Sedimentary rocks Flashcards
What is weathering?
The breakdown of rock materials in situ, at or near the earth’s surface, under the influence of low pressures, low temperatures and the presence of air and water.
Why do rocks and minerals weather?
They are out of equilibrium with the conditions under which they formed.
Why is quartz the only stable silicate mineral?
It does not have to try to readjust to new conditions at the earth’s surface.
What is erosion?
The removal or weathered products ex situ by agents such as gravity, water, wind and ice.
What is mechanical weathering?
-Leads to disintegration of the bedrock into smaller, angular, but chemically identical fragments.
-Results in an increase in surface area of rock expose to chemical weathering to act upon, but same volume.
What are the products of weathering?
-Rock fragments
-Unreactive quartz grains
-Clay minerals
-Ions in solution
What is freeze-thaw weathering?
-Water penetrates joints or cracks in the rock.
-Temperature drops and water freezes leaving ice.
-Ice melts and expands putting pressure on the rock and causing it to expand and forcing the cracks to widen.
-Repeated process may lead to fracturing of angular fragments.
What is exfoliation/onion skin weathering?
-Areas with large diurnal temperature ranges
-Outer layers or rock heat up and expand more rapidly than the layers at depth during daytime.
-At night, outer layers cool and contract quicker than layers at depth.
-Series of concentric fractures initiated.
-Rock peels off in layers.
What is granular disintegration?
-Areas with large diurnal temperature ranges.
-Affects coarse grained igneous rocks.
-Different coloured minerals in rock heat up and expand at different rates.
-Immense stresses at crystal boundaries.
-Black biotite mica may cleave and weaken whole rock structure.
-Rock crumbles into constituent grains.
What is salt crystal growth?
-Same effect as freeze-thaw but halite and gypsum crystallise instead of ice.
-Common in coasts.
-Sea spray penetrates rock structure.
-Evaporation occurs and halite/gypsum crystallise.
-Process repeated-crystals grow larger.
-Eventually internal stresses fracture the rock.
What is dilation/pressure release?
Rocks at depth under great confining pressure.
-Erosion removes overlying material.
-Removal of mass causes rock to expand parallel to its own surface.
-Rock fractures to form horizontal joints.
-Also occurs in quarries following blasting.
What is biological activity?
-The action of tree roots widening joints and bedding planes.
-Root growth exerts pressure within rock and widen lines of weakness.
-Burrowing animals create natural conduits for water to reach bedrock.
What is hydration?
-Chemical process.
-Minerals taking up water into their atomic structures.
-Only readily affects clay minerals produced by chemical processes.
-Minerals absorb water, expand and fall apart.
What is chemical weathering?
-Leads to decomposition of bedrock.
-Only quartz unreactive and not affected.
-Result is formation of clay minerals from breakdown of silicate minerals.
-Ions released into solution.
What is hydrolysis?
-Silicate minerals react with water.
-Clay minerals and ions in solution are produced.
-Orthoclase feldspar decomposes to kaolinite and releases potassium and silicon ions into solution.
-Biotite mica decomposes to chlorite and releases iron ions into solution.
What is carbonation?
-Rainwater falls through atmosphere, picks up CO2 and forms weak carbonic acid.
-Water infiltrates soil, picks up more CO2 from soil air.
-Weak carbonic acid dissolves carbonate minerals.
-Rocks containing calcite are most susceptible.
What is solution?
-Halite and gypsum most soluble minerals on earth’s surface
-Transports away products of other chemical processes.
What is oxidation?
-Minerals incorporate oxygen atoms into their atomic structure.
-Most susceptible are iron-rich minerals.
-Red, brown, orange and yellow soil colouration.
-Brown stain due to oxidation.
What is reduction?
-Loss of oxygen atoms from atomic structure of minerals.
-Quicker under anaerobic conditions.
-Iron compounds go ferric to ferrous state.
-Blue, grey, green colouration in soil profiles.
What is chelation?
-Rainfall percolates through humus, becomes organic acid.
-Organic acids attack clay minerals, releases iron and aluminium into soil.
-Organic acids combine with metallic ions forming organic metal.
-Chelates are soluble and accumulate at depth.
What is spheroidal weathering?
-Rectangular blocks chemically weathered, more rapid at corners and edges.
-Once spherical, even weathering on surface, so shape does not change.
What factors control the rate and type of weathering?
-Lithology (rock type)
-Rock structure
-Temperature
-Rainfall
-Relief
-Anthropogenic influence
-Time
What do sedimentary structures give information about?
Depositional environment and show the ‘way up’ of beds.
How can imbrication be measured?
A rose diagram.
What is bedding?
-A bed is a layer of rock separated by a bedding plane from the layer above and below.
-Bed represents unbroken episode of sediment deposition, whereas bedding plane represents a break in deposition.
-Beds 1cm+.
-Beds occur in uniform thicknesses over large areas or pinch out laterally.
What is lamination?
-Layer of sediment <1cm thick.
-Individual laminations 1mm or less.
What is cross bedding?
-Large scale=dune bedding, small scale=cross lamination.
-Sediment moved and accumulated at angle to principle bedding direction.
-Uni-directional current of water/wind moving sediment as series of asymmetrical rippeles/dunes.
How do you work out the current direction in cross bedding?
Look at the beddings and laminations to observe the angle that they are dipping at to determine current direction.
What is herringbone cross bedding?
-Current reversal through 180 degrees.
What are included/derived fragments?
-Older beds eroded before deposition of new bed in a sequence.
-Eroded fragments then included as clasts/fragments in beds above.
-A law.
What is imbricate structure?
-Deposited under influence of powerful current.
-Long axes of clasts lie sub-parallel with another ‘leading’ direction of current flow.
What are mud cracks?
-Formed when sediment exposed to atmosphere.
-Form as desiccation polygons.
-Sediment dries out and shrinks as water evaporates out.
-Develop contraction centres and then polygonal pattern of cracks.
-Widest at top, narrowest at bottom.
What are load casts and flame structures?
-Sandstone and shale= denser sandstone sinks into less dense shale as bulbous protrusions.
-Shale flows upwards into spaces.
-Rounded protrusions mark base of sandstone bed, flames mark top of shale bed.
-Possibly, globule of sandstone detaches from bed above and sinks into shale below= distortion, result is teardrop structure.
What is life position of fossils?
-Organisms preserved in life position(tree roots, corals), can indicate if beds are ‘correct way up’.
-Coral=clear water,<50m deep, well oxygenated environment, normal salinity, temps 22-28 degrees celcius, within 30 degrees latitude of equator.
What are sole structures?
-Formed in interbedded sandstone and shale sequences.
-Preserved in base of overlying sandstone bed.
-Classification=shape.
-Include flute casts+grooves.
What are flute casts?
-Scouring in fluted hollows in soft mud by current vortices.
What are groove casts?
-Formed by pebble rolling across sediment surface=groove cut into it.
-Preserved as cast on under surface of overlying bed.
-Palaoecurrent could be either way.
What are ripple marks?
-A system of parallel wavy ridges and furrows left on sand, mud, or rock by the action of water or wind.
What are concretions?
-Spherical/ellipsoidal body.
-Produced due to early localised cementation within sediment.
-Often found with fossil as nucleus of concretion.
What are burrows (trace fossil)?
-Organisms burrow from surface downwards into unconsolidated soft sediment.
-Burrows open at ancient sediment surface and taper downwards to a point.
-Can be horizontal.
What are trails and footprints (trace fossil)?
-Represents upper sediment surface, organisms walked/crawled across.
-Specific fossils hint at certain environments.
What are halite pseudomorphs?
-Original crystal dissolved away and mould infilled by mud.
-Preserved on base of overlying bed as a cast.
What is graded bedding?
-A bed displaying fining upwards sequence from the base.
-Sequence may be produced several ways.
How is graded bedding formed in relation to turbidity currents?
-Progressive settling of grade sizes from coarse to fine in calm waters.
-Larger, denser rock fragments and sand particles sink first, followed by less dense silt and clay particles.
How is graded bedding formed in relation to seasonal variation?
Example 1:
-Deposition from glacial meltwater in a pro-glacial lake.
-Spring/summer more meltwater+sand and gravel transported and deposited in lake.
-Autumn/winter meltwater is less, so finer silt and clay to settle out from suspension.
Example 2:
-Variation in river discharge.
-Winter coarse sand+gravel deposited during high discharges.
-Summer finer sand+silt deposited in low flow conditions.
What is hydraulic action (fluvial)?
-Force of water on the banks and bed of the river channel.
-Faster flow=larger volume of water and faster erosion.
-Water forces compressed air into the cracks causing them to widen.
What is corrasion (abrasion)(fluvial)?
-Action of material carried by the river in contact with banks of bed and channel.
-Most at times of high discharge.
What is solution (corrosion)(fluvial)?
-Slightly acidic river water.
-Dissolves minerals from banks and bed of channel.
-Especially carbonates.
What is attrition (fluvial)?
-River material gradually reduced in size.
-Changes from angular to rounded particles.
-By numerous collisions during transportations.
What is the landscape like in the upper course?
-Narrow and v-shaped.
-Interlocking spurs, waterfalls, potholes and rapids common.
What is the landscape like in the middle and lower courses?
-Wider valley.
-Meanders common.
-River begins to create a floodplain on either side of the channel often with levees.
What is plucking (glacial)?
-Water freezes in surface cracks in rocks below ice.
-Glacial ice moves, material around ice in cracks is pulled and plucked out.
What is abrasion (glacial)?
-Rocks embedded into base of ice.
-Due to plucking scour and erode bedrock.
-As ice moves downhill due to gravity.
What is attrition (glacial)?
-Rock materials transported within and under ice are ground down.
-To produce progressively smaller angular fragments .
-Due to pressure contact and movement.
-Produces texture of boulder clay.
What are the characteristics of glaciated landscapes in u shaped valleys?
-Glaciers widen, deepen and straighten the course of pre-existing river valleys.
-Steeper valley sides.
-Features include corries, aretes, truncated spurs and hanging tributary valleys.
What is hydraulic action (marine)?
-Effect of waves breaking on shoreline.
-Air trapped between breaking waves and rock.
-Forced into fissures and/or cracks in rock forcing the cracks to widen.
What is corrasion (abrasion)(marine)?
-Waves pick up sediment and throw it at the cliffs.
-Effective in storms and high tide.
What is solution (marine)?
-Sea water dissolves minerals from rocks.
-Especially like limestone.
What is attrition (marine)?
-Sediment that is transported is reduced in size.
-Becomes more rounded.
-As particles collide and rock fragments break apart.
-As it is moved by the backwash and swash.
What is wind abrasion (aeolian)?
-Wind erodes bedrock.
-Through contact with bedrock and rock particles carried by wind.
What is deflation (aeolian)?
-Concentrates coarser particles at surface.
-Result is a surface layer of coarser grained fragments.
-That cannot be transported by wind.
Order the process of diagenesis and lithification?
-Loose sediment/mud.
-Burial.
-Compaction.
-Dewatering.
-Reduction in porosity and volume.
-Grain welding.
-Cementation.
-Sedimentary rocks, e.g., sandstone and mudstone.
What is lithification?
-Physical change from loose sediment to solid rock.
What is diagenesis?
-Physical, chemical and biological changes that take place in a sediment after deposition.
-All the processes that enable loose sediment to be converted into solid rock.
-Diagenesis enables lithification.
What are the conditions needed for diagenesis and lithification and what do they exclude?
-Low temperatures and pressures.
-At shallow crustal depths.
-Excludes weathering, erosion and metamorphism.
What happens in burial and compaction?
-Air and water is squeezed out, porosity reduces and sediments become compacted/reduced in volume.
What happens when cementation from pore waters occurs?
-Minerals precipitate from pore waters around edges of grains to bind sediment together.
-Reduction in porosity.
-Common cements: calcite, quartz and haematite.
What happens in grain welding/fusion/pressure solution?
-Continuation of burial and compaction forces grain edges together and they fuse.
What do the following sediments turn into after lithification:?
-Gravel
-Sand
-Silt
-Mud
-Broken sea shells
-Coccoliths
-Dead plant material
-Gravel turns to conglomerate.
-Sand turns to sandstone.
-Silt turns to siltstone.
-Mud turns to mudstone/shale.
-Broken sea shells turn to limestone.
-Coccoliths turn to chalk.
-Dead plant material turns to coal.
What are the characteristics of sedimentary rocks?
-Formed at or very close to the earth’s surface.
-Deposited in layers or beds.
-Frequent fossils.
-3 groups classification:
Clastic/detrital=accumulation of weathered and eroded fragments of older, pre-existing rocks.
Organic=remains of organisms.
Chemical precipitates=rocks precipitated directly from solution.
What is a phenoclast?
-Large clast/rock fragment.
What is a matrix?
-Finer material surrounding phenoclasts.
What is cement?
-Material precipitated from a solution to sick sediment together.
What is well sorted?
-All clasts very similar size(unimodal).
What is poorly sorted?
-Clasts show wide range of particle sizes(polymodal).
What are the classifications of particles sizes of clastic rocks?
->256mm=boulder.
-256mm-64mm=cobble.
-64mm-4mm=pebble.
-4mm-2mm=granule.
-2mm-1/16mm=sand.
-1/16mm-1/256mm=silt.
-<1/256mm=clay.
What is grain size/shape?
-Shape of grains in sedimentary rock depends on the type of mineral that they are made from.
-Described as sphericity=low/high.
-Roundness depends on rate of attrition=more transportation results in more rounding.
-Described as very angular to very rounded.
-Larger grains=less transport, so more resistant than smaller grains.
How do you calculate grain size/shape?
-Zinng classification.
-Measuring a, b and c axes of clasts.
-A axis is longest dimension of clast.
-B axis is widest dimension at right angles to a axis.
-C is shortest axis on which clast sits vertically.
=b/a and c/b axial values are plotted as co-ordinates to identify individual clasts as spheres, discs, rods or blades.
What is sorting?
-Particles variations in size.
-Well sorted=a rock with similar sized grains.
-Poorly sorted=a rock with grains which vary widely in size.
What is textural and mineralogical maturity?
-Mineralogically mature=rock consists of one type of clast.
-Texturally mature=rock consists of all well rounded clasts.
-Mineralogically immature=rock consist of a wide range of clasts.
-Texturally immature=rock consists of clasts that are very angular.
-If a rock it texturally and mineralogically immature it has only been transported a short distance and suffered limited erosion.
What are some of the characteristics of desert sandstone?
-Well sorted.
-Mineralogy=quartz.
-Mineralogically and texturally mature.
-Haematite cement.
-Clastic.
-Cross bedding common but no fossils.
What are some of the characteristics of orthoquartzite?
-Well sorted.
-Mineralogy=quartz.
-Mineralogically and texturally mature.
-Quartz cement.
-Clastic.
-Very resistant to chemical and mechanical weathering.
-Ripple marks.
What are some of the characteristics of arkose?
-Moderately sorted.
-Mineralogy=quartz+ feldspar.
-Mineralogically and texturally immature.
-Haematite and feldspar cement.
-Clastic.
-Pinkish/purple colouring due to lots of haematite and feldspar cement.
What are some of the characteristics of siltstone?
-Well sorted.
-Mineralogy=clay and quartz.
-Mineralogically immature and texturally mature.
-Haematite cement.
-Clastic.
-Grain size mainly 1/16mm-1/256mm.
-Gritty when rubbed on teeth.
What are some of the characteristics of mudstone?
-Well sorted.
-Mineralogy=clay.
-Mineralogically and texturally mature.
-Clay cement.
-Clastic.
-Homogenous/structureless.
-Little evidence of laminations, but can be present.
What are some of the characteristics of shale?
-Well sorted.
-Mineralogy=clay.
-Mineralogically and texturally mature.
-Clay cement.
-Clastic.
-Splits into thin layers=fissile.
-Black due to organic matter.
-Often contains fossils.
What are some of the characteristics of chalk?
-Very well sorted.
-Mineralogy=coccoliths (calcite).
-Mineralogically and texturally very mature.
-Calcite cement.
-Organic.
-Reacts violently with dilute HCl.
-Made of coccoliths.
What are some of the characteristics of shelly/bioclastic limestone?
-Poorly sorted.
-Mineralogy=calcite.
-Mineralogically mature and texturally immature.
-Calcite cement.
-Organic/clastic.
-Often contains bivalve fragments.
What are some of the characteristics of oolitic limestone?
-Very well sorted.
-Mineralogy=calcite.
-Mineralogically and texturally mature.
-Calcite cement.
-Chemical.
-Made of ooliths(chemical).
What are some of the characteristics of conglomerate?
-Poorly sorted.
-Mineralogy=variable.
-Mineralogically and texturally mature, however this depends on what it erodes from.
-Variable cement.
-Chemical.
-Made of clasts.
What are some of the characteristics of breccia?
-Poorly sorted.
-Mineralogy=variable.
-Mineralogically and texturally immature.
-Variable cement.
-Clastic.
-Contains angular clasts.
What types of environments can sedimentary rocks be deposited in?
-Desert dunes.
-Lakes.
-Glacial environment.
-Fluvial environment.
-Beach.
-Delta.
-Tidal flat.
-Barrier island.
-Shallow marine environment.
-Deep marine environment.
-Submarine fan.
-Continental shelf.
-Lagoon.
-Alluvial fan.
What are evaporites(precipitates)?
-A water-soluble sedimentary mineral deposit that results from concentration and crystallisation by evaporation from an aqueous solution.
-Water in lagoons evaporate to precipitate thin beds of evaporates.
-Form basis of petro-chemical industry.
What is coal and what conditions are required for it to form?
-A carbon-rich mineral deposit formed from the remains of dead plant matter.
-Vegetative material must eventually be covered by sediment for coal to form.
-Most coal formed in Europe 280-300 Ma during Carboniferous.
-Hot, wet, tropical climates with stagnant anaerobic swamps most favourable coal-forming environments, e.g., Everglades of Florida and the Okefenokee Swamp in South Carolina, USA.
What are the different types of coal?
-Peat (50% C).
-Lignite/Brown Coal (60%-70%).
-Bituminous Coal (80%-85%).
-Anthracite (90%-95%).
