bedforms lecture 2

8/8/2019 Bedforms Lecture 2

1/35

Wind Ripples andDunes

Desert Environment: saltation

and surface creep Ripples are asymmetric, straight

crested, some bifurcation

R.I. much higher than subaqueousbedforms (less air resistance)

Most important structures in aeolian

environment: Dunes

Aeolian

Remember:

millet-seedgrains help

to identify

aeolian

environment


2/35

DistinguishingSubaqueous and AeolianDunes

Aeolian

Steeper foreset angle: 35

Thicker foresets: few ms to 30 ms


3/35

Aeolian Cross Bedding


4/35

Barchan Dunes Lunate

Form normal to wind direction Asymmetric cross section (shallow

stoss, steep lee)

Grains creep up stoss andavalanche down lee

Internally: cross-bedding facingwind direction

Aeolian


5/35

Barchan Dunes


6/35

Aeolian Seif Dunes Highly elongated in direction of

wind

Foresets perpendicular to flow

direction

Internally: chevron effect cross

bedding


7/35

Other Dune Shapes

Transverse: oriented perpendicularto wind direction

Star: occur where wind direction

changes frequently Draa: similar to dunes but much

larger: = kms, h = 10s m

Aeolian


8/35

Primary Dune Types


9/35

Erosional SedimentaryStructures

2 types:Object striking surface

Erosion by water flow (SoleStructures)


10/35

ImpactStructures Groove mark. Object impacts with surface and is

dragged along it.

Common on underside ofsandstone beds over muds.

Sometimes isolated, sometimes

parallel swarms. Common at turbidite bases and

river flood plains.


11/35

ImpactStructures

Groove Marks


12/35

ImpactStructures

Common in continental and

shoreline mud deposits

Rain pits on mudstone


13/35

StructuresFormed byFlow

Sole structures: General

Top of mud: erosion and infilling by

sand

Commonly see these on base of

sst beds


14/35


Channels erode base leaving

characteristic channel lag deposits

N.B. bedding truncated

Channel Scours


15/35


Heel-shaped sole structure with

bulbous end facing upstream

Frequently found in swarms

Swarm flutes generally all same size

and orientation

Formation:Flow detachment erosion by

turbulent sand laden flow over

cohesive muds

Flow leaves sed surface at

upstream rim of flute

Eddy forms preventing deposition

in eroding hollow

Flute Marks


16/35

Flute Marks: diagram


17/35

Very common at

turbidite bases

Very good flow

direction indicator

Flute marks:images


18/35

Soft SedimentDeformation Structures

Mainly sand-mud interfacephenomena

Load Casts Flame Structures

Ball and pillow structures

Dewatering structures:

Contorted bedding

Sand volcanoes

Dish and pillar structures


19/35

Soft

SedimentDeformation Sand overlying mud: mud contains

more water; sand more dense.

For equilibrium, sand movesdownwards, producing loads.

Load Casts


20/35

Soft

SedimentDeformation Load Casts


21/35

Soft

SedimentDeformation As sand sinks into mud: streamers

of mud move upwards: Flame

Structures

Flame Structures


22/35

SoftSedimentDeformation


23/35

Pseudonodules

As sand continues to sink into themud, eventual detachment to formPseudonodule.

Soft

SedimentDeformation


24/35

Whole layer collapses : mix ofround and tabular shapes: Ball andPillow Structures

Usually shock related:

Earthquake

Waves crashing onto sediment

Soft

SedimentDeformation

Ball & Pillow structure


25/35

Soft

SedimentDeformation Ball & Pillow: image


26/35

Soft

SedimentDeformation Common cause of soft sedimentdeformation

If rapid occurrence: can causeformation of Contorted or ChaoticBedding

Dewatering


27/35

SoftSediment

Deformation

Reverse situation: mud overlyingsand.

Mud: impermeable so water from

sand cannot escape vertically Pressure builds up

Water moves rapidly up a fault to

the surface forming a sand volcanoand dyke (Neptunian Dyke)

Sand Volcanoes


28/35

Sand Volcanoes: structure


29/35

SandVolcanoes:image


30/35

Water may not be able to moveupwards (compaction, low

permeability layer)

Moves laterally until it finds a breakit can exploit

Dish and Pillar structures result

Soft

SedimentDeformation Dish & Pillar Structures


31/35

Dessication Structures

Mud cracks: Mud dries out and cracks

Sand fills in the spaces

Indicator of subaerial exposure ofmuds.


32/35

Mud Cracks (Plan View)


33/35

Mud Cracks (Side View)


34/35

Beware of

Pseudostructures!

Leisegang Rings


35/35

Useful texts

Tucker, M.E. 1982. Sedimentary Petrology. Blackwell

Scientific Publications

Middleton & Hampton 1973: Flow Types

De Raaf 1977 Sedimentology 24, p. 451-483

Jones & Preston 1987. Geol. Soc. Ondon Special

Publication 29. : Soft sediment deformation

bedforms lecture 2

Documents