THE SEDIMENTOLOGY AND PETROLOGY OF THE MORONI FORMATION; INDIANOLA UTAH

The Moroni Formation covers a large area of central Utah, and only parts of it having been studied. This study was undertaken northeast of Indianola, Utah, in Little Clear Creek Canyon. This formation is a complex geological unit made up of coarse volcaniclastic sandstones and conglomerates, a thick air fall tuff, lava flows, and intrusive igneous bodies. This unit represents an unusual period in the regional geologic history when western North America underwent a long period of volcanic activity accompanied by relatively quiescent tectonics. The section in the study area records several pyroclastic eruptions and flows emitted from an unknown volcanic center. The petrology of these rocks indicates the sequence is dominated by pyroxene and amphibole bearing andesites. Between these eruptive pulses, fluvial reworking and deposition generated a thick (450 m) sequence of volcaniclastics. A single fossil was previously discovered at the top the formation which dates the uppermost part as early or middle Miocene. Attempts to locate additional fossils as part of this study were unsuccessful. There are several competing theories on the origin of these volcanics and the results of the chemical analyses (not yet available) will aide in identifying the actual volcanic center. The topography in the Miocene was similar to that of today, in that the region was mountainous with large valley drainage systems. The volcanic flows, ashfalls, and volcaniclastics infilled these valleys. The sequence found in Little Clear Creek Canyon records primarily fluvial infill of one of these valleys. The basal tuff unit in the Moroni Formation within the study area represents an ash fall that would have originally blanketed a larger area, but only parts of it have been preserved as valley fill. Higher in section are several very poorly sorted lahars that have had minor fluvial reworking. The thick uppermost layers consist of poorly sorted very lithic rich sandstones with thin conglomerate lens scattered throughout. This suggests that the eruptions had ceased and that reworking of the lahar and ash layers became the prominent process generating high energy, fluvially deposited coarse grained volcaniclastics.