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عدد الرسائل : 10
نشاطك :
تاريخ التسجيل : 04/04/2008
 |  موضوع: relative dating cont.  الجمعة أبريل 04, 2008 3:39 pm | |
| This exercise introduces the
concept of relative dating of geologic sequences. Basically, relative
dating means determining which rock units are older and which are
younger in some particular geologic setting. Stratification or bedding
is the most obvious large scale feature of sedimentary rocks. Bedding
is readily seen in a view of the Grand Canyon, or almost any other
sequence of sedimentary rocks. Each of the beds or strata (singular = stratum)
is the result of a natural event in geologic history, such as a flood
or storm. As time passes many such events occur and the sediment piles
up, layer upon layer. In this way, thick sedimentary sequences are
formed. STENO'S LAWS It was recognized in the 1600's that in a sedimentary sequence, the older beds are on the bottom, and the younger beds are on the top. This has come to be called the Principle of Superposition.
You can visualize how this occurs if you imagine a stack of newspapers
in the corner of a room. Every day you put another newspaper on the
pile. After several weeks have passed, you have a considerable stack of
newspapers, and the oldest ones will be on the bottom of the pile and
the most recent ones will be on the top. This fairly obvious, but very
important fact about layering was first noted by Nicholaus Steno, and
is the first of three principles which have come to be known as Steno's Laws. Vertical geologic section.Layer A is the oldest, layer C is the youngest. Steno's second law is the
Principle of Original Horizontality,
which states that sediments are deposited in flat, horizontal layers.
We can recognize this easily if we consider a sedimentary environment such as the sea floor or the bottom of a lake.
Any storm or flood bringing sediment to these environments will deposit it in a flat layer on the bottom
because of the sedimentary particles settling under the influence of gravity.
As a result, a flat, horizontal layer of sediment will be deposited. Steno's third law is the
Principle of Original Lateral Continuity.
If we consider again the sediment being deposited on the seafloor, the sediment will not only be
deposited in a flat layer, it will be a layer that extends for a considerable distance in all directions.
In other words, the layer is laterally continuous. In summary, the three principles which we call Steno's Laws are:
 The Principle of Superposition
The Principle of Original Horizontality
The Principle of Original Lateral Continuity
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Of the three, the Principle of Superposition is most directly applicable to relative dating.
We can examine any sequence of sedimentary strata and determine in a relative sense which beds are
older and which beds are younger. All that we need to know is whether the beds are right-side-up or not.
This complication comes because tectonic forces can cause sedimentary sequences to be tilted, folded, faulted,
and overturned. Although sediments are originally deposited in horizontal layers, they do not always remain horizontal.
A trip to the mountains or a quick look through your textbook is probably all that is needed to convince you that
any sedimentary sequences consist of beds which dip at some angle to the horizontal. And in some cases,
the beds are vertical or overturned.
LITHOLOGIC SYMBOLS
In diagrams that show the features of rock units (such as stratigraphic sections and geologic cross-sections),
there is a set of standard lithologic symbols that are
used to indicate the rock types. These are used throughout this lab manual, and you will also find them
used in your lecture textbook.
Take a look through your textbook and see how many of these symbols you can identify in the geologic
cross sections.  DETERMINING "UP DIRECTION" When you approach a sequence of beds which
has been tectonically deformed, before you can determine which beds are younger and which are older,
it is first necessary to determine the "[i]up direction[/i]". This is done by examining the sedimentary structures for clues.
Sedimentary structures such as graded beds, cross beds, mudcracks, flute marks, symmetrical (but not asymmetrical)
ripples, stromatolites, burrows, tracks, and others can be used to establish the original orientation of the beds.
(Fossils can also be used to establish up direction, if they are present in the rock in life position.)
You should examine carefully the sedimentary structures in any dipping sedimentary sequence,
because the rocks can be overturned by tectonic forces, and what initially appears to be younger
because it is on top, may in fact turn out to be at the bottom of the section!
Illustration of overturned beds.
OTHER BASIC PRINCIPLES OF GEOLOGYIn addition to Steno's Laws, there are a number of other basic geologic principles which can be used for relative dating.
1. The Principle of Intrusive Relationships Where an igneous intrusion cuts across a sequence of sedimentary rock, the relative ages of these two units can be determined. The sedimentary rocks are older than the igneous rock which intrudes them.
(In other words, the sedimentary rocks had to be there first, so that
the igneous rocks would have something to intrude.) Or, you could say, the intrusion is younger than the rocks it cuts.
Examples of types of igneous intrusions (or plutons) are dikes, sills, stocks, laccoliths, and batholiths.
Examples of plutons showing intrusive relationships.
Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B.
Sedimentary Rock D is younger than Erosion Surface C.
Diagram (2) Sill B is younger than Sedimentary Rock A. Dike C is younger than sill B.
Diagram (3) Stock B is younger than Sedimentary Rock A. Dike C is the youngest.
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