Sed Structures

8/13/2019 Sed Structures

1/18

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


2/18

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


3/18

Terminology of Bedsets


4/18

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


5/18

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


6/18

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


7/18

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


8/18

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


9/18

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


10/18

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


11/18

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


12/18

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


13/18

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


14/18

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


15/18

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


16/18

Massive Bedding

Bed appears

homogenous and lackinginternal structureTurbidite and/or

bioturbation generatedLiquefaction of

sediment by shock-wave


17/18

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


18/18

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

Sed Structures

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