RESEARCH ARTICLES CURRENT SCIENCE, VOL. 91, NO. 12, 25 DECEMBER 2006 1655 *For correspondence. (e-mail: [email protected]) Micropalaeontological investigations on tsunamigenic sediments of Andaman Islands S. M. Hussain 1, *, R. Krishnamurthy 2 , M. Suresh Gandhi 1 , K. Ilayaraja 2 , P. Ganesan 1 and S. P. Mohan 1 1 Department of Geology and 2 Department of Applied Geology, University of Madras, Guindy Campus, Chennai 600 025, India In order to understand the type of microfauna (Fora- minifera and Ostracoda), which might have been transported by tsunamigenic sediments that deposited on the beaches, estuaries/creeks and mangrove loca- tions of Andaman Islands, a detailed fieldwork has been carried out from these islands. The main objective of this study is to record and document the calcareous microfaunal assemblage and its distribution pattern in the tsunamigenic sediment samples collected from Andaman group of Islands. The pre-tsunami foramini- feral fauna of Andaman Islands reported by earlier workers is also compared. A total of 46 surface and nine core samples have been collected from various coastal geomorphological features such as beaches, estuaries/creeks and mangrove areas of Andaman Islands. These samples are analysed for Foraminifera and Ostracoda, by applying standard micropalaeon- tological techniques. A total of 87 species belonging to 74 genera of Foraminifera and 29 species belonging to 22 genera of Ostracoda have been encountered. Among Foraminifera, Assilina ammonoides, Amphistegina ra- diata and Calcarina sp. are widely distributed. Most of the forms are highly to moderately abraded and ap- peared in milky white colour, may be due to churning action and transportation. However, the Ostracod population is scanty; of these only Macrocyprina sp. is comparatively a deep water form. Ostracod fauna is reported in this study based on the recent tsunami- genic sediments of Andaman Islands. All the forms re- corded in the study area thrive in the shallow inner shelf (neritic) zone. From their distribution, it is in- ferred that the 26 December 2004 tsunamigenic sedi- ments deposited on the coastal landforms in Andaman group of islands have been derived from shallow litto- ral to neritic depths and not from deeper bathyal ter- ritories. Keywords: Andaman Islands, documentation and distri- bution, microfaunal assemblage, tsunamigenic sediments. RECENTLY, the application of microfossils has entered a new arena, as it emerges that they can be used as an ex- cellent monitoring as well as assessment tool in tsunami- genic impact studies 1 . Benthic Foraminifera and Ostracoda are also used as a tool for tracking transgression and re- gression events in lagoons connected to the open sea. Like storm events, tsunamigenics can also be recognized in the stratigraphical record from the identification of Fora- minifera and Ostracoda. However, on the basis of Fora- minifera alone, it is difficult to distinguish between storm and tsunamigenic deposits. Foraminifera can be used to identify early formed tsuna- migenic deposits. The most detailed investigation of Fora- minifera contained within tsunamigenic deposits has been undertaken by Dominey-Howes et al . 1 in western Crete. In particular, the presence of certain species indicated that the transport from deep water offshore, and this observation was used to support the concept of a clear identification of earlier tsunamigenic events. Studies of Foraminifera contained within tsunamigenic deposits are also found to be in their infancy and the value of this particular tech- nique remains to be discovered. Based on the results of the Q-mode cluster analysis and radiocarbon dating of cores along the coast of Kachchh, NE Arabian Sea, Nigam and Chaturvedi 2 hypothesized that the erosion and transportation of fine-grained sedi- ments from relatively deep water and their deposition in shallower water are under the influence of storm/tsunami events. Study area and field sampling The Andaman group of islands is situated in the Bay of Bengal, between peninsular India and Myanmar, spread- ing like a broken necklace in the north–south direction. A chain of about 349 islands is located between 6°45″– 13°41″ N lat. and 92°12″–93°57″ E long. The entire group of Andaman Islands has been taken up for this study. Extensive fieldwork was carried out from 8 May to 7 June 2005 for collection of tsunamigenic sediment samples from various sites (all possible and assessable coastal sites) as shown in Figure 1. The locations of sur- face and core samples are recorded using GPS and are given in Table 1. Surface sediment samples are collected from the study area from various tsunami affected geo-
Transcript
RESEARCH ARTICLES
CURRENT SCIENCE, VOL. 91, NO. 12, 25 DECEMBER 2006 1655
Micropalaeontological investigations on tsunamigenic sediments of Andaman Islands S. M. Hussain1,*, R. Krishnamurthy 2, M. Suresh Gandhi1, K. Ilayaraja 2, P. Ganesan1 and S. P. Mohan1 1Department of Geology and 2Department of Applied Geology, University of Madras, Guindy Campus, Chennai 600 025, India
In order to understand the type of microfauna (Fora-minifera and Ostracoda), which might have been transported by tsunamigenic sediments that deposited on the beaches, estuaries/creeks and mangrove loca-tions of Andaman Islands, a detailed fieldwork has been carried out from these islands. The main objective of this study is to record and document the calcareous microfaunal assemblage and its distribution pattern in the tsunamigenic sediment samples collected from Andaman group of Islands. The pre-tsunami foramini-feral fauna of Andaman Islands reported by earlier workers is also compared. A total of 46 surface and nine core samples have been collected from various coastal geomorphological features such as beaches, estuaries/creeks and mangrove areas of Andaman Islands. These samples are analysed for Foraminifera and Ostracoda, by applying standard micropalaeon-tological techniques. A total of 87 species belonging to
74 genera of Foraminifera and 29 species belonging to 22 genera of Ostracoda have been encountered. Among Foraminifera, Assilina ammonoides, Amphistegina ra-diata and Calcarina sp. are widely distributed. Most of the forms are highly to moderately abraded and ap-peared in milky white colour, may be due to churning action and transportation. However, the Ostracod population is scanty; of these only Macrocyprina sp. is comparatively a deep water form. Ostracod fauna is reported in this study based on the recent tsunami-genic sediments of Andaman Islands. All the forms re-corded in the study area thrive in the shallow inner shelf (neritic) zone. From their distribution, it is in-ferred that the 26 December 2004 tsunamigenic sedi-ments deposited on the coastal landforms in Andaman group of islands have been derived from shallow litto-ral to neritic depths and not from deeper bathyal ter-ritories.
Keywords: Andaman Islands, documentation and distri-bution, microfaunal assemblage, tsunamigenic sediments. RECENTLY, the application of microfossils has entered a new arena, as it emerges that they can be used as an ex-cellent monitoring as well as assessment tool in tsunami-genic impact studies1. Benthic Foraminifera and Ostracoda are also used as a tool for tracking transgression and re-gression events in lagoons connected to the open sea. Like storm events, tsunamigenics can also be recognized in the stratigraphical record from the identification of Fora-minifera and Ostracoda. However, on the basis of Fora-minifera alone, it is difficult to distinguish between storm and tsunamigenic deposits. Foraminifera can be used to identify early formed tsuna-migenic deposits. The most detailed investigation of Fora-minifera contained within tsunamigenic deposits has been undertaken by Dominey-Howes et al.1 in western Crete. In particular, the presence of certain species indicated that the transport from deep water offshore, and this observation was used to support the concept of a clear identification of earlier tsunamigenic events. Studies of Foraminifera contained within tsunamigenic deposits are also found to
be in their infancy and the value of this particular tech-nique remains to be discovered. Based on the results of the Q-mode cluster analysis and radiocarbon dating of cores along the coast of Kachchh, NE Arabian Sea, Nigam and Chaturvedi2 hypothesized that the erosion and transportation of fine-grained sedi-ments from relatively deep water and their deposition in shallower water are under the influence of storm/tsunami events.
Study area and field sampling
The Andaman group of islands is situated in the Bay of Bengal, between peninsular India and Myanmar, spread-ing like a broken necklace in the north–south direction. A chain of about 349 islands is located between 6°45″–13°41″ N lat. and 92°12″–93°57″ E long. The entire group of Andaman Islands has been taken up for this study. Extensive fieldwork was carried out from 8 May to 7 June 2005 for collection of tsunamigenic sediment samples from various sites (all possible and assessable coastal sites) as shown in Figure 1. The locations of sur-face and core samples are recorded using GPS and are given in Table 1. Surface sediment samples are collected from the study area from various tsunami affected geo-
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Figure 1. Map of study area showing field-sampling points.
morphological features such as beaches, estuaries/ creeks, backwaters and mangrove swamps, with the help of Van veen grab using motor launch. Sediment core samples are collected up to 1 m from the specified and re-quired localities in order to observe the vertical distribu-
tion of microfauna and to identify the post-tsunamigenic fauna. A total of 46 surface samples and nine core sam-ples collected during fieldwork were brought to the labo-ratory for further analysis during June 2005. All the nine cores are sub-sampled into 78 sections at an interval of
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DG, Diglipur; MB, Mayabander; RA, Rangat; BA, Baratang; SA, South Andaman; HA, Havelock Island; HB, Hut Bay and SC, Sippighat.
5 cm and all are analysed for microfossils. Scanning Electron Microscope (SEM; JEOL JSM-6360) photogra-phy was carried out at the Department of Geology, Uni-versity of Madras.
Results
The tsunamigenic sediment samples are subjected to standard micropalaeontological techniques to recover the
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Figure 2. 1. Assilina ammonoides, side view; 2. Nodosaria. sp., side view; 3. Textularia sp., side view; 4. Para-hauerinoides complanatiformis?, side view; 5. Adelosina laevigata, side view; 6. Calcarina calcar, side view; 7, Elphidium sp., side view; 8. Elphidium sp., side view; 9. Elphidium discoidale multiloculatum, side view; 10. Ammonia beccarii, side view; 11. Elphidium sp., side view; 12. Elphidium crispum, side view, 13. Recto-bolovina?, side view; 14. Orbulina universa, side view; 15. Pavonina flabelliformis, side view; 16. Quinqueloclu-ina sp., side view; 17. Elphidium crispum, side view; 18. Cancris ariculus, side view. (Bar scale is given in the photograph.) Numbers commence from left to right, row-wise.
microfauna (Foraminifera and Ostracoda)3,4. Based on the results obtained on species identification, a total of 87 species belonging to 74 genera of Foraminifera and 29 species belonging to 22 genera of Ostracoda have been recorded. Important microfauna are photographed using
SEM (Figures 2–7). Recent Ostracods have not been re-ported earlier from the Andaman Islands. The following Ostracod species are distributed in the tsunamigenic sediments collected from Diglipur, May-abundar, Baratang, South Andaman (Chidyatapu, Wan-
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door, Sippighat, Janglighat) and Havelock Islands: Cam-pylocythere sp., Copytus sp., Cytherella semitalis, Cytherelloidea leryoi, Cytherelloidea sp., Chrysocythere keiji, Chrysocythere sp., Loxocorniculum lilljeborgii, Loxocncha gruendeli, Hemicytheridea bhatiai, H. paiki, Hemitrachyleberis sp., Lankacythere coralloides, Macro-cyprina sp., Paracypris sp., Propontocypris sp., Stigma-tocythere indica, S. kingmai, Neocytheromorpha sp., Paranesidea sp., Tanella gracilis, Triebelina sp., Xestole-beris sp. and Venericythere reticulata. Results of this study show that Ostracods are more in samples collected at Mayabander coast compared to the other areas. These tiny crustaceans are not found in the samples collected from Rangat coast and Hut Bay, this may be due to en-ergy conditions. The surface ornamental studies of Ostracod fauna re-vealed that the following species are highly to moderately ornamented: Campylocythere sp., Cytherella semitalis, C. leryoi, Cytherelloidea sp., C. keiji, Chrysocythere sp., L. lilljeborgii , L. gruendeli, H. bhatiai, H. paiki, Hemi-trachyleberis sp., Neocytheromorpha sp., L. coralloides, S. indica, S. kingmai, T. gracilis, Triebelina sp. and V. reti-culata. The rest of the forms are smooth in ornamentation. The following foraminiferal species have been widely distributed in the Andaman group of islands, namely Textu-laria agglutinans, T. bocki, Adelosina laevigata, Spirolo-culina communis, S. nitida, S. orbis, Cycloforina semi-plicata, Quinqueloculina agglutinans, Q. kerimbatica, Q. lamarckiana, Triloculina insignis, T. schreiberiana, Am-phistegina radiata, Pararotalia nipponica, Ammonia bec-carii , A. dentata, A. tepida, E. craticulatum, E. crispum and Assilina ammonoides. Larger foraminiferal genera like Amphistegina and Assi-lina are widely distributed in most of the sampling sta-tions. In Hut Bay (HB) stations, the species Amphistegina radiata and Assilina ammonoides are widely distributed. In station HB-4, Amphistegina alone is widely distributed with more than 290 specimens, whereas in station HB-5, Assilina ammonoides is found to be widely distributed. The species Q. agglutinans is the only one dominant in Mayabander sampling station. The distribution of Foraminifera at Mayabandar Island is more or less similar to the Rangat coast. In Baratang Island, the species Cibicides soendacnsis and Amphistegina ra-diata are dominant. Discorbinella bertheloti, Elphidium crispum and Sigmavirgulina tortuosa are found lesser in amount. Textulari and Nodosaria are rarely distributed in this region. In Havelock Islands, Miliolids like Quinque-loculina and Triloculina are widely distributed. In South Andaman, the following species are widely distributed: Pararotalia, Elphidium crispum, E. craticulatum, Quinque-loculina and larger Foraminifera like Amphistegina les-soni, Sorites marginals and Cymbaloporreta bradyi. Here, the larger and smaller benthic Foraminifera are dis-tributed more or less similarly. In Hut Bay, A. radiata and A. ammoniodes are widely distributed. The core samples
collected at Sippighat, Hut Bay and Diglipur (varying in length from 75 to 90 cm) yield sporadic occurrence of benthic Ostracods such as C. leryoi, C. keiji, L. lilljeborgii, L. gruendeli, H. bhatiai, H. paiki, Hemitrachyleberis sp., L. coralloides, Macrocyprina sp., S. indica, S. kingmai, Neocytheromorpha sp., Paranesidea sp., T. gracilis and Triebelina sp. The following species of Foraminifera are also recorded in these core samples: Elphidium, Quinque-loculina, Florilus, Cibicides, Calcarina, Assilina, Ammonia, Amphistegina, Peneroplis and Globigerina. The distribu-tion of Foraminifera and Ostracoda in different islands is given in Table 2. From Table 2, it can be observed that the species diversity is more in Mayabander and Havelock Islands and Amphistegina and Globigerina are recorded in all the islands.
Discussion on ecological implications
The general distribution of the Foraminifers is characteristic of an assemblage under marine influence, and dominated by calcareous species. The occurrence of A. lessonii and A. radiata indicates tropical reefoidal environment. Some agglutinated foraminifera, e.g. Textularia are also abun-dant. T. agglutinans is characteristic of more open marine conditions. Distribution of species of Pararotalia, which is a good indicator of strong marine influence on environ-ments, is noteworthy. A. lessonii is an algal symbiont bearing phytal (attached or clinging) in its habitat. The important constituents of the larger Foraminifera in the beaches are Spiroloculia orbiculus, Amphistegina gibbosa, A. lessonii, A. radiata and Peneroplis pertusus. S. orbiculus is a circumtropical cosmopolitan taxa and indicates tropical reefoidal environment. The larger spe-cies Amphistegina is a shallow-water species reported in all parts of the Andaman Islands. Cibicides is more abun-dant in depths shallower than 300 m. Maximum concen-tration of Cibicides is recorded in the outershelf depths, where it is one of the most prominent genera5. Ammonia and Elphidium are common shallow-water species. In Hut Bay, Cassidulina delicata is common to abundant in oc-currence. The Cassidulina species is observed between 63 and 105 m in Andaman Sea4. The distribution of Cal-carina species indicates the shallow environments of high water energy on hard substrates, in particular on exposed reef flatforms. The deep-water species Globigerina bulloides distributed in the beach sediments may be due to the upwelling action. G. bulloides is a non-symbiotic, spinose planktonic Fora-minifera and is associated with temperature to sub-polar water mass. During deep mixing of the surface ocean, particularly during storms in spring, the living fauna can be distributed over the upper 200 m of the water column7. Rosalina globularis is a tolerant species in turbid waters. The prolific abundance of Quinqueloculina and Spirolo-culina directs the prevalence of cosmopolitan species. In-
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Figure 3. 1. Globigerina sp.?, side view; 2. Elphidium craticulatum, side view; 3. Triloculina terquemiana, side view; 4. Quinqueloculina kerimbatica, side view; 5. Spiroloculina sp., side view; 6. Textularia aggluti-nans, side view; 7. Massilina sp., side view; 8. Peneroplis pertusus, side view; 9. Calcarina sp., side view; 10. Globigerina bulloides, side view; 11. Amphistegina radiata, side view; 12. O. universa?, side view; 13. Alveolinella sp., side view; 14. Vertebralina striata, side view; 15. Amphistegina lessonii?, side view; 16. Heterolepa praecincta, side view; 17. Bolivina sp., side view; 18. Amphistegina radiata, side view. (Bar scale is given in the photograph.) Numbers commence from left to right, row-wise.
nershelf species like Bolivina, Orbulina, Globigerina and Textularia are widely distributed in all the beach samples. This could be attributed to deposition by drifting onshore currents. Bolivina prefers muddy substratum and is re-stricted to bathyal and marginal conditions.
Ammonia, Elphidium and Pararotalia are originally bi-convex; their ventral side is slightly flattened when com-pared to the other regular forms of the species for better attachment. Amphistegina and Operculina are rich and well preserved may be due, in part, to their survival over long
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Figure 4. 1. Calcarina calcar, side view; 2. C. calcar, side view; 3. Eponoides repandus, side view; 4. Planorbullinella larvata, side view; 5. A. leavigata, side view; 6. Sorites marginalis, side view; 7. Q. karimbatica, side view; 8. Assilina ammonoides, side view; 9. Quinquelocluina sp., side view; 10. Spiro-loculina nitida, side view; 11. Textularia agglutinans, side view; 12. Textularia dupla, side view; 13. Quinqueloculina sp., side view; 14. C. calcar, side view; 15. Prygo mascilenta, side view; 16. Amphiste-gina lessoni, side view (abraded form); 17. Fissurina. sp., side view and 18. S. tenuis, side view. (Bar scale is given in the photograph.) Numbers commence from left to right, row-wise.
periods of time and also to their continued existence in time. The larger benthic Foraminfera, namely Amphisorus hemprichii, Borelis schlumberrgerri, A. gibosa, A. les-soni, A. radiata and P. pertuses are reported in South Andaman Islands8.
In most of the islands, the smaller size of Ammonia, Elphidium, Quinqueloculina and Pararotalia species is widely distributed. Thomas and Schafer9 studied Fora-minifers from the Minas Basin in eastern Canada. They concluded that the presence of intact but small and rela-
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Figure 5. 1. T. agglutinans, side view; 2. Quinqueloculina bicostata, side view; 3. Textularia sp., side view; 4. Quiun-queloculina sp. 1., side view; 5. Quiunqueloculina sp. 2, side view; 6. Quinqueloculina coastata, side view; 7. Quinquelo-culina sp., side view; 8. Fissurina cuculatta, side view; 9. Rosalina macropora, side view; 10. Siroloculina costifera, side view; 11. Spiroloculina depressa, side view; 12. Prygo complentiformis, side view; 13. Cymbaloporecta bradyi, side view; 14. Neopateoris sp., side view; 15. Bolivina sp., side view; 16. Rectobolivina raphanus, side view; 17. Hanzawia concrentrica, side view. (Bar scale is given in the photograph.) Numbers commence from left to right, row-wise.
tively fragile tests of Eggerella advena and Elphidium frigidium suggested selective erosion and transport in suspension, since this mode of transport would have sub-stantially reduced damage to these tests. This view is also supported by Nigam and Chaturvedi2, who explained that the sediments are fine-grained, and they were probably
eroded from relatively deep water and transported in suspen-sion until they were deposited at the present shallower site. When compared to Foraminifera, Ostracods are rare in occurrence as well as population. They are smaller in size and morphologically well preserved. Only few Ostracod
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Figure 6. 1. Cytherelloidea leroyi, left valve external view; 2. Mutilus pentoekensis, right valve external view; 3. Trie-belina sp., right valve external view; 4. Macrocyprina sp.?, right valve external view; 5. Tanella gracilis, left valve exter-nal view; 6. Neocytheromorpha reticulata, right valve external view; 7. Keijcyoidea praecipua, right valve external view; 8. Keijella sp., left valve external view; 9. Keijcyoidea preacipua, internal view (specimen upside down); 10. Loxoconcha sp., left valve external view; 11. Hemicytheridea reticulate?, left valve external view; 12. Puricythereis whatleyi, right valve external view; 13. Caudites sublevis, right valve external view; 14. Lankacythereis coralloides, left valve external view; 15. Tanella gracilis, right valve external view; 16. Hemicytheridea sp., left valve external view; 17. Lankacythereis coralloides, right valve extenal view; 18. Bairdoppilata alcyonicola, left valve external view. (Bar scale is given in the photograph.) Numbers commence from left to right, row-wise.
forms are slightly corroded and abraded. Species like Cytherelloidea leryoi, Xestoleberis sp. occur generally in shallow marine conditions. However, C. keiji, H. bhatiai, H. paiki and T. gracilis prefer to thrive in mostly mar-ginal marine environments. The occurrence of Macro-
cyprina and Paracypris indicates comparatively deeper environments (outer shelf/neritic zones). The well-calcified and highly ornamented forms such as L. corolloides, Triebelina, Hemitrachyleberis, Neo-cytheromorpha and V. reticulata are restricted to shallow
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Table 2. Distribution of Foraminifera and Ostracoda in different islands
Foraminiferal species DG MB RA BA HA SA HB
Ammonia beccarri** X X X X X Ammonia dentata X X X X X Ammonia tepida X X X X X X X Amphistegina lessoni** # X X X X X Amphistegina radiata** # X X X X X X Amphistegina gibosa# X X X X X Amphistegina sp. X X Amphisorus hemprichii# X Ammobaculities exigus** X X X Alveolinella sp X X Adelosina leavigata X Assilina ammonoids X X X X Bolivina nobilus X X Bolivina sp. X X Borilus schlumbergerii# X Calcarina sp. X X X Calcarina calcar# X X X X Calarina unbilicata X X Cancris aricula X Cassidulina delicata X Cibicidus lobotulus X X Cibicides soendacnsis** X X Cibicides praecintus X Cycloferina semiplicata X Cymbaloporreta bradi# X Discarbinella bertheloti X X Edentostomina cultrata X X Elphidium craticulatum# X X X X X Elphidium crispum# X X X X Elphidium discoidale X X X Elphidium sp. X X Eponoids sp. X X X Eponides raphandus X X Fissurina marginata X Globigernia bulloides X X X X X X Globorotalia globotruncana X X Globorolatia tumida X X Globotruncana sp. X Glandulina laevigata X Hanzawia concrentrica X X X Massilina sp. Miliolinella circularis X X X Miliolinella labiosa X Nodosaria sp. X Nonion elongatum X X X Nonionella stella X X Neopateoris sp. X X Osangularia venusta X X Osangularia inherance X Orbulina universa X Peneroplis planatus# X X X Paralina hispidula X X Pararotalia nipponica X X X X Pavonina flabelliformis X X Prygo mascilenta X X X Prygo complentiformis X X X Pseudorotallia schoretorina X X X Rectobolivina raphanus X X X Quinqueloculina lamarckina X X X Quinqueloculina seminulum** X X X X X X X Quinqueloculina tropicalis X X X X Quinqueloculina parkeri X X X
Contd…
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Table 2. (Contd…)
Foraminiferal species DG MB RA BA HA SA HB
Quinqueloculina elongatum X X X X X X Quinquloculina agglutinans X X X Quinqueloculina semiplicata X X Quinquelocuoina pontoni X Quinqueloculina porrecta X X Quinqueloculina bicostata X Q. kerimbatica X Rosalina globularis X X X Ruesella simplex X X X Sigmavirgulina tortuosa X X X Spiroloculina afixa X Spiroloculina aquea X Spiroloculina depressa X X Spiroloculina nitida X Spiroloculina orbis# X X Spiroloculina tenuis X X X Spirolina aritnes X X Spriroloculina sp. X X Sorites marginals X Textularia agglutinans X X Textularia bocki X X X Textularia conica** X X X X Triloculina insigns X X X Triloculina trigonula X X X X Triloculina tricarinata X X X X X Triloculina scheriberina X X X Vertebralina striata X X X
Ostracoda species Campylocythere sp. X Chrysocythere keiji X Chrysocythere sp. X Cytherelloidea leroyi X X X Cytherelloidea sp. X X Cytherella senitalis Copytus sp. Hemikrithe peterseni X Hemicytheridea bhatia X Hemicytheridea paiki X Hemitrachyleberis sp. X Lankacythere coralloides X X Loxoconcha mandviensis X X Loxoconcha gruendeli X Loxoconcha sp. X X Loxocorniculum lilljiborchi X X X Macrocyprina sp. X X Mutilus pentoekensis X X X Mutilus sp. X Neocytheromorpha sp. X X X X Paranesidea sp. X X Paracypris sp. X Propontocypris sp. Stigmatocythere indica X X Stigmatacythere kingmai Tanela gracilis X X Triebelina sp. X Venericythere reticulata X X X X Xestoleberis sp. X
Total Foraminifera 24 45 41 24 54 17 25 Total Ostracod 8 14 7 2 12 1 1
Pre-tsunami species reported by earlier workers (**Frerichs6; #Rajshekhar and Reddy8).
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Figure 7. 1. Propontocypris (Schedopontocypris) bengalensis, left valve external view; 2. Hemikrithe sp., left valve external view; 3. Actinocytheereis scutigera, right valve internal view; 4. Keijella reticulata, left valve ex-ternal view; 5. Leptocythere pulchra, left valve external view; 6. Keijella neali, left valve external view; 7. Loxo-conchella sp., right valve extenal view. (Bar scale is given in the photograph.) Numbers commence from left to right, row-wise.
marine environments. Paranesidea and Propontocypris exist in reefoidal habitats4,10.
Conclusions
Foraminiferal forms belonging to Miliolids and Rotaliids are common and characteristically contain the species of Quinqueloculina, Ammonia, Elphidium and Pararotalia. In addition, the species of Amphistegina is common in the Andaman Islands. The larger benthic Foraminfera like A. hemprichii, B. schlumberrgerrii, A. gibossa, A. lessoni, A. radiata and P. pertuses are commonly distributed in Anda-man Group of Islands. Among Foraminifera, A. ammonoides, A. radiata and Calcarina sp. are widely distributed. The higher species diversity at Mayabaunder and Havelock Islands is correlated to the coastal geomorphology and topography. Foraminifers are highly abraded in Hut Bay. Most of the forms are highly to moderately abraded and may be due to high wave energy conditions. Compara-tively, the Ostracod population is scanty in Hut Bay sam-pling stations. All the forms recorded in this study are those that preferably thrive in the shallow inner shelf (neritic) zone. Hence, it is inferred that the tsunamigenic sediments deposited on the beaches, creeks/estuaries and mangrove swamps in the Andaman group of islands have been mainly derived from shallow littoral to neritic bathymetry and not from deeper bathyal territories. Similar
observations have also been made from the tsunamigenic sediments of Nagapattinam coast, Tamil Nadu11. The distribution of T. bocki and T. agglutinans is dominant only in few stations like Baratang and Havelock Islands, where the substrate is predominantly sandy with insignifi-cant mud content. Some species such as O. universa, G. bulloides, E. repandus, Bolivina sp. and Globorotalia sp. are more abundant in the deeper portions. However, O. universa, Globoratalia and Globigerina are also distrib-uted in the nearshore region in some areas, indicating that they have been transported from the deeper portions of the outer innershelf regions. The Ostracod taxa recorded generally thrive in the shal-low, inner-shelf environment. However, sediment samples have been collected from beaches, creeks and mangrove areas along the coastline. This indicates that the tsunami-genic sediments deposited on these coastal landforms, are derived from shallow inner-shelf environment due to the tsunami. The documentation and distribution of these tiny crustaceans and protozoans will serve as a reference for this catastrophic event and also be useful for future inves-tigations assessing the impact of natural hazards in the Andaman Islands.
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ACKNOWLEDGEMENTS. This article is based on the results obtained from a research project sanctioned and funded by the Department of Science and Technology, New Delhi, to R.K. and S.M.H. Thanks are also due to Dr M. Prithviraj, Director, Earth System Science Division, DST; Dr P. Periakali, Department of Applied Geology and Director, Guindy Campus, University of Madras and to Dr V. Krishnamurthy, Director, DST and Director, Department of Fisheries, Port Blair, and his colleagues particularly Mr Rishikesh, for support in successful completion of this project. We also thank Dr Rajiv Nigam, National In-stitute of Oceanography, Goa for valuable suggestions. We acknowl-edge the anonymous referees for their constructive criticism in revising the manuscript. Received 15 February 2006; revised accepted 17 August 2006
