West-Central Florida Coastal Transect # 2: Caladesi Island ... · Oblique aerial photograph (taken...
1
Vibracore USGS-95-43 Vibracore USGS-95-44 Sinkhole Deformed limestone bedrock Large sand waves form sand ridge Sand waves Hardbottom or sediment veneer St. Joseph Sound 10 20 15 5 25 0 Water depth (meters) 0 1 2 km Vertical exaggeration ~180 X Sea level +5 Caladesi Island Northwest-trending sand ridge Northwest-trending sand ridge Sea floor (see seismic data above) USGS-95-5 -7.61 m 9 8 1 USGS-95-21 -9.74 m 1 11 10 USGS-95-43 -13.69 m 1 2 14 15 16 USGS-95-44 -12.47 m 1 13 14 0 0 0 0 8,300 ± 90 YBP Standard C-14 date Articulated mollusc shell Elevation: -15.94 m Units in meters Tampa Bay References Cited Barnard, P.L., 1998, Historical morphodynamics of inlet channels: West-Central Florida: St. Petersburg, University of South Florida, unpublished M.S. thesis, 179 p. Brame, J.W., 1976, The stratigraphy and geologic history of Caladesi Island, Pinellas County, Florida: St. Petersburg, University of South Florida, unpublished M.S. thesis, 109 p. Brooks, G.R., Doyle, L.J., Suthard, B.C., and DeWitt, N.T., 1999, Inner West-Central Florida continental shelf: Sedimentary facies and facies associations: U.S. Geological Survey Open-File Report 98- 796, 124 p. Davis, R.A., 1994, Barriers of the Florida Gulf peninsula, in Davis, R.A., ed., Geology of Holocene Barrier Island Systems: Heidelberg, Springer-Verlag, p. 167-206. Davis, R.A. and Hine, A.C., 1989, Quaternary geology and sedimentology of the barrier island marsh coast, West-Central Florida: 28th International Geological Congress Guidebook T375, 38 p. Evans, M.W., Hine, A.C., Belknap, D.F., and Davis, R.A., 1985, Bedrock control on barrier island development: West-Central Florida coast: Marine Geology, v. 63, p. 263-283. Oblique aerial photograph (taken in 1992) of Dunedin Pass which closed in 1988. Vegetated beach ridges, washover, and flood-tidal deposits occur landward of the beach. Vibracore locations are shown for the island-transect portion (C-D) presented below. Depth to bedrock is 4 to 5 m below sea level along this section. J128 J40 J125 J126 J127 J155 CAL-3c CAL-3d J19 CAL-3b Line B94J-20 West-Central Florida Coastal Transect # 2: Caladesi Island - Clearwater Beach Island U.S. Geological Survey Open-File Report 99-506 S.D. Locker, 1 R.A. Davis, 2 G.R. Brooks, 3 A.C. Hine, 1 and G. Gelfenbaum 4 1 College of Marine Science, University of South Florida, St. Petersburg, FL 33701 2 Department of Geology, University of South Florida, Tampa, FL 33620 3 Department of Marine Science, Eckerd College, St. Petersburg, FL 33711 4 U.S. Geological Survey, Menlo Park, CA 94025 CAL-1 Elevation: -0.16 m Vegetated paralic USGS-95-43 Elevation: -13.69 m C-14 Date: 8,300 ± 90 YBP Darkness at top and bottom of core photo due to unequal light Burrowed sand (back-barrier) Side-scan sonar imagery overlain on bathymetry reveals a northwest-trending sand-ridge morphology common throughout the inner shelf in this region. Surface-sediment cover is thin and exhibits a patchy and discontinuous distribution. Low backscatter (light gray) areas correspond to sand ridges and flats dominated by quartz sand. The dark (high backscatter) areas are largely coarse sediment veneer with increased carbonate material (primarily shell material), or some hardbottoms. Landward of the 4-m isobath, a 1995 color infrared digital orthophotograph shows the extension and seaward accretion of the north end of Clearwater Beach Island – compare with the USGS quadrangle map in panel above. S 10 20 0 15 5 25 Water Depth (m) 10 20 0 15 5 25 South Water Depth (m) 10 20 0 15 5 25 Water Depth (m) Seismic-profile data Uninterpreted high-resolution "boomer" seismic profile illustrates the poor acoustic contrast between the Holocene sediment cover and the Pleistocene exposure surface. The poor contrast is typical throughout the region and is attributed to the karst and weathered nature of the underlying pre-Quaternary bedrock. The offshore Holocene sediment thickness is usually less than 2 to 3 m, corresponding with the higher relief portions of the sand waves or ridges seen here. Integrated stratigraphic cross section combining line-drawn interpretation of seismic data, ground truthed by coring, with a coastal cross section based on vibracores. Cores in the offshore transect have no cross-shelf correlation potential because they often contain different ridge deposits, shown in side- scan sonar imagery and bathymetry data. Most of the sediment volume in this coastal system is found in the barrier-island section. Side-scan sonar imagery Location map Caladesi Island One of the oldest islands in this system is Caladesi Island, a classic example of a drumstick barrier. The north end of the island was separated from what is now Honeymoon Island as the result of the hurricane of 1921, which broke through Hog Island to form Hurricane Pass (Brame, 1976). Caladesi Island experienced considerable progradation at its southern end due to the entrapment of sediment downdrift north of Dunedin Pass throughout most of the history of the island. Numerous beach/dune ridges separate swales and ponds. Dunedin Pass closed in 1988, three years after Hurricane Elena removed the ebb-tidal delta and facilitated longshore transport across the mouth of the small and unstable inlet (Davis and Hine, 1989; Barnard, 1998). Since that time, the southern end of the island has experienced erosion, and considerable accretion has taken place on the north end. Sediments that comprise the barrier/inlet system along this coast display little variety (Yale, 1997). Fine quartz sand dominates with carbonate skeletal debris and mud as subordinant constituents. Most sediments are distinctly bimodal, with a shell-gravel fraction and the sand fraction dominated by quartz with lesser amounts of fine carbonate skeletal material. Stratigraphically, Caladesi Island shows a sand-dominated series of lithofacies with a range of shell gravel and mud. Back-barrier facies that originated as washover deposits are bioturbated and contain a significant amount of mineral and organic mud. Shallow to intertidal shoals, beach, and dune environments are represented by clean sand and shelly sand. As progradation of beach/dune ridges took place with mangrove environments intercalated between them, the surface and near-surface sediments reflected the presence of mangrove environments. Grain-size and composition data for bottom grab samples are presented below the sonar imagery. Samples generally consist of quartz-rich sand with subordinate amounts of gravel and mud. Locally, samples are rich in carbonate gravel or sand. Side-scan sonar low backscatter correlates with medium to fine siliciclastic sand with minor carbonate grains and is associated with the thicker sand-ridge deposits. The higher backscatter areas correlate with coarse grain size and increased carbonates. The coarse-grained sediments are usually <10 to 20 cm thick, occupy low areas adjacent to thicker sand bodies, and form ripples oriented N-S with a 40- to 70-cm spacing. Surface sediments Projection: UTM, GRS 1980, NAD83, Zone 17. Coordinates: Geographic. Bathymetry in 2-m intervals starting at 4 m (modified from Gelfenbaum and Guy, 1999). Land areas represented by USGS 7.5-Minute Topographic Map. +2 +1 0 -1 -2 -3 -4 -5 -6 Elevation (m) CAL-5 CAL-1 CAL-4 CAL-3 CAL-2 Modern mangroves Back-barrier (Washover) Dune Channel Washover/Intertidal shoal Vegetated paralic Miocene limestone ? 100 200 300 400 0 meters Nearshore/Beach Back-barrier/Washover Caladesi Island MLLW 0 1 2 3 4 5 1 2 3 4 0 CAL-5 +0.82 m CAL-4 +0.26 m 0 1 2 3 1 2 3 0 CAL-3 +0.19 m 0 1 2 3 4 1 2 3 4 0 CAL-2 -0.26 m 0 1 2 3 1 2 3 4 CAL-1 -0.16 m 0 1 2 3 4 1 2 3 4 MLLW Units in meters Transect cross section Seven generalized sedimentary-facies types were defined for a unified comparison of core data from the entire study area. All seven color-coded facies for the entire study are shown in the Explanation below. However, not all facies necessarily are present on each transect. Core photographs present individual cores cut into 1-m sections from top (upper left) to bottom (lower right). Discrepancies in core length between the photographs and the diagrams are due to compaction during the coring process. Offshore cores (left) are aligned at core tops. Core locations were chosen to sample thicker Holocene sections and to aid in identifying pre-Holocene stratigraphy. Core elevations were determined from water depth and tide tables. The datum for the barrier-transect cores is the mean lowest low water (MLLW). Core photographs are shown for USGS-95-43 (most seaward) and CAL-1 (most landward). Offshore vibracore retrieval ranged from a little over 1 m to approximately 2.5 m in length. Although the vibracores penetrated to bedrock, there is often little indication of a bedrock reflection seen in the seismic data. A poor impedance contrast between well- sorted shelf sands and the underlying exposure surface appears to be responsible for the lack of a well-defined seismic boundary at the base of the Holocene section. Offshore cores contain a surface layer dominated by quartz sand. The facies is interpreted to have been deposited under modern open-marine conditions. In the seaward most core, the quartz sand unit is underlain by a muddy sand unit with numerous visible burrows. The muddy sand is interpreted to have been deposited in a back-barrier environment. Radiocarbon dates indicate that it was deposited 8,300 ±90 years before present (YBP). In the two landwardmost cores, the Miocene limestone surface was penetrated immediately below the surficial sediment/sand layer, once again indicating the paucity of sediments on this part of the shelf. The barrier-island transect cores (right), taken across the prograding southern end of the barrier, record a transition from back-barrier and overwash sedimentary facies containing a more abundant shell and mud fraction, to cleaner sands in the beach and dune sedimentary facies. On cross sections where cores do not penetrate to bedrock, the control is based on probe-rod data. The expanded coastal cross section to the right includes general environmental interpretations. Core Data Holocene sediment Gelfenbaum, G. and Guy, K.K., 1999, Bathymetry of West-Central Florida: U.S. Geological Survey Open-File Report 99-417, CD-ROM. Locker, S.D., Brooks, G.R., Hine, A.C., Davis, R.A., Twichell, D.C., and Doyle, L.J., 2001, Compilation of geophysical and sedimentological data sets for the West-Central Florida Coastal Studies Project: U.S. Geological Survey Open-File Report 99-539, CD-ROM. Lynch-Blosse, M.A. and Davis, R.A., 1977, Stability of Dunedin and Hurricane Passes, Florida. Coastal Sediments '77: American Society of Civil Engineers, New York, p. 774-789. Stapor, F.W., Mathews, T.D., and Lindfors-Kerns, F.E., 1988, Episodic barrier island growth in southwest Florida: A response to fluctuating Holocene sea level?: Miami Geological Society, Memoir 3, p. 149-202. Yale, K.E., 1997, Regional stratigraphy and geologic history of barrier islands, West-Central Florida: St. Petersburg, University of South Florida, unpublished M.S. thesis, 180 p. ? ? Core penetration 0 1 2 km 10 20 15 5 25 0 Water depth (meters) Note intermittent reflection at base of Holocene sediment Holocene sediment Sand waves Sea level 100 % 50 % 50 % Grain Size (wt. %) (right side) Bottom-Sediment Diagram 0 N104 Gravel Sample ID Composition (wt. %) (left side) Mud Insoluble Carbonate (CaCO 3 ) Sand Web Page Address: http://coastal.er.usgs.gov/wfla/ Data references: Color Infrared Digital Orthophoto Quarter Quadrangles (CIR DOQQ), (1994, 1995), USGS EROS Data Center, Sioux Falls, SD 57198. CD-ROMs. Landsat TM Image, February 18, 1997, path 17, row 40. USGS EROS Data Center, Sioux Falls, SD 57198. CD-ROM. 7.5-Minute Series (Topographic) Quadrangles, U.S. Geological Survey, Reston, VA 22092. Acknowledgments The large field program and combination of data sets brought to this compilation are the result of significant efforts by many people. Kristy Guy and Beau Suthard helped compile, process, and display much of the imagery presented. Significant contributions were made by Nancy DeWitt and Kristin Yale. We thank the following people for help in the field or laboratory: Patrick Barnard, Greg Berman, Jim Edwards, Brian Donahue, Larry Doyle, Dave Duncan, John Cargill, Tom Ferguson, Megan FitzGerald, Mark Hafen, Jackie Hand, Scott Harrison, Tessa Hill, Bret Jarrett, Jennifer Kling, Katie Kowalski, David Mallinson, John Nash, Steve Obrochta, Meg Palmsten, John Pekala, Boudewijn Remick, Peter Sedgwick, Brad Silverman, Darren Spurgeon, David Ufnar, Ping Wang, and Tao Yucong. We also thank the crews and support staff of the research vessels R/V Bellows, R/V Suncoaster (Florida Institute of Oceanography) and R/V Gilbert (U.S. Geological Survey) for their assistance. Technical reviews by Barbara Lidz and Bob Morton are greatly appreciated. List of west-Florida coastal-transect series maps (1 sheet each): Transect #1: Anclote Key, USGS Open-File Report 99-505 Transect #2: Caladesi Island-Clearwater Beach, USGS Open File-Report 99-506 Transect #3: Sand Key, USGS Open-File Report 99-507 Transect #4: Indian Rocks Beach, USGS Open-File Report 99-508 Transect #5: Treasure Island-Long Key, USGS Open-File Report 99-509 Transect #6: Anna Maria Island, USGS Open-File Report 99-510 Transect #7: Longboat Key, USGS Open-File Report 99-511 Transect #8: Siesta Key, USGS Open File-Report 99-512 Transect #9: Casey Key, USGS Open File-Report 99-513 Introduction A major goal of the West-Central Florida Coastal Studies Project was to investigate linkages between the barrier-island system along the west coast of Florida and offshore sedimentary sequences. High population density along this coastline and the resultant coastal-management concerns were primary factors driving the approach of this regional study. Key objectives were to better understand sedimentary processes and accumulation patterns of the modern coastal system, the history of coastal evolution during sea-level rise, and resource assessment for future planning. A series of nine "swath" transects, extending from the mainland out to a depth of 26 m, was defined to serve as a focus to merge these data sets and for comparison of different coastal settings within the study area. Transect #2 extends seaward from Caladesi Island and North Clearwater Beach (see location map to right). Information from seismic and vibracore studies is combined to derive a 2-D stratigraphic cross section extending from the offshore zone, through the barrier island, and onto the mainland. This stratigraphic record represents the late Holocene evolution of the coastal-barrier system and inner shelf following the last sea- level transgression and present highstand conditions. A comparison to surface-sediment distribution patterns indicated by side-scan sonar imagery and bottom grab samples illustrates the importance of spatial variability in sediment-distribution patterns offshore when considering stratigraphic interpretations of seismic and core data. Methods The primary data sets used in this study were collected from 1993 to 1998. Geophysical surveys included high-resolution single-channel "boomer" seismic data and 100-kHz side-scan sonar imagery (Locker and others, 2001). Most of the reconnaissance seismic and side-scan sonar data were acquired during two offshore cruises in 1994. Additionally, bottom samples were collected during the cruises using an underway grab sampler at 4-km intervals along track. Offshore core locations were selected based upon seismic data and were focused in areas likely to contain sufficient sediment thickness for core retrieval (Brooks and others, 1999). Vibracores and probe data provided stratigraphic control in the barrier-island and bay areas. The four panels showing location and side-scan sonar imagery, seismic data, and a stratigraphic cross section are at the same horizontal scale. The seismic profile and cross-section panels are constructed by fitting the data between the labeled cross-section turns (location map panel) that have been projected downward to the straight cross-section line. Subtle differences in the horizontal scale of segments in the cross section due to this projection are minimal. The horizontal scale, as well as vertical exaggeration of the seismic profile and cross section, are the same for all nine transects in the map series in order to facilitate comparison among transects. Geologic History and Morphodynamics of Barrier Islands Barrier islands on the west-central Gulf coast of Florida display a wide range in morphology along the most diverse barrier/inlet coast in the world (Davis, 1994). In addition, the barriers have formed over a wide range of time scales from decades to millennia. The oldest of the barriers have been dated at 3,000 years (Stapor and others, 1988) and others have formed during the past two decades. The barrier system includes long, wave-dominated examples as well as drumstick barriers that are characteristic of mixed wave and tidal energy. Historical data on the very young barriers and stratigraphic data from coring older ones indicate that the barriers formed as the result of a gentle wave climate transporting sediment to shallow water and shoaling upward to intertidal and eventually supratidal conditions. The barriers probably formed close to their present position and several have been aided in their location and development by antecedent topography produced by the shallow Miocene limestone bedrock (Evans and others, 1985). The two most important variables that control barrier-island development along the coast are the availability of sediment and the interaction of wave and tidal energy. Location of west-central Florida coastal-transect maps with Transect #2 shown in red. 1997 LANDSAT TM imagery of Florida's west coast is merged with a bathymetric-surface model (Gelfenbaum and Guy, 1999). Bathymetric trends offshore in part reflect sediment-distribution patterns. The study area extends from Anclote Key to Venice, FL. Dune Dune Projection: UTM, GRS 1980, NAD83, Zone 17. Coordinates: Geographic. Bathymetry (areas > 4 m) after Gelfenbaum and Guy (1999). Coastal areas (< 4 m) represented by Digital Orthophoto Quarter Quadrangle (1995). MULTIPLE Location of study area along the west-central Florida coastline showing cruise-track coverage in red. Data types include high- resolution seismic-reflection data, side-scan sonar imagery, surface-sediment samples, and vibracores. Blue-box areas identify continuous-coverage side-scan sonar mosaic areas. The bathymetry shown in 5-m intervals by gray shading is modified from Gelfenbaum and Guy (1999). Vibracore USGS-95-5 MULTIPLE Vibracore USGS-95-21 Transect boundary Vibracore Underway surface-sediment sample Surface-sediment grab sample Cruise track in red with yellow highlight indicating line of cross section or seismic profiles (below). CAL-5 +0.82 m 1 0 1 0 Depth from top of core Depth relative to mean sea level (MLLW) Core ID & Elevation Explanation: core logs and sedimentary facies gravel (> 60% gravel) muddy sand (3-50% mud) mud (>50% mud) Pleistocene facies Miocene (limestone and blue-green clay) gravelly sand (< 3% mud and 25-60% gravel) sand (< 3% mud and < 25% gravel) Units in meters North South North South North Vibracore Underway surface-sediment sample Surface-sediment grab sample Cruise track in red with yellow highlight indicating line of cross section or seismic profiles (below). Location map showing bathymetry, cruise-track coverage, core and sample locations, and location of figures. Evidence for shoreline change is shown by comparing the 1997 shoreline, drawn in black, with a 1974 USGS quadrangle map. Dunedin Pass is shown open on the quadrangle map, and the north end of Clearwater Beach Island was narrower. The closing of Dunedin Pass occurred in 1988. The line of section A-E is linked to the seismic profile and cross sections presented below. F-G, H-I and J-K identify seismic profile locations presented with offshore core data at the bottom of this sheet.
Transcript of West-Central Florida Coastal Transect # 2: Caladesi Island ... · Oblique aerial photograph (taken...
VibracoreUSGS-95-43
VibracoreUSGS-95-44
SinkholeDeformed limestone
bedrock
Large sand waves formsand ridge
Sand waves
Hardbottom orsediment veneer
St. Joseph Sound
10
20
15
5
25
0
Wat
er d
epth
(m
eter
s)
0 1 2 km Vertical exaggeration ~180 X
Sea level
+5Caladesi
Island
Northwest-trending sand ridgeNorthwest-trending sand ridge
Sea floor
(see seismicdata above)
USGS-95-5 -7.61 m
9
8
1
USGS-95-21-9.74 m
111
10
USGS-95-43-13.69 m
1
2
14
15
16
USGS-95-44-12.47 m
1
13
14
000 0
8,300 ± 90 YBPStandard C-14 dateArticulated mollusc shellElevation: -15.94 m
Units in meters
Tampa Bay
References Cited
Barnard, P.L., 1998, Historical morphodynamics of inlet channels: West-Central Florida: St. Petersburg,University of South Florida, unpublished M.S. thesis, 179 p.
Brame, J.W., 1976, The stratigraphy and geologic history of Caladesi Island, Pinellas County, Florida: St.Petersburg, University of South Florida, unpublished M.S. thesis, 109 p.
Brooks, G.R., Doyle, L.J., Suthard, B.C., and DeWitt, N.T., 1999, Inner West-Central Florida continentalshelf: Sedimentary facies and facies associations: U.S. Geological Survey Open-File Report 98-796, 124 p.
Davis, R.A., 1994, Barriers of the Florida Gulf peninsula, in Davis, R.A., ed., Geology of HoloceneBarrier Island Systems: Heidelberg, Springer-Verlag, p. 167-206.
Davis, R.A. and Hine, A.C., 1989, Quaternary geology and sedimentology of the barrier island marshcoast, West-Central Florida: 28th International Geological Congress Guidebook T375, 38 p.
Evans, M.W., Hine, A.C., Belknap, D.F., and Davis, R.A., 1985, Bedrock control on barrier islanddevelopment: West-Central Florida coast: Marine Geology, v. 63, p. 263-283.
Oblique aerial photograph (taken in 1992) of Dunedin Pass which closed in 1988. Vegetated beach ridges,washover, and flood-tidal deposits occur landward of the beach. Vibracore locations are shown for theisland-transect portion (C-D) presented below. Depth to bedrock is 4 to 5 m below sea level along thissection.
J128 J40 J125J126J127 J155 CAL-3cCAL-3dJ19 CAL-3b
Line B94J-20
West-Central Florida Coastal Transect # 2: Caladesi Island - Clearwater Beach Island
U.S. Geological Survey Open-File Report 99-506
S.D. Locker,1 R.A. Davis,2 G.R. Brooks,3 A.C. Hine,1 and G. Gelfenbaum4
1 College of Marine Science, University of South Florida, St. Petersburg, FL 33701 2 Department of Geology, University of South Florida, Tampa, FL 33620 3 Department of Marine Science, Eckerd College, St. Petersburg, FL 33711 4 U.S. Geological Survey, Menlo Park, CA 94025
CAL-1Elevation: -0.16 m
Vegetated paralic
USGS-95-43Elevation: -13.69 m
C-14 Date:8,300 ± 90 YBP
Darkness at top andbottom of core photodue to unequal light
Burrowed sand(back-barrier)
Side-scan sonar imagery overlain on bathymetry reveals a northwest-trendingsand-ridge morphology common throughout the inner shelf in this region.Surface-sediment cover is thin and exhibits a patchy and discontinuousdistribution. Low backscatter (light gray) areas correspond to sand ridges andflats dominated by quartz sand. The dark (high backscatter) areas are largelycoarse sediment veneer with increased carbonate material (primarily shellmaterial), or some hardbottoms. Landward of the 4-m isobath, a 1995 colorinfrared digital orthophotograph shows the extension and seaward accretion ofthe north end of Clearwater Beach Island – compare with the USGSquadrangle map in panel above.
S
10
20
0
15
5
25
Wat
er D
epth
(m
)
10
20
0
15
5
25
South
Wat
er D
epth
(m
)
10
20
0
15
5
25
Wat
er D
epth
(m
)
Seismic-profile dataUninterpreted high-resolution "boomer" seismic profile illustrates the pooracoustic contrast between the Holocene sediment cover and the Pleistoceneexposure surface. The poor contrast is typical throughout the region and isattributed to the karst and weathered nature of the underlying pre-Quaternarybedrock. The offshore Holocene sediment thickness is usually less than 2 to 3m, corresponding with the higher relief portions of the sand waves or ridgesseen here.
Integrated stratigraphic cross section combining line-drawn interpretation ofseismic data, ground truthed by coring, with a coastal cross section based onvibracores. Cores in the offshore transect have no cross-shelf correlationpotential because they often contain different ridge deposits, shown in side-scan sonar imagery and bathymetry data. Most of the sediment volume in thiscoastal system is found in the barrier-island section.
Side-scan sonar imagery
Location map
Caladesi IslandOne of the oldest islands in this system is Caladesi Island, a classic example of a drumstick barrier.The north end of the island was separated from what is now Honeymoon Island as the result of thehurricane of 1921, which broke through Hog Island to form Hurricane Pass (Brame, 1976). CaladesiIsland experienced considerable progradation at its southern end due to the entrapment of sedimentdowndrift north of Dunedin Pass throughout most of the history of the island. Numerous beach/duneridges separate swales and ponds. Dunedin Pass closed in 1988, three years after Hurricane Elenaremoved the ebb-tidal delta and facilitated longshore transport across the mouth of the small andunstable inlet (Davis and Hine, 1989; Barnard, 1998). Since that time, the southern end of the islandhas experienced erosion, and considerable accretion has taken place on the north end.
Sediments that comprise the barrier/inlet system along this coast display little variety (Yale, 1997).Fine quartz sand dominates with carbonate skeletal debris and mud as subordinant constituents.Most sediments are distinctly bimodal, with a shell-gravel fraction and the sand fraction dominatedby quartz with lesser amounts of fine carbonate skeletal material. Stratigraphically, Caladesi Islandshows a sand-dominated series of lithofacies with a range of shell gravel and mud. Back-barrierfacies that originated as washover deposits are bioturbated and contain a significant amount ofmineral and organic mud. Shallow to intertidal shoals, beach, and dune environments arerepresented by clean sand and shelly sand. As progradation of beach/dune ridges took place withmangrove environments intercalated between them, the surface and near-surface sediments reflectedthe presence of mangrove environments.
Grain-size and composition data for bottom grab samples are presented belowthe sonar imagery. Samples generally consist of quartz-rich sand withsubordinate amounts of gravel and mud. Locally, samples are rich incarbonate gravel or sand. Side-scan sonar low backscatter correlates withmedium to fine siliciclastic sand with minor carbonate grains and is associatedwith the thicker sand-ridge deposits. The higher backscatter areas correlatewith coarse grain size and increased carbonates. The coarse-grained sedimentsare usually <10 to 20 cm thick, occupy low areas adjacent to thicker sandbodies, and form ripples oriented N-S with a 40- to 70-cm spacing.
Surface sediments
Projection: UTM, GRS 1980, NAD83, Zone 17. Coordinates: Geographic.Bathymetry in 2-m intervals starting at 4 m (modified from Gelfenbaum and Guy, 1999).Land areas represented by USGS 7.5-Minute Topographic Map.
+2
+1
0
-1
-2
-3
-4
-5
-6
Ele
vatio
n (m
)
CAL-5 CAL-1CAL-4 CAL-3 CAL-2Modernmangroves
Back-barrier(Washover)
Dune
Channel
Washover/Intertidal shoal
Vegetated paralic
Miocene limestone?
100 200 300 4000
meters
Nearshore/Beach Back-barrier/Washover
Caladesi Island
MLLW
0
1
2
3
4
5
1
2
3
4
0
CAL-5+0.82 m
CAL-4+0.26 m
0
1
2
3
1
2
3
0
CAL-3+0.19 m
0
1
2
3
4
1
2
3
4
0CAL-2-0.26 m
0
1
2
3
1
2
3
4
CAL-1-0.16 m
0
1
2
3
4
1
2
3
4
MLLW
Units in meters
Transect cross section
Seven generalized sedimentary-facies types were defined for a unified comparison ofcore data from the entire study area. All seven color-coded facies for the entire study areshown in the Explanation below. However, not all facies necessarily are present on eachtransect. Core photographs present individual cores cut into 1-m sections from top (upperleft) to bottom (lower right). Discrepancies in core length between the photographs andthe diagrams are due to compaction during the coring process. Offshore cores (left) arealigned at core tops. Core locations were chosen to sample thicker Holocene sections andto aid in identifying pre-Holocene stratigraphy. Core elevations were determined fromwater depth and tide tables. The datum for the barrier-transect cores is the mean lowestlow water (MLLW). Core photographs are shown for USGS-95-43 (most seaward) andCAL-1 (most landward).
Offshore vibracore retrieval ranged from a little over 1 m to approximately 2.5 m inlength. Although the vibracores penetrated to bedrock, there is often little indication of abedrock reflection seen in the seismic data. A poor impedance contrast between well-sorted shelf sands and the underlying exposure surface appears to be responsible for thelack of a well-defined seismic boundary at the base of the Holocene section.
Offshore cores contain a surface layer dominated by quartz sand. The facies is interpretedto have been deposited under modern open-marine conditions. In the seaward most core,the quartz sand unit is underlain by a muddy sand unit with numerous visible burrows.The muddy sand is interpreted to have been deposited in a back-barrier environment.Radiocarbon dates indicate that it was deposited 8,300 ±90 years before present (YBP).
In the two landwardmost cores, the Miocene limestone surface was penetratedimmediately below the surficial sediment/sand layer, once again indicating the paucity ofsediments on this part of the shelf.
The barrier-island transect cores (right), taken across the prograding southern end of thebarrier, record a transition from back-barrier and overwash sedimentary facies containinga more abundant shell and mud fraction, to cleaner sands in the beach and dunesedimentary facies. On cross sections where cores do not penetrate to bedrock, thecontrol is based on probe-rod data. The expanded coastal cross section to the rightincludes general environmental interpretations.
Core Data
Holocene sediment
Gelfenbaum, G. and Guy, K.K., 1999, Bathymetry of West-Central Florida: U.S. Geological SurveyOpen-File Report 99-417, CD-ROM.
Locker, S.D., Brooks, G.R., Hine, A.C., Davis, R.A., Twichell, D.C., and Doyle, L.J., 2001,Compilation of geophysical and sedimentological data sets for the West-Central Florida CoastalStudies Project: U.S. Geological Survey Open-File Report 99-539, CD-ROM.
Lynch-Blosse, M.A. and Davis, R.A., 1977, Stability of Dunedin and Hurricane Passes, Florida. CoastalSediments '77: American Society of Civil Engineers, New York, p. 774-789.
Stapor, F.W., Mathews, T.D., and Lindfors-Kerns, F.E., 1988, Episodic barrier island growth insouthwest Florida: A response to fluctuating Holocene sea level?: Miami Geological Society,Memoir 3, p. 149-202.
Yale, K.E., 1997, Regional stratigraphy and geologic history of barrier islands, West-Central Florida:St. Petersburg, University of South Florida, unpublished M.S. thesis, 180 p.
?
?
Corepenetration
0 1 2 km
10
20
15
5
25
0
Wat
er d
epth
(m
eter
s)
Note intermittent reflection atbase of Holocene sediment
Holocenesediment
Sand waves
Sea level
100 %
50 %50 %
Grain Size (wt. %)(right side)
Bottom-Sediment Diagram
0N104
Gravel
Sample ID
Composition (wt. %)(left side)
MudInsoluble
Carbonate(CaCO3)
Sand
Web Page Address:http://coastal.er.usgs.gov/wfla/
Data references:
Color Infrared Digital Orthophoto Quarter Quadrangles (CIRDOQQ), (1994, 1995), USGS EROS Data Center, Sioux Falls, SD57198. CD-ROMs.
Landsat TM Image, February 18, 1997, path 17, row 40. USGSEROS Data Center, Sioux Falls, SD 57198. CD-ROM.
7.5-Minute Series (Topographic) Quadrangles, U.S. GeologicalSurvey, Reston, VA 22092.
Acknowledgments
The large field program and combination of data sets brought to this compilation are theresult of significant efforts by many people. Kristy Guy and Beau Suthard helpedcompile, process, and display much of the imagery presented. Significant contributionswere made by Nancy DeWitt and Kristin Yale. We thank the following people for helpin the field or laboratory: Patrick Barnard, Greg Berman, Jim Edwards, Brian Donahue,Larry Doyle, Dave Duncan, John Cargill, Tom Ferguson, Megan FitzGerald, MarkHafen, Jackie Hand, Scott Harrison, Tessa Hill, Bret Jarrett, Jennifer Kling, KatieKowalski, David Mallinson, John Nash, Steve Obrochta, Meg Palmsten, John Pekala,Boudewijn Remick, Peter Sedgwick, Brad Silverman, Darren Spurgeon, David Ufnar,Ping Wang, and Tao Yucong. We also thank the crews and support staff of the researchvessels R/V Bellows, R/V Suncoaster (Florida Institute of Oceanography) and R/VGilbert (U.S. Geological Survey) for their assistance. Technical reviews by BarbaraLidz and Bob Morton are greatly appreciated.
List of west-Florida coastal-transect series maps (1 sheet each):
Transect #1: Anclote Key, USGS Open-File Report 99-505Transect #2: Caladesi Island-Clearwater Beach, USGS Open File-Report 99-506Transect #3: Sand Key, USGS Open-File Report 99-507Transect #4: Indian Rocks Beach, USGS Open-File Report 99-508Transect #5: Treasure Island-Long Key, USGS Open-File Report 99-509Transect #6: Anna Maria Island, USGS Open-File Report 99-510Transect #7: Longboat Key, USGS Open-File Report 99-511Transect #8: Siesta Key, USGS Open File-Report 99-512Transect #9: Casey Key, USGS Open File-Report 99-513
IntroductionA major goal of the West-Central Florida Coastal Studies Project was to investigatelinkages between the barrier-island system along the west coast of Florida and offshoresedimentary sequences. High population density along this coastline and the resultantcoastal-management concerns were primary factors driving the approach of thisregional study. Key objectives were to better understand sedimentary processes andaccumulation patterns of the modern coastal system, the history of coastal evolutionduring sea-level rise, and resource assessment for future planning. A series of nine"swath" transects, extending from the mainland out to a depth of 26 m, was defined toserve as a focus to merge these data sets and for comparison of different coastalsettings within the study area.
Transect #2 extends seaward from Caladesi Island and North Clearwater Beach (seelocation map to right). Information from seismic and vibracore studies is combined toderive a 2-D stratigraphic cross section extending from the offshore zone, through thebarrier island, and onto the mainland. This stratigraphic record represents the lateHolocene evolution of the coastal-barrier system and inner shelf following the last sea-level transgression and present highstand conditions. A comparison to surface-sedimentdistribution patterns indicated by side-scan sonar imagery and bottom grab samplesillustrates the importance of spatial variability in sediment-distribution patternsoffshore when considering stratigraphic interpretations of seismic and core data.
MethodsThe primary data sets used in this study were collected from 1993 to 1998.Geophysical surveys included high-resolution single-channel "boomer"seismic data and 100-kHz side-scan sonar imagery (Locker and others,2001). Most of the reconnaissance seismic and side-scan sonar data wereacquired during two offshore cruises in 1994. Additionally, bottom sampleswere collected during the cruises using an underway grab sampler at 4-kmintervals along track. Offshore core locations were selected based uponseismic data and were focused in areas likely to contain sufficient sedimentthickness for core retrieval (Brooks and others, 1999). Vibracores andprobe data provided stratigraphic control in the barrier-island and bayareas.
The four panels showing location and side-scan sonar imagery, seismicdata, and a stratigraphic cross section are at the same horizontal scale. Theseismic profile and cross-section panels are constructed by fitting the databetween the labeled cross-section turns (location map panel) that have beenprojected downward to the straight cross-section line. Subtle differences inthe horizontal scale of segments in the cross section due to this projectionare minimal. The horizontal scale, as well as vertical exaggeration of theseismic profile and cross section, are the same for all nine transects in themap series in order to facilitate comparison among transects.
Geologic History andMorphodynamics of Barrier IslandsBarrier islands on the west-central Gulf coast of Florida display a wide range inmorphology along the most diverse barrier/inlet coast in the world (Davis, 1994). Inaddition, the barriers have formed over a wide range of time scales from decades tomillennia. The oldest of the barriers have been dated at 3,000 years (Stapor andothers, 1988) and others have formed during the past two decades. The barriersystem includes long, wave-dominated examples as well as drumstick barriers thatare characteristic of mixed wave and tidal energy. Historical data on the very youngbarriers and stratigraphic data from coring older ones indicate that the barriersformed as the result of a gentle wave climate transporting sediment to shallow waterand shoaling upward to intertidal and eventually supratidal conditions. The barriersprobably formed close to their present position and several have been aided in theirlocation and development by antecedent topography produced by the shallowMiocene limestone bedrock (Evans and others, 1985). The two most importantvariables that control barrier-island development along the coast are the availabilityof sediment and the interaction of wave and tidal energy.
Location of west-central Florida coastal-transect maps with Transect #2 shown in red. 1997 LANDSAT TMimagery of Florida's west coast is merged with a bathymetric-surface model (Gelfenbaum and Guy, 1999).Bathymetric trends offshore in part reflect sediment-distribution patterns. The study area extends fromAnclote Key to Venice, FL.
Dune
Dune
Projection: UTM, GRS 1980, NAD83, Zone 17. Coordinates: Geographic.Bathymetry (areas > 4 m) after Gelfenbaum and Guy (1999).Coastal areas (< 4 m) represented by Digital Orthophoto Quarter Quadrangle (1995).
MULTIPLE
Location of study area along the west-centralFlorida coastline showing cruise-trackcoverage in red. Data types include high-resolution seismic-reflection data, side-scansonar imagery, surface-sediment samples,and vibracores. Blue-box areas identifycontinuous-coverage side-scan sonar mosaicareas. The bathymetry shown in 5-m intervalsby g r ay shad ing i s mod i f i ed f romGelfenbaum and Guy (1999).
VibracoreUSGS-95-5
MULTIPLE
VibracoreUSGS-95-21
Transect boundary
Vibracore
Underway surface-sediment sample
Surface-sediment grab sample
Cruise track in red with yellow highlight indicatingline of cross section or seismic profiles (below).
CAL-5+0.82 m
1
0
1
0
Depth fromtop of core
Depthrelative tomean sealevel(MLLW)
Core ID & Elevation
Explanation: core logs and sedimentary facies
gravel (> 60% gravel)
muddy sand (3-50% mud)
mud (>50% mud)
Pleistocene facies
Miocene (limestone and blue-green clay)
gravelly sand (< 3% mud and 25-60% gravel)
sand (< 3% mud and < 25% gravel)
Units in meters
North South North SouthNorth
Vibracore
Underway surface-sediment sample
Surface-sediment grab sample
Cruise track in red with yellow highlight indicatingline of cross section or seismic profiles (below).
Location map showing bathymetry, cruise-track coverage, core and samplelocations, and location of figures. Evidence for shoreline change is shown bycomparing the 1997 shoreline, drawn in black, with a 1974 USGS quadranglemap. Dunedin Pass is shown open on the quadrangle map, and the north endof Clearwater Beach Island was narrower. The closing of Dunedin Pass
occurred in 1988. The line of section A-E is linked to the seismic profile and cross sectionspresented below. F-G, H-I and J-Kidentify seismic profile locations
p r e sen t ed w i thoffshore core dataat the bottom ofthis sheet.
