Sediments Flashcards
Describe the properties of a limestone
- Made of calcium carbonate
- Non-clastic
- Typically composed of shells of marine life etc
- Can be chemogenic (oolitic limestones)
Describe sediments deposited in an aeolian environment
- Fine-grained due to low density and velocity of air
- Grains move by saltation
- Repeated impact means grains well-rounded and high quartz content
- Dunes, often cross-bedding
- Common in deserts, usually red due to iron oxide (no bacteria to reduce)
Describe sediments deposited in a fluvial environment
- Braided rivers - river struggles to transport sediment (a lot or coarse - upstream)
- Transport during seasonal high water levels
- Movement as bedload
- Coarse grained
- Meandering river - flow more efficient
- Transport as suspended and bedload
- Alternating layers of mudrock and sandstone due to floods (heterolithic sediment)
- Calcretes: carbonate precipitates from ground water around roots, forming thick CaCO3 layers in mudrock
Describe sediments deposited in a deltaic environment
- Coarsening upwards - delta progrades (moves out to sea)
- Sediments sorted by distance travelled suspended in sea
- Hence as delta moves out coarser sediments overlay finer ones
Describe sediments deposited in a glacial environment
- Can be very large (>10m) - ice is low velocity and high viscosity
- Till - glacial sediment deposit when melts
- Poorly sorted matrix supported conglomerate (diamictite)
- Icebergs with large rocks float to sea - dropstones
What is a clastic sediment?
Composed of grains separated from a parent rock by erosion
What is a carbonate sediment?
Typically non-clastic, formed of CaCO3 grains, either from skeletons of organisms or precipitated from ocean (ooids)
What is a sedimentary log?
Graphical representation of vertical rock sections. x axis is grain size, y axis is time.
What is a regolith?
A rock loosened by erosion, available for transport by fluids
Describe sediments deposited in a turbidite environment
- Turbidite (water plus sediment) moves down shelf under gravity
- Deposits sediments in order (Bouma sequence) - graded bedding
- Fining up with time
- Submarine fan: sediment originates from point source (eg underwater canyon). Turbidites flow for long distances
Describe the physics of entrainment
- Entrainment - when the force is just great enough to move a sediment
- Lift force provided by Bernoulli effect
- Greater velocity required for less dense fluid (air)
- Drag and push force from fluid
- Suspended load maintained by buoyancy (Archimedes)
- Laminar flow better at entrainment (low Reynolds number)
Draw and explain the Hjulstrom diagram
- Shows relationship between critical flow velocity (v required to transport sediment) and grain size
- In general, larger grains = greater velocity needed
- Hence larger grains settle first as v decreases, this leads to sorting
- Erosion leads to roundedness
Describe erosive processes creating a regolith
- Chemical weathering - solution, hydrolysis, oxidation
- Physical weathering - salt growth, freeze-thaw, biological intrusion (roots)
Explain what leads to graded bedding
- Graded bedding - within a bed, grains become coarser/finer along an axis
- Due to changes in flow velocity leading to different-sized grains settling
- Decreasing velocity - fines upwards
- Increasing velocity - coarsens upwards
Which minerals are more resistant to weathering, so more likely to be found in sedimentary rocks?
- Felsic minerals more resistant - sedimentary rocks have high quartz content, some feldspars
- Clay minerals formed during erosion or diagenesis
