Chapter 8 – FROM SEDIMENT INTO

SEDIMENTARY ROCK

Sediments and Sedimentation• Deposition

– Laying down of sediment• Separated into 3 broad

categories– Clastic (Detrital)– Chemical– Biogenic (Biochemical)

• Clastic sediment– Fragmented rock &

mineral debris from weathering & erosion

– Described by particle shape, angularity, & size

– Volcani-clastic sediments• Pyroclasts

– Distinguished by size (bombs, lapilli, ash)

Sediments and Sedimentation

• Chemical sediment– Precipitation of minerals dissolved in lake, river, or sea

water• Limestone

– Shallow sea water evaporation causes dissolved salts to precipitate

Sediments and Sedimentation

• Biogenic sediment– Primarily composed of plant or

animal remains• Shells, bones, teeth• Wood, roots, leaves

Sediments and Sedimentation

Sedimentary Rocks• Lithification

– Processes where loose sediment transforms into sedimentary rock• Bedding

– Layers of strata in sediment/sedimentary rock

• Bedding surface– Top or bottom of rock

bed• Graded bed

– Bed with large clasts at bottom, fining toward top

• Cross bedding– Beds are inclined

relative to layer they occur in

Lithification Processes• Compaction

– Reduced pore space due to weight of sediments above

• Cementation– Dissolved substances

precipitate out & act as glue

• Recrystallization– Form new crystalline

mineral grains from old ones

• Diagenesis– Low

temperature/pressure changes

Types of Sedimentary Rocks• Clastic sedimentary

rock– Conglomerate or breccia

• Large fragments in fine grained matrix

– Sandstones• Medium grained (sand

sized)• Arkose or Quartz

Sandstone– Mudstones

• Very fine grained rock• Same composition as

shale but lacks fissility– Shale

• Very fine grained fissile rock

• Fissile = breaks into layers

Particle Sizes in Clastic Sedimentary Rocks

Diameter (mm)

Particle Type

Sediment Name

Sedimentary Rock

More than 256 Boulder

Gravel Conglomerate or Breccia

64 to 256 Cobble

4 to 64 Pebble

2 to 4 Granule

1/16 to 2 Sand Sand Sandstone

1/256 to 1/16 Silt

Mud Siltstone, Shale or MudstoneLess than

1/256 Clay

Types of Sedimentary Rocks• Chemical sedimentary rocks

– Classified mainly by composition– Evaporite

• Formed by lake or sea water evaporation

• Rock salt; rock gypsum

– Limestone & related• Oolitic• Dolostone

– Chert

• Biogenic sedimentary rocks– Limestone

• Mostly made of calcite• Coquina• Chalk

– Peat• Formed by

accumulation & compaction of plant remains

– Coal• Formed by lithification

of plant-rich sediment• Bituminous coal

Types of Sedimentary Rocks

Depositional Environments• Interpreting

environmental clues– Patterns made by air

& water that moved over sediments

– Ripple marks– Fossils– Mud cracks

Depositional Environments on Land• Streams

– Alluvial fan• Forms where stream is not

constrained by valley walls• Fan-shaped

• Lakes– Delta

• Forms where stream enters standing body of water

• Usually triangle-shaped– Playa

• Seasonal lake

Depositional Environments on Land• Glacier

– Till• Mixed particles

sizes & shapes• Wind

– Eolian sediment• Sediments carried & deposited by

wind– Loess

• Wind-blown silt & clay sediment

Depositional environments in & near the ocean• Delta• Estutary

– Semi-enclosed body of coastal water

– Fresh water mixes with sea water• Beaches

– Mixed material; quartz common• Shelves

– Silt or sand sediment; numerous fossils

• Carbonate platforms and reefs– Reef: wave-resistant structure built

of marine invertebrate skeletons

Depositional environments in & near the ocean• Turbidites & turbidity currents

– Turbulent, gravity driven flow of sediment & water

– Moves sediment from continental shelf to deep sea

• Seafloor– Nutrient-rich

• Calcareous ooze• Siliceous ooze

Depositional environments in and near the ocean• Sedimentary facies

– Changes in the character of sediment from one environment to another

How Plate Tectonics Affect Sedimentation• Divergent plate

boundaries– Rift valleys

• Convergent plate boundaries– Collisional type – Subduction type

• Back-arc basin• Accretionary wedges