DIRECT OBSERVATION OF TILT ASSOCIATED WITH GLACIO-ISOSTATIC REBOUND, WESTERN WHATCOM COUNTY, WASHINGTON

Estimates of tilt during glacio-isostatic rebound of the southern Salish Lowland have been questioned because they depend on uncertain correlation of landforms and deposits formed during a period of rapid relative sea level change. Furthermore, the underlying observations are distributed along a N-S corridor and provide little constraint on possible E-W tilt.

Subtle latest Pleistocene fossil shoreline benches (FSB) are evident in a 6-ft (XY resolution) DEM derived from a 1 pulse/m² lidar survey of western Whatcom County. Most FSB are short (233 m mean length, 2409 m maximum, n = 1693). Observations of tilt based on change of elevation along a single FSB are suspect because of probable error in the lidar survey, error associated with interpretation of the position of a FSB, and DEM artifacts related to misidentified and missing ground returns. However, by disaggregating each FSB arc into evenly-spaced points, calculating the XYZ centroid of each point group, and then translating the group such that its centroid is at (0,0,0), we discard the where and how high associated with each arc and preserve its shape, including any tilt. Least-squares fitting of a plane to the resulting point cloud gives the mean tilt. For the FSB analyzed, mean tilt is 1.2 m/km up to the NE (azimuth 38º). Higher-elevation FSB appear to be tilted more than lower-elevation FSB. This result is consistent with previous estimates, derived from correlation of discontinuous features 60–200 km farther south, of 0.6–1.15 m/km up to the N tilt (Thorson, 1989, Dethier and others, 1995; Kovanen and Slaymaker, 2004).

Using this mean tilt, we correlate fossil shoreline segments that trim early Sumas-age moraines of the Blaine upland, the Lake Terrell upland, and the Lummi Peninsula. Least-squares fit of a plane to this shoreline yields an estimated tilt of 0.9 m/km up to the NE. The low tilt may reflect low elevation (hence relative youth), or that the shoreline is diachronous, younger in the direction of ice retreat to the NE. North of Ferndale the fossil shoreline is consistently 2–4 m higher than the best-fit plane, suggesting Holocene deformation consistent with observed recent uplift of the late Holocene shoreline at nearby Birch Bay.