Depositional Environments and Their Facies

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

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

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

Deposition

• Gravity works to move sediments– Water– Wind

• Change in slope or change in climate impacts sediment transport and abundance

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”

Alluvial Fan Facies Models

• 3 types– Debris- flow dominated– Braided fluvial fans– Low- sinuosity/meandering fluvial fan

• Generally – lacking fossils– Fan shaped– Texturally immature

Death Valley

Martin Miller

http://wrgis.wr.usgs.gov/docs/parks/deva/galfan.html

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

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

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?)

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

River Basics• 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

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

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

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)

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

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

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

Fluvial Facies

• Braided streams– Straight channel streams

• Meandering streams• Share characteristics with stream-

dominated fan! – Associations!

• Large scale (mountains?)• Small scale (flood plain?)

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

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

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

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

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

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

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

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

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

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

Deposition

• Channel deposits– Channel lag– Channel bar (Braided River)– Point bar (Meandering River)– Channel fill

• Flood plain deposits– Flood plain– Marsh– Levees– Crevasse splay

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

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

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)

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

Braided Bar types• Longitudinal

– 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

Point Bar deposits

• Major process of sedimentation in meandering rivers– Usually that which is preserved– Thickness may equal depth of river

• Mississippi River= 20-25 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

Point Bar Facies

• Silts, clays• Planar 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

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

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

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

http://www.escalera.com/safelevee/1955flood.htm

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

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

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

Crevasse Splay

• Breach in natural levee– 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)

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

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

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

http://www.uga.edu/srel/ESSite/congaree_national_monument.htm