ABSTRACT The shelfbreak (SB) is a distinct, critical interface of continental margins which delineates the major physiographic boundary between two major submarine provinces, shelf and slope. The shelfbreak is defined as that point of the first major change in gradient at the outermost edge of the continental shelf, and its depth, distance from shore and configuration are highly variable. Although structural framework is a dominant controlling factor, depositional regime and consequent progradational and regradational development generally modifies, substantially, the shelf-to-slope configuration. These depositional considerations include, among others, sediment supplied by rivers, carbonate reef buildup, influence of ice transport at high latitudes, and the interplay of fluid-driven and gravitative processes active in environments at and adjacent to the break. Moreover, the imprint (relict) of earlier eustatic oscillations, particularly low stands when the SB was at or close to the coastline, is still in evidence. This overview, which incorporates observations in diverse geological and geographic settings, focuses on geomorphological aspects and is an attempt to synthesize shelfbreak type by means of a descriptive-genetic classification which takes into consideration the interaction of the dominant controlling factors. These are: (a) structural framework and rate of substrate motion which are functions of the larger-scale geological evolution of a margin, (b) the overprint of earlier (largely Quaternary) climatic and eustatic events, and (c) sediment supply and processes (reviewed in terms of climatic belts). The interplay of these three large-scale parameters has, of course, varied considerably in time and space, giving rise to a diverse suite of “end-member” break and transitional variants. In summary, we view the shelfbreak as a reworked palimpsest feature which has not yet attained complete equilibrium with presently active processes.

ABSTRACT Mid-range side-scan sonar images of the U.S. Middle Atlantic continental margin show the presence of a variety of erosional features. Amphitheatre-shaped scars are present on the continental slope near Carteret Canyon. Several slope canyons, whose heads do not appreciably breach the shelfbreak, have a pinnate drainage pattern on the upper, sediment-draped slope. The thalwegs of these canyons follow nearly straight paths down the slope. In contrast, Wilmington, a shelf-indenting canyon, follows a curved to meandering path down the slope. On the basis of these and other data, we propose a preliminary explanation of evolution of canyons on the middle Atlantic slope. In this model, localized slope failure begins the process of canyon formation. By headward erosion, these depressions extend upslope and form linear sediment chutes. These slope canyons represent the youthful phase in canyon evolution. Slope canyons begin the transition to a mature phase when the canyon heads breach the shelfbreak. Access to the continental shelf leads to the transport of shelf-derived materials through the canyon. During the youthful phase, the dominant mechanism of canyon erosion is the failure of the slope itself. In the mature phase, entrenchment is augmented by the episodic cutting by turbulent sediment suspensions enroute from the shelf to the continental rise and abyssal plain. The shelfbreak, in this model, is an important factor in the evolution of a passive continental margin.

ABSTRACT Subsidence of passive margins appears to be of thermal origin, and increases from near zero at a landward hinge zone to a maximum value at the ocean-continent boundary. At all points on the attenuated margin the driving subsidence is at a maximum immediately subsequent to rifting, and decreases thereafter as a function of time. The present driving subsidence at the ocean-continent boundary of the U.S. East Coast may be greater than 1 cm/1000 years. Rifting at this margin ceased at least 160 m.a. It is concluded that glacial fluctuation is the only known mechanism that can cause world-wide sealevel to change at rates in excess of 1 cm/1000 yrs and with a magnitude greater than 100 m. We do not preclude the possibility that mechanisms as yet unknown may be sufficient to change sealevel by rates and with magnitudes in excess of the above. Considered here are geological periods during which glacial fluctuation was minimal to non-existent. We have assumed that at most passive margins, the shelfedge lies at (or even seaward of) the ocean-continent boundary. Under these conditions it is not likely that the shoreline can move out over the shelfedge because the driving subsidence at the shelfedge is greater than the rate at which sealevel may fall. We have also shown that if by chance the rate of sealevel fall is several times greater than the rate of subsidence at the shelfedge it may take several million years to displace the shoreline seaward to the shelfedge. Alternatively, at a starved margin, the shelfedge may lie well landward of the ocean-continent boundary (well up toward the hinge zone). In this case, rate of subsidence may be much less than 1 cm/1000 yrs and rate of sealevel fall may be much greater than rate of subsidence at the shelfedge. In this case the coastline may move seaward over the shelfedge, but this will still require a time interval of a million years or greater. If sealevel drops sufficiently to allow the coastline to move seaward over the shelfedge and if at the same time sealevel ceases falling, then within a time interval of less than two million years the combined effects of both erosion and subsidence will generally cause the coastline to retreat onto the shelf and transgress toward the hinge zone.

ABSTRACT This study focuses on the origin of prograding shelfbreaks on passive margins, using as examples selected seismic sections recorded on Iberian Atlantic and Western Mediterranean margins. Five main factors appear to control shelf progradation: (1) the amount and nature of sediment contributed to the outer shelf; (2) the equilibrium depth H at which sedimentary particles come to rest, a factor depending on grain-size distribution and specific hydrodynamic conditions at the depositional site (according to the models presented herein, H is the shelfbreak depth); (3) the morphology of the margin, i.e. the shelfbreak is significantly prograding only where the shelf forms on a slightly inclined slope or a marginal plateau; (4) the geological activity of the margin, i.e. prograding shelves have a sigmoid configuration on young subsiding margins, and an oblique configuration on mature, slowly subsiding margins; (5) the eustatic sealevel changes, i.e. the shelfbreak is eroded during periods of low sealevel and is built-up and progrades during periods of high sealevel. In sum, the prograding outeredge of shelves provides a fairly reliable record of Quaternary sealevel changes and of the geological evolution of margins, of which they are an integral part.

ABSTRACT The paleogeography, paleotectonics, and paleoceanography of continental margins and shelfedges around the present western, southern, and eastern sides of the conterminous United States are reconstructed for a brief span (about 1.5 m.y.) of Mississippian time. The time is that of the middle Osagean anchoralis-latus conodont Zone (latest Tour- naisian, Mamet foram zone 9). At this time, a shallow tropical sea covered most of the southern North American continent and was the site of a broad carbonate platform. Bordering this platform were three elongate foreland troughs, each containing several bathymetrically distinct starved basins on their inner (continentward) sides. The foreland troughs were bordered on their outer sides by orogenic highlands or a welt that formed in response to successive collisions or convergences with North America by Africa and Europe to the east, by an oceanic plate to the west, and by South America to the south. During a eustatic rise of sealevel that accompanied the orogenies and culminated during the anchoralis-latus Zone, the carbonate platform prograded seaward while the troughs subsided and carbonate sediments were transported over the passive shelfedges to intertongue with thin carbonate foreslope deposits and thin (~10 m) phosphatic basinal sediments. Simultaneously, thick (~500 m) flysch and deltaic terrigenous sediments, such as the Antler flysch on the west and the Borden deltaic deposits on the east, were shed into the outer parts of the foreland basins from active margins along orogenic highlands. This Mississippian reconstruction provides a unique opportunity to compare and contrast passive and active shelfedges of a Paleozoic continent during a high stand of sealevel. The passive shelfedges can be recognized and mapped by application of a six-part sedimentation and paleoecologic model developed for the shelfedge of the Deseret starved basin in Utah, Idaho, and Nevada.

ABSTRACT About 40 subduction-zone segments have been identified worldwide on the basis of intermediate-focus earthquakes, calc-alkalic volcanic arcs, and lines of rapid tectonic uplift. The total length of these actively convergent plate boundaries is 57,000 km. Of this length, 42% is of the Japan type, in which the upper plate is relatively stable with respect to the subduction zone; 37% is of the Andes type, in which the upper plate actively overrides the trench; and 21% is of the Himalaya type, in which continental plates or microplates collide with other continental bodies. Subduction zones of both the Japan and Andes types are marked by basement highs at the trench-slope break. Uplift of the crust and upper mantle at the edge of the upper plate causes these basement highs where a relatively low-density prism of accreted oceanic material is emplaced below. The accretionary prism for each cycle of subduction forms within 5 m.y. after a new subduction zone is established, while the megathrust is evolving from an initial dip of about 30° near the zone's seafloor outcrop to a steady-state dip of about 10°. Except during this relatively brief period of accretion, most oceanic sediment at subduction zones is believed to be carried deeply into the lithosphere. Elongate sedimentary basins form on both sides of the uplift at the trench-slope break: forearc basins toward the arc, and trench-slope basins toward the trench. Depending on the balance between sedimentation and tectonic displacement, the topographic shelf-slope boundary may be located anywhere in the forearc basins, approximately to the upper edge of the trench-slope basins. At the lower trench slope, compression usually removes the seawater involved in primary oil migration before thermal maturation of oil precursors can occur. Elsewhere at the active margin, although a low geothermal gradient caused by the subduction of cool oceanic crust delays hydrocarbon maturation, such thermal maturation can nevertheless resume when a normal geothermal gradient is reestablished after continental collision or after the subduction-zone alignment moves to a new position.

ABSTRACT Tectonically active continental margins include transform, protoceanic and convergent settings. In transform settings, numerous small basins develop on oceanic, transitional and continental crust. Protoceanic gulfs may be formed by orthogonal or oblique rifting; both types are characterized by segmented crust with consequent juxtaposition of deep basins and active uplifts. Convergent margins include the following types of basins: intramassif basins, major forearc basins and accretionary basins (trench-slope basins). Extensional backarc margins have structural styles and histories similar to protoceanic gulfs. Shelf-slope breaks in these settings tend to be transient in time and space because of rapid vertical movements; abrupt facies changes are the result. Two types of shelf-slope breaks are common: elastic-starved and progradational. The former type is characterized by unconformities or biostratigraphically compressed intervals separating shallow-marine/nonmarine from slope/basinal deposits. Commonly, glauconitic and phosphatic lithologies mark the clastic-starved shelf-slope break. Progradational shelf-slope breaks are characterized by deltaic outbuilding, which results in coarse shallow-marine, shoreline and fluvial deposits prograding over slope mudrocks. Clastic-starved shelf-slope breaks tend to predominate in transformal and protoceanic settings due to the segmented nature of crust and the structural control on shelfedges. Most detritus bypasses outer-shelf and upper-slope environments. Deltaic progradation is the dominant process in forearc basins, with abundant detritus supplied by neighboring magmatic arcs. During the latest stages of filling of forearc basins, shelf-slope breaks may correspond with the structural boundary between forearc basins and subduction complexes. Shelf-slope break deposits involved in continental collision in general are destined to be destroyed during continental suturing.

ABSTRACT Large river systems deliver significant quantities of fine-grained sediment to continental shelf regions. In specific areas off deltas, deposition rates are rapid and the sediment may be involved in a variety of mass-movement processes on the subaqueous slopes (slumps and slides, debris flows, and mudflows), causing rapid sediment accumulation at shelfbreak depths and resulting in active progradation of the shelfedge. Seismically, the deposits appear as large-scale foresets and are commonly composed of in situ deep-water deposits alternating with shallow-water sediments transported by mass movement. On electric logs, sands within these units are sporadic and display sharp basal planes and blocky shapes. Progradation of the shelfedge deposits is generally accompanied by oversteepening and large-scale instability of the upper shelfbreak slopes. Deep-seated and shallow rotational slides move large volumes of sediments and deposit them on the adjacent slopes and upper rise. Extensive contemporaneous faults commonly form at the shelfedge. Continuous addition of sediment to the fault scarps, particularly by mass movement from nearby delta-front instability, causes large volumes of shallow-water sediment to accumulate on the downthrown sides of the faults, mostly forming large-scale rollover structures. Continued movement along the concave-upward shear planes commonly results in compressional folds and diapiric structures. Contemporaneous accumulation of shallow-water mass-movement deposits may occur in association with these structures. Massive retrogressive, arcuate-shaped landslide scars and canyons or trenches can also form at the shelfedge owing to slumping and other mass-movement processes. Such canyons and trenches can attain widths of 10–20 kilometers, depths of 800 meters, and lengths of 80–100 kilometers. The Mississippi Canyon probably originated in this manner. The creation of such features by shelfedge instability results in the yielding of exceptionally large volumes of shallow-water sediment to the deep basins in the form of massive submarine fans. The infilling of depressions by deltaic progradation is rapid, forming large foresets near the canyon heads. The low strength of the rapidly infilled, under-consolidated sediments causes downslope creep or reactivation of failure mechanisms, resulting in multiple episodes of filling and evacuation.

ABSTRACT In some continental margin basins such as the northwestern Gulf of Mexico and the Niger Delta, large-scale slumping of the continental slope disturbs the topset-foreset geometry of the prograding shelf margin and thereby inhibits recognition of ancient shelfedges. As a result, concepts of shelf-margin dynamics have been underemphasized in explaining the structure and stratigraphy of such basins. Nonetheless, ancient unstable clastic shelf margins can be approximately located by criteria such as isopach maxima, timing of growth faulting, and the stratigraphic top of geopressure. Gravity sliding of the continental slope creates a strongly extensional regime along the shelf margin, resulting in growth faulting and greatly enhanced subsidence rates. The corresponding compressional regime along the lower slope is important in initiating salt and shale structures; if the shelf margin progrades over these structures, diapiric activity can greatly complicate the style of growth faulting. High subsidence rates result in greatly expanded progradational cycles, which serve to distinguish shelf-margin deltaic sequences from deltas of the more stable shelf platform. Rapid fault movement along the shelf margin can hydraulically isolate shallow-water sandstones and juxtapose them against dewatering slope shales, thus allowing the development and maintenance of excess fluid pressure. These deep-water shales are probably a major source of both hydrocarbons and brines instrumental in diagenesis of geopressured deltaic sandstones.

ABSTRACT The Devonian Catskill Sea of the Appalachian region was stratified much of the time as recorded by the deposition in it of black shale and evaporites. Submarine topography which developed along the southeastern perimeter of the sea consisted of a basin margin and a gently sloping clinoform constructed by sediment progradation. Intersection of the surface of stratification (pycnocline) with the seafloor marked the basin margin-clinoform junction and it caused a separation of sedimentary processes. Shoreward of the intersection, on the basin margin, bottom flow driven by various current-producing phenomena, transported, deposited and reworked sediment. A complex mosaic of facies resulted, including a typical suite of lenticular sandstone, bioturbated shales and mudstones, most of which are abundantly fossiliferous. Basinward of the intersection, on the clinoform, density currents originating at or shoreward of the intersection moved down the clinoform onto the basin floor. Relatively simple facies accumulated, including turbidites and other, pelagic sediments, with very few fossils. At the intersection, internal waves moved along the pycnocline, shoaled, broke and reworked sediment, leaving a record of relatively thick sandstones, some with hummocky cross-bedding. This separation of process is likely to have been a hallmark of sedimentation in epicontinental seas where stratification can be inferred.

ABSTRACT Modem carbonate shelf-slope breaks are highly variable, complex features that are morphologically distinct from their siliciclastic counterparts because of the dominance of in situ organic sediment production and early diagenesis. Carbonate shelf-slope breaks are, in reality, carbonate margins commonly characterized by abrupt and rapid transitions of sedimentary facies, biological communities and physical energy. We recognize four major types of modern carbonate shelf-slope break margins: (1) reef-dominated rimmed margins; (2) atoll margins; (3) sand-shoal-dominated rimmed margins; and (4) non-rimmed margins. Reef-dominated margins usually occur along windward, open-ocean settings, and their associated shelf-slope breaks tend to be abrupt and precipitous, with steep, seaward slopes. Such margins are characterized by distinct morphological and ecological zonation as well as spur and groove structure. Atoll shelf-slope break margins are circular to elliptical in map view and overlie oceanic, volcanic basement. These margins are the most precipitous and reef-dominated of all carbonate margins. Distinctly zoned reefs, with algal ridges at their seaward edges, generally separate deep, open- marine lagoons from steep (>50°), seaward slopes mantled with reef debris and talus. Sand shoal-dominated margins may also be abrupt and precipitous, but are essentially devoid of reefs at the surface. Such margins typically occur along leeward settings and are characterized by bank-parallel sand bodies composed mostly of non-skeletal and degraded skeletal grains which have buried earlier Holocene reefs. Similar shelf-slope breaks are also found in tidedominated settings where strong tidal and storm currents flow on and off the shelfedge. These margins are characterized by bank-perpendicular sand bodies consisting mostly of oolitic grains. Shelf-slope breaks along non-rimmed carbonate margins are broad, subtle, non-reef features that occur in deeper waters (100–500 m). These carbonate shelfedges are characterized by a mix of non-skeletal and skeletal grains that grade up-dip into molluscan calcarenites and/or bioherms and down-dip into pelagic oozes. Four primary processes control the location and gross geomorphology of carbonate shelf-slope breaks: (1) tectonism; (2) physical energy flux; (3) antecedent topography; and (4) sealevel history. Secondary processes such as biogenic barrier development, in situ sediment production, sediment transport and cementation serve to modify the gross structure of carbonate shelf-slope breaks. Analogous shelf-slope breaks should be recognizable in the rock record. Rimmed margins will be the easiest to identify, particularly on seismic reflection profiles, because of their abrupt changes in depth, sediment facies and biological communities. However, non-rimmed margins should also be recognizable on the basis of careful examination of lateral and vertical facies relationships.

ABSTRACT The shelf-slope break is the zone which controls the evolution of fossil carbonate platforms and shelves because it is the locus of most rapid carbonate fixation, both organic and inorganic. The fossil record of carbonate platforms and shelf margins is biased. The best known and most studied shelf-slope breaks are of middle and late Paleozoic age, occuring in intracratonic basins. Those of early Paleozoic and Mesozoic through Cenozoic age, which occur mostly along continental margins, are poorly known. Shelf-slope breaks of Precambrian age are poorly known. Five recurring types of carbonate shelf-slope break are found in the fossil record: (1) stationary, (2) offlap, (3) onlap, (4) drowned and (5) exposed. The reefs and carbonate sand shoals at the break are the line source of most sediment deposited on the foreslope. In the case of drowned or exposed margins this sediment production is arrested, resulting in starved slope and basin margin sedimentation. Most examples in the rock record are a combination of these types. The nature of the break through time depends upon the types of organisms present and their paleoenvironment. When large skeletal metazoans were alive, barrier reefs formed at the margin and reef mounds grew on-shelf or downslope, but when only diminutive skeletal organisms occurred, the break is generally formed by sand shoals. Lithologies at the break are particularly prone to diagenetic alteration. Intensive early diagenesis tends to preserve texture but decrease porosity, whereas intensive late diagenesis generally destroys texture but creates good reservoir rock.

ABSTRACT Internal breccias caused by dilation of slightly lithified limestones have been investigated in the Triassic and Jurassic of the island of Hydra (Greece). They occur between shelf platform and rift basin of the early Tethys and are in general composed of monomictic and closely fitted angular clasts of platform carbonates. The matrix consists frequently of reddish deep-water limestones with thin-shelled molluscs (filaments) and radiolarians; it was sucked-in from above as a result of dilation during brecciation. The vertical succession includes about 20–30 m beginning with (a) fractured and fissured shallow-marine limestones grading upward into (b) internal breccias and (c) mass flows (characterized by a low degree of fitting and considerable roundness). Internal breccias imply a very small lateral displacement; they provide an important source for mass flows near shelf-to-slope breaks. Repeated brecciation is typical. The breccias are most frequently composed of shallow-water limestones, but are overlain by basin sediments. This indicates that the brecciation was connected with tectonic downwarping. We suggest that these breccias were produced by large migrating flexures, and that such flexures are a tectonic alternative or substitute for faults. In the Triassic and lower Jurassic limestones of the island of Hydra, five major breccia horizons are recognized. They correlate well with major tectonic phases in the early geosynclinal history of the northern and eastern Alps, in which internal breccias were found as well. This coincidence emphasizes the significance of such breccias in the evolution of geosynclines.

Abstract Because the shelfedge bridges shallow and deep ocean environments, sedimentary processes characteristic of each of these provinces interact at the shelfbreak to influence sediment transport in the benthic boundary layer. Processes at the shelfedge and mechanics of sediment transport have been inferred from data gathered in many regional shipboard sampling and surveying investigations. Grouping these processes into two major categories—geologic factors and oceanic factors—aids in conceptualizing the complex system of sediment dynamics at the shelfedge. Geologic factors include tectonism, sediment supply, sediment size, shelf width, depth of the break below sealevel, gradient of the upper slope, and bathymetric irregularities. Oceanic factors include fronts between water masses, boundary currents, meteorologically-induced currents, tides, internal waves, and surface waves. Although any of these factors may operate simultaneously on any continental margin, their relative importance varies with time and space; i.e., one, two, or several of these factors may dominate shelfedge sediment transport on a given continental margin or at any given time. Few investigators have actually measured the water and sediment motions in the benthic boundary layer at the shelfedge. Regional sediment-transport data are of limited value as long as the various factors of the forcing mechanisms have not been properly studied and correlated with the flow field and sediment activity at the bottom. Sophisticated instruments deployed for long periods of time are necessary to acquire data adequate for an assessment of the forcing mechanisms that control sediment transport. The few existing measurements of this type support the concept that shelfedge processes differ with place and time among continental margins and on any given continental margin. It follows that caution should be exercised when one attempts to generalize about the shelfedge transport system.

ABSTRACT A survey of fundamental physical oceanographic processes that may affect sediment distribution along shelfbreak regions is presented, emphasizing the Atlantic and Pacific coasts of the USA. The processes encompass the entire spectrum of known motions and are thus generic to all shelfbreak interfacial zones. These shelfbreak strips couple the bounded coastal oceans to the open seas, but there is no systematic pattern to this coupling. In the South Atlantic Bight, the Gulf Stream acts like a vibrating, permeable wall which can variously entrain shelf waters, flood the shelf with North Atlantic Central Water and violently mix shelf waters by towing whirling vortices through the outer shelf. Middle Atlantic Bight, New York Bight and Gulf of Maine shelfbreak processes contain many of the dynamic elements of their southeastern counterpart, but the relative importances of various random surface and offshore driving forces change. Pacific coast shelfbreak processes tend to be less energetic than those on the Atlantic coast since the Pacific coast is missing a Western Boundary Current and because the shelf is narrow and deep. Subinertial frequency shelfbreak motions on the west coast are typically manifested across the entire shelf, while those on the east coast tend to be confined to a loosely defined band, which brackets the break. Principal Pacific coast circulation elements include forms of continental shelf waves and thermohaline driven and mechanically wind forced currents, as well as the California Current System. While high frequency edge waves and inertial currents are indigenous in similar fashion to all coasts, east and west coast tides are shown to be quite disparate, given tradeoffs between dominance of diurnal and semidiurnal constituents as a function of topographic constraint and strength of density stratification. All of the shelfbreak zones are graced by thermohaline fronts. The fronts are progradational on the west and south-eastern coasts and retrogradational on the northeastern shelf. These fronts are an integral ingredient of all aspects of physical processes at the shelfbreak strip. The interplay of bottom topography with the physics of the outer continental margin is significant. Bottom features such as shoals, bumps, ridges and canyons are shown to be regions of sediment erosion, deposition and draping. Moreover, these features are shown to be causally related to upwelling and down-welling phenomena and to the deflection and scattering of waves and currents. Both subtidal and supertidal frequency events are shown to be capable of initiating sediment motion and of suspending sediments, but lower frequency events are shown to be responsible for the buLk of sediment migration on the outer shelf and upper slope environs.

ABSTRACT An investigation of shelfedge sedimentary processes in the Gulf of Mexico has been underway for the past five years. It has consisted of in situ bottom boundary layer (BBL) experiments, time series observations using moored instruments, and hundreds of hydrographic stations. A ubiquitous nepheloid layer exists over the outer continental shelf in the BBL. It reaches a maximum thickness of 30 m when offshore flow near bottom stacks detached bottom boundary layers at the shelfedge. The shear stresses which maintain the sediment suspension are contributed by a superposition of many modes of motion. In the northwestern Gulf of Mexico, surface gravity waves, high frequency internal waves and tides do not appear to contribute significantly to the sedimentary processes at the shelfbreak. However, diurnal inertial oscillations do resuspend silt and clay at the shelfedge and transport that sediment to the offshore. Winter storms produce three types of phenomena that influence sediment transport: (1) direct, energetic, cross-shelf wind-driven flow; (2) production of dense, cool, saline bottom water that flows offshore under the influence of gravity; and (3) inertial oscillations which propagate to the bottom. The mean shelfedge flow was found to be west to east in the interior, with bottom waters oriented more southeasterly. The latter should contribute to a long term advection of sediment off the shelf. Flow on the bottom of the upper slope has been observed to be oriented to the northeast, suggesting a convergence in the BBL near the shelfbreak.

ABSTRACT Long term current-meter data from outer shelf, shelfbreak and slope sites off Nova Scotia have been compared with sediment textures in the same area to assess whether they are in equilibrium. Currents on the shelf and shelfbreak are strong with maximum velocities exceeding 50 cm s −1 . Sediment grain-size distributions were dissected into near-Gaussian medium sand subpopulations and non-Gaussian tails. These subpopulations were interpreted dynamically as representing bed-load (coarse tail), suspended load with “dynamic settling” (central subpopulation) and suspended load with “passive settling” (fine tail). Below 500 m water depth, only the fine-tail subpopulation is seen. The modal size of the central subpopulation corresponded well, in most cases, to u * estimates from Shields' criterion and to the observed maximum currents. Sediment textures can be explained by modem dynamic conditions. Sand transport is dominantly in suspension and in an alongslope direction with a small downslope component. Medium sand is transported only during short periods of high flow, whereas fine sand transport is during more continuous weaker flow. Permanent deposition occurs at a point downslope where the currents rarely exceed the suspension criterion for the size of particle concerned. Slight differences between inferred u * gradients at two slope areas, separated by 150 km, are tentatively interpreted as reflecting the effects of topographic Rossby waves, formed by Gulf Stream eddies impinging on the slope.

ABSTRACT The mudline, the depth of substantially increased silt and clay content and the level below which deposition prevails on continental margins, often occurs near, but is only rarely coincident with, the shelf-to-slope transition. An evaluation of the mudline off the U.S. Mid-Atlantic States and northern Gulf of Mexico highlights marked differences between depth and position of this horizon and those of the shelfbreak, and is summarized in four relationships. Type I = Off Cape Hatteras and portions of the west Florida shelf, offshelf transport of sand-size material results in a mudline position well below the shelfbreak. Spillover at Cape Hatteras, where the mudline occurs at 800–1000 m, is a response to the powerful NE flow of the Gulf Stream that tangentially crosses the narrow, shallow shelf. Type II = The shallower depth of the mudline (200–400 m, or distinctly below the shelfbreak) off the Mid-Atlantic States between Norfolk and Wilmington canyons, and off Panama City, Florida, margin identifies the long-term signature of energy concentrated on the seafloor; erosion results from the interplay of several mechanisms, including fronts, tides, and internal waves. The mudline at these localities thus defines the position where, over time, shear-induced resuspension has largely exceeded the threshold required for sediment transport. Type III = The near-coincidence of the mudline (130–175 m) with the shelfbreak at the head of Hudson Canyon is a response to physical oceanographic parameters and to offshelf spillover; involved are the intersection of density fronts separating Shelf and Slope Water, and the channelizing effect of the canyon head cut deeply into the outer shelf. Type IV = Considerable shoaling of the mudline arid a marked departure between this level and the shelfbreak occur on margins where large amounts of sediment are supplied. Broad asymmetric shoreward swings of the mudline on the Gulf of Mexico margin west of DeSoto Canyon record Mississippi and other river input and its extensive lateral dispersal by regionally important water mass flow. Along many continental margin segments, the mudline is an erosion-deposition boundary whose position relative to the shelfbreak on a margin is the long-term resultant of several factors including sediment supply, offshelf spillover by a plexus of fluid-driven processes and gravity flows, shelfbreak morphology, structural framework, sediment stability and eustacism.

ABSTRACT Pacific-style continental margins, such as that of western North America, are marked by large contrasts in the type of shelfedge sedimentary deposits and the processes that form them. The Pacific shelves of the United States are generally much narrower than the Atlantic shelves, and the source areas exhibit more relief. The greater relief of Pacific coast source terranes results in a relatively high rate of sedimentation in humid areas and fluctuating (areally and seasonally) sedimentation patterns and rates in semiarid areas. Sediment shed from the adjacent landmass is discharged, generally seasonally, onto the Pacific Continental Shelf at point sources. Many of the sediment sources of the northwestern United States and southern Alaska feed directly onto swell- and storm-dominated shelves. On such narrow unprotected shelves, sediment has a short residence time in submarine deltaic deposits before being remobilized and dispersed to outer-shelf and upper-slope environments. Through study of high-resolution seismic-reflection profiles, we have identified four principal types of shelfedge deposits: (1) starved, (2) draped, (3) prograded, and (4) upbuilt and outbuilt. Each type of shelfedge deposit results from a characteristic balance between sedimentation rate and distributive energy (waves and currents) and is, therefore, characterized by distinctive seismic facies and bedding patterns. A special type, the cut-and-fill shelfedge, and a composite type consisting of two or more of the main depositional styles supplement the four principal types of shelfedge. Incorporated within each of these facies, especially on the upper slope, are chaotic deposits formed by slumps or slides, which are common along technically active margins.