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 Eocene mammals spanning much of the Wasatchian land-mammal age. The Wasatchian is an especially significant period in mammalian evolution since it marks the first appearance of several modem orders of mammals including Perissodactyla, Artiodactyla, and true Primates. Magnetostratigraphic analysis of paleosol horizons in the McCullough Peaks region is used to correlate Wasatchian land-mammal zones to the geomagnetic polarity time scale. A total of 135 paleomagnetic samples were analyzed from 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 Lostcabinian subage). Progressive thermal demagnetization of samples from red (B type) and mottled red/gray (AB type) soil horizons provides the 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 carry 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 represent as much as one-half of the Wasatchian land-mammal age, while earlier zones Wa-0 to Wa-6 (Sandcouleean-Lysitean subages) together span only 2.4 m.y. of early Eocene time.