Inferring Depositional Environments

Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States. 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

Why don’t they evolve the same amount of gas?

YOUR FAVORITE

CARBONATED BEVERAGE

Right out of the cooler.

Right out of a car that’s been sitting in

the sun all day.

3) Agitation

YOUR FAVORITE

CARBONATED BEVERAGE

Why don’t they evolve the same amount of gas?

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.

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?

CO2 + H2O chlorophyll Sugar + O2

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

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.

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.

4)

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.)

Map from the National Geographic Society

Equator

*

* * * * * * * *

*

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.

*

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

Image from Google Maps Street View. Looking north from I-80 toward Bonneville Speedway.

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.

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.

1A) Mineralogy (cont.) Sandstone Mineralogy

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

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.

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 3) On a corner (at any angle) (at any angle)

Which one of these will happen most often? Which least often?

1. Flat on a face 2) Flat on an edge 3) On a corner (very rare) (fairly rare) (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.

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).

GREATER TRANSPORT DISTANCE

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.

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.

x y

x y

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?

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. Stoss Side Lee Side

(Slipface)

Because of erosional loss on the stoss side and deposition on the lee, the ripple migrates downstream over time

1

2

3

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.)

CURRENT

The internal cross-lamination in a ripple (not distinctly visible in this photograph)

forms in the fashion outlined below.

~3cm

Small scale ripple cross-lamination in a Triassic rock in Canyonlands National Park, Utah.

Current direction indicated

Large scale dune cross-beds in a Jurassic rock in Zion Canyon National Park, Utah.

Which way did the wind blow?

3-5m

2B) (cont.) Ripple Marks and Cross-Beds b) Symmetric Ripples

x x

~3mm

POINTED CRESTS

ROUNDED TROUGHS

Symmetric ripples in Chickamauga Limestone, Davis Crossroads, GA

Symmetric ripples in sandstone from GSW teaching collection. Locality and age uncertain.

The same rock in cross-section view.

Wave Movement

Two Lee Slopes – one when the wave comes in,

one when it goes out

~3mm

~3mm

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.

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.

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

Thou

sand

s of

met

ers

Mud of previous turbidite

Mud Silt

Fine Sand

?Medium Sand

Basal sand of next 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 back-arc 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.

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.

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. 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”.)

scallop

oyster

sea urchin

sand dollar

shark teeth

3) Fossils (cont.)

Ferns and other plants

A large lizard-like reptile

lycopsids

sphenopsid

Trunks of three trees in a “logjam”. (Lens cap ~5cm.)

Contrast those fossils with these from Pennsylvanian rocks of the western Appalachians from northern Alabama to Pennsylvania. (Plants from Lookout Mountain, GA/AL.)

Where did these things live?

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.

4) Geometry of the Deposit

Cross-Section at Thickest Point

4) Geometry (cont.)

Shoestring and wedge geometries both occur in sedimentary deposits of various environments. A couple of other geometries are also common. 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.

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.)

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.

Two small patch reefs in the Florida Keys.

Image from Google Maps

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Image from highlandsbassangler.com

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

Valley Wall

Floodplain

Floodplain

Channel with Natural Levees on Both Sides

Inside

Inside

Outside

Outside

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)

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.

Inside (B’)

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.

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.

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.

First let’s think about the kinds of sediment that accumulate in each part of the river system.

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 fining-upward sequence.

Sea Level

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.)

(RIVER)

Sea Level

PROGRADATION

Let’s consider what is being deposited in various parts of the delta.

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.

Silt and fine sand brought by distributaries to the front edge of the delta

slump down into deeper water.

DELTA PLAIN DELTA FRONT PRODELTA

Channel gravel and sand are found only on the landward part of the delta.

FINER OFFSHORE

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.

Sea Level

PROGRADATION

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.

Delta Plain (Coarsest) Delta Front )Intermediate) Prodelta (Finest)

C O A R S E R

A delta environment stacks sediments in a coarsening-upward sequence.

Delta Plain

Delta Front

Prodelta

C O A R S E R

A delta environment stacks sediments in a coarsening-upward sequence.

Delta Plain

Delta Front

Prodelta

F I N E R

A meandering stream environment stacks sediments in a fining-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.

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