Subtask: Geologic Framework and Holocene Coastal Evolution – Eastern Louisiana Region
Subtask Leader: David Twichell - USGS Woods Hole Science Center for Coastal and Marine Geology, Woods Hole, Massachusetts
Aim
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Information on the shallow stratigraphy and geologic framework of the inner shelf of the northern Gulf of Mexico study area is needed to understand its influence on the evolution of coastal systems within this region. The shallow stratigraphy and geologic framework under the barrier island chains that characterize much of the NGOM coast is quite variable, and understanding these variations and how they influence different island chains is central to understanding the fate of these barrier island coasts and how to optimally manage them. Three island chains are receiving attention in this study: those that surround Apalachicola Bay, those along the Mississippi and Alabama coasts, and the Chandeleur Islands. Subtask 2.1 focuses on understanding the role the shallow stratigraphy and geologic framework of eastern Louisiana have played in the evolution of the Chandeleur Islands and the role this history plays in the future of these islands.
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| Figure 1: The Chandeleur Island chain is located in Eastern Louisiana and rest on top of the St. Bernard Delta Lobe Complex. The study area (black polygon) was divided into 3 subregions: North, Central, and South. The shoreline is pre-Katrina (2002). The MRGO (Mississippi River Gulf Outlet) navigation channel is shown cutting across the study area. The red line through the center of the study area is showing the separation
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Approach
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| Equipment used to collect the high-resolution chirp seismic-reflection profiles for this study. A, Chirp subbottom profiler on the after deck of the R/V Acadiana. B, Schematic showing deployment of the towfish astern of the vessel. C, An EdgeTech product detail photograph of the SB-512i tow vehicle.
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The approach taken to address the aims of the project has been to conduct a comprehensive geophysical mapping of the inner shelf surrounding the islands, collect cores to verify the geophysical observations, and link these offshore datasets with Lidar imagery and cores collected on the islands to provide baseline information for both the onshore and offshore components of this littoral system. These geological observations in concert with hydrodynamic measurements and modeling are used to define transport pathways within this littoral system.
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| Geophysical track lines and core locations used for analysis and interpretation of sediment distribution and facies maps.
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Accomplishments
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The Chandeleur Islands rest on the late Holocene St. Bernard Delta Lobe of the Mississippi Delta Complex in Eastern Louisiana (Fig. 1). High-resolution seismic-reflection, vibracore, and bathymetry data show the distribution, lithology, thickness, and volume of the sand sheet associated with the Chandeleur Islands. The interpretation of seismic and vibracore data reveal a barbell-shaped body of sandy material, the Chandeleur Island barrier lithosome, that sits unconformably on delta front, prodelta, and distributary channel deposits associated with the St. Bernard Delta. The barrier island lithosome is narrow (2-5 km wide) and thin (< 1m thick) in its central portion, and broader and thicker at the northern and southern ends (Fig. 2). The geometry of the lithosome is a function of net littoral sediment transport and accommodation space. The narrow thin nature of the central part of the island lithosome coincides with a zone of divergence in net sediment transport that has been identified in modeling studies with alongshore transport moving much of the sediment to depositional areas at the northern and southern ends of the chain. The geometry of the deposits at the northern and southern ends differ from each other because of the accommodation space which is controlled by the underlying geologic framework. The deposit at the southern end of the lithosome is a broad thin sheet that rests in shallow water on St. Bernard Delta Plain deposits. The deposit at the northern end of the lithosome is a narrow thick spit that extends beyond the edge of the St. Bernard Delta Plain and rests in deeper water on delta front and prodelta deposits (Fig. 3). Much of the St. Bernard Delta facies contain less than 20% sand and appear to be an inadequate source to maintain the islands in light of estimated long-term sediment transport rates.
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| Figure 2: Isopach map showing barrier lithosome thickness with 2005 shoreline (black). The dashed line is indicating the 2-m isopach.
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| Figure 3: Illustrative diagram showing the distribution of sediment in the lithosome of the Chandeleur Islands, La., and the inferred long-term sediment transport pathways (yellow arrows) based on sediment distribution. The deposit at the northern end of the islands transport path (Hewes Point) has accumulated beyond the edge of the original St. Bernard Delta, where water depths are deeper, and this increased accommodation space allows the deposit to be narrow but thick. The deposit at the southern end of the islands transport path (around the Mississippi River-Gulf Outlet [MRGO]) is broad and thin because it has accumulated in shallow water on top of the St. Bernard Delta, where the lack of accommodation space resulted in a sheet-like geometry to the deposit.
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Publications
Manuscripts
D. Twichell, L. Edmiston, B. Andrews, W. Stevenson, J. Donoghue, R. Poore, L. Osterman, 2010, Geologic controls on the recent evolution of oyster reefs in Apalachicola Bay and St. George Sound, Florida. Estuarine, Coastal and Shelf Science 88, 385-394. Flocks, J, Twichell, D, Sanford, J, Pendleton, E, and Baldwin, W, 2009, Chapter F, Sediment sampling analysis to define quality of sand resources, in Lavoie, D., editor, Sand resources, regional geology, and coastal processes of the Chandeleur Island coastal system - an evaluation of the resilience of the Breton National Wildlife Refuge: U.S. Geological Survey Scientific Information Report 2009-5252, p. xxx-xxx.
Flocks, J., Miner, M.D., Twichell, D.C., Lavoie, D.L., and Kindinger, J., 2009, Evolution and preservation potential of fluvial and transgressive deposits on the Louisiana inner shelf: understanding depositional processes to support coastal management: Geo-Marine Letters, v. 29, p. 359-378. Osterman, L.E., Twichell, D.C., and Poore, R.Z., 2009, Holocene evolution of Apalachicola Bay, Florida: Geo-Marine Letters, v. 29, p. 395-404. Rogers, B., and Kulp, M., 2009, Chapter G. The St. Bernard Shoals - an outer continental shelf sedimentary deposit suitable for sandy barrier island renourishment, in Lavoie, D., ed., Sand resources, regional geology, and coastal processes of the Chandeleur Islands coastal system - an evaluation of the Breton National Wildlife Refuge: U.S. Geological Survey Scientific Information Report 2009-5252, p. xxx-xxx.
Rogers, B., and Kulp, M.A., 2009, Late Holocene chronology, origin, and evolution of the St. Bernard Shoals, northern Gulf of Mexico, USA: Geo-Marine Letters, v. 29, no. 6, p. 379-394. Twichell, D, Pendleton, E, Baldwin, W, and Flocks, J, 2009, Geologic mapping of distribution and volume of potential sand resources, in Lavoie, D., editor, Sand resources, regional geology, and coastal processes of the Chandeleur Island coastal system - an evaluation of the resilience of the Breton National Wildlife Refuge: U.S. Geological Survey Scientific Information Report XXX, p. xxx-xxx.
Twichell, D., Pendleton, E., Baldwin, W., and Flocks, J., 2009, Chapter E. Geologic mapping of distribution and volume of potential sand resources, in Lavoie, D., editor, Sand resources, regional geology, and coastal processes of the Chandeleur Island coastal system: an evaluation of the resilience of the Breton National Wildlife Refuge: U.S. Geological Survey Scientific Information Report 2009-5252, p. xxx-xxx.
Twichell, D.C., Pendleton, E.A., Baldwin, W.E., and Flocks, J., 2009, Subsurface control on sea-floor erosional processes offshore of the Chandeleur Islands, Louisiana: Geo-Marine Letters, v. 29, no. 6, p. 349-358. Danforth, W.W., Denny, J.F., O'Brien, T.F., Bergeron, E., Twichell, D.C., Worley, C.W., Andrews, B.D., and Irwin, B.J., 2007, USGS shallow water geophysical mapping operations using a new autonomous surface vehicle, in U.S. Hydro 2007, 15-17 May 2008, Norfolk, VA, Hydrographic Society of America. Denny, J.F., Schwab, W.C., Twichell, D.C., O'Brien, T.F., Danforth, W.W., Foster, D.S., Bergeron, E., Worley, C.W., Irwin, B.J., Butman, B., Valentine, P.C., Baldwin, W.E., Morton, R.A., Thieler, E.R., Nichols, D.R., and Andrews, B.D., 2007, USGS advances in integrated high-resolution sea-floor mapping, in Coastal Sediments '07: Sixth International Symposium on Coastal Engineering and Science of Coastal Sediment Processes: Proceedings, New Orleans, LA, May 13-17, 2007, American Society of Civil Engineers, p. 2487-2500.
Lavoie, D., 2007, Current and future science plans for restoring a resilient coast, in Farris, G.S., Smith, G., Holm, T., Robbins, L., and Demas, C., eds., Science and the storms: the USGS response to the hurricanes of 2005: Lafayette, LA, U.S. Geological Survey, p. 265-269. Open File Report(OFR)
Pendleton, E.A., Barras, J.A., Williams, S.J., and Twichell, D.C., 2010, Coastal vulnerability assessment of the Northern Gulf of Mexico to sea-level rise and coastal change: U.S. Geological Survey Open-File Report 2010-1146. Twichell, D.C., Pendleton, E.A., Poore, R.Z., Osterman, L.E., and Kelso, K., 2009, Vibracore, radiocarbon, microfossil, and grain-size data from Apalachicola Bay, Florida: USGS Open-File Report 2009-1031. Baldwin, W.E., Pendleton, E.A., and Twichell, D.C., 2008, Geophysical data from offshore of the Chandeleur Islands, Eastern Mississippi Delta, U.S. Geological Survey Open-File Report 2008-1195 (DVD-ROM) Twichell, David C., Brian D. Andrews, Brian D., Edmiston, H. Lee, and Stevenson, William R., 2007, Geophysical Mapping of Oyster Habitats in a Shallow Estuary; Apalachicola Bay, Florida, U.S. Geological Survey Open-File Report 2006-1381 Abstracts and Presentations
Twichell, D.C. and nine others, 2009, Mapping the floor of Apalachicola Bay, Florida: Summary of acoustic mapping techniques and findings: Survey methods for shallow water habitat mapping in Northeast Department of Interior Holdings and Estuarine Research Reserves, Univ. New Hampshire.
Flocks, J., Twichell, D.C., Baldwin, W.E., Kulp, M., and Miner, M.D., 2008, Influence of antecedent geology on barrier island development, northern Chandeleur Islands, Louisiana [abstract #2462]in 2008 Ocean Sciences meeting, March 2-7, 2008, Orlando, FL, American Society of Limnology and Oceanography.
Flocks, J., Twichell, D.C., Baldwin, W.E., Pendleton, E.A., Miner, M.D., and Kulp, M., 2008, Shallow stratigraphic framework and evolution of the northern Chandeleur Islands, Louisiana [abstract]in Association of Environmental and Engineering Geologists 2008 annual meeting program with abstracts, New Orleans, LA, p. 63-64.
Miner, M.D., Kulp, M., Georgiou, I., Sallenger, A., FitzGerald, D., Flocks, J., and Twichell, D., 2008, Sediment transport trends along the Chandeleur Islands, Louisiana: Implications for island sustainability and barrier island management: AGU-ASLO Meeting Pendleton, E.A., Twichell, D.C., and Baldwin, W.E., 2008, Documenting erosion of the St. Bernard Delta: Past and present [abstract]: Eos, Transactions - American Geophysical Union, v. 89, no. 53, Fall meeting supplement, OS43B-1292. Twichell, D.C., Baldwin, W.E., Flocks, J.G., Pendleton, E.A., Miner, M.D., and Kulp, M., 2008, Subsurface control on sea-floor erosional processes offshore of the Chandeleur Islands, LA [abstract #1368], in 2008 Ocean Sciences meeting, March 2-7, 2008, Orlando, FL, American Society of Limnology and Oceanography. Twichell, D.C., Edmiston, L., Andrews, B.D., Stevenson, W., Osterman, L.E., Donoghue, J., and Poore, R.Z., 2008, The development of oyster beds in Apalachicola Bay, FL during the late Holocene [abstract and poster], in Lavoie, D., and Rosen, B., eds., USGS Gulf Coast Science Conference and Florida Integrated Science Center Meeting: Proceedings with Abstracts, October 20-23, 2008, Orlando, FL: St. Petersburg, FL, U.S. Geological Survey Open-File Report 2008-1329, p. 90. Twichell, D.C., Flocks, J.G., Miner, M.D., Baldwin, W.E., Pendleton, E.A., Kulp, M., and Georgiou, I.Y., 2008, Assessing the importance of shoreward vs. alongshore sand transport during the late Holocene evolution of the Chandeleur Islands, LA [abstract]: Geological Society of America Abstracts with Programs, v. 40, no. 6, p. 314. Twichell, D.C., Pendleton, E.A., Baldwin, W.E., and Flocks, J., 2008, Subsurface control on sea-floor erosional processes offshore of the Chandeleur Islands, LA [abstract and poster], in Lavoie, D., and Rosen, B., eds., USGS Gulf Coast Science Conference and Florida Integrated Science Center Meeting: Proceedings with Abstracts, October 20-23, 2008, Orlando, FL: St. Petersburg, FL, U.S. Geological Survey Open-File Report 2008-1329, p. 89. Danforth, W.W., Denny, J.F., O'Brien, T.F., Bergeron, E., Twichell, D.C., Worley, C.W., Andrews, B.D., and Irwin, B.J., 2007, USGS shallow water geophysical mapping operations using a new autonomous surface vehiclein U.S. Hydro 2007, 15-17 May 2008, Norfolk, VA, Hydrographic Society of America.
Denny, JF, Schwab, WC, Twichell, DC, O'Brien, TF, Danforth, WW, Foster, DS, Bergeron, E, Worley, CR, Irwin, BJ, Butman, B, Valentine, PC, Baldwin, WE, Morton, RA, Thieler, ER, Nichols, DR, and Andrews, BD, 2007, USGS advances in integrated high-resolution sea-floor mapping: Inner continental shelf to estuaries: Coastal Sediments 2007 Proceedings, v. 3, 14 p.
Twichell, D.C., Andrews, B.D., Bergeron, E., Edmiston, L., and Stevenson, W., 2007, Benthic mapping in Apalachicola Bay, FL: Summary of acoustic mapping techniques and findings [abstract], in Coastal GeoTools07 Proceedings, March 5-8, 2007, Myrtle Beach, SC, p. 53.
Denny, J.F., O'Brien, T.F., Bergeron, E., Twichell, D.C., Worley, C.W., Danforth, W.W., Andrews, B.D., and Irwin, B.J., 2006, Advances in shallow-water, high-resolution sea-floor mapping; integrating an autonomous surface vessel (ASV) into nearshore geophysical studies [abstract NS31B-1574]: EOS, Transactions AGU, v. 87, no. 52 Fall meeting suppl.
Other
Twichell, D.C., and Brock, J.C., 2011, Seafloor erosional processes offshore of the Chandeleur Islands, Louisiana: U.S. Geological Survey Fact Sheet 2011-3026, 2 p. Danforth, W.W., Denny, J.F., O'Brien, T.F., Bergeron, E., Twichell, D., Worley, C.R., Andrews, B.D., and Irwin, B., 2007, USGS shallow water geophysical mapping operations using a new autonomous surface vehicle: U.S. Hydro 2007, Norfolk, VA, Conference Proceedings, CD-ROM 4
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U.S. Department of the Interior |
U.S. Geological Survey