Sedimentary Rocks
Chapter 6
What Are Sediments?• Loose particulate material
• In order of decreasing size
Sources of Sediments1. From weathering & erosion
2. From chemical precipitation
What Happens to Sediments
1. 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: sandRemember 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. Marine
3. Transitional
Terrestrial Environments
• Aluvial Fan
• Braided stream
• Lakes
• Rivers
• Levees
• Swamps
• Deserts
• Glacial
Marine Environments
Seas & oceans
• Continental shelf
• Continental slope and rise (deep sea fans)
• Abyssal plain
• Reefs
Transitional Environmentsbetween the land and the sea.