Transcript Slide 1

GE0-3112
Sedimentary processes and products
Lecture 8. Lakes
Geoff Corner
Department of Geology
University of Tromsø
2006
Literature:
- Leeder 1999. Ch. 19. Lakes.
Contents
► 8.1
Introduction
► 8.2 Lake types
► 8.3 Hydrology
► 8.4 Sedimentation processes
► 8.5 Modern lakes
► 8.6 Ancient lake deposits
► Further reading
Geological importance of lakes
► Sinks
for water and sediment on continents.
► Presently comprise/contain:
 2% Earth’s area.
 0.02% Earth’s water volume.
► Sediments
are climate archives.
► Sediments host hydrocarbons, coal and
uranium.
Lake types (origin)
► Rift
basins
► Cratonic sags
► Volcanic
► Glacial
 overdeeped by scour
 moraine-dammed
► Other
Rift basin lakes
► African
rift valley
► Lake Baikal, Russia
► Basin and range, USA
Volcanic lakes
► Calderas:
 Crater Lake, USA
 Mono L, Yellowstone
 L. Taupo, NZ
Cratonic basin
lakes
Glacial lakes
Lake water (density) stratification
► Thermal
(seasonal)
► Haline (perennial)
Warm upper
Transition
Maximum gradient
Less warm
Lake types
Amictic – permanent ice cover.
► Monomictic – one season free circulation (summer or winter).
► Dimictic – two seasons of circulation (spring/autumn).
► Oligomictic - circulation rare (stable stratification).
► Polymictic – frequent or continuous circulation.
► Meromictic – salinity stratified.
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Lake circulation
► Thermally
driven (seasonal).
► Inflow driven
► Wind driven (intermittent).
Example
► Inflow
and wind-driven circulation,
Peyto Lake, Canada.
Wind driven circulation and mixing
Epilimnion
Mixing
Hypolimnion
Sedimentation
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Clastic input from rivers
Wave reworking
Downslope mass-movement
In-situ biological and chemical production
Clastic sediment input
► Points
sources via fan and river deltas:
 underflows (turbidity currents)
 interflows
 (overflows)
Wind-driven processes
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Waves along the shoreline.
Set up water gradient and generate currents.
Wind relaxation causes surface or internal oscillations (seiches).
Chemical processes
► Input
controlled by weathering and lithology.
► Ionic salinity dom. by:
 cations: Ca, Mg, Na, K
 anions: HCO3, CO3, SO4, Cl
► Carbon
cycle dom. by:
 precipitation of CaC03
 fixation of C by organisms.
► Si
fixed by diatoms.
► Seasonal variations.
Marl-lake facies common in
temperate dimictic lakes.
Saline lakes
► Solutes >5000 ppm (5 ‰).
► Playa: seasonally exposed evaporitic lake
► Lake levels much higher during pluvials.
► Examples:
 Death Valley
 Dead Sea
floor.
► Surface
and subsurface inflow.
► Dom. Na-Ca-Cl-S04
Biological processes
► Photosynthetic
plankton in the epilimnion.
► Diatoms important in nutrient-poor
(oligotrophic lakes).
► Bacterial decay of organic matter uses up
oxygen  anoxis at depth in chemically
stratified lakes.
► Seasonal oxygen fluctuation give organicrich/organic-poor laminae.
Organic
Minerogenic
Lake Nakkevatnet, Troms
Meromictic lake lamination
Modern lakes and facies
► Cool
dimictic lakes
 Lake Brienz, Switzerland
 Lake Zurich
► East
African rift lakes
 Lake Malawi
 Lake Tanganyika
 Lake Turkana
► Lake
Baikal rift lake
► Shallow saline lakes
Cool dimictic lakes
► Thermal
stratification summer and winter;
overturn in autumn and spring.
► Lake Brienz, Swiss Alps
 14 km long, 261 m deep.
 Turbidite sands and varves.
► Lake
Zurich
Lake Brienz
Reineck & Singh 1980
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Clastic deposition.
Deposition in seasonally stratified lake by overflows, interflows and
underflows.
High-density turbidity currents (extreme flood events)  thick (<1.5 m)
graded sand beds.
Low-density turbidity currents (seasonal flood events)  thin (cm’s) faintly
graded sand.
Summer settling of overflow/interflow silt  dark part of varve couplet.
Winter settling of silt/clay after overturn  light part of varve couplet.
Lake Zurich
► Flood
dams in 1900 have stopped most
clastic input.
► Dominant biogenic and chemical deposition.
► Chemical and biogenic cycles produce
chalky varves on lake floor.
► Cf. to Neogene lacustrine chalks in Black
Sea.
East African rift lakes
► Half-grabens
► Deep
lakes
permanently stratified
► Shallow lakes well
mixed
East African rift lakes
Lake Tanganyika
► 23000
km2, 1470 m deep.
► 4 km thick sediments, 1
Myr.
► Asymmetric basin form.
► Steep slopes: sediment
bypass and mass flow.
► Turbidity currents onto
lake bottom.
L. Tanganyika - facies
Lake Malawi
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45 000 km2, 730 m deep.
4.5 km thick sediments, 5 Myr.
Slope deposits and turbidites.
Side deltas common.
S. floor contains hemipelagic muds , diatom oozes and Feoolites.
NB. Variable facies due to major (>150 m) rapdid (~350 yr) lake level
fluctuations.
Lake Turkana
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5000 km2, 35 m ave. depth.
Well mixed.
Saline (2.5‰), alkaline (pH9.2), oxidizing (70-100%).
Clastic underflows during floods.
Deltas and beaches at different levels.
Little organic sediment.
Varve-like muds; some authogenic minerals.
Lake Baikal
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World’s largest: 23 000 km3, 1640 m deep.
Oligotrophic.
No dimictic overturn below 500 m.
<7 km thick sediments, 15 Myr.
Deltas and turbidity currents.
Fe/Mn cement horisons in muds.
Diatom-rich (>60%) sediments.
Hot-spring vents.
Shallow saline lakes
► Salinas
and playas
► Evaporite-clastic couplets.
► Halite, gypsum
► Sensitive to climate change (lake level
fluctuation
Facies successions in evolving lakes
► Pluvial
–interpluvial (100 kyr) fluctuations.
► Short-term fluctuations (e.g. during
Holocene) in warm environments.
Ancient lake facies
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Newark Supergroup
 Transgressive sands
 microlaminated black shales
 Highstand-lowstand 21 kyr cycles
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Devonian Lake Orcadia
 Fluviolacustrine sediments
 Carbonate-, organic rich and clastic laminites.
 Ripples and subaerial shrinkage cracks.
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Eocene Green River Formation
 950 m thick
 World’s largest Trona /NA2CO3) deposit.
 World’s single largest hydrocarbon reserve.
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Tertiary lake Madrid
 pedified mudrocks
Eocene Green River Formation
Further reading
► Galloway
► Reading
and Hobday