chapter 6 sediments and sedimentary rocks
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Chapter 6
Sediments and Sedimentary Rocks
GEOL 101
Introductory Geology
GEOL 101
Sediments
Collective name for loose, solid particles originated from weathering and erosion, thus Unconsolidated.
Classification of sediments based on sizes
Gravel > 2mm
Sand between 1/16 and 2 mm
Silt between 1/256 mm and 1/16 mm
Clay < 1/256 mm
GEOL 101
Change from Sediments to Sed Rxs
Transportation
Roundness: round – distance from source
Sorting: well sorted - distance from source
Deposition: settle of particles
Preservation
GEOL 101
Change from Sediments to Sed Rxs
LithificationCompaction – reduction in pore space
Cementation – seal out the pores
Crystallization – growth of crystals from solution.
GEOL 101
Sedimentation Process
GEOL 101
What is Sedimentary Rock?Sedimentary rocks are products of mechanical and chemical weathering
Account for about 5 % (by volume) of Earth’s upper crust
Contain evidence of past environments• Provide information about sediment transport• Often contain fossils
Economic importance• Coal
• Petroleum and natural gas
• Sources of iron, aluminum, and manganese
GEOL 101
Sedimentary Rocks
Rock types
Classification
Sedimentary Environments
Fossils
GEOL 101
Classified according to material type• Clastic• Chemical
Two major textures are used in the classification of sedimentary rocks
• Clastic– Discrete fragments and particles– All detrital rocks have a clastic texture
• Nonclastic– Pattern of interlocking crystals– May resemble an igneous rock
Sedimentary Rock Classification
GEOL 101
Sedimentary Rocks Types
Sediment originates from mechanical and/or chemical weathering
Rock types are based on the source of the material
• Clastic rocks – transported sediment as solid particles
• Chemical rocks – sediment that was once in solution
GEOL 101
Detrital Sedimentary RocksThe chief constituents of detrital rocks include:
• Clay minerals• Quartz• Feldspars• Micas
Particle size is used to distinguish among the various types of detrital rocksParticle shape and sorting give indication of transport distance (maturity)
GEOL 101
Particle Size Classification
GEOL 101
Sorting, Shape, Setting
Sorting
SphericityRoundness
DepositionalSetting
GEOL 101
Clastic Sedimentary RocksCommon Clastic sedimentary rocks (in order of increasing particle size)
• Shale– Mud-sized particles in thin layers that are
commonly referred to as laminea– Most common sedimentary rock
• Sandstone– Composed of sand-sized particles– Forms in a variety of environments– Sorting, shape, and composition of the grains
can be used to interpret the rock’s history– Quartz is the predominant mineral
GEOL 101
Detrital Sedimentary Rocks
• Conglomerate and breccia– Both are composed of particles greater than
2mm in diameter
– Conglomerate consists largely of rounded gravels
– Breccia is composed mainly of large angular particles
GEOL 101
Shale containing plant remains
GEOL 101
Sandstone
GEOL 101
Conglomerate
GEOL 101
Breccia
GEOL 101
Chemical Sedimentary RocksConsist of precipitated material that was once in solution
Precipitation of material occurs in two ways
• Inorganic processes• Organic processes (biochemical origin)
Common chemical sedimentary rocks
GEOL 101
Chemical Sedimentary RocksCommon Types
Limestone
Dolostone
Chert
Evaporites
Coal
GEOL 101
Limestone• Most abundant chemical rock
• Composed of calcite carbonate CaCO3
• Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)
• Inorganic limestones include travertine and oolitic limestone
GEOL 101
Coquina
GEOL 101
Fossiliferous Limestone
GEOL 101
Chemical Sedimentary Rocks• Dolostone
– Composed of calcite, magnesium carbonate CaMgCO3
– Typically formed secondarily from limestone
• Chert– Made of microcrystalline quartz
– Varieties include flint , jasper and agate (banded form)
• Evaporites– Evaporation triggers deposition of chemical
precipitates
– Examples include rock salt and rock gypsum
GEOL 101
ChertAgate – Banded Chert
GEOL 101
CoalCommon chemical sedimentary rocks
Different from other rocks because it is composed of organic material
Stages in coal formation (in order)• Plant material• Peat• Lignite• Bituminous
GEOL 101
Coal
GEOL 101
Coal Formation Stages
GEOL 101
Sedimentary Environments
A geographic setting where sediment is accumulating
Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)
Identified by sedimentary facies, fossils, and structures
GEOL 101
Sedimentary Environments• Continental
– Dominated by stream erosion and deposition– Glacial– Wind (eolian)
• Marine– Shallow (< 200 meters)– Deep (seaward of continental shelves)
• Transitional (shoreline)– Tidal flats
– Lagoons
– Deltas
GEOL 101
Depositional Environments
GEOL 101
Depositional Environments
Continental Marine
GEOL 101
Turbidity Currents
GEOL 101
Turbidity Currents
Graded Bedding
GEOL 101
Evaporite Formation
GEOL 101
Sedimentary FaciesDifferent sediments often accumulate adjacent to one another at the same time
Facies: possesses a distinctive set of characteristics reflecting the conditions in a particular environment
Merging of adjacent facies tends to be a gradual transition
GEOL 101
Sedimentary Facies
GEOL 101
Sedimentary Structures
Useful for interpreting sedimentary environment
• Strata: or beds, layers of similar sediment (most characteristic of sedimentary rocks)
• Lamina: thin layers (< 1cm)• Bedding planes separate strata• Cross-bedding• Graded beds• Ripple marks• Mud cracks
GEOL 101
Bedding
GEOL 101
Bedding Structures
Lamina Graded bedding
GEOL 101
Cross Beds
GEOL 101
Ripple Marks
unconsolidated sediment lithified rock
GEOL 101
Mud Cracks
GEOL 101
Grand Canyon
o
GEOL 101
Fossils: Evidence of past life
By definition, fossils are the traces or remains of prehistoric life now preserved in rock
Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)
GEOL 101
Dinosaur footprint in limestone
GEOL 101
Fossils: Evidence of past life
Geologically fossils are important for several reasons
• Aid in interpretation of the geologic past• Serve as important time indicators• Allow for correlation of rocks from
different places
Studied in detail in Historical Geology (GEOL 102), offered in Spring 2004
GEOL 101
Chalk Cliffs of Dover, England
Tiny planckon shells
Deposited on sea floorLater uplifted
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