PETROGENESIS OF THE EDIACARAN BROWNS HOLE FORMATION OF NORTHERN UTAH

The Neoproterozoic rifting of the western margin of Laurentia is recorded by Cryogenian to Cambrian volcanic and sedimentary units along the length of the North American Cordillera. The later stages of this rifting, interpreted as a second rifting event, occur in Ediacaran and Cambrian successions from the Southern Canadian Cordillera to the Caborca region in Northwestern Mexico. Along the Middle Fork of the Ogden River in Utah, the second rifting phase is manifested in the presumably Ediacaran Browns Hole Formation, overlain by the Cambrian Geertsen Canyon Quartzite. The Browns Hole Formation consists of thin layers of siliciclastic rocks interbedded with mafic agglomerates, volcanic flows and potentially tuffs and lapillistones with an estimated age of K-Ar in hornblende of ~580 Ma. Trace and major element chemistry of these volcanic rocks show low SiO₂ (39-54%) and high TiO₂ (0.52-5.46%) with Cr and Ni maximum concentrations of 930ppm and 356ppm respectively. These combined results characterize the flows as alkaline basalts that have within-plate characteristics and OIB signatures. These characteristics are similar to the Ediacaran volcanism of the Caborca region in NW Mexico, where similar pyroclastic and volcanic rocks occur, while also differentiating this volcanism from broadly coeval mafic volcanics of Nevada (Stirling Quartzite) and southern Utah (Tintic and Prospect Mountain quartzites), where the source of the volcanism may have been the same, but the geochemical characteristics are identified as E-MORB, tholeitic within-plate basalts. The Browns Hole Formation is thus likely a key locality for understanding the evolution of western Laurentia’s second Neoproterozoic rifting event.