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Sedimentary Rock Classification Depositional Environments Sedimentary Structures

Sedimentary Rock
+ 5% of rocks in the outer 10 miles of crust. + 75% of rocks exposed at the surface. + Preserve evidence of surficial processes...

February 14, 2002
Geology 211, Physical Geology Geology 211, Physical Geology ©Copyright, 2002, Ron Parker ©Copyright, 2002, Ron Parker

! Occurring today ! That occurred in the past
Geology 211, Physical Geology, ©, 2002, Ron Parker

Types of Sedimentary Rock
+ Sediment originates from mechanical and/or chemical weathering. + Two major divisions of sedimentary rock. ! Detrital – Made of particles. ! Chemical – Made from dissolved ions that crystallize into solids. Geology 211, Physical Geology, ©, 2002, Ron Parker

Detrital Sedimentary Rocks
+ Sedimentary particles are…
! Eroded from pre-existing rock.
: Mechanical weathering : Chemical weathering

! Transported by wind, water or ice. ! Retained in a depositional environment. Geology 211, Physical Geology, ©, 2002, Ron Parker

Transport Effects
+ Sediments are modified by transport. + Sorting – The uniformity of grain size. + Sorting indicates the constancy of environmental energy. ! Well-sorted – Uniform grain size. ! Poorly sorted – Wide variety of grain sizes.

Transport Effects
+ Rounding – Relative sphericity. + Sediment grains start out as angular grains. + With transport, sediments bocome more spherical. ! Well-rounded – Long transport distances ! Angular – Negligible transport

! Well sorted – Uniform energy i.e. a beach ! Poorly sorted – Variable energy i.e. an alluvial fan Geology 211, Physical Geology, ©, 2002, Ron Parker

Geology 211, Physical Geology, ©, 2002, Ron Parker

1

Transport Effects
+ Maturity – Unstable minerals (feldspars, micas) are removed with transport and by chemical weathering. ! Immature sediments ! Mature sediments
: Contain unstable grains : Indicate short transport or arid conditions : Only stable grains (quartz, clay) : Longer transport; humid conditions Geology 211, Physical Geology, ©, 2002, Ron Parker

W. W. Norton

Well-rounded, well-sorted sandstone Detrital Sedimentary Rocks + Minerals
! Quartz ! Clay Minerals

+ Particle size basis of classification.
Boulder gravel Cobble gravel Pea gravel Granule sand
S. C. Porter

Coarse sand Fine sand Silt Clay
Geology 211, Physical Geology, ©, 2002, Ron Parker

Sediments

Conglomerate
+ Conglomerate - Rounded gravel sized sediments cemented together.

B. Sedimentary Rocks

+ Indicate high-velocity water borne transport.

Geology 211, Physical Geology, ©, 2002, Ron Parker

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Conglomerate and Breccia
+ Both are composed of particles greater than 2mm in diameter + Conglomerate consists of rounded gravels + Breccia is made of large angular particles

Breccia
+ Like a conglomerate except the particles are angular, NOT rounded. + Indicates little transport from source.

Geology 211, Physical Geology, ©, 2002, Ron Parker

Geology 211, Physical Geology, ©, 2002, Ron Parker

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. + Siltstone is similar with smaller grains.

Shale
+ Composed of clay-minerals (extremely small particles). + Deposited only in quiet non-agitated water. Common in deep water basins. + Often contains organic matter (dark) and fossils.

Geology 211, Physical Geology, ©, 2002, Ron Parker

Geology 211, Physical Geology, ©, 2002, Ron Parker

Shale

Chemical Sedimentary Rocks
+ Consist of precipitated material that was once in solution (dissolved). + Precipitation of material occurs in two ways : Inorganic processes : Organic processes (biochemical origin)

Geology 211, Physical Geology, ©, 2002, Ron Parker

Geology 211, Physical Geology, ©, 2002, Ron Parker

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Chemical Sedimentary Rocks
+ + + +...
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