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GY111 Physical Geology
Lecture 6: Sediments &
Sedimentary Rocks
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
Stages of Sedimentary Rock
Formation
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Weathering
Erosion
Transport
Deposition
Burial
Diagenesis
Sediment Transport
• Suspended Load (sand size particles)
• Bed Load (Gravel)
– Saltation: bouncing action along bedding surface
• Dissolved Load (various ions: Na+, K+, Ca2+)
Stream surface
Suspended particles
Bed load particles
Stream Bed
Stream flow
saltation
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)
Rounding of Sediment Grains
• Time in transport will round transported
grains and decrease size
Sedimentary Basins
• Continental Rifts
(Divergent)
– Thermal
subsidence
• Flexural Basins
(Convergent)
– Loading of the
overriding plate
Sedimentary Depositional
Environments
• Continental
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–
–
–
Alluvial: 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
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Continental shelf
Continental margin and slope
Deep sea (abyssal)
Organic reefs
Sedimentary Depositional
Environments
Alluvial Depositional Environment
• Stream
meanders
• Cutoffs
• Point bar
• Flood Plain
• Oxbow
lake
• Levee
Desert (Eolian) Landform
Features
• Alluvial
Fans
• Playa
Lakes
• Inselbergs
• Bajada
• Pediment
(Bajada)
(Inselbergs)
Glacial Depositional Environments
•
•
•
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Glacial Till: extremely
unsorted material
Moraine: any deposit
generated directly by
glacial ice
End moraine: deposited at
terminus of glacier when
glacial front is static
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
Beach Depositional Environments
• Swash-backwash action
• Longshore current
Problems Associated with Beach
Deposition/Erosion
• Jetties
• Groins
• Seawalls/Bulkheads
Tidal Flat Depositional
Environments
• 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
Continental Shelf
• Redistribution of deltaic sediments by waves and
longshore current
• Carbonate banks: tropical climates distal to river deltas
Continental Margin/Slope
• Turbidites: deposits produced by
submarine landslides termed turbidity
flows
Abyssal Seafloor
• Chert: remains from diatoms
• Fe oxides: oxidized meteorite dust
• CCD: carbonate compensation depth (> 1km)
Organic Reefs
• Continental shelf edge: Bahamas
• Coral Atolls: develop on subsiding volcanic
islands
Sedimentary Structures
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•
•
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Bedding
Cross-bedding
Ripple marks
Bioturbation
Bedding example
• Layers of strata represent changes in
depositional environment
Sandstone=high energy
Shale=low energy
Cross-bedding Example
• Cross-bedding can be developed from
currents of wind or water
Ripple Marks
Bioturbation
Exam Summary
• Know the sources of sediment.
• Know the stages of sedimentary rock
formation.
• Know the 3 components of sediment
transport.
• Know the different names of sedimentary
basins.
• Know the various types of sedimentary
structures.