MAGNETIC STRATIGRAPHY AND GEOCHRONOLOGY OF THE BARSTOVIAN-CLARENDONIAN (MIDDLE TO LATE MIOCENE) PART OF THE MOONSTONE FORMATION, CENTRAL WYOMING

The Moonstone Formation in central Wyoming consists of about 400 m of fluvial and lacustrine deposits that mantle the Precambrian granites of the Sweetwater Arch and overlie the lower to middle Miocene Split Rock Formation with a slight unconformity. Previous age constraints were very imprecise (“Pliocene” mammals, now considered middle to late Miocene, and Miocene to Pliocene diatoms). Recent new collections show that the fauna is late Barstovian to earliest Hemphillian in age. Magnetic samples were taken through two fossiliferous sections at Castle Basin and Big Blowout. The samples showed a single component of remanence held mainly in magnetite, with only minor overprinting, and passed a reversal test for stability. Based on a U/Pb date of 11.3 ± 0.5 Ma on the base of the Castle Basin section and a geochemically correlated date of 9.8-10.2 Ma near the top of the same section, we correlate this section with Chrons C5r1r-C5r2r (9.9-11.4 Ma), which is latest Barstovian to earliest Clarendonian. The shorter Big Blowout section was entirely reversed in polarity, and probably correlates with Chron C5r2r (11.1-11.4 Ma). These results suggest that the time between the cessation of Split Rock deposition (earliest Barstovian, 15.9 Ma) and the earliest deposition of the Moonstone Formation (at least 11.4 Ma, and possibly earlier) is only about 4.5 million years or less, much shorter than previously thought.