Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | March 19, 2017

An Opportunity To Work At Slope Monitoring Radar Company: An Overlook

By Fery Andika Cahyo (March 19th 2017)

It has been a while since the last time I wrote for GPRG blog, and life happens for me at its usual rapid pace. After 3 years stints of mine geologist career at Vietnam, it appears that fate has brought another change toward my profession. This time it brings me back to a city where I started it all the very first time I’m working subsequently after my graduation, the one and only Balikpapan city. I’ve been given an intriguing chance to join a geotechnical consultancy company who fare in slope monitoring radar business.  So this piece of writing is only a simple embodiment of thought that I would like to convey to all GPRG blog reader, what it is all about to work as geotechnical engineer in slope monitoring radar company. I hope this article of mine will give the reader an ample portrayal of a field of work which is, in my opinion, not conventional among many choice of career that geological student could undertook.

To begin with, the job that I undertook now is in a whole different realms compared with my previous job as mining geologist back then in Vietnam. For an instance I have to learn all over again from the very beginning since this job as a slope monitoring radar geotechnical engineer has its own peculiarity and novelty which can’t be grasped in short time lapse only with the pretext that I have already possessed an experience of working in mining industry for more than 4 years. But that is the catch. It holds true that a relentless eagerness to learn is an indispensable disposition that you must acquire in order to survive in the industry. And as Darwin’s tenet say “the survival of the fittest”, the hostile mining and oil industry environment nowadays has inevitably entreat me to adapt at any rate I can.

The vocal point of my job is the radar itself. What is Radar? Radar is now has been well accepted as the acronym of “Radio Detection & Ranging”. From this terminology we can infer that radar is a technology which utilize radio wave for some technical purpose, i.e. detecting and object, determines the dimension of an object, calculate movement of an object, etc. The basic principle of how radar working is almost similar with the case when you shout in the direction of sound reflecting object such as a rocky mountain or wall. The sound (sound is one kind of wave) you shout will somewhat reflected back to you. This phenomenon is defined as an echo. If you know the speed of sound through air medium, you can then roughly calculate the distance of the object which turns out has reflected back your sound. The radar which being used for slope monitoring take the technology and concept to a whole new level rather than just calculate the flight time of wave.

In slope monitoring activity whether it is plainly at mining or other field it is precarious to determine deformation well into some extents. The magnitude of the extent we talk about is down to sub millimetre which hardly could be achieved if one is only utilize the radar concept to measure distance using time flight of wave. This is where interferometry takes its role. It might sound a highly sophisticated science, but I promise that it is not belong to some kind of rocket science whereupon geological student will have difficulties to understand. Interferometry is a very accurate method of displacement measurement using the phenomenon of interference of waves. Simply put, the radar will transmit a signal which will reflected back by the slope wall, and the return signal will be received by the radar’s receiver that will generated a phase angle. The slope stability radar, the terminology coined to specifically refer to the radar we used in the work, further utilizing the differential radar interferometry i.e. the differential between successive phase angle’s measurement of each scan(Φ1-Φ2) to detect deformation movement of  the slope wall with high spatial and temporal resolution.

The cutting edge of this slope stability radar technology compared to another slope monitoring method such as the use of extensometer, laser EDM, robotic total station, etc. is essential. The other slope monitoring methods usually emphasize on points or lines monitoring on the wall hence will give rise to some sets of limitation and uncertainty. Let say if we apply some prism on the wall and then we periodically monitor its location to detect any kind of movement, yet the movement is coincidentally occur in between the prism, hence will give an incorrect information regarding the deformation of the slope wall. The slope stability radar on the other hand could provide a closed to real time wall face deformation development which will enable the geotech engineer to understand the condition of the slope wall and further interpret the potential of hazard of the monitored slope wall. Further understanding on atmospheric correction, slope behaviour, velocity of slope deformation, and inverse velocity analysis will even allow one to predict an imminent slope failure with great accuracy with this state of the art slope monitoring radar.

The advantage of the application of the technology for mining industry, first and foremost is for safety, and second which also interesting is for the effectiveness of production. The former is very clear, which such an excellent slope wall deformation and hazard potential information provided by the slope monitoring radar we can prevent any undesirable accidents due to the collapse of the slope wall which is a usual suspect in mining industry. Citing from paper authored by David Noon in 2003, US Mining Safety and Health Administration report indicate that high-wall fatalities account for arounf 10% of surface fatalities in US coal mines. We can imagine if we can provide the correct information and guidance regarding the slope wall condition to the worker who involve in close proximity with the slope wall of mining area then we can save life of many. While for the latter, with such a highly confidence information of slope behaviour one can work at an area with distinguished hazard potential in term of the slope wall deformation up until the strict extent in which it is still deemed to be safe to work there. There is potential of maximizing the extraction of mining commodity on some particular area which previously has been identified as a highly unsafe area to work. Off course in the token of this condition, there must be a set of good procedure that must be set in place prior to the execution.

So there are lot of thing ahead for me to be studied in this new career of mine. The silver lining I can grasp currently perhaps is life happens and when it came down to choose you have to make a decision without ever looking back again. When I’m interviewed by the manager who eventually  become my current employer, He ask me a simple question “Don’t you mind to  restart into the entry level once again if you join our company?”. That one question leaves a lasting impression among other question that I answer that day. I came to this decision to start all over again in a new company is based on my contemplation that I can’t ride the wind of coal mumbo jumbo for all my life. The bravado of coal mining industry in my opinion has passed, and it is well worth to consider to have a new career path. Work for this company give me a broader industry overlook because the slope stability radar not only cover the mining industry but also other project such as infrastructure, underground, satellite imaging, etc. At the other hand I can put my endeavour to learn at a specific cutting edge of profession, which is a slope monitoring radar geotech engineer. An entry level means we have to study from the very beginning all over again despite the accumulation of experience we have possessed previously, and yet I don’t have problem with that. Stay hungry, stay foolish.

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Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | January 20, 2017

Statistik Blog GeoPangea Research Group: Lima Artikel terpopuler dari tahun 2012 hingga tahun 2016

Artikel pertama pada tahun 2017 ini kami akan memberikan statistik dari blog GPRG Indonesia selama blog ini online yakni 2012 hingga 2016. Setiap tahunnya blog GPRG selalu menyajikan artikel menarik bagi pembaca setia, dan dalam hal ini kami akan memberikan lima artikel yang paling banyak di akses pada tahun 2012 hingga tahun 2016 berdasarkan statistik yang diperoleh dari wordpress.com.

Blog GPRG sendiri memiliki 8 (delapan) sub-halaman yakni: home, profile, personel, fieldtrip, research, geotage (baru online pada 2015), publications dan contact us. Berdasarkan statistik, home adalah halaman yang paling banyak di kunjungi pembaca (50%), seperti terlihat pada gambar 1. Tingginya persentase terhadap home ini sangat wajar, karna merupakan halaman default blog GPRG, di bawah home terdapat sub-halaman fieldtrip (17%) yang menunjukkan banyak pembaca yang tertarik dengan aktifitas fieldtrip yang dilakukan GPRG.

1

Gambar 1. Statistik pengunjung blog GPRG pada 8 sub-halaman.

Seperti yang telah kami singgung sebelumnya, pada blog GPRG banyak memuat artikel yang menarik, sejauh ini artikel pada blog GPRG terbagi menjadi 6 (enam) kategori. Persentase artikel di masing-masing kategori dapat di lihat pada gambar 2, dimana terdapat keseimbangan dalam artikel yang disajikan di blog ini, terbukti dengan tidak begitu jauhnya presentase antar masing-masing kategori artikel. Berterima kasihlah kepada para author/penulis/kontributor yang telah menyajikan artikel-artikel ini, sebagai informasi, tidak kurang dari 17 (tujuh belas) orang yang aktif menyumbangkan artikelnya pada blog ini.

2

Gambar 2. Persentase artikel di 6 (enam) kategori pada blog GPRG.

Selajutnya kita akan mulai membahas 5 artikel terpopuler pada setiap tahunnya, mulai dari tahun 2012 hingga 2016.

A. Artikel Terpopuler Tahun 2012:

Pada tahun pertama blog ini online, ada cukup banyak artikel yang disajikan, di bawah ini terdapat 5 (lima) artikel populer tahun 2012 (Gambar 3) yang masih sangat menarik untuk dibaca dan dijadikan referensi pembacanya. Contohnya artikel yang di resume oleh Agung Budiman tentang klasifikasi basin, ditulis dengan simpel untuk memberikan penjelasan tentang klasifikasi basin. Artikel ini juga menjadi artikel paling sering di akses (31%).

  1. Classification Schemes of Sedimentary Basins
  2. “Karena Hidup Adalah Sikuen Pembelajaran Yang Tak Berhenti”: Tips Efektif Belajar Geologi
  3. Sedimentologi Perjuangan Quarry
  4. Pentingnya Pemodelan Analog Kuantitatif Sedimentologi & Stratigrafi Untuk Eksplorasi Migas
  5. Berawal Dari Mimpi Kecil Di Buku Catatan Lapangan Hingga Panggung IPA, IAGI Dan Selangkah Menuju AAPG ICE 2012: Sebuah Introspeksi Diri
3

Gambar 3. Persentase 5 artikel popular pada tahun 2012

B. Artikel Terpopuler Tahun 2013:

Semakin banyak artikel di tahun kedua blog ini online dan terdapat 5 (lima) artikel populer tahun 2013 (Gambar 4), namun pada tahun ini GPRG kehilangan salah satu anggota dan sekaligus salah satu pendiri GPRG, Zaenal Fanani. With his decisive spirit, and irrestible passion, not to mention his spartanism, he postulated what will UPN student now as GeoPangea Research Group nowadays, as bold as it is”, sepenggal kalimat dalam artikel populer urutan kelima pada tahun 2013.

  1. Paper Review: Gunung Samalas, Menyingkirkan Tambora dan Menjadi Gunung dengan Erupsi Paling Dahsyat yang Dicatat Dalam Sejarah Manusia, Jawaban Atas Misteri Erupsi Super Dahsyat di Tahun 1258
  2. Sekilas Geologi Afrika Selatan
  3. Geologi Maroko: Mengenal Domain Struktural Negara Penghasil Fosfat Terbesar Dunia
  4. PAPER REVIEW: RIFT SEQUENCE STRATIGRAPHY
  5. Zaenal Fanani: Memoar of A Man Who Think Imposible is Really Nothing.
4

Gambar 4. Persentase 5 artikel popular pada tahun 2013

C. Artikel Terpopuler Tahun 2014

Apakah yang dimaksud dengan Clastic Injection? “Shanmugham (2005) berusaha menjelaskan prinsip dasar pembentukan struktur ini dimana struktur ini kemungkinan besar terbentuk pada saat syn-depositional atau post-depositional”, fenomena struktur ini dijelaskan dengan detil pada artikel terpopuler 2013, yang ditulis oleh Okctavika Malda. Selain itu artikel populer ketiga merupakan hasil penelitian GPRG tentang Fluvial Geomorphology juga sangat menarik untuk dibaca. Persentase 5 (lima) artikel populer tahun 2014 dapat dilihat dibawah ini Gambar 5.

  1. Fenomena Injeksi Klastik (Clastic Injection) pada Endapan Turbidit Formasi Kerek, Bagian Barat Zona Kendeng.
  2. Apa Itu Petrofisika? (Bagian 1).
  3. Quantitative Application of Fluvial Geomorphology: Preliminary Analogue Study from Modern Mahakam River, East Borneo.
  4. Ringkasan Cekungan Sedimen based on Sam Boggs Jr. 2nd Vol.
  5. Ringkasan Cekungan Sedimen based on Sam Boggs Jr. vol 5.
5

Gambar 5. Persentase 5 artikel popular pada tahun 2014

D. Artikel Terpopuler Tahun 2015

Artikel yang sering di akses pada tahun 2015 berhubungan erat dengan sumber daya alam. Artikel populer urutan pertama menjelaskan batuan beku yang mengandung berlian, pada artikel urutan kedua menjelaskan tentang batubara, dan artikel urutan kelima menjelaskan shale oil, ketiga artikel tersebut ditulis oleh Fery Andika Cahyo. Persentase 5 (lima) artikel populer tahun 2015 dapat dilihat dibawah ini Gambar 5.

  1. Kimberlit: Batuan Beku Pembawa Berlian
  2. Terminologi Batubara Dalam Dunia Insustri: Thermal Coal, Coking Coal, & Antrasit
  3. Ringkasan Cekungan Sedimen based on Sam Boggs Jr. 1st vol.
  4. Diagram Hjulstrom dalam Hidrodinamika Sedimentologi
  5. Shale Oil: Sang Pengubah Tatanan Sumber Daya Energi Fosil Konvensional
6

Gambar 6. Persentase 5 artikel popular pada tahun 2015

E. Artikel Terpopuler Tahun 2016

Pada tahun 2016, banyak artikel tentang penelitian GPRG yang menjadi artikel populer (Gambar 6). Urutan pertama dan kedua, artikel tentang singkapan batuan karbonat pada Formasi Jonggrangan dan Formasi Rajamandala yang ditulis oleh Ari Wibowo. Tidak kalah menarik adalah artikel pada urutan keempat, artikel yang berisi curhatan geologist ditengah menurunnya hampir semua harga komoditi. “On the end, death come to all of us anyway. So, no matter what problem that currently oppress us in a such uncertainty, this life will still proceed on his gusto run and we need to jump into the wagon and keep on moving also until the very last moment of this life. Life is meant to be living.”, salah satu penggalan kalimat pada artikel yang ditulis oleh Fery Andika Cahyo.

  1. Outcrops Overview: Jonggrangan Carbonate Complex in Yogyakarta Area.
  2. Outcrops Overview of Rajamandala Limestone: World Class Outcrops in West Java.
  3. Back to Basic: Batuan Karbonat Seri III.
  4. The Iridescent of Geology Career: Contemplation During the Havoc of Commodity Price’s Slump.
  5. Extended Abstract “Alluvial – Fluvial Architecture of Synrift Deposits: An Observation from the Outcrops of Brani Fm., Ombilin Basin, West Sumatra.
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Gambar 7. Persentase 5 artikel popular pada tahun 2016

 

Semoga artikel yang sederhana  ini dapat memberikan manfaat pembaca, serta memberi motivasi lebih kepada seluruh author/penulis/kontributor kami untuk dapat terus menyajikan artikel ilmiah ataupun penelitian kebumiannya.

Salam Spartan.


*Teks dan Data statistik di kompilasi oleh Rian CR, data dihitung dari tanggal 3 Maret 2012 hingga 31 Desember 2016.

Authors: Ari Wibowo & Iqbal Fardiansyah

*Published on Berita Sedimentologi (Indonesian Journal of Sedimentary Geology) #36

ABSTRACT

Synrift sediments are currently a major focus of both academic research and industrial interest related to petroleum exploration, particularly with regard to their reservoir potential. Reservoirs are alluvial – fluvial deposits with a variety of sedimentary architectures. Their complexity requires good knowledge of basic sedimentology and use of outcrop analogies. The Brani Fm. of the Ombilin Basin has good outcrops that can be used as surface geology model. This paper uses measured outcrop data from the synrift, alluvial – fluvial sediments to characterize reservoirs, revealing that reservoir properties, such as porosities & permeabilities have wide ranges. Thicker sand body geometries in channel assemblages have good connectivity. Utilizing outcrop data is a useful analogue for the surrounding subsurface to tie into the geophysical data.

FIELD LOCATIONS

The outcrops are easily accessible and excellent in term of vertical extent of the sequences. Administratively, the outcrops belong to Payakumbuh Area, Limo Puluh Koto Regency, West Sumatra, Indonesia.

location-map-harau-final-2

Figure 1. Location of Brani Fm.’s outcrop from Google Earth

Braided-Fluvial Outcrop

Detailed sedimentary features observed physical sedimentary structures, the abundance of quartz, and existence of paleosol beds. There are two groups of facies association, namely: fluvial channel complex deposit, consisted of pebble lag (Sp) with trough cross-stratified sandstone (Stc), massive sandstone (Sm), and parallel cross stratified sandstone facies (Sps) and flood plain deposits, consisting of massive siltstone (Fm.) facies. These records may indicate that active migration of channel complex has occurred.

picture1

Figure 2. Detailed view of fining upward grain size succession with Channel Lag Deposits overlying Massive Siltstone overbank deposits.

It is interesting to note the existence of multiple paleosol beds, characterized by purplish color a few centimeters thick. The existence of paleosols represents temporary cessation of sedimentation (minor disconformity) that could be used as stratigraphic marker for correlation purposes.

picture4

Figure 3. Detailed view of purplish paleosol.

Six sandstone samples were taken from five sand bodies representing the channel assemblage deposit, and cores cut. Based on core analysis, ranges are:  grain density 1.72 to 2.72 gm/cc, effective porosity from 13.7 to 20.8 %, and permeability from 10.7 to 3294.8 mD.  Petrography shows mature sand, with composition dominated by quartz and some matrix. Minor monocrystalline quartz, indicated by wavy undulatory extinction under crossed polars, is interpreted as volcanic. Minor plagioclase and detrital clay content are observed. Grain size, roundness, sphericity and sorting depend on lithofacies type. Visible porosity is ranging from 10.5 – 14.2 % with mainly intergranular pore type.

sand-4

Figure 4. Petrographic view of sandstone sample

Trough cross-stratified sandstone facies demonstrates good reservoir properties, such as high porosity (up to 20.8%) and high permeability (up to 3294.8 mD). This facies can be interpreted as part of main channel with high energy turbulence that is proven by poor-well sorted, sub-well rounded, moderate-high sphericity, and moderately-well consolidated.

brani-outcrop-final-2

Figure 5. Sedimentary log analysis showing a vertical succession of interbedded sandstones with siltstone and thin layers of paleosol.

In plan (birds’ eye view), the outcrop is interpreted as most likely a channel evolved upwards from fine-grained sediment to soils, indicated by existence of fine-grained sediment below (massive siltstone facies) and paleosols upward. There is no indication of active channel switching to different direction.

outcrop-2-fluvial

Figure 6. Architecture from outcrop of Brani Fm. showing facies distribution as a part of braided-fluvial system.

DISCUSSION

The fluvial deposits are interpreted as a braided system, indicated by existence of multiple channels deposited along lower fan lobes. Sixteen sand bodies geometry were measured with widths (maximum, average and minimum) of 25.5m, 13.57m and 6.57m, respectively, and thicknesses max/ave/min, of 2.2m, 0.91m and 0.33m, respectively. Cross-plots of channel width vs sand body thicknesses show a linear trend with thicker the sand bodies corresponding to the wider channels. Sand-shale ratio indicates that shale composition is relatively high (50 to 65%), which indicates that the braided stream deposit belongs to a distal stage.

crossplot_rev

Figure 7. (Left) Cross-plot of channel width versus sand bodies thickness showing linear trend and (Right) Bar diagram of sand-shale ratio presenting a different composition ratio on each section.

CONCLUSION

The exposure of Paleogene outcrops along the Harau valley brings valuable insight to the complexity of fluvial reservoirs within a synrift system. The wide ranges sandstone porosities & permeabilities are due to weathering and diagenetic processes. However, these actual data can still be used for petrophysical assumptions. The best reservoir candidates regarding the lithofacies and properties are cross-stratified sandstone (Stc), massive sandstone (Sm), and parallel cross-stratified sandstone facies (Sps) which all are a part of channel assemblage. These measurements are very beneficial to understand the relationship of reservoir geometry, estimates of properties, to guide subsurface stratigraphic correlation, and variogram geo-modeling of Paleogene fluvial systems in the Central Sumatera and Ombilin Basins in particular.

ACKNOWLEDGMENTS

The authors would like to thank Agung Budiman (Founder member of GPRG) for the valuable contribution during fieldwork and laboratory analysis, GPRG members who supported this study and FOSI for publishing this article.

REFERENCES

Koesoemadinata, R.P. and Th. Matasak, 1981, Stratigraphy and sedimentation-Ombilin basin, Central Sumatra (West Sumatra Province). Proceedings of the 10th Annual Convention, Indonesian Petroleum Association p.217-247.

Miall, A.D., 2006, The Geology of Fluvial Deposits: Sedimentary Facies, Basin Analysis and Petroleum Geology, Springer.

Noeradi, D., Djuhaeni, and Batara Simanjuntak, 2005, Rift play in Ombilin basin outcrop, West Sumatra, Proceedings of the 30th Annual Convention, Indonesian Petroleum Association p. 39-51.

Zaim, Y., Habrianta, L., Abdullah, C.I., Aswan, Rizal, Y., Basuki, N. I, and Sitorus, F. E., 2012, Depositional History and Petroleum Potential of Omblin Basin, West Sumatra – Indonesia, Based on Surface Geological Data, Extended abstract, AAPG International Convention and Exhibition.

Full paper can be downloaded on:

Berita Sedimentologi No. 36

Authored By: Fery Andika Cahyo (2nd of May 2016)

Today I’m moved to write a mild topic which somehow has become a peculiar subject that bring us, geologist all over the world, into head ache (or heart ache if you are tend to be the more melancholic person). The slump of many, if not every, commodity prices which severely affect the mineral industry as general, and our profession as a geologist in more particular locus, in a bad way. You can take and perceived this writing as a scientific writing, since I will try to equipped this writing with some of the dependable “scientific” information as a consideration to think that this is a worthy piece of work to read. Or in the contrary you can take this article only as a trivial and venial writing (which I prefer you to choose) since this article is the resulted effort of my contemplation toward the chaotic circumstances our industry has to going through.  Some people say, the more provocative one, that this is the doomsday, the day that our grandfather and prophet (to dramatized it let us wonder Nicolas Steno as Moses, and William Smith as Paul) has forecasted and reminded all over the past day, that it will happen one day. But some people, maybe the wiser one, say that this is just another cycle of the ups and the down of the mundane mineral industry. But nevertheless we as geologist hate this moment. Really. Hate.

 The Inevitable Catastrophe

As we all aware that nowadays, almost all mineral commodity has fell into the harbinger of low. When we open up our internet, or just simply read our newspaper, it is become a usual thing to read the words “Price”, “Record” & “Low” in one coherence sentence added with others sophisticated word which bring more horrific taste, that as further as we read makes our dim hopes get more depressed. From coal, iron ore, to oil & gas are all fell in price slump. They all ends up(this idiom is kind a ironic) in the bottom valley of downturn. The coal firstly caught in the abyss, follow swiftly by iron ore, and finally the marque oil & gas commodity abruptly fell on the same circumstances subsequently. The reason to why each of the aforementioned commodities fell into price slump is essentially diver and varied, nonetheless of course we can find the line that relating the fall of one commodity with another. On the next few passage I will try to summarize from all dependable source and  convey, of course in as much as simple take, the background of each three of the biggest commodity in mineral industry (coal, iron ore and oil & gas. No. Gold. You motherfuc***. You still relatively sound and safe, but it doesn’t make the oracle of Omaha change his mind over you) stumble on the past few years.

Coal is the first wolf from the wolf pack who get shoot downed by this severe price slump. It seems the heaven of coal industry, the sweet 2011, is just yesterday, yet now the black gold has to kneel and prostrated on the eve of downturn. 2 phrase to make us understand. “Over supply” and “lower demand”. As 2011 brought coal price into the sky, the greed that inspired all investor and speculator to jump into the wagon of the coal. It is become inevitable then. As coal exporting nation racking up and escalate their production, in the light of optimism (it’s more like naïve and stupidity right now) that the price of 2011 will be steady, here come China & India tussle. Both of this 2 biggest thermal coal importer on recent year alter their perspective and policy about energy that lead them to overlook thermal coal  progressively, and pursue a more clean power alternative. The late 2012 has allude the premonition of the suffering that will approach, and on the early 2013 the doomsday come upon the coal industry. The coal price slumped in a hasty fashion, and from that point it is just getting worse. Oh yeah, let us not overlook the fact that the low price of natural gas make the life worst for coal. It is a blatantly fierce reciprocal relationship between coal and oil & gas isn’t it.

The result is staggering and mouthwatering. The thermal coal price which is peaked on September 2011 at 131 USD/MT is now touching the rock bottom at 43 USD/MT on April 2016. It is  a price prolapse of a fantastic 67% from the late of 2011 to early 2016. In the past old days, the expertise said the below 50 USD is a no man land for coal industry, yet now we have to struggle and barely survive below 45. All the big fish in the coal industry get suffocated on the brink of bankruptcy(some even, errr bankrupt). Peabody Energy has posted a series of massive lost, which inflict a downfall of their share price from $US 72  four years ago into only $US 2  on the present day. The Rome has been downed and burned. In the light of this circumstances On  15 March 2016  Peabody signaled that, in a very tragic way, they may seek bankruptcy protection in order to restructure their overwhelming debt. The biggest giant in the coal industry has been left in the brink of dead. In July 2015, The Japanese magnate Sumitomo and Brazilian company Vale sold their Isaac Plains coal mine in Queensland for just $US 1, only after 4 years before Sumitomo had purchased its 50% stake for $US 430 million. In August 2015, Vale offloaded a second mine for unrevealed price, leaving only 1 coalmine in Australia for them, a coking coal operation in Queensland. I’m not intended to make a horror novel so let stop here for coal.

Coal price chart

 

 

 

Picture 1. Yearly Thermal Coal Price Chart (source: infomine.com)

Iron ore, what we can say to our dear commodity. $US 180 a tones in 2011 into $US 40 a tones today, what a motherfuc**r achievement. If the question raised regarding the downfall of iron ore price, all finger will pointed to China, again. The performance of china economical growth, and subsequently the performance of their iron or trade is so under achieving and getting alleviate as the day progress. Chinese export going down around 18.1% on February 2014, in spite the fact the market was hoping an increased around 7.5%. Hence the China GDP is barely reach 7.5% in 2014, and of course there is a strong correlation between usage of steel and GDP growth levels.  Things get more complicated when the on early 2014 Chinese government decide to limit credit to steel mils which are not performing well, therefore resulting in limiting their ability to buy iron ore. The final nail in the coffin is when many steel mills have to purchase iron ore by borrowing money and keeping the commodity as collateral with lending institution. The dwindle of price inflict margin calls and dumping of inventory on the market by lenders in case of failure to meet those calls. The market get flooded, and the rest become a dark history.

iron ore price chart

Picture 2. Yearly Iron Coal Price Chart (source: infomine.com)

 

Finally now we come to oil & gas. The king of fossil fuel, the superstar who now going into a  dawn phase in the industry. From 2012 until the middle of 2014, world oil price has been relatively steady, at average $US 110 a barrel. But since June 2014, the price cut more than halved. Per June 2014 Brent crude oil was is below $US 50 a barrel, while US crude oil down until reach the level of $US 48 a barrel.  The story behind oil & gas price slump can also, even though somehow more intricate than coal and iron ore, be credited to simple supply and demand law. United state has increased his oil & gas domestic in production intensively on recent year, to the level that force Nigeria, Saudi Arabia, and Algeria (the traditional oil & gas importer country for United States) to find a new market, hence make the competition become fiercer in Asia market.

Oil Price Chart.png

 

 

Picture 3. Yearly Crude Oil Price Chart (source: Reuters)

 

In the light of this bare condition it seems that the conundrum of the revival of mining industry is still far from the horizon. Some expert has speculated that in the end of 2016 there will be a surge of coal price, while from the present time the rally of iron ore price seems steady even though still meager, at the other hand oil & gas still in halt to return on it’s stable price condition. To put it simply, all of us who took part in expertise career in mineral industry is still in the middle of uncertainty and still have to take this crazy ride over the course of some period. And what should we done as a geologist in this period of maladies? They say that there is always a silver line over the horizon, but what choice left for us to undertake?

 

Be still as a Geologist?

For some of us, geologist, who already have really probed into the mineral industry, and have done many sacrifices to get into this point of our career and life as geologist there is no other choice but to stand firm. The statistic quoted that there are approximately 250,000 jobs is under fire due to oil price slump alone (quoted from oilprice.com) and still counting. It is a very staggering number for any standard. It evokes a more severe competition among the geologist amid this massively uncertain condition. If even the experience geologist has to encounter harsh reality of losing their job and find it difficult to find job substitution, without speaking we can envisage the apprehension of a newly graduate geologist at the time of this calamity. But amid in this condition, it holds true that the versatility of geologist profession brings glimmer of hope to survive from this jeopardy. Maybe the thing that we need to improve at this moment is the ability to read between the lines better.

There is some possibilities even though the projected future of some commodity is on the verge of abandonement. Let’s take coal as example. Because I’m working in coal industry, in this writing I will exert more emphasize on coal industry. Arguably coal is the source of energy that gives the most hazardous effect toward environment. Coal is deemed as a dirty energy, which touted as the producer of more than a quarter of the nation’s total global warming emission(quoted from union of concerned scientist). No wonder that the Paris agreement, under the United Nation framework convention on climatic change ratified on 22 April 2016, is considered as the waning signed of coal industry. The second aim of the convention which is “Increasing the ability to adapt to the adverse impact of climate change and foster climate resilience and low greenhouse gas emission development, in a manner that doesn’t threaten food production” can be considered as a verdict to coal industry, a judgmental decision which will discourage the coal industry on the years to come. For good or worse, we have to accept the fact that  the future is belong to green and clear energy. But it is really the end for coal industry now? Is this recent price slump really signaling the diminishing fate of coal industry? It is apparently, and arguably, not.

We can debate all day whether coal industry will be no longer suitable for a long term career path, with each side of opinion, has their own convictions. The group who opted to side with contra of murky future of coal industry(hence let stay on this “seemingly” troubled ship, it’s on tremble but it will not sinking to the depth of the ocean) have several argument which can summarized by three words I describe fore ward. Cyclic, Efficiency, and Metallurgical Coal(that’s 4 words if “and” is not counted). 1st, All these thing that has happen, all the turmoil is just a part of cyclical downturn of coal price rather than long term shifting trend that move away from coal utilization. 2nd, Efficiency will still bring margin of profit, even though meager (yeah but it still there), by maximizing short run capacity utilization and revenue (even though it will make the market getting more and more saturated with coal stock which will alleviate the price further, but yes actually the profit is still dully there). 3rd, Metallurgical coal (coking coal, anthracite, etc) will not be affected by the demand and requirement to shift into greener and low carbon energy source. While on the other side, the group which has strictly castigated coal as an evil thing, thus the diminishing of it is creed that should be supported, will say(sarcastically) “Come on man you have to move on. Using this coal is costing the environment so much and can be deemed as a modern savagery that need to be neglected. Face the fact. World is changing, we are all now moving into a more environmental friendly option to fulfill our energy requirement. Let put coal on the museum, just next to the vestige of whale oil usage.” Is it really the case?

In my very personal humble opinion, the coal usage is still far from the end, even though it is an inevitable fact that coal will someday will be replaced(not in sense of an immense and a whole substitution that will make our grand-grand children will never now a damn thing about it, even in the very far future) by other commodity, just like oil will be. This global climate change is a REAL thing, something that must be perceived as an urgent global dissension which will affect the future of humanity (sorry Ted Cruz, that’s why I hate you, you simply overlooked global warming by a myriad of vague reason. But I don’t like Donald Trump either, he is lunatic. Hillary is a hypocrite and serial liar. I go for Bernie Sanders, an awesome man). But surely this world economy still depends, on a foreseeable portion of the future, on coal. Thermal coal, on the long run, will encounter a progressive decline even though in my opinion we still can hope for a cyclical price rebound(but yes, China & India has start to do the unprecedented things. Swiftly moving on from thermal coal to more renewable and green energy). But the future of metallurgical coal is still relatively more shinning. The steel industry will still keep their obedience toward the usage of metallurgical coal for a very long time, hence to keep in line with my jargon “read between the lines better”, as a geologist we must put more of our effort to scrutinize the opportunity in metallurgical coal industry. I won’t reveal the exact number, but without saying if you are an Indonesian geologist and pursue career in Indonesia, you will know that Indonesia have a meager number of coking coal reserve compare to the other coking coal producer country in the world such as Australia, USA, Canada, etc. If you still want to well develop your career in coal industry, then you have to take a particular step of measure and decide how you can improve yourself to be able to compete and involve in metallurgical coal industry overseas.

In oil & gas and iron ore industry of course you can always try to read between the lines better also. I have mentioned before that I’m not well exposed with this 2 industries, since all my experience in working so far is in coal industry, therefore I will just talk about it dully. In our global economy the two most stand out commodity that keep the economic wheel rolling is oil and iron ore. Oil arguably will face the same fate with thermal coal in my opinion, even though it will take more time for oil to be fundamentally replaced compare to thermal coal (some people even goes as far to say that Uncle Sam will never let you free from oil). But truth to be told you can never replace iron ore, because this commodity is the essence raw material for so much diverse product. But yes, even you work in one of these two industries, you also have to suffer on the mean times. In case to better improve your fortune in these two industries you can probe on the trend of its. Oil & gas company will try to pursue more viable and effective method to extract hydrocarbon from the sub-surface. In the light of this you must well exposed yourself with the latest method they use such as hydraulic fracking. While for the iron ore, which on the current condition they already start their recuperation from price slump, you can probe about the newest iron ore sorting technology which utilize sensor based technology. Actually since our profession and way of live is not a merely one handed science but it is versatile, geologist profession offer you so much possibility amid the difficulty of the present mineral industry.

Whether you are still a geologist or not

You can take this havoc in mineral industry as a contemplation of your life’s overview. If you possessed the positive thinking attitude, this period of decline can be considered also as a time for you to stop for a while and ask one simple question. Is it really what I like to do for my life and career?  If the answer is yes you can go back to the previous part of this article, but if the answer is the opposite you can read forth for some few minutes. Life will present an exquisite chance to you even in the time you conceived as the most inappropriate one. If the chance that this life provide is anything but geologist career, then so be it. I understand clearly the magnitude of difficulty to look for a job in geology nowadays, so if it is the time for us, whether it is temporary or permanently, to move on from geological career just embrace it dearly. We are still young isn’t it, and even though when you read this you already pass 40 something, believe me you are actually not as old as you thought. There is a time to really change the course of your career and walk of life into something completely terra incognita for you.

 Go back to universities is a good option now as well. Try to pull your brain for another exhausting effort to comprehensively learn again do not always correlate to pursue career in education. Not every single one of us is fated to be Albus Dumbledore, even if you are destined to be Tom Riddle you must remember that even this crazy bad guy, in some point of his wizardly criminal career, would like to go back to Hogwart School for some times (Harry Potter fans detected). Going back to study fundamentally means that you are trying to refine yourself, cut your teeth once more, hone your aptitude, in the light of hopes that it will give you more opportunities. If you already possessed a bachelor degree in geology, it is a good thing to add Master degree in geology or you can go for other lane such as Master in Business or Management. This tumultuous situation has strangle us hence in order to survive you have to really offer something different to the industry, re-finding your place once again in the ever changing realms of mineral industry, or by the simpler term redefine yourself and the path you are eager to walk on in the future.

On the end, death come to all of us anyway. So no matter what problem that currently oppress us in a such uncertainty, this life will still  proceed on his gusto run and we need to jump into the wagon and keep on moving also until the very last moment of this life. Life is meant to be living. This is a course of action that we must undertake because a true measure of human quality is his work in the mist of adversity. The quality that distinguished us against any other creature in this earth is our choice to do something that unprecedented, something that change the dimension of our cosiness and move to the frontier, a choice that will make us a better person by the end of the day. So can we climb this mountain? I don’t know, it’s higher now than ever before. No, we can make it, if we take it slow (That’s one from The Killers song “When You Were Young” for you).

Reference:

– Cleary P.(2014), Coal Crash- How Long can Australia Ride On Coal Wagon? , Accessed from TheMonthly.com on 24th April 2016

– Coal & iron ore prices chart is retrieved from Infomine.com on 23rd April 2016.

– Bhayani R.(2015), Coal Price Slump to Continue, Accessed from BusinesStandardNews.com on 4th April 2016.

– Gloisten H.(2015), Coal Price fall to 12-years lows As China, India Join Demand Slowdown, Accessed from Reuters.com on 22nd April 2016.

– Keane A.(2015), End of the Mining Boom: Iron Ore’s Price Slump Affect All Australians, Accessed from News.com.au on 23rd April 2016

– Krauss C.(2016), Oil Price: What’s Behind the Drop? Simple Economics, Accessed from TheNewYorkTimes.com on 24th April 2016.

– Wyco Researcher (2016), Peabody Energy Legal Bankruptcy Update, Accessed from SeekingAlpha.com on 27th April 2016.

 

Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | February 2, 2016

Outcrops Overview of Rajamandala Limestone: World Class Outcrops in West Java

Written by: Ari Wibowo

Observation team: Fajri, Sahid, Waladun.

Rajamandala Formation, as one of the oldest limestone outcrops in Java Island, has a good reef succession record that contains numerous building organisms and inherent mineralogy. Many studies are conducted related to sedimentology & stratigraphy, structural evolution, diagenesis development and moreover on geotourism point of view. A number of geologist thinks that it can be used as a guide to distribute analogue in subsurface prolific carbonate reservoirs. In addition, a beautiful Rajamandala karst landform could be considered as one of the best national geopark. It is worth it if some references called it as world class limestone outcrop. So, let’s we discuss more about Rajamandala Formation that well exposed on following locations below.

1

Figure 1. Distribution of Rajamandala Fm. shown on pink colour (Sujatmiko, 2003).

  • Gunung Manik Outcrop

The outcrop is located at steeply sloping limestone canyon. It shows a good structural geological feature which is normal right slip fault with almost vertically fault plane. This fault belongs to stratigraphic contact between Rajamandala unit toward Citarum sandstone. Fault breccia features are well recorded as a prove of deformation’s product. Coralline framestone facies appear dominantly which represent building reef’s part.

The outcrop belongs to core reef part consist of coral and algae aragonitic that got dissolution easily into fractures which recorded on the rock. Those fractures could be infilled by hydrocarbon if they were present (?). On bird view, we can observed the development of numerous pore system as result from dissolution, namely caves. In addition, Gunung manik outcrop is also often used for climbing sport because of its huge size and firm as a hard limestone.

2

Figure 2. Gunung Manik outcrop view; (a) a normal right slip fault feature (b) fault breccia zone (c) coralline framestone facies

  • Gunung Masigit Outcrop

The outcrop is located at northward of Gunung Manik outcrop. It has around 40 meters in height. Autochthonous group facies are dominant in the bottom part followed by bioclast-rich limestone upward. According to log analysis, it belongs to foreslope part which characterized by the occurence of debris lithoclast with matrix supported. Porosity do not develop well on several petrographic section caused by dominant matrix composition.

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Figure 3. Gunung Masigit outcrop view

4a

Figure 4. Sedimentary log of Gunung Masigit outcrop

  • Gunung Kencana Outcrop

The outcrop is located at lime quarry. Commonly it is difficult to observe the deformation evidence in limestone formation caused by dissolution on unstable part but Rajamandala limestone does the opposite. A complexity of rock deformation is observed. An anticline feature lies along to west-east characterized by different dip direction apart followed by a number of vertical bedding that may indicate a reverse fault feature. If we go round the outcrop, we may observe others namely strike slip fault and minor faults (?).

  • Stone Garden Outcrop

The outcrop is located at high-topographic karst landform. Pinnacle reef-like contain numerous type of corals and organism. Branching and head coral supported by laminated algae represent high-energy shallow marine condition. Large size and angular fragments indicate talus deposit characteristic. A number of stylolites, fractures, and vuggy pore are also observed indicate that there are strong post diagenetic event occur. The abundance of these carbonate fractures could lead us to know better about their relationship toward porosity development.

Recently, it has been transformed into geopark area so that help observer to pass through to the outcrop location. The existence of this geopark prove that geology’s thing could be enjoyed by anyone not for geoscientist only. Because of its geology phenomena, we may endorse Stone Garden outcrop as one of the best geoparks in the worldwide.

5

Figure 6. Stone Garden outcrop view; (a) geopark welcome gate, (b) a group of pinnacle reef like on small scale, (c) braching coral colonies, and (d) stylolite and fracture features.

Conclusions

Rajamandala limestone is preserved well on the several outcrops in Padalarang Area, West Java. It records good data that can be used for studying more about carbonate. Organism variety and structural geology influence enrich the carbonate complexity on Rajamandala limestone. More over, on geotourism point of view it can be good destination for vacation.

References:

  • Akbar, F.Z., 2015, Geologi dan Mikrofasies Formasi Rajamandala Daerah Gunung Masigit dan Sekitarnya. Skripsi (tidak dipublikasikan).
  • Harahap, H., B., et al., 2003, Stratigraphic Lexicon of Indonesia, GRDC, Bandung.
  • Maryanto, S., 2009, Pendolomitan Batugamping Formasi Rajamandala di Lintasan Gua Pawon, Bandung Barat. Jurnal Geologi Indonesia, Vol. 4 No.3 p.203-213.
  • Sudjatmiko, 2003. Peta Geologi Lembar Cianjur, Jawa, Skala 1:100.000. Pusat Penelitian dan Pengembangan Geologi, Bandung.
Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | January 8, 2016

Outcrops Overview: Jonggrangan Carbonate Complex in Yogyakarta Area

Written by: Ari Wibowo

Jonggrangan Formation, a part of Kulon Progo Sub-basin, is characterized by major occurence of reef limestone and small portion of bioclastic limestone that exposed at Samigaluh Area, Kulon Progo Regency, Daerah Istimewa Yogyakarta Province. These carbonate sediment were influenced greatly by sea level fluctuation which recorded on rock textures and also composition of organism. The heterogenity carbonate features could be observed well on outcrops data as result of this geological condition. Furthermore, geologists who concern on reef carbonate cases, should pay a visit to Kulon Progo area, so hopefully the doubt related to sub-surface problem could be steadily filled.

Picture1

Figure 1. Distribution of Jonggrangan Fm. shown on green colour (Rahardjo et al., 1995) (left) and Simplified Kulon Progo Stratigraphy (Budiadi, 2008) (right).

Now, let’s we discuss more about Jonggrangan Formation that outcropped well on following locations below.

  • Goa Maria Outcrop 

The outcrop is located at back-yard of local house, shows the intercalation of lignit seams with marl on the bottom part and overlied by bioclastic and coralline limestone. It has around 7.5 meters in height. The existence of lignit seams and blackish marl on bottom part may indicates the lower energy products with supported by terrestrial influence. It is followed by retrogradation features on upward proven by the presence of compact organism rich limestone. A lot of macrofossil could be found here, namely Coelenterata, Bryozoa and Molluscs particularly Gastropod and Pelecypoda (Figure 2).

 

Picture2

Figure 2. Goa Maria outcrop view; (a) a good carbonate stratification, (b) branching coral presence, (c) bioclast of Coelentrata’s part, (d) blackish marl supported by Bryozoa (upper pict.) and Gastropod (lower pict.).

  • Klendrekan Outcrop

The outcrop is located at high-topograhic karst hill which a good reef succesion is recorded. The abundant of organism varieties lead us to name the carbonates, namely massive head coral, branching coral, and red/green algae. These organism reflect the shallow clean water condition that reef distribution are occurred. Outcrop observation looks like no any geological structure trace or strong diagenetic influence yet. But when we get details on polished slab view, recrystallization and features were well recorded on several spot (Figure 3). Petrographic views present a lot of pore types and diagenetic products. Both primary and secondary pore type are well developed with good porosity and permeability value. Based on further laboratory analysis, non clastic carbonate facies has porosity up to 14.4 % and permeability up to 151.68 mili Darcy. This proves that carbonate reef may be considered as good reservoir for petroleum or great akuifer for ground water.

Picture3

Figure 3. Klendrekan outcrop view; (a) a karst hill landform, (b) massive head coral framestone, (c) branching coral bafflestone, (d) phylloid algal floatstone (from top to bottom: outcrop, polished slab, and petrographic view).

The abundant of large foraminifera are shown on petrographic view. Undamaged Lepidocyclina should belong to shallow subtidal environment. By using Letter Classification (Adams, 1970), the presence of Lepidocyclina prove that Jonggrangan Limestone are deposited on Middle Miocene (Lower Tf to Upper Tf). (Figure 4).

Picture4

Figure 4. Letter Classification (Adams,1970) of Jonggrangan Limestone

 

  • Goa Kisendo Outcrop

The outcrop is located at Dukuh area, near from Progo River. It has more than 15 meters in height. Whitish fresh limestone are displayed as result from infilling meteoric water into cave pore system. Algae and coral organism supported by crystalline calcite are observed here. The special one is a basal conglomerate on bottom part of carbonate body indicates that Jonggrangan limestone unconformably overlies the Oligo-Miocene Volcanic rock known as Kaligesing formation (Figure 5).

Picture5

Figure 5. Goa Kisendo outcrop view; (a) a whitish limestone climbing, (b) laminated algal bindstone, (c) basal conglomerate indicates an unconformity relationship.

Jonggrangan Carbonate Complex

There are lots of approaches on many stratigraphic cases especially on the cases to determine depositional environment from certain place. There will be a big hesitation, if we have to do the analysis by simply just matching the field observation result with the conventional facies model. A lot of carbonate facies models have been established, namely: Wilson (1975) for ramp facies belts, Richards (1989) for carboniferous carbonate ramp, Luis Pomar (2004) for sigmoidal reef-rimmed complex and so on. But if we made the depositional environment analysis on a Jonggrangan carbonate deposit in Indonesia, is it correct to use those facies model that created based on observation on other place/country? The fact is that carbonates are formed on spesific conditions such as temperature, agitation, salinity, light, and other. These result a variety on physical, chemical and biological aspects that are very close toward facies term.

Adopting a Uniformitarianism Low (James Hutton, 1785) that “the present is the key to the past”, therefore an option is using modern reef as analogue to ancient reef. Modern Pulau Seribu reef that located on North Java, can be used as a guide to distribute analogue in Jonggrangan Limestone. According to similar coral species found on Jonggrangan Fm toward Modern Pulau Seribu, we may assume that equal paleogeography formed both of them (Figure 6). The mineralogy composition and lithology distribution of Jonggrangan unit also lead us to interpret it as patch reef complex (Figure 7).

Picture6

Figure 6. Coral diversities on Modern Pulau Seribu Reef and Ancient Jonggrangan Reef.

Picture7

Figure 7. Pulau seribu reef model (Jordan, 1998 re-drawn by Ari Wibowo) as a guide to ancient Jonggrangan reef complex.

Conclusions

Jonggrangan Fm. is preserved well on the outcrops in Yogyakarta. It records good data that can be used for studying more about reef carbonate. The facies association conclude that Jonggrangan Fm belongs to patch reef complex that equal toward Modern Pulau Seribu reef.

References:

  • Ansori, C., dan Hastria, D., 2013, Studi Alterasi Dan Mineralisasi Disekitar Gunung Agung, Kabupaten Kulon Progo – Purworejo. Buletin Sumber Daya Geologi Vol. 8 no.2. p.75.
  • Harahap, H., B., et al., 2003, Stratigraphic Lexicon of Indonesia, GRDC, Bandung.
  • Jordan, C.F., 1998, The Sedimentology of Kepulauan Seribu: A Modern Patch Reef in The West Java Sea, Proceeding IPA Convention and Exhibition, p.81.
  • Lunt, P., Allan,T., 2004, A History and Application of Larger Foraminifera in Indonesian Biostratigraphy, Calibrated to Isotopic Dating, Museum of GRDC, Bandung.
  • Rahardjo, W., Sukandarrumidi, Rosidi, H.M.D., 1995. Geological Map of the Yogyakarta Sheet, Java, 1:100.000. Geological Research and Development Centre, Bandung.
  • Wibowo, A. 2014, Geologi, Studi Fasies, dan Porositas Batuan Karbonat Formasi Jonggrangan Daerah Purwoharjo dan Sekitarnya, Kecamatan Samigaluh, Kabupaten Kulon Progo, Provinsi Daerah Istimewa Yogyakarta. Skripsi (tidak dipublikasikan).
Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | January 3, 2016

Back To Basic: Batuan Karbonat Seri III

Translated by: Fery Andika Cahyo (3 Januari 2016)

Tekstur Dolomit

Dolomit atau disebut juga dolostone secara dominan disusun oleh mineral dolomit (CaMg(CO3)2). Tidak seperti batugamping, yang mempunyai ciri khas berupa kehadiran butiran, mikrit, dan atau semen sparry, dolomit memiliki tekstur kristalin yang dominan. Berdasarkan bentuk kristal, dua jenis dolomit dapat dikenali. Dolomit planar(atau idiotopic) yang disusun oleh kristal rhombic, euhedral(terbentuk secara baik), hingga anhedral (terbentuk secara buruk). Dolomit nonplanar(atau xenotopic) pada umumnya disusun oleh kristal anhedral nonplanar(Sibley dan Gregg, 1987). Tiap jenis utama dari dolomit ini dapat dibagi lagi menjadi beberapa subtipe seperti yang bisa dilihat pada gambar. Banyak dolomit dibentuk oleh proses replacement terhadap batugamping. Tekstur asli batugamping bisa terpreservasi secara parsial dengan tingkatan yang bervariasi, mulai dari tergantikan sebagian hingga tergantikan seluruhnya. Oleh karenanya, dolomit yang terbentuk dapat mempreservasi tekstur aslinya sebagai “ghost” atau tekstur asli bisa secara sepenuhnya hilang.

Tabel 1.1 Penjelasan Tekstur Dolomit

Tabel 1

 Tabel 1.1.jpg

Struktur Batuan Karbonat

Batuan karbonat mengandung banyak jenis struktur sedimen yang juga dapat dijumpai pada batuan silisiklastik. Struktur-struktur sedimen tersebut antara lain lapisan silang-siur, laminasi, lapisan lenticular, laminasi konvolut, flame structure, load cast, flute cast, dan mudcracks, begitu juga terdapat trace fossil seperti track, trail, dan burrow(Demicco dan Hardie, 1994). Batuan karbonat juga dapat mengandung stromatolites dan struktur cryptoalgal dan struktur yang tidak begitu umum seperti struktur teepe(struktur sedimen menyerupai bentuk busur atau polygon terbalik), solution cavities, dan stromatactis (masa kalsit sparry dan sedimen internal berbentuk irregular mulai dari berbentuk elongate hingga globose).

Klasifikasi Batuan Karbonat

Usaha untuk membuat klasifikasi batuan karbonat dapat ditelusuri dari tahun 1904 melalui publikasi buku teks klasik karangan Grabeau mengenai klasifikasi batuan sedimen. Klasifikasi tambahan diajukan oleh pengarang lain pada tahun 1930an, 1940an, dan 1950an. Kebanyakan dari klasifikasi yang ditemui pada masa awal ini didasarkan pada aspek genetik di mana nama sepeti ”fore-reef talus limestone” atau “low-energy limestone”digunakan untuk mengidentifikasi batugamping berdasarkan pada pre-asumsi lingkungan pengendapannya(Ham dan Pray, 1962). Klasifikasi ini gagal mengenali perbedaan jelas antara butiran karbonat dan lumpur karbonat atau untuk mengeksploitasi perbedaan berbagai jenis butiran karbonat. Publikasi pada tahun 1959 yang bersifat sangat deskriptif oleh Folk yang berjudul “Practical Petrographic Classification of Limestone” menandai awal era modern dari klasifikasi batugamping. Pada 1962, beberapa klasifikasi tambahan muncul(Ham, 1962) yang, dengan satu pengecualian, masih bersifat deskriptif secara umum. Tidak seperti kebingungan yang ada mengenai penyebaran klasifikasi batupasir, kemunculan beberapa klasifikasi deskriptif batugamping sepertinya memberikan dampak yang sangat positif karena ini memaksa geologist untuk lebih sadar akan variasi konstituen yang membentuk batugamping, begitu juga dengan signifikansi lingkungan pengendapan dari konstituen ini.

Mineralogi hanya memiliki sedikit peranan dalam klasifikasi batuan karbonat karena kebanyakan batuan karbonat secara esensi bersifat monomineralik. Mineralogi secara primer digunakan untuk membedakan dolomite dengan batugamping atau membedakan batuan karbonat dengan batuan nonkarbonat. Konstituen mendasar atau parameter yang digunakan pada klasifikasi batuan karbonat adalah jenis butiran karbonat atau allochem dan rasio antara butiran dengan mikirit. Sifat dari grain packing atau kemas juga digunakan pada beberapa klasifikasi, di mana kemas dianggap sebagai grain supported atau  mud supported. Kemas grain supported di mana void dapat diisi atau tidak diisi oleh mud(matriks). Pada kemas mud-supported, kebanyakan butiran tidak saling bersentuhan, dan kelihatan seolah-olah mengambang pada lumpur karbonat.

Klasifikasi Folk(1959,1962) pada umumnya merupakan klasifikasi batuan karbonat yang paling diterima karena cakupannya terhadap berbagai jenis batuan karbonat dan kemudahan yang diberikan dalam konteks pemahaman dan penggunaan istilah dalam klasifikasi tersebut. Klasifikasi Folks didasarkan pada jumlah relatif dari tiga tipe utama konstituen penyusun batuan: (1) butiran karbonat atau allochem, (2) lumpur mikrokristalin karbonat (mikrit), dan (3) semen kalsit sparry(Sparit). Seperti yang ditunjukkan pada table klasifikasi Folk di bawah, klasifikasi ini dibuat dengan pertama mendeterminasi jumlah relatif dari total allochem vs mikrit dan sparit. Subdivisi lebih jauh dilakukan berdasarkan jumlah relatif dari berbagai jenis butiran karbonat(gambar 1.1) dan jumlah relatif dari mikrit dibandingkan dengan sparit. Pendekatan klasifikasi ini menghasilkan nama bipartite yang merefleksikan baik jenis utama dari butiran karbonat pada batugamping dan jumlah relatif dari mikrit dan sparit. Oleh karenanya, oosparit adalah batuan kaya ooid yang disementasi oleh kalsit sparry dan mengandung sedikit mikrit, sementara oomikrit adalah batugamping kaya ooid yang didominasi oleh mikrit dan hanya sedikit mengandung sparit. Informasi tektural tambahan dapat ditambahkan dengan menggunakan terminologi maturitas tekstur seperti yang ditunjukkan pada gambar 1.2. oleh karenanya packed oomikrit mengindikasikan batugamping oolitik grain supported, sementara sparse oomikrit mengindikasikan batugamping oolitik mud supported. Perlu diperhatikan juga jika klasifikasi Folk dapat digunakan juga untuk klasifikasi dolomit, jika tekstur “ghost” dari allochem aslinya masih bisa diidentifikasi.

Tabel 1.2 Klasifikasi Batuan Karbonat Menurut Folks(1962)

Tabel Klasifikasi Folk.JPG

Gambar 1,1.jpg

Gambar 1.1 klasifikasi dasar Folk(1962, sumber:www.sepmstrata.org)

Gambar 1,2.jpg

Gambar 1.2 klasifikasi dasar Folk(1962, sumber:www.slideshare.net)

Terminologi yang dipakai oleh Folk (1959,1962) untuk membedakan tektur pengendapan didasarkan secara murni pada aspek deskriptif; kendati demikian, terminologi tersebut juga mengandung signifikansi lingkungan pengendapan. Terminologi biomikrit, sebagai contoh, mengungkapkan intrepertasi dari pengendapan pada keadaan air tenang di mana mikrit melimpah dan pemisahan lumpur karbonat bersifat minimal. Oleh karenanya, mikrit diendapkan bersama dengan partikel skeletal. Pada sisi lain, terminologi biosparit mengindikasikan pengendapan pada lingkungan dengan agitasi gelombang tinggi di mana mikrit dibersihkan oleh arus yang bertindak sebagai pemisah, mengijinkan karbonat yang bebas dari lumpur untuk terendapkan. Butiran-butiran ini kemudian disementasikan oleh sparit selama proses diagenesis.

Dunham (1962) memiliki tipe klasifikasi yang berbeda (tabel 1.2) yang menitik beratkan pada jumlah relatif allochem dan mikrit namun tidak memperhatikan identitas butiran karbonat yang berbeda-beda. Klasifikasi Dunham didasarkan sepenuhnya pada tektur pengendapan dan memperhatikan dua aspek dari tektur: (1) kemas butiran dan jumlah relatif antara butiran dengan mikrit dan (2) pengikatan butiran oleh proses pengendapan. Pengikatan oleh proses pengendapan memiliki artian apakah butiran karbonat menunjukkan kecenderungan terikat bersama-sama pada saat pengendapan, seperti pada kompleks terumbu kolonial, stromatolit(cyanobacteria), lapisan alga gampingan, atau tidak. Klasifikasi Dunham memisahkan komponen yang tidak terikat secara bersamaan pada saat pengendapan menjadi batuan yang mengandung lumpur karbonat atau batuan yang tidak mengandung lumpur karbonat. Batuan yang tidak mengandung lumpur karbonat tentu saja bersifat grain supported. Batuan yang mengandung lumpur karbonat bisa saja bersifat grain supported atau mud supported. Perlu dicatat, kendati demikian, bahwa sifat grain supported tidak tergantung secara absolut pada rasio butiran dan mikrit, karena grain supported juga merupakan fungsi dari bentuk butiran karbonat itu sendiri. Butiran dengan bentuk platy seperti cangkang bivalve dapat membentuk kemas grain-supported pada tingkat kelimpahan jumlah butir lebih rendah daripada partikel dengan bentuk lebih spherical seperti ooid. Klasifikasi Dunham telah dimodifikasi, dengan tambahan dua nama baru (floatstone, dan rudstone) oleh Embry dan Klovan(1972) dengan tujuan supaya lebih dapat merefleksikan kehadiran butiran karbonat dengan ukuran gravel (>2 mm). kedua pengarang tersebut juga membagi boundstone Dunham menjadi tiga tipe (framestone, bindstone, dan bafflestone) berdasarkan basis organisme penyusun awalnya yang bekerja menghubungkan material sedimennya secara bersama. Wright (1992) mengajukan klasifikasi dengan nama yang secara umum berasal dari klasifikasi Embry dan Klovan namun juga mengambil pertimbangan terhadap fitur yang dihasilkan oleh proses diagenetik.

                Karena klasifikasi Dunham tidak mempertimbangkan identitas dari butiran karbonat penyusunnya, ia mungkin diinginkan untuk digunakan dalam konjugasi bersama dengan klasifikasi lain seperti klasifikasi Folk. Oleh karenanya, suatu batugamping yang diidentifikasikan sebagai packed oomicrite menggunakan klasifikasi Flok secara alternatif dapat juga disebut oomicrite packstone menggunakan kombinasi klasifikasi Folk & Dunham.

                Terminologi coquina, chalk, dan marl pada umumnya digunakan sebagai nama informal untuk suatu batuan karbonat. Coquina adalah karbonat sedimen dengan ciri khas tersortasi secara mekanikal dan terabrasi dan terkonsolidasi secara jelek yang terutama disusun oleh debris fosil; coquinite adalah ekuivalen dari coquina yang telah terkonsolidasi. Chalk adalah batugamping bertekstur halus, menyerupai tanah, lembut, dan terutuma disusun oleh test kalsit organisme mikro yang dulunya hidup mengambang, seperti foraminifera. Marl adalah terminologi lawas, yang agak tidak tepat, untuk sedimen karbonat yang terkonsolidasi lepas dan campuran dari lempung silisiklastik dan kalsium karbonat.

Referensi:

Boggs, Jr. S.(2006): Principal of Sedimentology and Stratigraphy 4th edition, Hal 167-172, Pearson Education, inc., Upper Saddle River New Jersey.

Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | October 10, 2015

Ringkasan Kegiatan: Joint Convention HAGI-IAGI-IAFMI-IATMI Balikpapan

Oleh: Rian Cahya Rohmana

Pada tanggal 5 oktober hingga 8 oktober 2015 JCB 2015 digelar disalah satu kota di Pulau Kalimantan yaitu Balikpapan. Apa itu JCB? JCB adalah Joint Convention Balikpapan yang diselenggarakan oleh empat asosiasi kebumian/energi di Indonesia. Empat asosiasi itu adalah Himpunan Ahli Geofisika Indonesia (HAGI), Ikatan Ahli Geologi Indonesia (IAGI), Ikatan Ahli Fasilitas Produksi Minyak dan Gas Bumi Indonesia (IAFMI) dan Ikatan Ahli Teknik Perminyakan Indonesia (IATMI). Tema yang diangkat pada JCB 2015 adalah Empowering Marine Earth Resources, yang bertujuan agar pemerintah dan semua pihak yang terkait sadar akan potensi kelautan Indonesia (seperti energi, pelayaran, perikanan dll). Namun sadar akan potensi kelautan saja tidaklah cukup, harus ada aksi dan eksekusi yang tepat sasaran.

Gambar 1. Suasana sebelum pembukaan JCB 2015

Gambar 1. Suasana sebelum pembukaan JCB 2015

Pada JCB 2015 ini, Teknik Geologi UPN “Veteran” Yogyakarta dan Geopangea Research Group (GPRG) berhasil meloloskan beberapa paper dan dipresentasikan baik dalam oral presentation maupun poster presentation.

Berikut ini adalah daftarnya:

a. Oral Presentation

  • Mesozoic Metamorphism of the Meratus Mountains Southeast Kalimantan”, dipresentasikan oleh Bapak Dr. Joko Susilo.
  • “Prioritas Ekosistem Karst Dengan Perkembangan Ekonomi Masyarakat” dipresentasikan oleh Dwi Noviar Aditiya.
  • The Occurrence of Acid Mine Drainage at Coal Mining of Upper Warukin Formation; the Correlation with Clay Minerals Genesis in Coal Bearing Formation, Reviewed of Sedimentary Process with Geochemical Aspect” dipresentasikan oleh Dilla Artha F (Gambar 2).
Gambar 2. Dilla Artha sedang menjelaskan Geologi Regional daerah penelitian.

Gambar 2. Dilla Artha sedang menjelaskan Geologi Regional daerah penelitian.

  • Application of Modern Fluvial Geomorphology: Quantitative Approach of Uncertainty Prediction for Subsurface Interpretation” dipresentasikan oleh Pingkan Yessica (Gambar 2).
Gambar 3. Penyerahan momento dari JCB kepada Pingkan

Gambar 3. Penyerahan momento dari JCB kepada Pingkan

  • Geotage: Shareware Application for Record Indonesia Geological Heritage” dipresentasikan oleh Adi Gunawan (Gambar 4).
Gambar 4. Mas Adi Gunawan sedang menjelaskan keunggulan Geotage.

Gambar 4. Mas Adi Gunawan sedang menjelaskan keunggulan Geotage.

b. Poster Presentation

  • Tectonic Event Trailing Based On Fragments Of Waturanda Formation, Wadasmalang, Karangsambung, Central Java” Paper yang dipresentasikan oleh M. Gazali Rahman ini mendapatkan “BEST POSTER PRESENTATION-STUDENT”. Selamat Gazali…!!!
Gambar 5. BEST POSTER PRESENTATION-STUDENT, proud of you bro!!!

Gambar 5. BEST POSTER PRESENTATION-STUDENT, proud of you bro!!!

Selain itu, JCB 2015 menjadi lebih spesial karna di JCB-lah debut dari “Geotage”, aplikasi smartphone yang sedang di kembangkan oleh tim Geotage Startup dan GPRG. Geotage adalah aplikasi yang dapat digunakan oleh semua orang (bukan hanya geoscientist bro!!). Lalu apa kegunaannya? Silahkan klik link ini untuk penjelasan detilnya dan silahkan klik link ini untuk men-download Geotage. Cukup promosinya (hehe), kembali ke JCB 2015, Geotage dipresentasikan di dua sesi yang berbeda, sesi pertama tentu saja di Oral presentation (session 39) dan yang kedua Geotage dipresentasikan didepan para geoscientist muda Indonesia. Geotage mendapatkan undangan dari Forum Geosaintis Muda Indonesia (FGMI) untuk presentasi dan berdiskusi di acara “FGMI Talk: Yang muda Yang Berkarya”. Pada acara ini saya sendiri yang mempresentasikan Geotage (Gambar 6) didepan mahasiswa/mahasiswi dan turut hadir pula Ketua HAGI dan Ketua IAGI.

Gambar 6. Presentasi Geotage dan penyerahan sertifikat pada acara FGMI Talk

Gambar 6. Presentasi Geotage dan penyerahan sertifikat pada acara FGMI Talk

Tak terasa empat hari sudah kami di Balikpapan, banyak cerita, pengalaman dan motivasi yang kami semua dapatkan dan saya mengucapkan terima kasih kepada seluruh panitia JCB 2015 yang telah membuat acara berjalan dengan baik, lalu terima kasih pula kepada FGMI yang telah memberikan kesempatan kepada kami untuk mempresentasikan tentang Geotage dan tak lupa terima kasih yang sebesar-besarnya kepada mahasiswa STT-Migas Balikpapan (Ikhsan, Fadel, Jack, Andra, Lisa dan semua teman-teman lain) yang telah membantu kami selama di Balikpapan. Sampai jumpa lagi di event lainnya, sukses untuk kita semua!!!

Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | August 2, 2015

Learning Geology from Outcrops: More You Believe, More You Understand

Created by: Ari Wibowo (Geology Department ‘10)

What do you think about geology and the outcrop at the first time? A huge mountain range of rock? Strata of rock which laid down along river bank? An open pit mining of coal deposit? Or probably you think about the modern sediment? Yup, absolutely geology is related to the outcrop all above. The development of geology science has lead people to think more about the research and innovative exploration method. One of them, the use of newest geological tools to know the subsurface condition both in petroleum and also mining companies. In numerous petroleum companies, there are several division of geologist, namely exploration geologist who concern about the new field discovery related to its reserves in long term and development geologist who concern to develop the previously field by doing several exploration program, e.g. infill drilling, enhanced oil recovery (EOR), behind casing opportunity (BCO), etc. Knowledge, skill, and experience are fully needed to achieve the company’s goal right. But in fact, either of them today prefer computer workstation to fieldwork. They should realize that all the geological problems in subsurface both pure exploration or development project, must be validated toward surface data. Adopt a statement from a highly experiences Italian geologist, Professor Emiliano Mutti, said, “You must has very strong passion on field geology, or you will give up soon on fieldwork. I do not believe anything, but rocks!”. Good geologist should agree with his point of view. Basic geological concept is needed to interpret available data so then a framework of geological model could be steadily attained. So, let’s we discuss more about the advantages of using outcrop as our main data

  • Outcrop provides view of rock strata on specific scale

When we deal with sedimentary rock outcrop both siliciclastic and carbonate clastic, which show stratification features, directly we sketch them on field book with no doubt. In turbidite sequence, called flysch deposit, we face with interbedded of sandstone and shale in centimeter thickness or less. But Wireline log do the opposite. Log response have no ability to detect the thin layer of shale in few centimeter thickness (Figure 1). Does this condition have any effect in sub surface exploration? Sure. Definitely it does. It can effect the petrophysical value of reservoir rock while calculating oil & gas reserve. Moreover when we do the well correlation, the existence of interbedded sand or shale although in thin layer, could be considered as important data.

Figure 1. The outcrop view shows interbedded sand with shale and probably log response.

Figure 1. The outcrop view shows interbedded sand with shale and probably log response.

  • Outcrop presents high resolution data for interpreting depositional environment

Depositional environment is a key to reconstruct the paleogeography condition so then the geometry of sedimentary strata could be identified. There are many approaches to determine the depositional environment, which are physical aspect (rock texture and sedimentary structure), chemical aspect (composition of mineral), and biological aspect (fossil content). Recently, geologist utilize wireline log data to interpret the facies based on the only electrofacies pattern (blocky, funnel, bell, etc) and they do believe on log data without any doubt. In fact, no single environment has a unique grain-size profile (gamma ray pattern). Similar profiles may be produced by different environments (Figure 2). There are still so much integration of data needed by geologist to build the stratigraphic framework of target area.

Figure 2. Wireline log data with blocky pattern may be produced by numerous environment.

Figure 2. Wireline log data with blocky pattern may be produced by numerous environment.

With bird view, outcrop shows the lateral continuity so then rock’s geometry could be determined (Figure 3). If we get closer, we will face the numerous sedimentary features which could lead us on understanding the depositional environment better (Figure 4). For example, we meet the association of normal graded bedding, parallel lamiantion and convolute strcuture in a sequence, we may predict it as low density tubidity current (LDTC) products. On the opposite, dealing with reverse graded bedding, we may have a view of high density turbidity current (HDTC) products. And so on.

Figure 3. An outcrop view of Kaligesing Fm show the channelized of submarine fan feature.

Figure 3. An outcrop view of Kaligesing Fm show the channelized of submarine fan feature.

Figure 4. Sedimentary features record from the outcrop; (a) convolute lamination, (b) scour structure, (c) reverse graded bedding, and (d) volcanic mineral content.

Figure 4. Sedimentary features record from the outcrop; (a) convolute lamination, (b) scour structure, (c) reverse graded bedding, and (d) volcanic mineral content.

  • Outcrop demonstrates a certainty while describing rock name

Particularly when we deal with carbonate rocks, we probably get a little confused because of their complexities. Neither core data (polished slab) nor rock thin section can named rocks correctly. For example, we could give a name of boundstone (bafflestone, bindstone and framestone) in field outcrop certainly because it reflects the organism‘s act such as baffling, binding, or building a rigid framework. But when we do observation on core data or even in thin section, we may only see the matrix so then we name it as wackestone/mudstone. Simply put, the “scale” takes an important place in this case (Figure 5).

Figure 5. From left to right: outcrop, core (polished slab) and rock thin section photograph.

Figure 5. From left to right: outcrop, core (polished slab) and rock thin section photograph.

Conclusions

Validation of all data is a must. We realize that outcrop has limit in length and depth. Despite that, subsurface data still must be validated toward the surface condition. Pay a visit to the outcrop is the way to understand geology more. Because in rocks, geologist do believe.

References:

  • My undergraduate thesis photo album
  • Analisa log sumur handout by Teguh Jatmiko
  • Colleagues share
Posted by: GEOPANGEA RESEARCH GROUP INDONESIA | July 31, 2015

Back To Basic: Batuan Karbonat Seri 2

Translated by: Fery Andika Cahyo

Tekstur Batugamping

Seperti yang telah didiskusikan di bagian sebelumnya, batugamping purba disusun secara utama oleh kalsit. Kalsit dapat hadir setidaknya dalam tiga bentuk tektural yang berbeda: (1) butiran karbonat, seperti ooids, dan butiran skeletal, yang berukuran lanau atau agregat lebih besar dari kristal kalsit, (2) kalsit mikrokristalin, atau lumpur karbonat, yang secara tektural analog dengan lumpur pada batuan sedimen silisiklastik namun disusun oleh kristal kalsit dengan ukuran yang sangat kecil secara ekstem, dan (3) sparry calcite, terdiri butiran kristal kalsit yang lebih besar yang terlihat tembus pandang dan jernih di bawah cahaya nonpolarized.

Butiran Karbonat

Geologist awalnya cenderung mengira bahwa batugamping merupakan batuan kristalin yang umumnya mengandung fosil dan terbentuk oleh proses presipitasi dari air laut. Sekarang telah diketahui bahwa banyak, dan kemungkinan sebagian besar, batuan karbonat tidak terbentuk oleh proses presipitasi kristalin sederhana. Sebaliknya, batuan karbonat dibentuk oleh agregat partikel atau butiran yang bisa telah mengalami transportasi mekanikal sebelum terendapkan. Folk(1959) mengajukan terminology umum allochem untuk butiran karbonat untuk menekankan bahwa mereka tidak terbentuk oleh presipitasi normal. Butiran karbonat secara tipikal memiliki rentang ukuran dari lanau(0,02 mm) hingga pasir(di atas 2 mm), namun partikel yang lebih besar seperti cangkang fosil juga bisa hadir. Mereka dapat dibedakan menjadi lima tipe dasar, masing-masing dicirikan oleh perbedaan bentuk, struktur internal, dan mode pembentukan: pecahan karbonat, partikel skeletal, ooid, peloid, dan butiran agregat. Scholle dan Ulmer-Scholle(2003) telah menyediakan koleksi mengagumkan dari gambar ilustrasi bewarna dari semua jenis utama dari butiran karbonat.

Pecahan Karbonat(Lithoclast)

Pecahan karbonat adalah fragmen batuan yang berasal dari erosi batugamping purba di daratan atau erosi terhadap sedimen karbonat terlitifikasi di dalam cekungan pengendapan. Jika pecahan karbonat berasal dari batugamping lebih tua di daratan yang berada di luar cekungan pengendapan, maka akan disebut sebagai extraclast. Jika ia berasal dari dalam cekungan pengendapan oleh erosi terhadap sedimen karbonat yang tersemi-konsolidasi, tidal flats, atau pantai karbonat(beach rock), maka akan disebut sebagai intraclast. Perbedaan antara ekstraclast dan intraclast mempunyai implikasi penting dalam intrepertasi mekanisme transportasi dan sejarah pengendapan batuan karbonat. Extraclast dapat memililki iron stained rim yang merupakan hasil pelapukan, dapat mengandung urat terekristalisasi yang diwariskan dari batuan asalnya, atau juga dapat menunjukkan property lain yang membedakannya dengan intraclast(Boggs, 1992, hal. 425). Kendati demikian, pembedaan antara extraclast dan intraclast sering kali sulit dilkukan. Litoclast(atau limeclast) adalah termiologi non-spesifik yang dapat digunakan untuk pecahan karbonat saat pembedaan tidak dapat dilakukan.

Gambar 1.1 Kenampakan Intraklast pada oolitic-packstone(Sumber: wwwf.imperial.ac.uk)

Gambar 1.1 Kenampakan Intraklast pada oolitic-packstone(Sumber: wwwf.imperial.ac.uk)

Gambar 1.2 Kenampakan Extraclast pada sayatan tipis(Sumber: wwwf.imperial.ac.uk)

Gambar 1.2 Kenampakan Extraclast pada sayatan tipis(Sumber: wwwf.imperial.ac.uk)

Litoclast memiliki ukuran dengan rentang mulai dari pasir sangat halus hingga bongkah, mesikpun fragmen dengan ukuran pasir adalah yang paling umum dijumpai. Jenis butiran karbonat ini biasanya menunjukkan derajat pembundaran(gambar 1.1A) tertentu, mengindikasikan mekanisme transportasi, namun butiran angular dan sub-angular juga tidak jarang dijumpai. Beberapa pecahan menunjukaan tekstur internal atau struktur seperti laminasi, sisipan pacahan yang lebih tua, butiran silisiklastik, fosil, ooid, atau pellet, namun jenis lain secara internal bersifat homogen. Batugamping yang disusun oleh limeclast dengan ukuran bongkah merupakan suatu jenis dari konglomerat intraformasional. Limeclast bukan merupakan jenis butiran karbonat yang paling melimpah pada batugamping purba, namun mereka hadir dengan frekwensi yang cukup menonjol dalam sejarah geologi untuk menunjukkan bahwa mekanis pembentuk pecahan karbonat adalah seuatu mekanisme yang umum.

Partikel Skeletal

Partikel skeletal orgnanisme hadir pada batuan gamping sebagai mikrofosil utuh, fosil dengan ukuran besar yang realtif utuh, atau fragmen pecahan cangkang dari suatu fosil. Jenis butiran karbonat ini adalah yang paling dominan, dengan margin besar, ditemui pada batuan karbonat. Fosil yang mewakili semua kelas utama dari filum invertebrate karbonatan laut dapat dijumpai pada batugamping. Jenis spesifik dari partikel skeletal yang hadir tergantung baik oleh umur batuan dan juga kondisi paleoenvirmental dimana batuan karbonat diendapkan.Dikarenakan perubahan evolusi pada susunan fosil seiring berjalannya waktu, beberapa jenis sisa fosil tertentu mendominasi batuan karbonat pada umur tertentu pula. Sebagai contoh, sisa skeletal trilobite mencirikan batuan Paleozoikum awal, namun ia tidak lagi dijumpai pada batuan Kenozoikum, yang didominasi oleh kehadiran foraminifera. Demikian juga, beberapa jenis fosil tertentu mencirikan lingkungan tertentu. Sebagai ilustrasi, sisa koral yang berkoloni membentuk struktur rigit, tahan terhadap ombak, pada umumnya teretriksi pada batugamping yang diendapkan pada lingkungan laut dangkal, lingkungan dengan energi tinggi dimana air beragitasi baik dan level oksigennya tinggi. Secara kontras, bryozoan dengan tipe bercabang tumbuh, yang merupakan organisme yang rapuh terhadap gelombang pada lingkungan dengan energi tinggi, mencirikan lingkungan laut dalam. Tergantung pada kondisi paleoenviromental, sisa-sisa skeletal organisme pada suatu spesimen mungkin saja tersusun oleh hanya satu jenis organisme, namun mungkin juga tersusun oleh berbagai jenis organisme.

Ooid

Terminologi ooid diaplikasikan secara umum untuk butiran karbonat berlapis dengan nukleus di tengahnya yang bisa berupa pecahan cangkang, pellet, atau butiran kuarsa- dikelilingi oleh satu atau lebih lapisan tipis(korteks) yang terdiri dari kristal kalsit atau aragonit dengan ukuran kecil(pada beberapa ooid, nukleusnya kadang terlalu kecil untuk dapat dilihat). Butiran karbonat jenis ini kadang disebut juga sebagai oolith, kendati demikian terminology ooid lebih dipreferensikan. Batuan karbonat yang susunannya didominasi oleh ooid disebut sebagai oolites. Ooid dengan bentuk spherical atau subspherikal yang menunjukkan lapisan konsentrik internal dengan total tebal lebih besar dari pada nukleusnya disebut sebagai ooid dewasa. Ooid terbentuk saat kondisi arus bawah permukaan kuat dan air teragitasi terbentuk dan saat kadar saturasi kalsium bikarbonat tinggi didapati. Lapisan pada ooid modern pada umumnya berupa aragonit, sedangkan ooid purba dilapisi oeh kalsit pada umumnya. Banyak dari ooid purba ini lapisan-lapisan di bagian tepinya disusun oleh aragonit pada awalnya yang kemudian terubah menjadi kalsit, kendati demikian, bukti-bukti petrografis juga menyingkapkan bahwa ooid purba lain lapisannya disusun oleh kalsit dari awal. Presipitasi dari ooid kalsitik cukup signifikan terjadi pada Paleozoikum dan Mesozoikum(Morse & Mackenzie, 1990, hal,. 538). Variasai pada mineralogi ooid sepertinya memiliki kaitan dengan ketinggian muka air laut. Muka air laut pada keadaan high stand sepertinya lebih mempreferensikan pembentukan ooid kalsitik karena CO2 cenderung lebih tinggi dan rasio Mg/Ca relatif rendah pada keadaan tersebut, low stand lebih mempreferensikan ooid aragonitik karena kadar CO2 lebih rendah dan rasio Mg/Ca lebih tinggi(Wilkinson, Owen, dan Carrol, 1985). Seperti yang dijelaskan sebelumnya, rasio Mg/Ca yang tinggi akan lebih mendukung pembentukan aragonit karena kehadiran ion Mg akan menghalangi kristalisasi kalsit.

Gambar 1.3 Kenampakan Ooid pada sayatan tipis sampel batuan yang diambil dari Formasi Carmel, USA(Sumber: En.Wikipedia.org)

Gambar 1.3 Kenampakan Ooid pada sayatan tipis sampel batuan yang diambil dari Formasi Carmel, USA(Sumber: En.Wikipedia.org)

Meskipun kebanyakan ooid menunjukkan struktur internal yang terdiri dari lapisan konsentrik, beberapa ooid menunjukkan struktur internal radial. Ooid radial yang juga menunjukkan struktur konsentrik, kemungkinan terbentuk oleh proses rekristalisasi ooid normal, kendati demikian, ooid radial juga dapat terbentuk secara primer. Lapisan korteks pada beberapa ooid hanya terdiri dari satu atau dua lapisan yang sangat tipis, yang memiliki tebal lebih kecil dari pada nukleusnya. Ooid dengan karakteristik ini disebut sebagai ooid superficial atau pseudo-ooid. Butiran berlapis yang memiliki struktur internal serupa dengan ooid namun dengan ukuran lebih besar-lebih besar dari 2 mm- disebut sebagai pisoid(batuan yang disusun oleh pisoid disebut sebagai pisolites). Pisoid pada umumnya tingkat spherikalnya lebih rendah daripada ooid dan pada umumnya memiliki bentuk krenulasi. Beberapa pisoid dapat terbentuk oleh alga, yaitu oleh proses aktivitas penangkapan dan pengikatan oleh alga hijau-biru(cyanobacteria) dengan proses yang sama stromatolites terbentuk. Stromatolites spheroidal yang mencapai ukuran lebih dari 1 hingga 2 cm disebut sebagai oncoid.

Peloid

Peloid adalah terminologi butiran karbonat non-agregat yang disusun oleh kalsit/aragonit mikrokristalin atau kriptokristalin yang tidak menunjukkan kenampakan struktur internal. Peloid berukuran lebih kecil daripada ooid dan pada umumnya meiliki ukuran lanau hingga pasir halus(0.03-0.1 mm), meskipun beberapa bisa lebih besar. Jenis peloid yang paling umum adalah pellet fekal, yang diproduksi oleh organisme yang mencerna lumpur kalsium karbonat dan mengeluarkan lumpur yang tidak tercerna sebagai pellet. Pellet fekal cenderung berukuran kecil, oval hingga membundar, dan berukuran seragam. Pada umumnya ia memiliki kadar material organik yang cukup membuat kenampakannya menjadi opaque atau bewarna gelap. Pellet dapat dibedakan dari ooid oleh ketiadaaan struktur internal konsentrik atau radial, dan dibedakan dari intraklast membundar oleh keseragaman ukuran, sortasi yang baik, dan ukurannya yang kecil. Karena peloid dibentuk oleh organisme, ukuran dan bentuknya tidak berhubungan dengan mekanisme tranportasi oleh arus, meskipun pellet dapat ditransport oleh arys dan terendapkan kembalu setelah pengendapan awalnya oleh organisme.

Gambar 1.4 Kenampakan Peloid pada sayatan tipis(sumber: : En.Wikipedia.org)

Gambar 1.4 Kenampakan Peloid pada sayatan tipis(sumber: : En.Wikipedia.org)

Peloid juga dapat dihasilkan oleh proses lain, seperti mikritisasi dari ooid kecil atau fragmen skeletal membundar yang disebabkan oleh aktivitas boring oleh organisme tertentu, terutama alga endolitik. Aktivitas boring ini mengubah butiran karbonat asal menjadi masa kalsit mikrokristalin homogen dan seragam. Beberapa peloid marin dapat juga terbentuk oleh presipitasi di sekeliling gumpalan bakteria aktif(e.g, Chafetz, 1986). Peloid lain dapat juga berupa intraklast membundar dengan ukuran sangat kecil yang merupakan hasil reworking dari lumpur semikonsolidasi atau agregat lumpur.

Butiran Agregat

Butiran agregat adalah butiran karbonat dengan bentuk irregular yang terdiri dari dua atau lebih fragmen karbonat(pellet, ooid, dan fragmen fosil)digabungkan oeh matriks lumpur karbonat yang pada umumnya bewarna gelap dan kaya akan material organik. Karena bentuk dari butiran agregat pada beberapa endapan karbonat modern, seperti di Bahama Banks, menyerupai bentuk sekumpulun anggur, ia pada umumnya disebut sebagai grapestone (Illing, 1954). Butiran agregat lain dengan kenampakan yang lebih halus diberi terminologi sebagai lumps. Tucker dan Wright(1990, p.12) mengajukan pendapat bahwa lumps merupakan hasil evolusi grapestone yang terus mengalami sementasi dan mikritisasi pada butirannya. Butiran agregat pada lingkungan karbonat modern pada umumnya disusun oleh aragonit namun pada batugamping purba lebih didominasi oleh kalsit. Butiran agregat pada lingkungan modern dapat dikenali dari bentuk botroidal dan ketiadaan struktur internal, kendati demikian terkadang ia bisa salah diidentifikasi sebagai intraklast. Pada kenyataanya beberapa geologis menganggap butiran agregat sebagai bagian dari intraklast(e.g., Scholle & Ulmer-Scholle, 2003, hal. 246). Butiran agregat jarang dijumpai pada batugamping purba, kemungkinan karena bentuknya telah terdistorsi oleh proses kompaksi selama diagenesis.

Gambar 1.4 Kenampakan grapestone pada sayatan tipis(sumber:www.geol.umd.eduz)

Gambar 1.4 Kenampakan grapestone pada sayatan tipis(sumber:www.geol.umd.eduz)

Kalsit Mikrokristalin

Lumpur karbonat yang disusun oleh kristal kalsit dengan ukuran sangat halus banyak dijumpai pada batuan karbonat purba sebagai tambahan terhadap butiran karbonat dengan ukuran pasir. Lumpur karbonat atau lumpur gamping juga hadir pada lingkungan modern dimana ia hadir secara dominan berupa kristal-kristal aragonit berbentuk jarum dengan ukuran 1 hingga 5 mikron(0.001-0.005 mm). Lumpur karbonat pada batuan gamping purba disusun oleh kristal kalsit dengan ukuran sama. Lumpur karbonat juga dapat mengandung mineral detrital dalam jumlah kecil seperti mineral lempung, kuarsa, feldspar, dan material organik dengan ukuran kecil. Ia memiliki kenampakan keabu-abuan hingga kecoklatan, kenampakan subtranslucent di bawah mikroskop, dan dapat dibedakan dengan sangat jelas terhadap butiran karbonat dan sparry calcite oleh ukurannya yang sangat kecil. Folk(1959) mengajukan kontraksi mikrit untuk kalsit mikrokristalin, terminology yang secara universal diadopsi untuk menekankan signifikansi sedimen karbonat berukuran butir sangat halus.

Mikrit dapat hadir sebagai matriks di antara butiran karbonat, atau ia juga dapat menyusun sebagian besar hingga keseluruhan batugamping. Batugamping yang disusun sebagian besar oleh dapat dianologikan secara tekstural dengan serpih atau batulumpur. Kehadiran mikrit pada batugamping purba pada umumnya diintrepertasikan sebagai indikasi pengendapan pada lingkungan dengan kondisi air tenang. Secara kontras, sedimen karbonat yang diendapkan pada lingkungan  di mana arusnya tinggi akan bebas dari lumpur karbonat. Dengan dasar aspek kimia murni, lumpur karbonat atau mikrit  secara teoritis dapat terbentuk oleh proses presipitasi inorganic aragonit, yang kemudian terkonversi menjadi kalsit, dari air permukaan yang memiliki sifat jenuh kalsium bikarbonat. Geologist kendati demikian belum yakin, seberapa banyak aragonit yang sebenarnya dihasilkan melalui proses inorganik pada lingkungan modern. Banyak dari lumpur karbonat modern sepertinya dihasilkan melalui proses organik. Prosesnya mencakup peluruhan alga karbonatan pada air dangkal untuk menghasilkan lumpur aragonit, dan pengendapan nannofosil karbonat(<35 mm) seperti cocolith pada laut dalam untuk menghasilkan lumpur kalsit(chalk).

Kalsit Sparry

Banyak batugamping mengandung kristal kalsit dengan ukuran besar, pada umumnya dengan rentang 0.02 hingga 0.1 mm, yang kelihatan jernih atau putih di bawah pengamatan lensa atau mikroskop. Kristal dengan karakteristik tersebut adalah kalsit sparry. Ia dibedakan dari mikrit oleh ukurannya yang lebih besar dan kejernihannya, dan dibedakan dari butiran karbonat oleh bentuk kristal dan ketiadaan struktur internal. Beberapa kalsit sparry dapat diamati di bawah mikroskop sebagai pengisi pori antar butiran sebagai semen. Kehadiran semen kalsit sparry pada pori intergranular mengindikasikan bahwa void yang ada di antara rangka batuan kosong pada saat pengendapan, mengindikasikan bahwa pengendapan berlangsung dengan kondisi air beragitasi tinggi yang menghilangkan lumpur halus.

Kalsit sparry juga dapat terbentuk pada batugamping purba oleh proses rekristalisasi butiran primer dan mikrit selama proses diagenesis. Kalsit sparry yang dibentuk oleh proses rekristalisasi dapat sangat sulit dibedakan semen kalsit sparry(Boggs, 1992, hal 549). Menjadi sangat penting untuk membedakan dua jenis kalsit sparry ini karena kesalahan identifikasi spar rekristalisasi dengan semen kalsit sparry dapat menyebabkan kesalahan intrepertasi lingkungan pengendapan dan klasifikasi batugamping.

Referensi:

Boggs, Jr. S.(2006): Principal of Sedimentology and Stratigraphy 4th edition, Hal 161-167, Pearson Education, inc., Upper Saddle River New Jersey.

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