John E. Hughes-Clarke

HIGH-RESOLUTION SWATH SONAR INVESTIGATION OF SAND RIDGE, DUNE AND RIBBON MORPHOLOGY IN THE OFFSHORE ENVIRONMENT OF THE NEW JERSEY MARGIN

Abstract Sand ridges on the northeast US Atlantic shelf form in the near-shore environment, most likely in response to storm-driven flows. As the Holocene transgression has continued, the ridges have been transferred to an offshore hydrodynamic regime, where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly. Here, we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller-scale features in an effort to place constraints on bedform development in these deeper waters. A recent high-resolution swath sonar survey mapped a portion of the New Jersey shelf from ∼20 m water depth to the shelf break (∼120 m), imaging both sand ridges and smaller-scaled dunes and ribbons in far greater detail than has been previously possible. Using a robust statistical analysis, we find that the gross morphology of ridges (height, width, length) does not change with depth beyond ∼20 m water depth, and changes in ridge orientation generally mirror changes in regional contour orientation. Hence, ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime. However, on the inner shelf (∼20 m water depth to the Mid-Shelf shore), we do find evidence in the ridge shape, which has an asymmetry opposite to that seen near shore, and in the complex backscatter response that some important modifications to ridges are taking place at these water depths. In contrast, on the mid-shelf (from the Mid-Shelf shore to the Franklin shore), ridges tend to have higher backscatter at the crests, implying that these are largely winnowed, relict features. Lineated, smaller-scale (∼100–500 m wavelength,

Seafloor imagery from the BIG'95 debris flow, western Mediterranean

Seafloor backscatter data are used to image the product of one of the youngest major mass-wasting events in the northwestern Mediterranean Sea: a 26 km3 debris-flow deposit that covers 2000 km2 of the Ebro continental slope and base of slope, offshore Spain. Backscatter images provide unprecedented insights on debris-flow dynamics in the deep sea. A pattern of low-backscatter patches represents large sediment blocks that moved while keeping their internal coherence. High-backscatter alignments restricted to topographic lows that represent coarse sediment pathways separate the blocks. The results presented prove the occurrence of large catastrophic sediment failures near heavily populated coastal areas even in continental margins considered to be geodynamically quiet, such as those of the northwestern Mediterranean.

Application of Spatial Cross-Correlation to Detection of Migration of Submarine Sand Dunes

[1] Knowledge of migration rates of bedforms provides an indirect indication of the behavior of tidally averaged bottom currents, enables optimization of hydrographic survey frequency and may enable calculation of bedload transport rate. To measure bedform migration rate, we test the use of spatial correlation as a measurement method, which quantifies and locates a region of maximum similarity between two spatial variables. For the latter, we use consecutive eight-bit images of spatial gradient, derived from bathymetric digital terrain models, carrying out the correlation over this representation of the shape of the seabed rather than the bathymetric surface. The digital terrain models were compiled from six repeat multibeam surveys of a headland-associated bank near Saint John, New Brunswick, with a roughly 30-day interval. Vectors are drawn depicting the movement of a sand dune at time to toward a point in the spatial correlation array at a later time, t 1 . A number of different techniques of picking the end of the migration vector were used. The sinuosity of the dune crest at the scale of the correlation window has an impact on which migration vector is the better pick. Averaging of migration vectors from consecutive epochs diminishes random errors in the correlation picks using any single pair of images and creates a more accurate picture of the migration field. Migration rates and crest-relative migration directions vary substantially across the sand bank, reflecting the high gradients in bottom shear stress around the headland.

Sediment undulations on the Llobregat prodelta: Signs of early slope instability or sedimentary bedforms?

A field of sediment undulations has been mapped by means of high resolution multibeam bathymetry and seismic reflection profiles in the Llobregat River prodelta, off the city of Barcelona, Catalonia, Spain. Similar features had previously been recognized in other prodelta environments and interpreted either as downslope sediment deformation or sedimentary structures induced by bottom currents or hyperpycnal flows. Since the study area is undergoing significant offshore development, proper interpretation of such sediment undulations is needed for a correct risk assessment. The occurrence of the sedi- ment undulations is restricted to the prodelta front on slope gradients between 3 and 0.2o. The undulations have developed at the edge and atop an area of gas bearing sediments within the Late-Holocene high-stand mud wedge. An evaluation is made of the character- istics of the sediment undulations in order to determine the most likely process for the origin of these structures. Amongst these characteristics are the continuity of the reflec- tions and lack of diffractions in between different undulations, their size distribution (large to small) both from shallow to deep and with depth in section, the asymmetry (de- creasing from proximal to distal), the crest to trough vertical distance on the landward side of the undulations (up to 0.5 m), and the lack of features that could indicate a pro- gressive movement such as growth structures and drag folds. These characteristics indi- cate that the sediment undulations on the Llobregat River prodelta do not result from sediment deformation, but rather from the interaction of bottom currents generated by hyperpycnal flows from the Llobregat River with regional sea water circulation. Their identification as sediment waves implies that such features do not pose a major hazard for further offshore development.

Timing and patterns of basin infilling as documented in Lake Powell during a drought

Between 1999 and 2005, drought in the western United States led to a >44 m fall in the level of Lake Powell (Arizona-Utah), the nations second-largest reservoir. River discharges to the reservoir were halved, yet the rivers still incised the tops of deltas left exposed along the rim of the reservoir by the lake-level fall. Erosion of the deltas enriched the rivers in sediment such that upon entering the reservoir they discharged plunging subaqueous gravity fl ows, one of which was imaged acoustically. Repeat bathymetric surveys of the reservoir show that the gravity fl ows overtopped rockfalls and formed small subaqueous fans, locally raising sedi- ment accumulation rates 10-100-fold. The timing of deep-basin deposition differed regionally across the reservoir with respect to lake-level change. Total mass of sediment transferred from the lake perimeter to its bottom equates to ~22 yr of river input.

The western Fiji Transform Fault and its role in the dismemberment of the Fiji Platform

Abstract The Fiji Transform Fault (FTF) is a sinistral transform fault trending nearly E-W from the central North Fiji Basin triple junction at 17°S, 174°E to the northern Lau Basin and is characterized by very high bathymetric relief and dispersed seismicity. SeaMARC II and GLORIA sidescan imagery reveal the structural fabric along the fault trace. From the triple junction to 176°E, the fault location is uncertain but lies within an area having northeast and southeast trending structures. It is believed that a right step in the fault trace has produced a component of compression. From 176°E to 179°E, the fault is clearly delineated next to the Fiji Platform and within two left step offsets are short spreading segments. It is hypothesized that periodic propagation of these short segments into the Fiji Platform has accompanied changes in the location of the fault and in so doing has broken off parts of the Fiji Platform.

Holocene Muddy Bedforms on the Llobregat River Prodelta Wedge

A field of sediment waves on the Llobregat River prodelta, Catalonia, Spain, is examined using high-resolution multibeam bathymetry and seismic reflection profiles. The sediment waves develop on the prodelta front on slope gradients between 3° and 0.2°, within the Late-Holocene highstand mud wedge. Their characteristics and the most likely process at the origin of these structures are evaluated.