Sedimentary structures provide important information about the depositional environment and post-depositional changes to sedimentary rocks. Key structures discussed include beds and bedding planes, laminations, graded bedding indicating changes in grain size over time, cross-bedding reflecting currents, load casts and flame structures from density differences, sole structures on bed bases indicating erosion, and trace fossils providing evidence of organism behavior and helping determine correct bed orientations. Together, an understanding of these sedimentary structures allows reconstruction of the depositional environment and testing of the law of superposition.

1. Sedimentary
Structures
I.G.Kenyon

2. Give information about the
depositional environment
Allow the ‘way-up’ of
beds to be ascertained

3. A Bed
A layer of rock separated from the layer
above and below by a bedding plane
A bed represents a single unbroken
episode of sediment accumulation
Beds vary in thickness from
1cm to many tens of metres
Beds 2 to 5cm thick are
called flags or flagstones
Beds may occur in uniform thicknesses
over large areas or pinch out laterally

4. Beds & Bedding Planes, Blue Hills, Near St.Agnes
£1 coin
for scale
Bedding Planes One bed

5. Bedding Plane
Defines the top or bottom of a bed
Represents a change in the
nature of sedimentation
a change in the rate or
type of sedimentation
a pause where no sediment is deposited
a period of erosion where
some sediment is removed

6. Lamination
A layer of sediment <1cm thick
Common in argillaceous rocks
such as siltstone and shale
Individual laminations may be
just 1mm thick or even less
If the sedimentary unit
is >1cm thick it is a bed

7. Laminations in Devonian Mylor Beds, Porthleven
Laminations here are <1mm thick
Difference in colour explained by variation in amount of
organic/carbonaceous matter incorporated into the sediment
1cm Mineral content mainly clay minerals
such as kaolinite, illite and serecite

8. Competent Beds
A bed of rock, which during folding,
flexes and bends without appreciable
flow or internal shear to maintain its
original thickness
Mechanically strong rocks such
as limestones and sandstones
commonly show this behaviour

9. Incompetent Beds
A bed of rock that deforms internally
during folding, resulting in rapid
changes in lateral thickness
Mechanically weak rocks such
as clays, mudstones and shales
commonly show this behaviour

10. Competent and Incompetent Beds at Millook, Near Bude
Shale deforms and thins
on the fold limbs
1m
Tension cracks occur in sandstone
around the nose of the fold
Shale is much thicker in
the nose/core of the fold
Sandstone is competent, retaining original
thickness in the limbs and nose of the fold

11. The Law of Superposition
First proposed by Nicolaus
Steno in the 17th Century
If one bed of sediment lies on top of another,
then the one above must be the younger
This assumes the beds have not been
overturned due to earth movements
Sedimentary structures collectively known
as ‘way-up criteria’ can be used to decide
if the beds have been overturned or not

12. Graded Bedding
A bed which displays a fining
upwards sequence from the base.
3cm
3cm
The fining upward sequence may
be produced in several ways

13. The Formation of Graded Bedding 1
Progressive settling of grade sizes from coarse to
fine in comparatively calm bodies of water
Example-greywackes on the continental slope,
where a poorly sorted sediment is deposited rapidly
The larger, denser rock fragments and sand
size particles sink first, followed by the smaller
and lower density silt and clay particles
Greywackes are deposited by turbidity currents
which are often initiated by minor seismic events

14. Formation of Graded Bedding by Turbidity Currents

15. Graded Bed with an Erosional Base
Fining
upwards
Irregular surface with
laminations of shale
beneath truncated in places
Represents an abrupt change from the
much finer grained sediment underneath

16. The Formation of Graded Bedding 2
Variations in the seasonal supply of
sediment, for example deposition from
glacial meltwater in a pro-glacial lake
In Spring/Summer much meltwater is
available and coarse sand and gravel may
be transported and deposited in the lake
In Autumn/Winter, the meltwater will
be greatly reduced, the lake may even
freeze over allowing only the finer silt
and clay to settle out from suspension

17. Millstone Grit showing Graded Bedding
Deltaic deposit with seasonal
fluctuations in energy conditions
Particles 0.5 to 1.0mm at the top
Fining upwards
sequence
1cm
Particles mainly 2-4mm at the base

18. The Formation of Graded Bedding 3
Seasonal variation in river discharge-in
Winter coarse sand and gravel may be
deposited during high discharges, in summer
finer sand and silt may be deposited when low
flow conditions occur. Example Millstone Grit
The stirring up of bottom sediments by storms and
their subsequent differentiation on settling
The stirring up of already deposited sediment
by submarine slumping and sliding by turbidity
currents followed by gravity settling

19. Cross Bedding
Also known as Current Bedding and False Bedding
If very large scale it is termed Dune Bedding
If very small scale it is termed Cross Lamination
In each case the sediment is being
moved and accumulated at an angle
to the principal bedding direction
Produced by a uni-directional current of
wind or water moving sediment as a series
of asymmetrical ripples or dunes

20. The Formation of Cross Bedding
Topset beds are truncated
Erosion surface Foreset beds
2m
Bottom set beds are preserved
Layers curve in towards the horizontal (asymptotically)
at the base of a cross bedded unit
Erosion surface-truncated topset beds
Foreset beds
10cm
Bottom set beds

21. Dune Bedding – Large Scale Cross Bedding
2m

22. Large Scale Cross Bedding – Dune Bedding
Topset beds are truncated Palaeo-wind direction
indicated by yellow arrows
Foreset and bottom
set beds preserved
People for scale

23. Herring Bone Cross Bedding
Upper Unit
Middle Unit
Lower Unit
Penknife for scale
Represents a current reversal through 180°. Blue arrows indicate the
direction of sediment movement in each of the 3 units above

24. Cross Lamination (Very small scale cross bedding)
Truncation/erosion surface of topset beds
Current
direction
Approximate base of
Pen top cross laminated unit
for scale
Individual laminations 2 to 4mm thick
Fine sandstone unit, Compass Point near Bude

25. Convolute Bedding/Slump Bedding 1
Common on deltas
where sediment is
saturated with water
and easily mobilised
Occurs frequently in
interbedded sandstone
and shale sequences
Shales deform internally
and flow showing
incompetence
Sandstone layers break
into rigid blocks which
become displaced and
Incompetent shale Competent sandstone
show competence

26. Convolute Bedding/Slump Bedding 2
Incompetent shale
Often initiated by which has flowed or
deformed internally
a minor tectonic
disturbance or
slope failure
Can also be formed
by the rapid expulsion
of pore water
The example here is
from the Carboniferous
beds at Compass
Point near Bude
Rigid, competent sandstone blocks

27. Included/Derived Fragments
Younger upper series
Older beds may be
Derived fragments from
eroded before the
older lower series deposition of the next
bed in the sequence
The eroded fragments
unconformity
are then included as
clasts in the bed above
1m
Younger upper series
Lower older series Derived
Lower older series fragments

28. Imbricate Structure
Common in
rudaceous rocks
Deposited under
the influence of a
powerful current
Long axes of clasts
lie sub-parallel with
one another ‘leading’
in direction of
current flow
Arrows indicate
direction of flow

29. Mud Cracks
Formed when sediment is
exposed to the atmosphere
Common in tidal flats, Note how the edges curl
mudflats and playa lakes up to accentuate the V
shaped gap between them
Mud cracks form as
desiccation polygons
The sediment dries out
and shrinks as water
is evaporated from it
Contraction centres develop
and a polygonal pattern
of cracks develop
Analogous to columnar
jointing in cooling lavas 30cm

30. Mudcracks
The mud cracks are widest at
the surface tapering to a point
at a depth of 0.5 to 2.0 cm
Often later infilled with finer,
wind blown sediment of a
different colour or calcareous
material if in a playa lake

31. Mud Cracks and Rain Pits
Rain pits formed by impact of raindrops on an exposed
sediment surface. They appear as small rounded depressions
up to 1cm in diameter, sometimes with a small raised rim.
Rain pits mark the top of the sediment

32. Wash-Out in Fine Grained Sediment
Scremerston, Northumberland
Coin for
Small scale channel – base
scale
is convex downwards
Older laminations
truncated
Formed as a result of Scour and Fill

33. Load Casts and Flame Structures
Common in sandstone and shale sequences
Locally, the denser sandstone sinks down into the
less dense shale below as bulbous protrusions
The shale is incompetent and deforms/flows upwards into
the spaces between the bulbous sandstone protrusions
The rounded protrusions mark the base of the sandstone
bed, whilst the flames mark the top of the shale bed
Sometimes a globule of sandstone becomes completely
detached from the bed above and sinks into the shale below
distorting the laminations to form a teardrop structure

34. Load Casts and Flame Structures
Competent
sandstone
Incompetent shale

35. Load Casts and Flame Structures
Bulbous protrusions from base of
overlying sandstone bed
Competent sandstone
Incompetent shale
Shale squeezed up between sandstone
protrusions as a flame structure

36. The Life Position of Fossils
Organisms preserved in
life position such as trees
can indicate if the beds are
the ‘right way up’

37. The Life Position of Fossils
Algal mounds are convex upwards Top
Convex
upwards
growing
towards the
light
4cm
Bottom
Modern day stromatolites Stromatolites 2.5 billion years old
from Cordoba Provence, Argentina
Organisms preserved in life position such as stromatolites
(algal mounds) can indicate if the beds are the ‘correct way up’

38. The Life Position of Fossils
3cm
The Great Barrier Reef, Australia
Corals preserved
in limestone
Organisms preserved in life
position such as corals can
indicate if the beds are the
‘correct way up’
Corals indicate clear water less
than 50m deep, well oxygenated
envi. with normal salinity (3.5%),
Living corals form the
upper part of the reef
temperatures 22-28°C and located
within 30° latitude of the equator

39. Sole Structures
Formed in interbedded
sandstone and shale sequences
These are preserved on the base
of the overlying sandstone bed
Main processes are scouring and
erosion of the soft shale accumulation
surface by currents and tools
Classified according to shape
They include Flute, Groove,
Bounce and Prod/Tool casts

40. Flute Casts on the underside of a Greywacke Bed
Cowpeel Bridge, Peebleshire, Scotland
10cm
Palaeo-current Direction

41. Flute Casts
Plan View Cross section showing scouring
of fluted hollows in soft mud
by current vortices

42. Groove cast on the under surface of a
Greywacke bed, Hartland Quay, North Devon
Formed by a pebble rolling across a soft sediment
surface and cutting a groove into it. Preserved as
a cast on the under surface of the overlying bed
Pen top for scale
Possible palaeo-current directions

43. Prod/Tool Casts on the underside of a
Greywacke Bed, Hartland Quay, North Devon
Possible palaeo-
current directions
5cm

44. Bounce, Groove and Prod Casts
1cm
Prod Cast
Bounce cast formed by a
saltating fish vertebra
Groove cast

45. Geopetal Structures
Partially infilled shells
of marine organisms
Also known as
‘fossil spirit levels’
They indicate how
much tilting has taken
place since deposition
When mud originally entered the cavities it would have
settled horizontally due to the influence of gravity
Subsequent tilting results in the level of mud being moved to
a new inclination and different from today’s horizontal
If the cavities are empty or completely filled with sediment,
then they cannot be used as geopetal structures

46. Geopetal Structures – Brachiopods in Reef Limestone
Only partially infilled
brachiopod shells can be
used as geopetal structures
Emanual Range Western Australia

47. Ripple Marks-Symmetrical
Mark the top of the bed and
imply the sediment was under
the influence of wave action
Minibus key for scale
Carboniferous sandstones, Compass Point near Bude

48. Concretions
30cm
Concretion within a fine
sandstone bed, Compass
Point, Near Bude
A roughly spherical or ellipsoidal body produced as a
result of early localised cementation within a sediment.
Often found with a fossil as the nucleus of the concretion

49. Trace Fossils - Burrows
‘Right way up’
Organisms such as
bivalves and marine Open at the surface
worms burrow from the
surface downwards Tapers to a point
into unconsolidated
soft sediment
The burrows are open
at the ancient sediment
surface and taper
downwards to a point ‘Right way up’

50. Trace Fossils – Horizontal Burrows
Limestone showing tube-like traces of
burrowing animals, Port Issol, France

51. Trace Fossils-Trails and Footprints
Trilobite trail
Winding trails-Repichnia
Cruziana
and Pasichnia
Represents the upper
sediment surface over
which organisms
walked or crawled
Sauropod footprints

52. Relationships between types of Trace
Fossils and Sedimentary Environments

53. Halite Pseudomorphs
1cm
Halite pseudomorph, the original
crystal has been dissolved away and
the mould has been infilled by mud
Preserved on the base of the overlying bed as a cast

54. The End