One object of
geological mapping1) is to elucidate the structure2) and
structural history3) of the region studied. This can only be one if
measurements4) are made of: the attitude of planar structures5)
such as the bedding6) and foliation7) linear features8)
including the intersection of bedding6) and cleavage9);
the trends of minor folds10); and the directions11) of
overturning12). It is assumed that the reader already knows what
these structures2) are, although many budding geologists13)
do not always know the best way of measuring them. Measurements4)
once made must be plotted and recorded, and there are several ways of doing
this too, some easier than others. Structures5) must also be
investigated, specimens14) collected, photographs taken, and
possibly even soils15) panned to determine heavy mineral16)
suits where no rocks17) are exposed (see Section 4.5.3). These are
all part of the technique of mapping1).
5.1
Measuring Strike18) and Dip19)
Measurements4)
of strike18) and dip19) of bedding6), cleavage9),
foliation7) and jointing20) are fundamental. Without
them, a geological map21) means little. A useful rule of thumb is to
take readings to give an average density22) of about one for every 5
cm2 or 1 inch2 of map surface23) regardless of the scale of mapping24).
Naturally there will be greater concentrations of measurements4)
where strikes18) vary and fewer where structure is more consistent
or exposures25) poor. If there is a greater concentration, perhaps
you need to map those areas at a larger scale. Strikes18) and dips19)
can be measured in a number of different ways. Suit your method to the type of
exposure. Limestones26), for instance, often have uneven27)
bedding6) surfaces and a method which allows you to measure strike18)
and dip19) over a wide area of surface will give more representative
values than one where only a point on the surface is measured. Metamorphic
rocks28) offer additional problems. Measurements4) of
cleavage9) often have to be made on very small parts of a surface,
sometimes even overhanging29) ones. There may even be more than one
cleavage9) or foliation7) and at least one of them may be
obscure and difficult to measure. You must use your ingenuity. Many granite
gneisses29) crop out30) as pavements or turtle-backs
where the trace of foliation is clear enough but the dip is difficult to see.
Like limestone31) bedding planes32), joints20)
tend to have uneven27) surfaces; take this into consideration when
69
measuring
them. One point must be emphasised: you must plot measurements on to your map
immediately after you have taken them, so that any mistakes made in reading
your compass31), and they do happen, are obvious. Only in very bad
weather is it permissible to log readings in your notebook and plot them back
in camp32). Joints20) are an exception. They tend to
clutter a map without adding to a direct understanding of the structure. Record
joint directions in your notebook and plot them onto map overlays later, or
treat them statistically. Another exception to the rule of the immediate
plotting of structural measurements is where structures are locally complex:
then you may have to draw an enlarged sketch in your notebook and plot the
measurements4) on it. Several different methods of measuring strike18)
and dip19) are described below; modify them as occasion demands.
5.1.1
Method 1
This, the contact
method33), is commonest of all. Use it where the surface is smooth and
even. If there are small irregularities, lay your map case34) on the rock surface35)
and make your measurements4) on that, but sometimes such a small
area of bedding6) or cleavage9) is exposed that direct
contact is the only method that can be used. Place the edge of your compass31)
on the surface, hold it horizontally, align it parallel to strike18)
and read the bearing (Figure 5.1). Some compasses31) are provided
with a level bubble36) so that there is no difficulty in
establishing strike18). With others, you may first have to determine
strike18) with your clinometer37), as follows: rotate the
clinometer37) on the rock until it reads zero dip19) and,
if necessary, scratch a line parallel to it with your hammer39) or
lay your scale down beside it. With practice you can usually estimate strike19)
with sufficient accuracy, but where surfaces are close to horizontal, strike
may be more difficult to estimate. Then it may be easier to determine the
direction of maximum dip19), or if you have water to spare, let a
little run over the surface to determine the dip19) direction.
Measure dip19) with your clinometers37) at right angles
to the strike18) (Figure 5.2).
5.1.2
Method 2
On large
uneven planes40) of relatively low dip19), estimate a
strike18) line of a metre or more long (if necessary, mark it with a
couple of pebbles), then stand over it with your compass31) opened
out and held parallel with it at waist height (Figure 5.3). In a stream41)
or on a lake shore42) nature may help, for the water line makes an
excellent strike18) line to measure. The same method can be used to
measure the strike18) of foliation7) on turtlebacks, or
of veinlets on flat surfaces. Because you measure a greater strike length with
this method, it gives more accurate readings than the contact method, and it is
particularly useful where foliation7) is indistinct and seen better
in the rock17) as a whole. Dip19) is often difficult to
measure in some pavement exposures, because there (to be continued)
70
Glossary:
1) mapping :
proses pembuatan peta sebuah daerah
2)structure : adanya dislokasi pada
batuan yang umumnya disebabkan oleh gaya endogen
3) structural
history : sejarah pembentukkan
struktur geologi
4) measurements : pengukuran
5) planar
structures : struktur planar
6) bedding : susunan perlapisan pada
batuan sedimen
7)foliation : suatu bentuk planar pada metemorf
yang terbentuk oleh mineral sekunder
8) linear features :
roman perlapisan
9) cleavage : kemampuan batuan untuk
membelah sepanjang bidang lemahnya.
10)minor folds : lipatan batuan dalam ukuran
kecil yang masih berhubungan dengan lipatan major
11) directions : arah dari suatu kondisi batuan
12). Overturning : penjungkiran atau pembalikkan
batuan
13) geologists : orang yang bekerja sebagai ahli
geologi
14) specimens : sampel pada batuan
15 soils : tanah hasil dari
pelapukan batuan
16 mineral : komponen penyusun yang
menyusun batuan
17) rocks : kumpulan atau asosiasi
dari mineral-mineral
18)Strike : Arah jurus
dari perlapisan batuan
19)Dip :
arah kemiringan lapisan batuan
20)jointing : Retakan-retakan (kekar)
yang terbentuk dari pengaruh gaya Endogen
21)geological map : peta
Geologi yang mencerminkan strukur batuan
22)average density :
rata-rata densitas batuan
23)map surface :
permukaan atau sisi peta
24)scale of mapping : ukuran
pada peta yang mencerminkan jarak sebenarnya
25)exposures :
muncul di permukaan
26)Limestones :
Batuan Gamping
27)uneven :
tidak rata atau kasar
28)Metamorphic rocks : batuan
metamorfyang umumnya terbentuk adanya P&T tertentu
29)granite gneisses :
Batuan gneiss yang berkomposisi granitik
30)crop out :
Tersingkap atau muncul di permukaan
31)compass :
sebuah instrumen yang untuk mengukur jurus dan kemiringan Batuan.
32)camp :
tempat yang menjadi base bagi ahli geologis
33)contact method : Metode pengkuran untuk strike dan
dip
34)map case :
tempat penyimpanan peta
35)rock surface :
permukaan batuan
36)level bubble :
gelembung derajat pengukuran pada kompas
37)clinometers :
klinometer pada kompas geologi
39)hammer :
Palu geologi
40)uneven planes :
bidang permukaan yang tidak begitu halus
41)stream :
aliran air
42)lake shore :
garis tepi danau
Tidak ada komentar:
Posting Komentar