PROVENANCE ANALYSIS OF THE WINTHROP FORMATION OF THE METHOW BASIN, NORTHERN WASHINGTON: EVIDENCE FOR MINOR POST-DEPOSITIONAL DISPLACEMENT

U-Pb and Hf isotopic analysis of detrital zircon and sandstone petrography can be used to characterize the sediment sources of the Methow Basin of northern Washington in order to determine the basin’s probable location during deposition. The Cenomanian Winthrop Formation of the Methow Basin consists of east-derived fluvial deposits from late in the basin’s depositional history. Sandstone petrography data record a transition from a basement uplift source dominated by quartz and feldspar (57% plagioclase feldspar) for the lower Winthrop to a transitional arc source with increasing volcanic lithic input (57%) during deposition of the upper Winthrop. Detrital zircon age signatures from 10 samples (850 grains) show significant Middle Jurassic (ca. 160 Ma) and Early Cretaceous (ca. 115 Ma) age peaks, along with a smaller Late Jurassic component and a few Triassic and Precambrian grains. Jurassic grains outnumber Cretaceous grains in all samples, and the abundance of Cretaceous grains is highly variable. Hafnium isotopic analysis on grains from three detrital age populations (105-115 Ma, 135-150 Ma, and 160-170 Ma) reveal a consistent range of epsilon Hf values from +4 to +13 across all populations with a weighted mean of 7.5±0.4, suggesting a uniformly juvenile source for the Mesozoic grains.

The southern Canadian Cordillera and the Blue Mountains Province in Oregon contain significant Middle to Late Jurassic and Early Cretaceous components as well as sources for Triassic and Precambrian grains that match well with the Winthrop age peaks. Hf, Sr, and Nd studies indicate that plutonic rocks in these regions also have relatively juvenile geochemical signatures. Less likely potential source regions along the western margin of North America include the Sierra Nevada/Klamath Mountains magmatic arc and the Idaho Batholith, both of which are too young in their Cretaceous components, and the Peninsular Range and Mojave/Sonora province in Mexico which lack necessary Jurassic components. Although southern sources cannot be ruled out, these provenance results suggest that the Winthrop Formation underwent minor displacement since deposition near the southern Canadian Cordillera and restored Blue Mountains Province, and was not translated thousands of kilometers north from the latitude of Baja California.