Download - 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
Photo courtesy of Andrew D. Miall
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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
Photo courtesy of N. D. Smith
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Aeolian trough cross-stratification,
Arches National Park, Utah, USA
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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
Photo courtesy of N. D. Smith
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
Photo courtesy of N. D. Smith
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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!
Photo courtesy of Andrew D. Miall
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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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Tide-influenced sedimentary structures can
take different shapes:
Herringbone cross stratificationindicates bipolar flow
directions, but it is rare
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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Tide-influenced sedimentary structures can
take different shapes:
Herringbone cross stratificationindicates bipolar flow
directions, but it is rare
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)
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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Tide-influenced sedimentary structures can
take different shapes:
Herringbone cross stratificationindicates bipolar flow
directions, but it is rare
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)
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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Markings Generated by
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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Markings Generated by
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Markings Generated by
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