DO QUATERNARY SEDIMENTS REGISTER PALEOSEISMICITY OR NEOTECTONIC REJUVENATION OF ANCIENT FAULTS BENEATH LAKE ONTARIO?

Systematic surveys of the Quaternary cover in Lake Ontario using marine geophysical and geological techniques (high-resolution sleeve gun and boomer seismic profiling, sidescan and multibeam sonar, lithostratigraphic coring and analysis, and manned submersible observations) were conducted between 1992 and 2002, and showed areas of apparent faulting (ridges of relief >10 m, one with failed crestal sediments, in the eastern basin) and warping (convex upward bed configuration in the eastern basin). Broad elongated lows in the bedrock surface may have resulted from preferential erosion along ancient fault zones. However, a comprehensive review of the full data set does not validate an interpretation of neotectonism, but may indicate a paleoseismic event. The data revealed two drumlin fields aligned with onshore drumlin trends. The largest field trends WSW and is well-expressed in the lake bathymetry below 175 m in the deep eastern basin. A second field of drumlins is buried by glacio-lacustrine and younger lake sediments in western Lake Ontario in the area of apparent warping. These features are clearly imaged in N-S sleeve gun profiles and appear to trend westerly. In both fields, the drumlins occur as upstanding ridges (>10 m high) of an incoherently reflective material interpreted as glacial diamict (till). They are either rooted in a similar material or are standing on the bedrock surface. The Paleozoic bedrock surface passes beneath the drumlins with insignificant relief. Glacio-lacustrine beds were deposited in a continuous blanket over the high-relief surface of the drumlinized diamict with a generally conformable configuration. In the eastern basin where drumlin side slopes are steep, the glacio-lacustrine beds failed in at least two widely spaced locations. The failed sediments appear as debris flow deposits at the base of the drumlin slopes. Both debris flow deposits are interbedded with lake sediments of apparent early Holocene age, suggesting the slope failures may have been triggered simultaneously by the shaking of a major Holocene earthquake. Overall, the data indicate that a major paleoseismic event may be evident in the lake sediment, but that the large relief of the Quaternary diamict surface beneath Lake Ontario resulted from glacial processes rather than neotectonic displacement.