Sedimentary Structures

Sedimentary Structures I.G.Kenyon

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

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

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

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 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

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

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 Fining upwards Irregular surface with laminations of shale beneath truncated in places Represents an abrupt change from the much finer grained sediment underneath

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

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

Dune Bedding – Large Scale Cross Bedding 2m

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

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

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

Included/Derived Fragments Younger upper series Older 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 Derived fragments from older lower series unconformity 1m Younger upper series Derived fragments Lower older series Lower older series

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 Note how the edges curl up to accentuate the V shaped gap between them 30cm

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

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 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

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.

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Sedimentary Structures