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.
