TOWARDS THE DIFFERENTIATION OF THREE VOLCANICLASTIC UNITS IN SOUTHWESTERN UTAH, RESULTS OF PETROGRAPHIC AND GEOCHEMICAL ANALYSES

The Brian Head Formation (33-35 Ma), Bear Valley Formation (20-23.5 Ma), and Limerock Canyon Formation (20-21 Ma) are three volcaniclastic sedimentary rock units located in the High Plateaus of southwestern Utah. All three formations are similar in general appearance and often difficult to discern from one another in outcrop. In order to help improve the identification and mapping of the three volcaniclastic units new criteria was established to distinguish the units using petrographic analyses. Preliminary work has indicated that an apparent distinction can be made between units based on percentages of mafic minerals and composition of volcanic rock fragments (VRF). The Brian Head Formation averages about 2.6% biotite, 30% VRFs, and no pyroxene. The Limerock Canyon Formation averaged less than 1% biotite, 33% VRFs, and 1.8% pyroxene. The Bear Valley Formation averaged less than 1% biotite, 45% VRFs, and less than 1% pyroxene. The percentage of VRFs varies widely within each formation, however, the mafic composition of the Brian Head Formation is distinctive from those of the similar Limerock Canyon and Bear Valley Formations. This distinction is also recognized in the composition of the volcanic rock fragments; those in the Brian Head contain glass and plagioclase crystals, whereas those of the two other units typically also contain mafic minerals.

Using the petrographic criteria, the volcaniclastic units found at two sites on Hatch Mountain have been reinterpreted from previous studies. On the north side of Hatch Mountain an outcrop thought to be Bear Valley Formation is now interpreted as Brian Head Formation. On the south side, an interval of previously mapped Brian Head has been identified as non-volcanic Claron Formation, overlain by Limerock Canyon Formation. Furthermore, clasts of volcanic rocks at the base of the Limerock Canyon Formation have been identified using major- and minor-element x-ray fluorescence (XRF) analyses to be of the Isom Formation (26-27 Ma). Clasts of the Isom Formation have not been previously identified in the Limerock Canyon Formation. In the Hatch Mountain area, clasts of Isom Formation are useful for identifying the Limerock Canyon Formation.