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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 ƐHf 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 most likely source for the Winthrop Formation is the Coast Mountains Batholith, which intruded into the Intermontane superterrane in the southern Canadian Cordillera. The Blue Mountains of Oregon also match well with the latest Jurassic age peak in the Winthrop. Previous Hf, Sr, and Nd studies indicate that plutonic rocks in these regions also have relatively juvenile geochemical signatures. Although more 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.
Sickmann, Zachary T., "Provenance analysis of the Winthrop Formation of the Methow Basin, Northern Washington: Evidence for minor post-depositional displacement" (2012). Geosciences Student Honors Theses. 7.
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