Lectures on Sediments and Sedimentary Rocks for Physical Geology.
Sediment Sources
• Sediment is produced by weathering and erosion– Siliciclastic: composed of pre-existing rock
particles that are deposited as clastic sediments. The vast majority of rocks are dominated by silicate minerals.
– Chemical: chemical sediments are precipitated directly from water
– Biological: organisms may secrete a shell or other hard body part that is inorganic
Presenter
Presentation Notes
Sediment Sources: Siliciclastic: composed of pre-existing rock particles that are deposited as clastic sediments. The vast majority of rocks are dominated by silicate minerals. Chemical: chemical sediments are precipitated directly from water. Biological: organisms may secrete a shell or other hard body part that is inorganic.
Sediment/Sedimentary Rock Classification
• Classification has 3 main categories:– Clastic sediments/sedimentary rocks that are
composed of particles of weathered and transported pre-existing rocks.
– Biochemical/Chemical rocks are made of mineral material precipitated from water by biological or chemical processes, mostly coral reef material composed of aragonite (CaCO3), or evaporites.
– Organic sedimentary rocks are composed of organic materials such as coal or amber.
Presenter
Presentation Notes
Sediment/Sedimentary Rock Classification: Classification has 3 main categories: Clastic sediments/sedimentary rocks that are composed of particles of weathered and transported pre-existing rocks. Biochemical/Chemical rocks are made of mineral material precipitated from water by biological or chemical processes, mostly coral reef material composed of aragonite (CaCO3), or evaporites. Organic sedimentary rocks are composed of organic materials such as coal or amber.
Clastic Sediment Classification
• Based on grain size:– Gravel : > 2 mm average diameter.– Sand : 2- 0.062 mm average diameter.– Silt : 0.0063 – 0.0039 mm average diameter.– Mud: < 0.0039 mm average diameter.
• Note that particles in the silt range are not large enough to be observed with a hand lens but can be “felt” as a gritty material.
Presenter
Presentation Notes
Clastic Sediment Classification: Based on grain size: Gravel : > 2 mm average diameter. Sand : 2- 0.062 mm average diameter. Silt : 0.0063 – 0.0039 mm average diameter. Mud: < 0.0039 mm average diameter. Note that particles in the silt range are not large enough to be observed with a hand lens but can be “felt” as a gritty material.
Lithification Products• When sediment is lithified a sedimentary rock is
• Note that the majority of clastic sedimentary rocks are shale.
Presenter
Presentation Notes
Lithification Products: When sediment is lithified a sedimentary rock is produced: Gravel > Conglomerate. Sand > Lithic sandstone, Arkose, Qtz. Arenite, Arkose, Greywacke. Sandstone is a general term used to describe all of these types. Silt > Siltstone. Mud > Shale, Argillite, Claystone. Note that the majority of clastic sedimentary rocks are shale.
Stages of Sedimentary Rock Formation: 1. Weathering: breaking down existing rock chemically and physically. 2. Erosion: motion of weathering products under the influence of gravity. 3. Transport: transporting agents include running water, wind, glaciers, groundwater. 4. Deposition: depositional basins are sea level or local base levels. 5. Burial: over time deposition will bury deposits to greater depth. 6. Diagenesis: buried sediments undergo chemical (cementation) and physical modification (compaction) to become lithified sedimentary rock.
Sediment Transporting Agents
• Fluvial: transport by streams/rivers.• Glacial: transport by glaciers.• Eolian: transport by wind.• Note that size fractions for bed load and
suspended load are radically different for these different agents.
Presenter
Presentation Notes
Sediment Transporting Agents: 1. Fluvial: transport by streams/rivers. 2. Glacial: transport by glaciers. 3. Eolian: transport by wind. Note that size fractions for bed load and suspended load are radically different for these different agents.
Sediment Transport
• Suspended Load (sand size particles)• Bed Load (Gravel)
Sediment Transport via Streamflow: 1. Suspended Load (sand size particles). Velocity dependent. 2. Bed Load (Gravel). Velocity dependent. Saltation: bouncing action along bedding surface. 3. Dissolved Load (various ions: Na+, K+, Ca2+). The composition of the dissolved load is dependent only on the chemistry of the bedrock that the water has interacted with – the velocity of the stream has no effect on this component.
Sorting• Good sorting: all grains are approximately the same size• Poor sorting: large variation in grain size• Sorting tends to improve with:
– Transport time– Energy at the depositional site (depositional environment)
Presenter
Presentation Notes
Sorting: 1. Good sorting: all grains are approximately the same size. 2. Poor sorting: large variation in grain size. 3. Sorting tends to improve with: Transport time. Energy at the depositional site (depositional environment).
Rounding of Sediment Grains
• Time in transport will round transported grains and decrease size
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Presentation Notes
Rounding of Sediment Grains: 1. Time in transport will round transported grains and decrease size. 2. Softer minerals will round more rapidly than hard minerals. 3. Pronounced cleavage directions and other properties may degrade rounding.
Sedimentary Basins• Continental Rifts
(Divergent)– Thermal
subsidence• Flexural Basins
(Convergent)– Loading of the
overriding plate
Presenter
Presentation Notes
Sedimentary Basins: 1. Continental Rifts (Divergent). Thermal subsidence. 2. Flexural Basins (Convergent). Loading of the overriding plate by thrust sheets. Fore-arc and Back-arc basins.
Sedimentary Depositional Environments
• Continental– Alluvial/Fluvial: stream flow deposition– Lake (Lacustrine): lake deposition– Desert (Eolian): wind deposition– Glacial: deposition by glaciers
• Shorelines– Delta: river mouth deposition– Beach: wave and longshore current driven deposition– Tidal flat (Estuary): tide driven deposition
• Marine– Continental shelf– Continental margin and slope– Deep sea (abyssal)– Organic reefs
Presenter
Presentation Notes
Sedimentary Depositional Environments: 1. Continental a. Alluvial/Fluvial: stream flow deposition b. Lake (Lacustrine): lake deposition c. Desert (Eolian): wind deposition d. Glacial: deposition by glaciers 2. Shorelines a. Delta: river mouth deposition b. Beach: wave and longshore current driven deposition c. Tidal flat (Estuary): tide driven deposition 3. Marine a. Continental shelf b. Continental margin and slope c. Deep sea (abyssal) d. Organic reefs
Sedimentary Depositional Environments
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Presentation Notes
1. Schematic od various types of sedimentary basin environments.
Alluvial Depositional Environment
• Stream meanders
• Cutoffs• Point bar• Flood Plain• Oxbow
lake• Levee
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Presentation Notes
Alluvial Depositional Environment landform features: 1. Stream meanders 2. Cutoffs 3. Point bar 4. Flood Plain 5. Oxbow lake 6. Levee
• End moraine: deposited at terminus of glacier when glacial front is static
• Lateral moraine: continually deposited along flanks of glacier
Presenter
Presentation Notes
Glacial Depositional Environments and Landforms: 1. Glacial Till: extremely unsorted material. 2. Moraine: any deposit generated directly by glacial ice. 3. End moraine: deposited at terminus of glacier when glacial front is static. 4. Lateral moraine: continually deposited along flanks of glacier.
Delta Depositional Environments
• Distributaries: distribute the flow of a large river over the extents of the delta
Presenter
Presentation Notes
Delta Depositional Environments: 1. Distributaries: distribute the flow of a large river over the extents of the delta 2. Over the past 7,000 years the main “foot” of the Mississippi delta has shifted many times. 3. Currently the main flow of the Mississippi is trying to shift to the Atchafalaya distributary. 4. Deltaic environments are prefect for petroleum formation. The down-dip bottomset beds are rich in organic material, the topset beds are porous and permeable sands that trap petroleum.
Beach Depositional Environments
• Swash-backwash action• Longshore current
Presenter
Presentation Notes
Beach Depositional Environments: 1. Swash-backwash action migrate sand along beach face, usually several miles per day transport rate. 2. Longshore current is setup by prevailing wind direction. Current is strong enough to transport sand particles.
• Driven by tidal surges driven by tidal effects of Moon and Sun
• Spring versus Neap tides and the phases of the moon
• Tidal flat estuaries (Mobile Bay) are the most productive ecosystems on the planet
Presenter
Presentation Notes
Tidal Flat Depositional Environments: 1. Driven by tidal surges driven by tidal effects of Moon and Sun. 2. Spring versus Neap tides and the phases of the moon. 3. Tidal flat estuaries (Mobile Bay) are the most productive ecosystems on the planet.
Continental Shelf• Redistribution of deltaic sediments by waves and
longshore current• Carbonate banks: tropical climates distal to river deltas
Presenter
Presentation Notes
Continental Shelf: 1. Redistribution of deltaic sediments by waves and longshore current. 2. Carbonate banks: tropical climates distal to river deltas.
Continental Margin/Slope
• Turbidites: deposits produced by submarine landslides termed turbidity flows
Presenter
Presentation Notes
Continental Margin/Slope: 1. Turbidites: deposits produced by submarine landslides termed turbidity flows. 2. Glaciers may produce diamictites (bouldery mudstones).
Abyssal Seafloor
• Chert: remains from diatoms• Fe oxides: oxidized meteorite dust• CCD: carbonate compensation depth (> 1km)
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Presentation Notes
Abyssal Seafloor: 1. Chert: remains from diatoms. 2. Fe oxides: oxidized meteorite dust. 3. CCD: carbonate compensation depth (> 1km).
• Layers of strata represent changes in depositional environment
Sandstone=high energy
Shale=low energy
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Presentation Notes
1. Bedding represents gradual changes in depositional environment and/or provenance.
Cross-bedding Example
• Cross-bedding can be developed from currents of wind or water
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Presentation Notes
1. Schematic and actual example of cross-bedding. The transport direction is the down-dip direction of the crossbeds.
Ripple Marks
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Presentation Notes
1. Example of ripple marks. Steep side (lee) faces transport direction.
Bioturbation
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Presentation Notes
1. Example of bioturbation by worm tubes.
Exam Summary
• Know the transport agents of sediment.• Know the stages of sedimentary rock formation.• Know the 3 components of sediment transport in
a stream.• Know the different types of sedimentary
depositional environments.• Know the various types of sedimentary
structures.• Know the classification system for sedimentary
rocks.
Presenter
Presentation Notes
Exam Summary: 1. Know the sources of sediment. 2. Know the stages of sedimentary rock formation. 3. Know the 3 components of sediment transport. 4. Know the different names of sedimentary basins. 5. Know the various types of sedimentary structures.
