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SEDIMENT DEPOSITION &STRUCTURES:

USEFUL FOR INTERPRETING:• transport mechanism• current flow direction• relative water depth• relative current velocity

FLOW REGIMES

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FOUR DESCRIPTIVE CATEGORIES:

1. BEDDING AND LAMINATION• Parallel, Graded, Massive

2. BEDFORMS3. CROSS LAMINATION4. IRREGULAR STRATIFICATION

BEDDING AND LAMINATION:layers of strata that have lithologic, textural, structural unity

that clearly distinguishes them from layers above andbelow.

• by definition– Beds - > 1 cm– Laminae - < 1 cm

Bedding Planes

• Bedding Plane Surfaces– Represent

1. non-deposition or erosion2. abrupt change in

depositional conditions3. post depositional

– Surfaces - straight to wavy

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BEDDING AND LAMINATION: Time represented by beds?

• can accumulate rapidly– flood (hours to days)– debris flow (m/s)

• can accumulate slowly– clays in suspension - 1 year to 1000's of years

• pelagic sedimentation 1 to 50 cm/ky

LAMINATION:• LAMINAE - produced by short-lived fluctuations in

sedimentation conditions (physical, chemical, biological)which lead to variation in:1) grain size (always sand size or smaller grains)

• alternating smaller and larger grains (most common)• gradual or sharp boundaries - grading (decrease in size)

2) content of clay or organic material3) mineral composition4) microfossil content

Deposition by:A. Suspension mechanismsB. Traction mechanisms

varves

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LAMINATION:A. Deposition by Suspension mechanisms:

laminae of clay or fine silt generated by deposition ofsuspended sediment

Examples:1. slow suspension in lakes (seasonal variability)2. deposition on tidal flats due to changes in energy and

sediment supply during tidal cycles (dailyvariability)

3. Sub-tidal shelf - storm layers alternating with quietperiods (weekly or seasonal variability)

4. ocean upwelling regions - seasonally enhancedprimary production– coupled w/ low oxygen conditions on the seafloor (i.e.,

Arabian Sea)

LAMINATION:B. Deposition by traction mechanisms: involving sand size

sediment by traction transport by:• Swash & backwash - beaches (most common process)• Steady current flow during:

1. plane-bed phase (upper flow regime) ripples and dunesare removedoccurs in: stream channels, beaches

Upper flow regime : plane bed phase

laminae

2. shallow flow conditions (lower flowregime) - too shallow for avalanchefaces to form on the lee side of ripples,therefore no X-bedding

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Beach Face - Upper Plane BedStratification

FLOW REGIMES

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San Lorenzo River

• Ripples

BEDFORMS GENERATED BYUNIDIRECTIONAL FLOW

Vary primarily as afunction of1. velocity2. grain size3. depth of flow

A. Lower Flow RegimeB. Upper Flow Regime

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LOWER FLOW REGIME• RIPPLES:

– length: 10 to 20 cm or less,– height: a few cm– grain size: Sand < 0.7 mm– velocity: 0.2 to 1.0 m/s– depths: all

• SAND WAVES AND DUNES(Megaripples):– length: 0.5 to 10 m,– height: 10's of cm to meters– grain size : >0.2 mm,– velocity: 0.4 to 1.2 m/s– depths: >1 m

LOWER FLOW REGIME• Formation Process (applies to ripples, sand wave and

dunes):– Erosion on the stoss side:– Deposition on the lee side:

1.) avalanche of grains 2.) settling from suspension

– produces some size sorting point of separation

Inclined foreset beds

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Ripples: Foreset Configuration• Straight vs. Curved (tangential) foresets

– Bedload and Suspended load

bottomsets

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RIPPLE SYMMETRY, SHAPE

Ripple shape - controlled by velocity & depthFlow converges onScour Point

SYMMETRICAL STRAIGHTWAVE RIPPLES

Straight symmetrical waveripples (left) and slightly sinuouswave ripples (below)

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SINUOUS CURRENT RIPPLES

Point Bar, San Lorenzo River

Ripple Cross Bedding

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MIGRATION OF CROSS-BEDS

tabular cross bedding - planar bounding surfaces• Foreset laminae - straight, but can be tangential• formed by migration of ripples and dunes• bed thickness: 5 to 100 cm (as large as 10 m)

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Curved, Tangential foresets

Trough cross bedding - curve bounding surfaces• elongate scour filled with curved laminae, tangential to base of

set

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Sand Waves (Dunes) /Sinuous Ripples

Tangential foresets

S. Louisiana

AGGRADING RIPPLES(“Climbing ripple drift”)

Found in areas ofexcess sediment supplyin weakening flow

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PLANE (FLAT) BED: turbulent flowbecomes sheet like flow, erodingdunes, creating planar features

• velocity (V) 0.6 m/s to 1.5 m/sA) Lower plane bed -

slower V over coarse grains(>0.7 mm)

B) Upper plane bed -• faster V over finer grains• Parting lineations

UPPER FLOW REGIME (Fr>1)

ANTIDUNES:• stationary waves• Upper flow regime only• Dune affects flow velocities

• Stoss side - slows• Lee side - accelerates

• Low angle cross strata• Rarely preserved

UPPER FLOW REGIME (Fr>1)

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antidunes forming in asmall tidal channel

• migrate against the current

MULTIDIRECTIONAL FLOW1. Reversing tidal currents2. Reversing orbits of waves3. Fluctuating directions of flow in braided rivers• Predominately in intertidal settings• INCLUDES symmetrical ripples

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San Lorenzo River

Oscillating flow Upslope flow Breaking Wave

Increasing Energy Regime in Shoreface Settings

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INTERFERENCE RIPPLES

INTERFERENCE RIPPLES(“tadpole nests”)

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FLASER-WAVY-LENTICULARBEDDING

• Fluctuating hydraulic conditions (ie., changes in flow direction)• Sub-tidal environments, tidal flats, back bay lagoons, deltas

Lower energy, more mud Higher energy, more sand

FLASER vs. LENTICULARBEDDING

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FLASER vs. LENTICULAR

Graded Bedding

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Formation of Graded Beds

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Pyroclastic flow

• Gravity flow• Rapid changes in density/viscosity• Gas/fluid/particle mixture• Turbulent flow