Introduction to Sedimentary Rocks Flashcards
Sediment
material weathered from pre-existing rocks
Sedimentary rocks
rocks made up of sediments
Important factors characterising depositional environments
- type of transporting agent
- type and composition of sediment
- flow characteristics of transporting agent
- site of deposition
- types and abundance of organisms
Lithification definition .
Process of converting sediment to sedimentary rock
Lithification process
1) Deposition forms unconsolidated pile of sediment - lots of pore space
2) As pile grows thicker weight compacts lower layers - reduces pore space
3) Grains become cemented by:
- remobilisation of some minerals through pressure solution
- water moving through pores precipitating minerals
- commonly quartz or calcite
- also iron oxides, clay minerals
What are the five principal groupings of sedimentary rocks
1) Clastic
2) Biogenic/bioclastic
3) Organic
4) Chemical
5) Volcaniclastic
Clastic rocks
- made of clasts; detrital origin
- clasts are pieces of mineral or pre existing rock formed from weathering
- e.g. sandstone, siltstone. breccia
Biogenic/bioclastic rocks
- made of particles originally produced by living organism
- generally broken or complete shell material
- e.g. limestone, chalk, chert
Organic rocks
- made of compressed and heated plant material
- e.g. peat, coal
Chemical rocks
- made from precipitated dissolved particles, Ca, SiO2, FeO etc.
- e.g. evaporite, ironstone
Volcaniclastic rocks
- as for clastic but significant proportion of clasts are of volcanic origin
How is clast size determined
Energy of transporting medium determines clast size i.e. the closer to source the more coarse
Relationship between clast shape and transport distance and energy of depositional environment
Roundness of clast increases with transport distance and energy of depositional environment
Examples of clastic sedimentary rocks - Conglomerate
- contains large rounded clasts
- space between clasts contains much smaller grains and/or cement
Examples of clastic sedimentary rocks - Breccia
- contains large, angular clasts
- space between clasts contains much smaller grains and/or cement
Examples of clastic sedimentary rocks - Sandstone
- sand-sized clasts
- can see clasts with naked eye
Examples of clastic sedimentary rocks - Siltstone
- silt sized clasts
- cannot see clasts with naked eye
Examples of clastic sedimentary rocks - Claystone
- clay-sized clasts
- cannot see clasts with naked eye
Examples of clastic sedimentary rocks - Mudstone/shale
- mix of silt to clay-sized clasts
- cannot see clasts with naked eye
- mudstone breaks into blocks
- shales split easily along parallel planes
Examples of biogenic sedimentary rocks - limestone
- composed primarily of calcium carbonate
- can form from the accumulation of shell material
- can look crystalline
Examples of biogenic sedimentary rocks - chalk
- very fine-grained limestone
- formed from calcium carbonate skeletal remains of marine plankton
Examples of biogenic sedimentary rocks - coal
- forms from plant debris
- accumulates in swampy environments
- burial drives off volatiles, compacts
- higher grade means fewer volatiles
Examples of biogenic chemical rocks - Evaporites
- form from the evaporation of mineral rich water
- form in very arid climates
How and why does bedding occur
- formed during lithification process
- defined by changes in grain size, composition or colour
- upper and lower surfaces of a bed are known as bedding planes
- represent change in environment of deposition
- can be variable thickness
