16-19 sediment Flashcards
Sediment
Sediments are loose materials such as rock
fragments, mineral grains and shell fragments that have been moved by wind,
water or ice driven by gravity
Weathering
The process which breaks up solid rocks or converts them chemically
Physical - disintegration of rock without a change in chemical composition
eg. decrease in pressure resulting in joints, expansion of water when frozen, salt crystal formation and plant or animal
Chemical - decomposition of rock as a result of chemical attack. Chemical composition changes
eg. dissolution (no solids ions in solution), hydrolysis (new solids) and oxidation (rusting)
Influences on chemical weathering
Temperature: weathering rates double with 10°C rise
Precipitation: H2O is required for hydrolysis and H2O and CO2 form carbonic acid
Vegetation: respiration in soils produces CO2
Reactions are faster in warm wet environments
In what climate zones would you expect weathering to be dominated by chemical
processes?
Tropical and temperate environments
In what climate zones would you expect weathering to be dominated by physical
processes?
Polar and alpine environments
Mineral stability and weathering rates
Silicate minerals that crystalize at higher temperatures are generally less stable at earths surface
General rules:
Mafic minerals weather by oxidation
Felsic mineral weather by hydrolysis
Carbonates and salts weather by dissolution
Minerals containing Fe, Mg, U, Na and K weather faster
Sediment types
Clastic sediment
Biogenic sediment
Chemical sediment
Organic sediment
Lithification
Compaction: occurs as the weight of the accumulating sediment forces the grains together. water and air is expelled as pore space is reduced
Cementation: circulating pore water in sediments precipitate to form a cement that binds the grains together. Calcium carbonate, silica, and Fe compounds are common cements
Recrystallization: occurs due to increased heat and pressure with burial. development of stable minerals from unstable ones
Clastic sediment characteristics
Grain size: clay < silt < sand < gravel
Sorting: poorly sorted and well sorted
Rounding: angular and well rounded
Composition: the relative proportion
of resistant (quartz) vs. non-resistant (mafic) minerals
Graded bedding
A gradual change in
average grain size within a
single layer or bed
Normal grading: largest grains at bottom
Reverse grading: largest grains at top
Bedforms
Ripple marks:
Short, steep downcurrent slip face (lee side)
Long, gentle upcurrent ramp (stoss side)
Used to indicate current direction in ancient sediments
The steep side points away from current unless very strong eg \\\ would go»_space;>
Clastic sedimentary rocks
Conglomerate/Breccia:
- Contains rounded grains >2 mm (-1 φ)
- Finer matrix material cemented by silica, calcite, or Fe oxides
- Easy to ID parent rocks
- Clast texture and arrangement can be used to ID depositional process
Sandstone:
- Grain size ranges from .063 to 2.0 mm (-1 to 4 φ)
- Silica, carbonate, or Fe-oxide cement
Siltstone and shale:
- Grain size ranges from <.004 to .063 mm (< 8 to 4 φ)
- Consist largely of clay minerals and micas
- Siltstone has a gritty texture
- Shale has a smooth texture
Chemical sedimentary rocks
Halite and gypsum:
Evaporites—rock from evaporated sea or lake water
- Evaporation triggers deposition of chemical precipitates
- Thick deposits require large volumes of
water
Travertine:
Calcium carbonate (CaCO3) precipitated from ground water where it reaches the surface or the interior of a cave
- Dissolved calcium (Ca2+)reacts with
bicarbonate (HCO3-) to form calcite
- CO2 expelled into the air causes CaCO3 to precipitate.
Dolostone:
Carbonate rock composed of the mineral dolomite, a calcium magnesium carbonate: CaMg(CO3)2
Forms by the reaction of magnesium-bearing groundwater with calcium carbonate in limestone.
Chemical precipitates formed from biogenic or chemical sedimentary rocks
Biogenic (or biochemical) sedimentary rocks
Limestone:
Consists of CaCO3 shells/skeletons of marine organisms
Can form in shallow tropical waters or deeper marine because of the settling of foramminifera
Can form chert due to varying silica levels
Dolostone:
Carbonate rock composed of the mineral dolomite, a calcium magnesium carbonate: CaMg(CO3)2
Forms by the reaction of magnesium-bearing groundwater with calcium carbonate in limestone.
Chemical precipitates formed from biogenic or chemical sedimentary rocks
Organic sedimentary rocks
Coal:
- Organic sedimentary rock composed of plant remains
- Organic matter preservation requires rapid burial and low oxygen concentrations
- Plant material that died and accumulated in a swamp or forest floor
