EFFECTS OF THE READVANCE OF AN ICE MARGIN ON THE SEISMIC CHARACTER OFTHE UNDERLYING SEDIMENT

Seismic stratigraphy from a 65 km long transect of the Lake Melville f jord basin, southeast Labrador, Canada, has revealed seven cycles of i ce-front advance-retreat, from a stratigraphic interval which regional ly is correlated to 10-9 ka BP. A maximum thickness of 400 m of glacim arine sediments, ice-contact submarine fans and associated fine-graine d debris (totally 9.8 km(3)), was deposited. The thick ice-contact dep osits (terminal moraines) provide evidence that the Lake Melville basi n was occupied by a grounded temperate tidewater glacier during this e arly Holocene period of deglaciation. Detailed interpretation of the a coustic architecture of four associated ice-contact fans shows that th ey were formed mainly during advance of the ice-front; only thinner se diment packages were emplaced during ice-front stillstand episodes. A subglacial unit equivalent to these foreset subunits, mapped on the ic e-proximal side of the fans, is relatively uniform in thickness. The s ubglacial unit is interpreted as a basal deformation layer formed duri ng the ice-front advance. Seismic data from Lake Melville support the idea of asymmetric ice-front advance-retreat cycles: the individual cy cle, however, shows a more complex pattern than the simple 'slow advan ce-stillstand-rapid retreat' model, as suggested from many modern tide water glaciers. If the regional chronology is correct, these advance-r etreat cycles have an average duration of 150 yrs. They apparently occ urred during a period of drastic ablation of the Laurentide ice-sheet, as a result of an unknown combination of extrabasinal and intrabasina l factors. (C) 1997 Elsevier Science B.V.