3D seismic imaging of buried Younger Dryas mass movement flows: Lake Windermere, UK

Windermere is a glacially overdeepened lake located in the southeastern Lake District, UK. Using the threedimensional(3D) Chirp subbottom profiler, we image mass movement deposits related to the Younger Dryas(YD) within a decimetre-resolution 3D seismic volume, documenting their internal structure and interactionwith preexisting deposits in unprecedented detail. Three distinct flow events are identified and mappedthroughout the 3D survey area. Package structures and seismic attributes classify them as: a small (totalvolume of c. 1500 m3) debris flow containing deformed translated blocks; a large (inferred total volume ofc. 500,000 m3), homogeneous fine-grained mass flow deposit; and a debris flow (inferred total volume ofc. 60,000 m3) containing small (c. 8.0×2.0 m) deformed translated blocks. Geomorphological mapping oftheir distribution and interaction with preexisting sediments permit the reconstruction of a depositionalhistory for the stratigraphic units identified in the seismic volume.<br/

Estimating quality factor and mean grain size of sediments from high-resolution marine seismic data

Seismic quality factor has the potential to characterize sediment properties but seldom is used by the industry for offshore site investigations because of practical difficulties with reflection seismology (e.g., restricted bandwidth) and because of uncertainties in rock-physics models. A spectral-ratio analysis of high-resolution marine seismic data can determine a quality factor to within a 95% confidence of ±10 within the uppermost 30 m of unconsolidated marine sediments. Our spectral-ratio technique does not require assumptions on how attenuation scales with frequency. Emphasis is placed on interpretation of spectral signatures before applying an iteratively reweighted robust least-squares regression to subdue the effects of noise and local heterogeneities when determining the quality factor of a sediment package. We combined data from boomer and chirp sources toexamine attenuation over four octaves of frequency (0.5–8.0kHz) and to demonstrate that expanding the frequency range improves the precision and accuracy of quality-factor fits. We obtain frequency-independent quality factors with 95% confidence intervals of 135 (+12; ?10) and 107 (+6; ?5) for silty clays with mean grain sizes of 7.7 and 6.9 phi, respectively, and 63 (+10; ?7) for a modern sand deposit with mean grain size 2.5 phi, from the Solent (U. K.). Sediments with higher quality factors require more independent observations to achieve a desirable 95% confidence. We required only 45 traces over sands and 1250 traces over the lowest attenuating silty clays. By constructing an empirical model of quality factor against mean grain size from published sediment studies, the mean grain sizes of our Solent sediments can be located, and we find that quality factor can be used to distinguish between coarse grain-dominated and clay-dominated sediments

Deglacial history of glacial lake Windermere, UK; implications for the central British and Irish Ice Sheet

In the UK, a combination of outcrop mapping, satellite Digital Elevation Models, high-resolution marine geophysical data, and a range of dating techniques have constrained the maximum limit and overall retreat behaviour of the British and Irish Ice Sheet (BIIS). The changing styles of deglaciation have been most extensively studied in the west and north-western sectors of the BIIS, primarily using offshore geophysical surveys. The surviving record in the southern, terrestrial sector is fragmentary, permitting only large-scale (tens of kilometres) and longer timescale (c. 1 ka) reconstructions of ice-margin movement, with limited information on deglacial processes. Here we present a high-resolution study of the retreat behaviour for a section of the southern ice-margin from Windermere in the Lake District, using high-resolution 2D multi-channel seismic (MCS) data, processed using prestack depth migration. By combining the seismic stratigraphy with landform morphologies, extant cores, and seismic velocity measurements, we are able to distinguish between: over-consolidated till; recessional moraines; De Geer moraines; flowed till/ice-front fan; supra-/en-glacial melt-out till; and subsequent glaciolacustrine/lacustrine sedimentation. The results reveal a complex and active valley glacier withdrawal from Windermere that changed character between basins and produced two small, localised areas of ice-stagnation and downwasting. This study indicates that similar active ice-margin retreats probably took place in other valleys of the Lake District during the Late Devensian deglaciation rather than the previously held view of rapid ice-stagnation and downwasting. When combined with the regional terrestrial record, this supports a model of early ice loss in terrestrial England compared to other parts of the United Kingdom

Geomorphology and remote physical properties of late Quaternary slide structures using decimetre-resolution 3D seismic volumes: insights for deep water geohazard assessment

Traditional exploration methods, involving a combination of two-dimensional seismic profiles with cores and/or swath bathymetry/side-scan sonar, do not adequately sample all three spatial dimensions for true morphological mapping of submarine mass movement deposits. It is only with the acquisition of three-dimensional (3D) seismic volumes that the complex 3D nature of these features can be correctly imaged. Over the last 5-10 years, the interpretation of industry-scale 3D seismic volumes from numerous continental shelf locations has allowed effective mapping of the deposit morphology. Features such as head scarps, side walls, extensional/compressional ridges, striations on the basal surface, translated blocks, and preservation/deformation of internal reflectors have been shown to constrain the direction and method of material transport. Here we demonstrate the application of these techniques to the shallow-water environment using decimeter-resolution 3D seismic volumes, with case studies from Trondheimsfjorden (Norway) and Lake Windermere (UK Lake District). Through the mapping of top/base reflector morphologies, internal structure, translated blocks, and head scarps/side walls we demonstrate the same techniques can be used to differentiate flow mechanics (coherent slide blocks, slumped material, debris flows, and a mass flow) and quantify direction of motion at this radically different scale