The Sedimentology of Abyssal Plains

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The Sedimentology of Abyssal Plains

• Christina Marie Ebey Honeycutt

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An Overview of Abyssal Plains

• Defined by Heezen as areas of the deep ocean
  floor in which the bottom gradient is less than
  11000
• Deepest part of ocean
• More than 75 abyssal
• 4.4 of Earths surface
• Range in size from 1,000s to 100,000s square km
• Pilkeys 12 factors

Source Weaver et al, 1987
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Rule 1 Abyssal Plains are usually dominated by
turbidites

• Hemipelagic and pelagic sediments usually make up
  less than 20 of the sediment column
• Debris flows are almost always minor constituent

Sources Left Weaver Rothwell, 1987 Right
Pilkey, 1987
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Rule 2 Most sediment filling abyssal plains
arrives via submarine canyon mouths
Sources Left Wynn et al, 2002 Right Pilkey,
1987

• Passive vs. active margins
• Proximal vs. distal
• Eustatic change

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Rule 3 Pre-existing bathymetry plays a role in
flow path and turbidite thickness

• Bathymetry plays determines
• Thickness of turbidite
• Placement of turbidite
• Location of sand

Wynn et al, 2002
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Rule 4 Individual turbidity currents produce
elongate, tongue-shaped turbidites

• Assumptions
• turbidity current originated from a point source
• Large abyssal plain
• Small turbidity current
• Thin towards tip of tongue
• Proximal portion sandier than distal

Source Wynn et al, 2002
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Rule 5 Channelization of flow is unimportant in
most abyssal plains

• Makes abyssal plains plains
• Environment beyond canyon
• Exception
• Demerara

Source Pilkey, 1987
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Rule 6 Abyssal plains are depositional
environments

• Erosion is minimal on abyssal plains
• A few exceptions
• Turbidity currents erode original deposits
• Bioturbation can erode or blur boundaries
• Organics can, in oxic environments, aid in the
  erosion of carbonates

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Rule 7 The sediments of most abyssal plains
contain more sand than other off-shelf
environments

• Nearly always the case on plains very close to a
  major landmass, e.g. a continent
• Some plains have sorting systems

Wynn et al, 2002
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Rule 8 Giant turbidity events are necessary for
the formation and maintenance of abyssal plains

• Giant events allow for even basin filling
• Excellent example turbidite AB12 from the
  Madeira Abyssal Plain
• Over 300 cubic km deposited
• Began on Moroccan Shelf and ended on the Madeira
  Abyssal Plain

Source Wynn et al, 2002
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Rule 8 continued
Source Wynn et al, 2002
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Rule 9 The geometry of sand layers is dependant
upon the arrangement of point sources around the
edge of the basin and the ratio of the area of
the drainage basin area to the basin plain area

• There are several distinct geometries that a sand
  layer can take

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Rule 9 continued
Pilkey, 1987
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Rule 10 Overall accumulation content and
structure is a function of 6 factors

• Sea level and eustatic change
• Tectonic framework
• Ratio of the drainage basin area to the basin
  plain area
• Nature and extent of fan development
• Seasonal changes
• Climatic changes

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Summary

• Abyssal plains are primarily a depositional
  feature
• Abyssal plains are usually dominated by
  turbidites which originate from a submarine
  canyon mouth
• Turbidites are usually elongate structures, but
  pre-existing bathometry does play a significant
  role in turbidite structure
• Overall accumulation is dependant on sea level,
  tectonics, fan development and climate

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Thank You!