sedimentary structures smallas
TRANSCRIPT
Sedimentary Structures
I.G.Kenyon
Give information about the depositional environment
Allow the ‘way-up’ of beds to be ascertained
A BedA 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
Beds & Bedding Planes, Blue Hills, Near St.Agnes
One bedBedding Planes
£1 coin for scale
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
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
Laminations in Devonian Mylor Beds, Porthleven
1cm
Laminations here are <1mm thick
Difference in colour explained by variation in amount of organic/carbonaceous matter incorporated into the sediment
Mineral content mainly clay minerals such as kaolinite, illite and serecite
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
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
Competent and Incompetent Beds at Millook, Near Bude
Sandstone is competent, retaining original thickness in the limbs and nose of the fold
Tension cracks occur in sandstone around the nose of the fold
Shale deforms and thins on the fold limbs
Shale is much thicker in the nose/core of the fold
1m
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
Graded BeddingA bed which displays a fining
upwards sequence from the base.
The fining upward sequence may be produced in several ways
3cm
3cm
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
Formation of Graded Bedding by Turbidity Currents
Graded Bed with an Erosional Base
Represents an abrupt change from the much finer grained sediment underneath
Fining upwards
Irregular surface with laminations of shale
beneath truncated in places
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
Millstone Grit showing Graded Bedding
Fining upwards sequence
Particles mainly 2-4mm at the base
Particles 0.5 to 1.0mm at the top
1cm
Deltaic deposit with seasonal fluctuations in energy conditions
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
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
Foreset beds
Topset beds are truncated
Erosion surface
Bottom set beds are preserved
Erosion surface-truncated topset beds
Foreset beds
Layers curve in towards the horizontal (asymptotically) at the base of a cross bedded unit
Bottom set beds
The Formation of Cross Bedding
2m
10cm
2m
Dune Bedding – Large Scale Cross Bedding
Large Scale Cross Bedding – Dune Bedding
People for scale
Palaeo-wind direction indicated by yellow arrows
Topset beds are truncated
Foreset and bottom set beds preserved
Herring Bone Cross Bedding
Penknife for scale
Upper Unit
Middle Unit
Lower Unit
Represents a current reversal through 180°. Blue arrows indicate the direction of sediment movement in each of the 3 units above
Cross Lamination (Very small scale cross bedding)
Truncation/erosion surface of topset beds
Pen top for scale
Fine sandstone unit, Compass Point near Bude
Approximate base of cross laminated unit
Current direction
Individual laminations 2 to 4mm thick
Convolute Bedding/Slump Bedding 1Common 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
show competenceIncompetent shale Competent sandstone
Convolute Bedding/Slump Bedding 2
Often initiated by 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
Incompetent shale which has flowed or deformed internally
Included/Derived Fragments
unconformity
Lower older series
Younger upper seriesOlder beds may be eroded before the
deposition of the next bed in the sequence
The eroded fragments are then included as
clasts in the bed above
Lower older series
Younger upper series
Derived fragments
Derived fragments from older lower series
1m
Imbricate StructureCommon 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
Mud Cracks
Formed when sediment is exposed to the atmosphere
Common in tidal flats, mudflats and playa lakes
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
Note how the edges curl up to accentuate the V
shaped gap between them
MudcracksThe 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
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
Wash-Out in Fine Grained Sediment Scremerston, Northumberland
Formed as a result of Scour and Fill
Older laminations truncated
Small scale channel – base is convex downwards
Coin for scale
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
Load Casts and Flame Structures
Competent sandstone
Incompetent shale
Load Casts and Flame Structures
Bulbous protrusions from base of overlying sandstone bed
Shale squeezed up between sandstone protrusions as a flame structure
Competent sandstone
Incompetent shale
The Life Position of Fossils
Organisms preserved in life position such as trees
can indicate if the beds are the ‘right way up’
The Life Position of Fossils
4cm
Organisms preserved in life position such as stromatolites (algal mounds) can indicate if the beds are the ‘correct way up’
Modern day stromatolites
Algal mounds are convex upwards
Bottom
Top
Convex upwards growing
towards the light
Stromatolites 2.5 billion years old from Cordoba Provence, Argentina
The Life Position of FossilsThe Great Barrier Reef, Australia
Living corals form the upper part of the reef
3cm
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%),
temperatures 22-28°C and located within 30° latitude of the equator
Corals preserved in limestone
Sole StructuresFormed 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
Flute Casts on the underside of a Greywacke Bed
Palaeo-current Direction10cm
Cowpeel Bridge, Peebleshire, Scotland
Flute Casts
Plan View Cross section showing scouring of fluted hollows in soft mud
by current vortices
Groove cast on the under surface of a Greywacke bed, Hartland Quay, North Devon
Pen top for scale
Possible palaeo-current directions
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
Prod/Tool Casts on the underside of a Greywacke Bed, Hartland Quay, North Devon
5cm
Possible palaeo-current directions
Bounce, Groove and Prod Casts1cm
Bounce cast formed by a saltating fish vertebra
Groove cast
Prod Cast
Geopetal StructuresPartially 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
Geopetal Structures – Brachiopods in Reef Limestone
Emanual Range Western Australia
Only partially infilled brachiopod shells can be
used as geopetal structures
Ripple Marks-Symmetrical
Carboniferous sandstones, Compass Point near Bude
Minibus key for scale
Mark the top of the bed and imply the sediment was under the influence of wave action
Concretions
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
30cm
Concretion within a fine sandstone bed, Compass
Point, Near Bude
Trace Fossils - Burrows
Organisms such as bivalves and marine
worms burrow from the surface downwards into unconsolidated
soft sediment
The burrows are open at the ancient sediment
surface and taper downwards to a point ‘Right way up’
Open at the surface
Tapers to a point
‘Right way up’
Limestone showing tube-like traces of burrowing animals, Port Issol, France
Trace Fossils – Horizontal Burrows
Trace Fossils-Trails and Footprints
Trilobite trail Cruziana
Winding trails-Repichnia and Pasichnia
Sauropod footprints
Represents the upper sediment surface over
which organisms walked or crawled
Relationships between types of Trace Fossils and Sedimentary Environments
Halite Pseudomorphs
1cm
Preserved on the base of the overlying bed as a cast
Halite pseudomorph, the original crystal has been dissolved away and the mould has been infilled by mud
The End