CONTINENTAL DEPOSITIONAL ENVIRONMENTS • fluvial • desert • lacustrine (lake) • Glacial KEY PARAMETERS (VARIABLES) 1. Climate (Hydrologic Cycle) – Annual Precip vs. Seasonality 2. Tectonics (Mountains) The Hydrologic Cycle 1 U.S. Precipitation Map Notice the effect of the Rocky Mountains U.S. Runoff Map Longitudinal Stream Profiles Base level 2 Tranquil (turbulent) flow in the center of a river cShuapnenreclrtical flow in the headwaters of a rushing mountain stream Laminar vs. Turbulent Flow 3 Mountains to Plains: Relationship of Stream Shape, Discharge, and Gradient FLUVIAL SYSTEMS Systems: – alluvial fan – braided stream – meandering stream Processes: – Rivers – Streams – Gravity flows 4 Sub-aerial Sediment Gravity Flows • Debris & Mud Flows Characteristics: • Slurry of large particles, in finer grain matrix • Cohesive muddy matrix supports grains • Matrix lubricates grain irregularities • Flow may continue at low angle slopes (1-2°) Deposits: • Thick, poorly sorted • Massive - lack internal layering/structure – Reverse/normal grading “possible” Cretaceous, southern Mongolia, Alluvial Fans: Deposits w/ shape of a segment of a cone Depositional setting: –areas of high relief, active abundant sediment supply (ie., Fault scarps) Death Valley – sparse vegetation – dry (semi-arid) – episodic sediment transport 5 Black Mountains, Death Valley Alluvial Fans (cont.): Geometry - 1. cone shaped to arcuate, convex upward – Slope 1 to 25°, larger grains, higher slope 2. sinuous, branching channels proximal fan midfan distal fan X-section of fan (convex upward) radial x-section (concave upward) 6 Alluvial Fans (cont.): Sedimentary Processes and Deposits: 1. Stream flow -braided streams, typically ephemeral 3 deposits: – stream channel sediments • coarse, poorly sorted – sheetflood deposits - surge of sediment laden water “fan” out at terminus of channel • well sorted gravel, sand, silt • cross-beds, laminated, or structureless - sieve deposits - coarse gravel lobes, - channels emerge mid-fan (sand or silt rare) Alluvial fan, Death Valley, CA Represents waning stages of a single Fining upward flooding event Fine grain 7 Alluvial Fans (cont.): 2. Debris flow – sediment gravity flow - behaves plastically – slurry texture (mixture of mainly coarse sediment and water) matrix supported common on fans in arid or semi-arid regions where; 1) rainfall is infrequent, but intense 2) slopes are steep - Poorly sorted - Reverse grading Alluvial Fans (cont.): 3. Mud flow – sediment gravity flow - behaves plastically – slurry texture (mix of silt, clay & water) common on fans in arid or semi-arid regions where 1) infrequent rainfall, but intense 2) steep slopes 4. land sliding • rock falls, slumps, etc. Difficult to differentiate from debris flow 8 Alluvial Fans: Summary • Composition, Textures and Structures - compositionally and texturally immature: wide range of grain sizes 1. grain size - decreases upper to lower 2. sorting - increases upper to lower Distribution of Depositional Environments/Facies upper fan (proximal fan) - steepest slope, coarsest sediment, single channel • poorly sorted, conglomerates in stream channels midfan - gentler slope, branching network of channels, • stream flow & debris flow • finer grain sediments - gravel, sands • structures preserved in stream sediments (planar and trough x-bedding, imbricated pebbles in channels) distal fan - toe of the fan, gentlest slope, lack of well defined channels, sheet flood • finest sediment- sand and silt • well sorted • low angle x-stratification Alluvial Fan upper mid lower 9 Fluvial Systems • Braided Rivers • Meandering Rivers Longitudinal Stream Profiles 10 Braided Rivers Requirements: 1. High sediment load 2. High stream power (discharge) 3. Non-cohesive banks (i.e., constantly eroding) Son-Kul River, Tien Shan Mtns., Kyrgyzstan Braided Streams & Rivers Depositional setting: 1. distal end of alluvial fans 2. mountainous reaches of river systems (subsiding basins) 3. glacial drainage Resurrection River, Kenai Peninsula, Alaska 11 Braided Streams & Rivers Characteristics: • Low sinuosity (ratio of channel length to length of valley containing the river; >1.5 ~ meandering). • Anastamosing channels • Constantly changing course • mid-channel bars (islands) • grade downstream from gravelly to sandy 12 Depositional Features Characterized by • Channel lag deposits & sand waves • Gravel/sand bars (large bedforms) – 3 types - longitudinal, transverse, lateral – trough and tabular x-bedding, laminations • gravel (upper reaches), sand, little silt/clay 13 Longitudinal Bar Kicking Horse River, BC Bedforms and structures: 1. Longitudinal bars location: mid channel bars - coarsest part of the stream load shape: elongated, slightly convex in x-section orientation: long axis - parallel to flow direction grain size: gravel (imbricated) & sand • size decreases upward & downstream • coarsest material concentrated in central axis of bar structure: massive or crude horizontal bedding (upper flow regime conditions) , faint planar x- bedding deposition: primarily as flow begins to wane • High flow -Vertical, downstream accumulation • Low flow - lateral accretion (perpendicular to main flow direction). 14 Poorly sorted imbricated pebbles Glacier Bay Alaska Braided River: Depositional Features • Longitudinal Bar Deposit Graded? Decreasing velocity N. Fork Toutle R., WA 15 Longitudinal Bar • Poorly sorted planar- x-bedded gravels planar-bedding gravels Banff, Canada Longitudinal (Channel) Bar • Well sorted Kosi River, India Cross-bedded (ripple lamination) sands planar-stratification sands 16 2. Transverse Bar Slip face planar Sinuous ripples 2. Transverse (linguoid) bars - sides of the channel at an angle to stream flow shape: lobate or rhombic, steep downstream avalanche faces grain size: mostly sand, decreases upward and downstream structure: extensive planar /trough cross bedding, dip direction variable,but downstream, w/ numerous erosional surfaces deposition: start as large ripples (dunes) during high flood stage, followed by low velocity deposition 17 Transverse (linguoid) bars South Platte River 3. lateral (point) bars - sides of channel (inner bend) grain size: mostly sand, decreases upward structure: large ripples with planar & trough cross bedding, dip direction generally downstream deposition: areas of low energy, accrete laterally • More sand & x-bedding (planar/trough) than longitudinal bars 18 Braided River: Depositional Features Low angle cross strata Erosion Surface High angle cross strata Alsek River, Alaska 19 Braided Rivers Channel/Longitudinal Bars Brahmaputra River Braided Rivers Nelchina River, Copper River region, Alaska 20 Braided Rivers Alsek River, Alaska Braided Rivers: Sediment Load Transport Sediment Transport Relief Maps of the Waimakariri River, New Zealand • (-) Bank erosion • (+) Gravel lobes advance 21 Braided River Deposits: Summary • vertical succession of facies: highly variable, but usually dominated by coarse grained (little silt/clay) bed forms/sedimentary structures – vary as function of • size of bedload • depth • discharge rates and variability – deposits form downstream, laterally, or by vertical accretion on channel floors & bar tops. • Channels fill by aggradation during waning current flow Braided River: Pattern of Sedimentation 22 Braided River Deposits: Summary • Extensive x-stratification, avalanche faces • Fining upward within individual cross beds • Cross beds may decrease in size in overlying units • Repeated stacking/channeling Braided to Meandering 23 Meandering Rivers tend to be confined to a single channel, with cohesive banks that are difficult to erode, and show much greater sinuosity • Erosion - Cut bank • Deposition - Point bar • Change course gradually • Create floodplains much wider than the channel – Seasonal flooding – Fertile Meandering Rivers • Depositional Setting: distal end of river systems, low lands, reduced gradient. – smaller bedload, finer grains – water discharge is moderate, less variable • Main geomorphic features: – main meander channel – point bars – natural levees – flood basins – oxbow lakes 24 Meandering Rivers: Oxbow Lake 25 Point bar deposits Channel flow dynamics • Centripetal force - shifts fast upper flow toward outside of bend • Pressure force - slower bottom flow toward inside of bend 26 Meandering Rivers Moderate/High flow conditions: –transverse spiral or helical flow • deflects water along the stream bed - from outer bank to inner bank. • transports sediment across the channel up the slope of the adjacent point bar responsible for: –size sorting –lateral & downstream migration of bars Flood stage conditions: overbank flooding – deposition of fine silt and mud on the bank near the stream edge, building natural levees fine velocity slows coarse Point Bar Sedimentary Structures: • large dune bedforms on the lower part of the point bar (trough cross bedding) • ripples form on the higher part of the point bar (cross laminae) • plane bed laminations may form on the bar • Location depends on flow velocity • plane bed laminations may be interbedded with trough cross beds Plane bed laminations trough cross bedding 27 Meandering Rivers Point Bar Deposits Point bar deposits expand laterally 28 Vertical succession of Facies • trough erosion surface overlain by lag-gravel conglomerate • large scale cross-bedded point bar sands • small scale trough cross beds • fine grain overbank mud and silt Flood Plain Deposits Flooding produces overbank deposits • Fine grain sands, silts and clays – Lower velocity • Levee deposits – Form primarily on cut banks Mississippi R., MS • Crevasse