Lecture 3 geologic time, sediments, and sedimentary rocks

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


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


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

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


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


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

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


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


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


Sedimentary Rocks
• Clastics, Siliciclastics, Carbonates, and

Evaporites
• Clastic rocks, depositional medium, and
energy
• Diagenesis — chemical changes after
deposition


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)


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)


Weathering Cycle

Clastic Sediments



Classification of Sedimentary
Rocks (ex. evaporites and coal)


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


Clay Structure


Clay Structure Cont’d.
Kaolinite

Illite

Montmorillonite


Clay Plasticity


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


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

Carbonate Environments


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