CHRONOLOGY OF THE WASATCHIAN LAND-MAMMAL AGE (EARLY EOCENE) - MAGNETOSTRATIGRAPHIC RESULTS FROM THE MCCULLOUGH PEAKS SECTION, NORTHERN BIGHORN BASIN, WYOMING

The McCullough Peaks section of the Willwood Formation in the northern Bighorn Basin, Wyoming, has produced a rich fossil record of early Eo cene mammals spanning much of the Wasatchian land-mammal age. The Wasa tchian is an especially significant period in mammalian evolution sinc e it marks the first appearance of several modem orders of mammals inc luding Perissodactyla, Artiodactyla, and true Primates. Magnetostratig raphic analysis of paleosol horizons in the McCullough Peaks region is used to correlate Wasatchian land-mammal zones to the geomagnetic pol arity time scale. A total of 135 paleomagnetic samples were analyzed f rom 37 levels in a 1480 m section that ranges from Wasatchian zone Wa- 0 (earliest Sandcouleean subage) to the base of zone Wa-7 (earliest Lo stcabinian subage). Progressive thermal demagnetization of samples fro m red (B type) and mottled red/gray (AB type) soil horizons provides t he most reliable results. The characteristic magnetization is carried by hematite with unblocking temperatures between 400-degrees-C and 680 -degrees-C. Fine-grained hematite and magnetite are also present and c arry a strong present-day overprint. Magnetostratigraphic correlation indicates that Wasatchian zone Wa-0 (earliest Sandcouleean subage) to middle zone Wa-5 (Bunophorus interval-zone) correlate with Chron C24r, middle zone Wa-5 to middle zone Wa-6 (Lysitean subage) correlate with Chron C24n.3n, late zone Wa-6 correlates with Chron C24n.2, and early zone Wa-7 (Lostcabinian subage) correlates with Chron C24n.1r. These results indicate that the last zone, Wa-7 (Lostcabinian subage), may r epresent as much as one-half of the Wasatchian land-mammal age, while earlier zones Wa-0 to Wa-6 (Sandcouleean-Lysitean subages) together sp an only 2.4 m.y. of early Eocene time.