Sediments, Sedimentary Rocks and the Geological Record

Question 1

What are the three materials sediment % is measured on?

Answer 1

Gravel, mud and sand.

Question 2

What is the meaning of a Gaussian curve?

Answer 2

Bell shaped.

Question 3

What does a large standard deviation mean in terms of soil sorting and size?

Answer 3

• Many different sizes, meaning not very well sorted. Probably immature.

Question 4

Describe the amount of sorting occurring down down the rock cycle.

Answer 4

Top of river/lake: Increasing sorting, ongoing chemical weathering
Middle of river: Slower flow, coarser materials left behind. Only silt/clay left.
Mouth: Load deposition, well sorted. Flocculation removes clay.

Question 5

What are the five stages of sediment maturity?

Answer 5

Immature, submature, mature, supermature. Maturity measures grain size (mature = more even) and grain shape (mature = more round)

Question 6

How are sedimentary rocks arranged?

Answer 6

• Layered or stratified in planar horizontal beds
• Sequence of beds is called bedding and stratification

Question 7

What is the boundary between two sedimentary beds called?

Answer 7

Bedding plane.

Question 8

What are strata?

Answer 8

Several sedimentary beds together.

Question 9

Why does bedding form?

Answer 9

• Reflects changing depositional conditions
• Changing transporting speed, velocity, volume
• Changing sediment source: Sediment composition, grain size, sorting

Question 10

What are clastic sediments?

Answer 10

• Mineral grains are lithified (the process of transforming loose sediment into solid rock)
• Compacted through burial and cemented (Precipitate gluing sediments together) , often by silica or calcite

Question 11

What are biochemical sediments?

Answer 11

• Skeletal parts (opal)
• Calcite and aragonite carbonate

Question 12

What are chemical sediments?

Answer 12

• Non biogenic precipitates such as evaporites like gypsum

Question 13

How is dolomite formed?

Answer 13

• Interaction of calcite with Mg-bearing groundwater

Question 14

What does clast mean?

Answer 14

Grain.

Question 15

What do asymmetric ripples show about the flow directions?

Answer 15

• Used to indicate current direction in ancient sediments

Question 16

What are cross beds created by?

Answer 16

1. Sediment moves up gentle side of dune
2. Piles up, slips down steep face
3. Slip face continuously moves down current
4. Added sediment forms cross beds

Question 17

What is glacial till?

Answer 17

Sediment deposited by a glacier.

Question 18

What is a breccia?

Answer 18

A rock made of angular rock fragments cemented together. Deposited close to clast source.

Question 19

What is an arkose?

Answer 19

Sand and gravel with abundant feldspar (indicates short transport and arid weather)
- Common in alluvial fans

Question 20

What is conglomerate sediment?

Answer 20

• Rounded rock clasts
• Rounded from flowing water and deposited rather than breccia

Question 21

What is a delta?

Answer 21

Where sediment accumulates where a river enters the sea. Creates many sub environments present.

Question 22

What forms down slope turbidity currents?

Answer 22

1. Sediment moves on a slop as turbid water
2. As it slows, grains settle
3. Coarsest material settles first, then medium and fine.

Question 23

What is chert?

Answer 23

Rocks made of cryptocrystalline quartz.
- After burial opaline silica in bottom sediment dissolves
- Silica in pores forms a gel and precipitates chert

Question 24

What algae is calcite made of?

Answer 24

Coccolithophorids.

Question 25

Name the replacement cherts (other materials silica replaces).

Answer 25

Flint
Jasper
Petrified wood
Agate

Question 26

Describe the formation of Travertine.

Answer 26

• CaCO₃ precipitated from ground water
• Dissolved calcium reacts with bicarbonate
• Co2 expelled into air causes CaCO₃ to precipitate

Question 27

How do rift basins form?

Answer 27

• Divergent plate boundaries
• Crust thins, stretches and subsides
• Sediment fills down dropped basin

Question 28

What are foreland basins?

Answer 28

• Craton side of a collision mountain belt. The flexure of the crust from loading creates a downwarp.
• Fills with eroded mountain debris

Question 29

What is a relative age?

Answer 29

• Based on order of formation
• Qualitative
• Determines older vs younger relationships

Question 30

What is numerical age?

Answer 30

Actual number of years since an event.

Question 31

What is uniformitarianism?

Answer 31

The theory that the processes observed today were the same in the past.

Question 32

What is the principle of original horizontality?

Answer 32

Sediments accumulate horizontally so tilted sediment rocks must be deformed.

Question 33

What are formations in stratigraphic columns called?

Answer 33

Mapable rock units.

Question 34

What are the two types of stratigraphic correlation?

Answer 34

Lithologic: Based on rock type
Fossil: Based o fossils within the rocks

Question 35

When and what was the Phanerozoic?

Answer 35

• 542 Mya to present
• Visible life. Starts at Pre Cambrian - Cambrian boundary.
• First hard shells

Question 36

When and what was the Proterozoic?

Answer 36

• 2.5 to 0.542 Ga
• Development of tectonic plates and atmospheric O²
• Multicellular life

Question 37

When and what was the Archean?

Answer 37

• 3.8 to 2.5 Ga
• Birth of continents and initiation of plate tectonics

Question 38

When and what was the Hadean?

Answer 38

• 4.6 to 3.8 Ga
• Internal differentiation
• Formation of the oceans and secondary atmosphere

Question 39

What is numerical dating called? What is it based on?

Answer 39

Geochronology. Based on radioactive decay using U-238.