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Subbottom acoustic profiling in lakes on the Canadian Shield and in the Canadian Cordillera has revealed disruptions in deltaic and lacustrine sediments that can be attributed to large historical earthquakes. The Timiskaming earthquake of 1935 (Magnitude 6.25) and the Vancouver Island earthquake of 1946 (Magnitude 7.2) triggered extensive slumping and flow of postglacial sediments in lakes located within a few tens of kilometres of the epicentres of these events. There is some evidence in one of the surveyed lakes for in situ mobilization of sediment on relatively gentle slopes, with minimal horizontal displacement; this may be a product of liquefaction induced by coseismic shaking or displacements along faults beneath the basin. In another lake, widespread coseismic slumping resuspended large amounts of fine sediment, which then settled out onto the lake floor to produce a texturally anomalous layer. Features similar to those produced by the 1935 and 1946 earthquakes can also result from a variety of nontectonic processes, for example, the collapse of sediment over buried ice blocks, depositional oversteepening of deltas, groundwater sapping, and variations in pore-water pressures due to lake-level fluctuations. Records of sediment disruption in lakes therefore must be interpreted with caution. In general, many lakes in a region should be surveyed and a variety of different features identified before sediment disruptions are attributed to an earthquake.Un sondage acoustique sur des lacs dans le Bouclier canadien et dans la Cordillkre canadienne a devoilt des ruptures dans des dCp6ts deltaiques et lacustres, lesquelles auraient pu Ctre provoqukes par de puissants tremblements de terre historiques. Le tremblement de terre de Timiskaming, en 1935, de magnitude 6,25, et le tremblement de terre de l'fle Vancouver, en 1946, de magnitude 7,2, ont produit d'importants glissements et des coulCes de sediments postglaciaires dans des lacs localises B 1'intCrieur d'une region de quelques dizaines de kilomktres des epicentres de ces stismes. Dans un des lacs CtudiCs, les indicateurs suggkrent une remobilisation in situ des sediments sur des pentes relativement faibles, avec un deplacement horizontal minimum; il s'agit peut-Ctre d'un effet de liquefaction induite par une agitation cosismique ou de mouvements le long des failles existantes sous le bassin. Dzns un autre lac, un grand glissement cosismique a remis en suspension d'importantes quantites de matkriel fin, qui ont sediment6 sur le fond du lac pour former une couche presentant une texture anormale. Des phCnomknes analogues i ceux crCCs par les tremblements de terre de 1935 et de 1946 peuvent possiblement rCsulter de divers processus non tectoniques, par exemple, la chute de sediments accumulCs sur des blocs de glace enfouis, des dCp6ts instables sur les pentes de deltas trop raides, le sapement par les eaux souterraines, et les variations des pressions de l'eau interstitielle crkCes par les fluctuations du niveau du lac. Les Cvknements de rupture des sediments dans le...

The stratigraphic record in the James and Hudson Bay Lowlands indicates that the sequence of glacial events at the geographical center of the 12.6 × 106 km2 Laurentide Ice Sheet may have been more complex than hitherto imagined. Isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups. Two alternative explanations of the data are offered, of which we strongly favor the first. Hypothesis 1: Setting the age of the “last interglacial” marine incursion, the Bell Sea, at 130,000 yr B.P. results in a long-term average diagenetic temperature for the lowlands of +0.6°C. Using this temperature enables us to predict the age of shells intermediate in age between the “last interglaciation” and the incursion of the Tyrrell Sea 8000 yr ago. Between these two interglacial marine inundations, Hudson Bay is predicted to have been free of ice along its southern shore about 35,000, 75,000, and 105,000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments. These results suggest (1) that traditional concepts of ice-sheet build-up and decay must be reexamined; (2) that “high” sea levels may have occurred during the Wisconsin Glaciation; and (3) that a critical reappraisal is required of the open ocean δ18O record as a simple indicator of global ice volume. An alternative, Hypothesis 2, is also examined. It is based on the assumption that the 35,000-yr-old deposits calculated on the basis of Hypothesis 1 date from the “last interglaciation”; this, in effect, indicates that the Missinaibi Formation, commonly accepted as sediments of the “last interglaciation,” are about 500,000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130,000 yr has been close to −6°C. We argue for rejection of this alternative hypothesis.