Evolution of Earths Environments: Preservation of Sedimentary Environments as Stratigraphy

1. Evolution of Earths Environments:Preservation of Sedimentary Environments as Stratigraphy • What are the main sites of sediment accumulation on the globe? • What evidence can we use to distinguish different environments in the stratigraphic record? • How do environments get preserved through time? • What impact does this have for the stratigraphic record?

2. Where is the preservation potential of sediment highest? Is this reflected in the stratigraphic record?

3. Global denudation rates

4. Sediment discharge from the continents has also changed through time

5. Global climate change

6. How do even the biggest outcrops of stratigraphy in the world record such vast piles of sediment through time?

7. How do we reconstruct ancient environments from such minimal preservation? We use our understanding of sedimentary processes and how they characterise certain environments

8. Sedimentary structures that record environment-specific processes. • Wave ripples, wind-ripples • Mud-cracks • Rootlets • footprints

9. Modern mud-cracks only found In areas of periodic drying e.g. Deserts and intertidal mud flats.

11. Modern (left) and fossilised ancient (right) ripples generated by waves

12. Modern arid Floodplains where soils contain nodules of calcium carbonate due to seasonal drying

13. Calcrete accumulated on arid floodplain

14. Fossils are very important tool • Footprints • Roots • Fossils

15. Modern (left) and fossil ancient (right) roots

16. Fossil footprints

17. Fossil plankton preserved in marine mudstones Globigerina

18. But, we need to understand how these environments get preserved in three dimensions • Predict geometries is important for fossil fuels such as coal • Also important for understanding fluid flow in aquifers and in oil and gas reservoirs • Also important for quarrying • More on this next week………

19. Rates of burial in different sedimentary environments • Rates depend on whether the ground surface is subsiding and the mechanism that generates the subsidence • Sedimentary Basins are those regions of the globe where sediment accumulates on the long-term– next lecture • Rates range from 0.1 to a few mm/yr (which is same as km/million yrs)

20. Environments do not get preserved as they look today • Environments shift laterally faster than they subside. • Erosion and reworking of previously deposited material is common • Examples of rivers and coastlines

21. Example of meandering rivers

24. Eocene Montana Sandstones, Tremp Basin,Spanish Pyrenees

25. Example from present coastlines North Carolina, US

26. Cape Hatteras, N. Carolina

27. Chesil Beach, Dorset UK

29. Peat and fossil trees underlying beach sands, UK

30. Peat underlying beach sands, Coney Island New York

31. The sediments that accumulate in different environments end up overlying each other. But – they are often separated by surfaces that must represent part of the environment that is erosional (e.g. many beaches and outer bends of rivers). This is the basis of Walther’s Law

32. Walther’s Law • A conformable succession of sedimentary layers represents a neighbouring set of coeval sedimentary environments. But, abrupt surfaces in a succession may represent erosion, and may superimpose two spatially separated environments. Next lecture – punctuated stratigraphy, unconformities and catastrophism versus gradualism in geologic time