Lithification/Diagenesis

• From pile of sand to solid rock

Loose sediment, like that shown in (A) may someday become a rock like the one

in (B) if compacted and cement fills the spaces

between clasts

Diagenesis: How Sediment Becomes Rock• Diagenesis:

collective term for all the chemical, physical, and biological changes that affect sediment as it goes from deposition through lithification

Sand or othersediment, grainsseparate

After compaction,grains crushedtogether andinterlocked

After cementation,mineral crystalscemented grainstogether

Diagenesis: Processes Involved

1. Compaction - packing together of sediment grains

When sediment has been deposited we start with a pile of fragments that enclose a great deal of pore space

As sediment continues to accumulate the sediment undergoes compaction: the geometric arrangement of grains changes so that pore space is reduced

In principle, compacted sediment can sit there foreverwithout becoming a rock - normally this doesn't happen

Diagenesis: Processes Involved

2. Desiccation – loss of water from sediment pore spaces

Desiccation cracks

ie. Desiccation and Compaction of Shale

Diagenesis: Processes Involved

3. Cementation – ions precipitate out in the pore spaces to form a cement that binds clasts together

                                            

                

ie. Compaction and Cementation of Sand

Clastic Sedimentary Rocks

Conglomerate

Sandstone

Breccia

Shale

Angular fragments

Breccia

Conglomerate

Rounded particles & dissimilar lithologies

Sandstone Sandstone

Quartz Sandstone - > 90% qtz

Mudstone

Shale –fissile (layered)

Claystone –not fissile (not layered)

Non-Clastic Sedimentary Rock

• Organic sediments• Sediment precipitates from solution in

water originating from chemical and organic processes– These chemical precipitants settle to the

bottom of a body of water. When first deposited, these sediments are loose and non-structured. In time, they are slowly hardened by compaction, cementation, and re-crystallization

Non-Clastic Sedimentary Rock

• Classified based on composition:– Siliceous Rocks– Carbonates– Evaporites

Non-Clastic Sediments: Siliceous rocks

Siliceous rocks - The siliceous rocks are those which are dominated by silica (SiO2)

Chert from Washademoak Lake, NB

Chert - Palaeoindian projectile points

Non-Clastic Sediments: Carbonates

Carbonates - The carbonate sedimentary rocks are formed through both chemical and biochemical processes

Example: Limestone

Chalk cliffs - Dover, England

Non-Clastic Sediments: EvaporitesEvaporites form from the evaporation of water

(usually sea water or lake water)

Example: Rock salt (halite)

Bonneville Salt Flats of the Great Salt Lake, Utah.

Rock salt forms as a result ofchanging physical conditions(increasing temperature), whereminerals dissolved in seawater areprecipitated when the waterevaporates

Non-Clastic Sedimentary Rocks

Dolostone

Oolitic Limestone Fossiliferous Limestone

Chalk

Non-Clastic Sedimentary Rocks Fossil Fuels Deposits• Found in areas of sedimentary rock

formation– Basically, plant and animal remains were deposited

in swamp and mud areas.

– Result: Poor oxygen quality, hence they did not decay.

– Overtime, they accumulated and then sands, silts and clay buried them

– As lithification of the sediments took place, the organic remains beneath turned to valuable fossil fuels

Fossil Fuel Deposits: Example

Coal – made of organic matter which is the end product of large amounts of plant material buried for millions of years

Coal bed,Trinidad

Fossil Fuel Deposits: Example

• Natural Gas and Oil Deposits– Drops of oil were scattered throughout source rocks– As source rock was squeezed, this oil becomes

trapped in reservoirs / traps• These reservoirs make this an economic goldmine

– Common characteristics for reservoir / trap• Source rock can be squeezed• Reservoir rock is porous and permeable• Cap rock is impermeable• Groundwater pushes the lighter gas and oil to the top• When a trap is tapped, an extreme amount of pressure is

released– Think oil well

Fossil Fuel Deposits: Common Trap Types• Structural traps hold oil and gas because the earth

has been bent and deformed in some way.  The trap may be a simple dome (or big bump), just a "crease" in the rocks, or it may be a more complex.– Types: Anticline trap, Fault trap, Salt dome

• Stratigraphic traps are depositional in nature.  This means they are formed in place, usually by a sandstone ending up enclosed in shale.  The shale keeps the oil and gas from escaping the trap.– Types: Stratigraphic (pinch out) trap

Types of Traps

Sedimentary Rock: Non- Metallic Industrial Mineral Deposits

• Products of deposition, minerals in these rocks are non-metallic– Example:

• Thick salt beds are the result of the movement of ocean water into basins. As the water evaporates due to desert-like conditions, the salt is left behind

– These are Evaporites