Deep-sea Fan System Research Articles

Extending morphometric scaling relationships: the role of bankfull width in unifying subaquatic channel morphologies

Subaquatic channels, situated in lakes, fjords, submarine canyons and on deep-sea fan systems, exhibit diverse morphometric characteristics controlled by sediment transport processes, particularly turbidity currents. These processes play a significant role in transporting sediment, organic carbon, nutrients, and pollutants and pose hazards to critical infrastructure. This study examines a balanced set of subaquatic channels across various settings, sizes, and locations, employing novel data harmonization techniques to address biases toward larger channels. The analysis reveals consistent scaling relationships, particularly the dominant role of bankfull width, influencing bankfull depth, cross-sectional area, wavelength, and amplitude. The aspect ratio (width-to-depth) emerges as a significant parameter, reflecting a necessary channel compactness to maintain turbidity currents. Meandering development reveals relations between width, wavelength, and amplitude, with a preference for specific ratios. Meandering is most pronounced at channel beginnings, transitioning to straighter forms downstream, potentially influenced by flow dynamics and flow confinement. The study broadens the understanding of subaquatic channel evolution, emphasizing the importance of bankfull width and providing insights applicable across settings and scales.

Pre-salt clastic systems in the Herodotus Basin, SE Mediterranean Sea

This study is focused on the architectural characteristics of the offshore extension of a late Messinian deep-sea fan system through the analysis of a large set of multichannel seismic-reflection profiles. A 2 km thick Messinian Salinity Crisis sequence occurs on top of a clastic system, indicating that even during the maximum sea-level drop, this part of the basin was still a marine setting. The top of the clastic system is characterized by a sharp surface on which several shear surfaces within the overlying Messinian salt terminate. This is attributed to the northward gliding of the Messinian mobile salt layer from the Nile cone to the deep Herodotus basin. Mapping the base of the clastic system, it is concluded that the bathymetry on which it evolved was complex, forming valley-like structures in the southern half of the study area. The sediment source of this clastic system is interpreted to have originated from the Nile River through gravity flows during the early stages of the Messinian Salinity Crisis that resulted in thick deposits at the deepest parts of the Herodotus basin. Medium to high amplitude discontinuous reflections with multiple sudden terminations representing local (channel-like) to broader-scale erosional surfaces characterize the late Messinian clastic system. Such a seismic character, in combination with its confined nature, indicates the development of a submarine channel complex set. On the other hand, towards the north, the convex top surface of the clastic system and discontinuous internal reflections, exhibit in multiple occasions, bidirectional downlap geometries that point to the basinward evolution of the channel complex set into a lobe complex set at the most unconfined parts of the basin. This clastic system is time equivalent to the fluvio-deltaic deposits of the Abu Madi Formation of the Nile Delta region.

LA "GONFOLITE LOMBARDA": STRATIGRAFIA E SIGNIFICATO NELL'EVOLUZIONE DEL MARGINE SUDALPINO

The Oligocene-Miocene succession of the "Gonfolite Lombarda" outcrops in Western Lombardy and is largely represented in the subsurface of the northern PO Plain. In the this group was considered as a typical example of post- orogenic molasse deposit. The new data suggest that it represents a clastic wedge infilling a deep foreland basin flanking a zone of continental collision. The stratigraphic evolution of the succession strictly reflects the tectono-sedimentary evolution of the basin.
 The succession (almost 3000 m thick) between Como and Varese covers a time span from late early Oligocene (late Rupelian) at least to early Miocene (Burdigalian). It consists of clastic sediments deposited in a deep-sea environment. Both the benthic Foraminifera and the macrofossil assemblages indicate that these sediments were deposited in the epibathial zone (from 500-700 m to 1000-1300 m) throughout all the time span considered. The succession can be splitted in two main segments based on their different sedimentary characters and because they are separated by a regional unconformity corresponding to a hiatus of several million years. The lower one is represented by the Chiasso Formation (about 170m thick) and the upper one by the Gonfolite Group s. Str. (about 2800 m thick).
 The Chiasso Formation is a depositional sequence late Oligocene (late Rupelian to early Chattian) in age. It consists of mudstones and rare thin-bedded turbidites, representing slope or slope-base deposits. The overlying Gonfolite Group represents the build-up of several deep sea fan systems one on top of the other during the very late Oligocene (late Chattian) to the early Miocene (Aquitanian- Burdigalian) and perhaps to the middle Miocene. Resedimented conglomerates infilled deeply incised canyons confined by natural levees in their distal part. The canyons evolved laterally and distally to still partly confined sandy depositional lobes during the early Miocene.
 In the Gonfolite Group three depositional sequences could be recognized. The first one is represented by the Como Conglomerate (800-1500 m thick) and of some heteropic formations, covering a time span from late Chattian to early Burdigalian. This depositional sequence represents an asymmetric megacycle with an initillly very rapid progradation of the conglomeratic system followed by its gradual regression and, in the Como area, by a westward shift of the system axis. Moreover, it is worth mentioning that pebbles of granodiorite-diorite called "ghiandone and serizzo" of the Val Masin0-Val Bregaglia Massif make their appearance already in the basal part of the Como Conglomerate.
 The second depositional sequence consists of the Lucino Conglomerate (800-1000 m thick) and of some heteropic formations. It represents a gradual progradation of the whole deep sea fan system during the late earIy Miocene (Burdigalian).
 In the Varese area, the Gurone Sandstone (120 m thick) may represent a third depositional sequence, which because of its stratigraphic position is younger than the late Burdigalian, and it appears to be deposited prior to the late Miocene (Tortonian).
 The "Gonfolite Lombarda" basin was interested by an intense and continue tectonic activity with at least two periods of "crisis" occurring in the late Chattian and in the early Burdigalian. Because of the strong tectonic Control it's difficult to correlate the "Gonfolite Lombarda" depositional sequences with the generalized sea level curves.

New palaeobotanical data from Carboniferous Culm deposits constrain the age of the Variscan deformation in the eastern Pyrenees

The pre-Variscan rocks of the Pyrenees exhibit a polyphase deformation linked to the Variscan crustal shortening and a low-pressure–high-temperature metamorphism. However, there is scarce chronostratigraphic evidence of this Variscan deformation. In the Pyrenean low-grade metamorphic domains, maximum ages have been provided by the synorogenic Carboniferous Culm deposits. In medium- to high-grade metamorphic areas, the Variscan regional metamorphism or intrusive magmatic bodies constrain the age of the main Variscan deformation structures. However, these data usually provide a minimum age. Here, we present new palaeobotanical records that assign a Namurian age to the base of the Culm deposits of la Cerdanya in the eastern Pyrenees. This dating is based on the co-occurrence of the sphenopsids Archaeocalamites radiatus , Mesocalamites cistiiformis and the seed of Cardiocarpus sp. The plant remains were found in sandstone facies produced by high-density turbidity flows of a deep-sea fan system. The new biostratigraphic information constrains the age of the Carboniferous Culm succession in the eastern Pyrenees.

Depositional environments and ichnology of Upper Cretaceous deep-marine deposits in the Sistan Suture Zone, Birjand, Eastern Iran

The palaeoenvironmental significance of trace fossil assemblages in the flysch deposits of the Upper Cretaceous of the Sistan ocean – the Sefidabeh basin in the Sistan Suture Zone SSZ in Eastern Iran – has been assessed for the first time. The Sefidabeh basin of turbidite origin consists of 10 sedimentary facies, which can be grouped into 3 facies associations (FA) representing submarine channel-related facies associations (FA1), lobe-related facies associations (FA2), distal fan-basin floor facies associations of a deep-water turbidite system (FA3). Thirty three ichnogenera, with many ichnospecies, have been identified in this deep sea succession: Alcyonidiopsis, Arthrophycus, Asterostoma, Belorhaphe, Bergaueria, Cardioichnus, Chondrites, Cosmorhaphe, Desmograpton, Gyrophyllites, Halopoa, Helminthopsis, Helminthorhaphe, Laevicyclus, Lophoctenium, Mammilichnis, Megagrapton, Multina, Nereites, Ophiomorpha, Palaeophycus, Planolites, Phycodes, Phycosiphon, Paleodictyon, Rutichnus, Scolicia, ?Strobilorhaphe, Taenidium, Teichichnus, Thalassinoides, Zoophycos and Urohelminthoida. Their distribution is clearly linked with lithofacies and depositional palaeoenvironments. Changes in trace fossil assemblages and ichnocoenoses follow different environments of the turbidity system of the submarine channel to fan system of the Sefidabeh basin and are associated with variations in environmental controlling factors. Environmental controlling factors including hydrodynamic regime, oxygen level, organic content and sedimentation rates. Ten ichnocoenoses were recognized in the facies associations of the deep-sea fan system of this study. Taking into consideration the diversity, bioturbation level, and colonization order of bioturbated beds and the obvious deepening of the deep-sea depositional system from inner to outer parts of the succession, ichnocoenoses can express a bathymetric trend from shallower to deeper parts, and from higher-to-lower hydrodynamic condition of deep-sea fan systems of the Sefidabeh basin. This study reveals important sedimentological and ichnological features of turbiditic systems in deep sea settings of Iran and permits the development of predictive models for the palaeoenvironmental significance of trace fossil assemblages that can be readily translated to analogous depositional systems in the surface/subsurface.

Rheological Complexity In Sediment Gravity Flows Forced To Decelerate Against A Confining Slope, Braux, SE France

Abstract: Hybrid event beds are now recognized as an important component of many deep-sea fan and sheet systems. They are interpreted to record the passage of rheologically complex sediment gravity currents (hybrid flows) that comprise turbulent, transitional, and/or laminar zones. Hitherto, the development of hybrid flow character has mainly been recognized in system fringes and attributed to distal and lateral flow transformations and/or declining turbulence energy expressed over lateral scales of several kilometers or more. However, new field data show that deposition from hybrid flows can occur relatively proximally, where flows meet confining topography. Turbidity currents primed to transform to hybrid flows by up-dip erosion and incorporation of clay may be forced to do so by rapid, slope-induced decelerations within 1 km of the slope. Local flow transformation and deposition of hybrid event-beds offer an alternative explanation for unusual facies developed at the foot of flow-confining seafloor slopes.

Run-out distance and erosion of debris-flows in the Nile deep-sea fan system: Evidence from lithofacies and micropalaeontological analyses

Debrites with different volumes, depositional periods and setting are observed in the Nile deep-sea fan system. Here we present debrites from four late Quaternary environments: (i) within the Rosetta submarine channel at 1700 m; (ii) within small basins created by salt tectonics in the Eastern Province at 1300 m; (iii) on the open slope outside of channels at 2600 m; and (iv) in the distal part of the fan fringe beyond channel terminations at 3050 m water depth. The debrites comprise mud clasts in a generally ungraded muddy sand matrix. They are relatively thin (20–150 cm) in fan fringe, in channel and salt-tectonic basin settings, and somewhat thicker (>8 m) in the open slope setting. Benthic foraminiferal assemblages and variation in the planktonic/benthic foraminiferal ratios are considered as a useful tool for tentatively evaluating the origin of sediment in these debris flows and estimation of their run-out distance. Similar microfaunal contents of clast/matrix combinations document that the debrites in the Rosetta channel and salt-tectonic basin were locally sourced. The debrite from the open slope travelled farther (up to 150 km). Three separate debrites in a fan fringe core were deposited by debris flows that originated over 200 km away in water depths shallower than 1000 m. These far-travelled debris flows appear to have eroded remarkably little sediment as they travelled for ∼200 km across low (0.8–1.2°) gradient seafloor. This lack of erosion is attributed to hydroplaning or increasing pore-fluid pressure at the base of debris flows. This study confirms that remarkably mobile debris flows can travel long distances to produce thin debrites in the distal fringes of submarine fans.

Characterisation of a Campanian deep-sea fan system in the Norwegian Sea by means of ichnofabrics

Ichnofabrics are applied in concert with sedimentological data to discriminate sub-environments within a Campanian deep-sea fan system in the Norwegian Sea. Seven ichnofabric types are recognised in the studied cores, which correspond to specific architectural elements of the fan system, including amalgamated channels, lobate sand sheets, proximal and distal overbank, fan fringe, and hemipelagic basin plain environments. A unique observation is an ultra-deep Chondrites ichnofabric, interpreted to result from the activity of a chemosymbiotic tracemaker, possibly utilising hydrothermal vents or hydrocarbon seeps in the near vicinity of an active rift system. Mapping and inter-well correlation of ichnofabrics allow a better control of lateral and vertical facies changes, which are important to exploration and production strategies. This study demonstrates that ichnofabric analysis is proving to be a valuable tool for the characterisation and prediction of reservoir quality, the recognition of potential flow barriers and prediction of lateral depositional trends in deep-sea fan deposits.

Mesozoic sequence of Fuerteventura (Canary Islands): Witness of Early Jurassic sea-floor spreading in the central Atlantic

The Fuerteventura Jurassic sedimentary succession consists of oceanic and clastic deposits, the latter derived from the southwestern Moroccan continental margin. Normal mid-oceanic-ridge basalt (N-MORB) flows and breccias are found at the base of the sequence and witness sea-floor spreading events in the central Atlantic. These basalts were extruded in a postrift environment (post–late Pliensbachian). We propose a Toarcian age for the Atlantic oceanic floor in this region, on the basis of the presence higher up in the sequence of the Bositra buchi filament microfacies (Aalenian–Bajocian) and of clastic deposits reflecting tectono-eustatic events (e.g., late Toarcian to mid-Callovian erosion of the rift shoulder). The S-1 sea-floor oceanic magnetic anomaly west of Fuerteventura is therefore at least Toarcian in age. The remaining sequence records Atlantic-Tethyan basinal facies (e.g., Callovian–Oxfordian red clays, Aptian–Albian black shales) alternating with clastic deposits (e.g., Kimmeridgian–Berriasian periplatform calciturbidites and a Lower Cretaceous deep-sea fan system). The Fuerteventura N-MORB outcrops represent the only Early Jurassic oceanic basement described so far in the central Atlantic. They are covered by a 1600 m, nearly continuous sedimentary sequence which extends to Upper Cretaceous facies.

Quaternary sandy deposits and canyons on the Venezuelan margin and south Barbados accretionary prism

The geomorphological and tectonic setting, distribution and origin of Late Quaternary sandy deposits on the southern termination of the Barbados accretionary prism and the adjacent abyssal plain have been investigated using Seabeam and 3.5 kHz echosounding data, and lithological and mineralogical analyses of 28 Kullenberg cores. Quaternary sands on this prism are deposited in piggyback basins. They are transported along three major canyons, the morphology and trend of which are highly variable and which are tectonically controlled. The narrow and relatively shallow canyon sections are in some ways like the major distributary channels of deep sea fans, but they are true canyons in terms of the sedimentary processes taking place, namely active erosion associated with the continuous uplift of tectonic structures and deposition of only slightly elevated sediment levees. The sand is partly trapped on basin floors as flat sediment sheets composed of thinly and thickly bedded fine grained sandy turbidites. High deposition rates occurred during the last period of low sea levels. The sands originated from the Orinoco-Trinidad shelf, as inferred from geographical considerations and clay and heavy mineral provenance data (predominently kaolinite, illite, hornblende and epidote). Sands of the abyssal plain settled on a deep sea fan system that trends north-south, parallel to the toe of the prism. Most of the sediments are introduced at the southern end of the system where they are delivered by the three major canyons which merge, on the fan, in a northward braided channel system. This material consists of sandy turbidites that originated from the Orinoco-Trinidad margin. The grain size of the sands is similar to or coarser than the grain size of the prism sands. Sand components are identical and preclude supply from the Amazon river. The sedimentation rate, during periods of low sea level, seems to be lower. A significant percentage of smectite in the clay fraction suggests that a minor part of the material may be derived from the Demerara margin, located closer to the main smectite source, i.e. the Amazon delta. These results emphasize the difference between the sand distribution on active and passive margins.

Flexure Trend Slope Fan Deposition Gulf of Mexico

ABSTRACT Geological and geophysical data from the Gulf of Mexico flexure trend are used to interpret the depositional processes and sediment distribution of a slope fan system on the Louisiana slope. A schematic model developed from detailed subsurface study of one slope fan system is compared and contrasted to published descriptions of deep sea fan systems. These slope fan to deep sea fan comparisons allow a better understanding of the sediment transport mechanisms, depositional processes, external geometry, and internal facies variations possible in the slope environment. Results suggest that slope sediments in the Gulf of Mexico can originate from turbidity current processes. The external geometry of the resulting deposit may be adequately described using existing submarine fan terminology, including proximal and distal deposits that fit the morphological criteria of upper, middle, and lower fan systems. Internal facies characteristics, however, are found to vary significantly from existing descriptions of submarine fan systems.

Sediment gravity flow deposits of an Ordovician deep-sea fan system (western Precordillera, Argentina)—reply

Abstract Ordovician siliciclastic sequences of Portezuelo del Tontal Formation (PTF) in the western Precordillera, Argentina, are interpreted to be deep-sea deposits which formed on a passive continental margin. Several facies and layer divisions have been identified and interpreted as high- and low-density turbidites, debrites, and contourites-hemipelagites. Outer and inner (upper-middle) submarine fan subenvironments have been recognized. Outer-fan associations consist of lobe and lobe-fringe sequences. Inner-fan associations are defined by coarse-grained and channelized deposits. The vertical arrangement of facies associations is interpreted in terms of progradational and retrogradational processes. Some sharp progradational and retrogradational transitions were controlled by allocyclic factors, such as sea-level changes and tectonics of the source areas. The submarine fan model for the PTF is an intermediate type between the pelite-rich and the sand-rich fans. It was formed in a narrow and elongate deep-sea basin controlled by the contemporaneous evolution of an oceanic rift in the direction of progradation.

Lowstand Deep-Water Clastic Fans and Related Depositional Systems: Terminology, Characteristics, Processes, and Variability

ABSTRACT As presently used in sequence stratigraphic practice, lowstand clastic deep-water fans and related depositional systems consist of mainly "basin-floor fan," "slope-fan" and "shingled turbidites." Basin-floor fan refers to fan-shaped turbidites and related sedimentary gravity-flow deposits in the intraslope basins and basins of bathyal and deeper paleowater depths; the "basin-floor fan" is expected to have unchannelized and channelized sandy turbidites but should be devoid of channel-overbank deposits. "Slope fan" implies, in principle, fan-shaped, channel-overbank turbidites and related deposits that built the continental slopes, and slopes of the intraslope basins, outward and upward. The slope-fan system may also occur on the basin-floors of intraslope basins. The basin-floor fan is thought to be deposited during relative sea-level fall and maximum lowstand ("basin-floor fan time" or "basin-floor fan systems-tract time"), and the slope fan is thought to be deposited during subsequent sea-level still stand and slow rise ("slope fan time" or "slope fan systems-tract time"). "Shingled turbidites" are turbidites deposited at the toes of clinoforms of the prograding wedge following the time of slope fan deposition. Shingled turbidites may include massive sands like those in basin-floor fans, but are deposited closer to the sources of supply than basin-floor fans. Outside the realm of strict sequence-stratigraphic terminology, "deep-sea fan" usually refers to fan- or cone-shaped morphology of organized systems of turbidites and related deposits in basinal settings. The concept of the slope-fan in sequence stratigraphy extends the "deep-sea fan" usage to continental slopes also. In actuality, fans of any usage, may or may not be "fan-shaped" sedimentary bodies depending upon the tectonic and physiographic setting of the basins, but must consist largely of organized systems of turbidites and related deposits. In addition to basin-floor fan and slope-fan systems or "systems-tracts," mass-transport complexes, and slope-, or base-of-slope-, aprons (slides and slumps, debris flow and some turbidity current deposits) should be recognized as separate depositional systems. In the past, these depositional systems were included under "slope fan" or "slope-fan complex." However, the characteristics of aprons and mass-transport complexes, although highly variable and frequently gradational into turbidites, are sufficiently different from the slope-fan and therefore need separate identity. We discuss and illustrate in our presentation the sedimentological, well log and seismic characteristics of fans, slope-aprons and related systems, and their variabilities with some examples from the Gulf of Mexico and elsewhere. Outside the mold of sequence-stratigraphic practice, sedimentologists and geologists use other terminologies for deep-sea fan systems. We attempt to point out the equivalencies of these terminologies to the ones used in sequence stratigraphy. End_of_Record - Last_Page 373-------

Gas-Producing Submarine-Fan Channel-Levee Complexes in Forbes Formation, Arbuckle Field, Central Sacramento Basin, California: ABSTRACT

The Campanian Forbes Formation was deposited as a southprograding basin-plain, deep-sea fan and slope turbidite system in the Late Cretaceous Great Valley forearc basin of California. Gas production from the Arbuckle field is from the middle deep-sea fan deposits of the Forbes Formation at depths of 5,500-7,000 ft. The field is located approximately 7 mi east of the western erosional margin of the Sacramento basin, where the Forbes Formation crops out and dips steeply to the east. The field has a proven acreage of 3,450 and contains 25 producing wells. A north-trending anticline, which is present on the regional easterly dip, provides 150-200 ft of closure in the southern part of the field. Detailed maps and cross sections of the gas-producing intervals of the Forbes Formation in the Arbuckle field have been constructed from correlation of 77 well logs and detailed examination of the nearby outcrops. Well-log correlations and sedimentary facies identified in outcrop indicate that the gas-producing intervals are chiefly channel-levee complexes in a mud-rich deep-sea fan system. These channel-level complexes trend southward, are slightly sinuous, and are as much as 1 mi wide. A comparison of structure-contour maps with net-sandstone maps and gas production data indicates that notmore » all gas production is structurally controlled. Channel-axis sandstone that is as thick as 250 ft generally produces gas only within structural closure, whereas thinner, more shale-rich, channel-margin and levee deposits produce gas from updip stratigraphic pinch-outs into interchannel shale.« less

Late Quaternary sedimentation in Baffin Bay

Baffin Bay is a small ocean basin that connects the Arctic and Atlantic oceans. The adjacent continental shelves have been extensively reworked during Quaternary glaciation. The shelf break generally lies between 200 and 500 m. The continental slope passes directly into the abyssal plain of Baffin Bay basin without any major submarine canyon – deep-sea fan system being present, except for a large smooth sediment apron in northern Baffin Bay.On the basis of over 50 piston cores, six Quaternary sediment facies are distinguished from detrital mineralogy (reflected in colour) and sediment texture. Facies A, B, and C are predominantly ice-rafted or are debris flow deposits, each with a distinct mineralogy. Facies D is turbidites and bottom-current sorted sands, silts, and muds. Facies E is hemipelagic sediment. Facies F consists of sediments ranging from slumps, through debris flow deposits, to fine-grained turbidites, with a distinctive provenance in northern Baffin Bay.These sediment facies appear to be partly controlled by glacial conditions. Hemipelagic facies E predominates during the present interglacial. During glacial stages, facies D turbidites were deposited. They resulted from slumping of proglacial sediments on the continental slopes off Greenland and Baffin Island. Facies C and F occurred on the continental slopes at these times. Ice-rafted facies A and B predominate at several horizons, reflecting a rapid breakup of ice shelves in northern Baffin Bay and increased rates of iceberg melting within the Bay. Overall sedimentation rates are relatively low, reflecting dry-base ice sheets in source areas.Deep-sea channel systems floored by sorted coarse sediments and bounded by muddy levees are absent in Baffin Bay, in contrast to mid-latitude glaciated continental margins off eastern Canada. These channel systems are the result of melting of wet-base glaciers, which provide a localized supply of sediment that is sorted by ice margin processes. In Baffin Bay, most glacial sediments are derived by calving of icebergs, probably from dry-base glaciers. Sediments are gradually released over large areas as the bergs melt, and are subsequently redistributed by debris flows.

Slope and Deep-Sea Fan Facies and Paleogeography of Upper Cretaceous Chatsworth Formation, Simi Hills, California

The Upper Cretaceous Chatsworth Formation is a sand-rich deep-sea fan deposit consisting of thick channelized sequences and interchannel deposits typical of middle fan facies. It has poorly developed outer fan lobes and slope facies. Petrographic and x-ray diffraction (XRD) data indicate the arkosic deposits of the Chatsworth Formation were derived from an uplifted granitic terrane. Paleocurrent studies indicate northward sediment transport and an east-trending slope. Regional comparisons of the Simi Hills with the Santa Monica and Santa Ana Mountains suggest these terranes were once contiguous and covered part of a deep-sea fan system that comprised much of the southern California area. Subsequently, these terranes have been offset along the Malibu-Cucamonga and Elsinore faults, and rotated in response to the opening of the Los Angeles basin and to other adjustments in the tectonic framework of the area. The Chatsworth Formation is thought to be part of a forearc-basin deposit similar to the Great Valley sequence and not part of the Salinian borderland block. The original geographic position of the forearc basin was to the south of the Sierra Nevada Mountains and the Great Valley sequence.

A model of volcanogenic sedimentation in marginal basins

Summary Marginal basins adjacent to oceanic island arcs receive volcaniclastic debris from the bordering volcanic arc, back-arc spreading centre, and, to a lesser degree, the remnant arc. The volcanic arc is volumetrically the most important source, with abundant volcaniclastics being produced by explosive subaerial and/or subaqueous eruptions and secondary erosion of the arc complex. Transport of material to the deeper parts of marginal basins occurs by passive settling through the water column or by a variety of sediment gravity flows generated by primary eruptions and secondary remobilization processes. Deposition of arc-derived material at the base of the arc flank produces thick volcaniclastic aprons and results in a marked asymmetry in the accumulation of volcanogenic sediment within basins. The apron facies are complex and not analogous to typical deep-sea fan systems because of the influence of arc volcanism on the nature and delivery of sediment to the deep sea. Back-arc spreading centres supply a small volume of deep-water hyaloclastites and hydrothermally derived volcanogenic sediments which are incorporated into the basal parts of sedimentary sequences. The dispersal and deposition of volcanogenic sediments are profoundly influenced by geological processes associated with marginal basin evolution such as back-arc spreading, faulting, and island-arc splitting.

Petrology and Dispersal Pattern in the Marnoso-Arenacea Formation (Miocene, Northern Apennines)

ABSTRACT Paleocurrent, gross composition, rock-fragment data, and heavy mineral data from the Miocene Marnoso-arenacea Formation of the Northern Apennines suggest six source areas. Five are primarily extrabasinal (Alpine I and II; Apenninic I, II, and III) and one is intrabasinal. The identification of characteristic multicycle materials reinforces the idea that migration of foredeeps within a plate-tectonic framework occurred in the Apennines. Integration of petrofacies, paleocurrent, and sedimentary-facies-analysis data in the study of the Marnosoarenacea depositional system has proved a very powerful method for understanding the complexity of a deep-sea fan system.

Sedimentology of the Dezadeash flysch and its implications for strike-slip faulting along the Denali Fault, Yukon Territory and Alaska

The Dezadeash Formation of Late Jurassic – Early Cretaceous age is a flysch sequence with a thickness of about 3000 m. It consists of turbidites, mass-flow deposits, and argillite. Most of the formation was deposited on a deep-sea fan system fed by an uplifted volcanogenic terrane to the west. Along its western border, the Dezadeash flysch is truncated by the Denali Fault; towards the east, part of the Dezadeash Formation seems to have been metamorphosed into the Kluane Schist.It is suggested that both the Dezadeash flysch and Kluane Schist have been torn apart by the Denali Fault and that equivalent rock units are found within the Nutzotin Mountains Sequence and the McLaren Metamorphic Belt of Alaska. A dextral displacement of 300 km could account for the present distribution of these units along the Denali Fault. This offset was probably achieved during mid-Tertiary time.

Development of the Astoria Canyon-Fan physiography and comparison with similar systems

A detailed bathymetric study of Astoria Canyon and Astoria Fan provides a model for typical submarine canyon-fan systems. The present canyon head is 9 miles (17 km) west of the Columbia River mouth but buried Pleistocene channels appear to have connected the two features in the past. The canyon, which is distinguished by its relief, V-shaped profiles, and numerous tributaries, winds sinuously and is coincident with apparent structural trends across the continental shelf and slope. At the fan apex, the canyon mouth merges smoothly into Astoria Channel, which is characterized by its U-shaped profiles, lower walls of even height, and levee development. Astoria Channel and the fan valley at the base of the continental slope are the most recently active of a series of main fan valleys that appear to have: ( 1) progressively “hooked left”; ( 2) migrated from north to south across the fan during its formation; and ( 3) been partly responsible for the asymmetrical shape of the fan. The deep, narrow upper fan valleys that characterize the steep (> 1:100, or 0°35′) and rough (10–30 fathoms, or 18–55 m) upper fan surface break into distributaries on the middle fan, where there is the sharpest change in gradient. The main valleys become broader and shallower down the fan, while the generally concave fan surface grades to nearly a flat seafloor (to gradients < 1:1000, or 0°0.5′), (< 10 fathoms, or 18 m relief). Similarity of Astoria Canyon-Fan system with other deep-sea fan and alluvial fan systems, suggests the hypothesis that size of drainage basin, sediment size, and sediment load control the size, gradient, and valley development of any fan system. Data from bathymetry, seismic refraction stations, and sediment load of the Columbia River indicate that the cutting of Astoria Canyon and the deposition of the unconsolidated sediment layer forming Astoria Fan could have been accomplished during the Pleistocene. A similar history can be suggested for other major submarine canyon-fan systems.