Physical Geology Chapter
6 Sediments & Sedimentary Rocks
Sediments by general
type:
Siliciclastic broken
pieces of
silicate rocks
Chemical or biochemical
created by
chemical processes
commonly with the help of
organisms (especially bacteria)
Organic (or carbonaceous)
accumulations of organic material
peat coal petroleum (tar)
Breaking up rocks weathering
minerals that are unstable at Earth
surface temperatures and pressures,
and with water
chemical reactions:
oxidation
hydration
acid-base reactions (think limestone)
Bowens reaction
series run in reverse
effects of water, ice, wind, abrasion
** dont forget gravity **
What is happening in
a stream high in the Rocky Mountains?
What determines the
resulting particle size?
(Well come back to this question)
Sediments may be classified by
SIZE or ORIGIN
Size
classification:
boulders
cobbles
gravel
sand
silt
clay
Mud is a mixture of silt and clay
Each size category
spans one or more orders of magnitude
Major
categories:
Diameter
(mm) Sediment Particle
>256 Boulder
>64 Cobble
>2 Pebble
>1/16
Sand
>1/256 Silt
<1/256 Clay
Particle size Rock type
Sand Sandstone
Silt Siltstone
Clay Mudstone
Sediment
Sorting
poorly sorted well
sorted
close to source transported and
reworked
Sedimentation is controlled by particle size and energy (or turbulence)
Think of different depositional
environments:
beach
high-mountain stream
pond
marsh
flood plain of Ottawa River
erosion transport deposition
terrigenous
from land
biogenic
from organisms
a
uthigenic
from chemical reactions
volcanogenic
from volcanic sources
cosmogenic
from space
strongly reflect
their source
transported by:
wind
rivers
glaciers
Compare the sediments in the Maumee River with the Snake River in Idaho
Continental
glaciation Antarctica
Mountain
glaciation Alaska
British Columbia
How much rock was removed?
Where did it go?
Sources:
mountain ranges
uplifted plateaus
tropical humid climates
glaciated areas
Mississippi
River Drainage
Downstream change in sediment size
Mississippi Delta
Ganges-Brahmaputra Delta
Ganges drainage basin
n
south flank of the
Himalayas & Tibetan Plateau
Yellow River discharges to the East China Sea
River Input of Silt
global map
largest inputs:
Yukon
Amazon
Yangtze
Ganges
***
Southeast Asia ***
can be substantial from arid regions
Sahara
Desert
Gobi
Desert
Where in the world? Pacific Rim in particular
Mt.
Pinatubo, Philippines
Mount St. Helens, erupted August 1980
Biogenic Sediments
Open Ocean deposited in deep water
Carbonate producers
foraminifera
coccolithophorids
(nannoplankton)
Silica producers (siliceous seds)
radiolarians
diatoms
The White
Cliffs of Dover chalk from coccoliths
map of world-wide distribution
primarily in the tropics and subtropics
away from major rivers (too much suspended
sediment)
Sediment size and water velocity
Logarithmic
scale of particle size
Boulder
beach
What do the boulders tell you about the wave conditions?
Grain
size changes downriver
boulders gravel sand
silt & clay
High-relief,
high-energy streams
Braided
river more sand than water
Lower
Missouri River channel bars
Lower
Mississippi River lots of suspended silt
Deposition compaction
cementation
The
most common cements are:
calcite CaCO3
silica
SiO2
Deposit
of poorly sorted sediments
Example: alluvial fan in the Southwest
Death Valley alluvial fan
Inside an alluvial fan
Upper fan canyon feeding alluvial fan
conglomerates deposited
Breccia made of pieces of broken rock
Volcanic
explosions, faults, impact craters
Sandstones quartz arenite
90%
or more quartz grains
Beaches
and sand dunes
Sandstones
arkose
Mostly
quartz and feldspar grains
Forming an arkose
Deposited close to a
source such as granite in mountains
(Sierra Nevada)
Sandstones graywacke
Grains
in a dark gray, fine-grained matrix
Moderately
poorly sorted, fine-grained sediments mixed with sand
Silts
and clays deposited in a relatively
quiet basin a large lake, or the ocean
Commonly
forms thin beds
Forming a shale from a silt deposit
Current
velocity changed as sediments were deposited
Figure shows deposits from two distinct depositional events
Most
common process transporting sediments to deep ocean
also happens in lakes
Sections
of a turbidity flow
Tail Body
Neck Head
Low energy to very high energy
Turbidity
flow in a flume
Movie
University of Arizona
Turbidite in an outcrop
Sediments from top to
bottom:
fine laminae (thin layers)
small ripples
horizontally stratified
fining upward
trough cross-bedding
scoured base
Stacked turbidite sequences in the Alps
Alternating
layers of sandstone and shale relate to
changes in the surrounding region
Tectonic uplift (create mountains)
or subsidence (create basins)
Climate change (change rate of
erosion and transport)
Graded beds in other settings
Example
a beach with repeated storms
Produced
on the surface of the sediment by the movement of water or wind
Ripples on a bedding surface
from Capitol Reef National Park, Utah
Current ripples on a tidal flat
Baja California
Cross-bedding
Large sand dunes from the Jurassic
Internal bedding structures of a sand dune
Subaqueous cross-bedding
Such as channel bars in a large river
desiccation cracks in mud
Playa lakes in Death Valley
Flat basins at the base
of the alluvial fans
Silts with
evaporite salts
Coral reef and back-reef lagoon
Carbonate mud
sand gravel
Bioclastic limestone
Broken pieces of shells and coral cemented together
Carbonate mud from coralline algae
Most common in the back-reef lagoon
Coquina cemented carbonate beachrock
Oolites (egg stones)
Form on shallow, tropical carbonate banks such as Bahamas Banks