Adjacent Continental Shelf Research Articles

Subsurface Geology of Upper Tertiary and Quaternary Deposits, Coastal Louisiana and Adjacent Continental Shelf: ABSTRACT

Subsurface Geology of Upper Tertiary and Quaternary Deposits, Coastal Louisiana and Adjacent Continental Shelf: ABSTRACT

Late Quaternary development of left-bank-dominant levees in the Bounty Trough, New Zealand

The Bounty Trough encompasses an extensive submarine channel system that arises on the Otago fan-complex off the South Island, New Zealand, and extends 1000 km eastward to feed a large fan on the 4500 m-deep Pacific Ocean floor. Channels are bordered by prominent levees which, on the steep Otago fans, are mainly narrow sandy ridges. By comparison, the gently sloping floor of the trough supports broad, low relief, muddy levees. In most cases, levees display preferential development on the left side of the channel (facing down-channel). We argue this preference results not only from the Coriolis deflection of channel-full turbidity currents, but also from bottom currents and centrifugal forces operating at channel bends. Basement relief and locally high sedimentation rates at channel confluences may also influence levee height. Piston cores reveal two distinct phases of levee growth related to Late Quaternary climatic cycles. During glacial times rivers discharged directly into the Bounty Trough. Turbidity currents travelled and overspilled the entire length of the Bounty channels thereby producing the main phase of asymmetric levee growth and terrigenous deposition. Such turbidite deposition was punctuated by bouts of hemipelagic sedimentation. In interglacial periods river-borne sediment was diverted away from the trough which became the site of predominantly pelagic sedimentation represented by a drape of foraminiferal-nannoplankton ooze over channels and levees alike. However, levees on the Otago fan-complex still received terrigenous sediment presumably reworked from the adjacent continental shelf.

Late Quaternary sedimentation in Baffin Bay

Baffin Bay is a small ocean basin that connects the Arctic and Atlantic oceans. The adjacent continental shelves have been extensively reworked during Quaternary glaciation. The shelf break generally lies between 200 and 500 m. The continental slope passes directly into the abyssal plain of Baffin Bay basin without any major submarine canyon – deep-sea fan system being present, except for a large smooth sediment apron in northern Baffin Bay.On the basis of over 50 piston cores, six Quaternary sediment facies are distinguished from detrital mineralogy (reflected in colour) and sediment texture. Facies A, B, and C are predominantly ice-rafted or are debris flow deposits, each with a distinct mineralogy. Facies D is turbidites and bottom-current sorted sands, silts, and muds. Facies E is hemipelagic sediment. Facies F consists of sediments ranging from slumps, through debris flow deposits, to fine-grained turbidites, with a distinctive provenance in northern Baffin Bay.These sediment facies appear to be partly controlled by glacial conditions. Hemipelagic facies E predominates during the present interglacial. During glacial stages, facies D turbidites were deposited. They resulted from slumping of proglacial sediments on the continental slopes off Greenland and Baffin Island. Facies C and F occurred on the continental slopes at these times. Ice-rafted facies A and B predominate at several horizons, reflecting a rapid breakup of ice shelves in northern Baffin Bay and increased rates of iceberg melting within the Bay. Overall sedimentation rates are relatively low, reflecting dry-base ice sheets in source areas.Deep-sea channel systems floored by sorted coarse sediments and bounded by muddy levees are absent in Baffin Bay, in contrast to mid-latitude glaciated continental margins off eastern Canada. These channel systems are the result of melting of wet-base glaciers, which provide a localized supply of sediment that is sorted by ice margin processes. In Baffin Bay, most glacial sediments are derived by calving of icebergs, probably from dry-base glaciers. Sediments are gradually released over large areas as the bergs melt, and are subsequently redistributed by debris flows.

Tides of Mississippi Sound and the adjacent continental shelf

Hourly water level and current data were collected in Mississippi Sound and the adjacent continental shelf from April 1980 to October 1981. Amplitudes and epochs of the principal tidal constituents were calculated for each location. The data were used to construct cotidal charts and describe characteristics of the predominant diurnal tide. The pattern of the diurnal partial tides on the shelf is explained by reflection of first-class waves propagating normal to the coast. The period of the K 1 tide equals the inertial period just offshore of Mississippi Sound. The resulting diurnal shelf currents are Sverdrup waves with typical speeds of 5–10 cm s −1. The tidal wave is transformed into a rectilinear shallow water wave as it enters the tidal inlets of Mississippi Sound. The form number of the shelf currents changes from 8·5 to 2·1 in the inlets due to enhancement of the semidiurnal currents. This is a result of continuity constraints, which transform currents in the inlets. Within Mississippi Sound, the tidal waves shoal and become increasingly progressive in response to bottom friction.

Along-shelf evolution and sea levels across the continental slope

If wind-stress or a horizontal oceanic density gradient acts over an ocean basin with an adjacent continental shelf and slope, sea-surface slopes and currents are set up along the shelf and slope with a return flow in the ocean. The currents evolve from zero at blocked ends of the shelf and basin. Such evolution is essentially barotropic (even for baroclinic forcing) and is relevant to all flow adjustments after longshore changes of depth profile or forcing. The distance over which this evolution takes place is investigated analytically for simple geometries, and numerically for a range of shelf, slope and ocean widths, shelf/ocean depth ratios, frictional decay rates and oscillatory frequencies. A close correspondence is found with the decay distance (group velocity x decay time) for a lowest mode continental shelf wave, often exceeding 1000 km. This correspondence is used to interpret some published model calculations of shelf and slope currents or return flows resulting from wind-stress or alongshore pressure gradients. Where a slope current is evolving, coastal sea levels do not follow oceanic levels. Implications for coastal/oceanic level differences are discussed. Oceanic sea-level features of shorter scale than the above 1000 km (say) do not penetrate fully to the coast. However, coastal sea levels averaged around small islands without broad shelves well represent surrounding oceanic levels.

Weddell Fan and associated abyssal plain, Antarctica: Morphology, sediment processes, and factors influencing sediment supply

The newly discovered Weddell Fan, Antarctica, covers 0.75 million km2. The adjacent continental shelf is characterized by deep, rugged topography; the inner shelf is covered by a grounded polar ice sheet. The upper fan has numerous deep, V-shaped canyons that intersect a slope-base, leveed fan valley. Piston cores from the valley contain disorganized gravel grading upward into graded gravel and sand. Levee cores contain interbedded hemipelagic sediments and fine-grained turbidites. The lower fan is sand-rich. Sediment supply to the fan apparently occurred before development of glacial shelf topography and during a more temperate glacial setting.

The distribution and transportation of fine-grained sediments on the inner continental shelf off the Keum River estuary, Korea

A study of suspended particulate matter in the Keum estuary and the adjacent continental shelf, southeastern Yellow Sea, showed that most of the suspended matter (average amount of2.7 × 10 4 tons in October 1980) is derived from the Keum River and the major dispersal directions are westward and southwestward. Associated investigations of the clay mineral and trace element of the bottom sediment in the same study area confirmed that the bulk of fine-grained sediments derived from the Keum River and its estuary probably is dispersed essentially to the southwest, but indicated that less significant amounts are dispersed to the west or northwest.

The deep structure of the Moine Thrust, southwest of Shetland

The Caledonian orogenic front (the Moine thrust zone) has been described in detail from outcrops in the NW Highlands and Shetland (see [Flinn et al. 1979][1]; [McClay and Coward 1981][2]). Recently however, the use of deep seismic reflection profiling on the adjacent continental shelf has provided new information on the deep structure of the orogen ([Smythe et al. 1982][3]; [Brewer et al. 1983][4]). This note presents further evidence for the offshore position and orientation of the thrust plane as interpreted from seismic profiles in the vicinity of Foula. In the extreme NW of Shetland, Lewisian orthogneisses which crop out on North Roe and Ve Skerries belong to the Caledonian foreland (Fig. 1). These are separated from the younger metasediments of Shetland and Foula by a steeply (60°) east-dipping Caledonian thrust (the Wester Keolka shear), which is generally considered to be the northward extension of the Moine Thrust ([Pringle 1970][5]; [Flinn et al. 1979][1]). Further west, an area of strongly developed magnetic and gravity anomalies has been attributed by [Bott and Watts (1970)][6] to basement gneiss at or near the seabed (Fig. 1). A further prominent seabed feature is the Foula Ridge, which connects Foula and Ve Skerries (Fig. 1). This is composed of metamorphic rocks and has yielded samples of banded hornblendic gneisses and granulite ([May et al. 1984][7]). The youngest tectonic feature in the area is the Permo-Triassic West Fair Isle Basin which is preserved on the downthrown side of the Walls Boundary Fault (Fig. 1; Bott and Browitt . . . [1]: #ref-8 [2]: #ref-11 [3]: #ref-15 [4]: #ref-4 [5]: #ref-13 [6]: #ref-2 [7]: #ref-12

Probabilistic Estimates of Maximum Seismic Horizontal Ground Acceleration on Rock in Alaska and the Adjacent Continental Shelf

Estimates of ground motion hazard from earthquakes in Alaska and the adjacent continental shelf indicate that, for all the exposure times considered, the predicted values of peak acceleration are highest in the Gulf of Alaska and near the major active strike-slip faults of southern Alaska. The evaluations assume a Poisson model of earthquake occurrence and are based on seismic source zones delineated from regional geologic considerations and the historical record of earthquakes. Calculated peak acceleration values for a return period of 100 years range as high as 0.4 g in the Gulf of Alaska sector between Kodiak and Kayak Islands, are about 0.2 g near Anchorage, and 0.1 g near Fairbanks. Values for most of the rest of the state are estimated to be less than .04 g; however, most of the southern Alaska industrial and population base lies within the 0.2 g contour. For a return period of 500 years, peak accelerations are estimated as high as 0.8 g for parts of southeastern Alaska near the Fairweather fault, 0.6 g or greater for part of the Gulf of Alaska, and are about 0.45 g and 0.2 g, respectively, for the Anchorage and Fairbanks areas. Values of acceleration for a return period of 2,500 years exceed 0.6 g for much of southern Alaska and are 0.8 g or greater near the Fairweather and central Denali faults; estimated values are 0.1 g or greater for nearly all of onshore Alaska and for the continental shelf areas of the Bering Sea, Norton and Kotzebue Sounds, southern Chukchi Sea and southeastern Beaufort Sea.

Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea

The vertical distribution of macrobenthos and sedimentary structures in near-surface sediments of the East China Sea off the Changjiang has been assessed from analyses of boxcores. The geographic distribution of macrobenthos across the continental shelf reflects sediment type and sedimentation rate. In the rapidly accumulating silty-clays of the submarine delta, sparse populations of small polychaetes predominate. As sediment accumulation rate declines away from the river mouth, denser populations of more deeply burrowing polychaetes, crabs, and ophuroids occur. On the central and outer shelf, sparse populations of surface dwellers relect the sandy, mobile sediment conditions. Based on X-radiographs, five sedimentary facies are identified based on physical and biogenic sedimentary structures in the top 25 cm of sediment. These sedimentary fabrics reflect (1 and 2) high and low rates of accumulation on the submarine delta, in which physical and biogenic structures, respectively, predominate; (3) dynamic sands across the shelf; (4) a muddy, low-depositional environment south of Cheju Island; and (5) a shelf-break regime under the influence of the Kuroshio Current.

Transport and deposition of river sediment in the Changjiang estuary and adjacent continental shelf

Hydrographic observations, suspended-sediment measurements, and historical data indicate transport paths and sinks for sediment within the Changjiang estuary and adjacent shelf. Most of the sediment transported by the Changjiang to the ocean is carried through the North Channel of the South Branch. Sediment transport is directly related to river stage, but tidal phase (spring vs neap tides) also plays an important role. An estimated 40% of the sediment load in the river is deposited in the estuary, mostly in and seaward of the South Channel. The remaining sediment is deposited directly offshore during flood seasons, but much is resuspended and carried southward by subsequent winter storms.

Regional Variability of Beach and Foredune Characteristics Along the Texas Gulf Coast Barrier System

ABSTRACT A regional analysis of adjacent beach and foredune variability was conducted along the Texas barrier system. Evaluated parameters include beach morphology, dune internal structures, and comparative textural and heavy mineral characteristics. The most variable element of beach morphology is backshore width, which reflects both long-term erosional trends and local anthropogenic effects. Foredune-ridge height increases southward and is climatically controlled. Dune cross-bedding is highly variable in terms of both dip angles and azimuth distributions, indicating the presence of polygenetic cross-strata. The azimuth distributions are generally polymodal and do not demonstrate a direct relationship to prevailing wind direction. The azimuth distribution modes indicate the presence of both p ramidal dune structures generated by seasonal wind variations, and storm-related structures resulting from multiple episodes of ridge erosion and rebuilding phases. Regional constraints on foredune-ridge orientation imposed by the coastline have some polarizing influence on the cross-bedding azimuth distributions. Comparative textural characteristics of adjacent beach and foredune sediments show both local variations and regional trends. Beach textures are greatly influenced by shell admixtures and show relatively high local variability. In contrast, dune textures are not significantly influenced by shell admixtures and show a low degree of local variability, thus allowing the detection of regional trends. Foredune deposits show a northward reduction in grain size and improved sorting; these trends result mainly from a regional reduction in energy level and competency range of the eolian regime, but they also may be partly attributed to provenance. Comparative beach and foredune heavy mineral assemblages are variable in terms of total concentrations, grain morphology, and grain density. The foredunes generally contain higher total heavy mineral concentrations and higher proportions of dense opaque minerals than the beach population, whereas no apparent difference occurs in the proportion of elongate minerals. Beach and foredune heavy mineral assemblages show regional trends in total concentrations and opaque mineral content that correlate with similar trends on the adjacent continental shelf, indicating a constancy of provenance and regional dispersal patterns during the Holocene transgression.

The “insulating” effect of a steep continental slope

Geochemical evidence shows that the midcontinental slope off the Mid‐Atlantic Bight constitutes an important sink for substances attached to fine particles. It is reasonable to infer that near‐bottom currents in this region are generally much weaker than those over the adjacent continental shelf, allowing the settling out of fine particles and their incorporation into permanent sediment. A partial dynamical explanation of this phenomenon may be sought in the “insulating” effect of a steep slope. At levels below the main thermocline the bottom pressure perturbation associated with steady flow is governed by a parabolic equation which expresses vorticity tendency balance between bottom stress curl and vortex stretching. A heat conduction analogy is appropriate, in which the friction coefficient corresponds to conductivity and the bottom slope corresponds to heat capacity. Over the continental slope the latter is large, suppressing pressure gradients originating from the shelf. Thus steady, near‐bottom currents over the middle slope are generally weak. Exceptions to this rule are locations subject to intense winter cooling and deep convection, which may generate strong thermohaline near‐bottom flow. Oscillatory motions of importance (which may be taken to be coastally trapped waves and topographic waves propagating shoreward from deep water) are weak partly because an efficient local generation mechanism is lacking. Disturbances originating from the Gulf Stream decay as the depth reduces below 3 km, by a mechanism the details of which are not clear at present. The findings in the Mid‐Atlantic Bight are likely to apply to other continental slopes distant from western boundary currents and not subject to intense winter cooling.

Bathyal ostracodes from the Florida-Hatteras slope, the Straits of Florida, and the Blake Plateau

Epibathyal ostracodes from the Florida-Hatteras slope, the Blake Plateau and the Straits of Florida were studied to determine the relationship of numerous genera and species to bottom-water environmental conditions such as dissolved oxygen and bottom-water temperatures. From a total of 100 samples, 44 samples evenly distributed between 200 and 1100 m water depth and having an average of 325 specimens were examined in detail. Using occurrence data from the adjacent continental shelf, carapace preservation, Rose Bengal staining and population data, indigenous death assemblages were distinguished from transported or reworked fossil specimens. The percent of transported specimens varied as follows: Blake Plateau < 1%; Straits of Florida 10–60%; Florida-Hatteras slope 1–15%. Indigenous death assemblages contained between 10 and 61 species per sample, averaging 33.3 species. Krithe, Argilloecia and Pseudocythere occur in > 90% of the samples and usually constitute 10 to 30% of each. Trachyleberidea, Bairdoppilata, Saida, Paranesidea, Ambocythere, Bythocypris, Cytherella, Bradleya, Henryhowella, and Polycopidae occur in 45 to 80% of the samples in varying percentages. The upper depth limits of 39 taxa occur at or just below the thermocline suggesting a relationship to temperature. Australoecia, Quasibuntonia, Cytheropteron, Ruggieriella, Saida, Ambocythere, Trachyleberidea, Macrocypris, Krithe, “Thalassocythere”, and Cytherella are most common or restricted to the O 2 minimum zone. Conversely, Anchistrocheles, Bradleya, Henryhowella, and Rockallia are most common below 750 m in well oxygenated water with temperatures below 8°C. The results show that: (1) ostracodes display a narrow depth zonation controlled by dissolved oxygen and water temperature; (2) species diversity is very high for a bathyal zone; (3) ostracodes can be used to identify the source of sediment that has been transported downslope; and (4) some taxa are useful in recognizing low oxygenated water in Cenozoic deposits.

Sediment dispersal from source to shelf on an active continental margin, S. Turkey

Provenance and sediment dispersal has been investigated in the eastern part of the southern Turkish coast and its adjacent continental shelf. The physiographic and geological setting make this area especially interesting for the study of post-orogenic (Molasse-type) sedimentation. The siliciclastic fractions of river, beach and shelf-surface sediments were examined. Grain-size frequency and statistical analyses showed that the sands of these environments can be distinguished by their textural parameters. The sediments are rich in heavy minerals such as olivine, pyroxenes, amphiboles and opaques. Combined light- and heavy-mineral data indicate that the siliciclastic detritus was derived largely from the ultramafic suites of the ophiolite complexes exposed in the hinterland. The mineral spectra of the river sands and the heavy-mineral provinces delineated along the coast faithfully reflect the various petrographic units of the source area. In the coastal region a northeasterly longshore current plays an important role in the sediment dispersal. Two types of surface sediments are present on the shelf: A fine-grained, mineralogically fairly homogeneous and a medium grained. The latter is thought to be the deltaic deposits of the ancestral Seyhan and Ceyhan rivers during the low sea-level stages of the Quaternary, probably indicating the position of the coastline at that time. The distribution patterns of the principal heavy minerals indicate the major point-sources of sediment supply onto the shelf and reveal intensive mixing and sorting processes. As a result, the mineral distribution along depositional strike is homogeneous whilst down-dip, well-defined, grain-size-related mineral zones have developed.

Circulation in the Hudson Shelf Valley: MESA physical oceanographic studies in New York Bight, 1

Over 900 days of current velocity data were obtained at mainly two locations in the inner and outer Hudson Shelf Valley (HSV). The large cross‐axis depth gradients in the HSV, together with the strong winter cyclones and the baroclinic density distribution over the shelf, are primarily responsible for the major circulation features observed in the valley. CSTD data from 12 cruises and meteorological data from JFK International Airport and an environmental buoy were collected concurrently with the current meter data. Although the mean cross‐shelf pressure gradient is generally seaward in the Middle Atlantic Bight, it is shoreward in the HSV below the level of the adjacent continental shelf (shelf horizon), thus imposing a bias toward upvalley flow. The average velocity below the surrounding shelf horizon in the HSV is upvalley or shoreward (west‐northwestward ≈ 290° T) in the range of 2–5 cm/s. The circulation in the HSV is seasonal and individual events can drastically alter the mean picture. The several day average upvalley flow can sometimes approach 20 cm/s when intense winter cyclones pass over the bight and can sometimes also be directed downvalley depending upon the path of the winter cyclone. A topographically controlled barotropic flow commonly opposes the dominant (southeast‐ward) wind direction even near the surface in the winter. In the context of circulation on the open shelf, upvalley (downvalley) flow events generated by winter cyclones are associated with reduced (enhanced) southwestward flow or flow reversals that are northeastward in the lower half of the water column at LTM, a typical mid/shelf site (Mayer et al., 1979). Current meter data suggest that whether or not reversals occur on the open shelf depends upon the interannual variability of the winter wind regime. Upvalley flow events are not confined only to the winter (unstratified) season but are stronger in the winter and can last for several days and longer. During the summer (stratified) season the maximum horizontal KE in the upper part of the water column shifts from the meteorological forcing band, characteristic of winter, to diurnal inertial and semidiurnal frequencies. In the diurnal band there appears to be a strong relationship between the diurnal wind and currents near the surface in the HSV as well as on the open shelf (LTM). The structure of the semidiurnal motions in the inner valley where the depth gradients are larger than in the outer valley has a significant depth dependence unlike most regions on the shelf, i.e., during all times of the year the semidiurnal tidal ellipse is anticlockwise in the lower 20 m of the water column.

The Southern part of the Armorican orogeny: A result of crustal shortening related to reactivation of a pre-Hercynian mafic belt during Carboniferous time

This synthesis of detailed geological and geophysical information relates deep crustal structure to deformation and the emplacement of granites in the South Armorican Massif and the adjacent continental shelf. Near the continental margin a zone of geophysical anomalies is regarded as a nearly complete line of suture between a southern continent and America. A major mafic body in the trace of the supposed suture in interpreted as a relic of probable oceanic crust which has apparently acted as a deflector of regional strain. It is proposed that late in the Hercynian collision history, during late Carboniferous time, compressional strain acted across the mafic body and can be directly related to generation of younger regional structures in the metasediments and to localization of granite emplacement.

The subtidal circulation in Delaware Bay and adjacent shelf waters

Surface and seabed drifters were used to study the subtidal circulation in Delaware Bay and over the adjacent continental shelf of the Middle Atlantic Bight. Over 7000 drifters were released in eight experiments during the year‐long study period. Velocity vector maps representing the study period mean fields for the surface and bottom confirmed the pattern of classical, two‐layer, estuarine flow, with surface water moving seaward from the Bay onto the shelf and bottom water landward over the shelf from at least as far as 40 km seaward of the Bay mouth. Within the Bay, bottom water moved laterally toward the nearest shore, diverging horizontally along a line roughly marking the deep channels. The surface speeds were about an order of magnitude larger than the bottom. Significant variations from this mean pattern were seen between the eight experiments. Return percentages for surface and bottom drifters correlated inversely over the eight experiments. Correlations with wind stress were above the level of significance, and winds explained about two‐thirds of the total variance. The strong coupling found between the subtidal circulation in the estuary and over the shelf is consistent with recent observational and modelling studies.

A Numerical Study of the Kuroshio South of Japan

Abstract A numerical model is used to study the interaction of the Kuroshio with boundary constraints south of Japan. The model consists of two layers with the lower layer inert, and is both nonlinear and quasi-geostrophic. The boundary constraints are highly idealized. The Kyushu Peninsula and its adjacent continental shelf have the form of a V-shaped wedge on an otherwise rectilinear coastline; the Izu Ridge is represented as a square island. Solutions almost always form a meander downstream from Kyushu. The dynamics of meander formation involve the interaction of a Rossby lee wave generated by the Kyushu Peninsula and a westward disturbance forced by the Izu Ridge. Three different quasi-steady paths have been identified. The first path has a meander between Kyushu and the Izu Ridge, passes to the north of the ridge, and is associated with a current of low-volume transport. The second path has a large meander between Kyushu and the Izu Ridge, passes to the south of the Izu Ridge, and always occurs when ...

A comparison of lignin and stable carbon isotope compositions in quaternary marine sediments

Organic matter in four Quaternary sediment cores from the Gulf of Mexico and one core from the Washington State coast have been analyzed for lignin and stable carbon isotope compositions. Holocene sequences of all five cores contain organic matter with high relative abundances of 13C ( δ 13 C = −19.0 to −22.5% versus PDB ) and low lignin concentrations, both of which are consistent with a marine origin. Distinctly lower 13C concentrations ( δ 13 C = −24.0 to −25.5% ) occur in underlying glacial-age sequences from four of the five cores, including the core from the Washington coast where such trends are previously unreported. Although the carbon isotopic compositions of these Pleistocene sediments are typical of predominantly land-derived organic matter, they contain only about 5% of the lignin found in modern sediments of similar δ 13 C from adjacent continental shelves. The lignin-poor organic matter in the glacial-age deposits appears to be either marine-derived or terrigenous material that likely was depleted in vascular plant debris at the time of deposition.