Lecture 3 geologic time, sediments, and sedimentary rocks

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Lecture 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 • • • • • • 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 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 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) 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 Sediment Sorting Sedimentary Structures — Load Casts, Rip-ups, etc Sedimentary Structure — Cross Bedding 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 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) [...]... 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... 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.. .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... strong anisotropic and heterogeneous based on bedding Expanding Sedimentary Materials • Expanding clays (especially bentonite) • Gypsum-Anhydrite hydration (CaSO4) 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... (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 • Fluvial – Channelization – Complex and close interrelationship of fine and course sediments – Challenge for characterization due to high variability – Special... deposits at margins, finer materials in lake beds 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 ... 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... 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
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