Seds Flashcards
Ways that volume changes occur within fissures
1) frost weathering (H2O -> ice)
2) precipitate salts in fissures
3) biological (tree roots)
Types of physical weathering
1) pressure release
burial = pressure
then uplift = expand and break along planes of weakness
2) Insolation weathering
thermal expansion and contraction -> differential expansion between surface and middle creates stress
3) Hydration weathering
water absorbed and released by clay minerals, swelling and shrinking of rock
4) volume changes within fissures
methods of chemical weathering
1) acid hydrolysis
carbonic acid reacts with primary mineral to replace metal cation with H+ ion, forming hydrous mineral
e.g. acid hydrolysis of silicate minerals
carbonic aid + feldspar = hydrated aluminosilicate (+ metal ion + bicarbonate)
2) oxidation
metals lose e- and rust
methods of biochemical weathering
plant roots and bacteria produce organic acids which break down rocks
e.g. fungi penetrate and take nutrients directly from mineral
how does goldich’s weathering series relate to Bowens reaction series
as you do down Bowens reaction series, minerals become less susceptible to weathering, as were formed/are stable at conditions closer to those at the surface
4 categories of sediments/sedimentary rocks
1) clastic deposits
- particles from pre-existing rocks
2) biogenic/organic deposits
- resulting from biological materials/biologially mediated processes
3) chemical deposits
- precipitates from solution
4) volcaniclastic deposits
- products of volcanic eruption of breakdown of volcanic rocks
examples of each of the 4 types of sediments/sedimentary rocks
1) clastic depostis - breccia, sandstone, mudstone
2) biogenic - limestone, coal, CaCO3
3) chemical deposits - evaporite, BIF
4) volcaniclastic - tuff
difference between a matrix and a cement
matrix = fine grained granular
cement = precipitated out of fluid after deposition, crystalline
physical descriptors of clastic sediments
- grain size + size distribution
- grain morphology
- grain packing (fabric)
what factors effect grain size distribution
- nature of sediment source
- efficiency of transport medium (only carry specific sizes, wind, water, glacier)
- energy in deposition
what are patterns of grain morphology with distance transported
- roundness = increase rapidly
- size = slow decrease
- sphericity = not much change
define textural maturity
immature = high matrix proportion, poorly sorted, angular grains
mature = less matrix, well sorted, rounded grains
high maturity = porous + permeable
what makes up clastic rocks
quartz
feldspars
lithic fragments
micas + clay minerals
heavy minerals (accessory grains)
other detrital (fossils)
characteristics of quartz in clastic rocks
- v common, hard, no cleavage
- derived from granite + gneiss
- can be monocrystalline or polycrystalline
characteristics of feldspars important in clastic rocks
- common, less hard than quartz, cleaved
- chemically liable to be replaced by clay minerals = indicator of weathering intensity + environment
characteristics of lithic fragments important in clastic rocks
- type depends on geology of source and durability
- key in breccia, coarse sandstones
what is the pettijohn classification of sandstones
ternary diagram showing the relative proportion of quartz, feldspars, and lithic fragments in a rock
what is the importance of micas and clay minerals in clastic rocks
- common in matrix
- clay minerals small and fine grained
- muscovite > biotite (stability)
- matrix creates another scale on pettijohn classification (amount of matrix)
importance and characteristics of heavy minerals in clastic rocks
- density > 2.85 g/cm^3
- stable oxides and silicates
- evidence for origin
e.g. garnet = met, apatite = igneous
how does composition and texture evolve with maturity
composition:
unstable minerals (feldspar) -> stable + LFs -> entirely quartz
texture:
matrix, poor sorting, angular
-> decreasing matrix, increasingly sorted, increasingly rounded
difference between conglomerates and breccia
conglomerate = rounded clasts
breccia = angular clases
what is diagenesis
physical and chemical changes that alter the characteristics of a sediment after deposition
unconsolidated sediment -> consolidated rock
how does packing change during diagenesis, and example with different grains
increase pressure by overlying sediments -> change packing to increase density -> decrease porosity
fluid expelled and volume is reduced
muds = very compressible due to layered structure
sand = compacted much less
when are sutured contacts between grains seen
when grains fuse together due to temperature increase during diagenesis
how does cementation occur
- material dissolves at grain contacts
- solution percolated through pore spaces
- crystallise into cement
what are common cements
silica
calcite
name sediment transport media
- water
- air
- ice
- sed/water mixtures
- gravity
what are the 2 ways that fluids flow and what number defines this
laminar flow - in debris flow (high viscosity sed/water mixtures) and glaciers
turbulent flow - natural systems that transport sediments
Re (Reynolds no.):
high = turbulent flow
low = laminar flow
what are the 4 categories of sedimentary structures
1) depositional
2) erosional
3) post depositional
4) biogenic
difference between bedding and lamination
bedding > 1cm
lamination < 1cm
what controls change in bedding/lamination
- composition
- size
- shape
- orientation
- packing
of deposited sediments
what is the stoss side and lee side of an asymmetrical ripple
stoss = gentle slope, exposed, roll up
lee = steep, sheltered, fall down
why are preserved unidirectional currents useful
indicator of palaeocurrent direction
when are dunes created as opposed to ripples
- higher mean flow velocity
- larger grain size
what structures do waves produce
symmetrical ripples by oscillatory motion, no net movement of grains
examples of erosional sedimentary structures
- scour marks
- tool marks
how are scour marks identified (erosional sedimentary structure)
- flare downstream
- created by turbulent eddies
- cm scale on base of beds
name and describe post depositional sedimentary structures
- dewatering of sediment
- deposition onto a slope - slumps
- loading of higher density layer on lower density layer -> load balls, flame structures
what are biogenic sedimentary structures
- bioturbation
- trace fossils
what is a facies
a body of rock with specified characteristics that reflect the conditions under which it has formed
what are classes/examples of major depositional environments
marine
- shallow, deep
terrestrial
- deserts, lakes, rivers
marginal
- deltas, estuaries
describe the desert environment
- low precipitation <250mm/yr
- hot+cold arid regions
- lack of water = susceptible to movement by wind
- can have flash floods
distribution/types of deserts
subtropical
- descending Hadley cell
rain shadow
- continental topography forces air upwards
coastal
- cool air over ocean wicks water from land
interior
- air loses moisture as crosses continental interior
polar
- air above 66º holds little moisture, wicks water from land
characteristics of wind blown particles/deposits
- very well rounded (saltation impacts)
- frosted grains
- well sorted (air small transport range)
- compositionally mature (quartz + little matrix)
3 stages of pettijohn classification of sandstones with increasing percent matrix
arenite
wacke
mudrock
how to recognise deserts in the rock record
- clean, quartz rich sandstones
- large scale X-bedding
- rare fossils
- associated conglomerates from flash floods
what are the types of sediments seen in lakes
clastic
- supplied by rivers
biochemical sedimentation
- carbonates through biological processes and chemical precipitation
organic production
- photosynthetic organisms
how to recognise lakes in the rock record
- diverse lithologies
- ripples, cracks, stromatolites (near shore)
- rhythmic laminations (deep)
- fossils - non marine (anoxia preserved)
- shallowing upward cycles
- associated with fluvial facies
