Paper No. 2
Presentation Time: 9:15 AM
STRUCTURE OF THE SEATTLE FAULT ZONE IN THE BREMERTON EAST, WA 7.5' QUADRANGLE
Outcrops in the Bremerton East 7.5' quadrangle demonstrate a complex history of deformation in the Seattle fault zone. (1) Along the shores of Rich Passage, the map distribution of members of the Oligocene to latest Eocene(?) Blakeley Formation and bedding attitudes indicate a south-verging duplex with three slices of Blakeley above a footwall. Farther south, at Manchester, poor outcrops of conglomerate that dips 20° to the north may indicate an additional thrust at depth. (2) On southern Bainbridge Island, the poorly-exposed Fort Ward fault juxtaposes north-facing, near-vertical upper Blakeley sandstone and siltstone with Miocene conglomerate, sandstone, siltstone, and peat of the Blakely Harbor Formation. Immediately north of the fault, Blakely Harbor beds dip 40° south and are upright. Farther north, Blakely Harbor beds are steep to overturned and mostly north-facing. Available evidence permits interpretation of the Fort Ward fault as either a south-dipping thrust or a north-dipping normal fault. (3) Along Rockaway Beach, dips in the Blakely Harbor and overlying Pleistocene strata decrease to the north, consistent with long-lived deformation. At Bill Point and on the south shore of Eagle Harbor, ~18,000-year-old glacial outwash is mildly tilted and locally bedding is disrupted. (4) Six Holocene fault scarps are south facing. Where trenched (Nelson et al., 2002, 2003), the scarps are associated with N-dipping faults. Last movement on most of these faults predates (5) the 900-930 CE earthquake on the Seattle fault, when motion on a south-dipping blind thrust produced widespread uplift of as much as 9 m. Lidar topography shows that the uplifted beach flat extends from Illahee State Park south to Manette and from Bill Point south to Harper. Uplift is more extensive than previously recognized (e.g., ten Brink et al., 2006), as shoreline outcrops a mile south of Point Southworth display a well-preserved uplifted beach mantled with landslide debris. Overall, structures are consistent with the several km of contraction that would be expected if GPS-derived deformation rates applied throughout the late Cenozoic.