AN EARTHFLOW IN SENSITIVE CHAMPLAIN SEA SEDIMENTS AT LEMIEUX, ONTARIO, JUNE 20, 1993, AND ITS IMPACT ON THE SOUTH-NATION RIVER

A large (est. volume 2.8 X 10(6) m3) landslide occurred in sensitive L eda clay on the east bank of the South Nation River at Lemieux, Ontari o (45.4-degrees-N, 75.06-degrees-W), on June 20, 1993. The earthflow i nvolved an area of about 17 ha and retrogressed a total of 680 m, 555 m into the flat plain above the river. No lives were lost but a motori st was injured when he drove into the landslide crater. The 1993 lands lide occurred 4.5 km downstream of the well-known 1971 South Nation Ri ver landslide along a stretch of river that had experienced other hist orical landslides in 1895 and 1910. A band of earlier, undated, retrog ressive sliding, between 100-130 m in width, was present at the base o f the slope that failed in 1993, and the earthflow was probably trigge red by a reactivation of these failures. Borehole information obtained in 1986 and 1987 in the vicinity of the landslide indicates that a zo ne of soft, sensitive marine clay existed beneath the flat farmland, w hich was overlain by a stiffer cap consisting of laminated marine-estu arine sands and deltaic silts and sands. The morphology of the debris suggests a mechanism that involves the fluidization of much of the lan dslide mass and subsidence, translation, and rotation of cap blocks. T he stability number for the site was approximately 9.6, suggesting tha t the flow could have occurred as a result of extrusion of the soft se nsitive clay layer due to undrained cap loading. Landslide debris temp orarily blocked the South Nation River, causing flooding upstream and adversely affecting water quality downstream.