NEW DISCOVERIES OF VOLCANICS IN THE SEA OF LOST ROCKS, MARKAGUNT MEGABRECCIA (MIOCENE), HIGH PLATEAUS, SOUTHWESTERN UTAH

Reconnaissance of two peaks on the southeastern Markagunt Plateau, Little Creek Peak (LCP; 10,140 ft.) and Sandy Peak (SP; 9,537 ft.), has revealed that each summit is composed of a different volcanic formation, neither of which has been described or reported previously from these peaks. The upper 50 m of LCP is a mass of gray-green, porphyritic, intermediate lava that is arrowhead-shaped in map view and 250 m in length north to south. It overlies thick mafic lava flows. The upper 240 m of SP is a mass of tan-gray, feldspar-rich, platy, felsic lava about 340 m east to west by 320 m north to south, with a north-trending dike-like ridge that extends 540 m beyond the twin summit. Earlier geologic maps show both peaks mapped as Mount Dutton Formation alluvial facies (MD) or MD mapped as part of the Markagunt Megabreccia (MM). Thus, it is unclear if the summit volcanics are either post-MM flows or allochthonous masses of the MM.

Southeast of SP, several obsidian dikes (6 to 18 cm in width) with spherulitic texture have been discovered adjacent to another locale in the MM known as the Sandy Creek pseudotachylyte site (SC). An ⁴⁰Ar/³⁹Ar analysis of a whole rock sample of a spherulitic dike has yielded a total gas age of 22.59 ± 0.18 Ma (Nevada Isotope Geochronology Laboratory, 2018). Notably, the contact between the MD and the outcrop of Bear Valley Formation sandstone that includes the spherulitic dikes is cross-cut by thinner, glass-filled fractures that resemble those at the adjacent SC, a relationship which suggests that the dikes predate the MM. However, the age of the dikes correlates well with that of only one other volcanic formation in the area, the Haycock Mountain Tuff (HMT), a unit that overlies the MM throughout its known areal extent. Hence, this age correlation indicates that the dikes and HMT may have been derived from the same magmatic source. Thus, the age correlation and field relationships present a paradox because rocks of the same age and source cannot be both older (the dikes) and younger (the HMT) than the MM. If the age correlation is correct, then the SC glass (melt) must have been injected by intrusive igneous activity after the MM. Furthermore, the fact that the HMT overlies the extensive MM requires that, if it occurred somewhere in this area, the HMT eruption must have penetrated the MM to reach the surface, perhaps near the studied sites.