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 bandwidth͒ and because of uncertainties in rock-physics models. A spectral-ratio analysis of highresolution marine seismic data can determine a quality factor to within a 95% confidence of ‫01ע‬ 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 leastsquares 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 to examine attenuation over four octaves of frequency ͑0.5-8.0 kHz͒ 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 ‫;21ם͑‬ ‫͒01מ‬ and 107 ‫;6ם͑‬ ‫͒5מ‬ for silty clays with mean grain sizes of 7.7 and 6.9 phi, respectively, and 63 ‫;01ם͑‬ ‫͒7מ‬ 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 claydominated sediments.

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 two‐dimensional multi‐channel seismic 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, localized 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 with other parts of the UK. Copyright © 2012 John Wiley & Sons, Ltd.

13The stratigraphy and sedimentological processes operating over the last 15,000 years within 14 glacial lake Windermere (UK), at the mouth of Cunsey Beck, were imaged by a decimetre-15 resolution seismic reflection survey. A complex of fifteen mass movement deposits were 16 identified as contemporaneous with the Younger Dryas, and two within the overlying Holocene 17 drape. The high vertical resolution and dense grid of profiles allowed pseudo three-dimensional 18 mapping of individual events, along with the determination of their relative temporal 19 relationships. The size of the mass wasting deposits has been estimated to range between 20 2,100 m 3 and more than 100,000 m 3 . The geometry, structure and relationship to the existing 21 stratigraphy suggests a rapid emplacement of the Younger Dryas mass movement deposits, 22 facilitated by climatic changes making subaqueous slopes unstable, with possible triggering by 23 seismic activity. Morphometric parameters, such as volume and planar surface area, indicates a 24 greater mobility of the Younger Dryas mass movement deposits compared to the Holocene 25 events. The sediments of all imaged mass movement deposits are believed to originate from the 26 slope deposits of the lake. The age of two Holocene mass movement deposits, triggered by 27 flooding or terrestrial debris flows, is estimated to be 2,400 and 4,400 years B.P. 28