Contrasting Facies in Upper Mesozoic Strata of Pacific Northwest: ABSTRACT
Upper Jurassic to Lower Cretaceous sedimentary rocks in the San Juan Islands, Washington, can be grouped into two facies that are represented elsewhere along the Pacific margin from southeastern Alaska to California. The eastern facies comprises well-stratified volcaniclastic turbidites depositionally overlying mafic to felsic volcanic rocks resting on a mafic-ultramafic igneous basement. Sedimentary breccias derived from the basement and pelagic radiolarian argillites are locally End_Page 913------------------------------ interbedded with basal sandstones. This facies was deposited in part on the remnants of a Middle to Late Jurassic volcanic arc. It is laterally equivalent to the Nooksack Group and underlying Wells Creek volcanic rocks, and correlative with strata in the Methow trough. Overall, the stratigraphic sequence and basement rocks are remarkably similar to the basal Great Valley sequence and underlying rocks in the California Coast Ranges. The western facies consists of thinly bedded to massive volcaniclastic sandstones complexly interbedded with ocean-floor (MORB) basalts, basaltic tuffs, ribbon chert, and polymict pebbly mudstone. Clastic rocks contain rare fragments of amphibolite, blueschist, and upper Paleozoic limestone in addition to voluminous volcaniclastic debris. This clastic eugeo ynclinal association is a deposition unit rather than a melange formed by offscraping or accretion. In contrast to the eastern facies, basaltic rocks and pelagic cherts are distributed throughout the section and are interspersed with terrigenous clastic rocks. Coeval, lithologically comparable sequences are the Kelp Bay group (southeastern Alaska), the Pacific Rim complex (western Vancouver Island), and probably the Yolla Bolly terrane in the easternmost Franciscan. The western and eastern facies in northwestern Washington are presently separated by middle Cretaceous thrust faults. End_of_Article - Last_Page 914------------