William L. Stubblefield

Large-scale current lineations on the central New Jersey shelf: Investigations by side-scan sonar

Abstract Two morphological orders of ridge and trough topography can be recognized on a terraced segment (at 37 m) of the central New Jersey shelf: (1) a first-order system with ridges to 14 m high, 2–6 km apart, in a Z-shaped pattern trending to the NNE, and (2) a second-order system with ridges 2–5 m high, 0.5-1.5 km apart and trends to the NE. Side-scan mapping together with submersible observations and bottom samples indicate a third-order system of large-scale current lineations which has been imprinted across the first- and second-order systems. The lineations are low relief forms (to 1.5 m high) which occur as elongate zones of textural contrast arranged in furrows, bands, patches and ribbons and display a uniform directional trend to the ENE. The morphology of the lineations appear to vary in response to the nature of the bottom. The lineations are narrow (10–25 m apart) and have no detectable relief in troughs and wider (to 75 m apart) and higher (to 1.5 m high) on ridges, especially second-order ridges of fine sand. Also revealed are wave ripple patterns and a pattern related to the outcropping of Pleistocene/Holocene units in trough bottoms and lower flanks. It is suggested that the first- and second-order systems developed during earlier stages of the Holocene transgression in response to a hydraulic regime of the inner shelf. The first-order system may have inherited some of its morphology from an older system, but did respond to a Holocene tidal regime adjacent to a major estuary. The second-order system developed in slightly deeper water, subsequent to the resumption of the transgression after the 37-m stillstand. The third-order lineations appear to be a response to the helical-flow structure within the flow field of a major shelf storm. Ridges of the central shelf may be maintained by alternate periods of oblique sweeping during storms, resulting in a net transport of fine sand out of the troughs and up on the ridges. Subsequent wave reworking returns the fine sand to the troughs.

Wilmington Submarine Canyon: a marine fluvial-like system.

Midrange sidescan sonar data (swath width = 5 km) show that a system of gullies and small channels feeds into large submarine canyons on the Middle Atlantic Continental Slope of the United States. The surveyed canyons all have relatively flat floors, but they have different channel morphologies. Wilmington Canyon has a meandering channel that extends down the Continental Slope and across the Continental Rise, whereas two canyons south of Wilmington Canyon have straight channels that trend directly downslope onto the rise. The morphology of these submarine canyon systems is remarkably similar to that of terrestrial fluvial systems.

Reconnaissance in DSRV Alvin of a “fluvial-like” meander system in Wilmington Canyon and slump features in South Wilmington Canyon

Three dives in DSRV Alvin on the Atlantic Continental Rise in Wilmington and South Wilmington Canyons, off the east coast of the United States, allowed examination and sampling of morphological features, in water depths of 2,300 to 2,400 m, that were observed in midrange sidescan sonar data. In Wilmington Canyon, a “fluvial-like” meandering system was confirmed. The meandering channel had steep undercut outer banks and gently sloping inner banks. Localized slumping is inferred from many steplike depressions on the steep outer banks. Although the meander system is as much as 500 m wide and has many characteristics typical of a fluvial system, extreme depth and evidence of episodic ongoing sedimentary processes preclude a true fluvial origin. Currents of unknown origin—that is, downcanyon turbidity flows—appear to be the only agent capable of sculpturing the observed features. A meander system was not observed in South Wilmington Canyon. Channel-floor features, including deformed and displaced sediment, support a previous suggestion of large-scale slumping in the area. These sediments include upturned clay beds, disaggregated gravels, loosely bound gravel conglomerates in a reddish-brown matrix, and a tubular structure, resembling a tree-root cast, within a thinly bedded, reddish-brown sandstone.

Ridge development as revealed by sub-bottom profiles on the central New Jersey shelf

Abstract Closely-spaced 3.5 kHz seismic profiles were collected over the north-easterly trending ridge and swale system 50 km east-southeast of Atlantic City, New Jersey. They yield information on the Late Quaternary depositional history of the area, and on the origin of the ridge system. Four of the sub-bottom reflectors identified were sufficiently persistent to warrant investigation and interpretation. These reflectors, which have been cored, lithologically identified, and radiocarbon dated, are stratigraphically higher than the reflectors dealt with by the majority of previous studies. The upper three reflectors are definitely mid- and post-Wisconsin in age and present a record of the most recent glacial cycle. The upper three units associated with the observed reflectors appear to exert a pronounced influence on the bathymetry. The gently corrugated ridge system of Holocene sand is formed over the regionally flat-lying upper unit, an Early Holocene lagoonal silty clay. The characteristically flat, broad depressions of the area are floored by this lagoonal material. Locally, however, marine scour has cut through the silty clay into an underlying unit of unconsolidated fine Pleistocene sand. Several stages of trough development appear to be represented. After penetrating the lagoonal clay, troughs are initially narrow, but when incised through the sand into a lower, Pleistocene, silty-clay unit, the troughs become notably wider. As downcutting is inhibited by the lower clay, the upper clay is undercut as the trough widens in a fashion similar to a desert blowout. The sub-bottom reflectors indicate that ridge development on the central shelf has involved aggradation as well as erosion. Some ridges seem to have grown by vertical and lateral accretion from small cores. The internal structure of other ridges suggests that they formed by the coalescence of several small ridges. Others appear to have undergone appreciable lateral migration. The ridges appear to be in a state of continuing adjustment to the hydraulic regime of the deepening post-Pleistocene water column.

Grain size variation across sand ridges, New Jersey continental shelf

Cross-ridge grain size profiles were determined for a nearshore and an offshore sand ridge on the New Jersey shelf. On both, the coarset sands occur on the landward flank. The range and rate of change in grain size with horizontal distance are greatest on the nearshore ridge. The grain size gradient is believed to be a response of the bottom to the cross-ridge shear stress distribution during storm flows. The difference between the two ridges may be a consequence of the relict origin of the offshore ridge versus a modern origin for the nearshore ridge, or to the different flow climates of the nearshore and offshore environments.

Sediment Response to the Present Hydraulic Regime on the Central New Jersey Shelf

ABSTRACT Petrographic data, from vibracores and grab samples collected on the Central New Jersey Shelf, suggest a substrate still actively responding to the hydraulic regime. Radiocarbon dates of shell material from the ridge and swale topography indicates aggradation of the ridges crest during the last 500 years and exposure of earlier Holocene material in the deeper troughs of the area. The samples from both the cores and the surficial samples were investigated for heavy mineral percentages and grain size analysis in addition to radiocarbon dating. The concentration of heavy minerals into disseminated bands, as observed in the vibracores, is compatible with sediment transport by sand ripples on the ridges flanks. The grain size variation was subjectively analyzed by applying a Q-mode facto analysis which produced three distinct groupings of the grain size distribution. Each grouping is found to characterize a particular part of the ridge topography. Fine sand and moderate sorting occurs on the flanks, medium to fine sand and moderate sorting occurs on the crests whereas two populations are found in the troughs; coarse, poorly sorted sands and very fine, well sorted sands. This textural variation supports a hypothesis of up-flank rheologic and suspensive transport of medium and fine sand during intense storms and subsequent down-flank winnowing of fine sand during less intense meterological events. The radiocarbon dates indicate that size fractionation and heavy mineral concentrations are subsequent to isolation from a beach environment.

Geology of Great Abaco Submarine Canyon (Blake Plateau): Observations from the research submersible “Alvin”

Abstract Nine dives in the research submersible “Alvin” were made into Great Abaco Submarine Canyon to depths ranging from 1850 to 3666 m. Our observations indicate that the walls of this canyon are distinctly terraced, consisting of nearly vertical to overhanging rock cliffs and intervening, less steep sediment-covered slopes. The wall rock consists mostly of massive, shallow-water limestones and dolostones of Cretaceous age, coated on exposed surfaces with manganese oxides. These rocks are heavily jointed/fractured and thus very blocky to angular in appearance, with sponges and other sessile organisms commonly attached. Talus slopes and sedimentary breccia deposits containing angular boulders are present at the base of these steep escarpments. Short-term bottom current measurements in the axis of the eastern part of the canyon indicate that currents are relatively weak, reaching velocities of only 10 cm/sec. This relatively placid setting is further corroborated by the abundance of turtle grass ( Thalassia ) found along the canyon axis. However, abundant subdued, symmetrical ripple marks and large scour depressions at the base of boulders, indicate that high-energy events sporadically impact the canyon axis. Contemporary erosional activity along the axis of the western (headward) part of the canyon appears to be more significant, as evidenced by asymmetrical ripple marks, sand waves and bioerosion. Great Abaco Canyon has evolved with time via a variety of processes, including: (1) faulting: (2) subsidence; (3) defacement; and (4) erosional down-cutting. The location, orientation and initiation of this canyon appear to be structurally controlled by the Great Abaco Fracture Zone during pre-Santonian time. Regional subsidence during the Mesozoic allowed the walls of Great Abaco Canyon to build vertically by accretion of shallow-water limestones, whereas joint-controlled defacement has widened the canyon while maintaining steep walls. Erosional down-cutting in the canyon axis by carbonate sediment gravity flows also appears to have been important episodically, particularly during the Miocene and Pleistocene.

Petrologic and foraminiferal evidence for active downslope transport in Wilmington Canyon

Abstract The texture and composition of surficial sediment collected by DRSV “Alvin” in a meandering part of Wilmington Canyon, on the upper rise off the Mid-Atlantic States, indicate that deposition from gravity flow or bottom current, or both, has occurred in this sector in recent time. The marked petrologic and foraminiferal variations between closely spaced sample sites across the canyon axis show a distinct pattern that is closely related to the configuration of the thalweg. The sediment distribution pattern recalls that in sinuous fluvial systems: the presence of higher proportions of sand, terrigenous grains and displaced “shallow” water (shelf-upper slope) foraminifera close to the incised part of meanders is a result of active erosion. The irregular down-core variations of texture, terrigenous grains and biogenic components tend to favor a discontinuous “stop-and-go” transport process. The nature and distribution of mixed assemblages of “shallow” and “deep” (lower slope and rise) benthonic and planktonic foraminifera also indicate a downslope-directed transport process involving several pulse-like redepositional phases. Some material was introduced at the head of the canyon, but much sediment has also entered the canyon channel by displacement down canyon walls. It appears that sediment between the shelf-edge and the rise must reside sufficiently long in transit to pick up and incorporate an important mix of “deep” water faunas as the material is displaced seaward.

Development of Middle Continental Shelf Sand Ridges: New Jersey

Mid-shelf ridges on the New Jersey mid-continental shelf, in water depths of 20 to 40 m (65 to 130 ft), were recently reinterpreted as degraded barriers. These barriers are believed to have developed at various intervals between 8,000 and 14,000 years B.P. The 30 km (16.2 nmi) width of this barrier complex suggests: (1) two or more prograding barriers with the younger barrier being landward of the older barrier; or (2) successive small barriers forming landward of preexisting, partly preserved barriers. During periods when sand supply was in excess of the amount needed for maintenance, the barriers prograded. During this time the sea level rose, causing the barriers to both aggrade and prograde. As the barrier grew, an increasing amount of sand was needed to maintain the arrier. When the sea level rise exceeded the amount of available sediment, the barriers became submerged. Our model proposes that, during initial submergence, the dunes and upper shoreface of the barrier were removed by erosion, leaving only the lower portions of the shoreface. After isolation on the mid-shelf, the degraded barriers, which now constitute the mid-shelf ridges, continued to be modified by shelf currents. During winter northeasterlies, bottom currents deepened the preexisting topographic lows and carried the sediment upflank for deposition on the crest. During summer, mild storms and internal waves tended to degrade the upper portion of the ridges. The net result was slow aggradation of the ridges since isolation on the mid-shelf.

Use of Power Spectra to Estimate Characteristics of Sand Ridges on Continental Shelves

Attempts to explain the origin of sand ridges on continental shelves require objective means for quantifying their wavelengths and orientations. Two-dimensional power spectral analysis is such an approach. However, mathematical peculiarities may lead to apparent discrepancies between the results of spectral analysis and wavelength estimates from bathymetric maps. This discrepancy may explain much of the reported difference between wavelength predictions based on hydrodynamic stability theory and measurements from bathymetric maps. Such a discrepancy can be significantly reduced by a modified spectral analysis technique which utilizes power spectra of bottom slopes rather than bathymetry.