Intercanyon Areas Research Articles

Exploration of the Canyon-Incised Continental Margin of the Northeastern United States Reveals Dynamic Habitats and Diverse Communities.

The continental margin off the northeastern United States (NEUS) contains numerous, topographically complex features that increase habitat heterogeneity across the region. However, the majority of these rugged features have never been surveyed, particularly using direct observations. During summer 2013, 31 Remotely-Operated Vehicle (ROV) dives were conducted from 494 to 3271 m depth across a variety of seafloor features to document communities and to infer geological processes that produced such features. The ROV surveyed six broad-scale habitat features, consisting of shelf-breaching canyons, slope-sourced canyons, inter-canyon areas, open-slope/landslide-scar areas, hydrocarbon seeps, and Mytilus Seamount. Four previously unknown chemosynthetic communities dominated by Bathymodiolus mussels were documented. Seafloor methane hydrate was observed at two seep sites. Multivariate analyses indicated that depth and broad-scale habitat significantly influenced megafaunal coral (58 taxa), demersal fish (69 taxa), and decapod crustacean (34 taxa) assemblages. Species richness of fishes and crustaceans significantly declined with depth, while there was no relationship between coral richness and depth. Turnover in assemblage structure occurred on the middle to lower slope at the approximate boundaries of water masses found previously in the region. Coral species richness was also an important variable explaining variation in fish and crustacean assemblages. Coral diversity may serve as an indicator of habitat suitability and variation in available niche diversity for these taxonomic groups. Our surveys added 24 putative coral species and three fishes to the known regional fauna, including the black coral Telopathes magna, the octocoral Metallogorgia melanotrichos and the fishes Gaidropsarus argentatus, Guttigadus latifrons, and Lepidion guentheri. Marine litter was observed on 81% of the dives, with at least 12 coral colonies entangled in debris. While initial exploration revealed the NEUS region to be both geologically dynamic and biologically diverse, further research into the abiotic conditions and the biotic interactions that influence species abundance and distribution is needed.

Survival of a submarine canyon during long-term outbuilding of a continental margin

Net-depositional submarine canyons are common in continental slope strata, but how they survive and prograde on constructional margins is poorly understood. In this study we present field evidence for the coevolution of a submarine canyon and the adjacent continental slope. Using a three-dimensional seismic data cube that images the Ebro margin (northwest Mediterranean), we identify a preserved canyon on a middle Pleistocene paleosurface and relate it directly to its expression on the present-day seafloor. A subparallel stacking pattern of seismic reflectors, similar to that seen between prograding clinoforms in intercanyon areas, is observed between the modern and paleocanyon thalwegs. The concavity of the modern long profile differs from the convex-concave long profile on the middle Pleistocene surface, suggesting a long-term change in canyon sedimentation. We interpret this change as a shift to a canyon dominated by turbidity currents from one strongly influenced by the pattern of sedimentation that built the open-slope canyon interfluves. We find support for our interpretation in previous studies of the Ebro margin.

Glacial and glacially-related features on the continental margin of northwest Ireland mapped from marine geophysical data

Please click here to download the map associated with this article. Despite over a century of research, reconstructions of the western margin of the last British-Irish Ice Sheet remain controversial. This is mainly because the ice sheet extended offshore onto the continental shelf and therefore significant portions of the glacial record are below present sea level. The availability of high resolution multibeam swath bathymetric and geophysical sub-bottom data for over ∼30,000 km2 of the northwest Irish continental margin (between 55°40′ and 54°18′ N, and 7°50′ and 11°4′ W) has made it possible to map glacial features on the continental shelf and glacially-related features on the continental slope and rise. The resulting map, presented here at a scale of 1:260,000, shows that the major glacial features consist of subglacial bedforms and nested arcuate moraines of different orientations across the width of the shelf. Distal to these moraines, the outermost shelf and upper slope are crossed by subparallel and cross-cutting furrows created by iceberg scouring and meltwater discharge at the ice margin. A well-developed system of gullies and canyons and frequent escarpments incise the continental slope. Several large depositional lobes, likely related to the delivery of vast amount of sediment at the ice margin are found in inter-canyon areas and at the base of the slope. Collectively these data show an extensive ice sheet margin across the entire shelf and sedimentation on the continental margin was greatly affected by the presence of the ice margin at the shelf edge.

Continental slope sedimentation adjacent to an ice margin I. Seismic facies of Labrador Slope

Sleeve-gun, 3.5-kHz, and 12-kHz profiles from the Labrador Slope provide the basis for an analysis of sedimentary facies, processes, and evolution of a continental slope adjacent to an ice margin. The upper slope is deeply incised by numerous canyons reflecting headward canyon branching. The less rugged middle-slope topography has fewer canyons and large slide and slump scars followed downslope by debris-flow deposits. Echo character of seismic profiles reflects the difference in sediment types supplied from mud-dominated sources and sand-, gravel- and till-dominated sources. On the rise, debris-flow deposits are largely confined to canyons. Intercanyon areas are dominated by spill-over turbidites alternating with hemipelagic sediments, which on some of the southern to southwestern levees occur in sediment-wave fields formerly attributed to bottom-current activity.

Terraces on the Florida escarpment: Implications for erosional processes

SeaBeam bathymetric data and GLORIA (Geologic LOng-Range Inclined Asdic) sidescan sonar images of a 175-km-long section of the Florida escarpment in the eastern Gulf of Mexico show that this carbonate escarpment has been eroded since its initial formation, but its morphology suggests that erosional processes have not acted uniformly on the escarpment. Parts of the escarpment are notched by box canyons that have extremely steep headwalls and may be sites of active ground-water sapping. The intercanyon areas commonly have previously unrecognized terraces below 2,600 m. Above 2,600 m, the escarpment is steeper and has no terraces. The terraces may reflect differences in platform strata exposed at the escarpment that are responding differently to erosional processes.

Modern sedimentary processes in the Wilmington Canyon area, U.S. east coast

Sedimentary processes active in the Wilmington Canyon area during the past century are documented from geochronologic, sedimentologic, and radiographic analysis of sediment cores collected using primarily D.S.R.V. Alvin. Site selection for the Alvin dives, and for collection of box and kasten cores, was achieved using Sea MARC and SeaBeam data, allowing accurate sampling of numerous subenvironments from this extensively dissected area of the continental margin. 210Pb sediment accumulation rates from intercanyon areas are highest on the upper slope, 1.7 mm/yr (140 mg/cm 2/yr), and decrease with water depth to about 0.4 mm/yr (36 mg/cm 2/yr) on the upper rise. Within submarine canyons, accumulation rates vary dramatically, from a maximum of > 6 mm/yr (> 500 mg/cm 2/yr) to less than 0.3 mm/yr (22 mg/cm 2/yr). Higher accumulation rates in canyons occur as a result of additional supply of older sediment, probably from bioerosion of the canyon walls. This is supported by observations from D.S.R.V. Alvin, reduced 210Pb activities, and indurated mud clasts observed in several canyon cores. Extremely low accumulation rates in Wilmington and South Wilmington canyons are probably the result of strong episodic boundary currents. Non-steady-state 210Pb profiles and sedimentologic data indicate small-scale mass wasting in South Heyes Canyon and on the lower slope and rise just north of Wilmington Canyon.

Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and stratigraphy

The depositional bodies of the Chile Trench (trench fans, the axial channel, sheeted basins, ponded basins, and axial sediment lobes) control the spatial distribution of modem lithofacies in the basin. Sheeted basins (south of 41°S) are presumably fed by closely spaced submarine gullies that approximate a line of source of sediment supply along the base of the slope. Trench fans (41°S−33°S) are built at the mouths of major submarine canyon systems which act as point sources of sediment supply. The axial channel follows the northward gravitational gradient, draining the distributary networks of trench fans into the longitudinal transport system. Down-gradient (northern) fan lobes are severely dissected by erosional processes; evidently, periods of proximal deposition alternate with periods of massive sediment remobilization and progradation of the axial dispersal system into more distal environments. Channelized basins in the canyon-mouth areas yield to sheeted basins (depositional surface maintains an axial gradient) or ponded basins (depositional surface is strictly flat) in inter-canyon areas. Tectonic disruption of the oceanic basement can locally augment the axial gradient and stimulate flow channelization, or reverse the gradient and induce sediment ponding. A large, margin-parallel sediment lobe is built at the base of a high axial escarpment near 33°S, where the axial channel crosses a transverse discontinuity at the convergent plate boundary. Five lithofacies are defined by Q-mode factor analysis of sediment textures and hydro-dynamic structures in 27 cores from the Chile Trench. The Channel facies (thick, amalgamated sand, massive to laminated or cross-bedded) is deposited by high-energy processes within the coarse-grained bedload of turbidity currents; it forms in distributary and axial channels. The Levee facies (rhythmically bedded, internally structureless, graded sand and graded silt) is deposited from concentrated sediment suspensions that quickly lose momentum during channel spillover; it forms on channel flanks, although constructional levees are not always present. The Basin-1 facies (more complete Bouma sequences, both upper and lower flow regime structures) forms in ponded basins where flows are confined by a high-relief, seaward trench wall. The Basin-2 facies (graded and laminated silt, lower flow regime structures) forms in low-energy environments, such as interchannel areas, distal basins, trench walls, and elevated topographic features. The Contourite facies (silt and sand laminations winnowed from hemipelagic muds and distal turbidites) is best developed in sediment-starved basins where geostrophic currents are constricted and accelerated between the steep inner and outer trench walls. The trench wedge records a coarsening-upward sequence as the oceanic plate migrates toward and into the trench during plate convergence, and becomes more proximal to sediment sources along the base of the continental margin. Near canyon mouths, prograding trench fans drive the axial channel seaward into the trench wedge, and the coarsening-upward sequence is truncated by a time-transgressive erosional unconformity. Abandoned axial channel deposits are carried landward beneath prograding fans to record a fining-upward sequence above the basal unconformity. Channel migration and lobe aggradation may produce fining- and coarsening-upward sequences on depositional fan lobes, but sequences on the erosional lobes are fragmented by numerous truncation surfaces.

Recent Sedimentation on the New Jersey Slope and Rise

Radiocarbon dating and sedimentological studies of closely spaced cores indicate movement during the Holocene of sediments on the New Jersey continental slope and upper rise between Wilmington and Lindenkohl canyons. The uneven time-stratigraphic thickness of the late Quaternary sediment sections between cores and the nonuniform deposition rate at any given core site and among core sites show that the sediment blanket in canyon and intercanyon areas has been affected by downslope, gravity-driven pocesses during the Holocene to the present. The reduced rate of deposition on the slope and upper rise between the late Pleistocene and the present is largely due to decreased off-shelf transport in response to the eustatic rise in sea level. Very old radiocarbon dates at core tops result from emplacement of older reworked materials from upslope or from truncation of sections by mass wasting processes exposing older material at the sea floor. These processes also account for an irregular sequence of dated sections within cores and stratigraphic irregularities of the surficial cover from core to core. Marked variability in deposition rates on the slope and upper rise is largely a function of topographic configuration, proximity and accessibility to sediment source, and transport processes seaward of the shelf break. Moreover, higher accumulation rates on the upper rise are attributed primarily to slope bypassing. Bypassing, prevalent during the late Pleistocene, has continued periodically to the present.

Rhone Deep-Sea Fan: A review

The Rhone Fan is a large Plio-Pleistocene turbidite deposit in the western Mediterranean Sea. The fan is fed from the broad Rhone River delta, but only one canyon, the Petit-Rhone, has fed most of the major turbidite depositional sequences that have been mapped. Slumping of sediment from intercanyon areas on the delta slope also has provided much sediment for the fan. The lack of Recent turbidite deposition on the fan suggests that turbidite sedimentation dominates during glacial low stands of sea level, building major leveed valley sequences, while surficial slumping of the valley levee deposits and pelagic sedimentation seem to mark high stands of sea level during interglacial periods.

Variability of Sedimentary Textures and Processes on Continental Margin North of Wilmington Canyon: ABSTRACT

Sedimentary properties and processes of a 7,500-km2 corridor seaward of the Baltimore Canyon Trough off New Jersey were studied in detail using over 100 bottom samples consisting of grab samples and box, hydroplastic, and piston cores. The sediments in both canyon and intercanyon areas are primarily bioturbated, olive-gray, sandy silts with local features indicative of gravity-induced mass sediment movements (i.e., graded sequences and load structures). C14-based sediment accumulation rates vary by a factor of three in cores separated by as little as 6 km. These variations seem to be a function of shelf-edge spillover rates. Detailed analyses of the sand grain-size distribution throughout the corridor reveal that the sand component of the slope and rise sediments contains a high percentage of material currently present on the adjacent shelf. The relatively large percentage of sandy sediment in the upper parts of cores from Spencer Canyon suggests its recent role in transporting shelf sediments seaward. Sandy intervals in other slope cores are commonly obscured by intense bioturbation. Cores from the continental rise show an upward decrease in the number of sand layers and lenses. Active transport of sediment to the slope and rise occurred during the late Pleistocene and, although the intensity has declined, the slope is presently the site of deposition for both fine and coarse sediment. End_of_Article - Last_Page 1662------------

Slumping in Intercanyon Areas, Middle Atlantic Continental Slope: ABSTRACT

Analyses of high-resolution seismic reflection profiles show that slump deposits are ubiquitous within the intercanyon areas of the continental slope of the Middle Atlantic Bight. Of 15 widely spaced 3.5-kHz profiles obtained between Hudson Canyon and Chesapeake Bay, 12 define slump deposits that vary from thin, homogeneous or parallel-bedded lenses of sediments, to masses of intermediate thickness containing contorted bedding, to relatively thick slump blocks with discontinuous bedding. These deposits constitute the upper 10 to 90 m of sediments, extend downslope for 2 to 7 km, and are present at water depths ranging from 545 to 1,500 m. Minisparker profiles obtained during a detailed survey of a 9 by 28 km area of the slope between Hudson and Wilmington Canyons define 1 slump deposits in water depths of 398 to 2,190 m that comprise 12% of the survey area. Individual masses are as much as 50 m thick, cover as much as 5.3 sq km, and contain a maximum of 0.11 cu km of sediments. Although some of the slump deposits on the Middle Atlantic slope undoubtedly are relict, stemming from sediment instability produced by rapid deposition during Pleistocene sea-level regressions, the acoustic characteristics of others suggest recent formation. Data from this study indicate that slumping in the intercanyon areas may be quantitatively important in transporting sediments to the deep sea and suggest that recent mass movements may constitute a geologic hazard to future economic development of this part of the continental slope. End_of_Article - Last_Page 480------------

Small-scale slump deposits, Middle Atlantic Continental Slope, off eastern United States

Analyses of 24 high-resolution seismic-reflection profiles that were collected during local and regional surveys show that small-scale slump deposite are ubiquitous whthin the intercanyon areas of the Continental Slope of the Middle Atlantic Bight. The deposits involve the upper 10–90 m of sediments, extend downslops for 1.8–7.2 km, and are present at water depths ranging from 545 to 1500 m. The characteristics of the deposits vary from thin, homogeneous or fairly regularly bedded lenses of sediment, to masses of intermediate thickness with contorted bedding, to relatively large slump blocks. A detailed survey of one slump mass just south of Hudson Canyon (by means of close-spaced Minisparker profiles and sediment cores) showed that it had a thickness of about 30 m and a volume of at least 0.4 km 3 and consisted of homogeneous clay which accumulated rapidly during the late Pleistocene or Holocene. Although some of the slump deposits undoubtedly are relict, stemming from sediment instability porduced by rapid deposition during Pleistocene sea-level regressions, others were formed relatively recently. Possible causes of modern slumps include gas generation in the sediments, bottom-water turbulence on the upper slope, and shallow faulting. This study indicates that small-scale slumping in the intercanyon areas may be an important process in transporting sediments to the deep sea and suggests that recent mass movements may constitute a geologic hazard to future economic development of this part of the Continental Slope.

Submarine Canyon Origin on Upper Continental Slope off Cape Hatteras

The configuration of sedimentary strata revealed by continuous seismic-reflection profiles over submarine canyons on the upper continental slope off Cape Hatteras is best explained by the hy pothesis of composite origin of submarine canyons. The canyons were originally incised into a post-Miocene (?) surface of unconformity. The intercanyon areas were constructed by deposition of sediment up to several hundred meters thick forming the canyon walls while the canyons were maintained by sediment removal processes over the sites of original incision. Canyon heads are presently buried under a smooth sediment cover which extends from the shelf break at about 90 m down to about 400 m below sea level, where the canyons emerge from the cover. The depth of emergence of canyons approximately coincides with the inferred level of no motion between the northeast-flowing Florida Current and the southwest-flowing Western Boundary Undercurrent.

Types of Pediments in the San Pedro Valley Region, Arizona

1954of Pacific Coast Geographers17 TYPES OF PEDIMENTS IN THE SAN PEDRO VALLEY REGION, ARIZONA Tuan Yi-fu University of California, Berkeley The word pediment, as first defined by McGee (1) and as used in this paper, refers to an exposed bedrock surface at the foot of the mountain front, or a rock surface covered by a layer of alluvium no thicker than the depth of the stream scour. In igneous rocks of coarse texture, the pediment may be covered only by its own regolith — i.e, the material weathered in place rather than transported alluvium. Under this definition, which excludes all postulates of an extension of the surface beneath the alluvium, the pediment is only a fragmentai feature of the arid landscape in the American Southwest in contrast to the ubiquity of alluvial fans, bahadas, and bevelled piedmont surfaces or "peripediments". There are at least two major types of pediments in the San Pedro Valley region of Arizona: 1. The narrow rock benches or embayments which fringe a broad mountain mass, and are separated from the alluvial deposits of the basin by a high-angle fault; 2. The extensively bevelled bedrock surfaces out of which the mountains appear to rise as mere residuals. The¦Santa Catalina and Huachuca pediments belong to the first type, whereas the south Dragoon and Johnny Lyon Hills pediment belong to the second. The Santa Catalina Pediments The northwest and south fronts of the Santa Catalina Mountains show narrow pediments because they have had little time to develop since the latest major uplift along the normal faults in late Tertiary or early Quaternary time (?). The morphology of these pediments can be described best in the context of their adjoining mountain fronts. (Figure 1) The south front of the Santa Catalina Mountains is composed of gneiss. It is much ravaged by erosion, and has a series of parallel canyons separated by bare, sharp-crested ridges. The northwest front is divisible into two morphologic units along the Romero and Montrose canyons. The belt occupied by these canyons marks the transitional zone between gneiss of the southern forerange and an intrusive igneous rock of distinctly granitic texture in the northern portion of the northwest front, where it is known as the Samaniego Ridge. Canyons and sharp crests are absent in the Samaniego Ridge which is rounded at the top and has long, smooth, concave slopes leading down to the foot of the mountain. It is along this granite ridge that amphitheatre-like, incipient pediments have formed. (2) Wide pediments generally do not occur along a deeply dissected mountain front, and the southern forerange off the Santa Catalina Mountains is no exception. Here, large alluvial fans appear to emerge directly from the mouths 18 Yearbook of The Association Vol. 16 Figure 1. Diagram of the northwest corner of the Santa Catalina mountains. SW SANTA CATALINJA PEDIMENT BENCH NE Pediment Bench Fan A (Mesa) Fan B Fan C Fan B Gneiss . Conglomerate beds (Tilted and deformed) Figure 2. Diagrammatic cross-section of the south front of the Santa Catalina mountains showing the relationship between the pediment benches and the alluvial fans. (Modified from Blissenbach ) 1954of Pacific Coast Geographers19 of the canyons, and the steep mountain wall seems to be in sharp contact with the aggradational slope of the fans. But this simple arrangement is deceptive and fades upon closer scrutiny. The fans have been much eroded. Blissenbach recognizes three fan levels, the highest and oldest remnants of which are isolated knolls or mesas. (3) The highest fan mesa, when projected toward the mountain front and across the fault that separates the gneiss from the alluvium, correspond in elevation to the narrow pediment benches developed on gneiss, whereas a more recent fan (fan B) still covers much of the lower gneissic surface. It is outside the scope and purpose of this paper to trace the history of the uplift of the mountain front along the fault, and the relation of the succesive fans to the pediment benches. Figure 2 summarizes a possible interpretation in diagrammatic form. The rock benches are at most only a few hundred feet wide. They have suffered much from erosion, and are best preserved in the intercanyon areas...