Yes, absolutely, but it still is not just a feeling. It's an educated guess, backed by experience, too. And following the evidence. Not just any old feeling would do. We could call that a 'feeling' if you want too. But it's a very loose definition of the word 'feeling' then.
It's only after you've got enough evidence (well, you feel that you have enough evidence), to be able to relatively accurately model (another feeling) the deposit, where you can try and figure out where you made some wrong decisions. Even after that, you're still not 100% sure.
Don't even get me started on those Engineers. For them everything is either black or white; no grey areas inbetween. We drive them crazy and they drive us crazy.
Edited by Pressie, : Changed last paragraph as it didn't come across as intended
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.
Well, I would say that geologists' feelings have no place in that definition either. "Obscure" and "uncertain" are not so definitional of a paraconformity that it would be a contradiction in terms to say: "This is clearly a paraconformity and I am certain that this is the case".
What defines a paraconformity is that it's an unconformity without an erosional surface. Something either is a paraconformity or it isn't; it isn't made one by our being uncertain whether it is or not.
quote:BTW you can not see a jump in fauna across a noncomformity.....because there aren't any fossils in igneous or metamorphic rocks.
Not necessarily. You can preserve fossils in contact metamorphosed sedimentary units, and the thermal budgets of cooling plutons can be large enough for erosional surfaces to be identified as nonconformities.
Besides the principles already explained, we can determine the relative ages of various geological features by studying the way they intersect with one another: their cross-cutting relationships. In this article we shall explain how this can be done.
The principle of cross-cutting relationships
The principle of cross-cutting relationships may be stated as follows: when one geological feature cuts through another, the former is the younger and the latter is the older of the two features.
For example, consider the diagram below.
The brown sedimentary rocks (A) must be older than the dike (B) that cuts through the strata; the dike must be older than the erosional surface which truncates it (C) which is older (by the principle of superposition) than the gray sedimentary rocks (D) which overlie it; this rock must be older than the dike (E) cutting through the gray strata; and then this dike must be older than the fault which cuts through it (F).
One exception might occur to you: what if we have an outcrop of rock which is then covered over by sediment? The outcrop will cut through the resulting sedimentary strata, but be the older of the two. Whether or not this is really an exception to the principle depends on how you look at it; but if we consider it to be an exception, it is a recognizable exception, for the outcrop would be weathered and eroded where it formerly projected above the surface, and from this erosional surface we would therefore be able to see that it was the older component of an unconformity.
How do we know?
The reasoning behind these conclusions is actualism of the most straightforward sort. Is it really necessary to argue that an erosional surface is younger than the eroded rock, or that a fault is younger than the faulted rock? This is a matter as much of logic as of physics, for it is simply to assert that the surface of a thing cannot precede the thing.
Why must a dike or similar intrusion be younger than the rock into which it intrudes? Because a sheet of molten rock would not stand up by itself: where the magma forming a dike pierced through to the surface, then it would not go on up into the air buliding a dike, but rather it would ooze along the ground creating a lava flow.
Hence we can indeed use these cross-cutting relations to establish the relative ages of geological features.
In this article we shall briefly recap the facts about igneous rocks as they relate to stratigraphy. The reader may find it helpful to go back and re-read the main article on igneous rocks.
Igneous rocks and stratigraphy
From the facts about stratigraphy and igneous rocks that we have covered in previous articles, we may make the following statements:
When they are first formed, extrusive igneous rocks (lava flows and volcanic ash) will be younger than the sedimentary rocks below them, and will be older than the sedimentary rocks which subsequently form above them.
Why so? Well, by definition, extrusive igneous rocks are igneous rocks which are deposited on the surface, and so the principle of superposition applies to them.
Note that as usual the original position of the rocks can be altered by tectonic events, and if this has happened the original order must be recovered by studying way-up structures and observation of the faults and folds that have been formed. Using our < notation, we may write that if rock A < rock B < rock C, where rocks A and C are sedimentary and rock B is extrusive, then rock A was deposited before rock B, and rock C was deposited after it.
Now consider intrusive igneous rocks. If we have an intrusive sheet-like structure (a sill) then this is necessarily younger than the sedimentary rocks below it. But it is also younger than the sedimentary rocks that were above it when it formed. (Note that something must have been above it when it formed, otherwise it would not be intrusive.)
But it is only older than the sedimentary rocks above it when it formed. Suppose that igneous rock B intrudes between sedimentary rocks A and C. Then further sediment is deposited on top of C, forming sedimentary rock D. Then B is younger than C but older than D.
When considering intrusive igneous structures such as dikes and plutons, we may appeal to the principle of cross-cutting relationships to say that they are younger than the sedimentary rocks through which they penetrate.
The reader should bear in mind that we can tell the difference between intrusive and extrusive igneous rocks just by looking at them: intrusive igneous rocks are coarse-grained, and extrusive igneous rocks are fine-grained. So even though a sill and a lava flow are the same shape and may have just the same chemical composition, we can still tell the difference between them by their texture.
Why is this important?
So far, we have only been able to consider the relative ages of features in the geological record; we can say that this stratum is was deposited before that, and that the deposition of this fossil preceded the deposition of the other. But this only provides us with relative ages: we can say that A is older than B, but how much older? Older by a minute? A day? A million years? We have relative ages, but so far we have no actual dates.
Now the crucial thing about igneous rocks is that we can assign actual dates to them by analysis of the rocks, something we can't do directly with sedimentary rocks or fossils.
A geologist, if handed (for example) a fossil trilobite, cannot perform a physical or chemical analysis of the fossil and tell you how old it is. But if that same geologist finds the trilobite in the field lying in sedimentary rocks between two lava flows, then s/he can find the actual dates of the lava flows, and can then tell you that the trilobite is younger than the oldest lava flow and older than the youngest lava flow, and by assigning dates to the lava flows can tell you that the date of the fossil lies between them.
This brings us on to the topic of absolute dating, how it is done, and why it works.
In this article we shall define absolute dating, and shall discuss the conditions that we would require to use a geological process as the basis for absolute dating.
Absolute dating defined
In the articles on stratigraphy we looked at what is called relative dating, where we could say that one geological feature was older or younger than another but without actually putting dates on them. By contrast, absolute dating allows us to assign dates to geological features.
To avoid confusion later on, let us say at once that the "absolute" in "absolute dating" is not short for "absolutely correct". In this context, the word "absolute" is not the opposite of "approximate" but just of the word "relative". A method of dating which was accurate to within a billion years either way would still technically speaking be a method of absolute dating, it would just be a very bad one.
Conditions for absolute dating
Suppose that we wanted to find out how long it has been since an hourglass was set running by measuring the amount of sand in the lower bulb. To do so successfully, we would need to assure ourselves of the following conditions:
* That we can in fact accurately measure the amount of sand in the lower bulb.
* That we know the rate (or rates, if it may have varied) at which the sand has trickled from the upper bulb to the lower.
* That there is no hole in the lower bulb from which sand may have leaked out.
* That there was no sand in the lower bulb when the process started.
* That the process has been uninterrupted, and that since the hourglass was set running it hasn't spent time lying on its side, or standing the other way up.
Given these conditions, we can find out how long the hourglass has been running. If we wish to use a geological process as the geological equivalent of an hourglass, we would want to have similar conditions: we would like to find some quantity which we can measure reliably (corresponding to the condition that we can measure the amount of sand in the lower bulb of the hourglass); which increases or decreases from a known quantity (corresponding to the lower bulb of the hourglass being empty when it starts running) at a known rate (corresponding to knowing the rate of flow of sand); and so forth.
Note that the conditions we have given for the hourglass are ideal conditions which we would require to know exactly how long it is since the hourglass started running. If conditions are less than ideal, we may still be in a position to come up with an approximate figure which is better than nothing. If, for example, we cannot measure precisely the amount of sand in the lower bulb, but we have good reason to think that our estimate of it must be within 10% of the true value, then we also have a good reason to think that we can give the time it's been running to within 10%. Or again, if there may have been sand in the lower bulb when it started running, but we have a good reason to think that there can't have been very much, then we also have a good reason to think that the figure which we get for the time can't be very wrong; and so on.
The geological processes that we'll be discussing work rather like that. We have no reason to believe that nature will be so obliging as to always provide us with the ideal conditions that would provide us with ideal clocks; but by analysis of the geological conditions as they actually are, we can often set limits to how imperfect the geological clocks can be.
For educational purposes, we shall start with a method which barely works at all under any circumstances. This will be the subject of the next article.
One obvious way you might think of to estimate times is to look at a feature of erosion or deposition, measure how much erosion or deposition has taken place, measure the rate of erosion or deposition, divide the former measurement by the latter, and derive a length of time as our answer.
In this article we shall look at some of the problems involved in using such procedures as dating methods.
Rates of erosion and time
As an example, we might look at a wave-cut platform, see how far the waves cut away the cliff per year or per decade, see how extensive the platform is, and so find out how long the waves have been cutting the platform.
Or again, we might look at the depth of a canyon, measure the rate at which it is being cut, and then see how long it would have taken to cut it at that rate.
Of course this sort of reasoning could only work if as in these examples the erosional process only cuts away part of the landscape. We need to measure the depth of the canyon by comparison to the remaining rock; or the erosion caused by the waves by measuring the length of the remaining platform. If we were just looking at a horizontal erosional surface, then even if we knew the rate at which it was being eroded, we could not just by looking at the erosional surface figure out how much material was there originally and has been removed.
Rates of deposition and time
We might also look at the rates of deposition of sedimentary or igneous rock.
We could for example look at the sediment deposited at a mouth of a river, figure out how much sediment it transports each year, and see how long the river mouth has been in that location.
Or again, we might look at a volcano, see from historical records how often it erupts, see how large the typical lava flow is, and figure out how long it took to build the volcanic cone.
There are, however, problems with these approaches.
One problem is this: geological events vary in intensity, and the larger they are, the rarer they are. So, for example, geologists will talk of a "ten-year storm", one of an intensity that only occurs one year in ten; a "hundred-year storm", of a magnitude that only happens one year in a hundred; and so on. Similar things may be said of volcanic eruptions, of rivers flooding, of earthquakes, and of pretty much anything else.
Now this presents us with a difficulty. If we look at the rates of erosion and deposition as they are happening now, we may be discounting the largest events. In principle it might be possible (for example) that much or even most of the erosion forming a wave-cut platform is performed by thousand-year storms of a magnitude that we have never actually observed on that stretch of coast. Similarly, much or most of a volcanic cone might have been formed by eruptions of a magnitude that that particular volcano only ever undergoes every hundred thousand years.
Then again, when we look at erosion or sedimentary deposition, we must also consider long-term alterations in climate. A river, for example, eroding its banks and depositing sediment at its mouth may be a mere trickle in a cold dry climate when compared to its rate of flow in a hot moist climate; and as we shall see in our articles on paleoclimatology, climates have indeed undergone long-term variations such as these.
In some cases, deposition may have stopped altogether for a period, forming a paraconformity; and if the paraconformity is brief enough not to cause a significant discontinuity in the faunal succession, we might never know about it; and if there are enough such paraconformities, then we could be missing sizable chunks of time when we measure the thickness of sediment and try to estimate its age.
(Note that the problems we have been discussing do not all operate in the same direction: some would cause us to overestimate durations, and some to underestimate them.)
And one more problem: suppose we wanted to use these techniques to find the age of a fossil in (for example) the Tonto Group, which lies near the base of the Grand Canyon. Naively, we might try looking at the layers of sediment lying above it, estimating how long it took the limestone, sandstone, shale, etc to be deposited, add it all up, and arrive at a figure.
The trouble is that the rocks contain a number of unconformities between the bottom and the top, and the top itself, the Colorado Plateau, is also an eroded surface. Each of these surfaces represents vanished sediment which took a certain amount of time to be deposited which we cannot even estimate, because we don't know how much sediment there was, and a certain amount of time to be eroded which we also can't estimate for exactly the same reason.
So even if we managed to overcome all the other problems we have mentioned and produce good ball-park estimates for the length of time it took to produce each rock formation, we would be unable to put a date on the lowest rocks and the fossils they contained. The best this would do for us, even if we overcame all the other problems and got our figures exactly right, is supply us with a minimum date which could be too low by any quantity at all.
For these reasons, nineteenth-century geologists barely attempted to put dates on rocks. The best they could do was say that the Earth was old. How old? Very old. And a geological period (the unit of time corresponding to a system) was long. How long? Very long. That millions of years were involved rather than hundreds or thousands was very obvious to them; but so also was the fact that considerations of sedimentation and deposition would not permit them to perform absolute dating.
Dendrochronology is the technique by which we can identify the age of a piece of wood by studying the growth rings it contains.
In this article we shall examine how it works, how we know it works, and the limitations of the technique.
How dendrochronology works
It is a well-known fact that many tree genera will produce one new growth ring each year. This means that, as every schoolchild knows, you can find out how old such a tree is by chopping it down and counting the rings.
This in itself would not be particularly useful. However, it is also the case that the rings produced are of different thicknesses according to the weather in each particular year, with a good year corresponding to a thicker growth ring. This is of some interest to paleoclimatologists, but what is important from the point of view of absolute dating is that this produces a sequence of growth rings of different thicknesses which is almost as distinctive as a fingerprint. Imagine for the sake of simplicity that there are only two thicknesses of rings: large ones and small ones. Then over a mere twenty years, over a million different sequences of large and small rings could potentially form, and which one actually does will depend on the weather over those twenty years.
This still leaves one question. If we were to dig up a piece of wood with the distinctive dendrochronological "fingerprint" of (let us say) 10,000 years ago, how would we know that that was in fact the time that it was the fingerprint of? To recognize the fingerprints of a criminal at a crime-scene, we need to have his fingerprints in our files. In the same way, to recognize a 10,000 year dendrochronological "fingerprint" we would have to know what a 10,000 year fingerprint looks like. To know that, we would have to find a piece of wood which we knew to be 10,000 years old to take the fingerprint of. It seems, then, that we can't do dendrochronology unless we already have a way to determine the age of a piece of wood. If this was the case, it would be one of the more useless scientific techniques.
However, there is a way out. Suppose we take a core sample from a tree which grew between 1500 AD and the present; that gives us fingerprints for the past 500 years or so. Now suppose we find dead wood which, unknown to us, represents growth from 1100 AD to 1600 AD. This will have a fingerprint, and the last 100 years of its fingerprint will match the first 100 years of the tree we sampled. Observing the identities between these fingerprints, we can now put a date on each of the rings of the dead wood, which allows us to extend our knowledge of what the fingerprints look like back to 1100 AD, four hundred years before the living tree took us. Now if we find another dead sample which runs from 800 AD to 1250 AD, its tree-rings have a 150-year overlap with the known sequence, we can use this to date it, and then we can extend the sequence still further. By continuing this process with older and older samples of wood, we can build up data stretching back tens of thousands of years. This technique is known as crossdating; similar principles can be employed in other absolute dating methods.
Limitations of the technique
From the point of view of a geologist, tens of thousands of years is not very much. It is useful to an archeologist, but to a geologist that's just the recent past. And it seems very unlikely that the technique will ever take us much further.
The problem is that wood is not readily preserved; for it to last a long time, it must have been preserved under fairly unusual conditions; perhaps in an anoxic peat swamp, or buried under volcanic tuff. What's more, not all kinds of wood are suitable for the task. Some trees are complacent: that is, they produce growth rings of about the same thickness whatever the weather is like; whereas other kinds of trees don't produce exactly one growth ring per year, which also makes them unsuitable for denrochronology.
So while dendrochronology may be an excellent technique so far as it goes, its scope is limited by the ability of archeologists to locate the right pieces of old wood; and these are scarce and become progressively scarcer as we go back through the geological record.
Dendrochronology: how do we know?
We can check that trees, or at least the kinds of trees we use for dendrochronology, do in fact add one ring per year. We can also check that different trees do produce the same pattern of thick and thin rings. Such observations tell us that dendrochronology should work in principle.
And in practice, when we cut down a tree with a known date of planting and count its rings, we can verify that they do in fact give its age.
We can also look, for example, at the timbers in an old building of known date. If dendrochronology works, then we would predict that the dates it gives for the timbers should not be later than the date of construction.
Or we can look at the charred timbers from cities destroyed by a volcano with a known date of eruption. For example, we can look at Herculaneum, which was destroyed by the eruption of Pompeii in 79 AD, and we would predict that dendrochronological dates for the charred timbers would not post-date 79 AD. The success of such predictions confirms the accuracy of dendrochronology.
Finally, we can note that dendrochronology is in close agreement with other techniques described in this textbook; techniques which are based on completely different principles. Even if we can imagine some unusual conditions in the past that might have messed up dendrochronology in some undetectable way, we should also have to suppose that other unusual conditions messed up other dating methods in such a way that they would still concur with dendrochronology. This is an extravagant conjecture: it is more parsimonious to conclude that the reason that all the methods concur is that they all actually work.
We shall have more to say on this subject in later articles.
In this article we shall examine what a varve is, how they can be used for absolute dating, and when they cannot.
What is a varve?
In its original definition, a varve was a sedimentary feature in a proglacial lake, consisting of a couplet of coarse and fine sediment. Such varves are deposited in proglacial lakes annually because of the seasonal changes in the ablation of the glacier and the amount of meltwater feeding the lake.
Since then the definition of a varve has been extended so that it can be used to describe any layer which is deposited annually, the varves in proglacial lakes being only one example.
In this article we shall discuss varves in the wider sense, since they are equally good for absolute dating whatever the origin of the sediment.
Varves and absolute dating
Given such a situation, there is no difficulty in principle in finding the age of any varve; we just start from the one that was deposited this year and count backwards. (In practice there may be technical difficulties, but the principle is straightforward enough.)
Of course, this only works if there is still a source of sediment, so that we can identify this year's varve and know which year we're counting from. Once the source of sediment is cut off, the link with the present is severed, and unless we could find some other method to place an absolute date on one of the varves, the only thing we can tell from them is the difference in age between two varves, but not how old either of them is.
This limits the use of varves for absolute dating. We can count back thousands or tens of thousands of years, but as we shall see in later articles this is only a short span of time in proportion to the much longer history of the Earth.
Varves and cross-dating
We introduced the idea of cross-dating in the article on dendrochronology. The same principle can sometimes be applied to varves. In varves in proglacial lakes, for example, the thickness of the layers will vary yearly, as a hotter year will result in more melting, more outwash, and more deposition in the coarse-grained part of the varve.
However, it is difficult to use this principle as it is used in dendrochronology, to link together varve sequences in the present and in the past, and to associate varves in a lake currently being fed by glacial meltwater with a different lake in a different location where the glacier has melted entirely and the source of sediment has been removed.
Rather, the technique is more usually used on core samples taken from different locations in the same lake, in order to reduce error: if a minor disturbance has taken place in one location within the lake, removing one or more varves, then it is possible to use cross-dating with a sample taken from a different point in the lake to determine that this has taken place and to correct the chronology.
Varves: how do we know?
We can look in a lake or other environment of deposition, and see that varves are deposited on an annual basis; then we can take a core sample and see that the layer deposited in the past look just like those that are being deposited on an annual basis in modern times. It is an obvious conclusion that layers that look just like annually deposited layers were in fact annually deposited.
But what if the deposition has long ceased, and a succession of varves has been buried and lithified? How can we recognize them then? This question has various answers, depending on what sort of varves we're looking at. In classic varves, i.e. those found in a proglacial lake, the appearance of the sedimentary couplets is quite distinctive, and we may be fortunate enough to find striated dropstones, unambiguous evidence of a glacial depositional environment. In other cases it might not be so easy.
However, as we have pointed out, such ancient varves would usually not allow us to carry out absolute dating in any case, because they would have lost their link with the present: we could determine by counting varves the time interval between any two of them, but not the age of either of them.
quote:The 14C/12C and 13C/12C ratios of more than 250 terrestrial macrofossils (leaves, twigs, and insect wings) in the sediments were measured by accelerator mass spectrometry (AMS) at the Groningen AMS facility (13), after proper sample pretreatment (14). The floating varve chronology was connected to the old part of the absolute tree-ring chronology (2, 15) by 14C wiggle matching (16), resulting in an absolute calendar age covering the time span from 8830 to 37,930 cal yr B.P. (17). The age beyond 37,930 cal yr B.P. is obtained by assuming a constant sedimentation in the Glacial.