Comparative Analysis of Oncoids From the Upper Flagstaff Formation, Sanpitch Mountains, Central Utah

Abstract

Sevier and Laramide orogenesis produced regional highlands and intermontane basins, including Lake Flagstaff (Paleocene) in central Utah. Lake Flagstaff deposits are exposed on the Wasatch Plateau and San Pitch Mountains. The Flagstaff Formation is dominantly interpreted as a lacustrine limestone with fluvial influence, and it is divided into three members, the Ferron Mountain Member, the Cove Mountain Member, and the Musinia Peak Member, along its Wasatch Plateau depocenter. However, these members are not easily distinguishable in the San Pitch Mountains, which is the focus of this study.

This is an investigation of a locality that exposes unique oncoids in the Flagstaff Formation in the San Pitch Mountains. The locality, Radio Tower, is located 3.6 km W of Sterling, UT. Research objectives include use of the physical characteristics of the oncoids, including size, shape, and contents of the nuclei, to interpret the depositional environment and hydrodynamic characteristics at Radio Tower. Additional data obtained included petrographic (thin section, acetate peel, and XRD) analysis, specifically analyzing oncoids nuclei and cortices.

Oncoids from Radio Tower are small (<100 mm) to large (100-400 mm) in diameter (i.e., length of the long axis), yet some samples are quite large (30 cm x 41 cm; length of short and long axes, respectively). They are generally ellipsoidal in shape, and their nuclei are simpler, containing primarily terrigenous sediment. The nuclei and the host rock contain abundant spar in some stratigraphic intervals, but other host rock lithologies contain large influxes of quartz sand. We interpret the setting of these oncoids to be in an agitated, higher energy, marginal lacustrine environment in a fluvial channel system.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018