Sediments and sedimentary rocks |
Sedimentary part of the rock cycle:
o Weathering and disaggregation, and dissolved substances yield the raw materials for sediments.
o Erosion and transportation: This matter removed and transported, water or wind,
o Deposition in oceans (or lakes or rivers). Particulate matter is sedimented by gravity; dissoved material is deposited (taken out of solution) by chemical or biochemical processes.
o Burial
o Diagenesis - physical and chemical changes whereby sediment --> sedimentary rock, ie how sediment is lithified to rock.
Thus aspects of weathering, transportation, sedimentation and lithification processes are all preserved in the rock
Raw materials of sediments
Clastic sediments are made up of clastic particles (detritus) - pieces of preexisting rock, minerals. So contains partially weathered particles, new particles produced by waethering (clay minerals) etc. these seds accumulte rapidly and 10x more of these than chem/biochem seds.
Chemical and biochemical sediments - precipitation from solution. Ppt. due to changes in solubility, evaporation, etc. leads to precipitation. Biochemical precipitation predominantly produces limestones
Bioclastic particles - biochem ppt, followed by breaking of shells
Transportation of clastic particles
Gravity, moving water, wind, ice move particles downhill
Currents - in water (and air) work against natural tenendency of a particle to fall in fluid. Generally faster the current, larger the particle that can be carried. Such particles are in suspension.
Changes in current velocity, eg slowing, lead to deposition of particles of a certain size, others left in suspension -
sorting : _ well sorted, poorly sorted
Modification of sediment by transport - weathering continues.
Chemical weathering may induce feldspar ---> clays.
Physical weathering causes abrasion of particles. Angular --> rounded
depending on distance of transport
Deposition - 1. sedimentary environments
Kinds of places where sediments are deposited
Sedimenatry basin
Kind of water (marine, lake, land); topography (lowland coast, shallow ocean, deep ocean etc), biological activity. Some examples:
Continental - desert, alluvial, lake, glacial
Coastal (shoreline) - delta, tidal flat, beach
Marine - organic reefs, continental shelf, cont. margin, deep sea
Facies - different environments are characterized by different sediments - these features of sediments characteristic of an environment are called sedimentary facies.
Can divide broadly into clastic and chem/biochem classes
Deposition - 2. sedimentary stuctures
bedding (lamination) - all sed rx posses some kind of layering - basic feature
cross bedding - when deposited by currents
ripples
graded bedding
bioturbation structures - disturbance caused by organisms living on or within sediment (trace fossils)
Burial and diagenesis - sediment to sedimentary rock
How buried? - subsidence
Areas of accumulation called sedimentary basins - shapes of these are variable
Diagenesis - set of processes that lead to lithification.
compaction- water squeezed out,
pressure solution
cementation
(if time recrystallization and replacement)
Clastic (detrital) sediments and sedimentary rocks
Classification by particle size
| Particle Size | Sediment | Rock |
| Coarse >2mm | Gravel | conglomerate (round), breccia(angular) |
| Medium | Sand | sandstone |
| fine | silt | siltstone |
| v. fine | mud | mudstone, shale (bedding fissility) |
Chemical/Biochemical sediments and sedimentary rocks
roughly tend to divide by chemical compostion
Carbonates: limestone and dolostone
Mostly biochemical: forams, coccolliths, corals etc
Some limestones may have purely chemical (inorganic) origin:
Ca2(HCO3 )2 --> CaCO3 + H20 +CO2
Ooids and oolites
Carbonate platforms eg Bahamas mixed sources of carbonates
Dolostone formed from dolomite CaMg(CO3)2. Formed primarily in diagenesis
Evaporites
Gypsum CaSO4.2H2O
Halite NaCl
Formed in closed ocean basins, Red Sea or tidal flats;
Non marine: lakes with no output Great Salt Lake, Dead Sea
Siliceous sediments
Chert SiO2 radiolaria, diatoms or inorganic; flint
Iron formations
Organic
Coal (de-volatized wood).
Petroleum: decay of marine phyto-plankton; continental shelves