DETRITAL ZIRCONS IN THE GARTRA MEMBER OF THE CHINLE FORMATION IN NORTHERN UTAH: IMPLICATIONS FOR UPPER TRIASSIC PALEODRAINAGE PATTERNS

Detrital zircons were separated from a sample of conglomeratic fluvial sandstone collected near the base of the Gartra Member of the Chinle Formation near Vernal in northeastern Utah. U-Pb ages were determined for 95 individual zircon grains using laser ablation ICPMS with a beam diameter of 50 microns. Analyses of five grains were rejected due to >20% age discordance, leaving 90 grains that yielded reliable results. There are three main age populations of zircon grains in the Gartra sample: 480-580 Ma, 1320-1480 Ma, and 1600-1780 Ma. Subordinate numbers of grains are present in the age ranges of <480 Ma (n=3, Paleozoic), 620-760 (n=2, Pan-African), 880-1220 Ma (n=13, Grenville), and >1780 Ma (n=8, Paleoproterozoic and Archean). Approximately 40% of all the grains (n=36) lie within the 480-580 Ma age range, and half of that group have ages of approximately 520 Ma (Early Cambrian). The two other main age groups, with frequency peaks at 1433 Ma and 1715 Ma and making up approximately a third (n=29) of the total grain population, could have been derived from Precambrian rocks forming the cores of uplifts within the Ancestral Rocky Mountains to the southeast in Colorado, and Gartra paleocurrents imply derivation of detritus from the southeast. The prominent Cambrian zircon grains (~520 Ma) suggest derivation, however, from the Amarillo-Wichita uplift lying 750-1000 km southeast of the collecting locality. Their presence in the Gartra Member implies that a paleodrainage system with headwaters in the Amarillo-Wichita uplift passed through the Ancestral Rocky Mountains province to exit as the Eagle paleoriver responsible for deposition of the Gartra Member. Alternately, the Cambrian zircons could perhaps have been recycled from Paleozoic strata of the central Colorado trough that might have been derived in part from the Amarillo-Wichita uplift. In that case, the inferred Late Triassic transport distance would be reduced.