8: Sedimentary Rocks & Processes

Question 1

How do sedimentary rocks form?

Answer 1

Deposition: laying down of material by a natural agent (air, water, ice, gravity) or by precipitation of solution
- Often occurs in sedimentary basins

Lithification:
compaction + Cementification = lithification
- Compaction: The process by which pressure reduced the volume of sediments due to a reduction in void space
- The process by which clastic sediments are converted to rock by the precipitation of mineral cement (calcite, quartz, iron oxides) between grains)

Question 2

What are the three main categories of sedimentary rocks?

Answer 2

1. Clastic (detrital)
   - starts as clasts, aka fragments or particles
   - sandstone, siltstone, shale, mudstone…
2. Chemical (evaporites)
   - Formed by precipitation from solution
   - halite, gypsum, some limestones
3. Organic (biogenic)
   - Formed from previously living organisms
   - Chalk, coal, and most limestones

Question 3

Describe Clastic (detrital) sedimentary rock

Answer 3

Composed of fragments of weathered and eroded rocks, grains, and particles

The longer the rocks travel:
- the finer the grains
- the more rounded and less angular due to abrasion from transport

poorly sorted = well graded, you start with this

well sorted = poorly graded
you get this after a long transport

Question 4

For Clastic Sedimentary Rocks, what are the 4 groups? Give examples for the major 2:

Answer 4

Mudrock
- mudstone: 75% silt and clay, not bedded
- Shale: 75% silt and clay, thinly bedded
- siltstone: More silt than clay

Sandstone
- Quartz sandstone: dominated by sand, >90% quartz
- Arkose: dominated by sand, >10% feldspar
- Lithic wacke: dominated by sand, >10% rock fragments, >15% silt and clay

Conglomerate
- pebbles to boulders, rounded clasts

Breccia
- Pebbles to boulders, angular clasts

Question 5

Give me some characteristics of Shale, Mudstone, Sandstone, Conglomerate, and breccia:

Answer 5

Shale: splits easily along bedding and lamination

Mudstone: does not break in layers like shale

Sandstone: Dominated by sand sized grains that are cemented together

Conglomerate: rounded grains >2mm in a finer-grained matrix

(only diff is conglomerate is like rounded chocolate chips while Breccia is like chocolate flakes in mint ice cream)

Breccia: Angular grains >2mm in a finer-grain matrix

Question 6

Describe Chemical (evaporites) sedimentary rocks

Answer 6

Precipitation of minerals from solution
- Often requires restricted basin in warm arid climates

Question 7

What are some common evaporites?

Answer 7

Halite: NaCl (rock salt)

Gypsum (CaSO_4 2H_2O)
- used in “Gyprock” drywall

Speleothems: CaCO3
- precipitated in caves
Tufa: CaCO3
- precipitated from cold springs
Travertine: CaCO3
- Precipitated from hot springs

Question 8

Describe Organic (biogenic) sedimentary rocks. Name Some:

Answer 8

They are the remains of living things.

limestone
chalk: Another calcium carbonate from plankton
Coal: Formed from the remains of vegetation, composed mainly of carbon (fossil fuels)

Question 9

What is coalification?

Answer 9

Conversion of peat (organic soil) to sedimentary rock under pressure and temperature

Question 10

What are fossils in sedimentary rock? What do they do?

Answer 10

Fossils are the preserved traces of animals, plants, and other organisms,

They help us identify and correlate sedimentary rocks at different sites

Also help us date rocks

Question 11

What is sedimentary structures? Name all the layers:

Answer 11

Sedimentary structure is a feature formed during deposition that gives clues to the environment and the time and place:

TOP
- Planar bedding
- current bedding showing cross lamination
- ripple marked bedding
- imbricate (overlapping) fossil shells)
- Graded bedding
- Cut-and-fill bedding
BOTTOM

Question 12

What is Bedding?

Answer 12

A layer of sediment or sedimentary rock with a recognizable top and bottom called a bed

The boundary between 2 beds is a bedding plane

A series of beds that is distinct from beds above and below (and thick enough to be shown on a map) is called a formation

Question 13

What can ripples in sedimentary structures tell us?

Answer 13

There are symmetric and asymmetric ripples:

symmetric: still water like beach

Assymetric: a direction was prefered, like ripples in a river bed or wind in a desert

Question 14

What are facies?

Answer 14

They are a distinct rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment

Here, vertical and lateral differences reflect changes in conditions over time and space