Clastic Depositional Environments
The main environments for coarse clastic deposition
• Fluvial Environments• Deserts• Lacustrine Environment• Deltas• Shoreline Marine Environments• Shallow marine shelves and epeiric seas• Continental margins and deep water basins• Glacial Environments
Fluvial Sedimentary Environments
Alluvial Fans
The Start of the Sedimentary Cycle
• Bedrock weathered away from uplifted areas (mountain ranges)
• Carried away in mountain streams• Start the process of building up
sedimentary deposits.• First of these deposits to form: Alluvial
fans
How do alluvial fans form?
• When a narrow (confined) canyon stream disgorges onto a valley floor
• Sudden deceleration in flow and in gradient– Decreased ability in the stream to carry
coarser material: this is dropped.• Results in a cone-shaped deposit of
coarse stream sediments, sheet flood deposits and debris flows: Alluvial Fan
Typical structure of an alluvial fan
RADIAL FAN SECTION
FAN SURFACE
RADIAL PROFILE
• Alluvial fans best known from arid/ semi-arid environments, where periodic flow occurs in the canyons but also occur in humid environments.– Usually triangular in map view and wedge-shaped in
cross section.– Slopes range from 1 – 25°, average 5-10°.– The larger the particle size, the steeper the slope.
• Described as “active” when the fan is building or “inactive” when it is not.– To be active: must be continued uplift and erosion of
highlands to supply sediment: fault scarps are common sites of alluvial fans.
Typical structure
of an alluvial
fan
RADIAL FAN SECTION
FAN SURFACE
RADIAL PROFILE
• Alluvial fans can build out onto:– Playas– Lakes– Floodplains of permanent rivers– Coastal plains– Directly into the sea (fan deltas)
• The surface of the fan is dissected by radiating channels
What sort of sediments are found on alluvial fans?
• Generally coarser than other fluvial deposits (short transport distance)
• Compositionally immature• Grain size normally decreases down-fan• Upper- Mid- and Lower-Fan facies can be
distinguished.
Transport of material on alluvial fans
• Three methods:– Debris flow– Stream flow– Mud flow
Debris Flow on Alluvial Fans• When sediment becomes saturated with water: flows as
a viscous plastic mass: behaves like quicksand• High density – High Viscosity Flow• Debris flow can carry very large boulders and also clays
and fine particles• Results in very poorly sorted deposits with little or no
stratification• Sometimes the base of a debris flow shows inverse
grading (grain size increases upwards)• Generally form lobes in the upper reaches of the fan.
Debris Flow on Alluvial Fans
• Produces laterally extensive beds without erosive bases
• Matrix-support fabric
Stream Flow/ Stream Floods• In arid environments• Flash floods in canyons: extreme erosional power.• Low viscosity Flow• As flow velocity decreases, bounders, cobbles and
pebbles are dropped (clast supported fabric).• Results in a flow that is choked with more sediment than
it can carry: braided streams form on the fan (dry up quickly)
• Each flood cuts new channels, filling old ones with gravel.
• Produce cross-bedded pebbly sandstones with imbrication: generally upper / mid-fan deposits
Stream Flow: Sheetflood deposits
• At high flood levels, sand and gravel-rich flow covers the mid-fan (N.B. no fine material): Sheetflood Deposits
• Typically well sorted, stratified and cross-bedded.• Commonly form lobes than emerge from the channel at
the intersection point of the fan surface & the channel profile
• Little silt/ clay so water flows freely through these deposits without blocking the pores so these lobe deposits are commonly called sieve deposits
• These become progressively coarser towards the front of the lobe, where gravel accumulates.
• Sieve deposits are normally proximal/ upper mid-fan deposits.
Schematic profile of a sieve lobe
deposit
Mud Flow deposits on alluvial fans
• Where the debris flow is primarily fine particles
• Forms restricted narrow lobes like debris flows
• Mudflows that are more fluid can form enormous sheetflood deposits (>10km/h)
• Very fast moving, very dangerous deposits.
Typical depositional structure in alluvial fans
• Require rapid uplift: commonly found in– Rapidly downdropping grabens– Foreland basins– Strike-slip basins
• Typical profile:– Mixture of unsorted debris flows– Stream channel conglomerates (fanglomerates)– Cross-bedded sandstones– Sieve deposits– Commonly coarsen up in the stratigraphic record
• Sequence:– Cross-bedded ssts of distal fan at base– Overlain by coarser proximal fan deposits as uplift continues– Thin fining up sequence of fan decay on top
Can be very thick: 9000m of fanglomeratesat margin of San Andreas Fault.
Fluvial Sedimentary Environments
Braided River Systems
Basic features
• High discharge of water/ high sediment load / high topographic gradient
• Network of low sinuosity channels• Often originate on alluvial fans: rapid erosion and
little vegetation to stop run-off• Rapid erosion
– channels quickly become choked with sediment– Almost as soon as channel is scoured, it is infilled– So much sediment can fill the channel that it pokes out
of the surface: gravel bar (may become vegetated)– Rapid divergence of channels due to gravel bars
Braided river channels• Generally broad and shallow channels• Where sand grade dominates: commonly floored
by dunes• Sand bars and large dunes (straight crested)
divide the stream into smaller channels• Sand bars and dunes are exposed at low
discharge levels• Can have smaller ripples and dune developed
on them at this stage• The dunes and bars migrate downstream and
alter the positions of the channels
• In coarse grained braided systems:• Commonly have flat bedded imbricated
gravel channel deposits– These usually have thin, rippled and dune
sandstone tops (bar top sandstones)• Fine grained deposits are poorly
developed (only in abandoned channels)• Braided systems usually aggrade, but can
migrate laterally
Dominant components of braided facies
• Normally dominated by channel and bar facies with tabular, planar bedding (downstream migration of bars and dunes)
• Very little fine grained sediment• No overall fining up motif• Some trough cross bedding may be present• Internal erosion surfaces are common• Channels may show some fining upwards• Braided systems generate
– Elongate multistorey sand bodies with sheet geometry (depending on lateral migration)
– Mudrocks are missing or rare– Palaeocurrents are unidirectional with low dispersion
Fluvial Sedimentary Environments
Meandering Stream Facies
Meandering streams• Confined flow: Possess distinct channel &
overbank subenvironments• High sinuosity meandering streams develop in
regions of low discharge and low topographic gradient.
• Associated vegetation inhibits widespread erosion and limits sediment supply.
• Produced by confined flow with periodic overflow of banks
• The typical succession and general pattern of sedimentation is shown in the following diagram:
• The Channel– Formed by lateral movement erosion and deposition– generally has large dune structures on its floor– Often has coarse channel lag deposit– Generates a fining up motif with point bar above lag– Often contains mud pellets from river bank
• The Point Bar– Dunes also occur on lower part of point bars– These produce trough cross beds (compare with braided)– Flat bedded sands of upper flow regime may also occur on the
point bar – in the uppermost part, rippled sands produce cross laminations
and mud drapes
Gravel deposited in a channel
Graded Bedding in Fluvial Deposit
• Alluvial Ridges– Produced by migration: erosion and deposition of point bar
produces a region higher than the rest of the alluvial plain.• Levees
– are built up from fine sands and silts that are deposited duringriver high stages
• Crevasse channels– can be cut through the channel bank to bring coarser sands onto
the floodplain in crevasse splays– This produces thin sandstone/ siltstone interbedded deposits
• Overbank deposits– Overbank flooding carries suspended fine material onto the
floodplain– Often evidence of drying out– Floodplains can also be sites of soil formation, marshes,
swamps, lakes…
Mudcracks formed in overbank
• Meandering streams may abandon channels during their development.– Form Oxbow lakes gradually filled with fine
grained sediment (clay plugs)– These generally help to keep the river to its
alluvial ridge but occasionally the river breaches these to build a new alluvial ridge on the floodplain
• This process is known as avulsion
Meandering Stream Sedimentation Patterns
• Generates a fining upwards succession– through point bar migration
• Occasional floods smooth off the point bar to form epsilon cross bedding of the lateral accretion surface– Always dips normal to palaeoflow direction
• Thick floodplain deposits with channel point bar sandstones characterize the meandering environment
• Point bars may be connected to each other or may be separated by floodplain silts.
Fining up in a fluvial
system
Comparison: Meandering and Braided Rivers
BedloadSuspendedLoad transport
Sand and gravelMud and siltPredominant sediment load
Sand and gravel at margins
Mud & Silt mainly at margins
Channel cross section
Shallow but WideDeep but NarrowSediment cross section
LOWHIGHChannel sinuosity
YESNOAlluvial Island Bars formed
Desert Sedimentary Environments
Aeolian sands, playas & ephemeral lakes/ streams
• Arid deserts: 20° - 30° latitude in areas of constant high pressure or rain shadow (mountains)
• Wind less efficient: coarser material left as deflation lag or desert pavement, sand/ finer material transported.
• Desertic environments are NOT just sand dunes: a series of subenvironments exist.
• Desert character depends upon balance between – wind strength (can exceed 190kph), – rate of sediment supply– effectiveness of vegetation binding.
• Arid climate: particles are weathered in upland regions and move to lowland regions in ephemeral streams, which pass through wadis.
• Aeolian deposits will be developed in a relatively arid setting where there is more sand being transported into the system than is leaving it.
• Particles deposited in lowlands coalesce into sand seas or ergs.
• Ripples and dunes build up and migrate across the ergs (and the ergs themselves move) unless the sand is fixed by vegetation (savannah).
• Sand systems will eventually reach the sea (longest route=5000km).
• Particles move long distances, hence sphericity and roundness.
• Remember: sand can move uphill, unlike water.
High winds & Low Sediment Supply
• Rock & Gravel Pavements– Finer grains (clay and silt grades) are transported in
suspension as red dust storms, sometimes for 1000’s km before they settle from the air
• Where supply of sediment does not match the rate of erosion, – the sand may be removed down to the damp water-
table. – This deflation can give a very distinctive planar
erosion surface in aeolian deposits.– Sand transport will resume when more sediment is
supplied or the water-table drops still further.
Bedforms common in deserts
• Dunes, ripples…• Cross-bedding (very large scale)• Evaporites common in dried out Playa
lakes (interdune)• Reddening of sediments common.• Most importantly: aeolian sands are
marked by definite truncation surfaces
Saltating Sand
Big Dunes
• No limit on the overlying fluid so the dune size is limited only by wind strength.
• Each cross set can be up to 35m thick with foresets dipping 20-35° (steeper than water lain deposits)
• Cross beds are generally long sweeping features with asymptotic bases
Barchan Dunes
Aeolian Cross-bedding
Dune structures
• Low amplitude wind ripples can migrate up the lee faces of aeolian dunes: not found in fluvial systems.
Modern Wind Ripples
Ephemeral Streams & Lakes
• Between dunes• Thin mudstones with rain pits and
dessication features: ephemeral lakes• No regulr progression between dunes and
interdune deposits.• Ephemeral lake formation often a random
process
• Low, flat "interdune" areas lie between the crescentic dunes.
• Playa Lakes: commonly fine grained lacustrine style deposits in the interduneareas. Often dry out: evaporitic.
Recognizing aeolian systems
• Roundness, sphericity• High-angle cross beds• Very thick cross bed units• Reddened• Interdune deposits of muds and evaporites• Very texturally and mineralogically mature
sediments
Fossils in the desert
• Commonly reptile/ dinosaur trackways.
Shoreline Deposition
What governs the depositional style?
• Rate of water discharge from rivers• Rate of sediment influx from rivers• Tidal regime and action of waves• Larger scale marine currents• Climate and vegetation• Geometry of shelf slope/ marine basin• Relative movements of land and sea, rate of
marine basin subsidence
• Modern shorelines: some erosional/ some depositional.
Depositional Shorelines• Interplay between sediment supply from land
and ability of marine processes to remove it.• Produces a range of shoreline types from
sediment dominated to marine process dominated.
• Consequence of this interplay: Shoreline Shape• Low weight shorelines: Deltas built up• Linear Shorelines: Barrier Islands and lagoons.• Essential to distinguish between these in the
record.
River Mouth Flow Behaviour
• Water + Sediment in river enters the sea or lake: 3 possibilities
• 1. Inflow more dense: turbidity current deposits submarine fan
• 2. Equal densities: sediment dispersed radially into a narrow zone: gilbert delta
• 3. Inflow less dense: Build up of sediment due to deceleration: Deltas Build