TA2910 08 - Shelf Sediments

8/10/2019 TA2910 08 - Shelf Sediments

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Challenge the future

DelftUniversity ofTechnology

M.E. Donselaar

Shelf deposits


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Sedimentary environments

Continental: fluvial (braided, meandering)

aeolian

lacustrine

Coastal: deltaslinear (clastic, carbonate)

Marine: shelf

deep marine sands

pelagic


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Outline

Introduction

Ocean circulation patterns

Shelf sedimentation

Examples tide-dominated shelf Reservoir analogue


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Definition shelf

The part of the sea bordered by:

The wave base

The shelf break (usually at 100-200 m water depth)

http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStrat%207/SedStrat7.html


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Shelf types

Marginal (= peri-continental):

Shelf bordered by land on one

side, and the deep sea on the

other side

Epiric (= epi-continental):

Shelf bordered by land on

several sides

epiric

marginal

http://www.ruf.rice.edu/~feegi/coastal.html


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Factors that influence shelf sedimentation

Shape of the shelf

Type and intensity of the hydraulic regime

Amount and type of sediment supply

Sea level fluctuations Climate

Interaction sediment - fauna

Chemical factors


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Shape of the shelf

Wide shelf: large sediment storage capacity (largeaccommodation space). Trailing (passive) margins

Narrow shelf: low sediment storage capacity (smallaccommodation space).Active margins


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Plate margins

http://www.mc.maricopa.edu/academic/phy_sci/Geology/hernlund/

Active margin

passivemargins


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Active and passive plate margins


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Active margins narrow shelf

http://cache.eb.com/eb/image?id=4953&rendTypeId=4


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Passive margin wide shelf

http://www.utexas.edu/tmm/npl/mineralogy/Mineral_Genesis/PassiveMargin.jpg


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Hydraulic regime - 1

World wide circulation patterns caused by temperature andsalinity (=density) gradients

Tidal forces caused by gravity fields sun and moon

Meteorological currents


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Ocean circulation - 2

http://www.dkrz.de/dkrz/gallery/vis/ocean


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Ocean circulation - 3

Ocean currents (spiral eddies)

expand onto shelf

More spiral eddies:

http://www.navis.gr/meteo/ed

dies.htm

http://www.lpi.usra.edu/public

ations/slidesets/oceans/oceanviews/slide_01.html
http://www.navis.gr/meteo/eddies.htmhttp://www.navis.gr/meteo/eddies.htmhttp://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_01.htmlhttp://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_01.htmlhttp://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_01.htmlhttp://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_01.htmlhttp://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_01.htmlhttp://www.lpi.usra.edu/publications/slidesets/oceans/oceanviews/slide_01.htmlhttp://www.navis.gr/meteo/eddies.htmhttp://www.navis.gr/meteo/eddies.htm


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Meteorological currents

Air pressure gradients -> wind shear stress -> waves andcurrents

Animationhttp://www.weather.com/maps/maptype/satelliteworld/atlanticoceansatellite_large.html
http://www.weather.com/maps/maptype/satelliteworld/atlanticoceansatellite_large.htmlhttp://www.weather.com/maps/maptype/satelliteworld/atlanticoceansatellite_large.htmlhttp://www.weather.com/maps/maptype/satelliteworld/atlanticoceansatellite_large.htmlhttp://www.weather.com/maps/maptype/satelliteworld/atlanticoceansatellite_large.html


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Hydraulic regime - 2

Wave dominated shelf: seasonal fluctuation of wave heightand current strength - sediment transport only duringstorms

Tide dominated shelf: strong bi-directional bottom currents- active sand transport, large bedforms

Ocean current dominated shelf: strong uni-directionalcurrents


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Amount and type of sediment supply

Relict sediment: Non-reworked sediment that was depositedon the shelf as fluvial or coastal sediment in a previous sealevel lowstand phase

Palimpsest sediment: Reworked fluvial or coastal sedimentfrom a previous sea level lowstand phase

Modern sediment: Sediment that was recently added to the

shelf: clay and silt (suspension from rivers, etc.)

sand (coastal source, seaward transport during storms)


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Sea level fluctuations

Influence the shelf accommodation space

Influence the type of sediment supply:

Low sea level: coarser sediment

High sea level : finer sediment


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Sediment supply

Sea level highstand: coarse

sediment trapped in estuaries,

only fine-grained suspended

sediment on shelf

Sea level lowstand: shelf is

(partly) exposed, coarse

sediment is transported ontothe shelf by rivers

Jervey (1988)


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Example tide-dominated shelf - 1

Strait of Dover: Convergence of

tidal currents, hence deep erosion

of sea bottom

North and south of Strait of Dover:

Flow expansion, hence gradual

decrease of current speed

Deposition of eroded sea bottom

sediment

Reading (1986)


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Example tide-dominated shelf - 2

Sedimentation succession at decreasing

current speed:

> 150 cm/s: furrows and gravel waves.

Scour hollows up to 150 km long, 5 km

wide, 150 m deep ~ 100 cm/s: sand ribbons. Longitudinal

ridges parallel to flow. Up to 15 km

long, 200 m wide, 1 m high

~ 90 cm/s: barchan-like bedforms

~ 75-65 cm/s: sand waves. Straight-

crested large bedforms, crest

perpendicular to flow. 3-15 m high,

wave length 150-500 m

Reading (1986)


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Sandwave - example


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Tidal sand ridges - 1

Elongate ridges oriented

obliquely to tidal currents

Distribution related to

transgressive history of area

Do not follow sedimentation

succession (previous slides)

Up to 50 km long, 1-3 km wide

and 10-50 m high, spacing up

to 12 km Well-sorted, medium-fine sand

Reading (1986)


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Are formed between mutually

evasive ebb and flood currents

(stage a)

Secondary cross shoal currents

deform straight crest line(stages band c)

Ebb and flood channels develop

in double curve (stage d)

Original tidal sand ridge breaks

up into three parallel ridges(stages eand f )

From: Caston (1972)


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Active sand ridges:

Current speed > 50 cm/s

Sand waves superimposed on ridge

Asymmetrical in cross section, steep side up to 6o

Crest sharp to flattened (near sea surface)

Moribund sand ridges:

Current speed < 50 cm/s

Slope < 1o

Do not actively move Develop over sandy to muddy sea floor


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Eocene Brussels Sands

Fossil example of shallow-marine linear sand body deposits


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Tabular cross-bedded facies

(Houthuys, 1990)


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Sedimentary structures


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Transverse bar characteristics

Tabular cross bedding

Well-developed bottomsets

Set heights ~ 1m

Well-sorted fine to medium sand Glauconite coating

Iron oxide forms permeability baffles

Bioturbation restricted to low-energy parts bar


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Reservoir analogue

Shannon and Sussex Sandstones : producing linear sandridges

Additional reading:

http://pubs.usgs.gov/dds/1997/dds-

033/USGS_3D/ssx_txt/3dstart.htm

St ti h Sh d S
http://pubs.usgs.gov/dds/1997/dds-033/USGS_3D/ssx_txt/3dstart.htmhttp://pubs.usgs.gov/dds/1997/dds-033/USGS_3D/ssx_txt/3dstart.htmhttp://pubs.usgs.gov/dds/1997/dds-033/USGS_3D/ssx_txt/3dstart.htmhttp://pubs.usgs.gov/dds/1997/dds-033/USGS_3D/ssx_txt/3dstart.htmhttp://pubs.usgs.gov/dds/1997/dds-033/USGS_3D/ssx_txt/3dstart.htm


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Stratigraphy Shannon and SussexSandstones

From: http://pubs.usgs.gov/dds/dds-033/USGS_3D/ssx_txt/geology.htm


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Powder River Basin - Shannon Sst

Reservoir potential:

Sand ridges develop on muddy

sea floor (source)

After formation of sand ridge:

covered with fine-grained marine

suspension sediment

Hence, permeable sand ridge

enveloped in impermeable mud

(Tillman & Martinsen, 1985)


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Shannon Sandstone - palaeogeography


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Shannon Sst at Hartzog Draw



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Shannon Sst - Salt Creek area

Lower and upper unit separated by thin continuous shaleinterval

Total thickness ~ 50 m; length 45 km; width 1-4 km



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Shannon lithofacies

Coarsening-upward sequence from:

Bioturbated marine shaley

sandstone at base, to

Large-scale trough cross-bedded

medium-grained sandstone at top

Porosity: 14.8% ave.

Permeability: 0.01-143 mD; ave.

16.3 mD

From: Ranganathan & Tye (1986)


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Sussex Sandstone

Elongate sand ridge

Sediment is transported by

smaller transverse bars

migrating obliquely over the

sand ridge surface


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Stacking of Sussex-B tidal sand ridges

http://pubs.usgs.gov/dds/dds-033/USGS_3D/ssx_txt/depomod.htm


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Sussex Sandstone

http://pubs.usgs.gov/dds/1997/dds-033/USGS_3D/ssx_txt/3dstart.htm


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Links http://msx4.pha.jhu.edu/ssip/asat_int/ocean.html

http://topex-www.jpl.nasa.gov/overview/why-study.html
http://msx4.pha.jhu.edu/ssip/asat_int/ocean.htmlhttp://topex-www.jpl.nasa.gov/overview/why-study.htmlhttp://topex-www.jpl.nasa.gov/overview/why-study.htmlhttp://topex-www.jpl.nasa.gov/overview/why-study.htmlhttp://topex-www.jpl.nasa.gov/overview/why-study.htmlhttp://topex-www.jpl.nasa.gov/overview/why-study.htmlhttp://topex-www.jpl.nasa.gov/overview/why-study.htmlhttp://msx4.pha.jhu.edu/ssip/asat_int/ocean.html


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Compulsory reading

Chapter 11:

Section 11.4 Thermo-haline and geostrophic currents

Documents

TA2910 08 - Shelf Sediments