Sedimentology and Palaeontology Lecture 18: Sedimentary Materials and Processes

Question 1

Describe a grain flow

Answer 1

A grain flow is a type of sediment-gravity flow in which the fluid can be either air or water, acts only as a lubricant

• Transient - lasting only for a short time; impermanent.
• Well-sorted fine-medium sand
• Mutual grain collisions keep them temporarily suspended in the flow
• Deposition instantaneous
• Reverse-graded laminae due to kinetic seiving

Question 2

Describe a debris flow

Answer 2

Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock flow downward.

Subaqueous or subaerial

• Sediment > water
• Mud to boulder grain size
• Dense and viscous
• Laminar flow = no sorting
• Instantaneous deposition of parabreccia/ paraconglomerate

Question 3

Where do submarine debris flows take place

Answer 3

• Steep slopes offshore from rivers
• Near volcanic islands
• Tectonically active areas

Question 4

Define and describe a turbidity current

Answer 4

underwater current of usually rapidly moving, sediment-laden water moving down a slope

• Subaqueous (Takes place underwater)
• Turbulent
• Sediment deposited from bed load as flow decelerates

Question 5

How far, fast and how long do turbidity currents flow?

Answer 5

Deep-sea turbidity current events can be sustained and can last from several hours to 6 days

Travel on the seafloor for tens to hundreds of km at speeds like 10 to 30 km/h before stopping.

Question 6

Describe the anatomy/structure of turbidity currents

Answer 6

The main body is a slab like structure (similar to a Molluscs muscular foot) that sits on the seabed. In this slab there is head at the front, the middle is the body and the end is the tail.

Above the main body is a ‘cloud’ of entrained water/mixing water.

The flow erodes the seabed as well as depositing. In the concentrated part of the flow there is deposition of thick graded sand and silt onto the mud bed.

The less concentrated part of the flow deposits thin graded silt.

Question 7

Define a turbidite

Answer 7

A type of sedimentary rock composed of layered particles that grade upward from coarser to finer sizes.

Are thought to have originated from ancient turbidity currents in the oceans.

Question 8

Define a Desication crack

Answer 8

Desiccation cracks are cracks in mud or soil when it is particularly dry.

The development of cracks at the soil surface and throughout the depth of a cohesive soil layer as a result of moisture content loss.

Polygonal and taper downwards

Caused by subaerial exposure of mud

Question 9

Define Syneresis cracks

Answer 9

Syneresis is the expulsion of a liquid from a gel-like substance. Syneresis cracks are formed by the contraction of clay in response to changes in the salinity of a liquid surrounding a deposit.

Lenticular - shape of double convex lens

Also looks like it has worm like lines in various directions.

• = subaqueous salinity fluctuations

Question 10

Describe load casts and flame structures

Answer 10

Load casts are bulbous depressions formed on the base of a bed of sediment. Load casts develop by the differential sinking of the sediment, while still soft, into less dense sediment below.

Flame structures are found on the less dense sediment below, the weight of an overlying bed forces an underlying bed to push up through the overlying bed,