COWICHAN FOLD AND THRUST SYSTEM SOUTH AND EAST OF BELLINGHAM, WA

Paleocene and Eocene Chuckanut Formation in the Bellingham South and Lake Whatcom 7.5’ quadrangles is fully involved in NE-verging folds and thrusts that are the southeastern continuation of the Cowichan fold and thrust system of England and Calon (1991). Rootless folds and local bedding cutoffs within the Bellingham Bay Member (Tcbb) indicate 3-fold thrust thickening of Tcbb on Lookout Mountain east of Interstate 5. Similar thickening is evident, though less well defined, on Chuckanut Mountain west of I-5. Overlying Padden Member of the Chuckanut Formation is for the most part folded passively above imbricated Tcbb. Shortening postdates Chuckanut deposition, locally as young as 49.9 Ma (Breedlovestrout and others, 2013), and probably predates initiation of the Cascade magmatic arc at 35 Ma.

A best-fit fold axis plunges 32° towards azimuth 326°. Projection of map data onto a cross section normal to this axis shows that Tcbb, where not structurally thickened, is about 1 km thick, consistent with well and seismic data farther north (Hurst, 1991; Mustard and Rouse, 1994; Pelican Dome No. 1 well). Previously-reported (Glover, 1935; Johnson, 1982, 1984) ~2.7 km thicknesses for Tcbb were measured in thrust-repeated sections. Area balancing of the section indicates 21 to 28 km of shortening in Tcbb across the 17 km width of the map area, substantially more than inferred for the Cowichan system farther NW. The ~1 km (half-wavelength) folds and associated faults in the Chuckanut Formation piggyback on larger, ~8 km folds and associated faults that involve underlying Jurassic and Cretaceous Easton schist. East of Lake Samish, Easton schist has been thrust over Tcbb.

Eocene accretion of Siletzia involved substantial NE-directed shortening. These results from the Cowichan fold and thrust belt indicate that some deformation previously attributed to the mid-Cretaceous Northwest Cascades thrust system is likely Cenozoic.