Geology of the Park Butte-Loomis Mountain Area, Washington (Eastern Margin of the Twin Sisters Dunite)

Mappable units in the Park Butte-Loomis Mountain area of northwestern Washington are distinguished on the basis of age, lithologic association, structural position, and metamorphic recrystallization. There are four volcanic/volcaniclastic units: the Chilliwack Group, the Cultus Formation, the Elbow Lake-Haystack Mountain unit, and the Nooksack Group: and at least three allocthonous crystalline units: ultramafic rock (including the Twin Sisters and Goat Mountain dunite bodies), the Yellow Aster Complex, and the Vedder Complex. All units occur as tectonic fragments (fault bounded blocks) which are juxtaposed along anastomosing, horizontal to low angle, west dipping faults. The upper Paleozoic Chilliwack Group is represented by the lower clastic sequence, Red Mountain limestone, and part of the upper clastic sequence. Stratigraphy, chemical composition, point count data on sandstones, and compositions of magmatic clinopyroxenes suggest a volcanic arc environment of deposition. The Loomis Mountain dacite center is tentatively considered partly correlative with the 'Permian volcanic sequence exposed in the Chilliwack Valley, British Columbia. In the study area, this dacite center is interbedded with the Triassic Cultus Formation. The Elbow Lake-Haystack Mountain unit is recognized by the presence of ribbon chert and titanaugite-bearing basalt in a sequence of siltstone and minor sandstone-graywacke. This unit is the most highly deformed of all the volcaniclastic units. The age is questionably Jurassic based on work in the Haystack Mountain area of Cruver (thesis in progress). In this thesis and that of Cruver, it is a newly proposed lithologic-tectonic element in the North Cascade foothills. The Yellow Aster Complex in the study area is subdivided into the Yellow Aster Complex sensu stricto and the mafic/ultramafic Yellow Aster Complex on the basis of differing lithologies. The Yellow Aster Complex sensu stricto consists of pyroxene gneiss and related intrusives and is correlative with the Yellow Aster Complex at Yellow Aster Meadows. Geochemical data for the non-gneissic greenschist facies intrusives suggest a volcanic arc environment of intrusion. The mafic/ultramafic Yellow Aster Complex occurs as tectonic fragments separate from the Yellow Aster Complex sensu stricto, and consists of pyroxenite, cumulate gabbro, and gabbro. The mafic/ultramafic Yellow Aster Complex may represent the structurally lower portion of the Yellow Aster Complex at Yellow Aster Meadows or possibly even a separate unit. Amphibole schist and pelitic schist of the Vedder Complex crop out as tectonic fragments imbricated with Yellow Aster Complex and ultramafic rocks within the Bell Pass fault zone. The Twin Sisters Dunite is interpreted from map relations to be imbricated with other rock units of the study area along low angle faults. Numerous smaller fragments of dunite and serpentinized dunite-harzburgite are imbricated throughout the area. The distribution of allochthonous crystalline rocks delineates the major low angle, west dipping fault zones within the area. The direction of thrusting is interpreted to be northwest-southeast or east-west based on the orientation of a large recumbent fold within the area. This low angle fault zone corresponds to the imbricate zone of Misch (1966). The low angle structures are slightly modified by Tertiary folds and Tertiary (?) high angle faults in the study area.