Geologic structures within the Birdseye 7.5 minute quadrangle Utah County, Utah have been related by previous workers to both the Jurassic Arapien Shale diapirism and to the mid-Cenozoic extensional collapse of the Charleston-Nebo Thrust. Whichever model proves valid, it will have implications for oil exploration and interpretation of the subsurface geologic structure in the region. A detailed map of the quadrangle was constructed to better constrain which mechanism was responsible for the deformation. Exposures of Arapien Shale near, and within the Birdseye quadrangle show no evidence of diapiric movement. Arapien involvement in the deformation of Tertiary rocks in the center of the quadrangle is therefore unlikely. Changes in the pattern of sedimentation of Eocene age rocks suggest a change in tectonics during this time. Restoration of the Eocene strata shows that the most plausible mechanism for this deformation is extension along reactivated thrusts in the Arapien Shale, Thaynes Formation, and Woodside Shale, related to Basin and Range extension. The Moroni Formation, a prominent Tertiary volcanic unit present throughout the Birdseye quadrangle, has been used to justify Eocene extension. Deformation with the formation was found to be present only along the Thistle Canyon normal fault, constraining movement along the fault to the Eocene and later. Dip and facies relationships present within the formation mainly are a result of paleotopography rather than extension. Several distinctive units were mapped within the formation, including lahar and fluvial deposits, as well as two different ash-flow tuffs. A depletion in nickel and chromium, an unusually ferroan composition, and distinctive Fe/Ti ratios suggest that the volcaniclastic rocks of the Moroni Formation are similar to volcanic rocks in the Slate Jack Canyon and Goshen quadrangles which lie about 35 km to the west. This implies that the ignimbrites and volcanic clasts in the Moroni Formation were sourced from the East Tintic volcanic center. It further implies that any mid-Tertiary extension between the East Tintic center and the Birdseye quadrangle did not create barriers to sedimentation and was limited in extent.



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Physical and Mathematical Sciences; Geological Sciences



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Charleston-Nebo Thrust, Extensional collapse, Moroni Formation, East Tintic volcanic center, Birdseye quadrangle



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