DETRITAL ZIRCON U/PB AGES AND PROVENANCE OF THE TOFINO BASIN SEDIMENTARY SEQUENCE, OLYMPIC PENINSULA, WASHINGTON
In four of the five samples with known faunal ages, the U/Pb MDAs are significantly older than the faunal ages, which indicates a lack of volcanism in the source region. Thus, the source probably did not include the Cascade arc. The sample from the Hoko River Formation is distinctive, as it has a significant number of grains near its depositional age of ~35 Ma. Two zircons from the Blue Mountain sample are 48.0 and 50.5 Ma, which supports previous work that suggests a young basal age for the Crescent Formation of the Olympic Peninsula, and implies a short (<1 Myr) time for eruption of the Crescent basalts. Grain-age distributions for the younger samples typically have Eocene populations (~50 Ma), and one Makah Formation sample has a Miocene population; most samples also have Late Cretaceous (~72, 90 Ma) and Jurassic (145 – 200 Ma) populations. This is quite different than contemporaneous units in the CPW (the Orca Group), which typically have significant populations from 60 – 50 Ma. The Jurassic populations at about 175 Ma match the age of the Island-Bonanza plutons that intrude Wrangellia on Vancouver Island, and likely make up a significant portion of the source of the Tofino Basin.
Precambrian grains in nearly every sample define peaks at 1.4 and 1.7 Ga. These are indicative of a potential southern Laurentian source component (Yavapai and Granite – Rhyolite Provinces) of the Tofino Basin sedimentary sequence. The Blue Mountain unit also has a population at about 1.05 Ga, indicative of a Grenville source. This southern Laurentian signature is generally not present in similar age strata in the CPW, but it does occur locally in Alaska.