Seaward Movement Research Articles

Sand-dispersal patterns in an active-margin estuary of the northwestern United States as indicated by sand composition, texture and bedforms

Sediments found in Alsea Bay, Oregon, an active-margin estuary which experiences seasonally high fluvial discharge, are generally a mixture of river sands from the high-gradient Alsea River and beach sands carried into the bay by tidal currents. However, sands from these two sources are very distinctive, differing in both their heavy-mineral assemblages and in the degree of heavy-mineral grain rounding. Both a measure of the mean grain roundness and factor analysis of the heavy-mineral assemblages provides a direct indication of the relative proportions of beach and river sands in estuarine deposits. This accurate determination of sources together with a study of bedform asymmetry and migration and sediment grain-size distributions permits a detailed analysis of the patterns of sediment movement and deposition in Alsea Bay. Contours of sample composition indicate that the predominant routes of beach-sand intrusion occur along the south side of the east—west trending estuary, penetrating 1–2 km from the mouth, and along the north side of the estuary, reaching 2–4 km from the mouth. Asymmetrical, tidal-current transport is also indicated by megaripple orientations and is the predominant mechanism by which beach sand is transported up the estuary. In addition, the alignment and compression of composition contours along prominent tidal channels imply that: (1) channels serve as the major conduits of sand transport; and (2) channel margins are the predominant sites of recent sand deposition. In contrast to estuarine margins, the central channels of the bay are dominated by the seaward movement of river sand, silt and clay with silt and clay passing entirely through the estuary. Repeated sampling revealed that down-channel transport of river sands occurs during winter periods of high fluvial discharge while beach-sand intrusion occurs during periods of low fluvial discharge in summer months.

Seaward Movement of Coho Salmon (Oncorhynchus kisutch) Fry in Carnation Creek, an Unstable Coastal Stream in British Colombia

The seaward movement of coho (Oncorhynchus kisutch) fry was monitored over a 10-yr period as a part of a major watershed study. The time period over which downstream movement took place varied widely during different years of study. Coho fry moved seaward earlier, and terminated the main period of movement earlier, following winters in which stream temperatures were warmer. It is presumed that they underwent more rapid development during winters in which stream temperatures were relatively high. Winter stream temperatures were primarily dependent on air temperatures. During seaward movement daily numbers fluctuated widely. Peaks of movement were coincident with or slightly before freshet peaks. In 94 of 122 cases (77%) the number of fry moving seaward during the night of peak discharge, or during the night before, was higher than in any of the three preceding nights. Movement in these cases may have been initiated by rainfall or falling water temperature or a combination of both. Aggressive behavior among coho fry is considered to be an underlying cause of seaward movement. In Carnation Creek, particularly in the early part of the period of seaward movement, the effects of such social behavior on movement patterns may be masked by the effect of freshets and related conditions. By autumn the number of fry remaining in the stream ranged from 9000 to 13 000 over the 10-yr study. Much of the downward adjustment to this resident fry population size occurred after the most active period of seaward movement.Key words: coho fry, social behavior, seaward movement, stream ecology, coho management

Suspended-material transport in marsh tidal channels, Kiawah Island, South Carolina

Suspended-material concentrations in a salt-marsh tidal channel on Kiawah Island, South Carolina, ranged from 6 to 50 mg/l in March, 1977, and from 10 to 200 mg/l in July–August, 1977. Higher summer than winter concentrations are related to more biologic activity and increased thundershower frequency. Maximum suspended-material concentrations for both periods occurred during spring tides when tidal prisms were greater and, consequently, current velocities were stronger, or during high winds or heavy rains. Monitoring of the flux of suspended material during fifteen tidal cycles in March, 1977, indicated there was an export of combustible (organic) matter and a balance of noncombustible (inorganic) matter for that period. During eight tidal cycles in July–August, 1977, there was a net export of both combustible and noncombustible matter. Export of suspended material is attributed to the time—velocity asymmetry of the tidal currents. Maximum current velocity for both flood and ebb tide occur near high slack water (time asymmetry). Peak ebb velocity is, on the average, 20–30% stronger than peak flood velocity (velocity asymmetry). Consequently, higher concentrations of suspended material are transported for longer periods of time on the ebb tide, resulting in a seaward movement of material.

Seaward Migration of Juvenile Chinook Salmon without Elevated Gill (Na+K)-ATPase Activities

The hypothesis that juvenile anadromous salmonids which are migrating to sea have higher levels of gill (sodium and potassium) adenosine triphosphatase ([Na+K]-ATPase) activity than nonmigrants was examined for wild and hatchery spring and fall chinook salmon (Oncorhynchus tshawytscha) from the Rogue River, Oregon. Although migrant juvenile chinook salmon captured at river km 173 had elevated gill (Na+K)-ATPase activity, migration from the more upstream portions of the Rogue River occurred without significant changes in gill (Na+K)-ATPase activity. High water flows stimulated migration without increasing gill (Na+K)-ATPase activity. Tributary-to-main-stem migration of juvenile fall chinook salmon did not result in elevated gill (Na+K)-ATPase activities. Juvenile spring chinook salmon released from Cole Rivers Hatchery (km 254) migrated rapidly to km 18 on the Rogue River without changes in gill (Na+K)-ATPase activity. We conclude that, although the relationship between seaward migration and elevated gill (Na+K)-ATPase activity is demonstrable in chinook salmon in many cases, such elevated ATPase activity is not a prerequisite for seaward movement of juvenile chinook salmon.

Sedimentary Facies Relations and Inferred Dynamics of a Single-Barred Nearshore Environment, Atlantic Coast of Eastern Long Island, New York: ABSTRACT

Facies relations in a conglomeratic single-barred nearshore environment on the glaciated Atlantic Coast of eastern Long Island, New York, were determined by examination of 42 can cores, bed-form distributions, and sediment textures. Two principal subtidal zones are recognized. The shoreface zone (0 to 10 m water depth) is composed of clean fine to coarse sand and consists of the planar parallel to megaripple-laminated upper shoreface facies, the massively bedded longshore trough facies, and the megaripple to parallel-laminated longshore bar facies. The inner shelf zone (> 10 m water depth) is composed predominantly of organic-rich fine sand and consists of the parallel-laminated transitional facies and bioturbated offshore facies. Scattered throughout this zone are coarse sand outcrops of megaripple-laminated inner shelf lag facies. By deployment of dye tracer and by inspection of bed forms in the longshore trough (during low wave-energy, fair-weather conditions) little evidence of longshore or seaward flow could be found. Landward migration of bar deposits over trough deposits was observed over a 6-week period during fair-weather conditions. A core in the trough facies adjacent to the distinct bar-trough contact revealed an underlying bar facies. An equilibrium model of near-continuous fair-weather landward migration of the bar facies interrupted by high wave-energy pulses of seaward movement of trough sediment is believed to account for the stable position of the longshore bar. End_of_Article - Last_Page 783------------

Emigration of Juvenile Atlantic Croakers, Micropogon undulatus, from a Semi-impounded Marsh in Southwestern Louisiana

Abstract Descriptions of the emigration patterns of juvenile Atlantic croakers, Micropogon undulatus, from their nurseries have varied from (1) gradual seaward movements, to (2) mass outward movements, to (3) a “bleeding off” of the larger individuals. The present study was conducted at a semi-impounded marsh within a few kilometers of the Gulf of Mexico. Emigration patterns were monitored by trawling within the nursery and continuous trapping in the nursery outlet. Early in the year (February and March) croakers were small and tended to move in and out of the nursery with the tide. Thereafter there was an increasing divergence in the size of trawl- and trap-caught croakers; the fish taken in the trap were consistently the larger, indicating that the larger juveniles were continually “bleeding off” and emigrating back to the Gulf.

Ecology of a Late Silurian fauna of graptolites and associated organisms

The Ludlow Series of Wales and the Welsh Borderland represents an ocean-bordering basin and shelf which received a shoaling sequence of sediments in the late Silurian. Graptolites grade from highest diversity and density in basin areas to lowest diversity and density on the nearshore shelf. By analogy with modern zooplankton, this distribution is attributed to differentiation of surface water masses over basin and shelf. During Ludlow time, the center of graptolite abundance shifted northward, indicating a restriction and seaward movement of the basin water mass. Small nektic cephalopods and phyllocarids parallel distribution trends of the graptolites, as does a fauna of small bivalves and brachiopods which may have lived on floating plants. The water mass model accounts for most distributional patterns of pelagic organisms in this local area, and has broader application to the distribution and evolution of graptolites.

Late Quaternary sedimentation in the western equatorial Atlantic

Research Article| May 01, 1977 Late Quaternary sedimentation in the western equatorial Atlantic JOHN E. DAMUTH JOHN E. DAMUTH 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN E. DAMUTH 1Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (5): 695–710. https://doi.org/10.1130/0016-7606(1977)88<695:LQSITW>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JOHN E. DAMUTH; Late Quaternary sedimentation in the western equatorial Atlantic. GSA Bulletin 1977;; 88 (5): 695–710. doi: https://doi.org/10.1130/0016-7606(1977)88<695:LQSITW>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The late Quaternary terrigenous sediments of the continental margin and abyssal plains of the western equatorial Atlantic consist predominantly of hemipelagic silty clay rich in organic detritus with interbeds of redeposited silts and sands; deposition was cyclic and was controlled by glacial-interglacial climatic fluctuations. The redeposited beds are graded, as much as several metres in thickness, and were episodically deposited by turbidity currents and related mass flows. Although the terrigenous component of the hemipelagic sediments was also transported to abyssal depths via gravity-controlled bottom flows, the pelagic component (foraminifera) of these sediments indicates that accumulation occurred relatively slowly (5 to >30 cm/103 yr) and continuously for periods of as much as 100,000 yr. Thus some near-bottom process (other than simple downslope flow), such as redistribution and deposition by contour-following bottom currents, apparently was responsible for deposition of the hemipelagic sediment; however, evidence for such contour-current deposition is sparse.The hemipelagic and redeposited sediments were continuously deposited during relatively long intervals (∼90,000 yr) of low sea level that accompanied the Wisconsin and previous glacial phases. During these intervals the continental shelf was emergent, and river sediments were discharged directly into submarine canyons from where they could easily be transported downslope to the continental margin and abyssal plains. In contrast, during the relatively short intervals (5,000 to 20,000 yr) of high sea level that accompanied the Holocene, beginning of the Last Interglaciation (125,000 to 115,000 yr B.P.), and beginning of previous interglacial phases, the locus of river sedimentation migrated landward across the continental shelf. The low gradient and great width of the shelf plus strong longshore currents prevented seaward movement of sediment beyond the innermost shelf. Thus terrigenous sedimentation was briefly, but completely, halted throughout the entire basin and only pelagic sediments were deposited. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stratigraphy and Formation of Holocene Dolomitic Carbonate Deposits of the Coorong Area, South Australia

ABSTRACT Geologically significant and spatially extensive deposits of primary lacustrine dolomite are found over a 90 200 km region of the coastal plain in the vicinity of the Coorong Lagoon, in southeastern South Australia. The microcrystalline dolomites and associated cabronates, which range in age from Pleistocene to Recent, occur beneath interdune flats between stranded barriers. The most active but by no means only region of present-day microcrystalline dolomite formation is the linear zone of shallow. ephemeral alkaline lakes which occupies the regional groundwater discharge area, immediately landward of the Coorong Lagoon and along the southeastern extension of its interdune corridor. Comparative studies between sediment cores from th s region and a range of contemporary sedimentary environments show that it is possible to characterize the sedimentary sequence which is being generated by Holocene sedimentary regression. In its ideal form this sequence passes upward from a basal marine or lagoonal unit, through a protodolomite + Mg-calcite lacustrine unit, to culminate in an uppermost dolomite or dolomite + magnesite unit which may be capped by a soil. Hydrological considerations, which imply the presence of a landward-dipping continental groundwater-marine groundwater interface immediately inland from the coast, show the above sedimentary sequence to be the result of a progressive seaward movement of that interface, caused by sediment accretion and consequent shoreline regression. The position of a particular microcry talline carbonate-forming environment at any point in time, with respect to the interface, defines its carbonate mineralogy. These observations clearly indicate that optimal conditions for Coorong-type dolomite formation are found in ephemeral lakes which are fed solely by evaporitically-modified continental groundwater and which occur inland from the interface. The relatively large amounts of Quaternary primary dolomite in the area render it somewhat of a geological anomaly in terms of present-day dolomite-forming environments. This study implies that the main reasons for this anomaly may be a favourable combination of factors which include an abundant, renewable source of magnesium for the seaward-flowing groundwaters, an active grounwater regime, and a climate which seasonally varies rom humid to semiarid. The most likely source of magnesium is hypothesized to be from weathering of areally extensive Quaternary basic volcanics which occur in groundwater source areas marginal to the coastal plain. From a geological viewpoint it is suggested that many microcrystalline dolomite occurrences in the geological record may be of the Coorong-type, formed in shallow migrating ephemeral lakes in a relatively humid climate on regressive coastal plains, overlying beach deposits. Such dolomites, which contain no obvious traces of evaporite minerals, are contrasted with superficially similar sabkha-type dolomites which, at least in part, are early diagenetic replacement dolomites and which contain abundant evaporitic associates such as gypsum and anhydrite.

Seaward Movement of Juvenile Fishes, Including Lunar Periodicity in the Movement of Coho Salmon (Oncorhynchus kisutch) Fry

Following emergence, more than 51,000 coho salmon (Oncorhynchus kisutch) fry moved seaward from Lymn Creek. Peaks in movement occurred coincident with the new moon and progressively declined through four lunar cycles although 75% of the outmovement took place between early April and the middle of June. Coho smolt migration occurred simultaneously, the run peaking in late May during a period of maximum low tides between sunrise and sunset when the moon was full. Sticklebacks (Gasterosteus) moved seaward sporadically throughout the trapping period from April to September but their numbers increased markedly in August due to the appearance of underyearlings. The seaward movement of coho fry and smolts and of sticklebacks showed no apparent relationship to either stream discharge or temperature.Lunar rhythmicity in movement of coho fry and probable fate of fry are discussed within a framework of behavioral and ecologic concepts pertinent to the species’ life history in the juvenile stage and relevant to concern for enhancing coho salmon production.

Movement of spilled oil as predicted by estuarine nontidal drift

Information on water movement obtained from bimonthly releases of surface and seabed drifters in the San Francisco Bay and adjacent Pacific Ocean is used to understand major processes controlling dispersal of oil after a spill of 3,200 m3 of Bunker C in the bay in January 1971. River‐induced nontidal estuarine circulation was the dominant factor controlling net movement of the oil spilled at the entrance of the bay system, reinforcing ebbing tidal currents and causing the seaward movement of floating oil, which followed paths taken by surface drifters released 3 weeks before the spill. In contrast, some oil formed globules which sank to the near‐bottom waters, had the same relative buoyancy as seabed drifters, and moved similarly, beaching in eastern San Pablo Bay after being transported landward in the near‐bottom waters. No oil or surface drifters floated into the south bay because surface waters were drifting seaward, away from the south bay.Notable seasonally modulated phenomena which must be considered in predicting surface and near‐bottom oil drifts of future spills include a summer (low‐river discharge period) diminution of the estuarine circulation mechanism in the north and central bay‐adjacent ocean region and a seasonal reversal in two‐layer drift in the south bay.

Ground-water formation of dolomite in the Coorong region of South Australia

The 90 × 200 km coastal plain of southeastern South Australia provides a setting which throughout Quaternary time was ideally suited for the sedimentation of dolomite and other carbonate minerals. Comparative studies between various present-day sedimentary environments and cores taken from regions of active dolomite formation near the Coorong Lagoon have defined a typical Holocene regressive sedimentary cycle for the area. A basal restricted marine and lagoonal aragonite and Mg-calcite unit, approximately 6,500 yr old, passes upwards into ephemeral lake units with carbonate mineralogies ranging from protodolomite +Mg-calcite to ordered dolomite. Hydrologic data show that the present coastal zone is the main area where seaward-flowing shallow ground waters emerge from unconfined carbonate aquifers beneath the coastal plain. This fact, and the observation that modern dolomite is also forming in isolated areas of ground-water discharge and evaporation remote from the coast, suggests that dolomite ideally forms from ions provided by ground waters, whereas aragonite, Mg-calcite, and protodolomite assemblages form from a marine or mixed sea-water–ground-water reservoir. The observed Holocene cycle is therefore explained by progressive seaward movement of a sea-water-ground-water interface, due to a slight relative fall of sea level about 6,500 yr ago combined with lateral sediment accretion. That the bulk of the dolomite formation is presently occurring near the coast is explained by the fact that this is the site, near base level, of most ground-water discharge and evaporation. Possible sources of Mg ions for the extensive formation of dolomite throughout Quaternary time are Mg depletion of high Mg-calcite allochems in the carbonate aquifers and leaching of ash deposits around Quaternary volcanic centers near ground-water source areas.

Trends in the Abundance of American Shad, Alosa sapidissima, in the Delaware River Basin

The American shad,Alosa sapidissima, was very abundant in the Delaware River Basin until 1901. Annual commercial landings from 1890 to 1901 (about 11 to 17 million lb) were several times greater than in any other river system. Abundance declined rapidly after 1901, except for a resurgence in 1907 which may be interpreted as evidence that by 1903 pollution was affecting successful seaward movement of young produced in the Delaware River. Less than 0.5 million lb were landed in 1920. Abundance has remained low since 1920, although fluctuations have occurred.

Orientation of the sand-beach amphipod, Orchestoidea corniculata

Laboratory experiments demonstrated that Orchestoidea corniculata, a sand-beach amphipod, can orient to slopes of only 3°. Studies suggest they reach the upper interdial zone where they burrow by moving up a wet slope or down a dry slope. When they emerge from the sand at night, however, they either reverse this response or show no response to slope. This behaviour, enhanced by a preference for wet, surface sand, would account for the initial seaward movement observed in the field. Behavioural variations among differing segments of the population were investigated. Three additional beach inhabitants oriented to slope, thus indicating the mechanism of slope orientation as a possible generalization in the beach habitat.

The degradation of the Barton Clay Cliffs of Hampshire

Summary The Barton Clay crops out along a 4·8 km stretch of the Hampshire coast and together with its capping of Plateau Gravels forms a cliff profile averaging around 30 to 37 m high. The original classic work on the geology of the Barton Clay had suggested a total thickness of only 34·5 m; more recent work involving boring and levelling on the cliff face has shown the formation to be 46·4 m thick. Active degradation of the outcrop creates a markedly stepped profile, relatively flat bench levels being separated from one another by steeper scarp slopes. The benches are underlain by up to 6 m of slipped debris or bench rubble. The scarps are formed by exposures of the solid Barton Clay and Plateau Gravel in the case of the top scarp. The interface between the bench rubble and Barton Clay is formed by a shear surface, an underground continuation of the scarp surfaces, which has been polished and grooved as a result of the seaward movement of the bench rubble. These shear surfaces are developed along particular, identifiable stratigraphic horizons within the Barton Clay and tend to develop along these horizons irrespective of the height of the latter within the cliff profile. Variations in the morphology occur laterally along the outcrop and can be related to the erosional history and partly to the geology. Five individual processes of degradation are particularly active on the undercliffs; these comprise bench sliding which is sliding of the bench rubble over the shear surfaces, collapse of the scarp slopes or scarp slumping, wearing down of the scarp slope by weathering or scarp spalling, and erosion by mud-flows and running water. Both mud-flows and streams when well developed cut through bench rubble and into the scarp slope and locally destroy the stepped profile. The current activity of these processes is due to marine erosion of the toe, the net result of the processes being a progressive retreat of the cliff top averaging over one metre per year in some places.

Behaviour and distribution of the sand-beach amphipod Orchestoidea corniculata

The field distribution of Orchestoidea corniculata Stont a talitrid amphipod inhabiting the upper intertidal zone of sandy beaches, was measured by pitfall trap and subsurface core transects. Variations in the distribution of juveniles and adults were recorded. At Coal Oil Point (Goleta, California, USA), the population was concentrated on the leeward side of the Point and, with increasing distance from the Point, both the number and size of beachhoppers decreased. The amphipods burrowed in a 5 to 7 m wide band during the day, and were active over a 15 to 25 m area at night. When they emerged from their burrows, a net seaward movement of 3 m was observed for the population. Behavioural tests suggested that sand penetrability is an important factor determining the location of burrow sites. Diurnal orientation was investigated, and it was found that the amphipods oriented in a landward direction despite overcast sky conditions, beach slope, moisture gradients, or displacement to a new beach.

The influence of rainfall on the distribution and abundance of the school prawn Metapenaeus macleayi in the Hunter River region (Australia)

The effects of rainfall on the distribution and abundance of Metapenaeus macleayi in the Hunter River region (Australia) were examined as part of a study of the prawn fishery and biology in this region. A marked increase in the freshwater flow into the estuary of the Hunter River enhances the seasonal seaward movement of prawns and produces an initial increase in the abundance of adults in the adjacent oceanic waters; it is suggested that the disturbance to the estuarine sediments and to the prawns' normal burrowing and respiratory activity, as a result of the increased river flow, is responsible for this pronounced emigration. This initial increase in adult abundance enhances the prawns' reproductive potential, and the heavy rainfall indirectly assists the recruitment of young to the cotuary and their growth and survival, and thereby increases prawn abundance in the following year also. Prolonged dry weather adversely affects the prawns, and usually results in a smaller population. The annual fluctuations and absence of a downward trend in the catch statistics indicate that this stock has not been overfished.

Bristol Bay: Model Contemporary Graded Shelf

A veneer of contemporary sediments covers the continental shelf in the Bristol Bay region of the southeast Bering Sea. The continental shelf is broad, covering an area of 1,500,000 sq/km, and has a relatively smooth bottom and a gentle slope of about 2.4 percent. Nearshore sediments are extremely poorly sorted gravelly sands which grade to well-sorted fine sands in the central bay. The far offshore sediments are extremely poorly sorted muddy sands. The mean size of the sediments generally decreases with increasing depth and distance from the coast. An overall decrease in the weight percent of total heavy mineral contents also is observed from the coast to the outer bay. The mineralogy of the sand reflects a complex provenance--sedimentary, high-grade metamorphic and igneo s sources in the north and east, and a volcanic source in the south. The heavy mineral dispersal pattern presumably is influenced by fluvial and marine currents. The general paucity of clay minerals and the dominance of feldspars in the less than 2-micron fractions of Bristol Bay sediments indicate the absence of intense chemical weathering in the source areas. Organic carbon content in the sediments is uniformly low (0.5 percent), and is closely related to the amount of clays in the sediments. On the basis of scatterplots of grain-size parameters, the Bristol Bay shelf can be delineated into two broad depositional environments, the inner and the outer shelves. Textural differences in the two environments are believed to be caused by storm-generated waves. These waves may be as great as 200 m long and are believed to elutriate the silts and clays from sediments up to a depth of 80 m. Grading of the shelf also is achieved principally by these waves that bring about a net seaward movement of nearshore sediments.

Quaternary shelves and the return to grade

The first attempt to propose a general theory of shelf sedimentation was Johnson's (1919) concept of the equilibrium shelf profile, and its corollary, the size-graded shelf. The concept of the graded shelf was challenged when Shepard (1932) pointed out that chart notation indicated a non-graded condition, and was discredited when the field studies of many workers showed central and outer shelves to be mantled with sediments relict from Pleistocene low stands of the sea. Recently, however, Curray (1965) has pointed to nearshore belts of fine modern sediments, and has suggested that these will prograde seaward to form first a texturally graded shelf and finally an equilibrium profile. Sufficient data concerning the dynamics of shelf sedimentation has now accumulated so that some tentative models may be proposed for the processes that might drive a shelf towards grade. In the nearshore zone, a wave-driven null line mechanism for sediment sorting and dispersal has been proposed. Attempts to establish its validity in the field have not yielded consistent results. However, the generally recognized response model described here as the nearshore modern sand prism suggests that some such process model must be applicable to nearshore marine sedimentation. There is generally believed to be little sediment movement seaward of the nearshore zone. However, Moore and Curray (1964) have shown that in this area a “natural cyclic period” of shelf sedimentation must be considered, on the order of a century's duration. “Cataclysmic” storm sedimentation then becomes a steady-state process. It is suggested that on the central shelf where sedimentation occurs only during storms, wave-drift currents may be a dominant control of sediment dispersal. With conditions of a linear sediment source at the shoreline, a linear sediment sink at the shelf break, and a slowly rising sediment water interface, then random, intermittent movements of shelf sediment by storm-generated currents would result in net seaward movement. Shelf grading would then occur as a result of progressive sorting. Models of a shelf sediment dispersal based on tides or other agents are probably also valid, but are considered only briefly here. Shelves with relict sediments and relict morphology result when the sediment transport net has been disrupted by a sea-level rise which is rapid relative to the rate of sediment supply. Consideration of modern and ancient shelf deposits suggests that following such a rise a sequence of partially graded stages may ensue, which is terminated by the climax graded shelf. A first stage appears to be in situ grading, where central shelf sediments of relict origin become equilibrium sediments in terms of texture. In this stage, characterized by portions of the present Atlantic shelf, the nearshore lens of modern sediments is at least in part derived by winnowing of relict central shelf sediments. A second stage begins when estuaries bypass suspended fines in amounts adequate to generate seaward prograding blankets of mud. Climax occurs when sand as well as mud is bypassed, and fine sand spreads to all parts of the shelf. This hypothetical cycle is ideal rather than normative and its degree of realization depends on energy imput, rates of subsidence and sedimentation, and other factors which control shelf sedimentation.

Recent Sedimentation on Southern Bering Shelf: ABSTRACT

A veneer of contemporary sediments covers the entire continental shelf in the Bristol Bay region of the southeast Bering Sea. This broad shelf, covering an area of 1.5 × 105 sq km has a relatively smooth bottom and gentle slope of about 1:4,000. Nearshore sediment is gravelly, coarse sand which grades to fine sand offshore. In general, the mean size of the sediment is related to the depth. The nearshore and offshore heterogenous sediments are very poorly sorted whereas homogeneous sand in the center is well sorted. The mineralogy of sand is diverse, reflecting the complex sources: sedimentary, metamorphic, and igneous terrane of the hinterland in the north and east, and typically volcanics from the Aleutian Islands in the End_Page 2503------------------------------ south. The general paucity of clay minerals and dominance of feldspars in size fraction The sediment character and distribution in Bristol Bay reflects two distinct sedimentary environments of deposition. Sediments on the shallow shelf are controlled by the storm-generated-wave drift currents, are moderately well sorted, and are devoid of silt and clay. In protected bays, and at water depths prohibiting elutriation by storm waves, the silt-clay content in sediments increases significantly. Local influences of fluvial, tidal, and wind-driven currents on sediments are also apparent. Long waves generated by frequent storms roil shelf sediments with a net seaward movement. The absence of silt and clay sediments and abundance of fine sand on the shelf and seaward prograding blankets of mud near the slope reflect midstage of shelf grading. Sedimentation in Bristol Bay shelf ma well be an analogue to the widespread, uniform epicontinental clastic deposits that were formed in the past. End_of_Article - Last_Page 2504------------