SEDIMENT DEPOSITION & STRUCTURES jzachos/migrated/eart120/lectures_08/LECT... · SEDIMENT DEPOSITION

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Text of SEDIMENT DEPOSITION & STRUCTURES jzachos/migrated/eart120/lectures_08/LECT... · SEDIMENT...

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    SEDIMENT DEPOSITION &STRUCTURES:

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

    FLOW REGIMES

    Frou

    de #

    >1Fr

    oude

    #

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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

    Frou

    de #

    >1Fr

    oude

    #

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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 separationInclined 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