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 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

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 Meandering 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

http://www.utexas.edu/depts/grg/hudson/grg301c/hudson_grg_301c/schedule/4_water_geomorph_images/14_rivers/8.htm

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 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 Flood plain 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 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

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 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

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 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)

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