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Hybrid submarine flows comprising turbidity current and cohesive debris flow: Deposits, theoretical and experimental analyses, and generalized models
No full textHybrid flows comprising both turbidity current and submarine debris flow are a significant departure from many previous influential models for submarine sediment density flows. Hybrid beds containing cohesive debrite and turbidite are common in distal depositional environments, as shown by detailed observations from more than 20 modern and ancient systems worldwide. Hybrid flows, and cohesive debris flows more generally, are best classified in terms of a continuum of decreasing cohesive debris flow strength. High-strength cohesive debris flows tend to be clast rich and relatively thick, and their deposit extends back to near the site of original slope failure. They are typically confined to higher gradient continental slopes, but may occasionally form megabeds on basin plains, in both cases overlain by a thin turbidite. Intermediate-strength cohesive debris flows typically contain clasts, but their deposits may be <1 or 2 m thick on low-gradient fan fringes, and are encased in turbidite sand and mud. Clasts may be far-traveled, and meter-sized clasts can be rafted long distances across very low gradients if they are less dense than surrounding flow. Low-strength cohesive debris flows generally lack mud clasts, and as cohesive strength decreases further there is a transition into fluid mud layers that do not support sand. Intermediate- and low-strength cohesive debrites are consistently absent in more proximal parts of submarine systems, where faster moving sediment-charged flows are more likely to be turbulent. Intermediate-strength debris flows can run out for long distances on low gradients without hydroplaning. Very low strength cohesive debris flows most likely form through late-stage transformations near the site of debrite deposition, and emplaced gently to avoid mixing with surrounding seawater. The location and geometry of cohesive debrites in hybrid beds are controlled strongly by seafloor morphology and small changes in gradient. Debrites occur as fringes around raised channel-levee ridges, or in the central and lowest parts of basin plains lacking such ridges. Small variations in mud fraction produce profound changes in cohesive strength, flow viscosity, permeability, and the time taken for excess pore pressures to dissipate that span multiple orders of magnitude. Reduction in flow speed can also cause substantial increases in viscosity and yield strength in shear thinning muddy fluids. Small amounts of sediment can dampen or extinguish turbulence, especially as flow decelerates, affecting how sediment is supported or deposited. This ensures that cohesive debris flows and hybrid flows have a rich variety of behaviors
Multistage collapse of eight western Canary Island landslides in the last 1.5 Ma: Sedimentological and geochemical evidence from subunits in submarine flow deposits
Full text linkVolcaniclastic turbidites in the Madeira Abyssal Plain provide a record of major landslides from the Western Canary Islands in the last 1.5 Ma. These volcaniclastic turbidites are composed of multiple fining-upward turbidite sands, known as subunits. The subunits indicate that the landslides responsible for the sediment gravity flows occurred in multiple stages. The subunits cannot result from flow reflection or splitting because the compositions of volcanic glasses from each individual subunit in an event bed are subtly different. This indicates that each subunit represents a discrete failure as part of a multistage landslide. This has significant implications for geohazard assessments, as multistage failures reduce the magnitude of the associated tsunami. The multistage failure mechanism reduces individual landslide volumes from up to 350 km3 to less than 100 km3. Thus although multistage failure ultimately reduce the potential landslide and tsunami threat, the landslide events may still generate significant tsunamis close to source
Turbidite record of frequency and source of large volume (>100 km3) Canary Island landslides in the last 1.5 Ma: Implications for landslide triggers and geohazards
Full text linkDuring the last two decades, numerous studies have focused on resolving the landslide histories of the Canary Islands. Issues surrounding the preservation and dating of onshore and proximal submarine landslide deposits precludes accurate determination of event ages. However, submarine landslides often disaggregate and generate sediment gravity flows. Volcaniclastic turbidites sampled from Madeira Abyssal Plain piston cores represent a record of eight large-volume failures from the Western Canary Islands in the last 1.5 Ma. During this time, there is a mean recurrence rate of 200 ka, while the islands of El Hierro and Tenerife have individual landslide recurrences of 500 ka and 330 ka, respectively. Deposits from the 15 ka El Golfo landslide from El Hierro and 165 ka Icod landslide from Tenerife are examined. This study also identifies potential deposits associated with the Orotava (535 ka), Güímar (850 ka), and Rogues de García landslides (1.2 Ma) from Tenerife, El Julan (540 ka), and El Tiñor (1.05 Ma) landslides from El Hierro, and the Cumbre Nueva landslide (485 ka) from La Palma. Seven of eight landslides occurred during major deglaciations or subsequent interglacial periods, which represent 55% of the time. However, all of the studied landslides occur during or at the end of periods of protracted island volcanism, which generally represent 60% of the island histories. Although climate may precondition failures, it is suggested that volcanism presents a more viable preconditioning and trigger mechanism for Canary Island landslides
Basin plain deposits of the Marnoso-Arenacea Formation, Italy
No full textThe Marnoso Arenacea turbidite system outcrops across a large area (125 × 35 km [78 × 22 mi]) of the northern Apennines in Italy (1). It records sedimentation within a Miocene foreland basin and has been an area of turbidite research for more than fifty years. Correlations of individual flow deposits (beds) within the Serravallian, ponded basin-plain sequence are some of the most extensive in any ancient turbidite system. Long-distance (>100 km [>62 mi]) bed correlation was first established by Ricci Lucchi and Valmori (1980) for a 200-m (656-ft)-thick stratigraphie interval, between 18 measured sections. Correlation is aided in this system by the presence of carbonate-rich marker beds which have a distinct provenance and composition. This contribution presents an enhanced correlation framework for a 30-m (98-ft)-thick sub-interval, positioned above the most prominent marker bed (the Contessa). This new correlation framework is based on more than 100 measured sections (Figures 1,2). The additional sections were identified using new 1:10,000 geological maps (produced by the Emilia-Romagna, Marche, Toscana, and Umbria Geological Surveys) that show the position of the Contessa marker bed
Imaging bed geometry and architecture of massive sandstones in the Fontanelice Channels, Italian Apennines, using new digiscoping techniques
No full textIn this study we present digital images and sedimentological data from a channel fill succession in the Italian Apennines that is dominated by massive sandstones. Although the studied outcrop is largely inaccessible, valuable data have now been obtained using the new technique of ‘digiscoping’, which allows features of < 10 cm to be resolved from a distance of several hundred metres.About 75–80% of the channel fill is composed of massive sandstone beds > 1 m thick, with overall sandstone : shale ratios of 9 : 1. Massive sandstones are poorly sorted and overall show little or no normal grading. They are commonly amalgamated and always have sharp bed tops. Massive sandstone beds show abrupt pinch-outs at the channel margin, whereas overlying thin-bedded siltstone/mudstone layers taper gradually and drape up the margin more extensively. This suggests that the depositing flows were stratified into a lower, thin, (hyper)concentrated density flow and an upper, more dilute, turbidity current. In summary, the digiscoping technique is shown to be a cheap and efficient method for imaging distant and/or inaccessible outcrops and providing information on bed geometry and architecture
New insight into the evolution of large-volume turbidity currents: comparison of turbidite shape and previous modelling results
No full text.The Marnoso Arenacea Formation provides the most extensive correlation of individual flow deposits (beds) yet documented in an ancient turbidite system. These correlations provide unusually detailed constraints on bed shape, which is used to deduce flow evolution and assess the validity of numerical and laboratory models. Bed volumes have an approximately log-normal frequency distribution; a small number of flows dominated sediment supply to this non-channelized basin plain. Turbidite sandstone within small-volume (<0·7 km3) beds thins downflow in an approximately exponential fashion. This shape is a property of spatially depletive flows, and has been reproduced by previous mathematical models and laboratory experiments. Sandstone intervals in larger-volume (0·7–7 km3) beds have a broad thickness maximum in their proximal part. Grain-size trends within this broad thickness maximum indicate spatially near-uniform flow for distances of 30 km, although the flow was temporally unsteady. Previous mathematical models and laboratory experiments have not reproduced this type of deposit shape. This may be because models fail to simulate the way in which near bed sediment concentration tends towards a constant value (saturates) in powerful flows. Alternatively, the discrepancy may be the result of relatively high ratios of flow thickness and sediment settling velocity in submarine flows, together with very gradual changes in sea-floor gradient. Intra-bed erosion, temporally varying discharge, and reworking of suspension fallout as bedload could also help to explain the discrepancy in deposit shape. Most large-volume beds contain an internal erosion surface underlain by inversely graded sandstone, recording waxing and waning flow. It has been inferred previously that these characteristics are diagnostic of turbidites generated by hyperpycnal flood discharge. These turbidites are too voluminous to have been formed by hyperpycnal flows, unless such flows are capable of eroding cubic kilometres of sea-floor sediment. It is more likely that these flows originated from submarine slope failure. Two beds comprise multiple sandstone intervals separated only by turbidite mudstone. These features suggest that the submarine slope failures occurred as either a waxing and waning event, or in a number of stages
Timing and emplacement dynamics of newly recognised mass flow deposits at ~8–12ka offshore Soufrière Hills volcano, Montserrat: How submarine stratigraphy can complement subaerial eruption histories
No full textThis contribution describes two mass movement deposits (total volume ~ 0.5 km3) identified in seven marine cores located 8 to 15 km offshore southern Montserrat, West Indies. The deposits were emplaced in the last 35 ka and have not previously been recognised in either the subaerial or distal submarine records. Age constraints, provided by radiocarbon dating, show that an explosive volcanic eruption occurred at ca 8–12 ka, emplacing a primary eruption-related deposit that overlies a large (~ 0.3 km3) reworked bioclastic and volcaniclastic flow deposit, formed from a shelf collapse between 8 and 35 ka. The origin of these deposits has been deduced through the correlation of marine sediment cores, component analysis and geochemical analysis. The 8–12 ka primary volcanic deposit was likely derived from a highly-erosive pyroclastic flow from the Soufrière Hills volcano that entered the ocean and mixed with the water column forming a water-supported density current. Previous investigations of the eruption record suggested that there was a hiatus in activity at the Soufrière Hills volcano between 16 and 6 ka. The ca 8–12 ka eruptive episode identified here shows that this hiatus was shorter than previously hypothesised, and thus highlights the importance of obtaining an accurate and complete marine record of events offshore from volcanic islands and incorporating such data into eruption history reconstructions. Comparisons with the submarine deposit characteristics of the 2003 dome collapse also suggests that the ~ 8–12 ka eruptive episode was more explosive than eruptions from the current eruptive episode
Beds comprising debrite sandwiched within co-genetic turbidite: origin and widespread occurrence in distal depositional environments
No full textCo-genetic debrite–turbidite beds occur in a variety of modern and ancient turbidite systems. Their basic character is distinctive. An ungraded muddy sandstone interval is encased within mud-poor graded sandstone, siltstone and mudstone. The muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite. The mud-poor sandstone, siltstone and mudstone show features indicating progressive layer-by-layer deposition and are thus called a turbidite. Palaeocurrent indicators, ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event. Detailed field observations are presented for co-genetic debrite–turbidite beds in three widespread sequences of variable age: the Miocene Marnoso Arenacea Formation in the Italian Apennines; the Silurian Aberystwyth Grits in Wales; and Quaternary deposits of the Agadir Basin, offshore Morocco. Deposition of these sequences occurred in similar unchannellized basin-plain settings. Co-genetic debrite–turbidite beds were deposited from longitudinally segregated flow events, comprising both debris flow and forerunning turbidity current. It is most likely that the debris flow was generated by relatively shallow (few tens of centimetres) erosion of mud-rich sea-floor sediment. Changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition. The association with distal settings results from the ubiquitous presence of muddy deposits in such locations, which may be eroded and disaggregated to form a cohesive debris flow. Debrite intervals may be extensive (> 26 x 10 km in the Marnoso Arenacea Formation) and are not restricted to basin margins. Such long debris flow run-out on low-gradient sea floor (< 0.1?) may simply be due to low yield strength (<<50 Pa) of the debris–water mixture. This study emphasizes that multiple flow types, and transformations between flow types, can occur within the distal parts of submarine flow events
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