Sediments

Question 1

What is a sediment?

Answer 1

• collection of loose earth materials (rocks, minerals, soil, fossils
• Most of Earth’s surface is covered with at least some sediment

Question 2

Where do we find sediment-free surfaces?

Answer 2

Where sediments originate i.e mountains, basically very high points

Question 3

How do sediments form?

Answer 3

1. When older, bigger rocks break into smaller rocks
2. When minerals in rocks react with air/water and form new minerals (often occur as residue on rock i.e rust)
3. When minerals in rock dissolve in water and re-precipitate elsewhere (water dissolves some minerals, then transported in water and re-precipitate elsewhere)
   - Precipitate is often triggered by a change in conditions.
   - Minerals that precipitate directly from water are considered sedimentary minerals
   - If minerals are loose, they are considered a sediment.
   - If bound together they are considered a rock.
4. Minerals in rocks dissolve in water and animal extract the ions to build their shells, a pile of shells is sediment

Question 4

Often a combo of processes occur to

Answer 4

produce a sediment

Question 5

Dissolved minerals are suspended in water and may

Answer 5

precipitate out or be used to build shells

Question 6

Weathering:

Answer 6

describes processes that create sediments

Question 7

Physical weathering:

Answer 7

breaking big rocks into smaller rocks, i.e gravity or hammer

Question 8

Chemical weathering:

Answer 8

alters rocks and minerals at or near Earth’s surface as they react with air and water. This produces new minerals and dissolved elements and compounds.

Question 9

How many types of sediments are there?

Answer 9

• Two common types based on texture: chemical (crystalline, minerals precipitate out of solution) and clastic (broken pieces of earth materials)
• Each group can be further subdivided based on composition

Question 10

Compositions of chemical sediments:

Answer 10

• Calcite crystals, microcrystals or spheres
• Dolomite microcrystals
• Halite crystals
• Gypsum crystals
• anhydrite
• Iron-bearing minerals crystals or residues
• Microcrystalline quartz
• Can form as new minerals inside carbonate sediments. Others form from seafloor ooze
• Some chemical sediments are biogenic, produced through biological activity i.e shells

Question 11

Two types of clastic sediments based on composition:

Answer 11

• Siliciclastic (consists of quartz feldspar grains, rock fragments (lithics) and clay)
• Bioclastic (consists of fragments of transported skeletal material i.e shells)

Question 12

Where does weathering happen?

Answer 12

Sediment is created at source area through weathering, source: exposed rocks from which sediments form

Question 13

What happens to sediments after they form?

Answer 13

• Once sediment has formed through weathering, it can be picked up and transported through erosion
• Agents of erosion include: water, air, gravity, and ice
• Only solid clasts move under the force of gravity. Gravity has the power to move large and small clasts at the same time.
• Flowing air, water, or ice can pick up and move sediment. More viscous a fluid is, the more power (carrying capacity) it has to move a larger range of clast sizes

Question 14

In water and air, clasts will move through

Answer 14

rolling, saltation, or traction

Question 15

How do solid clasts change as they are transported further away from the source?

Answer 15

• Decrease in grain size (clasts become smaller)
• Rounding (sharp corners get knocked off and smoothed out)
• Sorting (increasing similarity in grain size), result of energy of the transporting medium and density of transported clasts: what is heaviest clast that gravity, ice, water, and air can move? -> depends on energy
• Textural maturity: Immature = poorly sorted angular, moderately mature = somewhat sorted, mature = well sorted and well rounded

Question 16

Mineralogical maturity:

Answer 16

measures the extent to which unstable grains have been destroyed and removed by weathering i.e first stuff to precipitate out of magma melt so olivine, pyroxene, hornblende, feldspars = most unstable, most easily weathered away. Quartz is most resistant to weathering.

Question 17

Clay and rust are a by-product of

Answer 17

weathering. Sediments rich in clay and mineralogically immature.

Question 18

How are sediments deposited?

Answer 18

Chemical sediment deposition and Clastic Sediment Deposition

Question 19

Chemical sediment deposition

Answer 19

1. Minerals form in situ from a chemical rxn b/w air or water and a rock
2. Minerals precipitate out of solution when conditions change
3. Ions are extracted from water to build shells or skeletons

Question 20

Clastic Sediment Deposition

Answer 20

• Solid clasts are deposited when agent of erosion loses the power to trans grains (decrease in carrying capacity) due to: drop in velocity, drop in viscosity (when ice melts)
  
  • Water and air deposit sediment when velocity drops below the critical threshold for give grain size
  • Hjulström diagram: predicts the critical velocities at which a given grain size will be picked up, moved, and deposited by flowing water

Question 21

Why is clay so hard to erode?

Answer 21

Some clays are platy and layered. Inter-particle forces hold plates together

Question 22

Where are sediments deposited?

Answer 22

• Many environments where sediment can be deposited
• Grouped into 3 main depositional environments: continental, coastal, marine.
• Because of gravity, sediment tends to accumulate in low spots
• Marine depositional environments include deposits on the continental shelf (flooded margins of continents) to the abyssal plain (deep sea).