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

4
(No Transcript)
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 Dovers 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
16
(No Transcript)
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
• montmorilloniteweak 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 Contd.
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)