COMPARISON OF DETRITAL ZIRCONS IN THE FLUVIAL KAYENTA FORMATION (LOWER JURASSIC) OF SOUTHERN UTAH WITH DETRITAL ZIRCONS FROM UNDERLYING AND OVERLYING EOLIAN STRATA

Detrital zircon grains for this study were collected from the Kayenta Formation of the Glen Canyon Group in North Wash southeast of Hanksville (Utah) on the Colorado Plateau. U-Pb ages for 100 detrital zircon grains were determined individually by LA-ICPMS using a beam diameter of 50 microns. Due to >20% discordance or poor precision, 7 grains were removed from consideration, leaving 93 reliable analyses. The majority of the zircons fall into three main age populations: 240-300 Ma (12%), 400-540 Ma (12%), and 1020-1340 Ma (32%), with specific frequency peaks at 259 Ma, 447 Ma, and 1082 Ma. The remaining zircons fall primarily into four subordinate age clusters: 780-960 Ma, 1380-1460 Ma, 1580-1720 Ma, and 2640-2720 Ma. The age distributions and frequency peaks of most detrital zircons in the Kayenta Formation closely resemble those in eolianites of the underlying Wingate Sandstone and the overlying Navajo Sandstone within the Glen Canyon Group. Reworking of eolian sand into the Kayenta fluvial system was the likely source of these zircon grains. The zircon age spike in the 240-300 Ma range, present in the Kayenta Formation but absent from both the Wingate and Navajo sandstones, suggests a Kayenta provenance partly in the Permian-Triassic East Mexico arc to the southeast. Paleocurrent directions in the Kayenta Formation of southern Utah indicate primary flow from the east but subordinate flow from the southeast as well. The Kayenta sample contains less quartz (74%) than either the Wingate or Navajo (82% and 85% respectively), and more lithic fragments (9%) than either Wingate or Navajo samples (4% in each case). The petrographic difference suggests a contribution of less quartzose and more lithic sand to the Kayenta fluvial system from 240-300 Ma source rocks that did not contribute to underlying and overlying eolian systems. Eolian reworking of fluvial sand could increase quartz content and reduce lithic content, but could not eliminate 240-300 Ma zircon grains which are absent from Wingate and Navajo eolianites.