DETAILED CORRELATION BETWEEN TWO SEPARATED LATE EOCENE - EARLY OLIGOCENE TERRESTRIAL DISTAL VOLCANICLASTIC SEQUENCES, THE WHITE RIVER FORMATION OF FLAGSTAFF RIM, AND DOUGLAS, WYOMING

Distal volcaniclastic terrestrial sequences can provide a wealth of information for very detailed stratigraphic correlations. Such deposits typically contain abundant vertebrate fossils for biostratigraphy and contain abundant volcanic tuff beds for correlation. Two such sequences are in the upper Eocene and lower Oligocene White River Formation at Flagstaff Rim, in Natrona County, and near Douglas, in Converse County, Wyoming. The two locations are 120 km apart and have White River sequences that are 236 and 231 m thick, respectively. Lithologically the two sequences are similar with a basal mudstone, and middle mudstone, and an upper siltstone sequence. Vertebrate fossils are common in both localities and have late Eocene (Chadronian) faunas in both and early Eocene (Orellan) faunas at Douglas. Only the lower 107 m have Chadronian fossils in the Douglas sequence, indicating deposition ended at Flagstaff Rim near the end of the Eocene but continued into the Oligocene at Douglas. 

Flagstaff Rim contains 16 tuff beds and the Douglas area contains 15 tuff beds, though only nine of the tuffs at Douglas occur in Chadronian rocks. Seven of the Chadronian tuffs can be matched between the two areas using a combination of mineral suites of accessory heavy minerals, geochemistry and magnetic properties of hemoilmenite, and magnetic polarities of the tuff beds. Published 40Ar/39Ar ages for the tuffs provide a chronostratigraphic framework allowing for a comparison of the rock accumulation rates between the two locations. Rock accumulation rates vary from 0.3 m/ky to 0.02 m/ky, with the two areas showing different accumulation rates at different times. The Douglas sequence preserves the transition between the Eocene and Oligocene, and provides an age for the end of the Chadronian Land Mammal Age at 33.7 Ma. This age is 200 ky after the global Eocene/Oligocene boundary, and correlates with the major drop in sea temperatures just after the boundary.