ENVIRONMENTAL AND MORPHOLOGIC DIVERSITY OF FLUVIAL AND LACUSTRINE ONCOIDS FROM TWO LOCALITIES IN THE FLAGSTAFF FORMATION (SAN PITCH MOUNTAINS, CENTRAL UTAH)

In central Utah, Lake Flagstaff was an intermountain basin produced by regional orogenic uplift. Lake Flagstaff deposits are exposed on the Wasatch Plateau and San Pitch Mountains. The resultant Flagstaff Formation is interpreted as a lacustrine limestone with fluvial influence and is the focus of this study.

This is a comparative investigation of two localities that expose oncoids in the Flagstaff Formation in the San Pitch Mountains. Radio Tower is located 3.6 km W of Sterling, UT, while Fayette is located 2 km NE of Fayette, UT. Research objectives use descriptive field and laboratory criteria of oncoids ― including size, shape, surface patterns, cortex/nucleus features, and distribution/position in the section ― to interpret the depositional environment and hydrodynamic regime at both localities. Additional data obtained involves field photographs, stratigraphic details, and petrographic analyses, specifically targeting differences between the oncoid nuclei, its cortices, and the host rock.

Locality results vary in sedimentologic contexts and provide statistical significance between oncoid morphologies. We infer that oncoids from these two localities were formed in distinct energy levels and paleoenvironments. Radio Tower oncoids are placed in a fluvial facies model near the shoreline, where small channels fed Lake Flagstaff. At Radio Tower, an oncoid-rich unit coarsens upwards; the unit exhibits rare and fragmented macrofossils. Oncoids range in size from small to giant and are mostly ellipsoidal with type C laminae. Nuclei contain subangular/subrounded quartz grains; cortices consist of flat to wavy, alternating dark and light calcite laminae. The long axis trends of oncoids measured in the field show a strong preferred orientation through the unit. Fayette oncoids are interpreted primarily from a shallow, freshwater lacustrine facies model, where Lake Flagstaff onlapped and flooded a local paleotopographic high. The micrite-rich Flagstaff contains mostly spheroidal, small oncoids with type C laminae. Nuclei often contain fragments of abraded molluscs and smaller oncoids, with the cortex showing flat calcite laminae. There is no preferred orientation to long axes in the field. At Fayette, there is a deepening of the lacustrine system with ooidal-ostracodal units above the oncoid sequence.