Sedimentary Structures

SEDIMENT SEDIMENTARY ARY STRUCTURES STRUCTURES By Dr Khai Khairu rull Az Azla lan n Mus Mustapha apha

SEDIMENTARY STRUCTURES •

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Primary structures are created by erosion and deposition of sediments (especially clastic) that reflects the depositional conditions under which they were laid down eg. bedding, cross-bedding, ripple marks, flute casts. Sedimentary structures can be found within, on top or on the bottom of a bed. Sedimentary structures are useful for helping us to determine normal and overturned beds, paleocurrent directions and interpret paleoenvironments. Syn-depositional sedimentary structures are created during deposition while post-depositional post-depositional sedimentary sedimentary structures structures are created shortly after deposition but before diagenesis.

Type of sedimentary structure 1. Syn-depositional On the bed surface •

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Within beds 1. 2. 3.

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Ripple mark Mud crack Graded bedding Bouma sequence Cross-bedding

Bottom bed 1. 2. 3. 4. 5. 6.

Grooves marks Chevron marks Prod Bounce Flute Sole

2. Post-depositional I. Pillow & ball II. Load cast & Flame III. Folded slump beds IV. Disc structure

Beds and Bedding •

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package of sediment arranged by the processes of sedimentation Represents a single event of deposition Distinguished by changes in texture of sediment within them

THICKNESS OF BEDS

TYPES OF BEDS

Structure within beds: Graded Bedding •

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Characterized by a systematic change in grain or clast size from the base of the bed to the top. Normal grading Inverse grading

Structure within beds: Turbidite beds

Bouma sequence (Overlying sand bed) Te= Shale Td=Parallel laminated Tc=Convolute laminated Tb=Parallel laminated Ta=Massive bedded

(Underlying shale bed)

Structure within beds: Cross bedding

Structure within beds: Cross bedding

Structure on bed surface: Ripple Marks

Structure on bed surface: Ripple Marks

Traction currents-Unidirectional •

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Bedforms developed in sediment of a given grain size depends on the flow velocity After reaching a certain critical velocity, sand will start to move and form ripples The ripples transform into larger dunes as flow velocity increases The sediment flattens with continued velocity increase Antidunes form at higher velocity

Current

Ripples

Flat beds

Ripples on sand dune

Structure on bed surface: Mud crack & tracks

Structure on bed surface: Raindrops imprint

On mud surface (mold)

Below sand bed (cast)

Structure bottom beds: Sole marks

Structure bottom beds: Flute marks

Post depositional structure:

Load cast & flame

Pillow & Ball

Post depositional structure: Slump beds

Application of sedimentary structure: 1) Determine Younging direction of beds 2) Determine Paleodepositional environment 3) Determine Paleocurrent

Determine younging direction •

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Sedimentary bedding can be deformed by tectonics How do we determine which beds were originally on top or originally at bottom? This question is important: in order to unravel Earth History, we need to know which bed is oldest and which bed is youngest

Determine younging direction •

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Younging direction: direction in which beds become younger (towards the top) Can be determined using certain sedimentary structures Top

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Ripples: crest always towards top

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Crossbeds: Dip angle of crossbed foreset (internal layers) becomes gentler toward base of bed

Younging direction

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Sole marks: at the base (bottom) of the bed

Bed is upside down (overturned)

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Load Structures: Sandstone bulges downwards

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