BEDROCK POP-UPS WESTERN LAKES ONTARIO, EASTERN LAKE ERIE AND EASTERN LAKE HURON: EVIDENCE OF NEOTECTONIC ACTIVITY ON THE LAKEBED OF THE SOUTHERN GREAT LAKES

Bedrock pop-ups have been investigated using both conventional marine survey equipment including echo sounders, sidescan sonar, shallow seismic reflection profilers and under water photography as well as new high resolution multibeam sonar systems. These lakebed instability features have been observed in eastern Lake Erie, western Lake Ontario and eastern Lake Huron in water depths ranging from 3 to 70 m. Pop-ups can be observed where relatively flat-lying Paleozoic strata outcrop on the lakebed or occur under a thin sediment cover. Pop-ups trend dominantly to the northwest. In Lake Ontario an orthogonal set of pop-ups have been mapped. Pop-ups are ridge shaped. They are linear to curvilinear along strike and vary in length from a few hundred to more than 2000m. Relief along strike varies from less than 0.5 to more than 3m. Pop-up width can range from a few metres to more than 10m. Crests are associated with axial cracks, open fractures or small grabens. In places the bedrock disruption can be observed to depths of 5m below lakebed. Axial fractures consist of open depressions to depressions partially infilled with sediment. Exposed axial fracture surfaces appear in some cases to be fresh, sharp and free of biological growth while in others the surfaces are oxidized. Where buried, the thin overlying sediments drape the pop-ups and do not appear to be disturbed. The regional northwest trend of the features in the 3 lake basins is sub-parallel to the Georgian Bay Linear Zone and perpendicular to the regional tectonic stress regime. The orthogonal set of pop-ups in the Lake Ontario basin is sub-parallel to the St Lawrence Rift System. Preservation considerations suggest the features post-date glaciation although their exact age is not clear. The undisturbed Holocene sediment drape would suggest the pop-ups are older and have not been reactivated over time. However, the fresh sharp fracture surfaces indicate a younger age. The features may be time transgressive.