Subsidence in the Mississippi Delta

8/12/2019 Subsidence in the Mississippi Delta

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Kolb and Van Lopik, 1966

Curtis, 1970

Holocene Deltas (6,000 ybppresent)

Miocene Deltas (1311 mmybp)

For at least the past 50 million years the

fundamental interconnection between the Gulf of

Mexico Basin and the Mississippi River has been

based on the ability of the basin to accept the

sedimentary load delivered by the river. The lateHolocene deltas that deposited the substrate of the

coastal wetlands now at the surface are only the

latest in a succession of historical deltas that has

been continuous throughout that time span. These

are shown in comparison with deltas of the

Miocene Epoch that were deposited in what is now

the south Louisiana coastal zone over 10 million

years ago

Each delta has represented the terminus of a river

system that carried the erosional detritus from the

North American Continent. The balance between

river and basin is the accommodation capacity of

the basin to accept the sedimentary load delivered

by the river. That balance is determined by the

subsidence of the basin. Geologic processes drive

the engine of subsidence, and as their activity

varies across the surface of the coastal plain, the

river moves back and forth always seeking

accommodation capacity for its delta. It is the

engine of subsidence that ultimately determines

how the patterns of delta-switching and land area

change at the surface are played out across the

coastal plain.

Geohistory diagram

Geohistory diagram


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Nelson, E.J. et.al., 2000, Timing of Source Rock Maturation in the

Northern Gulf of Mexico Basin: Results of Thermal Modeling of a Regional

Profile, GCAGS Transactions, v 50, p. 309-323

This is a geohistory diagram for a point in the

central Louisiana coastal plain in Lafourche Parish

(location indicated by a red dot on the delta maps

of the previous slide). The axes of the graph are

time in millions of years on the horizonal and depthin feet on the vertical. The lines crossing the body

of the diagram track the depth history of

sedimentary layers indicated by their age on the

right end of the diagram (at time 0, or the present).

In theory if a well were drilled at this location, it

would encounter sediments of the ages indicated

along the right column at the depths indicated.

All of the lines within the body of the diagram slopefrom left to right indicating that the depth of any

horizon always increases with time. In other words

every horizon has been continually subsiding since

the time of its deposition. The red line in the upper

right corner tracks the history of the Miocene

detlas on the previous slide. The slope of the line

is the rate of subsidence. This point on the

Louisiana coastal plain has been continuously

subsiding for millions of years.


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Recent Deltatic Deposits of the Mississippi River:Their development and chronologyDavid E. Frazier, 1967

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Thousand Years Before Present

4 3 The construction of the modern wetlands of coastal Louisiana also record a

history of subsidence that is a vital component of the delta cycle by the

Mississippi spreads its sedimentary load back and forth across the coastal

plain. David Frazier used an extensive collection of core samples from

across the coast to reconstruct the history of the Holocene deltas. He

numbered each delta, and the graph to the left show the lifespan of each of

those deltas from 1 to 16. At the end of the lifespan of each delta, it is

abandoned by a change in course of the river, and becomes subject to the

forces of subsidence. Most of these recent deltas of the Mississippi have

subsided below the surface and are preserved as sedimentary layers.


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4,500years before present

2

3

Fraziers Delta #3 was the first incursion of the Mississippi River to the

eastern part of the coastal plain in the late Holocene about 4,500 years ago.

It appears to have cut a fairly straight path across the shallow waters that

covered the area at the time. The area of the French Quarter and the West

Bank would have probably been cypress swamps. This course of the river

was used over and over again throughout the Holocene, as it is used by the

most recent course of the river. Delta #3 has entirely subsided below the

surface and today is found at depths of about 30 feet below sea level.

6 2 1 05


4 3


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6 2 1 05


4 3

6

4

5

7

Within the 1,000 year period between 4,500 and 3,500 years ago fourdelta systems were active across the coastal plain. The river switched

back and forth depositing sediment to build up the coastal wetlands. The

outer fringes of deltas 4 and 6 have now subsided below the surface. In

this natural system a no net loss of wetlands cover was maintained

because every acre of wetlands that subsided below the surface in one

area was matched by an acre of newly created wetlands at the site of the

active delta. The channel of the river that fed delta 5 ran along the front of

the barrier island chain, and it forms the backboneof the Metairie and

Gentilly ridges that are highpoints of elevation in the metro area.


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6 2 1 05


4 3

6

4

5

7

The inset photograph is profile of the subsurface from a pit dug off BullardRoad in New Orleans East. It clearly captures the superposition of

Mississippi delta deposits on the white sands of the barrier island. Most of

the marsh at the surface today around New Orleans was deposited by

delta 5, and this is a cross section of those deposits. Subsequent river

channels continued to provide sediment by overbank flooding until the

construction of levees beginning in the 19thcentury. The thickness of the

delta deposits (labeled B)shows that subsidence was active in the area

even 4,000 years ago. The islands sunk below the surface and the delta

built across them.


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6 2 1 05


4 3

6

7

9

8

Bayou Jasmine Archeological Siteca. 800 BC

By 2,500 years ago the marshes created by deltas 6 and 7 were

subsiding below the surface and deltas 8 and 9 had built a vastexpanse of wetlands across what are now Breton and Chandeleur

Sounds. Archeological sites in the area strongly suggest that this

complex ecological system of cypress swamps and fresh and saline

marshes was inhabited by native people. This vast ecosystem

succumbed to subsidence over the next few hundred years. The

cypress swamps were drowned and died off and the marshes went

through a natural progression from freshwater to brackish to saline, andfinally to open water bays as the subsided below the surface.

Outer Islands


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6 2 1 05


4 3

6

79

8

The red line across delta 9 shows the location of a cross section

constructed by David Frazier from cores taken in the sediments at thebottom of Breton Sound. These sediments are the deposits of delta 9

that extended across the area between 2,500 and 1,900 years ago.

The cores found an in situ cypress stump that was a part of the vast

cypress swamp that covered the delta. The stump was age-dated to

2,100 years old, and it is now 33 feet below sea level. This is a direct

measurement of the rate of subsidence that submerged this delta

system, and its about the same rate of subsidence that is measured atthe surface in parts of the Central Wetlands Unit today.

Outer Islands

Breton Sound

33 of subsidence in 2,100 years


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Rogers, et.al. used cores and sparker profiles to study the historyof the St. Bernard Delta (Fraziers #8 and #9 deltas. The delta

complex fed by Bayou LaLoutre is strikingly similar in size and

shape to the modern birdfoot delta. Rogerswork indicated that

it subsided below the surface in about 300 years. The modern

delta is about 200 years old, and it likely to subside below the

surface by the end of this century.

Rogers, B.E. et.al., 2009, Late Holocene chronology, origin, and evolution

of the St. Bernard Shoals, Northern Gulf of Mexico, USA, GeoMarine

Letters, v.29, p. 379394


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320 mm = 12.5 inchessubsidence in 50 years

= 0.25 in/year

BLUM & ROBERTS, 2011

Grand Isle Tidal Gauge normalized to Pensacola

The subsidence of the coastal wetlands of Louisiana can be observed several in several ways. The one that lends

itself to the most direct measurement is the comparison of apparent sea level rise between historical tidal gauges

across the Gulf Coast. The graphs at the right are tidal gauge records from Pensacola, Florida and Grand Isle,

Louisiana. Pensacola sits on the backbone of the Appalachian Mountain Chain, and is generally accepted to have

experienced no subsidence over the past century. The graph on the left shows that the Pensacola tidal gauge

record closely mirrors the accepted global mean sea level rise curve. The curve for Grand Isle is markedly

different, and the difference can be used to measure subsidence. The subsidence measured by this method at

Grand Isle is about 0.25 inches/year, which is very close to the value inferred by the depth of burial of the cypress

stump in Breton Sound indicated that these rates have been continuous for at least 2,000 years.


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The effects of subsidence are most obviously manifested by the submergence of the coastal wetlands over the past 80

years. The ability to measure land area change, or wetlands loss, is strictly a function of the availability of aerial

photography. The first aerial surveys were shot in this area in the early 1930s, and all measurements are made from

this baseline. An examination of the historical holocene deltas of the Mississippi River shows that thousands of square

miles of wetlands had already subsided below the surface before then. The distribution of wetlands loss across the

plain shows an obvious pattern of hotspotswhere subsidence is most active. These areas can be directly tied to the

activity of major fault systems at the surface. These are the same geologic mechanisms that have been driving

subsidence of the coastal plain since the deposition of the Miocene deltas.

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.

Surface Fault Traces


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Land Loss in Southeast Louisiana

The activity of the major faults at the surface of the coastal plain is obvious in its effect on the marshes. The

movement of the faults rotates the marsh surface inducing a slope toward the inland trace of the fault. This change in

slope allows for the flow of saltwater into the interior freshwater marsh, which changes the hydrology of the coastal

marshes. The subsiding areas along the fault traces follow a succession of hydrologic changes that ends with the

marsh drowning and converting to open water. These are likely the same processes by which the expansive St

Bernard Delta, that was covered with cypress swamps, progressively converted to open water.

The next two slides examine the measurable effects of subsidence in the area around the city of New Orleans, whichis affected by the activity of surface faults that cross the East New Orleans Land Bridge.

Surface Fault Traces


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This is the subsidence map of Dixon, 2006 with the patterns showing areas of relatively high rates of subsidence color-

filled in orange and red. These color-filled areas are used on the next slide to draw conclusions about the relationship

between the principal geologic features of the subsurface in this area, and the rates of subsidence that are being

experienced at the surface.

Dixon, T.H., 2006, Subsidence and flooding in New Orleans, Nature, v. 441, p. 587-588

< 20 mm/yr


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Fort Proctor stands as a measurable documentation of the effects of subsidence. The fort was constructed at Proctor

Landing on the shore of Lake Borgne in 1865, but never used. Historical documents show the fort was positioned 150

from the shoreline, and based on comparison with other forts around the Gulf, i t was almost certainly at least five above

sea level at the time of construction. The fort is now in Lake Borgne, and the foundation is about four feet below sea

level. The implied rate of subsidence is very similar to the values measured in 2006 by Dixon.

Dixon Area of Maximum

Subsidence ~ 0.8 in/yr

Fort Proctor


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The following sequence of three slides shows the effects of subsidence in coastal Louisiana over the course of te

rest of this century. These images were a part of numerous presentations by Dr. Roy Dokka, and LSU geology

professor prior to his untimely death in 2011. The images were created by the LSU Center for Geoinformatics.

They show the areas of coastal Louisiana that were below sea level in 2010 and projections for 2050 and 2100.

The striking increase in the area of south Louisiana that will be below sea level over the course of this century is

due entirely to the effects of subsidence. This subsidence is being driven by the same mechanisms that have

been in effect across this area for millions of years. There are deposits of the ancient Mississippi River delta that

are now three miles below the surface of coastal Louisiana. These deposits were subsided by the same

mechanisms that have submerged the historical delta deposits of the past 6,000 years.

The mechanisms of subsidence affecting coastal Louisiana are as fundamental as the movement of the crustal

plates and as simple as the compaction of surface sediments. Taken together these mechanisms produce an

unrelenting effect at the surface that cannot be reversed or even meaningfully offset by the actions of humans.

The Effects of Subsidence in Coastal Louisiana


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The subsidence of the Mississippi River Delta and the Louisiana coastal plain has been active and ongoing for

millions of years. The deltas deposits of the Mississippi from 10 to 15 million years ago have now subsided to

depths between one and three miles below the surface. The Holocene of the Mississippi River built up the

modern coastal wetlands by the progression of a delta cycle in which subsidence of the abandoned delta is an

equally important component to the aggradation of the new one. Delta deposits from the construction of these

wetlands over the past 6,000 years are now buried as deep as 35 feet below sea level. The sampling of a buried

cypress stump from a swamp of the former St. Bernard Delta indicates a subsidence rate of about 0.2 inches per

year over 2,100 years. This rate is very consistent with the subsidence rate measured by the comparison of

historical tidal gauge records at Grand Isle. Similarly the rates of subsidence measured by Dixon using GPStechnology around New Orleans are consistent with an inferred rate of subsidence that would be necessary to

leave Fort Proctor in its current state of submergence in Lake Borgne.

Subsidence is and has always been active on the coastal plain. There is no necessity to invoke any

anthropogenic mechanisms to explain it, and by the same measure the effects of subsidence fully explain the

rate and patterns of land loss in the coastal plain.


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This slide show was produced by Chris McLindon.

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Subsidence in the Mississippi Delta