sed structures
TRANSCRIPT
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Primary SedimentaryStructures
Mechanicalstructures resulting
from sedimentdeposition
Under unidirectionalflow, ripples begin
under criticalentrainment velocity is
reachedOther bedforms
require increasingvelocity and/or grain
size propertiesIncreasing velocitiesin sand produce
predictablesedimentary structures
McBrideRipples.mov
McBrideLamination.mov
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Stratification & Bedforms
Beds are tabular or lenticular layers of sedimentary rock withlithologic, textural, or structural unity
Distinguishable from subjacent and suprajacent layersUpper/lower surfaces are bedding planes or bounding planes
Marked discontinuities within beds are amalgamation surfaces
CHAPMANS PEAK, SOUTH AFRICA
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Terminology of Bedsets
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Plane Bedding Structures
Simple, horizontalbeds > 1 cm
Results fromsuspension
sedimentation,Horizontal accretion, Encroachment into
lee side of obstacle
Lamination < 1 cm
thick Absence of lamination may be due
to flocculation
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Low Flow Regime SedimentaryStructures
Ripple Index ratio of ripple length : ripple heightOut-of-phase wave propagation with bedforms
Ripples smallest bedform with RI ~8 (coarse) to 20 (finesand), forming in sand and silt
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Low Flow RegimeSedimentaryStructures
Dune larger bedform
with RI ~5 (fine sand) to 50(gravel)2D Dune
straight/sinuous and longcrested; 10 cm - 100s m3D Dune curved faces,
irregular and short crested;10 cm -10s m
DEATH VALLEY
Arabian Peninsula Dune Field
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Upper Flow Regime SedimentaryStructures
In-phase wavepropagation with
bedforms
Plane-bed flowstage with transportover a relatively flat
bed
Internal planar lamination (mm - cm laminae)Highest flow velocities may create antidunes
Antidunes low, undulating with RI ~7 to 100
Low angle cross beds directed upstream
AntiDunes.mov
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Multidirectional Flow-Generated Bedforms
Oscillation (wave) ripples generally symmetrical to slightlyasymmetrical due to eddies
Orbital velocity difference < 1 cm/s = symmetrical bedformsOrbital velocity difference > 5 cm/s = asymmetrical
bedformsCrests are straight to sinuous, bifurcate
Herringbone cross-beds; interference ripples; lenticular beds; flaser beds
CURRENT RIPPLES CURRENT-DOMINATED WAVE-DOMINATED OSCILLATION RIPPLES
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Geometries of Beds
Planar stratification internal layers and laminae that areparallel to bedding planes
Cross strata internal layers or laminae that are at an angle
to bedding planesBeds composed of cross-laminated or cross-stratified units
are cross bedsBedsets comprised of similar beds or cross beds
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Cross-Beds Result of ripple and/or dune migration; filling of scour pits & channelsForeset laminae develop
as avalanche or suspensionsettling phenomena; lee side
of ripple with steep and
straight laminaeBottomset laminae from
suspension load nearly at theangle of repose
Topset laminae rarelypreserved (sigmoidal cross
beds)Occur in cross-bed sets
small scale bedsets < 5 cm;large scale bedsets > 5 cm
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Cross-Bed Geometries
McKee & Weir (1953) Tabular & Trough Cross beddingTabular Cross beds units broad in lateral dimensions withrespect to set thickness with planar bounding surfaces.Migration of large-scale ripples and dunes; lower flow regime
Trough Cross beds units whose bounding surfaces arecurved, consist of elongate scour filled with curved laminaeMigration of small-scale or large-scale ripplesPaleocurrent measured in dip direction of foreset laminae
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Hummocky Cross StratificationUndulating sets of cross laminae both concave-up (swales)
and convex-up (hummocks)Common in 15 - 50 cm thick sets; wavy erosional bases and
rippled, bioturbated topsFine sandstone to coarse siltstone, micaceous with
dispersed plant debris
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Turbidites
Density current in ocean and/or lake flowing downslopeInitiated by short-lived catastrophic events (earthquake
trigger; storm)Flow divided into: Head 2x thick as remainder of flow with
turbulent flow; Body uniform thickness with uniform flow; Tail flow thins and becomes dilute
Thick-bedded (high density flow) and thin-bedded (dilutedensity flow) turbidites
WAKEMIXING
ROLLERS
HEAD
BODYSubAqueousDebrisFlow.mov
UnconfinedTurbidite.mov
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Bouma Sequences
Idealized turbidite sequencerecording decay of flow strengthSubdivided into Units A through E
A Massive graded bed (coarsestsettled); B plane laminated bed(high flow); C lower flow ripplesand wavy lamination; D laminatedsilt; E laminated mud
Hs believes can be divided intoonly 2 unitsLower, horizontally laminated unit;Upper, cross-laminated unit
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Graded Bedding
Vertical gradations in grain size within a bedBasal coarse particles that grade upwards to finer particles
at top is Normal (common)Basal fine particles that grade upwards to coarser particles
at top is Reverse (rare)Basal contacts are sharp; Attributed to turbidites
GRADED-STRATIFIED
GRADED
INVERSE TO NORMAL
DISORGANIZED
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Massive Bedding
Bed appears
homogenous and lackinginternal structureTurbidite and/or
bioturbation generatedLiquefaction of
sediment by shock-wave
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Soft-SedimentDeformationStructures
Convolute bedding &lamination due to
liquefaction processesComplex folding or
crumpling of semi-consolidated bedsFlame structures
wavy or flame-shapedtongues injected into
overlying layersBall and Pillow
hemispherical or kidney-shaped masses
into underlyingmudstone
SynsedimentaryFolds & Faults slump
units
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Bedding-Plane
MarkingsUnderside of beds aspositive-relief casts andirregular markings SoleMarkings
Current-formed structuresinclude: flute casts(elongate ridges, bulbousat one end and flare indirection); currentcrescents (obstacle scours)
Tool-formed structures include: groove casts (object dragged acrosssurface); bounce, brush, prod, roll, and skip marks (intermittent object
contact with bottom)
Load Casts irregularly shaped without current indicators