UNIVERSITY OF SOUTH ALABAMA
GY 111: Physical Geology
Lecture 14: Sedimentary Rocks Part 2:
Alluvial Fans and Rivers
Instructor: Dr. Douglas W. Haywick
Last Time
1) Types of Sediment
2) Sedimentary Rock Classification
3) Sediment Transport
Web notes 14
GY 111 Lab Manual Chapter 3
Types of Sedimentary Rocks
1) siliciclastic
2) biochemical
3) chemical
4) organic
Classification of Sediment and
Sedimentary Rocks
Classification of Sediment and
Sedimentary Rocks
Siliciclastic Rock Name
Classification of Sediment and
Sedimentary Rocks
Siliciclastic Rock Name
Quartz Arenite
Arkose
Lithic Sandstone
Greywacke
Sedimentary
Rocks
Four distinct classes:
1) Siliciclastic
2) Biochemical
3) Chemical
4) Organic
Sediment Transport
Source
transport
Sink
Sediment Transport
Bed load: sediment
(sand and gravel)
transported along the
bed of a river
Suspended load:
sediment (clay and silt)
transported within the
water column of a river
Solute load: dissolved
minerals transported in
a river
Sedimentary Structures
Current Ripples
http://www.wilderness-wales.co.uk/ww/october/october27.jpg
Today’s Agenda
1) Depositional Environments in general
2) Alluvial fans and their rocks
3) Meandering Rivers
Web notes 15a: Alluvial Fans
Web notes 15b: Rivers and Deltas:
Lab Manual Chapter 3
Depositional
Environments
Depositional (or sedimentary)
environments: Places where
sediment is deposited (i.e.,
sediment sinks).
Depositional
Environments
Depositional (or sedimentary)
environments: Places where
sediment is deposited (i.e.,
sediment sinks).
They are numerous.
Sedimentologists study
sedimentary rocks to
reconstruct past environments
of deposition
Depositional
Environments
Depositional (or sedimentary)
environments: Places where
sediment is deposited (i.e.,
sediment sinks).
They are numerous.
Sedimentologists study
sedimentary rocks to
reconstruct past environments
of deposition
Alluvial Fans
Fan-shaped deposits
of siliciclastic
sediment deposited
at the base of
mountains
Alluvial Fans
Fan-shaped deposits
of siliciclastic
sediment deposited
at the base of
mountains
Gravel to clay sized
Angular to sub rounded
immature
Alluvial Fans
1 km
Alluvial Fans
1 km
Alluvial Fans
1 km
Alluvial Fans
1 km
Alluvial Fans
Fan head
1 km
Alluvial Fans
Fan head
mid fan
1 km
Alluvial Fans
Fan head
mid fan
Fan toe
1 km
Alluvial Fans
Alluvial
plain
Fan head
mid fan
Fan toe
1 km
Alluvial Fans
2 km
Alluvial Fans
2 km
Alluvial Fans
2 km
Alluvial Fans
2 km
Alluvial Fans
2 km
Alluvial Fans
 Best developed in arid regions
Alluvial Fans
 Best developed in arid regions
 Best developed in “tectonically active areas”
Alluvial Fans
 Best developed in arid regions
 Best developed in tectonically active areas
From Walker, R.G. 1980. Facies Models. Geological Association of Canada
Alluvial Fans
 Best developed in arid regions
 Best developed in tectonically active areas
From Walker, R.G. 1980. Facies Models. Geological Association of Canada
Lateral fining
Alluvial Fans
 Best developed in arid regions
 Best developed in tectonically active areas
 Formed primarily through the action of water flow…
Alluvial Fans
 Best developed in arid regions
 Best developed in tectonically active areas
 Formed primarily through the action of water flow…
 …but there are also landslide deposits (mass flow)
Walker, R.G. and James, N.P., 1992. Facies Models. Response to Sea
Level Change.Geological Association of Canada, 409p.
Alluvial Fans
breccia
arkose
Red shale
Walker, R.G. and James, N.P., 1992. Facies Models. Response to Sea
Level Change.Geological Association of Canada, 409p.
Alluvial Fans
Alluvial Fans - sedimentary sections
Nemec, W. and Steel, R.J., 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravelly
mass-flow deposits. In Koster, E.H. and Steel, R. eds), Sedimentology of Gravels and Conglomerates. Canadian Society of
Petroleum Geologists Mem. 10, 1-31.
Alluvial Fans - sedimentary sections
Interbedded
brecccia/arkose
beds
Nemec, W. and Steel, R.J., 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravelly
mass-flow deposits. In Koster, E.H. and Steel, R. eds), Sedimentology of Gravels and Conglomerates. Canadian Society of
Petroleum Geologists Mem. 10, 1-31.
Types of Rivers
Braided
Anastamosing
Meandering
Walker, R.G. and James, N.P., 1992. Facies Models. Response to Sea Level Change.Geological Association of
Canada, 409p.
Meandering Rivers
•
Sinuous, single channel
drainage systems
Meandering Rivers
•
•
Sinuous, single channel
drainage systems
Typically form on low
gradient alluvial plains
Meandering Rivers
•
Are characterized by a
distinct suite of facies
and processes
Oxbow lakes
 Levees
 Floodplains
 Cut banks
 Point bars
 Yazoo streams
 Cutoffs

Meandering Rivers
The channel meanders
across the flood plain
Meandering Rivers
Deposition occurs on the
inside of meander loops
(point bar)
Meandering Rivers
Large point bars may
consist of numerous
accretionary ridges
Meandering Rivers
Erosion occurs on the
outside of meander loops
(cut bank)
Meandering Rivers
Meandering river channels are
asymmetrical (deepest near cut
bank)
Meandering Rivers
Water velocity is greatest where
the channel is deepest resulting in
a “corkscrew” flow pattern.
http://www.geocities.com/sogodbay/Images/SDK/Inecar03.jpg
Meandering Rivers
Vortices can be either singular or
complex.
Either way, it results in a
“sieving” action during point bar
deposition.
Meandering Rivers
Sediment eroded from
the cutbank is
transported onto the
point bar where the
current is slower
Deposition
Erosion
Meandering Rivers
The bottom of the channel is frequently
characterized by a channel lag of gravel
and/boulders (conglomerate)
Meandering Rivers
Apart from deposition in
channels, rivers periodically
flood resulting in
sedimentation on flood
plains
http://www.fes.uwaterloo.ca/geomatics/images/mississippi_flood_NASA.jpg
Meandering Rivers
Apart from deposition in
channels, rivers periodically
flood resulting in
sedimentation on flood
plains
Red shale
Today’s Homework
1. Download and read Web Lectures 15a,b
2. Call your parent(s) and say thank you for changing
all of those dirty diapers when you were a baby
3. Assignment 2: instructions
Next Time
Fall Break
Contour Map Homework
Contour Map Homework
Contour Map Homework
GY 111: Physical Geology
Lecture 14: Alluvial Fans and Rivers
Instructor: Dr. Doug Haywick
dhaywick@southalabama.edu
This is a free open access lecture, but not for commercial purposes.
For personal use only.