Structure of the Sunda Trench lower slope off sumatra from multichannel seismic reflection data

Multichannel seismic reflection profiles across the Sunda Trench slope off central Sumatra reveal details of subduction zone structure. Normal faults formed on the outer ridge of the trench offset deep strate and the oceanic crust, but die out upsection under the trench sediments. At the base of the inner trench slope, shallow reflectors are tilted seaward, while deeper reflectors dip landward parallel to the underlying oceanic crustal reflector. Intermediate depth reflectors can be traced landward through a seaward-dipping monocline. We interpret this fold as the shallow expression of a landward-dipping thrust fault at depth. Landward of this flexure, relatively undeformed strata have been stripped off the oceanic plate, uplifted 700 meters, and accreted to the base of the slope. The oceanic crust is not involved in the deformation at the toe of the slope, and it can be observed dipping landward about 25 km under the toe of the accretionary prism.The middle portion of the trench slope is underlain by deformed accreted strata. Shallow reflectors define anticlinal structures, but coherent deep reflectors are lacking. Reflectors 45 to 55 km landward of the base of the slope dip 4°-5° landward beneath a steep slope, suggesting structural imbrication.A significant sediment apron is absent from the trench slope. Instead, slope basins are developed in 375–1500 m water depths, with an especially large one at about 1500 m water depth that is filled with more than 1.1 seconds of relatively undeformed sediments. The seaward flank of the basin has recently been uplifted, as indicated by shallow landward-dipping reflectors. Earlier periods of uplift also appear to have coincided with sedimentation in this basin, as indicated by numerous angular unconformities in the basin strata.Contribution of the Scripps Institution of Oceanography, new series.     

Analysis of multi-channel seismic reflection and magnetic data along 13° N latitude across the Bay of Bengal

Analysis of the multi-channel seismic reflection, magnetic and bathymetric data collected along a transect, 1110 km long parallel to 13° N latitude across the Bay of Bengal was made. The transect is from the continental shelf off Madras to the continental slope off Andaman Island in water depths of 525 m to 3350 m and across the Western Basin (bounded by foot of the continental slope of Madras and 85° E Ridge), the 85° E Ridge, the Central Basin (between the 85° E Ridge and the Ninetyeast Ridge), the Ninetyeast Ridge and the Sunda Arc. The study revealed eight seismic sequences, H1 to H8 of parallel continuous to discontinuous reflectors. Considering especially depth to the horizons, nature of reflection and on comparison with the published seismic reflection results of Currayet al. (1982), the early Eocene (P) and Miocene (M) unconformities and the base of the Quaternary sediments (Q) are identified on the seismic section. Marked changes in velocities also occur at their boundaries.In the Western Basin the acoustic basement deepening landward is inferred as a crystalline basement overlain by about 6.7 km of sediment. In the Central Basin possibly thicker sediments than in the Western Basin are estimated. The sediments in the Sunda Arc area are relatively thick and appears to have no distinct horizons. But the entire sedimentary section appears to be consisting of folded and possibly faulted layers.The comparatively broader wavelength magnetic anomalies of the Central Basin also indicate deeper depth of their origin. Very prominent double humped feature of the 85° E Ridge and broad basement swell of the Ninetyeast Ridge are buried under about 2.8 km thick sediments except over the prominent basement high near 92° E longitude. The positive structural relief of the buried 85° E Ridge in the area is reflected in magnetic signature of about 450 nT amplitude. Flexural bulge of the 85° E Ridge and subsidence of the Ninetyeast Ridge about 24 cm my^–1 rate since early Eocene period have been inferred from the seismic sequence analysis.     

Bottom processes, morphology, and geotechnical properties of the continental slope south of Baltimore Canyon

The continental slope south of Baltimore Canyon seaward of the coasts of Delaware and Maryland has a different morphology and sedimentary structure than adjacent portions of the continental margin. Ridges of sediment 600 m thick and transverse to the slope contain many unconformities that can be traced from ridge to ridge. The age of the sediment is inferred to be late tertiary to recent with the morphology related to a major drainage system. Physical properties of a suite of sediment cores display a pattern that varies in relationship to the morphology and depositional environment. Sedimentary structures and low shear strengths indicate instability of surficial sediments present on the upper slope and can be correlated with regions where the seismic reflection profiles show slumping has occurred. A veneer of sand overlying the general silty clay of the area is present on the upper slope and on the ridges indicating sand spillover from the shelf with a recent change in deposition pattern.     

A high-resolution geological and geophysical investigation of the Dry Tortugas carbonate depositional environment

 Geophysical surveys and ground truth data are compared from a site in the Dry Tortugas, Florida. Seismic data reveal six depositional sequences bounded by high-amplitude reflectors interpreted as subaerial unconformities. Chirp sonar data reveal structure within the Holocene depositional sequence that is correlated to ground truth data. Sedimentary units within the Holocene sequence record a transition from a low-energy, lagoonal environment, to a high-energy, shallow marine environment, to a moderate-energy, slightly deeper marine environment. Forward modeling and impedance inversion reveal good agreement between sediment physical properties, acoustic properties measured by the electric logger, and the chirp sonar data.     

Submarine canyon development in the Izu-Bonin forearc: A SeaMARC II and seismic survey of Aoga Shima Canyon

SeaMARC II sidescan (imagery and bathymetry) and seismic data reveal the morphology, sedimentary processes, and structural controls on submarine canyon development in the central Izu-Bonin forearc, south of Japan. Canyons extend up to 150 km across the forearc from the trench-slope break to the active volcanic arc. The canyons are most deeply incised (1200–1700 m) into the gentle gradients (1–2°) upslope on the outer arc high (OAH) and lose bathymetric expression on the steep (6–18°) inner trench-slope. The drainage patterns indicate that canyons are formed by both headward erosion and downcutting. Headward erosion proceeds on two scales. Initially, pervasive small-scale mass wasting creates curvilinear channels and pinnate drainage patterns. Large-scale slumping, evidenced by abundant crescent-shaped scarps along the walls and tributaries of Aoga Shima Canyon, occurs only after a channel is present, and provides a mechanism for canyon branching. The largest slump has removed >16 km³ of sediment from an 85 km² area of seafloor bounded by scarps more than 200 m high and may be in the initial stages of forming a new canyon branch. The northern branch of Aoga Shima Canyon has eroded upslope to the flanks of the arc volcanoes allowing direct tapping of this volcaniclastic sediment source. Headward erosion of the southern branch is not as advanced but the canyon may capture sediments supplied by unconfined (non-channelized) mass flows.Oligocene forearc sedimentary processes were dominated by unconfined mass flows that created sub-parallel and continuous sedimentary sequences. Pervasive channel cut-and-fill is limited to the Neogene forearc sedimentary sequences which are characterized by migrating and unconformable seismic sequences. Extensive canyon formation permitting sediment bypassing of the forearc by canyon-confined mass flows began in the early Miocene after the basin was filled to the spill points of the OAH. Structural lows in the OAH determined the initial locus of canyon formation, and outcropping basement rocks have prevented canyon incision on the lower slope. A major jog in the canyon axis, linear tributaries, and a prominent sidescan lineament all trend NW-NNW, reflecting OAH basement influence on canyon morphology. This erosional fabric may reflect joint/fracture patterns in the sedimentary strata that follow the basement trends. Once the canyons have eroded down to more erosion-resistant levels, channel downcutting slows relative to lateral erosion of the canyon walls. This accounts for the change from a narrow canyon axis in the thickly sedimented forearc basin to a wider, more rugged canyon morphology near the OAH. About 9500 km³ of sediment has been eroded from the central, 200 km long, segment of the Izu-Bonin forearc by the formation of Aoga Shima, Myojin Sho and Sumisu Jima canyons. The volume of sediment presently residing in the adjacent trench, accretionary wedge, and lower slope terrace basin accounts for <25% of that eroded from the canyons alone. This implies that a large volume (>3500 km³ per 100 km of trench, ignoring sediments input via forearc bypassing) has been subducted beneath the toe of the trench slope and the small accretionary prism. Unless this sediment has been underplated beneath the forearc, it has recycled arc material into the mantle, possibly influencing the composition of arc volcanism.     

Seismic stacking pattern of the Faro-Albufeira contourite system (Gulf of Cadiz): a Quaternary record of paleoceanographic and tectonic influences

A Quaternary stratigraphic stacking pattern on the Faro-Albufeira drift system has been determined by analysing a dense network of high-resolution single-channel seismic reflection profiles. In the northern sector of the system an upslope migrating depositional sequence (elongate separated mounded drift) parallel to the margin has been observed associated with a flanking boundary channel (Alvarez Cabral moat) that depicts the zone of Mediterranean Outflow Water (MOW) acceleration and/or focussing. A consequent erosion along the right hand border and deposition on the left hand flank is produced in this sector. The sheeted aggrading drift is the basinward prolongation of the elongate separated mounded drift, and developed where the MOW is more widely spread out. The overall sheeted contourite system is separated into two sectors due to the Diego Cao deep. This is a recent erosional deep that has steep erosional walls cut into Quaternary sediments. Two major high-order depositional sequences have been recognised in the Quaternary sedimentary record, Q-I and Q-II, composed of eight minor high-order depositional sequences (from A to H). The same trend in every major and minor depositional sequence is observed, especially in the elongate mounded drift within Q-II formed of: A) Transparent units at the base; B) Smooth, parallel reflectors of moderate-high amplitude units in the upper part; and C) An erosional continuous surface of high amplitude on the top of reflective units. This cyclicity in the acoustic response most likely represents cyclic lithological changes showing coarsening- upward sequences. A total of ten minor units has been distinguished within Q-II where the more representative facies in volume are always the more reflective and are prograding upslope with respect to the transparent ones. There is an important change in the overall architectural stacking of the mounded contourite deposits from a more aggrading depositional sequence (Q-I) to a clear progradational body (Q-II). We suggest that Q-I and Q-II constitute high-order depositional sequences related to a 3rd-order cycle at 800 ky separated by the most prominent sea-level fall at the Mid Pleistocene Revolution (MPR), 900–920 ky ago. In more detail the major high-order depositional sequences (from A to H) can be associated with asymmetric 4th-order climatic and sea-level cycles. In the middle slope, the contourite system has a syn-tectonic development with diapiric intrusions and the Guadalquivir Bank uplift. This syn-tectonic evolution affected the overall southern sheeted drift from the A to F depositional sequences, but G and H are not affected. These last two depositional sequences are less affected by these structures with an aggrading stacking pattern that overlaps the older depositional sequences of the Guadalquivir Bank uplift and diapiric intrusions.     

Depositional characteristics and accumulation model of gas hydrates in northern South China Sea

The South China Sea (SCS) shows favorable conditions for gas hydrate accumulation and exploration prospects. Bottom simulating reflectors (BSRs) are widely distributed in the SCS. Using seismic and sequence stratigraphy, the spatial distribution of BSRs has been determined in three sequences deposited since the Late Miocene. The features of gas hydrate accumulations in northern SCS were systematically analyzed by an integrated analysis of gas source conditions, migration pathways, heat flow values, occurrence characteristics, and depositional conditions (including depositional facies, rates of deposition, sand content, and lithological features) as well as some depositional bodies (structural slopes, slump blocks, and sediment waves). This research shows that particular geological controls are important for the presence of BSRs in the SCS, not so much the basic thermodynamic controls such as temperature, pressure and a gas source. Based on this, a typical depositional accumulation model has been established. This model summarizes the distribution of each depositional system in the continental shelf, continental slope, and continental rise, and also shows the typical elements of gas hydrate accumulations. BSRs appear to commonly occur more in slope-break zones, deep-water gravity flows, and contourites. The gas hydrate-bearing sediments in the Shenhu drilling area mostly contain silt or clay, with a silt content of about 70%. In the continental shelf, BSRs are laterally continuous, and the key to gas hydrate formation and accumulation lies in gas transportation and migration conditions. In the continental slope, a majority of the BSRs are associated with zones of steep and rough relief with long-term alternation of uplift and subsidence. Rapid sediment unloading can provide a favorable sedimentary reservoir for gas hydrates. In the continental rise, BSRs occur in the sediments of submarine fans, turbidity currents.     

Geomorphology and sedimentology of the continental shelf adjacent to Mac. Robertson Land,East Antarctica: A scalped shelf

During the Quaternary, the Mac. Robertson shelf of East Antarctica was deeply eroded by glaciers and currents exposing the underlying basement, resulting in a scalped shelf. Major geomorphic zones are: (1) high-relief, ridge and valley topography (200–1400 m); (2) smooth sea floors associated with low-energy, depositional shelf valleys and basins (400–800 m); (3) low-relief, planated banktops (100–200 m); and (4) iceberg gouged and current reworked seaward-bank margins and upper slope (200 to < 630 m). About 90% of the shelf's surface has net erosional conditions and about 10% is net depositional. The sedimentary processes and deposits may be common to large areas of the East Antarctic margin.     

Neogene stratigraphy and the sedimentary and oceanographic development of the NW European Atlantic margin

A regional correlation of Neogene stratigraphy has been attempted along and across the NW European Atlantic continental margin, between Mid-Norway and SW Ireland. Two unconformity-bounded successions are recognised. These are referred to as the lower and upper Neogene successions, and have been dated as Miocene–early Pliocene and early Pliocene–Holocene, respectively, in age. Their development is interpreted to reflect plate-wide, tectonically driven changes in the sedimentary, oceanographic and latterly climatic evolution of the NE Atlantic region. The lower Neogene succession mainly preserves a record of deep-water sedimentation that indicates an expansion of contourite sediment drifts above submarine unconformities, within this succession, on both sides of the eastern Greenland–Scotland Ridge from the mid-Miocene. This is interpreted to record enhanced deep-water exchange through the Faroe Conduit (deepest part of the Southern Gateway), and can be linked to compressive inversion of the Wyville–Thomson Ridge Complex. Thus, a pervasive, interconnected Arctic–North Atlantic deep-water circulation system is a Neogene phenomenon. The upper Neogene succession records a regional change, at about 4 Ma, in the patterns of contourite sedimentation (submarine erosion, new depocentres) coeval with the onset of rapid seaward-progradation of the continental margin by up to 100 km. This build-out of the shelf and slope is inferred to record a marked increase in sediment supply in response to uplift and tilting of the continental margin. Associated changes in deep-water circulation may be part of an Atlantic-wide reorganisation of ocean bottom currents. Glacial sediments form a major component of the prograding shelf margin (shelf-slope) sediment wedges, but stratigraphic data indicate that the onset of progradation pre-dates significant high-latitude glaciation by at least 1 Ma, and expansive Northern Hemisphere glaciation by at least 3 Ma.     

Sedimentological processes in the southern margin of the Cretan Sea (NE Mediterranean)

The Cretan Basin can be characterized as a back-arc basin of the Hellenic Trench System, that is related to the subduction zone of the African Plate under the Eurasia Plate. The study area includes the narrow and relatively steep (gradient 1.5°) continental shelf of the island of Crete followed by the steep slope (2°–4°) and the rather flat deeper part of the Cretan basin (water depths >1700 m).Surficial sediments of the coastal zone are coarser and of terrigenous origin, while in deeper waters finer sediments, of biogenic origin, are more abundant. Sand-sized calcareous sediment accumulations, identified in middle-lower slope, may be attributed to the aggregation of seabed biogenic material related to the near bed current activity.High resolution profiles (3.5 kHz) taken from the inner shelf shows a typical sigmoid-oblique progradational configuration, implying prodelta sediment accumulation during the Holocene. In the upper-middle slope, sub-bottom reflectors indicate continuous sedimentation of alternating fine and/or coarse grained material. Small-scale gravity induced synsedimentary faults appeared, locally. In contrast, a series of gravity induced faults, identified in the lower slope, are associated with sediment instabilities due to seismotectonic activity. Sediment cores taken from the shelf-break consists of calcareous muddy sand with small amounts of terrigenous silt and fine sand, while the cores recovered from the middle slope has revealed a more homogeneous fine sediment texture of hemipelagic deposition.The prevailing accumulation processes in the southern margin of the Cretan basin are: (i) prodelta deposition in the inner-middle shelf; (ii) settling from bottom nepheloid layers in the shelf and upper slope; (iii) calcareous sediment formation due to settling from suspension and post accumulation aggregation (middle-lower slope); (iv) long-term episodic sediment gravity processes in the lower slope; and (v) to a lesser extent, redeposition from resuspension due to gravity processes and bottom currents.     

The importance of bioturbation to continental slope sediment structure and benthic processes off Cape Hatteras, North Carolina

Even though the continental slope off Cape Hatteras has sediment accumulation rates on the order of 1 cm/year, large areas of soft sediment are intensively reworked by infaunal organisms. Primary sedimentary structures have been completely replaced with biogenic structures. Surface sedimentary structures are dominated by the bioturbational activities of a deep burrowing infauna (to at least 30 cm). The layer actively mixed by the benthos, as evidenced by sediment profile and X-ray images, is estimated to range from 5 to 20 cm, with the residence time for particles within the surface mixed layer ranging from about 4.5 to 18 years. The biological mixing parameter (G) ranges from 0.4 to 5.5, which indicates moderate to strong biological mixing relative to accumulation and strata formation. Bioturbation contributes to the dynamic forces affecting the surface sediments by decreasing compaction of sediment layers and dilating sediment fabrics by sediment mixing, and introducing large water-filled burrows and voids to subsurface sediments. The sediment profile images captured numerous subsurface feeding voids, and worms in the process of making deep burrows, many of which extended below the 5 to 10 cm average depth of the apparent color redoxpotential discontinuity layer. High rates of accumulation of organic-rich sediment lead to high standing stocks of benthos and intensive feeding/burrowing activity that result in organic rich stratagraphic sequences that are thoroughly mixed. Cape Hatteras is a apparent focusing point for the transport of shallow water sediments to the deep sea. Sediments across other areas of the continental slope just 100's of kilometers south of Cape Hatteras are not as thoroughly mixed or biologically active.     

Seismic architecture and seismic geomorphology of heterozoan carbonates: Eocene-Oligocene,Browse Basin,Northwest Shelf,Australia

Seismic characterization of Eocene-Oligocene heterozoan carbonate strata from the Browse Basin, Northwest Shelf of Australia, defines marked progradation of nearly 10 km. Stratal terminations and stacking subdivide the succession into mappable seismic units. Stratal architecture and seismic geomorphology varies systematically through the succession.Individual surfaces, discerned by toplap, onlap, and truncation, outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18°. Sigmoidal clinoforms can include aggradation in excess of ∼200 m, prograde more than 500 m, and have slopes characterized by inclined, wavy to discontinuous reflectors that represent ubiquitous gullies and channels. In contrast, the overlying tangential oblique clinoforms include downstepped shelf margins, limited on-shelf aggradation (<100 m) and toplap, subdued progradation (<500 m), and continuous parallel inclined reflectors on the slope. Wedges of basinally restricted reflectors at toe of slope onlap surfaces of pronounced erosional truncation or syndepositional structural modification. The succession includes repeated patterns of seismic units that onlap, aggrade, and prograde, interpreted to represent sequence sets and composite sequences.The associations of shelf aggradation, shelf-margin progradation, and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation. This distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded, perhaps enhanced by downwelling. Comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture, interpreted to be driven by distinct characteristics of heterozoan sedimentary systems.     

Post-Calabrian sequence stratigraphy of the northwestern Alboran Sea (southwestern Mediterranean)

The post-Calabrian sedimentary column of the northwestern Alboran Sea comprises three depositional sequences. The two older depositional sequences are defined by lowstand systems tracts (shelf-margin deltas, slope, base-of-slope, and basin deposits, and the Guadiaro channel-levee complex). In contrast, the most recent depositional sequence also includes transgressive (relict shelf facies) and high-stand (the Guadalmedina-Guadalhorce prodelta and hemipelagic facies) systems tracts. The stratigraphic architecture of these depositional sequences is controlled by the synchronism between high frequency sea-level changes, variations in sediment supply, and sedimentary processes. The configuration of the depositional sequences is variable and their distribution is complex, as a result of the relative importance played by sea-level changes and tectonism through the area.

The sequence boundaries are represented by polygenetic surfaces in the proximal margin, and by monogenetic surfaces in the distal margin and basin. Each polygenetic surface results from the interaction between the sequence boundary with the lowstand erosional truncation surface and the transgressive surface, both developed during the previous sea-level cycle. The monogenetic surfaces correspond to unconformities and their correlative conformities, formed during sea-level lowstands. This pattern of depositional sequences developed in the margin and basin of the northwestern Alboran Sea shows differences with the Exxon Sequence Stratigraphy Model as traditionally applied: sea-level change control is essentially recognized through lowstand systems tracts, and sequence boundary coincides with lowstand erosional truncation surface and transgressive surface, both developed during the previous sea-level cycle.     

Bedforms and depositional sedimentary structures of a barred nearshore system, eastern Long Island, New York

The depositional sedimentary structures and textures of a single-barred nearshore system on the Atlantic coast of eastern Long Island, New York, were studied along seven shore-normal transects. Data along these transects consisted of textural analysis of 160 sediment samples, temporal bedform observations, and 42 can cores for the analysis of sedimentary structures.

Six sedimentary subenvironments were observed, based on distinct combinations of sediment color and texture, bedforms, physical, and biogenic sedimentary structures, and benthic infaunal communities. The shoreface environment is divided into the upper shoreface, the longshore trough, and the longshore bar. The divisions of the inner shelf environment are the shoreface-inner shelf transition, the offshore, and the coarse-grained deposit. The first five subenvironments are arranged in bands parallel to the shoreline, whereas the coarse-grained deposit occurs in patches across the inner shelf.

The location of fair-weather wave base, coinciding with a reduction in slope (3.0–0.3°) from the shoreface to the inner shelf, is characterized by the cessation of debris surge in the troughs of ripples, the formation of a “rust layer” of microorganisms over the bedform surface, and a sediment color change caused by an increase in organic detritus. The sequence of bedforms and physical sedimentary structures observed in this system fits well with existing wave-generated (oscillatory) flow regime models. These models explain the observed sequences as a response to the degree of asymmetric flow created by shoaling waves. Distribution of biogenic structures and assemblages of infaunal organisms is influenced by the distance landward or seaward of fair-weather wave base.

The overall relationships of this nearshore system can then be summarized as a hypothetical prograding stratigraphic sequence. The entire sequence is underlain by organic-rich, bioturbated, offshore deposits. Overlying the offshore is the planar-laminated sediments of the transition. Grading upward from the transition are the cleaner, planar-laminated, seaward slope deposits of the longshore bar. Above this, is a distinct erosional surface indicating the base of the massive to cross-laminated coarse sediments of the longshore trough. Capping the sequence are the cross- to planar-laminated, clean sands of the upper shoreface and foreshore.     

Sediment deposition in the northern basins of the North Atlantic and characteristic variations in shelf sedimentation along the East Greenland margin

New seismic data off East Greenland were acquired in the summer of 2002, between 77°N and 81°N, north of the Greenland Fracture zone. The data were combined with results from the Greenland Basin and ODP site 909, and indicate a pronounced middle Miocene unconformity within the deep sea basins between 72°N and 81°N. Seismic unit NA-1 consists of sediments older than middle Miocene age and unit NA-2 contains sediments younger than the middle Miocene. Classification of a thinly bedded succession in the Molloy Basin resulted in a subdivision into four units (unit I, unit II, unit IIIA and unit IIIB). A comparison of volume estimations and sediment thickness maps between 72°N and 81°N indicates differences in sediment accumulation in the Greenland, Boreas and Molloy basins. Important controls on the variation of accumulation included different opening times of the basins, as well as tectonic conditions and varying sources of sediment transport.Due to prominent basement structures and the varying reflection character of the sediments along the entire East Greenland margin, we defined an age model of shelf sediments on the basis of similar sediment deposit geometry and known results from other regions. The seismic sequences on the shelf up to an age of middle Miocene are divided into three sub-units along the East Greenland margin: middle Miocene–middle late Miocene (SU-3), middle late Miocene–Pleistocene (SU-2), Pleistocene (SU-1). The differences in the geometry of the sequences show more ice stream related sedimentation between 72°N and 77°N and more ice sheet related sedimentation north of 78°N. The region south of 68°N is dominated by more aggradational sedimentary strata so that a glacio-fluvial drainage seems the main transport mechanism. Due to the Greenland Inland–ice borderlines, we assume the glaciers between the Scoresby Sund and 68°N did not reach the shelf break. A first comparison of the sediment structure of the Northeast Greenland margin with the Southeast Greenland margin made it possible to demonstrate significant differences in sedimentation along this margin.     

Radiocarbon-derived sedimentation rates in the Gulf of Mexico

Sedimentation rates were determined for the northern Gulf of Mexico margin sediments at water depths ranging from 770 to 3560 m, using radiocarbon determinations of organic matter. Resulting sedimentation rates ranged from 3 to 15 cm/kyr, decreasing with increasing water depth. These rates agree with long-term sedimentation rates estimated previously using stratigraphic methods, and with estimates of sediment delivery rates by the Mississippi River to the northern Gulf of Mexico, but are generally higher by 1–2 orders of magnitude than those estimated by ²¹⁰Pb_xs methods. Near-surface slope sediments from 2737 m water depth in the Mississippi River fan were much older than the rest. They had minimum ¹⁴C ages of 16–27 kyr and δ¹³C values ranging from −24‰ to −26.5‰, indicating a terrestrial origin of organic matter. The sediments from this site were thus likely deposited by episodic mass wasting of slope sediment through the canyon, delineating the previously suggested main pathway of sediment and clay movement to abyssal Gulf sediments.     

Salt-controlled slumping on the Mediterranean slope of central Israel

The highly complex morphology of the continental slope of central Israel is the expression of deep-seated rotational block slumping. The overburden of 1.0–1.5 km thick Pliocene-Quaternary sediments that accumulated over 200 m thick evaporites deposited under the deeper portion of the present-day continental slope and in Late Miocene erosion channels, caused the evaporites to flow downslope. This flowage was presumably caused by excessive pore pressures generated by the Pliocene-Quaternary sedimentary overburden in confined layers of clastics embedded within the impervious evaporites.     

High‐frequency subbottom reflection types and lithologic and physical properties of sediments

Abstract

Eight types of reflections are interpreted from 3,800 km of 3.5 kHz profiles taken over a 25,000 km² area of the upper continental slope and shelf in the northeastern Gulf of Mexico off Panama City, Florida. The corresponding sediments in five of the reflection types were sampled in 77 piston cores from which data were obtained on in situ acoustic velocities (V), bulk densities (gr), sediment texture (mean grain size = Mz), CaCO₃ content (C), sedimentary structures, and gross sediment composition. A distinct bottom echo with numerous subbottom reflectors (Type I) is observed in deeper areas where terrigenous clay or lutite (Mgi = 9.9 to, gr = 1.4 g/cc, porosity (P) = 74 percent, C = 28 percent, and V (upper 2 m) = 1,435 m/s) predominates. Type I reflection grades upslope into Type IV, which shows a distinct bottom echo with fewer subbottom reflectors, and the corresponding sediment is a foraminiferal silty clay (mz = 9.4 to, gr = 1.43 g/cc, P = 73 percent, V = 1,447 m/s, and C = 37 percent). The uppermost slope gives indistinct, semiprolonged bottom echoes with faint subbottoms (Type VI) where calcareous silt (Mz = 6.6 to, gr = 1.57 g/cc, P = 65 percent, C = 70 percent, and V = 1,482 m/s) is the main sediment type. The shelf sediments (gr = 1.66 g/cc, P = 58 percent, V = sl1,530 m/ s), varying from coarse silt (Mz = 5.3 to) to very coarse sand (Mz = ‐0.3 to) and 25 to 100 percent carbonate, show indistinct, semiprolonged bottom echoes with intermittent or mushy subbottoms (Type VII). Prolonged echoes with no subbottoms (Type VIII) are observed in areas where algal sands of variable grain size (Mz ‐ ‐0.9 to 2.7 to, gr = 1.66 g/cc, P = 59 percent, V = 1,530 to 1,690 m/s) occur.

The major trends in reflection types (loss in depth of penetration, loss in number of reflectors, and prolongation of initial bottom reflections) follow gradients of sedimentary and physical properties of the sediments, which are increases in mean grain size, bulk density, in situ acoustic velocity, CaCO₃ content, and decrease in porosity. Increases in the reflection coefficient and attenuation of the sound energy in the shelf sediments are probably important factors in the observed decrease in the depth of penetration of the sound energy in the shelf sediments.     

Sedimentary geology of the Columbia River Estuary

A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m³s⁻¹). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging.Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5 phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5–2.0phi; 0.25–0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75–6.0phi; 0.016–0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephemeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions.The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6–8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations.A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these enviroments to the ecosystem of the estuary is discussed in subsequent papers in this volume.     

Correlation of shear strength,hydraulic conductivity,and thermal gradients with sediment disturbance: South Pass region,Mississippi Delta

The degree of sediment disturbance in the South Pass area is correlated to the average hydraulic conductivity, shear strength, and thermal gradient. Hydraulic conductivity averages 18, 6, and 4 × 10^–7 cm/s in the undisturbed, moderately disturbed, and most disturbed sediments, respectively. Shear strength also decreases with increasing disturbance, from 7.6 to 4.4 to 3.5 kPa. Excluding the four stations dominated by annual temperature variations, the remaining 19 thermal gradients correlate well with sediment disturbance. The average gradient is positive in all of the disturbed sediments (0.12 ± 0.07° C/m) and 0 in the undisturbed sediments (0.02 ± 0.05° C/m).