6 - Continental Clastic Environments

What is source to sink?

What it involves:

• The complete sediment routing system

• A holistic approach

• Integration of multiple data sources

• Focus on link between segments/  depositional environments

• Link between processes and stratigraphy

Why is it useful:

• Improve understanding of landscape and seascape evolution in 3D

• Encourage thinking across disciplines

• Improve predictability in ancient systems

  

Define ‘river’, ‘fluvial’ and ‘alluvial’

• River: a ‘large’ conduit for the flow of water and sediment

• Fluvial: processes and bedforms relating to rivers

• Alluvial: processes and deposits related to rivers but occuring outside of the channel itself (floodplains, deltas)

What are the key influences on fluvial sediment preservation?

• To preserve non-marine strata into the rock record there needs to be net subsidence

• Alluvial, fluvial, and megafans form a continuum of distributive bodies – issue using tributive systems

• The distributive system builds radially through repeated channel avulsion events.

• Palaeorivers classed as either perennial meandering or braided, but also ephemeral and ‘flashy’ types

• Fluvial overbank is an important component: palaeoenvironmental record

How is sediment transported in river?

Three modes:

• Dissolved load/wash load (ions in solution - pollution)

• Suspended load

  • Fine particles (sand, silt & clay)

  • Turbulent eddies pick up, carry upward if vel. > settling vel.

• Bedload

  • On/near bed; rolling, bouncing (‘saltating’), etc.

Suspended and bedload transport increase rapidly with flow strength (nonlinear relationship)

What are the two main kinds of rivers?

Bedrock rivers; part of the bed is bare rock, which the river has eroded into cutting down

• Generally in upper reaches of rivers

• Erosion rate depends on slope

• Presence of sediment (‘tools’) increases erosion

  

Alluvial rivers; bed consists of sediment (‘alluvium’ = river-associated sediment)

• Downstream reaches

  

What are alluvial fan depositional systems?

Form at exit of drainage basin

Mix of sediment processes

• Debris flows

• Hyperconcentrated flows

• Fluvial channels

• Sheet floods

Lobe-switching process produces composite cone

Radial sediment dispersal pattern

Decreasing grain size and gradient downslope

What are the alluvial fan end member types?

Debris-flow dominated alluvial fan:

• Small and steep catchments. High magnitude/low frequency events. Common debrite lobe features

  

Stream-flow dominated alluvial fan:

• ‘Wet’ fan that receive annual rains. Avulsion and migration of rivers dominate. Soil development

  

What are 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 & immature texture

• Evidence of subaerial emergence: palaeosols and desiccation cracks

• Unidirectional to radial palaeocurrents

• Nature of fan considered based on facies: stream- vs. debris-flow

  

What are the main types of (perennial) fluvial channel (in planform)?

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

  

What are the recognition criteria for ancient fluvial deposits?

Deposits usually of relatively low textural and compositional maturity

1. erosive-based coarse grained facies associations (palaeochannel belts) dominated by tractional, current-produced sedimentary structures with unidirectional palaeocurrent patterns

2. 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 features of meandering river?

Sinuous =  shape, meandering = process

Influences on sinuous river channels

• Sediment load: mixed load

• Channel slope: rel. low gradient

• Bank cohesion

• External forcing

  

Causes of meandering: secondary helical flow

• Secondary flow cells

  • Spiral flow (helical cells)

  • Outward at the surface, inward at the bed

• Erosion on outside of bends

• Deposition on inside of bends

  

Distinctive facies characteristics

• Fining-upward patterns

• Lateral accretion surfaces

• Mix of channel and overbank deposits

What are depositional features of meandering river?

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 > 1 m thick, with inclined bedding (5-15°)

• Small-scale structures on inclined beds indicating orthogonal flow

  

Scroll bars: record bend migration in a lateral and downstream direction

What are the facies models for meandering river?

In-channel facies

• Channel bed lags

• Lateral accretion units

• Fining upwards

• Abandoned channels

Floodplain facies

• Levees (a few metres)

• Overbank silts, peats/coals

• Crevasse-splay deposits

• Palaeosols

  

What are the recognition criteria for high sinuosity, meandering palaeochannels?

• Low relief basal erosion surfaces – migration leads to channel belts

• Tractional structures that record a systematic reduction in flow velocity upwards or laterally 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 the features of braided rivers

Complex multichannel systems of low sinuosity

• Bedload-dominated

  • Gravel or sand

• High gradient

• Dynamic and rapidly changing

Origins of braiding

• Variable discharge (seasonal)

• Slope/discharge & grain-size

• Sediment supply

• Bank erosion

• High width: depth ratio

  

Describe channel/point 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 are (sandy) braided river dynamics recorded in stratigraphy?

• 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 are the criteria for recognition of 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 are the sedimentary features of floodplains?

• Levees: low relief, fine and thin away

• Overbank fines: sediment storage

• Palaeosols: climate, organic traps, roots

• Crevasse splay deposits

  

What are the effects of channel crevasse and avulsion?

• Crevasse channel and splay – thinner than parent channel

• Crevasse deposits: evidence of rapid deposition

• Impact downstream on delta lobes