Dynamic Surface L2 Flashcards
What leads to the formation of poorly sorted and unstable sediment?
Rapid transportation and deposition of sediment
What can soft sediment be modified by to form soft sediment deformation structures?
Gravitation and fluid movement
What are the two scales of post-depositional modification?
Grain scale
Bed scale
What are 6 bed scale structures formed as a result of post-depositional modification?
Slump structures
Convolute bedding
Dish and pillar structures
Mud volcanoes
Flame structures/load casts
Diapiric structures
What is diagenesis?
Physical and chemical changes that alter the characteristics of sediment
In what conditions do diagenetic changes occur?
Low temperature
Low pressure
What is mechanical compaction?
Changes in packing that lead to a reduction in porosity and volume
What can mechanical compaction be caused by?
Seismic activity or waves on a shoreline
What are the 4 types of grain and clast contacts?
Point contacts
Long contacts
Concavo-convex contacts
Sutured contacts
In what order do grain contacts progress with increasing overburden?
1) Point contacts
2) Long contacts
3) Concavo-convex contacts
4) Sutured contacts
What do soft materials form at the point of contact with each other?
Pressure solution/a sutured boundary between grains
How can compaction pressure be compensated?
The presence of a matrix (e.g. mud) will prevent pressure grains from making surface contact.
What is cementation?
A chemical diagenetic process whereby crystals grow in pore spaces
What are the 4 types of cement fabric?
Overgrowth cement fabric
Poikilotopic cement fabric
Isopachous fabric
Meniscus cement fabric
What is overgrowth cement fabric?
Cement growth that is in optical continuity with the grain (grain forms a cement around itself)
What is poikilotopic cement fabric?
Grains are enveloped in large crystals of cement
What is isopachous cement fabric?
Crystals of cement grow uniformly into pores
What is meniscus cement fabric?
Cement develops from drops of pore water attached to grains
What are 3 common cements?
Calcite (CaCO3)
Quartz (SiO2)
Clay minerals
What does the precipitation of cement species depend on?
Solubility
pH
Composition of pore waters
Sediment grains
What are the characteristics of sparry calcite under xpl?
Oblique twinning
4th order interference colours
What are nodules and concretions?
Localised patches of extreme cementation
What 3 minerals commonly form nodules and concretions?
Calcium carbonate
Quartz
Pyrite
Where are silica concretions common and what are they known as?
Silica concretions are common in chalk where they are known as flint
What is a common example of a colour change due to diagenesis?
Reduction and oxidation
Fe3+ (red) –> Fe2+ (green)
What does limestone undergo when buried to 100 metres of depth and what does this form?
Pressure dissolution may form stylolites at distinct pressure solution surfaces
What is an example of organic diagenesis?
Formation of coal
Briefly describe the formation of coal.
1) Organic matter is buried as swamp deposits and partly decayed and compressed to peat
2) Peat transforms to lignite under shallow burial
3) Lignite transforms to soft coal under deeper burial
4) Soft coal metamorphoses into hard coal under continued burial and heat
Briefly describe the formation of hydrocarbons.
1) Organic matter (plankton and algae) accumulate in anoxic conditions
2) Burial diagenesis breaks down material to form kerogen
3) Kerogen heated at different temperatures can form either oil, methane or graphite
How are slump structures formed?
Sediment beds respond to gravity when formed on a slope
How is convolute bedding formed?
Formed when complex folding and crumpling of beds or laminations occur.
How are dish and pillar structures formed?
Formed by dewatering (escaping water) through sediment
How are mud volcanoes formed?
Subsurface layers of fluidised sediments (e.g. silt or clay) which are less dense are pressurised forcing them upwards to the surface.
How are load casts/flame structures formed?
Density contrasts in sediments of interbedded layers of sand and mud - wet dense sand sinks into less dense mud which escapes upwards
How are diapiric structures formed?
Regional scale density contrasts - differential loading causes underlying less dense salt deposits covered by overburden (overlying sediment) to rise upward toward the surface through fractures in the overburden.
