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Perpetaan Geologi(Tahap Pemetaan Geologi : Sampling, Penomoran & Plotting)
Oleh :
Irvani
Universitas Bangka Belitung Jurusan Teknik Pertambangan
Referensi :
Sukandarrumidi, 2011. Pemetaan Geolog : Penuntun Praktis Untuk
Geologist Pemula. UGM Press, Yogyakarta.
Coe, A.L., Argles, T.W., Rothery, D.A. And Spicer, R.A., 2010.
Geological Field Techniques, John Wiley & Sons, Inc., UK.
Bates, R.L. and Jackson, J.A., 1987. Glossary Geology. 3th Edition.
American Geological Institute Elexandria, Virginia.
Van Der Pluijm, B. A. and Marshak, S. 2004. Earth Structure. 2nd
Edition. W. W. Norton & Company, Inc., USA.
Dll
Universitas Bangka Belitung Jurusan Teknik Pertambangan
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Materi (Pokok Bahasan)I Pendahuluan (P.1)
II Pengenalan Kartografi (P.2)
III Jenis Peta (P.3)
Peta situasi, Peta topografi
& Peta tematik
IV Peralatan Pemetaan Geologi (P.4)
a. Macam dan jenis alat/bahan
b. Teknik pemakaian alat
V Metode Pemetaan Geologi (P.5)
a. Orientasi lapangan
b. Lintasan kompas
c. Lintasan kompas dan pita ukur
VI Tahap Pemetaan Geologi (P.6-13)
a. Persiapan (P.6)
b. Pengambilan data lapangan (P.7-9)
- Petrologi dan mineralogi (P.7)
- Stratigrafi batuan
- Struktur geologi (P.8)
- Sampling dan penomoran (P.9)
- Plotting data
c. Analisis laboratorium (P.10)
d. Penyajian peta (P.11-13)
- Rekonstruksi struktur geologi
- Satuan batuan- Kolom stratigrafi dan korelasi
- Sayatan geologi
- Penyajian peta & pelaporan
VII Keselamatan Kerja dan Etika Geologi/Etika Lapangan (P.14)
Universitas Bangka Belitung Jurusan Teknik Pertambangan
I Sampling & Penomoran/Labelling
Universitas Bangka Belitung Jurusan Teknik Pertambangan
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What are the objectives and therefore how much and what
type of sample is required?
Is the sample representative?
What does the sampling resolution need to be?
Is the sample fresh and unweathered? (Unless of course the
study is about weathering.)
Does the sample need to be orientated?
Do I have the appropriate tools for the job?
Have I recorded exactly where the sample is from?
Is there a record of how the samples were collected, forinstance the tools used (to eliminate possible contamination)
and stratigraphic precision?
What are the conservation issues? Am I taking only what is
necessary, have I got the necessary permission and have I
considered the visual impact on the environment?
The key points to consider when taking samples
Selecting and labelling samples
1. Samples for thin - sections
A sample of about 10 cm thick 5 cm 5 cm is
usually sufficient for producing one or more thin -
sections provided the rock is not too coarse -
grained.
In deformed rocks it is useful to cut two or even three
thin - sections at right angles to each other, so a
slightly larger sample may be required.
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2. Orientated samples
Younging direction and approximately orientated
samples
For sedimentary rocks the younging direction is often required
and is usually recorded on the face perpendicular to bedding
with an arrow. If the bedding orientation is not clear this may
also need to be recorded so that the section can be cut
perpendicular to the bedding. For igneous rocks the younging
direction may need to be recorded for cumulates, pyroclastic
rocks, and igneous rocks containing flow features and
vesicles. For samples from metamorphic rocks it is also often
necessary to record the top surface because of the need to
obtain thin - sections in a particular orientation relative to the
structural fabric(s).
Precisely orientated samples
This includes samples for palaeomagnetism studies and
some samples for more precise metamorphic and structural
analysis.
Using a compass -clinometer mark the rock exactly with theline of strike, direction of dip and either the way up or the top.
Take note of the dip and strike.
Orientated samples can also be obtained with a drill used for
palaeomagnetic studies.
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3. Samples for geochemical analysis
A 200 g sample is usually sufficient for a range of
major and minor element, trace element and isotopic
analyses, except for very coarse - grained or
heterogeneous rocks, where about 1 kg may berequired.
Particular care needs to be taken to ensure that the
sample is fresh.
Some caution may also need to be exercised if the
sample is being analysed for metal isotopes in order
to avoid contamination from metal chisels and
hammers.
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4. Samples for mineral extraction
The amount of sample that needs to be collected for
mineral analysis depends on the minerals to be
extracted and the composition of the rock. For
heavy minerals such as zircon that are present only
in low abundance, 1 2 kg of rock may be required.
For volcanic glass and feldspars for 40 Ar 39 Ar
and K Ar dating, generally at least 1 kg of rock is
required. For pyroclastic and sedimentary rocks it is
particularly important to collect from the base of the
bed as the coarser grained minerals that are more
easily picked out will be most abundant there.
5. Samples for fossils
Samples for megafossil analysis
Megafossil samples and their supporting rock
material tend to be large. They are best wrapped in
paper. If they are fragile or the rock needs to dry outslowly, wrap them in cling fi lm/plastic food wrap and
then paper. They commonly have commercial value
and therefore special permission may be needed to
remove and/ or ship them out of the locality or
country where they were collected.
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Samples for microfossil analysis
The size of samples for microfossil analysis depends on the
likely abundance of fossils within the sample. For an average
abundance of foraminifers 200 g should be suffi cient; for
nanofossils anddiatoms a small 10 g sample is ample. For
palynology samples of 0.5 1 kg are required.
Samples for molecular fossilsVery small samples are required for molecular fossils but the
sample needs to be large enough (2 5 g) to avoidcontamination. The samples should be packaged in metal foil
or glass or polythene bags of known composition. In addition
care needs to be taken not to cross - contaminate samples bycarefully cleaning the sampling instruments between each
sample.
6. Sampling for regional studies
Regional and low - resolution studies require samples
that are representative of the whole area. In addition
samples should be collected from units that need
laboratory analysis because their composition is
enigmatic.
Collecting a sample from each of the mainlithostratigraphic units is probably a good place to start.
Other samples that are worth considering are a set to test
for compositional variation across an igneous body,
representative fossils, tectonic contacts, representative
samples from metamorphic zones and representative
samples from a set of dykes.
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7. High - resolution sample sets
High - resolution sampling requires great care and
patience. A logical means of labelling the samples/sample
positions will need to be devised. In any situation where
closely spaced samples are required it is best to mark the
exposure clearly first with a scale (Figure 13.2 ) and, if
irregularly spaced samples are required, mark where the
samples will be taken from. The exposure can then be
photographed as a permanent record before extracting
the samples.
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Labelling samples and their packaging
For regional studies the locality number or an
abbreviation of the name and then sample number at
that locality is often used, but an alternative way is to
use the national grid square and then a sample
number
On the sample bag or outside of the packing
material itself it is useful to record the following as
appropriate:
sample number;
cross reference to fi eld notebook page and/or datecollected;
location;
whether or not the sample is orientated;
rock type;
associated samples.
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Practical advice
1. Packing and marking materials
Cling film/shrink wrap: This is very useful for fragile
specimens such as fossils and mudstones. As well as the
obvious advantage of helping to hold the specimen together
the fi lm also retains the moisture in the sample, allowing it to
be dried out under controlled conditions.
Paper: This is useful to help protect delicate specimens and
for packing. It should not be used in direct contact with the
rocks if the samples are also to be used for geochemicalanalysis, particularly organic carbon analyses.
Polythene sample bags: It is good practice to put each
sample into a new bag to avoid cross - contamination. Most
Polythene sample bags: It is good practice to put each
sample into a new bag to avoid cross - contamination. Most
bags come with write - on labels available from a variety of
suppliers. Check for rocks with sharp edges because they tend
to split the bag. If the rock has sharp edges either round them
by gently tapping them using the square end of your hammer orcarefully place the rock in a bag and wrap over this with paper.
Aluminium foil: This is useful for holding together very fragile
specimens such as mudstones. Wrap the foil around the
specimen immediately after extraction in a systematic manner
so that you are able to remove it sequentially in the laboratory.
Aluminium foil is best avoided if the samples will be stored for
any length of time as salts from the rock tend to corrode the
foil.
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Glue:A strong glue suitable for metal or wood can be used
for sticking samples together. If you need to protect fragile
specimens such as shelly material within mudstone or
vertebrate teeth before shipping or transporting them back,
soak the sample for several hours in a 50:50 mixture of PVA
glue and water. Then allow the specimen to dry.
Marker pens: Permanent marker pens provide a distinct
label. Note that wet, dusty or fi ne - grained dark coloured
rocks can be diffi cult to mark. It is worth carrying several
markers into the fi eld because they tend to wear out quickly.
2. Extraction of samples
Different rocks break in different ways, but skill and
experience
enable the geologist to obtain the best samples.
Look for somewhere that is safe and easy to access and
where there is a piece jutting out that might easily be
removed.
You should also consider the impact of extraction on the
environment and preferably choose a location that will not
be obvious.
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II Plotting
Information to record on field maps
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Mapping techniques
Traverse mapping
(a) A linear traverse mapped in the Alaknanda
valley, northwest India. In this complex orogenic
zone, the focus of this project was on gathering
structural data and collecting samples for
metamorphic and geochemical analysis. (b)
Correcting a closed compass traverse for minor
bearing errors. When the traverse was plotted
using the bearings and distances in the notebook
(red lines), the last leg (e to a) did not join up
exactly to the origin (Point A), by a closure error
of 43 m. The traverse was corrected by adjusting
each plotted point (a to e) parallel to the closure
error (green lines) by an amount proportional to
the cumulative distance travelled to reach that
point. So, for point d, correction = 43
(780/1435) = 23.4 m. The result is the corrected
closed traverse ABCDE (dashed blue lines).
Foto 1. Kenampakan lipatan antiklin dan sinklin pada litologibatulempung berseling batugamping pada daerah Salo Mapela,
difoto ke arah barat daya N210E disekitar stasiun 56
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http://dearthurjr.blogspot.com/2014/04/struktur-geologi-daerah-tompo-kecamatan_20.html
PETA STASIUN PENGAMATAN
PETA STRUKTUR GEOLOGI DAERAH PENELITIAN
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Gambar 4 Mekanisme pembentukan struktur geologi pada daerah penelitian
Gambar 2. Kenampakan foto udara yang memperlihatkan sesar geser (merah)daerah penelitian. Citra Google earth akuisisi 2013
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Foto 4. Kenampakan sesar minor pada litologi batugampingberseling batulempung pada daerah Salo Mapela, disekitar stasiun 64
Foto 2 Bentuk kekar tak sistematik pada litologi batulempung karbonatpada stasiun 30 di foto relatif ke arah barat laut
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Foto 5. Kenampakan bidang sesar pada litologi tufa halus disekitar stasiun 63
Tabel 1. Hasil pengukuran
kekar pada batulempung
stasiun 30
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Tabel 2 Data frekuensi pengukuran kekar pada litologi batugamping
pada desa Kalompie stasiun 30.
Gambar 1 Hasil pengukuran data kekar pada litologi batulempungstasiun 30 menggunakan diagram roset
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Terima Kasih
Universitas Bangka Belitung Jurusan Teknik Pertambangan