/pdf/landslide-inventory-maps-new-tools-for-an-old-problem-wkxin2jqxg.pdf

Landslide inventory maps: New tools for an old problem

Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.

Subaqueous sediment density flows: Depositional processes and deposit types

Huge landslides, mobilizing hundreds to thousands of km3 of sediment and rock are ubiquitous in submarine settings ranging from the steepest volcanic island slopes to the gentlest muddy slopes of submarine deltas. Here, we summarize current knowledge of such landslides and the problems of assessing their hazard potential. The major hazards related to submarine landslides include destruction of seabed infrastructure, collapse of coastal areas into the sea and landslide-generated tsunamis. Most submarine slopes are inherently stable. Elevated pore pressures (leading to decreased frictional resistance to sliding) and specific weak layers within stratified sequences appear to be the key factors influencing landslide occurrence. Elevated pore pressures can result from normal depositional processes or from transient processes such as earthquake shaking; historical evidence suggests that the majority of large submarine landslides are triggered by earthquakes. Because of their tsunamigenic potential, ocean-island flank collapses and rockslides in fjords have been identified as the most dangerous of all landslide related hazards. Published models of ocean-island landslides mainly examine ‘worst-case scenarios’ that have a low probability of occurrence. Areas prone to submarine landsliding are relatively easy to identify, but we are still some way from being able to forecast individual events with precision. Monitoring of critical areas where landslides might be imminent and modelling landslide consequences so that appropriate mitigation strategies can be developed would appear to be areas where advances on current practice are possible.

Submarine landslides: processes, triggers and hazard prediction

Landslide volumes and landslide mobilization rates in Umbria, central Italy

http://urbanrim.org.uk/cache/Storegga-Slide-Bryn-2005.pdf

Explaining the Storegga Slide

The Storegga Slide: architecture, geometry and slide development

The dating and morphometry of the Storegga Slide

The Holocene Storegga Slide is the last of a series of slides occurring in the same area during the last 500ky. The objectives of the present paper are to present the current understanding of the trigger mechanisms and development of the Storegga Slide, and to show the link between the sliding and Pleistocene climatic fluctuations in the area. Instability is created by the rapid loading of fine-grained hemipelagic deposits and oozes by rapid glacial deposition during peak glaciations. Postglacial earthquake activity was the most likely trigger. Although slide development is complicated and involves a number of slide mechanisms and processes, the overall development is retrogressive, starting at the mid- to lower slope. Sliding stops when the headwall reaches the flat lying, overconsolidated glacial deposits of the shelf.

https://link.springer.com/content/pdf/10.1007%2F978-94-010-0093-2_24.pdf

The Storegga Slide Complex; Repeated Large Scale Sliding in Response to Climatic Cyclicity

On the dynamics of subaqueous clay rich gravity mass flows—the giant Storegga slide, Norway

14C AMS-dated gravity cores reveal that the Traenadjupet Slide offshore Norway occurred about 4,000 14C years B.P. (ca. 4,000 cal. years ago). From 4,000 to 3,000 years B.P., minor areas of the newly formed slide scar were probably eroded, the result of smaller episodes of mass wasting caused by delayed collapse of part of the western, upper sidewall or by bottom currents. From about 3,000 years B.P. to the present, sediments were derived from alongslope-flowing, north-eastward-oriented ocean currents carrying sediments in suspension. These results demonstrate that large-scale mass wasting during sea-level highstand is rather common on passive continental margins.

R. Lien

Papers

The Storegga Slide: architecture, geometry and slide development

The dating and morphometry of the Storegga Slide

The Storegga Slide Complex; Repeated Large Scale Sliding in Response to Climatic Cyclicity

On the dynamics of subaqueous clay rich gravity mass flows—the giant Storegga slide, Norway

The Trænadjupet Slide: a large slope failure affecting the continental margin of Norway 4,000 years ago