Seismic geomorphology of buried channel systems on the New Jersey outer shelf: assessing past environmental conditions

Quantitative geomorphologic analysis of shallowly buried, dendritic channel systems on the New Jersey shelf provides estimates of paleo-hydrologic parameters needed to link channel morphology to the former hydrodynamic setting. These channels, observed in 1–4 kHz deep-towed chirp seismic data, formed presumably as fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). The presumed fluvial origin of these channels is supported by their incision into underlying Pleistocene strata, a chaotic seismic fill unit at their bases which may be indicative of non-marine gravel lag, and measured stream junction angles that are consistent with a riverine origin. The channels would also have been subjected to estuarine/tidal environments during ensuing sea-level rise. We employ empirically derived hydraulic equations for modern rivers and estuaries to estimate paleo-discharges, velocities and maximum shear stresses, using the preserved and interpolated paleo-channel geometries as a guide. Generally, trunk/main channels have box-like, symmetric cross-sections, with width/depth ratios of >100, whereas smaller, tributary channels have more v-shaped, asymmetric cross-sections with width/depth ratios of ∼40–70. The high width/depth ratios, along with low sinuosities (∼1.1) and slopes (<0.02°), are consistent with braided streams as specified by a modern river classification system. However, the channels show no evidence of braiding. We hypothesize instead that these channel systems are immature, having had insufficient time to develop high sinuosities that would otherwise be expected before they were drowned by the Holocene transgression. Mean paleo-flow estimates for these systems assuming a tidal environment (1.0–1.5 m/s) are consistent with modern tidal creeks comparable to the sizes of channels observed here (<2 km wide and <25 m deep). Estimated tidal shear stresses would be sufficient to initiate sediment transport of grains 2–8 mm in diameter (coarse sand and fine gravel) as bedload and finer grained material in suspension. However, paleo-flow estimates assuming a fluvial environment (1.1–2.0 m/s) are generally too high for a non-tidal creek, given the presumed low hydraulic gradients in this coastal plain setting. Retrodicted fluvial discharge and boundary shear stresses would have been sufficient to transport particles up to ∼15 mm in diameter (gravel) as bedload; these grain sizes are too coarse to be transported by sluggish coastal plain rivers. We conclude that either flows were quite high when this system was first incised fluvially, perhaps due to meltwater pulses following the LGM, or that tidal influences have modified the original fluvial geometry.     

Ridge development as revealed by sub-bottom profiles on the central New Jersey shelf

Closely-spaced 3.5 kHz seismic profiles were collected over the north-easterly trending ridge and swale system 50 km east-southeast of Atlantic City, New Jersey. They yield information on the Late Quaternary depositional history of the area, and on the origin of the ridge system. Four of the sub-bottom reflectors identified were sufficiently persistent to warrant investigation and interpretation. These reflectors, which have been cored, lithologically identified, and radiocarbon dated, are stratigraphically higher than the reflectors dealt with by the majority of previous studies. The upper three reflectors are definitely mid- and post-Wisconsin in age and present a record of the most recent glacial cycle. The upper three units associated with the observed reflectors appear to exert a pronounced influence on the bathymetry. The gently corrugated ridge system of Holocene sand is formed over the regionally flat-lying upper unit, an Early Holocene lagoonal silty clay. The characteristically flat, broad depressions of the area are floored by this lagoonal material. Locally, however, marine scour has cut through the silty clay into an underlying unit of unconsolidated fine Pleistocene sand. Several stages of trough development appear to be represented. After penetrating the lagoonal clay, troughs are initially narrow, but when incised through the sand into a lower, Pleistocene, silty-clay unit, the troughs become notably wider. As downcutting is inhibited by the lower clay, the upper clay is undercut as the trough widens in a fashion similar to a desert blowout.

The sub-bottom reflectors indicate that ridge development on the central shelf has involved aggradation as well as erosion. Some ridges seem to have grown by vertical and lateral accretion from small cores. The internal structure of other ridges suggests that they formed by the coalescence of several small ridges. Others appear to have undergone appreciable lateral migration.

The ridges appear to be in a state of continuing adjustment to the hydraulic regime of the deepening post-Pleistocene water column.     

Origin of late Pleistocene submerged marine terraces on the outer continental shelf,Northern Israel

A series of elongated hills on the outer continental shelf off northern Israel, between 100 and 120 m, rise several meters above their surroundings. They have steep escarpments seaward and gradual slopes landward. The escarpments are commonly covered by colonies of sponges. The biogenic cover is normally 1.0–1.5 m thick. The gentle slopes commonly consist of soft, unconsolidated sediment, although some bedrock outcrops occur. Stratification patterns along these outcrops suggest calcareous eolianite (“kurkar”). Coquina was encountered in several sites at the base of the biogenic cover, built of fragments of shells not observed at the present depth. The eolianite, coquina, and steep escarpments suggest a late Pleistocene nearshore terrace.     

Large-scale current lineations on the central New Jersey shelf: Investigations by side-scan sonar

Two morphological orders of ridge and trough topography can be recognized on a terraced segment (at 37 m) of the central New Jersey shelf: (1) a first-order system with ridges to 14 m high, 2–6 km apart, in a Z-shaped pattern trending to the NNE, and (2) a second-order system with ridges 2–5 m high, 0.5-1.5 km apart and trends to the NE.Side-scan mapping together with submersible observations and bottom samples indicate a third-order system of large-scale current lineations which has been imprinted across the first- and second-order systems. The lineations are low relief forms (to 1.5 m high) which occur as elongate zones of textural contrast arranged in furrows, bands, patches and ribbons and display a uniform directional trend to the ENE.The morphology of the lineations appear to vary in response to the nature of the bottom. The lineations are narrow (10–25 m apart) and have no detectable relief in troughs and wider (to 75 m apart) and higher (to 1.5 m high) on ridges, especially second-order ridges of fine sand. Also revealed are wave ripple patterns and a pattern related to the outcropping of Pleistocene/Holocene units in trough bottoms and lower flanks.It is suggested that the first- and second-order systems developed during earlier stages of the Holocene transgression in response to a hydraulic regime of the inner shelf. The first-order system may have inherited some of its morphology from an older system, but did respond to a Holocene tidal regime adjacent to a major estuary. The second-order system developed in slightly deeper water, subsequent to the resumption of the transgression after the 37-m stillstand.The third-order lineations appear to be a response to the helical-flow structure within the flow field of a major shelf storm. Ridges of the central shelf may be maintained by alternate periods of oblique sweeping during storms, resulting in a net transport of fine sand out of the troughs and up on the ridges. Subsequent wave reworking returns the fine sand to the troughs.     

Wave-induced bottom currents on the outer shelf

Estimates of the wave-induced oscillatory bottom currents available to move sediment on the continental shelf have been made using the Pierson—Moskowitz wave spectrum; the oscillations are substantially stronger than had previously been thought. Near a shelf edge with canyons the wave-induced currents will be locally enhanced by wave refraction by about four times under certain conditions for idealized topography. Local sediment-transport rates will be greater and will occur more frequently than had previously been supposed.     

Grain size variation across sand ridges,New Jersey continental shelf

Cross-ridge grain size profiles were determined for a nearshore and an offshore sand ridge on the New Jersey shelf. On both, the coarset sands occur on the landward flank. The range and rate of change in grain size with horizontal distance are greatest on the nearshore ridge. The grain size gradient is believed to be a response of the bottom to the cross-ridge shear stress distribution during storm flows. The difference between the two ridges may be a consequence of the relict origin of the offshore ridge versus a modern origin for the nearshore ridge, or to the different flow climates of the nearshore and offshore environments.     

Late Quaternary transgressive erosion and deposition in a modern epicontinental shelf: The Adriatic semienclosed basin

The Adriatic Sea is a modern epicontinental basin where the late Quaternary transgressive systems tract shows substantial variations within two contrasting shelf domains, separated by a 250-m-deep remnant basin: a lowgradient shelf in the north, and a steeper margin in the south. Four differentiated sedimentary responses reflect contrasting physiographic domains and differences in the ratio between oceanographic regime and sediment input during relative sea-level rise. The progressive widening of the Adriatic epicontinental shelf, up to seven times its low-stand extent, also determines variations in the style of transgressive deposition by controlling major changes in oceanographic circulation.     

Recent sedimentation and sedimentary budgets on the western Iberian shelf

Thus study is based on 31 box-core samples collected mainly from the mud-patches of the Galician and North-Portuguese shelf. Each of these cores, some decimetres in length, have been described, radiographied and sampled each centimetre for grain-size analysis and measurement of the particulate organic carbon (C_org) content. ²¹⁰Pb excess profiles have been measured using gamma spectrometry, which, combined with the analysis of the vertical variations of the sedimentary facies, have allowed estimates of sedimentation rates. These rates were found to vary between 0.05 and 0.40 cm yr⁻¹. Considering the latitudinal variation of sedimentation rates around the 100m contour, we note that the maximum rates occurred off the Douro estuary in the southern sector and also near the Galician rias to the north. The organic content (C_org) of sediments varied between 0.4 and 2%. Latitudinally C_org showed a decrease from the South to the North. These results coupled with the sedimentary map (see Dias, Gonzalez, Garcia & Diaz del Rio, 2001) have permitted us to establish the detrital mass budget and an inventory of C_org burial.     

Correlation of side-scan backscatter intensity with grain-size distribution of shelf sediments, New Jersey margin

The dependence of acoustic backscatter variations on sediment grain-size distribution is examined using a combined analysis of 95 kHz side-scan data and approx. 300 grab samples from the New Jersey margin. We show that, under well-controlled circumstances, an unprecedented degree of correlation can be obtained between backscatter intensity and mean grain size. However, backscatter is disproportionately affected by larger grain sizes. Addition of just a few extra weight percentage of the larger grain sizes (>4 mm), which in our study is typically represented by more abundant shell hash among the older sediments at greater water depths, can completely degrade this correlation. Received: 29 September 1999 / Revision accepted: 14 February 2000     

Tracking the last sea-level cycle: seafloor morphology and shallow stratigraphy of the latest Quaternary New Jersey middle continental shelf

Seafloor geomorphology and surficial stratigraphy of the New Jersey middle continental shelf provide a detailed record of sea-level change during the last advance and retreat of the Laurentide ice sheet (120 kyr B.P. to Present). A NW–SE-oriented corridor on the middle shelf between water depths of 40 m (the mid-shelf “paleo-shore”) and 100 m (the Franklin “paleo-shore”) encompasses 500 line-km of 2D Huntec boomer profiles (500–3500 Hz), an embedded 4.6 km² 3D volume, and a 490 km² swath bathymetry map. We use these data to develop a relative stratigraphy. Core samples from published studies also provide some chronological and sedimentological constraints on the upper <5 m of the stratigraphic succession.The following stratigraphic units and surfaces occur (from bottom to top): (1) “R”, a high-amplitude reflection that separates sediment >46.5 kyr old (by AMS ¹⁴C dating) from overlying sediment wedges; (2) the outer shelf wedge, a marine unit up to 50 m thick that onlaps “R”; (3) “Channels”, a reflection sub-parallel to the seafloor that incises “R”, and appears as a dendritic system of channels in map view; (4) “Channels” fill, the upper portion of which is sampled and known to represent deepening-upward marine sediments 12.3 kyr in age; (5) the “T” horizon, a seismically discontinuous surface that caps “Channels” fill; (6) oblique ridge deposits, coarse-grained shelly units comprised of km-scale, shallow shelf bedforms; and (7) ribbon-floored swales, bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits.We interpret this succession of features in light of a global eustatic sea-level curve and the consequent migration of the coastline across the middle shelf during the last 120 kyr. The morphology of the New Jersey middle shelf shows a discrete sequence of stratigraphic elements, and reflects the pulsed episodicity of the last sea-level cycle. “R” is a complicated marine/non-marine erosional surface formed during the last regression, while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum Wisconsin lowstand (i.e., marine oxygen isotope stage 3.1). “Channels” is a widespread fluvial subarial erosion surface formed at the late Wisconsin glacial maximum 22 kyr B.P. The shoreline migrated back across the mid-shelf corridor non-uniformly during the period represented by “Channels” fill. Oblique ridges are relict features on the New Jersey middle shelf, while the ribbon-floored swales represent modern shelf erosion. There is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession.     

Sediment distribution and transport processes on the outer continental shelf of the Hebridean Sea

The major topographical features of the study area within the Hebridean Sea consist of two submarine banks lying at the edge of the outer continental shelf, an offshore island slope surrounding the Island of Barra-Head, a basin crossing the area in a SSE—NNW direction, and a deep trench.The present investigation suggests that the area was covered with glacial drifts and morainic accumulations during the Pleistocene and that during a subsequent low sea level the drifts were reworked around the topographically high areas. Much of the clay material has been transported to and deposited in regions off the continental slope.Recent conditions are represented by an erosional and depositional environment. The erosional environment is represented by the summit and leeward side of the two banks and the offshore island slope. In these areas tidal-induced currents and south-westerly swells continue to sweep the remaining sand, resulting in the formation of lag gravel deposits. The depositional environment in the basin results in homogeneous sand cover.     

Pliocene to Recent shallow-water contourite deposits on the shelf and shelf edge off south-western Mallorca, Spain

This paper presents evidence for the presence of shallow-water contourite drifts on the south-western shelf and shelf edge off Mallorca in water depths between 150 and 275?m. These are called the Mallorca contourite depositional system (CDS). The elongate-mounded shallow-water CDS in this area is ascribed to an offshoot of the Balearic Current, which flows north to south through the Mallorca Channel as part of the overall thermohaline circulation in the Mediterranean Sea. Drift geometry suggests that the north?Csouth current is deflected into an east?Cwest flow pattern by interaction with a marked seafloor bathymetry, associated with major fault displacement. Four seismic units separated by three prominent discontinuities can be identified. The three internal discontinuities are correlated to large-scale basin-wide events: the lower Pliocene revolution (4.2?Ma), the upper Pliocene revolution (2.4?Ma) and the mid-Pleistocene revolution (0.9?Ma). The Plio-Quaternary succession has been deposited on top of a Miocene reef, which serves as an acoustic basement and is affected by a large fault, offsetting the basement on average by 150?m. Marked erosional features throughout and further incision of the Sant Jordi Channel along the basement fault in the Pleistocene deposits indicate stronger currents in this period. The Pleistocene deposits also show a pronounced cyclicity, which is tentatively ascribed to climatic variations and the effects of eustatic sea-level fluctuation over the south-western Mallorca shelf at that time.     

Characteristics of outer shelf water in the East China Sea

Water mass properties along cross-sections of the Kuroshio in the East China Sea (ECS) are investigated in detail. We used temperature, salinity and dissolved oxygen data from 2000 and 2002, together with historical temperature and salinity data from 1987 to 2004. Water properties were divided into two groups: high and low salinities or oxygen at temperatures warmer than 15 and 12 °C, respectively. We found the existence of outer shelf water W2, as defined by clear modes in frequency distributions of salinity and oxygen within various temperature segments. The outer shelf water was different from both Kuroshio Tropical Water (KTW) and coastal water. We mapped horizontal and vertical distributions of W2, along with W1 and KTW. The outer shelf water was distributed with density σ _t = 22.5–25.5 over a relatively broad area, from the outer continental shelf to the continental slope, particularly in autumn. Vertical distribution of the water suggests that W2 spread from the outer shelf to just the shelf side of the Kuroshio Current velocity maximum. Seasonal variations are examined with historical data along PN section over 17 years, and suggest that the appearance of W2 is distinct in summer and autumn. By comparing temperature–salinity (T–S) diagrams from Taiwan Strait and east of Taiwan, the outer shelf water (W2) originates from South China Sea Tropical Water (SCSTW), as suggested by Chen, J Geophys Res 110:C05012 (2005). The present study of the ECS clearly shows that SCSTW is transported along the east coast of Taiwan or through the Taiwan Strait into the ECS. It then spreads over a relatively wide area from the outer shelf to just the shelf side of the Kuroshio axis, and there is some horizontal mixing between SCSTW and KTW around the shelf break.     

Late Quaternary sediments on the outer shelf of the Korea Strait and their paleoceanographic implications

Sedimentological and micropaleontological characteristics of core sediments from the outer shelf of the Korea Strait, which connects the northern East China Sea and the East Sea (Sea of Japan), were investigated to elucidate the paleoceanographic environment, especially the timing of the Kuroshio inflow, since the last glacial maximum. The core sediments, containing continuous records of the last 15,000 years, are characterized by a relatively high mud content (more than 50%, on average) and well-developed tide-influenced sedimentary structures. Their mineralogy suggests that the material originated from the paleo-Nakdong River system, which extended across the shelf of the Korea Strait during low sea-level periods. Planktonic foraminifers reveal a series of well-defined changes in paleoceanographic conditions during the late Pleistocene–Holocene. Down-core variations in the abundance of four foraminiferal assemblages, i.e., cold, coastal, tropical–subtropical, and Kuroshio water groups comprising characteristic planktonic species, suggest the occurrence of a distinct paleoenvironmental change in the surface water at 7,000 years b.p., i.e., from 15,000 to 7,000 years b.p., the area was influenced by coastal waters whereas since ca. 7,000 years b.p., it has been under the influence of open-sea water related to the Kuroshio Current flow, associated with both higher temperature and higher salinity. In particular, Pulleniatina obliquiloculata increased markedly in abundance at this time, documenting the inflow of the Kuroshio into the study area. These data indicate that the coastal water stage terminated at ca. 7,000 years b.p. when the warm Kuroshio and its major branch, the Tsushima Current, began to flow into the East Sea, as is the case today. The intrusion of the Tsushima Current through the Korea Strait after ca. 7,000 years b.p. resulted in abrupt changes in sedimentation rates and a dramatic increase in abundance of the Kuroshio indicator species, P. obliquiloculata.     

Possible influence of salinity and temperature on modern shelf carbonate sedimentation

In modern, marine, carbonate sands from shelf areas between the equator and latitudes 60°S and 60°N several major grain associations can be distinguished.On open shelves (< 100 m water depth) there are two skeletal grain associations. One (chlorozoan) is virtually restricted to warm, tropical waters; the other (foramol) characterizes temperate waters but also extends into the tropics. The distribution of these two associations cannot be explained in terms of water temperature alone: salinity is suspected as being a further controlling factor. Indeed, a third skeletal association (chloralgal) appears to be characteristic of areas where salinity is higher than on open shelves.Non-skeletal grains, where present, can be grouped into two associations. In one, pellets are the only non-skeletal grains represented; in the other, ooliths and/or aggregate grains are also present. These non-skeletal associations are restricted to relatively warm waters, but temperature does not determine which one of the associations develops. Again, salinity seems important.As both salinity and temperature apparently influence the grain associations, an attempt is made to present the relationships diagrammatically. By using graph pairs of “maximum temperature/minimum salinity” and “minimum temperature/maximum salinity” (named S.T.A.R. diagrams after Salinity Temperature Annual Ranges), the various grain associations can be classed into separate salinity/temperature fields.Salinity and temperature often seem to have a mutual “compensating” effect. For example, even at high temperatures the chlorozoan association does not develop if the salinity falls below a certain value, but it develops at relatively low temperatures when salinity is sufficiently high.This “compensation” effect also appears on the S.T.A.R. diagram for non-skeletal associations. More striking here, however, is a relationship suggesting that development of the oolith/aggregate association is strongly dependent on salinity.Carbonate muds are not shown on the S.T.A.R. diagrams, but an attempt is made to assimilate them into the model.The S.T.A.R. diagrams have a predictive value. In principle, given salinity and temperature values for an area, the grain associations can be predicted. In fact, the prediction is one of “potential”, i.e. that which is to be expected provided any other necessary environmental conditions are satisfied. Predictions are presented for the shelves of an ideal ocean and of present-day oceans and seas. The S.T.A.R. diagrams thus provide the basis for a tentative global model of present-day shelf carbonate sedimentation.The special problems of land-locked seas are discussed with reference to the Mediterranean Sea and the Persian Gulf. Predictions are presented.To illustrate the possibilities of the S.T.A.R. diagram technique, an attempt at detailed prediction is given for an area — the Gulf of Batabano, Cuba — where the sediments are known and predictions can be checked.In conclusion, the problems inherent in applying the model to ancient sedimentary systems are briefly discussed.     

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.     

Spatial and temporal variation in surfclam (Spisula solidissima) larval supply and settlement on the New Jersey inner shelf during summer upwelling and downwelling

To examine the relationship between near-bottom larval surfclam concentrations and surfclam settlement at an inner continental shelf site off New Jersey (USA), four consecutive sets of settlement experiments were carried out at three stations at the Long-term Ecosystem Observatory (LEO-15) from 14 to 31 July 1997 during upwelling and downwelling. Two inshore stations were on the landward and seaward sides of Beach Haven Ridge at ∼12 m depth, and a third station was 8 km further offshore at ∼20 m depth. In each experiment, four replicate trays of azoic sand from Beach Haven Ridge were placed flush with the seafloor and exposed for 3–7 days. Larval surfclam concentrations were measured every 4 h at 1 m above the bottom (mab) using Moored, Automated, Serial Zooplankton Pumps at the three stations. At all three stations, larval surfclam concentrations (1 mab) were low during upwelling, and higher during and after downwelling. Pulses of highest larval surfclam concentrations coincided with the initial arrival of downwelled warm water. In addition, larval surfclam concentrations were higher at the two inshore stations than at the offshore station. Larval surfclam settlement in the trays was higher during and following downwelling than during upwelling at one inshore station and at the offshore station. At the other inshore station (landward of Beach Haven Ridge), surfclam settlement did not increase during and following downwelling. Overall, surfclam settlement was higher inshore than offshore. The results indicate that spatial and temporal variation in larval surfclam supply was controlled by upwelling and downwelling circulation and that surfclam settlement was influenced by larval supply. Bottom flows across Beach Haven Ridge during a storm may have reduced larval surfclam settlement on the upcurrent side of the ridge, affecting initial densities on a small (∼1 km) scale.     

海底扇对外大陆架划界的潜在影响

自《联合国海洋法公约》1994年生效以来,外大陆架划界工作引起了地质、法律等多学科研究者的共同关注。为论证海底扇这一特殊海底地形在外大陆架划界工作中可能产生的影响,通过对全球典型的10个海底扇共同特征的统计,运用CARIS LOTS 4.0划界软件,对《联合国海洋法公约》第76条中2大重要地质因素——大陆坡脚位置和沉积物厚度进行了分析和研究,对孟加拉海底扇、蒙特利尔海底扇及印度海底扇3个全球典型的海底扇进行了外大陆架区域的划界分析,探讨了海底扇的发育及其对地貌的改造作用,以及对划界结果可能造成的影响。结果显示,影响海底扇发育的3大主要因素——构造环境、沉积物供给和海平面变化,使大陆坡脚位置的确定成为大陆架划界工作中面临的首要且关键的问题,对外大陆架的划界结果有潜在影响。而海底扇复杂的地质背景和特征,无疑对划界时使用《联合国海洋法公约》中的规定条款提出了新的挑战。在考虑海底扇影响的前提之下,陆坡坡脚位置会因其地形的变化向前推进或后退,峡谷和水道的分布对选区剖面确定最大坡折点的位置也会带来较大影响。因此,在外大陆架划界工作中,不但需要考虑《联合国海洋法公约》中的法律条款,海底扇及影响其形态、分布等3大重要因素的规律性,也是未来划界工作中需要进一步深入研究的重要问题。     

Mississippi-Alabama outer continental shelf topographic features formed during the late Pleistocene-Holocene transgression

A geophysical survey of the outer Missisippi-Alabama continental shelf revealed three types of topographic features: 1) reef-like mounds (RLM), 2) isobath-parallel ridges, and 3) shallow depressions. RLM appear to be bioherms. Their bases cluster in two isobath bands, 105 to 120 m and 74 to 82 m, implying the late Pleistocene-Holocene transgression was nonuniform. The deeper RLM probably formed during a slow sea level rise at the beginning of the deglaciation. An 11 ka BP C¹⁴ age was obtained from the summit of one, at a depth of 90 m. Depth relationships suggest the shallower RLM and most ridges formed about the time of the mid-deglacial Younger Dryas cooling episode.