Sedimentary structures smallas

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