THE MORONI FORMATION IN THE BIRDSEYE, UTAH, 7.5' QUADRANGLE

The Eocene to Miocene Moroni Formation covers an area of approximately 350 km² in central Utah and consists of varying volcaniclastic strata, ash-flow tuffs, a rhyolitic dike and a trachyte dike. Mapping within the Birdseye, Utah quadrangle shows that the Moroni Formation exhibits significant lateral variation with at least six facies present. Recognized units are a basal ash-flow tuff with pumice fragments, an andesite-dominated lahar deposit, a pink welded tuff with flattened pumice lapilli, a tuffaceous limestone, a fluvially deposited conglomerate with dominant andesite clasts and minor Paleozoic clasts, and a pink sandstone with interbedded volcanic clasts. Fluvial and lahar facies were deposited as lenses and are of limited use for regional correlation. The Moroni Formation was deposited in a series of paleovalleys cutting through and along the front of the Charleston Nebo thrust, partially filling the Birdseye paleovalley. Later it was covered by younger alluvial sediments. Depositional relationships indicate that a monocline formed to the east at approximately 40-38 Ma, limiting the extent of Moroni volcaniclastic deposition. Comparative geochemistry shows that the andesite-dominated lahar facies of the Moroni Formation is similar to lahars and lavas southeast of Goshen Utah, implying a source in the East Tintic volcanic center. Clasts of distinctive volcanic rocks from the East Tintic volcanic center further support this conclusion.