Quaternary Uplifted Coral Reef Terraces on Alor Island, East Indonesia
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Coral Reefs (1994) 13:215-223 Coral Reefs Springer-Verlag 1994 Quaternary uplifted coral reef terraces on Alor Island, East Indonesia W. S. Hantoro 1'2, P. A. Pirazzoli 3, C. Jouaunic 4, H. Faure z, C. T. Hoang 5, U. Radtke 6, C. Causse 7, M. Borel Best 8, R. Lafont 2, S. Bieda 2 and K. Lambeck 9 1CGRD-LIPI, Jalan Cisitu 21/154D, Bandung 40135, Indonesia 2CNRS, Laboratoire de G~ologie du Quaternaire, Luminy, Case 907, F-13288 Marseille Cedex 9, France 3CNRS; Laboratoire de G~ographie Physique, 1 Place A. Briand, F-92190 Meudon-Bellevue, France *ORSTOM, Station de G~odynamique, BP 48, La Darse, F-06230 Villefranche-sur-Mer, France SCNRS-CEA, Centre des Faibles Radioactivit6s, F-91198 Gif-sur-Yvette Cedex, France 6Geographisches Institut, Heinrich-Heine Universit~it, D-40225 Dfisseldorf, Germany 7Museum National d'Histoire Naturelle, Laboratoire de G~ologie, 43 rue Buffon, F-75005 Paris, France 8National Natuurhistorisch Museum, PO Box 9517, NL-2300 RA Leiden, The Netherlands 9Institut de Physique du Globe, 4 Place Jussieu, F-75252 Paris Cedex 05, France Accepted 16 February 1994 Abstract. A flight of six major coral reef terraces, up to Introduction 700m in altitude, occurs along the eastern and northern sides of Kabola Peninsula, Alor Island, Indonesia. Some Sequences of superimposed raised marine terraces have radiometric dates have been obtained from unrecrystallized been reported from many coastal areas in the world. coral samples collected in growth position by three different In several cases dating methods have provided age estima- methods (14C, 23~ ESR). This enabled the identi- tions of middle- to late-Quaternary marine episodes. fication of the terraces corresponding to the Holocene and The best known examples are those reported from the to oxygen-isotope stages 5c, 5e and 7. According to the Huon Peninsula, Papua New Guinea (Bloom et al. 1974), present elevation of the dated terraces, a 1.0-1.2 mm/y Barbados, West Indies (Mesolella et al. 1969; Radtke et al. mean rate of uplift can be discerned. Extrapolation of this 1988; Bard et al. 1990) and the northern New Hebrides trend to the whole sequence of terraces reveals a good (Taylor et al. 1985). correlation between the development of major terraces In the Indonesian region, Chappell and Veeh (1978) and interglacial or interstadial stages corresponding to studied reefal terraces on Atauro Island where they dated astronomically calibrated oxygen isotope records, up to fossil corals belonging to the last interglacial period. They stage 13. The relatively rapid uplift rate in this region attempted to correlate by extrapolation the oxygen isotope minimized the possibility of polycyclic sea-level stands at records and upper marine terraces back to about 700 ka, the same levels and contributed to the good preservation on the assumption that uplift movements remained linear, of some morphological reef features. Two superimposed at rates in the range of 0.43-0.5 mm/y. On Sumba Island, marine notches are visible near the present shoreline, with coral-reef terraces corresponding to oxygen-isotope Stages retreat points at about 5.0 m and 8.6 m respectively above 15 (about 600 ka) and 9 (ca. 330 ka) have been dated using the present MLWST level. They can be interpreted as the electron spin resonance (ESR) method (Pirazzoli et al. corresponding to a glacial interstadial (the upper notch) 1991, 1993; Hantoro 1992). The resulting uplift rate of and to the Holocene sea-level peak (the lower one). As 0.49 ram/y, extrapolated to a one million-year sequence of Holocene emergence has been less than what could be spectacular terraces, strongly suggests a polycyclic geo- expected from a 1 mm/y rate of uplift, a major coseismic morphic development for some of these terraces. vertical displacement may occur in the future. In the present study, a sequence of coral-reef terraces on Alor Islar~d is described, which may have been the result of uplift rates about twice as fast as those for Atauro and Sumba, thus enabling the emergence and preservation of individual fossil shorelines corresponding to sea-level peaks during interstadials of the last glacial cycle. This work is a contribution to the IGCP Project 367 "Late Quater- nary coastal records of rapid change" and to the activities of the INQUA Commissions "Neotectonics" and "Shorelines", the task group "Paleoseismicity of the Late Holocene" of the Inter-Union Study area Commissionon the Lithosphere,and the UNESCO-IUGS cooperative programme "Earth Processes in Global Change" (CLIP Pilot Project) Alor is located north of central Timor (Fig. 1), on the Correspondence to: P.A. Pirazzoli volcanic chain of the Banda island arc. Most of the island 216 ,,.,~-n'rP"~'>:,-., . .....C. e:z.Sik~ _ ._. ............. , ....... j ..-v' i V"""~... i;, .% ,, ~ ~ , ~ e -" ~.;.-/zzzf" E ""/ "" ...r"',;-.,' " h..... , . .- I 661 m.i" VI.j")B~i~-'-'&-'-4-'4"~ i 'e~;'~;~r r~ s o / 2 km -'~//" ~.S" v ~. ~!c~," .,~. ~g, , , ,> .P .. / / '....... ~' NBu~ <- , .D.~..~,d~....L4-'" ,--.',,., ......,i i i~ ~ BUj anta 852 m .i ';t..')~4 'Kabo3.a )VY, ! t" ../k. ._.,_........,- / .. ,.,.,-- ..., .s' i t ....:i.-.: ,,. .......... ..,e i i ~', .;' > "~' '-"" III,-,.'~ "-" . .. d"'"~ ) 1 / ........... .~.f { ..-.' .......,-., . i t II ..................' //I .......... 7.L- ......... ....... ...................................... :<::::':.'-:.~; ........ .4"3 ~,2 (~,\'-~ .pJ I f'";''"v'".... :','t'\L~'./" ............ .'"j[2:::!~:'<e:., / "~-- ~ ~ ALOR I <-..5:J~-~"~~ <,' ~,...//~ ."..... ':;;~'.: / ~o~,J~ ==,o~ "'/'~'; "''-:' -" Kalabahi Bay ;'~/r / ~ ...." "~'/"o.. i ......... / ..... ........ e . i / ( i" 1: u , ,',,'..4' .... ' j .~ .~~ ~. q ....-"/.2 ,,I [ ,, .> " / '>',, '--- I .... Ombai Strait % ~./ . #" 1. Fig. 1. Location map and morphological analysis of coral reef terraces in the Kabola Peninsula area, AIor Island. a, main terrace identification; b, transect studied; c, lagoon morphology; d, cliff; e, canyon;f, structural line; 8, modern reef flat; h location of dated sample consists of submarine effusive rocks, interbedded with Material and methods calcareous tufs and marls containing Globigerinidae.As for the nearby Atauro and Wetar islands, volcanism in Alor Before field work, the areas to be investigated were studied on the has been inactive for about 3 Ma (Abbott and Chamalaun multispectral Landsat satellite scene E-30187-01254 of September 8, 1981). This seems to correspond to a deceleration and 1978, in which the main terraces around Kabola Peninsula appear virtual interruption of the subduction of the Australian clearly (Hantoro 1992), as well as on the aerial photographs of continental crust beneath the Banda arc (Johnston and Bakosurtanal Indonesia. This led to the identification of six major terraces and of some subterraces on the northern and eastern parts Bowin 1981; McCaffrey and Nabalek 1984). On the other of Kabola Peninsula, where their frontal crest or the cliff edge hand, this sector of the volcanic chain (from Alor to Wetar) appears to be marked by a slightly clearer line. The major terraces is affected by presently active back-arc thrusting, in which are numbered I to VI in Fig. 1. Higher terraces may exist inland, the ocean crust of the South Banda Basin underthrusts the above 600 m in altitude. arc southwards (Silver et al. 1983). A one-week field survey was carried out by three of us (WSH, A detailed geological description of Alor was provided PAP and CJ) in August 1988, mainly along five transects. It was by Van Bemmelen (1949). Raised coral reefs are common generally not possible to walk straight across the terraces and we had to take winding paths, often between two walls of vegetation, with in Kabola Peninsula, which is connected to the main island little or no lateral visibility. It was possible to have a wide view only in its northwestern part by a 3 kin-wide isthmus, consisting on the outer scarps of the terraces (Fig. 2). This was essential in of a small volcanic outcrop, rising up to 50m above order to localize observations and evaluate horizontal distances. present sea level. Limestones occur as nearly horizontal Altitudes were measured with a spirit level and a folding ruler strata that dip gently seawards. Early authors reported five for Holocene samples and estimated using three pocket altimeters terraces, up to 700 m in altitude, from the Kabola Peninsula, for higher terraces. Since interpolated corrections for barometric changes were often not possible, altitudes above 100 m are accurate eight terraces up to 700 m from the south coast of Alor and only to +10m. the highest occurrence of marine sediments was reported Regarding the analytical procedures for the U-series method, the at 1200 m in the middle of the island (for a review of early inner part of each clean coral sample was taken for isotopic analysis. studies, see Hantoro 1992, pp 294-297). The samples were broken into small fragments which were scrubbed 217 Fig. 2. View from Terrace V 1 of transect D towards Cape Sika (all the photos are by P.A. Pirazzoli) by ultrasonic vibration, dried at 100~ and then ground to a fine Seaward, live branching and massive corals occur imme- powder. An aliquot of each powder sample was taken for the diately below the mean low water spring tide (MLWST) determination of the relative abundance of calcite by X-ray diffrac- level (the local tidal range is of the order of 2.2 m). A strip tion. Only samples with ~ 3 4% calcite were chosen for analysis. The of sand beach or mangrove patches are usually found radiochemical procedure used is described in Causse and Hillaire- Marcel (1989) and Hoang and Hearty (1989). Uncertainty ranges of landwards. U-series data correspond to la counting errors. The U-series data The main data concerning the fossil samples dated are obtained display values of uranium concentration and 234U/23aU summarized in Table 1. Discontinuous remnants of a fossil activity ratios within the range of those generally observed in reliable reef flat capped by coral debris appear at some places at fossil corals.