Historical dynamics and depositional sequences in the Mississippi River delta

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Historical dynamics and depositional sequences in
the Mississippi River delta
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Mississippi River

• Drains an area of 3,344,560 km2
• Average water discharge of 15,360 m3/sec
• Annual sediment discharge of about 6.21 billion
  kilograms
• Bed load is 90 fine sand
• Suspended load is 65 clays and 35 silt and very
  fine sand

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James Coleman

• Coastal Studies Institute
• Louisiana State University

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Faces
Facies

• Prodelta
• Delta Front or Delta Platform
• Distributary Mouth Bar
• Subaqueous Natural Levee
• Subaerial Natural Levee
• Distributary Channel
• Interdistributary Bay
• Crevasses or Bay Fill

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Prodelta

• Basal Portion
• Consists of blankets of clays from suspension
• Parallel Laminae
• Thickness Ranges from 20 to 100 meters

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Delta Front

• Often referred to as the delta platform or distal
  bar
• Increased sedimentation rate
• Coarser materials deposited than in the prodelta
• Depositional slope is high compared to other
  facies
• Mississippi slope at .5 to .75 degrees

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Distributary Mouth Bar

• Shoaling occurs here
• Highest sediment accumulation rate of any other
  environment,
• Sediments are constantly reworked from stream
  currents and marine waves.
• Almost all the sand in the delta is deposited
  here and even some silts and clays
• Coarsening upward

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Subaqueous and Subaerial Natural Levees

• Subaqueous
• Border the channel
• Composed of very fine sand and silt
• The combined action of current and wave processes
  create complex forms of cross lamination.
• Climbing ripples are most common

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Distributary Channel

• Transports sediment from the parent river to a
  receiving basin
• Range from two meters to one kilometer in width
  and one to thirty meters in depth
• Continued progradation of the distributary
  channels eventually leads to abandonment
• Lower portions of the channel are filled with
  poorly sorted sands and silts.
• Deposits are lenticular bodies of fine-grained
  materials interfingering locally with peats and
  organic debris

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Interdistributary Bay

• Surrounded by marsh or distributary levees
• Rarely exceed more than four meters in depth
• Fine-grained sediments
• Deposited during high floods or abnormally high
  tides
• Lenticular laminae
• Short Life Cycle

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Crevasses

• Form when natural levees of the distributary
  channel are breached
• Infill interdistributary bays
• Extend seaward through a system of radial
  bifurcating channels

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Modern Crevasses Bays
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Delta Lobes

• Sale Cypremort
• Teche
• St. Bernard
• Lafourche
• Balize
• Atchafalaya

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• Began its deposition 7500 years ago
• Lasted for 2500 years
• Spread far out onto the continental shelf
• Thicknesses are on the orders of 10 to 20 meters

SALE CYPREMORT
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• Began 5500 years ago
• Lasted for 1700 years

TECHE
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ST. BERNARD

• Most eastern portion of the delta
• Started 4000 years ago
• Lasted for 2000 years

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• Characterized by a complex network of
  distributaries.
• Lasted from 2500 to 800 years ago

LAFOURCHE
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• Began 600-800 years ago.
• Birdfoot delta
• Covers an area of approximately 600 km2.
• Pre-modern lobes average 2700 km2
• Reason Balize delta thicker at approximately
  100-120 meters. Other lobes average about 20 m

BALIZE
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Four Principle Outlets
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• Youngest of all the delta lobes
• Flow is diverting from the main Mississippi River
  channel to the Atchafalaya River
• It branches from the Mississippi upstream from
  Baton Rouge.
• Cause Balize delta has built up so much that it
  now requires more energy for the river to
  continue in course to the Balize delta than the
  Atchafalaya delta.
• Route is 300 kilometers shorter than the present
  route

ATCHAFALAYA
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Old River Complex

• Constructed in 1959 by the Army Corps of
  Engineers
• Regulates the flow into the Atchafalaya
  distributary
• Allows the port of New Orleans to remain
  functioning.
• 30 percent of the flow is transported through the
  Atchafalaya River

http//civilu.ce.utexas.edu/stu/mcbraymc/Or04.jpg
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Finale

• Delta lobes alter their paths every 1500 years.
• Deltas are formed in a sequence of facies with
  finer sediments of base overlaid by coarser
  sediments and capped with organic soils.
• Studying the facies of the modern-day Mississippi
  delta helps to better understand ancient deltas
  and piece together their geologic history.