An evaluation of the historical effects of

Holocene faulting on the New Orleans East

Land Bridge and adjacent wetlands

Chris McLindon

February 2015

The August 28, 2014 publication “Losing Ground” by the online publications ProPublica and The Lens

referred to the New Orleans East Land Bridge as “the most valuable piece of real estate in southeast

Louisiana”. They characterized it as “a low narrow ridge of marsh and sand on the extreme eastern

flank of the Crescent City [that] separates Lake Pontchartrain from Lake Borgne, gateway to the open

waters of the Gulf of Mexico”. The strategic importance of this area is underscored by the fact that the

land bridge and the areas just to the south that comprise the Golden Triangle Marsh Area and the

Central Wetlands Unit have experienced some of the most dramatic changes in land area around the

greater New Orleans Metropolitan area. There are several restoration projects that are currently

underway or in the planning stages that are intended to address the issue of land area change. These

include the Louisiana Coastal Restoration and Protection Authority’s Golden Triangle Marsh Creation

Project, the New Orleans Sewerage and Water Board’s Central Wetlands Unit Assimilation Project, and

Chef Menteur Wetland Mitigation Bank Marsh Creation Project of the private enterprise Ecosystem

Investment Partners. The full cost of completing these projects as they are envisioned is more than

$200 million dollars.

Several key studies have determined that these areas are subsiding at measurable rates, and that

subsidence is most likely to be due to the vertical movement of faults crossing the Proposal Area. The

results of these studies can be combined to draw a reasonable conclusion that the New Orleans East

Land Bridge and the Golden Triangle Marsh Area are crossed by a set of an en echelon faults extending

southward from and approximately paralleling the north shore of Lake Pontchartrain. Dokka

(2006,2011), Haggar (2014), Yeager, et.al. (2012), and Lopez, et.al. (1997) have mapped out individual

elements of this set of faults and the impacts that are expressed in their specific areas of study. Lopez,

et.al. (1997) in particular showed that faults cutting the sedimentary layers immediately below the

bottom of Lake Pontchartrain could be defined with striking accuracy with high resolution seismic

profiles acquired in the Lake by the U.S. Geological Survey.

The portion of a high resolution seismic profile shown in Fig. 2, taken from Lopez, et.al. (1997), perfectly

illustrates the ability of the technology to allow for the accurate interpretation of the location of the

fault, the timing and rate of its vertical movement, and therefore an estimation of the rate of

subsidence caused by that vertical movement. It is clear that this fault, which extends upward to the

bottom surface of Lake Pontchartrain at the location shown of Fig. 6, both vertically offsets the

sedimentary layers beneath the surface of the lake bottom and causes a thickening of the sedimentary

layers in the “downthrown fault block” to the left of the fault line on the profile. The best interpretation

Fig. 1

of the thickening of the sedimentary layers by the fault is that gradual accumulation of muds and silts on

the bottom of the lake was contemporaneous with the vertical movement of the fault. The movement

of the fault created more accommodation space for the accumulation of sediments on the downthrown

side, and so each sedimentary layer is thicker on that side than it is on the “upthrown” side. If the age

of the sedimentary layers can be determined, then a rate of thickening, and therefore a rate of

subsidence, can be estimated for the time period. For example if the sedimentary layer between B and

C on this profile is 27 feet thick on the upthrown side of the fault and 38 feet thick on the downthrown

side of the fault, then the fault caused 11 feet of thickening (and by inference approximately 11 feet of

subsidence) during the time period in which those sedimentary layers were deposited. If it could be

determined that horizon C is 550 years old and horizon B is 220 years old then the subsidence of 11 feet

took place over 330 years, or it had an average rate of motion of 0.4 inches (or 10 mm) per year. This

evaluation process can be used to determine both the location of the fault and the average historical

rate of subsidence caused by a fault.

The importance of faulting in determining the geomorphology and ecology of the wetlands and the

evolution of that ecology has been demonstrated by the work of the authors Dokka (2011), Haggar

(2014), Yeager, et.al. (2012), and Lopez, et.al. (1997) Each of these authors used different

methodologies to examine the effects of faulting at the surface of the lake bottom and surrounding

wetlands. The fluvial/marsh settings at Bayou Lacombe and the Pearl River Delta on the north shore of

Lake Pontchartrain studied by Haggar (2014) and Yeager, et.al. (2012), respectively, are located along

the northernmost trend of faults that directly affect the Lake Pontchartrain Basin. This trend has been

recognized to be an extension of the larger Baton Rouge (or Tepate) Fault system. Haggar (2014) used

Lidar imagery to delineate the obvious vertical escarpment of a segment of this trend called the

Fig. 2

Lacombe Fault, and effectively related the landscape level changes in plant communities in the Goose

Point area to subsidence caused by vertical movement on the fault. Haggar (2014) also offered the

possibility of a causal relationship between this subsidence and the infestation of pine beetles in

Fountainbleau State Park in the mid-1990s due to the stress induced on the trees by a loss of elevation.

Yeager, et.al. (2012) successfully measured estimates of subsidence due to faulting in the Pearl River

Delta using high resolution seismic data, radiocarbon and optically stimulated luminescence dating and

sedimentological analysis from shallow cores. They found that “groups of active, near-surface growth

faults are critically important in terms of defining marsh accretion rates and how parts of a given marsh

will respond to external stress, including accelerated sea level rise.” Both Haggar (2014) and Yeager,

et.al. (2012) recognized that although the subsidence rates on the faults along the basin edge are lower

than those measured further within the basin, the patterns of change in the plant communities due to

subsidence are comparable in both areas.

Fig. 4

Fig. 5

Dokka (2011) and Lopez, et.al. (1997) used different methodologies to document subsidence due to

fault movement at locations further into the basin. Lopez, et.al. (1997) combined interpretation of the

U.S.G.S. high resolution seismic data with dramatic physical evidence in the form of vertical offset on the

Highway 11 Bridge across Lake Pontchartrain to illustrate the location and effects of the faults just to the

south of the trend of the Baton Rouge – Lacombe Faults. Lopez, et.al. (1997) estimated a rate of vertical

movement on these faults to be 2.5 mm/yr, which is more than 10 times greater than the estimate of

0.16 mm/yr derived by Yeager, et.al. (2012) in the more stable Pearl River Delta area. Lopez, et.al.

(1997) was also able to relate the location of the surface traces of these faults to much deeper cutting

faults that were imaged with conventional 2-D seismic data which had been acquired for oil and gas

exploration. This relationship established that these faults are fundamental components of geologic

structure of the area, and they are likely to have been actively moving over periods of several million

years.

Dokka used a combination of geodetic elevation studies and INSAR data collected by Canada’s

RADARSAT satellite to relate estimates of subsidence derived from these technologies to the vertical

movement of faults. It is significant that he was also able to infer the location of the faults from the

elevation data. The subsidence values measured by INSAR (Dixon, et.al., 2006) had a mean value of 5.6

mm/yr across the greater New Orleans area, the maximum rate observed by this method was 29 mm/yr.

It is clear that the results of these studies can be combined to illustrate both the striking relationship

between subsidence and fault movement and the progressive increase in the rate of subsidence from

Fig. 6

the basin edge inward toward the basin center. The faults affecting the Golden Triangle Marsh Area and

the Central Wetlands Unit are those for which the highest rates of subsidence have been estimated.

Taken together, these studies show a clear pattern of both the direct association of subsidence and the

vertical movement of active faults, and an increasing rate of vertical fault movement and resulting

subsidence moving from the basin edge inward – or west-southwestward across the New Orleans East

Land Bridge. The highest values of subsidence measured by Dixon, et.al. (2006) in the area of the

Golden Triangle and the Central Wetlands Unit are over 20 mm/yr, or about 8 inches per decade. The

methodology of measuring subsidence from INSAR data is limited by the fact that values must be

measured from locations that are exposed dry land over a period of several years. Dixon,et.al. (2006)

was able to get scattered measurements from within the wetlands area from the tops of spoil banks

along dredged canals in the marsh. Most of the area of the New Orleans East Land Bridge is not optimal

for this type of subsidence measurement.

Another means of assessing the effects of subsidence on the wetlands in this area is by the comparative

analysis of aerial photography and satellite imagery. A striking example of this type of analysis was

published in the previously mentioned article “Losing Ground” in the online journals ProPublica and The

Lens on August 28, 2014. The staff at these publications working cooperatively with the Tow Center for

Digital Journalism at Columbia University produced a set of 14 images from the New Orleans East Land

Bridge. These images, which show the area of wetlands cover, can be view in chronological succession

on the ProPublica website. They reveal the patterns of change, and in doing so they can also be used to

demonstrate how the U.S.G.S. Land Area Change Map was constructed. Fig. 8 shows two of the images

Fig. 7

from the “Losing Ground” article from 1932 and 2009. The yellow dashed fault traces and the red ovals

were added to the original images to emphasize the area of wetlands loss. It is located on the

downthrown side of the southern fault. This pattern of wetlands loss by the formation of small ponds

that progressively grow into larger ponds, lakes and bays is most common progression seen across the

coastal wetlands. A portion of the U.S.G.S. Land Area Change Map that covers this area is shown in the

lower right corner of the figure. The map (Couvillion, et.al., 2011) uses color codes for areas of wetlands

loss to indicate the time interval in which the marsh surface became submerged. This map shows that

the majority of the land loss on the New Orleans East Land Bridge has occurred over the last decade. A

comparison of the Couvillion, et.al. (2011) map with the location of the faults derived from existing

studies appears indicate that the recent wetlands loss is the result of the cumulative effects of

subsidence due to fault movement. The image in the lower left corner of the figure is a diagrammatic

representation of a profile across the fault. The downthrown side to the right of the fault surface in this

image is subsiding at a higher rate than the upthrown side creating the cluster of open water ponds that

are bounded by the fault trace on the map images.

The article “Sinking levee shows difficulty of protecting New Orleans from flooding” by Bob Marshall

published on February 17, 2014 in The Lens documented that a 1.1 mile long stretch of the levee along

the Gulf Intercoastal Waterway has sunk by 3 to 6 inches below its design height of 25 to 27.5 feet. The

article quoted then-president of the South Louisiana Flood Protection Authority-East Tim Doody as

saying “… the Corps has estimated it will probably cost about $35 million over the next 10 years to keep

this system certified for flood insurance.” The existing studies considered here point to a very strong

likelihood that the sinking of this stretch of levee measured by the Corps is the result of subsidence due

Fig. 8

to fault movement. The planning and maintenance of the flood protection infrastructure is arguably the

most important aspect of public works in the Greater New Orleans area.

The New Orleans Easts Land Bridge and surrounding wetlands are arguably the most important portions

of the coastal wetlands to understand in terms of the patterns and processes of land area change. The

conversion of marsh and swamp ecosystems to open bodies of water is occurring in close proximity to

the population centers of Greater New Orleans. Billions of dollars have been spent on flood protection

infrastructure in the area and restoration projects potentially costing hundreds of millions of dollars

have been planned or implemented in the area.

REFERENCES

Armstrong, C., et.al., 2014, Influence of growth faults on coastal fluvial systems: Examples from the late Miocene to Recent Mississippi River Delta, Sedimentary Geology, v. 301, p. 120-132. Couvillion, B.R.,et.al, 2011, Land area change in coastal Louisiana from 1932 to 2010: U.S. Geological Survey Scientific Investigations Map 3164, scale 1:265,000, 12 p. pamphlet. Dawers, N.H. and E. Martin. 2005. Fault-related changes in Louisiana coastal geometry, Louisiana Governor’s

Applied Coastal Research and Development Program, GACRDP Technical Report Series 05-000, 21 p

Dixon, T.H., 2006, Subsidence and flooding in New Orleans, Nature, v. 441, p. 587-588 Dokka, R.K., 2006, Modern-day tectonic subsidence in coastal Louisiana, Geology, v. 34, p. 281-284. Dokka, R.K., 2011, The role of deep processes in late 20

th century subsidence of New Orleans and coastal areas of

southern Louisiana and Mississippi, Journal of Geophysical Research, v. 16, 25 pgs. Gagliano, S.M., et.al., 2003, Neo‐tectonic framework of southeast Louisiana and applications to coastal

Restoration, Trans. G.C.A.G.S., v. 53, p. 262‐272

Gagliano, S.M., et.al., 2003, Active Geological Faults and Land Change in southeastern Louisiana, A Study of the Contribution of Faulting to Relative Subsidence Rates, Land Loss and Resulting Effects on Flood Control, Navigation, Hurricane Protection and Coastal Restoration Projects. U.S. Army C.O.E. Contract No. DACW 29-00-C-0034

Haggar, K.S., 2014, Coastal Land Loss and Landscape Level Plant Community Succession; an Expected Result of Natural Tectonic Subsidence, Fault Movement, and Sea Level Rise, Trans. G.C.A.G.S. v.64, p. 139-160

Lopez, J.A., et.al., 1997, Confirmation of Active Geologic Faults in Lake Pontchartrain in Southeast Louisiana, Trans.

G.C.A.G.S., v. 47, p. 299-303

Yeager, K.M., et.al., 2012, Significance of active growth faulting on marsh accretion processes in the

lower Pearl River, Louisiana, Geomorphology, v. 153-154, p. 127-143