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Cross bedding and other
indicators in sedimentary rocks
What is so important about
understanding how these sedimentary
rocks look the way they do?
Bedding, no it has nothing to do with
where you sleep
• Virtually all sediments clastic or chemical are deposited
in layers. This can be their most prominent
characteristic.
• Bedding that is very fine or thin is termed lamination.
Some of these may be many meters thick
• Each visually distinct layer that corresponds to a
particular period of deposition is a single stratum
(plural, strata)
• Do not confuse with “facies”: This is an aspect,
appearance and characteristic of a rock unit usually
reflecting the conditions of how it was made
What is this? What kind of environment does this
represent?
What is “cross bedding”
This is not a good example of “cross bedding”
Breaking the principle
• Every once in a great while the principle of original
horizontality gets broken.
• Take a sand dune. With each blow of wind sand grains
are dislodged down the slope of the dune, forming
layers that are inclined. The same thing can happen
under water (sub-aqueous dunes). As the dune
continues to form and advance sand from the top is
moved by down the slope, eroding the old top of the
dune slope. So over time a sedimentary deposit is
created with inclined surfaces bounded on the top by
an erosion surface. We can recognize this in the rock
record as cross-bedding.
Looking at this massive cross bedding, what can you say about
the size of the desert environment that left these?
Using the trees as a reference point gives you an idea of how deep this
ancient dune field was.
Another example of cross bedding.
Cross Bedding on another planet
This picture was taken by one of the mars rovers. If you were a scientist
that was looking for life how important would this image be to you?
Aeolian and Fluvial processes
• It is hard for geologists to determine that the some
cross bedding were once sand dunes(Aeolian), because
cross bedding is created in many different ways.
• Geologists have used the study of sedimentary
environments to understand for example “the Navajo
Sandstone”. They have discovered that the cross
bedding in the Navajo almost exactly resemble the
cross bedding that form in sand dunes. Sand dunes are
mounds of sand moved by the wind (Aeolian). They
commonly form in dry Earth surface environments.
Aeolian (wind)
• From observing active sand dunes, we know how they
form cross bedding.
• Viewed from one end, sand dunes have a flat back side
and a steep front side. The wind blows up the back side
of the dune. If the wind moves fast enough, it will pick
up and transport sand grains up the back of the dune.
When the wind goes over the top of the dune, it slows
down. This causes it to drop the sand grains it was
carrying. The cross-bed gets flatter at the bottom
because some of the sand rolls down to the bottom of
the dune and piles up.
Modern dune cross bedding
Fluvial (water)
• Cross-beds of many different shapes and sizes are
also made in water environments (Fluvial), such
as beaches, rivers, and the deep-sea.
• One of these is trough cross bedding. Crossbedding in which the lower surfaces are curved
erosional contacts which result from scour and
subsequent deposition. Large scale bed forms are
periodic and occur in the channel (scaled to
depth). Their presence and morphologic
variability have been related to flow strength
An outstanding example of Trough
Cross Bedding
Cross bedding not associated with sand dunes
This kind of cross bedding occurs when a stream deposits sediment into a
slow moving body of water.
Trough Cross bedding
Trough Cross bedding
Saltation and Creep
• Sand undergoes two physical processes. Sand
grains if they are heavy or the wind is weak
may only roll along the surface (creep). Grains
that become airborne will rise up, and then
return to the surface either bouncing off a
larger stone or dislodging another grain that
will also get airborne (saltation). About 75
percent of sand is moved by saltation and 25
percent by surface creep.
Saltation versus creep
Remember that the size of the particle determines the process of
transportation.
How do sand dunes become rocks?
• Sand dunes become sedimentary rock through
cementation. This happens in several stages.
• First, after the sand dune forms, it is usually buried below
the Earth’s surface as newer sand dunes cover it.
• Second, during and after the dune is buried, water within
the ground begins to move through it. The water flows in
the spaces or pores between individual sand grains. This
water often contains dissolved silica (SiO2) or calcium (Ca)
and carbonate (CaCO3). If the chemical conditions are right,
the third stage, cementation, will occur. Quartz or calcite
crystals will precipitate in the spaces between the sand
grains. They will grow until the spaces are filled up,
effectively cementing the sand together.
Photo taken at the Great Sand Dunes National Park in
Colorado
Well known Aeolian sedimentary rocks
• Sand dunes are common in two modern sedimentary
environments: beaches and deserts. The size and shape of
the beds in the Navajo Sandstone, along with other
sedimentary features found in it, indicate that it was
formed in a desert environment.
• Land fossils in the Navajo, including petrified wood and the
footprints of lizards and dinosaurs, further suggest that the
unit formed during the early Jurassic Period of the
Mesozoic Era, about 150 million years ago.
• The Navajo Sandstone covers an area of 1,500,000 square
miles of the southwestern United States. This indicates that
during the Jurassic Period, this part of the United States
was like the modern Sahara Desert of Africa.
This is the “Wave” very unique
depositional/erosional environment
This is the Sahara Desert of present day
This is what paleontologists think that a large area of what is now
Colorado, Arizona, Utah and New Mexico looked like more then 150
million years ago
Another view of the Sahara Desert
Modern ripple marks
Compare these to the next slide, are they familiar, could they have been
made in the same environments?
These ripple marks are over 160 million years old
Do these look like the marks in the previous slide? What kind of
ecosystem could have existed there?
Cross Bedding on another planet
This picture was taken by one of the mars rovers. If you were a scientist
that was looking for life how important would this image be to you?