Evidence for mid‐Cretaceous wrench‐faulting in the Methow Basin, Washington: Tectonostratigraphic setting of the Virginian Ridge Formation

Abstract

The Cretaceous Methow Basin of north‐central Washington is the southernmost of a series of Mesozoic successor basins in the Canadian Cordillera. The Albian‐Campanian sedimentary sequence there documents a restricted, asymmetric basin, filled by a west‐derived, laterally amalgamated fan‐delta system (the Virginian Ridge Formation), and an east‐derived, stable fluvial‐deltaic system (the Winthrop Formation), followed by a locally derived volcanic sequence (the Midnight Peak Formation). This study focuses on the depositional and tectonic setting of the Albian‐Cenomanian Virginian Ridge Formation. The mid‐Cretaceous Methow Basin is interpreted as a wrench‐fault basin generated in response to transpressive dextral offset and uplift of a highland to the west. Stratigraphy and sedimentology of the Albian‐Cenomanian Methow sequence indicates that (1) the basin was asymmetric normal to its axis, in that the western margin was a mobile zone generating considerable relief while the eastern margin was passive, stable, and of relatively low relief; (2) the basin was asymmetric along its axis, documenting transpression and dextral offset along the western margin; (3) the relatively short and steep fan‐delta system deposited along the western margin was laterally amalgamated and possibly time‐transgressive to the north. This basin and similar, coeval successor basins to the north may record extensive transtensile‐transpressive mobility along the Cordillera in middle to Late Cretaceous time.