George L. Freeland

Barrier island evolution, middle Atlantic shelf, U.S.A. Part II: Evidence from the shelf floor

Where the Atlantic coastal gradient is low and the substrate is unconsolidated, shoreface processes lead to an equilibrium shoreface surface, characterized by a straightened plan view and a concave-up profile. Maintenance of this surface during the post-glacial sea-level rise may lead to barrier formation by mainland beach detachment, where the outer oceanic shoreface is maintained by wave and current processes. Downwelling storm currents sweep sand from the shoreface of the middle Atlantic bight, transport it downcoast and seaward and deposit it on the adjacent inner shelf. Some of this sand is returned by the action of asymmetrical wave orbital currents during fair weather. However, there is a net loss in most areas. The loss is not made good by river sand input, since on the Atlantic Coast all river sand is trapped by estuaries. As a result the shoreface in these areas is undergoing erosional retreat. Mean horizontal retreat rates are on the order of 1–3 m yr−1 but most movement occurs in brief episodes with recurrence rates on the order of several events per century. Erosional shoreface retreat, coupled with aeolian overshoot, storm washover and the movement of sand through inlets into the lagoons, has lead to landward migration of the Atlantic barriers through the Holocene period of sea-level rise. The landward retreat of the barrier as a whole is as dependent on inlet formation as it is on erosional shoreface retreat. Repeated inlet breaching and the downdrift migration of inlets yields coalescing flood tidal deltas within the lagoon. The resulting surface forms a platform on which the subaerial barrier deposits can advance under the impetus of storm washover and aeolian action. The backbarrier deposits eventually re-emerge at the shoreface, where their upper beds are eroded and recycled. The barrier thus migrates over a pavement of its own flood tidal deltas and washover fans; sand is added to the system partly through the erosion of updrift headlands but mainly by means of scour in inlets, which reaches down to the underlying Pleistocene. The modern shelf sand sheet is the debris blanket resulting from the retreat process. The leading edge occurs as a thin veneer of rip-current fallout on the shoreface; this is periodically stripped off by winter storms to expose the underlying backbarrier strata. Further seaward, shelf sands eroded from the shoreface rest disconformably on backbarrier deposits; shoreface, beach and dune strata are always missing. Every grain of sand in this shelf sand sheet has occupied a position on the beach or upper shoreface within the recent past. However, the sheet as a whole has been reconstituted; its primary structures are those of the inner shelf floor. Backbarrier sand and mud deposits, characterized by channeling and landward-dipping reflectors, can be traced for over 100 km seaward across the middle Atlantic shelf, beneath the modern sand sheet. The backbarrier stratum together with the overlying shelf sand layer constitutes the record of barrier retreat across the Atlantic shelf surface during Holocene time. The sheet-like nature of this record indicates that, viewed at the appropriate time scale, barrier migration is a continuous process.

Quaternary rivers on the New Jersey shelf: Relation of seafloor to buried valleys

The Quaternary evolution of the stream net on the New Jersey shelf has been interpreted on the basis of bathymetric maps and also by means of seismic profiling, with somewhat different results. Maps show the most recent positions of seafloor shelf valleys, but these valleys may have been created by retreating estuary mouths rather than by subaerial stream erosion. Seismic profiles reveal buried valleys of subaerial fluvial origin, which may follow courses that diverge markedly from the trends of associated seafloor valleys. Shelf valleys must be understood in the context of erosional shoreface retreat, a process that largely remade the shelf surface during successive Quaternary transgressions. Most shelf landforms are marine and post-transgressional in origin, having been formed at the foot of the shoreface. Only very large and deeply incised subaerial landforms survive the shoreface-retreat process. The marine landforms that tend to replace or bury subaerial river valleys include shelf valleys created by estuary-mouth scour, shoal-retreat massifs, and shelf deltas. Three distinct shelf valley sets, including both seafloor and buried valleys, are attributable to the ancestral Delaware, Great Egg, and Hudson Rivers, respectively. Individual valleys within valley sets may follow markedly divergent paths. In the case of the Hudson, the estuary retreated up a deeply incised river valley and was confined by it; the shelf valley is a river valley only partially filled by estuarine deposits. In the case of the other two rivers, the estuary mouths became largely decoupled from the underlying river valleys during the transgression, and their retreat paths do not everywhere overlie the buried channels. In each valley set, divergent buried valleys apparently belong to periods of subaerial exposure of the shelf that occurred earlier in Holocene time.

Current lineations and sand waves on the inner shelf, Middle Atlantic Bight of North America

ABSTRACT Elongate bedforms of less than one meter relief are abundant on the North Atlantic shelf floor. Spacing between features, and width of solitary forms, ranges from 15 to 50 m. Length-to-width ratios observed by side-scan sonar are in excess of 10:1. The most common form consists of a band of coarse sand or shelly gravel that is depressed slightly below the level of the finer sand on either side. In some cases these bedforms appear to be erosional windows exposing the basal coarse sand or gravel of the Holocene Transgression; elsewhere they are merely localized lag concentrates. Bedforms on the shore face and adjacent inner shelf tend to be nearly shore-normal with slightly acute angles opening to the northeast. Further seaward most bedforms are parallel to the coast, and to the generalized trend of the isobaths. These relationships lead to the inference that the nearshore features are the troughs of low amplitude, flow-transverse sand waves, and that they are probably responses to the intense, downwelling, along-coast flows that occur during northeaster storms. The offshore bedforms may also be responses to storm flow, but their orientation suggests that they are flow-parallel current lineations, perhaps responses to longitudinal vortices in the flow. The bedforms indicate that the shelf floor is responding to the modern hydraulic regime. Time-averaged bed l ad transport is directed downshelf, to the south and west. On the inner shelf there is also an offshore transport component.

Rotation history of Alaskan tectonic blocks

Abstract Alaska is considered to be tectonically comprised of five elongate blocks separated by transcurrent faults formed prior to rotation which enter the state from the southeast and continue westward to the edge of the Bering Sea continental shelf. We propose an additional, inactive fault, indicated by gravity and magnetic data and other observations, to extend between the Bering Strait and the Arctic Ocean continental shelf east of the Northwind escarpment, separating northern Alaska from northeast Siberia. Near the center of the state the faults are bent, concave to the south, about the north-south axis of the so-called Alaska orocline. In our reconstruction the blocks have rotated from a position whereby the north slope was adjacent to Banks Island of the Canadian basin. During the rotation the northernmost, or Brooks block, was squeezed, up to 15% in the western end, to its present width. After rotation, when the three southern blocks were in their present position, the Brooks block and the next block to the south were pushed eastward by North America moving against Siberia, forming the bend in the British-Richardson-Ogilvie Mountains we call the Ogilvie orocline.

The erosion—deposition boundary in the head of Hudson Submarine Canyon defined on the basis of submarine observations

Abstract Submersible observations in the head of Hudson Canyon, to depths of 305 m, indicate that coarse relict sediment at and below the shelf edge (at about 105 m) has been reworked by currents and benthic organisms during the Holocene and at present. The sharp boundary between an upper gravel—shell—sand facies and a deeper mud facies is herein termed “mud line”. This “mud line”, present at depths ranging from 130 m to 175 m, appears to record a long-term separation of energy levels, i.e., below these depths, current speeds necessary to erode fine sediments decrease significantly in frequency and intensity. We observed that the shelf break and uppermost slope in the canyon is a zone of continuing resuspension where there is no permanent accumulation of fines above the “mud line”. Defining the “mud line” position relative to regional lithofacies patterns may provide an additional, albeit indirect, measure of depth of impingement of current speed sufficient to exceed the threshold of sediment transport on the outer continental margin.

The Hudson Shelf Valley: Its role in shelf sediment transport

Abstract Freeland, G.L., Stanley, D.J., Swift, D.J.P. and Lambert, D.N., 1981. The Hudson Shelf Valley: its role in shelf sediment transport. Mar. Geol., 42: 399-427. 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 current-meter 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.

Geotechnical properties of surficial sediments in a mega-corridor: U.S. Atlantic continental slope, rise, and deep-sea basin

Abstract Marine geotechnical studies at The National Oceanic and Atmospheric Administrations (NOAA) Atlantic Oceanographic and Meteorological Laboratories (AOML) for the past fourteen years have resulted in a considerable amount of sediment mass physical properties data, which have provided the basis for a comparative analysis of the surficial sediments of the U.S. Atlantic continental margin and deep-sea basin in a mega-corridor. A synthesis of available mass physical properties data for the Atlantic and Pacific basins was made by Keller and Bennett (1968, 1970). An extensive coring program initiated in 1974 by NOAA along the U.S. Atlantic outer continental margin (outer shelf, slope, and upper rise) between Hydrographer and Hatteras Canyons has provided data for an initial synthesis of the sediment geotechnical properties of the continental slope (Keller et al., 1979). Recent completion of laboratory analyses of the continental rise sediment cores (Lambert et al., in press) offer an opportunity to compare these suites of data. Although the data for the Atlantic deep-sea basin cover a considerably more extensive area than the data for the Atlantic continental margin, there is now adequate geotechnical properties information to show a sharp contrast between predominantly pelagic and hemipelagic deep-sea sediments and continental margin deposits which are primarily terrigenous in origin. In concert with the fining of sediments seaward off the continental margin (Bennett et al., 1977a; Keller et al., 1979), the mean water contents are higher on the rise (95%) than on the slope (88%) and deep basin (86%). In general, average water contents are significantly higher than the liquid limits for surficial slope, rise and basin sediments. Undrained shear strengths average 5–8 kPa for slope, rise and North Atlantic basin deposits, with values for the slope and rise sediments generally higher than the Atlantic basin deposits. Basin deposits have the greatest range in shear strength, as would be expected for such an extensive and sedimentologically diverse area. Sediments associated with the outer Hudson Canyon display the lowest average water content (73%), highest wet unit weight (1.60 Mg/m 3 , wet bulk density) and grain specific gravity (2.76, average grain density) as compared with continental slope, rise and deep-sea Atlantic basin deposits.

Dispersal of Mediterranean and Suez Bay sediments in the Suez Canal

Abstract This study determines the extent to which sediments of Mediterranean, Suez Bay, and in-situ (authigenic, eroded channel) derivation have been displaced along the Suez Canal. Sediment transport is largely a response to hydrodynamics controlled by markedly different oceanographic conditions at both ends of the channeled byway. Petrology of sand, silt and clay fractions determine distributions of diagnostic tracer minerals. These are used to identify five sediment provinces in the canal which indicate long-term dispersal patterns. Sediments of Mediterranean origin (largely terrigenous from the Nile River) are transported southward to Ballah Bypass, while those of Suez Bay—southern Canal derivation (mixed carbonate and terrigenous) are transported northward to the northern edge of Great Bitter Lake. In-situ derived Bitter Lakes sediments, mostly carbonates, are not transported extensively from the lakes area. Similarly derived Lake Timsah sediment is transported into the short stretches of the canal to the north and south of the lake. The canal south of Little Bitter Lake is a zone of erosion, while the Bitter Lakes are sinks for sediments from several sources.

In situ electrical resistivity measurements of calcareous sediments

A four-electrode resistivity probe was successfully designed for use in the upper layers of unconsolidated submarine sediments, with emplacement by vibratory driving. Measurements with this probe in calcareous sediments off the southeast coast of Florida indicate an average conductivity of 1.0 mho/m and a range of 0.4 to 1.4 mho/m.