1.3 Sedimentary Rocks And Their Fossil Content

Question 1

What does grain size, shape and sorting tell us?

Answer 1

Grain size - get smaller the further transported by attrition and abrasion
Grain shape - more rounded the further transported due to measured amounts of abrasion and attrition - angular, subangular/sub rounded, rounded
Grain sorting - rate of deposition. Poorly sorted grains in a rock means quickly deposited - if well sorted then deposited slowly

Question 2

Summarise grain size, shape and sorting of sedimentary rocks?

Answer 2

Mountain = angular, coarse grained, poorly sorted
Sea = rounded, fine grained, well sorted

Question 3

How to find texture of sedimentary rock?

Answer 3

Size - range of numbers and include units: large grains have undergone less transport and erosion
Shape - very angular -> Angular(little T and E) -> sub angular -> sub rounded -> rounded -> well rounded (lots of T and E)
Sorting - poorly sorted(little T and E) -> moderately sorted -> well sorted (lots of T and E)

Question 4

Explain porosity, permeability and impermeability?

Answer 4

Porosity = pores are gaps between grains - porosity is % of pores in a rock eg sandstone is very porous
Permeability = if pores are connected fluids flow through eg limestone
Impermeable = no water passes through a rock eg clay

Question 5

What are the diagnostic properties of breccia?

Answer 5

Size - 2mm - coarse grained
Shape - sub angular/angular 
Sorting - poorly sorted
Little transport and erosion, deposited quickly eg flash floods and scree slopes
Minerals -

Question 6

What are the diagnostic properties of limestone?

Answer 6

Size - 1-1.5mm
Shape - subrounded/ rounded
Sorting - well sorted
Grains deposited slowly so well sorted, with much transport and erosion as they are small and rounded
Minerals - calcite reacts with acid to give off carbon dioxide, colourless with white tints

Question 7

What are the diagnostic properties of sandstone?

Answer 7

Size - 1mm medium grained
Shape- subrounded to rounded
Sorting - well sorted
Large transport, erosion and slow deposit eg by wind, ice, water
Minerals - quartz (glassy), mica, feldspar, muscovite mica (silvery flakes)

Question 8

What are the diagnostic properties of shale?

Answer 8

Size - <0.1mm
Shape - well rounded
Sorting - very well sorted
Large amount of transport and erosion, slow deposit often in deep waters
Minerals - remains of plankton

Question 9

What are the diagnostic properties of conglomerate?

Answer 9

Size - 8mm coarse grained
Shape - rounded
Sorting - moderately sorted
Large transport and erosion but quickly deposited
Minerals - quartz in beach conditions

Question 10

Explain how weathering affects sedimentary rock?

Answer 10

Weathering is the breaking up (physical) or breaking down (chemical) of rocks insitu
There is also biological weathering

Question 11

Explain physical weathering?

Answer 11

Usually temperature changes eg freeze thaw and exfoliation

1. Freeze thaw produces screes of large angular poorly sorted rock
   Water enters crack, freezes and expands by 9% expanding crack, thaws and repeats as rock breaks up into smaller pieces
2. Exfoliation produces layers peeling away
   Occurs in diurnal temperature range eg deserts
   Rocks heat and expand in day, cool and contract at night, repeat and outside layer peels off

Question 12

Explain chemical weathering?

Answer 12

When minerals are out of equilibrium with surroundings
Some minerals dissolve in weak acids (rain water) eg calcite, halite and gypsum
Rocks which are formed by evaporation are soluble and so can dissolve in weak acids
Explains why marble fades with time
Granite crystallises from magma at 850C

Question 13

How do minerals breakdown with chemical weathering?

Answer 13

Quartz -> quartz fragments + ions in solution
Feldspar -> clay + ions in solution
Muscovite -> muscovite fragments + ions in solution
Biotite -> clay + ions in solution
Hornblende -> clay + ions in solution

Question 14

Explain biological weathering?

Answer 14

There are two types of biological weathering:

1. Roots and rootlets force their way into rocks (physical) and leach nutrients (chemical)
2. Organic acids (chelation) = leaves build on soil, break down to create organic top soil humous. Water flows and becomes acidic and breaks down (chemical) rocks below

Question 15

What is erosion?

Answer 15

The break up of rocks during transport

Question 16

What are the different types of transport?

Answer 16

Wind
Water
Gravity
Ice

Question 17

What are the 2 main methods of erosion?

Answer 17

Attrition = sediments colliding in liquid during transport
Abrasion = bed load dragged during transport (traction)

Question 18

What is weird ice?

Answer 18

No attrition because no collisions with other sediments during transport in solid ice
Only abrasion as bed load dragged
Deposits are angular

Question 19

What is saltation?

Answer 19

Transport of material by bouncing

Material that is too large to be suspended in water and too small to be dragged by traction

Question 20

What are the environments of deposition?

Answer 20

1. Shallow marine
2. Deep marine
3. Terrestrial:
   A) Rivers, deltas and swamps
   B) Evaporite basins
   C) Wind and water in deserts
   D) Ice

Question 21

Explain beach/shallow marine deposition?

Answer 21

Current flows both directions
Creates symmetrical ripples (oscillating current), deposits and erosion
3 types of rock sandstone, conglomerate on high energy beaches and limestone from shells, coral and fish deposits

Question 22

Explain abyssal plain deposition?

Answer 22

Low energy environment
Small grains deposited
Mudstone and shales will form eg clay and marls (shells of plankton)
Deposits show laminations - fine layers in muds due to slow deposit and low sediment input

Question 23

Explain reef deposition?

Answer 23

Uniformitarianism - the present is the key to the past
Oolitic limestone = calcite from reef sand, circular precipitate due to rolling waves and current - Ooliths cement together
Bioclastic limestone = shell fragments fall down tallus slope and cement together
Reef limestone = coral polyps secrete nutrients and fossilised and cement
Fossils = shells break on lines of symmetry but see crinoid for leaves

Question 24

Explain turbidity current in deep marine sediment deposits?

Answer 24

Turbidity currents are clouds of sediment carried down canyons in the continental slope
Graded bedding as high energy large grains deposited before low energy mud - creates rocks like sandstone (greywacke)
Flute casts carved as swirling sediments cut into soft mud
Flame and load structures created by heavy deposits forcing up less dense mud

Question 25

Explain river deposits?

Answer 25

Cross beds are the preserved deposition slope of the ripple as it moves down stream
Graded bedding as channel migration means high energy deposits of coarse grained sediments deposit in deeper water - before low energy in shallower water
Flood plain
Point bar
Sandstone if a lot of transport so coarser grain upriver
Mudstone clay left behind after flood

Question 26

Explain delta/swamp deposits?

Answer 26

River loses velocity as it reaches sea, heavy material deposited first creating deltas and river splits into distributaries
Graded bedding as channel migrates
Sandstone is well rounded and well sorted rich in quartz and mature texture
Mudstone between river channels
Coal from fossilised plants falling into swamps

Question 27

Explain evaporite basins?

Answer 27

Sea water level drops due to evaporation
Current causes sabkha - porous barrier - to build up until lagoon cut off
Minerals precipitated - calcite then gypsum then halite then potassium in order solubility

Question 28

Explain fossils as climate indicators?

Answer 28

Uniformitarianism = the present is the key to the past
Fossils buried quickly dissolve leaving cast rich in sediment minerals
Coral in tropical warm water with high oxygen levels - fossils eg septa
Trilobites with eyes on top of head fossilise on the sea bed
Ammonite is related to modern nautilus
Plant fossils preserved only where they fell as transport would break them up
Trace fossils like footprints and burrows show evidence of life without preserving organism