the Baton Rouge area (average displacement of 2.5 ft/century). Roger Saucier (1963) related geomorphic features on the north shore of Lake Pontchartrain to this fault system and traced the fault into the lake. Highway and railroad bridges built across Lake Pontchartrain cross faults of the Baton Rouge system (Figure 3-16). Surface offsets of bridge structures caused by fault movement, measured to be from 0.83 to 3.33 ft/century at various bridge locations, have been documented (Lopez, 1991; Lopez et al. 1997). There has also been "minor apparent earthquake activity" in the region associated with the Baton Rouge fault system (Stover, et al. 1987; Lopez 1991; Lopez et al. 1997). The pattern of faults in this system in the eastern end of Lake Pontchartrain is en echelon, indicating shearing (Lopez et al. 1997), with the southern block moving east in reference to the northern block. Individual faults in this system have been identified in the subsurface on subbottom and high-resolution seismic profiles (Kolb et al. 1975: Lopez et al. 1997). Fault traces in this system coincide with what E. G. Anderson (1979) referred to as the "inferred edge, Mesozoic shelf and Ouachita system." Spearing (1995) calls this the "Early Cretaceous Shelf Margin." This fault system is apparently deep seated and, at least in part, is a line of delineation between areas of uplift and subsidence. Fisk called this a "hinge line”; the fulcrum of isostatic adjustment to crustal loading. Landward of the hinge line the land is stable or rising and seaward of it the land is sinking. Figure 3-4 shows regional patterns of uplift and subsidence north and south of the hingeline faults, as determined from sequential survey of benchmarks (Holdahl and Morrison 1974; Watson 1982). Saucier (1994), in a synthesis of structural elements in the Mississippi River Valley, includes the Baton Rouge Fault Zone with the South Louisiana growth faults. He states that, "several lines of evidence suggest that most of the fault zones have had some noticeable but geomorphologically unimportant effect on near-surface deposits of Pleistocene age.” Only the Baton Rouge Fault Zone has had major geomorphic impact and is known to be currently active. Saucier (1994) considers the Baton Rouge fault zone to be second only to the Reelfoot Rift, (located in northeast Arkansas, southeastern Missouri and northwestern Tennessee and which was the locus of the New Madrid earthquake of 1811-1812), in the entire Lower Mississippi River Valley area in terms of the extent and recentness of Quaternary displacements.
Transcript
the Baton Rouge area (averagedisplacement of 2.5 ft/century). RogerSaucier (1963) related geomorphicfeatures on the north shore of LakePontchartrain to this fault system andtraced the fault into the lake.
Highway and railroad bridges built acrossLake Pontchartrain cross faults of theBaton Rouge system (Figure 3-16). Surface offsets of bridge structurescaused by fault movement, measured tobe from 0.83 to 3.33 ft/century at variousbridge locations, have been documented(Lopez, 1991; Lopez et al. 1997). Therehas also been "minor apparentearthquake activity" in the regionassociated with the Baton Rouge faultsystem (Stover, et al. 1987; Lopez 1991;Lopez et al. 1997). The pattern of faultsin this system in the eastern end of LakePontchartrain is en echelon, indicatingshearing (Lopez et al. 1997), with thesouthern block moving east in referenceto the northern block. Individual faultsin this system have been identified in thesubsurface on subbottom andhigh-resolution seismic profiles (Kolb etal. 1975: Lopez et al. 1997).
Fault traces in this system coincide withwhat E. G. Anderson (1979) referred toas the "inferred edge, Mesozoic shelf andOuachita system." Spearing (1995) callsthis the "Early Cretaceous Shelf Margin." This fault system is apparently deepseated and, at least in part, is a line ofdelineation between areas of uplift andsubsidence. Fisk called this a "hingeline”; the fulcrum of isostatic adjustmentto crustal loading. Landward of thehinge line the land is stable or rising andseaward of it the land is sinking. Figure3-4 shows regional patterns of uplift andsubsidence north and south of the
hingeline faults, as determined fromsequential survey of benchmarks(Holdahl and Morrison 1974; Watson1982).
Saucier (1994), in a synthesis ofstructural elements in the MississippiRiver Valley, includes the Baton RougeFault Zone with the South Louisianagrowth faults. He states that, "severallines of evidence suggest that most of thefault zones have had some noticeable butgeomorphologically unimportant effecton near-surface deposits of Pleistoceneage.” Only the Baton Rouge Fault Zonehas had major geomorphic impact and isknown to be currently active. Saucier(1994) considers the Baton Rouge faultzone to be second only to the ReelfootRift, (located in northeast Arkansas,southeastern Missouri and northwesternTennessee and which was the locus ofthe New Madrid earthquake of1811-1812), in the entire LowerMississippi River Valley area in terms ofthe extent and recentness of Quaternarydisplacements.
Figure 3-16. Baton Rouge fault system. A. Fault traces from the Mississippi River to Lake Borgne. Displacements have been
measured on bridges shown. Reported earthquake occurrences are also shown. B. Drawing showing fault offset in re-built section of Norfolk-Southern Railroad bridge in
eastern Lake Pontchartrain. C. Displacement of beds by south dipping normal fault as recorded on U.S.G.S. high
resolution seismic line from eastern Lake Pontchartrain. Abrupt terminations of shallowreflectors indicate that the fault is within 10 ft of the lake bottom (after Lopez et al. 1997).
SW-NE Fault Systems and Alignments
Three parallel fault systems cutdiagonally across south Louisiana: theCalcasieu Lake Fault Zone, the LakeSand-Frenier Alignment, and theMauvais Bois Alignment (Figure 3-15).
The Calcasieu Lake Fault System is along straight trend of faults cuttingacross the Uplands and Calcasieu Lakeand intersecting the Gulf of Mexicoshore in the Holly Beach area.
The Lake Sand-Frenier Alignment is astrong trend of faults, some of whichbranch or fan toward the southwest. Thetrend terminates at its northeast endunder Lake Pontchartrain where it runsinto northwest-southeast alignedsystems.
The Mauvais Bois Alignment is welldefined at its southwest end by faultsunder Point au Fer. The Mauvais Boisridge, a prominent landform, follows thealignment. Toward the northeast it cutsacross the ends of a series of east-westgrowth faults. The alignment terminatesunder Lake Borgne, where it runs intonorthwest-southeast aligned systems.
NW - SE Alignments or Shear Faults
Fault patterns, variations in subsidencerates, and other data examined during thecourse of this study indicate a regionallyimportant, apparently deep seated faultsystem herein called the Terre aux BoeufFault System (Figure 3-15). In the LakeBorgne area, patterns of splinter faultfans at the eastern end of growth-faultsterminate at the Terre aux Boeuf fault. The pattern suggests shearing, with thesouthern block moving east in reference
to the northern block. In the activeMississippi River delta, this fault systemmerges with a circular fault patternaround an apparent collapse feature. TheLafayette Fault System is anotherapparently deep-seated fracture or faultsystem that brackets the Deltaic Plain onthe west. It is defined by splinter faultfans on growth faults, which terminate atthe Lafayette Fault System. Nopublished references to these twopostulated fault systems have been foundin the literature.
E-W Growth Fault Systems
This is the predominant trend of growthfaults in the Gulf Coast Salt Dome Basin. A series of long and continuous faultsystems extend across the southern partof the state from Texas to the east andterminate at the Biloxi and Tere auxBoeuf Fault Systems. The Golden Meadow-Theriot-Forts FaultSystem is one of the most continuousand distinctive. From the standpoint ofcoastal erosion and deterioration, it is themost important fault system in theregion. The Golden Meadow FaultSystem, as depicted by Murray (1960;Figure 3-9) would include the Theriotand Forts faults as defined in this paper. The Golden Meadow trend is clearlyidentified on the Wallace map. Onshoreit extends from Point au Fer to BayouLafourche, where it branches to the east.Wallace classifies a segment of thesystem (at the Gulf of Mexico shore andunder Point au Fer) as a major fault(2,000 to 5,000 ft displacement). Immediately south of the fault, where itcrosses Bayou Lafourche, there is agraben structure. To the east, two saltdomes fall within the alignment in the
Barataria Bay area, and the area betweenthe two domes is classified by Wallace asa major fault. It is also interesting tonote that the fault cuts into the LakeWashington salt dome. East of thisdome, the fault trend continues as theForts Fault. It crosses the MississippiRiver and probably influenced theconfiguration of the Forts Bend, a sharpbend in the river. Wallace also classifiesthis as a major fault in the area where itcrosses the Mississippi River. It isclassified as a major fault along more ofits length than any other south Louisianafault. The Theriot fault is north of, andtrends subparallel to, the GoldenMeadow fault.
Surface traces of faults in these threesystems have become increasinglyevident on aerial photographs and imagesin recent decades. For example, thetrend is not evident on 1955 AmmannInternational Corporation aerialphotographs, but is clearly visible on theNovember 1990 Landsat TM SatelliteImagery, bands 4, 5 and 3. Traces aredefined by linear contacts between marshand open ponds and broken marshpatterns (land loss and marshdeterioration) on the down-thrownblock. Some traces are parallel to, butdo not coincide with, fault traces asshown on the Wallace map. This is dueto the fact that Wallace used subsurfacedata, which was not necessarily projectedto a surface datum.
The Lake Pelto Fault System is identifiedfrom the Wallace map. It contains sevensalt domes, including the LakeWashington dome, into which it anchorsat its eastern end and where it mergeswith the Golden Meadow-Theriot-FortsFault System. The system exhibitsreverse faulting and sets of fault tracesalong some segments. This system is less
important than the Golden Meadowsystem. Land loss and marshdeterioration patterns along the southside of the fault trace suggest rotation ofthe down-dropped block.
The Eugene Island Fault System isdefined primarily by a string of nine ormore salt domes extending generallyparallel to the Gulf shore, partiallyoffshore and partially onshore. Definedfaults between the domes tend to bereverse faults.
Sinking Land and Rising Sea
If fault bound blocks along the coast aresinking and are being inundated by thesea it becomes important to determinethe rate of change between the elevationof the land and the level of the sea, thecombined effect of which is relative sealevel rise.
As shown in Figure 3-17, the task ofdetermining rates of relative sea level riseis complicated by the large number ofprocess variables that contribute tovertical change. The land elevation onthe blocks, the rate of sinking of the landsurface (subsidence) and the rate of riseof the sea (eustatic sea level rise) areprimary factors. To further complicatethe task, subsidence has a number ofcomponents, the two principal of whichare compaction of poorly consolidatedsediments (compactional subsidence) andgeosyncline down-warping, oneexpression of which is fault movement(fault induced subsidence).
Figure 3-17. Factors contributing to relative sea level rise and subsidence in the Louisianacoastal region (after Penland et al. 1989:8; adapted from Kolb and Van Lopik 1958:95).
In their study of the geology of theDeltaic Plain, Kolb and van Lopik (1958)considered tectonic activity as acomponent of relative sea level rise (totalsubsidence). They noted that "mostmovement probably occurs in spasms,and average rates of movement, whichwould allow a prediction of the tectonicportion of total subsidence, would bevery difficult to establish". A furtherdiscussion of the components of relativesea level rise and methods and results ofmeasurements follows.
Rising Sea
The current average eustatic sea levelrise rate is 0.49 ft/century. Until recentlythe sea level rise rate has been low, butthe rate is increasing. The best estimateof sea level rise experts have provided isthat the level of the world’s oceans willincrease 0.67 ft over the next 50 years
and 1.53 ft during the next century(Wigley and Raper 1992).
Compaction
Compaction is related to the type andthickness of Holocene Period (modern)sediment that has accumulated on top ofthe weathered surface of the Pleistoceneformation during the past 7,500 years. This buried top of the Pleistocene is acontinuation of the upland surface, andprior to burial it was exposed by low sealevel stands during the last ice age. Aprism of modern sedimentary deposits(sand, silt, clay, peat beds and shell beds)accumulated above the weatheredsurface during the rise and the relative"still-stand" of the sea that followedglacial melting. The poorly consolidatedclay and peat beds had higher watercontent at the time of deposition. Afterburial, they compacted and lost volume.
This compaction process, which stillcontinues, contributes to subsidence.Where the Holocene deposits are thick,compaction and subsidence rates arehigher.
Ramsey and Moslow (1987) attribute80% of the observed relative sea levelrise in coastal Louisiana to"compactional subsidence." Del Britsch(personal communication) has compileddata from innumerable borings in thecoastal zone and from analysis of thisdata has concluded that subsidence ratesare directly related to thickness of theHolocene deposits, and compactionthereof. Kuecher (1994) has studiedrelationships between land loss, thicknessand characteristics of Holocene sediment,subsidence rates and faulting and has alsoconcluded that compaction is a primarycause of subsidence. Most researchershave recognized that fault inducedsubsidence is a contributing factor, butthe consensus has been that the majorityof relative sea level rise can be attributedto compactional subsidence.
Methods of Measuring Rates ofRelative Sea Level Rise and Subsidence
Data for measuring relative sea level riseand subsidence comes from a number ofsources. These include: 1) change inelevation of surfaces upon which humanstructures (prehistoric Indian villagesites, lighthouses, forts, roads, etc.) werebuilt, 2) radiometric dating of buried peatdeposits, 3) tidal gage records, and 4)sequential land surveying. The lattertechnique provides the best measure ofpresent day subsidence rates.
Tide Gage Data
Shea Penland, Tom F. Moslow, Karen E.Ramsey, and their colleagues, in animportant series of studies and papers,have grappled with the problems relatedto causes, effects, and rates of relativesea level rise in south Louisiana.(Ramsey and Moslow 1987; Penland etal. 1988; Penland et al. 1989; Ramseyand Penland 1989; Nakashima andLouden 1989; Penland and Ramsey1990). The team conducted acomprehensive study of historical waterlevel records from 78 tide gage stationsand 342 line miles of geodetic levelingdata from south Louisiana and adjacentareas of the northern Gulf of Mexicoregion for the period 1942-1982.
Figure 3-18 shows a typical water leveltime series from the Grand Isle gage, asanalyzed by the Penland et al. team(1989). Water levels generally "climb thegage" through time. The records fromeach state were analyzed to determinethe rise rate for the entire period ofrecord as well as for two twenty yeartime epochs. Epoch one included theperiod 1942 - 1962 and Epoch two theperiod 1962 - 1982. Records from manysouth Louisiana stations also showed adistinctive increase in rate of rise.
Figure 3-18. Water level time series from National Ocean Survey, Grand Isle, La. tide gagebetween 1947 and 1978. A change in the rate of rise between the period 1947 - 1962 (Epoch 1),and the period 1962 - 1978 (Epoch 2) has been found on many of the records from gages insouth Louisiana (after Penland et al. 1989:24).
Gage records from the northern Gulf ofMexico, from Cameron, La to CedarKey, Fl, were analyzed. Most recordsindicated a relative rise in the level of thesea through time, but as shown in Figure3-19, the rates of relative rise variedfrom east to west, with the lowest ratesbeing along the coasts ofFlorida-Mississippi and the highest beingalong the Deltaic Plain of Louisiana. Land leveling data indicated that thePensacola location has remainedrelatively stable and for this reason therate of relative rise at Pensacola wasselected as the best measure of eustaticsea level change for the northern Gulf ofMexico region (see Figure 3-3). Thus,the rate of 0.75 ft/century, as determinedfrom the Pensacola record, was used bythe Penland et al. team as a correctionfactor in adjusting relative sea level rise
rates to subsidence rates and vise versa. This same correction method and factorare also used in this paper.
As mentioned above, many of the tidegage records from coastal Louisiana alsoexhibit a distinctive increase in rate ofrelative rise, beginning in about 1962(Figures 3-18 and 3-20). This change ismost pronounced in three areas, theSouth Shore-Little Woods area in theeastern end of Lake Pontchartrain, theDeltaic Plain west of the MississippiRiver, and the Mermentau River area inthe Chenier Plain. These changes in ratesuggest fault movement. Further, theyspecifically suggest that the rate ofmovement on faults in the three areas hasincreased during the 1962-1982 interval.
Figure 3-19. Rates of relative sea level rise across the northern Gulf of Mexico region from Cameron, LA to Cedar Key, FLbased on records from the National Ocean Survey and U.S. Army Corps of Engineers tide gage stations. The Pensacola, FLgage land location is considered to be stable, and this gage provides a record of eustatic sea level rise in the Northern GulfRegion. The rates of rise of all stations in coastal Louisiana exceed the rate of eustatic rise. The differences are attributed tosubsidence (after Penland et al. 1988).
Figure 3-20. Relative sea level rise based on readings from U.S. Army Corps of Engineers tidegage stations in Louisiana. Note change in rate of rise between 1947-1961 (Epoch 1) and1962-1978 (Epoch 2), (after Penland et al. 1989).
Figure 3-21. Present and future trends of relative sea level rise based on tide gage records fromcoastal Louisiana (after Ramsey and Moslow 1987).
The statewide sea level rise rate wascalculated to be 1.148 ft/century for1942-1962 period and 3.67 ft/centuryfor 1962-1982 period. The relative sealevel rise rate for the study area wasfound to be 3.2 times greater in thesecond 20-year epoch. Projections offuture trends of relative sea level risewere made based on the tide gagerecords (Figure 3-21).
Ramsey and Moslow (1987) grouped thegage data into seven hydrographicbasins. The data show great variationboth temporally and spatially throughoutcoastal Louisiana. Using average valuesfor the entire period of record (1942through 1982) rates of rise of 3.28 to3.94 ft/century were found in the areas
immediately along the Louisiana coast. Relative sea level rise in the southwestportion of the Deltaic Plain wasdetermined to be 5.91 to 6.23 ft/century. In most cases there was a pronounceddecrease in rate landward.
Figure 3-22 depicts a map adapted fromRamsey and Moslow (1987). The mapshows a large area of high relative sealevel rise rates south of theTheriot-Golden Meadow-Forts faultsystems. A local area of high ratesoccurs along the south shore of theeastern end of Lake Pontchartrain. Thisis on the down-thrown block of theBaton Rouge Fault System, wheremovement has been documented byLopez et al. (1997). Another area of
Figure 3-22. Isopleth map of sea level rise rates in coastal Louisiana based on 1962-1982(Epoch 2) tide gage data (adapted from Ramsey and Moslow 1987). Locations of sequential landleveling lines are also shown. Line A is located along the Mississippi River natural levees (seeFigure 3-23). Line B is located along the Bayou Lafourche natural levees (see Figure 3-24).
relatively high rates is found at the mouthof the Mermentau River in the ChenierPlain. This is where the Grand ChenierFault System reaches the coast.
After subtracting the isostatic rate of riseof 0.75 ft/century, Ramsey and Moslowdetermined the "compactionalsubsidence" rate. From this analysis theauthors concluded that approximately80% of the observed relative sea levelrise in Louisiana was attributable tocompactional subsidence. They alsoconcluded that compaction and loadingaccount for the spatial variation in rate.
The implications of this map are farreaching. Do relative sea level rise rates
in the Terrebonne area meet the realitytest? The rate of 8.0 ft/century equalstwo feet of vertical change during twentyyears. During the 20 years from 1962 -1982 did the relative sea level rise rate inthe Barataria Basin exceed the rate in theBalize Delta Lobe area, wherehistorically relative sea level rates havebeen reported to be the highest in theregion?
Sequential Land Leveling
Perry C. Howard (in Van Beek et al.1986) studied subsidence in PlaqueminesParish, which includes the MississippiRiver from New Orleans to its mouth. Areview of the geological literature
Figure 3-23. Changes in land elevation alongMississippi River natural levees between Chalmetteand Venice. A. Benchmark movement between New
Orleans and Venice for period 1938 to1971.
B. Average movement of individualbenchmarks for period 1938 to 1971(after van Beek et al. 1986).
disclosed subsidence estimates for theActive Mississippi River Delta arearanging from 4 to 14 ft/century. Fromthe published estimates Howardconcluded that the minimum value ofsubsidence is 4 ft/century and the uppermaximum is probably about 8 to 10ft/century. In either case, the maximumsubsidence value for the delta exceedsthe Ramsey-Moslow rate for theBarataria Basin. It should also be notedthat "subsidence" as used by Howard isequivalent to "relative sea level rise" asused herein.
Howard also evaluated data fromNational Geodetic Survey verticalbenchmark surveys along the MississippiRiver natural levees between Chalmette,La and Venice, La. The dates of thesurveys were 1938, 1946, 1951, 1964,and 1971. Figure 3-23a shows verticalmovement between 1938 and 1971 forbenchmarks and Figure 3-23b, showsaverage movement for the period ofrecord. There is an apparent gradualdecrease in subsidence towards Venice. The highest rates were found atBraithwaite, with an average rate of 4.0ft/century, and just north of Phoenix,with an average rate of 4.5 ft/century. The average rate of benchmarkmovement for the entire section andperiod of record was determined to be2.2 ft/century. The National GeodeticSurvey data does not include the effectof sea level rise as the benchmarkelevations are determined by surveynetworks that are referenced to stablebench marks well outside of the coastalzone. To determine relative sea levelrise, an adjustment must be added for therate of eustatic rise. Howard added anadditional 0.5 ft/century for the rate ofsea level rise, and thus concluded that
the average rate of relative sea level risewas 2.7 ft/century for the line of section. When an eustatic sea level adjustment of0.75 ft/century is made the average rateof relative sea level rise for the section is2.95 ft/century.
Ramsey and Moslow (1987) and Penlandet al. (1988) also used sequential landleveling data to measure subsidence. Themost important traverse that they studiedfollows the natural levee ridges alongBayou Lafourche (Figure 3-24). In
Figure 3-24. Changes in land elevation along Bayou Lafourche natural levees between Raceland and Fourchon, (including the Grand Isle barrier island). A. Growth fault traces superimposed on
subsidence rates (cm/yr). Subsidence ratesincrease abruptly on downthrown side of fault (after Kuecher 1994, subsidence ratesfrom Penland et al. 1988).
B. Graph showing rates of land movement along Bayou Lafourche (Penland et al. 1988).
contrast to the section along theMississippi natural levee ridges south ofNew Orleans (Figure 3-23), the rates ofsubsidence down Bayou Lafourcheincrease toward the coast. Both theLafourche and the Mississippi sectionexhibit spikes and valleys in rates. Asshown in Figure 3-24b, Kuecher (1994)has compared the location of benchmarksshowing spikes along the Lafourchesection with locations of the GoldenMeadow and Lake Hatch fault traces. He concluded
that pronounced spikes occurimmediately south of the traces.
Buried Peat Deposits
Another important data set comes fromradiocarbon dating of buried organicdeposits, primarily peat. The advent ofradiocarbon dating in the 1950s made itpossible for the first time to dategeologic features and events. David E.Frazier (1967), working under thedirection of H. N. Fisk for the EssoProduction Research Company, collectedhundreds of samples of buried organicdeposits from the Deltaic Plain. Thesewere taken from undisturbed cores anddated at the Esso Production Laboratory. Not only did the dates provide the basisfor a more detailed understanding ofdelta building events, but the dates andother relevant data from the core holeswere published for use by otherresearchers.
Coleman and Smith (1964) usedradiocarbon dates of buried peat depositsfrom south central Louisiana todetermine the approximate time that sealevel reached its present stand followingthe end of the last continental glaciation. Using the Coleman and Smith techniqueand dates and sample data from Frazier’slist and other sources, Gagliano and vanBeek (1970) plotted radiocarbon datesagainst depth of burial. The resultingplot shows rates of relative sea level risefor the period 7,200 - 400 years beforepresent (yrs. B.P.). The data indicatethat between 7,200 and 4,256 yrs. B.P.the relative sea level rise rate was 0.83ft/century, and for the interval 4,256 to400 yrs. B.P. it was 0.35 ft/century. These are average rates for the DeltaicPlain area.
Penland et al. (1988:94-5) plotted age ofburied peat against rate of subsidence. They concluded that, "...a comparison ofdata sets from the youngest (0 - 500 yrs.B.P.) and the oldest (500 - 3,000 yrs.B.P.) portions of the Terrebonne DeltaPlain indicates that, if we assume a stableeustatic regime, the rate of compactionalsubsidence decreases with time afterdelta-plain abandonment. This decreaseoccurs because the sediment de-wateringthat begins upon abandonment diminisheswith time."
Del Britsch, a geologist with the USACENew Orleans District, has studied thisrelationship. He has a comprehensivecompilation of radiocarbon dates ofburied organic deposits and has usedthem to develop maps of the rates ofsubsidence in coastal Louisiana (Britsch personal communication).
H. Roberts (1995) used radiometricdating of buried organic deposits from aselection of core holes to determinesubsidence rates across the centralLouisiana coastal plain. The data showrates of 0.3 ft/century for a shallow areaof the Holocene (recent) sediment fillover the Pleistocene surface increasing to1.2 ft/century for an area of thick fill, afour fold increase. This section has beenused to illustrate the relationship betweenthickness of Holocene sediments andsubsidence rates (Reed, ed. 1995).
The relative sea level rise rates based ondates and depth of buried organicdeposits are considerably lower thanthose from tide gage and sequential landleveling data. However, they do provide a long-term base for evaluating bothtemporal and spatial changes in rates.
Summary of Relative Sea Level andSubsidence Data
The different data sets discussed aboveare each unique pieces in the relative sealevel rise puzzle, as numerousresearchers over a wide period of timeapproached the issue from a variety ofperspectives using different data sources. The data sets, while alone depictingdifferent figures for relative sea level rise,taken together, they demonstrate thesame trends in relative sea level rise andidentify important anomalies in the data.
The radiocarbon peat dates demonstratethat compaction rates slow with timeafter delta or depositional abandonment. The tide gage data demonstrate thespatial variation of relative sea level riserates across the Gulf of Mexico coast, aswell as the temporal increase in the rateof relative sea level rise from the first tosecond epoch. The land leveling data,the most verifiable data set, validate theother data sets and identify fault effectson subsidence.
The anomaly that these data sets identifyis the temporal change in relative sealevel rise demonstrated by Ramsey andMoslow. While the relative sea level riserate variation across the coast (spatialvariation) could be explained bycompaction due to respective variationsin Holocene sediment thickness, theincrease in rates over time (temporalvariation) at some locations can not beexplained in the same way. Sincecompaction at a given location has beenshown to decrease over time, thetemporal relative sea level rise rateincrease demonstrated at given locationscan not be due to compaction. Thecause of this regional, episodic variation
Table 3-1. Summary of published findings regarding rates of relative sea levelrise in coastal Louisiana.
in relative sea level rise is explainablewhen fault induced subsidence is takeninto account. Selected relative sea levelrise and subsidence rates are presented inTable 3-1.
Effects of Fault InducedSubsidence on Coastal Lowlands
Unmasking of Fault Displacement(Aggradation vs Subsidence)
Until the twentieth century, movement ofgrowth faults within the coastal area wasmasked by aggradation resulting fromriver derived sediment deposition andaccumulation of organic materials. Surface traces of faults became exposedby patterns of erosion and marshdeterioration.
Reduction of Overbank Flow andSediment Supply
Construction of flood protection leveesalong the Mississippi River and closureof distributary channels have cut offvirtually all over-bank flow into theestuarine basins of the Deltaic Plain(Gagliano et al. 1971; Gagliano and vanBeek 1976; Reed, ed. 1995). Theamount of sediment transported by theMississippi River has deceased by 50%since 1953 due primarily to constructionof five large dams on the upper MissouriRiver (Meade and Parker 1985). This inturn has reduced the river’s capacity tofill the holes resulting from relative sealevel rise. Much of the loss in the activedelta area of the Mississippi River (DeltaHydrologic Unit) can be attributed to thischange.
Reduction of Organic Matter Build upand Deterioration of Floating Marshes
Some swamp and marsh plants can adjustto subsidence and resulting increase inhydroperiod by elevating their root zone. This occurs where peat
and other deposits accumulate and theplants maintain their relative position tothe water level by constantly sproutingand seeding on the top of theaccumulating deposits. As long assubsidence rates do not exceed accretionrates of the swamp and marsh floor, theliving surface survives. In many areassubsidence rates have exceededaggradation rates (Nyman et al. 1990;Reed, ed. 1995; and others).
Floating marshes represent another wayin which vegetation responds tosubsidence. By producing andmaintaining a floating root mat, marshplants are able to maintain their positionrelative to water level independent of theelevation of the firm substrate. Floatingmarshes require freshwater conditions, afirm skeletal framework (natural levees,cheniers, spoil banks, lake rims, etc.) andlow water energy conditions. Alterationof required conditions has resulted inextensive breakup and loss of floatingmarsh mats (Sasser 1994).
Penland et al. (1988) compared rates ofsediment accumulation with subsidencerates in the Terrebonne region. Theyconcluded that, "...wetland sedimentationrates lag behind the rates of relative sealevel rise in Terrebonne Parish" (Figure3-25). The relationship between wetlandsedimentation and relative sea level risecontrols Deltaic Plain land loss. Whensedimentation rates exceed sea level riserates, the delta plain aggrades andmaintains its subaerial integrity. When
Figure 3-25. Comparison of relative sea level rise rates and wetlandsedimentation rates for the Terrebonne Parish region. Only in theAtchafalaya River Delta was land building up at rates higher thanrelative sea level rise. Wetland sedimentation rates are from DeLaune etal. 1985, and relative sea level rise rates based on records from USACEand NOS tide gages (adapted from Penland et al. 1988).
sedimentation rates fall below relative sealevel rise rates, land loss ensues. Themean modern (0-50 yr. B.P.) relative sealevel rise rate of 4.20 ft/century (basedon the average rate record at the HoumaUSACE tide gage station) exceeds themean sedimentation rate for theTerrebonne coastal region of 2.76ft/century. Under these conditons, whichhave existed for the last 25 years,
sedimentation cannot maintain theTerrebonne delta plain. The meansubsidence rate of 0.48 ft/century for0-500 yr B.P. calculated from theradiocarbon data indicates that wetlandsedimentation rates were previouslycapable of maintaining the stability of theDeltaic Plain. (Penland et al. 1988). Fora through review of the accretion processand their relationships to relative
sea level rise the reader is referred toReed (ed. 1995).
Other Processes Contributing to LandLoss and Coastal Erosion
There is a synergy, between subsidenceand hydrologic forces, that acceleratesland loss and erosion. Subsidence,whether due to compaction or faulting,undermines the foundation of coastallowlands by lowering land elevations andthus exposing wetlands, ridges, andhuman infrastructure to the forces of theGulf of Mexico that erode away the land. Fluid withdrawal has also been cited as acause for subsidence (Penland et al.1988; Coleman et al. 1998; Boesch et al.1994), but evaluation of this aspect ofthe problem is beyond the scope of thisstudy.
Of the variety of damaging forces,marine tidal invasion and storms areresponsible for removing a vast area ofLouisiana’s vulnerable coastal lowlands. Herbivory, the loss of marsh plants dueparticularly to intensive grazing by themultiplying nutria population, and dredgeand fill activities, are also responsible forcontinued losses. Navigation canalsdredged for oil and gas extraction, theMississippi River Gulf Outlet, theCalcasieu and Sabine ship, HoumaNavigation, and other channels have alldisrupted hydrology, resulted in saltwaterintrusion to fresh marshes, and causedextensive land loss through marineinvasion of fresh marshes. Storms causeland loss not only because of thetremendous forces they can wield onfragile wetlands, but also because thenatural systems that once protectedagainst extensive storm damage arepresently in a state of near collapse.
The protection offered by barrier islandsis disappearing as the islands themselvesdisappear, the weakened condition ofwetlands can not stand up to or recoverfrom intense storms, and the stormsaccelerate tidal intrusion, furthering tidalinduced loss. In addition to inundationof the land by water, all the forces thatcause land loss are exacerbated by thereduction of land elevation due torelative sea level rise.
Effects of Fault Induced Subsidence
The subsidence that is caused by faultmovement affects Louisiana landforms indefinable areas and in characteristicways. The following discussion outlineswhere fault induced land loss has thestrongest effects, and what landforms itmost seriously impacts.
Effects on Wetlands
The areas of highest land loss in theLouisiana coastal area, almost all ofwhich consists of wetland loss, occurssouth of the Golden Meadow-Theriotand Forts fault systems and appears to berelated to slump induced fault movement(Figure 3-26). Cumulative losses onthese fault blocks since 1930 total morethan 737 square miles. This is 46% ofthe total loss along the entire Louisianacoast, and 61% of the loss in the DeltaicPlain for that same period.
Effects on Barrier Islands and GulfShore
Louisiana’s barrier island systems haveundergone landward migration, area loss,and island narrowing as a result ofcomplex interaction among subsidence,sea level rise, wave processes,
inadequate sediment supply and intensehuman disturbance. Consequently, thestructural continuity of the barriershoreline weakens as the barrier islandsnarrow, fragment and finally disappear. In the past 100 years, the total barrierisland area in Louisiana has declined 55%at a rate of 155 acres/yr. Thisdeterioration will continue to destroyLouisiana’s coastline until coastalrestoration techniques that complementnatural processes are implemented torestore and fortify the shoreline (Williamset al. 1992).
Effects on Ridges and Fastlands
Ridges only aggrade or build up whenthey are being formed along the banks ofactive distributaries or as active gulfbeaches. Surface elevations of all relictnatural levee ridges, chenier ridges, manmade ridges, embankments, levees, andfastlands become lower through time inresponse to subsidence. Protectionlevees around fastlands preventaggradation; therefore, all fastland areaswithin the coastal zone are subsiding
(Figure 3-27, see also Figure 3-25). Theproblem of reduction of land surface isexacerbated in forced drainage districtswithin fastlands, where drained soilsshrink and compact. Surface elevationswithin some fastland areas in easternNew Orleans are more than 16 ft belowmean Gulf of Mexico level. Fastlandlevees are constructed of earth andcannot withstand the marine erosiveforces that are gradually approacingmany drainage levees. Furthermore, allinfrastructure along the corridors issubject to sinking and erosion. Transcoastal corridors, which crossmajor fault zones, are critically affectedby fault induced subsidence. Theseinclude: 1) the Mississippi River belowNew Orleans; 2) Bayou Lafourche-Louisiana Highway 1; and 3) naturallevee ridges south of Houma.
Figure 3-26. Birdseye view of southeastern Louisiana showing relationships between major faults and areas of high landloss.
Figure 3-27. Effects of subsidence on ridgelands andfastlands. A. Distributary natural levee corridor, natural
conditions. B. Subsided distributary natural levee corridor with
forced drainage and storm protection levees (afterGagliano 1990).
Delineation of Fault Bound Blocks
As shown in Figure 3-28, the majorfaults systems and alignments provide thebasis for dividing south Louisiana into sixmega blocks. Each has distinctivestructural and subsidence characteristics. The ability to identify and characterizethe conditions on these blocks is akeystone to the integrity of future coastalplanning. A brief description of thecharacteristics of each follows:
Block I Only the southern end of theblock lies within the coastal zone. Partof the Calcasieu Lake collapse structureis on this block.
Block II Several major east-west faultsrun across this block. The Five IslandSalt Trend is along the southwestboundary. The Weeks Island andCharenton collapse features are on thisblock. The Maurepas fault separatesuplands and wetlands at the western endof the Ponchatrain basin and the Baton
Figure 3-28. Mega blocks with major fault trends of south Louisiana.
Rouge fault forms the northern boundaryof the basin.
Block III This block is relatively stable,accounting for the low erosion rates inthe Biloxi marshes. The block is dividedby the Biloxi fault. The northernChandeleur Islands, which lie north ofthe fault were relatively stable untilimpacted by Hurricane Georges in 1998. The southern Chandeleurs and BretonIslands, on the south side of the Biloxifault, are eroding rapidly.
Block IV Active subsidence on thisblock is located near the coast. Growthfaults come into the Chenier Plain at anangle to the shore zone. These are older,less active faults than those in the deltaicplain.
The breakup of land between White Lakeand Grand Lake may be fault induced, asis shoreline erosion at RockefellerRefuge. Salt collapse feature underCalcasieu Lake area may be acontributing factor to the high historicland loss rates in that area (Figure 3-6).
Block V Fisk (1944) referred to this asthe Lake Borgne Fault Zone. It is slicedinto many smaller blocks by numerousfaults. Many of the large lakes in thiszone may reflect the intense faulting. The Chacahoula collapse feature is alsoon this block.
Block VI This is the area of most activeland loss. It is criss-crossed by severalmajor E-W fault zones, which subdivideit into smaller blocks. Three of thesmaller blocks are discussed below.
Block VI A. This is located on the down-thrownblock of Forts Fault system. This is theactive Mississippi Delta area, the area ofsecond highest land loss along theLouisiana coast, second only toneighboring block VI B. The gulf shoreand barrier islands along this block arebeing lost. The Balize collapse feature ison this block.
Block VI B.Located on the downthrown block of theGolden Meadow Fault System, the LakeWashington and Four Island salt collapsefeatures underlie this block and the Peltofault that cuts across it. This is the areaof highest total land loss along the entireLouisiana Coast. In addition, all barrierislands on this block are eroding. In thecase of the Golden Meadow Fault Zone,the active shore zone is in the process ofmoving inland from its present positionalong the barrier islands to a newposition against the fault trend. Allremaining features (landforms and humaninfrastructure) on the surface of thisblock are vulnerable to inundation anderosion.
Some of Louisiana’s most important andmost endangered barrier islands are onthe block, including the Derniers andTimbalier chains, the Fourchon headland,Grand Isle and Grand Terre Island.
Block VI C.On the down-thrown block of theTheriot Fault System, the zone of lakesin west Terrebonne Parish may be relatedto tilting of this block against thebounding fault on the north side of theblock.
Subsidence Rates by EnvironmentalMapping Unit
For the purpose of the Coast 2050planning process, a generalizedsubsidence map of the Louisiana coastalzone was prepared (Figure 3-29). Findings from this study were utilized inpreparation of the map. The primarysource of subsidence rates for the DeltaicPlain were land level data along naturallevee ridges from the most recent periodof record. The land level data isconsidered to be the most accuratemeasure of subsidence. Rates from thesurvey lines were extrapolated to themajor fault bound blocks. Boundariesfor "environmental mapping units"developed for the Coast 2050 projectwere then superimposed over the faultblocks map to determine applicable ratesfor each mapping unit. Average valuesfrom other data sources, as gleaned fromthe geological literature, were used formapping units where sequential landleveling data was not available, such asthe Pontchartrain Basin and the ChenierPlain. The map should be regarded as ageneral tool developed for the Coast2050 planning process, and not adefinitive work intended for engineeringdesign values of subsidence.
Summary and Conclusions
Faults, subsidence and land loss incoastal Louisiana have all been topics ofconsiderable study. Researchers agreethat land loss, particularly wetland lossand deterioration, is closely linked tosubsidence. They generally acknowledgethat geotechnical or fault inducedsubsidence is a contributing factor, butmost tend to agree that subsidence is
predominantly attributed to compaction. Even if compaction of sediments is themajor cause of subsidence, mostadjustments for compaction probablytake place along faults. Verticaladjustments to gravity induced earthmovement and isostatic down-warpingalso occur along fault planes. Cumulative displacement on growthfaults and episodic changes in subsidencerates support this conclusion. Much, ifnot most of the vertical adjustment takesplace along fault planes. Therefore, faultinduced or geotechnical subsidence, as ithas been used in the literature, is a majorcontributing factor to relative sea levelrise. This paper identifies the importanceof fault movement, the locations andtypes of major faults, and identifiesblocks bound by the major faults. Itestablishes a framework for further studyand application to coastal restoration. Geotechnical subsidence occurs asmovement along circular patterns offaults, which circumscribe collapsefeatures, and along linear growth faults. Collapse features may be induced by saltdepletion at depth and/or sedimentloading at the surface. Movement alonggrowth faults occurs in response tocompaction, geosynclinal downwarpingand gravity slumping.
The origin and locations of major growthfaults are related to basement topographyand earth crust movements. Onceestablished they become zones ofweakness where vertical displacement inresponse to sediment loading occurs.Cumulative displacement of bedsindicates that some have been activesince Cretaceous times. Thus, thedown-thrown blocks of growth faultsbecome depressions which "attract"
deposition, and in turn cause movementon the faults.
The coastal region is divided into amosaic of massive fault-bound blocks.Movement of the blocks is similar tomass movement along the delta front, buton a larger scale and over a longer timeperiod. Some blocks are moving andslumping into the deep Gulf of Mexicothrough a process of gravity inducedslumping which is occurring on a massivescale along the continental margin.
Not all fault-bound slump blocks move atthe same time. From the geological timeperspective, seaward blocks are moreactive than inland blocks. Movementalong the basin margin fault system(Baton Rouge Fault System) is anexception to this generality. Slumpinduced movement is episodic. Blocksare subject to abrupt short-term changesin subsidence rates. Rates have increasedfrom prehistoric to historic times. Aninferred rate increase occurred in the1890’s, initiating the Twentieth CenturyTransgression. In the early 1960’s,subsidence rates on some blocksincreased significantly, resulting inaccelerated land loss and barrier islanddeterioration.
Until recently, fault movement in thecoastal lowlands was masked and wentunnoticed because of accretionprocesses. However, within the last 40years the effects of fault movement havebecome more evident because ofincreased rates of sinking and reductionof accretion processes. Fault traces havebecome visibly delineated by patterns ofland loss and marsh deterioration.
Areas of high land loss occur on blockson the down-thrown side of theTheriot-Golden Meadow-Forts Faultsystems. The Baton Rouge FaultSystem, located along the rim of the GulfSalt Dome Basin, is active and hascaused structural damage to buildingfoundations and bridges. Some minorearthquake activity may be related tomovement along this fault system. Azone of intensive faulting (Lake BorgneFault Zone) occurs between the LakeSand-Frenier and Mauvais Boisalignments. Occurrence of numerouslakes in this zone may be related tofaults. Collapse features which may havecontributed to land loss include theCalcasieu Lake, Four Island, LakeWashington and active Mississippi Deltafeatures. The Hog Bayou (GrandChenier) Fault System may be affectingsubsidence in the Mermentau Basin area.
Fault movements of a fraction of an inchper year are almost imperceptible at thesurface in upland areas; however, inlow-relief coastal areas, small verticalmovement can result in subsidence ratesthat can upset natural system equilibriumand cause catastrophic loss of wetlandvegetation and accelerated erosion ofshorelines and barrier islands. Thesechanges in turn may make humaninfrastructure more vulnerable toflooding, storm surge and erosion.
Subsidence rates on these largefault-bound slump blocks showsignificant increases since the early1960’s. Areas north of the Gulf CoastSalt Dome Basin are being uplifted as aresult of isostatic adjustment. The ratesof uplift are approximately the same asthose of down-warp to the south.
Relative sea level rise rates along theentire Louisiana coast are higher than atPensacola, Fl, which is considered to be ageologically stable gage responding onlyto eustatic change. The highest rates arefound in the Deltaic Plain and areassociated with foundering fault-boundblocks. Rates in the Chenier Plain arehigher than at Pensacola. Some of thisdifference can be attributed to faultinduced subsidence. Rates that arehigher than the northern Gulf eustaticrise rate also occur in the eastern end ofLake Pontchartrain and appear to berelated to a block on the down-dip sideof the Baton Rouge Fault System.
Results of geological research in theLouisiana coastal area has beencumulative. A number of different linesof research have contributed to anunderstanding of the role of faultmovement in the Twentieth CenturyTransgression in the Deltaic Plain. Replication of some aspects of theresearch by different scientists providesimproved confidence in the findings regarding the role of fault movement incoastal change.
All features on the surface of subsidingblocks including wetlands, natural leveeridges, highways, and flood protectionlevees are affected. Location of faults,thickness of poorly consolidatedmaterials, and rates of relative sea levelrise are parameters that must beconsidered in evaluating and designingcoastal restoration projects. Theboundaries of the problem have beendefined. Nature’s driving forces can notbe changed, and if coastal sustainability isto be successful, planning and buildingneed to proceed with
Conversion Matrix
acknowledgment of, and considerationfor these critical natural parameters.
REFERENCES
Adams, R. L. 1997. "Microbasinanalysis of south Louisiana: AnExploration Model, or: Hurronand Lyell were wrong!" Transactions of the Gulf CoastAssociation of GeologicalSocieties, XLVII, 1-12.
Anderson, E.G. 1979. Basic MesozoicStudy in Louisiana, the NorthernCoastal Region and Gulf Basin. Plate 1, Folio Series #3.
Barras, J.A., P.E. Bourgeois, and L.R.Handley. 1994. Land loss incoastal Louisiana 1956-90. National Biological Survey,National Wetlands ResearchCenter Open File Report 94-01. 4 pp. 10 color plates.
Boesch, D.F., M.N. Josselyn, A.J.Mehta, J.T. Morris, W.K. Nuttle,C.A. Simenstad, and D.J.P. Swift.1994. Scientific assessment ofcoastal wetland loss, restorationand management in Louisiana.Journal of Coastal ResearchSpecial Issue No. 20. 103pp.
Britsh, L.D. and E.B. Kemp III. 1990. Land Loss Rates: MississippiRiver Deltaic Plain. GeotechnicalLaboratory, Department of theArmy, Waterways ExperimentStation, Corps of Engineers. Tech. Rept. GL-90-2, Vicksburg,MS.
Chabreck, R.H., T. Joanen and A.W.Palmisano. 1968. Vegetativetype map of the LouisianaCoastal Marshes. La. Wildlifeand Fisheries Comm., NewOrleans.
Chabreck, R.H. and G. Linscombe. 1978 Vegetative type map of theLouisiana coastal marshes. La.Wildlife and Fisheries Comm.,Baton Rouge, La.
Chabreck, R.H. and G. Linscombe. 1988 Vegetative type map of theLouisiana coastal marshes. La.Wildlife and Fisheries Comm.,Baton Rouge, La.
Coleman, J.M., H.H. Roberts and R.S.Tye. 1998. Causes of LouisianaLand Loss. A report preparedfor Louisiana Mid-Continent Oiland Gas Association. P:28.
Durham, C. O., Jr. and E. M., PeeplesIII. 1956. "Pleistocene faultzone in southeastern Louisiana," Transactions of the Gulf CoastAssociation of GeologicalSocieties 5, 65-66.
Fisk, H.N. 1944. Geologicalinvestigation of the alluvial valleyof the lower Mississippi River.U.S. Army Corps of Engineers,Mississippi River Commission,Vicksburg, MS.
Frazier, D.E. 1967. Recent deltaicdeposits of the Mississippi River:their development andchronology. Transactions/GulfCoast Association of GeologicalSocieties 17:287-311.
Gagliano, S.M., and J.L. van Beek. 1970.Geological and geomorphicaspects of deltaic processes,Mississippi delta system.Hydrologic and Geologic Studiesof Coastal Louisiana, Report 1,Coastal Resources Unit, Centerfor Wetland Resources, LouisianaState University, Baton Rouge,LA.
Gagliano, S. M. and J. L. van Beek 1973. New Era for Mudlumps. WaterSpectrum. Department of theArmy corps of Engineers, V. 5,no. 1, pp. 25-31.
Gagliano, S.M., and J.L. van Beek. 1976.Mississippi River sediment as aresource. Pages 102-126 in RamS. Saxena, editor, ModernMississippi Delta-DepositionalEnvironments and Processes.
Gagliano, S.M., K.J. Meyer-Arendt, andK.M. Wicker. 1981. Land loss inthe Mississippi River deltaicplain. Transactions/Gulf CoastAssociation of GeologicalSocieties 31:295-300.
Gagliano, S.M., and J.L. van Beek. 1993.A long-term plan for Louisiana’scoastal wetlands. LouisianaDepartment of NaturalResources, Office of CoastalRestoration, Baton Rouge, LA.
Galloway, W.E. 1986. "Growth faultsand fault-related structures ofprograding terrigenous clasticcontinental margins,"Transactions of the Gulf CoastAssociation of GeologicalSocieties, V. 36:121-128.
Halbouty, M. T. 1979. Salt Domes, GulfRegion of the United States andMexico, 2nd ed. Houston: GulfPubl. Co.
Holdahl, S. R. and N. L. Morrison,1974. Regional Investigations ofVertical Crustal Movements inthe U.S., Using PreciseRelevelings and MareographData." Tectnophysics, v. 23, p.375-390.
Kolb, C.R. and J.R. Van Lopik 1958. Geology of the Mississippi Riverdeltaic plain, southeasternLouisiana. Technical Report No.3-483, U.S. Army WaterwayExperiment Station, Vicksburg,MS.
Kolb, C., F. L. Smith and R. C. Silva,1975. Pleistocene Sediments ofthe New Orleans-LakePontchatrain Area. Soils andPavement Laboratory, U.S. ArmyEngineer Waterways ExperimentStation, Technical Report S-75-6,Vicksburg, MS.
Krinitzsky, E.L. 1950. Geologicalinvestigation of faulting in theLower Mississippi Valley,Technical Memorandum No.3-311, U.S. Army EngineerWaterways Experiment Station,Vicksburg, MS.
Kuecher, G.J. 1994. Geologic frameworkand consolidation settlementpotential of the Lafourche Delta,topstratum valley fill: implicationsfor wetland loss in Terrebonneand Lafourche Parishes,Louisiana.
Ph.D. dissertation, LouisianaState University, Baton Rouge,LA. 346 pp.
Lopez, J. A. 1991. Origin of LakePontchatrain and the 1987 IrishBayou Earthquake, Gulf CoastSociety of Sedimentologist andEconomic Paleontologist,Research Conference 1991.
Lopez, J.A., S. Penland and J. Williams 1997. Confirmation of ActiveGeologic Faults in LakePontchatrain in SoutheastLouisiana. Gulf CoastAssociation of GeologicalSocietists, Transactions of the47th Annual Convention. Pp.299-303.
Meade, R.H. and R.S. Parker. 1985.Sediments in rivers of the UnitedStates. National Water Summary1984. U.S. Geological Survey,Water Supply Paper 2275.
Morgan, J. P., J. M. Coleman and S.M.Gagliano, 1968. Mudlumps:Diapiric Structures in Mississippi. In Diapirism and Diapirs, AAPGMemoir No. 8, p. 145-161.
Murray, G.E. 1960. Geologic frameworkof Gulf Coastal Province ofUnited States. Pages 5-33 in F.P.Shepard, F.B. Phleger, and T.H.van Andel, editors, Recentsediments, northwest Gulf ofMexico. American Association ofPetroleum Geologists, Tulsa, OK.
Nakashima, L.D. & Louden, L.M, 1988,Water level change, sea level rise,subsidence, and coastal structuresin Louisiana, Technical Reportfor Coastal Engineering ResearchCenter, U.S. Army Corps ofEngineers, Vicksburg, MS,Louisiana Geological Survey,Louisiana State University, BatonRouge, LA.
Nyman, J.A., R.D. DeLaune and W.H.Patrick Jr. 1990 Wetland soilinformation in the rapidlysubsiding Mississippi Riverdeltaic plain: mineral and organicmatter relationships. Estuarine,Coastal and Shelf Science31:57-69.
O’Neil, T. 1949. The muskrat in theLouisiana coastal marshes: astudy of the ecological,geological, biological, tidal, andclimatic factors governing theproduction and management ofthe muskrat industry in Louisiana.Louisiana Department of Wildlifeand Fisheries, New Orleans, LA.152 pp.
Penland, S., K.E. Ramsey, R.A.McBride, T. Mestayer, and K.A.Westphal. 1988. Relative sealevel rise and delta-plaindevelopment in the TerrebonneParish region. Coastal GeologyTechnical Report No. 4,Louisiana Geological Survey,Baton Rouge, LA. 120 p.
level rise and subsidence inLouisiana and the Gulf ofMexico. Louisiana GeologicalSurvey, Baton Rouge, LA. 65 pp.
Ramsey, K.E. and T.F. Moslow. 1987. Anumerical analysis of subsidenceand sea level rise in Louisiana.Pages 1673-1688 in N.C. Kraus,editor, Coastal Sediments ’87:Proceedings of a specialtyconference on advances inunderstanding of coastal sedimentprocesses, New Orleans, La.,May 12-14, 1987. AmericanSociety of Civil Engineers, NewYork City, N.Y.
Reed, D.J., editor. 1995. Status andTrends of HydrologicModification, Reduction inSediment Availability, andHabitat Loss/Modification in theBarataria-Terrebonne EstuarineSystem. Publication #20,Barataria-Terrebonne NationalEstuary Program.
Roberts, H.H. 1985 A study ofsedimentation and subsidence inthe south-central coastal plain ofLouisiana. Coastal StudiesInstitute, Louisiana StateUniversity, Baton Rouge. Prepared for U.S. Army Corp ofEngineers, New Orleans,Louisiana, Final Report.
Roberts, H.H. 1998. Delta switching:early responses to theAtchafalaya River diversion.Journal of Coastal Research14(3):882-899.
Roland, Harry L., Tom F. Hill and PeggyAutin, 1981, The Baton Rougeand DenhamSprings-Scottlandville Faults,Louisiana Geological Survey,Dept of natural Resources
Salvador, A. ed. 1991. The Gulf ofMexico Basin. Geology of NorthAmerica., v. J, Plate 2, Geol. Soc.Amer.
Sasser, C.E. 1994. Vegetation dynamicsin relation to nutrients in floatingmarshes in Louisiana, USA.Center for Coastal, Energy, andEnvironmental Resources,Louisiana State University, BatonRouge, La. 193p.
Saucier, R.T. 1963. Recent geomorphichistory of the PontchartrainBasin. Coastal Studies Series 9,Louisiana State University Press,Baton Rouge, LA. 114 pp.
Saucier, R. T. 1994. Geomorphologyand Quaternary Geologic Historyof the Lower Mississippi Valley. U.S. Army Engineer WaterwaysExperiment Station, Vicksburg,Miss. Two volumes, V. 1: 364 p.+ appendices, V. 2: maps andplates.
Seglund, J.A. 1974. Collapse FaultSystems at Louisiana Gulf Coast.Amer. Assoc. Petroleum. Geol.Bull. 58(12) 2389-97
Spearing, D. 1995. Roadside Geologyof Louisiana. Mountain Press
Publishing Co., Missoula,Montana, 225 p.
Stover, C.W., B.G. Reagor, and S.t.Algermissen, 1987, Seismicitymap of the State of Louisiana:U.S. Geological survey,Miscellaneous field studies mapMF-1081
van Beek, J.L., T.J. Duenckel, P.C.Howard, K.J. Meyer-Arendt, andS.M. Gagliano. 1986. Long-termmanagement and protection ofPlaquemines Parish. CoastalEnvironments, Inc., BatonRouge, LA. 249 pp.
Wallace, W.E., Jr. 1957. Fault map ofsouth Louisiana.Transactions/Gulf CoastAssociation of Geology Society7:240.
Watson, C. C. 1982. An Assessment ofthe Lower Mississippi RiverBelow Natchez, Mississippi. Ph.D. Dissertation, ColoradoState University, Fort Collins,Colorado, 162 p.
Wigley, T. M. and S. C. B. Raper. 1992.Implications for climate and sealevel of revised IPCC emissionscenarios. Nature 357:293-300.
Winker, C.D. 1982. Clastic shelfmargins of the post-ComancheanGulf of Mexico; implications fordeep-water sedimentation: GulfCoast Sectl, Soc. Econ.Paleontologists and Mineralogists5th An. Research Conf., Prog.And abs., p. 109-117.
Williams, S. J., S. Penland, and A. H.Sallenger, Jr., editors, 1992. Louisiana Barrier Island ErosionStudy: Atlas of shoreline changesin Louisiana from 1853 to 1989. Prepared by the U.S. GeologicalSurvey in cooperation with theLouisiana Geological Survey. 103pp.
. . . . . . . . . . . . . . . . . . . . . .
SECTION 4
METHODOLOGY FOR ASSESSMENT OF FISHERIES
Identification of Guilds
In order to assess the recent trends andfuture projections of fishery populationswithin the Coast 2050 study area, fourbroad species assemblages wereidentified based on salinity preferences. These assemblages were marine,estuarine dependent, estuarine resident,and freshwater.
Within each of the four assemblages,guilds of fishery organisms wereestablished. As used in this document,guilds are groupings of ecologicallysimilar species identified by a singlerepresentative species and, hereafter, theterms “guild” and “species” are usedinterchangeably. Fishery guilds commonto coastal Louisiana, within eachsalinity-preference assemblage, are:
• Marine: Spanish mackerel guild,
• Estuarine dependent: red drum,black drum, spotted seatrout, Gulfmenhaden, southern flounder, whiteshrimp, brown shrimp, and blue crabguilds,
In a broad sense, each of the 12 guilds isuniquely identified by the combination
of the representative species’ habitatpreference, salinity preference, primaryhabitat function, seasonal occurrence inthe estuary, and spawning or migratoryseasons (Table 6-1, main report,reproduced as Table 4-1 of thisappendix). Habitat and life historyinformation is based on availablescientific literature specific to thenorthwestern Gulf of Mexico, but issomewhat generalized to accommodatethe establishment of guilds.
Trends and Projections forFisheries Populations
Once the species representing eachfishery guild was identified, populationchanges of each species were assessedand displayed by using a matrix for eachof the four coastal regions (Tables 4-2through 4-5). The matrices displaymapping units and guilds and, within themapping units, provide information onthe population stability (recent changetrends) and population projections foreach species group. Most of the recenttrend information was provided byfishery biologists of the LouisianaDepartment of Wildlife and Fisheries(LDWF). The assessments were basedon LDWF fishery independent samplingdata and personal observation by areafishery biologists, and generally span aperiod of 10 to 20 years. Staff of LDWFbelieve that, due to selectivity of sample
gear, the trend information is mostreflective of recent changes in thesubadult portion of each guild.
The projections of possible futurechanges in fishery production for coastalLouisiana, also shown in Tables 4-2through 4-5, are based solely onlandscape change model predictionsdiscussed in the main report. The keyparameters in making those projectionswere percent and pattern of wetland lossin each mapping unit. Numerous otherfactors which could not be forecast —changes in water quality, fishery harvestlevels, wetland development activities(e.g., dredging and filling), andblockages of migratory pathways —could negatively impact fisheryproduction. These factors and thepotentially great inaccuracy in predictingland loss 50 years into the future,especially when considering landscapechanges at a mapping unit scale, limitthe precision of the predicted changes infishery production.
Individuals Involved inApplication of Methodology
Information provided in the matrix wasdeveloped through the collaborativeeffort of the LDWF and the NationalMarine Fisheries Service (NMFS).
NMFS contributors were RicRuebsamen and Richard Hartman. LDWF personnel responsible forsynthesizing the information displayed ineach regional matrix are identifiedbelow.
Region 1: John F. Burdon, MarkLawson, and Glenn Thomas.
Region 2: Robert Ancelet, MarkSchexnayder, Greg Laiche, ClarenceLuquet, Keith Ibos, Randall Pausina,Brian McNamara and Glenn Thomas.
Region 3: Vince Guillory, RoyMoffet, Martin Bourgeois, SteveHein, Paul Meier, Pete Juneau, PaulCook and Glenn Thomas.
Region 4: Dudley C. Carver, JerryFerguson, Michael Harbison andGlenn Thomas.
The overall work effort was coordinatedby Ric Ruebsamen of NMFS and GlennThomas of LDWF.
NOTES: Steady=Sy, Decrease=D, Increase=I, Unknown=U, Not Applicable=NA
Fish and Invertebrate Guilds (Species)
Table 4-5. Region 4 fish and invertebrate population status and 2050 change (Cont.).
SECTION 5
METHODOLOGY FOR ASSESSMENT OF WILDLIFE
Species and Species Groups
Louisiana's coastal wetlands, extendingfrom the forested wetlands at the upperend to the barrier shorelines borderingthe gulf, provide a diverse array ofhabitats for numerous wildlifecommunities. In addition to fulfilling alllife cycle needs for many residentspecies, coastal wetlands providewintering or stopover habitat formigratory waterfowl and many otherbirds. The bald eagle and brown pelican,protected by the Endangered SpeciesAct, are recovering from very lowpopulations experienced over the lastthree decades. Increasing populationsfor those two species are projected tocontinue in the future, independent ofnear-term wetland changes. The fate ofother species groups in coastal Louisianawill be influenced by habitat conditionsthere. The prediction of extensive landloss and habitat change by the year 2050prompted an examination of the effect ofsuch losses and changes in theabundance of wildlife.
To assess habitat functions and thestatus, recent trends and futureprojections of wildlife abundance withinthe Coast 2050 study area, 21 prominentwildlife species and/or species groupswere identified on the basis of
prominence and/or availability ofinformation:
• Brown Pelican,• Bald Eagle,• Seabirds, such as Black Skimmer,
Royal Tern, Common Tern,Laughing Gull,
• Wading birds, such as Great BlueHeron, Snowy Egret, RoseateSpoonbill,
• Shorebirds, such as Piping Plover,Black-necked Stilt, AmericanAvocet, Willet,
• Dabbling ducks, such as Mallard,Gadwall, Mottled Duck, WoodDuck,
• Diving ducks, such as Greater Scaup,Ring-necked Duck, Redhead,Canvasback,
• Geese, such as Snow Goose, White-fronted Goose, Canada Goose,
• Raptors, such as Northern Harrier,Peregrine Falcon, American Kestrel,
• Rails, gallinules, and coots, such asKing Rail, Sora Rail, PurpleGallinule,
• Other marsh and open waterresidents, such as Anhinga, LeastBittern, Seaside Sparrow,
• Other woodland residents, such asPileated Woodpecker, CarolinaChickadee, Belted Kingfisher,
• Other marsh and open watermigrants, such as Tree Swallow,Barn Swallow, Savannah Sparrow,
• Other woodland migrants, such asHermit Thrush, American Robin,Cedar Waxwing,
• Nutria,• Muskrat,• Mink, Otter, and Raccoon,• Rabbit,• Squirrel,• White-tailed deer, and • American alligator.
Matrices
A matrix was developed for each regionto present the habitat function and thestatus, trend, and projection for theabove listed species and/or speciesgroups for each habitat type within eachmapping unit (Tables 5-1 through 5-4). Each matrix reflects available data andprofessional judgments.
“Habitat functions” considered were:nesting (Ne), wintering area (W),stopover habitat (St), and multiplefunctions (Mu). “Status” categoriesincluded the following: not historicallypresent (NH), no longer present (NL),present in low numbers (Lo), present inmoderate numbers (Mo), and present inhigh numbers (Hi). “Not historicallypresent” means that the species orspecies group has not been present in thegiven area for more than about 50 years. “No longer present” means that thespecies or species group was present inthe given area sometime during the last50 years, but is not currently present.
“Trend” refers to changes in abundanceover the last 10 to 20 years, and“projection” refers to a prediction ofchanges in wildlife abundance throughthe year 2050; “trend” and “projection”categories include steady (Sy), decrease(D), increase (I) and unknown (U).
“Habitat Types” reflect 1988 conditionsand include the following: open water(OW), aquatic bed (AB), fresh marsh(FM), intermediate marsh (IM), brackishmarsh (BM), saline marsh (SM), freshswamp (FS), hardwood forest (HF),barrier beach (BB), agriculture/upland(AU). Habitat types comprising lessthan 5% of a unit are shown only if thathabitat type is particularly rare orimportant to wildlife in the givenplanning unit.
“Habitat function,” “status,” and “trend”information displayed in each regionalmatrix represents commonunderstandings of the selected speciesand/or species groups, fieldobservations, data, and recent habitatchanges. “Projection” information isbased almost exclusively on thepredicted conversion of marsh to openwater and the gradual relative sinkingand resultant deterioration of forestedhabitat throughout the study area. Suchpredictions may or may not prove to beaccurate. Additionally, numerous otherfactors including water quality,harvesting level, and habitat changeselsewhere in the species’ range cannot bepredicted and were not considered inthese projections. Therefore, theprojections are to be viewed and usedwith caution.
Individuals Involved inApplication of Methodology
The individuals responsible forsynthesizing the information displayed ineach regional matrix are identifiedbelow.
The matrices were compiled by GerryBodin (U.S. Fish and Wildlife Service)and Quin Kinler (Natural ResourcesConservation Service).
Species or SpeciesGroup
Individuals Agency Affiliation
Brown Pelican, BaldEagle
Tom HessLarry McNease LDWF
Terry Rabot U.S. Fish and Wildlife ServiceSeabirds, wading birds,shorebirds, raptors, rails,gallinules, coots, othermarsh and open waterresidents, other woodlandresidents, other marsh andopen water migrants,other woodland migrants
Bill Vermilion LDWF
Dabbling ducks, divingducks, geese Robert Helm LDWF
