Charophyte flora from the Brian Head Formation, southwestern Utah, and its biostratigraphic implications

摘要

The age of Cenozoic non-marine rocks exposed in southwestern Utah is controversial because they generally have discontinuous exposures, marked lateral facics changes, post-depositional diagcnetic alteration, and scarce fossils. A well-preserved fossil charophyte assemblage is described here from the variegated unit of the Paleogene Brian Head Formation, along the western margin of the Sevier Plateau (Casto Canyon quadrangle). The charophyte assemblage is composed of three species previously documented in the European and Asiatic continents: Sphaerochara afT. major Riveline 1986. Hornichara aff. lagenalis (Straub 1952) Huang and Xu 1978, and Harrisichara tuberculata Grambast 1958. The biostratigraphic ranges of these three species suggest a relative age of late Eocene (Pnabonian)-early Oligocene (Rupelian) for the base of Brian Head Formation. These results are consistent with recent isotopic studies undertaken in the underlying conglomerate at Boat Mesa and the overlying volcaniclastic unit of the Brian Head Formation which are both late Eocene in age. Hence, in agreement with this isotopic data, the charophyte flora from the Brian Head is late Eocene in age. Moreover, this study provides valuable data about the paleogeographic distribution of biostratigraphie-key species of the European Paleogene that have been reported, for the first time, in North America.