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Sedimentary RocksSedimentary Rocks
Geology 200Geology for Environmental ScientistsGeology for Environmental Scientists
Major Concepts
• Sedimentary rocks form by the processes of weathering, erosion, transportation, and g, , p ,deposition, or by chemical precipitation.
• Two main typesTwo main types– clastic: pieces of older rocks (clasts)
h i l i i d f d i l– chemical: precipitated or formed in place• Sedimentary structures are critical to
interpreting sedimentary rocks.
Major Concepts• Major sedimentary environments:• Major sedimentary environments:
– fluvial and alluvialli– eolian
– glacial– delta– shoreline– organic reefs– shallow marine– deep marine
Major Concepts
Sedimentary rock layers are hierarchically grouped or subdividedhierarchically grouped or subdivided into both:
k i hi i– rock stratigraphic units– time stratigraphic unitsg pWe’ll cover this in the section on
t ti hi l tistratigraphic correlation.
Clastic Sedimentary Rocks
• Produced by weathering of rocks.• Breccia large angular grains• Breccia - large, angular grains• Conglomerate - large, rounded grains• Sandstone - sand sized grains• Siltstone silt sized grains• Siltstone - silt sized grains• Shale and Mudstone - silt and clay
sized grains
Grain Size
• Grain size can be an indicator of the energy of the environmentenergy of the environment.
• Generally speaking, higher energy i d i dwater or wind currents are required to
move larger grain sizes.
Grain Size cont’dGrain Size, cont dSize ranges: small to largeS e a ges: s a o a ge• boulders, >256 mm (>1 ft.)
bbl >64 (>3 i )• cobbles, >64mm (>3 in.)• pebbles, >4mm• granules, 2-4mm• sand 1/8-2 mmsand, 1/8 2 mm• silt, >1/256 mm
l 1/2 6• clay, <1/256 mm
Grain SortingGrain Sorting• Grain sorting can also be an indicatorGrain sorting can also be an indicator
of the energy of the environment.W ll d di d i d i• Well-sorted sediments are deposited in high energy environments. Currents sort the grains by size.
• Poorly-sorted sediments may indicatePoorly sorted sediments may indicate weak currents, or transport by glaciers.
Poorly sorted glacial deposit ranging from cobbles to mud-sized particlescobbles to mud-sized particles.
Chemical Sedimentary RocksChemical Sedimentary Rocks• Produced by chemical precipitation.• Evaporites - formed by evaporation of
seawaterseawater–Salt, NaCl–Gypsum, CaSO4
• Carbonates• Carbonates–Limestone, CaCO3
–Dolostone, CaMg(CO3)2
Carbonates• Typically, carbonates form in warm,
clear water free of clastic sediment.• Carbonate grainstone composed of• Carbonate grainstone - composed of
sand-sized grains from invertebrate k l t litskeletons or oolites.
• Carbonate mudstone - clay and silt-ysized grains from pellets and calcareous algae.calcareous algae.
Coal
• Classified as a chemical rock by most textbooks But it is not a chemicaltextbooks. But it is not a chemical precipitate. Instead it forms from plant debris not consumed by bacteria anddebris not consumed by bacteria and other organisms. It is an organic rock.
• Takes about 10 ft. of peat to form 1 ft. of bituminous coal.o b u ous co .
Sedimentary Structures
• Provide clues to depositional environments Some examples:environments. Some examples:
• Cross bedding - rivers, dunes, tidal h lchannels
• Graded bedding - storms and turbiditesg• Ripple marks - lower energy• Mud cracks - subaerial exposure
TOP
BOTTOM
Fig. 5.9. Large scale cross-bedding formed by eolian dune migration. Flow direction and top of bed indicated.
A meandering stream with a sandy point bar on the inside curve, and a cut bank on the
outside curve. The point bar dips toward the p pstream channel.
Color of Sedimentary Rocksy• Color can be useful in the
interpretation of depositional environments.
• Black color - indicates deposition in the absence of oxygen in either thethe absence of oxygen in either the ocean, lakes, or swamps.
• Red color - indicates deposition in the presence of abundant oxygen in a p ygwarm, humid terrestrial environment.
Sideling Hill Outcrop, I-68. What is the depositional environment of the black layer?depositional environment of the black layer?
Environments of Deposition
• 3 Major EnvironmentsC i l T i l• Continental or Terrestrial
• MarineMarine• Transitional
Continental or TerrestrialContinental or Terrestrial Environments
• Glacial - both Alpine and Continental• Alluvial Fans at the base of mountains• Alluvial Fans - at the base of mountains• Lakes• Rivers and Floodplains = Fluvial and
alluvial depositsalluvial deposits• Swamps• Deserts
M i E iMarine Environments• Continental Shelf - most common
marine environment in the rock recordmarine environment in the rock record• Shelf Edge• Slope• Ocean FloorOcean Floor
Transitional Environments
• Beaches• Barrier Islands• Barrier Islands• Tidal Flats• Lagoons and Bays• Estuaries• Estuaries• River Deltas
Marginal Marine Environments, a prograding Spit on a Barrier Islandprograding Spit on a Barrier Island
Sedimentary FaciesSedimentary Facies• Facies general appearance or aspect• Facies - general appearance or aspect
of sedimentary rocks. Often d t f ticorrespond to formations.
• A reflection of the depositional penvironment.
• Lithofacies defined by lithologic• Lithofacies - defined by lithologic features
• Biofacies - defined by organic features
Sedimentary Facies
• Facies occur laterally adjacent to one another just as do their depositionalanother just as do their depositional environments.
diff f i h i il• Two different facies can have similar lithofacies but different biofacies, or vice-versa.
Sedimentary Facies
• Facies migrate laterally with changes in sea levelin sea level.
• Rising sea level = transgression. i f i li iMarine facies overlie nonmarine
facies.• Falling sea level = regression.
Nonmarine facies overlie marineNonmarine facies overlie marine facies.
Causes of Changes in Sea Level
• Changing ice volumes at the poles.• Thermal expansion or contraction of• Thermal expansion or contraction of
the oceanic crust.• Continental tectonics or subsidence.• Shoreline progradation from river andShoreline progradation from river and
deltaic sedimentation.
P l i i t b d it WVPennsylvanian point bar deposits on WV Rt. 19 near Oak Hill.
coal seam point bar surfaces
A meandering stream with a sandy point bar on the inside curve, and a cut bank on the
outside curve. The point bar dips toward the p pstream channel.
A modern marine transgression is exposing Holocene peat deposits on the
Louisiana coastline.Louisiana coastline.
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