dalrymple et al., 1994; Riggs et al., 1995). Such areas have been fertile ground for the detection and measurement of ¢ne-scale (millennia) sea-level £uctuations and, conversely, have been used to link the sedimentary response to these £uctuations (e.g. Thomas and Anderson, 1994). However, all of these studies have occurred on the seaward extension of coastal plains, which have supported sizeable, well-developed £uvial drainage systems during sea-level lowstands. Our study area along the west-central coast of peninsular Florida (Fig. 1) is situated in a signi¢cantly di¡erent setting even though it is still classi¢ed as an autochthonous shelf (Swift, 1976) as is the more intensely studied US east coast shelf. Autochthonous shelves are those characterized by rapid transgression and sediment bypassing via shoreface erosion (Swift, 1976, p. 314). Alloch-
thonous shelves experience slow transgression, have signi¢cant river-mouth bypassing, support ¢ne-grained (large mud component), mobile sediment sheets, and have shorefaces that merge imperceptibly with the shallow inner shelf (Swift, 1976, p. 316). Tampa Bay has 1032 km2 of surface area, making it the largest estuary in Florida and one of the largest estuaries in the US. In spite of its areal extent, its average water depth is V 6 4 m (http://www.tbep.org/portrait/fastfacts.html). It has only several small streams feeding directly into it. However, it sits upon an impressive in¢lled valley system, which becomes a shelf valley o¡shore having as much as 40 m of subsurface relief (Herbert, 1985; Duncan, 1993; Ferguson and Davis, 2003; Duncan et al., 2003) and has been mapped approximately 40 km to the west (Fig. 2). This shelf-valley system lies in the middle of an ancient carbonate platform whose western portion is a gently sloping, former carbonate ramp. Peninsular Florida is entirely underlain by limestones, which crop out extensively or lie in the shallow subsurface beneath a thin quartz sanddominated veneer (see Randazzo and Jones, 1997; Hine, 1997; Hine et al., 2003). So the regional setting is unusual and contrasts sharply with the regional setting of other, better-studied shelf-valley, estuarine systems. Furthermore, antecedent topography, developed on this carbonate platform through karstrelated processes, has exerted strong controls on regional sedimentation (Locker et al., 2003). First and foremost is that £uvial-dominated coastal plains did not form on peninsular Florida in spite of its humid climate and relatively high rainfall (V120 cm/yr). The drainage basins on Florida are not large compared to those surrounding the rest of the Gulf of Mexico and those along the US east coast. This is because dominantly carbonate terranes do not support large rivers due to the highly permeable nature of karst features, which may redirect sur¢cial water £ow into the subsurface, particularly during periods of sea-level lowstand (Trudgill, 1985). Thus, along the west-central Florida coast we have the paradox of a large estuarine system situated on top of a large subsurface valley system
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221
Fig. 1. Location map of this and other survey areas on the West Florida shelf. Gray-scale background is the bathymetric map by Gelfenbaum and Guy (1999).
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MARGO 3352 25-8-03 Fig. 2. Seismic data collected west of Tampa Bay, indicating a shelf-valley system which decreases in relief to the west. Inset is a depth to seismic basement map from Duncan (1993), which indicates bedrock rising to the west.
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with no signi¢cant, modern, connected £uvial drainage system. In fact, the Charlotte Harbor estuary to the south along this same coastal system presents the same paradox (Evans et al., 1989; Evans and Hine, 1991). This apparent contradiction allows us to pose some fundamental questions, which form the basis of this manuscript. First, in the absence of a large modern £uvial drainage system, was there ever a paleo-£uvial drainage system that extended across the west Florida shelf during sea-level lowstands forming an extensive shelf valley carved into the interior of this ancient carbonate platform? Do seismic data reveal the existence of such a system? Second, as sea level rose since the last glacial maximum (Termination 2 deglaciation event) approximately 20 ka, did a paleo-Tampa Bay estuarine system track back up across the shelf leaving behind the tell-tale bathymetry and subsurface lithosomes of an estuarine retreat path? Third, conversely, are Tampa Bay and its associated barrier-island system relatively recent features, having formed at their present position only in the past one thousand years? If so, has the enormous ebb-tidal delta (1.4U108 m3 ; Hine et al., 1986), one of the largest shelf sand bodies in the Gulf of Mexico, recently prograded out onto the shelf? Or, has this sand sheet transgressed across the shelf as part of the estuarine retreat path? Does the modern estuarine system a¡ect sediment transport patterns on the inner shelf beyond the distal ebb-tidal delta? Fourth, how does this shelf sector, located seaward of a major estuary, compare to and contrast with the other well-studied shelf sectors located seaward of barrier islands on the west-central Florida shelf (Locker et al., 2003; Harrison et al., 2003; Twichell et al., 2003)?
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2. Methods The data used for this study consist primarily of remotely sensed geoacoustical data. These data sets include: bathymetric surveys, side-scan sonar surveys, and high-resolution seismic re£ection pro¢ling. In addition, sediment samples were collected using a bottom grab sampler deployed from a small boat and during underwater SCUBA transects. The sediment samples were analyzed with the primary goal of ground-truthing the sidescan sonar imaging. The geoacoustic data sets used are listed in Table 1. 2.1. Bathymetry Historical bathymetric data were previously collected in 1950, 1958, and 1975 in the study area by the National Ocean Service (NOS) to construct nautical charts. The 1950 and 1958 data sets were combined to yield a more complete coverage of the study area. A regional bathymetric map derived from NOS charts is presented in Gelfenbaum and Guy (1999) (Fig. 1). Bathymetric data were directly measured in sub-study areas with a Furuno model 667 echo sounder, which operated simultaneously with the side-scan sonar. Once all data sets were in X, Y, Z format, they were imported into MapInfo software. In MapInfo the areas between data points were interpolated by a nearest-neighbor analysis and coverages were created. The coverages were thematically mapped to generate color bathymetric maps (see Donahue, 1999 for details) for the purpose of detecting estuarine retreat path features. Finally, comparisons were made between maps of di¡erent time slices to see if there was measurable bathymetric change.
Table 1 Geoacoustic data sets used Data set
Date collected
Coverage
Resolution
Seismic survey Bathymetry 100 kHz side scan 500 kHz side scan
1982, 1994 1950, 1958, 1975, 1997 07/18^08/01/97 04/04/98
28.3 km variable 80 km2 2.4 km2
V0.5 m vert. res. variable 1 m/pixel 0.1 m/pixel
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Fig. 3. Main 100 kHz side-scan mosaic with rectangles showing the smaller 500 kHz survey locations. Westernmost rectangle is presented in Fig. 5, easternmost rectangle is presented in Fig. 4.
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2.2. Side-scan sonar data acquisition and processing Elics Delph sonar software was used to acquire and process all side-scan data in the three surveys conducted from a 7.5 m survey vessel equipped with a Trimble Navtrac XL global positioning system (GPS). The GPS was used in conjunction with a Trimble Nav Beacon to receive di¡erential corrections (DGPS). Typically this system has an accuracy of 2^7 m (Valldeperas, personal communication, 1998) in the selective-availability mode. Fig. 3 shows the ¢rst and largest side-scan survey of this study, which covered approximately 80 km2 . The survey was conducted over 10 days, from July 18 to August 1, 1997, and utilized an EGpG model 272-TD single-frequency (100 kHz) tow¢sh. The tow¢sh cable was attached to a ¢xed position on the vessel to limit the variation in position corrections required during processing. The ¢sh was towed approximately 1 m below the sea surface, with only slight variations in depth caused by changes in speed, heave, pitch and roll of the boat. The 100 kHz survey consists of 29 east^west lines of side-scan data, each approximately 12 km long with a full swath width of 300 m, and line spacing of 240 m. The line spacing and swath range allow for 60 m of overlap of adjacent swaths. The overlap provides full coverage for the study area to remedy any navigation errors or corrections for the outer areas of the side-scan record. The second and third surveys were conducted on April 4, 1998, with each covering approximately 1.2 km2 (see box inserts in Fig. 3). These areas were chosen to closely examine some of the major bedforms and other bottom features in the 100 kHz study area. Side-scan data were collected with an EGpG model 272-TD dual-frequency (100/500 kHz) tow¢sh set at 500 kHz. Each area was comprised of 13 north^south lines. The sidescan was con¢gured to collect a full swath of 100 m. Line spacing was 75 m for swath overlap of 25 m. The settings provided high-resolution data that were processed so that each pixel represented 0.1 m. As in the larger survey, the gains and tow¢sh depth were set at the beginning of the
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survey and were not changed. All of the mosaic images have the same shading scale, i.e. the darker the gray scale, the greater the amount of backscatter and the coarser the sedimentary material. This relationship was shown to be true in Harrison et al. (2003). 2.3. Sediment sampling and analysis Surface sediment samples were taken by deploying a clam-shell grab from the sea surface and by SCUBA divers deployed along 10 selected transects. A total of 63 samples were analyzed. Locations were determined by DGPS. Every attempt was made to document the dive areas with still photographs or video. All samples were treated with the same analysis method based on Folk (1980). Grain-size statistics and percent carbonate were generated. 2.4. Seismic data All seismic data utilized were acquired during 1982 and 1994 (Fig. 2). The earlier data, SC8202, were acquired with an ORE electromagnetic ‘boomer’ system using an eight-element EGpG streamer and recorded in analog format. The 1994 (cruise B94N) data were digitally acquired and processed using the Elics Delph2 system. The Elics software was used in conjunction with a Huntec Sea Otter ‘boomer’ sled powered at 135 J and a 10-element ITI hydrophone streamer.
3. Results 3.1. Bathymetry The bathymetry shown in Fig. 1 and the bathymetric changes shown in Donahue (1999) reveal that the dominant bathymetric feature is the distal, arcuate portion of the modern ebb-tidal delta cut by a dredged channel for shipping. This sand deposit grades seaward from 33 to 37 m water depth along a slope of 1.3 m/km (1:770) to the inner shelf along its distal edge over a distance of V3 km. Seaward, the inner shelf grades westward to 313 m water depth in the western study area
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Fig. 4. Detail of side scan with distinct NW^SE oriented sand bodies adjacent to the modern ebb-tidal delta (see Fig. 4). This is a spatially restricted ¢eld of these features in this study area. They do not occur elsewhere and are smaller and much straighter in appearance than the features described by Harrison et al. (2003) and Edwards et al. (2003). Their relationship to the main ebb-tidal delta is enigmatic (500 kHz side scan processed at 0.1 m/pixel).
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Fig. 5. Detail of side scan of lobate sand features located toward the western portion of the study area (see Fig. 3). These are aligned along a NE^SW trend. The areas of lower backscatter (light gray) are elevated and composed of ¢ne sand. These are interpreted as sediment-starved 3D dunes. The darker areas are bathymetrically lower and composed of coarse, small 2D dunes consisting of molluscan shells (500 kHz side scan processed at 0.1 m/pixel).
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Fig. 6. Detail of side-scan imagery showing ubiquitous small 2D dunes surrounding sediment-starved 3D dunes. Note the very distinct boundary between the ¢ne sand of the larger bedforms and the coarse molluscan gravel. The persistence of this sharp boundary indicates that physical processes maintain these features regularly, otherwise this boundary would become di¡used with bioturbation (side scan processed at 0.1 m/pixel).
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Fig. 7. Grain-size data and sample locations from the modern ebb-tidal delta. Side-scan data to the right are from the easternmost portion of the 100 kHz mosaic shown in Fig. 3.
along a £atter slope of 0.8 m/km (1:1250). This sand body measures approximately 8 km alongshore and extends about 6 km seaward. The bathymetric changes of the distal portion of the ebb-tidal delta suggest a slight contraction of this feature as the 38 m isobath is displaced to the east or toward the estuary nearly 1 km in some areas, but appears to be stable in others. There does not appear to be any systematic change in bathymetry on the inner shelf seaward of the ebb-tidal delta over the 39 year time frame. The appearance of the dredged channel and the corresponding dredge spoil deposits are clearly evident in the bathymetric change analysis, indicating that vertical changes on the order of several meters and horizontal changes on the order of 100^200 m can be detected using this methodology. If natural changes of this magnitude occurred they would have been detected by our bathymetric map comparison. However, smaller,
but undetected, bathymetric changes over this time frame probably did occur. Sand ridges, similar to those described by Harrison et al. (2003) and Edwards et al. (2003), are not shown in the bathymetric data. 3.2. Side-scan sonar mosaic The centerpiece of the data in this study is the side-scan mosaic presented in Fig. 3, which reveals complex patterns of light and dark gray shades. Acoustically low backscatter (light gray) represents topographically elevated areas consisting of ¢ne sands (see Section 3.3) and the acoustically dark areas represent topographically lower areas dominated by coarse shells or exposed bedrock similar to that shown in Harrison et al. (2003). From east to west, the mosaic reveals the following features: (1) an acoustically light and featureless modern ebb-tidal delta (Fig. 3),
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Fig. 8. Grain-size data from the deeper sand plain interpreted to be a relict ebb-tidal delta. Note that it is coarser, which is indicative of a larger carbonate content. Side-scan data to the right are from the center of the 100 kHz mosaic located in Fig. 3.
which transitions into (2) a restricted zone of distinct, but small NW^SE oriented sand ridges (Fig. 4), which are unlike the ones to the north; (3) a roughly circular, mostly featureless, 4 km wide zone having acoustic gray tones darker than the active ebb-tidal delta located just to the west of the ebb-tidal delta (Fig. 3); (4) at the west-central portion as well as the south-central portion of the mosaic, complex ¢elds exist of lobate-circular features (10^300 m diameter ; 50 cm relief) of low acoustic scatter surrounded by areas of high acoustic backscatter. The transition between these varying backscatter areas is very sharp, abrupt, and distinct (Figs. 5 and 6). These are interpreted to be sediment-starved 3D dunes (Ashley, 1990; Harrison et al., 2003). The acoustically darker areas are dominated by distinct small 2D dunes having 50 cm spacings and heights of 10 cm and whose crests trend N^S in very straight lines for many 10s to 100s of meters (Fig. 6). These features are identical to those small 2D dunes seen in
the sand-ridge ¢eld described by Harrison et al. (2003). 3.3. Sediments and benthic community Surface-grab samples from the modern ebb-tidal delta are ¢ne sands (mostly within 2^3 P interval) and 100% fall within the well to very well sorted category (Fig. 7). Percent CaCO3 was very low, varying from 1.0% to 7.7%, averaging 3.17%. This invariant nature of the surface sediment texture and composition also reveals itself in the evenness of the acoustic backscatter intensity shown in the side-scan data (Figs. 3 and 7). The roughly circular, mostly featureless 4 km wide zone just west of the active ebb-tidal delta is also a broad sand plain with similar grain-size textural and compositional characteristics (Fig. 8) as most of the sediments are ¢ne sands falling within the 2^3 P interval. However, due to a higher carbonate content (3.9^72%, averaging 12.5%),
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Fig. 9. Grain-size data from the sediment-starved sand waves and the surrounding coarser area. Side-scan data are a portion from the side scan shown in Fig. 5, where the dive transect was completed.
the sediment was less well sorted, with 92% of the samples being well sorted to moderately well sorted and 8% being poorly sorted. Within this group of samples, there is a trend of increasing CaCO3 content with increasing distance o¡shore. The increased CaCO3 content in this area could explain that, although acoustically featureless in side-scan data, the area does yield a darker pattern indicative of slightly higher acoustic backscatter intensity. A bottom-sampling strategy whereby divers obtained surface sediments along speci¢c transects to cross zones of highly contrasting side-scan backscatter intensity was conducted to determine the sedimentological cause of closely juxtaposed areas of high backscatter contrast (Fig. 9). As expected, samples from the high-backscatter (dark) area were distinctly coarser and consisted of high concentrations of molluscan shells and a hash of shell fragments. These shells were mostly Chione cancellata and Anadara transversa with some Atrina serrata, which were commonly arranged in the distinct, small 2D dunes. Also within the high acoustic backscatter area were hard-
ground areas ranging in size from 150 m2 to 3.4 km2 . The hard-grounds typically have 40 cm relief within pock-marked karstic limestone composed of friable, coquina-like rock that possibly had been phosphatized. These hard-bottoms attract ¢sh populations and support small corals, sponges (Xetospongia muta), gorgonians (Telesto riisei), nudibranchs (Hypselodoris edenticulata), and sea whips (Leptogorgia virgulata). To illustrate the correlation between side-scan acoustic intensity and sediment data, Fig. 10 was constructed to summarize the observation that low acoustic intensity was generally associated with the elevated, broad, ¢ne quartz sand plains having little carbonate content. The high acoustic backscatter areas were associated with the bathymetrically lower patches of higher carbonate content of coarser particles (molluscan shells). 3.4. Seismic data Previous seismic data have delineated the geometry and size of the shelf valley that lies beneath the Tampa Bay estuary extending to approxi-
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Fig. 10. Overall map of grain-size distribution showing broad spatial trends controlled by the presence of the modern and relict ebb-tidal delta.
mately 40 km seaward (Fig. 2 ; Duncan et al., 2003). Further west on the middle shelf, no data have been collected to map the full seaward extent of this shelf valley. However, our most seaward seismic line suggests that this basin disappears. Data from Duncan et al. (2003), also show that basin relief is reduced toward the west. Seismic lines cross the side-scan mosaic and reveal that the modern ebb-tidal delta and the sand plain to the west as well as the remaining sand cover rest upon a prominent re£ector which we interpret to be a limestone unit (Pleistocene?) capping the shelf valley (Fig. 11A,B). To the north, opposite Indian Rocks headland, the underlying limestones are most likely the Arcadia Formation of the Hawthorn Group (Green et al., 1995). However, this limestone cap is undoubtedly younger, probably Quaternary in age, and forms the scarps and ledges described by Obrochta et al. (1998, 2003).
Seismic Line 10 (Fig. 11A) is a W^E line, which traverses the inner-shelf, distal ebb-tidal boundary and runs across this sand body at the eastern end of this line. Here, the modern ebb-tidal delta is approximately 5 m thick and reveals some internal seismic re£ectors. The topography of the restricted zone of NW^SW trending sand ridges (Fig. 4) seen in the side-scan mosaic at the transition between the distal ebb-tidal delta and inner shelf is revealed in this seismic line and has about 1 m relief. At the extreme western end of this line, exposed hard-bottom is shown. Line SC84 (Fig. 11B) runs N^S, crosses Line 10, and illustrates the northward prograding clinoforms that have in¢lled the shelf valley. This line also shows several sinkhole deformational features, the dredged main shipping channel leading into Tampa Bay, a buried channel thalweg (3^ 4 m relief, 250 m wide) located just to the north of the dredged channel, and ¢nally, the £at, thin
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MARGO 3352 25-8-03 Fig. 11. (A) Seismic re£ection pro¢le of Line 10 oriented W^E (see Fig. 2 for location) illustrates the thin sediment cover in the most distal portion of the study area, the sand plain interpreted to be a relict ebb-tidal delta, some small sand ridges, and ¢nally, to the east, the distal portion of the modern ebb-tidal delta. Note the overall thin nature of the modern sedimentary cover with the modern ebb-tidal delta being the thickest. (B) Seismic re£ection pro¢le of Line SC84 oriented N^S (see Fig. 2 for location) illustrates the subsurface basin or shelf valley lying beneath the study area. One expanded view shows prograding clinoforms, indicating extensive lateral in¢lling that occurred in the Neogene. The other expanded view shows a buried channel that might represent paleo-£uvial drainage occurring during the late Pleistocene lowstand.
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(V2 m thick) featureless sand plain. The northern end of seismic line SC84 also reveals a marked rise in a re£ector to the north, which crops out on the sea£oor (not shown in Fig. 11B). This re£ector is limestone, probably of the Tampa Member of the Arcadia Formation (Hawthorn Group; Green et al., 1995), and its abrupt rise to the sea£oor marks the northern boundary of the shelf valley (Fig. 2).
4. Discussion 4.1. Interpretation of seabed features As mentioned above, the elevated sand plain on the easternmost and shallowest portion of the study area is clearly the modern ebb-tidal delta. Large ebb-tidal deltas are integral components of large inlet systems with large tidal prisms (Hayes, 1975; Nummedal et al., 1977). The bathymetric data show that a distinct boundary exists between the distal portion of this sand body and the inner shelf, except where the side-scan imagery reveals the restricted area of NW^SE trending sand ridges (Fig. 11A). These sand ridges appear quite di¡erent from the ones described in Harrison et al. (2003) in that they are straighter, more precisely aligned, more restricted in spatial extent, smaller, and not covered with multiple-scale bedforms. Nevertheless, they could represent ridges forming from ebb-tidal deltas ^ a process described by McBride and Moslow (1991). However, if this were a dominant process by which shoreline-oblique sand ridges are generated, why is not the entire distal margin of this huge ebbtidal delta marked by these sand bodies? In fact, there are no other modern sand ridges o¡ ebbtidal deltas along the entire west-central barrier island coast of Florida that transition into the inner shelf through a series of linear sand ridges. Conversely, there were probably very few inlets associated with the shoreline o¡ Indian Rocks headland, yet the inner shelf there is covered with sand ridges. The deeper, more carbonate-rich, featureless sand plain seaward of the modern ebb-tidal delta could be an earlier ebb-tidal delta that became
stranded on the inner shelf and is now relict. It is much thinner (2 m vs. s 5 m) than the active delta and smaller in area (by about 75%), suggesting that it was not active long enough to accumulate the volume of sand equal to the modern system. Its increased percentage in molluscan shell hash (Fig. 8) could be due to its older age, whereby this carbonate-producing infauna had more time to contribute its skeletal material to the overall mass of the sand body. If this sand plain is an older ebb-tidal delta, it formed earlier in the late Holocene marine transgression as a proto-Tampa Bay estuary was forming. The establishment of the modern ebb-tidal delta might represent a distinct back-step in the estuarine system and not a smooth, eastward horizontal translation. Possible causes of such an estuarine back-step include a sudden sea-level jump (Blanchon and Shaw, 1995) or changes in sand supply recon¢guring barrier islands at or near the mouth of the estuary forced by wave climate changes. The lobate sand features (Figs. 5 and 6) found in the western part of the side-scan mosaic are interpreted to be sediment-starved 3D dunes, which probably respond to £ows from a variety of directions, although their apparent alignment suggests dominant £ows from the NW. These features are similar to the sediment-starved 3D dunes described by Harrison et al. (2003). The distinct boundary between the ¢ne sand composing these bedforms and the coarse, shelly, much smaller 2D dunes surrounding them, formed by wave oscillatory motions on the seabed, suggests that sediment movement is active at least on a seasonal basis (Fig. 6). Otherwise, the small 2D dunes and the distinct textural boundary would be blurred by active bioturbation. Both of these bedform types respond to shelf processes and are not associated with ebb or £ood tidal currents emanating from or £owing towards Tampa Bay. 4.2. E¡ect of modern estuarine system on inner shelf What is striking about the inventory of features described above is the overall sediment-starved nature of this shelf sector seaward of Tampa Bay and the lack of tide-dominated, linear, sinu-
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Fig. 12.
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Fig. 12 (Continued).
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ous, and paired sand bodies commonly associated with the mouths of other large estuarine systems on the immediately adjacent inner shelf. The modern ebb-tidal jet seems not to have any in£uence on the inner shelf beyond the distal margins of the ebb-tidal delta. In fact, the overall lack of seaward-oriented bedforms, except in the dredged channel at the more proximal locations of this sand body, suggests that the ebb-tidal delta is no longer building seaward. The bathymetric change data (Donahue, 1999) indicate that it, in fact, might be contracting at least as far as its distal boundary is concerned. However, volume measurements (Hine et al., 1986) based on past bathymetric charts indicate that this sand body continues to accumulate sediments through aggradation and not progradation. So the western portion of the side-scan survey mosaic, dominated by these sediment-starved 3D dunes, probably responds to inner-shelf dynamics and is devoid of any in£uence from the modern estuary. 4.3. Estuarine retreat path? The distribution of sedimentary features shown in the bathymetry and side-scan sonar and the overall lack of sedimentary cover suggests that there is no paleo-estuarine retreat path. The subsurface data also indicate that one probably did not exist during the last rise of sea level. A strong seismic re£ector beneath the thin, modern sedimentary cover and the presence of limestone hard-bottom (Fig. 12B) both indicate that the shelf valley, so prominently seen in seismic data, was capped. As a result, very little accommodation space (V2^5 m) was available to create a deepwater paleo-estuarine embayment out on the
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present inner shelf as sea level rose. This is consistent with the present average depth of the modern Tampa Bay, which is about 5 m. Duncan et al. (2003) also indicate that most of the in¢lling of the subsurface shelf valley occurred well before the last glacial maximum. Their data and those of Hebert (1985) also indicate that the shelf valley decreases in relief to the west. Although we have few seismic data farther seaward of these datasets, it appears that the shelf valley essentially disappears or becomes very broad, having subsurface relief, perhaps, of no more than a few meters. From these observations, we suggest that the shelf valley is cross-shelf-restricted, does not extend westward to some sealevel lowstand point, but is contained to a regional depression in the middle portion of the carbonate platform. We further speculate that beneath the Tampa Bay area is a restricted zone of deepseated limestone dissolution resulting in stratigraphic collapse of the overburden, creating a spatially con¢ned, shallow basin at the ground surface. Although this basin has as much as 40 m of subsurface relief, the dimension of the Tampa Bay basin prior to Holocene £ooding was approximately 60 km in its longest direction with an average relief (now depth) of V5 m ^ a very broad, and a very shallow depression. Regardless of origin, the Tampa Bay area, including the inner shelf immediately to the west, was a lowrelief, sur¢cial depression before the last rise in sea level. Such a depression would have formed a wide, but shallow, embayment in the coastline as sea level was rising. It was this sur¢cial basin that caused local streams to £ow into it and out onto the shelf during sea-level lowstand. As has been mentioned,
Fig. 12. (A^D) The theoretical development of the inner shelf, Tampa Bay estuary, and ebb-tidal delta system. Panel A, around 10^11 ka, shows a shoreline consisting of low, overwash-dominated, transgressing barrier islands with sand ridges forming seaward. The mangrove/marsh mainland lagoon shoreline has intercepted the slight topographic depression (Tampa Bay Basin) lying on top of the buried shelf valley forming a slight embayment. The area of the modern Tampa Bay and area seaward is a freshwater swamp surrounded by uplands vegetated by trees. Panel B, around 8^9 ka, shows that the coastline has transgressed towards the east, with subtidal/intertidal swash bars and very low barrier islands forming the coastline. The embayment and headland morphology has become more prominent. Panel C, around 5^8 ka, shows further transgression. However, a small barrier island or swash-bar system has formed or migrated into the embayment, focusing an ebb-jet. This ebb-jet may create the lower sand plain, which we interpret to be a relict ebb-tidal delta. In panel D, around 3 ka, sea level has slowed down, the modern barrier system is becoming established. The modern ebb-tidal delta starts to prograde seaward.
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present-day streams are relatively small and probably were even smaller during the last sea-level transgression, as Florida was much more arid during the very early Holocene than today (Grimm et al., 1993; Wright, 1995). We suggest that the channel thalweg seen in Fig. 11B might have been the full extent of seaward-£owing stream activity when this portion of the inner shelf was subaerially exposed. Its cross-sectional dimension of V4 m of relief and V250 m in width is much larger than any single stream now £owing into the modern Tampa Bay. Thus, we speculate that Tampa Bay and the area just to the west was probably no more than a broad, very shallow depression that contained freshwater wetlands through which very small rivers passed, draining the central interior of peninsular Florida. This depression did not extend westward very far across the shelf and the rivers dissipated into smaller streams, creating thalwegs below seismic resolution, or simply disappeared down sinkholes ^ a common fate of streams in carbonate karsti¢ed terrain (Trudgill, 1985). 4.4. Late Holocene sea-level £ooding model Using a local, relative sea-level curve assembled by Wright (1995) and the Barbados curve (Fairbanks, 1989) as well as water depth and shelf gradient, we present a paleogeography/paleoenvironmental scenario depicting how this inner shelf developed during the last deglacial rise in sea level (Fig. 12A^C). Fig. 12A occurs at approximately 10^11 ka, when sea level was about 330 m below present and the shoreline was located some 50 km seaward (west) of its present position. Given an average sea-level rate of rise at that time (V10 m/1000 yr; Fairbanks, 1989) and the average gradient (1:2000), the eastward shoreline translation was approximately 20 km/1000 yr, transgressing a subaerial surface having a thin, quartz-sand veneer covering limestone bedrock. The coastline was probably a mixture of ephemeral washover-dominated barrier islands and/or subtidal swash bars. Perhaps a small river emanating from the upland Tampa Bay area discharged along this coast similar to the modern Suwannee River (Wright,
1995). More importantly, this coastline was oriented N^S and was approximately straight, not having signi¢cant headlands or embayments. A slight embayment seaward of the modern Tampa Bay might have started to form. The mid-platform basin (Tampa Bay Basin) occupying the Tampa Bay area had not yet been occupied or £ooded by the rising marine waters and was probably a freshwater wetland as mentioned above. By V8^9 ka (Fig. 12B), the marine transgression had reached the mid-platform basin, £ooding it and creating a broad embayment in the regional shoreline trend. The broad, elevated high forming Indian Rocks headland de¢ned the northern boundary of this embayment. As sea level rose, the embayment extended eastward very quickly as marine waters £ooded the basin. This was the proto-Tampa Bay, which probably did not have strong tidal £ow as the mouth was not constricted as it is today, and probably experienced very low wave energy as it was, essentially, a broad, shallow shelf embayment. Seagrass meadows probably dominated the embayment’s £oor and mangroves dominated the embayment’s shoreline. For this reason, no sand ridges formed, thus making it distinctive from the evolving shelves to the north and south. The development of the lower sand plain as an early precursor to the modern ebb-tidal delta is somewhat of an enigma. It lies at about 38 m water depth, whereas the top of the modern ebb-tidal delta lies at about 4^5 m depth. If subaqueous, subtidal ebb-tidal deltas build to an equilibrium depth and the modern system seaward of Tampa Bay is currently at this depth, then the sand plain must have been an active ebb-tidal delta approximately 5 ka. This was a period of deceleration in the rate of sea-level rise. Hence, the mouth of the estuary could have become partially closed o¡ by barrier islands, thus focusing the ebb-jet (Fig. 12C). This incipient barrier-island system could have stepped back to its modern position as a result of local changes in sand supply. Or, perhaps the relict ebb-tidal is older and was abandoned around 7.6 ka as a result of a sudden increase in the rate of sea-level rise (Blanchon and Shaw, 1995). This would have drowned the local coastal system and forced the
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estuarine mouth to retreat landward soon after the ¢rst ebb-tidal delta had formed. Either scenario, regardless of timing, would have left this sand body stranded and inactive on the inner shelf, never to obtain the size and thickness of its modern counterpart. It has become relatively enriched in molluscan shelly gravel through time by continuing to support a benthic infaunal molluscan community. By V3 ka (Fig. 12D), the rate of sea-level rise had slowed and sea level had nearly reached its current elevation. In response, the present-day barrier-island coastline was becoming established. The barriers changed from being less transgressive to more progradational/aggradational (Stapor and Mathews, 1980). The longshore transport system began to build spits into the broad mouth of Tampa Bay, restricting it laterally. Barrier islands became established on these spits. With this restriction came the focusing of the ebb-tidal jet, which started to develop a modern ebb-tidal delta. We speculate that this sand body prograded seaward and aggraded to its present depth as the barrier islands were forming around the mouth of Tampa Bay. Today, there is no obvious evidence of a sediment transport pathway (i.e. no persistent seaward-oriented bedforms) to continue to extend the ebb-tidal delta seaward, so it remains a huge subtidal sand body modi¢ed by the ambient wave climate, and acts as a signi¢cant wave-refracting agent controlling wave-energy £ux to the adjacent beaches. However, the net longshore sand transport continues today to introduce sand to the £anks of the ebb-tidal delta, probably making this an area of continued sand-body growth (Hine et al., 1986). 4.5. Comparison to other shelf sectors The inner shelf seaward of Tampa Bay is fundamentally di¡erent from the inner shelf o¡ the Indian Rocks headland (Harrison et al., this volume; Edwards et al., this volume) to the north. It is also fundamentally di¡erent from the inner shelf o¡ Sarasota (Twichell et al., this volume) to the south, indicating that distinct shelf sectors or provinces can be de¢ned. Seaward of Tampa
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Bay are no closely spaced, narrow (V500 m), bedform-dominated, well-oriented NW^SE sand ridges as seen to the north. The boundary between these shelf provinces is de¢ned by the northern margin of the shelf valley (Fig. 2). As mentioned earlier, these well-de¢ned sand ridges could not form in the broad, shallow, low-energy embayment that occupied the area seaward of Tampa Bay during the mid-Holocene. See Brooks et al. (2003) and Locker et al. (2003) for a broader discussion on shelf sedimentology and geology variability.
5. Conclusions There was not a large paleo-£uvial system, nor a high-relief valley system that extended across the exposed west Florida shelf during the last sea-level lowstand. Instead, a broad mid-platform depression existed in the proto-Tampa Bay area with its western boundary located probably within 40 km west of the present-day estuary. As sea level started to rise after the last glacial maximum, the shoreline, starting at the 3120 m isobath, transgressed rapidly across the shelf but did not track up a shelf-valley system until it reached the western, most seaward limit of the mid-platform depression. Hence, there is no estuarine retreat path on the west Florida shelf well seaward of Tampa Bay similar to those described on the shelf o¡ the US east coast. The inner shelf seaward of Tampa Bay is dominated by two sand bodies, the present-day modern ebb-tidal delta and a more distal and deeper sand plain, which we interpret to be an older, but short-lived ebb-tidal delta. The modern ebb-tidal delta has progaded out onto the inner shelf during the past few thousand years as the rate of sealevel rise decelerated, the modern barrier island system formed and narrowed the mouth of Tampa Bay. This, in turn, focused the ebb-jet, which exported sands seaward to form the ebb-tidal delta. A model of sea-level rise, coastal retreat, and shelf evolution is presented in the text and not summarized here. The most seaward part of the study area consists of sediment-starved 3D dunes surrounded by coarse, molluscan shelly material
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and/or exposed hard-bottom. These bedforms respond to modern shelf hydraulics and do not appear to be in£uenced by the modern estuarine system. The inner shelf seaward of Tampa Bay did not form sand ridges as seen in the other two provinces, located to the north and to the south. Instead, a broad, shallow, low-energy embayment, forming proto-Tampa Bay evolved into the modern Tampa Bay and was not conducive to creating such sand bodies.
Acknowledgements This work was funded by the US Geological Survey’s Center for Coastal and Regional Marine Studies in St. Petersburg, FL, as part of the WestCentral Florida Coastal Studies Project. This work was also partially funded by the O⁄ce of Naval Research Contract #N00014-96-1-5032. We thank Drs. Guy Gelfenbaum and Terry Edgar, both of the USGS, for their program management skills in running this project. We thank Mark Hafen for enormous assistance in the ¢eld as well as in the computer lab. We also thank Dr. David Naar and Dr. David Duncan for thoughtful advice and technical assistance. We appreciate the graphics assistance provided by Chad Edmisten. Finally, we thank Drs. Charles (Chip) Fletcher and Steven Goodbred for providing key advice. References Ashley, G.M., 1990. Classi¢cation of large-scale subaqueous bedforms: a new look at an old problem. J. Sediment. Petrol. 60, 160^172. Belknap, D.F., Kraft, J.C., 1985. In£uence of antecedent geology on stratigraphic preservation potential and evolution of Delaware’s barrier systems. Mar. Geol. 63, 235^262. Blanchon, P., Shaw, J., 1995. Reef drowning during the last deglaciation: evidence for catastrophic sea-level rise and icesheet collapse. Geology 23, 4^8. Brooks, G.R., Doyle, L.J., Suthard, B.C., Locker, S.D., Hine, A.C., 2003. Facies architecture of a mixed carbonate/siliciclastic inner continental shelf of west-central Florida: Implications for Holocene barrier development. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00190-7, this issue.
Dalrymple, R.W., Boyd, R., Zaitlin, B.A., 1994. Incised-valley systems: origin and sedimentary sequences. SEPM Spec. Publ. No. 51. Soc. Sediment. Geol., Tulsa, OK. Donahue, B.T., 1999. Modern morphodynamics and Holocene evolution of an inner shelf/estuarine transition zone on the west Florida shelf. M.S. Thesis, Department of Marine Science, University of South Florida, St. Petersburg, FL. Duncan, D.S., 1993. Neogene to Recent seismic stratigraphy of the lower Tampa Bay estuary: West-Central Florida. M.S. Thesis, Department of Marine Science, University of South Florida, St. Petersburg, FL. Duncan, D.S., Locker, S.D., Brooks, G.R., Hine, A.C., Doyle, L.J., 2003. Mixed carbonate-siliciclastic in¢lling of a Neogene carbonate shelf valley system: Tampa Bay, west-central Florida. Mar. Geol. 200, doi:10.1016/S00253227(03)00180-4, this issue. Edwards, J.H., Harrison, S.E., Locker, S.D., Hine, A.C., Twichell, D.C., 2003. Stratigraphic framework of sedimentstarved sand ridges on a mixed siliciclastic/carbonate inner shelf o¡ west-central Florida. Mar. Geol. 200, doi:10.1016/ S0025-3227(03)00183-X, this issue. Evans, M.W., Hine, A.C., Belknap, D.F., 1989. Quaternary stratigraphy of the Charlotte Harbor estuarine-lagoon system, southwest Florida: Implications of the carbonate-siliciclastic transition. Mar. Geol. 88, 319^348. Evans, M.W., Hine, A.C., 1991. Late Neogene sequence stratigraphy of a carbonate-siliciclastic transition: southwest Florida. Geol. Soc. Am. Bull. 103, 679^699. Fairbanks, R.G., 1989. A 17,000 year glacio-eustatic sea-level record: in£uence of glacial melting rates on the Younger Dryas Event and deep ocean circulation. Nature 342, 637^ 642. Ferguson, T.W., Davis, R.A. Jr., 2003. Post-Miocene stratigraphy and depositional environments of valley-¢ll sequences at the mouth of Tampa Bay, Florida. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00181-6, this issue. Folk, R.L., 1980. Petrology of Sedimentary Rocks. Hemphill Publ. Co., Austin, TX. Gelfenbaum, G., Guy, K., 1999. Bathymetry of west-central Florida. USGS Open File Report 99-417. Green, R., Arthur, J.D., DeWitt, D., 1995. Lithographic and hydrostratigraphic cross section through Pinellas and Hillsborough Counties, southwest Florida. Open File Report 61, Florida Geol. Surv., Tallahassee, FL. Grimm, E.C., Jacobson, G.L., Watts, W.A., Hansen, B.C.S., Maasch, K.A., 1993. A 50,000 year record of climate oscillations from Florida and its temporal correlation with the Heinrich Events. Science 261, 198^200. Harrison, S.E., Locker, S.D., Hine, A.C., Edwards, J.H., Naar, D.F., Twichell, D.C., 2003. Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf o¡ west-central Florida. Mar. Geol. 200, doi:10.1016/ S0025-3227(03)00182-8, this issue. Hayes, M.O., 1975. Morphology of sand accumulations in estuaries. In: Cronin, L.E. (Ed.), Estuarine Research, vol. 2, Geology and Engineering. Academic Press, New York, pp. 3^22.
MARGO 3352 25-8-03
B.T. Donahue et al. / Marine Geology 200 (2003) 219^241 Hebert, J.A., 1985. High-resolution seismic stratigraphy of the inner west Florida shelf, west of Tampa Bay. M.S. Thesis, Department of Marine Science, University of South Florida, St. Petersburg, FL, 52 pp. Hine, A.C., 1997. Structural and paleoceanographic evolution of the margins of the Florida Platform. In: Randazzo, A.F., Jones, D.S. (Eds.), The Geology of Florida. University Press of Florida, Gainesville, FL, pp. 169^194. Hine, A.C., Brooks, G.R., Davis, R.A., Jr., Duncan, D.S., Locker, S.D., Twichell, D.C., Gelfenbaum, G., 2003. The west-central Florida inner shelf and coastal system: A geologic conceptual overview. Mar. Geol. 200, doi:10.1016/ S0025-3227(03)00161-0, this issue. Hine, A.C., Snyder, S.W., 1985. Coastal lithosome preservation: Evidence from the shoreface and inner continental shelf o¡ Bogue Banks, North Carolina. Mar. Geol. 63, 307^330. Hine, A.C., Mearns, D.L., Davis, R.A., Jr., Bland, M., 1986. Impact of Florida’s Gulf coast inlets on the coastal sand budget. Final Report to The Division of Beaches and Shores, Florida Dept. of Natural Resources, Tallahassee, FL. Kraft, J.C., Sheridan, R.E., Moose, R.D., Strom, R.N., Weil, C.B., 1974. Middle-late Holocene evolution of the morphology of a drowned estuary system - the Delaware Bay. In: International Symposium on Interrelationships of Estuarine and Continental Shelf Sedimentation, Institute Geol. Du Bassin d’Aquitaine, July 9^14, 1973, pp. 297^ 306. Locker, S.D., Doyle, L.J., 1992. Neogene to recent stratigraphy and depositional regimes of the northwest Florida inner continental shelf. Mar. Geol. 104, 123^138. Locker, S.D., Hine, A.C., Brooks, G.R., 2003. Regional stratigraphic framework linking continental shelf and coastal sedimentary deposits of west-central Florida. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00191-9, this issue. McBride, R.A., Moslow, T.F., 1991. Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A. Mar. Geol. 97, 57^85. Niedoroda, A.W., Swift, D.J.P., Figueredo, A.G., Jr., Freeland, G.L., 1985. Barrier island evolution, middle Atlantic shelf, USA part II: Evidence from the shelf £oor. Mar. Geol. 63, 363^396. Nummedal, D., Oertel, G.F., Hubbard, D.K., Hine, A.C., 1977. Tidal inlet variability - Cape Hatteras to Cape Ca-
241
naveral. Coastal Sediments ’77 Proceedings, 5th Symposium. Am. Soc. Civ. Eng., Charleston, SC, pp. 543^562. Nummedal, D. Pilkey, O.H., Howard, J.D., 1987. Sea-level £uctuation and coastal evolution. SEPM Spec. Publ. No. 41. Society for Sedimentary Geology, Tulsa, OK. Obrochta, S.P., Duncan, D.S., Brooks, G.R., 1998. Hardbottoms of the inner West-Central Florida continental shelf. Trans. Gulf Coast Assoc. Geol. Soc. 48, pp. 312^319. Obrochta, S.P., Duncan, D.S., Brooks, G.R., 2003. Hardbottom development and signi¢cance to the sedimentstarved, west-Florida inner continental shelf. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00188-9, this issue. Penland, S., Suter, J.R., Boyd, R., 1985. Barrier island arcs along abandoned Mississippi River deltas. Mar. Geol. 63, 197^233. Randazzo, A.F., Jones, D.S., 1997. The Geology of Florida. University Press of Florida, Gainesville, FL, 327 pp. Riggs, S.R., Cleary, W.J., Snyder, S.W., 1995. In£uence of inherited geologic framework on barrier shoreface morphology and dynamics. Mar. Geol. 126, 213^234. Stapor, F.W., Mathews, T.D., 1980. C-14 chronology of Holocene barrier islands, Lee County, Florida: a preliminary report. In: Tanner, W.F. (Ed.), Shorelines Past and Present. Department of Geology, Florida State University, Tallahassee, FL, pp. 47^67. Swift, D.J.P., 1976. Continental shelf sedimentation. In: Stanley, D.J., Swift, D.J.P. (Eds.), Marine Sediment Transport and Environmental Management. Wiley, New York, pp. 311^350. Thomas, M.A., Anderson, J.B., 1994. Sea-level controls on the facies architecture of the Trinity/Sabine incised-valley system, Texas continental shelf, 1994. In: Dalrymple, R.W., Boyd, R., Zaitlin, B.A. (Eds.), Incised-Valley Systems: Origin and Sedimentary Sequences. SEPM Spec. Publ. No. 51, pp. 63^82. Trudgill, S., 1985. Limestone Geomorphology. Longman Group, Essex, 196 pp. Twichell, D., Brooks, G.R., Gelfenbaum, G., Paskevich, P., Donahue, B., 2003. Sand ridges o¡ Sarasota Florida: a complex facies boundary on a low energy, inner shelf environment. Mar. Geol. 200, doi:10.1016/S0025-3227(03)00185-3, this issue. Wright, E.A., 1995. Sedimentation and stratigraphy of the Suwannee River marsh coastline. Ph.D. Thesis, University of South Florida, St. Petersburg, FL.
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