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Author Topic:   Introduction To Geology
Perdition
Member (Idle past 1471 days)
Posts: 1593
From: Wisconsin
Joined: 05-15-2003


Message 76 of 293 (645755)
12-29-2011 5:32 PM
Reply to: Message 75 by Dr Adequate
12-18-2011 8:59 AM


Re: Volcanic Ash
I just found this thread and I'm finding it very interesting, but it appears to have fizzled for some reason. Can I bump this as a means to see if Dr. Adequate is interested in rehefting this heavy load?

AbE: I realize this time of year can lead to pauses in discourse, but seeing as how it had dropped to the second page of the recent topics list, I thought a bump was in order.

Edited by Perdition, : No reason given.


This message is a reply to:
 Message 75 by Dr Adequate, posted 12-18-2011 8:59 AM Dr Adequate has responded

Replies to this message:
 Message 77 by Dr Adequate, posted 12-29-2011 9:10 PM Perdition has not yet responded

    
Dr Adequate
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Posts: 16097
Joined: 07-20-2006


Message 77 of 293 (645775)
12-29-2011 9:10 PM
Reply to: Message 76 by Perdition
12-29-2011 5:32 PM


Re: Volcanic Ash
Further installments will follow shortly. Watch this space.
This message is a reply to:
 Message 76 by Perdition, posted 12-29-2011 5:32 PM Perdition has not yet responded

  
Dr Adequate
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Posts: 16097
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Message 78 of 293 (646099)
01-03-2012 4:50 AM


Soils and Paleosols
Soils and Paleosols

Introduction

In this article we shall discuss soils, their formation, and their preservation as fossil soils (paleosols).

Soil

The reader will of course have seen plenty of soil. But what exactly is it?

Sediments lying on the surface of the land will undergo chemical weathering as the result of rain falling on them. As this sediment is on the surface it will not have been compacted and lithified, so it will be highly porous as compared to a sedimentary rock, and the rainwater will easily be able to seep through it. This will cause the sort of changes in the composition of the sediment that you would expect if you have read the article on chemical weathering: feldspar minerals, for example, will be converted into clays; soluble minerals will be dissolved, and, depending on the climate and drainage of the local environment, can be deposited lower down in the soil.

The other thing that will happen to such sediment is, of course, that plants will grow in it. This has various effects: first, it means that decaying organic material will be deposited in the upper layer of the sediment. Second, the actions of plant roots and of burrowing worms and so forth will keep the soil loose and porous. Third, the decaying plant material will release organic acids into the soil, which increases the rate of chemical weathering. In fact, a rock will actually undergo chemical weathering faster buried in soil than it will exposed to the open air.

These processes, known as soil formation or pedogenesis cause the sediment to become soil. The reader will notice from this discussion that a soil is defined rather differently from the other sediments that we've discussed in our articles on geology. Other sediments are defined chiefly by their origin and mode of deposition. Soil, on the other hand, is defined by what happens to it after deposition: a soil is a sediment that has undergone pedogenesis, and this can happen to all sorts of sediments, from volcanic ash to glacial outwash.

The picture above shows you the results of pedogenetic processes on one particular soil. You will observe a number of distinct layers ("horizons") in the soil. On top, just under the grass cover, is a black horizon which gets its color by being rich in organic material. The white-ish horizon below that has had minerals leached out of it by chemical weathering of the soil. And the pinkish horizon below that takes its color from the iron oxides that have been deposited in it by the rainwater seeping through the soil.

This one example is just an illustration of the sort of thing that can happen in soil formation: the horizons will differ from soil type to soil type, and, indeed, identifying the different horizons is the first step towards identifying a soil type. We shall not here discuss all the different types of soil, because this would be something of a digression from the main of thrust this article.

Many determining factors combine to influence how a soil develops and so what type of soil it becomes. These include:

* The original sedimentary material. Obviously, for example, something that starts off with no iron-bearing minerals in it is not going to end up with a horizon rich in iron oxides.

* The climate. Chemical weathering acts much more vigorously in warm climates with plenty of rainfall.

* Drainage. For example, in waterlogged soil the decay of organic material is retarded by the fact of being waterlogged, causing the accumulation of such material.

* The vegetation type. For example, pine forests produce particularly acidic leaf litter, accelerating chemical weathering.

* Time. Since pedogensis takes time, it is clearly going to be the case that sediments of recent origin will not be so well developed as soils.

* Human activities, such as the addition of manure to fields.

Paleosols

In geology, a paleosol is a fossilized soil. Note that this does not necessarily mean that it has been lithified, merely that it has been preserved by burial, perhaps by volcanic ash, or a lava flow, or windborne sediment, or by peat, or what-have-you.

We should note that in pedology (soil science) the word paleosol has a different meaning: it that context it means a soil which developed under a set of conditions that are no longer obtaining, for example a soil which develops under tropical conditions in a country that later acquires an arid climate.

In the remainder of this article, we shall be using the word "paleosol" exclusively in the first sense.

Paleosols: how do we know?

As usual in this series of articles, we ask: how can geologists recognize paleosols when they see them?

The identification of rocks as paleosols is not at all challenging. For one thing, paleosols, though necessarily buried, are not necessarily lithified. So some paleosols can be recognized as once having been soil because they still are soil. When they have been lithified, they often retain a superficially soily appearance. The photograph below shows a paleosol found near Mexican Hat, Utah: note its distinctly soil-ish appearance.

Whether or not a paleosol has been lithified, it will retain the mineralogical changes caused by pedogenesis, and this allows geologists not just to recognize the fact that the paleosol was once a soil, but also to identify the soil type, and so to come to conclusions about the climate at the time it formed.

Paleosols will also typically show distinctive signs of biological activity, such as animal burrows and casts, and roots or root casts (these are the white features in the photograph above): sometimes one can even find tree-stumps rooted in paleosols, leaving one in no doubt that they were once fertile soil. The exception to this is one that proves the rule: obviously we are not going to find such signs of biological activity in soil which was buried before life evolved to live in it: so when we find paleosols that are dated from the Cambrian (for example) we find no such signs of life, just as one would expect.

The stratigraphy of paleosols is also consistent with geologists' theories of their origin. It would be peculiar to the point of inexplicable to find that a paleosol had been buried by (for example) a distinctively marine sediment such as a turbidite, because what would soil, which forms on land, be doing getting itself buried under a marine sediment? What we expect to find, and do find, is that the sorts of rocks that geologists identify as paleosols are found buried under volcanic ash, or lava flows, or coal, or wind-borne sediment, or sediment deposited by rivers; or such sediments which have undergone pedogenesis themselves and become soil.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Posts: 16097
Joined: 07-20-2006


(2)
Message 79 of 293 (647508)
01-10-2012 4:22 AM


Rivers
Rivers

Introduction

In this article we shall discuss the action of rivers on the landscape, and show how the characteristic sediments deposited by them can allow us to identify ancient river courses in the geological record.

The reader should note that there is no qualitative difference between a stream and a river; a stream is simply a small river, or, to put it another way, a river is a big stream. Rather than write "rivers or streams" over and over again, we shall write about rivers, and the reader may assume that what we have to say applies on a smaller scale to streams.

The subject of river deltas will be dealt with in a subsequent article.

Braided and meandering rivers

Rivers, unless artificially banked, rarely flow in completely straight lines. At low gradients, two characteristic forms they can take are braided and meandering rivers.

Braided rivers, as the name suggests, consist of a number of channels which separate and rejoin around bars of sediment. They are formed when a river with a lot of sediment repeatedly deposits the sediment and erodes it; so the braids and bars (i.e. the ridges of sediment breaking the surface of the water) are not permanent features, but shift around over time.

The video below, produced by Dr. Paul Heller of the University of Wyoming, shows the process of formation of braided channels in miniature: it was produced in a flume 3 meters wide and 6 meters long fed with sediment-laden water. The video took 45 minutes to make, but has been sped up by a factor of 30:1.

Meandering rivers wind from side to side in large loops. As a consequence of the hydrodynamics of this situation, the current is faster and causes more erosion on the outside of the loop, while sediment will tend to be deposited on the inside of the loop, forming point bars. The result of this is that a meandering river will become more meandering over time.

If this tendency goes far enough, the meander approaches a loop doubling back on itself. Eventually the meander will touch itself and the river will suddenly find itself with a new, straighter, shorter path, leaving the meander isolated as an oxbow lake. The photograph below shows a river with meanders, oxbow lakes, and dried-up, sediment-filled oxbows.

The shifting of meandering and braided rivers across the landscape as a result of their own deposition of sediment produces a flattened, sediment-rich landscape known as a flood plain.

Sedimentary structures

The sedimentary structures formed by a river at a particular point will depend on its velocity, its depth, and the sediment type. Geologists can discover the relationship between these factors both by observing actual rivers, and by laboratory experiments using flumes.

For example, consider the effect that a river's velocity has on a bed consisting of average-sized sand grains. At low velocities, the creep of sand along the bed will, if anything, tend to smooth out the bed.

At higher velocities, sand ripples begin to form: small ridges of sand with the ridge at right-angles to the current. These ripples have a characteristic profile with a shallow slope on the upstream side and a steeper slope on the downstream side. Saltation bounces particles of sand up the shallow upstream side and over the peak of the ripple, eroding the upstream side and depositing sand on the downstream side. This has the effect that the ripples march downstream; it also produces cross-bedding.

At a higher velocity still, dunes (essentially, big ripples) will form; as with ripples, they have a shallow slope on the upstream side and a steeper slope on the downstream side. Dunes formed in this way have ripples on their shallow upstream side; these are known, logically enough, as rippled dunes. As with ripples, transport by saltation moves the dunes and ripples downstream (with the ripples moving rather faster than the dunes) and produces cross-bedding.

At greater velocities still, especially when the sand is fine, the increased current will flatten out the ripples, resulting in a flat, stratified surface known as an upper plane bed.

At still higher velocities, antidunes form. These have a rounded undulating cross-section. While in dunes sand is eroded from the upstream face of the dune and deposited on its downstream face, in the case of antidunes, sand is eroded from the downstream side of the antidune and deposited on the upstream side of the next antidune downstream. This has the effect that although the sand is moving downstream, the antidunes, being eroded on their downstream sides and built up on their upstream sides, move upstream: this is why they are called antidunes. These may show some slight cross-bedding, which, if it occurs, will slope up in the downstream direction; again, the opposite direction to that seen in ripples and dunes.

At greater velocities still, the current is strong enough to carry the sand in suspension, moving it downstream, leaving only gravel, cobbles, or just plain bedrock, depending on what other sediments, if any, are present on the river bed.

As we have indicated, the type of sediments involved affect these processes: in fine sediments, which "flow" more easily, dunes will not be formed; in coarser sediments, especially in shallower water, the formation of an upper plane bed is less likely, and the sequence as velocity increases will skip straight from dunes to antidunes.

Some photographs of these structures in modern sediments and in ancient sedimentary rocks can be found here.

Vanished rivers: how do we know?

Geologists can reconstruct the courses of long-vanished rivers. The method by which they identify them should be obvious and familiar to anyone who has read this far in the textbook. If we take away a river, we are left with its sediments, which will eventually lithify. This will leave us with a set of rocks which look just like the lithified sediments of a river. As usual, we apply the rule that "if it looks like a duck and it quacks like a duck, it's a duck". In the case of rivers there are some very clear indications in the remaining sediment that allow us to identify what it once was.

In the first place, the sediments will be arranged in the long thin form of a river (what is sometimes called a shoestring topography). Note that since rivers shift, the "shoestring" will not be as narrow as the river was; but it will still be a shoestring. Depending on the depth of river and the rate of flow, the river bed will remain as a shoestring of gravel or cobbles or of duned or rippled sand. The ripples, of course, will cut perpendicular to the direction of the shoestring, making them distinct from beach ripples, which would be parallel to it. As the ripples in rivers are not upstream-downstream symmetric, it also is possible to use them to determine the direction of flow.

Looking horizontally at the different sedimentary types in a line cutting across the direction of flow, we will see coarse sand or gravel at the middle, then finer sand representing point bars, and then the mud of the flood plain. Because rivers gradually shift their course and their banks, we will also be able to see exactly the same sequence going vertically as over time river bed is replaced with point bar is replaced with flood plain. This is known as a fining-up sequence.

The types of sediment will be consistent with the hypothesis of a river. For example, we will not find gypsum or halite, because these would require totally different depositional environments.

Such fossils as we find will be of freshwater plants and animals, or of land plants and animals, but not marine forms. Similarly on the banks of the river we expect such fossils as are present to be of land animals or their footprints, and of land plants.

In short, we see exactly what we should expect to see as the remains of a former river. As usual in such cases, we are left either with the reasonable deduction that these features are, in fact, the results of the action of a river now vanished; or with bizarre hypotheses such as that a malevolent god is playing pranks on geologists.

Note on superposed and antecedent rivers

In many places in the world, we can find rivers which have cut channels through hills or mountain ranges. This may seem odd at first, since naively one might think that since rivers can't flow uphill, they could never have cut the gorges through which they flow in the first place. However, the thing is quite practicable so long as the rivers were there before the mountains.

In the case of antecedent rivers, tectonic uplift slowly raises hills or mountains across the path of the river. So long as the river can erode away the uplifted rock and soil as fast as the rate of uplift, it will maintain its course. As rates of uplift are small compared to the erosional powers of rivers, this should present no problem to a river of reasonable velocity.

In the case of superposed rivers (also known as superimposed rivers) a river flows over a plain, subject to weathering and erosion, beneath which are humps (antisynclines) of rock more resistant to erosion than the overlying rock. As erosional processes reveal the resistant antisynclines, the river cuts its way through them, resulting in a river that cuts through hills consisting of the exposed resistant antisynclines. An aerial view of the Susquehanna River, Pennsylvania, can be seen here; the exposed ridges of resistant rock are clearly visible.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Joined: 07-20-2006


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Message 80 of 293 (648477)
01-16-2012 12:14 AM


Deltas
Deltas

Introduction

A delta is a fan of braided streams and sediment formed when a river discharges into a larger body of water: a sea or a lake.

The dynamics of a delta

As a river discharges into a larger body of water, the current disperses and loses energy, and so the river dumps its sedimentary load: naturally, it will shed the heaviest sediments first, where the energy is highest, with progressively lighter sediments being carried further out into the sea or lake.

This means that the river will be constantly blocking up its own course with sediment, causing it to fan out into a web of distributary streams interspersed by bars of sediment. The process is not unlike the mechanisms which produce braided streams along the course of a river, and which produce alluvial fans as mountain streams disperse into a desert.

The appearance of a delta in profile will consist of flat topset beds of sediment, then foreset beds which slope down into the lake or sea; then horizontal bottom-set beds deposited on the floor of the sea or lake. Over the course of time, the delta will build out ("prograde") into the lake or sea, forming a characteristic sedimentary sandwich of coarser topset beds overlying sloping forset beds overlying finer bottomset beds.

Because the distributary streams of a delta will be constantly dumping sediment in their own path, the pattern of streams and sedimentary bars will not be static, but shift and change, producing a complex pattern of sedimentary deposition in the topset beds.

Indeed, given long enough, the whole course of a river may shift as the delta silts up. The mouth of Mississippi, for example, is known to have shifted several times over the course of the last few thousand years, and it is only by the unstinting efforts of the U.S. Corps of Engineers that the waters of the Mississippi still flow to the sea via the Mississippi River Delta.

Types of delta

Deltas may be categorized as freshwater or marine, depending on whether they discharge into a lake or a sea. Almost everything we have to say about deltas will apply equally to marine deltas and freshwater deltas. However, there is one notable difference in their dynamics. In a marine delta, the river will not be as salty as the sea into which it is discharging, and so the river water will be less dense than the seawater, and so will flow along the surface of the sea, mixing with the sea water in a horizontal layer, resulting in slower mixing and slower dissipation of the current than in a freshwater delta. The practical upshot of this is that the foreset beds of a freshwater delta will slope down at a much greater angle (up to 25 degrees from the horizontal) whereas in a marine delta the foreset beds will have a slope of only a few degrees from horizontal.

Marine deltas may further be categorized by their dynamics as tide-dominated, wave-dominated, or stream-dominated, according to the main factor affecting their form.

Stream-dominated deltas, such as the Mississipi River Delta, have long distributary channels extending seawards, as shown in the photograph below.

Tide-dominated deltas have, offshore, long bars of sand parallel to the direction of the tide. Inshore, in the main body of the delta, they have tidal flats: beds of mud deposited by the action of the tide. These exhibit cross-bedding produced by the tidal currents: because tides flow in two directions, they will exhibit herringbone cross-bedding, a distinctive sedimentary pattern where alternate sets of cross-beds slope in opposite directions.

In wave-dominated deltas, longshore drift (a current parallel to the shore) smears the deposed sediment across the face of the delta, so that instead of the tidal bars found in wave-dominated deltas, we get a set of barrier islands at right-angles to the direction of the distributary streams.

Former deltas: how do we know?

Where a river has only recently shifted from disgorging via a delta, then it is perfectly obvious that it used to be a delta: not only does it look just like one in terms of its form and position, but also aerial photographs will reveal the former course of the river and its distributaries.

But what of ancient deltas that have been buried and then lithified? Well, in that case they will look just like lithified deltas.

So, looking through a vertical section of the rock, we should expect to see coarser topset beds, with complex patterns of sedimentation caused by the shifting of streams and bars, overlying sloping forset beds, overlying finer bottomset beds. This is a very characteristic pattern of deposition produced by no other process.

Looked at horizontally, we expect to see a complex pattern of interfingering of land and marine sediments reflecting the complex ragged shape of the edge of a delta.

When we find fossils, they will reflect the nature of the beds in which they were deposed: so we expect to see land plants and animals in the topset beds, and the fossilized footprints of marine birds and suchlike fauna. In the bottom-set beds we may, to be sure, find a few remains of land plants and animals carried out to sea by the current: but we would expect the fossils in these beds to be dominated by aquatic fossils and by trace fossils such as the burrows of marine worms; we cannot, of course, find fossil footprints in these beds.

Further indications may be given according to the type of the delta: if, for example, it was tide-dominated, then in the topset beds we will find mudrocks displaying the herringbone cross-bedding characteristic of tidal flats.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 81 by RAZD, posted 01-16-2012 9:23 AM Dr Adequate has responded

  
RAZD
Member
Posts: 20069
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 3.3


Message 81 of 293 (648514)
01-16-2012 9:23 AM
Reply to: Message 80 by Dr Adequate
01-16-2012 12:14 AM


Re: Deltas ... picture?
... as shown in the photograph at the start of this article.

?

Thanks


This message is a reply to:
 Message 80 by Dr Adequate, posted 01-16-2012 12:14 AM Dr Adequate has responded

Replies to this message:
 Message 82 by Dr Adequate, posted 01-16-2012 2:02 PM RAZD has acknowledged this reply

  
Dr Adequate
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Posts: 16097
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(1)
Message 82 of 293 (648555)
01-16-2012 2:02 PM
Reply to: Message 81 by RAZD
01-16-2012 9:23 AM


Re: Deltas ... picture?
Oh, thank you. Yes, I was going to put in a picture of the Mississippi Delta 'cos it's pretty.
This message is a reply to:
 Message 81 by RAZD, posted 01-16-2012 9:23 AM RAZD has acknowledged this reply

  
Dr Adequate
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Posts: 16097
Joined: 07-20-2006


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Message 83 of 293 (649676)
01-25-2012 12:53 AM


Peat Swamps And Coal
Peat swamps and coal

Introduction

In this article we shall use the term swamp as a catch-all term for an area of waterlogged ground in which the water is shallow enough for land plants to grow: an ecologist might distinguish more carefully between swamps, marshes, bogs, fens and so forth.

From our point of view, swamps become of interest when the swamp plants deposit plant matter faster than it can completely decay: in that case the partially decomposed plant matter, known as peat, will become coal on lithification.

The reader should note that there are two types of coal: humic coal, produced by the deposition of the remains of land plants in swamps; and the rarer and less economically important sapropelic coal, formed by the deposition of algae in lakes. The processes of formation are similar, but what we have to say in this article will refer specifically to humic coal.

Deposition of peat

In peat swamps organic matter accumulates faster than it can decays: this is what makes them peat swamps. But why? On the one hand, peat swamps deposit a lot of matter, but perhaps no more than is deposited in an ordinary forest in the form of fallen leaves, branches, and so forth. The crucial difference is that in swamps the deposited vegetation is waterlogged.

The oxygen content of air is about 20%, and this allows aerobic bacteria and fungi and suchlike organisms of decay to function. On the other hand, the oxygen level in oxygenated water can be more conveniently measured in parts per million, and this is the limiting factor in the decay of organic matter in water. The deposited organic matter provides the aerobic bacteria with a potential feast, but in the process of metabolizing the available nutrients they must use up oxygen; and when there are so many of them, dining so heartily, that they are using up all the available oxygen, then that is as fast as they can decompose the matter.

There will be less oxygen available in the water surrounding more deeply buried plant matter, so that once the accumulation of such matter has started, it is likely to continue; in the same way, at a greater depth in the accumulated pile of sediment, the water will be more acidic, which also retards decay. The power of peat swamps to prevent decay is well-demonstrated by the discovery of well-preserved corpses thousands of years old in the peat bogs of Europe; for example the Danish "Tollund Man", dated to the fourth century B.C, shown in the photograph below.

Not all swamps will be peat swamps: this depends on factors such as the rate of deposition of vegetable matter and the rate at which oxygen comes into the system, which will vary according to the rate of flow (if any) of the water. We know of no general formula for determining whether a swamp will be a peat swamp: for our purposes, it is sufficient to note that in some swamps this build-up of plant matter does indeed take place.

Peatification and coalification

Peatification is the process of partial decay that we have described above. The action of bacteria destroys the weaker polymers making up the cell walls, such as cellulose, leaving behind mainly lignin, which is tougher. Because cellulose and lignin share a structural role in the cell walls of plants, the removal of the celluslose leaves the cell structures intact: a look at peat though an electron microscope reveals that even fine details of cell structure are preserved. The resulting matter is known as peat. The reader should note that this term does not refer only to gardeners' peat, which is peatified sphagnum moss, but to any plant matter that has undergone the peatification process.

Coalification is a chemical process in which hydrogen and oxygen are lost from the original peat, increasing the ratio of carbon to other elements. This involves alteration to the remaining molecules of the material, in particular the conversion of lignin to vitrinite. Coalification is not an all-or-nothing process: rather it produces coal of various ranks having a prgressively greater proportion of carbon, from lignite through sub-bituminuous coal through bituminous coal to the highest rank, anthracite.

In early coalification the process is carried out by bacterial action; when the material is so compacted that water cannot percolate (and so bacteria cannot penetrate) the later stages of coalification are produced by the action of heat and pressure (both of which are produced by sufficiently deep burial). In practice the processes distinguished as early and late coalification can overlap somewhat.

During the coalification process, pressure removes water from the material: peat has 95% water, anthracite less than 1%. At the same time, of course, the material is compressed, so that it may end up with as little as one-twentieth of its original volume.

Because the coalification process involves modification of the chemistry of the coal, all coal might, in a sense, be considered a metamorphic rock; however only anthracite is usually classified as such, since only anthracite approaches the temperatures and pressures that we associate with metamorphism.

As this text is intended to be read by readers with little knowledge of chemistry, we have treated the details of peatification and coalification as briefly as possible. The subject has been studied in considerable detail: the reader who is interested will find more information here and here.

Coal from swamps: how do we know?

In the lowest grades of coal, cell structures are still visible under the microscope, revealing their plant origins clearly. As for the higher grades of coal, we should note that from a chemical point of view, the various ranks of coal form a continuum: our divisions of coals into lignite, sub-bituminous, and so forth are, as usual in geology, artifical divisions of a continuum. This is illustrated well by localities where the upper coal beds are lignite and the lower coal beds progress through sub-bituminous to bituminous coal: which fits well with the theory that it is the same substance modified by the increasing heat and pressure associated with burial.

Furthermore, it is possible to simulate coalification in a laboratory. No-one has ever taken a piece of wood and turned it into anthracite coal, since one vital ingredient, that of time, must necessarily be lacking. But it is possible to show that the application of heat and pressure to peat will make it chemically more like lignite, and that similar treatment of lignite will make it more like sub-bituminous coal.

There is, then, no real doubt that coal has its origins as plant matter. We turn now to the question of why geologists ascribe the source of this plant matter to swamps.

In the first place, coalfields are just what we would expect to see if peat deposits were buried to a sufficient depth, since, as we have observed, heat and pressure, which are both produced by deep burial, cause the chemical changes involved in coalification. Coal fields therefore look like peat deposits should look after sufficient heat, time, and pressure.

We can then ask ourselves: how else can such deposits form? To produce the extent and thickness of coal beds that we observe, we require a lot of plant matter to be deposited over a wide area in anoxic conditions, so that it doesn't rot. No other environment fits the bill. On a forest floor, for example, although plant material will be deposited over a long period of time and a wide area, it will be decomposed fairly rapidly, and never attains any great thickness, as can be easily verified with a trowel. When we see a coal seam ten meters thick, which is not unusual, and when we consider how much it has been compacted down (maybe ten or twenty times) from its original volume, we can see that the original plant matter must have been deposited in conditions where only partial decay took place: i.e., in a swamp.

We can imagine peatification taking place in other environments besides swamps: for example, we can imagine a landslide transporting trees down a hillside into a so-called "dead" lake. The trees then might conceivably peatify and, if buried deeply enough under other sediments, coalify. But once again, we find that this would not account for the great depth and lateral extent of coal beds.

Peat swamps therefore stand out as the one plausible explanation for coal. This is confirmed by exampination of the beds of rock underlying and overlying coal beds.

Immediately underlying coal beds, we find paleosols, deposits which, as discussed in a previous argument, geologists identify as fossilized soils: most obviously, because they have fossilized roots in them. Indeed, the paleosols will sometimes have trees or tree-roots rooted in them, projecting up through the coal beds: furthermore, the trees are consistent with the fossil vegetation found in the coal. This fits well with the swamp theory of the origins of coal. The paleosols underlying coal beds (which are known as seat-earth, or underclay) also show the sort of soil characteristics we should expect to find in waterlogged soils.

The picture below, taken from Dawson's Acadian Geology, shows the relation of coal beds to paleosols.

The key from the original text identifies the strata from top to bottom as follows:

1. Shale.
2. Shaly coal, 1 foot.
3. Underclay with rootlets, 1 foot 2 inches.
4. Gray sandstone passing downwards into shale, 3 feet. Erect tree with Stigmaria roots (e) on the coal.
5. Coal, 1 inch.
6. Underclay with roots, 10 inches.
7. Gray sandstone, 1 foot 5 inches. Stigmaria rootlet continued from the bed above; erect Calamites.
8. Gray shale, with pyrites. Flattened plants.

The beds overlying coal beds are also consistent with the swamp theory: they are aqueous deposits, either freshwater or marine, depending on the location and nature of the swamp.

For these reasons we can be confident that humic coal does indeed have its origins in the deposition of plant matter in swamps.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 85 by RAZD, posted 01-29-2012 5:43 PM Dr Adequate has responded

  
Dr Adequate
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Message 84 of 293 (649787)
01-25-2012 2:18 PM


We're Getting There
I think I'm about done with sediments formed on land. The next article will take us to beaches and other nearshore sediments, and then we'll head out to sea.

At this point I'd like to say a few words.

There is an inordinate number of different kinds of sediment. This is just how it is. This makes the study of geology different from studying (for example) the theory of gravity. Instead of Einstein's single equation, geologists must study a vast variety of things that happen on the face of the Earth. Dust-storms blow, trees fall, the tide goes in and out, turbidity currents do their thing, glaciers do theirs, peat-swamps form, rivers dposit point-bars, inevitable chemical processes gnaw at the rocks, desert sand is piled up by the wind, coccoliths drift with immense slowness towards the seafloor, the tide makes flaser deposits ... and so on and so on.

And you don't quite see it all until you see it all. Before I undertook my own study of geology, I regarded sediment as the dirt one finds fossils in. Now I see landscapes. "Here" (we say) "are the remains of ancient mountains, long gone. Over their cloud-capped heads, the storm broke, and angry torrents flowed down and dwindled into the rain-shadowed desert when dinosaurs walked --- look, here are their footprints around the ancient oases. Vast was that expanse of sand, which the wind sifted for tens of millions of years. It was bordered by a great yet shallow sea ..."

In writing this textbook, I have to deal with this one sediment at a time, one ripple in the mud, one lamina in the sand, but when we look at it all together, we see vanished landscapes, lost worlds. Piece by piece we put it together, until we see the whole.

"Science is the poetry of reality."


  
RAZD
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Message 85 of 293 (650282)
01-29-2012 5:43 PM
Reply to: Message 83 by Dr Adequate
01-25-2012 12:53 AM


Re: Peat Swamps And Coal
Hi Dr Adequate, excellent.

Just a couple of small quibbles:

We can then ask ourselves: how else can such deposits form? To produce the extent and thickness of coal beds that we observe, we require a lot of plant matter to be deposited over a wide are in anoxic conditions, ...

[pendantmode] ... a wide area in anoxic conditions? [/pendantmode]

We can imagine peatification taking place in other environments besides swamps: for example, we can imagine a landslide transporting trees down a hillside into a so-called "dead" lake. The trees then might conceivably peatify and, if buried deeply enough under other sediments, coalify. But once again, we find that this would not account for the great depth and lateral extent of coal beds.

[creationistmode] or we can imagine great piles of organic debris deposited by the great flood of ~4kbc ... and compressed by the weight of the water during the flood ... [/creationistmode]

quote:
Pratt CC361.1
Response:

Coal deposits show evidence of a history. Most coals are found in sedimentary rocks deposited in flood plains. They often contain stream channels, roots, and soil horizons. Long time may not be necessary to form the coal itself, but it is necessary to account for the context where coal is found.


Rapid flood burial does not explain the great volume of coal world-wide that would have had to cover a pre-flood world, nor does it provide the heat necessary, nor does it provide a means to compress water out of the material (when the compression is accomplished by water), nor does it explain the gradations of coal deposits with depth (similar to the sorting of fossils problem).

Immediately underlying coal beds, we find paleosols, deposits which, as discussed in a previous argument, geologists identify as fossilized soils: most obviously, because they have fossilized roots in them. Indeed, the paleosols will sometimes have trees or tree-roots rooted in them, projecting up through the coal beds: furthermore, the trees are consistent with the fossil vegetation found in the coal. This fits well with the swamp theory of the origins of coal. The paleosols underlying coal beds (which are known as seat-earth, or underclay) also show the sort of soil characteristics we should expect to find in waterlogged soils.

[creationistmode] The existence of polystrate fossil trees is evidence of the great flood -- the tree should have decayed in one layer, not extend through many. [/creationistmode]

quote:
Pratt CC331:
Response:

Sudden deposition is not a problem for uniformitarian geology. Single floods can deposit sediments up to several feet thick. Furthermore, trees buried in such sediments do not die and decay immediately; the trunks can remain there for years or even decades.


Swampy areas can be periodically buried by a number of events (see above on coal formation context), including flash floods upstream. Some trees species are more tolerant of their roots being buried than others, and can continue to grow in such situations.

The "polystrate" trees (that are known) consist of trunks and roots, not full trees.

Enjoy.

NOTE: these comments should not be taken as a point to debate here - rather anyone interested in trying to rationalize creationist models should start a new thread.

Edited by Zen Deist, : no debate thread


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This message is a reply to:
 Message 83 by Dr Adequate, posted 01-25-2012 12:53 AM Dr Adequate has responded

Replies to this message:
 Message 86 by Dr Adequate, posted 01-29-2012 7:34 PM RAZD has acknowledged this reply

  
Dr Adequate
Member
Posts: 16097
Joined: 07-20-2006


(1)
Message 86 of 293 (650294)
01-29-2012 7:34 PM
Reply to: Message 85 by RAZD
01-29-2012 5:43 PM


Re: Peat Swamps And Coal
I fixed the typo, thanks.

I don't think it's necessary to talk about creationist blather. The picture I present speaks for itself in any case --- roots in a paleosol, coal, treestump, more paleosol, more roots, more coal. Anyone wishing to try to explain this in terms of "flood geology" is welcome to try (on another thread) while I giggle at their ineptitude. Meanwhile it's clear that the mechanisms I've presented do explain it.

Edited by Dr Adequate, : No reason given.


This message is a reply to:
 Message 85 by RAZD, posted 01-29-2012 5:43 PM RAZD has acknowledged this reply

  
Dr Adequate
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Posts: 16097
Joined: 07-20-2006


(2)
Message 87 of 293 (651299)
02-06-2012 10:24 AM


Nearshore Sediments
Nearshore sediments

Introduction

In this article we shall discuss the sediments of the nearshore, their origin, characteristics, and sedimentary structures; and, as usual in these articles, we shall discuss how we can identify sedimentary rocks as being lithified nearshore sediments.

Waves and the nearshore

We should first remind the reader of certain facts about waves and tides. First, the reader should bear in mind that the energy of waves only goes down so far: the rule of thumb usually given is that their action affects the water beneath them to a depth about equal to half the wavelength (where the "wavelength" is the distance between two consecutive waves). Out at sea, the wave base (the lowest depth at which the wave has any effect) will be well above the sea-bed, and will move with a regular rolling motion known as swell. However, as waves come into shallower coastal waters, the wave base eventually hits the sea bed. This leaves the energy of the wave with no place to go but up. Therefore the wave rises higher and higher until it becomes unstable and breaks as surf. The consequence of this is that waves will have no effect on the marine sediments of the deep sea.

Tide-generated waves do not always move at right-angles to the beach: the result of this is to generate a longshore current which moves parallel to the beach. This is often confused with longshore drift, which is also caused by waves approaching the beach at an angle. As they wash up the beach, they travel at an angle, moving the sediment with them; but they tend to roll straight back down the beach, again taking the sediment with them. This means that the sediment on such a beach will be transported along the beach in a zigzag path.

A note on terminology

We can divide the sea and shore up into zones according to the action of the waves on the sea bed. Unfortunately, geologists do not do so consistently, so the same word may mean different things according to which geologist is using it. Indeed, I have seen one textbook in which the set of definitions supplied in the text contradicted the accompanying diagram intended to illustrate them. My advice to readers who wish to pursue their study of nearshore sediments further is that for each book they read they should pay careful attention to how each particular author defines his or her terms.

For the purposes of this article, I shall use the term nearshore to describe the zone in which the sea bed is affected by waves; the term foreshore to describe the part of the nearshore which is uncovered at low tide; and the term backshore to describe the area higher up the beach than the foreshore, i.e. that part of a beach which is above the high-water line. Other writers will differ, especially as to the proper meaning of the term "nearshore".

Varieties of nearshore environment

Nearshore sedimentary environments are very variable in their nature. The sediments can consist of mud or sand or pebbles, or any combination of the three, which may be mixed or sorted according to the action of the waves.

These sediments can be deposited by rivers; they can have their origin in clasts broken from a rocky coast; they can be carried around the coast by longshore currents and longshore drift; they can have their origin as broken fragments of shells or coral.

The sedimentary structures will depend on such factors as the energy of the tide in that locality, the slope of the nearshore, and whether or not there is a significant longshore current.

In short, a whole book could be written on this subject enumerating the various characteristics of, for example, a high-energy muddy nearshore with a longshore current; and a low-energy nearshore consisting mainly of sand and gravel with no longshore current; and so forth.

Because of this, this article can only be a first sketch of the subject.

Some nearshore sedimentary structures

The reader should note that because of the variability discussed in the previous section, we would not expect to find all the structures listed below in one single nearshore environment.

* Wave ripples. These are superficially similar to the ripples produced in desert sand, but they tend to be more symmetrical in cross-section, because they are formed by waves going in both directions (oscillatory flow).

* Interference ripples. These are formed when the tide goes out at, or nearly at, ninety degrees from the angle at which it comes in. A particularly striking example can be seen, preserved in sandstone, in the photograph to the right.

* Herringbone cross-bedding. This is one of the most distinctive types of coastal sediment. You should recall that cross-bedding is produced by the action of a current, and that the beds slope down in the direction of the current. But the current produced by a tide runs in two ways. The consequence of this is that the sets of cross-beds slope in opposite directions, producing a herringbone pattern.

* Flaser deposits. Since the action of the tide is weaker at high and low tide than in between, sediments affected by the tide can in effect be in a high-energy environment and a low-energy environment alternately. This can result in a situation where, during the high-energy period of the tidal cycle, the waves shape sand into ripples, and during the low energy period of the cycle, the waves deposit mud in the depressions of the ripples. The resulting pattern of sediment is known as a flaser deposit.

* Deltas. A river disgorging into the sea will often form a delta: a fan of shifting bars and channels. There is enough to say about these structures that we have already covered them in a separate article on deltas. The reader should recall that we can tell a marine delta from the case where a river is flowing into fresh water by measuring the slope of its foreset beds.

* Longshore bars. At the point on the shoreface where the landward force exerted by the wave base is equal to the seaward force of the backwash of breakers, sand will accumulate in longshore bars: that is, bars of sediment running parallel to the beach. On the seaward side these bars typically have layers of crossbeds sloping gently seaward; on the landward side they exhibit ripple-formed laminae and trough cross-bedding (that is, crossbedding formed by waves repeatedly scouring out and filling in troughs in the bar). Broken shells are collected in the longshore bar by the same mechanism that accumulates the sand itself, and so longshore bars are typically abundant in layers of broken shells.

* Sand dunes. As the wind tends to blow off the sea, a sandy beach will have sand dunes pile up at the back of it. These will be similar to the wind-formed dunes found in deserts. However, unless the beach borders on a desert, there is no reason that it shouldn't get rain, and so beach dunes will tend to have plants growing in them, and often animals such as crustaceans burrowing in them; these will leave root traces and traces of burrows visible in the geological record, if the dunes are preserved. Another difference between these dunes and those of a sandy desert is of course that a desert will be spread across a large area; beach dunes may extend a long way along a coastline, but will form only a thin strip.

* Bioturbation. The fauna that burrows in the shoreface leave burrows of characteristic shapes. Indeed, the inhabitants of different zones of the shoreface leave different traces.

* Pebbles. The action of the waves and abrasion by other sediments will rapidly convert gravel to smooth pebbles. This can happen in other environments, such as rivers, but there is a tendency for nearshore pebbles to be flat on one axis.

Lithified sedimentary rocks: how do we know?

We can identify sedimentary rocks that originated as nearshore sediments using a number of criteria.

First, many of the sedimentary structures that occur on the nearshore are unique: there is, for example, no other way of producing flaser bedding. Others are very rare in any other context: for example, interference ripples can very occasionally be produced by the flooding of rivers, but they cannot be continuously produced except in a tidal environment. The picture below shows, on the left, fossilized interference ripples, and on the right, interference ripples on a beach.

Second, there is the fossil fauna of these sediments. The fauna of the nearshore is quite different both from the fauna of the land and the fauna of the deeper sea. For example, on the foreshore we find those creatures which can survive some exposure to the open air, when the tide is out, but cannot permanently survive such conditions.

Another consideration is the topography of the sediments. The coastline is a line, so if geologists are right in identifying certain kinds of rocks as lithified near-shore sediments, then we should expect to find these rocks in long thin strips running between rocks formed by the sorts of sediments we find on land and the sort of sediments we find further off shore: and this is indeed what we find, confirming the theory that these are nearshore sediments.

Furthermore, we should expect the different sorts of sedimentary structures and fossils to be arranged within each strip as they are in nearshore sediments. For example, if we find sedimentary rocks which we identify as backshore dunes, they should form a still narrower strip on the landward side; on the seaward side we should then find the characteristic fauna and bioturbation of the foreshore, followed by those of the deeper nearshore.

Finally, there are chemical clues, as explained here, for example. Coastal sediments have consistent chemical differences from those of deeper waters, and these differences are preserved in sedimentary rocks.

The picture below shows a slab of sandstone exhibiting the ripples characteristic of a beach and also the typical bioturbation caused by the invertebrates of the foreshore.

Certainly it looks exactly like a lithified section of beach to anyone who has ever been to the seaside. The reasonable conclusion is that that's because that's exactly what it is.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Message 88 of 293 (654212)
02-28-2012 1:22 AM


Marine Sediments
Marine sediments

Introduction

This article contains a short general discussion of marine sediments and some important terms and concepts relating to this field.

We need not in this article ask and answer our usual question: "How do we know?" since in the case of the facts given in this article the answer is simply and uniformly: "We looked at the bottom of the sea."

The sea in profile

Before we discuss marine sediments, we should introduce a few terms from oceanography. The diagram below shows a cross-section of the sea and land. Though it has been simplified and stylized, it is based on actual data. Do note, however, that the vertical scale has been exaggerated by a factor of 25 compared to the horizontal scale, and that this consequently exaggerates all the angles of slope.

Features to note are:

* The continental shelf. This may be regarded as the part of the continent which just happens to be underwater. Its true angle of slope is rarely more that half a degree.

* The continental slope. As you can see, this is formed of land-derived sediment which has piled up at the foot of the continental crust. Its angle of slope rarely exceeds ten degrees, and is more typically around four degrees.

* The continental rise. This has an average inclination of about half a degree from vertical, and flattens out into ...

* The abyssal plain. As you can see from the diagram, this tends to be flat (hence the name), because all but the most prominent topographical features are obliterated by sediment.

The shelf, slope, and rise are known collectively as the continental margin.

Distribution of marine sediments

The map below shows which marine sediments are deposited where.

The key is as follows:

* Gray: land.

* White: Sediments of the continental margin.

* Blue: glacial sediments.

* Orange: land-formed sediments.

* Brown: pelagic clay.

* Green: siliceous sediments.

* Yellow: calcareous sediments.

The nature of these sediments will be discussed in more detail in the section below.

There are a few ways in which this map may be misleading. First, note that this is a Mercator projection map, since, for technical reasons, this was the easiest map projection for me to use. All map projections distort reality, since they involve representing a spherical surface as a flat plain. In the case of the Mercator projection, its besetting fault is that it exaggerates north-south distances near the poles.

The second way in which the map does not truly represent reality is that it divides the sea bed into distinct regions of carbonates, silicates, pelagic clay, and so forth. In fact, the sediments will rarely be pure: the sediments that form pelagic clay, for example, get just about everywhere. So really every region represents a mixture of sediments, of which the color used on the map represents the one that predominates. This inaccuracy will, of course, be especially severe at the borders between regions, where one sediment type will grade into another.

Thirdly, note that this map represents sediments on top of the sea bed: they are not the same all the way down, as we shall discuss in the final section of this article.

Types of marine sediment

* Sediments of the continental margins. These include sand and mud from rivers; material eroded from cliffs; in some latitudes, material deposited by glaciers during ice ages; material carried down to the continental rise by turbidity currents; and, in some cases, calcareous ooze (see below).

* Glacial sediments. These are transported out to sea by icebergs calving from continental glaciers.

* Land-formed (or "terragenic") sediments. Technically, both glacial sediments and the sediments (other than limestone) of the continental margin can be considered land-formed. However, the areas marked as such on the map are regions where the abyssal plain has been covered by land-formed sediments either borne down to the abyssal plain by turbidity currents, or borne out to sea by river currents powerful enough to carry the sediment beyond the continental shelf.

* Siliceous ooze. This is formed from the silica shells of microscopic organisms: diatoms and radiolarians. These are common only in the most nutrient-rich and biologically productive parts of the ocean, such as the polar oceans and upwelling zones near the equator. If lithifed, this would by definition be chert.

* Calcareous ooze. This is formed from the remains of tiny organisms such as foraminifera, coccolithophores, and pteropods. Its peculiar pattern of deposition can be explained by the fact that this sediment tends to dissolve in deep cold water. Chemically, it is composed of calcium carbonate; hence when lithified it would by definition be limestone.

* Pelagic clay (also called "red clay" or "brown clay"). This consists chiefly of particles fine and light enough to be borne out to sea by currents of wind or water. As such, you might expect these sediments to get pretty much everywhere, and you would be right. The areas on our map showing pelagic clay are not so much areas in which lots of this sediment are deposited as areas in which nothing much else is deposited.

Marine sediments and plate tectonics

We mentioned above that the sediments found in a given region of the sea floor are not the same all the way down.

This would be puzzling if the ocean bed stayed still: for in that case, why should not calcareous ooze (for example) be deposited on the same spot for ever?

But in light of the theory of plate tectonics, this observation makes perfect sense. Given that the sea floor moves, and has been moving for a long time, we should expect it to become layered as the same bit of continental crust passes under successive different regions of deposition. Indeed, given that we know how the sea floor is moving, and given a map such as the one presented above, we can say what pattern of layers we should expect to see: and this is confirmed by observation.

We shall discuss this point in more detail in our articles on plate tectonics.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
Member
Posts: 16097
Joined: 07-20-2006


Message 89 of 293 (654931)
03-05-2012 5:11 PM


Glossary
I've posted the first installment of the glossary in message #26; it is, or should be, complete up to the end of the article on nearshore sediments.

Many thanks to Pressie for having the patience to read all the way through it and check it.

It contains, by my count, 293 entries so far. If anyone would like to drop out and become a Young Earth Creationist instead, I sha'n't go so far as to say that I sympathize, but I do understand.


  
Dr Adequate
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Posts: 16097
Joined: 07-20-2006


Message 90 of 293 (656407)
03-18-2012 2:04 PM


Turbidites
Turbidites

Introduction

Turbidites are sedimentary rocks caused by the lithification of turbidite sediments, that is, sediments deposited by turbidity currents. In this article we shall review what is known of their sedimentology, and discuss how we know their mode of deposition.

Turbidity currents

When a denser fluid flows through a lighter one, the difference in density prevents them from mixing, so that the denser fluid forms a current within the less dense fluid. In particular, turbidity currents in water are currents which are denser than the surrounding water as a consequence of being turbid (loaded with sediment). Because the turbidity current only mixes gradually with the surrounding water, its energy only dissipates very gradually into the larger body of water. This means that a turbidity current can flow for great distances --- hundreds of kilometers --- as a distinct current within the clearer water. Being denser than the surrounding water, it will flow downhill and along the bottom of the surrounding fluid: one might think of such a current as a sort of underwater river, although the analogy is not quite exact in that a turbidity current can flow up and over obstacles in its path.

The turbidity currents of interest to us in this article are those caused by slope failure, where sediment on the continental slope begins to slide down it, either as a result of a submarine earthquake or simply as a result of sediment accumulating on the slope until gravity alone is sufficient to start it sliding. This initiates a turbidity current, which flows down the slope accelerating as it goes: also, as it flows down the slope, it churns up more turbidity, increasing the difference in density between the current and the surrounding water.

By the time such a current reaches the ocean floor, it can be traveling at upwards of 100 kilometers per hour. As we have noted, the dynamics of a turbidity current ensure that it only loses energy very slowly, and so such a current can travel hundreds of kilometers before giving out.

Because these currents carry their loads of sediments at such high speeds, they must surely have a powerful erosional effect: they are thought to be the main cause of many underwater canyons. However, we are more concerned here with their role in the deposition of sediment, which will be discussed in the next section.

Turbidity sediments and turbidites

The sediments deposited by turbidity currents are known as turbidity sediments. The rocks formed from these sediments on lithification are known as turbidites.

At any particular point over which a turbidity current passes, it will start off strong and gradually weaken until its energy is entirely dissipated. The consequence of this will be that the sediment will grade upwards from coarser to finer sediments. How coarse the sediment at the bottom is will depend on the source of the sediment: it may be as coarse as boulders and cobbles, or as fine as sand. The thickness of the deposit is also variable, from meters to centimeters in scale.

Note that the current fails not only over time, but also spatially, as it loses energy the further it gets from its origin. So at the extreme distance from the origin, only mud will be deposited; closer to the origin than that, we would see silt overlain by mud; and so forth.

After the deposition of the turbidity sediments, there will usually be a more tranquil regime of deposition, during which ordinary marine clay-sized articles will be deposited on top of the turbidity sediments proper. The entire sequence of sediments produced by these two mechanisms is known as a Bouma sequence. Note that although the top of the Bouma sequence is not deposited by turbidity currents, the term "turbidite" is used to include the whole Bouma sequence and not just the part of it so deposited.

The photograph below shows several successive turbidites, each grading upwards from sand to clay.

While the ordinary marine clay in the Bouma sequence will contain fossils from the deep waters in which they were deposited, the turbidity sediments will typically contain fossils from the shallower waters in which they originated, and these fossils will typically be fragmented by the violence of the process which transported them. The current-deposited sediments will often display sedimentary structures associated with flow, such as ripple marks. When a fresh sequence is deposited on top of the previous one, the force of the turbidity current will erode the layers of fine clay at the top of the previous sequence, producing what are known as sole marks.

In the photograph below, you should be able to make out the climbing ripple marks in the current-deposited sediments.

The typical place to find a Bouma sequence is underneath one Bouma sequence and on top of another; although slope failures are intermittent, they are plentiful, and over a sufficiently long period of time great stacks of them will be deposited.

The picture below shows part of one of these stacks.

Turbidites: how do we know?

Offshore drilling on the continental margin finds sequences of unlithified sediments which look just like the sequences of lithified sediment found on dry land. To identify the latter as the lithified counterpart of the former is trivial; and so we can be confident that the lithified sediments we marine in origin and were formed by the same processes as the marine sediments sampled from the sea floor.

A more difficult question is how we know that sediments of this type really were deposited by turbidity currents. So far as I know, at the time of writing, no-one has ever been at the right place at the right time to see a turbidity current depositing its load of sediment; this is unsurprising, since the phenomenon is intermittent and unpredictable, so no-one knows what the right time is; and the right place is at the bottom of the sea.

For this reason turbidites were for a long time a puzzle for geologists. But when they started taking turbidity currents into consideration, suddenly everything became clear.

Note first of all that turbidity currents themselves are not hypothetical. They can be produced in the laboratory in tanks of water and their action observed. Furthermore, laboratory experiments confirm that the waning of a turbidity current does indeed result in graded sediments, as we would expect. Slope failures are also not hypothetical, and turbidity currents have been observed flowing down the continental slope through marine canyons; it is only the actual deposition of the sediments that has so far gone unrecorded. The video below shows turbidity currents in the ocean and in the lab.

We know that whatever leaves these sediments flows along the bottom of the sea, because it leaves ripple marks in the sediment and because it leaves sole marks gouged out of the previous layer of sediment. In order for something to flow at the bottom of the sea it has to be denser than seawater --- like a turbidity current is by definition.

One frequently cited observation is the aftermath of the Grand Banks earthquake of 1929. In the hours following this, a number of transatlantic cables were severed. Their position was known, as were the exact times when they were cut. It is therefore possible to say that something capable of severing cables moved from near the epicenter of the earthquake at a speed of approximately 100 kilometers per hour, and that it moved along the sea floor where the cables were laid. A turbidity current with its abrasive load of sediment would be a highly plausible candidate.

We know that whatever process forms the deposits that we're trying to explain must be happening in the present, because we can see freshly deposited turbidite sediments in the present day. But we also know that the process must be intermittent, partly because we can't see any continuous process forming these deposits on the sea floor, and partly because the sedimentology shows the effects of a high-energy current waning to a low-energy current followed by a period of ordinary marine deposition, followed by the same thing happened over and over again. The turbidity currents generated by slope failure would fit this bill.

Moreover, we know of no other cause that could transport such large clasts so far out to sea. This may seem like a mere argument from ignorance, but it gains force when combined with the following argument. We know that there are failures of the continental slope causing currents which are by the nature of their origin turbid. Therefore, these currents must transport sediment and depose it in some form. If it is not deposited in the form of turbidity sediments, in what form is it deposited and where is it?

The fossils found in turbidites are another important point. The alternation of shallow-water with deep-water fossils was a complete and baffling mystery. The theory of turbidity currents makes everything clear: the shallow-water fossils are carried by the turbidity current from shallow to deep water, and what was an inexplicable anomaly becomes an expected consequence of the theory.

Perhaps the closest anyone has got to direct observation of turbidite formation is the events in Lake Brienz in 1996. The lake showed distinct signs of an underwater landslip, including a sudden increase in the turbidity of the lake waters, a small (half-meter high) tsunami wave, and the release of a 200-year old corpse from the lake bed. Taking sediment cores from the lake revealed that an abnormal layer of sediment, 90cm thick at its thickest part, had been laid down concurrent with this event: the sediment graded vertically upwards from sand through silt to clay: that is, it looked just like turbidite sediment should, apart from not being marine in nature. Further investigation suggested that the 1996 event was caused by accumulated sediment sliding down the slope of the Aare delta.

In the light of all these facts, it seems to be a safe bet that turbidity sediments are indeed caused by turbidity currents.

Edited by Dr Adequate, : No reason given.


  
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