Development Of Submarine Canyons Research Articles

Development of submarine canyons on the continental slope of the Okinawa Trough with potential origin related to methane seepage

Submarine canyons serve as conduits to transport terrigenous detrital sediment and organic carbon to the deep sea. Herein, we carried out a detailed analysis of the geomorphological and erosional-depositional features of submarine canyons on the middle and southern slopes of the Okinawa Trough based on the established isochronous chronostratigraphic framework. According to sequence stratigraphy and well-seismic calibration technologies, the ages of six stratigraphic boundaries are redetermined, which are aged: Seafloor (0 Ma), TLGM (23kyr B. P.), T0 (1.8 Ma), T10 (6.0 Ma), T12 (11.02 Ma), and T16 (16.12 Ma), respectively. The submarine canyons mainly began to develop during the last glacial maximum (LGM), and some might initiate earlier in the early Pleistocene (<1.8 Ma) and further evolved after the LGM. Along the axial direction, the heads of these shelf-indenting canyons were mostly inherited from the ancient incised valleys developed on the shelf edge during the LGM; and the canyon channels were filled with multi-stage turbidites and mass transport deposits (MTDs). The multi-BSRs (bottom simulating reflectors), arched or imbricated and chaotic or twisty reflections, as well as pipe, notch, and mud volcano structures on seismic profiles indicate widespread gas-bearing fluid migration and methane seepage that is possibly related to gas hydrate dissociation. Besides the sea-level change and tectonic movements, we find that the slope failure (gravity flow) induced by gas hydrate dissociation accompanied by methane seepage during the glacial epoch (mainly the LGM) is a main controlling factor for canyon initiation in the Okinawa Trough. The link between canyon development and methane seepage is confirmed by characteristic seismogeologic expressions, such as the MTDs associated with BSRs, the imbricated, twisty or chaotic seismic reflections inside the MTDs, the liquefaction deformation structures, and the fluid transport pipes. In addition, the notches (maybe pockmarks or seabed depressions) that indicate methane escaping on the canyon sidewalls, and the truncated relationship between the BSRs and canyon channels further indicate that there is a complex relationship between the submarine canyons and the methane seepage associated with gas hydrate. Finally, a coupled model was established to illustrate the four evolutional stages of the submarine canyons, which provide an attractive case of how the submarine canyon dynamically responds to gas hydrate system. Therefore, our new findings are of great significance for understanding the mechanism of the submarine canyon and reconstructing the evolutionary history of gas hydrate modulated by sea-level changes in geological history.

How do tectonics influence the initiation and evolution of submarine canyons? A case study from the Otway Basin, SE Australia

The architecture of canyon-fills can provide a valuable record of the link between tectonics, sedimentation and depositional processes in submarine settings. We investigated the role of plate tectonics in the initiation and evolution of submarine canyons. We demonstrate that events at the plate tectonic scale (i.e. continental break-up and shortening) have a first-order influence on the initiation and development of submarine canyons. The Late Cretaceous ( c. 65 Ma) separation of Australia and Antarctica initially resulted in extensional fault systems, which then formed a steep stair-shaped palaeoseabed. Subsequently, the Late Miocene ( c. 5 Ma) collision of Australia and Eurasia resulted in substantial uplift and exhumation on the SE Australian continental margin. These tectonic events led to elevated seismicity, which ultimately gave rise to gravity-driven processes (i.e. turbidity currents and mass wasting processes) and formed the base of the canyon. The inherited stair-shaped topography then facilitated further gravity-driven processes that established a mature sediment conduit extending from the shallow marine shelf to the abyssal plain. We suggest that the stratigraphic architecture of canyons can be used as an archive to record tectonic movements. Moreover, the factors that precondition and trigger gravity-driven processes can induce canyon initiation and facilitate canyon development. Supplementary material: A PDF file containing all the clean, uninterpreted seismic sections is available at https://doi.org/10.6084/m9.figshare.c.5937760

Controls on submarine canyon morphology along a convergent tectonic margin. The Southern Caribbean of Colombia

Submarine canyons are major conduits for sediment transfer from continental shelf to deep marine environments. Mass failures and faults play a key role in the initiation and evolution of submarine canyons along convergent tectonic margins, in addition to continental sediment supply. Here we analyze high-resolution bathymetry and seismic data from the Colombian southern Caribbean to investigate the controls on submarine canyon morphology along a convergent tectonic margin. We propose three end-member types of canyons: Type I canyons have a direct connection to a major river associated with high sediment supply. These canyons are highly erosive and cut topographic barriers created by tectonic deformation. Type II canyons are associated with faults at the canyon's head, where the location, orientation, and sinuosity are controlled by fault strike. Also, the presence of faults is related to an increase in the occurrence of mass failures from the steep canyon borders, increasing the depth and width asymmetry. Finally, Type III canyons evolve toward the continental shelf edge through retrogressive mass failures. These mass failures produce erosional scars that merge downslope to create submarine canyons in early stages of development, which later evolve to wider but shallow canyons due to the progressive basinward merging. We argue that the morphology of each canyon type is controlled by the interaction between continental sediment supply, mass failures, shelf width, and fault occurrence. We also demonstrate that seafloor topography influences the size of submarine fan deposits at the mouths of canyons. We identified large fan deposits (>650 km2) on the continental rise, whereas in piggyback sub-basins we found smaller fans (<60 km2). Our analysis of downdip canyon morphology and associated deposits can be used to predict the development of submarine canyons and associated fans in convergent tectonic margins worldwide.

Morphology and evolution of submarine canyons on the northwest South China Sea margin

Submarine canyons are observed along both passive and active continental margins, but the factors controlling their complex morphology are still poorly understood. Here, we use high-resolution multibeam bathymetric and 2D seismic data to investigate an area of the northwest South China Sea in which 48 submarine canyons are identified. These previously unstudied submarine canyons incise the continental shelf, being located at a water depth between 200 m and 2200 m. Canyon morphology varies from southwest to northeast, namely in what their length and incision depth are concerned. We therefore divide these canyons into four main types: a) Types A, B and C showing a predominant NW-SE direction, and b) Type D canyons striking to the north. By analysing their internal architectures, we propose that submarine canyons along the northwest South China Sea margin were initiated in the Late Miocene by retrogressive slope failure in response to the gradual build-up of sediment on the continental slope. Differences in sediment supply and fault activity are recognised here as the main factors controlling the morphology of the investigated submarine canyons. In addition, recurrent mass-transport deposits (MTDs) fed sediment from the northwest South China Sea margin into the study area, accelerating the filling of the Central Canyon system, a giant submarine canyon located to the south of the investigated continental slope. The discovery of gas fields (LS22–1, LS17–2) and a gas hydrate drilling zone (GMGS5) in the Central Canyon system proves that MTDs comprise good reservoir intervals. Our results contribute to a better understanding of the origin and development of submarine canyons and highlight the role of sediment supply and tectonic events in controlling canyon morphology.

Along‐strike variability in shelf‐margin morphology and accretion pattern: An example from the northern margin of the South China Sea

AbstractInterplays among diachronous tectonism, uneven sediment supply, and local marine hydraulic processes make the northern margin of the South China Sea (SCS) an ideal location to investigate the complexity of along‐strike variability in shelf margins. This study examines shelf‐margin morphology, stratigraphy, and sedimentation from the northern SCS using multichannel seismic reflection profiles complemented with the data from commercial and ocean drilling sites. Analysis of seismic reflection profiles reveals three categories of shelf‐margin cross‐sectional profiles, the concave‐up, linear, and sigmoidal, according to which five margin sectors were recognized. Results show that these margin segments differ in relief, shelf‐edge trajectory, submarine canyon development, and long‐term accretion pattern. The westernmost margin sector, or the Yinggehai (YGH)‐western Qiongdongnan (QDN) margin, has appeared to be supply dominated since its commencement at ca. 10.5 Ma, which is characterized by well‐developed prograding clinoforms, low‐angle shelf‐edge trajectories, and an absence of canyons. Presence of concave‐up profiles is also suggestive of high sediment influx. In contrast, the eastern QDN margin was primarily regulated by local subsidence and faulting, leading to a stationary shelf‐edge migrating pattern and linear upper‐slope morphology. Densely distributed slope‐confined gullies indicate the margin’s disequilibrium and erosive nature. Further east, the Pearl River Mouth (PRM) margin formed much earlier (ca. 30 Ma) and experienced a more complicated accretion history, including three phases which were dominated by sequential marginal faulting (before ca. 30 Ma), basement structure (ca. 30–23 Ma), and sediment supply (ca. 23 Ma to the present). The overall sigmoidal morphology and truncated stratigraphy of this margin probably resulted from the sculpting of local marine processes, especially ocean currents and internal waves. The exception of the central PRM margin where concave‐up profiles develop is mainly related to canyon erosion. Overall, this study highlights the vital role of local forcing factors in controlling along‐margin variations and determining the final fates of different margin segments. A comparison between the northern SCS and other well‐established examples reveals that concave‐upward shelf‐margin shapes, which are usually associated with high sediment supply, little influence from hydraulic regimes, or sometimes, high degree of canyon development, may be an indicator of good reservoir potential beyond the shelf edge.

Earthquakes drive large-scale submarine canyon development and sediment supply to deep-ocean basins.

Although the global flux of sediment and carbon from land to the coastal ocean is well known, the volume of material that reaches the deep ocean-the ultimate sink-and the mechanisms by which it is transferred are poorly documented. Using a globally unique data set of repeat seafloor measurements and samples, we show that the moment magnitude (Mw) 7.8 November 2016 Kaikōura earthquake (New Zealand) triggered widespread landslides in a submarine canyon, causing a powerful "canyon flushing" event and turbidity current that traveled >680 km along one of the world's longest deep-sea channels. These observations provide the first quantification of seafloor landscape change and large-scale sediment transport associated with an earthquake-triggered full canyon flushing event. The calculated interevent time of ~140 years indicates a canyon incision rate of 40 mm year-1, substantially higher than that of most terrestrial rivers, while synchronously transferring large volumes of sediment [850 metric megatons (Mt)] and organic carbon (7 Mt) to the deep ocean. These observations demonstrate that earthquake-triggered canyon flushing is a primary driver of submarine canyon development and material transfer from active continental margins to the deep ocean.

Forearc Basin Stratigraphy and Interactions With Accretionary Wedge Growth According to the Critical Taper Concept

Forearc basins are important constituents of sediment traps along subduction zones; the basin stratigraphy records various events that the basin experienced. Although the linkage between basin formation and accretionary wedge growth suggests that mass balance exerts a key control on their evolution, the interaction processes between basin and basement remain poorly understood. This study performed 2‐D numerical simulations in which basin stratigraphy was controlled by changes in sediment fluxes with accretionary wedge growth according to the critical taper concept. The resultant stratigraphy depended on the degree of filling (i.e., whether the basin was underfilled or overfilled) and the volume balance between the sediment flux supplied to the basin from the hinterland and the accommodation space in the basin. The trenchward progradation of deposition with onlapping contacts on the trenchside basin floor occurred during the underfilled phase, which formed a wedge‐shaped sedimentary unit. In contrast, the landward migration of the depocenter, with the tilting of strata, was characteristic for the overfilled phase. Condensed sections marked stratigraphic boundaries, indicating when sediment supply or accommodation space was limited. The accommodation‐limited intervals could have formed during the end of wedge uplift or when the taper angle decreased and possibly associated with the development of submarine canyons as conduits for bypassing sediments from the hinterland. Variations in sediment fluxes and their balance exerted a strong influence on the stratigraphic patterns in forearc basins. Assessing basin stratigraphy could be a key to evaluating how subduction zones evolve through their interactions with changing surface processes.

Development of submarine canyons after the Mid-Pleistocene Transition on the Ebro margin, NW Mediterranean: The role of fluvial connections

After the Messinian Salinity Crisis (MSC), the Ebro Margin, like other Mediterranean deltaic margins, rebuilt through progradation and increasingly significant aggradation. The Plio-Quaternary transition from a ramp-like system to a “new” margin with large clinoforms is an opportunity to understand the processes that govern canyon initiation and evolution. We used a 3D seismic data set located at the outer shelf - upper slope of the Ebro margin. We tied major bounding surfaces to chrono-stratigraphic constraints from borehole data or, for the most recent interval, from averaged accumulation rates derived from borehole stratigraphy. The major shelfal erosion surfaces are interpreted as sequence boundaries, tied to major glacial maxima. Along these surfaces, seismic attributes characterize the fluvial/canyon connection, viewed as one of the key factors in canyon development. The first appearance of “proto-canyons” (dense networks of channels and gullies 50–100 m deep and 1–2 km wide) occurs in the late Zanclean. Their size increased progressively throughout the Pliocene and early Quaternary, in relation to the increase in clinoform heights. “True canyons” (with distinct interfluves, more than 200 m deep and 3–4 km wide) appeared during the Mid-Pleistocene Transition (MPT, 1250–700 ka BP). The first evidence of direct connection between canyon heads and fluvial systems is observed during Marine Isotope Stage 22, one of the most pronounced glacial stages of the Quaternary. Similar connections also existed, at least, during MIS 16 and MIS 12, which are also stages of pronounced low sea level. The topography of fluvial channels in the outer shelf is not imaged in detail in the picked horizons at the resolution of our seismic data, but sinuous fluvial systems are very well imaged through amplitude and coherency attributes. Direct connection of fluvial systems during and after MIS 22 also favored headward erosion and the formation of shelf-indenting canyons, probably during early phases of sea-level rise.

Numerical models of continental and submarine erosion: application to the northern Ligurian Margin (Southern Alps, France/Italy)

AbstractWe present a new numerical surface process model allowing us to take into account submarine erosion processes due either to submarine landslides or to hyperpycnal currents. A first set of models show that the frequency of hyperpycnal flows influences the development of submarine canyons at the mouth of continental rivers. Further experiments show that an increase in submarine slope leads to faster regressive canyon erosion and a more dentritic canyon network, whereas increasing the height of the unstable sediment pile located on the shelf break leads to wider and less dendritic canyons. The models are then applied to the western segment of the north Ligurian margin (northwestern Mediterranean), which displays numerous submarine canyons with various sizes and morphologies. From west to east, canyon longitudinal profiles as well as margin‐perpendicular profiles progressively change from moderately steep, concave‐up shapes to steeper linear to convex‐up shapes suggesting increasing eastward margin uplift. Moreover, the foot of the margin is affected by a marked slope increase with evidences of mass transport due to landslides. Numerical models which reproduce well the North Ligurian margin morphologic features indicate that the western part of the margin is submitted to rather low (i.e. 0.4 mm yr−1) uplift and intense submarine erosion due to frequent hyperpycnal currents, whereas the eastern part bears more rapid (i.e. 0.7 mm yr−1) uplift and has little or no hyperpycnal currents. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Formation of pockmarks and submarine canyons associated with dissociation of gas hydrates on the Joetsu Knoll, eastern margin of the Sea of Japan

This study, based on 3.5kHz SBP, 3D seismic data and long piston cores obtained during MD179 cruise, elucidated the timing and causes of pockmark and submarine canyon formation on the Joetsu Knoll in the eastern margin of the Sea of Japan. Gas hydrate mounds and pockmarks aligned parallel to the axis on the top of the Joetsu Knoll are associated with gas chimneys, pull-up structures, faults, and multiple bottom-simulating reflectors (BSRs), suggesting that thermogenic gas migrated upward through gas chimneys and faults from deep hydrocarbon sources and reservoirs. Seismic and core data suggest that submarine canyons on the western slope of the Joetsu Knoll were formed by turbidity currents generated by sand and mud ejection from pockmarks on the knoll. The pockmark and canyon formation probably commenced during the sea-level fall, lasting until transgression stages. Subsequently, hydropressure release during the sea level lowering might have instigated dissociation of the gas hydrate around the base of the gas hydrate, leading to generation and migration of large volumes of methane gas to the seafloor. Accumulation of hydrate caps below mounds eventually caused the collapse of the mounds and the formation of large depressions (pockmarks) along with ejection of sand and mud out of the pockmarks, thereby generating turbidity currents. Prolonged pockmark and submarine canyon activities might have persisted until the transgression stage because of time lags from gas hydrate dissociation around the base of the gas hydrate until upward migration to the seafloor. This study revealed the possibility that submarine canyons were formed by pockmark activities. If that process occurred, it would present important implications for reconstructing the long-term history of shallow gas hydrate activity based on submarine canyon development.

Geomorphic expression of collapsing ice streams revealed by latest generation swath bathymetry images

Submarine canyons are observed along both passive and active continental margins, but the factors controlling their complex morphology are still poorly understood. Here, we use high-resolution multibeam bathymetric and 2D seismic data to investigate an area of the northwest South China Sea in which 48 submarine canyons are identified. These previously unstudied submarine canyons incise the continental shelf, being located at a water depth between 200 m and 2200 m. Canyon morphology varies from southwest to northeast, namely in what their length and incision depth are concerned. We therefore divide these canyons into four main types: a) Types A, B and C showing a predominant NW-SE direction, and b) Type D canyons striking to the north. By analysing their internal architectures, we propose that submarine canyons along the northwest South China Sea margin were initiated in the Late Miocene by retrogressive slope failure in response to the gradual build-up of sediment on the continental slope. Differences in sediment supply and fault activity are recognised here as the main factors controlling the morphology of the investigated submarine canyons. In addition, recurrent mass-transport deposits (MTDs) fed sediment from the northwest South China Sea margin into the study area, accelerating the filling of the Central Canyon system, a giant submarine canyon located to the south of the investigated continental slope. The discovery of gas fields (LS22–1, LS17–2) and a gas hydrate drilling zone (GMGS5) in the Central Canyon system proves that MTDs comprise good reservoir intervals. Our results contribute to a better understanding of the origin and development of submarine canyons and highlight the role of sediment supply and tectonic events in controlling canyon morphology.

Geomorphological development of the Goto Submarine Canyon, northeastern East China Sea

The geomorphological development of submarine canyons is controlled by multiple factors, such as sea-level fluctuations, tectonic activity, and ocean currents, but the relative contributions of each factor vary among these canyons. The development of submarine canyons along active rift margins is not fully understood because there are relatively few documented examples of such phenomena. The Goto Submarine Canyon offers a good example of this type of structure because it is located on the flank of the Okinawa Trough, which is an active back-arc basin. Through multi-beam bathymetry, multi-channel seismic reflection profiling, and remotely operated vehicle surveys, we reveal that this canyon is more than four times wider than other submarine canyons and has a gradient that is half as deep and a remarkably flat canyon floor. A series of upstream-facing cliffs with reliefs up to 110 m and longitudinal lineaments are also associated with this canyon. These morphological features can mainly be attributed to direct sediment inputs from the Yellow River during glacial stages, vertical incision prevented by erosion-resistant beds, and faulting and tectonic tilting of resistant strata. The possibility of tidal current-induced erosion is also suggested. These processes have been influenced primarily by regional factors, such as the wide continental shelf, carbonate cementation, and active rifting of the Okinawa Trough. The contribution of rifting, including faults and tectonic tilting, to the canyon's morphology is expected to be a general characteristic of submarine canyons along active rifting margins.

Plio–Quaternary prograding clinoform wedges of the western Gulf of Lion continental margin (NW Mediterranean) after the Messinian Salinity Crisis

In contrast to the much-studied onshore and deep offshore post-Messinian sedimentary history of the Gulf of Lion, the continental shelf had been poorly explored until recently. New seismic data, acquired by ELF Oil Company on the Languedoc–Roussillon shelf (Western Mediterranean Sea), from Cap Creus in the SW to Cap d’Agde in the NE, together with data from previously drilled exploratory wells, allow us to propose a scenario for margin reconstruction following the Messinian Salinity Crisis. The seismic data display a complex pattern of prograding clinoforms and buried submarine canyons. Following the crisis that eroded the previous margin, clinoforms developed as the new Pliocene margin prograded. Three major periods characterize the evolution of the post-Messinian margin. During the first period, after the filling of the Early Pliocene rias which are now above sea level, the prograding sedimentary prisms rapidly migrated seaward and filled the underlying Messinian topographic lows. The second period consists of a transitional interval, which began with a pronounced fall in sea level that probably corresponded with the end of the Lower Pliocene. Deposits were disturbed by large slumped structures. The third period is characterized by the appearance and development of submarine canyons near the subsequent shelf edges, maybe as a result of the increased glaciations and related sea-level changes. From that time onward, most of the Late Pliocene and Quaternary sediments were directly transferred down to the deep basin.

Glaucony in Ocean-Margin Sequence Stratigraphy (Oligocene-Pliocene, Offshore New Jersey, U.S.A.; ODP Leg 174A)

ABSTRACT Glaucony occurs in abundance in clinoform-top and clinoform-toe positions within Atlantic-margin depositional sequences (offshore New Jersey, U.S.A.; Oligocene to Pliocene). Thin section and backscatter scanning electron microscope (BSEM) analyses indicate that grains of glaucony commonly formed in situ within burrows in deep-water (600-1000 m), clinoform-toe settings; fragmentation of glaucony grains in the matrix is most likely to have occurred through animal disturbance (e.g., ingestion and displacement by burrowers). Deep-water glaucony occurrences in the distal clinoform-toe positions show a pattern of association with quartz-sand abundance: in the most distal settings quartz sand is a minor component and its importance increases with proximity to the clinoform fronts. We hypothesize that these glauconitic sands, which commonly have erosional bases, formed by sediment starvation during relative sea-level rise and highstand, when the sandy clinoform fronts (deposited during sea-level lowstand) were abandoned. During particular times of regional sediment starvation over Oligocene to Middle Miocene time, biologically mediated erosion and transport were thus dominant processes in distal clinoform-toe settings, and in situ glaucony grains were mixed with quartz sand grains derived by degradation of the clinoform front. From Middle Miocene time onwards, development of submarine canyons may have restricted redistribution of quartz sand to discrete conduits. Sedimentary fabrics exhibited by shallow-water (

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.

Accretionary Wedge Convexity and Submarine Canyon Development: Two Consequences of Fluid Escape: ABSTRACT

Accretionary Wedge Convexity and Submarine Canyon Development: Two Consequences of Fluid Escape: ABSTRACT

A large‐scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

ABSTRACTThe late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn‐rift or early post‐rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation.The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low‐stand of sea‐level. They were subsequently filled by a fining‐upwards suite of sediments which reflects subsequent relative rise of sea‐level and carbonate platform development. Ultimately the canyon complex was buried by north‐westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle.It is considered possible that the series of isolated outcrops of canyon cross‐sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge‐like Patsy Springs Canyon lies in more proximal regions of the basin‐slope, whereas 40 km to the north‐east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel‐fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg.The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel‐fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.

Morphology, distribution, and development of submarine canyons on the United States Atlantic continental slope between Hudson arid Baltimore Canyons

The distribution and morphology of submarine canyons off the eastern United States between Hudson and Baltimore Canyons have been mapped by long-range sidescan sonar. In this area canyons are numerous, and their spacing correlates with overall slope gradient; they are absent where the gradient is less than 3°, are 2 to 10 km apart where the gradient is 3° to 5°, and are 1.5 to 4 km apart where the gradient exceeds 6°. Canyons range from straight to sinuous; those having sinuous axes indent the edge of the continental shelf and appear to be older than those that head on the upper slope and have straighter axes. A difference in canyon age would suggest that canyons are initiated on the continental slope and only with greater age erode headward to indent the shelf. Shallow gullies on the middle and upper slope parts of the canyon walls suggest that submarine erosion has been a major process in a recent phase of canyon development.

Saharan Glaciation Dated in North America

THE Lower Palaeozoic glaciation in the Sahara is rapidly becoming among the best known of fossil glaciations1–3 but because rocks within and below the glacial sequence are poorly fossiliferous it has proved difficult to estimate the duration of glacial conditions. We suggest that the duration of the glacial episode is dated as extending from Lower Cambrian to Middle and perhaps Upper Ordovician by the occurrence in eastern North America of resedimented deposits indicating widespread simultaneous development of submarine canyons in response to glacio-eustatic lowering of sea level.

Newport Submarine Canyon, California: An example of the effects of shifting loci of sand supply upon canyon position

Newport Submarine Canyon, located off Newport Beach, California, is typical of the small shelf canyons of the California continental borderland. The canyon has formed at a point where the shelf is narrow and sand moved by waves in longshore drift is concentrated at a convergence zone. This situation has apparently existed for much of the Pleistocene although the loci of sand concentration have shifted in response to changes in position of stream input, or local coastal configuration due either to sea level changes or as a result of coastal erosion or deposition. The present topographic expression of the submarine canyon represents its most recent position, but one that is now in process of abandonment as a result of shifts in loci of sand input. The Santa Ana River is the principal local sand source. Changes in the location of the river mouth during the past century have shifted the point of longshore drift convergence and sand concentration approximately one kilometer northwest of the present canyon head. The canyon head is now receiving only fine organic-rich sediments that are building an apparently stable deposit that will ultimately fill the head. The sand deposition necessary for canyon development is now being distributed as a lobe over the shelf north of the canyon and this lobe is just beginning to extend over the local shelf edge. Thus, the locale illustrates the process of canyon origin, development and ultimate abandonment. Evidence from local bathymetry and sedimentary distributions on the shelf and slope suggest that the cycle also occurred in Holocene and Late Pleistocene time and that the canyon has shifted laterally several times in response to the changing point of input of sand to the canyon system. Newport Canyon is therefore an exception that proves the rule for the current theories regarding small coastal submarine canyon development by mass movements of sand as described by Dill, Chamberlain and others over the past five years.