More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments. The processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized. The three types are differentiated mainly by motion, architecture, and shape of failure surface. For slides, the morphology of deposits can usually be linked to a process, but for plastic flows and turbidity currents, information about the motion is mainly provided by the sedimentary record. A static classification based on these features is given, and is related to a dynamic classification system to try to underline the morphological transformation of an offshore event from initiation to deposition.

Classification of offshore mass movements

The processes that control the formation, intensity and track of hurricanes are poorly understood. It has been proposed that an increase in sea surface temperatures caused by anthropogenic climate change has led to an increase in the frequency of intense tropical cyclones, but this proposal has been challenged on the basis that the instrumental record is too short and unreliable to reveal trends in intense tropical cyclone activity. Storm-induced deposits preserved in the sediments of coastal lagoons offer the opportunity to study the links between climatic conditions and hurricane activity on longer timescales, because they provide centennial- to millennial-scale records of past hurricane landfalls. Here we present a record of intense hurricane activity in the western North Atlantic Ocean over the past 5,000 years based on sediment cores from a Caribbean lagoon that contain coarse-grained deposits associated with intense hurricane landfalls. The record indicates that the frequency of intense hurricane landfalls has varied on centennial to millennial scales over this interval. Comparison of the sediment record with palaeo-climate records indicates that this variability was probably modulated by atmospheric dynamics associated with variations in the El Nino/Southern Oscillation and the strength of the West African monsoon, and suggests that sea surface temperatures as high as at present are not necessary to support intervals of frequent intense hurricanes. To accurately predict changes in intense hurricane activity, it is therefore important to understand how the El Nino/Southern Oscillation and the West African monsoon will respond to future climate change.

https://www.fs.fed.us/global/iitf/pubs/nature_donelly_woodruff.pdf

Intense hurricane activity over the past 5,000 years controlled by El Niño and the West African monsoon.

Reassessment of hydrodynamic equations: Minimum flow velocity to initiate boulder transport by high energy events (storms, tsunamis)

Boulder and fine sediment transport and deposition by the 2004 tsunami in Lhok Nga (western Banda Aceh, Sumatra, Indonesia): A coupled offshore–onshore model

Historical and geological evidence of boulders deposited by tsunamis, southern Ryukyu Islands, Japan

The role of porosity in discriminating between tsunami and hurricane emplacement of boulders — A case study from the Lesser Antilles, southern Caribbean

Preservation potential of tsunami deposits on arid siliciclastic coasts

The TsuSedMod inversion model applied to the deposits of the 2004 Sumatra and 2006 Java tsunami and implications for estimating flow parameters of palaeo-tsunami

Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain-size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea-level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea-level. Backflow re-mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross-bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2·5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low-gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits.

Michaela Spiske

Papers

The role of porosity in discriminating between tsunami and hurricane emplacement of boulders — A case study from the Lesser Antilles, southern Caribbean

Preservation potential of tsunami deposits on arid siliciclastic coasts

The TsuSedMod inversion model applied to the deposits of the 2004 Sumatra and 2006 Java tsunami and implications for estimating flow parameters of palaeo-tsunami

Sedimentology of tsunami inflow and backflow deposits: key differences revealed in a modern example

A quasi-experimental setting of coarse clast transport by the 2010 Chile tsunami (Bucalemu, Central Chile)