GIANT PALEO-LANDSLIDES OF THE TYEE FORMATION NEAR EDDYVILLE, OREGON COAST RANGE: COMPLEX HISTORY FROM LIDAR AND RADIOCARBON TESTING

Giant paleo-landslides have been uncovered at the US Highway 20 construction project between Corvallis and Newport, Oregon, in the turbidite beds of the Tyee Formation. Geotechnical models for landslide stability analysis have been developed based on the design geotechnical investigations, plus additional investigations to supplement landslide specific data, including: subsurface explorations, geotechnical instrument monitoring, construction outcrops, radiocarbon testing, and geomorphology revealed by Light detection and ranging (LIDAR). The process of predicting the landslide boundaries (head scarps, toes, lateral and basal shear zones) for the stability analysis models has revealed details of their geologic history.

The oldest slides are not readily visible; most of their boundaries have been removed or masked by surficial processes. They appear to have been giant translational-block slides controlled by bedding planes, high-angle fault zones and paleo-topography. Current erosion has been significant enough to divide some of the giants into multiple landslides, and the landforms also reflect multiple episodes of sliding.

Trees buried by one slide, identified as Eddy Creek Tributary C&D, were excavated from a road cut and radiocarbon tested at >46,600 yr B.P. The landforms at that location reflect multiple episodes of sliding, which is also indicated by radiocarbon tested 40,920±570 yr B.P. organic sediment buried within the same slide. Portions of the giant Tributary C&D slide continued to move during late Pleistocene as smaller block slides and earthflows, also evidenced with radiocarbon tests of wood from other excavations at 29,820±100 and 17,850±100 yr B.P. Local Holocene movement has also occurred in the giant slide areas.

An exploratory drill hole cored organic sediment near the bottom of an over 5,000-foot long paleo-earthflow, identified as Eddy Creek Tributary A&B, that tested at 36,850±380 yr B.P. Erosion since has incised 60- to 70-foot deep ravines on both lateral margins of the earthflow. These giant paleo-landslides are part of a complex geologic history of evolving landslide terrain that covers vast areas of the Oregon Coast Range. On-going studies by others should reveal more.