Peter J. Talling

Onset of submarine debris flow deposition far from original giant landslide

Submarine landslides can generate sediment-laden flows whose scale is impressive. Individual flow deposits have been mapped that extend for 1,500 km offshore from northwest Africa. These are the longest run-out sediment density flow deposits yet documented on Earth. This contribution analyses one of these deposits, which contains ten times the mass of sediment transported annually by all of the world’s rivers. Understanding how this type of submarine flow evolves is a significant problem, because they are extremely difficult to monitor directly. Previous work has shown how progressive disintegration of landslide blocks can generate debris flow, the deposit of which extends downslope from the original landslide. We provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide, encased within deposits of a more dilute flow type called turbidity current. Very little sediment was deposited across the intervening large expanse of sea floor, where the flow was locally very erosive. Sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea-floor gradient from 0.05° to 0.01°. This debris flow was probably generated by flow transformation from the decelerating turbidity current. The alternative is that non-channelized debris flow left almost no trace of its passage across one hundred kilometres of flat (0.2° to 0.05°) sea floor. Our work shows that initially well-mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean.

How and where do incised valleys form if sea level remains above the shelf edge

Sequence stratigraphic models predict that during relative sea-level fall, rivers will cut incised valleys across the exposed continental shelf. Incision is driven by the exposure of convex-up topography and consequent downstream increases in stream power. Modern shelf profiles from passive-margin and foreland-basin settings show that profile convexities occur at the shelf edge and at the current highstand coastline (the coastal prism). In nonglacial times, which form much of Earth9s history, it is unlikely that the shelf edge was subaerially exposed. During such periods, patterns of incision would have been primarily determined by the shape of the coastal prism. On the basis of modern shelf profiles from passive-margin and foreland-basin settings, river incision of as much as 20 to 70 m would be centered upon the preceding highstand coastline and would be restricted to the inner part of the shelf. Deeper incised valleys would only result from exposure of the shelf edge or from the processes involved in the formation of submarine canyons.

Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003): What happens when pyroclastic flows enter the ocean?

The Soufriere Hills volcano, Montserrat, West Indies, has undergone a series of dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the ocean. Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow over distances >30 km from the source. The total volume of pyroclastic material off the east coast of Montserrat exceeds 280 × 106 m3, with 65% deposited in proximal lobes and 35% deposited as distal turbidites.

Deposits of flows transitional between turbidity current and debris flow

The relationship between submarine sediment gravity flows and the character of their deposits is poorly understood. Annular flume experiments were used to investigate the depositional dynamics and deposits of waning sediment-laden flows. Decelerating fast (>3 m/s) flows with fixed sand content (10 vol%) and variable mud content (0–17 vol%) resulted in only four deposit types. Clean sand with a mud cap that resembled a turbidity current deposit (turbidite) formed if the flow was turbulent when deposition began, or if the muddy fluid had insufficient strength to suspend the sand. The clean sand could contain structures if mud content was low ( 300 s. Ungraded muddy sand with a mud cap that resembled a debris-flow deposit (debrite) formed if the flow became laminar before sand could deposit. Clean sand overlain by ungraded muddy sand and a mud cap formed either from a transitional flow or by late-stage settling of sand from a muddy suspension. These deposits resemble enigmatic submarine flow deposits called linked debrite-turbidites. The experiments provide a basis for inferring flow type from deposit character for submarine sediment-laden flows.

Heat flow in the Lesser Antilles island arc and adjacent back arc Grenada basin

Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to 15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present-day volcanism is confined to the region with the highest heat flow.

Braided stream and flood-plain deposition in a rapidly aggrading basin: the Escanilla formation, Spanish Pyrenees

Abstract Models of braided stream deposition have largely been developed from studies of regionally degrading and laterally confined alluvial environments. Glacial outwash streams, in particular, have supplied important and widely cited descriptions of intra-channel processes. These fluvial systems are typically confined within quite narrow valleys. It is felt that such systems have low long-term preservation potential and are unlikely to be present in the geologic record in large quantities. Therefore, the study of these modern laterally confined degradational systems may not provide holistic analogs of the larger-scale alluvial architecture developed in braided river environments in the ancient. The Escanilla Formation of the Spanish Pyrenees provides a well-exposed example of an Eocene fluvial system flowing axially within the Pyrenean foreland basin. Sedimentologic study shows coarse channelized deposits of braided character wholly enclosed within large amounts of fine-grained overbank mudstones and siltstones (>40% by volume), with both being deposited coevally across the Escanilla floodplain. A new depositional model is proposed that combines facets of existing models derived from other fluvio-morphologic systems. This consists of a laterally confined channel belt, internally preserving a braided stream character, capable of rapid vertical aggradation on short geological time-scales (about a thousand years). Avulsion processes are used to explain finer sediment deposition in interfluve settings, as well as the large-scale architectural geometries within the lower Escanilla Formation. This new model illustrates that discrete channel belt avulsion, and the preservation of thick sequences of overbank material are not exclusively characteristics of higher sinuosity fluvial systems.

Geomorphic evidence for tear faults accommodating lateral propagation of an active fault-bend fold, Wheeler Ridge, California

Abstract Wheeler Ridge is the topographic expression of an actively growing fault-bend fold developed in the hanging walls of a system of mostly north-vergent blind thrusts. The eastern tip of the fold system has migrated laterally by approximately 3 km during the last 120 ka perpendicular to the regional shortening direction. Comparison of surface topography and structural cross-sections indicates that eastern Wheeler Ridge comprises three fold segments, whose topographic expression steps southward towards the eastern end of the structure. Lateral propagation of the fold is associated with numerous tear faults, expressed as fault scarps that face towards the eastern end of the fold. We suggest that these tear faults accommodate earthquake-related processes that collectively build plunge of the fold and lateral propagation on blind thrusts. We envisage that the fold will grow primarily by thrust faulting events with similar displacement(s) along strike that are terminated abruptly at tear faults, build displacement over multiple earthquake cycles and then step eastward to form a new tear fault. Formation of especially large tear faults is inferred to be in response to an increase in rock strength governed by lithology. The high fault displacement vs length ( D L ) for Wheeler Ridge may be related to high strain rates across a restraining bend in the nearby San Andreas strike-slip fault system.

Hybrid submarine flows comprising turbidity current and cohesive debris flow: Deposits, theoretical and experimental analyses, and generalized models

Hybrid 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.

Multiple widespread landslides during the long-term evolution of a volcanic island: insights from high-resolution seismic data, Montserrat, Lesser Antilles

New high‐resolution multichannel seismic data (GWADASEIS‐2009 and JC45/46‐2010 cruises; 72 and 60 channels, respectively) combined with previous data (AGUADOMAR‐1999 and CARAVAL‐ 2002; 6 and 24 channels, respectively) allow a detailed investigation of mass‐wasting processes around the volcanic island of Montserrat in the Lesser Antilles. Seven submarine deposits have sources on the flanks of Montserrat, while three are related to the nearby Kahouanne submarine volcanoes. The most voluminous deposit (∼20 km 3) within the Bouillante‐Montserrat half‐graben has not been described previously and is probably related to a flank instability of the Centre Hills Volcano on Montserrat, while other events are related to the younger South Soufriere Hills‐Soufriere Hills volcanic complex. All deposits are located to the south or southeast of the island in an area delimited by faults of the Bouillante‐Montserrat half‐graben. They cover a large part of the southeast quarter of the surrounding seafloor (∼520 km 2), with a total volume of ∼40 km 3. Our observations suggest that the Bouillante‐Montserrat half‐graben exerts a control on the extent and propagation of the most voluminous deposits. We propose an interpretation for mass‐wasting processes around Montserrat similar to what has happened for the southern islands of the Lesser Antilles.

Submarine deposition of volcaniclastic material from the 1995–2005 eruptions of Soufrière Hills volcano, Montserrat

Abstract: Soufrière Hills volcano, Montserrat, has been erupting since 1995. During the current eruption, a large part of the material produced by the volcano has been transported into the sea, modifying the morphology of the submarine flanks of the volcano. We present a unique set of swath bathymetric data collected offshore from Montserrat in 1999, 2002 and 2005. From 1999 to 2002, pyroclastic flows associated with numerous dome collapses entered the sea to produce 100 Mm3 deposit. From 2002 to 2005, the 290 Mm3 submarine deposit is mainly from the 12–13 July 2003 collapse. These data allow us to estimate that, by May 2005, at least 482 Mm3 of material had been deposited on the sea floor since 1995. We compare on-land characteristics and volumes of dome collapse events with the submarine deposits and propose a new analysis of their emplacement on the submarine flanks of the volcano. The deposition mechanism shows a slope dependence, with the maximum thickness of deposit before the break in the slope, probably because of the type of the dense granular flow involved. We conclude that from 1995 to 2005 more than 75% of the erupted volume entered the sea.