Sedimentary

• Weathering - the breakdown in situ of rock at or near the earth's surface under the influence of low pressures/temperatures and the presence of air and water.

  • Rocks weather as they are out of equilibrium with the conditions under which they formed; all silicate minerals unstable at surface, eg. minerals in granite formed at >700$^oC$ and 5-15km depth.
  • Products of weathering; rock fragments, unreactive quartz grains, clay minerals (kaolinite, illite and smectite) and ions in solution.
• Freeze Thaw (P) - water penetrates joints, bedding planes, cleavages, faults, and pore spaces and as temperatures fall below freezing water turns to ice (9% greater volume than water); exerts internal stresses within rocks and when process is repeated, angular fragments can fracture off - forms graded bedded scree slopes.
• Exfoliation (P) - outer layers of rock heat up and expand more rapidly than layers at depth during the day. At night, outer layers cool and contract more rapidly than those at depth; series of concentric fractures are created and rocks peel off in layers - common in areas with large diurnal temperature ranges over 24hrs.
• Granular Disintegration (P) - different coloured minerals in rock heat up and expand at different rates; immense stresses are set up at crystal boundaries. Rock then crumbles into constituent grains - occurs in areas with large diurnal temperature ranges and affects coarse grained igneous rocks like granite (biotite mica cleaves and weakens rock).
• Salt Crystallisation (P) - sea spray penetrates rock structure; evaporation occurs and forms salt crystals. Process is repeated and crystals grow larger until internal stresses fracture rocks - common in coastal areas (eg. halite and gypsum).
• Dilation/Pressure Release (P) - rocks at depth are under great confining pressure; erosion removes overlying material and this sudden removal of mass causes rock to expand parallel to its own surface. Rock fractures to form horizontal joints.
• Biological Activity (P) - action of tree roots widening joints and bedding planes; root growth in confined spaces can exert immense stresses within rocks and widen natural lines of weakness + burrowing animals can create natural conduits for water to reach bedrock.
• Hydration (P) - involves minerals absorbing water into their atomic structures; minerals absorb water, expand and fall apart - hydration affects clay minerals produced by chemical processes (hydrolysis).
• Hydrolysis (C) - silicate minerals react with water to produce clay minerals and ions in solution; orthoclase feldspar decomposes to kaolinite and releases potassium/silicon ions in solution + biotite mica decomposes to chlorite and releases iron ions in solution.
• Carbonation (C) - rainwater picks up CO2 in atmosphere to form weak carbonic acid. Water infiltrating into soil picks up more CO2; this weak carbonic acid (pH5.5) can dissolve carbonate minerals (limestones).
• Solution (C) - soluble minerals dissolve and are carried away in solution (water); halite/gypsum most soluble - solution helps transport products of other chemical processes away.
• Oxidation (C) - ability for minerals to incorporate oxygen atoms into their atomic structures; affects iron rich minerals (biotite mica, hornblende, augite and olivine) resulting in red/brown/orange/yellow colouration of the soil/rock.
• Reduction (C) - the loss of oxygen atoms from the atomic structure of minerals (often aided by anaerobic bacteria) resulting in blue/grey/green colouration of soil/rock; converts iron compounds from ferric to ferrous state.
• Biological Chelation - rainfall percolating through humus (decaying organic material) becomes an organic acid (fulvic acid); organic acids or chelating agents attack clay minerals releasing iron/aluminium into soil. Chelating agents combine with metallic ions to form chelates; these are soluble, washed down profile and accumulate at depth.
• Physical Weathering (P) - leads to disintegration of bedrock into smaller, angular but chemically identical fragments.

  Chemical Weathering (C) - leads to decomposition of bedrock resulting in formation of clay minerals from breakdown of silicate minerals with ions released in solution (quartz is unaffected).

  • Factors affecting rate/type of weathering: lithology and rock structure, temperature, rainfall, influence of man and time.
• Erosion - the wearing away of the land surface and removal of sediment by transportation.

  • Abrasion - wearing away of the earth's surface by the action of wind, water or ice dragging sediment over or hurling it at the surface.
  • Attrittion - wearing down of sedimentary grains due to collisions with other grains during transport.
• Weathered material is transported away by:

  • Solution - transport of ions dissolved in water.
  • Suspension - transport of finer grains in water/air without it touching the surface or bed.
  • Saltation - transport of smaller grains by bouncing along the surface.
  • Traction - transport of larger grains (pebbles/boulders) by rolling and sliding along a surface.
• Graph shows relationship between sediment size and the velocity needed to erode, transport and deposit it.

  • Critical erosion line shows erosional velocity needed to initiate erosion (minimun velocity needed to lift particle of certain size).
  • Critical deposition line shows fall/settling velocity which rivers can flow before a particle of certain size is deposited.
  • Zone in-between represents transportation of sediment (large gap means sediment transported further).

  
• • Entrainment - the process of making something part of a flow and carrying it along.

  Sand is easiest to entrain as its incohesive, but finer-grained clays resist erosion as particles are cohesive. Clay is difficult to deposit as fine grains need extremely low velocities to be deposited. Hjulstrom's curve deglects other factor that affect deposition/erosion.
• • Clastic rocks represent accumulation of weathered and eroded fragments of older, pre-existing rocks of all types.
  • Organic sedimentary rocks are formed from the remains of living organisms.
  • Chemical sedimentary rocks are rocks precipitated directly from solution.
• Particle sizes of Clastic rocks:

  • >256mm - Boulder
  • 256-64mm - Cobble
  • 64-4mm - Pebble
  • 4-2mm - Granule
  • 2-1/16 - Sand
  • 1/16-1/256 - Silt
  • <1/256 - Clay
• • Rudaceous Rocks - over 50% of clasts are over 2mm in diameter primarily consisting of rock fragments, eg. conglomerate/breccia
  • Arenaceous Rocks - over 50% of particles 1/16-2mm in diameter; comprise high % of quartz grains (sandstones), eg. arkose
  • Argillaceous - over 50% of particles are <1/16mm in diameter; consist of clay minerals and small quartz grains, eg. shale
• • Immature - sediment that is a mixture of minerals of different grain sizes (often angular).
  • Mature - rounded grains that have been well-sorted into certain grain sizex after transportation.
  • Phenoclast - large clast or rock fragment.
  • Matrix - finer material often sand, silt or clay surrounding the phenoclasts.
  • Cement - material precipitated from solution to stick the sediment together (eg. quartz, calcite or haematite).
  • Well sorted - all clasts similiar in size (unimodal).
  • Poorly sorted - clasts wide range of particle sizes (polymodal).
• • Oligomict - all clasts of same type.
  • Polymict - clasts are of a variety of types.
  • Mineralogically Mature - the rocks consists of clasts of just one type.
  • Mineralogically Immature - the rock consists of a wide range of clast types.
  • Texturally Mature - all of the clasts are well rounded.
  • Texturally Immature - all of the clasts are very angular.
• Conglomerate (Clastic)

  • coarse grained + poorly sorted (clasts 1mm-3cm)
  • fine-grained matrix + siliclastic rock
  • clasts well-rounded/texturally mature (transported for long time)
  • typical deposit of high-energy shallow marine environment (beach/river channel deposit)

  
• Breccia (Clastic)

  • angular fragments (not transported for long time) + siliclastic
  • coarse grained (>2mm) + poorly sorted (clasts 1mm-3cm)
  • finer grains cemented in a red (iron oxide) haematite cement
  • produced by flash flood in desert environment; form as scree, alluvial fans and wadi deposits

  
• Greywacke/Turbidite Sandstone (Clastic)

  • range of angular to sub-angular clasts + fine to coarse grain sizes (poorly sorted) + texturally/mineralogically immature
  • sandstone with muddy matrix up to 40% + graded bedding common + fossils rare
  • formed in turbidity currents; large volumes of sediment are deposited rapidly in fan-shaped structures on continental slopes

  
• Arkose (Clastic)

  • angular feldspar/quartz grains (texturally/mineralogically mature)
  • medium/coarse grained sandstone (absence of fine material as blown away) + moderately sorted + pink/purple due to feldspar
  • sandstone with over 25% feldspar from physical weathering of granite/gneiss under arid conditions
  • formed in alluvial fan environments in arid areas

  
• Orthoquartzite (Clastic)

  • well sorted grains (0.25-0.5mm) + well-rounded + oligomict and unimodal - texturally/mineralogically mature
  • quartz grains held together by quartz cement (>90% quartz) + white/grey colour + absence of fossils due to erosion
  • forms in beach/shallow marine deposits + resistant to mechanical and chemical weathering

  
• Mudstone (Clastic)

  • fine grained (<1/256mm) + dark grey, very fine siliclastic rock
  • comprises of clay minerals (kaolinite/illite/serecite), mica and quartz + minerals no have preferred alignments (no laminations)
  • may contain fossils + not lot of cement between grains
  • deposited in a low-energy environment (marine and fluvial)

  
• Black Shale (Clastic)

  • fine grained (<1/256mm) + fissile (splits into thin layer) and well-laminated due to mineral alignment
  • shale is hard/brittle and impermeable + may contain fossils
  • composed of clay minerals (kaolinite/illite) and carbonaceous material (creates dark colour); minerals are flat/platy and align parallel to beds at 90$^o$ to pressure from overlying rocks
  • formed in deep sea, low energy environments

  
• Limestones: chemical limestones formed from the precipitation of calcium carbonate + biological limestones formed from organic matter - a rock containing more than 50% of calcium carbonate is a limestone.

  • Crinoidal Limestone (Bioclastic) - over 75% of rock made up of broken crinoid stems cemented together by calcium carbonate
  • other fossiliferous limestones; reef/coral limestone and algal limestone
  • limestone form in range of environments; low-energy freshwater lakes to marine beds + whole fossils in low energy
• Chalk (Organic)

  • made up of microscopic marine phytoplankton shells (coccoliths; microscopic calcite discs) + over 95% calcium carbonate
  • reacts with HCl + white/pure form of limestone + high porosity and permeability + very friable
  • formed in deep sea deposits and low-energy environments

  
• Bioclastic/Shelly Limestone (Organic)

  • calcium carbonate cement + silty matrix + reacts with HCl
  • formed in shallow water marine environments + high-energy conditions (inter-tidal zone)

  
• Coal

  • carbon-rich mineral deposit formed from the remains of dead plant matter; most coal in Europe formed 280-300Ma during the Carboniferous
  • coal-forming environments are hot, wet and tropical with stagnant anaerobic swamps, eg. Everglades, Florida
• Coal Formation

  • plant remains fall into swamps and the decay process uses up oxygen, so anaerobic bacteria changes plant material to peat; woody material/lignin, resins and waxes are preserved
  • vegetative material is covered by cement; peat is buried beneath sediment and comes under increased heat/pressure (expels water and volatiles like CO2/CH4) - reduces volume and increases proportion of carbon
• Increased depth of burial and carbon content:

  • Peat - semi-decomposed plant material + vegetation structure visible + 50% carbon content + burns poorly + spongy plant debris
  • Lignite (Brown Coal) - dark brown colour + 60-70% carbon content + woody look + generates ash/smoke when burnt
  • Bituminous Coal - black colour + 80-85% carbon content + little evidence of vegetation structure + breaks into cuboidal fragments
  • Anthracite - vitreous/metallic lustre + 90-95% carbon content + burns slowly with hot, bright flame + no vegetation structures
• Oolitic Limestone (Chemical)

  • concentric shells of calcium carbonate precipitate around a nucleus to build up the oolith; each one has a nucleus of a small sand grain/shell fragment
  • uniform texture/composition + reacts with dilute HCl
  • tropical, shallow water marine deposits in high energy conditions with abundance of calcium carbonate

  
• Evaporites (Chemical); material precipitated from seawater - % water needing evaporating for minerals to precipitate: K/Mg Salts >95%, Halite >90%, Gypsum >80%, and Calcite >60%.
• Bar Theory (Evaporite Formation)

  • lagoon is created by waves crashing over a bar during spring tides/storm surges; the shallow playa lake (1-2m deep) covers a large area
  • water in the lagoon evaporates to precipitate thin beds of evaporites; 3m of sea water produces 5cm of evaporites - cycles of replenishment/evaporation are needed for thick beds