Composite tuff units up to 17 m thick are exposed in strata of the Summit Creek Formation, south of Fort Norman, Northwest Territories. The age of the tuffs is bracketed between late Maastrichtian and Early Paleocene based on palynology. The geometry of the tuff beds, their consistently fine grain size (maximum particle size is 0.25 mm), their composition (less than 5% dense juvenile ash particles), and burial of plants in growth position suggest deposition by fallout from a high-altitude column of ash. Between 100 and 300 eruption–deposition events are preserved in the sequence.The nearest known volcanic source vents are located in central Yukon, about 600 km from the present exposures. Estimates of dispersal area and of volume of single ash layers and the degree of particle fragmentation compare favourably with similar parameters that have been determined from geologically recent plinian and ultraplinian eruptions.

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