Historical Subsidence and Wetland Loss in the Mississippi Delta Plain Robert A. Morton1, Julie C. Bernier2, John A. Barras3, and Nicholas F. Ferina1 1U.S. Geological Survey, 600 4th St. S., St. Petersburg, FL 33701 2Environmental Careers Organization, 600 4th St. S., St. Petersburg, FL 33701 3U.S. Geological Survey, National Wetlands Research Center, Baton Rouge, LA 70894 Abstract Five representative areas of the Mississippi River delta plain were investigated using remote images, marsh elevations, water depths, sediment cores, and radiocarbon dates to estimate the timing, magnitudes, and relative rates of marsh erosion and land subsidence at geological and historical time scales. In the Terrebonne-Lafourche region of rapid interior-wetland loss, former marshes are now submerged beneath water that averages 0.5 to 1.0 m deep. Most of the permanent historical flooding was caused by rapid subsidence and collapse of the delta plain that occurred during the late 1960s and 1970s. Subsequent erosion of the submerged delta-plain marsh was relatively minor at most of the coring sites. Widespread nearly simultaneous collapse of marshes across the Mississippi delta plain appears to be unprecedented and not repeated in the geological record of the past 1,000 years. Surface and subsurface data strongly indicate that the rapid subsidence and associated wetland loss were largely induced by extraction of hydrocarbons and associated formation water. Average historical rates of subsidence between 1965 and 1993 were about 8 to 12 mm/yr, whereas average geological rates of subsidence for the past 5,000 years were about 1 to 5 mm/yr. Natural processes such as deep-seated salt migration and fault movement cannot be discounted entirely, but there is no compelling evidence that these processes were responsible for the observed historical changes. Results of this study provide a basis for determining the relative importance of subsidence and shoreline erosion as causes of past wetland loss and for predicting sites and probable mechanisms of future wetland loss. This information should improve the selection of project sites and designs for wetland-loss mitigation and coastal restoration in south Louisiana. Introduction The magnitude, rate, and timing of wetland loss in south Louisiana and the identification of the underlying processes that cause historical wetland loss have been high-priority topics of scientific investigation since the 1980s. These issues take on even greater importance and urgency considering that the state is seeking substantial federal funding to restore parts of coastal Louisiana and to compensate for some of the historical wetland loss. There are two major challenges for researchers responsible for providing the scientific data used to formulate public policy regarding wetland loss and coastal restoration in Louisiana. The first is generating subsidence estimates for wetland areas that are not immediately adjacent to benchmarks and tide gauges, which is where subsidence rates have been determined previously. The second challenge is developing accurate models for predicting areas and rates of future subsidence and wetland loss. This paper addresses the general lack of subsidence estimates away from levee roads and marinas by applying the field and laboratory methods of Morton et al. (2003) to four additional areas of historical wetland loss. The purpose of the study is to examine further the timing and processes involved in subsidence and wetland loss in coastal Louisiana. This was accomplished by: (1) estimating magnitudes of recent subsidence and erosion at these selected areas and (2) comparing the temporal and spatial trends of wetland change to historical trends of subsurface-resource extraction in the same areas. Methods Vibracores and water depths were obtained at the Bay St. Elaine, DeLarge, Pointe au Chien, and Bully Camp study areas (Figs. 1 and 2), where historical wetland loss has been rapid and widespread. Core pairs provided close correlation between delta-plain sediments from the emergent marsh and adjacent open water. Ten additional vibracores had previously been collected from the Madison Bay area (Figs. 1 and 2, and Morton et al., 2003). The cores provided a basis for identifying the predominant sedimentary facies and for selecting stratigraphic contacts and surfaces that could be correlated between cores and used to estimate magnitudes of wetland subsidence and erosion (Table 1). The core locations, descriptions, and photographs, detailed descriptions of historical land-water changes, and histories of nearby resource extraction (oil and gas, sulfur) for the five study areas are reported in Morton et al. (2005). Water depths at open-water coring sites and along bathymetric profiles were measured from the coring barge with a graduated rod, while the geographic coordinates of each depth measurement were obtained simultaneously with a GPS receiver. Movements of water levels at the coring sites during the field operations were assumed to be comparable to those recorded at nearby tide gauges (Fig. 2, and Morton et al., 2003, 2005). Sub-regional water depths and marsh elevations (Table 1) can be compared only if they are corrected for any local conditions (e.g., tidal stage) that would bias the water-level data. The U.S. Army Corps of Engineers (USACE) New Orleans District and Louisiana Department of Natural Resources (LDNR) operate independent networks of tide gauges located throughout the coastal waters of south Louisiana. The tide gauges at Cocodrie (USACE #76305) and near Montegut (LDNR #TE01-12R) are located less than 20 km from the coring sites (Fig. 2). The Cocodrie gauge was used to correct measured water levels at Bay St. Elaine, Madison Bay, and DeLarge, and the Montegut gauge was used to correct measured water levels at Pointe au Chien and Bully Camp. Beta Analytic, Inc. (Miami, FL) conducted isotopic analyses of peat samples from the vibracores and provided radiocarbon ages (14C) and the corresponding _13C values for the remains of former delta-plain marshes (Table 2). Ranges and means of _13C ratios for the fresh, intermediate, brackish, and saline marshes of the Barataria Basin (Chmura et al., 1987) were used to interpret the types of marshes recovered in the vibracores. Average long-term geological rates of delta-plain subsidence can be inferred from burial histories of peats, using depths of peat below the surface and the 14C peat ages (Penland et al., 1988; Roberts et al., 1994; Kulp and Howell, 1998). Results of those calculations (Tables 3 and 4) can also be expressed as average long-term geological rates of sediment aggradation. For this report, burial histories of peats are expressed as subsidence rates rather than rates of sediment aggradation. Historical Subsidence and Erosion of Delta-Plain Marshes Methods of Estimating Subsidence and Erosion Magnitudes of marsh subsidence and erosion were estimated by comparing the elevations and vertical offsets (Table 1) of sediment surfaces and stratigraphic contacts correlated between adjacent core pairs. The relative subsidence and erosion between emergent marsh and open-water cores assumes that marsh-sediment thickness and stratigraphic positions of correlative contacts are uniform over short distances (tens to hundreds of meters). The amount of erosion at the open-water core site is equal to the difference in marsh-sediment thickness between the open-water core and the adjacent marsh core. The amount of subsidence at the open-water core is equal to the elevation difference between the correlated stratigraphic markers between the two adjacent cores. To be precise, the core sections being correlated must not be deformed (shortened), and the erosion and subsidence estimates must equal the vertical displacement between the cores (Table 1). This technique provides a minimum estimate of total subsidence because there is no measurement of the absolute amount of historical subsidence of the marsh surface relative to some standard vertical datum. Stated another way, the former marsh preserved beneath open water has subsided more than the adjacent emergent marsh, but the emergent marsh also has subsided some unknown amount. Pointe au Chien Area The Pointe au Chien (PAC) study area (Figs. 1 and 3) was selected to represent the results presented by Morton et al. (2005) because, in many respects, it is similar to the other four delta-plain settings. Extant marsh elevations at PAC range from 32 to 39 cm above NAVD88, and water depths where marsh formerly existed range from 24 to 68 cm and average about 48 cm below NAVD88 (Fig. 4). The Pointe au Chien study area is located within an east-west regional trend of historic wetland loss that extends from Lake De Cade to Bayou Lafourche (Fig. 2). Wetland loss at PAC is nearly complete, with isolated marsh patches surrounded by open water. Much of the wetland loss occurred between 1969 and 1974 (Fig. 3). There is no obvious surface expression of faults or other structures controlling patterns of wetland loss, although the projected surface trace of the Golden Meadow fault (Kuecher et al., 2001) occurs to the south of the core sites (Fig. 3D). Six stratigraphic units were identified in the PAC cores: (1) dark olive-gray peat, (2) gray to olive-gray, massive to laminated mud, (3) olive-gray to black peat, (4) gray to olive-gray or black, massive to laminated mud and organic mud, (5) olive-gray to gray, massive to laminated silt, sand, and/or mud, and (6) olive-gray laminated mud and sand. The unit 3 peat represents the first subdelta marsh. Radiocarbon ages and carbon-isotope ratios of peat samples indicate that freshwater plants established the first marsh at PAC about 950 BP. The duration of this wetland is uncertain because the ages of samples near the top of the peat are within the error range of ages from the base of the peat (Fig. 4). After the first marsh was flooded, as much as 50 cm of mud was deposited before the last marsh was established about 300 to 400 BP. Since then, long-term rates of marsh aggradation have averaged about 1 mm/yr (Table 3). The magnitudes of land subsidence are similar across the Pointe au Chien area of wetland loss (Fig. 4), and the highest marsh elevations coincide with the areas of least subsidence. Analysis of marsh cores PAC-01B and PAC-05 suggest that the base of the last marsh was near NAVD88 before the area subsided. Consequently, comparisons of open-water cores with marsh core PAC-02B may underestimate total subsidence because the marsh remnant at core PAC-02B has subsided more than the adjacent emergent marsh. Core PAC-02B has subsided 16 to 37 cm relative to the adjacent marsh at cores PAC-01B and PAC-05. Subsidence at the open-water sites ranged from 75 to 117 cm and averaged about 88 cm (Table 1). The variable thickness of marsh sediments across the Pointe au Chien area makes estimates of erosion at the open-water sites imprecise. Nevertheless, erosion of the last marsh surface ranged from 0 to 14 cm, which is minor compared to magnitudes of subsidence. Patterns of wetland loss in the Pointe au Chien area do not coincide with the projected extent of any single oil-and-gas field, but the area of wetland loss is surrounded by the Bayou Jean la Croix, Lirette, and Montegut fields (Fig. 2). Initial discovery of gas in the 1920s at Lirette was attributed to surface seeps, whereas deep hydrocarbons at Lirette were discovered in 1937 (Troutman, 1956) and at Montegut in 1957 (Silvernail, 1967). These fields produce from rollover anticline structures associated with a family of growth faults (Piaggio, 1961; Lyons, 1982). Peak hydrocarbon production from these fields occurred between 1965 and 1980 (Fig. 5). The combined cumulative production through 2002 from the three fields was 35.2 million bbls of oil, 1.7 Tcf of gas, and 103 million bbls of water. Regional depressurization of subsurface reservoir strata may be a contributing factor to surface subsidence in this area. The projected surface trace of the Golden Meadow Fault extends through the southern zone of greatest wetland loss between Bayou Terrebonne and Bayou Pointe au Chien (Fig. 2), but spatially it does not appear to correlate with any limits to wetland loss. The projected surface trace of the Lake Hatch spur fault, however, approximates the northern boundary of extensive wetland loss. Geological and Historical Rates of Subsidence Geological Subsidence Rates Rates of vertical sediment accumulation have been used as a proxy for subsidence rates based on the assumption that the accommodation space necessary for vertical sediment accumulation (aggradation) was provided by subsidence regardless of the specific process (crustal loading, sediment compaction, fault activation). For wetland sediments and static sea-level conditions, this assumption appears to be valid as a first approximation. The condition of constant sea level equivalent to modern sea level is not difficult to achieve for recent periods, such as decades or a few centuries, but would not be a reasonable assumption for periods encompassing several millennia. To avoid potential inaccuracies associated with eustatic fluctuations, only published subsidence rates for periods less than 5,000 years were included in the comparison (Table 4). Geological rates of subsidence calculated for this study range from 0.5 to 4.4 mm/yr (Table 3) and average about 2 mm/yr (Table 4). Historical Subsidence Rates Historical changes in land elevation relative to a standard vertical datum can be measured directly from controlled benchmarks or inferred from long-period tide-gauge records (Holdahl and Morrison, 1974). Both of these methods have been used to approximate subsidence rates in south Louisiana (Penland et al., 1988; Morton et al., 2002). Shinkle and Dokka (2004) re-analyzed historical leveling data along Bayou Lafourche and Bayou Petit Caillou and calculated revised subsidence rates between 1965 and 1993. The spatial trends of the revised subsidence rates (Fig. 6) are identical to those presented by Morton et al. (2002); however, they also allow comparison of subsidence rates for two periods (Fig. 6). Within the context of generally increased subsidence in a seaward direction, highest rates of subsidence coincided locally with faults and producing oil-and-gas fields. Between the fields and faults, subsidence rates were lower. There is no evidence of uplift across the known salt domes (Valentine, Bully Camp, and Leeville) that would indicate historical dome growth. From 1965 to 1982, subsidence rates between Raceland and Leeville ranged from 1.6 to 12.0 mm/yr and averaged about 7.6 mm/yr. From 1982 to 1993, subsidence rates ranged from 8.2 to 18.9 mm/yr and averaged about 12.1 mm/yr. Although subsidence rates accelerated between the two periods, the spatial order of higher and lower rates was maintained, indicating that subsidence is strongly controlled by subsurface geological processes. Comparison of Subsidence Rates Short-term historical rates of geological processes are commonly higher than the long-term average rates of those same processes, and subsidence rates are no exception. The important question to answer is whether the temporal differences are related to actual differences in the driving forces, or whether they are simply related to timing of the observations or sampling frequency. Some geological processes, such as fault slip, are intermittent, and their instantaneous rates may be very high, but the duration is short and the frequency of recurrence is low. These processes typically produce low long-term average rates of change. High instantaneous rates measured for these processes cannot be sustained indefinitely; therefore, those rates should not be extrapolated for predictive purposes. For example, if the historical rates of subsidence had persisted for the past 1,000 years, the Mississippi delta would have been deeply inundated long ago. Historical subsidence rates are roughly an order of magnitude higher than geological subsidence rates (compare Fig. 6 and Table 4). One explanation would be that natural faulting and subsidence are active at a time when monitoring is being conducted, and the methods of detection can resolve and measure the movement. Another explanation is that the rates actually are much higher than normally would be expected because subsidence and/or fault activation have been induced by subsurface-resource extraction. Whether the high rates of historical subsidence and associated wetland loss are natural or induced is still somewhat controversial. Gagliano et al. (2003) concluded that historical subsidence and wetland losses in south Louisiana were caused naturally by sediment loading, salt evacuation, and gravity gliding. All of these processes are known to be responsible for the overall tectonic regime of the Gulf Coast Basin, but Gagliano et al. (2003) presented no evidence to substantiate their claim that the recent timing (post-1960s) and rates of subsidence south of New Orleans were attributable to natural salt migration and faulting. They also did not consider that (1) major decreases in formation pore pressure, such as those reported by Morton et al. (2002) around hydrocarbon producing fields in south Louisiana, have the same effect as sediment loading, or that (2) changes in subsurface stress induced by fluid withdrawal are capable of accelerating movement of potentially active faults (Chan, 2005). Gagliano et al. (2003) also argued that the 1964 Alaskan earthquake was largely responsible for the timing of fault reactivation in south Louisiana, again without presenting any scientific evidence of transitory changes in subsurface stress that would support their speculation. The 1964 Alaskan earthquake was not felt in Louisiana, although seiches were generated in water bodies by the passing surface wave (Stevenson and McCulloh, 2001). Perhaps more important is the fact that the massive wetland losses in the delta plain (Figs. 2 and 7) were mostly initiated more than 5 years after the 1964 Alaskan earthquake. Significant reductions in subsidence rates are expected in the Terrebonne-Lafourche Basins because the rates of subsurface-fluid withdrawal that were largely responsible for the rapid induced subsidence have markedly declined (Fig. 7). Moreover, whatever contribution fault reactivation may have made, fault movement likely has already relieved the stress differential created by subsurface pressure reductions, and the state of stress has returned to near-equilibrium conditions. If this is true, then additional subsidence related to fault reactivation would not be expected because the subsurface perturbation caused by peak fluid production has passed (Fig. 7). Conclusions and Implications Historical wetland losses in the Mississippi delta plain have been classified on the basis of morphology and interpreted physical processes (Penland et al., 2000a, 2000b). Wetland losses around the margins of interior water bodies were attributed to shoreline erosion based on the inferred erosional capability of storm waves and field observations of local marsh erosion. Results of our study indicate that most of the wetland losses around open-water bodies at the coring sites are due to subsidence, and erosion is only a minor process contributing to the conversion of wetlands to open water. At most of the open-water sites that were formerly continuous emergent marsh, extant water depths are greater than the thickness of the delta-plain marsh. This physical relation is clear evidence that wetland loss resulted from subsidence, because it is impossible to erode to those depths and still preserve some of the marsh deposits. Emergent-marsh elevations, used as the standard for subsidence estimates, are significantly lower where subsidence has been greatest, such as at Madison Bay and the marsh-island remnants of Pointe au Chien and Bully Camp. The magnitudes and similarities of subsidence around the perimeters of water bodies that were former marshes provide compelling evidence that the subsidence is not largely related to fault reactivation, because it is not geologically reasonable to infer a fault between each emergent-marsh and open-water core pair. The similarities of subsidence magnitudes across the delta plain, regardless of position relative to a fault plane, are further evidence that recent subsidence is not locally fault controlled. Lithologic and chronostratigraphic similarities of peat deposits from Bay St. Elaine, DeLarge, Pointe au Chien, and Bully Camp indicate that processes that influenced the organic accumulation and influx of clastic sediments operated over large portions of the delta plain, and not just locally. This implies that fault reactivation is not a likely mechanism to explain the alternating deposition of peat and mud several hundred years ago. Furthermore, there is no unequivocal evidence of a fault influencing the thickness or number of peat beds at any of the coring sites. This includes Bay St. Elaine, where cores were deliberately taken across the marsh-water lineament that appears to be the surface expression of a fault. The fault may have moved recently, but there is no evidence of recurrent motion in the recent geologic past that has resulted in stratigraphic expansion, which is typical of an active growth fault that moves frequently. The types of core data and imagery used by Gagliano et al. (2003) and Morton et al. (2002, 2003, 2005) are similar, and yet their interpretations with regard to past and future subsidence and wetland loss are quite different. These differences are not academic, because they have profound implications with regard to predicting future subsidence and its impact on coastal-restoration projects. Gagliano et al. (2003) attributed the historical subsidence and wetland loss to natural processes deep within the Gulf Coast Basin that are random and unpredictable as to future occurrences. In contrast, Morton et al. (2002, 2003, 2005) concluded that historical subsidence and wetland loss was primarily induced by fluid withdrawal, and therefore the future impacts are qualitatively predictable. Results from this study confirm that the most likely explanation for historical wetland losses in south-central Louisiana is regional subsidence and local fault reactivation induced by hydrocarbon production. There is no compelling evidence of historical salt dome growth in the area before, during, or after the period of rapid subsidence and the pore-pressure reduction in the reservoirs is equivalent to sediment loading across the delta plain. Furthermore, it is clear that offsets in stratigraphic marker beds observed in shallow cores taken in marsh and adjacent open-water sites are a result of subsidence, not fault slip. There is no evidence that such widespread instantaneous subsidence occurred in the past few thousand years as a result of natural deep-basin processes (sediment loading, salt migration, gravity gliding). The results of this study give guidance to future research directions and the development of datasets that could facilitate resource-management decisions and coastal-restoration planning efforts in south Louisiana. The conclusion that some interior water bodies are expanding as a result of subsidence rather than shoreline erosion needs to be tested systematically in the field. Shoreline erosion seems to be an intuitively correct explanation for water-body expansion where fetch and water-body orientation with respect to predominant wind directions are sufficient to generate erosive waves. This hypothesis can be tested easily by taking core pairs around the perimeters of some of the largest water bodies. Also, there are several wetland-loss mitigation sites where riprap was used to dampen wave energy, but the shoreline continued to retreat. Elevation profiles and cores taken landward of the riprap would offer a way of determining which processes were primarily responsible for the wetland loss and shoreline retreat. If shoreline erosion is not the primary cause of water-body expansion, then hard structures may not be an appropriate method of mitigating wetland loss at those sites. Monitoring the rates and trends of delta-plain subsidence is necessary for accurately predicting future subsidence rates. Evaluating the relative vulnerability of coastal-restoration projects to potential subsidence is an objective of state officials who are charged with the responsibility of managing coastal resources. In the absence of a sophisticated numerical model for predicting subsidence, historical subsidence records can serve as indicators of regions of higher and lower risk. This approach becomes even more powerful when the subsurface processes causing subsidence are known and future trends can be predicted. References Britsch, L.D., and Dunbar, J.B., 1993, Land-loss rates: Louisiana coastal plain: Journal of Coastal Research, v. 9, p. 324-338. Chan, A.W.K., 2005, Production-induced reservoir compaction, permeability loss and land surface subsidence: unpublished Ph.D. dissertation, Stanford University, Stanford, California, 176 p. Chmura, G.L., Aharon, P., Socki, R.A., and Abernathy, R., 1987, An inventory of 13C abundance in coastal wetlands of Louisiana, USA: vegetation and sediments: Oecologia, v. 74, p. 264-271. DeLaune, R.D., Smith, C.J., and Patrick, W.H., 1985, Land loss in coastal Louisiana: effect of sea level rise and marsh accretion: Louisiana State University Final Report, Board of Regents Research and Development Program. Gagliano, S.M., Kemp, E.B., Wicker, K.M., Wiltenmuth, K., and Sabate, R.W., 2003, Neo-tectonic framework of southeast Louisiana and applications to coastal restoration: Transactions, Gulf Coast Association of Geological Societies, v. 53, p. 262-272. Hatton, R.S., DeLaune, R.D., and Patrick, W.H., Jr., 1983, Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana: Limnology and Oceanography, v. 28, p. 494-502. Holdahl, S.R., and Morrison, N.L., 1974, Regional investigations of vertical crustal movements in the U.S., using precise relevelings and mareograph data: Tectonophysics, v. 23, p. 373-390. IHS Energy Group, 2003, PI/Dwights Plus U.S. Production Data on CD: available from IHS Energy Group, 15 Inverness Way East, D205, Englewood, CO 80112. Kuecher, G.J., Roberts, H.H., Thompson, M.D., and Matthews, I., 2001, Evidence for active growth faulting in the Terrebonne delta plain, south Louisiana: implications for wetland loss and the vertical migration of petroleum: Environmental Geosciences, v. 8, p. 77-94. Kulp, M.A., and Howell, P.D., 1998, Assessing the accuracy of Holocene subsidence rates in southern Louisiana as indicated by radiocarbon-dated peats: Geological Society of America, Abstracts with Programs, v. 30, p. 142. Lyons, W.S., 1982, Subsurface geology and geopressured/geothermal resource evaluation of the Lirette-Chauvin-Lake Boudreaux area, Terrebonne Parish, Louisiana: unpublished M.S. thesis, University of Southwestern Louisiana, Lafayette, 125 p. Morton, R.A., Buster, N.A., and Krohn, M. D., 2002, Subsurface controls on historical subsidence rates and associated wetland loss in southcentral Louisiana: Transactions, Gulf Coast Association of Geological Societies, v. 52, p. 767-778. Morton, R.A., Tiling, G., and Ferina, N.F., 2003, Causes of hotspot wetland loss in the Mississippi delta plain: Environmental Geosciences, v. 10. p. 71-80. Morton, R.A., Bernier, J.C., Barras, J.A., and Ferina, N.F., 2005, Rapid subsidence and historical wetland loss in the Mississippi delta plain: likely causes and future implications: U. S. Geological Survey Open-file Report 2005-1216. Penland, S., Ramsey, K.E., McBride, R.A., Mestayer, J.T., and Westphal, K.A., 1988, Relative sea-level rise and delta-plain development in the Terrebonne Parish region: Louisiana Geological Survey, Coastal Geology Technical Report No. 4, 121 p. Penland, S., Wayne, L., Britsch, L.D., Williams, S.J., Beall, A.D., and Butterworth, V.C., 2000a, Geomorphic classification of coastal land loss between 1932 and 1990 in the Mississippi River delta plain, southeastern Louisiana: U.S. Geological Survey Open-File Report 00-417, 1 sheet. Penland, S., Wayne, L., Britsch, L.D., Williams, S.J., Beall, A.D., and Butterworth, V.C., 2000b, Process classification of coastal land loss between 1932 and 1990 in the Mississippi River delta plain, southeastern Louisiana: U.S. Geological Survey Open-File Report 00-418, 1 sheet. Piaggio, A.D., 1961, The Montegut-Lirette-Bay Baptiste structural complex: Terrebonne Parish, Louisiana: The Compass, p. 157-171. Roberts, H.H., Bailey, A., and Kuecher, G.J., 1994, Subsidence in the Mississippi River delta - Important influences of valley filling by cyclic deposition, primary consolidation phenomena, and early diagenesis: Transactions, Gulf Coast Association of Geological Societies, v. 44, p. 619-629. Rybczk, J.M., and Cahoon, D.R., 2002, Estimating the potential for submergence for two wetlands in the Mississippi River delta: Estuaries, v. 25, p. 985-998. Shinkle, K.D., and Dokka, R.K., 2004, Rates of vertical displacement at benchmarks in the lower Mississippi Valley and the northern Gulf Coast: National Oceanic and Atmospheric Administration, Technical Report 50, 135 p. Silvernail, J.D., 1967, Lirette and Montegut fields, in Braunstein, J., ed., Oil and Gas Fields of Southeast Louisiana, v. II: New Orleans Geological Society, p. 109-115. Stevenson, D.A., and McCulloh, R.P., 2001, Earthquakes in Louisiana: Louisiana Geological Survey Public Information Series No. 7, 8 p. Troutman, A., 1956, The oil and gas fields of southeast Louisiana: Five Star Oil Report, Houston, p. 101-104. Figure 1. Regional map of south-central Louisiana showing locations of coring sites and subdeltas of the Lafourche delta system. Geologic ages of the Lafourche subdeltas after Penland et al. (1988). Landsat TM 5 image acquired Nov. 7, 2004. The RGB visual display uses bands 4 (near-infrared), 5 (mid-infrared), and 3 (visible red). Figure 2. Regional map of south-central Louisiana showing locations of coring sites, the USACE Cocodrie tide gauge, the LDNR Montegut tide gauge, and the distribution of wetland losses (1956-2004) relative to producing oil-and-gas fields and potentially active faults. Land-water classification and wetland loss from Morton et al. (2005). Fault projection from Kuecher et al. (2001). Figure 3. Locations of sediment cores and sediment-surface profiles from the Pointe au Chien area superimposed on aerial photographs taken in (A) 1969, (B) 1974, and (C) 1998. (D) 1956-2004 wetland loss at Pointe au Chien and the surrounding area superimposed on the 1998 image. The 1998 DOQQ imagery was obtained from the Louisiana Oil Spill Coordinator's Office (LOSCO). Figure 4. Combined bathymetric profile and stratigraphic cross section for marsh and open-water cores illustrate the magnitude of subsidence and wetland erosion (in cm) at the Pointe au Chien area. Locations shown in Figure 3, 200x vertical exaggeration. Figure 5. Annual fluid production through 2002 from the Bayou Jean la Croix, Lirette, and Montegut fields in Terrebonne Parish. Data from the Louisiana Department of Natural Resources and the PI/Dwights PLUS database (IHS Energy Group, 2003). Figure 6. Plots of historical subsidence rates along (A) Bayou Lafourche and (B) Bayou Petit Caillou calculated by the National Geodetic Survey from re-leveling of benchmarks (Shinkle and Dokka, 2004). The plots show a close spatial correlation between highest subsidence rates, hydrocarbon-producing fields (delineated in tan), and the projected intersection of deep faults. They also show that subsidence rates accelerated between 1965-82 and 1982-93. Modified from Morton et al. (2002). Revised subsidence rates provided by Kurt Shinkle (NGS). Figure 7. Composite histories of fluid production from oil-and-gas fields and wetland loss in south Louisiana. Production data from the Louisiana Department of Natural Resources and the PI/Dwights PLUS database (IHS Energy Group, 2003). Wetland loss values were determined by Britsch and Dunbar (1993) and John Barras (personal communication, 2005). These historical data, integrated across the delta plain, show close temporal and spatial correlations between rates of wetland loss and rates of fluid production. Tables Table 1. Core depths and NAVD88 elevations of stratigraphic markers correlated between core pairs. The most prominent markers are contacts between predominantly organic and predominantly clastic sediments. Positive marsh-minus-water (M-W) depth-difference values indicate erosion, and negative M-W depth-difference values indicate sediment accumulation. M-W elevation-difference values represent estimated subsidence. Core locations are presented in Morton et al. (2005). Core ID Core location Core Elevation (cm NAVD88) Depth in Core Barrel – Base Last Marsh (cm) Elevation – Base Last Marsh (cm NAVD88) Depth in Core Barrel – Base First Marsh (cm) Elevation – Base First Marsh (cm NAVD88) Bay St. Elaine Area composite BSE-04 marsh 49 105 -56 BSE-05 water -7 112 -119 difference (M-W) 56 -7 63 composite BSE-04 marsh 49 105 -56 BSE-01 water -35 150 -185 difference (M-W) 84 -45 129 BSE-01 water -35 150 -185 composite BSE-03 marsh 50 96 -46 difference (M-W) 85 -54 139 BSE-02 water -8 111 -119 composite BSE-03 marsh 50 96 -46 difference (M-W) 58 -15 73 Madison Bay Area MB-10 marsh 30 153 -123 198 -168 MB-06 water -58 129 -187 175 -233 difference (M-W) 88 24 64 23 65 MB-10 marsh 30 153 -123 198 -168 MB-05 water -92 111 -203 134 -226 difference (M-W) 122 42 80 64 58 MB-10 marsh 30 153 -123 198 -168 MB-04 water -108 90 -198 134 -242 difference (M-W) 138 63 75 64 74 MB-01 water -46 125 -171 186 -232 MB-07 marsh 24 115 -91 192 -168 difference (M-W) 70 -10 80 6 64 MB-05 water -92 111 -203 134 -226 MB-09 marsh 17 146 -129 168 -151 difference (M-W) 109 35 74 34 75 MB-03 water -77 118 -195 131 -208 MB-09 marsh 17 146 -129 168 -151 difference (M-W) 94 28 66 37 57 MB-02 water -59 134 -193 151 -210 MB-08 marsh 20 161 -141 180 -160 difference (M-W) 79 27 52 29 50 DeLarge Area DL-01B marsh 32 30 2 110 -78 DL-01A water -49 28 -77 97 -146 difference (M-W) 81 2 79 13 68 Pointe au Chien Area PAC-05 marsh 33 41 -8 PAC-04 water -41 42 -83 difference (M-W) 74 -1 75 PAC-05 marsh 33 41 -8 PAC-06 water -54 33 -87 difference (M-W) 87 8 79 PAC-05 marsh 33 41 -8 PAC-02A water -41 46 -87 difference (M-W) 74 -5 79 PAC03-05 marsh 33 41 -8 PAC03-02B marsh 32 56 -24 difference (05-02B) 1 -15 16 PAC-02A water -41 46 -87 114 -155 PAC-02B marsh 32 56 -24 99 -67 difference (M-W) 73 10 63 -15 88 PAC-02B marsh 32 56 -24 99 -67 PAC-03 water -62 42 -104 129 -191 difference (M-W) 94 14 80 -30 124 PAC03-02B marsh 32 56 -24 99 -67 PAC03-01B marsh 39 26 13 103 -64 difference (01B-02B) 7 -30 37 4 3 PAC-03 water -62 42 -104 129 -191 PAC-01B marsh 39 26 13 103 -64 difference (M-W) 101 -16 117 -26 127 PAC-01A water -38 39 -77 110 -148 PAC-01B marsh 39 26 13 103 -64 difference (M-W) 77 -13 90 -7 84 Bully Camp Area SM-02B marsh 49 59 -10 93 -44 SM-02A water -45 33 -78 80 -125 difference (M-W) 94 26 68 13 81 SM-02B marsh 49 59 -10 93 -44 SM-05 water -50 56 -106 92 -142 difference (M-W) 99 3 96 1 98 SM-02B marsh 49 59 -10 93 -44 SM-03 marsh -8 61 -69 103 -111 difference (02B-03) 57 -2 59 -10 67 SM-02B marsh 49 59 -10 93 -44 SM-04 water -135* 24* -159 51* -186 difference (M-W) 184 35 149 42 142 SM-04 water -135 24* -159 SM-01B marsh 28** 46** -18 difference (M-W) 163 22 141 SM-01A water -67 40 -107 SM-01B marsh 28** 46** -18 difference (M-W) 95 6 89 *excluding 5-cm recent sand deposition **excluding uppermost 27-cm recent (muddy) marsh deposition Table 2. Radiocarbon ages and carbon-isotope data for organic samples. Core locations are presented in Morton et al. (2005). Core ID Sample Depth (cm) Stratigraphic Horizon Conventional Age (BP) d13C (per mil) Bay St. Elaine Area BSE-01 146-147 base first marsh 820 ± 40 -25.6 BSE-02 73-74 top first marsh 400 ± 40 -23.4 BSE-02 110-111 base first marsh 850 ± 40 -26.6 BSE-04 21-22 top first marsh 320 ± 40 -24.6 BSE-04 71-72 base first marsh 680 ± 40 -24.8 BSE-05 37-38 base last marsh 200 ± 40 -13.9 Madison Bay Area MB-02 133-134 base last marsh 840 ± 40 -26.0 MB-02 145-146 top first marsh 940 ± 40 -25.6 MB-02 150-151 base first marsh 930 ± 40 -25.8 MB-04 107-108 top intermediate marsh 720 ± 40 -26.6 MB-04 113-114 base intermediate marsh 700 ± 40 -27.0 MB-04 133-134 base first marsh 960 ± 40 -26.7 MB-07 114-115 base last marsh 600 ± 40 -25.8 MB-07 186-187 top first marsh 980 ± 40 -26.3 MB-07 191-192 base first marsh 950 ± 40 -26.5 MB-09 46-47 base recent marsh 150 ± 40 -14.1 MB-09 145-146 base last marsh 680 ± 40 -26.7 MB-09 167-168 base first marsh 920 ± 40 -26.3 MB-10 152-153 base last marsh 660 ± 40 -26.4 MB-10 197-198 base first marsh 970 ± 40 -26.5 DeLarge Area DL-01A 26-27 base last marsh 510 ± 40 -27.4 DL-01A 56-57 top first marsh 840 ± 40 -26.4 DL-01A 95-96 base first marsh 1050 ± 40 -26.9 Pointe au Chien Area PAC-01A 109-110 base first marsh 900 ± 40 -27.2 PAC-01B 25-26 base last marsh 280 ± 40 -26.1 PAC-02A 91-92 top first marsh 930 ± 40 -27.3 PAC-02A 112-113 base first marsh 980 ± 40 -28.0 PAC-02B 55-56 base last marsh 430 ± 40 -26.2 PAC-03 99-100 first marsh 940 ± 40 -27.4 PAC-03 128-129 base first marsh 950 ± 40 -19.4 Bully Camp Area SM-01B 27-28 base recent marsh 90 ± 40 -26.3 SM-01B 72-73 base last marsh 420 ± 40 -27.4 SM-02B 58-59 base last marsh 450 ± 40 -26.5 SM-02B 85-86 top first marsh 860 ± 50 -27.0 SM-02B 92-93 base first marsh 900 ± 40 -27.3 Table 3. Minimum subsidence rates inferred from minimum aggradation rates based on marsh thickness (interval rate) and sample depth (depth rate). Core locations are presented in Morton et al. (2005). Core ID and Sample Depth (cm) Stratigraphic Horizon 14C Age (BP) Marsh Thickness (cm) Interval Rate (mm/yr) Sample Depth (cm) Depth Rate (mm/yr) Bay St. Elaine Area BSE-01-146/147 base first marsh 820 147 1.8 BSE-02-073/074 top first marsh 400 38 0.8 74 1.9 BSE-02-110/111 base first marsh 850 111 1.3 BSE-04-021/022 top first marsh 320 51 1.4 54* 1.7 BSE-04-071/072 base first marsh 680 105* 1.5 BSE-05-037/038 base last marsh 200 38 1.9 Madison Bay Area MB-02-133/134 base last marsh 840 134 1.6 MB-02-145/146 top first marsh 940 145 1.5 MB-02-150/151 base first marsh 930 151 1.6 MB-04-107/108 top intermediate marsh 720 108 1.5 MB-04-113/114 base intermediate marsh 700 114 1.6 MB-04-133/134 base first marsh 960 134 1.4 MB-07-114/115 base last marsh 600 115 1.9 MB-07-186/187 top first marsh 980 187 1.9 MB-07-191/192 base first marsh 950 192 2.0 MB-09-046/047 base recent marsh 150 47 3.1 MB-09-145/146 base last marsh 680 100** 1.9 146 2.1 MB-09-167/168 base first marsh 920 168 1.8 MB-10-152/153 base last marsh 660 153 2.3 MB-10-197/198 base first marsh 970 198 2.0 Delarge Area DL-01A-026/027 base last marsh 510 27 0.5 DL-01A-056/057 top first marsh 840 40 1.9 57 0.7 DL-01A-095/096 base first marsh 1050 96 0.9 Pointe au Chien PAC-01A-109/110 base first marsh 900 110 1.2 PAC-01B-025/026 base last marsh 280 26 0.9 PAC-02A-091/092 top first marsh 930 22 4.4 92 1.0 PAC-02A-112/113 base first marsh 980 113 1.2 PAC-02B-055/056 base last marsh 430 56 1.3 PAC-03-099/100 first marsh 940 100 1.1 PAC-03-128/129 base first marsh 950 129 1.4 Bully Camp Area SM-01B-027/028 base recent marsh 90 27 3.0 SM-01B-072/073 base last marsh 420 46** 1.4 73 1.7 SM-02B-058/059 base last marsh 450 59 1.3 SM-02B-085/086 top first marsh 860 8 2.0 86 1.0 SM-02B-092/093 base first marsh 900 93 1.0 * depth to contact from composite core description for BSE-04 ** thickness excludes the overlying recent marsh Table 4. Rates (mm/yr) of sediment accumulation (sed) and inferred rates of subsidence (sub) for the Terrebonne and Barataria Basins estimated from isotopic ages (< 5000 BP) and direct field measurements (feldspar marker). Method Type Period Range (mm/yr) Mean (mm/yr) Reference marker sed years n/g 22 "Rybczyk and Cahoon, 2002" 137Cs sed decades 11-17 13* "Hatton and others, 1983" 137Cs sed decades 3-10 7** "Hatton and others, 1983" 137Cs sed decades 6-8 7 "DeLaune and others, 1985" 14C sub centuries 1-16 6 "Penland and others, 1988" 14C sub centuries 3-7 5 "Roberts and others, 1994" 14C sub centuries 0.5-4 2 this study 14C sub millennia 1-5 2 "Penland and others, 1988" 14C sub millennia 3-5 4 "Roberts and others, 1994" 14C sub millennia 0.1-8 1 "Kulp and Howell, 1998" n/g = not given * levee ** back marsh