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.
Research Article| July 01, 2006 Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003): What happens when pyroclastic flows enter the ocean? J. Trofimovs; J. Trofimovs 1Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar L. Amy; L. Amy 1Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar G. Boudon; G. Boudon 2Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France Search for other works by this author on: GSW Google Scholar C. Deplus; C. Deplus 2Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France Search for other works by this author on: GSW Google Scholar E. Doyle; E. Doyle 3Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar N. Fournier; N. Fournier 4Seismic Research Unit, The University of the West Indies, St Augustine, Trinidad, West Indies Search for other works by this author on: GSW Google Scholar M.B. Hart; M.B. Hart 5School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Search for other works by this author on: GSW Google Scholar J.C. Komorowski; J.C. Komorowski 6Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France Search for other works by this author on: GSW Google Scholar A. Le Friant; A. Le Friant 6Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France Search for other works by this author on: GSW Google Scholar E.J. Lock; E.J. Lock 7School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK Search for other works by this author on: GSW Google Scholar C. Pudsey; C. Pudsey 8British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK Search for other works by this author on: GSW Google Scholar G. Ryan; G. Ryan 9British Geological Survey, Natural Environment Research Council, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar R.S.J. Sparks; R.S.J. Sparks 10Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar P.J. Talling P.J. Talling 10Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information J. Trofimovs 1Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK L. Amy 1Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK G. Boudon 2Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France C. Deplus 2Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France E. Doyle 3Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK N. Fournier 4Seismic Research Unit, The University of the West Indies, St Augustine, Trinidad, West Indies M.B. Hart 5School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK J.C. Komorowski 6Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France A. Le Friant 6Institut de Physique du Globe de Paris et Centre National de la Recherche Scientifique (CNRS) Case 89, 4 Place Jussieu, 75252 Paris, Cedex 05, France E.J. Lock 7School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK C. Pudsey 8British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK G. Ryan 9British Geological Survey, Natural Environment Research Council, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK R.S.J. Sparks 10Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK P.J. Talling 10Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK Publisher: Geological Society of America Received: 11 Nov 2005 Revision Received: 09 Feb 2006 Accepted: 13 Feb 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2006) 34 (7): 549–552. https://doi.org/10.1130/G22424.1 Article history Received: 11 Nov 2005 Revision Received: 09 Feb 2006 Accepted: 13 Feb 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. Trofimovs, L. Amy, G. Boudon, C. Deplus, E. Doyle, N. Fournier, M.B. Hart, J.C. Komorowski, A. Le Friant, E.J. Lock, C. Pudsey, G. Ryan, R.S.J. Sparks, P.J. Talling; Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003): What happens when pyroclastic flows enter the ocean?. Geology 2006;; 34 (7): 549–552. doi: https://doi.org/10.1130/G22424.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Soufrière 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
This contribution provides an analysis of the 1995–2009
eruptive period of Soufriere Hills volcano (Montserrat) from
a unique offshore perspective. The methodology is based on
five repeated swath bathymetric surveys. The difference
between the 2009 and 1999 bathymetry suggests that at
least 395 Mm3 of material has entered the sea. This proximal
deposit reaches 95 m thick and extends ∼7km from shore.
However, the difference map does not include either the
finer distal part of the submarine deposit or the submarine
part of the delta close to the shoreline. We took both
contributions into account by using additional information
such as that from marine sediment cores. By March 2009,
at least 65% of the material erupted throughout the eruption
has been deposited into the sea. This work provides an
excellent basis for assessing the future activity of the
Soufriere Hills volcano (including potential collapse), and
other volcanoes on small islands.
Submarine landslides can be far larger than those on land, and are one of the most important processes for moving sediment across our planet. Landslides that are fast enough to disintegrate can generate potentially very hazardous tsunamis, and produce long run-out turbidity currents that break strategically important cable networks. It is important to understand their frequency and triggers. We document the distribution of recurrence intervals for large landslide-triggered turbidity currents (>0.1 km3) in three basin-plains. A common distribution of recurrence intervals is observed, despite variable ages and disparate locations, suggesting similar underlying controls on slide triggers and frequency. This common distribution closely approximates a temporally-random Poisson distribution, such that the probability of a large disintegrating slide occurring along the basin margin is independent of the time since the last slide. This distribution suggests that non-random processes such as sea level are not a dominant control on frequency of these slides. Recurrence intervals of major (>M 7.3) earthquakes have an approximately Poissonian distribution, suggesting they could be implicated as triggers. However, not all major earthquakes appear to generate widespread turbidites, and other as yet unknown triggers or sequential combinations of processes could produce the same distribution. This is the first study to show that large slide-triggered turbidites have a common frequency distribution in distal basin plains, and that this distribution is temporally random. This result has important implications for assessing hazards from landslide-tsunamis and seafloor cable breaks, and the long-term tempo of global sediment fluxes.
Seafloor sediment flows (turbidity currents) form some of the largest sediment accumulations on Earth, carry globally significant volumes of organic carbon, and can damage critical seafloor infrastructure. These fast and destructive events are notoriously challenging to measure in action, as they often damage any instruments anchored within the flow. We present the first direct evidence that turbidity currents generate seismic signals which can be remotely sensed (~1-3 km away), revealing the internal structure and remarkably prolonged duration of the longest runout sediment flows on Earth. Passive Ocean Bottom Seismograph (OBS) sensors, located on terraces of the Congo Canyon, offshore West Africa, recorded thirteen turbidity currents over an 8-month period. The occurrence and timing of these turbidity currents was confirmed by nearby moorings with acoustic Doppler current profilers.Results show that turbidity currents travelling over ~1.5 m/s produce a seismic signal concentrated below 10 Hz with a sudden onset and more gentle decay. Comparison of the seismic signals with information on flow velocities from the acoustic Doppler current profilers demonstrates that the seismic signal is generated by the fast-moving front of the flow (frontal cell), which contains higher sediment concentrations compared to the slower-moving body. Long runout flows travelling >1000 km have a fast (3.7-7.6 m s-1) frontal cell, which can be 14 hours, and ~350 km long, with individual flows lasting >3 weeks. Flows travelling >1000 km eroded >1300 Mt of sediment in one year, yet had near-constant front speeds, contrary to past theory. The seismic dataset allows us to propose a fundamental new model for how turbidity currents self-sustain, where sediment fluxes into and from a dense frontal layer are near-balanced.Seismic monitoring of turbidity currents provides a new method to record these hazardous submarine flows, safely, over large areas, continuously for years yet at sub-second temporal resolution. Monitoring these processes from land would considerably ease deployment efforts and costs. Thus, work is underway investigating if terrestrial seismic stations can record submarine seafloor processes in Bute Inlet, a fjord in western Canada where independent measurement of delta-lip failures and turbidity currents can be compared to a passive seismic dataset.
[1] Volcanic island landslides can pose a significant geohazard through landslide-generated tsunamis. However, a lack of direct observations means that factors influencing tsunamigenic potential of landslides remain poorly constrained. The study of distal turbidites generated from past landslides can provide useful insights into key aspects of the landslide dynamics and emplacement process, such as total event volume and whether landslides occurred as single or multiple events. The northern flank of Tenerife has undergone multiple landslide events, the most recent being the Icod landslide dated at ∼165 ka. The Icod landslide generated a turbidite with a deposit volume of ∼210 km3, covering 355,000 km2of seafloor off northwest Africa. The Icod turbidite architecture displays a stacked sequence of seven normally graded sand and mud intervals (named subunits SBU1–7). Evidence from subunit bulk geochemistry, volume, basal grain size, volcanic glass composition and sand mineralogy, combined with petrophysical and geophysical data, suggests that the subunit facies represents multistage retrogressive failure of the Icod landslide. The basal subunits (SBU1–3) indicate that the first three stages of the landslide had a submarine component, whereas the upper subunits (SBU4–7) originated above sea level. The presence of thin, non-bioturbated, mud intervals between subunit sands suggests a likely time interval of at least several days between each stage of failure. These results have important implications for tsunamigenesis from such landslides, as multistage retrogressive failures, separated by several days and with both a submarine and subaerial component, will have markedly lower tsunamigenic potential than a single-block failure.
Turbidity currents are the principal processes responsible for carving submarine canyons and maintaining them over geological time scales. The turbidity currents that maintain or "flush" submarine canyons are some of the most voluminous sediment transport events on Earth. Long-term controls on the frequency and triggers of canyon-flushing events are poorly understood in most canyon systems due to a paucity of long sedimentary records. Here, we analyzed a 160-m-long Ocean Drilling Program (ODP) core to determine the recurrence intervals of canyon-flushing events in the Nazaré Canyon over the last 1.8 m.y. We then investigated the role of global eustatic sea level in controlling the frequency and magnitude of these canyon-flushing events. Canyon-flushing turbidity currents that reach the Iberian Abyssal Plain had an average recurrence interval of 2770 yr over the last 1.8 m.y. Previous research has documented no effect of global eustatic sea level on the recurrence rate of canyon flushing. However, we find that sharp changes in global eustatic sea level during the mid-Pleistocene transition (1.2–0.9 Ma) were associated with more frequent canyon-flushing events. The change into high-amplitude, long-periodicity sea-level variability during the mid-Pleistocene transition may have remobilized large volumes of shelf sediment via subaerial weathering, and temporarily increased the frequency and magnitude of canyon-flushing turbidity currents. Turbidite recurrence intervals in the Iberian Abyssal Plain have a lognormal distribution, which is fundamentally different from the exponential distribution of recurrence intervals observed in other basin turbidite records. The lognormal distribution of turbidite recurrence intervals seen in the Iberian Abyssal Plain is demonstrated to result from the variable runout distance of turbidity currents, such that distal records are less complete, with possible influence from diverse sources or triggering mechanisms. The changing form of turbidite recurrence intervals at different locations down the depositional system is important because it ultimately determines the probability of turbidity current–related geohazards.
IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. These cores provide evidence and tests of previous hypotheses for the composition and origin of those deposits. Sites U1394, U1399, and U1400 that penetrated landslide deposits recovered exclusively seafloor sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. This supports the concepts that i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of preexisting low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique (U1399 and 1400), the landslide deposits comprised blocks of parallel strata that were tilted or microfaulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site U1394 offshore Montserrat penetrated a flat-lying block of intact strata. The most likely mechanism for generating these large-scale seafloor sediment failures appears to be propagation of a decollement from proximal areas loaded and incised by a volcanic debris avalanche. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits composed of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial block-rich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution data set to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes.
Abstract The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μ m < d < 250 μ m and volume fractions, φ s , up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 μ m and volume fractions, φ m , up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φ s ∼ 0·2 and entirely inhibited at φ s ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations ( φ s < 0·2), particle segregation was initially suppressed at φ m ∼ 0·07 and entirely suppressed at φ m ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φ s ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations.
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