Rates of sedimentation were calculated for the floor of Astoria Canyon and the adjacent continental slope based on layers datable by C¹⁴, the occurrence of volcanic glass from the Mount Mazama eruption (approximately 6,600 yr B.P.), and an increase in the ratio of radiolarians to planktonic foraminifers (approximately 12,000 yr B.P.).

Deposition rates calculated from the first occurrence of Mazama ash to the modern surface are highest (78 cm/1,000 yr) on the floor near the head of the canyon (water depth 100 fm), and lower (53 cm/1,000 yr) in two piston cores nearer the canyon mouth (water depths 1,000 and 700 fm). In the 700-fm core the ash is present at a depth of 350 cm. A C¹⁴ age of 5,620 ± 145 yr B.P. for sediment at a depth of 205 cm in the same core shows a change in deposition rate from 145 cm/1,000 yr in the lower 145 cm to 37 cm/1,000 yr in the upper 205 cm--a change probably caused by the great influx of Mazama ash.

Slower deposition occurred on the adjacent continental slope. Rates based on the faunal change range from 36 cm/1,000 yr on the upper slope to < 10 cm/1,000 yr at the base of the slope. Rates calculated using all three dating methods on a single core from the lower slope are (1) C¹⁴, 12 cm/1,000 yr; (2) radiolarian-foraminifer change, 15 cm/1,000 yr; and (3) Mazama ash, 19 cm/1,000 yr. The uniformity of the lithology also indicates a slower rate of deposition on the slope than on the canyon floor.

End_of_Article - Last_Page 560------------