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
What is clay?
- Clay is an ultrafine-grained, cohesive sediment made of clay minerals
- Formed during erosion of parent rock or diagenesis of sedimentary rock
- Sheet silicates with Si and Al tetrahedra
- eg chlorite, kaolinite
What is the Uden-Wentworth scale?
- Classifies rocks by grain size
- Based on logarithmic phi scale
- phi = -log2(grain size)
Describe the flow regime leading to ripples and cross-lamination
- Low velocity flow regime
- Erratic eddy currents cause grains to cluster
- Moving fluid pushes sediment up the slope (stoss side)
- Sediment crests and flow separates, causing avalanches down lee slide, accumulation
*Cluster grows into a ripple - New ‘slopes’ preserved as downstream-dipping laminae (cross-lamination - at an angle to bedding)
- Either planar or concave-upwards (trough)
Describe the flow regime leading to dunes and cross-bedding
- Low velocity flow regime
- Stronger flow than ripples
- Ripples reworked into dunes (different features - discontinuity)
- Dunes have larger wavelength and height
- Eddy currents push sediment up the steep face of the dune (reverse flow)
- Dunes create cross-bedding
Describe the flow regime leading to plane bedding and planar lamination
- Could be zero flow (lower plane bed)
- Could also be intermediate flow regime
- Stronger flow washes out dunes (entrainment)
- Increased sediment (bedload and suspended) dampens the effect of turbulence
- Forms a flat upper plane bed with planar lamination
Describe the flow regime leading to antidunes
- High flow regime
- Rare in sedimentary record
- Flow so high that fluid moves faster than waves on its surface
- Hence waves move upstream, steepen and then break (collapse on themselves)
- Moves sediment to form antidunes with laminae dipping upstream
Describe bedforms created in bidirectional flow regimes
- Symmetrical
- Formed if water depth less tha wavebase
What determines if an environment is depositional or erosional?
Evidence of erosional environment:
* Dessication cracks
* Raindrop impressions
What are the types of sedimentary basin?
Describe sediments deposited in lacustrine environments
- Organic rich sediments (anoxic seafloor)
- Clastic sediment from feeder river (fluvial)
- Evaporites (gypsum, halite) when lake dries up
Describe sediments deposited in alluvial fan environments
- Water exits mountain valleys and fans out
- Common at margins of basins
- Debris flows - water/sed mix flows short distance under gravity
- Coarse grained
- Poorly sorted matrix supported conglomerate
- Imbrication: pebbles dip upstream
Describe sediment deposited in coastal environments
- Sediment supplied by rivers, moved by longshore drift
- Carbonate coasts: ooids/biogenic material deposited
- Arid climate: hypersaline lagoons. Evaporites gypsum and anhydrite.
Describe sediment deposited in marine carbonate environments
- Shallow marine - seds have biological origin
- Complete skeletal remains or bioclasts (fragments)
- Stromatolites - calcified microbial mats
- Ooids - carbonate precipitated around a nucleus (usually a bioclast)
Describe sediments deposited in shallow marine environments
- Dominated by tide and wave processes
- Herringbone crossbedding - dip in opposite directions due to bidirectional tide
- Storm waves generate Hummocky cross-stratification (mounds)
