FINDING SOURCE REGIONS FOR OLIGOCENE WELDED TUFF CLASTS IN PUGET SOUND, AND IMPLICATIONS TO LITHOSPHERIC BLOCK ROTATIONAL MODELS

Multiple field trip reports (ie. Haugerud & Haeussler, 2000) have described “An enigmatic” welded tuff boulder, 2-3 m in diameter, located within the Blakely Formation at Manchester State Park near Port Orchard, Washington. The Blakeley formation, which is constrained to be ~23-33 Ma. and roughly 3,000 m thick, consists of laminar sandstone deposits as well as turbidite sequences of alternating sandstone to siltstone and shales with lesser conglomerate layers that include clasts of sand and siltstone as well as rare mafic to intermediate volcanic debris. This sequence includes the boulder that is an order of magnitude larger than other clasts in the unit. This boulder, and several smaller pieces near the boulder, has provided a way to investigate the possible depositional process and source region for the material. This constraint aids in assessing lithospheric-scale block rotational models for Puget Sound proposed to explain sedimentary bed orientation anomalies.

Bulk composition of the boulder attained using XRF and ICP-AES was compared to published data from the USGS and authors Dubray and Johns (2011). Portions of Ancestral (Challis) Cascade Arc (55-60 wt%. silica and 2-7 wt% Na2O + K2O) rocks west and south of the boulder locality are possible source regions.Rocks from the Ancestral Cascade Arc show a much lower amount of Sr relative to the SiO2 contents (~300-500ppm Sr to 50-65%wt SiO2) as well as a significantly lower Ba/Nb ratio at any given SiO2 amount (~ 17-30 ppm Ba/Nb to 58-65% wt SiO2)(Du Bray & Johns, 2011) which align better with the boulder composition than other regional units that include those from the Olympic Peninsula. This suggests that the source region of Manchester State Park was to the east and or south, which lines up with flow directions in the Blakeley formation, suggesting very small or almost complete 360˚ block rotation.

Further in-situ bulk compositional analysis of detrital tuff clasts is being explored using a hand-held Laser Induced Breakdown Spectroscopy (LIBS) instrument to avoid destroying the limited outcrop. Current analytical procedure required the integration of >100 spot analyses over several mm area of welded tuff in order to reproduce the reported compositions. Further refinement of the procedure and analysis of representative sample volume may reduce the analytical overhead.