1. Sedimentary Structures
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2. Primary Sedimentary Structures
Mechanical
structures resulting
from sediment
deposition.
v
Under
unidirectional flow,
ripples begin under
critical entrainment
velocity is reached.
v
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v

3. 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
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planes or bounding planes

4. Terminology of Bedsets
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5. Plane Bedding Structures
Simple,
horizontal beds >
1 cm
vResults from
suspension
sedimentation,
vHorizontal
accretion,
vEncroachment
into lee side of
obstacle
vLamination < 1
cm thick
vAbsence of
lamination may
be due to
flocculation
v
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6. 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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7. 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
•
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8. 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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9. 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
OSCILLATION RIPPLES
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WAVE-DOMINATED

10. 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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11. 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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12. 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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13. 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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14. 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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15. 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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§
Bouma Sequences

16. Graded Bedding
GRADEDSTRATIFIED
INVERSE TO
NORMAL
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
•
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17. Massive Bedding
Bed appears homogenous and lacking internal
structure
vTurbidite and/or bioturbation generated
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v
v

18. Soft-Sediment Deformation
Structures
Convolute bedding & lamination due to liquefaction
processes
vComplex folding or crumpling of semiconsolidated
vBeds
vFlame structures – wavy or flame-shaped tongues
injected into overlying layers
vBall and Pillow – hemispherical or kidney-shaped
masses
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vinto underlying mudstone
v

19. Bedding-Plane Markings
Underside of beds as positive-relief
casts and irregular markings – “Sole
Markings”
v
Current-formed structures include:
flute casts (elongate ridges, bulbous at
one end and flare in direction); current
crescents (obstacle scours)
v
Tool-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
v
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