Sedimentary Processes and Rocks

Question 1

How do sedimentary rocks form?

Answer 1

At/near the Earth’s surface as a result of:
Weathering of pre-existing rocks -> specific minerals, ions
Erosion & transportation of sediment -> rounded clasts
Deposition -> layers (beds) of sediment reflect different depositional events and environments
Burial
Compaction & cementation turn sediment into rock

Question 2

What minerals occur in sedimentary rocks?

Answer 2

Minerals that survive weathering (residual) e.g. quartz
Minerals produced in weathering reactions (secondary) e.g. clay minerals, iron oxides
Minerals that precipitate from dissolved products of weathering e.g. calcite, dolomite, gypsum and halite

Question 3

Where do sediments come from?

Answer 3

Plate Tectonics produces: Uplift

• > topographic relief for erosion & transport
• > rock sources of sediment

Question 4

Where do sediments collect?

Answer 4

Plate Tectonics produces: Subsidence

-> depressions, basins for deposition

Question 5

how are sediments transported?

Answer 5

Rivers, streams
Mass wasting
Glaciers
Wind
Ocean currents

Question 6

what are the different types of sedimentary rock?

Answer 6

Clastic
Chemical
Organic

Question 7

what are clastic rocks formed from?

Answer 7

formed from fragments of rocks or minerals

e.g. mudstone, sandstone, conglomerate

Question 8

what are chemical rocks formed from?

Answer 8

formed when dissolved ions carried in solution (usually seawater) are precipitated

e.g. limestone, chert

Question 9

what are organic rocks formed from?

Answer 9

formed from the dead remains of plants or animals

e.g. coal

Question 10

what are the steps to forming clastic sedimentary rocks?

Answer 10

1) Weathering and erosion -> sedimentary detritus (clasts = grains)
2) Transportation of sediment grains
3) Deposition of sediments
4) Lithification: unconsolidated sediment -> rock

Question 11

how are terrigenous sediments transported?

Answer 11

by rivers to continental margins

Question 12

what are turbidity currents? what do they do?

Answer 12

underwater mass flows

transport sediment down continental slope to floor of ocean basin

Question 13

Grain sizes deposited vary with…

Answer 13

velocity/energy and become sorted

larger grains are deposited first

Question 14

which clasts would be more mature?

Answer 14

more rounded ones (as opposed to more angular ones)

Question 15

what is lithification?

Answer 15

loose sediments -> rock

Question 16

what are the stages of lithification?

Answer 16

(1) Compaction:
As the pile of sediment grows, water is squeezed out from sediment at the base of the pile -> grains forced closer together (compacted)
(2) Cementation:
Fluids in the pile precipitate minerals -> forms a cement - binds the grains together

Question 17

what is cementation?

Answer 17

minerals (carbonates/silica/clays) precipitate and fill pore space between the clasts (grains)

Question 18

what are the steps to forming chemical sedimentary rocks?

Answer 18

1) Chemical weathering -> ions in solution
2) Transportation of dissolved ions
3) Precipitation of minerals from ions in solution (usually seawater) – due to changing physical conditions or due to actions of living organisms (biogenic)
4) Lithification

Chemical sedimentary rocks: classified as non-biogenic or biogenic

Question 19

what are non-biogenic chemical sedimentary rocks?

Answer 19

(no organisms involved)

Travertine (precipitation of calcite)
Tufa (calcite)
Evaporites (halite; gypsum)

Question 20

what are biogenic chemical sedimentary rocks?

Answer 20

Corals, colonial invertebrates, precipitate CaCO3 from sea water to build their skeletons/shells
Accumulation of dead corals and other fragments, compaction, cementation -> limestone

Question 21

where do most coral reefs form? why?

Answer 21

Most coral reefs form in the “photic” zone since they need light
They are restricted to warm, shallow sea

Question 22

how do atolls form?

Answer 22

Reef forms at edges of newly formed volcano
As volcano subsides, reef grows upward
As volcano continues to subside, reef continues to grow upward, forming an atoll

Question 23

what are deep-sea biogenics?

Answer 23

slow accumulation of silica (quartz=SiO2) skeletons of micro-organisms: diatoms, dinoflagellates, and radiolaria
e.g. siliceous ooze -> chert
Chert can also be made of glass sponges (silica skeletons)

Question 24

what is an example of an organic sedimentary rock?

Answer 24

Carbon-rich rock composed of plant and animal remains: coal.
Buried, decaying organic matter
Maturity increases with depth/ duration of burial
Depositional environment - Swamp

Question 25

Why are sedimentary rocks important?

Answer 25

Resources
–Host rocks for oil, gas, water
–Source of potash, phosphate, iron, aluminium, salt…
–Building stone, ceramic clays, aggregates
Provide a record of past conditions & processes on the Earth’s surface
They reflect energy of environment and type of depositional environment

Question 26

rocks record changes in sea level such as…

Answer 26

Transgression (sea level rises)
Regression (sea level falls)
Uplift/Erosion

Question 27

how are sediments deposited?

Answer 27

in layers (beds or strata)

Provide a record of Earth processes over geological time: climate, biologic evolution, environment, plate movement

Question 28

Sedimentary rocks preserve fossils and thus…

Answer 28

a record of prehistoric life & conditions