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Recognizing Sedimentary Structures
Flaser bedding
Plane beds
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Bi-directional pattern
Intermittent flows cause alternating sand and mud layers
Tidal environments, high energy
Individual ripples and mud drapes
Lenticular bedding
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Alternating sand and mud
Tidal stands before flow reverses
More mud than flaser beds
Intertidal (foreshore)
Laminar flows; parallel particle movement
‘Lamination’ in rocks
Coarser grains, low velocities
Finer grains, high velocities
Current lineations
Massive beds
• No stuctures
• Bioturbated zones
Graded bedding
• Normal; fining up
• Settling out or decrease flow
• Reverse; coarsening up
Cross Stratification
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Inclined layering that dips toward flow direction
Cross laminations from current ripples
Straight/sinuous or lingoid (unconnected)
Planar-same direction; trough- sinuous to linguoid
Dunes creates larger scale cross-beds
Flow Regime Diagram
• Can be used to estimate velocity, change in velocity of
depositional flow.
• A bedform stability diagram shows how the type of bedform
that is stable varies with both grain size and flow velocity.
• Boundaries change with water density and depth.
Lower flow regime
• Subcritical flow and change lies within antidune field.
• Ripples dunes and lower plane beds.
Upper flow regime
• Plane beds and antidunes form.
• Supercritical Flow
• Bedforms in-phase with wave propagation.
Vertical Sequences of Deltas
Delta Plain
Progradation
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• Sediment building out; progressively shallower
• Shallowing up sequence; Coarsening up.
• Sea level fall, change in channel path Avulsion.
Transition between fluvial/alluvial environment.
Branching and meandering of river as distributary channels.
Channel sands and plain muds
Brackish water
Delta Front
• Immediately forward of the channel mouth.
• Coarsest drops first into progressively finer
• Delta slope: 1-2 degrees (fine)
Prodelta
30 degrees (coarse)
• Suspended plume in front of the delta.
• Lower density river waters
• Fine grained sediment settles furthest from
delta front
• Gravity currents bring coarse down
Retrogradation
• Sediment erosion exceeds accumulation
• Deepening up sequence; fining up
• Sea level rise, constricted channel system
River dominated
River dominated
Tide dominated
Wave dominated
Anthropogenic Factors
Humans have been known to constrict the
outflow of water and sediment.
Preventing accumulation and channel
migration that keeps the delta in
equilibrium. As subsidence continues, and
the channel is unable to avulse,
the delta plain goes into retrogradation.
Ice transport/deposition
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High viscosity allows larger particles
Produces laminar flow.
Long period transport
Significant around polar caps
Greatest at times of extended glaciation.
Air transport/deposition
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Low density limits load capacity.
High viscosity limits grain size in load.
Up to fine grained sand.
Turbulent flow.
Water transport/deposition
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The most significant medium.
Viscosity allows up to cobble and boulder size
Density allows for greater capacity of load.
Turbulent flow to laminar flow
Wave-Dominated Coastal Environment
Continental shelf:
• Offshore Zone: Below storm-wave base
• Silt and mud, Extensive Bioturbation
• Storm-silt layers; Tempesites
• Weakly graded; suspension settling
• Offshore Transition:
• Sand, fine-grained, some silt, bioturbation,
• sedimentary structures include hummocky cross
stratificaton.
• Transported shells; bivalves
• Wave and current
• Nearshore (shoreface): region between low-tide
and fair weather base.
• Mainly fine sand to medium
• Fine laminations, crossbeds
• Longshore bar/ridges oblique to coastline
• Oscillatory wave
• Foreshore: Surfzone, swash, between mean high and
mean low
• Sand, medium-fine grained
• Land and seaward dipping laminae
• Shoaling wave
• Herringbone cross
• Backshore: from berm to dunes
• Fine grained sand with heavy minerals
• Wind ripples up to dunes