Lecture 3 Geologic Time, Sediments, and Sedimentary Rocks

1. Lecture 3 Geologic Time, Sediments, and Sedimentary Rocks CEE 437 Engineering Geology I Oct. 8, 2002

2. Sedimentary Rocks and Geologic Time • Geologic Time Scale and it Origins • Sedimentary Rock Types Depositional Environments • Engineering Properties

3. Relative Time • Principle of Superposition • Fossil Evidence • Cross Cutting Relationships • Unconformities • Alteration • Fracture Termination

5. Geologic Time Scale — Eras • Precambrian — Minimal fossil record • Era, Period, Epoch • Based on major changes — extinctions, mountain building events

6. Paleozoic (Old Life) — Brachiopods, Trilobites, Fish • Periods based on English Geology • Cambrian for Latin Wales • Ordovician and Silurian for ancient Welsh Tribes • Devonian for Devon • Carboniferous for Coal Measures (also Mississippian and Pennsylvanian in US) • Permian for Perm Basin in Ukraine

7. Mesozoic (Middle Life) — Ammonites, Dinosaurs • Triassic based on distinctive three-layer stratigraphy in southern Germany • Jurassic based on Jura Mountains in France and Switzerland • Cretaceous (Latin for Chalk) based on chalk unit that forms Dover’s cliffs

8. Cenozoic (Recent Life) — Mammals, Modern marine fauna (foraminifera) • Periods are Tertiary (before Ice Ages) and Quaternary (ice ages) • Primary and secondary have been long replaces • Rocks of western Washington are primarily Tertiary and Quaternary in age

9. Age of the Earth • Kelvin and a basis in heat flow (set at 20 million years) • Problem of fitting all of evolution in this time • Rutherford and the introduction radioactive decay • Added a head source, pushed ages back to 4.5 billion years

10. Absolute Time • Basis on radiometric dating Common dating tools • 14C, K-Ar, Rb-Sr,Uranium decay series

11. Sedimentary Rocks • Clastics, Siliciclastics, Carbonates, and Evaporites • Clastic rocks, depositional medium, and energy • Diagenesis — chemical changes after deposition

12. Clastic Sedimentary Rocks • Clastic — broken like iconoclast) • Often referred to as Siliciclastics as having Si based rock forming minerals • Based on grain size and to a lesser extent composition • Grain size related to energy of depositional environment • Relationship of medium velocity to maximum grain size)

13. Clastic Sedimentary Rocks • Clay, muds  shales, mudstones, claystones (difference based on fissility) • Silts  siltstones • Sands  sandstones • Gravels  Conglomerates (Breccia if angular, breccia may also be a term for tectonically fragmented rock)

14. Weathering Cycle

15. Clastic Sediments

17. Classification of Sedimentary Rocks (ex. evaporites and coal)

18. Clay Minerals • Sheets of linked silica tetrahedra sandwiching octahedral layers of gibbsite composition, Al2(OH)6, or brucite Mg3(OH)6 • Major Clay Groups • kaolinite: single gibbsite layer • montmorillonite:weak water bonding between layers, moderated by Ca, Na, or K (near-shore environments) • illite: K bonds between layers (off-shore environments) • bentonite: highly expansive, volcanic-derived, Na-rich montmorillonite

19. Clay Structure

20. Clay Structure Cont’d. Kaolinite Montmorillonite Illite

21. Clay Plasticity

22. Lithification • Cementation • deposition of a material different from clasts • Crystallization • crystal growth on clasts to fill pore space • Compaction • Diagenesis • Early post-depositional chemical transformation of sediments, e.g. calcite to dolomite

23. Carbonates • Generally like siliciclastics — carbonate muds, sands, etc. • Often deposited in reefs • Major portion of world oil deposits • Properties depend strongly on post-depositional pore chemistry • Cementation • Dissolution

24. Carbonate Environments

25. Evaporites • Rock salt (NaCl), Gypsum-Anhydrite (CaSO4), Sylvite (KCl) • Deposition in regions where evaporation exceeds recharge • desert lakes • restricted seas (Mediterranean) • lagoons, back-reef areas • Subject to flow and diapirism

26. Other Sedimentary Rocks • Chert: finely crystalline silica • as replacement/diagenetic nodules • as bedded material from silica-shelled biota • Coal • Derived from vegetation • Banded Iron Formation • Likely bacteria derived, mainly Pre-Cambrian

27. Sedimentary Rocks and Rock Properties • Properties for a given geologic description vary wildly based on cementation, porosity and other diagenetic factors. • Properties can be strong anisotropic and heterogeneous based on bedding

28. Expanding Sedimentary Materials • Expanding clays (especially bentonite) • Gypsum-Anhydrite hydration (CaSO4)

29. Depositional Environments • Synchronicity of deposition of different rock types • Sedimentary facies • A rock unit is not everywhere the same age: Bright Angel Shale • Related to energy of environment • (example channels and banks in fluvial systems) • Energy related to topography, climate, and tectonic activity

30. Sediment Sorting

31. Sedimentary Structures — Load Casts, Rip-ups, etc.

32. Sedimentary Structure — Cross Bedding

33. Fluvial and Lacustrine Environments • Fluvial • Channelization • Complex and close interrelationship of fine and course sediments • Challenge for characterization due to high variability • Special examples: glacial environments • Lacustrine • Deltaic deposits at margins, finer materials in lake beds

34. Deltaic Environments • Variability based on proximity to source • Stratigraphy effected by progradation

35. Deltaic Development and Sedimentary Facies

36. Continental Slope Environments • Turbidites and turbidity currents • Graded bedding • poor sorting • vertical zonation with fining upwards

37. Turbidites and Turbidity Currents • Formed by mobilization of sediments on slopes • Graded bedding (coarse at bottom, fining upwards)