Geological Oceanography Section II Lecture 3

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Geological Oceanography Section II Lecture 3

• Continental Shelf Sedimentation 14 February 2008

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Sedimentation on Continental Shelves and Margins

• Sedimentary Provinces - Shelf
• Beaches and barrier Islands (covered previously)
• Estuaries
• Deltas
• Shelves
• Sedimentary Provinces - Margins
• Slopes
• Submarine Fans
• Sediment Drifts

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Recommendation

• The following CMS seminars presented last spring
  are pertinent to this lecture!!!
• Charlie Paull (March 2, 2007)
• Detecting and Monitoring Recent Sediment
  Transport Activity in Monterey Canyon
• Mike Blum (March 9, 2007)
• Holocene Sea-Level Change, Northern Gulf of
  Mexico
• Dave Cacchione (March 23, 2007)
• Sediment Movement by Internal Waves and Tides on
  Continental Margins

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Single most important factor influencing
continental shelf and margin sedimentation

• Rate of sediment supply

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Major Mechanisms Influencing Shelf Sedimentation

• Tectonics
• Watershed characteristics
• Shelf/margin characteristics
• Climate
• Weathering mechanisms and rates
• Transport mechanisms and rates
• Biogenic and authigenic processes
• History
• Paleoclimate
• Sealevel history
• Oceanography
• Prevailing winds/fetch
• Oceanic and local geostrophic currents
• Tides and tidal currents

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Tectonics influence

• Watershed
• Area
• Topography
• Geology
• Climate
• Adiabatic
• Rain shadows
• Shelf and margin
• Area
• Topography
• Geology

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What do I mean by watershed?
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Dr. Naar discussed tectonic processes

• Why do continental margins exist?
• Types of continental margins
• Convergent/active
• Divergent/passive/trailing
• Conjugate, shear, etc.
• Margin structures
• How margins form
• How do margins mature
• Isostatic loading

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Tectonic Influence on Climate
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Primary Mechanisms Influencing Shelf
Sedimentation

• Tectonics
• Watershed characteristics
• Shelf/margin characteristics
• Climate
• Weathering mechanisms and rates
• Transport mechanisms and rates
• Biogenic and authigenic processes
• History
• Sealevel history
• Paleoclimate
• Oceanography
• Prevailing winds/fetch
• Oceanic and local geostrophic currents
• Tides and tidal currents

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Climate Zones
http//www.blueplanetbiomes.org/climate.htm
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Weathering and climate chemical weathering and
frost action require moisture
Revised from Peltier 1950, Annals, Association of
American Geographers. v. 40
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Relative depth of weathering with climate and
vegetation assumes minimal relief.
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Sediments reflect climatic conditions
(Schopf 1980)
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Primary Mechanisms Influencing Shelf
Sedimentation

• Tectonics
• Watershed characteristics
• Shelf/margin characteristics
• Climate
• Weathering mechanisms and rates
• Transport mechanisms and rates
• Biogenic and authigenic processes
• History
• Sealevel history
• Paleoclimate
• Oceanography
• Prevailing winds/fetch
• Oceanic and local geostrophic currents
• Tides and tidal currents

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Sea Level Changes Through Geologic Time
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James (1997)
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Last Glacial Maximum18-20,000 ybp
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Holocene sea-level rise

• Low-moderate rates of sediment supply
• Sediments trapped in estuaries
• Shelf has relict, biogenic or authigenic
  sediments
• Sediment delivery to margins diminished
• Examples much of shelf of eastern US
• High rates of sediment supply
• Deltas prograding over shelf
• Sediments being delivered to margins and deep-sea
  fans
• Examples
• Mississippi delta, Mississippi fan
• Ganges-Brahmaputra delta, Bengal fan

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Chesapeake Baydrowned valley sea level rise
outpaced sediment supply
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Mississippi Delta high rate of sediment supply
http//www.ssec.wisc.edu/gumley/modis_gallery/ima
ges/MississippiDelta_20040127_1940_250m.jpg
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Paleoclimate

• In areas of low-moderate sediment supply
• Shoreline and relict shelf sediments may reflect
  Pleistocene conditions and sea levels
• Examples
• Relict glacial features and sediments
  characterize shelves of northeastern US and
  eastern Canada
• Areas not currently glaciated
• Paleoshorelines and relict barrier islands on
  west Florida shelf
• More discussion later in lecture

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Primary Mechanisms Influencing Shelf
Sedimentation

• Tectonics
• Watershed characteristics
• Shelf/margin characteristics
• Climate
• Weathering mechanisms and rates
• Transport mechanisms and rates
• Biogenic and authigenic processes
• History
• Sealevel history
• Paleoclimate
• Oceanography
• Prevailing winds/fetch
• Oceanic and local geostrophic currents
• Tides and tidal currents

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Fetch distance over which winds blow
unobstructed
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http//www.bergey.com/Maps/World.Wind.Lg.htm
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Tidal ranges worldwide
Open University Waves, Tides and Shallow-water
Processes (1989)
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Shelf Sedimentary Provinces

• Beaches/barrier Islands
• covered later by Dr. Hine
• Estuaries
• Deltas
• Reefs (later lecture)
• Shelves

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• An estuary is an inlet of the sea reaching into a
  river valley as far as the upper limit of tidal
  influence

Open University Waves, Tides and Shallow-water
Processes (1989)
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Estuary Types

• Salt-wedge estuaries
• River discharge rate much stronger than tidal
  activity
• Sharp halocline with freshwater at surface
• Partially mixed estuaries
• Discharge into moderate tidal range
• Well-mixed estuaries
• Tidal currents strong relative to river discharge
• Negative-circulation estuaries
• Occur in arid regions
• Hypersaline water at the head of the estuary
• Flows seaward along estuary floor

Figures Open University Waves, Tides and
Shallow-water Processes (1989)
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Salt-wedge estuary
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Partially mixed estuary
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Well mixed estuary
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Well mixed estuary
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Negative estuarine circulation
Open University Waves, Tides and Shallow-water
Processes (1989)
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River Deltas

• Main conduit of terrigenous materials to ocean
• Suspended load-muds
• Bedload-sands
• Dissolved load - cations/anions

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Deltas

• Provide sediments to coastlines, barrier islands
  and marshes
• Huge wetlands--key environment
• Sediments can be transported great distances down
  into ocean basin from deltas
• Deltas can be enormously thick
• undergo compaction, isostatic adjustment
• Deltas impacted by sea-level change.

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Societal Significance of Deltas

• Modern and ancient deltas are great producers of
  oil/gas
• Ancient deltas are major producers of coal
• Natural hazards -- support huge populations
  impacted by typhoons
• Bangladesh Nov 1970 155 mph typhoon, 7m storm
  surge 400,000 drowned
• Bangladesh Apr 1990, 145 mph typhoon, 6 m
  surge, 140,000 drowned, 500,000 large farm
  animals drowned, 10 million homeless

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Basic structure of a delta
Open University Waves, Tides and Shallow-water
Processes (1989), p. 131
Delta front
Pro-delta
Note error in text
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River-dominated delta

• Tidal range is low
• Tidal currents weak
• Wave action relatively weak
• Density stratification may occur
• Speed of river flow is low to moderate
• Distributary mouth deep enough for salt-water
  penetration
• Fresh water plume over salt water
• A type of salt-wedge estuary

Figures Open University Waves, Tides and
Shallow-water Processes (1989)
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Turbulent mixingrapid fluvial discharge
precludes density stratification
Into deep water
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Turbulent mixingrapid fluvial discharge
precludes density stratification
Into shallow water
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Progradation of sedimentsupply- dominated delta
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Wave-dominated delta
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Wave-dominated delta
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Delta classification based on sediment supply and
influence of waves and tides
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Tidal ranges worldwide
Open University Waves, Tides and Shallow-water
Processes (1989)
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Recall what I said previously about tidal
transport and deposition
More sand
Channel
Rippled fine, muddy sands
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Changes in current velocity during a complete
tidal cycle.
Changes in rate of bedload transport during same
tidal cycle. Shaded areas are proportional to
total sediment transported.
Incoming
Outgoing
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Tide- dominated delta

• Sediments delivered by river are reworked by
  tides
• Form linear, subaqueous ridges
• Former ridges become vegetated, linear islands

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The Continental Shelf

• Marine terraces that surround continents
• Very narrow (1 km) to very wide (100s km)
• Sediments range from relict to massive deltas
• Dominated by terrigenous or carbonate or mixed
• Wave or tide dominated
• Effects of sea level change enormous
• Coastal plains are exposed continental shelves
• Archeological/paleontological implications
• Sharks teeth and mammoth teeth on west Florida
  shelf

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Terrigenous Continental Shelves--Key Features

• Sand ridges
• Cape retreat massifs
• Estuarine retreat massifs
• Paleo-shorelines
• Buried river valleys
• Scarps
• Mid-shelf deltas

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Relict morphologic elements of the Middle
Atlantic Bight, North America
Estuaries
Sand bodies Sand ridges
Relict deltas
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Proposed origin of some sand ridges
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Carbonate Shelves

• Subpolar to tropical
• Absence of terrigenous influences
• Reefal, skeletal, or non-skeletal sediments
• Attached or non-attached
• Rimmed or ramps

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Schematic section across Great Bahama Bank from a
seismic profile
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Carbonates can dominate where terrigenous input
is minimal
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Sediment transport to the deep sea
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Slopes

• Sediment gravity processes active
• Canyons
• may feed submarine fans
• rivers discharge at canyon heads during sea level
  lowstands
• Slumps, slump blocks, slides
• may generate turbidity currents
• Turbidity flows
• turbidites - primary sedimentary deposit
• predictable vertical sequence of primary
  sedimentary structures--Bouma Sequence

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Submarine Fan

• Major sediment accumulation at base of slope
• Fed by canyon system turbidity currents and
  debris flows can no longer entrain sediments
• Similar features as rivers
• levees, meandering systems
• Repository of great mountain systems
• E.g., Bengal Fan

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Bengal Fan
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BENGALFAN
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Sediment Drifts-Contourites

• Western boundary currents rework turbidites,
  which are redeposited as large mounds of
  fine-grained materials
• High sedimentation rates, have great
  paleoceanographic record

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Location Map of ODP Leg 194
NMP Northern Marion Platform SMP Southern
Marion Platform
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Food for thought by the end of this series of
lectures

• Given any continental shelf/margin worldwide, you
  should be able to give a first-order explanation
  of
• What kind of margin occurs there
• What kinds of sediments occur there, e.g.,
• Deltaic
• Relict siliciclastic
• Carbonate (lectures on carbonates will come
  later)
• Mixed carbonate/siliciclastic

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Reading assignments

• The Open University (either edition) Waves, Tides
  and Shallow-Water Processes.
• Chapter 7. Deltas
• Chapter 8. Shelf Seas
• Seibold, E. and W. Berger (any of the 3 editions)
  The Sea Floor
• Chapter 3. Sources and compositions of marine
  sediments
• Chapter 4. Effects of waves and currents

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Reading assignment for next Tuesday

• Seibold, E. and W. Berger (any of the 3 editions)
  The Sea Floor
• Chapters 6, 7 and Appendix 9