1.3 Sedimentary Rocks And Their Fossil Content Flashcards
What does grain size, shape and sorting tell us?
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
Summarise grain size, shape and sorting of sedimentary rocks?
Mountain = angular, coarse grained, poorly sorted Sea = rounded, fine grained, well sorted
How to find texture of sedimentary rock?
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)
Explain porosity, permeability and impermeability?
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
What are the diagnostic properties of breccia?
Size - 2mm - coarse grained Shape - sub angular/angular Sorting - poorly sorted Little transport and erosion, deposited quickly eg flash floods and scree slopes Minerals -
What are the diagnostic properties of limestone?
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
What are the diagnostic properties of sandstone?
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)
What are the diagnostic properties of shale?
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
What are the diagnostic properties of conglomerate?
Size - 8mm coarse grained Shape - rounded Sorting - moderately sorted Large transport and erosion but quickly deposited Minerals - quartz in beach conditions
Explain how weathering affects sedimentary rock?
Weathering is the breaking up (physical) or breaking down (chemical) of rocks insitu
There is also biological weathering
Explain physical weathering?
Usually temperature changes eg freeze thaw and exfoliation
- 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 - 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
Explain chemical weathering?
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
How do minerals breakdown with chemical weathering?
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
Explain biological weathering?
There are two types of biological weathering:
- Roots and rootlets force their way into rocks (physical) and leach nutrients (chemical)
- 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
What is erosion?
The break up of rocks during transport
What are the different types of transport?
Wind
Water
Gravity
Ice
What are the 2 main methods of erosion?
Attrition = sediments colliding in liquid during transport Abrasion = bed load dragged during transport (traction)
What is weird ice?
No attrition because no collisions with other sediments during transport in solid ice
Only abrasion as bed load dragged
Deposits are angular
What is saltation?
Transport of material by bouncing
Material that is too large to be suspended in water and too small to be dragged by traction
What are the environments of deposition?
- Shallow marine
- Deep marine
- Terrestrial:
A) Rivers, deltas and swamps
B) Evaporite basins
C) Wind and water in deserts
D) Ice
Explain beach/shallow marine deposition?
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
Explain abyssal plain deposition?
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
Explain reef deposition?
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
Explain turbidity current in deep marine sediment deposits?
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
