Week 3: Sedimentary rocks

Question 1

State the 2 processes where loose sediment becomes sedimentary rocks?

Answer 1

Diagenesis is chemical, physical and biological changes that take place after sediments are deposited. Including recrystallisation (developing more stable minerals from less stable ones) and lithification.

Lithification is two main steps. Compaction and cementation. Where ions precipitate in pore spaces and glue the minerals together.

Question 2

Describe how loose sediment becomes a sedimentary rock.

Answer 2

Mechanical & chemical weathering forms sediment from solid minerals and dissolved ions.

Question 3

What are the two big categories of sedimentary rocks? How do they differ.

Answer 3

Clastic sediment - Mineral grains weathered from pre-existing rock and cemented together by precipitated minerals. (e.g. quartz and clay).
Chemical - Composed of minerals/ions precipitated from water by biologic or inorganic processes. Or the
remains of organisms. (e.g. of an ionic dissolved sediment: halite, calcite.)

Question 4

Name and describe the classification of clastic sedimentary rocks on the basis of texture and composition.

Answer 4

GRAVEL:
- Breccia: Angular fragments.
- Conglomerate: Rounded fragments.

SAND:
- Sandstone: Varying degrees of quartz and feldspar grains.

SILT/(MUD)/CLAY:
- Siltstone, mudstone, claystone: Blocky
- Shale: Sheets.

Question 5

State the different grain sizes for clastic sedimentary rocks.

Answer 5

The average diameter of
grain sizes range from very coarse to very fine.
Gravel - Boulder, cobble, pebble & pea.
Sand - Course, medium & fine.
Silt - Course, medium & fine.
Clay - Course & fine.

Question 6

Say how to distinguish between these clastic sedimentary rocks (c.s.r.)
- Course c.s.r
- Fine c.s.r
- Sandstone c.s.r

Answer 6

Course c.s.r. - Gravel sized clasts. [Breccia, Conglomerate and Diamictite]

Fine c.s.r. - Silt & clay. [Siltstone, mudstone, shale and claystone]

Sandstone c.s.r. - Sand sized particles. [Quartz arenite, arkose, lithic sandstone and wacke]

Question 7

What is the importance of the grain or clast composition?

Question 8

What is the importance of the clastic texture and how it can be used to
interpret the deposition environment?

Answer 8

Angularity - To which degree grains have smooth/angular edges.
Sphericity - To which degree clasts are equidimensional.
Sorting - The uniformity of the grain sizes.

Question 9

What is the difference between chemical and biochemical sedimentary rocks.

Answer 9

Chemical rocks are formed by Inorganic processes, minerals precipitating from water.
Biochemical rocks are formed with organic processes. The involvement of living organisms.

Question 10

Name and describe the classification of the following chemical sedimentary rocks on the basis of their composition. How do you distinguish them from one another?

• Coral reefs, coquina and chalk.
• Travertine.
• Oolitic limestone.
• Dolostone.
• Chert varieties.
• Biochemical chert.
• Evaporites.

Answer 10

CHEMICAL
Evaporites are from evaporated seawater. (NaCl)
- Gypsum (80% of H20 evaporates)
- Halite (90% of H20 evaporates)

Travertine (CaCO3 precipitates)

Dolostone (CaMg(CO3)2 ; Mg rich limestone)

Oolitic limestone (CaCO3 particles)

Replacement chert are non biogenic
- Flint (Black/gray from organic matter.)
- Jasper (Red/yellow from Fe oxides.)
- Petrified wood (Wood grain preserved by silica.)
- Agate (Concentrically layered rings.)

BIOCHEMICAL
Limestone formed from skeletons
- Coquina
- Chalk (Plankton shells)

Chert (From Si rich shells)

ORGANIC
Coal ( more than 50% C)

Question 11

What are the different stages of coal formation?

Answer 11

Peat –> Lignite –> Bituminous coal –> Anthracite

Question 12

How are clastic and chemical sedimentary rocks primarily named and grouped?

Answer 12

Clastic - Based on texture.
Chemical - Based on composition.

Question 13

How do we get different sized grains? And rough or smooth and rounded grains?

Answer 13

The more it is transported the smaller the grain size. If the grains are very rounded they have been transported in water.

Question 14

Mineral identity is important.
Stable end-products of weathering include Quartz, Fe-oxides and clays. So Unstable minerals signify special
conditions. (e.g. Feldspars.)

Answer 14

Cool.

Question 15

What does sorting indicate?

Answer 15

Sorting indicates the constancy of environmental energy.
Well sorted - uniform energy.
Poorly sorted - Variable energy.

Question 16

Sedimentary rocks are normally layered. (stratified)

Answer 16

The oldest is at the bottom.