--> Abstract: Geochronology and Magnetostratigraphy of Paleogene North American Land Mammal "Ages": An Update, by D. R. Prothero and C. C. Swisher, III; #90987 (1993).

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PROTHERO, DONALD R., Department of Geology, Occidental College, Los Angeles, CA; and CARL C. SWISHER, III, Institute of Human Origins, Berkeley, CA

ABSTRACT: Geochronology and Magnetostratigraphy of Paleogene North American Land Mammal "Ages": An Update

Laser-fusion {40}Ar/{39}Ar dating and magnetostratigraphy have radically changed our conception of the temporal duration and correlation of Eocene and Oligocene North American land mammal "ages." The Bridgerian and Uintan land mammal "ages" are both middle Eocene. The Bridgerian/Uintan boundary occurs as the base of magnetic Chron C20R, about 46 Ma. The Uintan/Duchesnean boundary occurs within Chron C18N, and

lies above an ash dated at about 40 Ma. It appears to correlate with the middle/late Eocene boundary. For 50 years the Chadronian was thought to be early Oligocene in age, but it now appears that the Duchesnean and Chadronian are both late Eocene. The Duchesnean/Chadronian boundary lies within Chron C16N about 37 Ma. The Chadronian/Orellan transition occurs just above a date of 33.9 Ma, late in Chron C13R; it appears to correlate with the Eocene/Oligocene boundary. The Orellan/Whitneyan boundary occurs in the middle of Chron C12R, just below a date of 31.8 Ma. The Whitneyan/Arikareean boundary occurs within Chron C11N, above a date of 30.0 Ma. This makes the Orellan and Whitneyan (once considered middle and late Oligocene) both early Oligocene, and correlates the Arikareean (long cons dered early Miocene) with the late Oligocene. These changes, along with recent data which place the Paleocene/Eocene boundary within the Wasatchian land mammal "age" (thereby making the Clarkforkian completely late Paleocene) have radically realigned the correlations published as recently as 1987. Implications for our conception of faunal and climatic changes in North America will be discussed.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.