Geology and jungle fieldwork in eastern Indonesia

SIMON BAKER, ROBERT HALL & EMILY FORDE

Collecting geological information from the jungles and islands of eastern Indonesia depends largely on traditional
fieldwork methods involving much footwork and messing about in boats. Coupled with modern laboratoy and
coniputinE- techniques, information gained on the ground is being used to interpret the tectonic development of east
Indonesia, the Philippine Sea and the western Pacific.

F o r the geologist, south-east Asia is one of Fig. 2.Geographic Australia separated from Antarctica at about
the most intriguing areas of the Earth. T h e map of east Indonesia the same time as India but moved less quickly
mountains of the Alpine-Himalayan belt make and the Philippine Sea northwards; instead of a direct collision with
a major bend into Indochina and terminate in region, showing the another continent, Australia is now making a
a region of continental archipelagos, island principal tectonic glancing collision with a composite south-east
features; heavy lines Asia, which already includes some of the ear-
arcs and small ocean basins. To the south, west with ticks are
and east the region is surrounded by subduction lier Gondwana fragments to arrive. In east
subduction zones,
zones where Pacific and Indian Ocean rocks heavy lines without Indonesia, the area of our field projects, north-
are currently being subducted at a high rate, ticks are major strike- wards movement of Australia during the Ter-
accompanied by intense seismicity a nd slip faults, stars tiary has been marked by arc-continent colli-
spectacular volcanic activity. T h e abrupt represent major sion and major strike-slip motion at the north
division between Eurasian and Australian flora volcanic centres in Australian continental margin. During thc same
and fauna in Indonesia, described by Wallace active arcs. Top inset period the Philippine Sea Plate has been
in the nineteenth century, has its origin in the shows the study area. subducting beneath the east Asian margin,
rapid plate movements and reorganization of
landmasses in this region.
In very general terms, the region owes its
origin to the break-up of the Gondwana
supercontinent, the subsequent movement of
Gondwana fragments northwards, and their
eventual collision with Eurasia (Fig. 1). India
completed its passage in the Tertiary and col-
lided head-on with the Asian landmass. Ac-
cording to Tapponnier and colleagues, the
impact of the rigid Indian indentor caused
extrusion and rotation of continental frag-
ments and opening of some of the small oce-
anic marginal basins of south-east Asia.
Fig. 1.Map illustrating the break-up of
Gondwana and the subsequent movement of
India and Australia northward from the Mesozoic
to the present day.

18IGEOLOGY TODAY J a n u a y - February 1994

producing a chain of island arcs surrounding
the Philippine Sea. T h e north Australian
orogenic belt and the western Pacific arc sys-
tems link together in a complex knot in east
Indonesia which we have been attempting to
unravel during several field expeditions in the
last few years (see Fig. 2. for location).

Field operations
Our work has been very dependent upon
established geological methods. The region
was practically unknown from a geological
point of view until about 20 years ago; now we
probably know about as much as Highland
geologists knew of the Scottish mountains 100 Fig. 4.Investigating centre of power and population in the north
years ago. T h e advantages we have over the west coast of Moluccas, along with its adjacent and rival
nineteenth-century geologists are some modern Halmahera by island Tidore. Despite their small size (both
tools such as aerial photographs, satellite images longboat. T h e sheer are only 10 km in diameter), they have been
cliffs are formed from bitterly fought over b y the Spanish, the Portu-
and satellite location devices, but like them we Oligocene
often have relatively poor topographic maps. guese and later the Dutch and British to con-
volcaniclastic
Laboratory techniques definitely give us an sedimentary rocks of
trol the precious trade in cloves. Ternate town
edge when we have collected the rocks: isotopic the Tawali Formation is still a busy port and makes the ideal base for
dating, biostratigraphy, geochemistry, and illustrate some of our investigations of the region. For several
microprobe, palaeomagnetism, gravity and the problems in seasons we have hired a 25-m boat with eight
seismic tools are now routine. However, first sampling. crewmen and stocked it with fuel and provi-
we have to get our material. Twentieth-century sions for a 50-day field expedition. T h e Sznar
aircraft may get us to east Indonesia quickly, Bahtera is a local, general-purpose boat nor-
but once there transport is essentially nineteenth mally used for fishing and transportation; the
century; we move about using small slow boats crew members are mainly young, hard-work-
Fig. 5. Lowering skies
and on foot. over the rugged ing and wonderfully friendly. Provisions avail-
After obtaining government clearances and topography of north- able without refrigeration are basic: largely
meeting our Indonesian colleagues, we fly to east Halmahera rice, noodles, sardines and corned beef along
the main administrative centre in Halmahera, indicate a few wet, with garlic, tea, coffee, sugar and a few goodies
Ternate. Ternate has long been the major exhausting days at the to ward off mutiny amongst the geologists. All
site of our next
traverse. A low-lying
Fig. 3.View of the Halmahera volcanic arc alluvial strip (-5 km
looking south from Ternate; the islands of wide) terminates
Makian, Moti and Mare are just visible in the abruptly against
distance. Tidore displays the classic conical shape upfaulted ophiolitic and
often developed by mature arc volcanoes, while sedimentary basement
the drowned tree in the foreground attests to rocks that have been
neotectonic Earth movements. Makian was the deeply incised by the
site of the last major eruption in the arc in 1988. Akelamo River.

GEOLOGY TODAYJanuay-Februay 1994119

 Fig. 6.Climbing out of collection of geological data and rock samples
the Tapaya River and, especially for plate reconstructions,
system in central palaeomagnetic sampling. Coastal gravity sur-
Halmahera. The veys of the islands have also been completed.
density and darkness Three methods of fieldwork have proved the
of the rainforest is well
shown here; there are most efficient ways of covering the ground:
at least five people in coastal investigation by longboat, traverses u p
this photograph. rivers on foot accompanied by local guides
and, where possible, excursions along logging
tracks by jeep or truck.

Coastal work
Investigation of coastal exposure provides a
good method of rapid coverage of large areas.
Exposures are identified whilst sailing along
the coast and geologists are sent ashore in the
longboat. T h e longboat is used throughout the
islands for short-distance transport and is a
highly practical craft despite feeling alarmingly
unstable at first. It is made of a hollowed-out
tree trunk with the addition of an outboard
motor a n d is an example of Stone Age
technology yet to be improved (Fig. 4). T h e
biggest problem encountered with coastal
studies is trying to land in heavy swells, and on
many occasions the longboat and all aboard
the food is stored in tins or heavy plastic to keep have been dumped on a reef or submerged
out the damp and the boat's ravenous popula- during landing - luckily without injuries.
tion of rats and cockroaches. Consequently, tantalizing exposures can go
Fig. 7.Drilling
Sailing from Ternate harbour is spectacular palaeomagnetic cores unrecorded except for notes made using
on a clear morning. Gamalama (Ternate's vol- in the Cretaceous binoculars.
canic peak) looms 2 km high in the back- Gowonli Formation, T h e usual routine consists of one or two days
ground, the perfect cone of Tidore is to the Gowonli River, E examining coastal exposures as we move by
south and the mountains of central Halmahera Halmahera. Calc- boat to the site of the next traverse identified
rise mistily to the east. On particularly limpid alkaline debris flows from aerial photographs. Despite the heavy
days the active arc can be seen stretching south suggest these rocks jungle cover, aerial photographic interpreta-
to Moti and Makian islands (Fig. 3 ) and north formed in the forearc tion can be useful in mapping. Ophiolitic and
into the north-west arm of Halmahera. This of an active arc; ultrabasic rocks give rise to a high but rounded
arc marks the approximate surface trace of the palaeomagnetic
analysis of these landscape whose vegetation cover appears
100-km Benioff zone contour on the eastward rather less luxuriant than other areas. In con-
samples indicates
subducting Molucca Sea Plate. -180" of rotation since trast, limestones often produce a very distinc-
Fieldwork has three main objectives: the the Cretaceous. tive, jagged karstic topography that is obvious
both from aerial photographs and from off-
shore. Quaternary limestones are typically
found along the coasts and may achieve eleva-
tions above 100 metres, indicating very young
uplift along active faults. Fig. 5 shows the
rugged basement topography in north-east
Halmahera.

River traverses
Once at the site of the traverse, a team of two
or three geologists, several crew members and
up to eight porters from a nearby village will set
off up river for several days, taking supplies, fly
sheets and bed sleeves. Traverses can vary
from pleasant perambulations in cool, ankle-
deep water under the high rain-forest canopy
to thrashing through vicious rattan thickets on

ZOIGEOLOGY TODAY January - February 1994

 next to the river above flood level and away
from rotted trees is chosen, the fly sheets and
beds are set up, and we then bathe in the river
before a supper of rice and sardines. This may
be supplemented by river prawns, eel or, occa-
sionally, wild pig caught by the porters using
dogs and spears. Mornings are cold before the
sun rises and it is hard to drag oneself from a
warm sleeping bag into damp, muddy field
clothes. Geologists and porters alike are kick-
started into life by fearsomely strong Indone-
sian coffee and the ubiquitous rice and sar-
dines.
Wildlife in the jungles of the Halmahera
region is relatively benign. Scorpions, some
snakes, wild pigs and even crocodiles are the
rare dangers, whilst leeches and mosquitoes
are unpleasant irritations. However, any
steep mountainsides (Fig. 6). Walking through Fig. %Home sweet trepidations are soon forgotten, for the pri-
limestone areas can be especially hard going, home. The Sinar mary rainforest is a fascinating place. Great
Bahiera arrives to fluted trees with buttressed roots rise vertically
with many gorges, waterfalls and sink holes to
collect the field team to the canopy 50-60 m above the ground, lianas
be skirted, often by climbing muddy ravines in from a traverse.
a downpour. Heavy rainstorms upstream can loop and dangle from this roof, and strange
often cause rivers to rise suddenly by several fruits abound. In many areas the forest floor is
metres, making it essential that camps are relatively clear but in others dense foliage ob-
situated well above high water levels. However, scures everything beyond about 10 m, and it is
although sudden thunderstorms can fill rivers easy to become separated from the party. Lo-
rapidly and halt movement for the day, the cating them again depends on making loud
flood also subsides quickly and the next morning whooping noises that carry well, and following
will normally see a return to sunshine and blue the answering whoops. Multicoloured parrots
skies. and kingfishers are common, streaking across
If suitable, any exposures are drilled for the rivers against the deep green backdrop;
palaeomagnetic cores (Fig. 7). Palaeomagnetic colonies of large fruit bats hang in limestone
work is dependent on finding suitably mag- caves whilst the rivers teem with fish, prawns
netic, well-lithified rocks with good structural and eels. Swarms of insects at night produce a
control. Particular attention is given to mate- cacophony of bizarre sounds, and fireflies or
rial that can be dated either by luminous mushrooms emit spots of light into
micropalaeontological or isotopic methods. The
tropical climate means that many exposures
are deeply weathered and time must be spent
finding the freshest specimens. However, the
heavy rains do have some advantages and,
despite thick soils and rotted vegetation cover
in the forest, many river sections include excel-
lent water-polished outcrops ofvery fresh rock.
Each site is located as precisely as possible on
a map or aerial photograph. Existing maps
often bear little resemblance to reality in the
interior, and position fixing by conventional
methods in thick forest can be a nightmare.
Enlargements of aerial photos, where avail-
able, make good base maps and often allow
identification of every bend in steep but sur-
prisingly meandering rivers. Recently, hand- Fig. 9.Loading a
logging truck on the
held satellite navigation devices have become
return from a traverse,
available and can locate a position to within 20 Woi River, Obi. In wet
m ifyou can see enough of the sky through the conditions the rear
jungle canopy to find three satellites. By using wheels shower all
satellite fixes in conjunction with topographic aboard with huge clods
maps and aerial photos, we are able to locate of mud, making the
most positions with a high degree of accuracy. task of hanging on
In the late afternoon, a suitable camp site harder still.

GEOLOGY TODAYJanuay-February 1994121

the gloom. At first, sleeping is difficult in such Sea Plate arc crust. Palaeomagnetism is
a strange environment, but this is soon over- detecting amounts of rotation and translation
come by fatigue and growing familiarity. of crustal fragments. We now have new
Most field work is carried out on the way up- geological maps of all of the major islands and
river and the final day usually consists of a a good understanding of the stratigraphy of the
forced march downstream, only pausing for region. Integrating the results from various
lunch or any missed exposure, to the agreed geological disciplines and using computer
rendezvous with the Sinar Bahtera. After sev- software, we are now able to model the tectonic
eral days in the forest the boat starts to assume development of this corner of the globe. In
an air of untold luxury and comfort! (Fig. 8) particular, we have been able to understand
the development and movement of the
enigmatic Philippine Sea Plate since the
Road sections Mesozoic.
There is limited logging in parts of Halmahera T h e Philippine Sea Plate, unlike any other
and Obi, and in these areas it is often possible plate, is surrounded by subduction zones and
to use logging vehicles for excursions along dirt separated from the oceanic ridge system; con-
roads (Fig. 9). The nature of the terrain means sequently, its motion history is not well known.
that logging on a large scale is not possible (and Palaeomagnetic studies could help, since dec-
long may it remain so), but some roads provide lination changes can provide information on
a way of reaching parts of the interior very rotations whilst inclination changes give infor-
rapidly. We have used both jeeps and enormous mation on latitudinal shifts. However, the plate
flat-bed trucks, and the very steep and uneven includes very little land area for sampling, and
tracks can mean bone-shaking hours clinging palaeomagnetic data from the region are sparse
to hot metal. The weather is often wet, and dirt and difficult to interpret. Our early geological
roads swiftly become skid pans, even for the work indicated that large parts of the Halmahera
largest trucks. Tracks deteriorate, becoming Fig. 10. Cartoon group belong to the Philippine Sea Plate and
rapidly overgrown, and wooden bridges rot so reconstruction of the would be ideal for new studies. T h e results
that, once disused, roads can be barely passable Mesozoic to Holocene show that the Philippine Sea Plate has rotated
after a few months. T h u s the apparent evolution of eastern clockwise about 90" in the last 50 million years.
Indonesia and the T h e discontinuous rotation, accompanied by
attractions of high-speed transport to the northern Australian
interior sometimes seem less compelling than shifts in latitude and arc volcanism at the plate
margin, based on margins, can be understood in terms of the
several days on wet feet in a river. published data (for
Australian movement) movements of plates and crustal fragments in
and our new data (for the western Pacific.
Results Philippine Sea Plate During the Mesozoic, as Australia drifted
rotations). Arrows north, fragments of the northern margin rifted
Once the expedition members and, equally indicate directions of off and moved ahead of the main continental
importantly, the rocks are safely home, the movement and mass. These continental fragments are repre-
many aspects of laboratory work can begin. rotation; PSP = sented by siliciclastic rocks, ammonite-bear-
Isotopic dating is providing a history of island- Philippine Sea Plate, ing black shales, granite and high-grade meta-
arc volcanic activity. Trace element and isotopic EM = Eurasian morphic rocks. Continental lithologies con-
studies are distinguishing areas underlain by margin, MSP = trast with 'oceanic' rocks of the Philippine Sea
Australian continental crust and Philippine Molucca Sea Plate.
Plate, which consist predominantly of ophiolitic
Mesozoic Late Rleogene Early Neogene Late Neogene

ZZIGEOLOGY TODAY January - Februay 1994

 Fig. 11. The crewmen Thus from relatively ‘low tech’, traditional
looked after us very methods of fieldwork we have progressed from
well during our stay on the construction of a stratigraphy and geologi-
the Siiiar Bahtera, and cal maps to a point where we are now able to
fishing provided a determine the tectonic environments in which
welcome respite from
tinned sardines as well
many of the rocks formed and model the
as a bit of extra evolution of a large area of the western Pacific
excitement while we throughout the Tertiary. This, in turn, has
sailed from place to broader implications for the evolution of east-
place. Ito, Djamire, ern Indonesia and the Pacific as a whole. Many
Soultan and Wallace scientists and non-scientists have been in-
preparing lunch. volved in the project and contributed to its
success, but special thanks must go to the
Indonesian crewmen and village guides with-
out whom we would never have been able to
negotiate the seas and jungles of the North
Moluccas (Fig. 11).

Suggestions for further reading

Hall, R. & Nichols, G. J. 1990. Terrane
amalgamation in the Philippine Sea margin,
Tectonophysics, v. 18 1, pp. 207-222.
Hall, R., Audley-Charles, M. G., Banner, F.
T., Hidayat, S. & Tobing, S. L. 1988. T h e
basement rocks of the Halmahera region,
rocks and island-arc sequences (lavas, east Indonesia: a Late Cretaceous-Early
volcaniclastic conglomerates, sandstones). Arc Tertiary forearc. Journal of the Geological
activity in the southern part of the Philippine Society of London, v. 145, pp. 65-84.
Sea Plate was caused by subduction of the Hall, R., Nichols, G . J., Ballantyne, P. D.,
Australian plate as it moved north. In the early Charlton T. & Ali, J. 199 1. T h e character
Neogene, the Australian continental margin and significance of basement rocks of the
collided with the volcanic arc, terminating arc southern Molucca Sea region. Journal of
activity and causing the Philippine Sea Plate to South-East Asian Earth Science, v. 6, pp. 249-
begin rotating clockwise. As a result of this 258.
rotation, the former subduction boundary de- Katili, J. A., 1980. Geotectonics of Indonesia: A ,
veloped into a major left-lateral strike-slip sys- Modern View. Printed by the Directorate of
tem (the Sorong Fault Zone) which has since Mines, Jakarta, Indonesia. 27 lpp.
moved Australian continental fragments and Katili, J. A. & Reinemund, J. A. 1984. Southeast
Philippine Sea crust into the Molucca Sea Asia: Tectonic Framework, Earth Resources
area. From about 25 million years ago, Philip- and Regional Geological Problems. International
pine Sea and Australian fragments have a simi- Union of Geological Sciences Publication
lar history with, for example, the same regional 13, 68pp.
unconformities and major periods of carbon- Hamilton, W. B. 1988. Plate tectonics and
ate deposition. island arcs. Geological Society of America
As the Australian and Philippine Sea plates Bulletin, v. 100, pp. 1503-1527.
impinged upon the Molucca Sea Plate, the Hamilton, W. B . 1979. Tectonics of the
latest episode of arc activity was initiated at Indonesian region. USGS Professional Paper
around 12 million years as Molucca Sea crust 1078, 345pp.
began to be eliminated by subduction at its The Moluccas: The Spice Islands (2nd ed).
eastern and western edges, forming the Sangihe Muller, K. 1993. Periplus Publications.
and Halmahera arcs (see Fig. 10 for plate 200pp.
reconstruction cartoons). This process is con- Indonesia: A Travel Survival Kit. Lonely Planet
tinuing at the present time; the Sorong Fault Publications.
Zone is taking up clockwise Philippine Sea
Plate motion, the Molucca Sea Plate is bent
into an inverted U-shape and is nearly elimi-
nated, whilst the Philippine Trench is propa- Simon Baker is a research assistant on the Sorong
gating south as subduction of the Molucca Sea Fault Project, and Robert Hall and Emily Forde
becomes increasingly unable to accommodate are, respectively, Senior Lecturer and research
the clockwise movement of the Philippine Sea student at University College London.
Plate.

GEOLOGY TODAYJanuay-February 1994123