Chuckanut Formation - Geology

Geology

The Chuckanut and related formations "are all composed predominantly of fine- to medium-grained sandstones with lesser amounts of interbedded shale, conglomerate, and coal." The sandstones consist of sand eroded from the Mount Stuart massif and probably from uplifted metamorphic sources in northeastern Washington (this was well before the rise of the Cascade Range), and distributed by rivers across a low-lying coastal plain starting about 54 Ma. These were laid over a metamorphic suite of rocks, those under the Chuckanut now known as the Shuksan, and its correlate under the Swauk known as the Easton. Near Vancouver parts of the Huntingdon Formation lie disconformably on parts of the Cretaceous Nanaimo Group.

Deposition of the Chuckanut continued as convergence of the Crescent Terrane (Olympic Peninsula) initiated strike-slip motion on the Straight Creek Fault (48 Ma), displacing much of the original formation to the north. Continued deposition across the Straight Creek Fault formed the Raging River and Puget Group formations east of Seattle; the former is partly marine, indicating it was probably a large delta, and locating the Eocene coast line. Deposition is believed to have largely ended in the late Eocene (around 42 Ma?) when regional uplift diverted the rivers supplying the sediments, but some deposition may have continued, supplied from local sources.

Johnson (1984) estimated a total thickness of 6,000 m (about 20,000 ft), which would make the Chuckanut Formation one of the thickest nonmarine sedimentary sequences in North America. But more recent work suggest that, at least in parts, it may be only 2,500 m thick.