Central Sumatra Research Articles

Tropical vegetation mapping with combined visible and SAR spaceborne data

Abstract Within the tropics, due to high cloud cover, vegetation monitoring can only be achieved by using both visible and microwave spaceborne data. In the present study, Shuttle Imaging Radar (SIR-B) data and SPOT data acquired over central Sumatra are used in a supervised classification of vegetation. Emphasis is put on SAR data utilization. Assessment of SAR data potentialities is carried out by using (1) SIR-B imagery alone, and (2) combined SJR-B and SPOT data. The article describes the methodology developed for exploiting the SIR-B image including raw data analysis, image filtering and image classification. Finally, combined filtered SIR-B imagery and SPOT data are used jointly in a supervised classification leading to a better discrimination of surface units. These results indicate the unique capabilities of combined visible and SAR data for monitoring tropical vegetation, especially for forest/non-forest discrimination.

Phaeoceros hirticalyx(Steph.) Haseg. (Anthocerotae) new to New Zealand

Abstract Phaeoceros hirticalyx (Steph.) Haseg. is reported new to New Zealand based on specimens from Reefton. It has been reported previously from Java, from Ambon, and doubtfully from western central Sumatra. A comparison is made with Leiosporoceros dussii (Hässel) and P. squamuligerus (Spruce) Hässel.

Plate tectonics and the development of sedimentary basins of the dextral regime in western Southeast Asia

The western part of Southeast Asia is comprised of three tectonic plates: the Southeast Asia Plate, the Burma Plate, and the Australia-India Plate. The present regime of oblique subduction and partial coupling of the combined Australia-India Plate to the Southeast Asian part of the Eurasia Plate was initiated in the Middle Eocene. Resulting dextral shear interacting with the inertia of the largely continental plate were the driving forces of basin genesis and development, setting in train a sequence of temporally and spatially variable tectonic events, diminishing in intensity away from the active plate boundary. The effects can be identified in the east from the Malay Basin to Central Thailand. A Late Eocene to Oligocene rifting phase formed rift troughs in central Sumatra, later spreading north to the Mergui Basin and south to the Sunda Basin. The Sunda Volcanic Arc was initiated. As subduction proceeded, deformation spread out from the vicinity of the plate boundary. In the Oligocene, dextral shear, substantially along preexisting faults and consequential extensional faulting initiated the Thailand basins and the Malay Basin. Subsidence and extension continued until late Middle Miocene time, possibly with some thermal relaxation. In the Late Oligocene to Early Miocene, the back arc basins close to the plate boundary began to subside, extending out from the initial rift troughs during a quiescent interval transition between the rifting and later wrenching phases. Subsidence may have been the result of the withdrawal of heat from the asthenosphere beneath the basins by the cold subducted slab. As the Sunda arc-forearc developed by accretion and magmatism, partial coupling in the plate boundary region was enhanced and transpressional deformation commenced in the Sumatra basins in the Middle Miocene. This Sea defining the northern part of the Burma Plate. South of the Andaman Sea, deformation persisted in a broad zone along the margin of the Southeast Asia Plate. Dextral wrenching in the late Middle Miocene phase affected much of the eastern region, extending south into the Natuna Basin. Deformation diminished markedly at the end of the Middle Miocene with the inception of the Burma Plate. Transpressional tectonics continued through Late Miocene and Pliocene time in Sumatra, in a broad zone as an ill defined southern extension of the Burma Plate. The continuing movement resulted in the uplift of the Barisan Mountains and the deformation of the onshore Sumatra basins. The Sumatra forearc was transferred to the Burma Plate with the establishment of the dextral Sumatra Fault probably in the Pliocene.

Gesneriaceae of the Old World II. A new Didymocarpus from Sumatra

Didymocarpus koerperi B. L. Burtt is described from near Pajakumbuh, central Sumatra. It belongs to a group of species that is chiefly distinguished by having a single nectarial gland lying below the ovary, instead of the usual cupular disc.

Influence of topsoil removal and fertilizer application on peanut yields from an Indonesian Ultisol

Experiments were conducted on a tropical Ultisol in west central Sumatra, Indonesia, to evaluate the effects of topsoil removal and the subsequent application of phosphorus and potassium fertilizers, and calcium lime, on crop productivity. Peanut ( Arachis hypogaea L.) yields and total above-ground biomass production were determined in field plots and potted soils from which 0, 3, 7 and 15 cm of topsoil had been removed. The removal of 3 cm of topsoil, which was equivalent to the entire A-horizon resulted in a significant reduction in peanut yields and above-ground biomass in both field and potted soils. No significant yield differences in peanut yields or biomass were observed between] treatments from which ⩾3 cm of topsoil had been removed. The application of fertilizers and lime restored peanut yields to levels observed in unfertilized soils from which topsoil had not been removed. However, the use of off-farm inputs was economically unfeasible for most hillside farmers in the research area.

An event in the ethnography of ritual

Professor Schefold's description and analysis of Mentawei religion, feli citously combining broad scholarship with the results of thorough field work, is more than just another specimen of good ethnographic work. It has human significance. Living in close contact with a small and isolated group of simple horticulturalists, the author was able through meticulous observation to explore the basic needs and drives motivating human behaviour when and where man is left to his own devices, thrown back on his own resources. Schefold's report on this aspect of his experiences called for a voluminous book, which he divided into seven parts. In the introductory part (covering some forty-odd pages) the author discusses the theoretical implications of the aim of his fieldwork: unearth ing the meaning which the Mentaweians' elaborate ritual has to the performers themselves. Then follows a critical discussion of the available literature, and part I ends with a concise account of the place and the course of the fieldwork conducted. This involved an initial period of almost two years' residence on the island of Siberut, the biggest and northernmost of the Mentawei Islands, a small archipelago in the Indian Ocean some 90 miles west of Central Sumatra. The better part of this period was passed among the Sakuddei, a group (urna) of some 35 souls, women and children included. In later years three short re-visits followed. Part II (230 pp.) starts with a general description of the thinly populated island, followed by a discussion of the prehistory of the Mentaweians and the differences between their culture and the pattern of culture of the Indonesians living on the main islands. A detailed review of Mentaweian ethnography follows this: their material culture and economy, their social organization, and their religious concepts and specialists are discussed. Special attention is given to the absence of any form of social organization beyond the urna, the small patrilineage rarely counting more than 60 individuals, and usually far fewer. Their communal house (also called urna) is the centre of their ritual activities. Here the members of the urna,

Sedimentology of Upper Jurassic Deposits in the Tembesi River Area, Central Sumatra

Sedimentology of Upper Jurassic Deposits in the Tembesi River Area, Central Sumatra

Structural Influences on Seismic Stratigraphy of Nias Forearc Basin, West of Central Sumatra: ABSTRACT

The Nias forearc basin is a 250-km long, 100-km wide bathyal depression located between the mainland of west-central Sumatra and the island of Nias. A grid of multichannel reflection seismic data covering the basin and adjacent shelves was used to map Neogene structure and to delineate the Neogene seismic stratigraphy. Six major structural features, including the continental margin, strike-slip faults traversing the forearc, and uplifts within the accretionary wedge, were mapped, while 11 seismic sequences were identified within Neogene reflectors. Positive correlations between the structure and seismic facies distributions indicate structure has influenced subsidence and depositional patterns through the developmental history of the basin. From a developmental standpoint, the Nias forearc basin can be subdivided into two smaller basins, the Singkel basin to the north and the Pini basin to the south. During the Miocene and early Pliocene, distinct facies distributions indicate that these two basins developed independently of each other. Subsidence in the Singkel basis was initiated between two parallel strike-slip faults that traverse the forearc. The geometry of the Pini basin, on the other hand, was controlled by the local character of the continental margin and by the uplift of the developing accretionary wedge. In the late Pliocene,more » a more unified facies distribution across the study area indicates subsidence of a more regional character and an increased regional structural influence over the entire Nias basin area.« less

Epilithic freshwater diatoms in Central Sumatra

Epilithic freshwater diatoms in Central Sumatra

Stratigraphy, paleomagnetism, and magnetic fabric of the Toba Tuffs: Constraints on the sources and eruptive styles

The Toba depression in north central Sumatra is a complex of several overlapping calderas resulting from three major ignimbrite‐forming eruptions. Within the depression, the upland masses of Samosir and northern Uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment, hitherto interpreted to be two parts of a single resurgent dome. This study has demonstrated that the welded tuffs of Samosir and Uluan have different magnetic polarities and therefore at least two different ignimbrites are present; the Samosir/Uluan massif may consist of parts of two resurgent domes. The first ignimbrite eruption occurred at 0.84 Ma and produced a very thick (>400 m), densely welded unit having a reversed polarity. Anisotropy of magnetic susceptibility (AMS) flow direction and lithic size data indicate that the source lies in the southern part of the Toba depression, and the thick deposit of Uluan is thought to have ponded in a 40‐km‐wide caldera. The second ignimbrite is normally magnetized. AMS flow direction data indicate two separate source vents, one to the north in the Haranggaol caldera, and another to the south. The thick deposit at Samosir is thought to have ponded in the southern caldera. Coarse sediments then accumulated over Samosir and northern Uluan and were capped by lacustrine deposits. A renewed episode of resurgence then uplifted Samosir Island and possibly the northern part of Uluan. At approximately 0.075 Ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the Toba depression. This ignimbrite is mostly nonwelded and normally magnetized. Part of the Uluan dome was destroyed by collapse of the Sibandung caldera and Latung graben and concomitant with renewed subsidence of the Haranggaol and Porsea calderas.

Sedimentology and Diagenesis of a Rift Basin Lacustrine Sandstone: Pematang Group, Central Sumatra, Indonesia: ABSTRACT

The Central Sumatra basin is a back-arc basin that formed during the Paleocene as a series of half-graben structures. These early formed half-graben structures were filled with nonmarine clastics and lacustrine sediments of the Pematang Group, sourced from local highland areas. The Pematang Group consists of the Lower Red Beds, Brown Shale, Coal Zone Member, and Lake Fill Formation (in ascending order). The Pematang Group sedimentology is intimately related to the regional and basinal tectonic development and history. Sandstones of the Pematang Group are predominantly sublitharenites and litharenites; feldspars are rare. Sandstone diagenesis is relatively uniform regardless of the depositional environment. Diagenetic modifications include compaction, early calcite and dolomite cementation, quartz overgrowth formation, unstable rock-fragment dissolution, kaolinite precipitation, siderite formation, and local illite formation. Secondary porosity accounts for up to 50% of the effective porosity and was created by rock-fragment dissolution. This dissolution is probably caused by the organic acids released during kerogen maturation. Stable isotope data from diagenetic siderite are presented.

An investigation of the relationship between the geology of indonesian sedimentary basins and heat flow density

Geothermal data, collected from 929 wells drilled by oil companies in 20 Tertiary basins of Indonesia, have been related to the geology and tectonics of the area. It is found that the thermal conductivity increases with the depth of burial and degree of compaction in both transgressive and regressive sediments but decreases with increases in impurities and heterogeneous grain sizes. The temperature gradient is controlled mainly by the depth and temperature of a heat source beneath a basin. Very high heat-flow densities (> 125 mW m −2) found in central Sumatra, South Sumatra, Salawati Basin and Bintuni Basin may be caused by relatively shallow magmatic diapirism.

Man's Impact on the Rainforests of West Malesia: The Palynological Record

Palynological evidence for man's impact on the rainforests of West Malesia (Sumatra, West Java, Borneo, Peninsular Malaysia, the Philippines) is reviewed. The problems of interpretation in anthropogenic terms, identification of cultivars and difficulties occasioned by incomplete publications of results, sparsity of absolute dating and low sampling resolution are stressed. Man's impact may have begun as early as 17 800 B.P. in highland north Sumatra but the firmest evidence post-dates 6200 B.P. Clearance may have begun before c. 7500 B.P. in central Sumatra and certainly seems to pre-date c. 4000 B.P. Both areas show strong evidence for major clearance around 2500 B.P. which may relate to the introduction of irrigated rice cultivation. The west Javan forest also exhibits a long record of disturbance, perhaps initially due to vulcanicity, but clearance phases beginning before 4800 B.P. are evident. Data from very low lying land (below c. 100 m altitude) are sparse but forest clearance pre-dates 600 B.P. at Tasek Bera, Peninsular Malaysia. There is a need for multi-disciplinary studies of basin environment history in West Malesia to refine the results of pollen analysis.

A gravity and volcanostratigraphic interpretation of the Lake Toba region, North Sumatra, Indonesia

In northern and central Sumatra a wide distribution of ignimbrite and volcanic sediments is observed. Some geologists consider that these volcanic products were erupted from Lake Toba. The subduction of the Indian Ocean plate along the trench of central Sumatra started at least as long ago as the Late Miocene. Fitch's model (1972) obtained the plate convergence vector (67 mm/yr), decomposed into its components perpendicular (58 mm/yr) and parallel (34 mm/yr) to the trench axis. Tears seaward of the mountain range created the Sumatran fault about 2 Ma ago. At that time, the edge of the plate reached a depth of almost 100 km, and a large amount of dacite magma was generated. The Tuk-Tuk dacite, a kind of ignimbrite, was erupted along the Sumatran fault and covered a broad area. The age of this ignimbrite was obtained as 1.9 ± 0.4 Ma ( K- Ar age) and 2.0 ± 0.3 Ma (Fission Track age). Since this eruption, the hinterland of this fault has been changed to a tensional field forming a graben structure. At the northern end of this graben, west of Harranggaoul, the ignimbrite (Haranggaoul formation) was erupted (1.3-0.3 Ma ago) forming a caldera. The lacustrine sediments of Samosir Island have a normal magnetization and overlie the Tuk-Tuk dacite unconformably. From this we can conclude that the graben structure has been covered with water (Lake Toba) at 0.7-0.6 Ma ago. This tectonic movement has continued, so that it is difficult to distinguish the caldera structure on the gravity anomaly map of the northern part of Lake Toba. From the results of the gravity survey it can be concluded that at 0.1 Ma ignimbrite flowed down along the Asahan river and filled the old river along the lowland between Parapat and Porsea, forming a caldera. 30,000 years ago, dacite and and ésite eruptions have occurred around Lake Toba, forming the volcanic cones.

Oil fields—A source of heat flow data

A method of calculating terrestrial heat flow from oil fields is evaluated by reviewing its application to twelve petroliferous basins, three of them in Brazil and nine in Indonesia. In each well, terrestrial heat flow was calculated by multiplying the temperature gradient (obtained by commonly used procedures) by the effective thermal conductivity. The latter was calculated from quick thermal conductivity measurements on core samples by a transient hot-wire method and from the lithologic log. Typically, in each basin the conductivity of about 300 core specimens was measured and logs of 50–150 wells were examined. In each formation penetrated by a well, the total thickness of each rock type such as shale, sandstone, etc., was determined from the composite lithologic log. The harmonic mean of the thermal conductivity of a formation in the well was computed from these thicknesses and the average values of the conductivities of the rock types measured on cores collected from that formation. The conductivity of the well is the harmonic mean of the conductivities of the formations. The average precision of the determination of formation conductivity can be expressed as the α 95 limits divided by the average value. For twelve basins this measure of precision for formation conductivities averages 3.6% and the average precision of the conductivity of a basin is 2.0%. On the same basis, the precision of the determination of the temperature gradient in a basin averages 4.7% and that of the heat flow 5.8%. Expressing the variability of the geothermal quantities in a basin determined at discrete locations as the standard deviation divided by the average value, we get for the mean variability in the twelve basins: 6.7% for conductivity, 15.0% for the gradient and 16.4% for the heat flow. The variability must come in part from natural causes. In general, at the present level of accuracy no correlation between anomalous heat flow and the occurrence of oil in a basin was found. Values of thermal conductivity of sedimentary rocks and their variation with depth are useful in calculating paleotemperatures. The magnitude of the mean heat flows of the basins is normal (63 mW/m 2) for the Jurassic basins of the stable continental margin of Brazil and somewhat elevated in the Tertiary basins of Indonesia, north of the Sunda trench subduction zone, the values decreasing from 130 mW/m 2 in Central Sumatra to 70 mW/m 2 in eastern Kalimantan. Future improvements in the procedures for determining conductivity might possibly bring out local anomalies in heat flow that might be caused by vertical seepage of water. This would support the hydraulic theory of accumulation of oil and gas.

Sunda Strait and Central Sumatra fault

Sunda Strait marks the limit between Java trench frontal subduction and Sumatra trench oblique subduction. The right-lateral Central Sumatra fault accommodates the oblique subduction. It does not pass Sunda Strait but ends between Sumatra and Java in a complex pattern of dominantly normal faults associated with subsidence, seismicity, and volcanism. We examine the implications of this phenomenon in Sunda Strait and in the adjacent fore-arc basin and accretionary prism, which underwent north-south compression due to subduction, as well as east-west extension. We propose that the strait is a consequence of the northwestward motion of the southwestern Sumatra block along the Central Sumatra fault. We also propose that the extensional zone in the Sunda Strait widens considerably to the southwest, toward the trench, and changes into a composite zone of deformation affected by strike-slip as well as normal faulting.

Modeling of large‐scale accretionary wedge deformation

Instantaneous stress and velocity fields within an active accretionary wedge can be calculated using plastic slip line theory, given the assumptions of a perfectly plastic rheology and plane strain conditions. After selecting a plastic yield stress and an average density, a stress field (slip line field) can be computed for a wedge of known cross‐sectional configuration from known stress boundary conditions on the upper surface. Basal surface tractions are derived without basal boundary conditions. The geometry of the computed slip line field can then be used to constrain the lower limit of the yield stress. The known velocity boundary conditions are insufficient for calculation of a velocity field because the coupling between the wedge and the subducting plate is unknown. Consequently, basal velocity distributions must be assumed to permit the construction of velocity fields. These assumed distributions are constrained by known rates of surface uplift and rates of tilt as well as inferred patterns of instantaneous strain rates within a wedge. The Sunda accretionary wedge west of central Sumatra is used to illustrate the plasticity approach. A minimum average yield stress of 20–30 MPa and a weaker basal layer of variable effective strength are indicated for this example. Surface uplift rates may be affected significantly by regions of high basal strain rate, finite velocity discontinuities, and underplating. The known pattern of uplift rates near Nias Island, located on the outer arc ridge, is consistent with the constraint that 90% of the total incremental shortening of the wedge is concentrated within 20 km of the trench. Given a sufficient number of uplift and tilt rate observations across a particular wedge, the relative influence of underplating, tectonic erosion, and basal strain rate variations could be assessed using the perfectly plastic model.

Force against Force: Configurations of Martial Art in European and Indonesian Cultures

Sociology of non-European sport cultures is faced by exceptional methodolo gical difficulties. Social sciences (above all quantifying ones) of the western type cannot cope with it, since it is a question here of complex alien structures and one cannot count on universal aspects of the basic categories. That is why a different interdisciplinary approach is needed in the case of Indonesian art of fight "pencak silat", which would continue with an analysis of configurations of the anthropology of culture (Ruth Benedict) of the sociology of civilization (Norbert Elias) and historical structuralism (Michel Foucault). In this way one gets an idea about the art of fighting thas is typical of Indonesian peoples and which is in clear contrast to the historical development of European fencing and boxing. The different exercise patterns reveal most clearly the completely different traditions and ways of socio-cultural development. It should not be permitted to reduce such complexity to a one-dimensional direction in historical development, in the light of which Indonesian forms may appear as "archaic" or "mediaeval". The more so since we live in times when there exists fascination with Asiatic forms of physical culture (Karate, Aikido, Taekwon, Do, Tai, Chi Chuan, Yoga), which is growing in industrial metropolises. And thus the colonial perspective of science about evolutionism becomes doubtful. There is no single modernization. It is a fact well known to all biologists that in the individual development of an organism its phylogeny is repeated with more or less accuracy. (...) And a similar principle can be found not only in the biological, but also in the cultu ral-historical development of mankind. (...) For that reason we see conditions in the social and other cultural institutions of more primitive races of man that the European peoples have gone through during the course of their cultural de velopment and left far behind them. (...) During my journeys through East- and Central Sumatra (1967), for example, I came into areas at the shores of the two Tapungs, the sources of the river Siak, where conditions and institutions re minded me most strongly of Freytag's "Ahnen", the eagerly devoured reading of my boyhood.1

Crustal structure of the Sunda Forearc Region west of central Sumatra from gravity data

Free‐air gravity anomalies near Nias Island include a −100 mGal low centered 10–20 km landward of the trench axis and a +80 mGal high over the outer arc ridge but also show a few large anomalies unrelated to topography. A −90 mGal low is found on the structural high between the Banyak Islands and Sumatra, north of Nias. An 80 mGal rise (at a gradient of 8 mGal/km) is modeled by a near‐surface body of high‐density material (oceanic crust?) 4 km wide and tens of kilometers long. This slab may be exposed on the southwest coast of Nias, where ultramafic bodies have been mapped, and its northerly trend (compared with northwesterly bathymetric trends) may be related to other obliquely trending structures near here, to a proposed strike slip fault which crosses Nias, and/or to the basement high topped by the Batu Islands. A −30 mGal free‐air low over the forearc basin can be modeled best if pre‐Miocene mélange or continental crust underlies the basin, and it seems unlikely that the basin can be underlain by a large thickness of oceanic crust.

Seismic-stratigraphic framework of the forearc basin off central Sumatra, Sunda Arc

New multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central Sumatra. We recognize six seismic-stratigraphic sequences that reflect the Cenozoic history and development of the outer continental shelf and forearc basin southeast of Nias Island. These sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin. Early in the development of this margin, Paleogene slope deposits prograded onto the adjacent basin floor. Onlapping this assemblage are two units interpreted as younger Paleogene(?) trough deposits. Uplift associated with rejuvenation of subduction in the late Oligocene led to erosion of the Sumatra shelf and formation of a regional unconformity. The early Miocene was a period of significant progradation. A Miocene limestone unit partly downlaps and partly onlaps the older Paleogene deposits. It is characterized by shallow shelf and oblique progradational facies passing into basin floor facies. A buried reef zone occurs near the shelf edge. The cutting of an erosional unconformity on the shelf and slope in late Miocene/early Pliocene time culminated this episode of deposition. In the late Pliocene, a large flexure developed at the western boundary of the basin, displacing the outer-arc ridge upward relative to the basin. Over 1 km of Pliocene to Recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer-arc ridge. These deposits are characterized by flat-lying, high-amplitude, continuous reflections that overstep the late Miocene unconformity. Up to 800 m of shallow-water limestone have been deposited on the shelf since mid-Pliocene time.