Placer Deposits of Southern Tamil Nadu Coast, India

Exploration for placer deposits, especially for ilmenites, has been undertaken systematically in the southern coast of Tamil Nadu, India. On the basis of drainage network. Coastal landforms, lithology, and neotectonics, the study area has been grouped into five sectors: Mandapam, Valinokkam, Tuticorin, Manappad, and Kanyakumari. Ilmenites are abundant in Kanyakumari (Max. 53.39 wt%) and Tuticorin (Max. 20.88 wt%) sectors, negligible in Manappad (Max. 0.60 wt%) and Valinokkam (Max. 3.88 wt%) sectors and absent in the Mandapam sector. The abnormal enrichment of ilmenites in the Kanyakumari sector in the absence of any drainage network points to the possibility of a relict source. Literary and scientific evidence corroborates the existence of an Eastern Gondwana land called Lemuria, South of Kanyakumari which was later submerged in the Flandrian transgression. The association of other heavy minerals with ilmenites like overgrown and outgrown zircon supplements the idea of a longer stay of ilmenites in the depositional basin. Ilmenites from these fossil strandlines must have been reworked by the present day coastal processes and deposited in favorable bay-like NE-SW coastal configurations in Kanyakumari-Kuttankuli and Kallar-Vaippar regions under high energy wave conditions. The present study has disclosed the enrichment of ilmenites up to 1 m in depth in the above two zones which can be commercially exploited. The present study also calls for confirming part of the lost continent called Lemuria or Eastern Gondwana by systematic scientific investigations.     

Surficial Sediments along the Inner Continental Shelf of Maine

Through 10 years of support from the Minerals Management Service Association of American State Geologists' Continental Margins Program we have mapped along the Maine coast, seaward to the 100 m isobath. In all, 1,773 bottom sample stations were occupied, 3,358 km of side-scan sonar and 5,011 km of seismic reflection profiles were gathered. On the basis of these data, a surficial sediment map was created for the Maine inner continental shelf during the Year 8 project, and cores and seismic data were collected to evaluate sand thickness during Years 9 and 10. Sand covers only 8 % of the Maine shelf, and is concentrated seaward of beaches off southern Maine in water depths less than 60 m. Sand occurs in three depositional settings: (1) in shoreface deposits connected dynamically to contemporary beaches; (2) in submerged deltas associated with lower sea-level positions; and (3) in submerged lowstand shoreline positions between 50 and 60 m. Seismic profiles over the shoreface off Saco Bay, Wells Embayment, and off the Kennebec River mouth each imaged a wedge-shaped acoustic unit which tapered off between 20 and 30 m. Cores determined that this was sand that was underlain by a variable but thin (commonly 1 m) deposit of estuarine muddy sand and a thick deposit of glacial-marine mud. Off Saco Bay, more than 55 million m3 of sand exists in the shoreface, compared with about 22 million m3 on the adjacent beach and dunes. Seaward of the Kennebec River, a large delta deposited between 13 ka and the present time holds more than 300 million m3 of sand and gravel. The best sorted sand is on the surface nearshore, with increasing amounts of gravel offshore and mud beneath the surficial sand sheet. Bedforms indicate that the surficial sand is moved by waves to at least 55 m depth. Seaward of the Penobscot River, no significant sand or gravel was encountered. Muddy estuarine sediments overlie muddy glacial-marine sediment throughout the area offshore area of this river. No satisfactory explanation is offered for lack of a sandy delta seaward of Maine's largest river. Lowstand-shoreline deposits were cored in many places in Saco Bay and off the Kennebec River mouth. Datable materials from cores indicated that the lowstand occurred around 10.5 ka off the Kennebec. Cores did not penetrate glacial-marine sediment in the lowstand deposits, and seismic profiles were ambiguous about the vertical extent of sand in these units. For these reasons, no total thickness of sand was determined from the lowstand deposits, but given the area of the surficial sand, the volume is probably in the hundreds of millions of cubic meters.     

Reconstructing tsunami run-up from the characteristics of tsunami deposits on the Thai Andaman Coast

Tsunamis can leave deposits on the land surface they inundate. The characteristics of tsunami deposits can be used to calculate tsunami run-up height and velocity. This paper presents a reconstruction of tsunami run-up from tsunami deposit characteristics in a simple mathematical model. The model is modified and applied to reconstruct tsunami run-ups at Ao Kheuy beach and Khuk Khak beach, Phangnga province, Thailand. The input parameters are grain-size and maximum run-up distance of the sediment. The reconstructed run-up heights are 4.16–4.91 m at Ao Kheuy beach and 5.43–9.46 m at Khuk Khak beach. The estimated run-up velocities (maximum velocity) at the still water level are 12.78–19.21 m/s. In the area located 70–140 m inland to the end of run-up inundation, estimated mean run-up velocities decrease from approximately 1.93 m/s to 0 m/s. Reasonably good agreements are found between reconstructed and observed run-up heights. The tsunami run-up height and velocity can be used for risk assessment and coastal development programs in the tsunami affected area. The results show that the area from 0 to 140 m inland was flooded by high velocity run-ups and those run-up energies were dissipated mainly in this area. The area should be designated as either an area where settlement is not permitted or an area where effective protection is provided, for example with flood barriers or forest.     

粤西海岸全新世中期以来海平面升降与海岸沙坝潟湖发育过程

沙坝湖海岸是一种重要的海岸类型。文章论述了粤西海岸全新世中期以来在海平面升降影响下沙坝湖地貌的发育过程,得出如下结论：距今7000a至5000a．海平面曾上升至＋4m,由于大陆架泥沙的向陆堆积,形成了揭湖拦湾沙坝的后缘高突部分：以后海平面逐步下降至现今位置,它所形成的海退沙楔发育成低矮的向海沙坝和海滩;距今2000a,海平面趋于相对稳定;现今海平面微升,出现了海岸侵蚀。文章还对该地在海平面升降影响下的海岸泥沙运移机制以及海岸高海面堆积和现今海岸侵蚀现象作了解析。     

Tsunami deposits of the Shikotan earthquake of 1994

This paper describes the results of the grain-size and mineralogical studies of the deposits of the tsunami of 1994 on Shikotan, Tanfil’ev, and Kunashir islands. The studies were carried out within the portions of the coast with different configurations, geomorphologic structures, lithodynamical environments, and character of the tsunami manifestation. The composition of the tsunami deposits is shown to be controlled mainly by erosion-accumulative processes during the tsunami events and is in many respects inherited from the matter sources. The tsunami deposits contain marine diatom species, whose richest assemblages were found within the areas where the material from the underwater coastal slope was redeposited. The data concerning the deposits of earlier historical tsunamis encountered in the same cross sections are discussed too. Their examination points to a similar development of the erosional-accumulative processes during tsunami events with the same intensity and an entrainment of the matter from the same sources.     

Coastline sand waves on a low-energy beach at “El Puntal” spit, Spain

Coastline sand waves have been observed at “El Puntal” spit, located on the north coast of Spain. The spit has been monitored by an Argus video system since 2003 and the formation and destruction of sand waves has been observed. Coastline data from the video images are analyzed by means of principal components analysis, obtaining a mean sand wave length of 125–150 m and a maximum amplitude of ≈ 15 m. It is also observed that sand waves reach their maximum amplitude at about 15 days. No propagation of these sand waves is noticed during the approximately two-month-long events analyzed. Sand wave formation and evolution are examined in relation with the prevailing local wave conditions during that period. Incident waves at the west end of the spit approach from the east–northeast, with a very high angle with respect to the shoreline. Field observations suggest that sand waves may result from an instability in alongshore sediment transport caused by moderate-energy waves with a high-angle incidence.     

Sediment grains moved by passing tsunami waves: Tsunami deposits in deep water

Tsunamis propagating in the open ocean have associated horizontal particle velocities that do not change with depth — yet the limiting water depth where a tsunami of given characteristics will initiate sediment motion remains unknown. Based upon linear wave theory and a parametrization of the Shields curve, equations are derived and solved, using an iterative scheme, to address the topic of grain movement by tsunami waves as a function of water depth and wave amplitude. The focus is on waves in deep water where tsunami waves behave linearly and on non-cohesive sediment grains. Furthermore, the question is addressed of which grain sizes are picked up on a sloping beach as the wave shoals. According to the results, even the Boxing Day tsunami in 2004 was incapable of moving fine sand in water deeper than 985 m in the Bay of Bengal and 335 m in the Indian and Pacific oceans. The results suggest that tectonic tsunamis of size equal to or smaller than the Boxing Day tsunami cannot initiate motion of deep-water cohesionless sediments that can be correlated on an oceanic basin-wide scale.     

粤东后江湾海进海岸沉积特征和地貌发育

依据地质钻探和海滩观测资料，分析了后江湾海岸在海进作用下，海岸形成海进地层层序。滨面遭受侵蚀并正在后退和变陡。晚更新统陆相杂色粘土层和砾砂层直接暴露于海底。在海域供沙不足的情况下，整个海湾的海滩被侵蚀后退，而海滩各岸段侵蚀程度存在差异。     

Analysis of the Tsunami of December 26, 2004, on the Kerala Coast of India—Part I: Amplitudes

The Indian Ocean tsunami of December 26, 2004, not only affected the Bay of Bengal coast of India but also part of the Arabian Sea coast of India. In particular, the tsunami caused loss of life and heavy damage on some parts of the Kerala coast in southwest India. The tsunami traveled west, south of Sri Lanka, and some of the tsunami energy was diffracted around Sri Lanka and the southern tip of India and moved northward into the Arabian Sea. However, tsunami, being a long gravity wave with a wave length of a few hundred kilometers, has to take a wide turn. In that process, it missed the very southern part of the Kerala coast and did not achieve large amplitudes there. However, further north, the tsunami achieved amplitudes of upto 5 m and caused loss of life and significant damage. Here we identify the physical oceanographic processes that were responsible for selective amplification of the tsunami in certain locations.     

Analysis of the Tsunami of December 26, 2004, on the Kerala Coast of India—Part I: Amplitudes

The Indian Ocean tsunami of December 26, 2004, not only affected the Bay of Bengal coast of India but also part of the Arabian Sea coast of India. In particular, the tsunami caused loss of life and heavy damage on some parts of the Kerala coast in southwest India. The tsunami traveled west, south of Sri Lanka, and some of the tsunami energy was diffracted around Sri Lanka and the southern tip of India and moved northward into the Arabian Sea. However, tsunami, being a long gravity wave with a wave length of a few hundred kilometers, has to take a wide turn. In that process, it missed the very southern part of the Kerala coast and did not achieve large amplitudes there. However, further north, the tsunami achieved amplitudes of upto 5 m and caused loss of life and significant damage. Here we identify the physical oceanographic processes that were responsible for selective amplification of the tsunami in certain locations.     

The anatomy and provenance of thick volcaniclastic flows in the Cretan Basin, South Aegean Sea

Santorini volcano has been the largest source of volcaniclastic sediment in the Eastern Mediterranean during the late Quaternary. A dozen cores from the Cretan Basin, south of Santorini, have sampled two megabeds that consist of gravity emplaced volcaniclastic sequences. The uppermost megabed U consists of a succession of five (U₅–U₁) base cut out turbiditic units. Lower megabed A is a single turbiditic event. Only the uppermost U₂ and U₁ turbidites are separated from the underlying beds by hemipelagic marls. The texture and composition of the U and A megabeds closely match the texture and composition of the fine, vitric ash of the “Minoan” deposits on Santorini islands, dating from about 3500 yr BP. These megabeds are therefore attributed to rapid accumulation of separate gravity flows fed by the “Minoan” eruption, except for the upper U₂ and U₁ turbidites deposited from subsequent gravity flows transporting eroded volcaniclastic sediments. With the exception of the margin south of Santorini, dozens of cores retrieved around the margins of the Cretan Basin have a continuous late Quaternary succession that shows no evidence for massive sediment remobilization into the deeper basin, including the passage of the “Minoan” tsunami.

Extensive high-resolution 3.5 kHz records revealed the acoustic character, architecture and distribution of the U and A megabeds and four underlying late Quaternary volcanogenic megabeds in the Cretan Basin. The acoustic facies of megabeds are typical of megaturbidites and consist of an upper, transparent, lower velocity layer that corresponds to the fine-grained upper turbiditic silt and clay section and a lower, strongly reflective higher velocity section that corresponds to the lowest, coarser-grained base of the turbidite that is developed over a sharp erosional surface. Penetration of the high-resolution records reveals the existence of at least six megabeds. Correlation with core lithology and the physical properties of the various lithofacies, including down-core velocity profiles, has allowed us to determine the thickness and volumes of the upper four megabeds which are: U ≤ 9 m thick, volume 3.7 km³; A ≤ 25 m thick, volume 12.2 km³; B ≤ 22 m thick, volume 10.3 km³; C ≤ 15 m thick, volume 8 km³. These thick megabeds are the uppermost products of repeated explosive eruption of Santorini in the late Quaternary. Calculated sedimentation rates from and after the “Minoan” eruption are 9.4 m/1000 yr that rise to over 15.7 m/1000 yr if megabed B was also deposited during this eruption.     

Deposits of the 1983 and 1993 tsunamis on the coast of Primorye

Deposits of the two strongest tsunamis of the 20th century have been found on the eastern coast of Primorye. The tsunamis had epicenters in the Sea of Japan west of the coast of Hokkaido. The distribution and preservation of deposits in bays of different geomorphological structure have been analyzed. The best defined sedimentary covers occur in the upper part of sections in low-lying areas of bay shores, where the wave runup was more than 3 m. The best preserved deposits have been observed in bays attributed to loworder streams. Variations of the structural composition of tsunami deposits formed by traction processes associated with the tsunamis have been analyzed depending on distance from the shoreline; the sources of material have been identified. Tsunami waves transported sand not only from beaches, ancient storm ridges, and terraces, but also from the underwater coastal slope; waves also grabbed material from estuarine lagoons and lakes located in the shore inundation zone. Deposits include marine diatoms with dominant sublittoral planktonic and benthic species, which suggests that the material was transported from a depth no more than 15 m. Deep-sea species of diatoms and their fragments have been encountered. Among freshwater diatoms are species with different ecological identities, indicating erosion and redeposition of material transported from various sources.     

Postglacial depositional environments and sedimentation rates in the Norwegian Channel off southern Norway

Based on seismic profiles, multibeam bathymetry and sediment cores, an improved understanding of the deglaciation/postglacial history of the southern part of the Norwegian Channel has been obtained. The Norwegian Channel Ice Stream started to recede from the shelf edge ca. 15.5 ka BP (¹⁴C ages are used throughout). Approximately 500–1000 years later the ice margin was located east of the deep Skagerrak trough. At that time, the Norwegian Channel off southern Norway had become a large fjord-like embayment, surrounded by the grounded ice sheet along the northern slope and possibly stagnant ice remnants at the southern flank. The Norwegian Channel off southern Norway has been the main sediment trap of the North Sea, and south of Egersund more than 200 m of sediments have been deposited since the start of the deglaciation. Five seismic units are mapped. The oldest unit E occurs in some of the deepest troughs, and was deposited immediately after the ice became buoyant. Unit D is acoustically massive and comprises mass-movement deposits in eastern Skagerrak and south of Egersund. Unit C (in the channel southwest of Lista/Egersund) is interpreted to comprise mainly bottom current deposits derived from palaeo-rivers, e.g. Elben. During deposition of unit C (ca. 14.5–13 ka BP), there was limited inflow of Atlantic water. A change in depositional environment at ca. 13 ka BP is related to an increased inflow of saline water and more open hydrographic circulation. Widely distributed, acoustically stratified clays of unit B were deposited ca. 13–10 ka BP. The Holocene Unit A shows a depositional pattern broadly similar to that of unit B.     

Facies architecture of the mixed carbonate/siliciclastic inner continental shelf of west-central Florida: implications for Holocene barrier development

Sediment vibracores and surface samples were collected from the mixed carbonate/siliciclastic inner shelf of west–central Florida in an effort to determine the three-dimensional facies architecture and Holocene geologic development of the coastal barrier-island and adjacent shallow marine environments. The unconsolidated sediment veneer is thin (generally <3 m), with a patchy distribution. Nine facies are identified representing Miocene platform deposits (limestone gravel and blue–green clay facies), Pleistocene restricted marine deposits (lime mud facies), and Holocene back-barrier (organic muddy sand, olive-gray mud, and muddy sand facies) and open marine (well-sorted quartz sand, shelly sand, and black sand facies) deposits. Holocene back-barrier facies are separated from overlying open marine facies by a ravinement surface formed during the late Holocene rise in sea level. Facies associations are naturally divided into four discrete types. The pattern of distribution and ages of facies suggest that barrier islands developed approximately 8200 yr BP and in excess of 20 km seaward of the present coastline in the north, and more recently and nearer to their present position in the south. No barrier-island development prior to approximately 8200 yr BP is indicated. Initiation of barrier-island development is most likely due to a slowing in the Holocene sea-level rise ca. 8000 yr BP, coupled with the intersection of the coast with quartz sand deposits formed during Pleistocene sea-level highstands. This study is an example of a mixed carbonate/siliciclastic shallow marine depositional system that is tightly constrained in both time and sea-level position. It provides a useful analog for the study of other, similar depositional systems in both the modern and ancient rock record.     

A unique Yellow River-derived distal subaqueous delta in the Yellow Sea

Newly acquired high-resolution Chirp sonar profiles reveal a unique Yellow River-derived, alongshore distributed, bidirectional (landward and seaward) across-shelf transported, omega-shaped (“Ω”) distal subaqueous deltaic lobe deposited around the eastern tip of the Shandong Peninsula in the Yellow Sea. This clinoform deposit directly overlies the postglacial transgressive surface, featured by convex-up seafloor morphology, up to 40 m thick locally. Radiocarbon-14 dates from the underlain pre-Holocene and transgressive sediments indicate this distal lobe has formed since the middle-Holocene highstand under a relatively stable sea level. This along-shelf distributed distal clinoform has been deposited mainly by the resuspended Yellow River sediments carried down by the coastal current, interacting with the local waves, tides and upwelling. Collectively, over the past 7000 years, nearly 30% of the Yellow River-derived sediment has been re-suspended and transported out of the Bohai Sea into the Yellow Sea. Overall, the Yellow River-derived sediment could reach the − 80 m water depth in the central South Yellow Sea, about 700 km from the river mouth; in contrast, a very small fraction of the modern riverine sediment could escape the outer shelf or reach the Okinawa Trough.     

Traces of ancient tsunamis in the coastal parts of the South China Sea

The results of geological and geomorphological investigations have shown that the South China Sea and its close coastal margins are in danger because of the possibility of a tsunami arriving from seismically active zones in the Philippines and Taiwan to the west and northwest coastal zones of the South China Sea. The collected data on paleotsunamis make it possible to date at least three such ancient catastrophes that have occurred in the last 1000 years (about 350, 650, and 960 years ago). The height of splashes of these ancient tsunami waves can be estimated. It was more than 7 m and in some places exceeded 15 m. The length of the sea coast covered by the paleotsunamis is estimated at several hundred (up to a first thousand) kilometers, which gives the opportunity to distinguish them from the ancient typhoons that are also typical of the region.     

Late Holocene coastal sand movements in the Outer Hebrides, N.W. Scotland

Lithostratigraphical and biostratigraphical investigation of coastal marshes along the Atlantic coast of the Outer Hebrides from Lewis in the north to Barra in the south discloses inland-tapering sand units within marshland areas. The inland extent of each sand unit has been radiometrically dated and the units have been collectively interpreted as a proxy for past coastal storminess. The data appear to indicate that for the study sites investigated, the majority of the sand units were produced during episodes of climate deterioration both prior to and after the well-known period of Medieval warmth (MWP). Many were produced after ca. AD 1400. It is argued that the episodes of sand blow indicated by the deposits may reflect periods of increased cyclogenesis in the Atlantic associated with increased sea ice cover and an increase in the thermal gradient across the North Atlantic region.     

连云港海域废黄河水下三角洲北翼的沉积特征与空间分布

基于连云港海域浅地层剖面资料解释和16个站位柱状岩心的剖面观察、粒度分析、微体化石鉴定、有孔虫和腹足类纹沼螺的AMS 14C年龄，分析废黄河水下三角洲北翼的沉积结构、厚度和沉积物组成，划分沉积单元及其分布范围；识别海侵沉积区的沉积相，认识其沉积环境特征。结果表明，水下三角洲沉积体可分为三角洲前缘和前三角洲两个沉积单元，两者沿连云港海岸线呈并置关系。三角洲前缘位于埒子口至新淮河口外，为一NNE向进积、NW向倾斜的扇形楔状体，可识别出灌河叶瓣及其以东的叶瓣两个沉积中心。该沉积单元以砂质沉积为主，两个叶瓣之间的分流间湾分布有泥和砂质泥。前三角洲位于三角洲前缘西北侧，大致从埒子口外侧沿海岸带向海州湾展布。该单元为泥质沉积，厚度较薄，其北部的两个柱状岩心所揭示的沉积厚度分别为1.8 m和1.5 m。水下三角洲以北，海侵沙席分布广泛，厚度通常在20 cm左右。其分布在本区具有普遍性，只是在连云港海域被黄河泥沙所覆盖，由此也构成了废黄河水下三角洲的底界面。该界面具有侵蚀残留地貌特征。海侵沙席之下为晚更新世MIS 3阶段中早期的海陆过渡相沉积。沉积物多由黄色粉砂或砂质粉砂组成，含有较丰富的有孔虫、介形虫等，可见淡水纹沼螺。     

Extreme storms in 2006–2007 on Shikotan Island and their impact on the coastal relief and deposits

This paper describes the results of investigations of the consequences of the storms on the Pacific coast of Shikotan Island that occurred on October 7–10, 2006 and January 6–8, 2007. These storms and their impact on the coastal zone can be considered as extreme events for the last 40–50 years. The heights and flooding area of the storm surges within bay coasts of different types were measured. The coastal relief’s changes are described. During the storms, a cover of deposits was formed having a size of up to 30 m outside the beach zone and up to 52 m in the near-mouth zones. The grain-size composition of the storm deposits is analyzed and their difference from other coastal facies, including tsunami sands, are established.     

Observations of Tsunami Impact on the Coast of Kerala, India

The tsunami generated by the December 2004 Sumatra-Andaman earthquake had a devastating effect on some parts of Kerala coast, which is a coast located in southwest India. Results of post-tsunami field surveys carried out to understand the changes in coastal morphology and sediment characteristics in the worst affected Kayamkulam region of Kerala coast are documented in this study. Analysis of offshore bathymetric data indicates the shifting of depth contours towards shore, indicating erosion of sediments and deepening of innershelf due to the tsunami. Depth measurement along the backwater (T-S canal) in the hinterland region indicates siltation due to the inundation of the canal.