David B. Prior

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

Morphology of a submarine slide, Kitimat Arm, British Columbia

A digitally acquired, scale-corrected side-scan sonar survey yielded high-resolution imagery of a submarine landslide in British Columbia. The landslide, in a fjord-head setting at Kitimat, was last active in 1975 and created a wide area of deformed sea floor. The sediment failure involved shallow rotational movements on the slopes of a fjord-head delta, marginal tearing, translational sliding, compressional folding, and block gliding of fjord-bottom marine clays. The slide is shallow and elongate and appears to have been produced by failure in mobile, low-strength sediments.

Hyperpycnal plumes and plume fronts over the Huanghe (Yellow River) delta front

The Huanghe (Yellow River) discharges extremely high suspended sediment concentrations (25 to 220 g/l) which favor sustained hyperpycnal plumes (underflows). Observations of weakly hyperpycnal unchannelized plumes and indirect evidence of strongly hyperpycnal channelized underflows over the delta front indicate the importance of these modes of sediment dispersal. The weakly hyperpycnal plumes occupy the entire water column over the shallow (<5 m) delta top. From a pronounced front near the break in slope at about 5 m depth, they descend over the delta-front slope as wide-spread underflows. Evidence of strongly hyperpycnal underflows was shown from subaqueous valleys partly filled with low-density mud.

Sedimentary framework of the modern Huanghe (Yellow River) delta

The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]

Active Slides and Flows in Underconsolidated Marine Sediments on the Slopes of the Mississippi Delta

On the continental shelves off large deltas, rapid progradation and deposition result in highly underconsolidated marine sediments. These deposits, which are often also rich in interstitial methane gas, can be subject to widespread and active mass movement downslope. For example, the submarine slopes of the Mississippi River delta are affected by a variety of sediment instability processes. Geologic and geophysical surveys using side-scan sonar, subbottom profilers, and precision depth recorders have been completed for the entire subaqueous delta. Survey lines were spaced at 240-m intervals, and water depths ranged from 5 m to 20 m. Bottom morphology, including sediment deformations indicative of instability, has been mapped at a scale of 1:12,000, and large-area, scale-corrected sonar mosaics have been constructed. The features identified include collapse depressions, bottleneck slides, shallow rotational slides, mudflow gullies, overlapping mudflow lobes, and a wide variety of faults. The slides and mudflows are extremely active, and movement rates of several hundred metres per year have been recorded. Damage to offshore oil and gas pipelines and platforms has occurred. Also, the concept of slow, continuous deltaic progradation must be modified to include the effects of these processes. For example, on the shelf, normal settling of suspended clays averages only a few millimetres per year, whereas at the front of the delta slope more than 30 m of sediment has been deposited by mudflows and slides since 1875.

Evidence for Sediment Eruption on Deep Sea Floor, Gulf of Mexico

A large crater has been discovered on the sea floor, Gulf of Mexico, in a water depth of 2176 meters. Deep-tow high-resolution imagery shows that the crater is cut into a low hill surrounded by near-surface concentric faults. Approximately 2 million cubic meters of ejected sediment forms a peripheral debris field. The low hill and faults may be related to mud diapirism or intrusion of gas hydrates into near-surface sediments. A recent eruption evacuated sediments from the crater, apparently because of release of overpressured petrogenic gas.

Generation of Martian chaos and channels by debris flows

Abstract A debris flow mechanism is proposed to account for the formation of chaos and the large channels debouching into Chryse Planitia from the adjacent southern uplands of Mars. The debris is thought to have originated through a mechanism of collapse in the chaotic terrains which exist at the head of these channels as well as locally along the channels. This proposition is based on the detailed morphologic similarities between Martian channel source areas and the heads of both subaerial and subaqueous terrestrial debris flows. The downslope movement of the debris produced the channels through (a) modification of earlier collapse areas, (b) active bed erosion, and (c) loading-induced collapse. The large-scale channel geometry and the assemblage of related morphologic features on Mars correspond tto that observed in subaqueous debris flow chutes on the Mississippi delta front. Through various mechanisms of strain-dependent viscosity decrease the debris flow gained mobility downstream, turned into a debris avalanche, and moved onto Chryse Planitia at very high velocities. This high-velocity avalanche eroded a series of streamlined remnants near the channel mouths and deposited its load as a thin blanket over a large area of the basin creating virtually no depositional relief.

Disintegrating retrogressive landslides on very‐low‐angle subaqueous slopes, Mississippi delta

Abstract Side‐scan sonar records from the interdistributary bay areas of the Mississippi delta (East Bay, Garden Island Bay, and shallow water areas adjacent to Pass a Loutre) have shown widespread subaqueous disturbance of the bottom sediments. These occur in shallow water and on slopes with very low inclinations (0.01°‐0.45°). The morphology of the features is indicative of mass movement processes involving subsidence and downslope translatory movements. The precise conditions under which failure occurs have not been fully documented, but a conceptual model of potential factor interaction can be formulated.

Active sand transport along a fjord-bottom channel, Bute Inlet, British Columbia

An underwater channel system is incised in the Holocene fjord basin sediments of Bute Inlet, British Columbia. High-resolution side-scan sonar swaths and seismic profiles reveal two channels within a zone of extensive rotational sliding on the slopes of a fjord-head delta. The channels coalesce into a single sinuous channel that extends 14 km downfjord on a 0.9° slope where it splits into two distributaries, which continue another 18 km, ending within stacked depositional lobes. Piston cores show sands in the channels and lobes in contrast to deep-water silty clays in the basin floor. The sea-floor features and sediment distributions result from active, highly mobile slide-generated sediment flows, which transport sands via the channels from the delta to the fjord basin.

Active slope failure, sediment collapse, and silt flows on the modern subaqueous Huanghe (Yellow River) delta

Post-depositional slope instability and bottom mass-movement processes strongly modify the progradational subaqueous slopes of the modern Huanghe (Yellow River) Delta. Wide, shallow gullies dissect the submarine slopes with gradients of 0.3 to 0.4°. Lower delta-front sediments experiencein situ subsidence, forming numerous collapse depressions. These processes are pronounced over much of the delta, incising and redistributing the most recently deposited silt-rich sediment. Principal causative factors include low sediment strengths created by rapid deposition in the delta during annual peak discharges from the river and severe bottom perturbations by surface storm-generated waves.