Alluvial/Braided Systems Flashcards
What are the main Continental depositional systems?
- ALLUVIAL FANS
- BRAIDED FLUVIAL
- low sinusoidal channels, high grad, high stream, bed load
- MEANDERING FLUVIAL
- ANASTOMOSING FLUVIAL
- EOLIAN
- LACUSTRINE FAN DELTAS
Alluvial fans
- deposited by a stream adj to upland region (fault-line uplift)
- unchannelized deposits
- coarse grained
- steep slope
- subsidence
- theres fan incision and reactivation during tect uplift
- arid fans have mudflow-debris deposits and waterlain sediments
- wet fans have anastomosing
- sediments can be ortho or para conglomerates, thin bedded fine conglomerates (cx bed), thinly bedded ortho conglomerates and sandstone lens
- ortho has cx bedding, lens
- para has inverse grading
- thin bed fine cong: ephemeral shallow channels on fan surface
- thin bed orthos: sheetwash deposits (unchanneled flow on surface)
what are two major groups of rivers?
- alluvial rivers: flow over own deposits, thick and extend laterally
- incised (bedrock) rivers: persist during tectonic uplift: have erosional and aggradational valley-fill deposits
what determines the type of alluvial systems?
- processes and flow during seasonal flooding
- flow in all systems meanders because of internal shear, turbulence, bank/bed friction
tell me the link between fluvial style and emergence of land plants
braided existed all the way through, mixed showed up in silurian, meandering showed up in devonian, anabranching in the carboniferous (bc of grass)
-bank stability in the silurian allowed the others to form
what are lithofacies and fluvial deposits mainly based on?
- bedding, grain size, texture, structures, biogenic, local structures
- sediment transport from traction currents and sed gravity flows
Describe alluvial deposits and what you can determine.
- matrix massive gravel=debris flow
- matrix gravel=grading=low strength debris flow
- clast gravel=inv grade=high strength or low strength
- crude bedded gravel=imbrication, horiz bed=lag deposits, sieve deposits
- strat gravel=trough cx beds, =minor channel fills,
- planar cx beds= deltaic growths
- sand that is f to c (pebbly)=grped cx beds=3D dunes
- above has planar cx bed grps=2D dunes
- sand v fine to coarse=low angle cx beds=scours,humpback dunes, antidunes
- above that has broad shallow scours=scour fill
- sand silt or mud=massive, fine lamination, sm ripples=overbank, abandoned channels, waning flood deposits
- just silt and mud=massive=swamp, abandoned channels
- above that has dessication cracks=overbank, abandoned, drapes
- above that has bioturbation=root bed
- coal/carbonates=swamp
- calcite, siderite=precipitation
what is an architectural element?
-part of a depositional sys same size or smaller than a channel fill and larger than an indiv lithofacies unit. has distinctive facies assemblage, internal geometry, etc
classification of within-channel architectural elements
channels: finger, lens, sheet
Gravel bars: lens, blanket, wedge, interbedded with sand bedforms
sandy bedforms: lens, sheet, blanket, wedge, crevasses splays
Upstream accretion: accretion gently dipping upstream, lens on bar
Downstream accretion: lens on channel base, accretion downstream
Lateral Accretion: wedge, sheet, lobe, accretion across channel, downlaps into flat basal erosion
Scour hollows:
Architectural elements of overbanks
levee=flooding=wedge=large
crevasse channel=break in main channel=ribbon=large
crevasse splay=delta-like progradation=lens=large
floodplain=sheet=swamp=large
abandoned channel=ribbon=chute, neck cutoff=active channel size
What are the characteristics of braided streams?
- close to areas of high relief
- low sinusosity
- poor channel stability
- high width/depth ratio
- bedload transport
- gravel, sands
- channels and bars are dominant
- vertical accretion
morphological features of a braided river
floodplains, mid channel bars, veg former bars, overbank deposits, channel deposits, bar surfaces
