Sedimentary Rocks
Geology 200
Geology for Environmental
Scientists
Major Concepts
- Sedimentary rocks form by the processes of
weathering, erosion, transportation, and deposition, or
by chemical precipitation.
- Two main types
- clastic: pieces of older rocks (clasts)
- chemical: precipitated or formed in place
- Sedimentary structures are critical to
interpreting sedimentary rocks.
Major Concepts
- Major sedimentary environments:
- fluvial and alluvial
- eolian
- glacial
- delta
- shoreline
- organic reefs
- shallow marine
- deep marine
Major Concepts
Sedimentary rock layers are hierarchically
grouped or subdivided into both:
- rock stratigraphic units
- time stratigraphic units
We’ll cover this in the
section on stratigraphic correlation.
Clastic Sedimentary Rocks
- Produced by weathering of rocks.
- Breccia - large, angular grains
- Conglomerate - large, rounded
grains
- Sandstone - sand 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 environment.
- Generally speaking, higher energy water or
wind currents are required to move larger grain sizes.
Grain Size, cont’d
Size ranges: small to large
- boulders, >256 mm (>1 ft.)
- cobbles, >64mm (>3 in.)
- pebbles, >4mm
- granules, 2-4 mm
- sand, 1/8-2 mm
- silt, >1/256 mm
- clay, <1/256 mm
Grain Sorting
- Grain sorting can also be an indicator of
the energy of the environment.
- Well-sorted sediments are deposited
in high energy environments. Currents sort the grains by
size.
- Poorly-sorted sediments may
indicate weak currents, or transport by glaciers.
Chemical Sedimentary Rocks
- Produced by chemical precipitation.
- Evaporites - formed by evaporation
of seawater
- Salt, NaCl
- Gypsum, CaSO4
- Carbonates
- Limestone, CaCO3
- Dolostone, CaMg(CO3)2
Carbonates
- Typically, carbonates form in warm, clear
water free of clastic sediment.
- Carbonate grainstone - composed of
sand-sized grains from invertebrate skeletons or oolites.
- Carbonate mudstone - clay and silt-sized
grains from pellets and calcareous algae.
Coal
- Classified as a chemical rock by most
textbooks. But it is not a chemical precipitate. Instead
it forms from plant debris not consumed by bacteria and
other organisms.
- Takes about 10 ft. of peat to form 1 ft.
of bituminous coal.
Sedimentary Structures
- Provide clues to depositional environments.
Some examples:
- Cross bedding - rivers, dunes,
tidal channels
- Graded bedding - storms and
turbidites
- Ripple marks - lower energy
- Mud cracks - subaerial exposure
Color of Sedimentary Rocks
- Color can be useful in the interpretation
of depositional environments.
- Black color - indicates deposition
in the absence of oxygen in either the ocean, lakes, or
swamps.
- Red color - indicates deposition in
the presence of abundant oxygen in a warm, humid
terrestrial environment.
Environments of Deposition
- 3 Major Environments
- Continental or Terrestrial
- Marine
- Transitional
Continental or Terrestrial
Environments
- Glacial - both Alpine and Continental
- Alluvial Fans - at the base of mountains
- Lakes
- Rivers and Floodplains = Fluvial and
alluvial deposits
- Swamps
- Deserts
Marine Environments
- Continental Shelf - most common marine
environment in the rock record
- Shelf Edge
- Slope
- Ocean Floor
Transitional Environments
- Beaches
- Barrier Islands
- Tidal Flats
- Lagoons and Bays
- Estuaries
- River Deltas
Sedimentary Facies
- Facies - general appearance or
aspect of sedimentary rocks. Often correspond to
formations.
- A reflection of the depositional
environment.
- Lithofacies - defined by lithologic
features
- Biofacies - defined by organic
features
Sedimentary Facies
- Facies occur laterally adjacent to one
another just as do their depositional environments.
- Two different facies can have similar
lithofacies but different biofacies, or vice-versa.
Sedimentary Facies
- Facies migrate laterally with changes in
sea level.
- Rising sea level = transgression.
Marine facies overlie nonmarine facies.
- Falling sea level = regression.
Nonmarine facies overlie marine facies.
Causes of Changes in Sea Level
- Changing ice volumes at the poles.
- Thermal expansion or contraction of the
oceanic crust.
- Continental tectonics or subsidence.
- Shoreline progradation from river and
deltaic sedimentation.