GEOL1002 Lecture Notes - Lecture 16: Mudcrack, Flame Structure, Bouma Sequence
5 Jul 2018
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Sedimentary processes and structures
The Earth has a dynamic surface, especially when considered on geological timescales. To
interpret the geological record, we must understand the range of processes that are in
action from source to sink and from loose sediment to exposure of lithified sediment.
Bedding and laminations
Bedding: stratification at greater than 1cm
thickness.
Lamination: stratification at less than 1cm
thickness.
Bedding
Layers in a sedimentary sequence
represent individual depositional event.
Changes in sediment supply/deposition
rate or environment/climate can all result
in bedding planes.
Graded bedding
Gradational grain-size variation within a bed. Typically beds grade with the coarsest
particles at the base and the finest on the top. This is because the coarsest grains are
deposited first as per Stokes Law and currents lose energy progressively, ie. If energy
increases then sediments will likely be eroded.
Reverse graded bedding
Graded bedding can indicate which way up a sequence was deposited. However
reverse grading and overturning of beds can occur.
Turbidite sequence
Density current downwards to the oceanic abyssal plain produces a sequence from coarse
to fine sediments (Bouma sequence).
Laminations
Fine (<1cm) layering typically defined by slight variations in composition.
Ripples
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Document Summary
The earth has a dynamic surface, especially when considered on geological timescales. To interpret the geological record, we must understand the range of processes that are in action from source to sink and from loose sediment to exposure of lithified sediment. Layers in a sedimentary sequence represent individual depositional event. Changes in sediment supply/deposition rate or environment/climate can all result in bedding planes. Typically beds grade with the coarsest particles at the base and the finest on the top. This is because the coarsest grains are deposited first as per stokes law and currents lose energy progressively, ie. if energy increases then sediments will likely be eroded. Graded bedding can indicate which way up a sequence was deposited. However reverse grading and overturning of beds can occur. Density current downwards to the oceanic abyssal plain produces a sequence from coarse to fine sediments (bouma sequence). Fine (<1cm) layering typically defined by slight variations in composition.
