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Middle Jurassic radiolarian chert, Indarung, Padang District, and itsimplications for the tectonic evolution of western Sumatra, Indonesia
Andrew J. McCarthya,*, Basir Jasinb, Neville S. Hailec,1
aSoutheast Asia Research Group, Geology Department, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UKbGeology Programme, Faculty of Science and Technology, Universiti Kebangasaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia
cGeology Division, School of Biological and Molecular Sciences, Oxford Brookes University, Oxford, UK
Received 9 November 1998; accepted 10 January 2000
Abstract
Radiolaria from chert in the Indarung Area belong to the Transhsuum hisuikyoense Zone, indicating an Aalenian, lower Middle Jurassic,
age. Carbonate in the area has been dated as Upper Jurassic to Lower Cretaceous from the occurrence of Lovcenipora, and overlying tuff has
given a radiometric K/Ar age of 105 ^ 3 (Albian, uppermost Lower Cretaceous). The chert and carbonate are probably in tectonic contact,
with the chert faulted into the limestone during ENE-directed compression. This comprises one of the best dated occurrences of allochtho-
nous material in Sumatra and con®rms the accretion of oceanic material along the Sunda margin during Mid- to Late-Cretaceous times.
q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
Mesozoic sediments crop out below an unconformable
Tertiary volcanic cover in the Indarung area, 10 km E of
Padang city, West Sumatra (Figs. 1 and 2). Interpretations of
complex relationships of limestone, radiolarian chert, and
tuff, in this area, have given rise to varying views in the past.
A new age determination and additional ®eld evidence
presented here go some way to clarifying the stratigraphy,
although uncertainties remain. The relationships in this area
probably bear on those of similar complexes of Mesozoic
rocks along the West coast of Sumatra. Specimen numbers
referred to, preceded by UM, are stored in the Depart-
ment of Geology, University of Malaysia, Kuala
Lumpur; prepared radiolarian specimens are archived in
the Department of Geology, Universiti Kebangsaan Malay-
sia, Bangi, Malaysia.
2. Previous work
The geology of an area around Indarung, east of Padang,
West Sumatra, was mapped and described by Yancey and
Alif (1977). They recorded limestone with Lovcenipora, indi-
cating an Upper Jurassic to Lower Cretaceous age. They
described associated radiolarian cherts and terrigenous strata,
and suggested that these together formed a Mesozoic forma-
tion unconformably overlain by Cenozoic volcanic rocks.
Haile (1978) disagreed with some aspects of the interpre-
tation of Yancey and Alif (1977), based on ®eld observa-
tions he had made in the course of a palaeomagnetic
reconnaissance in 1975. In particular:
1. He considered that the inclusion of radiolarian cherts,
presumably formed in deep water, with shallow-marine
limestones in a single formation was inappropriate.
2. His reconnaissance had indicated that the cherts were less
extensive than shown by Yancey and Alif (no chert was
seen as outcrops or even as boulders further east than
about 0.5 km east of Ngalau Quarry), and he maintained
that some of the rocks at Ngalau Quarry are not chert, but
weathered strati®ed rock, possibly tuff.
3. He suggested that the undoubted radiolarian cherts seen
by him were probably large blocks rather than in situ
exposures.
4. Limestone and shale mapped by Yancey and Alif (1977)
downstream from the road bridge where the Golok High-
way crosses the Lubuk Peraku River, were mapped by
Haile as calcareous crystal-lithic tuff (con®rmed micro-
scopically in thin section), which youngs to the south, as
shown by small-scale cross-bedding.
Journal of Asian Earth Sciences 19 (2001) 31±44
1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S1367-9120(00)00009-2
www.elsevier.nl/locate/jseaes
* Corresponding author. Fax: 144-1224-286-045. Present address: Enter-
prise Oil plc, Victoria Tower, 62 Market Street, Aberdeen, AB11 5PJ, UK.
E-mail address: [email protected]
(A.J. McCarthy).1 Present address: 13 Talbot Road, Oxford, OX2 8LL, UK.
3. Additional ®eld evidence
This area was re-investigated by McCarthy during
research with the University of London Southeast Asia
Research Group in 1995 (McCarthy 1997).
3.1. Lubuk Peraku River and Golok Highway
In the E part of the area a carbonate sequence is overlain
by tuff, here named informally, the Lubuk Peraku Lime-
stone and the Golok Tuff respectively. The Lubuk Peraku
Limestone is a mildly deformed, apparently conformable
sequence of limestone, 200±250 m thick, exposed over a
250±300 m length in the Lubuk Peraku River, upstream
from the bridge carrying the Padang-Solok Road (Fig. 3).
The sequence dips steeply to the SW, and from scours and
cross-bedding demonstrably youngs in that direction. Most
of the sequence consists of limestone conglomerate/breccia
and massively bedded limestone (Fig. 4). The lowest brec-
cias exposed contain a small percentage of ®ne-grained
basic igneous clasts (Fig. 5a), but further up the section
the clasts are almost all carbonate (Plates 5b and c), and
the proportion of massively bedded limestone increases.
The congomerates/breccias are poorly sorted, clast
supported, and include subrounded to subangular clasts up
to several metres in diameter, containing a wide variety of
textures, from shelly marls to oolite. The matrix is sandy
and locally oolitic limestone (UM8457;UM8458). Dykes
and volcanic interbeds are common towards the top of the
section, particularly in the last 50 m before the road bridge
(Fig. 5d). Both dykes and interbeds are heavily altered and
dominated by quartz and feldspar, with little ma®c mineral
intact. Present day compositions are acid/intermediate but
may be signi®cantly more silicic than the original composi-
tions, since there is evidence of replacement of other phases
by ®ne-grained quartz.
A 6 m interval of ®nely interbedded limestone and shelly
marl shows boudinage, shale (marl) ¯owage and calcite
veining. (Fig. 5e). Otherwise the only deformation seen,
apart from the steep bedding dips, is one signi®cant fault
and minor brittle deformation along bed and dyke bound-
aries. The fault strikes at 0108 across the river (Fig. 3) and is
accompanied by extensive fracturing and veining; displace-
ment is unlikely to exceed 10 m. Massively bedded unde-
formed S-dipping limestone (shelly algal biosparite) occurs
along a 45 m length in the gorge below the bridge. N.R.
Cameron (personal communication, 1998) noticed possible
altered pillow basalts and limestone breccia below the road
bridge along a stretch of the river which is no longer
accessible.
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±4432
Fig. 1. Location of the study area and the present day plate tectonic setting.
Fig. 2. Sketch map showing distribution of the studied outcrop areas of Jurassic/Cretaceous units. Note that apart from Karang Putih and Ngalau quarries,
exposure is limited to river and road cuttings beneath a cover of Quaternary alluvium or Tertiary volcanics. The structural data shown outside the areas covered
by Figs. 3 and 7 are from Yancey and Alif (1977), from which this ®gure is adapted (with a corrected scale).
A.J.
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Fig. 3. Geological sketch map of the Lubuk Peraku river section and adjacent road section. See Fig. 2 for location.
The Golok Tuff, calcareous crystal vitreous tuff
(UM8456), containing sand-grade beds dipping and young-
ing to the S, was observed by Haile (1978) overlying the
limestone along the road and in the gorge below the bridge.
He recorded that along the road the transition between lime-
stone breccia with altered crystal clasts, possibly volcanic,
occurred along a steep irregular contact, possibly faulted.
The tuff there is intruded by a cross-cutting light-coloured
altered dyke at least 2 m thick. These exposures were not
visible in 1995, probably having become overgrown in the
intervening 20 years. The tuff sequence, however, was seen
along the road for about 600 m, and showed occasional
graded bedding and ripple cross-lamination con®rming
general younging to the S, and consistent with deposition
in water. Dips are mostly steep to both E and W, and the
strata appear to be folded on the 50±100 m scale.
3.2. Karang Putih Quarry and Ngalau Quarry Area
Carbonate (limestone and marble) and chert are exposed
in the W part of the area, about 1.5 km WSW of the bridge
over the Lubuk Peraku River (Fig. 7). These two lithologies
are here referred to by the informal names Karang Putih
Carbonate and Ngalau Chert respectively.
The Karang Putih Quarry is situated on and around a
single steep-sided hill which consists almost entirely of
variably metamorphosed limestone, which from the topo-
graphy appears to represent at least 300 m vertical
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±4434
Fig. 4. Schematic stratigraphic log of the Lubuk Peraku River section.
(stratigraphic?) thickness (Fig. 6a). All the carbonate seen in
this quarry in 1995 is apparently recrystallized, hard, and
compact, with little or no indication of bedding or other
fabric. Kendarsi (1964) suggested that a broad zoning exists,
with `hard baked limestone' in the N, through marble in
central parts to calcite in the S, re¯ecting increasing degrees
of thermal alteration due to the emplacement of a (poorly
mapped) granitic body to the S. No mention is made of
fossiliferous strata in this publication or in a report by
Padang Semen (1994).
By contrast Yancey and Alif (1977) describe a range of
lithologies `from micrite to biosparite to oolitic calcarenite,
while the most common rock type is biosparite containing
abundant bioclasts', which they assign to the Karang Putih
Member of their Indarung Formation (Fig. 8). These obser-
vations seem inconsistent with the observations made by
McCarthy in this study, and by the other studies quoted
above, that the hill is composed largely of massive pure
light-coloured recrystallized limestone. Possibly Yancey
and Alif collected material from cliff exposures on the
banks of the Padang Idas River, rather than from the hill
itself, or less altered limestone could have been exposed in
the quarry at that time.
A number of small isolated lenses or sliver-like bodies of
®ne-grained siliceous rock have been observed at various
levels within the limestone in the quarry. The chert out-
crop studied (Fig. 7) measured 25 m long (in an ENE
direction) £ 5 m wide £ 3 m high. In contrast to the
surrounding carbonate, the chert is intensely deformed by
disharmonic folding and some associated contractional
faulting (Fig. 6b) at this place, indicating folding about
shallow-plunging N±S or NNW±SSE axes (Fig. 7). The
boundary between the chert and limestone is not clearly
exposed, but ®eld evidence suggests that it is a low angle
contact. A low angle contact, such as a bedding plane or
thrust plane, must be of irregular geometry, since carbonate
nearby and along strike from the chert occurs at the same
elevation (Fig. 6c). A dyke or stock of andesitic igneous
rock (UM 8455; seen as 2 m thick in 1975) intrudes the
limestone on the W side of the quarry.
At Ngalau Quarry (Fig. 2) Yancey and Alif (1977) show a
crescent-shaped outcrop of limestone, overlain by cherts.
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±44 35
Fig. 5. Lubuk Peraku Limestone: (a) Subrounded clasts of bioclastic limestone and minor dark volcanics; (b) Bioclastic limestone with ?brachiopod shell;
(c) Shelly limestone; (d) Lenticular intrusion or interbed of ®ne-grained igneous rock in limestone conglomerate; (e) Interbedded limestone and marl showing
ductile deformation and incipient boudinage.
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Fig. 6. (a) View north across the Karang Putih Quarry from the chert outcrop at its southern end. The hill in the foreground is Karang Putih carbonate. Foothills of the Barisan Range in the distance rise from the
W Sumatra coastal plain at left. (b) Folding and faulting in bedded Ngalau Chert, Karang Putih Quarry, looking north. (c) Obscured junction of Ngalau Chert (right of track) and Karang Putih carbonate (left),
consistent with a tectonic contact, looking west, nearly perpendicular to structures within the chert. (d) Irregular folding of Ngalau Chert at Ngalau Quarry.
Neither Haile nor McCarthy could ®nd limestone at that
locality; McCarthy recorded only folded well bedded cherts
forming cliffs 15 m high extending for approximately 200 m
(Fig. 6d). Haile recorded weathered bedded rocks at Ngalau
Quarry, which he suggested might be weathered tuff, and
cherty rocks to the north, possibly large blocks, and includ-
ing some reddish-brown radiolarian chert (specimen
UM8428, see below). However, since Yancey and Alif
(1977) recorded a fossiliferous sample, presumably lime-
stone, from Ngalau Quarry, there must be some limestone
at or near this locality, unless their sample was wrongly
located.
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±44 37
Fig. 7. Topographic and geological map of the Karang Putih Limestone quarry. Mapped area of siliceous rocks and joint measurements are from Padang
Semen (1994). See Fig. 2 for location.
4. Age of the units
4.1. Karang Putih Carbonate and Golok Tuff
The carbonate from Lubuk Peraku, Karang Putih, and
Ngalau have all, according to Yancey and Alif (1977),
yielded Lovcenipora, which indicates an Upper Jurassic to
Lower Cretaceous age. Koning (1985) cites a Caltex Paci®c
Indonesia internal report recording a radiometric (K/Ar) age
of 105 ^ 3 Ma (Albian, uppermost Lower Cretaceous) for
tuffs at Lubuk Peraku, although Cameron (pers. comm.)
urges caution in the use of this date.
4.2. Radiolaria and age of the Ngalau Chert
A sample of chert (UM 8428) collected by Haile in 1972
has been examined recently by Basir Jasin, with the follow-
ing results:
Six radiolarians were identi®ed to species level:
Eucyrtidiellum disparile Nagai and Mitzutani (Fig. 8-1)
Eucyrtidiellum unumaense s.l. (Yao) (Fig. 8-2)
Hsuum exiguum Yeh and Cheng (Figs. 8-3, -4)
Hsuum fukazawaense (Sashida) (Fig. 8-5)
Parvicingula gigantocornis Kishida and Hisada (Fig. 8-6)
Transhsuum hisuikyoense (Isozaki and Matsuda) (Fig. 8-7)
These species indicate that the assemblage belongs to the
Transhsuum hisuikyoense Zone (Yao, 1990; Takahashi and
Ishii, 1995). The zone is de®ned by Nishizono et al. (1997)
as Zone J4 which is Aalenian, lower Middle Jurassic. Hori
(1990) suggested that the Transhsuum hisuikyoense Zone is
Aalenian-lower Bajocian. The occurrence of Eucyrtidiellum
disparile and Eucyrtidiellum unumaense is indicative of an
Aalenian-lower Bajocian age (Nagai and Mizutani, 1990).
We thus conclude that the chert is Aalenian to lower Bajocian.
Other taxa identi®ed are:
Angulobracchia sp. (Fig. 8-8)
Archicapsa cf. pachyderma (Tan) (Fig. 8-9)
Bernoullius sp. (Fig. 8-10)
Bistarkum sp. (Fig. 8-11)
Elodium sp. (Fig. 8-12)
Homoeoparonaella cf. elegans (Pessagno) (Fig. 8-13)
Hsuum cf. matsuokai Isozaki and Matsuda (Fig. 8-14)
Katroma sp. (Fig. 8-15)
Orbiculiforma sp. (Fig. 9-1)
Parahsuum sp. (Fig. 9-2)
Paronaella cf. corpulenta De Wever (Fig. 9-3)
Paronaella cf. skowkonaensis Carter (Fig. 9-4)
Paronaella sp. (Fig. 9-5)
Parvicingula sp. (Fig. 9-6)
Parvicingula cf. spinata (Vinassa) (Fig. 9-7)
Praeconocaryomma cf. decora Yeh (Fig. 9-8)
Praeconocaryomma sp. (Fig. 9-9)
Pseudoeucyrtis sp. (Fig. 9-10)
Spongotrochus sp. (Fig. 9-11)
Tetratrabs sp. (Fig. 9-12)
Transhsuum sp. (Fig. 9-13)
5. Relationship of the units
The bedded conglomeratic carbonate sequence (Lubuk
Peraku Limestone) appears to be internally conformable.
The appearance of volcanic inclusions near the top of the
sequence suggests that the carbonates pass conformably, or
at least without major structural discontinuity, into the over-
lying Golok Tuff. Accepting the radiometric age for the tuff
as Albian (uppermost Lower Cretaceous), and the limestone
as Upper Jurassic to Lower Cretaceous, the age of both is
probably Lower Cretaceous (at most the carbonate could
range down into the uppermost Jurassic, but this seems
unlikely). The association of carbonate breccia containing
volcanic clasts decreasing in quantity up-sequence is remi-
niscent of a carbonate cap formed on a seamount. The over-
lying Golok Tuff thus represents a renewal of volcanic
activity (possibly from the same seamount).
The lower Middle Jurassic (Aalenian) age of the Ngalau
Chert, based on radiolaria, indicates that it is signi®cantly
older than the carbonate at Karang Putih. If these age deter-
minations are accepted, then the chert and limestone must
be in tectonic contact, which is supported by:
1. The probable shallow water depositional environment
indicated for the carbonate, compared with deep-water
for the radiolarian chert.
2. The intense deformation of the bedded chert compared
with the carbonate.
Thus it is probable that the chert is faulted into the lime-
stone, and compressional emplacement, probably along
low-angle faults, is proposed. It is noteworthy that no
chert clasts are found within the conglomeratic limestones,
further supporting an allochthonous origin for the chert.
A proposed revised stratigraphy for the Mesozoic of the
Indarung Area is shown in Fig. 10. In view of the remaining
uncertainties, and the fact that the base of the carbonates is
not exposed, we consider it would be premature to de®ne
formal stratigraphic units (formations), and have thus
suggested informal names.
The structures within the cherts, the style of deformation,
and the general orientation of all structures (bedding, fold
axes, faults, joints) in the Indarung Area are consistent with
prolonged or intermittent ENE±WSW directed compression
(Figs. 2, 3 and 7). Such a regime could have emplaced older
SW-derived oceanic chert into carbonate sequences along
the active margin and folded the Mesozoic sequence of the
area into its present con®guration (Fig. 11). Deformation of
this style and intensity is unknown in the Tertiary of Suma-
tra. Furthermore, emplacement of the chert must have
predated the Tertiary development of the modern forearc
basin which now separates the Sumatran coast from the
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±4438
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±44 39
Fig. 8. Radiolaria from the Ngalau Chert (Scale bar in mm is indicated in parentheses). (1) Eucyrtidiellum disparile Nagai and Mitzutani (83 mm). (2)
Eucyrtidiellum unumaense s.l. (Yao) (59 mm). (3,4) Hsuum exiguum Yeh and Cheng (83 mm and 100 mm respectively). (5) Hsuum fukazawaense (Sashida)
(227 mm). (6) Parvicingula gigantocornis Kishida and Hisada (100 mm). (7) Transhsuum hisuikyoense (Isozaki and Matsuda) (100 mm). (8) Angulobracchia
sp. (156 mm). (9) Archicapsa cf. pachyderma (Tan) (100 mm). (10) Bernoullius sp (100 mm). (11) Bistarkum sp. (119 mm). (12) Elodium sp (100 mm). (13)
Homoeoparonaella cf. elegans (Pessagno) (119 mm). (14) Hsuum cf. matsuokai Isozaki and Matsuda (100 mm). (15) Katroma sp. (78 mm).
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±4440
Fig. 9. Radiolaria from the Ngalau Chert (Scale bar in mm is indicated in parentheses). (1) Orbiculiforma sp. (156 mm). (2) Parahsuum sp. (78 mm). (3)
Paronaella cf. corpulenta De Wever (227 mm). (4) Paronaella cf. skowkonaensis Carter (119 mm). (5) Paronaella sp. (100 mm). (6) Parvicingula sp.
(100 mm), (7) Parvicingula cf. spinata (Vinassa) (100 mm). (8) Praeconocaryomma cf. decora Yeh (119 mm). (9) Praeconocaryomma sp. (78 mm).
(10) Pseudoeucyrtis sp. (156 mm). (11) Spongotrochus sp. (119 mm). (12) Tetratrabs sp. (100 mm). (13) Transhsuum sp. (100 mm).
A.J. McCarthy et al. / Journal of Asian Earth Sciences 19 (2001) 31±4442
Fig. 11. Tectonic model/schematic cross-section of the Mesozoic of the Indarung area. Note that the east end of the section is constrained by structural data
(see Figs. 2 and 3). The west end is less so, and the larger structure is speculative although consistent with ®eld and biostratigraphic data.
Fig. 12. Distribution of Jurassic/Cretaceous units in Sumatra and their spatial relationship to the Sumatran Fault System. References: 1 Cameron and Djunuddin
(1980); 2 Wajzer et al. (1991); 3 McCarthy (1997); 4 Rosidi et al. (1976); 5 Fontaine and Gafoer (1989); 6 Gafoer et al. (1992b); 7 Gafoer et al. (1992a); 8 Gafoer
et al. (1993).
reported in several areas south of the Batang Natal section
(e.g. McCarthy, 1997; McCourt et al., 1996). The Siguntur
Formation to the south of Indarung is unusual among Juras-
sic/Cretaceous sections of western Sumatra in being domi-
nated by mildly metamorphosed ®ne-grained clastic
sediments and volcanics (McCarthy, 1997). The minor
carbonates are reported to contain Lovcenipora (Rosidi et
al., 1976) as reported near Indarung by Yancey and Alif
(1977), but the occurrence is otherwise very different from
the nearby Indarung cherts and carbonates. A turbidite
origin has been proposed by De Smet (1991).
Serpentinised ultrama®cs outcrop south of Galagah and
are sheared and almost certainly fault-emplaced (McCarthy,
1997), as are other mapped occurrences in southern Suma-
tra. The Gumai Mountains of southern Sumatra reportedly
contain serpentinite and radiolarian chert, the latter within
the Cretaceous Lingsing Formation (Gafoer et al., 1992a).
Similarly the Garba Mountains are reported to contain
Jurassic/Cretaceous serpentinite, melange and basalt
(Gafoer et al., 1993).
The Jurassic/Cretaceous occurrences lie dominantly to
the W of the active right-lateral Sumatran Fault System
(SFS) Ð a lithosphere-scale structure which dates from at
least the mid-Tertiary. (McCarthy, 1997; McCarthy and
Elders, 1997). It has been suggested (e.g. Bennett, 1978;
Pulunggono and Cameron, 1984) that the SFS lies along a
late Mesozoic suture separating these Jurassic/Cretaceous
occurrences from older pre-Tertiary rocks to the E.
It is clear that if the serpentinites, cherts, and ultrama®cs
do mark a suture, the SFS does not follow it for its entire
length, particularly in southern Sumatra. Furthermore the
relations between the original occurrences of Mesozoic
pelagic sediments and ultrama®c rocks are at best confused.
Pre-Tertiary basement is unproven west of the SFS and
south of the Siguntur section. Ultrama®c rocks are not
con®ned to areas of Mesozoic basement (as south of Gala-
gah where they are found amid carbonates of probable
Permian age). This may suggest that the suture(s) has
been disrupted.
The Indarung units comprise one of the best dated occur-
rences of allochthonous material in Sumatra. The associa-
tion of chert and thick, massive limestone in the area seems
to have no direct extensions in Sumatra, and it is unclear
why it appears to be so isolated. It is possible that strata of
similar age and derivation underlie much of the western
coast of Sumatra, or that an originally more extensive
body was dismembered by late strike-slip faulting. Alterna-
tively, the area may represent an original distribution related
to a feature of limited extent, such as a seamount, which
collided with or was telescoped against the Sumatran
margin during late Mesozoic times.
This study con®rms that Mesozoic tectonism in Sumatra
took the form of the accretion of allochthonous oceanic
material (and possibly small continental or ocean island/
plateau fragments) and the consequent deformation of auto-
chthonous sediments along the Sunda margin. This
tectonism imparted a strong structural grain to the margin.
Compression was directed nearly perpendicular to the
margin, as it seems to have been through much of the
Cenozoic (McCarthy and Elders, 1997).
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