1. Depositional Environments and Their Facies

2. Environments of Deposition
Continental
Fluvial alluvial fan, braided stream, meandering stream
Desert dunes, playa lakes, salinas
Lacustrine
Glacial
Deltaic delta plain, delta front, prodelta
Marginal- Marine
Beach/ barrier island
Estuary/ lagoon
Tidal Flat
Neritic continental shelf, organic reef
Marine
Pelagic continental slope and rise, deep- ocean floor

4. Continental (Terrigenous)
Journey from source to sink
Plate tectonics lead to mountain building
Slope! Exposure! Produces rock
Climate causes weathering
Surface processes move sediments, sculpt landscape
Weathering, erosion, transport, deposition

5. Deposition Requires place to put sediments Depression/ basin
Deep Marine is best
Lakes are good for short records
Terrestrial is difficult!
Glacial sediments are misleading
Abundant because they are recent

6. Deposition Gravity works to move sediments
Water
Wind
Change in slope or change in climate impacts sediment transport and abundance

7. Alluvial Fan
Localized areas of enhanced sedimentation downstream of points where laterally confined flows expand
Narrow valley or gorge opens up, onto piedmont or coastal plain
At basin margin
Often structurally controlled (fault or mountain front)
Contribute greatly to the rock record!
10’s of meters - >100 km cone
“Fanglomerate”

8. Alluvial Fan Facies Models
3 types
Debris- flow dominated
Braided fluvial fans
Low- sinuosity/meandering fluvial fan
Generally
lacking fossils
Fan shaped
Texturally immature

9. Death Valley Martin Miller

10. Alluvial Fan Facies Models
Morphology
Proximal, mid-fan, distal
Streamflow processes dominate
Stream- channel sediments
Long narrow bodies of coarsest materials
Poorly sorted
Sheetflood deposits
Sediments settle out of suspension
VERY high sediment load
Deposit gravels even!
Sieve deposits
Very coarse grains are deposited (> sand)
Fines (and water) settle through

11. 3 Alluvial Fan Facies Models
Debris- flow dominated
Poorly sorted
Pebbles, gravels, boulders dominate
No sedimentary structures
Possibly reverse graded bedding in base
Flow is initiated when strength is exceeded
Freezes after short distance
Although some travel up to 24 km (Sharp and Nobles, 1953)
Mud- flow dominated
Similar but sand and muds dominate
Braided Fluvial Fan
Meandering Fluvial Fan

12. Associated with Fluvial Facies
Braided streams
Straight channel streams
Meandering streams
Share characteristics with stream- dominated fan!
Associations!
Large scale (mountains?)
Small scale (flood plain?)

13. Alluvial to Fluvial Transition
Move downslope, from coarse to fine
Low sediment supply: scoured between systems
High sediment supply: continuous transition to braided streams, then meanders
Steeper slopes
coarser sediments, ephemeral flow
Mountain edges, ice sheets
Rivers are overloaded with sediments
Tend toward BRAIDED streams
Shallower slopes
finer sedimentss, lower velocity flow
Vegetation slows flow on slope; no pre-Devonian meandering streams

14. River Basics Drainage basin (= catchment area); contributive network
Stream
Part of a watershed
Drainage basin (= catchment area); contributive network
Provides water and sediment to stream
Adjacent basins separated by divide
Many scales; continental divide
Stages
Young: Mountain regions
Erosional features dominate
Mature
Formation of flood plains and point bars
Depositional features dominate (old, also)
Old: Coastal regions
Rivers coalesce; Channels reduced in size due to division
Tie in to GA

17. Channel Patterns Channels define rivers
Function of adjustment of channel to changes
Gradient, Cross-section
Controlled by
Sediment load
River characteristics
Amount and nature of discharge
Three basic channel patterns
Straight
Braided
Meandering
Types Change! Along length or at flood stage

21. Fluvial Facies
Means by which sediment is transferred across the land surface towards its eventual resting place (Collinson, 1996)
Characteristics of streams
gradient- water flows downhill
increase slope, increase velocity
Three styles of flow
alluvial fan- debris accumulates at base of slope
braided stream- high bedload; multiple channels
meandering stream

22. Fluvial Processes Erosion Provenance Transport and deposition
Textural, compositional maturity
Wide range of associated sedimentary structures
Post-depositional alteration/ pedogenesis
Often associated with soils
Alluvium
Paleosols (in floodplains)

23. Fluvial Processes- Erosion
Wide range of scales! (spatial, temporal)
Incision
Vertical cutting of substrate that deepens the channel
Single flood event
Progressively increasing discharge
May be accompanied by widening
Drop in base level
Strands river banks as terraces
Migration of channels
Lateral erosion of channels
Scouring of loose grains
Mass movement when banks are oversteepened/ undercut
Function of cohesive strength
Overhang
more cohesive material overlies less cohesive

25. Fluvial Processes- Transport/ Deposition
Debris flow
Freeze when applied shear < strength
Cohesionless debris flow (liquifaction)
Freezes as pore water escapes
Cohesive debris flow (mudflows)
Viscosity incr. by fine grain matrix (+bouyancy inhibits settling)
Bedload
Movement of non-cohesive grains by rolling or bouncing
Mostly sand and gravel sized grains in rock record
Suspended load
Transport due to fluid turbulence (Can includes sands!)
Some deposited on flood plain; most deposited at end of river
Wind

27. Basic River Morphology
Thalweg
Deepest part of channel
Point bars
Built on inside of curve (depositional)
Cut bank
On outside of curve (erosional)
Pools
Deep area opposite to point bar
Riffle
Shallower area (cross-over area)
Levee and Flood Plain
Borders river

28. Fluvial Facies Braided streams Meandering streams
Straight channel streams
Meandering streams
Share characteristics with stream- dominated fan!
Associations!
Large scale (mountains?)
Small scale (flood plain?)

29. Braided Channel Main channel consists of several channels Bars
Major depositional feature
Often submerged at high flow
Development
Grow downstream and on lateral areas
Often eroded upstream
Stabilized by deposition of fine grains on top + vegetation

30. Braided Channel
Successive division and rejoining of flow around islands
Most common in mountain reaches, on alluvial fans and glacial outwash
Requires
high bedload
Steeper gradient (aids in higher bedload)
Relative ease of bank erosion
Grain size
Gravels most common
Can braid fines if discharge is high and banks are weak

33. Straight channels
Channel has low to no sinuosity over its length (relative to its width)
Thalweg is sinuous
Rare!
Exist only over short distances
Limited to 10x the channel width
Leopold et al, 1964

34. Straight channels
Channel has low to no sinuosity over its length (relative to its width)
Thalweg is sinuous
Rare!
Exist only over short distances
Limited to 10x the channel width
Leopold et al, 1964

35. Distinctive and Common Sedimentary Facies Associations
Vertical successions principally identified by lithology, associations and vertical arrangement of sedimentary structures
indicative of particular sedimentary depositional environments

36. Meandering Channel Point bars dominate
Helical circulation leads to meandering
In curves, two flow directions
Strong downstream
greatest at 3, towards the cut bank
Weaker cross-flow at depth
Towards the point bar
When cut bank slumps, material is carried downstream to next point bar

37. http://www.unomaha.edu/geomorf/Todd/page3.html, Pidwirny, 2000

39. Note point bars along inner bends
Note point bars along inner bends. The channel belt is defined as the region between the red lines wihtin which the active channel tends to meander. Photo from R.H. Meade, USGS

40. Flow Regime and Bedforms
Bedload and suspended load
Suspended load
Much moved in suspention; Important in flood plains
Bedload
Dominant in channels and point bars; coarsest grains lag
Floods increase it
Noncohesive materials on stream bed lead to easy formation of bedforms
Lower flow regime is common
Ripples abundant
Upper flow regime is also common
Highest flow ever recorded
7-8 m/s (25-29 km/hr - Leopold et al., 1964

41. Deposition Rates Most deposition occurs during flood stage
Coleman, 1969
1m thick in 24 hours
Migrating dunes
5-6m thick in 24 hours
Non- flood periods
Non-depositional
Often erosional

42. Deposition Channel deposits Flood plain deposits Channel lag
Channel bar (Braided River)
Point bar (Meandering River)
Channel fill
Flood plain deposits
Flood plain
Marsh
Levees
Crevasse splay

43. Channel deposits Channel lag Coarsest sediment available in stream
Accumulates in lenses
Generally gravels, pebbles
May include
Wood
Rip-up clasts
Fossils
Blocks of sediment
Often preserved as fine grains cover it

44. Channel Bars Most deposition occurs during floods
Two types based on grain size
Braid bars (coarse material)
Pebbles, etc
Mountain streams
Braid bars (fine material)
High sediment load (seasonal discharge)
Lower reaches of rivers, near deltas
Brahmaputra River

46. Channel bar (= channel islands)
Dominant depositional feature in braided rivers
Controlled by
Lateral and vertical deposition
Cutting and abandonment
Upstream end
Steep
Often has pool in front of it
Downstream end
Gentler slope
Migrates downstream (foreset bedding)
Sides
Steep concave (foreset bedding/ laminae)
Gentle convex (foreset bedding/ laminae)

47. General Channel Bar Facies
Upper
Fine sand, mud in horizontal layers
Muds
Mudcracks, raindrop imprints
Sometimes scour and erosional markings
Convolute bedding
Bioturbation (plant growth)
Lower
Large- scale cross-bedding
Medium sands
ripples and dunes
Coarsest grains at base (pebbles)
Often rests on muds, silts of previous cycle
scour and fill structures

48. Braided Bar types Longitudinal Linguoid and transverse Lateral
Mid-channel (coarsest sediment)
Deposition occurs as stream loses velocity
Long axis // to flow
Grow downstream fill in with finer sediments
Decrease grain size downstream
Massive bedding; cross- bedding
Linguoid and transverse
Lobate or straight; at angle to flow
Found in sandy streams
Steep face downstream
High flood conditions
Lateral
Along sides of channel attached to bank

49. Point Bar deposits Major process of sedimentation in meandering rivers
Usually that which is preserved
Thickness may equal depth of river
Mississippi River= m (Fisk, 1944, 1947)
Lateral migration of point bar (and river!) during flooding
But the channel width is maintained (erosion= deposition)
Generally fining upward
Deposition
channel lag at base and mud drape on top
Preservation
Fines at top eroded

52. Point Bar Facies Silts, clays Planar bedding Cross-bedding
May be upper - flow
likely it is sediments settling out of suspension as flood wanes
Often interbedded with ripples
Cross-bedding
Ripples and climbing - ripples
Dune common in lower part
Beds ~ 1m
Scour, fill common
Channel lag at base

53. Point Bar Facies
Larger scale point bars develop scroll bars and swales
Scroll-shaped ridges with swales between
Each scroll = migration during one flood
Swales infilled with muds
Mississippi
Few hundred meters wide, 4-5 m thick
Base is concave-upward

54. Point Bar Life History Rapid, periodic deposition!
Arkansas River, Oklahoma (Steinmetz, 1967)
May 19-22, 1957 to October 3-6, 1959
459,000 m3 sed deposited; max = 13 m in 156 hours
Mostly its large-scale cross-bedding
overlain by silts (settling) and sand dunes (1 m!)
Sedimentation in a single flood (p. 235, R/S)
Klarälven River, Sweden (Sundborg, 1956)
Develop a longitudinal bar on the meander
Slowly migrates bank-ward
Accumulates drape
New bar begins, migrates

55. Downtown Yuba City December 1955 (Seepage Related Levee Break in upper center) View Southerly

56. Note the many scroll bars marking the former positions of channels (point bars) across the flood plain.

59. Levee Deposits (Natural)
Wedge- shaped ridges of sediment bordering stream channels
Formed as flood waters top banks
Sediment in suspension settles out as velocity drops
Highest at stream banks
Slopes away towards flood plain
Best development on point bank side (concave)
Grades into point bar
Mississippi River
Max: 1.5 km wide, 5-6 m above flood basin

60. Levee Deposit (Natural)
Sediments grade laterally
Decrease in size away from channel
Reflects rapid drop in velocity
Coarsest sediments nearest levee
Rate is reduced also
Maximum height of levee
= maximum height of floodwaters

61. Crevasse Splay Breach in natural levee Tongues of sediment
Occurs in floods
Creates drainage network in flood basin
Generally small
10’s cm to 10’s meters
May be several hundred meters across
Tongues of sediment
Taper in direction of flood basin
Coarser sediments than that of flood plain
Small- scale cross-bedding
Scour and fill
Fossils (plant and animal)

64. Channel- fill (Oxbows!)
Stream channels that have been abandoned
Cut-off abandonment
Avulsions: sudden abandonment
Very high rate of sedimentation
Meandering can cause if
Chute cut-off: stream shortens its course
Neck- cut off: new channel but between 2 meander loops

65. Channel- Fill deposits
At first,
Rapid and focused at ends
Sands concentrated here
Slower later on
Fill is similar to flood- plain deposits
Clay and silt dominated
Plants also present

66. Flood Basin Deposits
Poorly drained, flat, featureless with little or no relieft adjacent to active or abandoned channels
(Rheinhold and Singh, 1975)
SLOW accumulation
1- 2 cm per flood period
(compacts over time!)
Controlled by channel form and pattern
Stationary streams develop thick flood plains
Migrating streams have poorly developed plains
Braided streams, active meanders
Backswamps
may form if the region is humid
Accumulates muds, peats
Congaree National Monument