Submarine Slope Stability of a Fjord Delta: Bella Coola, British Columbia

Abstract

Recent evidence indicates that the submarine slopes of the Bella Coola Delta, a fjord delta in British Columbia, are subject to mass movements. Mass movements originate in the source areas of chutes (gullies) and transfer coarse sediment downslope. Stability analyses indicate that earthquakes, depositional loading and wave loading are capable of causing slope failures in chute source areas. Gas generation and tidal drawdown appear to reduce sediment strength by increasing pore water pressures, increasing the potential for failures initiated by other mechanisms. Failure related to slope over-steepening at distributary mouths would require much steeper slopes than those encountered. The most unstable areas are those at distributary mouths where most of the mechanisms of failure generation could occur simultaneously. With the exception of earthquakes, the failure mechanisms examined would result in high frequency, low magnitude slides that are most likely to occur in spring and summer. Earthquakes would cause high magnitude, low frequency failures.Des observations récentes démontrent que les pentes sous-marines du delta de Bella Coola sont sujettes à des mouvements de masse. Ces mouvements prennent leur origine dans les zones de formation de ravines et provoquent le transfert des sédiments grossiers vers le bas. Les analyses de stabilité montrent que les tremblements de terre, la charge d'accumulation et la charge des vagues peuvent causer des décrochements. La formation de gaz et la baisse des marées semble réduire la résistance au cisaillement des sédiments en augmentant la pression interstitielle, multipliant ainsi les possibilités de ruptures amorcées par d'autres mécanismes. Les pentes observées à l'embouchure des effluents ne permettent pas qu'il y ait ruptures provoquées par une trop forte inclinaison. Les régions les plus instables sont situées à l'embouchure des effluents où la plupart des mécanismes à l'origine des décrochements peuvent s'exercer simultanément. Sauf en ce qui à trait aux tremblements de terre, les mécanismes étudiés provoqueraient de fréquents glissements, mais de faible importance, plus susceptibles de se produire au printemps et en été. Par contre, les tremblements de terre seraient à l'origine de glissements importants, mais plus rares.Neuere Belege zeigen, dass die untermeerischen Abhange des Bella Coola-Deltas, einem Fjord-Delta in British Columbia, Bewegungen von solidem Material ausgesetzt sind. Die Bewegungen von solidem Material gehen von Schluchten aus und befôrdern grobe Sedimente hangabwârts. Stabilitàts-Analysen zeigen, dass Erdbeben und die Belastung durch Ablagerung und Wellen Hangbrùche Rutschungen bewirken kônnen. Die Bildung von Gas und das Nachlassen der Gezeiten scheinen die Sediment-Stàrke zu verringern, indem sie den Druck von Poren-Wasser erhôhen und damit die Môglichkeit fur Bruche, die durch andere Mechanismen eingeleitet wurden, noch erhôhen. Die vorgefundenen Abhànge sind nicht steil genug, um die Bruche auf zu steile Abhànge an den Mundungen der Abflùsse zurûckz...

Axial and transverse deep‐water sediment supply to syn‐rift fault terraces: Insights from the West Xylokastro Fault Block, Gulf of Corinth, Greece

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Processes and typology in Gilbert-type delta bottomset deposits based on outcrop examples in the Corinth Rift

“…The geometries, sedimentary processes and stratigraphic architectures in topsets and foresets are well known (e.g. Prior, 1981;Lowe, 1982;Postma, 1984a, b;Bornhold and Prior, 1988;Massari and Colella, 1988; Kostaschuk and McCann, 1989;Nemec, 1990;Horton and Schmitt, 1996;Massari, 1996;Breda et al, 2007;Rohais et al, 2008). Conversely, toesets and bottomsets are poorly charaterized and constrained.…”

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