A2 Sedimentary Processes

Question 1

Turbidity current

Answer 1

high-velocity current that flows down gentle gradients because the sediment dispersed within it makes it denser than sea water. These currents are triggered by earthquakes or slope instability.

Question 2

Turbidites

Answer 2

an upward fining deposit of greywacke deposited from a turbidity current.

Question 3

Bouma sequence

Answer 3

an idealised sequence of sediments and sedimentary structures seen in a turbidite deposit.

Question 4

Climbing ripples

Answer 4

a series of cross-laminae formed by superimposing migrating ripples. They form when deposition exceeds the rate of migration of ripples.

Question 5

Tool marks

Answer 5

impressions made on the surface of soft sediment by the dragging or bouncing of an object (tool) in a current.

Question 6

Rip-up clasts

Answer 6

pieces of shale or mudstone eroded by a current containing suspended sediment. They are preserved when the current deposits its sediment.

Question 7

Siliceous ooze

Answer 7

pelagic clay containing >30% biogenic skeletal material made of silica.

Question 8

Plankton

Answer 8

minute organisms living in the surface layers of the ocean, that are transported by currents.

Question 9

Diatoms

Answer 9

planktonic algae that secrete siliceous material.

Question 10

Distributary

Answer 10

a stream channel that takes water away from the main stream channel

Question 11

Cyclothems

Answer 11

represent layers repeated due to cyclic sedimentation.

Question 12

Topsets

Answer 12

the uppermost horizontal layers of a delta commonly consisting of channel sandstones, coal and seat earth.

Question 13

Seat earth

Answer 13

a sandy or clay-rich fossil soil found beneath a coal seam. It represents the soil in which coal-forming plants grew and frequently contains carbonised traces of plant roots.

Question 14

Foresets

Answer 14

the inclined layers formed on the delta front, commonly consisting of cross- bedded sandstones.

Question 15

Mouth bars

Answer 15

crescent-shaped deposits of sand and silt forming below sea level, where distributaries enter the sea.

Question 16

Bottomsets

Answer 16

the lowest horizontal layers of a delta, commonly consisting of shales.

Question 17

Bioturbation

Answer 17

the disturbance of sediment by the activities of organisms (e.g. burrowing).

Question 18

Banded iron formations

Answer 18

units of sedimentary rock of Precambrian age consisting of alternating layers of iron oxides and chert.

Question 19

Archean

Answer 19

an era within the Precambrian, covering a period from 4.0 billion to 2.5 billion years ago.

Question 20

Palaeoproterozoic

Answer 20

an era within the Precambrian, covering the period from approximately 2.5 billion to 1.6 billion years ago.

Question 21

Great Oxygenation Event (GOE)

Answer 21

the biologically induced appearance of dioxygen (02) in Earth’s atmosphere. It occurred around 2.3 billion years ago.

Question 22

Photoferrotrophs

Answer 22

photosynthetic bacteria that oxidise ferrous iron Fe2+ into ferric iron Fe3+ using sunlight to create iron oxides and hydroxides instead of molecular oxygen.

Question 23

Pycnocline

Answer 23

the boundary separating two liquid layers of different densities in the oceans. It forms a lower limit to turbulence caused by mixing processes at the surface.