J Coast Conserv DOI 10.1007/s11852-013-0272-1
Assessment of coastal dune characteristics using georadar imaging and sedimentological analysis: Odisha and Visakhapatnam, India K. Devi & M. C. Raicy & Deshraj Trivedi & P. Srinivasan & S. G. N. Murthy & Ronald J. Goble & Rajesh R. Nair Received: 17 November 2012 / Revised: 18 July 2013 / Accepted: 19 July 2013 # Springer Science+Business Media Dordrecht 2013
Abstract The East coast of India is subject to continuous changes by high energy events. We sought to assess the depositional conditions along the coast from the geophysical and sedimentological character of the dune sands of the Gopalpur and Paradeep coast of Odisha, and the Sagarnagar coast of North Visakhapatnam. Quartz layers of the heavy mineral-rich zone collected at a depth of ~2 m from the landward foot of the dunes in the Visakhapatnam and Odisha coast, gave the OSL age estimates as 1,050±50 and 260±10 years respectively, revealing that the age of the dunes in Visakhapatnam are older than those on the Odisha coast. Episodic high energy events have affected the coast. Evidence from ground penetrating radar data consists of three stratigraphic units. The upper unit consists of vague reflections, parallel to the ground in continuous manner, most probably formed by wind action. On the other hand, the middle layer shows high amplitude reflections of heavy mineral-rich massive layers, possibly the result of tsunami activity. The lower massive layer parallel to the ground surface shows a low reflection pattern. The GPR studies showed that the thickness of the heavy mineral layers is greater on the landward foot of the dune as compared to that on the seaward side. According to the grain size analysis, the dune is composed of both wind generated and tsunamigenic sediments. K. Devi : M. C. Raicy : R. R. Nair (*) Department of Ocean Engineering, IIT Madras, Chennai 600 036, Tamil Nadu, India e-mail:
[email protected] D. Trivedi Department of Geology and Geophysics, IIT Kharagpur, Kharagpur 721 302, West Bengal, India P. Srinivasan : S. G. N. Murthy Structural Engineering Research Centre, CSIR Campus, Chennai 600 113, Tamil Nadu, India R. J. Goble Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, 225 Bessey Hall, Lincoln, USA
The scanning electron microscope studies revealed that the heavy minerals present in the dunes are mainly sillimanite, ilmenite, garnet, pyroxene, rutile, sphene, biotite, hornblende, zircon, monazite and magnetite. The study demonstrates the origin of sand dunes in different ages along the East Coast of India by the effect of various natural phenomena. Keywords Heavy minerals . Tsunami . Depositional history . Stratigraphy . Sedimentology
Introduction Coastal dunes are dynamic systems that undergo constant changes with cycles of stabilization and reactivation, and there have been many discussions and speculations about the source of sand, age, and nature of episodic events that have shaped the dune systems (Jungner et al. 2001). Transverse dunes can act as effective barriers against storms and tsunamis and can record the alternate events of dune activity and stabilization over geologic time. Here, we studied the geophysical properties, chronology and sediment parameters of the dunes along the coast of Odisha and Andhra Pradesh by means of ground penetrating radar (GPR) investigations, optically stimulated luminescence (OSL) dating, grain size analysis and scanning electron microscope (SEM) analysis. Established records of previous tsunamis and storms are geologically important. Both tsunami and storm deposits can be generated by overwash surges and backwash flows; simultaneous erosional and depositional features may form by means of outwash flow; as storm deposits are the result of high-energy processes, they may have characteristics similar to tsunamigenic deposits and leave marine traces in coastal stratigraphic sequences like those of tsunami deposits (Phantuwongraj and Choowong 2012). Overwash is the flow of a mixture of water and sediment over a beach crest that does not directly return to the water body where it originated
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(Donnelly and Woodruff 2007). This occurs only when the run-up level of waves, usually coinciding with a storm surge, exceeds the local beach or dune crest height. A decrease in overwash flow velocity on the landward side of the beach or barrier results in deposition bodies as sediment, the washover deposit, which is one of the most commonly observed depositional features related to extreme storm events (Morton and Sallenger 2003; Wang and Horwitz 2007). General washover sedimentary structures are normal graded, reverse graded, laminae of shell and heavy minerals, and planar laminae. The average slope of the beach profile is an important parameter in the morphologic characterization of a beach and related to the granulometry of the sediments and the beach energy level (da S. Alves and El-Robrini 2006). For the past few decades, GPR has been increasingly applied in sedimentary facies analysis (Van Heteren et al. 1998; Vijaya Lakshmi et al. 2011; Nair et al. 2010; Winslow et al. 2009; Davis and Annan 1989), considering its potential as a high resolution stratigraphic tool. The basis of interpretation of GPR records is the behavior of electromagnetic waves (Daniel et al. 1988). Ground penetrating radar (GPR) has been used successfully to image the internal structures of coastal sand dunes (e.g., Bristow et al. 2000) because they have a low conductivity (Bristow 2009). The analysis of radar facies helps in the description and interpretation of GPR images using differences in reflection pattern (Gawthorpe et al. 1993; Van Heteren et al. 1998). The diagnostic subsurface GPR signatures have been studied by many researchers along the south east coast of India (Nair et al. 2011). Mohanty et al. (2004, 2003a, b, c) measured the radio activity of the heavy mineral sands along the Ganjam Coast of Odisha and studied their geochemistry. Rao (1957) reported that the sorting and concentration of heavy mineral sands along the east coast of India is primarily due to the beach erosive and regressive processes associated with annual sea level changes and storms in the Bay of Bengal. Heavy minerals tend to concentrate in relatively high energy environments (Paine et al. 2005). The accumulation of heavy minerals in the form of beach and dune placers along the coastline is attributed to the unique geographical location, favorable hinterland geology, humid tropical climate aided by wind, well developed drainage systems and coastal processes like waves and currents in the area. Scanning electron microscope (SEM) is the most frequently used auxiliary instrument in mineral studies for examining the surface textures of grains, imaging the effects of sub-aerial or subsurface dissolution processes, and aiding the assessment of post depositional diagenetic modifications (Setlow and Karpovich 1972; Morton 1984). Earlier studies by Mahadevan and Anjaneyalu (1954) observed that the sorted and concentrated heavy mineral patches are the product of storms in the Bay of Bengal. Optically Stimulated Luminescence dating (OSL) is an ideal method for age determination of recently deposited sediment
layers, as it has a wide dating range and mostly adopts methods to date clastic inorganic sediments, mostly quartz, abundant in coastal environments. However care must be taken with respect to collecting sands deposited immediately by events such as tsunamis, storms and hurricanes (Murari et al. 2007). Tsunami deposits comprise the accumulation of sediments consisting of a single or more layers; in which one layer represents a normal graded or reverse graded depositional unit; different units are separated by an erosional surface with sharp contact between adjacent layers (Choowong et al. 2008a). However, we recognized that high energy flows, both tsunami and storm, exposed a discrepancy in the style of deposition that generally depends on (1) the frequency of inflow waves, (2) the difference in the source of the deposit that is reflected in the difference in grain size and grain concentration in the flows, and (3) the local change in micro-topography.
Study area and regional setting The study areas are situated in East coast of India and cover the coastal tracts of Gopalpur (19°15′ N; 84° 54′ E) and Paradeep (19° 25′ N; 85° 06′ E), in Ganjam district of Southern Odisha and Sagarnagar coast (17° 45′44″ N; 83° 21′38″ E) in Visakhapatnam district of Northern Andhra Pradesh. The sediment characteristics and subsurface records acquired from GPR, and sediments collected along two topographic profiles (Figs. 1 and 2) on the dunes, stretch out parallel to sub parallel to the coast of Sagarnagar, Visakhapatnam (Fig. 1a). Four GPR profiles along the Gopalpur coast and one from Paradeep, from sea to land, were mapped in order to investigate the internal sedimentological structure during February, 2011. Prominent and well developed transverse dunes and in some places, barchans, are seen along the Gopalpur coast. The coastal areas of Paradeep are low lying and marshy with mangrove forests formed by the deltaic sediments of the Mahanadi river system. Wider sand bodies are found both in the barrier beach and along the western margin towards the inland (Rao et al. 2001) of Paradeep. Palaeo sand ridges of 10– 15 km inland from present day shorelines were reported above the cultivated lands (Rao et al. 2001). These deltaic sediments are underlain by Gondwana sediments with a minimum thickness of 2,000 m in the Paradeep area (Vaidyanathan and Ghosh 1993). In the Paradeep coast, poor presentation of beach ridges has been attributed to the presence of fluvial action. Continuous and thin parallel bars of heavy minerals are distributed along the shoreline of Odisha coast. The geomorphic province is drained mainly by the Rishikulya River, originating from the Eastern Ghats and joining the Bay of Bengal. Either sides of the river are characterized by an old flood plain, stretching up to the Eastern Ghats. Active flood plains are seen adjacent to the water channels. The river is seasonal and exceptional amounts of sediments are carried during monsoon, forming an extensive
Assessment of coastal dune characteristics using georadar imaging
Fig. 1 Location map of the Sagarnagar, north of Visakhapatnam indicated by red solid circle in India and enlarged study area marked near (modified from Subramanian and Mohanachandran 1990). Study area showing two GPR profiles (orange color arrows) (P1 and P2) of E-W direction, within those profiles black and white rectangles indicates pits (P1p1 (on dune), P1p2 (on slope) and P2p3 (on dune)). Study area
bounded west by beach road which is parallel to the coast and east Bay of Bengal (Source: Google earth 2011). The topography of the study area, view from sea to land (a). Sand dunes running parallel to sub parallel to the coast (yellow line) and the profiles have taken perpendicular to the long axis of the dune indicated by orange arrow, Eastern Ghats can be seen as the background view (a); inset is the closest view of the dunes (b)
estuarine type delta in the mouth. The coastal plain is characterized by several creeks. Extensive laterite cappings of Tertiary ages (Mohanty and Devdas 1989) can be seen close to the coastal plains at Parampet. These are the products of the
weathering of the khondalites and charnockites of the Eastern Ghat group of rocks. The geologic set up of the Eastern Ghat rocks in this area was studied by many workers (Pascoe 1950; Krishnan 1982; Mohanty et al. 1988; Rao 1989). The coastal
Fig. 2 The study area at Gopalpur and Paradeep, Odisha is marked in the figure as red squares. The satellite imageries of Gopalpur and Paradeep from where the GPR data were collected are shown in the insets. GPR profiles are marked as red lines and the pits are marked as black circles
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dunes are aligned parallel to the coast, lack of vegetation and have a maximum height of 15 m in the Sagarnagar coast of Visakhapatnam. The study area constitutes khondalite of the Eastern Ghat complex generally trending in a NE-SW direction with some hills on the Visakhapatnam coast protruding into the sea in a E-W direction (Fig. 1a), thereby forming shallow water bays in between them; in some places, charnockite intrusions are reported instead of khondalite (Srirama Das and Rao 1979).
which have estimated in quartz grains immediately overlying heavy mineral rich layer, is the result of erosional events (Buynevich and Donnelly 2006). The samples were collected by light-proof plastic pipes were inserted horizontally into the wall of the pits at different depths, just above the heavy mineral-rich layer. To prevent the exposure to the light during the sampling pipes were sealed on-site. The sediments at the end of the pipe, which may have been exposed to light during collection, were discarded in the laboratory.
Methodology Results and discussion The sedimentary features are able to picturize clearly with good depth of penetration and less attenuation by high resolution GPR profiling, along with pit sediment sampling, SEM analysis, OSL dating and granularity analysis were carried out for the present study GPR data were collected in common-offset continuous mode along two transects of GPR profiles (E-W) across the dunes (Fig. 1b), for which we used the Geophysical Survey System, Inc., USA (GSSI) SIR 3000 synchronized with 200 MHz antenna in monostatic mode. The antenna gave the penetration depth as about 8–10 m in both the study areas. In the GPR system, the antenna characteristics and the electrical properties of sedimentary units such as dielectric constant and conductivity are the important attributes of electromagnetic waves in case of sediment attenuation. RADAN 6.6 software was used for processing using appropriate steps, such as topographic correction, stacking and filtering. Also, the profiles were correlated with the available trench section. The lithological boundaries and sedimentary structures can be easily distinguishable in radar profiles (Figs. 5, 6 and 7) from the discrepancies of dielectric constant of the media. For the sedimentological studies, we dug pits of depth ~1.8 m after the preliminary investigation of GPR data. The sediments collected and well packed in small polythene cover can carry more than 500 g from different depths of pits, excavated on and along the slope of the dunes were subject to sieve analysis using different mesh sizes (8 μm to 2,000 μm). The weighing percentage of the samples remaining in each sieve was measured after shaking them in a sieve shaker for 10 min. The statistical parameters such as mean, standard deviation, skewness and kurtosis were then estimated using GRADISTAT software. Scanning Electron Microscope (SEM) analysis of the sediment samples was undertaken in order to study the morphological and textural studies of the heavy mineral assemblages of the study area. For that, JSM JEOL 6490 Scanning Electron Microscope (SEM) with a high resolution revealing details less than of 3.0 nm carried out a microscopic analysis of grains. Optically Stimulated Luminescence (OSL) dating of sediments gives the age of the most recent burial event. OSL can be used for estimating the time since the sediments were last exposed to sunlight or to estimate the total radiation exposure,
Grain size The tsunami and storm flow varies their depositional characteristics from place to place. Textural attributes of sediments such as mean, standard deviation, skewness and kurtosis are widely used to reconstruct the depositional environments of sediments (Krishnan 1968). Many sedimentologists have correlated the grain size parameters and trasportational-depositional mechanisms in various studies (Nigam et al. 1992; Sahu 1964; Psuty 1965; Collinson and Thompson 1989; Amaral 1977). In accordance with the grain size analysis (Tables 1 and 2), the phi values of samples range from 1.46 to 1.866 in the first profile and from 1.651 to 1.882 in second profile of the Odisha coast with medium grain size. The mean grain size of the coastal dunes of Visakhapatnam varies from fine to medium sand with phi values ranging from 1.858 to 2.305. The occurrence of medium- to coarse-grained sands along the coastal beaches after the 2004 Indian Ocean Tsunami was also reported by Hawkes et al. (2007). The variation in size of sediments is influenced by the source of supply, transporting medium and the energy conditions of the depositing environment (Folk and Ward 1957). The size of the grains depends upon the grain size of the sediment available for transportation (Srinivasalu et al. 2007). Mean values increase from top to bottom in both the profiles of Odisha coast (Fig. 4) and towards the land, the grain size tends to decrease. The existence of fine sand might be due to the uniform suspension mechanism of transport during the high energy event (Vijaya Lakshmi et al. 2010). The standard deviation values range from 0.454 to 0.721 in the first profile and from 0.488 to 0.846 in the second profile in Odisha, and from 0.543 to 0.702 in Visakhapatnam. The particle dispersion is in the range of moderately sorted to well sorted, and the samples show a wide range of variation in sorting. Variation in sorting reflects the continuous addition of fine and coarse materials in various proportions. The studies carried out by Sundararajan et al. 2009 along the southeast coast of India divulges the moderate sorting of sediments by waves and long shore currents resulted by fine to very fine sands gave rise to the unimodality in frequency curve (Fig. 3). The well sorted grains obtained might be due to the sudden
Assessment of coastal dune characteristics using georadar imaging Table 1 Statistical analysis results of sediments collected from different depths in three pits of two profiles of Visakhapatnam coast (P1and P2) (Samples abbreviations: P1p1T profile1 pit1top; P1p1M profile1 pit1 middle; P1p1B profile1 pit1 bottom; P1p2T profile1 pit21 top; P1p2M profile1 pit2 middle; P1p2T profile1 pit2 top; P2p3T profile1 pit3 top; P2p3B profile2 pit3 bottom)
Samples
Mean
Mode
Sorting
P1p1T
2.2
2.487
0.638
P1p1M
2.199
2.487
P1p1B
2.185
P1p2T
Kurtosis
Sample description
0.013
0.986
0.684
0.002
1.030
2.487
0.673
0.128
0.956
2.305
2.487
0.656
−0.043
1.086
P1p2M
1.905
1.736
0.688
0.149
1.078
P1p2B
1.736
1.736
0.580
0.022
0.970
P2p3T
1.977
1.736
0.702
0.142
1.04
P2p3B
1.858
1.736
0.543
−0.012
0.883
Fine sand, unimodal, moderately well sorted, symmetrical, mesokurtic Fine sand, unimodal, moderately well sorted, symmetrical, mesokurtic Fine sand, unimodal, moderately well sorted, Finely skewed, mesokurtic Fine sand, unimodal, moderately well sorted, symmetrical, mesokurtic Medium sand, unimodal, moderately well sorted, fine skewed, mesokurtic Fine sand, unimodal, moderately well sorted, fine skewed, mesokurtic Medium sand, unimodal, moderately sorted, fine skewed, mesokurtic Medium sand, unimodal, moderately well sorted, symmetrical, platykurtic
winnowing action or back and forth motion of the sediments by the depositing agent (Singarasubramanian et al. 2009), and could explain the positive skewness and unidirectional transport or deposition of sediments in a low energy environment (Brambati 1969). Values of skewness range between 0.021 and 0.234 in first profile and between 0.171 and 0.265 in the second profile of Odisha. Values between −0.012 and 0.149 indicate that the symmetry of samples varies from finely skewed to symmetrical nature. All the samples are characterized by positive skewness in the Odisha coast. The values of kurtosis range from 0.829 to 1.082 in the first profile and from 0.728 to 1.212 in the second profile of Odisha and the values ranges from 1.086 to 0.883 in Visakhapatnam coast. The samples fall under mesokurtic to platykurtic nature of distributions. The characteristic upward fining sequences of tsunami deposits were reported by Vijaya Lakshmi et al. (2010) along the coast of Tamil Nadu. The sediments of storm and tsunami deposits can be well to poorly sorted, and have heavy mineral layers at the base and within the deposit. (Hayes 1967; Schwartz 1975; Morton 1978; Leatherman and Williams 1983; Shi et al. 1995; Gelfenbaum and Jaffe 2003; Jaffe et al. 2003).
Table 2 Statistical analysis results of sediments collected from different depths in two pits of two profiles of Orissa coast (P1 and P2) (Samples abbreviations: P1B profile 1 bottom, P1T Profile 1 top, P2B profile 2 bottom, P2T Profile 2 top)
Skewness
Ground penetrating radar (GPR) studies The well processed GPR data, defined exclusively in terms of two-dimensional external facies form, were studied in detail for the prehistoric extreme event investigation in the study areas. A total of four shore normal profiles (among which two are displayed in this paper) from the coast of Goplapur and one profile from the Paradeep and two from Sagarnagar coast, Visakhapatnam were collected (Figs. 1 and 2). The data were then subjected to surface normalization in order to correct the topographic elevations. Background removal and auto gain adjustments were done to increase the amplitudes of later events. The horizontal distance and an average depth of penetration of the first profile are 165 m and 10 m respectively and those of the second profile are 225 m and 10 m respectively in Gopalpur, while in Visakhapatnam, the profile length from sea to land is 110 m with a penetration depth of 8 m. The profiles showed higher cyclicity of parallel dipping reflections below a depth of 2 m and the water table at a depth of 7.5 m. The radar facies reveals the internal stratigraphy of the coastal dunes. In this study, we try to disclose three stratigraphic units (U1, U2 and U3) (Fig. 5) of depth up to 7.5 m
Samples
Mean
Mode
Sorting
Skewness
Kurtosis
Sample description
P1T
1.46
0.986
0.454
0.021
0.829
P1B
1.866
1.736
0.721
0.234
1.082
P2T
1.651
0.986
0.488
0.171
0.728
P2B
1.882
1.736
0.846
0.265
1.212
Medium sand, unimodal, well sorted, fine skewed, platy kurtic Medium sand, unimodal, moderately sorted, fine skewed, mesokurtic Medium sand, unimodal, moderately sorted, fine skewed, platy kurtic Medium sand, unimodal, moderately sorted, fine skewed, lepto kurtic
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Fig. 3 Graph shows the frequency distribution of coastal dune (Sagarnagar coast, Visakhapatnam) samples collected from top, middle and bottom of pit1, pit2 (P1p1 and P1p2) and pit3(P2p3), indicates unimodal distribution and grain size (μm) increases upwards of the sequence
shown in the radar data from Sagarnagar coast, delineated from reflection patterns. Thick heavy mineral layers, which are the indicators of erosion as well as a proxy for sediment transport (Buynevich et al. 2004) are indicated by the high amplitude reflections. The heavy mineral concentrations comprising of finer fractions are exposed on the deflation surface which coincides with prominent reflections attenuating the electromagnetic GPR signals (Buynevich et al. 2007). In the second profile of Gopalpur, high amplitude hyperbolic reflections (Fig. 7) are recognized along the shore line at a horizontal distance of 5 to 45 m. The physical observations from the trial pits excavated at 25 and 200 m distances from the shoreline in the second profile and at 15, 105 and 148 m distances from the shoreline in the first profile (Gopalpur) coincide with the heavy mineral layers in the GPR data. Well defined subsurface cross bedding is seen in pit 4 and is marked in Fig. 7. Pits of depth 2 m were excavated in the first profile of Sagarnagar coast at distances of 50 m and 66 m from sea to land and coincide with the thick heavy mineral layer (Fig. 5). The thickness of heavy mineral layers varies considerably along the entire profile. At the foot slope of the dunes in the landward end, the thickness is double that of the rest of the profiles in both the Odisha and Visakhapatnam coasts. Two types of reflections are identified in the all profiles (Figs. 5, 6 and 7) — continuous and discontinuous. Data collected from the Paradeep coast were also concentrated by high amplitude reflections and indicative of heavy minerals layers, although the thickness of the layers at Paradeep is less as compared to those at Gopalpur.
The information from the ground reveals that the top layer consists of closely spaced laminae extending to a depth of ~1.5 m, and below that massive layer with thick dark patches of heavy minerals which thickness determination in direct way is limited by excavation depth, but from the radargram, thickness of layer estimated as ~3.5 m (pit photographs in Figs. 5, 6 and 7). The first unit is characterized by weak, continuous reflections, the direct observation in pit shows thin, continuous, closely spaced laminae in all the profiles. Sometimes, cross bedding is also seen (Fig. 7), which might have been formed by an undisturbed and calm environment, probably by wind action. Conversely, the second unit consists of a thick layer of heavy minerals evidenced by high amplitude, parallel to subparallel continuous reflections without structures like cross bedding, probably the result of a high energy event. The third unit has weak reflections parallel to the ground surface. Single, homogenous deposits without any structures are indicative of rapid deposition, such as flow deceleration between up rush and back wash phases (Morton et al. 2007). The watertable encountered at a depth of 7.5 m from the ground level in both the study areas is marked by a blue line (Figs. 5, 6 and 7). In general, the reflections in three facies are in a sequence of subhorizontal reflectors that are almost parallel to the ground surface (Figs. 5, 6 and 7). Tsunami deposits are thin, 7.5 m. Unit1 (U1), unit2 (U2) and unit3 (U3) are stratigraphic layers separated by orange and red lines; watertable marked by blue line and direct wave on the top in grey color b schematic representation of ‘A’ shows reflection pattern and arbitrary stratigraphic boundaries marked by dotted lines; P1 and P2 are two pits dug in the profile1 at distances of 50 m and 66 m respectively towards the inland from sea. c Pit1 and pit2, in which black rectangles shows heavy mineral rich layers and red rectangle shows the quartz layer, few centimeters above the heavy mineral layer taken for OSL dating
Odisha coasts. The composition of the wash-over deposits varied as a result of the difference in local source materials (Sedgwick and Davis 2003; Morton et al. 2007). Sedimentological exploration shows that the heavy mineral sands formed due to the mechanical disintegration of source rocks are dominated by finer size fractions. When the sand populations are reshuffled by wave action they are segregated into traction, swash saltation, back-wash saltation and suspension loads which show log-normal size distributions (Patro et al. 1989). Mineralogical assemblages in beaches vary from place to place depending on the host rocks in their provenance, climatic conditions prevailing in the area, agent and mechanism of transport, mechanism of transport and hydraulic conditions during deposition (Rao 1957). The heavy mineral sands of both the coasts (Odisha and Visakhapatnam) have been derived from the Eastern Ghat group of rocks comprising granites gneisses, Meta sediments of Achaean to Proterozoic age and Gondwana, exposed in the hinterland (Rao et al. 2001). The distribution pattern exhibited by ilmenite, garnet, sillimanite and pyroxene suggest that differences in specific gravity, settling velocity and differential transport have played an important role in their distribution at Gopalpur and Paradeep (Komar and Wang 1984). The studies in paleodune deposits along the Visakhapatnam-Bhumunipatnam
coastal tract consists of 10–15 % of heavy minerals with a substantial amount of sillimanite, garnet, magnetite, ilmenite, zircon, monazite (Rao 1978) and rutile (Rao and Raman 1986). The concentration of heavy minerals depends on the sediment influx from the hinterland, wave energy and velocity, long shore currents and wind speed which controls littoral transport, sorting and deposition of placer minerals in suitable locales (Rao et al. 2001). The source for the heavy minerals is believed to be khondalites, charnockites, granite gneisses, pegmatites, Precambrian crystalline, and the Deccan traps, laterites and Quaternary sediments occurring on the coastal plains (Rao et al. 2001; Mohanty et al. 2004, 2003a, b, c). The heavy mineral sands are derived during the weathering of high-land rocks followed by their deposition and concentration along the shore-lines by wave action of oceans (Rao et al. 2001; de Meijer et al. 2001; Anjos et al. 2006). The sediments that are delivered to the sea by the river will be redistributed by the waves and currents according to their densities, size and shape (Komar and wang 1984). Both wind and waves play important role in the redistribution of sediments along the coast of Gopalpur and Paradeep. Many authors (Reddy and Prasad 1997; Reddy et al. 2001; Dhana Raju 2005) reported that the coastal tract up to ~120 km north of Visakhapatnam having the heavy minerals
Assessment of coastal dune characteristics using georadar imaging
Fig. 6 Figure shows processed GPR data profile and diagrammatic representation of the GPR data collected along the first profile from sea to land at Gopalpur. Two types of reflections are recognized in the dataupper continuous and lower discontinuous reflections. Hyperbolic
reflections are seen along the berm of the profile. Ground water table is also visible in the data. The boundaries if thick layers of heavy mineral sands are marked as white doted lines in the photographs
like ilmenite, garnet, sillimanite, rutile, monazite, zircon, etc. and these concentrations are of >50 % quantity in finer fractions with dune sands (Reddy and Prasad 1997). Quartz grains are angular to subangular showing conchoidal fractures, which might be due to the transportation of sediments from offshore to land which is not subjected to the beach rounding of sand grains (Dahanayake and Kulasena 2008). Angular to sub angular nature of grains might have been due to the less of time for erosion, since the rapid deposition by high energy waves. The SEM studies of quartz grains from the beaches of Tuticorin to Thiruchendur on the south coast of Tamilnadu show both conchoidal fractures and etched marks, which are indicative of a high energy environment as well as the longer time of residence of sediments in the depositional basin (Cherian 2003; Suresh Gandhi et al. 2008). Higgs (1979) and Mallik (1986) opinioned that mechanical features like V-shaped pits and
conchoidal fractures formed in quartz by grain to grain collision in an aquatic medium (Fig. 8b, e). Krinsley and Doornkamp (1973) studied the surface textures of quartz grains over a decade. The presence of platy minerals (immature) indicates low energy conditions, and is found to co-exist with mature sediments from the near shore, and may be indicative of high energy event (Switzer et al. 2005). Mechanical features like ‘V’ marks, regular breaking along or across cleavage planes, and abrasion marks show that the heavy minerals are being transported in an aqueous medium (Suresh Gandhi et al. 2008). Subrounded zircon grains may the result of the prolonged transportation either in aqueous or land medium (Fig. 8h). The percentage of heavy minerals were significantly high in the 2004 tsunami deposits in Kerala in India and Kho Khao in Thailand (Babu et al. 2007; Jagodziski et al. 2009), but extensively lower at the southwest coast of Sri Lanka (Dahanayake and Kulasena 2008).
K. Devi et al.
Optically stimulated luminescence dating Luminescence dating has been applied successfully on coastal deposits (Clarke et al. 2002; Sommerville et al. 2003; Murari et al. 2007). Samples were collected in stratigraphic succession from depths of 1 m and 2 m depth from pit 1 and pit 2 of Sagarnagar, Visakhapatnam (Fig. 5, P1 and P2) for the OSL dating. The dates are estimated as 29±2 and 1,050±50 years before present (Nair et al. 2011) in pit 1 and pit 2 respectively. Optically stimulated luminescence (OSL) dating of quartz
Fig. 7 Figure shows a diagrammatic sketch of the processed GPR data along the second profile at Gopalpur, Odisha from sea to land, along with photographs of pits. The orange coloured line in the diagram and the white lines in the photographs indicate the boundaries of heavy mineral
mineral separated from sediments provided the depositional ages of the samples of pits 1, 2 and 3 as 13±20, 44±2 and 52±2 years respectively on the first profile (Fig. 6) and that of pit 5 as 260±10 years before present (Fig. 7) in Gopalpur, Odisha. The samples for OSL datings were taken from a depth of 1 m in the first profile and 2 m from the second profile. It is observed that the ages of sediments increase from sea to land. The comparison of the OSL data of Odisha and Visakhapatnam coast reveals that the dune in the former (260±10 years BP) is younger than that of the latter (1,050±50 years BP).
deposits; blue line indicates upper limit of water table. Cross bedding layers of sands are well marked in the bottom middle photograph. The grading of thin heavy mineral layers is clear from the photographs
Assessment of coastal dune characteristics using georadar imaging Fig. 8 SEM microphotographs of minerals collected from pits, of Sagarnagar coast, Visakhapatnam, mostly heavy minerals which are underwent past high energy events. 1. quartz 2. sillimanite 3. ilmenite 4. garnet 5. titaniferous magnetite 6. rutile 7. Zircon 8. monazite 9. Hornblende 10. magnetite. a, d, g overview of grains with angular to subrounded mixed grains; b Quartz with triangular, ‘V’ shapedpits at the edges; c depression filled with precipitation in Ilmenite; e Quartz with triangular pits with sharp edges; f elongated, smooth surfaced monazite, pits at the edges; h subrounded elongated Zircon; i Rutile with smooth edges
The widespread dune formation and reactivation along the Eastern coast of India during the last two centuries was reported by Kunz et al. (2010a, b) and Alappat et al. (2011). The OSL records of dune formation at around 3.3 ka and 1.2 ka were reported by Kunz et al. (2010a) along the coast of Tamil Nadu. So the coastal dunes of Gopalpur might have been formed even earlier than 260±10 years before present. The type of sand accumulation over geologic time varies in different regions. Cross examination of the OSL results revealed that in both profiles that the age of sediments increases from top to bottom, indicating the sequential deposition of sediments. The considerable difference in the ages calculated for the samples taken from the coarse silt layer in unit A and unit Fig. 9 Figure shows the SEM photographs of samples of heavy mineral layers of Gopalpur in the order of pit 1, 2, 3, 4 and 5. The minerals are marked as 1. quartz, 2. sillimanite, 3. ilmenite, 4. garnet, 5. titaniferous magnetite, 6. rutile, 7. zircon, 8. monazite, 9. hornblende and 10. magnetite. Quartz grains are of angular in nature. Due to the mixing of sands with onshore sediments, most of the sands have been altered
B indicate the time interval between depositions of sediments in a landward direction.
Conclusion In the present study, we could see that sand dunes are formed as a result of both storm and tsunamis. The grain size of quartz-rich sediments varies slightly within the dune sequence which is characterized by the presence of finer heavy minerals. The textural group assigned for the sediments is medium sand. The medium-sized sands in the Odisha coast are sediments carried by the rivers into the sea, followed by the
K. Devi et al.
mechanical disintegration of hinterland rock types. These sands are further sorted, concentrated and deposited along the coast by the combined action of waves and winds and thus these sediments may generally contain rich concentrations of heavy minerals (Davis 1968) especially along the Paradeep coast. The winnowing of these sediments is characterized both by strong tidal currents and littoral drift. Controversially, the increasing thickness of heavy mineral layers at the foot of the sand dunes in the landward end in the Gopalpur and Visakhapatnam coast implicate the higher run up of tsunami waves; thick deposition of overwash sediments are seen in the GPR profiles (Figs. 5, 6 and 7), indicating a tsunamigenic deposition. The tsunami waves might have inundated over the dune and the sediments deposited at the foot of the dunes. The OSL dates have shown that the sediments were deposited at around 260±10 years BP (1742–1762 AD) in the Gopalpur coast and 1,050±50 years BP (912–1012 AD) in the Visakhapatnam coast. The GPR data have revealed the stratigraphy of the dune as three units, in which detailed sedimentological analysis was done only in first unit (to a depth of ~1.8 m). Its characters were analyzed as closely spaced, continuous laminae, indicating its formation from undisturbed calm environment, probably by wind. The second unit was found to be rich in heavy minerals with layers of about 3.5 m thickness, perhaps formed by high energy event of age 1,050±50 and 260±10 years BP; the third unit consists of continuous reflections parallel to the topography and is not clear, may be due to the attenuation of signals. Scanning electron microscopic study of the heavy minerals revealed assemblages of sillimanite, ilmenite, garnet, titaniferous magnetite, rutile, zircon, monazite, hornblende and magnetite along with quartz, in which the presence of angular to subrounded grains indicate rapid deposition of minerals by a high energy event. However, some zircon grains show subrounded form, signifying transportation from longer distances and influence by the weathering mechanism. From these evidences, we conclude that both wind and high energy events contributed to the building of the present dune structure. Acknowledgments We are thankful to INCOIS for the funding of this project. We express our gratitude to our colleague, Mr. Sidhesh Kumar Pandey, who assisted in data collection during the field work, and also Department of Civil Engineering, IIT Madras for providing the facility for sieve analysis. We also express our sincere thanks to Dr. A. P. Pradeep Kumar for offering pertinent suggestions.
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