STRATIGRAPHY OF MIDDLE CENOZOIC STRATA NEAR HATCH, UTAH

Recognition of Cenozoic stratigraphic boundaries and units on the Markagunt Plateau is perplexing, but volcaniclastic strata of both the Eocene Brian Head Formation and the Miocene Limerock Canyon Formation have been identified on the south slope of Hatch Mountain near Hatch, Utah. Although fieldwork has reduced the overall thickness of the Limerock Canyon Formation to 53 m, previous radiometric dates of early Miocene (20 Ma) are likely valid and confirm the presence of the unit. Ages derived from invertebrate and vertebrate micro-sites indicate that the late-middle Eocene Brian Head Formation locally underlies the Limerock Canyon Formation in Limerock Canyon. Petrologic studies have helped constrain the actual stratigraphic boundary between the formations, however the Limerock Canyon Formation sits with an unobvious paraconformity on the Brian Head Formation. Locally, missing volcanic strata of the regionally present Wah-Wah Springs Formation, Isom Formation, and Quichapa Group supports a paraconformable relationship that represents more than 10 million missing years. Positive field idenitification of the units remains difficult, but the lithostratigraphic boundary is placed at the first appearance of volcanic rock fragments sourced from local and regional units that post-date the Brian Head Formation. The regional extent of the Limerock Canyon Formation is unknown, and has only been identified near Hatch Mountain and adjacent areas. Vertebrate fossils were recovered from the Limerock Canyon Formation, but they were undiagnostic of age.