DETRITAL ZIRCON U/PB AGES AND PROVENANCE OF THE TOFINO BASIN SEDIMENTARY SEQUENCE, OLYMPIC PENINSULA, WASHINGTON

The Tofino Basin sedimentary sequence of the northwestern Olympic Peninsula is a Paleocene to Miocene forearc basin that sits stratigraphically above the Crescent terrane and adjacent to Wrangellia. The clastic strata are thought to be derived, at least in part, from the unroofing of the Coast Plutonic Complex (CPC), and the CPC is also a primary candidate for the source area of the flysch of the Chugach-Prince William Terrane (CPW) in southern Alaska. In this study, samples from the Miocene to Paleocene Blue Mountain, Lyre, Hoko River, and Makah Formations are analyzed for detrital zircon U/Pb ages to compare maximum depositional ages (MDAs) and grain-age distributions of the Tofino Basin to those of the CPW.

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.