Sedimentary rocks Flashcards
weathering definition
the in situ chemical alteration + mech and bio breakdown of rocks by exposure to atmosphere, water and organic matter
types of weathering
chem
bio
mechanical
chem weathering
chemicals in water making changes to rocks
carbonation- CO2 + H20—-> carbonic acid
ground water = more acidic than rainwater
limestone susceptible, dissolves to clay particles
hydrolysis- water reacts with silicate minerals e.g. feldspar
left over often clay
speed up with carbonic acid
mechanical weathering
crumbling due to rain, wind ect
exfoliation- outer layers breakdown first, day and night temps cause expansion and contraction
outer layers most effected so break first
Frost shattering-
water enters cracks/ joints and bedding planes
expands by 9%
creates pressure
causes cracks and fractures
creating scree
pressure release-
rocks under pressure
buried
pressure reduces due to weathering of overlays
expands in direction of pressure
Bio weathering
weakening and disintegration of rocks by plants, animals and microbes
root action-
grow along bedding planes, joints amd cracks
force rock apart widening/ causing fractures
more susceptible to carbonation and hydrolysis
trees swing I’m wind and prise apart fractures
burrowing-
bring rocks to surface
more easy to weather
more carbonation + hydrolysis
and more atmospheric gases in contact with rock
Examples of weathering in diff environments
Arctic = mostly frost shattering
temperate = everything
warm arrid = exfoliation
humid tropical = burrowing and root and chemical
Types of sed transport
Attrition- carried by water and broken down by hitting eachother repeatedly, frequent collisions
abrasion- grinding and wearing down of material by dragging and hurling
solution-when dissolved material is carried by a river- transport of ions dissolved in water
erosion- transport of rocks/fragments by rivers, wind, sea, gravity and ice
saltation- the bouncing of sand grains as they are picked up carried along and dropped repeatedly by water
traction- rolling/ sliding of large grains along river bed/shore
suspension- very fine sed transported without touching Earth’s surface
grain shape/ roundness
spherictity- measure of how close to spherical something is
grains have a higher or lower spherictity
they are also either
very angular, angular, sub angular, sub rounded, rounded or well rounded
this describes the smoothness
Grain size
Phi scale
logarithmic - used for large ranges
phi decreases size increases
diameter phi example
> 2 -2–8 gravel
2 -1 very coarse sand
1 0 coarse sand
0.5 1 med sand
0.25 2 fine sand
0.125 3 very fine sand
0.0625 4 silt
0.0039 8 clay
Sorting of sed
very well sorted (all same size) , well sorted, mod sorted, poorly, very poorly (all diff sizes)
grain size analysis
- Weigh sed
2.arrange sieve stack largest at top to smallest with solid pan at bottom
3.place dry sed sample in top pan
4.agitate sieve stack for 1min
5.seperate sieves and weigh mass of sed in each - use data to produce graphs/ cumulative freq curves to determine degree of sorting
Bar graphs: y axis- relative mass (g)
x axis- sieve size (phi)
poorly sorted would be evenly spread
Well would be mostly 1 bar
Cumulative frequency graphs: calc to relative percentage mass of each sieve
calc cumulative percentage mass by adding up % as you go down table
Y axis - cumulative mass (%)
X axis- sieve size (phi)
draw curve
calc coefficient of sorting
(84phi - 16phi)/2
<0.5 well
0.5-1 mod
>1 poorly
characteristics of sed transported by wind
Wind-
higher energy
small+ med grains
mostly quartz as min mature
Well rounded+ high spherictity
very well sorted
characteristics of sed transported by ice
low energy
large range of grain sizes
mineralogically immature
angular
poorly sorted
characteristics of sed transported by rivers
high energy
large range of grain sizes
finer further down
mostly quartz- more mature down stream
angular to rounded downstream
progressively more sorted
Characteristics of sed transported at beach
high energy
majority coarse
some fine/med
quartz- shell fragments and pebbles
sub rounded - rounded
poorly sorted
characteristics of sed transported by gravity
low energy
large range of grain sizes
minerals varied- whatever in og rock
angular
poorly sorted
Process of diagenisis
all the changes that take place in sed at low temp and pressure, at or near to Earth’s surface
Compaction-
layers of sed acculate one on top of another
mass produces pressure- deeply buried
causes Compaction
pressure dissolution (points of grains resting on eachother dissolve away in pressure solution) and crystallisation occur
grains become closely packed and porosity decreases
mud can be Compacted by 80% as flat platy minerals align
sand only 40% as grains have to bend and fracture as more equi
cementation-
High permeability (sandstone and limestone)
groundwater containing minerals I’m solution flows through pore spaces
minerals are precipitated
form cement binding grains together
sandstone- quarz/silicate cement
limestone- calcite cement
mineralogical changes-
limestone made of unstable aragonite
changes to become more stable calcite during diagenesis (often seen in fossils(
what happens to plants in diagensis
plants decay anaerobically form peat
if peat is buried and pressure increases it expels water and other volatiles
reduces volume and increases proportion of carbon
as diagenesis continues gradual becomes coal
cementation in sandstone
silica (as sandstones mostly quartz) dissolves in pressure sol
comes out of sol and fills pore-spaces with silica cement
hold together like interlocking crystals
Not always silica
cementation in limestones
pressure sol
pore-spaces saturated with CaCO3 so calcite cement forms
limestone= mostly CaCO3 so usually calcite but not always
Folk classification- for limestones
prefixes-
oo - ooliths
intra- intraclast (mix of 3)
bio- fossil remains of living things
pel- remains of excrement
suffixes-
micrite- super fine calcite cement cant see grains
sparite- coarser calcite cement
Dunham classification- limestones
Mudstone - Contains mud, mud supported and <10% grains
wackestone- Contains mud, mud supported, >10% grains
packstone- contains mud, grain supported
Grainstone- Lacks mud, grain supported
