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(1)AGH University of Science and Technology in Kraków, Poland Faculty of Geology, Geophysics and Environmental Protection Department of Energetic Resources. Doctoral dissertation. Morphology and origin of modern seabed features in the central basin of the Gulf of Thailand. Radosław Puchała 1,2. Supervisor: Prof. Dr. Szczepan Porębski3. Kraków, 2014. 1 2 3. Fugro Survey B.V., 2260 AC Leidschendam, The Netherlands Fugro Survey Africa Pte Ltd, Cape Town, South Africa AGH University of Science and Technology, Poland.

(2) Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie Wydział Geologii Geofizyki i Ochrony Środowiska Katedra Surowców Energetycznych. Rozprawa doktorska. Morfologia i pochodzenie współczesnych form dna w centralnej części Zatoki Tajlandzkiej. Radosław Puchała1,2. Promotor: prof. dr hab. Szczepan Porębski 3. Kraków, 2014. _______________________ 1 2 3. Fugro Survey B.V., 2260 AC Leidschendam, Holandia Fugro Survey Africa Pte Ltd, Kapsztad, Republika Południowej Afryki Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie, Polska.

(3) ACKNOWLEDGEMENTS. I acknowledge Fugro Group of Companies for making available all of the geophysical datasets, provision of sediment cores, and financial patronage. My special thanks go to Fugro directors: Wilhard Kreijkes, Peter Aarts and Mark Heine. I wish to thank thesis supervisor Prof. Szczepan Porębski, AGH University of Science and Technology in Kraków, for full support, shared knowledge and valuable input to the research. I would like to extend my gratitude to the staff of the Institute of Geological Sciences, University of Wrocław, especially to Krzysztof Moskwa for managing the laboratory tests, Dr. Czesław August for performing X-ray analyses, Dr. Wojciech Śliwiński for scientific and logistical support, and to Prof. Andrzej Solecki for supervision during my pre-doctoral studies. I am also in debt to Joachim Ariel Kloskowski for photographing the microfossils and to my colleagues from Fugro, Simon Ayers and Dr. Marcin Jankowski for revising the text and many useful suggestions. I am grateful to the Institute of Geological Sciences, Polish Academy of Sciences, for providing scientific supervision during the years 2009-2010, and to the Gliwice Radiocarbon Laboratory for performing radiocarbon dating. Finally, I must thank my wife Anna for patience and a lot of sacrifice, which helped me to finalise the work.. 3.

(4) ABSTRACT. Three main stratigraphic units have been identified in the shallowest deposits of the central part of the Gulf of Thailand. Unit C represents an upper Pleistocene transgressiveregressive sequence. Unit B comprises marine sediments which were subjected to lateritization during the last glacial period and then flooding by the sea. Unit A consists of Holocene marine muds and clays (A1), which are in places underlain by muds (A2) that originated during an early transgressive stage of the Pleistocene/Holocene transition period. This tri-partite stratigraphy is consistent across the whole basin. The erosional unconformity between the two uppermost soft marine muds of Unit A and underlying stiff silt of Unit B is a ravinement surface (R1) associated with early Holocene marine transgression. Radiocarbon ages date this surface at 10.4-10.6 cal kyr BP for the centralsouthern part of the basin and more than 6.5 cal kyr BP for the northwestern area, which reflects a diachronous spread of the transgression in the landward (northward) direction. In water depths beyond 50 m, the Gulf of Thailand displays unique seafloor morphology, which is typified by the occurrence of elongated soft-mud mounds and depressions, as well as numerous pockmarks. These features result from a combination of fluid seepage, sediment dehydration processes and erosion by marine currents. Sediment dewatering has led to the formation of numerous, widely distributed, small pits and pockmarks within the unconsolidated muds of Unit A. The superimposed, long-term erosive activity of bottom currents modify gradually these features into small elongated pockmarks, long runnels and depressions, and ultimately into large fields of elongated mounds and ridges, as well as residual fragments of as yet un-eroded mud and clay sheets. Mud mounds represent a half-way stage in the spectrum of erosionally-controlled seabed morphology, encompassing continuous mud sheets at one end and isolated mud residues at the other, and all intermediate morphological forms have been found on the modern seafloor of the Gulf of Thailand. The occurrence of these features below the 50 m isobath reflects water stratification at a thermohalocline separating two water layers with different physical properties. The unidirectional bottom circulation operating within the lower water layer is interpreted to reflect a combination of tidal and density-driven currents associated with water exchange between the South China Sea and Gulf of Thailand.. 4.

(5) TABLE OF CONTENTS. ACKNOWLEDGEMENTS .............................................................................................................................. 3 ABSTRACT ............................................................................................................................................. 4 TABLE OF CONTENTS ................................................................................................................................ 5 LIST OF FIGURES .................................................................................................................................... 7 LIST OF TABLES .................................................................................................................................... 11 1. INTRODUCTION AND PURPOSE OF RESEARCH ....................................................................... 12. 2 GULF OF THAILAND – BASIN MORPHOLOGY AND STRUCTURAL, OCEANOGRAPHIC AND CLIMATIC SETTINGS ......................................................................................................................... 14 2.1 2.2 2.3 2.4 2.5 2.6 2.7. 3 3.1 3.2. 3.3 3.4 3.5. 3.6 3.7 3.8 3.9 3.10 3.11 3.12. Location and Topography ....................................................................................................... 14 Geological Background .......................................................................................................... 16 Structural Setting ................................................................................................................. 17 2.3.1 Pre-Quaternary Stratigraphy ..................................................................................... 17 2.3.2 Upper-Pleistocene and Holocene Sediments................................................................. 18 Climate ................................................................................................................................ 20 2.4.1 Monsoons ............................................................................................................... 22 Water Circulation .................................................................................................................. 22 2.5.1 Tides ...................................................................................................................... 23 2.5.2 Water Temperature and Salinity ................................................................................ 24 Water Currents ..................................................................................................................... 25 Seabed Morphology, Sedimentation and Environment ................................................................ 26 2.7.1 Accumulation Rates and Sediment Mixing ................................................................... 28 2.7.2 River Deltas ............................................................................................................ 28 2.7.3 Mangroves .............................................................................................................. 31 2.7.4 Coral Reefs ............................................................................................................. 32 METHODOLOGY ...................................................................................................................... 33 Survey Vessels ..................................................................................................................... 33 Positioning and Navigation ..................................................................................................... 34 3.2.1 Fugro Starfix HP DGPS ............................................................................................. 34 3.2.2 Fugro Starfix MRDGPS .............................................................................................. 34 3.2.3 Dynamic Heading Reference System .......................................................................... 34 3.2.4 Underwater Positioning ............................................................................................. 35 3.2.5 Navigation .............................................................................................................. 36 Sound Velocity Measurements ................................................................................................ 36 Single Beam Echo Sounder (SBES).......................................................................................... 36 Multi Beam Echo Sounder (MBES) ........................................................................................... 38 3.5.1 Simrad EM1002 MBES .............................................................................................. 38 3.5.2 Reson SeaBat 8101 MBES ......................................................................................... 38 3.5.3 MBES System Calibration and Processing .................................................................... 39 3.5.4 Tidal Reduction ........................................................................................................ 39 3.5.5 Refraction Reduction ................................................................................................ 39 3.5.6 Post-processing Analysis and Data QC ........................................................................ 39 Motion Sensors ..................................................................................................................... 40 Side Scan Sonar (SSS) .......................................................................................................... 41 3.7.1 System Description and Data Acquisition .................................................................... 41 3.7.2 Side Scan Sonar Data Processing ............................................................................... 41 Sub-bottom Profiler (SBP) ...................................................................................................... 43 3.8.1 Acquisition Parameters ............................................................................................. 44 3.8.2 Data QC and Processing ............................................................................................ 44 Gravity Coring ...................................................................................................................... 45 Laboratory Analyses .............................................................................................................. 46 Borehole Log Analyses ........................................................................................................... 47 Survey Coverage and Methods for Each Area ............................................................................ 48. 5.

(6) 4 4.1 4.2. 4.3. 4.4. 4.5. 4.6. 4.7. 5 5.1. 5.2. 5.3. 5.4. 6 6.1. 7. RESULTS ................................................................................................................................. 49 General Overview of the Whole Area ....................................................................................... 49 4.1.1 Seabed Morphology of the Gulf of Thailand.................................................................. 49 4.1.2 Stratigraphy of Shallow Sediments in the Gulf of Thailand ............................................. 49 Central-South Basin .............................................................................................................. 52 4.2.1 Bathymetry and Seabed Morphology .......................................................................... 52 4.2.2 Lithology and Sub-bottom Features ............................................................................ 53 4.2.3 Seabed Features ...................................................................................................... 59 Central-West Basin ............................................................................................................... 66 4.3.1 Bathymetry and Seabed Morphology .......................................................................... 66 4.3.2 Lithology and Sub-bottom Features ............................................................................ 66 4.3.3 Seabed Features ...................................................................................................... 89 South-West Margin (Offshore Songkhla) .................................................................................. 90 4.4.1 Bathymetry and Seabed Morphology .......................................................................... 90 4.4.2 Seabed Features ...................................................................................................... 90 4.4.3 Lithology................................................................................................................. 90 4.4.4 Sub-bottom Features................................................................................................ 92 North-South Profile ............................................................................................................... 92 4.5.1 Bathymetry and Seabed Morphology .......................................................................... 92 4.5.2 Lithology................................................................................................................. 92 4.5.3 Seabed Features ...................................................................................................... 94 4.5.4 Sub-bottom Features................................................................................................ 94 Mouth of the Gulf (Offshore Vietnam) ...................................................................................... 97 4.6.1 Bathymetry and Seabed Morphology .......................................................................... 97 4.6.2 Seabed Features ...................................................................................................... 97 4.6.3 Lithology and Sub-bottom Features ............................................................................ 97 Central Basin (Offshore Cambodia) ......................................................................................... 97 4.7.1 Bathymetry and Seabed Morphology .......................................................................... 97 4.7.2 Seabed Features ...................................................................................................... 97 4.7.3 Lithology and Sub-bottom Features ............................................................................ 98 INTEPRETATION ................................................................................................................... 105 Stratigraphy of Upper Pleistocene and Holocene of the Gulf of Thailand ..................................... 105 5.1.1 Unit C - Upper Pleistocene sedimentary sequence ...................................................... 105 5.1.2 Unit B – Uppermost Pleistocene lateritic palaeosoil ..................................................... 106 5.1.3 Reflector R1 – Lower Holocene transgressive surface .................................................. 107 5.1.4 Unit A – Transgressive systems tract and Holocene marine muds ................................. 109 The origin, shape and distribution of pockmarks of the Gulf of Thailand ..................................... 110 5.2.1 Pockmarks morphology and fluid seeps..................................................................... 110 5.2.2 Pockmarks morphology and marine currents ............................................................. 115 5.2.3 Processes driving to further changes in pockmarks morphology ................................... 117 Holocene marine mud mounds and ridges .............................................................................. 118 5.3.1 Soft mud mounds and ridges – morphology and spatial distribution ............................. 118 5.3.2 Orientation of mud mounds and ridges ..................................................................... 119 5.3.3 Age of the mud mounds and ridges .......................................................................... 122 Evolution of seabed morphology ........................................................................................... 122 5.4.1 Pockmark cluster evolution ..................................................................................... 122 5.4.2 Evolution of large pockmarks................................................................................... 123 5.4.3 Unit pockmark evolution and further development of the seabed morphology ................ 123 DISCUSSION OF RESULTS .................................................................................................... 130 Oceanographic conditions and formation of elongated pockmarks ............................................. 130 6.1.1 Bottom currents and wind direction .......................................................................... 130 6.1.2 Tidal currents – theory and definitions ...................................................................... 131 6.1.3 Tidal currents and seabed morphology...................................................................... 132 6.1.4 Density-driven currents and seabed morphology ........................................................ 134 6.1.5 Seabed morphology and thermohalocline .................................................................. 135 6.1.6 Seabed morphology and internal waves .................................................................... 135 6.1.7 Bottom currents and the nature of oceanographic data ............................................... 135 6.1.8 Summary .............................................................................................................. 136 CONCLUSIONS ...................................................................................................................... 137. REFERENCES ...................................................................................................................................... 140. 6.

(7) LIST OF FIGURES. Figure 2.1.. Location of study area. The Gulf of Thailand. ....................................................................... 15. Figure 2.2.. Regional tectonic map of western Southeast Asia (after Watcharanantakul and Morley, 2000, modified from: Packham, 1996; Leloup et al., 1995; Oudom-Ugsorn et al., 1986, Polchan and Sattayarak, 1989). ..................................................................................................... 16. Figure 2.3.. Stratigraphy of the Cainozoic sequences in the Gulf of Thailand (after Watcharanantakul and Morley, 2000, modified from Lian and Bradley, 1986). .......................................................... 17. Figure 2.4.. Correlation of Quaternary deposits in the coastal parts of Thailand (modified from Dheeradilok, 1995). ......................................................................................................... 19. Figure 2.5.. Composite stratigraphic succession of unconsolidated sediments in the Lower Central Plain (not to scale) (after Sinsakul, 2000). .................................................................................. 20. Figure 2.6.. Sea-level curve for the Gulf of Thailand and Sunda Shelf during the past 140 kyr based on oxygen isotope (Shackleton, 1987) and coral reef records (Chappell et al., 1996). Compiled by Hanebuth et al., 2003. ................................................................................................. 20. Figure 2.7.. Climate of the Gulf of Thailand (after Encarta® 2006a). ....................................................... 21. Figure 2.8.. Sketch of the water circulation at 5 m below the surface in the Gulf of Thailand, 1993-1994, deduced from the oceanographic data. The numbers indicate the distance in km travelled by a particle of water in 30 days (after Wattayakorn et al., 1998). ............................................. 23. Figure 2.9.. Seasonal variations of vertical profiles of water temperature, salinity and density at the mouth of Gulf of Thailand (after Yanagi et al., 2001). ........................................................... 24. Figure 2.10. Schematic representation of seasonal variation in wind, heat flux through the sea surface, river discharge, stratification, density-driven currents and wind driven currents in the Gulf of Thailand (after Yanagi et al., 2001). ................................................................................... 25 Figure 2.11. Surface winds and surface sea currents during July and November in the South China Sea region. Thickness of arrows indicates the constancy of the predominant surface current directions (after Koompans, 1972). .................................................................................... 26 Figure 2.12. Graphical presentation of the coastal morphology of the south-east coast of Thailand (modified from Dheeradilok, 1995)..................................................................................... 27 Figure 2.13. (A) Geomorphology and sediment distribution of the Chao Phraya delta plain and the adjacent region. (B) Index map of Chao Phraya delta (after Tanabe et al., 2003)..................... 29 Figure 2.14. Geomorphology and Late Holocene evolution of the Mekong delta. The dashed lines indicate estimated location and age (in years from present) of palaeo-offshore break. (after Ta et al., 2002a). .......................................................................................................................... 31 Figure 3.1.. Summary of the analogue system setup on-board MV Geo Surveyor (after Fugro, 2007). ......... 33. Figure 3.2.. Starfix Reference Stations Coverage in Southeast Asia (after Fugro, 2007). ............................ 35. Figure 3.3.. Generalised example of SBES system configuration. ............................................................. 37. Figure 3.4.. Example of SBES data. Horizontal lines spacing: 2.5 m, Vertical lines spacing: 125 m (1 fix-25 m). Top channel: 200 kHz, Bottom channel: 38 kHz, Delay: 45 m/55 m. ................... 37. Figure 3.5.. Example of processed MBES data. Approximate size of area 4x3 km. ..................................... 40. Figure 3.6.. Example of SSS system configuration. ................................................................................ 42. Figure 3.7.. Example of slant corrected SSS data. ................................................................................. 42. Figure 3.8.. Simplified diagram of Pinger (SBP) system configuration....................................................... 43. Figure 3.9.. Example of pinger (SBP) data. Horizontal lines spacing: 10 ms, ............................................. 45. Figure 3.10. Gravity corer model (after Fugro NV, 2001). ....................................................................... 46 Figure 4.1.. Sub-bottom Profiler (SBP) data showing major units and reflectors of the central part of the Gulf of Thailand. Water depth about 70 m below MSL. .......................................................... 50. 7.

(8) Figure 4.2.. MBES Image, showing morphology of seabed in SW part of study area, seabed consists of very soft clay, including minor depressions and large-scale isolated pockmark “P1”. ................. 52. Figure 4.3.. MBES image, showing seabed lithology at NE part of study area. The brown ridges on the right represent soft mud mounds, while the blue areas show isolated eyed pockmarks within the stiff silty sediment, covered by thin layer of soft clay. ..................................................... 53. Figure 4.4.. Photography of sample DC 40 (Unit B) showing bioturbation structures developed over lateritic soil features. ........................................................................................................ 54. Figure 4.5.. Grain size distribution of sample DC40 (Unit B).................................................................... 55. Figure 4.6.. XRD patterns of the clay fraction from sample DC 40 (Unit B) (after Puchała et al., 2011). ....... 55. Figure 4.7.. Grain-size distribution of sample DC 66B representing lower part of Unit A. ............................ 58. Figure 4.8.. XRD patterns of the clay fraction from sample DC 66B representing lower part of Unit A (after Puchała et al., 2011). .............................................................................................. 58. Figure 4.9.. MBES Image, showing pockmark cluster and other seabed features at and around coring location 37A, central Gulf of Thailand. ................................................................................ 60. Figure 4.10. Sub-bottom Profiler (SBP) image, showing shallow geology at and around pockmark cluster (37A), Gulf of Thailand. Red rectangles represent gravity corer sampling locations. .................. 61 Figure 4.11. Side Scan Sonar image showing pockmark cluster at and around coring location 37A. SSS range: 200 m per channel, frequency: 100 kHz. .................................................................. 62 Figure 4.12. Microscope images in cross-polarized light showing internal structure of ferruginous concretions: A, C, D, E & F - sample DC 37A (pockmark cluster 37A); B – sample DC 40 (Unit B). ......................................................................................................................... 63 Figure 4.13. Sub-bottom Profiler image, showing cross-section through the large-scale pockmark P1, Gulf Of Thailand. .................................................................................................................... 64 Figure 4.14. MBES image showing double pockmark P2, central Gulf of Thailand........................................ 65 Figure 4.15. Single channel seismic (SBP) image showing a cross-section through twin pockmark P2. The red rectangle indicates the location of gravity core DC 66B. .................................................. 65 Figure 4.16. Model of shallow geology of Gulf of Thailand central west basin, based on SBP data. ................ 66 Figure 4.17. Gravity core logs oriented in reference to sea level, showing detailed location of XRD sections and lab analysed samples. Cores GC 3 and GC 4 represent upper parts of the silty mounds, while cores GC 1 and GC 2 represents areas between the mounds, and core GC 5 represents the pockmark cluster area. ................................................................................ 68 Figure 4.18. X-radiograms and photographs of selected GC 3 core slabs (Unit A): RTG 5 (left), RTG 2 (middle), RTG 1 (right). .................................................................................................... 69 Figure 4.19. X-radiograms and photographs of selected GC 4 core slabs (Unit A): RTG 3 (left), RTG 4 (middle), RTG 6 (right). .................................................................................................... 70 Figure 4.20. X-radiogram and photograph of GC 1 core slab (Unit A/Unit B): RTG 7. .................................. 71 Figure 4.21. Grain size distribution curve (left) and table (right) of samples G3 and G5 from core GC 3, showing vertical variation of sediments within the upper part of a mud mound. Sediment type defined in the table is based on BS 5930:1999, Folk (1974), and Shepard (1954) classifications, respectively. .............................................................................................. 72 Figure 4.22. Grain size distribution curve (left) and table (right) of samples G7, G8, and G9 from core GC 4, showing vertical variation of sediments within the upper part of a mud mound. Sediment type defined in the table is based on BS 5930:1999, Folk (1974), and Shepard (1954) classifications respectively. ............................................................................................... 73 Figure 4.23. Grain size distribution curve (left) and table (right) of samples G10a, G10b, G14 and G12 from core GC 1, showing vertical variation of sediments on the boundary between Units A and B. Sediment type defined in the table is based on BS 5930:1999, Folk (1974), and Shepard (1954) classifications, respectively. ....................................................................... 74. 8.

(9) Figure 4.24. Grain size distribution curve (left) and table (right) of sample G15 (core GC 2), and samples G16 and G19 (GC 5 core), showing sediment types of Unit B (G15, G19) and muddy matrix from pockmark cluster (R1). Sediment type defined in the table is based on BS 5930:1999, Folk (1974), and Shepard (1954) classifications, respectively. ............................................... 75 Figure 4.25. Grain. size. distribution. triangle. of. all analysed. samples assigned. to corresponding. units/seabed forms (based on Udden-Wentworth grain scale). ............................................... 75 Figure 4.26. Microscope images showing internal structure of carbonate concretions: A & D - sample G21b (Unit B); B – sample G18a (pockmark cluster); C – sample G20 (R1 surface). ................ 78 Figure 4.27. Plate showing foraminifers found within sediment samples. Identification of these fossils is presented in Table 4.6. ..................................................................................................... 83 Figure 4.28. Plate showing gastropods found within sediment samples. Identification of these fossils is presented in Table 4.6. ..................................................................................................... 84 Figure 4.29. Plate showing bivalves found within sediment samples. Identification of these fossils is presented in Table 4.6, ..................................................................................................... 85 Figure 4.30. Plate showing micro-molluscs found within sediment samples. Identification of these fossils is presented in Table 4.6. .................................................................................................. 86 Figure 4.31. Gravity core logs oriented with reference to sea level, showing locations and ages of radiocarbon dated samples................................................................................................ 88 Figure 4.32. SSS record showing pockmark cluster within central west basin. ............................................ 89 Figure 4.33. Model of shallow geology of south-west margin of Gulf of Thailand, based on SBP data and drilling log. ...................................................................................................................... 91 Figure 4.34. Shallow seismic cross-section 170 km from northern coastline. Central basin of Gulf of Thailand. ........................................................................................................................ 93 Figure 4.35. Presence of mud mounds and ridges along North-South profile across Gulf of Thailand. ............ 95 Figure 4.36. Shallow seismic cross-section 450 km from northern coastline showing a growth fault within Unit C. Central basin of Gulf of Thailand. ............................................................................. 96 Figure 4.37. Shallow seismic cross-section 600 km from northern coastline showing a gas chimney below a palaeochannel displaying symmetrical fill. Central south basin of Gulf of Thailand. ................. 96 Figure 4.38. Borehole logs of the central basin, Gulf of Thailand – part I. .................................................. 99 Figure 4.39. Borehole logs of the central basin, Gulf of Thailand – part II. ............................................... 100 Figure 4.40. Shallow geological model for the central basin, Gulf of Thailand, based on SBP data (line SL-06) and drilling logs. The inclined stratification of Unit C4 represents deltaic clinoforms. .................................................................................................................... 101 Figure 4.41. Shallow geological model for the central basin, Gulf of Thailand, based on SBP data (line SL-09) and drilling log. ............................................................................................ 102 Figure 5.1.. Distribution pattern based on Average Nearest Neighbour method (top) and Standard Directional Ellipse (bottom) of unit pockmarks, calculated in ArcGIS software. ....................... 111. Figure 5.2.. Distribution pattern based on Average Nearest Neighbour method (top) and Standard Directional Ellipse (bottom) of eyed pockmarks, calculated in ArcGIS software. ..................... 112. Figure 5.3.. Model of pockmark cluster 37A. Central Basin, Gulf of Thailand. .......................................... 114. Figure 5.4.. Orientation of longest axis of unit pockmarks (violet roses), seabed depressions (red roses) and mud ridges (green roses) indicating dominant direction of bottom current within the Gulf of Thailand, against predominant surface wind direction (The wind directions based on Koompans 1972). .......................................................................................................... 116. Figure 5.5.. Diagram showing how fluidization velocity depends on grain size and consolidation of sediment (from Hovland and Judd 1988, redrawn from Lowe 1975). .................................... 117. 9.

(10) Figure 5.6.. Spatial distribution of mud mounds and ridges in the Gulf of Thailand. The magenta hatch indicates areas outside survey corridors where occurrence of mud mounds and ridges is expected. ...................................................................................................................... 120. Figure 5.7.. Evolutionary stages of mud mounds and ridges formation in the Gulf of Thailand, central basin. A - elongated pockmarks within soft clay - stage 2; B - elongated depression within soft clay - stage 3; C - mud mounds and ridges - stage 4; D – flay stiff seabed with eyed pockmarks– stage 5). ..................................................................................................... 126. Figure 5.8.. Model of seabed morphology development in the Gulf of Thailand, central basin. ................... 127. 10.

(11) LIST OF TABLES. Table 2.1.. Temperature values for selected locations around Gulf of Thailand (after Encarta®, 2006b). ..... 21. Table 2.2.. Precipitation values for selected locations around Gulf of Thailand (after Encarta® 2006b). ....... 21. Table 3.1.. List of utilised methods and survey coverage of each study area. ........................................... 48. Table 4.1.. Shallow stratigraphy of the Gulf of Thailand and correlation with the adjacent areas. ............... 51. Table 4.2.. Radiocarbon ages of plant matter and shelly fauna (after Puchała et al., 2011)........................ 56. Table 4.3.. Lithology of gravity core samples at and around pockmark cluster 37A. Detailed coring locations are presented on Figure 4.10. .............................................................................. 61. Table 4.4.. Statistical parameters of the grain size of analysed sediment samples. Methodology of calculations after Folk (1974). ........................................................................................... 76. Table 4.5.. Interpretation of XRD analyses of carbonate-ferruginous concretions. Detailed locations of the samples are presented on Figure 4.17. .......................................................................... 77. Table 4.6.. List of fossils identified within the sediment samples. ........................................................... 80. Table 4.7.. Radiocarbon ages of shells and microfauna of the central-west basin. ..................................... 87. Table 5.8.. Orientation of elongated pockmarks and seabed depressions indicating dominant direction of bottom current, southern central basin, Gulf of Thailand. .................................................... 117. Table 5.9.. Summarised. results. of. mud. mounds/ridges. and. depressions/subaqueous. channels. orientation for three different areas of the Gulf of Thailand. ................................................ 121. 11.

(12) 1. INTRODUCTION AND PURPOSE OF RESEARCH. The Gulf of Thailand is a shallow epicontinental sea. The location of the gulf within the monsoon winds zone, lack of major fluvial sediment supply, dominant muddy sedimentation, and the unique pattern of marine currents, have resulted in a specific seabed morphology. Seabed sediments consist of Holocene silts and clays, which form a regular continuous mantle within shallower parts of the basin, but in deeper waters these youngest sediments form elongated mounds arranged parallel to the axis of the basin. This morphology, which is globally unique, is widespread within the Gulf of Thailand in water depths in excess of 50 m. It covers an area of tens of thousands of square kilometres. The presence of these features indicates that there are unusual geological and oceanographic conditions within the basin. The morphology, distribution and origin of mud mounds have not been examined in detail so far and this forms one of the main objectives of this study. Other interesting features common on the Gulf of Thailand continental shelf are pockmarks. They are recognised as cone-shaped circular or elliptical depressions related to processes of dehydration and fluid escape from the sediments. Although pockmarks themselves are rather well known, the author’s interest focused on the sediments, which occur in close proximity to major clusters of pockmarks. These sediments are a mixture of mud, carbonate shells and goethite concretions. This portion of research has been largely. centred. on. erosive. processes. associated. with. early. Holocene. marine. transgression, which resulted in reworking of the bottom sediments in coarse-grained lags, and subsequent processes that prevented any significant deposition of marine mud within these areas. The study also aims to characterise the Late Quaternary seabed succession within the Gulf of Thailand with particular attention given to palaeoenvironmental reconstruction of the Upper Pleistocene and Holocene basin-fill increments. The three main hypotheses tested in this dissertation are as follows: . An erosive unconformity between the two uppermost stratigraphic horizons of the Gulf of Thailand, i.e., uppermost soft marine muds (Bangkok Clay Formation) and underlying stiff silt (Stiff Clay Member) is a ravinement surface associated with early Holocene marine transgression.. . The unique seabed morphology within the deeper part of sublittoral zone of the Gulf of Thailand, characterised by pattern of elongated soft mud mounds and depressions as well as numerous pockmarks, is a result of a combination of fluid seepage, sediment dehydration processes, and marine current influence.. . The mud mounds and ridges are an intermediate stage of seabed morphological development in a specific erosive marine current regime, where the original stage 12.

(13) is a non-eroded soft mud mantle and the final stage is a flat seabed with all soft mud cover washed away. A wide spectrum of methods has been utilised to achieve the goals of the study. These methods mainly included geophysical data acquired during marine acoustic surveys such as: side scan sonar imagery, sub-bottom profiling and swathe bathymetry. The geoacoustic records have been ground-truthed with numerous gravity cores and supported by geotechnical drilling logs. Selected samples were subject to laboratory tests including X-radiography of sediment slabs, radiocarbon dating, sieve-pipette analyses, XRay diffraction and thermal (TGA, DTA and DTGA) techniques, analyses of microscopic images of thin sections of iron-oxide concretions, and identification of extracted fossils. All of these methods are described in detail within Chapter 3. The continental shelf of the Gulf of Thailand has been chosen as an investigation area. Desk studies cover the whole Gulf of Thailand area including islands, river deltas and coastal areas. The field investigation covered selected parts of the basin, mostly within central part of the Gulf of Thailand and was limited to the continental shelf only.. 13.

(14) 2. GULF OF THAILAND – BASIN MORPHOLOGY AND STRUCTURAL, OCEANOGRAPHIC AND CLIMATIC SETTINGS. 2.1. Location and Topography The Gulf of Thailand, also known as the Gulf of Siam, is an inlet of the South China. Sea (Figure 2.1). It is located between the Malay Peninsula and the Southeast Asian mainland. Geographically, the gulf is located between 5° and 14° latitude (N), and from 99° to 105° longitude (E). The most northern fragment of the basin, located near the Chao Phraya River delta, is called the Bight of Bangkok. The Chao Phraya River is the largest river flowing directly into the gulf. The Mekong River, the biggest river in the region, flows into the South China Sea, east of the Gulf of Thailand; however, due to the area’s water circulation patterns, it is an important source of fresh water for the Gulf of Thailand (Stansfield and Garrett, 1997). The Gulf of Thailand is a slightly elongated basin, with an axis oriented SE-NW. The length of the inlet along the axis is around 750 km and the width across its mouth is approximately 360 km (Encarta, 2006a). It is a shallow shelf reservoir with a maximum water depth of 86 m (Srisuksawad et al., 1997). Two ridges separate the deeper central Gulf from the South China Sea. The south-western ridge, located about 100 km from Cape Camau, is less than 25 m deep. The north-east ridge is approximately 150 km offshore Kota Bahru, water depth over this feature is less than 50 m. The water depth within the narrow channel between these two ridges reaches 67 m (Robinson, 1974). Six separate survey areas, located across the Gulf of Thailand basin, were subject to the detailed analyses presented in this dissertation. These areas, presented on Figure 2.1, are considered to be representative of seafloor conditions in each section of the basin. The central south basin exhibits the greatest water depths within the Gulf of Thailand as well as an area of palaeo-prodelta related to the Kelantan River. The west basin is located approximately 100 km north of Ko Samui Island within the central-west Inner Gulf. The western margin of the Gulf of Thailand is represented by a site situated approximately 35 km north-east of the city of Songkhla. This area is characterised by a generally flat part of the basin slope, approximately 20 km off the west coast. A north-south cross-section is provided by an almost 600 km long and 150 m wide survey corridor passing across the Gulf of Thailand. This route starts at the northern coastal town of Sattahip and runs south to the central south part of the Gulf. The route shows a north-south profile through the seafloor of the northern and central part of the. 14.

(15) basin. At its northern extreme, the corridor begins in water depths of around 15 m and ends within the deepest part of the basin at a water depth beyond 80 m.. Figure 2.1. Location of study area. The Gulf of Thailand.. The survey area within the gulf mouth is situated offshore Vietnam on the most southern part of the Gulf of Thailand, close to the boundary with the South China Sea.. 15.

(16) The last analysed area is situated offshore Cambodia within the central part of the Gulf of Thailand. The area represents a deep water section of the basin near its axis and is located around 200 km from the nearest shoreline.. 2.2. Geological Background The Gulf of Thailand is located between the Thai-Malay Peninsula, the Indochina. Massif and the eastern part of the South China Sea. It is a typical shallow epicontinental sea, which is underlain by twelve sedimentary Cainozoic sub-basins separated by northtrending linear ridges (Pigott and Sattayarak, 1993). This succession rests unconformably on a pre-Palaeogene basement (Jardine, 1997). The Quaternary history of the Gulf of Thailand was largely controlled by frequent sea level changes related to glacial and interglacial periods. Upper Pleistocene to Holocene marine sediments known from the Lower Central Plain and Chao Phraya delta areas are the Holocene Bangkok Clay Formation (Cox, 1968, Moh et al., 1969, fide Tanabe et al., 2003; Sinsakul, 2000) unconformably overlying the Upper Pleistocene Stiff Clay Member (Rau and Nutalaya, 1983; Tanabe et al., 2003). A map illustrating the regional geology and tectonics of the study area is shown on Figure 2.2.. Figure 2.2. Regional tectonic map of western Southeast Asia (after Watcharanantakul and Morley, 2000, modified from: Packham, 1996; Leloup et al., 1995; Oudom-Ugsorn et al., 1986, Polchan and Sattayarak, 1989).. 16.

(17) 2.3. Structural Setting. 2.3.1 Pre-Quaternary Stratigraphy The geology of the pre-Tertiary basement has been established mostly on the basis of onshore studies (Suensilpong et al., 1982, fide Watcharanantakul and Morley, 2000). The basement of the Pattani Basin, which is located in the central part of the Gulf of Thailand, is most likely composed of the following elements: . Precambrian crystalline basement;. . Lower Palaeozoic highly folded and foliated green schist grade metasediments (Suensilpong et al., 1982, fide Watcharanantakul and Morley, 2000) including preDevonian mafic-ultramafic intrusions (Tan 1996);. . Permo-Triassic granitic belt. The Carboniferous, Permo-Triassic and Mesozoic clastics and carbonates (Ratburi) have been noted in the southern part of Thailand Peninsula and Chumpon Basin in the western part of the Gulf (Heward et al., 2000, fide Watcharanantakul and Morley 2000);. . Late Cretaceous and Early Tertiary granites, which flank and source the sediments of the Pattani Basin (Hutchison, 1983; Watcharanantakul and Morley, 2000).. The stratigraphy of the Cainozoic sequences in the Gulf of Thailand is presented on Figure 2.3.. Figure 2.3. Stratigraphy of the Cainozoic sequences in the Gulf of Thailand (after Watcharanantakul and Morley, 2000, modified from Lian and Bradley, 1986). 17.

(18) The thickness. of. Cainozoic. sediments in the Gulf. of. Thailand varies from. approximately 8.5 km in the deepest parts of the Pattani Basin to less than 300 m in areas where shallow subcropping, pre-Tertiary highs occur (Watcharanantakul and Morley, 2000).. 2.3.2 Upper-Pleistocene and Holocene Sediments The Pleistocene sequence on the eastern coast of the Thai Peninsula consists of sands, clays, gravels, and several lateritic layers, interpreted as colluvial and fluvial in origin (Dheeradilok, 1995). The sediments of the Lower Central Plain are composed of an Upper Pleistocene Stiff Clay Member (Rau and Nutalaya, 1983; Dheeradilok, 1995; Sinsakul, 2000) and Holocene marine clays, called the Bangkok Clay Formation (Cox, 1968, Moh et al., 1969, fide Tanabe et al., 2003; Dheeradilok, 1995; Sinsakul, 2000). An erosional unconformity between these units marks the Pleistocene/Holocene boundary. The Holocene sediments of Chao Phraya Delta are divided into lower transgressive peaty (mangrove swamp) sediments and upper regressive deltaic sediments (Somboon, 1988, Somboon and Thiramongkol, 1992, Songtham et al., 2000, Woodroffe, 2000, fide Tanabe et al., 2003; Sinsakul, 2000). The composite stratigraphic succession of the Lower Central Plain is presented on Figure 2.5. The correlation of Quaternary deposits in the coastal part of Thailand is shown in Figure 2.4. The Late Pleistocene and Holocene history of the Gulf of Thailand is determined by climate changes and marine transgression, related to the close of the glacial epoch. The sea-level curve for the Gulf of Thailand during the past 140 000 years is shown on Figure 2.6. The Pleistocene/Holocene transition is marked by expansion of arboreal vegetation in response. to. increased. precipitation. and. climate. warming. (Kealhofer,. 2002;. White et al., 2004). The Holocene marine transgression resulted in a gradual flooding of the gulf area. The Stiff Clay Member representing Pleistocene deposits, is unconformably covered by several peat lenses, intertidal and marine clay horizons (Sinsakul, 2000). The peat lenses originated from flooded forests and mangrove swamps. Intertidal and marine clays mark respectively the middle and final stages of marine transgression. The Middle and Late Holocene period, contrary to rapid post-glacial sea level changes, is characterised by the relatively low amplitude variations. Water level within Gulf of Thailand remained quite steady, with the highstand maximum at 8000-7000 years BP (Tanabe et al., 2003). Slight variations in vegetation occurred due to monsoon flow changes and human agricultural activity (White et al., 2004).. 18.

(19) Central Plain Lower Central Top soil alluvial. HOLOCENE. Meander belt Subtidal shell & peat (4-5 kyr). Soft Clay Member. Flood plain Terrace I. Intertidal. Southern Thai Peninsula (Songkhla Lake Basin) (marine) Bangkok Clay Fm. Upper Central. Fluvial/Recent beach deposit Channel/Lacustrine Old beach ridge/Tidalflat peat (4.3-6.6 kyr) Flood plain. Estuarine Stiff Clay Member. Alluvial fan Fm. MIDDLE LOW. PLEISTOCENE. ?. Kam Phoeng Phet Fm ?. Redsoil Fm Lacustrine marl Fm ? Phra Pradaeng Member. Fluviatile coarse sand and gravel with remains of (Terrace III). Ping Fm. Fluviatile deposit. Phra Nokhan Member. Chao Phraya Fm (non-marine). UPPER. Deltaic sand/silt. Laterite. Pediments / gravel beds (Terraces). Samut Prakan Member. Laterite ?. PLIOCENE. Plio-Miocene. Weathered Older rocks. Figure 2.4. Correlation of Quaternary deposits in the coastal parts of Thailand (modified from Dheeradilok, 1995).. 19.

(20) Figure 2.5. Composite stratigraphic succession of unconsolidated sediments in the Lower Central Plain (not to scale) (after Sinsakul, 2000).. Figure 2.6. Sea-level curve for the Gulf of Thailand and Sunda Shelf during the past 140 kyr based on oxygen isotope (Shackleton, 1987) and coral reef records (Chappell et al., 1996). Compiled by Hanebuth et al., 2003.. 2.4. Climate The Gulf of Thailand has a moist, tropical climate with two monsoonal winds: the. north-east during mid October to March and the south-west during May to September (Thampanya et al., 2006). In the southern part of the Gulf of Thailand, the climate is more humid, with average precipitation around 2000 mm (Table 2.2, Songkhla). The northern part is more affected by seasonal changes due to monsoons; thus, dry and wet. 20.

(21) seasons occur. The winds from the south-west produce a rainy season from mid April to mid October (Encarta, 2006c). The average precipitation in the northern part of the inlet is around 1500 mm (Table 2.2, Bangkok). There are no major seasonal temperature changes within Gulf of Thailand. Slightly higher temperatures are observed when area is under the influence of the south-west winds (Encarta, 2006b). The average annual air temperature ranges between 27°C and 28°C (Table 2.1). A map showing climatic zones in South East Asia is presented in Figure 2.7.. Figure 2.7. Climate of the Gulf of Thailand (after Encarta® 2006a).. Table 2.1. 2006b).. Temperature values for selected locations around Gulf of Thailand (after Encarta®,. Bangkok. Songkhla. Ho Chi Minh City. 28. 28. 27. January [°C]. 21-32. 24-30. 21-32. July [°C]. 25-33. 24-33. 24-31. Annual average [°C]. Table 2.2. 2006b).. Precipitation values for selected locations around Gulf of Thailand (after Encarta®. Annual average [mm] No of days with precipitation. Bangkok. Songkhla. Ho Chi Minh City. 1498. 2035. 1861. 100. 124. 134. 21.

(22) Two main rivers deliver fresh water into Gulf of Thailand; the Chao Phraya and Mekong. The annual peaks of water runoff occur between August and November. The Chao Phraya River mouth is located in northern part of the gulf. The area drained by this river is relatively small, thus, the influence of Chao Phraya River is rather local and dilution of sea water is restricted to northern coastal areas of the basin. The Mekong River, despite its delta being located outside the Gulf of Thailand, influences coastal waters offshore Vietnam. The Mekong is one of the world’s largest rivers carrying large amounts of fresh water derived both from rainfall and Tibetan snow melt. The river flow is subject to strong seasonal variation. The peak rate of flow in August (rainy season) is 16-19 times greater than during the dry season (Robinson, 1974). The climate of the Gulf of Thailand area has changed since the Last Glacial Maximum. During the Late Pleistocene, the climate was drier and cooler than that at present, which resulted in a wide range of vegetation types from dense forests (pine/oak dominated) to savannas (White et al., 2004). The Pleistocene to Holocene transition coincided with a change in the climate towards moist and hot.. 2.4.1 Monsoons The Monsoon is the wind that changes direction with the change of the season (Encarta® 2006d). It affects mostly the interior of Asia and the Indian Ocean. The wind usually blows from the south-west from May to September and from the north-east from November to February (Figure 2.11). The south-west wind, also called the “summer monsoon” blows across the Indian Ocean and Bay of Bengal carrying warm and moist air. It causes a rainy season in the Gulf of Thailand between July and October. The dry season is caused by the north-east “winter monsoon” and occurs when cold and dry air flows from Asian interior (China) towards the Indian Ocean. This period is one of variable moderate winds over the Gulf and mild temperatures on land. The direction of the north-east monsoon is relatively steady over the South China Sea; however, it may be variable within Gulf of Thailand (Robinson, 1974). These two seasons are divided by two periods of transition between the opposing monsoons, one of two months duration in March-April and the second one in October (Robinson, 1974).. 2.5. Water Circulation Based on oceanographic data collected during the NAGA Expedition (Robinson,. 1974), the Gulf of Thailand is a classical two-layered, shallow-water estuary. The upper layer is composed of low salinity water, diluted due to precipitation and fresh water runoff from rivers. The lower layer comprises high-salinity, relatively cold water delivered. 22.

(23) from the South China Sea. This high salinity layer usually occurs within the deepest areas of the gulf in water depths greater than 50 m below sea level. Superimposed on this twolayered system is water circulation generated by wind-driven currents related to monsoons and tides. Interactions between the variable winds, tidal currents, fresh water runoff and excessive precipitation may locally cause anomalies, which result in upwelling of cold salty water and sinking of warm, low-salinity and highly oxygenated waters. The general circulation and physical properties of the Gulf’s water undergo large seasonal as well as short-period variations. The water circulation in the Outer Gulf is principally related to the South China Sea; consequently, the circulation in the Inner Gulf is related to the Outer Gulf (Figure 2.8).. Figure 2.8. Sketch of the water circulation at 5 m below the surface in the Gulf of Thailand, 19931994, deduced from the oceanographic data. The numbers indicate the distance in km travelled by a particle of water in 30 days (after Wattayakorn et al., 1998).. 2.5.1. Tides. Tidal motion in the South China Sea and Gulf of Thailand is largely maintained by the energy flux from the Pacific Ocean through the Luzon Strait (Fang et al., 1999). The phase of semi-diurnal tides (M2 and S2) propagates clockwise in the central part of Gulf of Thailand, opposite to the phase of diurnal tides (K1, O1, P1), which are counter clockwise (Yanagi and Takao, 1998). The tidal cycle in the Gulf of Thailand consists of irregular tides with average amplitude of 2.7 m. Regular daily tides are observed on the eastern coast of the gulf. The amplitude of tides increases from 1.5 to 3.5 m towards the central part of the gulf, where irregular daily tides with amplitude of about 4 m can be observed (Loi, 1965; Gorshkov et al., 1974; Latypov 2003). Tidal currents in the Gulf of Thailand may exceed one knot (~0.5 m/s) (Srisuksawad el al., 1997).. 23.

(24) 2.5.2 Water Temperature and Salinity The average salinity of the Gulf of Thailand’s water varies between 30.06 and 31.26‰. Water salinity changes according to the season. In the rainy season, salinity within the inner part of the basin may drop down to 28‰ (Emery and Niino, 1963; Latypov, 1995, fide Latypov, 2003). The saline water migrates from the South China Sea, while the main sources of fresh water are river discharges. Lower seawater salinities were. noted. on. eastern. side. of. the. Lower. and. Middle. Gulf. of. Thailand. (Srisuksawad et al., 1997), most probably related to Mekong River or/and Cambodian river input. The temperature of the Gulf of Thailand surficial waters varies from 24 to 30°C, being at the maximum during May–August and minimum in November–February (Pham, 1985 fide Latypov, 2003). During seasons when winds are relatively light, stratification develops due to significant heating of the sea surface, (Yanagi et al., 2001). The stratification is additionally influenced by salinity changes caused by seasonal river discharge variation (Figure 2.9).. Figure 2.9. Seasonal variations of vertical profiles of water temperature, salinity and density at the mouth of Gulf of Thailand (after Yanagi et al., 2001).. A schematic representation of seasonal variation in wind, heat flux through the sea surface, river discharge, stratification, density-driven currents and wind driven currents in the Gulf of Thailand is presented on Figure 2.10. The greatest stratification and estuarine circulation in the Gulf of Thailand takes place in April. The stratification develops due to the occurrence of strong surface heating and relatively light winds, which results in formation of density driven currents. The estuarine circulation is intensified by surface Ekman transport related to the south-west monsoon. Moderate stratification remains until September and is maintained by high levels of river discharge and moderate heat flux. Cold and relatively undisturbed water masses remain in the central part of the Gulf of Thailand, where the water is deep and amplitudes of tidal currents are. 24.

(25) low. In December-January, the stratification is absent due to the disruptive influence of the cold, strong, north-east monsoon, (Yanagi et al., 2001).. Figure 2.10. Schematic representation of seasonal variation in wind, heat flux through the sea surface, river discharge, stratification, density-driven currents and wind driven currents in the Gulf of Thailand (after Yanagi et al., 2001).. 2.6. Water Currents Studies performed by Wattayakorn and others (1998) illustrate the domination of. strong, predicable, generally shore-parallel tidal currents within the Gulf of Thailand. The mean currents, monthly averaged, are usually less than 0.07 m/s in speed (max 0.12 m/s). The strongest net currents occur near the centre of the Gulf during the peak of monsoon season, while weak and variable currents predominate during the rest of the year. The north-east monsoon generally sets in during November and affects the Gulf of Thailand at full force during December and January. In May, the prevailing wind direction changes towards the south and in subsequent months the wind blows mainly from south to south-west (Koompans, 1972). The graphical presentation of winds, and related surface currents, is shown on Figure 2.11. Monsoons have a significant influence on surface currents. During the south-west monsoon season, the surface current moves clockwise and during the north-east monsoon season it moves counter clockwise. According to Robinson (1974), the windinduced motion appears to be a major component of the circulation in the Gulf of Thailand. The monsoon winds over the Gulf, however, are not simply north-east or south-west, but vary widely around these primary directions making interpretation of the results of in situ winds more difficult. The vertical wind-induced water motions that affect the density structure of the water column are costal upwelling, coastal sinking, open sea convergence and divergence (Robinson, 1974).. 25.

(26) Figure 2.11. Surface winds and surface sea currents during July and November in the South China Sea region. Thickness of arrows indicates the constancy of the predominant surface current directions (after Koompans, 1972).. 2.7. Seabed Morphology, Sedimentation and Environment The south-east coast of Thailand is characterised by a broad coastal plain with long. and wide mainland beaches of sand and dunes. Tidal flats with large lagoons and sand spits are also common. Close the mouth of the Tiger River (Surat Thani Province), prodeltaic. deposition. occurs.. High. sediment. supply. resulted. in. Holocene. coastal. progradation. The sandy material delivered by rivers is distributed by a northerly-directed. 26.

(27) longshore current (Figure 2.12). On the other hand, large stretches of the coastline have been found to be subject to erosion (Dheeradilok, 1995; Thampanya el al., 2006). The northern coast of the Gulf of Thailand is characterised by a coastal floodplain landscape. The floodplain, called the Lower Central Plain, has been formed by fluvial and deltaic deposition by the Chao Phraya, Mae Klong, Mae Nam Thachin and Bang Pakong rivers. Coastal sediments of the northern gulf are a mixture of mostly fluvial, muddominated material and minor amounts of sand transported from the south by littoral currents. Landscape features also include tidal flats and beaches (Dheeradilok, 1995).. Ko Samui Island (coral reefs) Tiger River Delta (mangrove swamps). Figure 2.12. Graphical presentation of the coastal morphology of the south-east coast of Thailand (modified from Dheeradilok, 1995). 27.

(28) In the central part of the Gulf of Thailand, seabed morphology is dominated by a flat central depression. The maximum depth of this depression is 86 m below mean sea level. Buried palaeo-river valleys radiating into the central depression may be observed in the seabed morphology (Srisuksawad et al., 1997). The bottom sediments within the Gulf of Thailand are dominated by clay and sandy clay. Locally, especially in nearshore areas, sands and clayey sands occur. A clay deposition was reported also inshore near major river. mouths. in. the. upper. Gulf. of. Thailand. (Emery. and. Niino,. 1963,. fide. Srisuksawad et al., 1997).. 2.7.1 Accumulation Rates and Sediment Mixing Sediment accumulation rates measured by Srisuksawad and others (1997) amount 270-490 mg/cm2/year in the upper Gulf of Thailand and 64-190 mg/cm2/year in the central basin. The influence of sediment mixing and re-suspension by storms and bottom currents seems to be more significant in the upper Gulf of Thailand. Biological mixing and trawling activity may also cause higher mixing coefficients. Mixing from storms and currents are probably the most significant factors in the southern Gulf.. 2.7.2 River Deltas Chao Phraya Delta The Chao Phraya is the biggest river flowing into Gulf of Thailand. It is located on the northern coast of the Gulf. The mouth of the Chao Phraya together with its tributaries, Mae Klong, Tha Chin and Bang Pakong rivers forms the Chao Phraya delta system. Detailed studies of the structure and evolution of the delta have been published by Sinsakul (2000) and Tanabe and others (2003). The geomorphology and sediment distribution of the Chao Phraya delta is shown on Figure 2.13. The coast on the Chao Phraya delta is a low-energy depositional environment. From the north towards the Gulf of Thailand, the entire system has been subdivided into delta plain, tidal flat, river mouth flat, delta front, and pro-delta portions (Tanabe et al., 2003). Clay is the dominant sediment on the floodplain. Along river channels mainly silt or sandy clay occurs. Well-sorted medium sand forms a 30 km-long beach ridge, which is located within the delta plain about 3 m above Mean Sea Level (Rau and Nutalaya 1983; Somboon and Thiramongkol 1992, fide Tanabe et al., 2003). A significant part of the delta plain is covered by mangrove forests, which form a 2030. km. wide. belt. along. the. coast. (Somboon. 1988,. Woodroffe,. 2000,. fide. Tanabe et al., 2003). A 1-5 km wide zone of tidal (mud) flats occurs between the. 28.

(29) mangroves and the delta front. This area lies approximately 1 m below mean sea level (MSL) and is exposed during low tide (Royal Thai Navy, 1995, 1996, fide Tanabe et al., 2003).. Figure 2.13. (A) Geomorphology and sediment distribution of the Chao Phraya delta plain and the adjacent region. (B) Index map of Chao Phraya delta (after Tanabe et al., 2003).. The boundary between the delta front and pro-delta is defined by the slope gradient break point, which is located approximately along the 11-13 m below MSL isobath (Tanabe et al., 2003). The delta front and pro-delta consists mostly of clay (Srisuksawad et al., 1997).. 29.

(30) Kelantan River Delta The Kelantan is 355 km long and is the third largest river in the Malay Peninsula. It is located on the south-western coast of the Gulf of Thailand near the entrance of the gulf (Figure 2.1). The Kalantan Delta was described by Koompans (1972). The delta debouches into the sea through two main channels. The mouth of the Kelantan River has gradually shifted to the west due to the influence of westward orientated beach drift generated by the north-east monsoon. Coastal erosion and river sedimentation within the delta are in a delicate balance. Retreat of the coastline is counterbalanced by accretion of land elsewhere. The river sedimentation within its mouth is mostly composed of clay with a silt fraction. A sand fraction is delivered occasionally during flooding events. The river’s sediments are mostly laid down within the coastal plain, causing its rapid seaward growth. Mekong River Delta The Mekong is one of the largest rivers in the world with a length of 4200 km, drainage area of 0.79 x 106 km2 and an annual water discharge of 470 km 3 (Wolanski et al., 1996). The Mekong River forms the delta that is located in the most southern part of Vietnam near the entrance of the Gulf of Thailand into the South China Sea. The present Mekong Delta system has two major distributary channels, both discharging directly into the South China Sea. However, presently no major channels supply the Gulf of Thailand, the Holocene history of the Mekong Delta described by Ta et al., 2002a shows delta progradation of about 200 km during the last 6 kyr. This means that during the Middle Holocene the Mekong River was discharging waters into both the South China Sea and the Gulf of Thailand. The water entering the Gulf of Thailand was flowing via a palaeochannel located within the western part of the delta; north of the Camau Peninsula (Figure 2.14). The present Mekong Delta Plain can be divided into two parts: an upper delta plain influenced by fluvial processes and a lower delta plain where marine processes dominate (Ta et al., 2002a). The lower delta plain is characterised by numerous beach ridges and inter-ridge swamps. The most common environments within the sub-aerial part of the delta are mangroves, beach ridges and tidal flats. The Late Holocene evolution of the Mekong River Delta shows the change of the delta system from tide-dominated to mixed, tide and wave influenced (Ta et al., 2002b). The dominant type of suspended sediment carried by Mekong River is fine silt, while the clay fractions do not exceed 30% by volume (Wolanski et al., 1996). At least 95% of that sediment is exported to the sea and deposited within 20 km of the coast (Anikiyev et al., 1986, fide Wolanski et al., 1996).. 30.

(31) Figure 2.14. Geomorphology and Late Holocene evolution of the Mekong delta. The dashed lines indicate estimated location and age (in years from present) of palaeo-offshore break. (after Ta et al., 2002a).. 2.7.3 Mangroves Mangroves are trees or shrubs that grow in shallow and muddy salt or brackish water, especially along quiet shorelines, deltas or estuaries in tropical regions, where they collectively form mangrove swamps (Encarta, 2006e). A significant concentration of mangroves occurs on the Chao Phraya delta plain (Figure 2.13). Mangroves occupy approximately 10% of the south-eastern coast of Thailand (Thampanya et al., 2006). The development of mangroves is related mostly to the positioning of river mouths and sheltered bays. The spread of mangrove forests can be fast and has been measured to be as high as 140-1200 m over the period of 31 years around some river mouths (Thampanya et al., 2006).. 31.

(32) Mangrove forests, which provide significant protection against coastal erosion and form an important habitat for countless aquatic species, have been seriously reduced in spread during last decades (Cheevaporn and Menasveta, 2003). The existing mangrove forest area in Thailand has decreased more than 50% in the past 32 years (Kongsangchai, 1995, fide Cheevaporn and Menasveta, 2003). Historically, mangroves occupied large bands of Southeast Asia’s coasts (Rao, 1986, Aksornkoae, 1993, fide Thampanya et al., 2006).. 2.7.4 Coral Reefs The reefs of the Gulf of Thailand develop essentially around archipelagos and single islands. Most of the islands are situated in the eastern part of the Gulf. The islands are relatively high mountainous plateaus with steep, marine-cut rockfall slopes. The underwater slopes consist of boulder–block pavements in the shallowest parts, stony and gravel-prone alluvial bedrocks in the middle zone, and sandy and coral deposits with a high content of organogenic detritus in the deepest parts (Latypov, 2003).. 32.

(33) 3. METHODOLOGY. Results presented in this dissertation are mainly based on a combination of geophysical and geological data acquired by Fugro’s designated survey vessels from the Gulf of Thailand area during the period 2006-2007. Apart from these marine geophysical records, additional utilised data include borehole logs, results of laboratory analyses of gravity core samples, and Fugro’s geophysical survey reports prepared before 2006. The author of this paper has been involved in data acquisition and interpretation as a field geophysicist. Selected sediment samples, collected by gravity corer, have been subjected to laboratory analyses carried out or assigned by the author, who is also responsible for the final data interpretation.. 3.1. Survey Vessels Two Fugro survey vessels, Geo Surveyor and Geo Eastern operated by Fugro Survey. Pte Ltd have been used during geophysical data acquisition and gravity coring. Both vessels were equipped with a Differential Global Positioning System (DGPS), Single Beam Echo sounder (SBES), Multi Beam Echo sounder (MBES), Pinger Sub-bottom Profiler (SBP), Side Scan Sonar (SSS), Ultra-short baseline (USBL) system for SSS fish positioning, Sound Velocity Probe (SVP), and a 3 m barrel Gravity Corer. A brief summary of the geophysical and geotechnical systems used during survey operations is given below (Figure 3.1).. Figure 3.1. Summary of the analogue system setup on-board MV Geo Surveyor (after Fugro, 2007).. 33.

(34) 3.2. Positioning and Navigation A Fugro Starfix HP DGPS with corrections received via Starfix Spot Multiple Reference. Station DGPS and the APSat satellite system was used as the primary positioning system. Fugro’s Starfix MRDGPS system was used as a secondary system and for DGPS data QC. Position of GPS antennas as well as all mounted survey sensors were calculated based on measured offsets in relation to the vessel Central Reference Point (CRP). DGPS verifications have been performed at the start of every project.. 3.2.1 Fugro Starfix HP DGPS The Starfix HP DGPS is a dual frequency GPS augmentation service that provides positioning for marine users. Accuracy for the Starfix-HP service is 20 cm, 95% for the North. and. East. components. with. a. vertical. accuracy. of. 30. cm,. 95%. (Fugro/Starfix.HP, 2003). By using dual frequency GPS receivers Starfix-HP can measure the true ionosphere at the reference and user locations, substantially eliminating this error. Using these iono-free measurements with information contained in the receiver carrier phase data, it is possible to create wide area positioning results of unmatched accuracy and performance.. 3.2.2 Fugro Starfix MRDGPS A Starfix is a multiple reference station Differential GPS (DGPS) system that uses the Inmarsat communication satellites as the downlinks for the correction data from each reference station and a Multi Reference Differential Global Positioning System (MRDGPS) position solution. The primary function of MRDGPS is to use all available DGPS data collected and computed at the mobile unit to provide the user with real-time indication of the position performance. Prior to the final position computation, all pseudo ranges are statistically tested for gross errors using the w-test method. Each observation is carefully corrected and weighted considering satellite elevation, age of differential corrections, distances from the reference station (Figure 3.2) and rate of change correction.. 3.2.3 Dynamic Heading Reference System Two Brown Meridian gyrocompasses were installed to provide vessel heading input to the Starfix.Seis systems and to enable real time calculation of vessel offsets. Gyro calibrations have been performed at the start of every project. These portable survey gyros are designed specifically for survey type operations allowing easy interfacing to navigation computers and have a digital output of the bearing to 0.17 of a degree. 34.

(35) Figure 3.2. Starfix Reference Stations Coverage in Southeast Asia (after Fugro, 2007).. 3.2.4 Underwater Positioning A Sonardyne Ultra Short Baseline (USBL) system was used to accurately track the towed side scan sonar fish position. The Sonardyne USBL system consisted of a pole mounted transceiver, pitch and roll sensor and transponder beacons. The system measured ranges and bearings between the transducer and the transponder beacon attached to the sonar fish. Data were transmitted via an RS232 line to the navigation computer that calculated and logged the position of the tow fish at every fix point. During survey operations, the USBL beacon was mounted on the side-scan sonar cable close by the side-scan sonar tow fish.. 35.