MAGNETIC STRATIGRAPHY OF THE MIDDLE HEMINGFORDIAN (LOWER MIOCENE) SPLIT ROCK FORMATION, CENTRAL WYOMING

The Split Rock Formation consists of several hundred meters of gray volcaniclastic sandstones and tuffs with minor claystones that mantle the Granite Mountains in Fremont and Natrona Counties in central Wyoming. It yields a famous vertebrate fauna, including 13 species of reptiles, and 46 taxa of mammals. Among these are the best specimen of the dome-skulled chalicothere Tylocephalonyx skinneri, and type specimens of several other mammals, including rodents, oreodonts and carnivores. The fauna has long been considered middle Hemingfordian (older than the Sheep Creek fauna, but younger than the Runningwater fauna of Nebraska), but the only age constraint is a K-Ar date of 17 Ma at the base of the section. Magnetic samples were taken through the thickest, most complete section (UCMP V69190 and 69191) which produces the best-constrained fossil localities. After both alternating field and thermal demagnetization, the samples contained a single component of remanence held in magnetite, which passed a reversal test. The lower part of the section is reversed in polarity, while the top half is normal. Based on the K-Ar date, we correlate the section with Chrons C5Cn-C5Cr (16.0-17.3 Ma), or latest early Miocene in age.