Lecture 3 geologic time, sediments, and sedimentary rocks

Lecture 3 Geologic Time, Sediments, and Sedimentary Rocks CEE 437 Engineering Geology I Oct... Sedimentary Rocks and Geologic Time • Geologic Time Scale and it Origins • Sedimentary Rock

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

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

Age of the Earth

• Kelvin and a basis in heat flow (set at 20

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

– Early post-depositional chemical

transformation of sediments, e.g calcite to dolomite

• 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

Carbonate Environments

– restricted seas (Mediterranean)

– lagoons, back-reef areas

• Subject to flow and diapirism

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

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

Expanding Sedimentary

Materials

• Expanding clays (especially bentonite)

• Gypsum-Anhydrite hydration (CaSO4)

• Related to energy of environment

– (example channels and banks in fluvial systems)

• Energy related to topography, climate, and

tectonic activity

Sediment Sorting

Sedimentary Structures — Load

Casts, Rip-ups, etc.

Sedimentary Structure — Cross

Bedding

Fluvial and Lacustrine Environments

Deltaic Environments

• Variability based on proximity to source

• Stratigraphy effected by progradation

Deltaic Development and

Sedimentary Facies

Continental Slope Environments

• Turbidites and turbidity currents

• Graded bedding

– poor sorting

– vertical zonation with fining upwards

Turbidites and Turbidity Currents

• Formed by mobilization of sediments on

slopes

• Graded bedding (coarse at bottom, fining upwards)