West-Central Florida Coastal Transect # 6: Anna … 20 0 15 5 25 Water depth (meters) 30 35 40...
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10 20 0 15 5 25 Water depth (meters) 30 35 40 West-Central Florida Coastal Transect # 6: Anna Maria Island S.D. Locker, 1 G.R. Brooks, 2 R.A. Davis, 3 A.C. Hine, 1 and G. Gelfenbaum 4 Side-scan sonar imagery overlain on bathymetry reveals a mixed pattern of bedform orientation. Offshore ridges appear to trend NW-SE. In contrast, the nearshore area associated with ebb-tidal deposits exhibits more NE-SW-oriented bedforms, including an area of shore-normal sand bars adjacent to the shoreface immediately south of Southwest Channel. Although thicker and more abundant sediment accumulation characterizes the shelf in this area, hardbottom areas are still present offshore. Low backscatter (light gray) areas correspond to sand ridges and flats dominated by quartz sand. The dark (high backscatter) areas are mainly a coarse sediment veneer containing carbonate material (primarily shell material) or hardbottoms. Oblique aerial photograph of Anna Maria Island looking south (taken in 1993). The island transect portion (B-C) is shown here with vibracore interpretations presented below. Uninterpreted seismic profile reveals a variety of morphology associated with the sediment cover in this area. Large and relatively thick sand ridges offshore contrast with sand flats and localized sandwave fields related to the Tampa Bay ebb-tidal delta nearshore. Overall, the base of the Holocene is extrapolated from vibracore data that supports the seismic interpretations. Additional evidence includes hardbottoms (pre-Holocene bedrock) and probe-rod measurements of sediment thickness. Typically, there is poor acoustic contrast between the Holocene sediment cover and the Pleistocene exposure surface, which is attributed to the karstic and weathered nature of the underlying pre-Quaternary bedrock. Integrated stratigraphic cross section combining line- drawing interpretation of seismic data, ground-truthed by coring, with a coastal cross section based on vibracores. The modern sediment cover can be over 4 m thick offshore, corresponding with the higher-relief portions of the sand waves or ridges seen here. Over 8 m of Holocene sediment are indicated beneath Anna Maria Island. The subsurface is highly deformed due to karstic processes forming structures ranging from sinkholes to shelf valleys. The base of the Holocene is a major unconformity that truncates these structures, indicating that most, if not all of the deformation occurred after deposition of the Miocene limestone. Subsequently, the major hiatal surface, reworked during Quaternary transgressions and regressions of sea level, has not been significantly deformed. Side-scan sonar data Anna Maria Island Anna Maria Island is a drumstick barrier island that is the first barrier south of the expansive mouth of Tampa Bay. This island has been thoroughly developed, primarily with residential and small tourist facilities. The island has been fairly stable in its morphology over historical time, with the exception of significant beach erosion along the southern half or so of the island. This was remedied with a nourishment project in 1994. The stratigraphy of Anna Maria was investigated in detail by Pekala (1996) who took 35 vibracores throughout the island and adjacent areas. He determined that the island was a maximum of 3,000 years old and has extended itself to the south only in the past few hundred years. The transect across the north end of the island is taken from his work and modified by Yale (1997). Miocene bedrock was not penetrated by any of the cores; it has been shown to be at a depth of about 11 to 12 m below the northern part of the island (Ferguson, 1997) where this stratigraphic cross section is located. The basal unit recovered in cores is a brown, organic-stained Pleistocene sand. It is unconformably overlain by an organic-rich, muddy sand containing scattered shell debris which is interpreted to represent a vegetated paralic environment. Above this is a muddy and shelly sand that was probably originally deposited by swashover and/or washover processes and has subsequently been reworked extensively by bioturbation. Beach, nearshore, and dune deposits represent the island facies and are characterized by well-sorted sand and shelly sand. The prograding beach ridges on this end of the island are typical of drumstick barriers with beach and eolian components. 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. The carbonate gravel primarily occurs offshore, but also is found in areas such as the troughs between the nearshore sand bars. Surface sediments Transect cross-section Projection: UTM, GRS 1980, NAD83, Zone 17. Coordinates: Geographic. Bathymetry in 2-m intervals starting at 4 m. Land areas represented by USGS 7.5-Minute Topographic Map. Core Data Tampa Bay U.S. Geological Survey Open-File Report 99-510 Location map J56 J6 J71 J72 J73 J74 J75 J76 J165 N1 AMI-4c AMI-E -1 -2 -3 -4 0 1 2 1 2 0 AMI-3 +0.11 m 0 1 2 3 AMI-1 -1.97 m 0 1 2 3 4 5 6 1 2 3 4 5 6 7 AMI-4 -0.24 m 0 1 2 3 4 5 6 1 2 3 4 5 6 7 AMI-5 -0.27 m 0 1 2 3 4 5 1 2 3 4 5 6 AMI-6 -0.41 m 0 1 2 3 AMI-2 -1.89 m MLLW +2 +1 0 -5 -6 -7 Elevation (m) Tidal channel Nearshore/Beach Nearshore Vegetated paralic Back-barrier Pleistocene strandplain Dune Anna Maria Island AMI-1 ? ? AMI-2 AMI-3 AMI-4 AMI-5 AMI-6 200 0 400 600 800 meters MLLW 0 1 13 USGS-95-55 -12.19 m 0 1 2 13 14 15 USGS-95-56(2) -12.8 m 0 20 USGS-95-57 -19.2 m 3 0 USGS-96-206 -2.41 m 5 6 7 0 1 2 WF-93-13A -4.42 m 0 1 8 9 WF-93-20A -7.92 m 0 AM-95-1 0 AM-95-2 0 AM-95-3 50 cm 100 cm 0 cm USGS-95-56(2) Elevation: -12.8 m Top Bottom Quartz sand with shell fragments and black grains Holocene sediment Sea floor 10 20 0 15 5 25 Water depth (meters) 30 35 40 ? Deformed limestone bedrock Vibracore USGS-96-206 Vibracore USGS-95-56 Hardbottom or sediment veneer Sinkhole ? Top Bottom AMI-6 Elevation: -0.41 m 100 cm 0 cm 50 cm References Cited 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. 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. FitzGerald, M.V., 1995, Stratigraphy of tidal and fluvial paleochannel sequences beneath west-central Florida Gulf Coast barrier islands: St. Petersburg, University of South Florida, unpublished M.S. thesis, 134 p. Ferguson, T.W., 1997, Post-Miocene stratigraphy and sedimentary environments of valley-fill sequences at the mouth of Tampa Bay: St. Petersburg, University of South Florida, unpublished M.S. thesis, 118 p. Gelfenbaum, G. and Brooks, G. R., 1997, Long-term observations of migrating shore- normal bars: Proceedings Coastal Dynamics 97, Plymouth, England, p. 654-663. 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. Pekala, J.M., 1995, Stratigraphy and geologic history of Anna Maria Island, Manatee County, Florida: St. Petersburg, University of South Florida, unpublished M.S. thesis, 144 p. 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. 10 20 0 15 5 25 Water depth (meters) 30 35 40 Vibracore AM-95-3 J56 Line B94J-8 transect cross section South Vertical exaggeration ~90X North Multiples Seafloor 0 0.5 1 km Line B94J-2d 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. 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 Web Page Address: http://coastal.er.usgs.gov/wfla/ 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 sediment 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 the data sets and for comparison of different coastal settings within the study area. Transect #6 crosses the northern end of Anna Maria Island located on the south side of the mouth of Tampa Bay. Deposition associated with the large Tampa Bay ebb-tidal delta is an important process in this location. 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 sam - ples illustrates the importance of spatial variability in sediment-distribution patterns off - shore when considering stratigraphic interpretations of seismic and core data. 1 College of Marine Science, University of South Florida, St. Petersburg, FL 33701 2 Department of Marine Science, Eckerd College, St. Petersburg, FL 33711 3 Department of Geology, University of South Florida, Tampa, FL 33620 4 U.S. Geological Survey, Coastal and Marine Geology Program, Menlo Park, CA 94025 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-95-56 (offshore sand ridge) and AMI-6 (back-barrier). Offshore vibracore retrieval ranged up to 2.7 m in thickness. Sediments consist dominantly of quartz sand on the surface, occasionally overlying mud and muddy sand, which probably represent back-barrier deposits. The occasionally thick quartz-sand units are probably related to the Tampa Bay ebb-tidal delta and shore-normal bars lying immediately seaward of Anna Maria Island (Gelfenbaum and Brooks, 1997). Seawardmost cores contain a considerable amount of blackened sands. The island cross section shown to the right (modified from Yale, 1997) shows sandy nearshore and dune deposits overlapping muddier back-barrier sands to landward. The boundary between “nearshore” sediments overlying “back- barrier” sediments represents a ravinement or maximum flooding surface, and suggest more open marine or lower bay conditions existed in this location prior to the late-Holocene development of northern Anna Maria Island. Beneath northern Anna Maria Island a significant hiatus may be associated with this boundary. Radiocarbon dates from a core located on the landward side of the island, about a kilometer north of the transect yielded a date of 1,040 60 YBP (standard C-14 date) from the base of the nearshore facies, directly and unconformably overlying a brown quartz sand that yielded an AMS C-14 date of 9,730 60 YBP. The brown quartz sand is considered Pleistocene in age, therefore the early Holocene date may reflect some reworking of the surface of this unit during sea-level transgression and ravinement. ± ± Shelf valleys Vertical exaggeration ~180X Underway bottom sample Sea level Sea level 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 Nearshore Continuous sediment cover associated with Tampa Bay ebb-tidal delta Northwest-trending sand ridges Vibracore USGS-95-57 Sinkhole 0 1 2 km 0 1 2 km Vibracore AM-95-2 Vibracore WF-93-13A Core penetration Core catcher Vibracore USGS-96-206 Vibracore USGS-95-55 Vibracore WF-93-20A Shore-normal sand bars off Anna Maria Island Line B94J-8 Sand waves Pleistocene channel? Miocene (?) shelf valley Holocene sediment Vibracore AM-95-2 Vibracore WF-93-13A Back-barrier Vibracore USGS-95-55 Vibracore WF-93-20A Vibracore USGS-95-56 Vibracore USGS-95-57 Location map showing bathymetry, cruise-track coverage, vibracore and surface-sediment sample locations, and location of figures. The full transect cross section A-D is presented below. An expanded view of the island portion of the transect B-C is shown at lower right. Line E-F locates the seismic profile shown below the offshore core data at lower left. 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 of west-central Florida coastal-transect maps with Transect #6 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. Seismic-profile data 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. ? ? ? 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 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. 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. 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 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). Units in meters 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 Units in meters
Transcript of West-Central Florida Coastal Transect # 6: Anna … 20 0 15 5 25 Water depth (meters) 30 35 40...
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West-Central Florida Coastal Transect # 6: Anna Maria Island S.D. Locker,1 G.R. Brooks,2 R.A. Davis,3 A.C. Hine,1 and G. Gelfenbaum4
Side-scan sonar imagery overlain on bathymetryreveals a mixed pattern of bedform orientation.Offshore ridges appear to trend NW-SE. In contrast,the nearshore area associated with ebb-tidal depositsexhibits more NE-SW-oriented bedforms, includingan area of shore-normal sand bars adjacent to theshoreface immediately south of Southwest Channel.Although thicker and more abundant sedimentaccumulation characterizes the shelf in this area,hardbottom areas are still present offshore. Lowbackscatter (light gray) areas correspond to sandridges and flats dominated by quartz sand. The dark(high backscatter) areas are mainly a coarse sedimentveneer containing carbonate material (primarily shellmaterial) or hardbottoms.
Oblique aerial photograph of Anna Maria Island looking south (taken in 1993). The island transectportion (B-C) is shown here with vibracore interpretations presented below.
Uninterpreted seismic profile reveals a variety ofmorphology associated with the sediment cover in thisarea. Large and relatively thick sand ridges offshorecontrast with sand flats and localized sandwave fieldsrelated to the Tampa Bay ebb-tidal delta nearshore.Overall, the base of the Holocene is extrapolated fromv ib raco re da t a t ha t suppo r t s t he s e i smicinterpretations. Additional evidence includeshardbottoms (pre-Holocene bedrock) and probe-rodmeasurements of sediment thickness. Typically, thereis poor acoustic contrast between the Holocenesediment cover and the Pleistocene exposure surface,which is attributed to the karstic and weathered natureof the underlying pre-Quaternary bedrock.
Integrated stratigraphic cross section combining line-drawing interpretation of seismic data, ground-truthedby coring, with a coastal cross section based onvibracores. The modern sediment cover can be over 4m thick offshore, corresponding with the higher-reliefportions of the sand waves or ridges seen here. Over 8m of Holocene sediment are indicated beneath AnnaMaria Island. The subsurface is highly deformed dueto karstic processes forming structures ranging fromsinkholes to shelf valleys. The base of the Holocene isa major unconformity that truncates these structures,indicating that most, if not all of the deformationoccurred after deposition of the Miocene limestone.Subsequently, the major hiatal surface, reworkedduring Quaternary transgressions and regressions ofsea level, has not been significantly deformed.
Side-scan sonar data
Anna Maria IslandAnna Maria Island is a drumstick barrier island that is the first barrier south of the expansive mouth of TampaBay. This island has been thoroughly developed, primarily with residential and small tourist facilities. Theisland has been fairly stable in its morphology over historical time, with the exception of significant beacherosion along the southern half or so of the island. This was remedied with a nourishment project in 1994.
The stratigraphy of Anna Maria was investigated in detail by Pekala (1996) who took 35 vibracores throughoutthe island and adjacent areas. He determined that the island was a maximum of 3,000 years old and hasextended itself to the south only in the past few hundred years. The transect across the north end of the island istaken from his work and modified by Yale (1997). Miocene bedrock was not penetrated by any of the cores; ithas been shown to be at a depth of about 11 to 12 m below the northern part of the island (Ferguson, 1997)where this stratigraphic cross section is located.
The basal unit recovered in cores is a brown, organic-stained Pleistocene sand. It is unconformably overlain byan organic-rich, muddy sand containing scattered shell debris which is interpreted to represent a vegetatedparalic environment. Above this is a muddy and shelly sand that was probably originally deposited byswashover and/or washover processes and has subsequently been reworked extensively by bioturbation. Beach,nearshore, and dune deposits represent the island facies and are characterized by well-sorted sand and shellysand. The prograding beach ridges on this end of the island are typical of drumstick barriers with beach andeolian components.
Grain-size and composition data for bottom grabsamples are presented below the sonar imagery.Samples generally consist of quartz-rich sand withsubordinate amounts of gravel and mud. Locally,samples are rich in carbonate gravel or sand. Thecarbonate gravel primarily occurs offshore, but also isfound in areas such as the troughs between thenearshore sand bars.
Surface sediments
Transectcross-section
Projection: UTM, GRS 1980, NAD83, Zone 17. Coordinates: Geographic.Bathymetry in 2-m intervals starting at 4 m.Land areas represented by USGS 7.5-Minute Topographic Map.
Core Data
Tampa Bay
U.S. Geological Survey Open-File Report 99-510
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J56J6 J71J72J73J74J75J76 J165 N1 AMI-4cAMI-E
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Anna Maria Island
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References Cited
Davis, R.A., 1994, Barriers of the Florida Gulf peninsula, in Davis, R.A., ed., Geologyof Holocene Barrier Island Systems: Heidelberg, Springer-Verlag, p. 167-206.
Evans, M.W., Hine, A.C., Belknap, D.F., and Davis, R.A., 1985, Bedrock control onbarrier island development: West-Central Florida coast: Marine Geology, v. 63,p. 263-283.
FitzGerald, M.V., 1995, Stratigraphy of tidal and fluvial paleochannel sequencesbeneath west-central Florida Gulf Coast barrier islands: St. Petersburg,University of South Florida, unpublished M.S. thesis, 134 p.
Ferguson, T.W., 1997, Post-Miocene stratigraphy and sedimentary environments ofvalley-fill sequences at the mouth of Tampa Bay: St. Petersburg, University ofSouth Florida, unpublished M.S. thesis, 118 p.
Gelfenbaum, G. and Brooks, G. R., 1997, Long-term observations of migrating shore-normal bars: Proceedings Coastal Dynamics 97, Plymouth, England, p. 654-663.
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-FileReport 99-539, CD-ROM.
Pekala, J.M., 1995, Stratigraphy and geologic history of Anna Maria Island, ManateeCounty, Florida: St. Petersburg, University of South Florida, unpublished M.S.thesis, 144 p.
Stapor, F.W., Mathews, T.D., and Lindfors-Kerns, F.E., 1988, Episodic barrier islandgrowth 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.
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Data references:
Color Infrared Digital Orthophoto QuarterQuadrangles (CIR DOQQ), (1994, 1995),USGS EROS Data Center, Sioux Falls, SD57198. CD-ROMs.
Landsat TM Image, February 18, 1997, path17, row 40. USGS EROS Data Center, SiouxFalls, SD 57198. CD-ROM
7.5 Minute Series (Topographic) Quadrangles,U.S. Geological Survey, Reston, VA 22092.
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 99-509
Web Page Address:http://coastal.er.usgs.gov/wfla/
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 this regionalstudy. Key objectives were to better understand sedimentary processes and sedimentaccumulation 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 the data sets and for comparison of different coastal settingswithin the study area.
Transect #6 crosses the northern end of Anna Maria Island located on the south side of themouth of Tampa Bay. Deposition associated with the large Tampa Bay ebb-tidal delta isan important process in this location. Information from seismic and vibracore studies iscombined 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 thelate Holocene evolution of the coastal-barrier system and inner shelf following the lastsea-level transgression and present highstand conditions. A comparison to surface-sediment distribution patterns indicated by side-scan sonar imagery and bottom grab sam-ples illustrates the importance of spatial variability in sediment-distribution patterns off-shore when considering stratigraphic interpretations of seismic and core data.
1 College of Marine Science, University of South Florida, St. Petersburg, FL 33701 2 Department of Marine Science, Eckerd College, St. Petersburg, FL 33711 3 Department of Geology, University of South Florida, Tampa, FL 33620 4 U.S. Geological Survey, Coastal and Marine Geology Program, Menlo Park, CA 94025
Seven generalized sedimentary-facies types were defined for a unifiedcomparison of core data from the entire study area. All seven color-codedfacies for the entire study are shown in the Explanation below. However, notall facies necessarily are present on each transect. Core photographs presentindividual cores cut into 1-m sections from top (upper left) to bottom (lowerright). Discrepancies in core length between the photographs and the diagramsare due to compaction during the coring process. Offshore cores (left) arealigned at core tops. Core locations were chosen to sample thicker Holocenesections and to aid in identifying pre-Holocene stratigraphy. Core elevationswere determined from water depth and tide tables. The datum for the barrier-transect cores is the mean lowest low water (MLLW). Core photographs areshown for USGS-95-56 (offshore sand ridge) and AMI-6 (back-barrier).
Offshore vibracore retrieval ranged up to 2.7 m in thickness. Sediments consistdominantly of quartz sand on the surface, occasionally overlying mud andmuddy sand, which probably represent back-barrier deposits. The occasionallythick quartz-sand units are probably related to the Tampa Bay ebb-tidal deltaand shore-normal bars lying immediately seaward of Anna Maria Island(Gelfenbaum and Brooks, 1997). Seawardmost cores contain a considerableamount of blackened sands.
The island cross section shown to the right (modified from Yale, 1997) showssandy nearshore and dune deposits overlapping muddier back-barrier sands tolandward. The boundary between “nearshore” sediments overlying “back-barrier” sediments represents a ravinement or maximum flooding surface, andsuggest more open marine or lower bay conditions existed in this location priorto the late-Holocene development of northern Anna Maria Island. Beneathnorthern Anna Maria Island a significant hiatus may be associated with thisboundary. Radiocarbon dates from a core located on the landward side of theisland, about a kilometer north of the transect yielded a date of 1,040 60 YBP(standard C-14 date) from the base of the nearshore facies, directly andunconformably overlying a brown quartz sand that yielded an AMS C-14 dateof 9,730 60 YBP. The brown quartz sand is considered Pleistocene in age,therefore the early Holocene date may reflect some reworking of the surface ofthis unit during sea-level transgression and ravinement.
±
±
Shelf valleys
Vertical exaggeration ~180X
Underway bottom sample
Sea level
Sea level
Transect #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
Nearshore
Continuous sediment cover associatedwith Tampa Bay ebb-tidal delta
Northwest-trendingsand ridges
VibracoreUSGS-95-57
Sinkhole
0 1 2 km
0 1 2 km
VibracoreAM-95-2
VibracoreWF-93-13A
Corepenetration
Corecatcher
VibracoreUSGS-96-206
VibracoreUSGS-95-55
VibracoreWF-93-20A
Shore-normalsand bars offAnna Maria
Island
Line B94J-8
Sand waves
Pleistocenechannel? Miocene (?)
shelf valley
Holocenesediment
VibracoreAM-95-2
VibracoreWF-93-13A
Back-barrier
VibracoreUSGS-95-55
VibracoreWF-93-20A
VibracoreUSGS-95-56
VibracoreUSGS-95-57
Location map showing bathymetry, cruise-trackcoverage, vibracore and surface-sediment samplelocations, and location of figures. The full transectcross section A-D is presented below. An expandedview of the island portion of the transect B-C is shown
at lower right. Line E-F locates theseismic prof i le shown below the
offshore coredata a t lowerleft.
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 of west-central Florida coastal-transect maps with Transect #6 shown in red. 1997 LANDSATTM 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 extendsfrom Anclote Key to Venice, FL.
Seismic-profile data
Acknowledgments
The large field program and combination of data sets brought to thiscompilation are the result of significant efforts by many people. Kristy Guy andBeau Suthard helped compile, process, and display much of the imagerypresented. Significant contributions were made by Nancy DeWitt and KristinYale. We thank the following people for help in the field or laboratory: PatrickBarnard, Greg Berman, Jim Edwards, Brian Donahue, Larry Doyle, DaveDuncan, John Cargill, Tom Ferguson, Megan FitzGerald, Mark Hafen, JackieHand, 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, DavidUfnar, Ping Wang, and Tao Yucong. We also thank the crews and support staffof the research vessels R/V Bellows, R/V Suncoaster (Florida Institute ofOceanography) and R/V Gilbert (U.S. Geological Survey) for their assistance.Technical reviews by Barbara Lidz and Bob Morton are greatly appreciated.
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100 %
50 %50 %
Grain Size (wt. %)(right side)
Bottom-Sediment Diagram
0N104
Gravel
Sample ID
Composition (wt. %)(left side)
MudInsoluble
Carbonate(CaCO3)
Sand
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. Vibracores and probe data providedstratigraphic control in the barrier-island and bay areas.
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 shallowwater and shoaling upward to intertidal and eventually supratidal conditions. Thebarriers probably formed close to their present position and several have been aidedin their location and development by antecedent topography produced by theshallow Miocene limestone bedrock (Evans and others, 1985). The two mostimportant variables that control barrier-island development along the coast are theavailability of sediment and the interaction of wave and tidal energy.
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 QuarterQuadrangle (1995).
MULTIPLE
MULTIPLE
Location of study area along the west-central Florida coastline showing cruise-track coverage in red. Data types includehigh-resolution seismic-reflection data, side-scan sonar imagery, surface-sedimentsamples, and vibracores. Blue-box areasidentify continuous-coverage side-scansonar mosaic areas. The bathymetry shownin 5-m intervals by gray shading is modifiedfrom Gelfenbaum and Guy (1999).
Units in meters
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
Units in meters
