Sultanate of Oman

Block and boulder accumulations along the coastline between Fins and Sur (Sultanate of Oman): tsunamigenic remains? G. Hoffmann, K. Reicherter, T. Wiatr, C. Grützner & T. Rausch

Natural Hazards Journal of the International Society for the Prevention and Mitigation of Natural Hazards ISSN 0921-030X Nat Hazards DOI 10.1007/s11069-012-0399-7

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Author's personal copy Nat Hazards DOI 10.1007/s11069-012-0399-7 ORIGINAL PAPER

Block and boulder accumulations along the coastline between Fins and Sur (Sultanate of Oman): tsunamigenic remains? G. Hoffmann • K. Reicherter • T. Wiatr • C. Gru¨tzner T. Rausch

•

Received: 23 October 2011 / Accepted: 10 September 2012 Springer Science+Business Media B.V. 2012

Abstract The rocky coastline of the Sultanate of Oman between Fins and Sur is decorated by a number of large blocks and boulder accumulations forming ramparts. The blocks occur as individual rocks of up to 40 tons, as imbricated sets and as ‘‘boulder trains.’’ Landward, the deposits change into a sand/boulder mixture and distal into sands. The coast is made up of Tertiary folded limestones and beach rock of Quaternary age, both also constitute the megaclasts. The transport distance from the fractured seaward platform of 6–10 m above mean sea level varies between 20 m and more than 50 m. We found individual blocks of recent corals and overturned blocks with attached oysters and rock pools. Terrestrial laser scanning was used to analyze geomorphologic features as well as for volumetric estimates of the block weights. Tropical cyclones such as Gonu in 2007 or Phet in 2010 are known to have affected Oman’s coastline in the past. The coastal changes during recent cyclones were minor; therefore, we interpret the block deposits as tsunamigenic. However, this interpretation is not unambiguous. The most likely source area for a tsunami is seen in the Makran Subduction Zone situated in the northern Indian Ocean. Here, at least 4–5 tsunamigenic earthquakes are documented. Keywords

Block deposits Tsunami LiDAR Makran earthquakes Coastal changes

1 Introduction Recent tsunami events like the Indian Ocean tsunami on December 26, 2004, and the To¯hoku earthquake and tsunami on March 11, 2011, resulted in large numbers of casualties and immense damage to infrastructure. These events underline the need for tsunami hazard

G. Hoffmann (&) K. Reicherter T. Wiatr C. Gru¨tzner T. Rausch Institute of Neotectonics and Natural Hazards, RWTH Aachen University, Lochnerstr. 4-20, 52056 Aachen, Germany e-mail: [email protected] G. Hoffmann German University of Technology in Oman (GUtech), PO Box 1816, 130 Athaibah, Sultanate of Oman

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Author's personal copy Nat Hazards

assessment for any potentially vulnerable region. In most cases this can only be done by studying past tsunami records. The coastlines of the Sultanate of Oman are prone to various natural hazards such as tropical cyclones, landslides and tsunamis. The devastating effects of cyclone Gonu, due to flash floods and landslides in June 2007, illustrated the need to investigate recurrence intervals of such events in order to assess the vulnerability and to mitigate damages (Nott 2007). So far, scientific research concerning recurrence intervals of natural hazards in this region is limited. Byrne et al. (1992) estimate repeat times of large earthquakes to be more than 300 years for the eastern Makran region. Page et al. (1979) conclude recurrence intervals of large earthquakes to be in the order of 125–250 years. Studies published by Heidarzadeh et al. (2008a), Jordan (2008), Rastogi and Jaiswal (2006) and Shah-hosseini et al. (2011) reveal past tsunami events in the western Indian Ocean with possible effects for the coastline of Oman. A holistic scientific approach to decipher the geological record of past extreme events is overdue, as the population of Oman and the neighboring countries is concentrated along the coastline and large infrastructure projects are planned or completed. Until the 1970s, the Sultanate of Oman was internationally isolated. Very poor living conditions prevailed, and no modern technology was in use. As a consequence, there are almost no historical documents available. Donato et al. (2008, 2009) published evidence for past tsunami events based on sediment cores derived from a lagoon in the southeastern part of the country. The authors conclude that these tsunami deposits are related to an earthquake in the Makran Subduction Zone (MSZ), which occurred on November 27, 1945, at 22:00. Thus, tsunami waves arrived just after midnight on November 28 (Pendse 1948) along the coast of Oman. The December 2004 Indian Ocean tsunami was also recorded on the southeastern shores of the Sultanate of Oman (Okal et al. 2006). So far, site-specific components (maximum run-up heights, area of inundation and erosion potential) have never been calculated nor have the recurrence intervals. To achieve this goal, local studies are necessary. We used sediments as a proxy to derive information of past extreme events. Depending on the coastal setting and the height of the wave, tsunami deposits show a huge variety in grain size, thickness, etc. (Dawson and Stewart 2007; Kortekaas and Dawson 2007; Morton et al. 2007), and thus are difficult to distinguish from storm deposits (Shanmugam 2011). Block and boulder deposits are recognized as tsunamigenic in various settings worldwide (e.g., Mastronuzzi and Sanso` 2000; Nott 2003a; Goto et al. 2007; Scicchitano et al. 2007; Imamura et al. 2008; Frohlich et al. 2009; Bourgeois and MacInnes 2010; Goto et al. 2010a; Paris et al. 2010; Watt et al. 2010; Etienne et al. 2011; Engel and May 2012). Storm-induced transport of the coarse-grained material landward cannot be excluded in every case (e.g., Jones and Hunter 1992; Nott 2003b; Goff et al. 2004; Williams and Hall 2004; Hansom et al. 2008; Spiske et al. 2008; Goto et al. 2009; Barbano et al. 2010; Benner et al. 2010; Goto et al. 2010b; Khan et al. 2010; Lorang 2010; Switzer and Burston 2010; Buckley et al. 2011; Keating et al. 2011; Nandasena et al. 2011; Shanmugam 2011; Vella et al. 2011). The displacement of blocks and boulders along the Iranian shores of the Indian Ocean has recently been documented by Shah-hosseini et al. (2011). The authors conclude that a tsunamigenic origin is most plausible. With this paper we aim at documenting and discussing possible coarse-grained tsunamigenic deposits on the rocky coastline of Oman. The forces necessary to quarry and dislodge boulders and blocks are revealed by analyzing their size and location. Following the classification of Blair and McPherson (1999), we

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define particles of a length ranging from 0.25 to 4.1 m as boulders and particles ranging from 4.1 to 65.5 m as blocks.

2 Setting 2.1 Plate tectonic setting The Sultanate of Oman borders the Indian Ocean and the Gulf of Oman and is part of the Arabian Plate (Fig. 1). The Owen Fracture Zone marks the boundary to the Indian Plate in the east. Fournier et al. (2007, 2011) demonstrate direct evidence for active strike-slip motion along the Owen Fracture Zone and argue that large-scale earthquakes ([MW 7) are infrequent but to be expected. The Arabian Plate is further confined by active spreading axes in the Gulf of Aden and the Red Sea defining the southern and southwestern boundary. The Dead Sea transform fault acts as the western border (Hempton 1987). The Makran and Zagros fold and thrust belts (Ross et al. 1986; Vernant et al. 2004; Vita-Finzi 2001) mark the northern boundary and are characterized by a continent–continent, partly continent–oceanic collision zone, that is, the Makran Subduction Zone (MSZ) situated in the Gulf of Oman (White and Ross 1979). Glennie et al. (1990) assume that the MSZ has been active since the Miocene; Byrne et al. (1992) see the subduction process as having been active since the Early Cretaceous. The MSZ stretches from the Sendan Fault system in the Strait of Hormuz to Karachi in Pakistan and has an along-strike extent of about 1,000 km (Mokhtari et al. 2008). DeMets et al. (1990) and Regard et al. (2005) estimate

Fig. 1 Historical epicenters of earthquakes, which caused tsunamis along the Makran subduction zone in the Indian Ocean (see also Table 1) and major tectonic features. Topography is SRTM based, inset shows the study area in the Sultanate of Oman

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the convergence vector to be in the order of 40 mm/year. The sedimentary input is very high resulting in one of the largest accretionary wedges observed at convergent margins (Kopp et al. 2000). 2.2 Historic tsunami events in the western Indian Ocean The southern coast of Sumatra, the Andaman-Myanmar province, the southern coast of Java and the Makran coast offshore Pakistan and Iran are major seismic zones surrounding the Indian Ocean. Simulations show that the coastlines of Iran, Pakistan and western India (Jaiswal et al. 2009) as well as the coastlines of Oman are threatened by tsunamis generated in the MSZ and partly by tsunamis generated on the southern coast of Sumatra (Heidarzadeh et al. 2008a, b, 2009; Okal and Synolakis 2008). The Sunda trench in the eastern Indian Ocean offshore of Sumatra is the most prominent source area regarding magnitude and frequency of tsunamis. Several events are documented as predecessors of the 2004 event (e.g., Dominey-Howes et al. 2007; Jankaew et al. 2008; Monecke et al. 2008; Fujino et al. 2009). Okal et al. (2006) surveyed the effects of the 2004 Sumatra tsunami along the coastline of Oman. The portion of coastline which was affected by the tsunami is limited to the southern parts of the country from the border to Yemen in the south to Masirah Island in the north. A maximum inundation of 447 m and maximum runup heights of 3.25 m are documented. No casualties were reported during this event in Oman, and only minor damages, mostly to fishing boats, were noticed. These observations demonstrate that the northern shores of the Gulf of Arabia are sheltered by the Indian subcontinent. Therefore, the impact of these teletsunamis is limited, and it is apparent that the MSZ is the most prominent source area for tsunamis in the western Indian Ocean. However, it remains unclear which coastal areas have been affected by tsunami events in the geological past. Information on historical earthquake and tsunami events in the western Indian Ocean is given by Berninghausen (1966), Ambraseys and Melville (1982), Murty and Rafiq (1991), Murty and Bapat (1999), Rastogi and Jaiswal (2006), Dominey-Howes et al. (2007) and Jordan (2008). The most detailed catalogue of historical earthquakes in the MSZ is summarized by Heidarzadeh et al. (2008a): 13 events since 326 BC are listed, of which 4–5 possibly generated a tsunami (Table 1). The MSZ is characterized by an eastern and a western part, which differ in seismic activity; large thrust earthquakes are frequent in the eastern part, whereas the western part experienced no great earthquakes in documented historic times (Byrne et al. 1992; Pararas-Carayannis 2006). The two parts of the MSZ are divided by the Sonne fault, which is characterized as a sinistral strike-slip fault by Kukowski et al. (2000). There is an ongoing discussion (Musson 2009) whether or not the western part of the MSZ is seismically active. The lack of historical earthquakes suggests that the western MSZ is almost aseismic. Bayer et al. (2006) argue that the aseismicity can be explained by large amounts of unconsolidated water-rich sediments. Kopp et al. (2000) revealed that the sediment thickness at the deformation front is 7 km. These sediments probably lubricate the subduction process. However, if it is active and currently locked, there is the potential of a great earthquake (see Byrne et al. 1992; Wyss and Al-Homoud 2004; Bayer et al. 2006). The first instrumentally documented tsunami generated in the MSZ is the event on November 28, 1945 (Pendse 1948). However, in the catalogue the earthquake occurred on November 27, 1945, at 21:56. Taking into account different local times and the tsunami travel time by modeling (Heidarzadeh and Kijko 2011), the waves hit the coast of the

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Author's personal copy Nat Hazards Table 1 Historical tsunami catalogue for the Indian Ocean, compiled by data of (1) Heidarzadeh et al. (2008a, b), (2) Dominey-Howes et al. (2007), (3) Jordan (2008), (4) Murty and Rafiq (1991), (5) Rastogi and Jaiswal (2006), (6) Pendse (1948), (7) Lisitzin (1974), (8) Murty and Bapat (1999), and (9) Byrne et al. (1992) Number

Date

Latitude (N)

Longitude (E)

Reference

Remarks

1

326 BC

24

67,3

1,2, 5

Macedonian fleet destroyed? (2,7)

2

May, 1008 AD

25

60

1,2

Storm? (3)(8)

3

1524

22

68

1,2,3

Probably did not affect Arabia (3)

4

1765

?

11

Hearsay

5

06.16.1819

21,5

70,5

3,4,5

1,543 deaths (4)

6

1845

7

04.19.1851

25,1

62,3

1,3, 10

8

1897

24,5

62

1

Volcanic? (1) eventually climatic?

9

11.27.1945

24,5

63

1,2,3,4,6, 12; 13; 14

4,000 deaths (1,2,3,4), in 28.11.1945?

10

1983

11

12.26.2004

3,316

98,854

2

[200,000 deaths, Oman run-up 3,52 m

? Several houses destroyed, tsunami?

Diego Garcia

Sultanate of Oman early in the morning of the November 28, 1945. The location of the epicenter was 408 km SSW of Karachi, 465 km NNE of Muscat, and the magnitude of the earthquake is given as 8.1 (Pacheco and Sykes 1992). According to Mokhtari et al. (2008), the earthquake can be categorized as an interplate thrust event that ruptured one-fifth of the length of the MSZ. Rajendran et al. (2008) analyzed the arrival time of the tsunami waves along the coastline of Pakistan and India and compared them to model results. Significant discrepancies are seen as evidence for landslides triggered by the earthquake (Rajendran et al. 2008). Neetu et al. (2011) explain the late arrival of the tsunami wave in some places along the eastern coastline of the Arabian Gulf as trapping of the wave energy on the continental shelf off the Makran coast. The number of casualties along the coastlines of Pakistan and India related to both the earthquake and the tsunami is estimated at 4,000. However, no historical documents which describe such an event in Oman are available to us. Evidence of fine-grained event layers in coastal lagoons on the coast of the Sultanate of Oman has been described. They have been attributed to the 1945 event. Donato et al. (2008, 2009) analyzed shallow sediment cores from the lagoon in Sur and recorded a 5–25cm-thick shell bed at a depth of c. 50 cm below the present-day seafloor. Based on the taphonomy and fragmentation, a tsunamigenic origin is discussed as the most likely form of deposition. The shell bed is seen as geological evidence for the 1945 tsunami along the coastline of Oman. Despite earthquake-related tsunami generation, there is the possibility of landslidegenerated tsunamis. Fournier et al. (2011) analyzed the Arabia–India plate boundary (Owen Fracture Zone) and revealed an active fault system of 800 km in length. They found morphological evidence for giant landslides on the slopes of the southern Owen Ridge and conclude that the area presents a potential source area for tsunamis, which would affect the coastline of Oman.

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2.3 Coastal effects of recent tropical cyclones in the Sultanate of Oman The largest storm events along the coastline of Oman are associated with tropical cyclones. These develop over the north Indian Ocean and either dissipate or turn toward India (Murty and El-Sabh 1984; Fritz et al. 2010a, b). Arabia is only rarely affected. However, recently the coastline was struck by two cyclones: cyclone Gonu (June 5–6, 2007) and cyclone Phet (June 4–5, 2010). Cyclone Gonu is the most intense cyclone on record in the Arabian Sea (Dibajnia et al. 2010; Fritz et al. 2010a, b). The effective wave height is important in terms of coastal change. Wave heights exceeding 6 m are reported for a cyclone in 1890 (Membery 2002). Dibajnia et al. (2010) report wave heights in excess of 9 m for cyclone Gonu. Fritz et al. (2010b) documented onshore flooding values of up to 200 m due to the storm surge during Gonu. Furthermore, intense precipitation that in some cases exceeded the yearly amount of rainfall tenfold leads to wadi floods which resulted in sediment input into the coastal system. The coastal area of Quriat and Sur was surveyed by one of the authors (GH) on June 7, 2010, immediately after cyclone Phet tracked over the area. Small pocket beaches along the coast were severely affected by erosion, with sand being washed offshore. The rocky coastlines showed swash lines in cliff-top positions of ?10 m above sea level (asl). Here wood debris and other floatable objects (rubbish) accumulated. Only a minor amount of gravel was encountered forming small ridges. Most visible changes were noticeable at the wadi mouths, as literally all wadis discharged into the sea resulting in erosion at the wadi mouth. These floods also caused the most damage to infrastructure. Boulder transport was not documented. 2.4 Study area in the Sultanate of Oman The study area in the Sultanate of Oman stretches along the coastline between Muscat and Sur and is directly exposed to the MSZ (Fig. 1). Here, we concentrate on the rocky coast with steep cliffs between the village of Fins and Tiwi (Fig. 2). The tidal range is 2.5 m. On this coastal strip, the shelf area is rather narrow as it extends to a maximum of 15 km. The passive margin of the Arabian Peninsula compromises continental slopes with an average dip exceeding 5.5, which is comparably steep for continental margins (Pratson and Haxby 1996). The area between the cities Quriyat and Sur (Fig. 2) is sparsely populated as is most of the country; small fishing villages are scattered along the coast. A paved road has only been in existence since 2008, connecting the cities of Quriyat in the north and Sur in the south (Fig. 2). Behind staircase-like marine terraces of different ages, the relief rises from the coast up to more than 1,500 m in the hinterland, which is formed by Paleogene to Neogene limestone formations. Geomorphologic evidence of Quaternary land-uplift is obvious along the entire coastline, parallel to the coast; wave-cut terraces are encountered up to elevations of 300 m asl. Within the study area, these terraces are cut into Paleocene– Early Eocene limestone (Fig. 3), which is either developed as black, bioclastic limestone or thin-bedded, micritic marly limestone (Wyns et al. 1992). Quaternary deposits within the area are either of fluvial origin (Qgx) or ancient (Qmx) to subrecent (Qmy) marine deposits. The latter two are usually preserved as beach rock. In most cases only erosional remnants of the beach rock are found, and the underlying older strata dominate. Quaternary slope deposits (Qcy-z) are encountered where the gradient is steep. Numerous wadis cut through the terraces and form small rias along the coast. Several generations of channel formation are obvious and related to the uplift. The deeply incised wadis have their catchments in the adjacent mountains.

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Fig. 2 Study area in the Sultanate of Oman north of Sur city, including the location of blocks between Fins and Tiwi. Inset: bathymetric profiles show variable extension of the shelf in three sections, but relatively steep slopes of an average dip larger than 5.5 in front of the study area. Vertical exaggeration is 94

Fig. 3 Geological map of the coastal area south of Fins (see Fig. 2 for location) based on Wyns et al. (1992). Note that the boulder ridge is discontinuously developed on larger creek outlets

The coastline within the study area is characterized by a cliff with heights in the range of 5–10 m. The seaward platform is almost bare of sediment with the exception of boulders and blocks of various sizes forming a narrow rampart and individual blocks situated some

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10–20 m from the cliff edge. A landward-thinning layer of boulders in a gravel-sand matrix grades in about 40–50 m from the cliff into sands. These sands show structures of eolian transport, that is, mini dunes and ripples.

3 Methods The field survey took place in spring 2011. We mapped the extent of boulder accumulations along the coastline of the Sultanate of Oman. Additionally, we investigated the boulders in terms of lithology, stratification, biogenic remains (e.g., corals, oysters), and rock pools. The orientation of the long axis of 50 boulders was measured (where a is long axis, b is intermediate axis, and c is short axis), to obtain paleocurrent and boulder volume/ weight information. In a further step samples were taken, and the landward finer-grained sediments were investigated. We measured the cliff height and transport distance by GPS and terrestrial laser scanning. We took samples of attached oysters for dating. Dating is in progress. The density of the blocks was determined in the laboratory by standard methods (Archimedes’ principle). Terrestrial laser scanning (TLS) or LiDAR is a comparably new ground-based active surveying method. The advantages of the method are rapid, accurate and nondestructive data acquisition. Measurements are based on a coherent light which is produced by stimulated emission in a defined wavelength of 1,500 nm with a small laser beam that has a divergence of 0.00974. The emitted monochromatic light of the electromagnetic spectrum is scattered on the surface of the scanned object, and the backscattered signal is detected by a receiver. The range between sensor and object is calculated by timing the two-way travel time of each laser pulse. The scattering of the footprint depends on the surface condition (e.g., color, roughness and wetness) of the object, on the wavelength, on the distance between sensor and target and on the angle of incidence. Precision for each point is within millimeters for scan positions of less than 100 m. Despite X–Y–Z information, intensity and range are recorded. The results are 3D models of the scanned objects with a high spatial resolution. TLS is widely used in the field of natural hazards research. Especially mass movements may be monitored and quantified. Application of TLS includes landslide monitoring (Travelletti et al. 2008; Prokop and Panholzer 2009), snow avalanche mapping (Deems et al. 2006; Trujillo et al. 2007) as well as rockfall reconstruction and simulation (Abellan et al. 2009; Nguyen et al. 2011). In studies addressing tsunami deposits, TLS has been used to estimate the volume of boulders and run-up distances of tsunami waves (Mastronuzzi and Pignatelli 2011; Hoffmeister et al. in press). For this study we used an Optech ILRIS-3D instrument. The three largest blocks observed in the study area were scanned in a field campaign in February 2011. The data allow the documentation and volume estimation of each individual block as well as a morphology analysis (Mastronuzzi and Pignatelli 2011). The system scanned the objects with a sampling rate of 2,500 points per second, based on the time-of-flight measurement. The objects were scanned with a point-to-point spacing of 20 mm. The infrared laser scanner detected the monochromatic information of the backscattered intensity in 256 gray values. Additionally, the point cloud was combined with panchromatic information by using a digital camera in order to achieve the RGB color-coding. Five different scan positions around the blocks were used. The positions two and three were scanned twice at different scan angles. The seven scan sequences are composed of around 1.2 million points. A 20 % overlap of the scan windows allows the alignment of the different scans. UTM

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coordinates were taken for each scan position. For the correct rotation of the point cloud, pitch, roll and heading angle of the TLS system was measured. After data validation and data cleaning, the point cloud was imported into a geographical information system (GIS). Here the scans were converted into a triangulated irregular network and into raster format.

4 Results 4.1 Field observations The rocky coast along the strip between Sur and Quriyat appears relatively ‘‘cleaned,’’ that is, only sparse rubble and plant remains are found in a distance of approximately 10 m to the cliff front. In this area huge, angular blocks and boulders of Tertiary limestones and Quaternary beach rocks are found (Fig. 4a). Most of these blocks and boulders are tilted, toppled and partly overturned, which is proven by (bio-)erosional features on the lower side and smooth surfaces (rock pools). We also observed hit marks on the surface of the bedding planes and on the blocks and boulders. The origin and provenance of the clasts suggest short transportation distance. They derive proximally from the cliff top of Tertiary limestone and Quaternary beach rock. These rocks show joints, fractures and bedding planes that pre-shape the clasts. The thick-banked (0.5–1.5 m) angular blocks are made up of Eocene limestone (Fig. 4b). These are partly karstified and decorated with marine sessile fossil remains (Fig. 4c). The subangular, platy or elongated boulders with a thickness of approximately 0.5 m consist of Quaternary beach rock. The estimated weights are as high as 500–1,000 kg, some even higher. The beach rock covers the Eocene limestone close to the cliff (Fig. 4d). Various species of oysters are attached on subangular to rounded coarse-grained sediments, indicating an origin from the immediate shoreline, where these organisms usually live (Fig. 4e). Furthermore, this quarrying and landward dislocation is evidenced by Lithophaga sp. borings, which also point to a tidal position before transport. A horizontal trend in grain size distribution (becoming finer inland) is observed; mixed gravel-sand deposits form the distal part grading into sand. These deposits are clearly of marine origin and resemble wash-over fans. We interpret this accumulation as a remnant matrix of the boulder deposits. Most probably, the proximal deposits have been washed away, so that only large boulders remained on the cliff top and appear now as individual boulders. The wavy pattern or lobe-like structure of the boulder accumulations (Fig. 4f) forms a rampart parallel to the coast. This rampart was mapped, including the orientation of the transported and partly toppled blocks. Most of the boulder deposition occurred between 15 and 25 m from the cliff front. In general the more platy boulders are imbricated and form ‘‘boulder trains’’ (Fig. 4f). Most of the boulders are aligned, the a-axis facing toward N30E (Fig. 4g). The N30E direction is at a high angle to the present-day coastline stretching approximately N–S. This boulder accumulation forms high ramparts of approximately 2–3 m (Fig. 5a). The ridges consist of reworked beach material, mainly sand with floating boulders and gravels (Fig. 5b) with oysters, scaphopods, Tridacna sp., coral fragments and gastropods (Fig. 5c). The marine faunal remains testify a near-shore origin; also Lithophaga sp. borings in pebbles support this assumption. The entire deposit apparently has inverse grading as the largest boulders are found on the top of it. However, the sedimentary sequence shows internal fining-up cycles (Fig. 5c). One boulder (c. 1.0 9 1.0 9 0.8 m) with attached

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Fig. 4 Field aspects of the block deposits, little arrows point to view direction. a Large blocks I, II and III of which we achieved t-LiDAR scans, persons on top for scale, note ‘‘cleaned’’ cliff front. b Block I with attached oysters (c). d Platy block with oyster growth, note driftwood of cyclone Phet (2010) above hammerhead. e, f Imbricated boulders forming a rampart. g Rose plot of the long axis of boulders, maximum interpreted as paleocurrent direction from N30E

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Fig. 5 a Rampart of imbricated boulders south of Fins, note ‘‘cleaned’’ cliff front, little arrows point to view direction. b Landward mixture of boulder and pebbles and sand in the intermediate zone (see Fig. 8 for map), with oysters, shells and coral debris. c Detail of b showing a fining-up sequence with oysters, shells and coral debris in a sandy matrix (arrows marked with c)

oysters was found 80 m from the cliff edge and 8 m asl. This may reflect the maximum transport distance of boulders. Another important observation is that major wadi inlets have no boulder accumulations, neither on sandy beaches at the wadi mouths, nor on the adjacent cliff tops, which by far do not reach the height of the studied sections. 4.2 Quantitative assessment of the block deposit The quarrying and subsequent transport of a block depends on several parameters such as weight (mass of block), shape, density (of water and block), porosity, water velocity, water depth, duration, roughness of platform, steepness of beach, direction and rate of flow (Keating et al. 2011). Additionally, the height of uplifted blocks above present sea level, wave length and the coefficients of drag and friction have to be taken into account and considered in calculations (Goto et al. 2009; Hoffmeister et al. in press). Three blocks (I, II and III; Fig. 4a, b) were sampled and scanned with TLS. These blocks appear relatively isolated on the platform area at elevations of 8–9 m asl (Table 2a). These values may tentatively be interpreted as minimum run-up height for any event capable of moving the clasts. A run-up height of 10–12 m seems more realistic, because finer-grained deposits are found in this elevation. Also, the inundation reaches as far as approximately 80–90 m inland for this depositional event. The three blocks (Fig. 6) are currently placed 22.4–33.3 m inland, away from the present cliff (Table 2a). The blocks were also measured to obtain weight information. The measurements of the a-, b- and c-axes were scored against high-precision TLS volume estimates. Mean densities of around 2.2 g/cm3 were measured in laboratory tests and were assigned to the blocks including rock

123

123

7.50

9.51

8.76

III

Mean

%-deviation

4.4

3.1

3.15

b-axis in m

Eq. (1) with axes Ht in m

9.28

III

II

3.42

II

I

3.3

3.6

I

a-axis in m

Block

81

7.10

7.34

6.17

7.81

Eq.(1) with Lidar Ht in m

1.99

1.63

1.95

c-axis in m

6.80

7.02

6.41

6.96

Eq. (2) with axes Vt in m/s

29.94552

18.1908

20.27025

Vol in m3

87

5.91

5.91

5.63

6.20

Eq. (2) with Lidar Vt in m/s

65.880144

40.01976

44.59455

Mass in tons

17.83

12.32

14.34

152.32

5.89

6.58

5.52

5.57

81

4.77

5.08

4.54

4.68


167.95

Mean:

147.65

141.35

8

9

9

H uplift (m)

10.12

12.13

9.45

8.77


Difference Vol and Mass in %

39.226

27.104

31.548

Mass in kg (Lidar data)


Vol in m3 (Lidar data)

57

5.78

6.62

5.50

5.22


22.40

23.55

33.30

X transport (m)

Table 2 (a) Comparison of weight estimation and calculation of blocks (I, II and III) taking axes and TLS measurements, note overestimation of c. 50 % taking the axes method; H vertical uplift (in m position above mean sea level on cliff top) and X (transport distance from cliff front); (b) results of wave transport equations: tsunami wave height and wave velocity calculation after Hoffmeister et al. (in press), Eqs. (1) and (2) by Benner et al. (2010); Eq. (3) by Paris et al. (2010) and Eq. (4) by Etienne et al. (2011). Equations and abbreviations are to be found in the ‘‘Appendix’’



Fig. 6 Terrestrial laser scanning (t-LiDAR) images of blocks and the calculation of volumes. a Mapped blocks from five scan positions with seven alignments scan windows using the whole sequence in a 3D view (gray is an artificial plane as reference horizon; a- and b-axes are N35E and E125S oriented). Note that the blocks are resting on a small elevated area. The absolute z-values of three blocks I, II and III are colored. The estimation of the volume was calculated by using the mean extension of the blocks from several dimensions in x, y and z. b Gray scale values (256 values, 8-Bit) of the point cloud include all scan windows, cuboid shape at block I shows overestimation of volume and weight due to irregular shape of the blocks

type. The TLS of the three blocks results in a relatively exact determination of the volume of the clasts. Although the lower side of the blocks could not be scanned, the TLS data give a realistic model of the blocks as these sides are flat (Figs. 4a, 6b). The volumes obtained by the TLS measurements are in the range of 12–18 m3. The calculated mass ranges from 27 to almost 40 t (see Table 2 for details).

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One of the simplest formulas, the ‘‘Transport figure’’ (Tf) by Scheffers and Kelletat (2003) and Scheffers (2008), gives first indication of the transport process: Tf ¼ W D V where W is weight of the boulder (in tons); D is the distance moved from the cliff line (m), and V is the vertical distance, that is, cliff height (m). A value of 2,000 is considered the upper limit of storm wave transport energy (Scheffers 2008). The transport figure Tf calculated for blocks I, II and III reveals numerical values of 9,454, 5,745 and 7,029, respectively (data for calculation in Table 2a). According to the Tf, the blocks seem to have been transported by tsunami rather than by storm waves. However, the hydrodynamics of boulder and block transport during a high-energy event are a complex process which this equation does not mirror in every account. Important factors that have an influence on the dislocation, transport and deposition of coarse clasts are friction forces, boulder shape and geometry, setting of the boulder before transport and re-arrangement by backflow, among others (Keating et al. 2011). Therefore, the Tf value is only used as a first hint in the field. The hydrodynamics of boulder and block transport during tsunami has also been discussed by Nott (2003a, b) and Noormets et al. (2004) or Pignatelli et al. (2009). Their formulas have been repeatedly revised. Although the numerical modeling of boulder transport by tsunami waves has its limitations (see Bourgeois and MacInnes 2010), we used the most recent ones in our approach (see summary in Hoffmeister et al. in press and formulas in the ‘‘Appendix’’): • Benner et al. (2010) calculated wave velocities based on the conservation of energy, • Paris et al. (2010) created a formula for the minimum wave velocity of a sliding boulder, and • Etienne et al. (2011) chose an approach to calculate the minimum wave velocity necessary for overturning a boulder. All approaches consider a submerged boulder as start condition as it is expressed by the boulder mass equation. Marine organisms such as piddocks indicate that the boulders were submerged. However, the pre-transport location could not be revealed for all individual blocks. Complex transport mechanisms including sliding, overturning, intense rotations and the interaction between numbers of boulders are complicated to be modeled and need further research. The obtained results are listed in Table 2b. For comparison ‘‘traditional’’ volume calculations were used in order to score TLS-obtained data for minimum wave height estimations. LiDAR-estimated values are only 80 % of the heights obtained by the axis method, but still in the order of 6.17–7.81 m; as a mean 7.10 m was determined. As the cliff height varies by several meters, with a maximum of between 9 and 10 m, the results obtained by this method seem reasonable. For the calculation of flow velocities, we used three different equations (see ‘‘Appendix’’) and additionally used the TLS and axis methods (Table 2b); again variations are obvious. TLS-based volume and weight of blocks give lower velocities in the order of around 4.5–6.6 m/s, depending on the formula.

5 Discussion The forces necessary to dislodge, uplift and transport a block high above the surf line, emplacing it onto the platform, as is proven along the Omani coastline, is an evidence of high-energy wave events.

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Fig. 7 Schematic cross section of the platform and wash-over deposits with boulder trains and blocks. MHW mean high water, MLW mean low water (tidal range)

The observed features of the deposits between Sur and Quriyat typically resemble lobelike boulder ramparts. The boulders were commonly deposited on an uplifted terrace flat within 80–100 m of the cliff edge as an exponentially fining landward deposit (Fig. 7). A plausible explanation for the observation that no blocks have been found at wadi inlets and on the nearby smaller cliffs may be provided by the erosional power of storm surges of the tropical cyclones (2007 Gonu, 2010 Phet) as well as associated flash floods rushing down the wadis. Severe damage has been observed in the fishing villages, which is rather due to flash-flooding than storm surges. In contrast to the common measurements of a-, b- and c-axes for volume estimates, we are able to show that volumes of the blocks are significantly smaller than calculated by the traditional method (Table 2a). This overestimation in volume has also been documented by Engel and May (2012). As a mean value an overestimation of the volume by 50 % was calculated. Taking 2.2 g/cm3 as density of the rocks, the mass of the blocks is also smaller. To give an example, block III has a mass of 39,336 kg determined with TLS in contrast to 65,880 kg by application of the traditional method, which is only c. 60 % of the traditionally determined mass (Table 2a). This observation is of importance, because all formulas to calculate transport figures (i.e., wave height or wave velocity, Table 2b) depend on the mass of the blocks. The velocities obtained by application of the formula (Table 2) are in good concordance with the data presented by Dawson and Stewart (2007), who reported relatively slow velocities of tsunamis after landfall of around 5 m/s, and Fritz et al. (2006), who documented velocities of 2–5 m/s for the 2004 event in Banda Aceh. However, an outflow velocity of 11 m/s was determined by Fritz et al. (2012) for the 2011 Japan event. Also, Heidarzadeh et al. (2009) modeled wave heights of tsunamis originating in the MSZ, which are around 5 m in height when arriving at the Omani coast. Tsunami waves of this size and velocity would be capable of moving and overturning boulders due to their higher flow velocity much further than storm waves, largely related to the longer period of the tsunami wave (Switzer and Burston 2010). As the coastal area under study is characterized by uplift throughout the Quaternary with uplift rates in the range of 1 mm/year (Wyns et al. 1992), the results obtained have to be treated as valid for a recent event. In case of older events, these movements as well as possible eustatic sea-level variations will have to be taken into account. Still, as discussed by Keating et al. (2011), mathematical formulas should not be taken as sole criteria to determine the transport process. Individual blocks and boulders deposited on the cliff top on beach rock were measured, and paleocurrent information was obtained by boulder train alignment and orientation of

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long axes of the platy boulders. It is evident that boulders are orientated. However, not all published papers agree with the alignment of the long axes of boulders parallel to the paleocurrent direction (e.g., Imamura et al. 2008; Watt et al. 2010). Orientations are important to discuss because boulders have a tendency to align either parallel or perpendicular during transport by current. For the platy boulders measured in the study area, the a-axis/b-axis ratio is generally high, that is, the length difference between a- and b-axis is significant. Beach rock clasts usually have prolate shapes (cigar shape). An orientation of the long axis perpendicular to the flow direction, as discussed by Imamura et al. (2008) for tsunamigenic deposits, contradicts the observations in our study area as the boulders are located on top of a cliff. Due to hydrological reasons, a coast-parallel flow is unlikely. Also, as evidenced by other authors, the flow in a turbulent suspension or saltation results in an orientation of the a-axis parallel to the flow (Inman 1949; Collinson and Thompson 1982; Williams and Hall 2004; Goto et al. 2007, 2010b). Boulders ‘‘floating’’ in the matrix as well as hit marks support this hypothesis. The evaluation of paleocurrent direction,

Fig. 8 Map of the wash-over and block deposits (yellow sand, reddish boulder/sand mixture, white ‘‘cleaned’’ platform with joints), note that the cliff is dependent on the joint system. Google Earth time series (2003–2010 images), including the tropical cyclones Gonu and Phet, show only minor changes along the coastline and vegetation. I, II, III = blocks in Fig. 6

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derived from measurements of the long axis of platy beach rock boulders, results in a predominant direction of 30 (a-axis, Fig. 4f, g). The observed lobe-like boulder rampart is a geomorphologic indicator for extreme waves. The deposition of these ramparts as either storm- or tsunami-induced is discussed controversially (e.g., Morton et al. 2008; Richmond et al. 2011). It is evident that both boulder trains and the wash-over fans point to an identical direction in our study area. A tsunami source in the northeasterly MSZ as described and modeled by several authors (Heidarzadeh et al. 2008a, b, 2009; Heidarzadeh and Kijko 2011; Jordan 2008; Rastogi and Jaiswal 2006; Jaiswal et al. 2009) accurately fits the observed imbrication and alignment of boulders parallel to the suspect flow direction in the study area. These observations support the interpretation as tsunamigenic deposits. Satellite images of the study area taken before and after cyclone Gonu were compared to further support a tsunamigenic origin of the deposits described (Fig. 8). The analysis of the images covering the period 2003–2009 shows no changes in the location of the three blocks. We attribute small changes to inaccuracies in the georeferencing of the images. Minor boulders disappeared or moved slightly; vegetation (i.e., small shrubs) was washed away, possibly by the tropical cyclones described. As no historical documents are available, interviews with old inhabitants (a fisherman and a ship carpenter) in the Sur and Fins villages were carried out and valuable information obtained on the extreme wave event in the dawn of November 28, 1945. Both descriptions of the event are sound and describe: ‘‘first the sea fell dry, two waves came, there was no warning, because the weather was fair’’ and continue ‘‘there was loss of land, the graveyard of Tiwi was flooded, fish and shellfish were found after the event in the lagoon of Wadi Tiwi.’’ No information regarding casualties was obtained.

6 Conclusions The boulder and block deposits investigated indicate a tsunamigenic origin. This interpretation is based on precise volume and mass measurements of three blocks and the application of transport equations. Maximum masses of 40 tons were calculated. Sedimentological evidence like boulder imbrication and boulder orientation further supports this interpretation. The quantitative assessment makes tsunami waves likely; however, storm waves cannot be entirely excluded as a transport agent. Yet, the interpretation as tsunamigenic deposits is backed by observations after recent cyclone events where no transport of blocks was observed. The MSZ is seen as the most likely source area for tsunami. The last major event occurred in 1945. However, the eye-witness reports collected indicate a minor wave arriving on the shores of Oman, as no casualties were reported. Therefore, we assume multiple tsunami events for the past as the run-up height within the study area is estimated to 10 m. Future tsunami events generated at the MSZ will undoubtedly occur. Though, so far the recurrence interval is not known. We assume recent ages (Late Holocene) as no form of pedogenesis was observed; the blocks show fresh hit marks as well as marine organism attached to them. Future research needs to concentrate on dating evidence. Additionally, our findings of block and boulder deposits along the coastline of the Sultanate of Oman are the first description of tsunami-related coarse-grained sediments in the western Indian Ocean (see map in Scheffers 2008). Similar deposits have recently been published by Shah-hosseini et al. (2011) for the northern Indian Ocean. In conclusion, by analyzing geological archives, the information regarding recurrence intervals and potential

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damages is revealed. This allows the assessment of risk and estimation of the vulnerability of the coastline of the Sultanate of Oman to future events. Acknowledgments This study was financially supported by the German Research Foundation (DFGproject Re 1361/14-1) and by The Research Council Oman (TRC-grant ORG GUtech EBR 10 013). Dirk Hoffmeister (University of Cologne) is thanked for calculation of wave heights and velocities. The two librarians Cornelia Lutter (RWTH Aachen University) and Barbara Wolf (Kiel University) helped a lot in organizing relevant references which is highly appreciated. The authors are grateful for the English review done by Stefani German and Magdalena Rupprechter (GUtech, Muscat). Two anonymous reviewers are thanked for their constructive feedback which helped to improve the manuscript.

Appendix Equations For the mass of a submerged boulder being: mb ¼ Vbðqb qwÞ : sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ekin Ht ¼ 0:125qw agl

ð1Þ

34 23 and where Ekin ¼ Ef þ Eh ¼ lmb gXtransport þ mb gHuplift and L ¼ L0 tanh 2pd L0 2

L0 ¼ gT 2p

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ekin 3 vt ¼ 0:5qw acT sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2lmb g vt ¼ Cd acqw sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:5bmb g vt ¼ 0:5Cd ac2 qw Mathematical symbols

a, b, c Cd d Ekin Ef Eh g Ht Huplift L L0

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Axes of a boulder Coefficient of drag (1,2) Water depth Kinetic energy Energy of friction Potential energy Earth’s gravity (9.81 m/s2) Height of tsunami wave Height of an uplifted boulder (asl) Tsunami wave length Tsunami wave length in deep water

ð2Þ

ð3Þ

ð4Þ


mb l qb qw T tanh vt Vb Xtransport

Mass of boulder Coefficient of friction Density of boulder Density of seawater (1,020 kg/m3) Tsunami wave period Tangens hyperbolicus Tsunami wave velocity Volume of boulder Distance of boulder to coastline

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