3. Continental Clastic Environments Flashcards
What are examples of continental clastic environments?
1) Rivers - fluvial, alluvial
2) Lakes
- Lacustrine
3) Glacial - moraine, outwash
4) Desert - aeolian
5) Volcanic - Pyroclastic, debris flow
6) soils
7) Natural, urban and in-between
What is source to sink
- the complete sediment routing system
- a holistic approach
- intergration of multiple data sources
- focus on link between segments/depositional environments
- link between processes and stratigraphy
Why is source to sink usefull?
- improve understanding of landscapes and seascape evolution in 3D
- encourage thinking across disciplines
- improve predictability in ancient systems
Why are fluvial systems important?
- deliver vast majority of terrestrial sediment, organic carbon, and pollutants to the coast
- provide water
- flooding
- power generation
- fishing
define river
a ‘large’ conduit for the flow of water and sediment
define fluvial
processes and bedforms relating to rivers
define alluvial
processes and deposits related to rivers but occuring outside of the channel itself (floodplains, deltas)
What are the three modes of sediment transport in rivers?
1) Dissolved load
2) Suspended load
- Fine particles (sand,silt,clay)
- Turbulent eddies pick up, carry upward if vel. > settling vel.
3) Bedload
- On/near bed: rolling, bouncing (‘saltating’)
Suspended and bedload transport increase rapidly with flow strength (nonlinear relationship)
What are the two main types of rivers?
1) Bedrock rivers
- Part of the bed is bar rock, which the river has eroded into cutting down
- Generally in upper reaches of rivers
2) Alluvial rivers
- Bed consists of sediment (has a floodplain)
- Downstream reaches
Describe bedrock river
- Part of the bed is bar rock, which the river has eroded into cutting down
- Generally in upper reaches of rivers
- erosion rate depends on slope
- presence of sediment ‘tools’ (clasts) increases erosion
What is an alluvial fan depositional system?
They form at the exit of a drainage basin, has a radial sediment dispersal pattern and decreasing grain size and gradient downslope
Mix of sedimentary processes
- Debris flow
- Hyperconcentrated flows
- Fluvial channels
- Sheet floods
Fan is build of lobes and lobe-switching processes produces a composite cone
Describe a debris-flow dominated alluvial fan
- small and steep catchments
- High magnitude/low frequency events
- Common debrite lobe features
- big flash flood
describe a stream-flow drominated alluvial fan
- ‘Wet’ fan receive annual rains
- Avulsion and migration of rivers dominate
- Soil development
- better sorted
- finer grained
What is the recognition criteria for ancient alluvial fans
- Ancient fan deposits located adjacent to, and tilt towards, normal faults at basin margins
- Upward changes in grain-size and facies reflect cycles of growth/shrinkage(allogenic) and/or lobe switching(autogenic)
- Absence of marine fauna and immature texture
- Evidence of subaerial emergence: palaeosols and desiccation cracks
- Unidirectional to radial paleocurrents
Main types of (perennial) fluvial channel
Straight
Braided
- degree of channel subdivision by large migrating bedforms
Meandering
- Planform description of channel deviation from straight
Anastomosing
- More permanent distributive channel subdivision into smaller channels
Recognition criteria for ancient fluvial deposits
Deposits usually of relatively low texturally and compositional maturity
- Erosive-based,coarse grained facies associations (palaeochannel belts) dominated by tractional, current produced sedimentary structures with unidirectional paleocurrent patterns
- Fine grained facies associations (overbank: levee and floodplain deposits) include evidence of emergence
Marine fauna are lacking, but freshwater body and trace fossils may be present
What are sinuous river channels and what influences them?
Sinuous = shape
Meandering = process
Influences:
- sediment load: mixed load
- channel slope: rel. low gradient
- bank cohesion
- external forcing
What are the distinctive facies characteristics of sinuous/meandering rivers?
- Fining-upward patters
- Lateral accretion surfaces
- Mix of channel and overbank deposits
What causes meandering?
Secondary helical flow
- spiral flow (helical cells)
- outward at the surface, inward at the bed
Erosion on outside of bends
Deposition on inside of bends
What are lateral accretion surfaces?
- Main deposition in meandering fluvial channels is point bar on the inner bend
- migration of the point bar is lateral with a downstream component
- fining-up, and >1m thick, with inclined bedding (5-15degrees)
- small-scale structures on inclined beds indicating orthogonal flow
What are scroll bars
record bend migration in a lateral and downstream direction
seen on mars and detectable using seismology
Describe an in-channel facies from a meadering river
- channel bed lags
- lateral accretio units
- finig upwards
- abandoned channels
describe a floodplain facies
- levees (a few metres)
- overbank silts, peats/coals
- crevasse-splay deposits
- palaeosols
What is the recognition criteria for high sinuosity, meandering palaeochannels?
- low relief basal erosion surfaces - migration leads to channel belts
- tractional structures that record a sytematic reduction in flow velocity upwards or laterallt through the channel fill: lateral accretion surface
- relatively high preservation of fine-grained overbank facies
- sheet-like channel sandstone bodies with relatively low width:depth
What are braided rivers?
- Complex multichannel systems of low sinuosity
- Bedload dominated - gravel or sand
- High gradient
- Dynamic and rapidly changing
What are the origins of braiding?
- Variable discharge (seasonal)
- Slope/discharge & grainsize
- Sediment supply
- Bank erosion
- High width:depth ration
Describe bar formation/migration
- Flow convergence causes scour
- Flow divergence causes deposition
- Bar growth
- Dune amalgamation
- Lateral Accretion
- Causes flow deflection
- Bars grow and migrate at high stage
- Dissected and reworked at low stage
- Dominated by cross-beds
How is there a record of (sandy) braided river dynamics?
- Braid bars migrate downstream whilst sub-channels shift laterally: downstream accretion
- Deepest channels produce composite basal master erosion surface
- Remnants of braid bars and channel-fills are preserved between the internal erosion surfaces
- Braided rivers tend to sweep laterally with time, unless they are confined within valleys, to form multistorey channel belts (sheets)
What is the recognition criteria for low sinuosity braided rivers?
- High relief, composite erosion surfaces resulting in sheet-like channel belts
- Widespread planar and trough cross bedding, commonly coarse-grained
- Palaeocurrent variance between high and low discharge periods
- Low preservation of fine-grained overbank facies: lateral mobility of channels
What is preserved from a floodplain?
- Levees: low relief, fine and thin away
- Overbank fines: sediment storage
- Palaeosols: climate, organic traps, roots
- Crevasse splay deposits
Describe fluvial channel crevasse and avulsion
Aulsion is the rapid abandonment of a river channel and the formation of a new river channel
Crevasse channel and splay - thinner than parent channel
Crevasse seposits: evidence of rapid deposition
Impact downstream on dleta lobes
What are the summary end-members for fluvial facies models?
Braided River: at low flow stage. Deposits are dominated by downstream accreation. Deposits from low sinuosity, braided rivers are commonly wider & thicker than meanering rivers
High sinuosity meandering perennial river. Sand sheets comprise lateral accretion dominated deposits (point bars) sperated by extensive floodplain mudstones and splay sands
What are the facies models for the sub-humid and semi-arid seasonal tropics?
Rivers with extremely variable discharge, have distinctive deposit characteristics:
1. erosionally based channel-fill that exhibit complex lateral facies changes,
2. complex internal architecture that may lack macroform elements
3. an abundance of sedimentary structures formed under high flow stage (planar lamination)
