Adjacent Continental Shelf Research Articles

Sea-bird affinities for ocean and ice boundaries in the Antarctic

We have observed more sea birds near the Ross Sea continental slope and near ice than over the adjacent continental shelf, deep ocean, and ice-free regions. Bird concentrations near the continental margin occurred above th subsurface Antarctic Slope Front, defined by increased horizontal gradients in water temperature, salinity, density, chemistry, color, and transparency. A convergence between easterly and westerly currents near the continental shelf break may accumulate plankton and other positively bouyant material from oceanic gyres to the north and south. In addition, some sea-bird prey are believed to have life cycles adapted to the continental margin regime of upwelling deep water and sinking shelf water. Order-of-magnitude changes in bird abundance over relatively small distances indicate the importance of hydrographic and cryographic features to sea-bird distributions in the Southern Ocean.

The effect of continental shelves on tides

Coastal tides are influenced by several factors and one of the most important of these is the character of the adjacent continental shelf. A continental margin theory is derived and used to discuss several different aspects of the effect “smooth” continental shelves have on tides. The main results are as follows. (a) The theory suggests, in accordance with observations, that semi-diurnal tides should be amplified on wide shelves in mid and low latitudes, but that diurnal tides should not be amplified. (b) Continental shelf tidal resonance occurs when the shelf scale ( gα/( ω 2− f 2)( α=shelf bottom slope, ω = tidal frequency) is approximately equal to the shelf width. Theoretical arguments and observation can be used to show that shelf resonance occurs, for example, along sections of the northwest Australian shelf. (c) Given the easily obtained coastal tide, theory shows that tides over the continental shelf and slope can be approximately estimated analytically. Calculations using simple prediction formulae can be made on a hand calculator. Subject to some restrictions, a simple and inexpensive method is thus available for estimating barotropic tides on continental shelves. (d) An appropriate boundary condition for global numerical tidal models, which cannot resolve the continental margin region, is derived. For the diurnal tides, the boundary condition can be well approximated by an impermeable wall condition at the deep-sea continental slope boundary. For the semi-diurnal tides, the impermeable wall condition usually, but not always, suffices; it can break down on wide continental shelves.

Unusual Ponding of Sediments on Deep-Water Reef: ABSTRACT

The Florida Middle Ground is a deep-water coral reef on the outer continental shelf in the northeastern Gulf of Mexico. The reef consists of two parallel north-trending ridges, each about 50 km long and rising 11 m from the shelf to a depth of about 26 m. The ridges are separated by a broad, flat, sediment-filled valley about 8 km wide. The south end of the valley is partly occluded by irregular knolls. Station data (in transects) were collected over several seasons by Shipek grab, scuba team, and manned submersible. Detailed textural and constituent analyses of valley sediments reveal sandsized carbonate material unlike the finer sands and silts found on the adjacent continental shelf. The percentage of terrigenous material in the valley is substantially less than that o the surrounding shelf. Hurricanes, frequent storm fronts, the Gulf Loop Current, and semidiurnal tidal currents together comminute and erode ridge constituents. Those constituents transported between the ridges are mixed and trapped with shelf sediments. An accumulation of up to 5 m of sediment, over twice the thickness on the adjacent shelf, is ponded in the valley. End_of_Article - Last_Page 682------------

Recent Benthonic Foraminiferal Biofacies in the Arctic Ocean

Analysis of Recent foraminiferal distribution patterns for the Amerasian Basin and the adjacent continental shelves indicate 3 major biofacies: the Trochammina nana biofacies (17-350 m), the Cassidulina teretis biofacies (350-900 m) and the Stetsonia horvathi biofacies (below 900 m), with the T. nana and S. horvathi biofacies divided into subfacies. Water mass properties appear to play the dominant role in controlling the distribution of the biofacies. Recent benthonic foraminiferal biofacies in the Arctic Ocean

A simple theoretical model for the flow of an estuary onto a continental shelf

A simple theoretical model is developed to describe the steady flow of an estuary onto an adjacent continental shelf. A two‐layer density stratification is assumed for the shelf water, and the fluid motion is driven by the positive (upper layer) and negative (lower layer) mass fluxes associated with a pair of point sources located at the mouth of the estuary. The dynamics are linear and include the effects of Coriolis acceleration, turbulent friction, and bottom topography. Analytic solutions for the one‐layer single‐source problem are found for two special depth profiles. A boundary layer solution is found for the power law depth profile h(y) = αyk, and a global solution is found for the log law profile h(y) = H0 [1 + α 1n (y/ Lb)] for small α ≪ 1. Both solutions indicate that the far‐field flow is asymmetrically concentrated toward the right‐hand coast in the northern hemisphere, a consequence of the basic balance between topographic vortex stretching and bottom friction. This mechanism also applies when a constant along‐shore current is present. In the two‐layer case the flow in the upper layer generally tends to be concentrated toward the left‐hand coast, since the upper fluid feels (1) the interface and not the bottom topography and (2) the interfacial drag exerted by the lower fluid toward the right‐hand coast. A brief comparison is made between model predictions and observations for the Hudson and Chesapeake Bay estuaries.

Geologic structure and evolution of the Keta basin, Ghana, West Africa

The Keta basin, covering approximately 2,200 km 2 , lies along the east coast of Ghana. It is one of the chains of Mesozoic and Tertiary sedimentary basins in the Gulf of Guinea. The basin is filled with 870 m of Paleozoic marine and nonmarine sediments that were deposited in an interior basin that once occupied the present site of the Keta basin. These sediments are unconformably overlain by 3,600 m of Mesozoic-Tertiary deposits. Three major post-Paleozoic depositional cycles are recognized in the basin. Jurassic dolerite sill from well samples in the basin are correlative with dolerite exposures in other coastal regions of Ghana and West Africa. Radiometric ages of these dolerites (162 to 192 m.y.) agree with the presumed initial time of separation of Africa and South America. Bouguer gravity anomalies show that the Keta basin is dominated by two major northeast-trending anomalies, with an axial high of 36 mgal to the north and a low of −15 mgal to the south, where the sediments are thickest. The prominent northeast-southwest orientation of the basinal structure, faults, and gravity anomalies coincide with the orientation of the Romanche Fracture Zone. The structure exerts considerable influence on sediment accumulation in the basin. Seismic data from the adjacent continental shelf and gravity data from the basin suggest that the Romanche fracture extends into the West African continent as the Fenyi-Yakoe fault. The Keta basin is a graben that is modified by basement faulting.

An Analysis of the Formation and Development of Avalanches on Submarine Slopes

ABSTRACT Terrigenous material from river-input accumulates on the adjacent upper continental slope and shelf edge and eventually moves to the deep sea. A set of geomorphic and hydrologic conditions determines the types of movement. The motion can be quantitively described by a set of derived equations. In addition, the force of the sliding loads on obstacles and structures can be quantified.

Fishes of the Indian River Lagoon and Adjacent Waters

A qualitative analysis of the ichthyofauna of east central Florida including the Indian River lagoon, its tributaries, and the adjacent continental shelf has accumulated records for 609 species. These species are listed in tabular form, including biotopic distribution and relative abundance. It is predicted that 704 species should eventually be collected from this area. The transitional nature of the Indian River ichthyofauna is emphasized, as tropical Caribbean and warm temperate Carolinian faunas overlap considerably within the Cape Canaveral area. Recorded as new range extensions into this area are 135 fishes, mostly of tropical origin. A physical description of the region is also given with a brief discussion of geology, water salinities, and temperature.

Comparative petrology and source of sediments in Newfoundland fiords

Comparative petrology of the gravel- and sand-sized fractions of sediments from Conception Bay, Placentia Bay, and Halls Bay, Newfoundland, has shown gravel clasts to be of the same composition as local bedrock, and sands to range in composition from litharenite to lithic arkose depending upon the proportion of fine and coarse grained local bedrock. Sand-sized lithic fragments comprise the same lithologies as local bedrock. The petrology provides conclusive evidence that almost all of the sediments in each fiord are glacially eroded from local bedrock; they are deposited in the fiords and subsequently reworked. However, the occurrence and distribution of garnet and other less frequently occurring mineral and lithic species indicates that a minor proportion of the sediments has been transported into the fiords from offshore by ice-rafting and/or from the adjacent continental shelf by bottom currents.

Submarine canyons: Internal wave traps?

Kinetic energy of water currents near the axis of La Jolla submarine canyon is enhanced in a broad frequency band above tidal frequencies and below the Väisälä frequency when compared with water motions nearby but not within the canyon. This energy appears to be derived from first mode internal waves over the adjacent continental shelf.

Geophysical evidence on the nature of the Hebrides Terrace Seamount

Synopsis A magnetic and seismic reflection survey of the Hebrides Terrace Seamount (56°28′N., 10°17′W.) is supplemented by gravity data obtained by the Ministry of Defence (Hydrographic Office). The Seamount has a guyot-like morphology, and is supposed to be a volcanic feature. Seismic reflection profiles suggest that it ante-dates a succession of sediments on the continental slope to which a tentative Eocene–Recentage is assigned; and there are indications that its flat summit may represent an erosional bevel coeval with a (?Palaeocene) phase of erosion of the adjacent continental shelf margin. The associated magnetic anomaly is complex and cannot be simply modelled, but its general character is consistent with the presence of basic igneous rocks emplaced during an epoch of reversed polarity. The gravity profile is consistent with a model invoking a deep-rooted cylindrical core of density 3.0 gm cm −3 , flanked by pyroclastics and sediments of density 2.4 gm cm −3 . Thus the assembled data tend to confirm that the Seamount is an early Cenozoic volcanic feature.

The seismicity of the norwegian and Greenland Seas and adjacent continental shelf areas

The seismic activity of the Norwegian and Greenland Seas and adjacent areas has been examined in view of the tectonic evolution of the North Atlantic. The 529 earthquakes used covered the period 1955–1972, and for fifteen of these events fault-plane solutions were available. An analysis was made of the location precision which turned out to be better than 20 km in most cases. Expectedly, little new evidence was obtained at the midoceanic ridges and major fracture zones, with possible exceptions of the Knipovich Ridge showing a well-defined seismicity belt supporting the idea of an active spreading ridge, and the Spitsbergen Fracture Zone, which seems to be a system of en-echelon faults. Most interesting is a weak linear event pattern in the Lofoten Basin, possibly giving evidence of unknown structures parallel to the Greenland and Senja Fracture Zones, although sediment loading also may be important. Earthquakes along the shelf edge off Norway are located at or near isostatic gravity belts which may act as hinge lines for the marginal subsidence, thus implying stress release caused by differential subsidence of the continental crust. Part of the seismicity of eastern Greenland and western Norway appears to be related to zones of weakness of pre-Cenozoic age. The seismic activity along the edges of the Norwegian Channel is very limited.

Stratigraphy of Atlantic Coastal Margin of United States North of Cape Hatteras--Brief Survey

A synthesis of studies of sea-floor outcrops of the sedimentary wedge beneath the northeastern United States continental shelf and slope and a reassessment of coastal plain Mesozoic stratigraphy, particularly of the coastal margin, provide insight for estimating the oil and gas potential and provide geologic control for marine seismic investigations of the Atlantic continental margin. The oldest strata known to crop out on the continental slope are late Campanian in age. The Cretaceous-Tertiary contact along the slope ranges from a water depth of 0.6 to 1.5 km south of Georges Bank to 1.8 km in Hudson Canyon. Few samples are available from Tertiary and Late Cretaceous outcrops along the slope. Sediments of the Potomac Group, chiefly of Early Cretaceous age, constitute a major deltaic sequence in the emerged coastal plain. This thick sequence lies under coastal Virginia, Maryland, Delaware, southeastern New Jersey, and the adjacent continental shelf. Marine sands associated with this deltaic sequence may be present seaward under the outer continental shelf. South of the Norfolk arch, under coastal North Carolina, carbonate rocks interfinger with Lower Cretaceous clastic strata. From all available data, Mesozoic correlations in coastal wells between coastal Virginia and Long Island have been revised. The Upper-Lower Cretaceous boundary is placed at the transition between Albian and Cenomanian floras. Potential hydrocarbon source beds are present along the coast in the subsurface sediments of Cretaceous age. Potential reservoir sandstones are abundant in this sequence.

Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf

Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude. Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.

Surficial Sediments of Barkley Sound and the Adjacent Continental Shelf, West Coast Vancouver Island

The bathymetry and sediment distribution of Barkley Sound and the adjacent continental shelf off the west coast of Vancouver Island have been markedly affected by the late Pleistocene glaciation and modern sedimentary processes. Several fjords widen and coalesce to form the sound, which is continuous with glacially eroded basins on the inner continental shelf. These basins are flanked by flat-topped banks, the larger of which merge with the outer shelf—a gently sloping plain that terminates at the 200 m isobath, 58 km from shore.Modern sediments are restricted mainly to Barkley Sound where the glaciated "basin and sill" bathymetry and an estuarine circulatory system prevent the predominantly muddy detritus from reaching the continental shelf. Relict sands and gravels cover most of the shelf except within basins and drowned river valleys where muds prevail. This relict cover was initially dispersed by glaciers and meltwater streams, then later inundated during the Holocene Transgression, and is now being partly reworked by the present hydraulic regime. Near the shelf-break relict sediments are sparse and authigenic sands (glaucontized mudstone pellets) predominate together with residual sediments derived from submarine exposures of Tertiary mudstone.

Distribution and Composition of Suspended Sediment in the Bottom Waters of the Washington Continental Shelf and Slope

ABSTRACT In order to study the dispersal of river-borne suspended sediments through the marine environment, suspensate samples were collected at 26 stations from the lower Columbia River and bottom waters of the adjacent continental shelf and slope during the summer and early fall of 1971. X-ray diffraction analysis has shown the montmorillonite/chlorite (M/C) ratio to be a useful indicator of Columbia River-derived sediments. M/C values vary from high (> 1.0) in the Columbia River and shelf bottom waters, through moderate (0.96-0.65) in the head and upper reaches of three major submarine canyons on the slope, to low (<0.48) in samples from the distal portions of the canyons and the open slope areas between the canyons. These trends appear to develop from physical sedimentary processes, such as differential sedimentation and resuspension, which result in a continually changing clay mineral composition of the Columbia River suspended load as it diffuses into the coastal ocean environment. There is no evidence to indicate that these changes in clay mineralogy are due either to dilution of the Columbia River suspensate by other continental or marine sources, or transformation of the clay minerals due to reactions with seawater. Comparison of the M/C values of the suspended sediment with surficial bottom sediment collected at the same stations indicates that naturally occurring differences between the two can be falsely exaggerated if similar particle size ranges are not used in each analysis. Agreement of the clay mineral trends is best when comparing the M/C ratios of the total inorganic suspended sediment to the M/C ratios of the 0-62µ, rather than the 0-20µ or 0-2µ, fraction of the bottom sediments.

A pattern of rupture of the Eastern North American-Western European paleoblock

Eastern North American and Western European paleomagnetic data indicate that, during most of the Upper Paleozoic, these regions and their adjacent continental shelves were parts of a single block situated near the paleoequator. With respect to a stationary paleopole, a north-north-west movement of the block during the Carboniferous can be detected. Comparison with polar displacement relative to other continents indicates that continental drift and possibly a pole displacement of about 15° occurred at that time. The Mesozoic results indicate that the rupture of the block and the opening of the North Atlantic did not follow each other closely in time. A latitudinal displacement of Eastern North America with respect to Western Europe places the time of rupture during Lower or Middle Triassic. A slight separation of the blocks may have occurred at that time. It is suggested that the rupture of the initial block occurred by strike slip with Eastern North America and Western Europe slipping one against the other in a north-south direction (by about 20° in latitude) but without pulling apart by any considerable distance. While this event took place before the Upper Triassic, it is probable that a substantial opening of the North Atlantic did not occur until after the Triassic. It appears that, in Cretaceous time, the longitudinal separation was considerable and that the North Atlantic Ocean was then partially open. Some of the relevant geological features and ocean floor spreading studies are discussed. A rupture by strike-slip in latitude mainly can be supported geologically and a post-Triassic time of opening of the North Atlantic is in agreement with the opening time obtained from certain interpretations of magnetic anomaly lineations.

Surficial Sediments of Barkley Sound and Adjacent Continental Shelf, Vancouver Island, British Columbia: ABSTRACT

The bathymetry of Barkley Sound and the adjacent continental shelf off Vancouver Island, has been affected by glacial erosion. Several fiords widen and coalesce to form the sound, which is continuous with glacially eroded basins on the inner continental shelf. Basins are flanked by flat-topped banks, the larger of which merge with the gently sloping outer shelf which terminates at the 200-m isobath, some 58 km from shore. Studies of surficial sediments for size, color, mineralogy, organic carbon, CaCO3, and fauna, led to recognition of 5 sediment types: (1) modern sediments, at present accumulating in Barkley Sound, are littoral sands and gravels, and deeper water organic-rich muds; (2) relict sands and well-rounded gravels mantle banks and parts of the outer continental shelf; (3) authigenic sands composed of mixed-mineral glauconite pellets are present near the shelf break, where they are closely associated with (4) residual sediments derived from submarine exposures of Tertiary mudstone; (5) organic sediments, composed of calcareous invertebrate End_Page 607------------------------------ remains, are present on small banks and beaches in Barkley Sound. Mineralogically, relict and modern sands are similar, consisting mainly of detrital plagioclase and lithic fragments. However, there are marked differences between heavy mineral suites, which led to the establishment of the Barkley Sound and continental shelf provinces. The ultimate sources of the sediments are mainly Mesozoic diorites and intermediate-basic volcanic rocks. End_of_Article - Last_Page 608------------

Major Heavy Mineral Assemblages and Heavy Mineral Provinces of the Central California Coast Region

Five distinct heavy mineral assemblages are found in late Cenozoic and modern sediments along the central California coast. Nine heavy mineral provinces distributed in the central California Coast Ranges and adjacent continental shelf are formed by these assemblages. They are: (1) hornblende-sphene, derived from granodiorite and quartz diorite rocks; (2) hornblende-garnet, derived from mixed granitic and metamorphic rocks; (3) glaucophane-jadeite, derived from Franciscan rocks; (4) augite, derived from Cenozoic volcanic rocks; and (5) hornblende-augite-hypersthene, derived from volcanic and plutonic rocks. These assemblages are based on nonmicaceous, nonopaque minerals in the sand-size fraction of sediments. Micaceous and opaque minerals differ greatly in hydraulic properties from most heavy minerals. They are generally not indicative of provenance but instead are useful indicators of depositional environment.

Holocene Evolution of a Portion of the North Carolina Coast: Reply

The presence of a soil zone under a portion of the large barrier chain, which makes up much of the North Carolina coast, and the absence of this zone under the remainder of the barrier has led to a re-evaluation of the mode of formation and the evolutionary development of this stretch of coastline during Holocene time. A primary barrier, formed during a rising sea, became detached by flooding of the area behind a mainland beach. This primary barrier apparently reoccupied the position of a barrier formed during a stillstand within the general Wisconsin regression. The present coastline has evolved from the primary barrier by retreat and migration of headlands as well as formation of secondary barriers, by spit elongation, over the adjacent continental shelf. Approximately 61 percent of the present barrier chain is secondary in nature; whereas the remaining 39 percent is modified primary. Continued retrogression of the barrier islands eventually will superimpose the secondary barriers over the primary. When this happens, evidence that a secondary barrier existed will be obliterated. The extreme case results when this compound barrier is driven landward until it eventually reaches the mainland, hence a mainland beach. At this time, no evidence remains that any barrier previously existed. The presence of a secondary barrier would suggest the landward presence of a primary barrier or a mainland beach.