The Hydrologic Cycle

Transcription

1 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

2 U.S. Precipitation Map Notice the effect of the Rocky Mountains U.S. Runoff Map Longitudinal Stream Profiles Base level 2

3 Tranquil (turbulent) flow in the center of a river channel Supercrtical flow in the headwaters of a rushing mountain stream Laminar vs. Turbulent Flow 3

4 Mountains to Plains: Relationship of Stream Shape, Discharge, and Gradient Systems: FLUVIAL SYSTEMS alluvial fan braided stream meandering stream Processes: Rivers Streams Gravity flows 4

5 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) sparse vegetation dry (semi-arid) episodic sediment transport Death Valley 5

6 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

7 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 flooding event Fining upward Fine grain 7

8 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

9 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

10 Fluvial Systems Braided Rivers Meandering Rivers Longitudinal Stream Profiles 10

11 Requirements: Braided Rivers 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

12 Characteristics: Braided Streams & Rivers 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

13 Characterized by Depositional Features 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

14 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

15 Poorly sorted imbricated pebbles Glacier Bay Alaska Braided River: Depositional Features Longitudinal Bar Deposit Graded? Decreasing velocity N. Fork Toutle R., WA 15

16 Poorly sorted Longitudinal Bar planar- x-bedded gravels planar-bedding gravels Banff, Canada Well sorted Longitudinal (Channel) Bar Kosi River, India Cross-bedded (ripple lamination) sands planar-stratification sands 16

17 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

18 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

19 Braided River: Depositional Features Low angle cross strata Erosion Surface High angle cross strata Alsek River, Alaska 19

20 Braided Rivers Channel/Longitudinal Bars Brahmaputra River Braided Rivers Nelchina River, Copper River region, Alaska 20

21 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

22 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

23 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

24 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

25 Meandering Rivers: Oxbow Lake 25

26 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

27 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

28 Meandering Rivers Point Bar Deposits Point bar deposits expand laterally 28

29 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 Crevasse splay Levee breach Coarse to fine grain (~graded) Oxbow deposits Fine grain muds Organic rich layers Mississippi R., MS 29

30 Idealized Meandering Stream Profiles A. Vertical stacking of fining upward cycles for sandy stream system B. Vertical stacking of fining upward cycles for fine grained stream system Each cycle capped by overbank (flood) muds Meandering Rivers: Depositional Features Channel deposits overlain by migrating point bar x-stratification Oberlin,OH 30

31 floodplain (overbank) deposits, with thin laterally extensive sands Channel Sand Floodplain Muds 31

32 Case Study: Bighorn Basin Paleocene-Mid -Eocene Basin Bighorn Mtns Beartooth & Bighorn Absaroka Mtns Basin Windriver Mtns Case Study: Bighorn Basin Paleocene to mid-eocene Basin axis subsiding uplift (east, north and west) Extensive erosion and fluvial deposition Rivers meandering across the basin floor Accumulation of 3 km of sediment 32

33 Case Study; Bighorn Basin, WY Overbank flood basin muds, dissected occasionally by channel sands, and sand lenses. Carbonaceous shales and lignites (ponds and backswamps) Well developed paleosols 33

34 Thick Sheet (Channel) Sandstones Lateral accretion (Point Bar deposits) Thin Sheet sandstones (lateral accretion) Small channels Crevasse splay 34

35 Geometry of River Deposits: BRAIDED RIVERS:due to extensive lateral channel migration and aggradation 1. sheet sandstones (in channel and bar top deposits) or 2. conglomerates 3. w/ thin lenses of shales (waning flood deposits) MEANDERING RIVERS: limited to narrow, sandy meander belt of stream flood plains: 1. linear "shoestring" sand bodies 2. surrounded by finer grain overbank flood deposits (more mud than braided systems) avulsion (breaching of levees) - lead to the formation of a new channel Idealized Braided Stream Profiles A. Vertical succession of beds in gravel dominated, proximal stream system B. Vertical succession of bedding cycles in sand dominated, distal stream system 35