PROVENANCE ANALYSIS OF THE WILLWOOD AND WAPITI FORMATION (EOCENE) SANDSTONES IN THE ABSAROKA BASIN, WY USING DETRITAL ZIRCON GEOCHRONOLOGY

The transition from Laramide syntectonic sedimentation of the Lower Eocene Willwood formation to the post-Laramide volcanogenic sedimentation of the Middle Eocene Wapiti Formation was studied in the Shoshone River Valley, Wyoming. To better understand the regional paleogeography and provenance of these sandstones, we sampled five units: two in the Willwood Formation (n=179), and three in the Wapiti Formation (n=241). The DZ pattern for the lower Willwood Formation is complex, with frequency peaks of ca. ~160 Ma, 380 Ma, 1050 Ma, 1450 Ma, 1650 Ma, and 2750 Ma. The upper Willwood Formation is dominated by 2500-3250 Ma zircons, and other frequency peaks occur at ca. 70 Ma and 1825 Ma. The complex DZ pattern of the lower Willwood indicates a provenance of recycled Paleozoic and Mesozoic sandstone units shed as the adjacent Laramide uplifts were being unroofed. The DZ pattern of the upper Willwood indicates that the basement rocks of adjacent Laramide uplifts were largely exposed. In addition, the Absaroka Basin was open to the west, as some sediment was derived from the distal Mesozoic orogen. The Wapiti Formation sandstones were sampled from a 300 m section of epiclastic volcanic rocks in the upper South Fork Shoshone river valley. Overall, the DZ pattern for these rocks is similar to the upper Willwood, with peaks at ca. 50 Ma,1650-1850 Ma and 2500-2800 Ma. The proportion of Eocene (syn-volcanic) zircon ranges from 12% in the basal sandstone to 61% in the upper sandstone. The provenance of the Wapiti formation was initially dominated by recycled and primary sands derived from Laramide uplifts to one dominated by primary volcaniclastic materials.