Transcript Sedimentary Rocks

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)
Sediments and Sedimentation
• 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
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
More than
256
Boulder
64 to 256
Cobble
4 to 64
Pebble
2 to 4
Granule
1/16 to 2
Sand
1/256 to
1/16
Silt
Less than
1/256
Clay
Sediment
Name
Sedimentary
Rock
Gravel
Conglomerate
or Breccia
Sand
Sandstone
Mud
Siltstone, Shale
or Mudstone
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
Types of
Sedimentary Rocks
• 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
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