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Sedimentary Structures

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Primary Sedimentary Structures

vMechanical structures resulting from sediment deposition.

vUnder unidirectional flow, ripples begin under critical entrainment velocity is reached.

vOther bedforms require increasing velocity and/or grain size properties

vIncreasing velocities in sand produce predictable sedimentary structures

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Stratification & Bedforms

• Beds are tabular or lenticular layers of sedimentary rock with lithologic, textural, or structural unity.

• Distinguishable from subjacent and suprajacent layers

• Upper/lower surfaces are bedding planes or bounding planes

• Marked discontinuities within beds are amalgamation surfaces

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Terminology of Bedsets

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Plane Bedding Structures

vSimple, horizontal beds > 1 cmvResults from suspension sedimentation,vHorizontal accretion,vEncroachment into lee side of obstaclevLamination < 1 cm thickvAbsence of lamination may be due to flocculation

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Low Flow Regime Sedimentary Structures

• Ripple Index – ratio of ripple length : ripple height

• Out-of-phase wave propagation with bedforms

• Ripples – smallest bedform with RI ~8 (coarse) to 20 (fine sand), forming in sand and silt.

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Low Flow Regime Sedimentary Structures

•Dune – larger bedform with RI ~5 (fine sand) to 50 (gravel)

• 2D Dune – straight/sinuous and long crested; 10 cm - 100s m

•3D Dune – curved faces, irregular and short crested; 10 cm -10s m

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Upper Flow Regime Sedimentary Structures

§In-phase wave propagation with bedforms§Plane-bed flow stage with transport over 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

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Multidirectional Flow- Generated Bedforms

•Oscillation (wave) ripples generally symmetrical to slightly asymmetrical due to eddies•Orbital velocity difference < 1 cm/s = symmetrical bedforms•Orbital velocity difference > 5 cm/s = asymmetrical bedforms•Crests 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 are parallel to bedding planes•Cross strata – internal layers or laminae that are at an angle to bedding planes•Beds composed of cross-laminated or cross-stratified units are cross beds•Bedsets comprised of similar beds or cross beds

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Cross-Beds•Result of ripple and/or dune migration; filling of scour pits & channels•Foreset laminae develop as avalanche or suspension settling phenomena; lee side of ripple with steep and straight laminae•Bottomset laminae from suspension load nearly at the angle of repose•Topset laminae rarely preserved (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 bedding•Tabular Cross beds – units broad in lateral dimensions with respect to set thickness with planar bounding surfaces.•Migration of large-scale ripples and dunes; lower flow regime•Trough Cross beds – units whose bounding surfaces are curved, consist of elongate scour filled with curved laminae•Migration of small-scale or large-scale ripples•Paleocurrent measured in dip direction of foreset laminae

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Hummocky Cross Stratification•Undulating 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 tops•Fine sandstone to coarse siltstone, micaceous with dispersed plant debris

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Turbidites•Density current in ocean and/or lake flowing downslope•Initiated 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 (dilute•density flow) turbidites

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Bouma Sequences§Idealized turbidite sequence recording decay of flow strength§Subdivided into Units A through E§A – Massive graded bed (coarsest settled); B – plane laminated bed§(high flow); C – lower flow ripples and wavy lamination; D – laminated§silt; E – laminated mud§Hsü believes can be divided into only 2 units§Lower, horizontally laminated unit; Upper, cross-laminated unit

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Graded Bedding

•Vertical gradations in grain size within a bed•Basal 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

INVERSE TO NORMAL

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Massive Bedding

vBed appears homogenous and lacking internal structurevTurbidite and/or bioturbation generatedvLiquefaction of sediment by shock-wave

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Soft-Sediment Deformation Structures

vConvolute bedding & lamination due to liquefaction processesvComplex folding or crumpling of semiconsolidatedvBedsvFlame structures – wavy or flame-shaped tongues injected into overlying layersvBall and Pillow – hemispherical or kidney-shaped massesvinto underlying mudstonevSynsedimentary Folds & Faults – slump units

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Bedding-Plane Markings

vUnderside of beds as positive-relief casts and irregular markings – “Sole Markings”

vCurrent-formed structures include: flute casts (elongate ridges, bulbous at one end and flare in direction); current crescents (obstacle scours)

vTool-formed structures include: groove casts (object dragged across surface); bounce, brush, prod, roll, and skip marks (intermittent object contact with bottom)vLoad Casts – irregularly shaped without current indicators