Download Inferring Depositional Environments

Inferring Depositional Environments
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WtoW globe image by Tatiana Baeva, to whom I am grateful.
This is a long slide show. You should budget at least an hour and a
half for it. There are several sections and this should allow you to
cover it in small breaks.
This notion of interpreting sedimentary environments from the facies
they deposit is the most important part of studying sedimentary rocks.
It is the Bowen’s Reaction Series of sediments, if you will.
There is a lot of detail here, but it is all here to help you see the basic
points – the basic ways we look at rocks to interpret their
environments.
Remember that there are 5 steps in the creation of sedimentary rocks:
1) Weathering of source rocks
2) Erosion of regolith
3) Transport of sediment
4) Deposition of that sediment in its final position
5) Lithification of the sediment to make a rock.
In this presentation we are interested in understanding the environment – the
sum of physical, chemical, and biological conditions in the environment where
the sediment was deposited -- # 4 above.
This environment is called, variously, the “environment of deposition”, the
“depositional environment” or the “sedimentary environment”.
There are five characteristics of sedimentary rocks that help us with this:
1) Rock Type (with several subordinate ideas)
2) Sedimentary Structures
3) Fossils
4) Geometry of the Deposit
5) Sequence of Rock Types
The first three will be our main focus because we can see them in a single
rock sample. The other two we will mention briefly because they require that
we examine the 3-dimensional shape of all the rocks deposited by the
environment (#4) or the different rocks encountered from bottom to top
throughout the stack of rocks created by the environment in at least one place
(#5). That is, we must see and compare many rocks for these to work.
1) Rock Type
Several separate ideas are included in the notion of “rock type”:
A) What is the general mineralogy (mineral content) of the rock?
B) What are the grain size characteristics of a detrital rock?
a) Average and maximum grain size
b) Range of grain sizes (sorting)
C) What are the shapes of clasts/grains – angular or rounded?
(Plus others that are beyond the scope of this class.)
1A) Mineralogy
Mineralogy of chemical sedimentary rocks is important because
precipitation of those minerals requires certain environmental conditions.
We will see some examples in the following slides.
Mineralogy of detrital sediments is important because it tells us something
about conditions in the source environment. Remember that intense
chemical weathering of source rocks should only yield quartz, clay, and
hematite for transport away from the source.
This means that if we find feldspars, femag minerals, and pieces of
aphanitic rocks (or similarly non-resistant metamorphic minerals and rocks)
in a detrital sediment that the source area was NOT intensely weathered.
Either it was very dry, very cold, very near, had very steep, mountainous
terrain, or some combination of those things.
1A) Mineralogy (cont.)
We recognize the mineral content of sandstone in the names of the major
sub-types of that rock.
1) A quartz sandstone has only quartz grains,
2) An arkose has quartz and feldspar, and
3) A lithic sandstone has those things plus femags, other non-resistant
minerals, and rock fragments (aphanitic rocks, for example).
Conglomerate or breccia can be similarly subdivided, but we will not go into
that in this course.
(Note that you do not need to subdivide sandstones on the specimen
identification part of the test, but I might ask you to distinguish them from
descriptions.)
1A) Mineralogy (cont.)
Limestone
Among chemical sedimentary rocks we’ll think about
four major rock types and the conditions necessary
for the creation of their minerals. We’ll begin with
limestone.
Limestone is made of calcite (or the chemically
identical aragonite) and requires Ca+2 ions
(introduced from a source rock by water) and CO3=
ions (from atmospheric CO2 reacting with water.) the
calcium is generally the limiting factor because the
carbonate is so easily made from scratch.
In fact, it is a little too easily made. Remember from
the reactions we saw in class that a by-product of its
production is H+, which makes the water acidic. Too
much of this and calcite will dissolve, not precipitate.
Surface water is, in general, a little too rich in H ions
for calcite precipitation. That reaction requires that
we remove a little CO2 from the water, driving the
reaction away from the production of H+.
How do you get CO2 out of solution in water?
We will do some thought experiments to understand the important ways.
We begin with a single bottle of carbonated beverage.
1) Pressure
YOUR
FAVORITE
CARBONATED
BEVERAGE
YOUR
FAVORITE
CARBONATED
BEVERAGE
Where do the bubbles/foam come from and why didn’t you see them in the capped bottle?
2) Temperature
YOUR
FAVORITE
CARBONATED
BEVERAGE
YOUR
FAVORITE
CARBONATED
BEVERAGE
Right out of
the cooler.
Right out of a car
that’s been sitting in
the sun all day.
Why don’t they evolve the same amount of gas?
3) Agitation
YOUR
FAVORITE
CARBONATED
BEVERAGE
YOUR
FAVORITE
CARBONATED
BEVERAGE
Right out of
the cooler.
Right out of the cooler, but
shaken by some fool with a
sick sense of humor.
Why don’t they evolve the same amount of gas?
There are three relevant physical factors that control the escape of CO2 gas
from water in the natural world:
1) Pressure. Hydrostatic pressure is higher at greater depths in a body of water,
allowing more gas to remain in solution. This makes the water more acidic and
calcite precipitation less likely. (Calcite dissolution is more likely, and we will return
to this point.)
2) Temperature. Cooler water retains more gas, making it more acidic. Water
temperature depends upon both latitude (warmest at the Equator) and depth (only
warmed by sunlight in the upper 200m or less). Below a few tens of meters or so
the water is noticeably cooler and therefore more acidic.
3) Agitation. Quieter water retains more gas, making it more acidic. Waves and tide
currents only affect the upper few meters to tens of meters of water, so deeper
water is quieter and more acidic.
There is one more factor that removes CO2 from natural waters but we cannot consider
this from the carbonated beverage perspective.
This is an aerial photograph taken across the continental shelf of southern Florida toward Cuba. The
waters off the Florida Keys are a site of active lime sediment production. The dark water along the horizon
is the deep water over the Florida Straits. The light colored patches are bare sand bottom, no more than
7-10m (20-30’) deep. The extensive darker areas around and between them are bottom covered with the
seagrass Thalassia. How do grasses and other plants and algae make their food?
So we can add a fourth, biological factor to our list of controls of CO2 gas in water
in the natural world:
1) Photosynthesis. Plants and algae remove the gas from the waters around them to
4)
manufacture sugar. Most photosynthesis takes place in about 10m of water or less
because the red light that is most efficient for the process is all absorbed by the water by
the time it reaches that depth. Some algae can function at depths as great as about
200m, but they do so very slowly and very inefficiently.
CO2 + H2O
chlorophyll
Sugar + O2
As with the other factors, it is shallow water where this process favors CO2 removal and
decrease in acidity. Deeper water, where photosynthesis cannot occur, does not experience
this mechanism of gas removal.
Indeed, respiration – the opposite of photosynthesis – actually produces excess CO2 in
deeper water, so the CO2 concentration doesn’t simply remain at some “normal” level, it
actually builds up. Cooler temperatures, higher pressure, and less agitation allow it to
become distinctly higher than it ever is in shallower water.
10m
Pervasive warmth, wave and other agitation, low pressure
and photosynthesis remove CO2 from the water and drop the
acidity, allowing calcite or aragonite to precipitate. Usually
the warmth is adequate only in tropical waters (or currents
that move those waters poleward, like the Gulf Stream)
200m
Less light penetration, increasing pressure, less agitation and
less photosynthesis down to about 200m removes less CO2
from the water and drop the acidity, making calcite or
aragonite precipitation harder, even in the tropics.
Below ~200m CO2 removal is essentially impossible and its production by
respiration ensures an increasing acidity all the way to bottom. Carbonate
production is impossible in this setting and carbonate dissolution is very
likely – becoming more likely with increasing depth.
Thus we expect limestone deposition in shallow, tropical water!
There are two other things to note about this picture. First, this is sea water. Though some fresh waters can
contain enough calcium to allow carbonate deposition, only seawater provides it in large enough and reliable
enough quantities to ensure abundant CaCO3 precipitation.
Thus limestone is almost always a marine deposit!
Second, you can see all the way to the bottom of this water – it is clear enough for the light to go to the
bottom, reflect off the white carbonate sand, come back through the water, and reach your eyes. Suspended
sediment in the water would block the sunlight to some degree, not allowing the light to warm the water and
not allowing photosynthesis. This negates two of the necessary conditions for limestone deposition.
Thus limestone is almost always deposited in clear, shallow, tropical, marine environments!
Let’s Recap:
Extensive limestone
deposits (like we see in
southwestern Georgia) are
deposits of clear, shallow,
tropical marine
environments.
Americus and Albany and
Newton are not on a shallow
tropical continental shelf
now, but they used to be.
Upper Eocene Ocala Limestone on the Flint River near Newton, GA
You are here (or would have
been ~35,000,000 years ago.)
Principle sites of limestone formation on Earth. Most carbonates are found between the Tropics of
Cancer and Capricorn (dashed lines) and in the western parts of ocean basins, where warm currents
turn and flow poleward. Waters on eastern sides of oceans (west coasts of continents) is generally
cooled by cold currents flowing from the poles.
Asterisks in the central Pacific and Indian Oceans reflect many, many reefs at small islands.
* *
*
Map from the National Geographic Society
*
Equator
*
*
**
*
* *
1A) Mineralogy (cont.)
Biogenic Chert
Most chert forms as a replacement of a pre-existing rock
(usually limestone) by SiO4 rich water. Some chert,
however, is biogenic or biochemical – it is the remains of
plankton that made their shells of silica.
Plankton all over the ocean surface are a mix of species
with calcite and silica skeletons. Though the ratios vary
from region to region, the calcite plankton is always
dominant.
On the abyssal plains, far from the reach of even the
finest detrital clays, biogenic oozes accumulate as the
shells of these plankton sink to the bottom. In most
places these oozes are calcareous ooze, but in the
deepest parts of the abyssal plains they are siliceous
ooze. The former can be lithified to limestone (chalk)
and the latter to chert.
The problem to solve is how to accumulate siliceous
shells on a seafloor when the plankton above is mostly
calcareous?
Any ideas?
Chert (replacing limestone) from
southwestern Georgia
10m
The plankton live and grow their shells, calcareous and silica
alike, in the upper level of the ocean, where CaCO3
precipitation is easy.
200m
However, as the shells sink after death they enter water that is colder,
darker, under higher pressure, and less agitated than the surface water – all
the wrong things for calcareous minerals because they lead to increased
acidity.
If the water is deep enough, by the time the shells have reached bottom all
the calcite will have dissolved. This leaves only silica shells to
accumulate on the bottom as a siliceous ooze – the unlithified
forerunner of biogenic chert.
Thus biogenic chert indicates deposition in extremely deep seawater!
1A) Mineralogy (cont.)
Evaporites
The third rock we’ll consider is rock salt, the
mineral in which is halite. Gypsum forms in the
same way.
Both seawater and desert lakes have sodium and
chloride ions in them from chemical weathering of
older rocks (and calcium and sulfate for gypsum).
In typical seawater there are not enough of these
ions to force (or even simply allow) precipitation of
halite. In desert lakes the same is true when the
runoff first enters the lakes in a rainy season. To
get the mineral to form, the concentration of the
ions must be increased. It does no good to bring in
more dissolved ions because that also necessarily
brings in more water too!
The concentration of ions cannot be increased
adequately by adding more ions – the trick is to get
rid of water. This happens by evaporation. As the
water volume goes down the ratio of ions to water
goes up, until the concentration is high enough to
drive precipitation.
Rock Salt, rock gypsum, and related rocks are
called evaporites.
The Great Salt Lake region is one of the most
conspicuous places on Earth viewed from space
because of the vast expanse (~3500 mi2) of salt flats
in the Great Salt Lake Desert west of it – a huge
white spot on an otherwise colorful planet.
The Bonneville Salt Flats and the raceway there is at
the western edge of the desert. The halite and
gypsum here were formed in a much larger lake
(Lake Bonneville) of which the Great Salt Lake is only
a small remnant.
Image from Google Maps Street View.
Looking north from I-80 toward
Bonneville Speedway.
Image from Google Maps
The Inland Sea is a shallow bay off the Persian Gulf in Qatar. At spring high tides (twice a month)
seawater reaches ponds along its shore and are trapped until the next spring tide, two weeks later.
The water evaporates and halite and gypsum precipitate from it.
1A) Mineralogy (cont.)
Sandstone Mineralogy
Remember the discussion of how sediment gets
from Stone Mountain to the coast of Georgia down
the Altamaha River. This entire system, as well as
the other main tributary to the Altamaha – the
Oconee River (dashed line), begins in the
temperate lowlands of the Piedmont. The
elevation and steepness of Stone Mountain are
unusual among the source rocks that supply
sediment to the rivers. Most are gently rolling hills.
Consequently, by the time sediment is washed into
the system most of it has already been deeply
weathered by chemical processes. Except at a
few places like Stone Mountain almost nothing but
clay, hematite, and dissolved ions are supplied to
the rivers. The only sand is quartz sand.
Even after the sand is in a river the trip to the sea
is not fast. The rivers wind between sandy banks
and any given grain can take centuries, even
millennia, to get to the coast. Even the feldspars
and other weatherable minerals that come from
Stone Mountain will be weathered in the course of
such a trip.
Consequently the sand arriving at the mouth of
the Altamaha and being moved along the coast
on barrier islands is almost pure quartz sand.
Even a complex mineral suite in the source
area cannot supply anything else.
Contrast that with the drainage in California, almost al of which reaches the sea at San Francisco. The
source area is the granite mountains of the Sierra Nevada. These are much higher, steeper, and more
rugged mountains than the Appalachians and they supply sand that is primarily feldspar to the mountain
rivers – there is little chance for chemical weathering in the source area before entering a stream.
Even though the rivers cross the
Great Valley of central California
before passing through the
Coast Ranges (which also
supply sediment of complex
mineralogy) and reaching the
coast, the streams have steep
gradients and the residence
time of a sand grain in them is
substantially less than in a
stream of the Georgia
Piedmont.
Consequently the sand
delivered to the coast is a mix
of minerals – mainly feldspar
and quartz from the Sierra –
an arkose, in other words.
N
~50 miles
In some ways the sediment of the river complex that drains through Bangladesh is similar to that of the
California rivers, but in many ways it is a more extreme case. The rivers do not even have to be
highlighted on this image – their widths and their extreme sediment loads are in stark contrast to the
forested and agricultural lands around them. That huge sediment load results from weathering and erosion
of rocks in the Himalaya to the north. The great heights and extreme slopes in those mountains mean that
chemical weathering of the source is minimal. Unlike the Sierra Nevada there is a complex array of source
rocks, not just granite.
Though the rivers flow a great
distance to the Indian Ocean their
gradients are so high that sediment
moves through them surprisingly
quickly. Even though they are
almost clogged with sand bars those
bars wash out and move
downstream every spring when the
snowmelt floodwaters arrive.
Residence time of any given sand
grain is probably decades here –
much shorter than in California and
Georgia.
Consequently the sand delivered
to the coast is a mix of many
minerals and rock fragments – a
lithic sand, in other words.
N
~50 miles
1B) Grain Size – Maximum and Average Grain Size
For detrital rocks the idea is very simple. It takes either a very steep slope or a very dense medium (glacial Ice) or very
fast water to transport gravel. A less energetic system can transport sand. Finally, it takes nearly stagnant water for
clay particles to settle. In fact, they probably have to clump together into larger masses before they will settle at all,
even in perfectly still water.
When we see a conglomerate we can rule out any transport mechanism – and therefore any sedimentary environment –
that doesn’t have the energy required to bring such large particles. This conglomerate was not deposited in the middle
of a lake because there is no way to get grains that size into that position. Similarly, the shale was not deposited in a
stream channel because it would have never settled in moving water.
1B) (cont.) Grain Size – Range of Grain Sizes (Sorting)
Both of these rocks are sandstone. The one on the right is arkose. It’s pink color is partly a result of the abundant Kspar in
it. The one on the left is quartz sandstone. Its color is strictly a result of hematite staining. There is no feldspar.
The arkose has grains a little coarser than the
quartz sandstone, but there is another noticeable
difference as well.
Some of the grains in the arkose are actually fine
gravel, not sand. This is still sandstone because
the average grain size is sand and the most
common grain size is sand. In fact, there are
grains in the arkose that are even finer than the
grains in the quartz sandstone.
That is, the range of sizes is different in the two.
The range of grain sizes in a rock is called its sorting. The quartz sandstone is well sorted and the arkose is poorly sorted.
Sorting tells us about the consistency of energy in the depositional environment. The quartz sandstone is much better sorted
that is typical of a stream sediment. Streams sometimes experience low flow levels (and energy) and sometimes they flood
(and the energy increases tremendously). Furthermore, as we’ll see when we talk about streams, different parts of the
channel have different energies at the same time. The arkose is almost certainly a stream deposit, judging from its poor
sorting. The quartz sandstone almost certainly is not, based on the same criterion.
Sorting like what you see in the quartz sandstone is typical of a beach (on a coast like the Atlantic coast of Georgia or Florida)
or a sand dune on such a beach or in a desert. The wave energy on beaches and in wind are much more consistent at a
given location than that of rivers. Consequently their sand deposits are much better sorted.
C) Grain Shape – Angularity and Roundness
Imagine a Cube. Imagine dropping it from some height and recording how it lands, recognizing three possibilities:
1.
Flat on a face
2) Flat on an edge
(at any angle)
3) On a corner
(at any angle)
Which one of these will happen most often? Which least often?
1.
Flat on a face
(very rare)
2) Flat on an edge
(fairly rare)
3) On a corner
(very common)
This matters because as time goes by and the particles are transported farther and farther the
impacts they experience are much more likely to occur on their corners, less commonly on their
edges, and hardly ever on their faces. Any mechanical weathering because of these impacts will
therefore tend to break off the angles, and particularly the corners, rather than affect the faces.
(Also, an impact on a face spreads the force over a much larger area, reducing the likelihood of
breakage anyway.)
Knocking the corners and edges off of something, of course, makes it less angular and more
rounded.
Obviously, different minerals will respond differently, but clasts of any one mineral should get
rounder the farther they are transported.
GREATER
TRANSPORT
DISTANCE
Both these rocks have quartz gravel in them. The one on the left has very sharp angular corners
(see arrows for example). The one on the right has clasts that are very well rounded (upper
right). Even the ones that are not almost spherical do not have sharp corners (bottom center).
2) Sedimentary Structures
Sedimentary structures are arrangements
of grains in a bed that are created during
or soon after deposition of the bed.
There are hundreds of different types that
have been described and most have
interpreted by comparison with Recent
counterparts or understanding of the
mechanics of their formation. The origin
of a few are still not well understood.
We will look at three types, with two
variants of one of them.
2A) Mudcracks
2B) Ripple Marks
a) asymmetric
b) symmetric
2C)
2A) (cont.) Mudcracks
We have examined these things in an
earlier slide show. In order to form
muddy sediments have to get wet and
then dry out. Contraction during drying
breaks the mud layer into polygonal
blocks and differential drying on the
top causes each resulting “mudchip” to
curl upward to a greater or lesser
degree.
A couple of ancient examples are shown. The red rock (on
Taylor Ridge in northwestern Georgia) has Ordovician
mudcracks in it. The tan rocks on the vertical wall behind Mike
Beckwith is covered with Silurian mudcracks. In both cases you
are looking at the bottoms of the beds.
Both deposits are geographically widespread. The Silurian
mudcracks are stacked for 10’s of meters. The isolated mudchip
is from that same rock at a different locality.
There are three natural environments in which mud can be alternately wet and
dried, and only one sees those conditions reliably and repeatedly enough to
stack up meters of mudcracks in its deposit.
The ones shown here were on the floodplain of the
Suwannee River near White Springs soon after high
water had introduced wet mud into the floodplain.
Between that flood and the next one these mudcracks
sat exposed. They were walked on by animals,
overgrown by plants, abraded by wind, and so on, and
may not have survived.
Similar mudcracks occur in lakes that frequently dry out,
but they are also subject to rapid destruction.
So even though mudcracks can form in river floodplains and ephemeral lakes they do not form reliably or frequently,
and are likely to be destroyed rather than preserved.
When we see mudcracks in an ancient sediment they are almost always from the upper part of a tidal flat, where
spring tide brings seawater only twice a week, and where drying occurs the rest of the time. This happens both
frequently and reliably, and the harsh environment keeps most animals from treading on the mudcracks and
destroying them and plants (which need fresh water) from overgrowing them.
You might recall that this same environment is a likely site of evaporite deposition. If so it will not surprise you to learn
that there are molds of both gypsum and halite crystals in the Silurian rocks with the mudcracks. Sedimentologists
are always looking for collaborative evidence like this to test their hypotheses about depositional environments.
2B) Ripple Marks and Cross-Beds
The friction of water passing over sand (and gravel) grains on the bottom of a stream or the sea, or
wind blowing across sand on a beach or in a desert causes the grains to move, but also causes
them to move in a certain way. As the grains move they form more or less linear ridges of sand at
right angles to the current direction. Both the size and the linearity of the ridges are controlled by
grain size and current speed. In higher speed/finer grain conditions the ridges get larger and more
sinuous. Lower speed/coarser grains create smaller, straighter ridges.
Small versions of these ridges are called ripple marks and larger ones are megaripples or dunes,
depending on size. (Dunes can form under water too – sand can pile high enough for this if the
water is deep enough and the current fast enough.)
The origin, movement, and evolution of dunes and ripples means that there is an interesting internal
arrangement of grains in them called cross-bedding or cross-lamination depending on scale.
This is a relatively thin layering of sand internally that is parallel to one or both of the slopes on the
ripple (or dune) face, depending on the type of ripple.
There are two major types of ripples. One forms in streams and subaerial dunes, where current
typically flows in one direction. The other forms in nearshore marine sediments where wave and
tide movements are in two opposite directions.
2B) (cont.) Ripple Marks and Cross-Beds
a) Asymmetric Ripples and Dunes
These ripples, on the bed of the Suwannee
River near Fargo, are asymmetric. The
steeper side is to the right in every one of
them. Dark organic matter accumulates
between them making it easy to see where
one ripple ends and the one bedside it
begins. The highest point on each ripple
looks lighter in color because the tannin-rich
water is not as deep there and so more light
reflects back to your eyes. Notice that the
dark stuff is just to the right of the crests of
the ripples, showing the asymmetry nicely.
x
y
O
O
O
The lower schematic shows the asymmetry of the ripples and why the organics (“O”) are closer to the crest on the
right side than on the left side. In other words the distance from a trough to a crest (red double arrows) is shorter on
the right than on the left. This asymmetry tells us the flow direction large red arrow on photo). The steeper faces
always form downcurrent (or downwind for a dune).
The height of a ripple (trough to crest difference) in comparison to its length (crest to crest or trough to trough)
distance is related to the speed of the water for sand of a given average grain size. The ratio of x:y tells us the same
thing – faster water makes higher, steeper, more nearly symmetric ripples.
The internal cross-lamination in a ripple
(not distinctly visible in this photograph)
forms in the fashion outlined below.
CURRENT
x
1
Stoss Side
y
2
Here is a “fossil” version of asymmetric ripples,
probably from Pennsylvanian rocks in the
Appalachians somewhere. (Probably Lookout or
Sand Mountain.)
Which way did the current flow?
3
Lee Side
(Slipface)
As water flows over the ripple
sand is eroded from the
upstream (stoss) side. It rolls,
of slides, or bounces, or floats to
the crest where it either
continues downstream, slips
down the steep slipface, or is
caught in an eddy and returned
to the slipface.
Because of erosional loss on the stoss side and deposition on
the lee, the ripple migrates downstream over time
The cross-lamination
forms in this way – each
thin layer marks a former
position of the slipface (as
indicated by the red line).
The upper end of all the
old slipface beds are
eroded away except for
the currently active one.
(Pun absolutely intended.)
~3cm
Large scale dune cross-beds in a Jurassic rock
in Zion Canyon National Park, Utah.
Which way did the wind blow?
Small scale ripple cross-lamination in a Triassic
rock in Canyonlands National Park, Utah.
Current direction indicated
3-5m
2B) (cont.) Ripple Marks and Cross-Beds
b) Symmetric Ripples
Wave Movement
x
x
POINTED CRESTS
ROUNDED TROUGHS
Two Lee Slopes –
one when the wave comes in,
one when it goes out
~3mm
~3mm
Symmetric ripples in sandstone from GSW teaching collection.
Locality and age uncertain.
Symmetric ripples in Chickamauga Limestone,
Davis Crossroads, GA
~3mm
The same rock in cross-section view.
This diagram gives you an idea of the complex form of
cross-lamination in ripples created by wave action in a
nearshore marine environment.
Note that you can identify dips in both directions, though
only the most recent set (left side) is continuous.
This is an Ordovician
sandstone from Horseleg
Mountain at Rome, GA. It
shows a larger-scale
version of the bi-directional
crossbeds common in
nearshore environments.
The cross-bedding is not
easy to see so I have
traced a couple of example
beds in three cycles for
you. Note that alternate
ones have dips in opposite
directions.
These were probably
formed because of wave
activity on an offshore bar,
but similar herringbone
crossbeds occur in the
deposits of tidal channels.
2C) Graded Beds
Sediment often washes or slumps from
near the top of a steep subaqueous
slope like the continental slope.
The result is a density current or turbidity current – the mixture of sediment and water moves in a
layer beneath the overlying water because together they make a denser fluid than water alone.
As the flow continues several things happen. It builds momentum (and speed), the internal flow
becomes more turbulent, and the coarser and finer particles become segregated to the bottom and
top of the fluid respectively.
The momentum can carry the flow far out beyond the bottom of
the slope, onto the abyssal plain, for example. As it moves here
the coarser sediment is progressively left behind and only clay
makes it to the greatest distance. Eventually the flow loses
energy and everything settles
Resulting Layer of Sediment
Because the coarser sediment settles from the turbidity
current first and the mud finally settles last, the bed is graded
from coarser to finer sediments upward. Notice that farther
out in the abyssal plain the beds bed thinner, losing the
progressively losing the relatively coarser basal sediments
with distance from the source – a shelf edge or volcanic arc.
Mud
Silt
Fine Sand
?Medium Sand
Mud of previous turbidite
The deposit of a single turbidity current is called a turbidite.
It may range from a few millimeters to a few centimeters in
thickness. On the continental rise and in trenches and backarc basins they can stack up for thousands of meters as slump
after slump arrives from the shelf or volcanic arc above. They
are also found in the water in front of deltas.
Thousands of meters
Basal sand of next turbidite
3) Fossils
This is sensible and straightforward.
Each living species requires certain
environmental conditions to live.
There are aquatic and terrestrial
organisms. Among aquatic ones
some require salt water (marine)
and some fresh water. Among
marine organisms some like cold
and some warm water, some like
deep and some shallow water, and
so on.
sea
urchin
scallop
We can generally work out the
environmental preferences of fossil
species, even extinct ones, in
several ways. You will examine
these in the second geology course.
For now, suppose that it is possible.
Once we have done that then the
fossil species become indicators of
the environment they require.
These fossils were all found in
Eocene (middle Paleogene) rocks
within 50 miles or so of Americus.
What was the environment like
when they lived here? (Coin is 1”.)
shark
teeth
oyster
sand
dollar
3) Fossils (cont.)
Contrast those fossils with these from Pennsylvanian rocks of the western Appalachians from northern
Alabama to Pennsylvania. (Plants from Lookout Mountain, GA/AL.)
Ferns and other plants
A large lizard-like reptile
sphenopsid
Trunks of three
trees in a “logjam”.
(Lens cap ~5cm.)
lycopsids
Where did these things live?
4) Geometry of the Deposit
Consider just the Altamaha River and its
delta, ignoring the two main tributaries. If the
deposits of this system are preserved in the
rock record, what will their shapes be like?
The river is long, but it is narrow. Its deposits
are not particularly thick – perhaps about as
thick as the valley is wide. Thus the deposits
of the entire valley – floodplain and channel –
will be long and narrow (map view) and thin
(cross section). The result is a shoestring
deposit characteristic of streams.
In contrast, the delta is not constrained at all
in width because at the mouth the river’s
banks no longer exist. It is also not so
constrained in its thickness, with the sloping
continental shelf on which to build. The result
is a broad wedge of sediment characteristic
of deltaic sediments.
Cross-Section
at Thickest Point
4) Geometry (cont.)
Image from Google Maps
Shoestring and wedge geometries both occur in
sedimentary deposits of various environments. A
couple of other geometries are also common.
The great Ergs (sandsheets) of North Africa and Arabia are,
at most, tens of meters thick, but cover thousands to tens of
thousands of square kilometers. (The Grand Erg Oriental is
120,000 km2.)
Image from Google Maps
Lake Okeechobee, FL is about 16’ deep near the center
and shoals toward its banks. When it ultimately fills with
sediment the shape of that sediment will be a lens.
Sheet geometry refers to a deposit that is both wide
and elongate – in both map directions in other
words – but thin in the vertical dimension. They
cover a lot of territory, but not very thickly. Desert
dune fields (which are also called “sand sheets”),
continental shelf, and abyssal plain deposits are all
generally sheets.
A lens deposit is thickest near the center and thins
in every direction from there. Lake deposits are
generally lenses that are flat on the top and small
patch reefs are often lenses that are flat on the
bottom.
Image from highlandsbassangler.com
Two small patch reefs in the Florida Keys.
5) Sequence
Let us consider again the deposits of the
Altamaha, a meandering stream and its
delta.
Notice that the rock type (or sediment
type, since it isn’t lithified) is the same
because the same sediment that comes
down the river builds the delta. The
sedimentary structures are also likely to
be the same because one-way flow of
water is the predominant transport
mechanism in both.
As we have seen, the geometry of the
deposits differ, but we have to visualize
the entire deposit to see that.
Another characteristic we can use, and
can see at a single place, is the sequence
of the deposit – how it changes from
bottom to top. This reflects the history of
deposition at a single locality.
5) Sequence (cont.)
Let’s begin with the river. We will study meandering streams in more detail later, so for now we’ll just look at a couple of
aspects of its behavior that influence its sequence.
We begin with a map view to learn the parts.
The single river channel occupies only a small part of the valley at any one time, though it moves around in the valley in
two different ways as time goes by. It is flanked on both sides by a ridge of sand called the natural levee unless the levee
has been lost on one side to erosion. It can erode into the valley walls, widening the valley, but otherwise never reaches
beyond them, even at maximum flood levels. At those times water can cover the floodplain between the valley walls.
The name of this type of river refers to its tendency to meander around on the floodplain. each loopy bend in the river is
called a meander or a bend (or meander bend). We can think of each meander as a portion of a circle with an inside and
outside of the bend. Which side is inside and which outside switches back and forth as the river bends in opposite
directions.
Valley Wall
Outside
Floodplain
Inside
Channel with
Natural Levees
on Both Sides
Inside
Outside
Valley Wall
Floodplain
5) Sequence (cont.)
Let’s begin with the river. We will study meandering streams in more detail later, so for now we’ll
just look at a couple of aspects of its behavior that influence its sequence.
Outside (B)
Inside (B’)
On the outside of every bend is a cutbank, so called because
the fast deep water at its foot undercuts it and causes it to fall
into the channel. (The natural levee might not occur here.)
The cutbank, in other words, is erosional.
On the inside is a depositional pointbar – an accumulation of
sand. Notice the resulting asymmetry of the channel.
First let’s think about the kinds of sediment that accumulate in each
part of the river system.
MUD (lines) is deposited on the floodplains when the water reaches them, spreads
across them, and slows. The natural levees are made of the coarser sediment
suspended in the floodwater. That settles immediately upon leaving the channel
because of the slower flow in the floodplain.
SAND (dots) occurs on the pointbar. The
grainsize gradually decreases up the pointbar to
the natural levee because the average water
speed decreases that way.
GRAVEL (big chunks or circles) is restricted to the deepest part of the channel, where the
fastest flow occurs. At a meander this is immediately beside the cutbank.
Second we’ll think about how the river changes over time.
Erosion operates to
widen the channel in
the direction of the
cutbank.
However, if the channel
were actually to get wider
the flow would slow and
sediment would have to be
deposited to partially fill it,
bringing it back to its
former size.
That deposition occurs
preferentially on the
pointbar. This keeps
the channel size and
shape consistent.
The result is that the channel simply migrates in the direction of the cutbank until something halts
the process. We’ll get back to that later in the term. The dotted line shows the form of the channel
at some time in the past.
Now lets put the two things together.
All the environments (and corresponding sedimentary facies) migrate with the channel. What is
the result over time?
You get some idea by looking right here.
As the channel and the facies move, pointbar sands
come to overlie gravel from the deepest channel.
You should be able to predict that as the process
continues finer sands of the upper pointbar will arrive
at the same place. Finally, when the channel has
migrated more than its width, the floodplain will arrive
here and deposit mud on top of pointbar sands!.
F
I
N
E
R
A meandering stream
environment stacks
sediments in a finingupward sequence.
5) Sequence (cont.)
Now let’s move on to the delta. When the river reaches the sea and is no longer constrained to its
banks it deposits all but its finest sediment immediately. The finer material can be carried in
suspension farther offshore to settle on the shelf.
As sediment is dropped and clogs the channel the flow divides around the clog. This is repeated
over and over as time goes by, The consequence is that the delta builds outward (“progrades) over
time and develops distributary channels that carry water across its surface.
As we have seen, this builds a wedge of sediment seaward from the shore. (The slopes are very
much exaggerated in the cross-section.)
Sea Level
(RIVER)
Let’s consider what is being deposited in various parts of the delta.
DELTA PLAIN
DELTA FRONT
Channel gravel and sand are found
only on the landward part of the delta.
PROGRADATION
PRODELTA
Silt and fine sand brought
by distributaries to the
front edge of the delta
slump down into deeper
water.
FINER
OFFSHORE
Sea Level
Clay and fine silt are carried
beyond the delta and settle on
the seafloor (or lake floor,
perhaps). A few sandy
turbidites might reach parts of
this environment.
The small stream you see here runs into the Suwannee River about 100m from here. The photograph was taken in the
floodplain of the river. (The mudcracks you saw earlier are very nearby).
When the river was high, a few weeks before the picture was made, this was underwater. The small stream built a
delta into that standing water and has cut down into its own delta deposits now that the river is back to a lower stage.
The delta plain and delta front deposits are obvious on the photo. The prodelta sediments are in or below the bed of
the stream.
Let’s consider what is being deposited in various parts of the delta and
how progradation distributes those facies within the deposit.
As the diagram suggests, most of the delta’s deposit will eventually be delta front facies – silt
and sand, mainly. This will prograde outward over the top of prodelta muds and will be capped
with a layer of delta plain sand and gravel.
PROGRADATION
Sea Level
Delta Plain (Coarsest)
Delta Front )Intermediate)
Prodelta (Finest)
Delta Plain
Delta Front
Prodelta
C
O
A
R
S
E
R
A delta environment
stacks sediments in a
coarsening-upward
sequence.
Now contrast the two sequences. Even though rivers and deltas traffic in
the very same sediment they sort it in exactly opposite ways. Stream
deposits fine upwards and deltas coarsen upwards.
Delta Plain
F
I
N
E
R
A meandering stream
environment stacks
sediments in a finingupward sequence.
Delta Front
Prodelta
C
O
A
R
S
E
R
A delta environment
stacks sediments in a
coarsening-upward
sequence.
We’ve already seen that the geometries are different in the two facies.
What other differences might you expect (of the five things we’ve seen can
distinguish environments)?
Recap
There are five characteristics of sedimentary rocks that help us interpret
sedimentary environments from the rocks deposited by those
environments:
1) Rock Type (mineralogy, grain size and shape)
2) Sedimentary Structures
3) Fossils
4) Geometry of the Deposit
5) Sequence of Rock Types