Continental clastic environments Flashcards
Give six examples of continental clastic environments
Rivers, lakes, glaciers, desert, volcanos, soils
What processes operate in rivers?
Fluvial (the action of a river) and alluvial (related to rivers but outside the channel, e.g. floodplains, deltas)
What process operates in lakes?
Lacustrine (the action of a lake)
What processes operate in glaciers?
Moraine (material left by a moving glacier), outwash (sediment carried by water from a melting glacier)
What process operates in deserts?
Aeolian (wind)
What processes operate around volcanoes?
Pyroclastics, debris flows
Describe source-to-sink
Holistically describes a complete sediment routing system using integrated data from multiple sources
What is the ultimate sink for sediment?
Deep sea, but much doesn’t make it there
What are the two end members for sources in source-to-sink?
Typical passive/mixed systems, typical small/large tectonically active systems
Why is source-to-sink useful?
Improve understanding of landscape/seascape evolution in 3d, thinking across disciplines, improves predictability in ancient systems
What do fluvial systems do?
Deliver the majority of terrestrial sediment, organic carbon, and pollutants to the coast
Why is there so much focus on fluvial systems?
Provide water, power generation, fishing, can cause flooding
Define river
A large conduit for the flow of water and sediment
When does there need to be net subsidence?
To preserve non-marine strata in the rock record
i.e. to be in a sedimentary basin
Give three examples of distributive bodies
Alluvial, fluvial, and megafan processes
How do distributive systems build?
Radially through repeated channel avulsion events
Describe a channel avulsion event
The rapid abandonment of a river channel and the formation of a new one
How are palaeorivers classified?
Meandering or braiding
May also be ‘flashy’ (ephemoral)
Describe fluvial overbank
Deposits accumulated as a result of overbank flooding and avulsion
An important part of the palaeoenvironmental record
What are the three modes of sediment transport in rivers
Dissolved load/wash load, suspended load, bedload
Describe dissolved load/wash load in rivers
Ions in solution
Pollution
Describe suspended load in rivers
Fine particles (sand, silt, clay)
Turbulent eddies pick up and carry upward (if velocity>settling velocity)
Describe bedload in rivers
On/near bed
Rolling or bounding (saltating)
Describe the relationship between suspended load and bedload transport with flow strength
Suspended load and bedload transport increase rapidly with flow strength
This is a non-linear relationship
What are the two main kinds of rivers?
Bedrock rivers and alluvial rivers
Describe bedrock rivers
Part of the bed is bare rock that the river has eroded into
Generally in upper reaches of rivers
What increases erosion in bedrock rivers?
Steeper slopes and the presence of tools (sediment)
Describe alluvial rivers
The bed consists of sediments (alluvium, river-associated sediment)
Generally in downstream reaches of rivers
What forms at the exit of drainage basins?
Alluvial fan deposition systems
These are distributive
What sediment processes are combined in alluvial fan depositional systems?
Debris flows, hyperconcentrated flows, fluvial channels, sheet floods
What processes produces the composite cone of an alluvial fan?
Lobe-switching
Describe depostion at an alluvial fan
Radial sediment dispersal pattern with decreasing grain size and gradient down slope
What are the two end member types of alluvial fans?
Debris-flow dominated alluvial fan and stream-flow dominated alluvial fan
Describe a debris-flow dominated alluvial fan
Small and steep catchments, they have high magnitude/low frequency events and common debrite lobe features
Describe a stream-flow dominated alluvial fan
‘Wet’ fan that receives annual rain
Avulsion and migration of rivers dominate
Development of soil
Give three examples of recognition criteria for ancient alluvial fans
Absence of marine fauna, evidence of subaerial emergence (e.g. desiccation cracks), and unidireciton to radial palaeocurrents
Define perennial rivers
Those that cut through the soil profile, removing fine particles
What are the four main types of perennial fluvial channel in planform?
Straight, braided, meandering, anastomosing
Define a braided fluvial channel
Degree of channel subdivision by large migrating bedforms
Complex multichannel systems of low sinuosity
Describe the characteristics of a braided river
Bedload-dominated (gravel/sand), high gradient, dynamic and rapidly changing
Give three examples of origins of braiding
Variable (seasonal) discharge, bank erosion, high width:depth
Describe the dynamics of braided rivers
Braid bars migrate downstream and sub-channels shift laterally
Overall downstream accretion
When do multistorey belts (sheets) form?
When braided rivers are prevents from sweeping laterally over time because they are confined within valleys
Give two examples of recognition criteria for low sinuosity braided rivers
Low preservation of fine-grained overband facies (bc of lateral mobility of channels), palaeocurrent between high and low discharge periods
Describe a meandering fluvial channel
Planform deviations from straight channel
Sinuous shape
Describe the overbank of a meandering river
Strength is needed to stop it washing away
Usually comes from clay and flora
Give three distinctive facies characteristics of meandering rivers
Fining-upward patterns, lateral accretion surfaces, mix of channel and overbank deposits
Give four examples of influences on meandering (sinuous) river channels
Sediment load (mixed load), channel slope (relatively low gradient), bank cohesion, external forcing
What causes meandering?
Secondary helical flow
Describe secondary flow cells
Spiral (helical) flow, flowing outward at the surface and inward at the bed
Causes erosion on the outside of beds and deposition on inside of beds
Describe lateral accretion surfaces in meandering river channels
Main deposition is at point bar (inner bend)
Migration of the point bar is lateral with a downstream component
Describe scroll bars
Record bend migration in a lateral and downstream direction
Give four characteristics of in-channel facies of meandering rivers
Channel bed lags, lateral accretion units, fining upwards, abandoned channels
Give four characteristics of floodplain facies of meandering rivers
Levees (few m), overbank silts/peats/coals, crevasse-splay deposits, palaeosols
Give four recognition criteria for high sinuosity, meandering palaeochannels
Channel belts, lateral accretion surface, high preservation of fine-grained overbank facies, sheet-like channel sandstone bodies with low width:depth
Describe an anastomosing fluvial channel
A more permanent distributive channel subdivision into smaller channels
Describe fluvial deposits
Usually of relatively low textural and compositional maturity
Give two examples of facies as recognition criteria for ancient fluvial deposits
Erosive-based coarse-grained facies associations dominated by tractional, current-produced sedimentary structures with unidirectional current
Fine-grained facies association with evidence of emergence (overbank: flood plain and levee)
Describe features of fluvial deposits used to recognise ancient ones
Lack of marine fauna
Freshwater body and trace fossils may be present
Define a channel bar
Elevated region of sediment that has been deposited by the flowing water
What causes channel bar formation/migration?
Flow convergences causes scour
Flow divergence causes depostion
Give three aspects of channel bar growth
Dune amalgamation, lateral accretion, causes flow deflection
Describe channel bar growth/formation
Bars grow and migrate at high stage
Bars are dissected and reworked at low stage
They are dominated by cross beds
When does a crevasse splay form?
When a flow breaks its levees and deposits sediment on the flood plain
What are crevasse deposits evidence of?
Rapid depostion
What impact can crevasse splays have?
Impact on downstream delta lobe
What are the end member fluvial facies models (Miall)
Low sinuosity braided river
High sinuousity meandering river
Give two examples of deposit characteristics of rivers with extremely variable discharge
i.e. sub-humid, semi-arid seasonal tropics
Complex internal architecture that can lack macroform elements
Abundance of sedimentary structures formed under high flow stage (planar lamination)
