Hudson Shelf Valley Research Articles

Wind-Driven Circulation in a Shelf Valley. Part I: Mechanism of the Asymmetrical Response to Along-Shelf Winds in Opposite Directions

AbstractMotivated by observations in Hudson shelf valley showing stronger onshore than offshore flows, this study investigates wind-driven flows in idealized shallow shelf valleys. This first part of a two-part sequence focuses on the mechanism of the asymmetrical flow response in a valley to along-shelf winds of opposite directions. Model simulations show that (i) when the wind is in the opposite direction to coastal-trapped wave (CTW) phase propagation, the shelf flow turns onshore in the valley and generates strong up-valley transport and a standing meander on the upstream side (in the sense of CTW phase propagation) of the valley, and (ii) when the wind is in the same direction as CTW phase propagation, the flow forms a symmetric onshore detour pattern over the valley with negligible down-valley transport. Comparison of the modeled upstream meanders in the first scenario with CTW characteristics confirms that the up-valley flow results from CTWs being arrested by the wind-driven shelf flow establishing lee waves. The valley bathymetry generates an initial excessive onshore pressure gradient force that drives the up-valley flow and induces CTW lee waves that sustain the up-valley flow. When the wind-driven shelf flow aligns with CTW phase propagation, the initial disturbance generated in the valley propagates away, allowing the valley flow to adjust to roughly follow isobaths. Because of the similarity in the physical setup, this mechanism of arrested CTWs generating stronger onshore than offshore flow is expected to be applicable to the flow response in slope canyons to along-isobath background flows of opposite directions.

Quaternary fluvial history of the Delaware River, New Jersey and Pennsylvania, USA: The effects of glaciation, glacioisostasy, and eustasy on a proglacial river system

Fluvial, glacial, and estuarine deposits in the Delaware Valley record the response of the Delaware River to glaciation, sea-level change, and glacioisostasy during the Quaternary. Incision following an early Pleistocene glaciation created the present valley, which is inset into a Pliocene strath and fluvial plain. Middle and upper Pleistocene and Holocene deposits were laid down in this inset valley. Estuarine terraces in the lower valley and bayshore at +20m (probably Marine Isotope Stage [MIS] 11), +8m (MIS 5e), and +3m (MIS 5a or c), and a fluvial deposit that correlates to offshore MIS 3 marine deposits at −20m are at elevations consistent with glacioisostatic models. Successive incisions during lowstands in the middle and late Pleistocene lengthened, deepened, and narrowed the channel in the lower valley and shifted the channel westward in Delaware Bay. During MIS 2 glaciation, from 25 to 18ka, the Delaware was diverted to the Hudson Shelf Valley by glacioisostatic tilting. Most glacial sediment was trapped in fluvial-lacustrine valley fills north of the terminal moraine. Incision of the valley fill was accomplished during the early stage of rebound, between 17 and 12ka. Drainage to the Delaware shelf was restored between 15 and 13ka as the forebulge collapsed. During incision, multiple postglacial terraces formed where the valley was perpendicular to rebound contours and so was steepened and elevated northward; and a single terrace formed where the valley paralleled the contours, and there was no differential elevation or steepening. About 65% of the original volume of MIS 2 glacial sediment remains in the main valley, and most of the eroded volume is in the channel in the lower valley beneath Holocene estuarine fill. Little glacial sediment reached the Delaware or Hudson shelf. Overbank deposition on the lower postglacial terrace and modern floodplain spans the Holocene. The volume of Holocene sediment in the estuary and bay yields a basinwide denudation rate of about 20m/my.

The vertical structure of the circulation and dynamics in Hudson Shelf Valley

AbstractHudson Shelf Valley is a 20–30 m deep, 5–10 km wide v‐shaped submarine valley that extends across the Middle Atlantic Bight continental shelf. The valley provides a conduit for cross‐shelf exchange via along‐valley currents of 0.5 m s−1 or more. Current profile, pressure, and density observations collected during the winter of 1999–2000 are used to examine the vertical structure and dynamics of the flow. Near‐bottom along‐valley currents having times scales of a few days are driven by cross‐shelf pressure gradients setup by wind stresses, with eastward (westward) winds driving onshore (offshore) flow within the valley. The along‐valley momentum balance in the bottom boundary layer is predominantly between the pressure gradient and bottom stress because the valley bathymetry limits current veering. Above the bottom boundary layer, the flow veers toward an along‐shelf (cross‐valley) orientation and a geostrophic balance with some contribution from the wind stress (surface Ekman layer). The vertical structure and strength of the along‐valley current depends on the magnitude and direction of the wind stress. During offshore flows driven by westward winds, the near‐bottom stratification within the valley increases resulting in a thinner bottom boundary layer and weaker offshore currents. Conversely, during onshore flows driven by eastward winds the near‐bottom stratification decreases resulting in a thicker bottom boundary layer and stronger onshore currents. Consequently, for wind stress magnitudes exceeding 0.1 N m−2, onshore along‐valley transport associated with eastward wind stress exceeds the offshore transport associated with westward wind stress of the same magnitude.

Pleistocene sequence stratigraphy of the shallow continental shelf, offshore New Jersey: Constraints of Integrated Ocean Drilling Program Leg 313 core holes

We used cores and logs from Integrated Ocean Drilling Program (IODP) Expedition 313 to generate biostratigraphic, lithofacies, biofacies, and geochemical data that constrain the ages and paleoenvironments of Pleistocene sequences. We integrate sequence stratigraphy on cores with new seismic stratigraphic data to interpret the Pleistocene history of the Hudson shelf valley and paleoenvironmental and sea-level changes on the inner to middle continental shelf. Improved age control compared to previous studies is provided by integrated calcareous nannofossil biostratigraphy, Sr isotopic stratigraphy, and amino acid racemization. We recognize four upper Pleistocene–Holocene sequences: sequence uP1 is correlated with Marine Isotope Chrons (MIC; “chron” is the correct stratigraphic term for a time unit, not “stage”) 7 or 5e, sequence uP2 with MIC 5c, sequence uP3 with MIC 5a, and sequence uP4 with the latest Pleistocene to Holocene (MIC 1–2). However, within our age resolution it is possible that sequences uP2 and uP3 correlate with MIC 4–3c and 3a, respectively, as suggested by previous studies. Lower Pleistocene sequences lP1 and lP2 likely correlate with peak interglacials (e.g., MIC 31 and MIC 45 or 47, respectively). Thus, we suggest that preservation of sequences occurs only during peak eustatic events (e.g., MIC 45 or 47, MIC 31, and MIC 5), unless they are preserved in eroded valleys. The architecture of the Pleistocene deposits at Sites M27 and M29 is one of thin remnants of highstand and transgressive systems tracts, with lowstand deposits only preserved in the thalwegs of incised valleys. Incised valleys at the bases of sequences uP3 (IODP Site M27) and uP2 (IODP Site M29) document more southward courses of the paleo–Hudson valley, compared to the more southeastward course of the MIC 1–2 paleo–Hudson valley. The patchy distribution of Pleistocene sequences beneath the New Jersey inner-middle continental shelf is due to low accommodation during an interval of large eustatic changes; this predicts that sequences in such settings will be discontinuous, patchy, and difficult to correlate, consistent with previous studies in Virginia and North Carolina.

Simulation of Water Age and Residence Time in New York Bight

Abstract The time scales on which river inflows disperse in the coastal ocean are relevant to a host of biogeochemical and environmental processes. These time scales are examined in a modeling study of the Hudson River plume on its entry to the New York Bight (NYB). Constituent-oriented age and residence-time theory is applied to compute two time scales: mean age, which is calculated from the ratio of two model tracers, and residence time, which is calculated using the adjoint of the tracer conservation equation. Spatial and temporal variability associated with river discharge and wind is investigated. High river discharge lowers surface water age and shortens residence time in the apex of the NYB. Easterly winds increase surface water age and extend the duration waters along the Long Island coast remain in the NYB apex. Southerly winds increase age along the New Jersey coast but drive a decrease in age of offshore surface waters and prolong the time that surface waters close to the New Jersey coast stay in the NYB apex. Residence time along the Long Island coast is high in spring and summer because of the retention of water north of the Hudson shelf valley. Patterns of modeled surface water age and an age proxy computed from the ratio of satellite-measured irradiance in two channels show qualitative agreement. A least squares fit gives a statistically significant empirical relationship between the band ratio and modeled mean age for NYB waters.

Shallow stratigraphy and complex transgressive ravinement on the New Jersey middle and outer continental shelf

The ravinement surface is a key marker for examining the effects of the most recent sea-level transgression on development of the latest Pleistocene–Holocene stratigraphic record of the New Jersey middle and outer continental shelf. This surface is marked by a moderate-amplitude reflector, identified as the “T” horizon in chirp seismic data, which caps filled incised valleys formed during or soon after the Last Glacial Maximum (LGM) and eustatic lowstand, and also serves as the base of a Holocene sand sheet. The “T” horizon is clearly erosional; we interpret it as marking landward migration of the shore zone across the coastal plain during the earliest-mid Holocene. We use chirp seismic surveys acquired in 1999 and 2001 to map the geomorphology of the “T” horizon over a ~ 100 by 60 km region. Shallow, bathymetric contour-parallel depressions, ~ 10 km wide and ~ 8 m deep, are present in the horizon at ~ 50–60 m and ~ 70 m water depths; we interpret them as preserved lagoonal/back-barrier topography; portions of filled tidal channels occur in these depressions. Perhaps these depressions represent pauses in the retreat of the shoreline, either as a result of eustatic fluctuations or slope steepening, allowing for formation of back-barrier/lagoonal features substantive enough to resist complete destruction by subsequent ravinement. Alternatively, they may be locations of local flattening in the antecedent topography that forced rapid drowning and preservation of back-barrier morphology. These paleo-estuarine/back-barrier features may also be more susceptible to erosion during transgression, so the presence of these depressions might have focused marine erosion in some way, perhaps by promoting near-bottom turbulence during storm events. Seismic stratigraphy and sediment cores show that the mid-shelf wedge (MSW) is composed of clay-rich latest Pleistocene and sand-rich Holocene components. Latest Pleistocene sediments of the MSW are complex, representing both erosional and depositional processes. In part, the seaward boundary of this part of the MSW is defined by the mid-shelf scarp (MSS), an erosion product of transgressive ravinement ~ 11.4 ka. The Pleistocene MSW appears to predate post-LGM glacial lake collapses to the northwest, which have been suggested as sources for subaerial, lobate deposition north of the Hudson Shelf Valley (HSV). The primary origin of this part of the MSW may instead be subaqueous deltaic formation associated with fluvial drainage, perhaps part of the ancestral Hudson River system, prior to the LGM. In contrast, the Holocene, sandy part of the MSW must have been deposited in a marine setting, because this deposition postdates transgressive ravinement. The Holocene MSW may be derived from Hudson river sediments, or from continuing shallow submarine erosion following landward shore-zone passage. The preexisting Pleistocene MSW provided accommodation and a source of reworked sediment for this deposit at its toe. Published sea-level curves confirm that Holocene deposits postdate known glacial lake collapses behind terminal moraines to the north. Erosion on this margin continues and, as a result, the maximum flooding surface has not yet formed.

Bulge Formation of a Buoyant River Outflow

Observations taken during the Lagrangian Transport and Transformation Experiment (LaTTE) in 2005 indicated that the Hudson's river outflow formed a bulge of recirculating fluid that limits the volume of fresh water that is advected away in a coastal current. Focusing on an event that began with downwelling winds we made estimates of the fresh‐water flux in the coastal current and the fresh water inventory of the bulge. The coastal current was characterized by a surface advected plume in thermal wind balance. However, the freshwater transport in the coastal current was less than 1/2 of the total freshwater outflow. The bulge extended 30 km from the coast and 40 km in the along‐shore direction and was evident in ocean color imagery. Recirculation in the bulge region was also apparent in daily averaged surface current radar data, but this flow pattern was obscured in the hourly data by tidal and wind‐forcing even in the diurnal band. Nevertheless, many aspects of the Hudson's outflow are consistent with recent laboratory experiments and numerical simulations of buoyant discharges. The growing bulge transports the river's outflow to the head of the Hudson shelf valley where it crosses the 50 m isobath. Previous work in this region indicates that frontal features reside along this isobath. We observed fresh water being transported along this isobath and is suggestive of a rapid cross‐shelf transport pathway for fresh water. Both the bulge formation and cross‐shelf transport have significant biogeochemical implications.

A catastrophic meltwater flood event and the formation of the Hudson Shelf Valley

The Hudson Shelf Valley (HSV) is the largest physiographic feature on the U.S. mid-Atlantic continental shelf. The 150-km long valley is the submerged extension of the ancestral Hudson River Valley that connects to the Hudson Canyon. Unlike other incised valleys on the mid-Atlantic shelf, it has not been infilled with sediment during the Holocene. Analyses of multibeam bathymetry, acoustic backscatter intensity, and high-resolution seismic reflection profiles reveal morphologic and stratigraphic evidence for a catastrophic meltwater flood event that formed the modern HSV. The valley and its distal deposits record a discrete flood event that carved 15-m high banks, formed a 120-km 2 field of 3- to 6-m high bedforms, and deposited a subaqueous delta on the outer shelf. The HSV is inferred to have been carved initially by precipitation and meltwater runoff during the advance of the Laurentide Ice Sheet, and later by the drainage of early proglacial lakes through stable spillways. A flood resulting from the failure of the terminal moraine dam at the Narrows between Staten Island and Long Island, New York, allowed glacial lakes in the Hudson and Ontario basins to drain across the continental shelf. Water level changes in the Hudson River basin associated with the catastrophic drainage of glacial lakes Iroquois, Vermont, and Albany around 11,450 14C year BP (∼ 13,350 cal BP) may have precipitated dam failure at the Narrows. This 3200 km 3 discharge of freshwater entered the North Atlantic proximal to the Gulf Stream and may have affected thermohaline circulation at the onset of the Intra-Allerød Cold Period. Based on bedform characteristics and fluvial morphology in the HSV, the maximum freshwater flux during the flood event is estimated to be ∼ 0.46 Sv for a duration of ∼ 80 days.

Winter-time circulation and sediment transport in the Hudson Shelf Valley

The Hudson Shelf Valley is a bathymetric low that extends across the continental shelf offshore of New York and New Jersey. From December 1999 to April 2000 a field experiment was carried out to investigate the transport of sediment in the shelf and valley system. Near-bed tripods and water-column moorings were deployed at water depths from 38 to 75 m in the axis of the shelf valley and at about 26 m on the adjacent shelves offshore of New Jersey and Long Island, New York. These measured suspended sediment concentrations, current velocities, waves, and water column properties. This paper analyzes observations made during December 1999 and January 2000, and presents the first direct near-bed measurements of suspended sediment concentration and sediment flux from the region. Sediment transport within the Hudson Shelf Valley was coherent over tens of kilometers, and usually aligned with the axis of the shelf valley. Down-valley (off-shore) transport was associated with energetic waves, winds from the east, moderate current velocities (5–10 cm/s), and sea level setup at Sandy Hook, NJ. Up-valley (shoreward) transport occurred frequently, and was associated with winds from the west, low wave energy, high current velocities (20–40 cm/s), and sea level set-down at the coast. Within the shelf valley, net sediment flux (the product of near-bed concentration and velocity) was directed shoreward, up the axis of the valley. Current velocities and suspended sediment fluxes on the New York and New Jersey continental shelves were lower than within the shelf valley, and exhibited greater variability in alignment. Longer term meteorological data indicate that wind, setup, and wave conditions during the study period were more conducive to up-valley transport than seasonal data suggest as average. To relate the observed up-valley sediment flux to observed accumulation of contaminants within the Hudson Shelf Valley requires consideration of transport over longer timescales than those observed here, and methods that account for the region's complex bathymetry, sediment distribution, and circulation.

A numerical model of the time‐dependent wintertime circulation of the New York Bight

The time‐dependent flow in the New York Bight from March 7 to April 30, 1975, is calculated from approximations to the barotropic vorticity equation subject to forcing by winds observed at the John F. Kennedy airport and by an upstream flow across the Long Island shelf. Comparisons of major axis velocities to observations throughout the bight show reasonable agreement. There is also considerable model skill in the prediction of alongshore variations in the major axis velocity with relatively low absolute errors. In general, associated with northeastward winds are northeastward shelf flows and upvalley currents in the Hudson Shelf Valley (HSV). Downvalley flows in the HSV occur when the northeastward wind abates and the flow on the shelf turns southwestward. The mechanism responsible for the behavior of the valley flow appears to be topographic steering with moderation due to bottom friction. The time rate of change of relative vorticity does not appear to have large net effect over the duration of an event Consequently, steady state models represent well the dynamics for an event‐averaged flow. Discrepancies between model results and observation suggest that there is indeed a southwestward pressure gradient force operating in the New York Bight, and an adiabatic pressure condition offshore may be an over simplification that need be removed.

An externally forced barotropic circulation model for the New York Bight

The sea level and the barotropic, frictional circulation response for the New York Bight are used to demonstrate the effects of external sea-level forcing, bathymetry, and variable friction. The governing equation is the steady, integrated vorticity equation and is computed by finite differencing over a curvilinear grid conforming to the 10- and 100-m isobaths and extending for 250 km alongshore. The boundary conditions are based on the hypothesis that the dynamics of the shelf are driven by the external sea-level gradient and the coastal no-flux condition; and consequently the conditions at the lateral boundaries are dependent thereon. Therefore, the external sea-level slope must be independently specified, and the lateral boundary conditions must be dependently generated. The diabathic component of the external sea slope forces the calm wind circulation by its effect on the transport through the upstream boundary; and the parabathic component has also an important modifying effect by forcing a shelf convergent transport. The parabathic sea slope at the coast is independent of its offshore value, being instead a direct product of the coastal boundary condition. The bottom friction is expressed as related to the sea level through a bottom length parameter and a veer angle, both of which are taken to increase shoreward. An additional bottom stress component, related to the surface stress, is determined for bottom depths less than the Ekman depth. Such bottom stress variability produces significant alterations in the nearshore flow field, over the constant bottom stress formulation, by reducing it and causing it to veer downgradient and downwind in the nearshore. The model is forced by different wind directions and the results are discussed. The circulations generally conform to the observed mean flow patterns, but with several smaller-scale features. The strong bathymetric feature of the Hudson Shelf Valley causes a polarized up- and downvalley flow for winds with an eastward or westward component, respectively. Under mean westerly winds, there is a divergence in the shelf valley flow at about the 60-m isobath. The Apex gyre existing off the western tip of Long Island becomes more extensive for winds changing from northeast to southwest. Mean flow reversals (to the northeast) occur off both Long Island and New Jersey for wind directions changing counterclockwise from northwest to southeast and from west to east, respectively. Southeastward transport over the outer New Jersey shelf tends to be enhanced by wind and external sea-level conditions; and the transport over the New Jersey midshelf, particularly in the lee of the shelf valley, tends to be weak and variable also under these mean conditions.

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

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

Sediment transport in the New York Bight, North American Atlantic Shelf

Aanderaa current-meter data collected during the course of NOAA's New York Bight Project are sufficiently extensive in time (135 000 observation hours) and space (41 stations) to permit an estimate of the sediment transport in this area. The analysis is based on mean flow observations; the role of wave oscillatory currents in resuspending sediment is considered but not explicitly included in the sediment transport calculations. An estimate of the potential sediment transport rate Q B at each station for three different threshold velocities U TH (12, 16, and 20 cm s −1) were made using a Bagnoldtype transport equation: Q B = const (U c−U TH) 3 for U c > U TH Q B = 0 for U c > U TH where U c is the mean current speed. Comparison was made between the results from this simple equation and other bedload and suspended-load transport rates (which include wave data) for a limited subset of the Bight data. This comparison indicates that the Bagnold results are reasonable for the along-isobath transport estimates, but are not reliable for the cross-isobath transport. In the cross-isobath direction, waves appear to play a dominant role in determining the transport rate. The comparison also indicates that the Bagnold approach tends to overestimate transport in deep water. In shallow water, resuspension of sand and rotor pumping of current meters both may become important, and sediment transport calculations may not be accurate. Fine-sediment suspended transport is approximately double the potential bedload transport for fine sand (0.02 cm), but does not dominate the total transport as is believed to occur on the Washington Shelf (Smith and Hopkins, 1972). The data show that the New York Bight is a storm-dominated shelf, with the general southwestward drift of water (∼6 cm s −1) modulated by more intense storm-induced flows. The transport calculations indicate that the bulk of sediment transport occurs during the winter months when wind stress is at a maximum and the water column is well mixed. Two major transport provinces are present; (a) the Hudson Shelf Valley, and (b) the rest of the shelf. The Shelf Valley acts as a sink and a barrier to the continuity of the mainly southwestward transport of sediment (∼10 4 g cm −1 yr −1) along the shelf. Within the valley the potential transport of material is of the order of 10 5 g cm − yr −1 along the valley axis. Progressive vector transport paths for fine sediment indicate that removal of sediment out of the ends of the valley can occur rapidly. However, consideration of the fine-sediment budget for the whole Hudson Shelf Valley and of the attenuation of wave activity with depth show that these results are not inconsistent with the presence of high mud concentrations in the valley. Up-valley sediment rates predominate during the period for which data are available.

The Hudson Shelf Valley: Its role in shelf sediment transport

The Hudson Shelf Valley was deeply incised by Pleistocene lowstands of the sea. During the Holocene transgression, fluvial, estuarine, and estuary-mouth depositional environments were displaced landward, and sedimentation in these environments partially filled the valley. Holocene sediment thickness in the seaward half of the valley is about the same as on the shelf to either side, but in the landward half of the valley, 22 m of sediment is present versus 5–10 m on the adjacent shelf. Sediment volume calculations in the upper shelf valley show that there is about three times the infill volume east of the thalweg as compared to the infill to the west. This configuration indicates that the valley has served as a sediment trap and records the westward migration of the thalweg during the Holocene. Most of the infill was the result of littoral drift which transported sand westward along the south shore of Long Island during the Holocene transgression. Modern sediment transport, as calculated from currentmeter observations and inferred from bedform observations, is mainly south westward, along the regional trend of the isobaths and across the shelf valley. Therefore, infill on the eastern side of the shelf valley is continuing at present. Observations of the regional distribution of sediment types indicate that the asymmetrical valley fill is part of a pattern of bottom response to flow. The former drainage divides on both sides of the shelf valley are surfaced by coarse lag deposits on slopes facing northeast, into the direction from which the major storm flows come, while the down-current sides are mantled with fine sand.

On the theory of deep flow in the Hudson Shelf Valley

The time averaged near‐bottom current in the New York Bight for a selected 3‐day period is idealized as a steady state response to the mean wind at the John F. Kennedy International Airport and the mean inflow condition across the Long Island shelf. The idealization invokes potential vorticity conservation in a homogeneous ocean subject to a linear bottom friction and results in shelf currents whose longshore components are in general agreement with the averages of the observed. In the Hudson Shelf Valley, qualitative agreement with the observation is achieved in the onshore component which is directed nearly along the valley axis. Nearshore, within 40 km or so of the coast, the response is, to a substantial degree, wind driven. Nearshore upvalley mean flow is due partly to shore‐parallel wind stresses that are directed away from the valley and is due partly to topographic deflection of an alongshore current that is driven in part by the inflow condition and in part by the alongshore component of the mean wind stress. Because of the important role of bottom friction, an alongshore current driven by longshore winds undergoes sharper (and more realistic) deflection across the valley than does an alongshore current driven by the inflow condition.

Hudson River: Evidence for extensive migration on the exposed continental shelf during Pleistocene time

Analyses of seismic-reflection profiles collected off New Jersey reveal a large buried channel that splits from the topographic Hudson shelf valley beneath the inner shelf and extends southward for at least 80 km. The buried valley has a flat bottom, a width of 2 to 17 km, and a relief of 3 to 15 m, all features similar to those of the Hudson shelf valley. The buried valley apparently is an ancestral pathway of the Hudson River that has been filled with heterogeneous fluvial deposits and capped by an additional 10 to 30 m of sediments. Vibracores from over or near the ancestral valley show that interbedded marine sand and mud layers constitute the upper part of the sedimentary fill. Radiocarbon ages, geotechnical properties, and micro-paleontological analyses of the core sediments indicate that the valley was formed and filled sometime prior to 28,000 yr ago and then was exposed subaerially during at least one sea-level regression. These results are the first clear subbottom evidence that the ancestral Hudson River flowed south of the Hudson shelf valley on the exposed continental shelf during Pleistocene time.

Aspects of Geologic History and Marine Geology of New York Bight: ABSTRACT

The New York Bight consists of two shelf sectors: (1) New Jersey shelf, trending NE-SW; and (2) Long Island shelf, trending east-west, which are separated by the Hudson shelf valley (extending southeast from outer New York Harbor to the Hudson apron at the shelf edge near the head of the Hudson submarine canyon). Although the present shoreline configuration of the bight results from the Holocene submergence, 2 important earlier developments include post-Miocene uplift of the Appalachians and especially of New England, and Quaternary glaciations. During the Pliocene, uplift inland created the present regional dips of the coastal-plain strata and supplied sediment to fans which prograded a thin sheet of gravel across the marine coastal-plain formations. There followed a great drop in base level and deep cutting of major river valleys. The inner cuesta and adjoining inner lowland of the coastal plain, part of which is now filled by the water of Long Island Sound, were formed and large quantities of sediment built the shelf edge seaward. Erosion in submarine canyons was especially active at this time. During the Pleistocene, in connection with continental glaciations, many changes of base level and climate occurred. During the Wisconsin Stage, ice flowing from about N20°W deposited 2 prominent terminal moraines atop Long Island's coastal-plain cuesta--the Ronkonkama (older and more southerly) and Harbor Hill (younger and more northerly) moraines. During each deglaciation, wide outwash plains spread across parts of what is now the Long Island shelf and extensive lakes floored by varved clays formed north of each moraine ridge. Rapid drainage of these moraine-dammed lakes through the Narrows (between Staten Island and Brooklyn) is thought to have been responsible for final erosion of the Hudson shelf valley and for major deposition on the Hudson apron. As the sea transgressed across sandy outwash (Long Island shelf) and fluvial gravels (New Jersey shelf), it formed barrier chains and reworked the sediments. During parts of the submergence, some barrier chains retreated progressively landward, and the surf zone reworked all shelf sediments. At other times the barriers were drowned in place. End_of_Article - Last_Page 780------------