Sedimentary Rocks

Chapter 6

What Are Sediments?• Loose particulate material• In order of decreasing size

Sources of Sediments1. From weathering & erosion2. From chemical precipitation

What Happens to Sediments1. transported by:

– Water.– Ice.– Wind.– Gravity.

• Most sediment is buried and converted to sedimentary rock.

2. Deposited by• Rivers• Wind• Glaciers• Other water

• When the transportation vector has run out of energy, the sediments are deposited (dropped).

What Happens to Sediments

River Sorting- Transporting & Depositing

Headwaters Mouth

Grade = change in elevation/length

Principle of Original Horizontality Most sedimentary layers of rock are deposited in a horizontal

position, with older rocks laid down first

Lithification Sediment becomes sedimentary rock

through lithification, which involves: • Compaction • Cementation • Recrystallization (of carbonate sediment) • Ex: sand

Remember that SiO2 is released into groundwater from the chemical weathering of quartz and feldspars.

3 Classes of Sedimentary RockClastic - fragments of

rock debris produced by physical weathering. Ex. Sand & clay.

Chemical - sediment precipitates from solution in water. Ex. Calcium carbonate & salt.

Biogenic (organic) - sediment composed of the fossilized remains. Ex. Coal, oil, & natural gas.

Clastic Sedimentary Rock• From the weathering of

other rocks – broken texture– Clasts (larger pieces, such

as sand or gravel) – Matrix (mud or fine-grained

sediment surrounding the clasts)

– Cement (the glue that holds it all together), such as:

• calcite • iron oxide • silica

Gravel Clastic Rocks• If rounded clasts = conglomerate • If angular clasts = breccia

Sand Clastic RocksDifferent Sandstones based on

dominate grains

• quartz grains = quartz sandstone

• feldspar grains = arkose

• sand-sized rock fragment grains = graywacke

Silt Clastic Rocks

• Siltstone - Grain size 1/256 to 1/16 mm (gritty)

Clay Clastic Rocks• Grains less than 1/256 mm

(smooth) • Shale (if fissile – splits) • Kaolinite (if massive)

also called Claystone

• Note: Mud is technically a mixture of silt and clay. (Mudstone)

Chemical Sedimentary RocksChemicals removed from seawater and made into

rocks by chemical processes, or with help of biological processes (such as shell growth).

3 types– Evaporites– Carbonates – Siliceous

Chemical EvaporitesFrom the evaporation of

water (usually seawater). • Rock salt - composed of

halite (NaCl). • Rock gypsum -

composed of gypsum (CaSO4.2H20)

• Travertine - composed of calcium carbonate (CaCO3), and therefore, also technically a carbonate rock

Chemical Carbonates• Formed through both chemical & biochemical processes. • Include the limestones (many types)• Two minerals are dominant:

– Calcite (CaCO3) – Dolomite (CaMg(CO3)2)

Chemical SiliceousDominated by silica (SiO2). • From diatoms, radiolarians,

or sponges.

• Diatomite - looks like chalk, but doesn’t fizz in acid. Made of diatoms. Also referred to as Diatomaceous Earth.

• Chert - Massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

Biogenic (Organic) Sedimentary Rocks Coals • organic matter (plants).• Increasing depth of burial

(temperature and pressure): – Peat (porous, brownish

plant fragments)– Lignite (crumbly and

black) – Bituminous (dull to shiny

and black; sooty; layers may be visible)

– Anthracite (extremely shiny and black; low density; not sooty

Formations from Sediments• Large enough to be recognized.• Ex.- haystack rock, sand dunes, delicate arch,

balanced rock

Sedimentary Environments (Sinks) Places where sediments accumulate and

sedimentary rocks form

3 Major Groups1. Continental 2. Marine3. Transitional

Terrestrial Environments• Aluvial Fan• Braided

stream • Lakes • Rivers• Levees• Swamps • Deserts• Glacial

Marine EnvironmentsSeas & oceans• Continental shelf • Continental slope and

rise (deep sea fans) • Abyssal plain • Reefs

Transitional Environmentsbetween the land and the sea.