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1.
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.  相似文献   

2.
Fine sediment dynamics were recorded in February 2007 in coastal waters of the Great Barrier Reef during a moderate flood of the Tully River. An estuarine circulation prevailed on the inner continental shelf with a surface seaward velocity peaking at 0.1 m s−1 and a near-bottom landward flow peaking at 0.05 m s−1. Much of the riverine mud originating from eroded soils was exported onto a 10 km wide coastal strip during the rising stage of the river flood in the first flush. In coastal waters, suspended sediment concentration peaked at 0.2 kg m−3 near the surface and 0.4 kg m−3 at 10 m depth during calm weather, and 0.5 kg m−3 near the surface and 2 kg m−3 at 10 m depth during strong winds when bottom sediment was resuspended. Diurnal irradiance at 4 m depth was almost zero for 10 days. The sedimentation rate averaged 254 (±33) g m−2 d−1 over the 28-day study period, and concentrations of dissolved and particulate nutrients originating from the river were high. The observed low irradiance would have prevented coral photosynthesis, while the sedimentation rate would have been lethal to some juvenile corals. The mud may ultimately be minnowed out over long periods, however, flushing of the mud occurs at time scales much longer than the flood event and the mud is likely to affect coral physiology for significant periods after the flood has subsided. The data show the need to better control erosion on farmed land for the conservation of coral reefs on the inner shelf of the Great Barrier Reef.  相似文献   

3.
Late Cenozoic seismic stratigraphy of the Vøring continental margin has been studied in detail, with emphasis on the geological development of the Naust Formation deposited during the last 3 million years. The Kai Formation (15–3 Ma) comprises mainly biogenic ooze deposited in the Møre and Vøring Basins. In Naust time, there was a marked increase in supply of sediments from the inner shelf areas and the western part of the Scandinavian mountain range, and glaciers expanded to the shelf and reached the shelf edge several times during the last 1.5–2 million years. During early to mid Naust time the shelf was widened by westerly prograding sediment units, but for a long period the shallowest part of the Helland-Hansen Arch (HHA) formed a barrier preventing glacially derived debris from being distributed farther west. West of the HHA, mainly stratified marine and glacimarine sediments were deposited. A substantial part of these sediments were transported by the north-flowing Norwegian Atlantic Current, which redistributed suspended particles from ice streams, rivers, coastal erosion and seabed winnowing. After burial of the crest of the HHA at c. 0.5 Ma, glacial debris and slide deposits were also deposited west of this high. In the north, massive units of glacial debris were distributed beyond the crest of the HHA, also in mid Naust time, thinning westwards and interfingering with fine-grained sediments on the lower slope. The Sklinnadjupet Slide, inferred to be c. 250,000 years old, corresponds in age with an earlier huge slide in the Storegga area. An elongated area of uneven seabed topography previously interpreted as diapirs (Vigrid diapirs) on the slope west of the HHA is shown to be formed by ooze eruption from the crest of the arch and submarine sliding.  相似文献   

4.
The oceanographic conditions of the Mackenzie River plume in the Arctic Ocean were examined during a 12-day period in August 2007. Field observations in the river channel and the delta region (2–6 m depth), ship-based observations on the shelf and satellite observations of sea surface temperatures indicate that movements of plume density fronts cause changes in water temperatures of over 10  C over a few days. We used a 1D model to compare the strength of stratification versus surface wind stress, and a 3D numerical model to simulate the plume motions under forcing from the river flows, local wind and water level variations from tides and wind-driven surge. The results indicate that the coastal region is stratified with a ∼2 m thick surface plume even in water depths of 3–4 m, resulting in strong vertical variation of horizontal currents. Moderate easterly winds of 5–10 m/s are sufficient to induce offshore transport of the surface plume and onshore transport of the deeper shelf water, leading to large fluctuations in temperature and salinity in the coastal region. This study examined a period of offshore transport and mean water level set-down, and indicates the rapid response of the plume to wind over the shallow delta.  相似文献   

5.
《Marine Geology》2005,216(4):239-247
The Ayeyarwady continental shelf is a complex sedimentary system characterized by large sediment influx (> 360 million ton/yr), a wide shelf (> 170 km), a strong tidal regime (7 m maximum tidal range), and incised by the Martaban Canyon. Grain size distribution on the Ayeyarwady shelf reveals three distinct areas in terms of sediment texture (i) a near-shore mud belt in the Gulf of Martaban and adjacent inner shelf (ii) outer shelf relict sands and (iii) mixed sediments with varying proportions of relict sand and modern mud in the Martaban Canyon. The bulk of the terrigenous sediment discharged by the Ayeyarwady River is displaced eastwards by a combination of tidal currents and clockwise flowing SW monsoon current and deposited in the Gulf of Martaban resulting in shoaling of its water depths. Part of the sediment discharge reaches the deep Andaman Sea via the Martaban Canyon and the rest is transported westward into the Bay of Bengal by the counter-clockwise flowing NE monsoon currents.  相似文献   

6.
Seismic characterization of Eocene-Oligocene heterozoan carbonate strata from the Browse Basin, Northwest Shelf of Australia, defines marked progradation of nearly 10 km. Stratal terminations and stacking subdivide the succession into mappable seismic units. Stratal architecture and seismic geomorphology varies systematically through the succession.Individual surfaces, discerned by toplap, onlap, and truncation, outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18°. Sigmoidal clinoforms can include aggradation in excess of ∼200 m, prograde more than 500 m, and have slopes characterized by inclined, wavy to discontinuous reflectors that represent ubiquitous gullies and channels. In contrast, the overlying tangential oblique clinoforms include downstepped shelf margins, limited on-shelf aggradation (<100 m) and toplap, subdued progradation (<500 m), and continuous parallel inclined reflectors on the slope. Wedges of basinally restricted reflectors at toe of slope onlap surfaces of pronounced erosional truncation or syndepositional structural modification. The succession includes repeated patterns of seismic units that onlap, aggrade, and prograde, interpreted to represent sequence sets and composite sequences.The associations of shelf aggradation, shelf-margin progradation, and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation. This distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded, perhaps enhanced by downwelling. Comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture, interpreted to be driven by distinct characteristics of heterozoan sedimentary systems.  相似文献   

7.
A high-resolution marine geophysical study was conducted during October-November 2006 in the northern Gulf of Aqaba/Eilat, providing the first multibeam imaging of the seafloor across the entire gulf head spanning both Israeli and Jordanian territorial waters. Analyses of the seafloor morphology show that the gulf head can be subdivided into the Eilat and Aqaba subbasins separated by the north-south-trending Ayla high. The Aqaba submarine basin appears starved of sediment supply, apparently causing erosion and a landward retreat of the shelf edge. Along the eastern border of this subbasin, the shelf is largely absent and its margin is influenced by the Aqaba Fault zone that forms a steep slope partially covered by sedimentary fan deltas from the adjacent ephemeral drainages. The Eilat subbasin, west of the Ayla high, receives a large amount of sediment derived from the extensive drainage basins of the Arava Valley (Wadi ’Arabah) and Yutim River to the north–northeast. These sediments and those entering from canyons on the south-western border of this subbasin are transported to the deep basin by turbidity currents and gravity slides, forming the Arava submarine fan. Large detached blocks and collapsed walls of submarine canyons and the western gulf margin indicate that mass wasting may be triggered by seismic activity. Seafloor lineaments defined by slope gradient analyses suggest that the Eilat Canyon and the boundaries of the Ayla high align along north- to northwest-striking fault systems—the Evrona Fault zone to the west and the Ayla Fault zone to the east. The shelf–slope break that lies along the 100 m isobath in the Eilat subbasin, and shallower (70–80 m isobaths) in the Aqaba subbasin, is offset by approx. 150 m along the eastern edge of the Ayla high. This offset might be the result of horizontal and vertical movements along what we call the Ayla Fault on the east side of the structure. Remnants of two marine terraces at 100 m and approx. 150 m water depths line the southwest margin of the gulf. These terraces are truncated by faulting along their northern end. Fossil coral reefs, which have a similar morphological appearance to the present-day, basin margin reefs, crop out along these deeper submarine terraces and along the shelf–slope break. One fossil reef is exposed on the shelf across the Ayla high at about 60–63 m water depth but is either covered or eroded in the adjacent subbasins. The offshore extension of the Evrona Fault offsets a fossil reef along the shelf and extends south of the canyon to linear fractures on the deep basin floor.  相似文献   

8.
Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.  相似文献   

9.
《Marine Geology》2005,214(4):411-430
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.  相似文献   

10.
《Marine Geology》2001,172(3-4):331-358
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.  相似文献   

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