Submarine landform assemblages and sedimentary processes related to glacier surging in Kongsfjorden, Svalbard

High-resolution swath-bathymetry data from inner Kongsfjorden, Svalbard, reveal characteristic landform assemblages formed during and after surges of tidewater glaciers, and provide new insights into the dynamics of surging glaciers. Glacier front oscillations and overriding related to surge activity lead to the formation of overridden moraines, glacial lineations of two types, terminal moraines, associated debris lobes and De Geer moraines. In contrast to submarine landform assemblages from other Svalbard fjords, the occurrence of two kinds of glacial lineations and the presence of De Geer moraines suggest variability in the landforms produced by surge-type tidewater glaciers. All the landforms in inner Kongsfjorden were deposited during the last c. 150 years. Lithological and acoustic data from the innermost fjord reveal that suspension settling from meltwater plumes as well as ice rafting are dominant sedimentary processes in the fjord, leading to the deposition of stratified glacimarine muds with variable numbers of clasts. Reworking of sediments by glacier surging results in the deposition of sediment lobes containing massive glacimarine muds. Two sediment cores reveal minimum sediment accumulation rates related to the Kongsvegen surge from 1948; these were 30 cm a-1 approximately 2.5 km beyond the glacier front shortly after surge termination, and rapidly dropped to an average rate of 1.8 cm a-1 in ∼ 1950, during glacier retreat

Modern and possible paleotsunami deposits in Samenoura, Sanriku Coast, and their relation to tsunami source mechanisms

Samenoura is situated in the bay head of a small inlet on the Pacific coast of Oshika Peninsula, one of the nearest places to the epicenter of the 2011 Tohoku-oki Earthquake. According to the Joint Survey Group, wave heights were measured at more than 20 m near the coastline. This area was severely damaged as a result of both co-seismic subsidence and tsunami inundation. We carried out field surveys of the Tohoku-oki and paleotsunami deposits at Samenoura in March, May and October 2013. Sandy deposits laid down by the Tohoku-oki tsunami were up to 20 cm thick at locations with an elevation greater than 10 m, and were several cm thick within the forest higher up. The tsunami deposit also contained numerous shell fragments and foraminifera. Although some possible sources of the tsunami deposits can be attributed to narrow sandy beaches near the study area, the deposition of such a thick sandy deposit is more or less enigmatic, considering the steep Ria-type coastal topography.Using a gouge auger and geoslicer, we found at least two sand layers intercalated within muddy sediments. A volcanic ash layer, which corresponds to the AD 915 Towada-a tephra, was also identified from a horizon between these sand layers. The underlying sand layer was most probably laid down by the 869 Jogan earthquake tsunami, one of the large-scale events known to have affected the region. Previous studies of the Jogan tsunami have proposed several possible source models that involve an interplate thrust earthquake. Given that the local bathymetry and topography of Samenoura Bay may be sensitive to the waveform of a large-scale tsunami, paleotsunami deposits found from this area may be the key to determining the source mechanisms of events on the Sanriku Coast.In this presentation, the possible correlation of the sandy deposits with known paleotsunami events based on detailed radiocarbon dating is discussed. The hydrodynamic character and processes of tsunami sediment erosion and deposition in Samenoura Bay are analyzed using numerical modeling of both interplate and outer-rise earthquake scenarios.Copyright on Japan Geoscience Union Meeting, 2014

Sedimentary record of 2010 and 2011 Warta River seasonal floods in the region of Poznań, Poland

The Warta River near Poznań (West Poland) represents a meandering lowland river modified by hydro-engineering projects. Recently, two large floods occurred during the summer of 2010 and the winter of 2011. Rapid response surveys were conducted to document erosion and sedimentary records of the floods (spatial extent, grain size, composition, grain microtextures). Follow-up studies, which were conducted during the two years after the floods, assessed postdepositional changes in the flood deposits. A thick sand layer formed that ranged from an average of 10–15 cm (summer) to 30–35 cm (winter), building natural levees, side bars and crevasse splays. The sand layers consisted of fine- and medium-grained sands that were well-sorted, fine skewed and sourced from the river channel. The longer but smaller winter flood produced thicker and coarser deposits at different sites compared with the summer flood. An organic-rich mud layer and algal mats, which were short-existing, were also detected on the floodplain. The study proved that the flood record on engineered rivers may be only in the sand bodies near the river channel and their preservation is mainly controlled by their thickness. A single site may not record all floods, as particular events tend to leave deposits in various places

Sedimentary evidence of extreme storm surge or tsunami events in the southern Baltic Sea (Rogowo area, NW Poland)

The Baltic Sea is not typically considered as an area affected by tsunamis. However, during the Late Pleistocene and Holocene several tsunami events have been interpreted from the sedimentary record, mainly in Sweden and Estonia. Furthermore, on the southern coast of the Baltic Sea, there are historical accounts of catastrophical marine floodings called “der Seebär” (“the Sea Bear”). Their descriptions reveal many features typical for tsunami, but their genesis remained unknown and sedimentary evidence for such events has not been found. Here we provide evidence of sandy event layers from the area of Rogowo, NW Poland – the area of historical catastrophic storms as well as “der Seebär” events. The study area is a low-lying coastal plain with an average elevation of –0.5 to +0.5 m a.s.l., protected from the open sea by beach and coastal dune systems up to 5 m high. Sedimentological, micropalaeontological and geochemical analyses along with AMS 14C dating were applied to sedimentary successions seen in 5 major trenches and 198 sediment cores up to 1.5 m long. Two sandy layers were identified in the peat deposits that developed on the plain during the last ~2000 years. They reveal a number of typical features of tsunami deposits (significant lateral extent and thickness, rip-up clasts, chemical and micropalaeontological evidence of marine origin), however, 14C dating along with the historical accounts revealed that the major layer, extending at least 1.2 km from the modern coasts, was probably deposited by arguably the largest storm surge during the last 2000 years, which took place in 1497 AD. These storm deposits were likely formed during inundation of the low-lying coastal plain after major breaching of coastal dunes resulting in tsunami – like flow pattern and thus similar sedimentological effects. A discontinuous sand layer of younger age (18th century) and sharing similar properties to the previous one may be related to “der Seebär” event or another storm surge. The study revealed that the southern Baltic Sea coast may be affected by much greater coastal flooding than known from more recent accounts and observations. Thus, the presented geological record should be taken as an example of a worst-case scenario in coastal zone risk assessment from natural hazards. These events left sedimentary deposits that resemble tsunami deposits. It is likely that, in similar settings where storm surges cause unidirectional inundation of a coastal plain, it may not be possible to establish whether the resulting deposits were laid down from storms or tsunamis

Transferability of a calibrated numerical model of rock avalanche run-out: application to 20 rock avalanches on the Nuussuaq Peninsula, West Greenland

Long run‐out rock avalanches are one of the most hazardous geomorphic processes, and risk assessments of the potential threat they pose are often reliant on numerical modelling of their potential run‐out distance. The development of such models requires a thorough understanding of past flow behaviour inferred from deposits emplaced by previous events. Despite this, few records exist of multiple rock avalanches that occurred in conditions sufficiently consistent to develop a set of more generalised, and hence transferrable, rules. We conduct field and imagery‐based mapping and use numerical modelling to investigate the emplacement of 20 adjacent rock avalanches on the southern flanks of the Nuussuaq peninsula, West Greenland. The rock avalanches run out towards the Vaigat Strait, and are sourced from a range of coastal mountains of relatively uniform geology. We calibrate a three‐dimensional continuum dynamic flow code, VolcFlow, with data from a modern, well‐constrained event that occurred at Paatuut (AD 2000). The best‐fit model assumes a constant retarding stress with a collisional stress coefficient, simulating run‐out to within ±0.3% of that observed. This calibration was then used to model the emplacement of deposits from five other neighbouring rock avalanches before simulating the general characteristics of a further 14 rock avalanche deposits on simplified topography. Our findings illustrate that a single calibration of VolcFlow can account for the observed deposit morphology of a uniquely large collection of rock avalanche deposits, emplaced by a series of events spanning a large volume range. Although the prevailing approach of tuning models to a specific case may be useful for detailed back‐analysis of that event, we show that more generally applied models, even using a single pair of rheological parameters, can be used to model potential rock avalanches of varied volumes in a region and, therefore, to assess the risks that they pose