FISSION TRACK AGES OF TERTIARY BEDROCK IN THE HANGING WALL OF THE SEATTLE FAULT ZONE

In the Seattle fault zone, the leading edge of the hanging wall is comprised of two Tertiary sedimentary units – Blakeley Formation and Blakely Harbor Formation. The marine Blakeley Formation consists of a conglomeratic lower member and a fine-grained upper member. A zircon fission-track age of a pumiceous tuff layer from the lower member of the Blakeley Formation at Manchester State Park show that most of the unit is younger than 31.6±2.1 Ma. On southern Bainbridge Island, continental deposits of the Blakely Harbor Formation consist of tuffaceous siltstone and sandstones, lignites and thin tephras, overlain by conglomerate. A zircon fission-track age of a thin tephra layer near the base of the Blakely Harbor Formation shows that most of the unit is younger than 13.3±1.3 Ma. Blakely Harbor conglomerates contain abundant basalt, and lesser amounts of dense metamorphic rocks, andesite, basaltic sandstone, hornblende dacite, vein quartz, and green chert. A prominent aeromagnetic anomaly clearly associated with the Blakely Harbor conglomerate at Bainbridge Island extends ~50 km along the Seattle fault zone from Bremerton to east of Lake Sammamish. Coincident with the magnetic anomaly at Vasa Park, on the shoreline of Lake Sammamish, tuffaceous siltstone and conglomerate crop out in ravine bottoms. A K-Ar age of siltstone shows that the Vasa Park section is older than 9-10 Ma (Yount and Gower, 1991, USGS OF 91-147). Conglomerates at Vasa Park contain abundant basalt, and lesser amounts of andesite, granite, diorite, green chert, basaltic sandstone, vein quartz, and quartzite. Pollen extracted from lignites and siltstones at Bainbridge Island and Vasa Park are from a late Middle Miocene flora. We infer that the Blakely Harbor Formation extends eastward from Bremerton to at least Lake Sammamish based on similarities in lithology, paleobotany, radiometric ages, and continuity of aeromagnetic anomalies between Miocene deposits at Bainbridge Island and Vasa Park.

Radiometric ages from the Blakeley and Blakely Harbor Formations constrain the timing of slip on strands of the Seattle fault zone. The Orchard Point strand displaces Blakeley Formation rocks at Manchester, indicating that faulting initiated after 32 Ma. Displacement of Blakely Harbor rocks at Vasa Park suggests that slip on the Frontal fault began after 9-10 Ma.