Sedimentary Rocks - OCR A-Level, Clastic sedimentary rocks, describing sediment, and classifying sedimentary rocks.

3 ways sedimentary rocks may form

Physically, Biologically, Chemically

How are sedimentary rocks typically formed?

Grains, eroded from previous rocks, are compacted/cemented together (lithification).

2 informal groups of sedimentary rocks

Clastic and carbonates

What is soil made of?

Rock fragments, minerals, water, mud and organic matter

Weathering

The in situ breakdown of rocks into pieces or solution.

Exfoliation

Minerals expand and contract as they heat and cool, causing the rock to break in onion-peel like layers.

Frost shattering (freeze-thaw)

Water gets into cracks, freezes and expands, causing the crack to expand, this repeats until the rock breaks.

Pressure release (unloading joints)

Vertical pressure causes breakage in horizontal lines.

Carbonation

Carbonic acid in rain reacts with the minerals in the rock, weathering it.

Hydrolysis

The reaction between water and feldspars breaks them down resulting in clay minerals.

Biological weathering

Plant roots force rocks to break over time, animals burrow into the ground and rocks break.

Name the products of weathering

Rocks and grains, solution, clay minerals

Describe clay minerals (mud)

Very fine grain size, platy structure, charge makes them sticky.

Mass Wasting

Movement due only to gravity

Give 3 examples of mass wasting

Any 3 from: rockfalls, rockslides, slumping, debris flows or rotational slips

Fluid Transport

Transport by water or wind

Solution

Transport of particles dissolved in the water

Suspension

Transport of particles that are suspended in the water or the air.

Saltation

Transport of particles bouncing along the ground (river bed).

Where is saltation most commonly seen?

Wind transport

Scale used for describing grain size

Udden Wentworth Scale

Boundary between gravel and sand

2mm

Boundary between sand and silt

0\.0625mm

Boundary between silt and clay

0\.039mm

3 ways to measure grain size

Callipers (for large grains)

Grain size comparator with a hand lens

Sieving

What is sorting

The range of grain sizes in a sample

2 ways to determine sorting

A comparator chart

Sieving

Sorting ranges from ? to ?

Very well sorted to very poorly sorted

Roundness ranges from ? to ? The middle two options are ? and ?

Very angular to well rounded

Sub-angular and sub-rounded

What is the difference between roundness and sphericity?

Roundness is how angular or not angular a grain is, sphericity is how close in shape it is to a sphere.

A factor of which number is used in the phi scale?

2

Which formula is used to convert mm to phi?

Ø=-log2(D/D^0) where D is the size in mm

What is the formula to convert phi into mm?

D=2^ø where D is the size in mm

What equipment is needed for sieving?

Sieve stack, weighing boats, a balance (2 d.p.) and a small brush

What is the cone and quartering method? Why is it used?

Pour sediment out to form a cone, then split the sediment cone into quarters.

To get a well-mixed and representative sample.

How do you normalise data from sieving?

Percentage on sieve=\[Amount on sieve(g)/Total of all sieves (g)\] x 100

What must your results table from sieving include?

The sieve size in both mm and phi, the raw data and the normalised data

How do you plot data from sieving?

As a histogram and then as a cumulative frequency graph

Which rocks are Rudaceous?

Conglomerate and Breccia

Which rocks are Arenaceous?

Orthoquartzite, Arkose and Greywacke

Which rocks are Argillaceous?

Siltstone and Mudstone

Orthoquartzite is over ?% quartz?

90

Difference between a conglomerate and a breccia?

Breccia’s have angular grains whilst conglomerates have rounded grains

Key feature of Arkose?

Over 25% feldspar

Key feature of greywacke?

Over 15% clay matrix

What is the boundary between siltstone and mudstone?

0\.039mm

Conglomerates and Breccia’s have a grain size of?

Over 2mm

Sandstones (Arenaceous rocks) have a grain size of?

2mm to 0.0625mm

Detrital

“Those that are deposited” (grains)

Authigenic

Cements

How are Authigenic mineral cements formed?

They are precipitated from pore water and form crystals.

Name 4 common Authigenic cements

Quartz cement, calcite cement, iron oxide cement, clay-minerals