Journal of Coas tal Researc h Fort Laud erd ale, Florida

Differential Vertical Crustal Movements Deduced from Late Holocene Coral-Rich Conglomerates: Farquhar and St. Joseph Atolls (, Western )

P.A. Pirazzoli", P.A. Kaplinl and L.F. Montaggioni' tCNRS-URA141 ' F acu lty of Geograp hy ' CNRS-URA 1208 Labora toire de Geographie Moscow State University Ce n tr e de Sedimentologie­ Phy siq ue Moscow 119899 Paleontolog ic 1 Place Aristide-Briand U.S .S.R. Universrte de Provence 9219 5 Me udon-Bellevue Place Victor Hugo 13331 Marseill e Cedex 03 France

AB STRACT_••••••••••••••••_

PIRAZZOLI , P.A.; KAPLIN, P.A. and MONTAGGIONI , L.F., 199 0. Differenti al vertical crusta l movements ded uced from la t e Hol ocen e co ra l- r ich co ng lome rates: Farquhar a nd St. Joseph .tftllllllt. atolls

ADDITIONAL INDEX WORDS: Cora l reef. petrologi cal analysis, atoll , S eychell es , In dian Ocean , sea level, vertical movements .

INTRODUCTION Indian Ocean have been outlined by RABINOV­ {' ITZ et at. (1983), BRAINTHWAITE (1984) a nd In Jan uary a n d Februar y 1986 , under t he MART (1988). In the Ami rante Are, only coral auspices of Project IGCP -200 "Sea level corre­ atolls and sa nd cays are expose d today and no lati on and applications ," an inte rnational ex pe­ ign eou s ro cks are known (BA K E R, 1963). dition, aboa rd the R/V Professor S htokman of Between the Amirante Isl a nds (Seychelles) an d the Institute of Oceanology of the Acad emy of Mad agascar , small marine plateaux, t he origin Sciences of the US SR , was dev ot ed to pal aeo­ of whic h is not clear , rise from dep ths of over geographic an d geo mor pho logical investiga­ 2000 m. They bear Farqu har and a few ot her tions in she lf and coastal areas of the wes te rn small is lands (Providence , St . Pierre, etc.) (Fig­ Indian Ocean (Kapli n et al ., 1986 ). Th is gave an ure 1) . Farquhar Ato ll consists of a small num­ opportu ni ty for a s hort visit t o a few small ber of coral sand is lands located near the north­ oceanic islands, re mote and difficul t to access. In this pap er a comparison is ma de between east rim of a slig htly submerged bank, 9 to 21 results obtaine d in Farquhar (10°11 'S, 51°07'E) km wide, and is locate d abo ut halfway between and in St . Joseph (5°25'S, 53°20' E) Isl ands, sug­ Cape Ambe r, the nort h point of Mad aga sca r , gesting 't h e occ urrence of slight di ffer ential and the Amirante Trench. It was discovered by crustal movements durin g the late Holocen e. J oao de Nova in 1504 and renamed in 1824 after Si r R. Farquhar, Gove rnor of Ma uritius (Lion­ GEOLOGICAL SETTING AND net, ,970). In 1905 it was visited by the Percy PREVIOUS WORK Slade n Trust Expedit io n. Geomorpho logical information on t he atoll has been provide d The geodynamic h istory a nd the geologic and main ly by GARDINER and COO PE R (1907), tectonic settings of t his a rea of t he western GARDINER (1936), BAKER (1963), an d es pe­ 89019 received 28 March 1989; accept ed in revisi on 14 August 1989. cia lly by STODDART and POORE (1970), BAT- Col.:) 00 t\:)

(, I ;s:~s i C- 300 km \ 50° I o I (' I ~~( -- I I \ I , ( / .i~) ,. c: 'J~ St. ~osePh (-" r I'" \ ) ( " ~ "(,.... \",r\_, I '/ fA\ '---'r):_.~:~ ~ I \~:\,. a.. \ I ,.f}/~. -; _._.... J , • .., \ , ... 6JSt. Joseph ~ N '.. N SON AL I 8ASI H ",,-' f) o J \ \ ~ t , "\ ttl '"'l 2km , ::s , ,.'" II: \ ... " II! e:- II~ O = G ""0 I-'+') ~I ,". ::;. (1 ~l ,;: ~ N o ~J ,l N ~ ...... ("""I 1\\~ ~ ~ ••••••• ~ e:- North Island ~ \ ,~. ~ ff : ~ ~ ~~ ~.," . ~ ~ Farquhar Is. 'E.. (t) ~..... ~ \"~ ~ ..... , t ..... :.~? S' '"'l ~ n '<~~.;.~ ::s ?"" '-:<:., ( 0.. _~.., Aldabra ... ~ -< ~ \...... ~ I. des Deposes IS2 VI;{ '.' ~O'J o " \''', Z 10° Q C)-~OrqUhOr ~ q '/.~ . ~t\:) .~~.::" ~; .: -, .:'. ~ (.0 (.0 o a»

-- 0 to rnA [Z]B[2JC Vertical Crustal Movements in the Seychelles 383

TISTINI (1977) and BATTISTINI and rante Trench. Geomorphological information is JOUANNIC (1979). extremely scarce and fragmentary in the Ami­ Evidence of emergence in Farquhar has been rantes. From St. Joseph, ecology and geomor­ reported by GARDINER (1936) from the north­ phology observations have been reported by western part of the atoll, where isolated masses STODDART and COE (1979). Hoa-like features of rock on the outer sides of the surrounding have been described in St. Joseph by BAKER reef and on small islets (Ile du Milieu, Ile des (1963, p. 54) as follows: "The reef has a well Lapins, l le des Deposes), sparsely scattered developed outer ramp on all sides except the with small encrusting corals, are interpreted west.... There are openings in the reef margin (p. 432) as remnants "of an almost or quite con­ but no deep passes communicating with the tinuous reef that stood up for 10 feet or more lagoon, which spills its water at tide changes above the water level and formerly surrounded through a number of narrow channels." the whole bank." STODDART and POORE (1970), though they did not visit these islets, METHODS estimate that "they may be storm-cast reef­ blocks." BATTISTINI (1977) and BATTISTINI According to the method used by MONTAG­ and JOUANNIC (1979) describe these rocky GIONI and PIRAZZOLI (1984) in coral reef masses as compact limestone, including large areas of the South Pacific, especially near the coral heads, some of which are in growth posi­ Tuamotu hoa, relative sea-level drops can gen­ tion, and recemented coralline breccias. On the erally be determined from petrological analysis small islets they are capped with storm ram­ of exposed skeletal conglomerates. This is parts and vegetation. Two coral samples, col­ obtained by sampling the conglomerates on ver­ lected from this compact limestone on the Ile tical sections, at several levels between the des Deposes at the highest tide level, were present-day low-tide level and the upper part of dated 3100 ± 700 (Th/U) and 3640 ± 100 e4 C) the outcrops, and identifying by petrological yr BP respectively. A third sample, collected 0.5 analysis the possible diagenetic boundary m higher than the highest tide level, was dated between two different marine environments of 4900 ± 600 yr BP. This would indicate, accord­ cementation (phreatic and vadose) for the first ing to BATTISTINI and JOUANNIC (1979), a generation of intergranular cements. In the former sea-level stand about 2 m above present. absence of moating phenomena, this boundary In the eastern part of , indi­ is considered to be related to the former mean cations of recent emergence are less elevated. low-tide level at the time of cementation. The They consist of slightly emerged beachrocks difference in elevation between this boundary and conglomerates around the northern part of and the present-day mean low tide level is North Island and especially in the Manaha regarded as corresponding to the relative sea­ Islands, where the surface of conglomerate flag­ level change which has occurred since the early stones stands about 0.5 m above high water lithification of the rubbly deposit concerned. level (STODDART and POORE, 1970). This Radiocarbon dating of aragonitic coral frag­ surface could correspond, according to BATTIS­ ments, sampled from the deposit near the TINI and JOUANNIC (1979), to a former sea­ boundary level, will provide useful indications level stand a little over one metre higher than of the maximum age of the former sea level. at present, i.e., about one metre lower than in This method, which has been applied success­ the northwest part of the atoll. As noted by fully to atolls and barrier reefs of French Poly­ STODDART and POORE (1970), there is a clear nesia (for detailed references, see PIRAZZOLI similarity between the conglomerates and shal­ and MONTAGGIONI, 1988), is used here for low channels around the Manaha Islands and the first time on Indian Ocean reef material. the hoa in the Tuamotu Islands (South Pacific). Measurements of altitude have been referred St. Joseph is an atoll, 4 to 6 km in diameter, to the tide levels predicted by the Admiralty located on the east side of the Amirante Bank, Tide Tables at D'Arros Island (for St. Joseph) which is situated near the western tip of the and at Farquhar, with reference to Zanzibar. Seychelles Bank, on the east side of the Ami- However, no tide gauge is in operation now in

Figure 1 (facing page). Location map. (A) emerged land; (B) outer either of the two atolls. Possible deviations of reef margin; (C) lagoon reef margin. meteorological origin from the tide levels pre-

Journal of Coastal Research, Vol. 6, No.2, 1990 384 Pirazzoli, Kaplin and Montaggioni

FARQUHAR m +1 - MHWS 1~--r--L--.--L...,...-...l.-.-~""""1160~ 230 BP

- MHW

c o -t MSL o

o l(' > ~ w

- MLW

- MLSW -1

0.8 ST.JOSEPH - MHWS

- MHW -.--...... -.,....~-"'--...... ---.--J~~ ~ 1070 :!: 220 BP --L-~..,..-J--r-~....l.-..,,"""'--FIG. I

------1-- --- M 5 l o

~hNO.8 m

1_ ---- MlW

- MLSW -0.8

.::······..:-:.::. c w.....•..t, Figure 2. Position of the samples analysed in Farquhar and St. Joseph, in relation to the former marine environments of cemen­ tation and of the present tide levels. (A) zone of former marine phreatic environment; (B) zone of former marine vadose environ­ ment; (C) recent sand deposits. dieted during the field work could not be RESULTS checked. Nevertheless, it seems unlikely that such deviations, ifthey existed, could have been In spite of the brevity of our visit, which of the same order as the differences in level prevented us from carrying out systematic geo­ measured between the two islands. morphological surveys, and particularly unfa­ vourable meteorological conditions in Far-

Journal of Coastal Research, Vol. 6, No.2, 1990 I Vertical Crustal Movements in the Seychell es 385

Figure 3. Sampling of the Man ahas conglomera te in Fa rquhar Atoll (photo P.A. Pirazzoli ). quhar, which made it impossible to reach the The lower four samples, up to + 0.4 m, sh ow islets dated by BATTISTINI and JOUANNIC only one generation of cements, with a dense, (1979), detailed observation and sampling could structureless micrite matrix, typical of the be carried out on conglomerate sect ions at Far­ marine phreatic zone (Figure 4). The next two quhar and at St. Joseph. samples, between + 0.45 and + 0.55 m, exhibit At Farquhar, nine samples have been col­ two successive generations of cements: (1) a lected from a single vertical profile of a con­ dense, structureless micrite matrix; and (2) the glomerate formation developed on the lateral development of micrite crystals in a northernmost side of the Manahas (F igures 2 "grain- cont act" a tt it ude, or thin rims of ara­ and 3), and then analyzed. The samples consist gonite fibres with truncated cry stal ends. Here of calcirudite rich in foraminifers, coral and cementation has occurred twice , within the coralline algal particles. Their vertical position marine phreatic, then in the marine vadose onthe profile is indicated in Figure 2 in relation zone . to the present mean sea level (MSL) . Above + 0.6 m, samples show diagenetic fea -

J ournal of Coas ta l Research , Vol. 6, No. 2, 1990 386 Piraz zoli , Kaplin and Montaggioni

Figure 4. Typical marine phreatic cem ent-matrix infilling, fr om the lower seque nce of exposed ong lome ra tes in Farquhar Atoll (sample 6FA2, situat ed at ab out + 0. 2 m abov e pr esent MSL ): occurre nce of a den se, structu re less micr it e matrix (Me), comple te ly infilling a ll th e interp articular pore spa ces availabl e. (Sca le = 250 11m).

tures typical of the marine vadose zone only: lected from a conglomerate exposure near one or several successive generations of Resource Islet and analyzed (F ig u re 2) . The cements in a grain-contact attitude, fibrous lower five samples, up to about +0.25 m, are rims with truncated crystal ends in the form of typified by very poorly sorted, mainly silt-sup­ meniscus and/or local microstalactites or drip­ ported material (wackstone, wackstone-pack­ ston es (F igur e 5). Porosity, which is usually stone), one single generation of dense micritic less than 25-30% in the lower six samples, matrices and low residual porosity (10- 25%). increases to 30-40% in the upper three. The three upper samples, above + 0.35 m, are The boundary between the two former envi­ characterized by less poorly sorted, silt- to ronments of cementation (phrea tic and vadose) grain-supported material (pa cks t one, pack­ for the first generation of intergranular stone-grainstone), two generations of cements cements, can be situated between 0.55 and 0.60 both exhibiting typical non-phreatic features m above the present MSL in the Manahas con­ (grain-contact, bridge-contact and/or microsta­ glomerates and the relative sea-level drop since lactites) (Figure 6) and high residual porosity early lithification can be estimated at about 1.2 (30-50%). m . Two coral samples, collected at about + 0.3 The boundary between the former marine m and + 0.85 m (in relation to MSL) have been phreatic and vadose zones should be localized dated 1120 ± 220 yr BP (MGU-1094) and 1160 between 0.25 and 0.35 m above the present MSL ± 230 yr BP (MGU-1092), respectively. in the St. Joseph's conglomerate and the rela­ At St. Joseph, eight sa mples have been col- tive sea-level drop since early lithification ca n

J ournal of Coa stal Research , Vol. 6, No.2, 1990 Vertical Crustal Movements in th e Seychelles 387

Figure 5. Typic al marine vadose cements from the upper sequence of exposed conglomerates in Farquhar Atoll (sample 6FA 7, situated at about + 0.65 m above present MSL): occurrence of cement rim s in a pend ant (microstalactitic) attit ude (ms) and in the form of meniscus (me) . Not e the remaining high residual porosity. (Scale = 100 urn ). be estimated at about 0.8 m. Two coral samples, ates. Since that time, differential vertical crus­ collected at about - 0.4 m and at + 0.45 m (in tal movements have occurred, making emer­ relation to MSL) , have been dated 900 :!: 230 yr gence in Farquhar greater than in St. Joseph BP (MGU-ll01) and 1070 :!: 220 yr BP (MGU­ and in other Seychelles Islands. According to 1097), respectively. the data provided by BATTISTINI and JOUANNIC (1979), this emergence trend is at DISCUSSION AND CONCLUSION least 4000 years old in Farquhar. Eustatic fluctuations may have contributed to the relative sea-level changes observed in this The radiocarbon ages obtained from the coral part of the Indian Ocean. BADYUKOV et al, conglomerates of Farquhar and St. Joseph are (1986), in particular, mention the possibility of very near to each other and statistically indis­ a small sea-level drop around 1000-500 yr BP cernible. This suggests that the skeletal debris in this area. It is worth noting, however, that forming the conglomerates was deposited on the indications of Holocene emergence are conspic­ reef flat of the two atolls at about the same uously absent not only throughout the granitic time, ca. one thousand years ago, during a short islands of the Seychelles (STODDART, 1984; period of increased storminess, or during a MONTAGGIONI and HOANG, 1988), but also major tropical cyclone or tsunami. The mean in isolated atolls such as Coetivy (KAPLIN and low water level at that time corresponds to the PIRAZZOLI, 1989). In contrast, elevated coral boundary between the former marine phreatic formations dating from the last interglacial and vadose zones identified in the conglomer- period have been reported from several granitic

Journal of Coasta l Rese arch, Vol. 6, No.2, 1990 Figure 6. Typical marin e va dose matrices from the upp er seq ue nce of exposed rubbly deposits on St . Joseph Atoll (sa mple RS 1.50, collec te d at + 0.45 m ab ove pr esent MSL ): occurren ce of scattered patches of den se micrite matrix, laterally ch anging into thin r ims in a gra in-cont act attitude (gc) or in th e form of bridges betw een adj acent grains (b). (Scale = 100 11m).

islands (VEEH, 1966; MONTAGGIONI and Oceanographique, Paris) for revising the Eng­ HOANG, 1988), but are completely a bsent from lish text. atolls such as Farquhar, St. Joseph or Coetivy. This tends to confirm the conclusion reached by LITERATURE CITED MONTAGGIONI and HOANG (1988) of rela­ BADYUKOV, D.D .; DEMIDENKO, Y.L.; KAPLIN, tive tectonic instability in the Seychelles area P.A. a nd NIKOLAEV, S.D. , 1986 . Geomo rphology in late Quaternary times, with local movements a nd late Pleistocene development of the Seychell es a nd Ami rante shoal-banks (in Russian with En gli sh presumably caused by either hydroisostatic or abstract). Okeanologija, 26(2), 273-281. thermal processes and changing from place to BAKER, B.H ., 1963 . Geolo gy a nd mineral resources of place. the Sey chelles arch ipelago. Geological Survey Kenya, Mem oir n° 3, 140p. BATTISTINI, R., 1977. Un atoll a morphologie eni g­ ACKNOWLEDGMENTS matiqu e: Farquhar (Seychelles). Bull. A ssoc. Geogr. Franc., 444 , 195-204. BATTISTINI, R. a nd JOUANNIC, C., 197 9. We thank S. Nikolaev (Moscow State Univer­ Recherch es sur la geomorphologie de l' atoll Far­ sity) for help in the field and in laboratory quhar (Archipel des Sey ch elles) . Atoll R esearch analysis, B. Franchon and J . Itef(SHOM, Brest) Bulletin , 230, 1-20. for very useful information on tide predictions BRAINTHWAITE, C.J.R., 1984. Geolo gy of the Sey­ chell es. In : D.R. Stoddart, (Ed .), B iogeography and in the areas investi ga ted, E. Badyukova (MSU) Ecology ofth e S eychelles Islands. Junk, Th e Hague: for discussions and Ms . M. Delahay e (Instit ut pp.17-38 .

J ourn al of Coastal Research, Vol. 6, No.2, 1990 Vertical Crustal Movements in the Seychelles 389

GARDINER, J.S., 1936. The reefs of the western from French Polynesia (Pacific Ocean) as indicators Indian Ocean. I. Chagos archipelago; II. The Mas­ of recent relative sea-level changes. Coral Reefs, 3, carene region. Trans. Linnean Society London, 19, 29-42. 391-436. PIRAZZOLI, P.A. and MONTAGGIONI, L.F., 1988. GARDINER, J.S. and COOPER, C.F., 1907. Descrip­ Holocene sea level changes in French Polynesia. tion of the Expedition (Percy Sladen Expedition to Palaeogeograp hy, Palaeoclimatology,Palaeoecology, the Indian Ocean in 1905). Transactions Linnean 68, 153-175. Society London (Zoology), 2, 12, 1-55, 111-175. RABINOWITZ, P.D.; COFFIN, M.F. and FALVEY, D., KAPLIN, P.A. and PIRAZZOLI, P.A., 1989. A geo­ 1983. The separation of Madagascar and Africa. Sci­ morphological reconnaissance of'Coetivy Atoll (Sey­ ence, 220, 67-69. chelles, Indian Ocean). Atoll Research Bulletin, 319, STODDART, D.R., 1970. Coral islands of the western 1-8. Indian Ocean. Atoll Research Bulletin, 136, 1-5. KAPLIN, P.A.; PIRAZZOLI, P.A.; PAVLIDES, Y.; STODDART, D.R., 1971. Environment and history in MEDVEDEV, V.S. and BADENKOV, Y., 1986. Sea Indian Ocean reefmorphology. Symposia Zoological level and environmental changes in shelf areas of Society London, 28, 3-38. the western Indian Ocean. Journal of Coastal STODDART, D.R., 1984. Coral reefs of the Seychelles Research, 2(3), 363-367. and adjacent regions. In: D.R. Stoddart (Ed.), Bio­ LIONNET, J.F.G., 1970. Names of islands. Atoll geography and Ecology of the Seychelles Islands. Research Bulletin, 136, 221-224. Junk, The Hague: pp. 63-81. MART, Y., 1988. The tectonic setting of the Sey­ STODDART, D.R., and COE, M.J., 1979. Geography chelles, Mascarene and Amirante plateaus in the and ecology of St. Joseph Atoll. Atoll Research Bul­ western equatorial Indian Ocean. Marine Geology, letin, 223, 27-42. 79,261-274. STODDART, D.R. and POORE, M.E.D., 1970. Geog­ MONTAGGIONI, L.F. and HOANG, C.T., 1988. The raphy and ecology of Farquhar Atoll. Atoll Research last interglacial high sea level in the granitic Sey­ Bulletin, 136, 7-26. chelles, Indian Ocean. Palaeogeography, Palaeocli­ VEEH, H.H., 1966. 230Th/238U and 234U/238U ages of matology, Palaeoecology, 64, 79-91. Pleistocene high sea level. Journal Geophysical MONTAGGIONI, L.F. and PIRAZZOLI, P.A., 1984. Research, 71, 3379-3386. The significance of exposed coral conglomerates

o RESUMEN D Los analisis petrol6gicos asi como las dataciones radiometricas efectuadas a las muestras de coral tomadas en dos atolones de Seychelles, sugieren una emergencia diferencial (1.2 m en Farquhar, 0.8 en St. Joseph) dado que la deposicion del material se realize simultaneamente hace unos mil aiios. Esto confirma que tectonicamente el area de Seychelles fue relativamente inestable durante el ultimo periodo del Cuaternario, como se ha sugerido en estudios anteriores.-Department afWater Sciences, University of Cantabria, Santander, Spain.

D RESUME D L'analyse petrographique et la datation radiornetrique d'echantillons preleves de conglomerats recifaux en deux atolls eloignes des iles Seychelles indiquent une difference dans l'emersion (1,2 m a Farquhar, 0,8 m a Saint-Joseph) depuis Ie depot presque simultane des materiaux des conglomerats, il y a environ mille ans. Ceci confirme la relative instabilite tectonique de la region des Seychelles a la fin du Quaternaire, qui avait ete deja suggeree par une etude anterieure.

D ZUSAMMENFASSUNG D Petrographische Analysen und radiometrische Datierungen von Proben, welche aus korallenreichen Konglomeraten von 2 abseits liegenden Seychellen-Atollen geborgen wurden, weisen auf unter-schiedliche Hebung (1,2 m in Farquhar, 0,8 m in St. Joseph) seit gleichzeitiger Ablagerung der Konglomerate vor ca. 1000 J ahren hin. Das bestatigt, daB das Seychellen-Gebiet im Spatquartar relativ instabil war, wie bereits friihere Studien angedeutet haben.-Dieter Kelletat, Essen/FRG.

Journal of Coastal Research, Vol. 6, No.2, 1990