Sedimentary structures

unidirectional flow

fluid flows in consistent direction and sed structures form in response

flow separation

caused by obstacles on the bed due to lower pressure and pressure drag behind the object after a separation point

what does flow separation cause

differential erosion and deposition along a bed

• origin of a ripple

cross beds

structures that can be used to infer the younging direction in sed sequence via stenos law of superposition

• truncated tops and asymptotic toes

what structures occur when Fr<1

ripples, sand waves, dunes

what structures occur when Fr>1

plane flat beds

antidunes

chutes and pools

ripples

• larger with larger grain sizes

dunes

• larger with larger flow depths

upper stage plane beds

no flow separation since too turbulent

• no leeward deposition

antidunes

• form inphase with standing wave
• low relief (~5 meter spacing)
• migrate upstream
• shallower with higher velocity
• deeper with lower velocity

multidirectional flow

cause symmetrical waves

two distinct directions of unidirectional flow

herring bone

currents go in equal directions

positive relief

wave and current ripple bedforms

negative relief

fluid current makes holes

sole marks

features on bottom of beds that are molds or casts

flute casts

formed by eddies touching down bed bases and taper down current

tool marks

form from objects dragging along by current then molded

SECONDARY

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soft sediment deformation

forms from rapid sedimentation rates, leading to unstable sedimentary layer

• water saturation leads to liquefaction
• most common = load structures
• ball and pillow

slumping

• most extreme soft sediment deformation
• soft layers are stuck together and preserved

burrows

soft sediment deformation

• animals distrubing wet sediments
• horizontal (sweeping)
• vertical (pipes)