Sedimentation in Cascadia Deep-Sea Channel
Cascadia Channel is the most extensive deep-sea channel known in the Pacific Ocean and extends across Cascadia Basin, through Blanco Fracture Zone, and onto Tufts Abyssal Plain. The channel is believed to be more than 2200 km in length and has a gradually decreasing gradient averaging 1:1000. Maximum channel relief reaches 300 m on the abyssal plain and 1100 m in the mountains of the fracture zone. The right (north and west) bank is consistently about 30 m higher than the left (south and east). Turbidity currents have deposited thick, olive-green silt sequences throughout upper and lower Cascadia Channel during Holocene time. The sediment is derived primarily from the Columbia River and is transported to the channel through Willapa Canyon. A cyclic alternation of the silt sequences and thin layers of hemipelagic gray clay extends at least 650 km along the channel axis. Similar Holocene sequences which are thinner and finer grained, occur on the walls and levees of the upper channel and indicate that turbidity currents have risen high above the channel floor to deposit their characteristic sediments. A thin surficial covering of Holocene sediment along the middle channel demonstrates the erosional or non-depositional nature of the turbidity currents in this area. The Holocene turbidity current deposits are graded texturally and compositionally, and contain Foraminifera from neritic, bathyal, and abyssal depths which have been size-sorted. A sequence of sedimentary structures occurs in the deposits similar to that found by Bouma in turbidites exposed on the continent. There is a sharp break in the textural and compositional properties of each graded bed. The coarser grained, basal zone of each bed represents deposition from the traction load; the finer grained, organic-rich, upper portion of each graded bed represents deposition from the suspension load. Individual turbidity current sequences are thinnest in the upper and thickest in the lower channel. Recurrence intervals between flows range from 400 years in the upper to 1500 years along portions of the lower channel. Evidently each flow recorded near shore did not extend its entire length. Turbidity currents have reached heights of at least 117 m and spread laterally 17 km from the channel axis. Calculated flow velocities range from 5.8 m/sec along the upper channel to 3.3 m/sec along the lower portion. Pleistocene turbidity currents within Cascadia Basin were much more extensive areally than the Holocene flows, and they deposited sediment which was coarser and cleaner. Pronounced levees which border the upper channel are due chiefly to Pleistocene overflow. Coarse gravels and ice-rafted pebbly clays were also deposited along Cascadia Channel during Pleistocene time.