Chapter 5

1. Chapter 5 Sedimentary Environments

2. Paleogeography • Reconstruction of ancient environments from the stratigraphic record • Distribution of land and sea • Identification of localized environmental features • Framework for interpretation of past life • Employ actualism

3. Nonmarine Environments • Soil • Loose sediment containing organic matter and accumulated in contact with atmosphere • Topsoil • Upper zone of many soils • Sand and clay mixed with humus • Humus • Organic matter that gives topsoil its dark color • Derived from decay of plant debris by bacteria

4. Nonmarine Environments • Soils • Type of soil depends on climatic conditions • Caliche • Calcium carbonate produced by evaporation of groundwater • Laterite • Iron oxide rich soil produced in moist tropical regions

5. Nonmarine Environments • Burrows • Plant • Animal • Aid in identifying ancient soils • “Devil’s corkscrews”

6. Nonmarine Environments • Lakes • Lower elevation, more likely preservation • Indicates abundant precipitation • Sediments • Coarsest around lake margin • Finest at center • Often layered • Freshwater fossils

7. Nonmarine Environments • Glaciers • Indicate cold climates • Scratches produced by glacial motion • Record direction of motion

8. Nonmarine Environments • Till • Unsorted, heterogeneous material • Tillite • Lithified till • Moraine • Ridges of till plowed up at the farthest edge of the glacier

9. Nonmarine Environments • Meltwater • Transports sediments from glacier • Forms streams and lakes • Outwash • Well-stratified layers of sediment • Varve • Annual layers of alternating coarse and fine sediments

10. Nonmarine Environments • Varves • Annual record • Count! • Dropstones • Scattered coarse sediments found in sediment matrix • Ice-rafted debris

11. Lacustrine Environments (Lakes) • May be large or small • May be shallow or deep • Filled with terrigenous, carbonate, or evaporitic sediments • Sediments are typically fine grained but may be coarse near the edges • Fine sediment and organic matter settling in some lakes produced laminated oil shales • Playa lakes are shallow, temporary lakes that form in arid regions They periodically dry up as a result of evaporation

12. Nonmarine Environments • Desert soils • Little organic matter • Evaporite minerals • Interior drainage • Precipitation does not leave the basin • Playa lake • Temporary lakes • Associated with evaporites

13. Nonmarine Environments • Dunes • Piles of sand • < 1% of deserts • Moves with prevailing wind direction • Associated with deserts and beaches

14. Eolian Environments • Windis the agent of sediment transport and deposition • Dominated by sand and silt • Common in many desert regions

15. Dune migration Moves downwind Sands move up and over top; accumulate on downwind side Trough cross- stratification Direction changes with prevailing winds Beds accumulate on curved surface cut through older beds Nonmarine Environments

16. Nonmarine Environments • Alluvial fans • Low, cone-shaped structures • Develop where mountain slope meets valley floor • Change in slope reduces stream velocity • Sediments settle out • Braided streams form

17. Alluvial Fans • Fan-shaped deposits at base of mountains. • Most common in arid and semi-arid regions with rapid erosion. • Sediment is coarse, poorly- sorted gravel and sand.

18. Nonmarine Environments • Mudcracks • Polygonal shape • Form from alternate wetting and drying • Associated with evaporites • Halite • Gypsum • Anhydrite

19. Nonmarine Environments • Waters move from mountains to sea through a variety of depositional environments • Braided streams • Meandering rivers • Marginal marine systems

20. Fluvial Environments • Braided and meandering river and stream systems • River channels, bars, levees, and floodplains are subenvironments • Channel deposits are coarse, rounded gravel, and sand. • Bars are sand or gravel. • Levees are fine sand or silt. • Floodplains are covered by silt and clay.

21. River FeaturesMain Channel fed by tributaries

22. River FeaturesErosion dependent on rock type

23. SEDIMENT TRANSPORT BY STREAMSBecause of the role of rivers in erosion of the continents, they must also carry this material downstream and finally to the oceans, in order to remove it from the continents. Rivers transport erosion products in three ways:A) In Suspension (clay, silt; kept in suspension by turbulence) B) In Bedload (Gravel, sand, and silt) C) In Solution (the components that enter solution during erosion, dissolved salts)

24. River FeaturesTransport of particles via Gravity

25. Stream ErosionAndDeposition

26. Stream ErosionandDepositionUpland region

27. Stream Erosion and DepositionAlluvial Fans

28. Stream Erosion and DepositionBraided Streamsabundant supply of coarse sediment

29. Stream Erosion and DepositionBraided Streamsabundant supply of coarse sediment

30. Nonmarine Environments • Braided streams • More sediment available than the water can transport • Forms numerous channels and bars

31. Nonmarine Environments • Point bar • Slowest flow on inner bend • Accumulate sands • Fastest flow on the outer bank • Cuts away bank • Natural levees • Form during floods • Coarsest sands deposited first, then fines

32. Rivers

33. Stream Erosion and DepositionMeandering StreamsPoint Bar

34. Stream Erosion and DepositionMeandering StreamsPoint Bars

35. Stream Erosion and DepositionMeandering StreamsPoint Bars

36. Stream Erosion and DepositionMeandering StreamsFlood Plain

37. Stream Erosion and DepositionMeandering StreamsFlood Plain

38. Stream Erosion and DepositionMeandering StreamsFlood Plain

39. Nonmarine Environments • Meandering River • Abundant water relative to sediment • Backswamps • Flood plain • Mud settles out when stream overflows

40. Nonmarine Environments • Vertical sequence of accumulation • Coarse channel sediments at base • Fine backswamp muds at top • Illustrates Walther’s Law

41. Transitional Depositional Environments Environments at or near the transition between the land and the sea. • Deltas • Beaches and barrier Islands • Lagoons • Tidal flats • Estuaries

42. Depositional Environments

43. Deltas • Fan-shaped accumulations of sediment • Formed where a river flows into a standing body of water, such as a lake or the sea • Coarser sediment (sand) tends to be deposited near the mouth of the river; finer sediment is carried seaward and deposited in deeper water. • The delta builds seaward (or progrades) as sediment is deposited at the river mouth.

44. Marginal Marine • Delta • Depositional body of sand, silt, and clay formed when river empties into the sea • Sediments settle out in sequence

45. Marginal Marine • Delta plain • Layers of sand and silt deposited as river nears sea • Distributary channels • Separated by levees • Delta front • Silt and clay slope deposits • Prodelta • Clays often deposited by a freshwater plume • Progrades into basin • Sediments coarsen upward

46. Marginal Marine • Mississippi River Delta • River dominated delta • Progrades into Gulf of Mexico • Lobes • Growing portion of the delta

47. Marginal Marine • Mississippi River Delta • Active lobe • Growing portion of delta • Switched lobes in the past • Abandoned lobe • Sediments compact • Lobe sinks • New lobe forms on top

48. Marginal Marine • Deltaic Cycles • Sequence of deposition • Coarsens upward • Erosion can remove tops

49. Deltas Mississippi River delta Niger River delta