LATE CRETACEOUS TO POST-EOCENE DEFORMATION AND TRUNCATION OF THE EASTERN NANIAMO BASIN, NORTHERN SAN JUAN ISLANDS, WASHINGTON

From 3.5 to possibly 5.0 km of Turonian to Maastrichtian rocks of the Nanaimo Group were deposited in a clastic marine-basin unconformably overlying Wrangellia of Vancouver Island. The rocks stretch from northern Vancouver Island nearly 425 km southeast to the San Juan Islands and are deformed in the southwest-directed Cowichan fold and thrust belt. At the eastern end of the basin, the Nanaimo rocks were uplifted and eroded by as much as 2.0 km prior to deposition of the unconformably overlying Eocene Chuckanut Formation. East of the San Juan Islands, Nanaimo rocks do not extend beyond the northerly trending Rosario Strait fault and the Chuckanut Formation rests directly on metamorphic tectonites of the western Cascades. In the northern San Juan Islands, the Nanaimo Group is deformed in three generations of folds and is structurally overlain to the south by metamorphic rocks of the San Juan thrust belt. First folds in Nanaimo rocks are west-northwest trending, locally exhibit penetrative axial-planar cleavage, verge to the southwest, and are cross-cut by shallow northeast-dipping thrust faults. These structures are deformed by north-northeast trending mesoscopic and megascopic folds with locally developed axial-planar cleavage. Superposed west-northwest and north-northeast folds of the Nanaimo were eroded and overlain in angular unconformity by the Chuckanut Formation, which is deformed in a younger generation of west-northwest trending folds. Juxtaposition of Nanaimo clastic rocks and metamorphic tectonites across Rosario Strait fault occurred prior to deposition of the Eocene Chuckanut Formation. Early deformation of the Nanaimo Group occurred during imbrication of the Cowichan fold and thrust belt and younger north-northeast folds are related to imbrication of San Juan thrust belt over the clastic basin. Both southwest and northwest periods of shortening in the Nanaimo predated deposition and subsequent deformation of the Eocene Chuckanut Formation.