The Geology and Structure of the Canyon Creek-Church Mountain Area, North Cascades, Washington

Four major rock assemblages ranging from Precambrian to Lower Cretaceous occur in the Canyon Creek/Church Mountain area of northwestern Washington. Lithologic, structural, and fossil evidence were used to discriminate among the assemblages and to correlate them with the Chilliwack Group, the Nooksack Group, the Yellow Aster Complex, and a unit previously included in the Chilliwack Group, referred to here as the chert/basalt unit. These assemblages were juxtaposed during middle Cretaceous tectonism along low angle and high angle faults. There is some evidence for additional structural deformation after the middle Cretaceous. The Chilliwack Group, consisting of volcanic rocks, volcaniclastic rocks, and limestones, were found to dominate the four assemblages. Limited stratigraphic control is provided by three limestone units in the Chilliwack Group that have been dated on fossil evidence as Devonian, Mississippian-Pennsylvanian, and Permian. On Mount Liumption, volcanic rocks are in flow contact with the Mississippian-Pennsylvanian limestone unit. Similar relationships are seen on the north side of Church Mountain between volcanic rocks and a limestone of uncertain age. The volcaniclastic rocks are laminated to thinly bedded siltstone and argillite, and thickly bedded volcanic arenite and cobble conglomerate. The Chilliwack Group may be interpreted as a volcanic-arc assemblage. The Upper Jurassic to Lower Cretaceous Nooksack Group is restricted to the southern portion of the study area. These stratified rocks consist of massive pebbly siltstone, volcanic arenite, cobble conglomerate, and minor andesitic volcanic rocks. Belemnite molds and Pleuromya and Buchia fossils are fairly abundant in most lithologies of this group. The Church Mountain fault, which is well exposed on the south face of Church Mountain, separates the Chilliwack Group from the underlying Nooksack Group. The northeast striking fault dips up to 75o to the northwest on Church Mountain, and flattens considerably a few kilometers to the northeast. The fault zone is manifested by mylonitized scaley argillite, sandstone boudins, and sporadic serpentinite. This fault has previously been mapped as a thrust (Misch, 1966), however its steep dip in the Church Mountain area raises some question as to this interpretation. More study is needed to determine the sense of motion along the fault. It could be a folded thrust fault, a strike-slip fault, or a normal fault. Rocks of the Yellow Aster Complex exist as tabular fault-bounded bodies within the Chilliwack Group. These rocks consist of felsic gneiss that has been intruded by gabbro, diorite, quartz diorite, trondhjemite, granite, and greenstone. Serpentinite is commonly associated with the Yellow Aster Complex, but it is restricted to fault zones within and surrounding it. Closely associated with the fault slabs of the Yellow Aster Complex, and occuring along the western margin of the area, is the chert/basalt unit. This is an assemblage consisting of alternating layers of basalt and ribbon chert. The basalt bears titaniferous augite and well developed quench textures, which are not recognized in the Chilliwack Group. The association of argillaceous chert with basalt may represent a deeper marine facies of the Chilliwack Group, or, alternatively, these rocks could be unrelated to the Chilliwack Group