THE YOUNGEST KNOWN VOLCANIC ROCKS OF THE MARKAGUNT MEGABRECCIA (LATEST OLIGOCENE OR EARLIEST MIOCENE), HIGH PLATEAUS, SOUTHWESTERN UTAH: THE MISSING LINK TO THE BIRTH AND TRUE AGE OF THE MYSTICAL CREATURE

New investigations of the Markagunt Megabreccia (MM) deposits exposed in roadcuts of state highway 143 along the northeastern margin of Panguitch Lake have revealed a distinct volcanic component which has yielded the youngest known rocks of the MM. This newly recognized volcanic component is interpreted as evidence of an eruption that was coeval with the MM depositional event, thus it is the missing link that connects the formation of the MM to a previously unknown volcanic event and its age is the true age of the MM.

These youngest volcanic rocks are composed of gray-black, poorly sorted (pebble to boulder), minutely vesicular, glassy, phenocryst-rich clasts in a gray-tan, fine- to coarse-grained, crystal-rich, pyroclastic matrix. X-ray diffraction indicates that the clasts are hypocrystalline and composed mainly of plagioclase phenocrysts (microphenocrysts, microlites), with augite and enstatite, and 35% glass. The rocks are andesite (geochemistry: total alkali - silica). The ⁴⁰Ar/³⁹Ar isochron ages of plagioclase from samples collected at two sites are 22.99 ± 0.21 Ma and 23.02 ± 0.21 Ma (Nevada Isotope Geochronology Laboratory).

The lithologic characteristics and field relationships of the volcanic component indicate that it may be part of a block-and-ash flow or pyroclastic density current from an eruption that occurred during the formation of the MM. The contacts between the volcanic component and the adjacent, previously lithified, allochthonous blocks of the MM (mostly Bear Valley Formation volcaniclastic arenite and tuff) indicate that it was neither lithified nor consolidated prior to formation of the MM because it appears to have filled the spaces between the lithified blocks during the MM depositional event. Therefore, the volcanic component is interpreted as evidence of an eruption that was concurrent with the fragmentation, transportation and deposition of the allochthonous MM blocks.

In summary, this newly studied volcanic component of the MM is significant because it is the missing link that connects the genesis of the MM to an eruptive event during the early developmental history of the unit and its age constrains the oldest possible depositional age of the MM to about 23 Ma. Based upon this new evidence, the 23 Ma age of these volcanic rocks is here proposed to also be the true depositional age of the MM.