Palaeomagnetic chronostratigraphy of Holocene sediments, Niagara basin, Lake Ontario, Canada

The North American Great Lakes contain 18% of the world's fresh water. Lake Ontario is the lowermost (downstream) of the Great Lakes and has existed for the past 13 000 yrs, yet details of the evolution of this lake during this period remain sketchy. Over the summers of 1992 and 1993, piston cores sampled the late Pleistocene and Holocene sediments within the Niagara and Mississauga basins of Lake Ontario. The history of late Pleistocene and Holocene environments and deposition rates are assessed by palaeomagnetic secular variations and stratigraphical descriptions of the sediment. Pollen samples were taken from the sediment to better constrain the palaeomagnetic ages and to establish the boundary between Pleistocene and Holocene deposition at 10 500 yr BP. Secular variation type curves were used to provide estimates of the age of the late Pleistocene and Holocene sediments. A description of the stratigraphical sequence is included. The sediments can be divided into six stratigraphical units for which ages have been derived. The lowermost unit is a diamict (upper contact 12 500 yr BP). Next in the sequence is a deformed unit (upper contact 12 300 yr BP), followed by a thinly laminated unit (upper contact 12 000 yr BP), a massive unit (upper contact 11 800–11 000 yr BP), and an irregularly laminated unit (upper contact 10 500 yr BP). The unit deposited in the deep water of the basin since 10 500 yr BP is an iron‐banded mud. Sedimentation rates of the basin compiled from pollen and from magnetic data are compared and the post‐glacial geological history of the Niagara basin of western Lake Ontario is discussed.