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Author Topic:   Introduction To Geology
Dr Adequate
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Message 166 of 293 (673407)
09-19-2012 12:47 AM


Steno's Principles
Steno's principles

Introduction

Steno's principles of stratigraphy are among the oldest of geological principles; they explain how we can look at sedimentary rocks today and figure out facts about the sediment at the time of deposition.

They are named for Nicholas Steno (1638 - 1686), a Catholic bishop and polymath scientist who set forth his principles in his Dissertationis prodromus of 1669. He was beatified in 1988, making him officially just one step away from being a saint; the scientific world has shown its admiration for him by naming craters on Mars and the Moon in his honor.

In this article we shall explain, discuss, and illustrate his principles.

The principle of superposition

The principle of superposition simply says that when sediments are deposited, those which are deposited first will be at the bottom, and so the lower sediments will be the older. This is because sediment is deposited from above, because gravity operates in a downward direction, and because sediment does not readily pass through other sediment.

Note that this only applies to sediment; it does not necessarily apply to igneous rock, because whereas sediment is deposited from above, magma oozes up from below. So, for example, an igneous sill intruding into sedimentary rock will be younger than the rocks immediately above it.

One snag may occur to you. It is all very well to say that the sediment, when originally deposited, was laid down from the bottom up. But does it necessarily follow from this that when we look at sedimentary rocks the lowest is the oldest? For tectonic processes such as folding can completely overturn a section of rock, or, just as bad, turn it on its side so that we can't tell which was originally up and which was down.

Fortunately, there are many clues within the rocks which allow us to discover which way up they were originally; these are known as way-up structures and will be the subject of our next article. Once we have used these indications to discover which way up the rocks were when the sediment was deposited, we can then apply Steno's principle to sort out their relative ages.

The principle of original horizontality

The principle of original horizontality states that sediment is originally laid down flat.

This needs some qualification. After all, some sediment is not originally deposited in flat beds, as you will know from our articles on sedimentology: for example aeolian sand is laid down in cross-beds, and the angle of repose of a sand-dune can be as much as 34º from the horizontal.

Yet you will also recall that the cross-beds are laid down in sets formed as the dune rolls across the desert landscape, and these are much more nearly horizontal. Consider the photograph below of aeolian sandstone in Utah.

The cross-beds themselves reflect the angle of repose of the original sand-dunes; but the sets of cross-beds are horizontal.

To take another example, you may remember that the progradation of a delta forms foreset beds, which show the slope of the delta into the lake or sea into which it builds. And yet these foreset beds considered as a whole will form a roughly horizontal layer sandwiched between the topset and bottomset beds.

So even apparent exceptions can, looked at the right way, be seen as instances of Steno's rule; and having taken them into account, we find that we can look at rock formations and identify the plane of horizontality at the time that the sediment was originally deposited.

The principle of original continuity

The principle of original continuity states that sediment is originally deposited in a continuous horizontal sheet until it meets some obstacle, such as the base of a cliff, or it tapers off because of its distance from its source, or grades laterally into another sort of sediment.

The qualifications in that sentence may seem at first to make the principle futile. What is it saying but "sediment will go on until it stops"?

An example will make the concept clearer. Consider this photograph of hoodoos at Drumheller, Canada.

You should easily be able to make out three kinds of sediment: there is hard dark mudstone at the bottom, followed by lighter-colored softer sandstone, and then the "caps" visible on the taller hoodoos are formed from sandstone which is harder and darker than that which underlies it.

Now, it is inconceivable that geological processes originally deposited these sediments in the forms of hoodoos: for one thing, there are no processes that would do that; and for another thing, the sediment, when unlithified, wouldn't have stayed standing if it had been deposited in such a way.

What must have happened is that each of the three layers was originally deposited as a continuous sheet, which subsequently underwent lithification and then erosion, producing the forms visible today.

Steno's principles and actualism

As we have just explained the principle of actualism, it is worth taking a while to examine how Steno's principles are just special cases of this more general principle.

For example, when discussing the processes that formed the hoodoos above, we appealed twice to actualism; we wrote: "It is inconceivable that geological processes originally deposited these sediments in the forms of hoodoos: for one thing, there are no processes that would do that, and for another thing, the sediment, when unlithified, wouldn't have stayed standing". We appealed to what we know of geological processes to say that the hoodoos couldn't have been deposited as hoodoos; we appealed to known laws of physics to point out that even if there was such a process, the unlithified sediment would have slumped and collapsed.

Again, consider the principle of superposition as it applies to the hoodoos. It seems plain that mud must have been deposited first, then the paler sand, and then the darker sand. In saying so, we are tacitly taking it as axiomatic that science-fictional processes such as levitation or teleportation did not come into play. If they did, then we have no way of knowing which rock layer came first: maybe the dark sand was deposited first but hung unsupported meters above the sea floor until the gap was filled in by the lower sediments which somehow managed to pass through the existing layer.

We can imagine such a thing happening, just as we can imagine a dragon or a unicorn, but the only basis for practicing geology is to ignore this hypothetical magical event and to work on the basis that this did not in fact happen.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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(1)
Message 167 of 293 (673966)
09-25-2012 11:31 AM


Way-Up Structures
Way-up structures

Introduction

As we noted in the previous article, it is perfectly possible for rocks to be overturned by tectonic processes. Hence, before we can apply the principle of superposition to discover the relative ages of the strata, we must first identify which way up the rocks were when they were formed.

Fortunately there are many indications we can use to find this out, known as way-up structures. In this article we shall list some of them

Some way-up structures

Note that we do not claim that this list is complete; these are simply some of the most commonly cited way-up structures.

Mud cracks

Mud cracks (also known as dessication cracks) are formed in mud when it dries, and examples can be found preserved in the geological record. These form a distinctive structure, with their polygonal forms and the roughly V-shaped cross-section of the cracks; not only is there nothing else like them, but also there is nothing that looks exactly like mud cracks apart from going up where mud cracks go down. Hence mud cracks can be used as way-up structures.

Ripple marks and cross-bedding

Ripples have curved troughs and sharp crests, and a convex shape as seen from above; as with mud cracks, we may note that there is nothing that looks exactly like a ripple only upside-down. Hence they form way-up structures.

The cross-bedding in aeolian sand is also convex on the upper side, as sand-dunes are steeper at the top and have a shallower curve near the base (see, for example, the picture in the previous article).

Flame structures

Flame structures are formed when a denser sediment, (typically sand) is deposited on top of a less dense sediment (typically mud). The difference in density forces the mud to flow upward in what are known as diapirs, producing a distinctive flame-like structure in which the "flames" are always at the top. The photograph below shows an unlithified example.

(We may note in passing that diapirs are technically a counterexample to the principle of superposition: some of the earlier sediment has managed to rise above some of the later sediment. However, as diapirs are fairly easy to identify, they cause little confusion in practice.)

Graded beds

Some forms of deposition produce graded beds: for example, turbidity currents produce beds which grade upwards from coarse to fine material. Although occasionally reverse grading can be seen in turbidites, one typically finds turbidites in large stacks, so there is no difficulty in discerning the general trend and identifying the occasional example of reverse grading as being the odd man out.

Again, in meandering rivers we find fining-up sequences at point-bars; rivers do not produce fining-down sequences.

Erosion

Currents will often incise structures into the sediment over which they flow: flute marks, scour marks, sole marks, etc, producing distinctive impressions in the underlying beds which are then filled in with other sediments (we have discussed this particularly in our article on turbidites, but such structures are produced in other environments such as rivers).

In general, it is easy to look at a surface where two sediments meet and determine which of the two sedimentary rocks was eroded, and which was laid down over the eroded surface. If, for example, potholes formed by a river or glacier are subsequently filled up with mud, it is not difficult to conclude that the mud conformed itself to the potholes rather than the potholes to the mud, and that consequently the potholes indicate the lower surface.

Fossils attached to the surface

Some fossils form attached to the ground, and display a distinct difference between up and down: so, for example, branching corals will branch upwards, not downwards; a tree-stump found still with its roots intact and embedded in seat-earth shows that the roots were down and the stump was up; stromatolites will have a flat base and a convex top.

Fossils not attached to the surface

Even when fossils are mobile and can be overturned, we can sometimes learn from them. For example, trilobites, being broad, flat creatures, are not readily overturned, and are usually found belly-side down as they were in life. Although one or two might get flipped over, if we have a fair number of trilobites we can distinguish the statistical trend, and say which direction was most likely up.

Bowl-shaped shells such as individual valves of bivalves will, in a current (such as that produced on beaches by the tide) tend to come to rest convex-side up, as the reader may easily observe by taking a stroll on a beach. Again, this will not be true of every single shell, but so long as we have a fair number of shells, we can gauge the general trend and use it to figure out which way was up.

Trace fossils

The casts thrown up by invertebrates as they burrow are naturally found on the surface. If we consider the picture below, of lithified beach sand, we can see that we must be looking at the upper surface (note also the ripple marks).

Some burrowing invertebrates make burrows which serve as way-up structures. For example, some make distinctive U-shaped burrows: naturally the openings are at the surface, so the prongs of the U point up and its bowl points down.

The indentations made by footprints are necessarily convex, and any sediment which fills them in will be concave, forming a way-up structure.

The photograph below shows the footprints of some large theropod dinosaur.

Note also the mud cracks. Clearly this particular piece of rock is the right way up. Conversely when we look at the footprint below, we are clearly looking at it from the underside.

Geopetal structures

These are formed when a hollow object (such as a shell) becomes partly filled with sediment (such as mud). This provides us with a naturally occurring spirit level allowing us to tell up from down, and indeed the plane of horizontality, at the time when the sediment was deposited.

Bubbles in igneous rock

When igneous rock is formed, bubbles of trapped volcanic gas will, of course, rise through the still-molten lava because of their lower density; for this reason, if we find bubbles in a solidified lava flow, they will tend to be at the top rather than the bottom.

Structures in lava flows

While the base of a lava flow will conform itself to the ground over which it flows, the top of the flow will usually not be flat, but may take on a number of distinctive forms: pillows if it was formed underwater, and aa or pahoehoe if it forms on land (the unusual terms are Hawaiian in origin).

The photograph below shows the distinctive "ropy" structure characteristic of the surface of pahoehoe: clearly this rock is the right way up.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Message 168 of 293 (674356)
09-28-2012 3:15 AM


Fossils
Fossils

Introduction

In this article we shall discuss the processes by which fossils are formed, and the circumstances under which this occurs.

Formation of fossils

In the process of replacement, the original material of bone, shell, or other tissue is replaced by minerals from the surrounding sediment. In permineralization minerals (typically silica) fill in the spaces and voids within a fossil, including the interior of cells. Permineralization and replacement often go hand in hand (in which case the organism is said to have undergone petrification) but it is possible to have one without the other.

A fossil mold is produced when sediment is packed around a organic remains which are then destroyed by decay, leaving a void in the shape of the organism; a cast is produced from a mold when minerals fine enough to percolate through the gaps in the sediment then fill in the void left where the organism used to be.

There are also unaltered fossils; although in popular usage to say that something is a fossil is often to imply that it has been mineralized, in scientific usage "fossil" can refer to any remains which are prehistoric: that is, which are older than any written human history. So, for example, people speak of "fossil mammoths" even though their bones are unaltered by mineralization.

Small organisms such as insects and spiders can become trapped in the sticky resin of trees, which hardens to become amber; the magnified image below of a tiny gnat trapped in amber shows the exquisite level of preservation that this process achieves.

Finally, we may note the existence of trace fossils such as footprints or worm casts: these are preserved like other sedimentary structures by the lithification of the sediment in which they are formed.

Conditions for fossilization

Following the death of an organism, several forces contribute to the dissolution of its remains. Decay and predators and/or scavengers will typically rapidly remove the flesh; the hard parts, if they are separable at all (i.e. if the organism doesn't just have one big hard part such as the shell of an ammonite) can be dispersed by predators, scavengers or currents; the individual hard parts are subject to chemical weathering and erosion, as well as to splintering by predators and/or scavengers, which will crunch up bones for marrow and shells to extract the flesh inside. Also, an animal swallowed whole by a predator, such as a mouse swallowed by a snake, will have not just its flesh but some and perhaps all its bones destroyed by the gastric juices of the predator.

It would not be an exaggeration to say that the typical vertebrate fossil consists of a single bone, or tooth, or fish scale. The preservation of an intact skeleton with the bones in the relative positions they had in life requires a remarkable and fortuitous circumstance: burial in volcanic ash; burial in aeolian sand due to the sudden slumping of a sand-dune; burial in a mudslide; burial by a turbidity current; and so forth.

The mineralization of soft parts is even less common, and is seen only in exceptionally rare chemical and/or biological conditions. How rare? Well, consider the story of the condont animals.

From 1856 onwards paleontologists noticed that the fossils record from the Cambrian period right through to the end of the Triassic was littered with microfossils known as condont structures. They guessed, correctly as it happens, that these were the scattered teeth of some unknown type of otherwise soft-bodied organism. The photomicrograph below shows some conodont structures.

In 1934 Schmidt and Scott discovered conodont structures grouped together on the same bedding plane, arranged in symmetric pairs; that is, in this case the conodont structures had presumably not been scattered but lay in the relative positions they would have had in life. But since the soft tissues were not preserved the nature of the condont animals remained a mystery.

Not until 1983 did paleontologists discover a specimen in which the soft tissues of a conodont animal had been preserved, when they finally found a Lagerstätte --- one hundred and twenty-seven years after the very abundant conodont structures had first been described.

Mineralized fossils: how do we know?

We take it for granted today that mold, cast and mineralized fossils are the relics of organic life. It may surprise the reader to learn that this was once a minority view, verging literally on heresy. Instead, it was widely believed that they were not: the most common view being that the fossils grew in the rocks as the result of a mysterious force known as vis plastica.

This explanation fitted nicely with the religious views of the time. Many fossils, if interpreted as the relics of once-living organisms, would have to represent species that had gone extinct, since no-one could find their modern equivalents. Now theologians argued that God, being perfect, would not have made any species so badly that it would go extinct; dissenting scientists such as Robert Hooke were obliged to guard themselves carefully against accusations of impiety.

It is then at least possible to suppose that mineralized fossils are not in fact mineralized remains of organisms. How would we argue against someone who was inclined to doubt it?

We might point out the existence of partially mineralized fossils: in fact, this was pointed out to supporters of vis plastica, who replied that the sequence was in fact the other way round, from rock to mineralized fossil to unmineralized fossil.

We might point out, as was pointed out at the time, the similarity between some fossils and living organisms. It would be remarkable if a process occurring in the rocks should produce just the same sorts of forms as are also produced by organic processes, giving rise to things that look exactly like sharks' teeth or sea-urchins.

We might also look at the consistency within rock formations. For example, one sort of sandstone will contain terrestrial plants and animals and the large cross-beds found in aeolian sand; another will contain seashells and the same sort of symmetrical ripples found on a beach. On the basis of the mineralization theory, this is explicable and indeed expected: but how strange it would be if the vis plastica somehow managed to bring forth just those tableaux that would look like (but not be) the relics of former ecosystems.

From our perspective in the twenty-first century we might appeal to actualism. We know the composition of rocks down to the very arrangement of atoms in their constituent minerals, and there seems to be no room for a mechanism for vis plastica. However, there is a mechanism for mineralization: the process of mineralization only requires the sediment and organism to become more chemically homogeneous, which is much more chemically plausible than the reverse.

This seems a fair reply to the proponents of vis plastica. Another view proposed at about the same time was that the fossils were created by God when he created the Earth. This is certainly conceivable (an omnipotent God can do what he wants) but it might be answered in a similar way. From an actualistic view, we would point out that this invokes a miracle where none is apparently necessary, violating the scientific method. And from consideration of the nature of the fossil record we would have to say that in that case God has gone to extraordinary lengths to deceive us by making it look exactly like we are looking at the lithified relics of times gone past; we may imagine this, but it is grossly inconsistent with the traditional view of God, which supposes honesty to be among his virtues.

Since the hypotheses of initial creation or of vis plastica as the origin of fossils are, so far as I know, currently held by no-one whatsoever, the foregoing discussion may seem somewhat in the nature of a needless digression. However, it does emphasize the point, which I feel is worth making, that any statement in a geology textbook, no matter how much we take it for granted today, had to be discovered by someone; evidence had to be produced, and arguments had to be made --- often against determined and dogmatic opposition.

So if we now take it for granted that mold, cast, and mineralized fossils are what we think they are, then this is not an unfounded assumption: we can afford to take it for granted because the case has been so well-made that perhaps no textbook except this one takes the trouble to review the question.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Message 169 of 293 (674613)
10-01-2012 12:54 AM


The Principle Of Faunal Succession
The principle of faunal succession

Introduction

In this article we shall introduce the principle of faunal succession, and discuss how we know it is valid and why it should be so. In subsequent articles we shall discuss further how it can be applied to stratigraphy.

The principle of faunal succession

By using the principle of superposition and by using way up structures to determine the up and down directions at the time of deposition, we can find the order of deposition in an assemblage of strata. This means of course that we can also find the order of deposition of the fossils within the strata.

Now, looking at some particular assemblage of strata, we may find that the deposition of species A stopped before the deposition of species B started. (We can write this for short with the notation A < B.)

Then the principle of faunal succession says that if we look at a different location and find species A and B, then we will also find that A < B. This is not to say that we will always find A in the same assemblage of strata as B, or vice versa, but it says that if we do, then we will find that A was deposited before B; that the order is the same in different locations. While, as discussed later in this article, exceptions to this are certainly possible (which is why we do not call the principle a law) the principle is very generally applicable, and so it does deserve to be called a principle.

Note that the relation < is transitive: that is to say if A < B and B < C, then we will also find that A < C. (Again, this is not to say that we always will find A and C in the same location, it just tells us what order they will be in if we do.) This allows us to establish a linear order on the fossil record, as we shall discuss further in our article on the geological column.

How do we know, and why is it so?

How do we know? Because we looked. The principle is a simple one which tells us directly what we should see if we look, so it is easy to verify that it is generally valid.

The question of why it is so is more interesting. It is also in a sense irrelevant, since it is possible to verify the principle and apply it without having the faintest idea why it should be true; and in fact this is what geologists did when the principle was first discovered.

However, we do now understand the reason why the principle holds good; it is an elementary consequence of the theory of evolution. It would take a textbook equal in length to this one to explain the theory of evolution and to sketch out the nature of the evidence for the fact of evolution. However, for the purposes of understanding the principle of faunal succession, the reader really needs to understand only one corollary of the theory: that any particular species will only evolve once.

So it is not possible for species A to evolve before species B in North America and for species B to evolve before species A in South America; each species must arise at one time in one place.

Nor is it possible for species A to arise and go extinct, for species B to arise and go extinct, and then for species A to arise a second time; again, species A can only evolve once, and extinction is final.

(Those readers, if any, who do not believe in evolution will just have to look on the validity of the principle of faunal succession as one of life's little mysteries, or to be more accurate as one of life's huge, gigantic, preposterously enormous mysteries; but the practical validity of the principle is beyond question.)

Now, the underlying mechanism of the principle does allow for the principle to be violated in particular instances. It would for example be possible to have A < B in one location and B < A in another, through some such scenario such as the following.

* Species A arises at location X.
* Species A spreads to location Y.
* Species A goes extinct at location X.
* Species B arises at location X.
* Species B spreads to location Z.
* Species B goes extinct at location Z.
* Species A spreads to location Z.

It would then be the case that at location X we have A < B, but at location Z we have B < A. This sort of elaborate dance is quite unlikely to happen, and if it ever does, it can't happen very often, since the principle of faunal succession does in fact generally hold good.

In summary, the principle of faunal succession should work in theory and does work in practice.

Note on vocabulary

The principle of faunal succession does not just apply to fauna (i.e. animals), so strictly speaking the principle should be called: "The principle of faunal, floral, fungal, and everything-else-al succession", but that would be inconvenient; and in any case, the name of the principle is so well established that it is too late to do anything about it now.


  
Dr Adequate
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Message 170 of 293 (674764)
10-02-2012 5:16 PM


Index Fossils
Index fossils

Introduction

In this article we shall discuss what an index fossil is, why they are useful, and what qualifies a fossil to be an index fossil.

Why do we need index fossils?

Consider the fauna living on the continental shelves of the east and west Atlantic ocean (the littoral fauna). Obviously these faunae will be different --- they have had 130 million years to follow separate evolutionary trajectories, and since they require shallow water they aren't going to cross from one side of the ocean to the other.

So when we look at the successions in the fossil record of the littoral faunae on each side of the Atlantic, we will see two faunal successions, an eastern and a western. What's more, the sediments in which they are deposited will not particularly correlate, since they will have different points of origin.

However, mixed in with the littoral fauna there will also be fossils of free-floating or free-swimming surface organisms: pelagic fauna. Now such species will spread throughout the ocean, since there's nothing to stop them from doing so.

So we can and do find the same pelagic species on both sides of the Atlantic. We can use such species to correlate the other littoral species and the sediments in which they are deposited. For if we find a pelagic species (call it S) somewhere in the fossil record on both coasts, then this gives us a way to divide up both successions into species that were deposited before S, along with S, or after S.

Given enough such species, we can find many such correlations, and this would allow us to represent the eastern and western deposition on the same timeline.

Such species would be examples of index species, and their fossils would be examples of index fossils, which we may define generally as follows: an index fossil is a fossil of a species that was sufficiently widely distributed that its fossils can be used to correlate the deposition of fossils and sediments in widely separated locations.

What makes a good index species?

Obviously, the first requirement for an index species is that it should indeed be widely distributed. Pollen makes a good index fossil, being wind-borne; so do foraminifera, which are pelagic species as in our example.

An index species should also be readily preserved in the fossil record. Birds, for example, would make bad index fossils, because although there are many species which have wide (indeed intercontinental) ranges, they fossilize very poorly: their skeletons come apart easily, and then their delicate honeycombed bones are highly susceptible to decay. Typically, only fragments will survive, and even if these were common, which they are not, it would still be hard to tell one species from another by studying them.

Finally, we would ideally like an index fossil to have a short time of deposition as a proportion of the fossil record, since we want to use it to identify a particular chapter in the history of deposition: it should represent a geographically broad but temporally narrow slice of the record.

Some examples of marine index fossils are shown in the table below. Note that they are not claimed to be an evolutionary sequence, any more than a list of Presidents is: they are a temporal sequence, and should be understood as such.


  
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Message 171 of 293 (674991)
10-04-2012 8:01 PM


The Geological Column
The geological column

Introduction

In this article, we shall explain what the geological column is, how it is constructed, and what relationship it bears to the geological record. We shall also provide a rough description of the geological column summarizing some of the major trends observed in it.

Construction of the geological column

By using the principles explained in the previous articles (superposition, faunal succession, the use of index fossils) it is possible to produce an account of the order of deposition of the organisms found in the fossil record, noting that one was deposited before the other, that the deposition of this group starts after the deposition of that group ceases, and so forth.

This means that fossils and the sedimentary rocks that contain them can be placed in order of their deposition. The resulting table is known as the geological column.

Prolog to a sketch of the geological column

Below, I sketch out the major geological systems from the Vendian onwards. Note that it is written from the bottom upwards, so that the earliest-deposed fossils are at the bottom; the reader may therefore find it tells a more coherent story if read from the bottom upwards.

It is no more than a sketch: it records the appearance and disappearance of major groups, rather than individual species; and it has been divided into the large stratigraphical units known as systems, which geologists would divide into series, which they would further divide into stages, which they would then further subdivide into zones. I am, then, only giving the broadest outline of the geological column; those who require the finer details must look elsewhere.

I have not attached any dates to the geological systems discussed here, because as we have not yet reached our discussion of absolute dating, it would be premature to do so. All that our study of fossils and their faunal succession tells us is the order of deposition. (It is for this reason that I have used the increasingly obsolete term "geological column" rather than "geological timeline"; it is not a timeline until we get round to attaching dates to it.)

I have also avoided using terms such as "evolution" and "extinction". From a biological standpoint, it is obvious that these are the underlying cause of the patterns in the fossil record; but as with the evolutionary explanation of the principle of faunal succession this biological explanation is irrelevant to the practice of geology. For the purposes of doing stratigraphy it doesn't really matter if dinosaurs appear in the geological column because they evolved from more basal archosaurs or because they parachuted out of the sky from an alien spaceship, and it doesn't matter if they disappear from the geological column because they went extinct or because they all went to live in cities on the Moon; what matters is that we can find out where their fossils come in the sequence of deposition.

A sketch of the geological column

Quaternary

Marked by the existence and spread of modern humans and the decline and disappearance of many groups of large fauna extant in the Neogene.

Neogene

Contains recognizable horses, canids, beaver, deer, and other modern mammal groups. The Neogene also contains many large mammalian fauna no longer extant: glyptodonts, ground sloths, saber-toothed tigers, chalicotheres, etc. First hominds found in Africa.

Paleogene

Marked by the diversification of mammals and birds. Among the mammals we see the first that can be easily identified with modern mammalian orders: primates, bats, whales, et cetera. Similarly representatives of many modern bird types are identifiable in the Paleogene, including pigeons, hawks, owls, ducks, etc. Now-extinct groups of birds found in the Paleogene include the giant carnivorous birds known colloquially as "terror birds".

Cretaceous

Here we see the diversification of angiosperms (flowering plants) from beginnings around the Jurassic-Cretaceous boundary; representatives of modern groups of trees such as plane trees, fig trees, and magnolias can be identified in the Cretaceous. Here also we see the first bees, ants, termites, grasshoppers, lepidopterans. Dinosaurs reach their maximum diversity; some of the best known dinosaurs such as Triceratops and Tyrannosaurus are found in the Cretaceous. Mosasaurs appear near the end of the Cretaceous, only to disappear at the Cretaceous-Paleogene boundary, which also sees the last of the dinosaurs (excluding birds, which biologists classify as dinosaurs) and the last pterosaurs, plesiosaurs, ichthyosaurs, ammonites, rudists, and a host of other groups.

Jurassic

This system is notable for the diversification of dinosaurs. It has the first short-necked plesiosaurs (pliosaurs); first birds; first rudists and belemnites. Mammals are certainly present, but tend to be small and insignificant by comparison with reptile groups. The first placental mammals are known from the Upper Jurassic.

Triassic

The Triassic contains the first crocodiles, pterosaurs, dinosaurs, lizards, frogs, snakes, plesiosaurs, ichthyosaurs, and primitive turtles. Whether or not there were mammals in the Upper Triassic depends on what exactly one classifies as a mammal. The Triassic-Jurassic boundary sees the loss of many groups, including the last of the conodonts, most of the large amphibians, and all the marine reptiles except plesiosaurs and ichthyosaurs.

Permian

This system is noted for the diversification of reptiles: the first therapsids (mammal-like reptiles) and the first archosaurs (the group including crocodiles and dinosaurs). It also has the first metamorphic insects, including the first beetles. It has the first trees identifiable with modern groups: conifers, ginkgos and cycads. Many species and larger groups come to an end at or shortly before the Permian-Triassic boundary, including blastoids, trilobites, eurpterids, hederellids, and acanthodian fish.

Carboniferous

This system contains the first winged insects. Amphibious vertebrates diversify and specialize. The Carboniferous has the first reptiles, including, in the Upper Carboniferous, the first sauropsid, diapsid, and synapsid reptiles. Foraminefera become common. All modern classes of fungi are present by the Upper Carboniferous.

Devonian

The Devonian has the first (wingless) insects; the first ammonites; the first ray-finned and lobe-finned fish; the first amphibious vertebrates; the first forests. Terrestrial fungi become common. The first seed-bearing plants appear in the Upper Devonian. The last placoderms are found at the Devonian-Carboniferous boundary. Almost all groups of trilobite have disappeared by the Devonian-Carboniferous boundary, but one group (Proetida) survives until the Permian-Triassic boundary.

Silurian

In the Silurian, coral reefs are widespread; fish with jaws are common; it has the first freshwater fish; first placoderms (armour-plated fish); the first hederellids; the first known leeches. Diversification of land plants is seen.

Ordovician

In the Ordovician system we see the first primitive vascular plants on land; jawless fishes; some fragmentary evidence of early jawed fishes. Graptolites are common, and the first planctonic graptolites appear. Bivalves become common. The first corals appear. Nautiloids diversify and become the top marine predators. Trilobites diversify in form and habitat. The first eurypterids ("sea scorpions" appear in the Upper Ordovician. Trilobite forms such as Trinucleoidea and Agnostoidea disappear at the Ordovician-Silurian boundary, as do many groups of graptolites.

Cambrian

This system sees the first animals with hard parts (shells, armor, teeth, etc). Trace fossils reveal the origin of the first burrowing animals. Trilobites are common; chordates exist but are primitive. Archaeocyathids are common reef-forming organisms in the Lower Cambrian and then almost completely vanish by the Middle Cambrian. Condonts are first found in the Upper Cambrian. Many groups of nautiloids and trilobites disappear at the top of the Cambrian, but some groups survive to diversify again in the Ordovician.

Vendian

This system contains the first complex life, including sponges, cnidarians, and bilaterians.

The geological column and the geological record

We should distinguish between the geological record and the geological column. The geological record is a thing: it is the actual rocks. The geological column is not a thing, it is a table of the sort given above. To ask questions such as "where can I go to see the geological column?" or "how thick is it?" is therefore a category error along the lines of asking how many people can be seated around the Periodic Table.

The relationship between the geological column and the geological record is this: when we look at a series of strata in the geological record and use the principle of superposition and way-up structures to discover the order deposition of the fossils in it, then if we find that A < B in the strata, this will correspond to B being shown above A in the geological column. The geological column is therefore a particularly simple and neat way of recording what relationships we do and don't find in the geological record.

However, the geological column is not a picture of what we find in the geological record. There are three reasons for this.

First, as we know, the geological record is folded and faulted in some places. Recall that when we write A < B we are talking about the original order of deposition of fossils, as reconstructed by using the principle of superposition and way-up structures: it does not necessarily mean that A is actually below B; whereas the geological column is always depicted as a vertical column with A below B when A < B.

Second, by using index fossils geologists produce a single time-line for the entire planet; but clearly any particular location will only have local fossils: the column, if written out in full, would show exclusively South American Cretaceous dinosaurs above exclusively North American Jurassic dinosaurs, but these will not in fact be found in the same assemblage of strata.

The third reason is that deposition will typically not happen continuously in one place: sediment is deposited in low-lying areas; it would not be deposited on top of a mountain. What's more, an elevated area will typically undergo erosion: not only will fresh sediment not be deposited, but existing sedimentary rocks and their fossils will be destroyed. Also, marine sediment will be destroyed by subduction, so the sediment of the oceanic crust will be no older than the ocean that it's in, and even then only at the edges --- it will be considerably younger near the mid-ocean rifts.

Consequently, the meaning of the geological column is not that any location in the geological record will look like the geological column: the column is merely an elegant way of representing the facts about faunal succession.

The geological column: how do we know?

As we explained at the start of this article, the geological column is constructed using ideas introduced in previous articles: the principle of superposition, the principle of faunal succession, and the use of index fossils.

Note that the geological column does not represent a scientific theory. It does resemble one, because there is a weak sense in which it suggests what we are likely to observe, which is the role of a theory; but it is essentially descriptive in nature. That is, it does not really predict the sequence of fossils that we will find, it is determined by, and summarizes, the sequences of fossils that we have found. Since it is likely that what we will find tomorrow will be similar to what we have been finding for the past couple of hundred years, the column is in that sense predictive, but its predictive power goes no further than that.

So if tomorrow we found that some trilobites were deposited above the Permian system, we should simply amend the geological column to reflect this, and it would be surprising not because it contradicted the geological column as such, but because in centuries of paleontology no-one has yet made such a discovery.

Compare this with how we would feel if we consistently found violations of the principle of faunal succession. This would present a difficulty in theory, and would require us to give up on the principle of faunal succession (and to give up on using it to construct a geological column, something that would then become impossible). But finding something that contradicts the geological column as it stands is merely unlikely in practice, not in theory, and would only require us to revise the geological column in one particular detail (i.e. to take the new discovery into account) without requiring us to rethink any fundamental ideas.

So the geological column is trustworthy simply because it is no less, but no more, than an up-to-date summary of our knowledge, and so it can be taken as such. To which we might add that after all these years of looking at the fossil record it is extremely unlikely that we'll find anything so unusual as to require any major revision of the column.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
Dr Adequate
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Posts: 16107
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(2)
Message 172 of 293 (675135)
10-06-2012 2:16 PM


Unconformities
Unconformities

Introduction

In this article we shall look at the various types of unconformity in the geological record, and discuss why they exist and how they can be recognized.

What are unconformities, and why do they exist?

As we mentioned in the previous article, we do not expect the fossil sequence to be complete in any given location, because there will be times when no sediment is deposited in that location, and/or times when it is destroyed by erosion after deposition.

Such an episode will show up in the geological record as an unconformity: a surface between successive strata representing a period of erosion or of no deposition. These come in several varieties, listed below.

What do unconformities look like?

In an angular unconformity the underlying beds meet the overlying beds at an angle. The diagram below shows the stages of the process.

First, sediment is laid down. Then tectonic events destroy its original horizontality, in this case by creating folds. Then an erosional surface is produced, truncating the beds. Then more sediment is laid down in horizontal beds on top of the erosional surface. In the example in the diagram, this has produced two sets of angular unconformities, one at each side of the diagram.

In a disconformity, the same thing happens, except that the underlying beds are not distorted by tectonic events, so that the first and second collection of beds lie parallel to one another rather than meeting at an angle, but they are still separated by an erosional surface.

A nonconformity is like an unconformity, except that the underlying rock is igneous or metamorphic rather than sedimentary. The presence of an erosional surface indicates a time when this basement rock was exposed to weathering and erosion, and so cannot have been protected by a blanket of sediment.

Finally, consider what would happen in a location where sediment is deposited, then it ceases, and then erosion does not take place, and then deposition starts again. This would produce what is known as a paraconformity. What would that look like? There would be no meeting of beds at an angle, there would be no erosional surface, there would just be beds of sediment lying on top of more beds of sediment.

Except that if the period of non-deposition lasted for any significant amount of time we would see a sudden jump in the faunal succession: where the geological column shows fossils in order (for example) P, Q, R, S, T, U, then at the location of the paraconformity we would see P, Q, T, U, where P and Q correspond to the first episode of deposition, T and U correspond to the second episode of deposition, and the missing fossils R and S correspond to the time at which no deposition was taking place. Note that if this jump forward in the faunal succession was just because there was no fauna around to be deposited, then we would see non-fossil-bearing sediment between Q and T; but instead we see nothing at all.

Obviously we will see a similar jump in the faunal succession in the case of angular unconformities and disconformities, but in the case of a paraconformity this is all that there is to see. This makes the identification of paraconformities less blatantly obvious than the identification of other unconformities, but since they look just like what we should see in locations where deposition stopped for a while and then restarted, it is reasonable to conclude that that's what they are.

Note that this only serves to identify sufficiently long periods of non-deposition. In cases in which the interruption of deposition only lasted for a small period of time then all we would see would at most be a bedding plane, and perhaps not even that.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


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petrophysics1
Inactive Member


(1)
Message 173 of 293 (675137)
10-06-2012 5:01 PM
Reply to: Message 172 by Dr Adequate
10-06-2012 2:16 PM


Re: Unconformities
Why don't you go and rewrite this. Begin by explaining and defining AN UNCONFORMITY. Then everyone will see that an angular unconformity, a disconformity, a nonconformity, and a paraconformity are all types of an unconformity.

Nonconformity is rarely used outside of a classroom and the British do not recognize "disconformity" or "paraconformity".

By what you've said, about any bedding boundary can be called a paraconformity (basically a useless word, and what are the chances of NOTHING happening, no deposition, no erosion, no rain, no wind, no plants growing, no creature walking across it, shiting or pissing on it or any one of a thousand things which would leave a mark).

BTW you can not see a jump in fauna across a noncomformity.....because there aren't any fossils in igneous or metamorphic rocks.

While you're at it fix this too.

Dr.A writes:

The picture below shows large-scale cross-bedding in sandstone. (To give you an idea of the scale, the small patches of green visible in the top right corner are not moss, as you might guess, but bushes.)


The scale of the cross-bedding depends on the process that produced it: this was produced by wind; smaller-scale cross-bedding would be produced by the formation of tidal ripples.

This was not deposited by wind.

Edited by petrophysics1, : add ,

Edited by petrophysics1, : remove nd

Edited by petrophysics1, : No reason given.


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Dr Adequate
Member
Posts: 16107
Joined: 07-20-2006
Member Rating: 8.3


Message 174 of 293 (675143)
10-06-2012 6:44 PM
Reply to: Message 173 by petrophysics1
10-06-2012 5:01 PM


Re: Unconformities
Thanks for your feedback.

Why don't you go and rewrite this. Begin by explaining and defining AN UNCONFORMITY. Then everyone will see that an angular unconformity, a disconformity, a nonconformity, and a paraconformity are all types of an unconformity.

Ithought the article made it clear. What would you recommend? "A surface separating rocks of different ages", something like that?

Nonconformity is rarely used outside of a classroom and the British do not recognize "disconformity" or "paraconformity".

Oh, some of them do. One of the textbooks I own (Doyle, Bennett and Baxter) is British and does use those terms.

By what you've said, about any bedding boundary can be called a paraconformity ...

Well, yes. I was wondering whether to elaborate on that and discuss the difference between a big paraconformity and a mere brief diastem.

(basically a useless word, and what are the chances of NOTHING happening, no deposition, no erosion, no rain, no wind, no plants growing, no creature walking across it, shiting or pissing on it or any one of a thousand things which would leave a mark).

Well, that depends on whether it's subaerial or not.

It's not like I just invented the concept, there seem to be plenty of geologists who think that there are paraconformities in the geological record.

BTW you can not see a jump in fauna across a noncomformity.....because there aren't any fossils in igneous or metamorphic rocks.

Ah, very true.

This was not deposited by wind.

Oops, thank you. I bow before the master. How did you know?

---

ETA: I've fixed it up a bit, let me know what you think. Thanks.

Edited by Dr Adequate, : No reason given.


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Dr Adequate
Member
Posts: 16107
Joined: 07-20-2006
Member Rating: 8.3


Message 175 of 293 (675215)
10-08-2012 2:23 PM
Reply to: Message 173 by petrophysics1
10-06-2012 5:01 PM


Petrophysics
Please get back to this. I am always eager for critical feedback.

---

But also I am really really interested to know how you know that that wasn't a photograph of aeolian sand. I just thought that it was because the sets of the crossbeds were so darn big. You apparently know better just by looking at a fairly small-scale photo. How? If you can explain how we know the difference, I am extremely eager to add it to the articles. If you haven't noticed, my recurrent theme in these articles is explaining "how do we know?" Now it seems that you know something that I don't know, and I don't know how you know it. So please ...

* gets on all fours and grovels *

... what do you see there that tells you for certain that that isn't aeolian?

---

And I would also welcome feedback in general. As they said in Latin 2000 years ago, non omnia possumus omnes --- that is, "we can't all be good at everything". I do not claim to have intensive knowledge in all or indeed any of the subjects that I am discussing in brief. I hope that people such as yourself will set me straight if I commit an error.

Edited by Dr Adequate, : No reason given.


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Dr Adequate
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(1)
Message 176 of 293 (675278)
10-09-2012 4:52 PM


Walther's Principle
Walther's principle

Introduction

In this article we shall explain what Walther's principle is and why it works.

The principle

Walther's principle can be stated as follows: If sediment A is succeeded vertically by sediment B without an unconformity between them, then sediment A will also be succeeded horizontally by sediment B in some direction.

The somewhat abstract statement of the principle will be clarified by a few of examples.

Examples

Walther himself was led to formulate his principle by looking at fluvial rocks. As a meandering river shifts its course, the riverbed (consisting, for example, of gravel) is overlaid by the sand of a point bar and then the mud of a flood plain. But this is exactly the same sequence as one would see moving laterally outward from the center of the river out to the bank: gravel, then sand, then mud. It is this lateral succession of sediments, plus the fact that the river shifts, which causes the vertical succession in a fining-up sequence, so naturally they are going to be the same.

Similar effects are caused by shifts in sea level. Consider the various depositional environment and their associated types of sediments (facies) that we would see on the sea floor as we move further from the shore and out to sea: a typical progression would be sand grading into mud which grades into calcareous ooze.

Now consider what happens if the sea level rises and so the shoreline moves inland (a marine transgression). The sandy facies would move landwards; so would the muddy facies, and similarly with the calcareous ooze. Now this means that in some locations we will see a vertical succession of sand giving way to mud; and further seaward a succession of mud giving way to limestone.

In a marine regression, when the sea level falls, the facies would of course move in the opposite direction, reversing the vertical succession.

The reader should note that this is why finding a physically continuous rock formation (of sandstone, for example) does not indicate that it was all laid down at the same time. The seaward end of it could have been laid down at the beginning of a marine transgression, and the landward end of it at the end of the transgression. So long as the transgression was sufficiently gradual, this would produce a continuous layer of sandstone (perhaps sloping slightly upwards from the seaward to the landward direction) and yet one end of it would have been laid down at a different time from the other. Such a transgression can be detected in the geological record because although the sandstone will remain sandstone throughout its length, the fossils at the landward end will be younger than those at the seaward end.

As a final example, consider the deposition of marine sediment on a moving oceanic plate. A particular section of the plate moves (let us say north, for example) from a location where pelagic clay is deposited to a location where calcareous ooze is deposited. Then it follows that when mud was being deposited on that part of the plate, calcareous ooze was being deposited to the north of it, and when calcareous ooze is being deposited on it, pelagic clay will be deposited to the south of it. The horizontal succession from south to north is pelagic clay followed by calcareous ooze, which is just the same as the vertical succession from bottom to top.

Walther's principle: how do we know?

The principle is a simple one: all it does is tell us what we should see if we look; and we do in fact see what the principle tells us we ought to see.

What's more this is what we ought to see in theory, because as you can see in our examples it is the horizontal succession of sediments that causes the vertical succession of sediments, so naturally the two successions are going to be the same.

The principle can have exceptions, but they will necessarily be rare. If there is sudden shift in the mode of deposition (such as a tsunami rapidly laying down a bed of sediment known as a tsunamite) then because of the underlying mechanism the succession of sediment will not obey Walther's principle. However, such an event will tend to be so violent in its nature as to be erosional and to produce an unconformity; and so it would not constitute a counterexample to Walther's principle, which by definition only applies to beds of sediment without an unconformity between them.

It is usually the case that a change in the mode of deposition at a given place will correspond to a gradual geographical shift in the places in which different types of sediment are deposited (or, as in our final example above, a shift of the geography itself beneath the different areas of deposition). For this reason Walther's principle will be generally if perhaps not universally applicable.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


  
petrophysics1
Inactive Member


(1)
Message 177 of 293 (675280)
10-09-2012 6:05 PM
Reply to: Message 175 by Dr Adequate
10-08-2012 2:23 PM


Re: Petrophysics
Dr.A. writes:

Except that if the period of non-deposition lasted for any significant amount of time we would see a sudden jump in the faunal succession: where the geological column shows fossils in order (for example) P, Q, R, S, T, U, then at the location of the nonconformity we would see P, Q, T, U, where P and Q correspond to the first episode of deposition, T and U correspond to the second episode of deposition, and the missing fossils R and S correspond to the time at which no deposition was taking place.

Get rid of "nonconformity" in this sentence. It makes no sense.

Paraconformity= "A term introduced by Dunbar & Rodgers(1957, p.119)* for an obscure or uncertain uncomformity in which no erosion surface is discernable or in which the contact is a simple bedding plane, and in which the beds above and below are parallel"(Glossary of Geology 4th ed, p.464, Julia A. Jackson).

In your example above you have a break in the fauna. Now I don't consider that to be either "obscure" or "uncertain", so it doesn't match the definition. I'd call it an unconformity.

HOWEVER, do you FEEL it's obscure or do you FEEL uncertain about it? If you do and also think that the geologists feelings have a place in geologic descriptions by all means call it a paraconformity.

Most (but not all) unconformities are angular unconformities. If you call something a disconformity make sure you have measured the strike and dip of the beds above and below and that they are parallel. Don't do this by eyeball, if you don't, just call it an unconformity since otherwise you are claiming something you actually don't know.

Your example of a disconformity is poor. If that structure is an anticline the beds above and below are not parallel anywhere. If it's a fold there is a line along the crest which can be said to be parallel to the beds above. I've never seen anyone do that. I suggest a drawing with parallel beds and an erosion surface or a paleosol. Something simple and to the point.

Unconformity= A substantantial break or gap in the geologic record where a rock unit is overlain by another that is not next in stratagraphic succession, such as an interruption in the continuity of a depositional sequence of sedimentary rocks or a break between eroded igneous rocks and younger sedmentary strata. It results from a change that caused deposition to cease for a considerably span of time, and it normally implies uplift and erosion with loss of the previously formed record. An unconformity is of longer duration than a diastem. ..........Local, contemporaneous erosion and deposition associated with geological processes such as point-bar development or aeolian dune migration are excluded from the definition of unconformity. (same ref. as above, p.689)

I'll be back in a bit to talk about that picture.

*Dunbar, C.O., and Rodgers, John(1957) Principles of Stratigraphy. New York: Wiley.

Edited by petrophysics1, : typos & insert spaces

Edited by petrophysics1, : Waste time, adding a ref.

Edited by petrophysics1, : typo

Edited by petrophysics1, : insert spaces

Edited by petrophysics1, : No reason given.

Edited by petrophysics1, : No reason given.


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Dr Adequate
Member
Posts: 16107
Joined: 07-20-2006
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Message 178 of 293 (675286)
10-09-2012 9:00 PM
Reply to: Message 177 by petrophysics1
10-09-2012 6:05 PM


Re: Petrophysics
Get rid of "nonconformity" in this sentence. It makes no sense.

You're quite right. It should have been "paraconformity". Thanks. The fact is that I'm a bit verbally dyslexic, if that makes any sense. When it comes to words which sound the same and have related meanings such as "convex" and "concave" I get all confused and have to start counting on my fingers.

In your example above you have a break in the fauna. Now I don't consider that to be either "obscure" or "uncertain", so it doesn't match the definition. I'd call it an unconformity.

HOWEVER, do you FEEL it's obscure or do you FEEL uncertain about it? If you do and also think that the geologists feelings have a place in geologic descriptions by all means call it a paraconformity.

Well, so far as I can find out paraconformities exist. So I have to talk about them. Now, you say that you'd call it an "unconformity". But this is a generic term. I want a word for something where there is a period of non-deposition, but there isn't an erosional surface. That would be "paraconformity".

Your example of a disconformity is poor. If that structure is an anticline the beds above and below are not parallel anywhere. If it's a fold there is a line along the crest which can be said to be parallel to the beds above. I've never seen anyone do that. I suggest a drawing with parallel beds and an erosion surface or a paleosol. Something simple and to the point.

Arguably it is a poor example. Also arguably it's a very good one. Having thought about it, I'll delete that bit. I'm still right, though --- there are angular unconformities at either side of the diagram, and a disconformity in the middle.

I'll be back in a bit to talk about that picture.

Thank you.

Edited by Dr Adequate, : No reason given.


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Pressie
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Posts: 2082
From: Pretoria, SA
Joined: 06-18-2010


Message 179 of 293 (675291)
10-10-2012 12:40 AM
Reply to: Message 177 by petrophysics1
10-09-2012 6:05 PM


Re: Petrophysics
quote:
Paraconformity= "A term introduced by Dunbar & Rodgers(1957, p.119)* for an obscure or uncertain uncomformity in which no erosion surface is discernable or in which the contact is a simple bedding plane, and in which the beds above and below are parallel"(Glossary of Geology 4th ed, p.464, Julia A. Jackson).

In your example above you have a break in the fauna. Now I don't consider that to be either "obscure" or "uncertain", so it doesn't match the definition. I'd call it an unconformity.


No, you have to realise that Dr Adequate tried to clearly explain to the readers what a paraconformity is, he used an extreme example, but gets the message across to as many readers as possible.

quote:
HOWEVER, do you FEEL it's obscure or do you FEEL uncertain about it? If you do and also think that the geologists feelings have a place in geologic descriptions by all means call it a paraconformity.
I completely disagree with this. It's not a "feeling" at all. It's following the evidence.

As a person who does a lot of core-logging, I often get contacts where the unconformity surface resembles a simple bedding plane; the strata are parallel, their's no apparant erosion, etc.

I call it a paraconformity in my logs, although I'm unsure of whether it really is one. That core has a very, very small diameter of just a few millimeters, remember, but I follow the evidence....


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roxrkool
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Posts: 1495
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Message 180 of 293 (675386)
10-10-2012 8:58 PM
Reply to: Message 179 by Pressie
10-10-2012 12:40 AM


Re: Petrophysics
Geology can be pretty "touchy feely," which is what drives the engineers crazy. Occasionally, there just isn't enough evidence to be sure, as you very well know, so we have to go with our gut. The longer you've been a geologist and the more rocks you've seen, the better your chances of being closer to the truth.

I'm modeling an ore deposit right now and I can tell you I'm digitizing a lot of touchy feely lines at the moment.

Edited by roxrkool, : No reason given.

Edited by roxrkool, : No reason given.


This message is a reply to:
 Message 179 by Pressie, posted 10-10-2012 12:40 AM Pressie has responded

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 Message 181 by Pressie, posted 10-11-2012 12:03 AM roxrkool has not yet responded

  
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