GY 402: Sedimentary · PDF filelaminations versus bedding; it’s all a matter of scale:...

GY 402: Sedimentary Petrology

Lecture 5: Bedform Development (Flume Studies)

Instructor: Dr. Douglas W. Haywick

UNIVERSITY OF SOUTH ALABAMA

Today’s Lecture

1. What’s a flume?

2. Traction induced bed forms (sedimentary structures)

3. Flow regime

Flume Studies

A flume is an elongated plastic or glass tank through which a water current flows. Sediment is placed at the bottom of the flume and moves down current as bed load.

Flume Studies

A flume is an elongated plastic or glass tank through which a water current flows. Sediment is placed at the bottom of the flume and moves down current as bed load.

Recall what this is

Sediment loads Sediment moving along the base of a channel* that mostly stays in contact with the substrate is called…

Current

* A channel here is defined as a moving column of water that is confined to a narrow pathway

Sediment loads Sediment moving along the base of a channel that mostly stays in contact with the substrate is called…

Current

Bed load

Sediment loads Sediment moving along the base of a channel that mostly stays in contact with the substrate is called…

Current

Bed load saltation rolling sliding

Types of bed load transport (traction)

Flumes come in different sizes; from a few cm long to a few hundred metres long.

Flume Studies

small

Flume Studies

Big

Flume Studies

Flume Studies

Sand (bed load)

Sediment movement results in bed forms

(sedimentary structures)

Flume Studies

Increasing current velocity under controlled situations (fixed water depth, constrained grain size) yields the following structures:

Flume Studies


Plan lamination (lower)

Flume Studies



Small current ripples

Flume Studies




Large current ripples (dunes)

Flume Studies





Plan lamination (upper)

Flume Studies






Antidunes

Flume Studies






Antidunes

Increasing water velocity

Flume Studies

A do it yourself recipe to make your own flume

Flume Studies

Take one large tank….

Flume Studies

The best flumes are capable of passing a consistent current from one end to the next.

Water is recycled through a recirculation pump

Sediment/water movement is monitored through the glass sides of the flume

Add water….

Flume Studies

Add sediment….

Flume Studies

Add a current….

current

Flume Studies

Add a current….

…. Observe!

current

Flume Studies

very slow currents (or none at all) U = very low

Flume Studies: 1) lower plan lamination

very slow currents (or none at all) U = very low

Flat bed, containing finely laminated* parallel sedimentary structures


laminations versus bedding; it’s all a matter of scale:



laminations: parallel layers less than 1 cm apart: thick (1cm-5mm spacing);

medium (5mm-1mm spacing);

thin (<1mm spacing)



bedding: parallel layers more than 1 cm apart: thick (> 50 cm spacing);

medium (10 to 50 cm spacing);

thin (1cm-10 cm spacing)


Flume Studies: 2) small current ripples

slow currents: U = low

Something wonderful happens….

Laminations start to fade…


Flat bed, passes gradually into a “rippled” bed form


Asymmetrical



Lee side slopes range from 20-34o


ripple height: 3 to 5 cm


ripple wavelength: 4 to 40 cm


ripple wavelength: 4 to 40 cm

Ripple index (height to wavelength ratio): 10 to 40

Flume Studies: 2) summary ripple morphology

From Collinson, J.D. and Thompson, D.B. 1982. Sedimentary Structures. George Allen & Unwin, 194p.

Ripples migrate down current


current

Net result is that an inclined lamination (marking the former lee side of the ripple) develops


current

Net result is that an inclined lamination (marking the former lee side of the ripple) develops

= cross stratification


?


Bypass fraction

Dune fraction


Big quartz

Small quartz

Heavy minerals

Which grains become part of the dune fraction; which ones are part of the bypass fraction?


Big quartz

Small quartz

Heavy minerals

Heavy and larger grains usually become part of the dune fraction; smaller and lighter grains become part of the bypass fraction


Migration direction


Migration direction


ripple cosets

Migration direction


shallow angle

steep angle

Migration direction

depositional “up”


shallow angle

steep angle

Video of ripple migration in a

flume (Bird’ eye view)

Source: USGS Coastal & Marine Geology Web page (http://walrus.wr.usgs.gov/seds/).

The movie was compiled from 1161 video images collected at the rate of 1 per minute for a duration of approximately 19 hours.

38 cm

59 c

m

Click image to start


Computer animation of

ripple migration

Source: USGS Coastal & Marine Geology Web page (http://walrus.wr.usgs.gov/seds/).


Click image to start

Flume Studies: current ripple crest morphology


straight sinuous linguiodal

increasing velocity (or decreasing water depth)

Flume Studies: 3) large current ripples

Moderate currents U = moderate

Moderate currents U = moderate

Small current ripples gradually pass into larger ones


Lee side angle: 10 to 34o

Ripple height: >5cm (commonly exceeds 10’s of m)

Wavelength: 60 cm to 100’s of m

Note: also known as megaripples and dunes


Flume Studies: 4) upper plan lamination

High currents; U = fast

High currents; U = fast

Something wonderful happens again! Large current ripples start to fade and….


…. plan lamination forms again


This form of plan lamination forms only during very fast currents; sediment is literally streaming along the substrate.


Flume Studies: 5) antidunes

Very high currents; U = very fast


Upper plan lamination fades and is replaced by….



Upper plan lamination fades and is replaced by….

…. antidunes


Flume Studies summary of structures

From

Col

linso

n, J

.D. a

nd T

hom

pson

, D.B

. 198

2. S

edim

enta

ry S

truct

ures

. Geo

rge

Alle

n &

Unw

in, 1

94p.

Flume Studies

I

ncre

asin

g ve

loci

ty

Plan lamination (lower) Small current ripples Large current ripples (dunes) Plan lamination (upper) Antidunes Chutes & pools (erosion) Fr

om C

ollin

son,

J.D

. and

Tho

mps

on, D

.B. 1

982.

Sed

imen

tary

Stru

ctur

es. G

eorg

e A

llen

& U

nwin

, 194

p.

Flume Studies and Flow Regime



Lower



Upper


Modified Hjustrom’s diagram C

urre

nt v

eloc

ity (c

m/s

)

Grain size (mm)

From

Col

linso

n, J.

D. a

nd T

hom

pson

, D.B

. 198

2. S

edim

enta

ry S

truct

ures

. G

eorg

e A

llen

& U

nwin

, 194

p.

Water depth as a variable

Wat

er d

epth

(cm

)

From

Col

linso

n, J.

D. a

nd T

hom

pson

, D.B

. 198

2. S

edim

enta

ry S

truct

ures

. G

eorg

e A

llen

& U

nwin

, 194

p.

Upcoming Stuff Homework

1) Activity 2 (Rock descriptions Due Thursday) 2) Writing Assignment 2-redo (Hypothesis and methods: Due Thursday)

Today’s Lab

Grain size analysis

Online: Lecture 6: More Sedimentary Structures

Thursday: Lecture 7: Sedimentary Sections

GY 402: Sedimentary Petrology

Lecture 5: Bedforms

Instructor: Dr. Doug Haywick [email protected]

This is a free open access lecture, but not for commercial purposed. For personal use only.

GY 402: Sedimentary · PDF filelaminations versus bedding; it’s all a matter of scale:...

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