Middle Jurassic radiolarian chert, Indarung, Padang ...directory.umm.ac.id/Data...

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


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.


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



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).


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).


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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