sedimentary stuctures knowledgebase

8/12/2019 Sedimentary Stuctures Knowledgebase

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Sedimentary Structures

Knowledgebase

Dr Kathryn AmosAustralian School of Petroleum,

University of Adelaide


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UNI-DIRECTIONAL

FLOW


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Flow ripples


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Ripples from interfering flow

produces ladderback ripples

Photo courtesy of Andrew D. Miall


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Linguoid Ripples (3D)


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Cross stratification

Cross lamination (small-scale crossstratification) is produced by ripples

Cross bedding (large-scale cross stratification)

is produced by dunes

Cross-stratified deposits can only be preserved

when a bedform is not entirely eroded by the

subsequent bedform (i.e., sediment input >

sediment output)

Straight-crested bedforms lead to planar cross

stratification; sinuous or linguoid bedforms

produce trough cross stratification


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Block model of 2D dune formation

Courtesy of Andrew D. Miall, modified after Harms et al. (1982)

Dunes scale to flow depth


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Modern planar cross-stratification

in a 2D Dune

Photo courtesy of N. D. Smith


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Ancient planar cross-stratification



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Block model of 3D dune formation

Courtesy of Andrew D. Miall, modified after Harms et al. (1982)


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Exposed modern 3D dunes



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Aeolian trough cross-stratification,

Arches National Park, Utah, USA


15/70http://www.umt.edu/geology/faculty/hendrix/g432/g432_L7.htm

stoss

lee

The angle of climbof cross-stratified

deposits increaseswith deposition rate,

resulting inclimbing ripple

cross lamination


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Planar stratification

Planar lamination (or planar bedding) is

formed under both lower-stage and upper-

stage flow conditions

Planar stratification can easily be confused

with planar cross stratification, depending

on the orientation of a section (strikesections!)

U Fl R i


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Upper plane bed flow conditions in a modern river


Upper Flow Regime


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Courtesy of Andrew D. Miall, modified after Simons et al. (1965)


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Sedimentary structures associated

with upper plane bedParting lineation (top)

Planar stratification (side)


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In-phase waves form antidunes



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Supercritical flows (Froude number > 1; ratio between

inertial and gravity forces)

respond to changes in bed elevation differently than

subcritical flows: for antidunes, the flow velocity in the trough

is greater than the flow velocity at the crest, and vice-versa.

As a result, the bed shear stress maximum (max) and

minimum (min

) move in a downstream direction from thetrough and crest.


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Erosion occurs where bed shear stress isincreasing

Thus, they migrate upstream

Since the trough is in a zone of erosion and

the crest is in zone of deposition, the

bedform will build in amplitude over time.


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Antidunes accrete sand at the upstream end of the

dune rarely preserved in the stratigraphic record!



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BI-DIRECTIONAL FLOW

Waves= oscillatory flow

Tides = unsteady bidirectional flow


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Waves are formed by wind

Allen and Chambers (1998)

Fetch is the distance of water the

wind has blown over.

A Tsunami is an ocean surface wave

not generated by wind, e.g.

landslide or earthquake.

"Ocean-surface waves"


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Gravity pulls water in the crests downward

The water forced out from under the falling crestpushes the trough upwards

- as the wave approaches you're carried up and

forward; down and back as it passes


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Oscillatory flowSymmetrical ripples

asymmetric

symmetric

Waves can also generate

larger scale bedforms

(dunes and bars)


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Hummocky cross stratification(HCS) Undulating sets of cross-laminae that are both

concave-up (swales) and convex-up (hummocks)

The cross-bed sets cut gently into each other with

curved erosion surfaces

Commonly occurs in 15-50cm thick sets with sharp,

wavy erosional bases and rippled bioturbated tops.

Spacing of hummocks and swales is 50cm to several

metres. Typically fine sandstone to coarse siltstone.

Forms below the fair-weather wave base duringstorm events with combined wave and current

activity in shallow seas, and is the result of

aggradation of mounds and swales

H k


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Hummocky

Cross-

stratification


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Possible model for the creation of

hummocky cross-stratification on the shelf

Duke et al. (1991)


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Hummock cross-

stratification from storm

waves in a shelfal setting

Convex - up surfaces


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TIDES

UNSTEADY

BI-DIRECTIONAL FLOW


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Allen and Chambers, 1998

Tidal Processes

Daily Tidal Cycles

~ 2 high tides per day


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Tide-influenced sedimentary structures can

take different shapes:

Herringbone cross stratificationindicates bipolar flow

directions, but it is rare


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Mud-draped cross strataare much more common, and are the

result of alternating bedform migration during high flow velocities

and mud deposition during high or low tide (slackwater)


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Dalrymple (1992)


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Heterolithic stratificationis

characterized by alternating

sand and mud laminae or beds

Flaser bedding is

dominated by sand with

isolated, thin mud drapes

Wavy bedding contains

approx equal amounts of

sand and mud

Lenticular bedding is mud-

dominated with isolated

ripples

Flaser Bedding W B ddi


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Flaser Bedding

Lenticular Bedding

Wavy Bedding

W b ddi


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Wavy bedding

Photo courtesy of George Allen


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Tidal bundlesare characterized by a sand-mud couplet with

varying thickness; tidal bundle sequencesconsists of a series of

bundles that can be related to neap-spring cycles


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Tidal bundle

"Clay Doublet" - corresponds to mud depositions during low and high tides.


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heterolithic facies showing cross-

bedded sandstone with tidal bundlesand double mud layers

Remember: mud couplets represent

a single tide; there are two tides per

day

Shanmugam et al.,AAPG Bulletin; May 2000; v. 84; no. 5; p. 652-682; DOI: 10.1306/C9EBCE7D-1735-11D7-8645000102C1865D


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Effects of Lunar

Cycles onSedimentation

~ 15 day cycle

- tidal bundle

sequencesAllen and Chambers, 1998

Thicker, sandier

Thinner, muddier

Thicker, sandier


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Tidal bundlesare characterized by a sand-mud couplet with

varying thickness; tidal bundle sequencesconsists of a series of

bundles that can be related to neap-spring cycles

Tidal rhytmitescan form in fine-grained facies that aggrade

vertically and/or laterally, to a large part from suspension, and

consist of commonly very thin (mm-scale) beds or laminae, usually

of sandstone, siltstone and mudstone, that exhibit rhythmic

thickness variation as a consequence of lunar-solar tides

Tid l R th it


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Tidal Rythmite

Spring

Neap

Photo courtesy of George Allen


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IRREGULAR

STRATIFICATION


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Convolute bedding and lamination

Plastic deformation of partially liquefied

sediment

S


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Flame Structures

Tongues of mud that project up into overlying

layer. Commonly mud projecting up intosandstone. Commonly associated with

loading.

Ball and Pillow Structures


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Ball and Pillow Structures

A load detatches and sinks into the underlying

sediment, probably due to partial liquefaction

of underlying mud.

S di t f ld d f lt


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Synsedimentary folds and faults

Slump structures. Rapid sedimentation and

oversteepened slopes led to instability.

Dish and Pillar Structures


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Dish and Pillar Structures

Indicate rapid deposition and form by escape

of water during consolidation of sediment

Dish structures are thin, dark,

subhorizontal, flat to concave-

upward, clayey laminations.

Pillar structures generally occur in

association with dish structures.

Vertical to near-vertical, cross-

cutting columns and sheets of

structureless or swirled sand.

Erosion Structures


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Erosion Structures

Channels

Scour-and-fill structures(similar to but smaller than

channels)


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BEDDING PLANEMARKINGS

Markings Generated by


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Erosion and Deposition

Often referred to as Sole Markings are bedding planemarkings that have been preserved on the soles or undersidesof beds.

Sole markings are irregular markings, and positive relief casts,which are typically preserved well in sandstones and coarsergrained sedimentary rocks overlying mudstone or shale beds.

Sole markings are formed by erosion of a cohesive sediment

bottom (mud) to produce grooves or depressions, followed bydeposition of a coarser sediment filling the negative reliefformations.

Sole markings include:


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Sole markings include:

Remember: we are looking at the


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Remember: we are looking at the

evidenceof these erosion features

Flute casts


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Flute casts form when a scour is formed in

cohesive sediment by a current

eddy created behind someobstacle or by chance eddy

scour. The scour is then filled in

forming a cast. Flute casts are

excellent paleoflow indicators,

indicating true flow direction.

elongated welts or ridges with a

bulbous nose forming the head

followed by tapering in to form a

tail with an overall shape of a

comet. They can occur singularly

or in groups where all the casts

are oriented in the same

direction.

From: www.earth.rochester.edu/ees201/Bren/BeddingPlane.html

Tool

Flutes

K, Utah

Groove casts Tools are objects carried byth t th t i t itt tl


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Groove casts the current that intermittentlyor continuously make contactwith the bottom and aredragged along causing a long

continuous erosional scour.Tools can be pieces of wood,or shell fragments.

Tool marks are mostcommonly preserved as

casts, and are excellentpalaeoflow indicators.

When tool marks arepreserved as casts they arecalled Groove Casts. Groove

casts are elongate nearlystraight ridges that resultfrom the infilling if erosionalrelief as a result from a toolbeing dragged across thesurface of a cohesive

sediment.From: www.earth.rochester.edu/ees201/Bren/BeddingPlane.html

Bounce/Brush/Prod/Roll/Skip Marks


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Deformation: Load Casts

are slight bulges or shallow rounded sacks, which commonlyoccur on the soles of sandstone beds overlying mudstones orshales. Load casts may resemble flute casts, however theylack definite upcurrent and downcurrent ends.

Load casts are not really casts, because they have not filled ina preexisting cavity. They are formed by deformation fromunequal loading of overlying sand layers.

Load casts can occur in any environment where watersaturated muds are quickly buried by sand before dewateringcan occur.


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Wrinkle marks


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Wrinkle marks

Miniature scale load casts

Bi i St t


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Trace fossils and bioturbation: tracks, trails,burrows, borings

Biogenic Structures

Other Bedding Plane


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Other Bedding Plane

MarkingsDesiccation cracks

Raindrop prints

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