Recent Sediment Stratigraphy and Geotechnical Characteristics of Foreset and Bottomset Beds of the Niagara Bar

Abstract

Forest beds of the Niagara Bar are composed of fine sands and sandy silts. Their thickness is uncertain, but probably 6 to 8 m have accumulated on the upper slope since the last low water stage in Lake Ontario. The maximum thickness of these beds occurs near the foot of the outer slope (water depths 80 to 90 m) where accumulation exceed 20 m north of the present river mouth, and 10 to 12 m to the northwest. The bottomset beds of fine silt and silty clay thin rapidly offshore and decrease to as little as 0.5 m, at about 5 km northwest of the Outer Bar. There is a fining upward lithology in many cores, and this is consistent with continuing deposition under rising water levels. Geotechnical measurements indicate that bottomset and lower foreset beds have low undrained shear strengths (typically less than 4 kPa) and high water contents. Upper foreset beds, of coarser composition, have somewhat higher, undrained shear strengths (20 to 90 kPa, and greater) and much lower water contents. The steepest parts of the Outer Bar lie between 20 and 60 m water depth, where slope angles vary between 2 and 10°; maximum local values reach 14 to 15°. Despite the presence of gas in sediments of the lower foreset beds the slope shows no evidence of slumping.