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.

2. 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

3. 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.

4. Lake types (origin) ► Rift basins ► Cratonic sags ► Volcanic ► Glacial  overdeeped by scour  moraine-dammed ► Other

5. Rift basin lakes ► African rift valley ► Lake Baikal, Russia ► Basin and range, USA

6. Volcanic lakes ► Calderas:  Crater Lake, USA  Mono L, Yellowstone  L. Taupo, NZ

7. Cratonic basin lakes

8. Glacial lakes

9. Lake water (density) stratification ► Thermal (seasonal) ► Haline (perennial) Maximum gradient Transition Less warm Warm upper

10. 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.

11. Lake circulation ► Thermally driven (seasonal). ► Inflow driven ► Wind driven (intermittent).

12. Example ► Inflow and wind-driven circulation, Peyto Lake, Canada.

13. Wind driven circulation and mixing Hypolimnion Epilimnion Mixing

14. Sedimentation ► Clastic input from rivers ► Wave reworking ► Downslope mass-movement ► In-situ biological and chemical production

15. Clastic sediment input ► Points sources via fan and river deltas:  underflows (turbidity currents)  interflows  (overflows)

16. Wind-driven processes ► Waves along the shoreline. ► Set up water gradient and generate currents. ► Wind relaxation causes surface or internal oscillations (seiches).

17. Chemical processes ► Input controlled by weathering and lithology. ► Ionic salinity dom. by:  cations: Ca, Mg, Na, K  anions: HCO 3, CO 3, SO 4, Cl ► Carbon cycle dom. by:  precipitation of CaC0 3  fixation of C by organisms. ► Si fixed by diatoms. ► Seasonal variations. Marl-lake facies common in temperate dimictic lakes.

18. Saline lakes ► Solutes >5000 ppm (5 ‰). ► Playa: seasonally exposed evaporitic lake floor. ► Lake levels much higher during pluvials. ► Examples:  Death Valley  Dead Sea

19. ► Surface and subsurface inflow. ► Dom. Na-Ca-Cl-S0 4

20. 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 organic- rich/organic-poor laminae.

21. Organic Minerogenic Lake Nakkevatnet, Troms

22. Meromictic lake lamination

23. 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

24. 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

25. Lake Brienz ► 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. Reineck & Singh 1980

26. 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.

27. East African rift lakes ► Half-grabens ► Deep lakes permanently stratified ► Shallow lakes well mixed

28. East African rift lakes

29. Lake Tanganyika ► 23000 km 2, 1470 m deep. ► 4 km thick sediments, 1 Myr. ► Asymmetric basin form. ► Steep slopes: sediment bypass and mass flow. ► Turbidity currents onto lake bottom.

30. L. Tanganyika - facies

31. Lake Malawi ► 45 000 km 2, 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 Fe- oolites. ► NB. Variable facies due to major (>150 m) rapdid (~350 yr) lake level fluctuations.

32. Lake Turkana ► 5000 km 2, 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.

33. Lake Baikal ► World’s largest: 23 000 km 3, 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.

34. Shallow saline lakes ► Salinas and playas ► Evaporite-clastic couplets. ► Halite, gypsum ► Sensitive to climate change (lake level fluctuation

35. Facies successions in evolving lakes ► Pluvial –interpluvial (100 kyr) fluctuations. ► Short-term fluctuations (e.g. during Holocene) in warm environments.

39. Ancient lake facies ► Newark Supergroup  Transgressive sands  microlaminated black shales  Highstand-lowstand 21 kyr cycles ► Devonian Lake Orcadia  Fluviolacustrine sediments  Carbonate-, organic rich and clastic laminites.  Ripples and subaerial shrinkage cracks. ► Eocene Green River Formation  950 m thick  World’s largest Trona /NA2CO3) deposit.  World’s single largest hydrocarbon reserve. ► Tertiary lake Madrid  pedified mudrocks

40. Eocene Green River Formation

41. Further reading ► Galloway and Hobday ► Reading