Structure and Stratigraphic Development of Continental Margins

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Structure and Stratigraphic Development of
Continental Margins
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Reading assignmentKlitgord, K.D., Hutchinson,
D., and Schouten, H., 1988, Atlantic continental
margin Structure and tectonic framework, in,
Sheridan, R.E., and Grow, J.A., eds., The
Atlantic continental margin, U.S., Geological
society of America, v. I-2. p. 19-55.
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Continental Margins

• What are continental margins?
• Transition from continental crust to oceanic
  crust
• May be tectonically passive or active
• May be divergent, convergent, or transcurrent
• May be mature or immature
• Contain thickest sediment accumulations on earth
  (22 km thick in NE Bay of Bengal)

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Why Are Continental Margins Important?

• 1. Support most of earths population
• 2. Contain most hydrocarbon deposits
• 3. Contains large supply of fresh water
• 4. Commercial entry points to continental
  interiors through harbors and seaports
• 5. Pose significant geohazards storms coming off
  oceans, plate boundaries-earthquakes, volcanoes
• 6. Contain large record of earths geologic
  history

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Origin of Sediments on Continental Margins

• Terrigenoussediments eroded from the
  land--siliciclastic sediments
• mechanical and chemical weathering of continental
  rocks to form clays, sands, gravels (quartz and
  feldsparsilicate minerals)
• transported to continental margin by rivers
• Authigenicsediments formed in the ocean
• ---carbonates
• ---evaporites
• ---phosphates

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Wilson Cyclethe quick version of birth/death of
continental margins
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Structure of Passive and Active Margins

• Passive margins not located on plate boundaries,
  mid plate location
• Deep structure determined by gravity,magnetics,
  and seismic data
• Active margins are plate boundaries either
  subduction zones or transform faults

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Passive Margins--The Deep Structure What are our
tools?

• Magnetic anomalies
• Gravity anomalies
• Seismic reflection
• Seismic refraction
• Boreholes

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Cartoon based on lots of geophysical data
(magnetics, gravity, seismics) and some
drilling). Note crustal boundaries, buried
topography, and rock types. Rift-stage crust same
as transitional crust.
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Paleo-mag allows to map the continent-ocean
structural boundariesthe conjugate passive
margin appears. Note Blake Spur, East Coast, and
Brunswick Magnetic Anomalies.
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How do passive margins work? Rifting Processes

• Low angle detachment surfaces--plastic
  deformation at depth brittle deformation near
  surface
• High angle normal faults

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First evidence of break-up is development of rift
valleys.
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Transition to seafloor spreadingformation of
oceanic crust (basalts). Development of
transitional crustcontinental crust with basalt
intruded into joints (cracks) and faults.
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Conjugate Margins and Aulacogens

• Platforms and basins separated by transform
  faults during rifting failed rifts forming
  basins occupied by major rivers.
• Controls fundamental shape/geometry of a passive
  margin.

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Cape Hatteras projects into the Atlantic because
it rests upon the deep-seated Carolina Platform.
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Baltimore Canyon Troughwide basin
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Baltimore Canyon Trough sits opposite Mazagan
Platforma narrower basin hence an asymmetry in
basin size juxtaposed.
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A extremely wide basin
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West margin of Florida Platform formed on the
topographic relief resulting from rifting of
basement rocks.
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Sequence of events along a passive margin

• Rifting, uplift, erosion,rift valleys--East
  African Lakes example, non-marine sediments first
  sedimentary cover.
• Post-rift unconformity formed-widespread
  erosion--provided sediments for non-marine
  basins.
• Paleodrainage away from new basin.
• New ocean basin formed, oceanic crust, evaporites
  and organic rich sediments (sapropelites) 1st
  marine sediments.

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Sequence of Events Continued

• Open marine sedimentation brought great
  thicknesses of carbonates due to rapid
  subsidence.
• Continued subsidence reverses paleo-drainage
  rivers now flow into new ocean basin, deltas,
  terrigenous influence, fans, turbidites
• Continued expansion of ocean over latitudinal
  gradients brings geostrophic and thermohaline
  circulation western boundary currents/undercurren
  ts leading to sediment drift formation

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Marine Flooding of Atlantic and Sedimentary
InfillingA Specific Example
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Tethysa circum-global tropical ocean no longer
exists today. Oceans today are N-S oceans, not
W-E oceans.
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Tethys is significant in that it provided an
environment for carbonate platforms to form
extensively along its marginsincluding the
Florida/Bahama carbonate platform. Tethysa Greek
sea goddess. Located at the Equator.
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Some more complicated, detailed diagrams. Last
stage of forming the Appalachians 225 my. Note
Yucatan and Florida Straits blocks.
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Spreading center propagating N-S unzipping Pangea
and creating early Atlantic Ocean 180my start
of widespread evaporite deposition in GOM
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Mid-Jurassic 170 mycontinuation of evaporite
(halite, gypsum, anhydrite) deposition the
evaporites.
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Ridge propagation around Florida/Bahamas basement
rocks.
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Active Margins

• Volcanic arcs at seaintra-oceanic arc-trench
  systems
• Volcanic arc on land--continental margin
  arc-trench system

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Basic Components

• Back-arc basin--Sea of Japan
• Volcanic arc itself--Aleutian Islands Central
  America line of volcanoes
• Forearc basin
• Outer ridge
• Accretionary prism
• Trench
• Arc-Trench gap function of dip steepness of
  descending plate

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Trench Infilling

• Submarine canyons
• Large mass movements-melanges
• Volcaniclastics
• Turbidites
• Pelagics radiolarian cherts, carbonate ooze

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Accretionary Prism Structure

• Landward dipping thrust faults
• Imbricate thrust sheets between faults, highly
  distorted
• Migration of fluids, seeps support strange life
  forms (e.g.,tube worms), carry metal-rich cations
  creating mineral rich deposits.

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Stop
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Transition to seafloor spreadingformation of
oceanic crust (basalts). Development of
transitional crustcontinental crust with basalt
intruded into joints (cracks) and faults.
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