Sedimentary Structures (F2)

Question 1

What is bedding?

Answer 1

• Layered structure

• Separated from layer above and below by bedding plane

•A bed = a single unbroken episode of sediment accumulation

•May be same thicknesses over large area ,or pinch out laterally
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Question 2

What are the Competent beds and Incompetent beds?

Answer 2

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

Incompetent:
• 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 = strong, incompetent = weak)

Question 3

What is graded bedding?

Answer 3

Bed which displays a fining upwards sequence from the base.

Most graded beds are coarsest at the bottom and finest at the top.
………….
,,,,,,,,,,,,,
•••••••••

Question 4

What are ripple marks?

Answer 4

Ancient ripples that form on bedding planes

• Indicate beach, shallow sea, or desert dune environment
• Can indicate palaeocurrent orientation or direction
• Can be symmetrical or asymmetrical

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Question 5

What is Cross Bedding?

Answer 5

• Usually seen in sandstones and limestones
• Forms within beds, not along bedding planes like ripples do
• AKA Current Bedding and False Bedding
• Very large scale = Dune Bedding
• Very small scale = Cross Lamination
• Sediment is moved and accumulated at an angle to the principal bedding direction
• Produced by a uni-directional current of wind or water
• Sediment is moved as a series of asymmetrical ripples or dunes downstream

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—— ——-
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Question 6

What is convolute bedding?

Answer 6

Sediment is saturated with water and easily mobilised
Muds (now shales) deform internally and flow (incompetence)
Sands (now sandstone) layers break into rigid blocks which become displaced (competence)

c~zzcxzcz (squiggles)
~~xvxvz

Question 7

What are Included/Derived fragments?

Answer 7

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

Question 8

What are Imbricate structures?

Answer 8

Clasts deposited by powerful current
Lead in direction of flow

Question 9

What are flame and load clasts?

Answer 9

They are lithified (turned to rock) later which preserves the structure
• The sand is deposited on top of the mud
• The denser sand sinks down into the less dense mud below as bulbous protrusions (the mud is loaded by the sand)

🔥

Question 10

What is Life Position of Fossils?

Answer 10

Organisms preserved in life position such as trees can indicate if the beds are the ‘right way up’

ie roots facing down

Question 11

What are tool marks?

Answer 11

Tool marks are produced as “tools” (such as sticks, shells, bones, or pebbles) carried by a current bounce, skip, roll, or drag along the sediment surface.
They are commonly preserved on the lower surfaces of sandstone beds as thin ridges.

Tool marks are generally aligned parallel to the direction of current movement.

Question 12

What are flute clasts?

Answer 12

Produced by erosion or scouring of muddy sediment, forming “scoop- shaped” depressions.

«««
««&laquo_space;
(means current direction goes ->)

Question 13

What are Geopetal structures?

Answer 13

Partially infilled shells of marine organisms

Question 14

What are trace burrows?

Answer 14

Organisms such as bivalves and marine worms burrow from the surface downwards