Subsurface control on seafloor erosional processes offshore of the Chandeleur Islands,Louisiana

The Chandeleur Islands lie on the eastern side of the modern Mississippi River delta plain, near the edge of the St. Bernard Delta complex. Since abandonment approximately 2,000 years b.p., this delta complex has undergone subsidence and ravinement as the shoreline has transgressed across it. High-resolution seismic-reflection, sidescan-sonar, and bathymetry data show that seafloor erosion is influenced by locally variable shallow stratigraphy. The data reveal two general populations of shallow erosional depressions, either linear or subcircular in shape. Linear depressions occur primarily where sandy distributary-channel deposits are exposed on the seafloor. The subcircular pits are concentrated in areas where delta-front deposits crop out, and occasional seismic blanking indicates that gas is present. The difference in erosional patterns suggests that delta-front and distributary-channel deposits respond uniquely to wave and current energy expended on the inner shelf, particularly during stormy periods. Linear depressions may be the result of the sandy distributary-channel deposits eroding more readily by waves and coastal currents than the surrounding delta-front deposits. Pits may develop as gas discharge or liquefaction occurs within fine-grained delta-front deposits, causing seafloor collapse. These detailed observations suggest that ravinement of this inner shelf surface may be ongoing, is controlled by the underlying stratigraphy, and has varied morphologic expression.     

Sedimentology of the New Hampshire Inner Continental Shelf Based on Subbottom Seismics,Side-Scan Sonar,Bathymetry, and Bottom Samples

Typical of glaciated environments, the inner continental shelf of New Hampshire is composed of bedrock outcrops, remnants of glacial deposits (for example, drumlins), sand and gravel deposits, as well as muddier sediments farther offshore. A number of previous studies have defined the general trends of the New Hampshire inner shelf from the coarser deposits nearer the shore to the muddier outer basins. Most recently, a seismic survey (150 km of side-scan sonar and subbottom seismic profiles), as well as bottom sediment sampling (74 stations), has provided a detailed bottom map of the southern New Hampshire shelf area (landward of the 30-m contour). The surficial sediments within this area range from very fine sand to gravel. Bedrock outcrops are common. The seismic survey indicated several large sand deposits exceeding 6-8 m in thickness that occur relatively close to the coast. These sedimentary units, which are within 3 km of the shoreline, are composed of fine to medium sands. Examination of the general morphology and depositional setting indicates at least some of these features are probably relic ebb tidal delta shoals. However, a large eroding drumlin occurs between two of the sand bodies and may represent the source of these deposits. Additional work is needed to verify the origin of these sediment bodies.     

冬季南海上层环流动力机制的数值研究

通过利用一个分区性的正压-斜压衔接模式来探讨冬季南海的上层环流特征及其动力机制,结果表明:(1)在南海北部,流态主要受黑潮的影响,除了东沙群岛西南的大陆架海域以及吕宋岛北部西岸附近各为一反气旋涡外,整个南海北部为一气旋式大环流所控制.(2)在南海南部主要是风生环流,源自粤西沿岸的水体在东北季风的作用下顺南海西边界岸线向南流动,形成一支相当强的西边界流;同时,由于受北康暗沙以南的陆架坡底形效应和β效应的作用,使得在南海南部出现以一个反气旋涡在南沙海槽处产生、发展并向西传播乃至衰减的约50d的周期性过程     

High-resolution analysis of the deposition pattern on the Amazon sub-aquatic delta and outer continental shelf

Continental shelves off major river deltas or estuaries act as an interface between terrestrial environments and marine depo centers like the Amazon Fan. In order to understand sedimentary processes on the Amazon sub-aquatic delta and outer shelf, Late-Quaternary erosional and depositional structures have been investigated with an ultra high-resolution seismic survey system (4 kHz). The wavy morphology of the outer shelf implies the presence of tidal sand ridges or meandering channels. In the latter case, the local presence of channel fill deposits suggests several cut-off loops (ox bows). The inter-channel areas are consequently interpreted as levees, which reach their maximum height of 10 m in the area between the Amazon river mouth and the Amazon canyon. The morphology of terraces in front of the sub-aquatic delta reflect erosional processes which presumably occurred during accelerated sea level rise at the beginning and the end of the Younger Dryas. A carbonate platform is present in the northwestern survey area close to the shelf break. Its water depth of 120–130 m implies an evolution during the last glacial maximum. The asymmetric shape of the Cabo Norte Shoal on the topset of the sub-aquatic delta is typical of bottom current produced shoals. The steeper northwestern flank lies in the lee position of the northwest flowing coastal current. Southeast of the shoal the <10 m thick uppermost sequence of presumably Holocene age consists of lobes that dip with the same apparent angle as the foreset towards the offlap break. Within the area 20 km northwest of the shoal the uppermost sequence forms a smooth depression and may be erosionally truncated at the seafloor. Here, the prograding direction includes a northwest component. A shale diapir, which rises from a transparent underlying sequence, marks the transition to an area where the upper sequence is not resolved in the data.     

Ancient buried submarine trough,northwest Gulf of Mexico

A large buried submarine trough crosses the seaward margin of the continental shelf off the southwest coast of Louisiana. Original length was about 90 km, and width at the shelf edge was 16 km. Maximum eroded depth may have been as much as 305 m. Seismic characteristics of the prograded fill indicate cyclically repeated sequences of retrogressive deltaic and partly slumped sediments overlain by well-layered transgressive deposits. Slumping was increasingly prevalent toward the shelf edge. The cyclic sequences indicate that the trough was a passageway for large volumes of sediment onto the continental slope during several stages of lowered sea level.     

Submarine Sand Resources,Southeastern Virginia-Contributions from Year 9 and Year 10 of Virginia's Continental Margins Program

Virginia's year 9 and year 10 funds from the Continental Shelf Program were used to supplement other work funded by the Minerals Management Service in an ongoing Cooperative Agreement focused on the area offshore of southeastern Virginia. Year 9 and year 10 funds facilitated interpretation of subbottom profiles and the analysis of sediment samples from cores and grabs. On Virginia's sediment-starved continental shelf, deposits of material potentially suitable for use as beach nourishment or, perhaps, as construction aggregate occur in three stratigraphic settings, each with specific characteristics of morphology, grain-size gradients, likelihood of discovery, and physical ease of exploitation. All must be verified with a careful program of coring. Modern shoals generally are easier to identify, prove, and access than either filled channels or lenticular facies. Shoals usually are identifiable on nautical charts and characteristically have a definite lower boundarythat can be seen in subbottom profiles. In most cases, the base of the shoal coincides with the level of the surrounding sea floor. Filled channels are readily identifiable on subbottom profiles but may have a narrow, sinuous form and steep lateral gradients in sediment properties. Buried lenticular facies of good-quality sand usually are found only fortuitously. As the lateral and often vertical gradients in geotechnical properties usually are low, the lenticular facies can be mined with a lesser concern for the consequences of violating the deposit's limits than with the other two types of deposit. There are three types of filled paleochannels in the study area. (1) Relatively near-surface, generally small, roughly shore normal channels most likely mark the migration of tidal inlets across the shelf during the most recent transgression. (2) Small, relatively wide and relatively shallow generally shore parallel channels may be filled back-barrier or lagoonal channels. (3) Larger channels trending across the shelf probably result from riverine flow. The complexity of the seismostratigraphy of the Quaternary deposits on south eastern Virginia's inner continental shelf is a result of series of high-frequency (fifth-order, 10-20,000 y), low-amplitude (20-30 m) variations in sea level that occurred during the last highstand, roughly 80,000 to 130,000 BP. The evidence of the small oscillations in sea level is best seen in the regions that were between the shoreline and wave base, today's inner shelf; however, the very low rates of deposition on the shelf make it difficult to correlate specific reflectors or beds or, at times, to distinguish between fifth-and fourth-order changes. Results for the continuing studies already have been used in the determination to mine several hundred thousand cubic meters of sand from Sandbridge Shoal for use on a Navy-owned facility and in consideration of mining greater quantities of sand from Sandbridge and other shoals for use in local beach nourishment and hurricane protection efforts.     

Submarine Sand Resources, Southeastern Virginia-Contributions from Year 9 and Year 10 of Virginia's Continental Margins Program

Virginia's year 9 and year 10 funds from the Continental Shelf Program were used to supplement other work funded by the Minerals Management Service in an ongoing Cooperative Agreement focused on the area offshore of southeastern Virginia. Year 9 and year 10 funds facilitated interpretation of subbottom profiles and the analysis of sediment samples from cores and grabs. On Virginia's sediment-starved continental shelf, deposits of material potentially suitable for use as beach nourishment or, perhaps, as construction aggregate occur in three stratigraphic settings, each with specific characteristics of morphology, grain-size gradients, likelihood of discovery, and physical ease of exploitation. All must be verified with a careful program of coring. Modern shoals generally are easier to identify, prove, and access than either filled channels or lenticular facies. Shoals usually are identifiable on nautical charts and characteristically have a definite lower boundarythat can be seen in subbottom profiles. In most cases, the base of the shoal coincides with the level of the surrounding sea floor. Filled channels are readily identifiable on subbottom profiles but may have a narrow, sinuous form and steep lateral gradients in sediment properties. Buried lenticular facies of good-quality sand usually are found only fortuitously. As the lateral and often vertical gradients in geotechnical properties usually are low, the lenticular facies can be mined with a lesser concern for the consequences of violating the deposit's limits than with the other two types of deposit. There are three types of filled paleochannels in the study area. (1) Relatively near-surface, generally small, roughly shore normal channels most likely mark the migration of tidal inlets across the shelf during the most recent transgression. (2) Small, relatively wide and relatively shallow generally shore parallel channels may be filled back-barrier or lagoonal channels. (3) Larger channels trending across the shelf probably result from riverine flow. The complexity of the seismostratigraphy of the Quaternary deposits on south eastern Virginia's inner continental shelf is a result of series of high-frequency (fifth-order, 10-20,000 y), low-amplitude (20-30 m) variations in sea level that occurred during the last highstand, roughly 80,000 to 130,000 BP. The evidence of the small oscillations in sea level is best seen in the regions that were between the shoreline and wave base, today's inner shelf; however, the very low rates of deposition on the shelf make it difficult to correlate specific reflectors or beds or, at times, to distinguish between fifth-and fourth-order changes. Results for the continuing studies already have been used in the determination to mine several hundred thousand cubic meters of sand from Sandbridge Shoal for use on a Navy-owned facility and in consideration of mining greater quantities of sand from Sandbridge and other shoals for use in local beach nourishment and hurricane protection efforts.     

中国陆架潮流沉积体系和模式

在1991年中法合作渤海潮流沉积研究的基础上,查阅了国内外有关研究成果,分析了中国陆架的水深地形、沉积地貌与潮流动力的关系,认为潮流对中国陆架的海底地貌和沉积的形成发育起了主导作用。当潮流流速大于3节时,潮流的侵蚀作用是主要的,往复潮流多形成冲刷深槽,大大刷深了海峡或水道。当潮流流速1－3节时,潮流的沉积作用是主要的,多形成浅滩,即潮流沙脊和潮流沙席。以M2分潮椭率绝对值0．4为界,大于0．久者意味着潮流旋转性强,多形成潮流沙席;小于0．4者意味着潮流往复性强,多形成潮流沙脊。提出了我国邻近陆架发育了5个现代潮流沉积地貌体系：（1）黄海东部潮流沉积体系,它由西朝鲜湾潮流沙脊和其南部的沙席两者组成;（2）渤海东部潮流沉积体系,它由老铁山水道冲刷槽,辽东浅滩沙脊和渤中浅滩沙席三者组成;（3）长江口外潮流沉积体系,它由江苏滨外潮流沙脊和长江口浅滩潮流沙席组成;（4）台湾滨外潮流沉积体系,它由台湾海峡冲刷槽、台湾浅滩沙脊、澎湖水道冲刷槽和台中浅滩沙席四者组成;（5）琼州海峡潮流沉积体系,它由琼州海峡冲刷槽、东浅滩沙脊和西浅滩沙脊三者组成。此外,在东海陆架上还有冰后期海侵早期形成的残留潮流沉积体系。全新世陆架浅海潮流沉积模式可分海峡一浅     

Plio–Quaternary sedimentation processes and neotectonics of the northern continental margin of the South China Sea

Analyses of over 6600 km of reflection seismic profiles on the northern continental margin of the South China Sea permit the recognition of four Quaternary high-frequency type 1 sequences of the 4th order, deposited during the past ca. 690 kyr. At the present-day shelf edge, only lowstand systems tracts characterised by a prograding clinoformal internal reflection pattern are preserved. The prograding complexes can be considered as regressive units accumulated during relative sea-level falls. They exhibit internal discontinuities which might point to minor sea-level fluctuations of the 5th order. A preliminary regional relative sea-level curve for the past 630 kyr is established using the present positions of the delta fronts mapped. The neotectonics curve derived by subtracting eustatic sea-level changes from the relative sea-level curve shows that the depths of the delta fronts today are controlled primarily by regional tectonic movements and the global sea-level.Our seismo-stratigraphic interpretation documents that the area off Hong Kong and around the Dongsha Islands experienced two uplift episodes during the past 5 ma, namely at the Miocene/Pliocene boundary and at the end of the lower Middle Pleistocene, respectively. These uplift events which are centred on the Dongsha Rise led to its subaerial exposure and to the erosion of the Pliocene and most of the Pleistocene strata. The change from thermal subsidence of the continental margin initiated at the end of the drift phase to the phase of magmato-tectonic uplift was caused by a reorientation of the tectonic regime.The Recent depositional environment on the northern continental margin of the South China Sea is dominated by sediment accumulation within the inner shelf and the Zhujiang (Pearl River) estuary. The outer shelf and upper slope, especially around the Dongsha Islands, are characterised by bypass of terrigenous material.The sedimentary column in the deepsea basin has a thickness of more than 2 km and comprises 14 depositional units starting with terrestrial rift deposits. It overlies oceanic as well as transitional crust.     

Subaqueous deltaic formation on the Atchafalaya Shelf,Louisiana

The Atchafalaya River in Louisiana shares the third largest drainage basin in the world with the Mississippi River. Sediment cores and seismic profiles were used to examine the development and impact on land accretion of an early-stage subaqueous delta accumulating on the shallow (<25 m water depth) continental shelf seaward of the Atchafalaya River mouths in the period (∼100 years) since the Atchafalaya has captured a significant fraction of the overall Mississippi discharge. The subaqueous clinoform is muddy (70–100% finer than 63 μm) and extends approximately 21–26 km seaward of the shell reef (to 8 m water depth) across the mouth of the Atchafalaya Bay, with a discontinuous, and, in places, mobile modern mud layer <20 cm thick covering a relict deltaic shoal area further seaward. The sigmoidal clinoform has a topset surface that steepens from east to west (1:2500 to 1:1600), a foreset with maximum slopes of about 1:550, and a limited bottomset region (<0.5 km wide). ²¹⁰Pb and ¹³⁷Cs geochronology show maximum sediment accumulation rates (>3 cm/year) correspond to the foreset and bottomset region, with rates decreasing to as low as 0.9 cm/year on the shelf topset region and its extension inside Atchafalaya Bay. Seven sedimentary facies are observed in the subaqueous delta, with differences created by degree of biological destruction of physical stratification, which is inversely related to sediment accumulation rate, and by the proximity of an area to the riverine sand source. There is a marked alongshore sediment dispersal pattern observed by the progressive winnowing of sand and coarse silt to the west. There is also a significant increase in shell content in Atchafalaya Bay relative to shelf facies. The resulting sigmoidal clinoform deposit (<3 m thick) more closely resembles strata geometries of subaqueous mud deltas associated with energetic systems (e.g., Amazon, Ganges–Brahmaputra, Fly), than it does the mature Mississippi delta 180 km to the east, albeit on a smaller scale and in shallow water.     

210Pb as a tracer of shelf–basin transport and sediment focusing in the Chukchi Sea

Activities of dissolved, particulate, and sedimentary ²¹⁰Pb were measured in the shelf-slope region of the Chukchi Sea. Samples were collected as part of the Shelf–Basin Interactions (SBI) Phase II process study (6 May–15 June, 2002) along three shelf–basin transects identified as West Hanna Shoal, East Hanna Shoal, and Barrow Canyon. Distributions of ²¹⁰Pb and suspended particulate matter indicate efficient removal of ²¹⁰Pb over the shelf by particle scavenging. Low ²¹⁰Pb activities measured throughout the halocline of the Canada Basin are attributed to shelf scavenging and subsequent advective transport into the interior basin. Additionally, ²¹⁰Pb inventories were used to construct a water-column-sediment budget of ²¹⁰Pb and determine regions of particle export and deposition on the continental shelf and slope. Sediment focusing calculated with this ²¹⁰Pb budget was observed throughout the shelf-slope region, particularly in shallow (∼100 m) shelf waters at Barrow Canyon. Despite elevated concentrations of suspended particulate matter in Barrow Canyon, the ²¹⁰Pb budget does not indicate that sediment transport occurred from the West and East Hanna Shoals into Barrow Canyon.     

A seismic geomorphology study of the fluvial and lacustrine-delta facies of the Cretaceous Quantou-Nenjiang Formations in Songliao Basin,China

With the aid of seismic geomorphology method, depositional systems such as fluvial-delta and turbidity current of the Cretaceous Quantou-Nenjiang Formations in Sangliao Basin were studied in detail as well as the morphology, dimension, depositional structures and boundaries of these sedimentary bodies. A geological model of a point bar in a fluvial system and the method to identify paleocurrent direction were proposed. The delta of Yaojia Formation with a gentle slope less than 1.4‰ in trangressive system tract (TST) was controlled by lake waves. Subaqueous distributary channels were widely developed in large-scale delta front with a width more than 16 km. Fluvial-dominated deltas with leaf and bird's foot shapes were widespread in highstand system tract (HST) of Qinshankou and Nengjiang Formation with moderately high slope gradients of 4‰ and 6.7‰.The width of the leaf-like and bird's foot delta front ranges from 1 km to 4 km and 3 km–5 km respectively. The mouth bar were well preserved due to the poorly developed widely distributed subaqueous distributary channels and the delta plains having widths of 9 km–15 km, 16 km-25 km respectively. Many turbidity current systems were recognized in the TST of the first member of Nengjiang Formation and the distribution area is about 10,000 km². Fourteen sublacustrine fans with different dimension were formed by ten fluvial-delta systems extending 30 km–70 km toward the lake from the west and south. With the application of seismic geomorphology, the analysis of fluvial system, delta system and turbidity current system could switch from approximation to quantitative analysis. Sedimentary model proposed for point bars and the quantitative analysis of delta system could provide new technical support for exploration and drilling plan. The research on turbidity current systems will also be beneficial for new exploration targets.     

Holocene evolution of Apalachicola Bay,Florida

A program of geophysical mapping and vibracoring was conducted to better understand the geologic evolution of Apalachicola Bay. Analyses of the geophysical data and sediment cores along with age control provided by 34 AMS ¹⁴C dates on marine shells and wood reveal the following history. As sea level rose in the early Holocene, fluvial deposits filled the Apalachicola River paleochannel, which extended southward under the central part of the bay and seaward across the continental shelf. Sediments to either side of the paleochannel contain abundant wood fragments, with dates documenting that those areas were forested at 8,000 ¹⁴C years b.p. As sea level continued to rise, spits formed of headland prodelta deposits. Between ∼6,400 and ∼2,500 ¹⁴C years b.p., an Apalachicola prodelta prograded and receded several times across the inner shelf that underlies the western part of the bay. An eastern deltaic lobe was active for a shorter time, between ∼5,800 and 5,100 ¹⁴C years b.p. Estuarine benthic foraminiferal assemblages occurred in the western bay as early as 6,400 ¹⁴C years b.p., and indicate that there was some physical barrier to open-ocean circulation and shelf species established by that time. It is considered that shoals formed in the region of the present barrier islands as the rising sea flooded an interstream divide. Estuarine conditions were established very early in the post-glacial flooding of the bay.     

长江口表层沉积物矿物磁性分区特征及其沉积环境指示意义

本研究旨在揭示现代长江口不同沉积环境铁磁性矿物的分布差异,寻找有效识别河口-陆架沉积环境的磁学指标,以便更好地将环境磁学应用于河口古环境研究。在长江口及邻近陆架的6个沉积环境:汊道、拦门沙、三角洲前缘斜坡、前三角洲、前三角洲-陆架过渡区和残留砂区,采集表层沉积物样品,进行粒度和磁性测量。结果显示,χ和SIRM在汊道和拦门沙呈现显著高值,HIRM、χfd%、χARM、χARM/χ和χARM/SIRM在前三角洲和前缘斜坡呈现显著高值,反映了陆源物质输运距离和河口沉积动力对磁性矿物分布的控制作用。因此,参数组合HIRM、χARM、χARM/χ和χARM/SIRM可用于识别全新世地层前缘斜坡和前三角洲-陆架;参数组合χ、SIRM和S-20mT可尝试用于识别汊道和拦门沙环境。     

南黄海西部浅部地层地震层序及其沉积特征

南黄海陆架在第四纪具有海侵—海退交替发生、海陆相地层交互沉积的特点,沉积相及其分布主要受海平面变化与古河流及其三角洲的控制。对南黄海西部地区采集的5600km浅层地震剖面进行了层序划分与解释,得到以下认识:(1)早更新世晚期以来的南黄海地层可分为U1—U14共14个地震地层单元,组成6个地震层序,代表了6次大规模海侵-海退过程,其中氧同位素5期海相地层在该地区最发育;(2)根据全新世地层厚度变化,将该区分为6个沉积区:东部弱沉积区、中北部沉积发育区、海州湾外侵蚀区、废黄河口沉积区、射阳河口岸外侵蚀区和苏北浅滩沉积区;(3)氧同位素6期至2期,南黄海西部陆相层内发育了3期埋藏古河道:上层古河道发育于氧同位素2期,古长江水系、古黄河水系分别自南黄海南部、北部流入古黄海;中层古河道发育于氧同位素4期,仅分布于近岸地区;下层古河道发育于氧同位素6期,分布于123°E以西地区,其南部古河道应为古长江水系,规模较大,其北部古河道规模较小,分布范围有限;(4)古长江第四纪早期在古黄海入海,古黄河在氧同位素5期中后期也开始自古黄海入海,其古河道发育具有一定继承性,后期发育规模逐渐增大;(5)气候温暖的高海面时期发育于构造沉降...     

800年来苏北废黄河三角洲的演变模式

根据历史资料、数据和相关研究,结合研究区域背景,分析苏北废黄河三角洲的演变。结果显示,岸线演变在发育阶段和侵蚀阶段分别为向海延伸约90 km和侵蚀后退约22 km,面积相差约800 km2,三角洲地貌演变表现为岸线平直-曲折-平滑-平直的过程。在废三角洲陆海相互作用的基础上,运用演化模式分析三角洲的演变过程。该三角洲演变可以分为7个演变阶段,发育期在径流和潮流作用下以沙洲并陆淤积延伸方式进行,侵蚀期在波浪和潮流作用下以沙洲合并侵蚀后退和淤积外长交替侵蚀的方式。泥沙输运、人类活动和气候变化对废三角洲的演变有重要影响,巨量的来沙是三角洲发育的原因,泥沙平衡被打破是侵蚀的主要原因。发育期中,泥沙输运影响淤积速度和位置,人类活动和气候变化影响黄河河道迁移、输沙量和产沙量;侵蚀期中,泥沙输运影响侵蚀状态,人类活动在一定程度上影响海岸带冲/淤,气候变化将影响三角洲的演变趋势。     

基于RS的辽河口三角洲现代沉积体形态演变研究

辽河口是渤海重要的入海口之一,河口拦门沙浅滩发育。文章利用20世纪40年代、50年代军用地图和70年代以来Landsat系列卫星MSS、TM、OLI遥感影像,通过人工解译与水边线相结合的方法来提取海岸线和拦门沙浅滩形态信息,分析近70年来辽河口三角洲现代沉积体的形态演变。结果表明,近70年间,辽河口海岸线受河流沉积影响不断向海淤进,80年代以来,由于人类活动的影响使得海岸线变迁愈加显著;口门处小型拦门沙浅滩始终受河流和海洋水动力作用而频繁变动;盖州滩位置和主体形态无较大变化,仅滩体南部因受海洋动力改造而变化明显。     

Evolution and preservation potential of fluvial and transgressive deposits on the Louisiana inner shelf: understanding depositional processes to support coastal management

The barrier-island systems of the Mississippi River Delta plain are currently undergoing some of the highest rates of shoreline retreat in North America (~20 m/year). Effective management of this coastal area requires an understanding of the processes involved in shoreline erosion and measures that can be enacted to reduce loss. The dominant stratigraphy of the delta plain is fluvial mud (silts and clays), delivered in suspension via a series of shallow-water delta lobes that prograded across the shelf throughout the Holocene. Abandonment of a delta lobe through avulsion leads to rapid land subsidence through compaction within the muddy framework. As the deltaic headland subsides below sea level, the marine environment transgresses the bays and wetlands, reworking the available sands into transgressive barrier shorelines. This natural process is further complicated by numerous factors: (1) global sea-level rise; (2) reduced sediment load within the Mississippi River; (3) diversion of the sediment load away from the barrier shorelines to the deep shelf; (4) storm-induced erosion; and (5) human alteration of the littoral process through the construction of hardened shorelines, canals, and other activities. This suite of factors has led to the deterioration of the barrier-island systems that protect interior wetlands and human infrastructure from normal wave activity and periodic storm impact. Interior wetland loss results in an increased tidal prism and inlet cross-sectional areas, and expanding ebb-tidal deltas, which removes sand from the littoral processes through diversion and sequestration. Shoreface erosion of the deltaic headlands does not provide sufficient sand to balance the loss, resulting in thinning and dislocation of the islands. Abatement measures include replenishing lost sediment with similar material, excavated from discrete sandy deposits within the muddy delta plain. These sand bodies were deposited by the same cyclical processes that formed the barrier islands, and understanding these processes is necessary to characterize their location, extent, and resource potential. In this paper we demonstrate the dominant fluvial and marine-transgressive depositional processes that occur on the inner shelf, and identify the preservation and resource potential of fluvio-deltaic deposits for coastal management in Louisiana.     

高邮凹陷阜三段沉积相分析

根据岩心微相分析、砂体形态和测井剖面对比分析 ,对阜三段各亚段的沉积微相进行了研究 ,掌握了沉积微相的时空展布和变化规律。在三角洲前缘发育区 ,由东北向西南方向 ,相带分异比较明显 ,由三角洲前缘的水下分流河道 -河口砂坝变为前缘席状砂和前三角洲及较深水湖泊相沉积。水下分流河道和河口砂坝砂体相互连接 ,形成厚度较稳定的三角洲前缘复合体 ,储油物性较佳。     

Possibility of Large Deposits of Gas Hydrates in Deeper Waters of India

The available geological and thermodynamic data, essential for the formation and accumulation of gas hydrates, have been integrated and broadly interpreted for the deeper waters of India. The preliminary studies indicate that, in all probability, vast gas hydrate resources exist in the shallow sediments under deep waters. The area of the Bay of Bengal and Arabian Sea, off the coast of India and Andaman Islands, have accumulated thick sediments, over 22 and 10 km, respec tively, during collision of the Indian Plate with the Tibetan Plate. Bottom Simulating Reflectors (BSRs), indicating the likely presence of gas hydrates, have been observed from multichannel and single-channel seismic reflec tion data west of the Andaman Islands and Kerala-Konkan offshore. The Indian continental shelf, slope, and rise areas have, at places, shown the presence of gas-charged sediments and gas seeps through faults. There are commercial oil and gas fields in the shallow waters of both the east and west coasts of India. These are indicative of generation of both biogenic as well as thermogenic gases in the offshore areas of India. For the first time, an attempt has been made to estimate in-place gas hydrate resources under deep waters of India beyond 600 m water depth to the legal continental shelf boundary, and to the Andaman Islands. The gas hydrate resources appear to be vast, and require extensive exploratory efforts for their precise mapping and quantitative assessment.