Paper No. 4-6
Presentation Time: 9:25 AM
POST-MID CRETACEOUS STRUCTURAL IMBRICATION OF THE CYPRESS PERIDOTITE, VENDOVI ASSEMBLAGE, AND LUMMI FORMATION ON CYPRESS ISLAND, NORTHWESTERN WASHINGTON
Over a region covering 200 kilometers squared in the eastern San Juan Islands Jurassic(?) to Cretaceous clastic and volcanic rocks have uncertain structural and paleogeographic relations to the Fidalgo ophiolite exposed to the south. The rocks are composed of massive to well bedded turbiditic feldspathic arenite, wacke, and mudstone of the Lummi Formation and a heterolithic succession assigned here to the Vendovi assemblage consisting of argillite, siltstone, sandstone, conglomerate, pillow basalt, basalt breccia, and olistolithic blocks of bedded chert, silicified argillite, and basalt. The Lummi Formation is thought to depositionally overly Late Jurassic – Early Cretaceous bedded chert and contains detrital zircons as young as 135 Ma. The Vendovi assemblage contains blocks of bedded chert with Tithonian – Hauterivian radiolaria but also contains pillow basalt with a poorly constrained Albian to mid-Cenomanian foraminifera. On Cypress Island, the Cypress peridotite, representing the basal part of the Fidalgo ophiolite, structurally overlies sheets of the Vendovi assemblage and Lummi Formation. The serpentinized dunite and harzburgite of the peridotite are separated from the underlying Vendovi assemblage by a 50 to 20 degree south-dipping fault. The Vendovi structurally overlies the Lummi rocks on a locally exposed fault that dips south at 30 to 40 degrees. Rocks of the Lummi and Vendovi are deformed in four generations of penetrative structures. The first consists of a shallowly south-dipping penetrative cleavage, which in the Lummi is axial-planar to pervasive tight-to-isoclinal folds. In the Lummi Formation the folds locally have a sheath morphology and are associated with stretching lineations indicating a north-south axis of elongation thought to be related to the tectonic transport direction. In the Vendovi, penetrative fabrics vary by lithology from highly penetrative foliations to poorly developed. E-W trending folds reorient first-foliations and locally fold bounding thrusts. Younger E-W, WNW, and NNE striking steeply-dipping foliations locally crenulate first foliations. Imbrication of the structural sheets occurred during the first-generation of structures in the late Cretaceous or possibly early Cenozoic.