Volcanogenic Tonsteins from Bukit Asam Coalfield, South ...

1Sriwijaya International Conference on Engineering, Science and Technology (SICEST) 2016

1.1

Volcanogenic Tonsteins from Bukit Asam

Coalfield, South Sumatra Basin, Indonesia

Ferian Anggara1*, Amanda A. Sahri2, Zain A. N. Asa2, D. Hendra Amijaya1

1Geological Engineering Department, Gadjah Mada University, Yogyakarta 52884, Indonesia

2Undergraduate program Geological Engineering Department, Gadjah Mada University, Yogyakarta 52884, Indonesia

*Corresponding Address: [email protected]

In the Muara Enim Formation, at least 18 coal layers are found, however only five main coal

seams layer in Bukit Asam coalfield e.g the A1 and A2 (Mangus) seams, B1 and B2 (Suban) seams

as well as C (Petai) seams are considered economic to be mined. A major influence on the

evolution of peat mires and coal formation in the Muara Enim Formation was volcanic activity

during the deposition. Volcanic eruptions in the surrounding area produced ash which accumulated

as claystone layers or tonsteins in the coal seams. Based on systematic field mapping, several

tonsteins layers were founded and sampled in the research area, e.g. tonsteins in interseam layer of

A1, A2, B1, B2 and C seam. This paper only focused on the tonstein in between B and C seam. The

mineralogy of the tonsteins has been evaluated using thin section and qualitative X-ray diffraction

(XRD) techniques. Tonstein in Bukit Asam coalfield, consisting mainly of kaolinite, quartz,

dickite and calcite. The results suggest that these tonsteins were derived from acid volcanic ash

fallout, which was subsequently altered into relatively kaolinitic rocks through hydrolysis and

diagenesis in a peat-bog environment. The tonsteins in Muara Enim Formation are significance in

providing distinctive horizons for stratigraphic correlation purposes.

Keyword: Tonstein, Bukit Asam Coalfield, Muara Enim Formation

1. Introduction

Tonsteins are widespread clay bands or partings

as the alteration product of volcanic ash that

associated with coal beds, contains dominantly

glass from the original ash and phenocysts of

quartz, magnetite/ilmenite, kaolinite, smectite

and other minerals [1]. Tonsteins layer were

founded in almost tertiary coal basin in

Indonesia including Bukit Asam coalfield, South

Sumatera, Indonesia. Characteristics of tonstein

in Bukit Asam coalfield based on [2] showed

that Rb, Cs and major element K are highly

leached from tonstein while elements Ga and Ti

were immobile and residual in tonstein.

Tonsteins has been used widely in coal geology,

e.g. as a tool for radiometric dating, identify

paleo-depositional environments, and parameter

to identify coalification, diagenesis and coal

quality [1]. Nowdays, some tonstein may

contain valuable trace elements that could be

potential in searches for alkali ore-deposits [3].

The presence of tonsteins also influence the

composition of macerals in coals. Coals were

enriched in desmocollinite, tellinite and

detrocollinite at top of the tonstein layer as

result of poorly-drained swamp conditions,

while coals below the tonsteins layer were

enriched in semifusinite, inertodetrinite and

fusinite as result of well-drained swamp

condition [4].

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1.1 Thus, study on tonsteins characteristics in the

research area especially for stratigraphic

correlation purpose is becoming very important

2. Geological Background

The Muara Enim Formation (MEF) was

deposited during regressive period of a

transgressive-regressive cycle in the South

Sumatera Basin [5]. The thickness of Muara

Enim Formation is around 500 – 700m, about

15% of which is coal. Top and bottom of this

unit are defined by the upper and lower

occurrence of laterally continuous coal beds.

The formation consists of stacked shallowing-

upward parasequences, typically 10 – 30 m

thick, with shallow marine or bay clays at the

base, and shoreline and delta plain facies (sand,

clay, coal) at the top, see Figure 1.

Figure 1. South Sumatra Basin Stratigraphy [6]

Coals rank in the MEF of Bukit Asam coalfield

are vary from sub-bituminous to semi-anthracite

as the result of igneous intrusion of andesitic

composition [2]. The MEF can be divided into

two units, the lower unit are the most economic

coal in Bukit Asam coalfield, consists of A

seams (Mangus), B seams (Suban) and C seams

(Petai). This unit dominantly consists of coal,

dark grey to black shale, brownish – grey

claystone and sandy claystone. While the upper

unit consists of shale, coal, tuff, claystone and

sandstone. The boundary between lower unit

and upper unit is the top of A1 seam (upper

Mangus). The age of this formation is Late

Miocene – Early Pliocene [6].

Figure 2. Measured section of West Banko coal

field indicated samples used in this study.

3. Sample and Method

Systematic field mapping has been conducted in

the Banko field and several tonsteins layers in

interseam layer of A1, A2, B1, B2 and C seam

have been founded. In this paper, we only

focused on collecting and analyzing tonstein

samples in between B and C seam (Figure 2).


1.1 There were 6 samples for thin section and XRD

(bulk powder and oriented clay) analysis. XRD

analysis with oriented clay method (air dried,

ethylene glycol and heated) is very useful to

analyze clay mineral that cannot be determined

by thin section analysis.

Figure 3. Outcrop of tuffaceous coal (a) of C seam,

claystone (b) and sandy tuff (c) tonstein layers

of interburden B1-B2 seam on the Banko field.

4. Result

4.1 Systematic Field Mapping

Based on Figure 2, there are very thin layer of

tuffaceous coal (sample TS-1 and TS-3) on the

top of C seam. The thickness of the two layers

are 5 cm to 15 cm, respectively. Thin clean coal

seam in between those layer is also sampled and

analyzed and called as TS-2 sample.

Megascopically, TS-1 and TS-3 samples are

light gray in colour and the roof this layer is

sandstone. TS-2 is dull banded friable coal.

Claystone (TS-4), coal (TS-5) and sandy tuff

(TS-6) in between B2 and B1 seam are sampled

and analyzed. TS-4 is 20 cm to 25 cm, grey to

the yellowish grey with laminated sedimentary

structure. TS-5 is 25 cm, black, and dull banded.

Sandy tuff is 20 cm, light grey with clay to sand

grain size (Fig. 3)

4.2 Petrographic Analysis

Tonsteins samples are examined under the

microscope and shows granular with a median

grain size of about 100 µm and a maximum of 1

mm. The type of this tonsteins based on [8] is

called pellet (graupen) tonsteins. The grains are

dominant composed of feldspar as well as quartz

and opaque minerals in minor composition (Fig.

4). Most of the feldspar are altered to kaolinite.

Quartz grains have low sphericity and

subangular and indicate as a product of

explosive volcanism rather than normal

sedimentary processes. Kaolinite grains are

mainly consist of fine-grained kaolinite with

orientated inclusions, and appears isotropic or

with weak aggregate polarization. This is

suggests an original altered volcanic rock

fragments or plagioclase feldspar.

Figure 4. (a) thin section of samples TS-3 (b)

thin section of samples TS-1. (c) quartz as

filling fractures in samples TS-6. Opq = opaque

mineral, Fsp = feldspar, Qz = quartz


1.1

Figure 5. XRD of samples TS-3 (a), TS-2 (b)

and TS-1 (c). K = kaolinit, Q = quartz

Figure 6. (a) XRD of samples TS-6 (a) and TS-4

(b). K = kaolinit, Q = quartz, D = dickite

4.3 XRD Analysis

The results show that kaolinite is the dominant

mineral found in XRD analysis, with only small

amounts of other minerals (Fig 5 and 6). Based

on Figure 5 and 6, tonsteins shows well-ordered

kaolinite. Spears [7] mentioned that tonsteins

from an altered volcanic ash is characterized by

well-ordered kaolinite in XRD peak data.

Quartz, feldspars, dickite, and calcite are found

as minor constituent.

5. Discussion

The megascopic description and laboratory

analysis shows tonsteins in the reseach area is

mostly composed of volcanic material. Altered

feldspar and clay mineral are observed from thin

section analysis. Based on XRD analysis, most

of the clay mineral is composed by well-ordered

kaolinite that is indicated volacanic ash origin.

Previous study indicate that the presence of

tonsteins in coal influence the percentage of

maceral composition in coal seam [2]. The plies

macerals and megascopic characteristics of the

plies immediately below and above the tonsteins

in the adjacent coal layers. The plies below the

tonstein generally have a ~10 cm of bright

banded coal, have higher liptinite, inertrinite,

detrovitrinite, and detrital macerals higher

mineral content. On the other hand, the plies

above tonstein show less convincing trends. In

some areas, samples from this plies contain less

detrital macerals than the coals immediately

above but on the other hand, some of ply

samples have higher.

6. Conclusion

A major influence on the evolution of peat mires

and coal formation in the MEF was volcanic

activity during the deposition. Volcanic

eruptions in the surrounding area produced ash

which accumulated as tonsteins in the coal

seams. The study of the tonsteins in Bukit Asam

coalfield has revealed their macroscopic

characteristic such as have 15 to 20 cm in thick,

light colored and mostly composed of kaolinite

as well as quartz, dickite, feldspar, and calcite as

minor constituents. Based on the mineralogical

composition, tonsteins in the research area were

derived from acid volcanic ash fallout, which

was subsequently altered into relatively

kaolinitic rocks. detrital maceral contents.

Within the distinctive character in term of

megascopic characteristic as well as mineralogy

composition, tonsteins layers in the research

area is useful to be used as a stratigraphic

marker for correlation purposes.

References

[1] Triplehorn, D.D., 1990. Applications of

tonsteins to coal geology: some example

from western United States. International

Journal of Coal Geology (16), pp: 157-

160.


1.1 [2] Pujobroto, A.,1997. Organic Petrology and

Geochemistry of Bukit Asam coal, South

Sumatera, Indonesia. Doctor of

Philosophy thesis, School Geosciences,

University of Wollongong.

[3] Dai, S., Wang, X., Zhou, Y., Hower, J.C.,

Li, D., Chen, W., Zhu, X., 2011.

Chemical and mineralogical

compositions of silicic, mafic, and alkali

tonsteins in the Late Permian coals from

Songzao Coalfield, Chongqing,

Southwest China. Chem. Geol. 282, 29-

44

[4] Crowley, S.S., Stanton, R.W. and Ryer,

T.A., 1989. The effects of volcanic ash

on the macerals and chemical

composition of the C coal bed, Emery

Coalfield, Utah. Organic Geochemistry,

14, 105-156

[5] de Coster G.L., 1974. The geology of

Central and South Sumatra Basins:

Proceedings 3rd Annual Convention

Indonesian Petroleum Association, pp:

77-110.

[6] Darman, H., and Sidi, F.H., 2000. An

outline of the geology of Indonesia.

Indonesian Association of Geologists,

Jakarta, 254 p.

[7] Spears, D.A., 2012. The origin of tonstein,

an overview, and links with seatearths,

fireclays and fragmental clay rocks.

International Journal of Coal Geology

(94), pp: 22-31.

[8] Diessel, C.F.K., 1992. Coal-Bearing

Depositional Systems. Springer Berlin

Heidelberg, Berlin, Heidelberg.

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