Abstract

Exposures of volcanic-sedimentary strata are widely distributed within central Utah. We believe that these volcanic and stratified sedimentary rocks, known by different formational names in different parts of this region, are, in fact, segments of one and the same suite of rocks that formed during the early and middle Tertiary.

The volcanic-sedimentary complex is exposed on both sides of a north-trending lowland formed by the collinear Juab and Sevier Valleys. West of the lowland, the complex has been named the "Goldens Ranch Formation" east of the lowland, it has been called the "Moroni Formation."; Both formations are stratigraphically alike in that each consists of a lower unit composed predominantly of water-laid, variably cemented sediments and sedimentary rocks with some tuff beds near the base, and an upper unit of intermediate-composition volcanic rocks, chiefly ash-flow tuffs, and volcanic breccias. Both formations contain abundant exotic clasts of andesite, tan and purple quartzite, and dark blue limestone and dolomite. Both formations are folded and faulted along with the underlying sedimentary units.

Potassium-argon ages indicate that both the Goldens Ranch and Moroni Formations formed during the late Eocene to middle Oligocene. The geochronology and stratigraphic relations are strong evidence that the Goldens Ranch and Moroni Formations are correlative, and that they are one and the same depositional unit.

During the latest Oligocene-earliest Miocene, minor monzonitic bodies intruded sedimentary units in the area.

The new K-Ar data bear on the matter of the origin of the complex structural deformation in central Utah. Different workers have attributed the singular deformation either to recurrent episodes of compression stemming from the Sevier orogeny, or to repeated episodes of salt diapirism. We recognize two sequences of repeated deformation: one that occurred prior to deposition and consolidation of the Goldens Ranch and Moroni Formations, and a second that occurred after these formations were emplaced, in essence, after early Oligocene time. The Sevier orogeny ended in Paleocene time; thus, the compression and thrusting stemming from the Sevier orogeny could be responsible for the structural complexity that marks pre-Paleocene units. These same orogenic forces do not seem to be viable explanations for the broad flexures and monoclinal downwarps that mark the Goldens Ranch, Moroni, and younger formations. In our view, multiple episodes of salt diapirism more reasonably explain the structural complexity in central Utah.

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