Eocene and Oligocene floras of the western United States show a climatic deterioration from warmer conditions to much cooler and drier conditions. Recent 40 Ar/ 39 Ar dates and magnetic stratigraphy have greatly improved their correlation. In this study, the uppermost Eocene Antero Formation, Colorado, is entirely reversed in polarity, and is correlated with late Chron C13r, based on 40 Ar/ 39 Ar dates of 33.77–33.89 Ma. The early Oligocene Pitch-Pinnacle flora of Colorado is within rocks of normal polarity, and best correlated with Chron C12n (30.5–31.0 Ma), based on 40 Ar/ 39 Ar dates of 32.9–29.0 Ma (although correlation with Chron C11n is also possible). The late Oligocene (40 Ar/ 39 Ar dated 26.26–26.92 Ma) Creede flora of southwestern Colorado is correlated with Chron C8r. The early Oligocene (40 Ar/ 39 Ar dated at 31.5 Ma) Granger Canyon flora in the Warner Mountains, near Cedarville, northeastern California, is correlated with Chron C12r. These results are compiled with previously published dates and magnetic stratigraphy of the Eugene-Fisher floral sequence in western Oregon, the Bridge Creek floras in central Oregon, other floras in the Warner Mountains of northeast California, and the Florissant flora of central Colorado. In Colorado, the climatic change seems to have occurred between the Florissant and Antero floras, and is dated between 33.89 and 34.07 Ma, or latest Eocene in age, although the Pitch-Pinnacle flora suggests that the deterioration was less severe and took place in the early Oligocene. In northeast California, the dating is not as precise, so the climatic change could have occurred between 31.5 and 34.0 Ma (probably early Oligocene). In western Oregon (Eugene and Fisher Formations), the change occurs between the early Oligocene Goshen flora (33.4 Ma) and the early Oligocene Rujada flora (31.5 Ma). In the John Day region of Oregon, it occurs before the oldest Bridge Creek flora, dated at 33.62 Ma (right after the Eocene-Oligocene boundary). Thus, only two of these four floral sequences (Eugene, Oregon, and Cedarville, California) clearly show the early Oligocene climatic change consistent with that documented in the global marine record, whereas the climatic change was seemingly abrupt in the late Eocene in Colorado between 33.89 and 34.07 Ma, and also sometime during the late Eocene (before 33.62 Ma) in central Oregon.

Full article available in PDF version.

Abstract Laser-fusion 40 Ar/ 39 Ar dating and magnetostratigraphy have significantly changed our conception of the temporal duration and correlation of Paleogene North American land mammal "ages." The Wood Committee (1941) originally divided the Paleocene Epoch into five land mammal "ages." Current age estimates of their time spans are: Puercan, 65–63.8 Ma; Torrejonian (including the "Dragonian"), 63.8–61 Ma; Tiffanian, 61–56 Ma; and Clarkforkian, 56–55.2 Ma. The Paleocene/Eocene boundary, long placed in the Clarkforkian, occurs in the earliest Wasatchian, based on correlations using mammals, pollen, and terrestrial carbon isotopes. The Wood Committee (1941) divided the North American Eocene Epoch into four land mammal “ages”: Wasatchian (originally thought to be early Eocene), Bridgerian (thought to be middle Eocene), and Uintan and Duchesnean (both once thought to be late Eocene). The earliest Wasatchian is now considered Paleocene age, and the Wasatchian/Bridgerian boundary is about 50.4 Ma in age. The Bridgerian, Uintan and Dudnesnean land mammal “ages” are all middle Eocene age. The Bridgerian/Uintan boundary occurs in magnetic Chron C21n, about 47 Ma. The Uintan/Duchesnean boundary occurs within Chron C18n, and lies above an ash dated at about 40 Ma. The Duchesnean/Chadronian boundary lies within Chron C16n, about 37 Ma. Finally, the Wood Committee (1941) divided their concept of North American Oligocene sequence into three land mammal “ages”: the Chadronian, Orellan and Whitneyan (supposedly early, middle, and late Oligocene). The Chadronian/Orellan transition occurs just above a date of 33.9 Ma, late in Chron C13r; it is slightly younger than the Eocene/Oligocene boundary, and this makes the Chadronian mostly late Eocene, not early Oligocene age. 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. Consequently, the Orellan and Whitneyan are both early Oligocene, and most of the Arikareean (long considered early Miocene) is late Oligocene age. These new age estimates and correlations differ greatly from the time scales published as recently as 1987.

Abstract Archaeologists use various properties of stone (lithic) artifacts to determine their sources, which can suggest trade or procurement patterns for the artifacts at a particular site. Sometimes source identification is relatively straightforward, because the ways in which certain artifacts were manufactured are often diagnostic of a particular place and time. In addition to the manmade properties of artifacts, the natural physical properties are often distinctive as well. In the past, chert artifacts were assigned a source based on macroscopic features such as color, luster, inclusions, and texture. Yet these features are not always diagnostic of a single source, and are difficult to assess objectively (Lavin, 1983a; Lavin and Prothero, 1987; Luedtke, 1976). For example, yellow, brown, and red cherts in New Jersey are usually attributed to the "Pennsylvania Jasper," yet the color is deceptive. Many cherts attributed to this source are in fact from other sources, and others from the "Pennsylvania Jasper" source are not yellow, brown, or red (Lavin and Prothero, 1981, 1987). There are several techniques that can be used for a more refined analysis of chert artifacts. One, neutron activation analysis, has been used successfully by a number of workers (Aspinall and Feather, 1972; De Bruin and others, 1972; Ives, 1974, 1975; Luedtke, 1976, 1978, 1979; Miller, 1982; Sieveking and others, 1972; Spielbauer, 1976). Other geochemical techniques require far more equipment and expertise than are available to most archaeologists. However, pétrographie, or thin-section, analysis is a simple, inexpensive technique that has been standard in geology for over