The Chumstick Formation: Exploring the sedimentary and structural history

The Chumstick Formation (Gresens and others, 1981a,b) comprises Eocene (49 to ~45 Ma) fl uvial sandstone with lesser conglomerate, mudstone, and tuff  currently exposed on the east fl ank of the North Cascade Range (fi g. 1). The unit is restricted to the northwest–southeast-trending region Willis (1953) called the Chiwaukum graben. To the northeast, Chumstick is separated from Swakane Biotite Gneiss and Napeequa Schist by the Entiat fault. To the southwest, the Leavenworth fault juxtaposes Chumstick strata with the 90 Ma Mount Stuart batholith, prebatholith rocks of the Chiwaukum Schist, and the Ingalls Tectonic Complex, and, farther south, ~61–51 Ma fl uvial strata of the Swauk Formation. Within the Chiwaukum graben lies a northwest-trending uplift of Swakane Biotite Gneiss termed the Eagle Creek anticline (Willis, 1953) or Eagle Creek fault zone (Evans, 1994).