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

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

Cross section

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

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

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

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

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

Ripple Marks- Asymmetrical

Formed by a uni-directional current such as a river or the wind, the downstream or downwind side will have the steeper face

Ripple Marks- Asymmetrical

Asymmetrical ripples in the flood sediment covering the footpath by the

River Ouse, York. The direction of current is from top right to bottom left. 

The End