Inventaire des formes littorales du Néogène, la côte de l’île Neil, Sud Andaman, Inde
SWATI Ghosh, ASHIS Kumar Paul
Abstract: The Coast of Neil Island, a part of Ritchie’s Archipelago of Andaman and Nicobar group of islands, exhibits varied coastal landforms and sedimentary environments. Field trip studies conducted and analysis of satellite images and SDI maps complimented this endeavor. The major coastal landforms identified in the study area are: cliff, wave-cut platform, sea notch, sea cave, sea arch, sea stack, beach, and sandy cay formation. The erosional features are mostly concentrated in the western part of the island whereas the sandy cay is found in the northern part of the island. The study reveals that factors like marine processes along with solution process have played significant role in the evolution of erosional landforms. The cay formation is mainly due to the result of sediment accumulation.
Keywords: Neogene, Arch, Cay, Coastal landforms, India
Résumé: Le littoral de l’île de Neil, une partie de l’archipel de Ritchie du groupe Andaman et de Nicobar, présente une variété de reliefs côtiers et des environnements sédimentaires. Des sorties sur le terrain, l’analyse des images satellites et des cartes SDI ont facilité l’analyse géomorphologique du littoral de l’île de Neil. Les principaux reliefs côtiers identifiés dans la zone d’étude sont: la falaise, le platier, les encoches, la grotte, l’arche marine, l’éperon d’érosion marine, la plage et la formation sablo-argileuse. L’activité érosive concerne surtout la partie ouest de l’île alors que la caye se trouve dans la partie nord de l’île. L’étude révèle que des facteurs comme les processus marins ainsi que des processus de dissolution ont joué un rôle important dans l’évolution des reliefs et l’érosion littorale. La formation de la caye résulte de l’accumulation de sable coquiller.
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INTRODUCTION
Neogene period had been one of the most crucial times in earth’s history (Sharma & Srinivas, 2007). During this period there had been major changes in the tectonics, physiographic, faunal and past sea level history, which had influenced the geomorphic and sedimentation processes in the coastal regions. Another major event recorded during this time was the upliftment of deep water marine sediments in the fore arc basin of Andaman Nicobar accretionary prism (Sharma & Srinivas, 2007). This gave rise to an inner sedimentary arc. Neil Island, a part of Ritchie’s Archipelago Group ranges in age from Early Miocene to Pleistocene. Chalk and limestone are the major rocks in the Archipelago group. Most of the coastal landforms of this group had been carved out during Holocene time. A coastline of 19.3km length, shows a complex nature, and exhibits a wide range of erosional landforms in the western part. The evolution and subsequent changes of coastal landforms were influenced by various factors like tectonics, coastal, and weathering processes (Bird, 2007).
This study identifies and describes various aspects of the Neogene coastal landforms determined during field trips and stratigraphic investigations.
Figure 1: (a) Location of Neil Island, (b) Location of various coastal landforms
PHYSICAL SETTING
TOPOGRAPHY
The island is part of Ritchie’s Archipelago of South Andaman division, situated in the Andaman Sea (Figure 1). It is about 32km east of Port Blair and lies in the southern-most part of the Archipelago between Havelock Island in the north and Sir Hugh Rose Island in the south. Neil Island is subdivided into two broad physiographic regions. The hills lying in the eastern part are mainly composed of mudstone, and the western part is the lowland, comprised of limestone.
BEDROCK GEOLOGY
The bedrock geology of the island has been described by many geologists (Curray, 2005, Oldham, 1885, Tipper, 1911, Karunakaran, Ray, Sen, & Saha, 1968).The two major formations of Neil Island are: (i) ‘Sawai Bay Formation’, which occupies most of the eastern part of the island and composed of mudstone, and (ii) ‘Neil West Coast Formation’ occupying the western part and consists of two units: a lower unit named Silty Mudstone Member, and an upper unit, the Limestone Member lying disconformably over the former (Srinivasan & Azmi, 1976a, Srinivasan & Azmi, 1976b). From afar, Earth looked much as it does today when the Neogene period began. The rocks belonging to this age are broadly classified into three categories: 1) continental, 2) shallow marine or midfacies and 3) deep marine facies (Sharma & Srinivas, 2007). This deep marine facies belongs to the Archipelago series and Nicobar series (Srinivasan, 1978). The strata exposed in the Ritchie’s Archipelago is proposed as Archipelago Series by Oldham (1885). The Neil West Coast Formation is the upper part of the Archipelago Group is predominantly grey, calcareous mudstone and limestone. The silty mudstone member forms the lower unit and the limestone member forms the upper unit disconformably over the former (Figure 2).
All along the west coast from Cape Mears to south of Natural Bridge, the limestone member is exposed. Neil West Coast Formation is overlain unconformably by recent to subrecent deposits comprising of shell limestone with boulders, pebbles, coral rags and beach sands (Sharma & Srinivas, 2007).
Figure 2: Location and lithology of West Coast section, Neil Island, the type section of Neil West Coast Formation. The formation is divisible into two members – the lower ‘Silty Mudstone Member’ and the upper ‘Limestone Member’ (Sharma & Srinivas, 2007)
PREVIOUS WORK
Research activities on Neil Island are limited to certain aspects like geology, hydrology. Geologists have published several artifacts on various geological features on Neil Island. Preliminary investigations were done by Oldham (1885), Tipper (1911), Gee (1927). All of them have identified several distinct groups of rocks in Andamans Islands. Recently Chandra and Guha (1963), Chatterji (1964), Chatterjee (1967), Karunakaran (1968), Bandyopadhyay et al. (1973), Srinivasan and sharma (1973), Srinivasan and Azmi (1976), Pandey et al (1993), Rajshekhar and Reddy (2003) have worked on the geological aspects of Neil island. V Sharma and M S Srinivasan (2007) gave a detailed account of lithostratigraphy, biostratigraphy and chronology of Neogene period. The Neogene sediments are found in the Archipelago group of the Andaman and Nicobar Islands. V.S. Singh, M.V. Nanda Kumar (2005) assessed the changes in the groundwater regime at Neil Island caused by Dec 26/2012 earthquake and tsunami. The fresh and saline water interface was delineated by N. C. Modal and V. S. Singh (2012) in Neil Island. Although till date there had been no evidence of any investigation on geomorphic features of this island.
METHODS
The SOI topographical maps (87 E/I) and satellite images of Resourcesat-1 LISS III false colour composites (FCCs) on a scale of 1:50,000 are used in the prefield session. The topographical maps and the images are superimposed in order to identify the location of the cliffs and beaches. After subsequent identification of the landforms in the field, various measurements were taken using GPS, tape, clinometer. The coordinates obtained from the field using GPS were imposed on the administrative map of Neil Island to produce the final output as depicted in figure 1, using ARCGIS software.The sand samples were dried in room temperature and then the thin sections were prepared. Polarized light microscopy (PLM) method was applied to identify the mineral composition of the sands in thin-section.
COASTAL LANDFORMS
The coastal landforms identified in the study area are: cliff, sea notch, sea cave, sea arch, sea stack, and wave cut platform, beach and sand cay.
CLIFF
Cliffs are produced by the upliftment of the land margin as a result of complex faulting. The seaward slope represents the plane of fault along which the displacement has been occurred. Most of the cliffs of the study area are usually near vertical to vertical and sometimes overhanging coastal slopes cut into various rock formations. Several limestone cliffs are found along the western coast of Neil Island. Vertical cliffs are best developed on homogeneous and well- stratified rock formations. When the cliff base is undercut by the hydraulic pressure due to wave action the original cliff retreats. The abrasive actions of water laden rock fragments (such as sand, gravel etc.) erode the limestone cliff of Lakshmanpur Beach on Neil Island. As a result, basal abrasion notches are formed. Length, width and height of the notches are measured within a range of 2-4 m, 6-10m and 2-8m respectively. The vertical cliff of Lakshmanpur beach is impacted due to marine action. Sub-aerial weathering accompanied by salt crystallization, produces taffoni (large cavities) and intricate honeycomb structure in the spray zone. The vertical section of the limestone cliff of Lakshmanpur beach shows the following layers (Photo 1): Limestone soil with gravellypherous coral, organic acid debris and accumulation of vegetation roots.
Photo 1: Vertical section of limestone cliff, Lakshmanpur beach
1. Shelley fragments with coralline debris and fossil, light gray in color with organic acid.
2. Potholes signifying former sea levels.
3. Very compact, thrusted limestone with lineation marks with smaller and finer debris, very hard and cemented.
4. Compact coralline debris with fossils and present notch marks.
5. Modern coralline raised reef platform (now intertidal to supratidal).
Some cliffs show vertical grooves and buttresses which cut out along joint planes. Mud caves in Sitapur beach at Neil Island is a very good example of this phenomenon (Figure 1b). Powerful wave action has excavated the cliffs along the weak zones (joints, fault planes, etc). As a result vertical grooves have been formed. The cliffs are impacted by erosion and sub-aerial weathering. The vertical section of the cliff of Sitapur beach on Neil Island shows the following stratification (Photo 2):
Photo 2: Vertical section of mud cliff, Sitapur beach
1. Immature soil with high organic content and vegetation root zone.
2. Fossilized coral debris with beach rock.
3. Highly jointed and weathered white clay rock.
4. Moderately jointed white clay rock with evaporites and percolation marks.
5. Compact white clay rock, algae mat and little reaction of sea water, caving structure.
Notch, cave, natural bridge and stack have been formed by the dissection of limestone cliffs in the western part of the island. The cliffed coast of the study area shows an array of such features in the limestone rock from Miocene to Pliocene age.
SEA NOTCH
Basal abrasion notches are formed due to the abrasive action of water-laden rocks accompanied with solutional processes. This phenomenon is active in the limestone coast of Tropical region on Neil Island. Notches are well-developed behind the shore platform of Lakshmanpur beach (Photo 3). Length, width and height of the notches are measured within a range of 1-10m, 1-5m and 0.5-10m respectively. When water from low tide and high tide gets thrown into the base of the cliff, it weakens the same, thus a wave cut notch is formed.
Photo 3: Notches at the base of limestone cliff, Lakshmanpur beach
SEA CAVES
Caves are commonly found in the rocky regions with numerous joints, faults or segments of weaker zones. Caves are commonly found in the Lakshmanpur Beach on Neil Island (Photo 4).
Photo 4: Cave at the base of cliff at Lakshmanpur beach
The main driving force in the development of cave is wave action. The primary process involved is erosion. Caves are usually formed in cliffs and are also known as littoral cave. The sea caves occur on the cliffed coast of the study area where the waves break directly on the rock cliff. Mechanical erosion along with chemical solution process is responsible for the formation of the caves. Deep inlets are formed through the zones of weakness under the force of waves and are eroded out. As time progresses, these cavities are enlarged by the hydraulic pressure generated by each wave and wave-splash action and finally forms the caves. Height, width and depth of the caves are measured within a range of 2.5-3m, 3m and 2-3m respectively.
SEA ARCH
It is a unique landform found at Lakshmanpur Beach on Neil Island which is formed in massive, horizontally-bedded limestone of Holocene time (Photo 5). Due to powerful wave action the caves are further excavated and extended through the headlands resulting into the formation of a natural bridge. Wave erosion is mostly concentrated at the headlands due to wave refraction. Destructive waves erode along the line of weakness that lies across the headland. Continuous wave impact and marine abrasion results into widening and deepening the line of weakness. Primarily, sea caves are formed at the two sides of the headland. Due to further erosion, the sea caves enlarge and finally join together to form the arch. These types of arches are called ‘‘vertical crevices’’ arch. When the vertical crevice in the rock is enlarged due to weathering it creates an arch. Geologically, these are ephemeral features because after a few centuries the sea arches collapse due to the same process by which it was formed. A sea arch consists of a few elements. The longest horizontal dimension of the opening is called the span. The span of the natural bridges of separate Lakshmanpur varies between 10-30m. The longest vertical dimension i.e. the clearance ranges from 10-25m. The remnant of bedrock which defines the top of the opening is called the lintel. The length of the lintel is determined by the span of the opening. The bedrock remnants on either side of the opening, holding up the lintel, are called the abutments. Mainly two natural bridges are found at Lakshmanpur beach in the south western part on Neil Island.
Photo 5: Sea arch formed at the headland at Lakshmanpur beach
SEA STACK
They are found in the western part on Neil Island. A weakness in an exposed headland becomes a cave, the cave is enlarged from both ends to form an arch, and finally the top of the arch collapses leaving the seaward pillar as a stack. A combination of wave attack at the base of the rock and sub-aerial weathering (wind, rain, sea spray) at the roof of the arch weakens the structure. Eventually the roof of the arch collapses leaving a stack.The stacks are gradually worn down and reduced by marine erosion to a short pillar known as a stump. Stumps are hidden at high tide and only become visible at low tide.
WAVE CUT PLATFORM
Cliffs of the Lakshmanpur Beach on Neil Island are bordered by a shore platform that extends across the intertidal zone. The platform is sub-horizontal, terminating in a seaward or low tide scarp. The processes that are active on the rocky shores are generally destructive (erosional) of which some erosional processes lead towards shore planation or the formation of a shore platform. Abrasion, wave quarrying, weathering, solution process and bio-erosion operate to shape the shore platform and various features across the intertidal zone. A schematic profile across the platform of Lakshmanpur Beach shows a range of features that naturally occur over the inter-tidal zone. The cliff face is marked by taffoni or honeycombing and notches are present at the base of the cliff. Rock pools and potholes occur on almost all lithologies throughout the inter-tidal flat. Potholes are deepened by the scooping action of individual boulders that are often retained in the potholes. On the seaward margin of the sub-horizontal platform, there is a rampart, which is called a lithothamnion ridge in Lakshmanpur beach (Photo 6).
Photo 6: Lithothamnion ridge on the seaward margin of wavecut platform, Lakshmanpur beach
This is a cuesta-like structure composed of moderately well-developed buttresses separated by broad, deep surge channels that make the seaward margin very irregular (Figure 3).
Figure 3: Plan and section through spurs and channels of Lakshmanpur beach. Elongate spurs separated by narrow channels
Seaward parts of the buttresses are cavernous and irregular, their upper surface form a knobby, pavement-type algae. The highest part of buttresses rise 0.60 to 0.91 m above low tide and the lower parts rise about 0.30 m above low tide covered by a globular type of Lithithamnion. This algae forms a hummocky surface
BEACH
A narrow stretch of sandy beach is observed all along the coastline except for the north-eastern part and the south-western part where the cliffs are backed by the wave cut platforms. The beach is composed by milky white sands. The sand samples were collected randomly from the beach, from backshore (supratidal) region, in 2 locations (Lakshmanpur and Sitapur) using global positioning system (Photo 7).
Photo 7: Sample site
The mineral composition of the sands was identified under microscope using Polarized light microscopy (PLM) method. The sediment of Lakhshmanpur beach is coarse to fine-grained, inequigranular carbonate sedimentary rock. It has two main components; orthochem and allochem. Orthochemical components (nearly 55%) are mainly fine to very-fine muddy carbonate and in few places fine crystalline sparite is also seen. The allochemical components (nearly 45%) are made up of 3 parts; a) Intraclasts (75% of allochem) which is coarse, polygonal to irregular shaped fragments of crystalline limestone. Most of these intraclasts are made up of sparite (both calcite and aragonite). Few intraclasts are dark grey to greyish brown in colour and are micrite. b) Fossil fragments (25% of allochem) they are medium to coarse-grained and are mainly fossil shell and made up of sparry calcite and have various shapes. c) Pelloid (5% of allochem) is medium-grained, sub elliptical and has no internal structure. They all are made up of micritic carbonate. Apart from orthochem and allochem very few angular to sub-angular clastic fragments of quartz and feldspar were found floating within the micrite mass. The rock shows an overall clastic texture. Within the orthochem a pure non-clastic texture is found. From the petrographic studies it is evident that the rock is deposited in a shallow shelf environment where impact of wave is not high. The presence of high amount of micrite and absence of ooid indicate this. This type of sediment is known as Intramicrite (after Folk). Whereas sediment from Sitapur beach shows that it is medium to fine-grained, the proportion of intraclasts are few in numbers while the proportion of fossil fragments are much higher (>55%). The percentage of pelloid is more that 5% of the allochem. A very little clastic silicate (1-2%) is found. The petrographic analysis shows that these sediments are also deposited in shallow marine environment. And due to higher proportion of fossil fragments these sediments are called Biomicrite (after Folk).
SAND CAY
A small, low island, usually sandy, known as a cay, is found on the carbonate platform of Bharatpur Beach of this island. They are mostly built on the edge of the coral platform. Clastic materials are scooped off the front of a coral reef and off reef-talus slopes and dumped on reef surface by large amplitude waves. Primarily this accumulation of sand is found to move around but gradually becomes stabilized.
DISCUSSION
The study area consists of Quaternary sediments overlying the Neogene sediments (Table 1). The surface sediments of the island are predominantly Holocene limestone. Rainwater in combination with organic matter produces acid which dissolves the limestone to form various landforms.
In the coastal areas it is marine processes which have shaped the various landforms. The coastal landforms particularly notch, cave, arch and stack along the south western coast of Neil Island have association with each other in their formation and evolution. The morphology of cliff is the result of a variety of tectonic, marine and solution processes of deposition and erosion.
Table 1: Coastal landforms and associated sedimentary deposits of Neil Island
In the course this study, it became clear that costal erosion is very active. The coastal zone of Neil Island is therefore notable for numerous remarkable geomorphological features. The geomorphological analysis of the island showed a perspective of long term coastal evolution. The identification and description of the various landforms suggest that there is an assemblage of processes and landforms which are unique features in this tropical region. Though the effects of tectonics occur regardless of latitude or climate, the rate of chemical reactions is accelerated by increasing temperature and precipitation of the tropics. It can be therefore concluded that the rapid rate of weathering in the tropics causes the formation of a wide range of erosional and depositional features along the island margin.
References
BANDYOPADHYAYA, S. SUBRAMANYAM, M., R., and SHARMA, P. N. (1973). The Geology and mineral resources of the Andaman and Nicobar Islands. Rec Geol. Surv. India, 105(2), pp. 25-68
BIRD, E. (2008). Coastal Geomorphology. An Introduction. England, John Wiley & Sons Ltd.
CHANDRA, P. K. and GUHA, D. K. (1963). The Neogene rocks of Andamans, Sci. and Cult. 29(4), pp. 202-203
CHATTERJEE, P. K. (1967). Geology of the main islands of the Andaman area. Proc. Symp. On upper mantle project N.G.R.I., Hyderabad, India, Session V, pp. 348-362
CHATTERJI, A. K. (1964). The Tertiary fauna of Andaman. Proc. 22nd Int. Geol. Cong, New Delhi, Section 8, pp. 303-318
GEE, E. R. (1972). The Geology of the Andaman and Nicobar Island with special reference to Middle Andaman, Rec. Geol. Surv. India, 59(2), pp. 208-232
INTERNET : http://science.nationalgeographic.com/science/prehistoric-world/neogene/
KARUNAKARAN, C., RAY, K. K. and SAHA, S. S. (1968). Tertiary Sedimentation in the Andaman- Nicobat geosyncline. Jour. Geol. Soc. India, 9, pp. 32-39
KENNETH, O. EMERY, TRACEY, J.T, Jr. and LADD, H.S. (1954). Geology of Bikini and Nearby Atolls, Washington, United States Government Printing Office
MONDAL, N. C. and Singh, V.S. (2012). Delineating the Fresh and Brackisk/Saline Aquifer Interface in Neil Island of Andaman- Nicobar from India: A Prelude Geoelectrical Investigation, Journal of agricultural Science and Applicatons, Vol 1, Issue 4, pp. 86-92
NARAYANA, A.C. and PRIJU, C.P, (2006). Evolution of Coastal Landforms and Sedimentary Environments of the Late Quaternary Period along Central Kerala, Southwest Coast of India, Journal of Coastal Research, Special Issue 39, pp. 1898-1902
OLDHAM, R. D. (1885). Notes on the Geology of Andaman Islands. Rec. Geol. Surv. India, 18(3), pp. 135-145
OLLIER, C. D. (1983). Tropical Geomorphology and Long-Term Landscape Evolution, Finisterra, Vol 18, 36, pp. 203-221
PANDEY, J., AGARWAL, R.P., DAVE, A., MAITHANI, A., TRIVEDI K.B., SRIVASTAVA A. K., and SINGH, D. N. (1993). Geology of Andaman Bulletin ONGC, 29(2), pp. 19-103
RAJSHEKHAR, C. and REDDY, P. P. (2003a). Quaternary Stratigraphy and micropaleontology of rocks of South Andaman, Bay of Bengal, Gond, Geol, Magz. Spec. 6, pp. 33-38
RAJSHEKHAR, C. and REDDY, P. P. (2003b). Quaternary Stratigraphy of Andaman- Nicobar Islands, Bay of Bengal, Jour. Geol. Soc. India, 62, pp. 485-493
SHARMA, V. & SRINIVASAN, M.S. (2007). Geology of Andaman- Nicobar: The Neogene, New Delhi, Capital Publishing Company
SINGH, V. S., NANDAKUMAR, M. V., SARMA, M. R. K., and DIMRI, V. P. (2005). Changes in groundwater regime at Neil Island (South Andaman) due to earthquake and tsunami of 26 December 2004, Current Science, Vol 88, No 12, pp. 1984-1987
SRINIVASAN, M. S. & Azmi, R.J. (1976a). Contribution to the stratigraphy of Neil Island, Ritchie’s Archipelago, Andaman Sea. Proc. VI Indian Colloq. Micropal. and Strat. , pp. 283-301
TIPPER, G. H.(1911). The Geology of the Andaman Islands with special reference to the Nicobar. Mem. Geol. Surv India, 35(3), pp. 135-145
WOODROFFE, C. D. (2002). Coasts: form, process and evolution, United Kingdom, Cambridge University Press
Acknowledgements
The authors acknowledge Swagata Bera, Somnath Ghosh for their assistance during field work. Infrastructural facilities were provided by Vidyasagar University and thanks are due to those who helped in preparing the thin section slides and analysis of the samples.
To cite this article
Electronic reference
Ghosh Swati and Kumar Paul Ashis. «Inventory of the Neogene Coastal Landforms along the Coast of Neil Island, South Andaman, India». Canadian journal of tropical geography/Revue canadienne de géographie tropicale [Online], Vol. (2) 1. Online in May 5, 2015, pp. 28-37. URL: http://laurentian.ca/cjtg
Authors
SWATI Ghosh
Department of Remote Sensing & GIS
Vidyasagr University, India
Email: sgswatighosh@gmail.com
Dr. ASHIS Kumar Paul
Department of Geography and Environment Management
Midnapore, west Bengal, India
Email: sgswatighosh@gmail.com
