Located adjacent to the paleoanthropological site of Hadar in Afar, Ethiopia, the Ledi-Geraru project area preserves multiple tephra deposits within the Pliocene sediments of the hominin-bearing Hadar Formation. Tephra deposits of the Bouroukie Tuff 2 volcanic complex (BKT-2) are important regional markers, and here we provide correlations between the Hadar and Ledi-Geraru project areas using major-element glass chemistry, stratigraphic relationships, outcrop characteristics, and 40 Ar/ 39 Ar dates. These correlations greatly expand existing temporal and spatial resolution, aid in interpretations of regional depositional environments, and increase the documented extent of BKT-2 to ~600 km 2 . BKT-2 exposures at Ledi-Geraru are the thickest and most complete yet observed. There, the BKT-2 complex is preserved as two air-fall lapilli layers, BKT-2U (<97 cm thick) and BKT-2L (<9 cm thick), separated by <2.5 m of silts and clays or diatomite that overlie the Green Marker Bed (GMB), a laminated ash tuff. Measured sections were evaluated to create a stratigraphy-based model of paleolandscape variations using BKT-2 tephra as laterally extensive isochronous surfaces. BKT-2 was mainly erupted into lacustrine and nearshore environments. The eastern Ledi-Geraru region was likely located at the depocenter of an expansive fluviolacustrine network. Representing the last major lacustrine phase of the Hadar Formation, lateral facies variations show the westward expansion of a lacustrine setting ca. 2.96 Ma followed by eastward regression initiated sometime prior to the eruption of BKT-2U ca. 2.94 Ma. High-resolution, well-correlated, and temporally constrained stratigraphic records are key to the interpretation of paleoenvironmental variation in East Africa.

40 Ar/ 39 Ar dates, field observations, and geochemical data are reported for Irazú volcano, Costa Rica. Volcanism dates back to at least 854 ka, but has been episodic with lava shield construction peaks at ca. 570 ka and 136–0 ka. The recent volcanic record on Irazú volcano comprises lava flows and a variety of Strombolian and phreatomagmatic deposits, with a long-term trend toward more hydrovolcanic deposits. Banded scorias and hybridized rocks reflect ubiquitous magma mixing and commingling. Two distinct magma batches have been identified. One magma type or batch, Haya, includes basalt with higher high field strength (HFS) and rare-earth element contents, suggesting a lower degree melt of a subduction modified mantle source. The second batch, Sapper, has greater enrichment of large ion lithophile elements (LILE) relative to HFS elements and rare-earth elements, suggesting a higher subduction signature. The recent volcanic history at Irazú records two and one half sequences of the following pattern: eruptions of the Haya batch; eruptions of the Sapper batch; and finally, an unusually clear unconformity, indicating a pause in eruptions. In the last two sequences, strongly hybridized magma erupted after the eruption of the Haya batch. The continuing presence of two distinct magma batches requires two active magma chambers. The common occurrence of hybrids is evidence for a small, nearer to the surface chamber for mixing the two batches. Estimated pre-eruptive temperatures based on two-pyroxene geothermometry range from ∼1000–1176 °C in basalts to 922 °C in hornblende andesites. Crystallization occurred mainly between 4.6 and 3 kb as measured by different geobarometers. Hybridized rocks show intermediate pressures and temperatures. High silica magma occurs in very small volumes as banded scorias but not as lava flows. Although eruptions at Irazú are not often very explosive, the pervasiveness of magma mixing presents the danger of larger, more explosive hybrid eruptions.

Abstract Since the publication of our previous time scale (Berggren and others, 1985c = BKFV85) a large amount of new magneto- and biostratigraphic data and radioisotopic ages have become available. An evaluation of some of the key magnetobiostratigraphic calibration points used in BKFV85, as suggested by high precision 40 Ar/ 39 Ar dating (e.g., Montanari and others, 1988; Swisher and Prothero, 1990; Prothero and Swisher, 1992; Prothero, 1994), has served as a catalyst for us in developing a revised Cenozoic time scale. For the Neogene Period, astrochronologic data (Shackleton and others, 1990; Hilgen, 1991) required re-evaluation of the calibration of the Pliocene and Pleistocene Epochs. The significantly older ages for the Pliocene-Pleistocene Epochs predicted by astronomical calibrations were soon corroborated by high precision 40 Ar/ 39 Ar dating (e.g., Baksi and others, 1992; McDougall and others, 1992; Tauxe and others, 1992; Walter and others, 1991; Renne and others, 1993). At the same time, a new and improved definition of the Late Cretaceous and Cenozoic polarity sequence was achieved based on a comprehensive evaluation of global sea-floor magnetic anomaly profiles (Cande and Kent, 1992). This, in turn, led to a revised Cenozoic geomagnetic polarity time scale (GPTS) based on standardization to a model of South Atlantic spreading history (Cande and Kent, 1992/1995 = CK92/95). This paper presents a revised (integrated magnetobiochronologic) Cenozoic time scale (IMBTS) based on an assessment and integration of data from several sources. Biostratigraphic events are correlated to the recently revised global polarity time scale (CK95). The construction of the new GPTS is outlined with emphasis on methodology and newly developed polarity history nomenclature. The radioisotopic calibration points (as well as other relevant data) used to constrain the GPTS are reviewed in their (bio)stratigraphic context. An updated magnetobiostratigraphic (re)assessment of about 150 pre-Pliocene planktonic foraminiferal datum events (including recently available high southern (austral) latitude data) and a new/modified zonal biostratigraphy provides an essentially global biostratigraphic correlation framework. This is complemented by a (re)assessment of nearly 100 calcareous nannofossil datum events. Unrecognized unconformities in the stratigraphic record (and to a lesser extent differences in taxonomic concepts), rather than latitudinal diachrony, is shown to account for discrepancies in magnetobiostratigraphic correlations in many instances, particularly in the Paleogene Period. Claims of diachrony of low amplitude (<2 my) are poorly substantiated, at least in the Paleocene and Eocene Epochs. Finally, we (re)assess the current status of Cenozoic chronostratigraphy and present estimates of the chronology of lower (stage) and higher (system) level units. Although the numerical values of chronostratigraphic units (and their boundaries) have changed in the decade since the previous version of the Cenozoic time scale, the relative duration of these units has remained essentially the same. This is particularly true of the Paleogene Period, where the Paleocene/Eocene and Eocene/Oligocene boundaries have been shifted ~2 my younger and the Cretaceous/Paleogene boundary ~1 my younger. Changes in the Neogene time scale are relatively minor and reflect primarily improved magnetobiostratigraphic calibrations, better understanding of chronostratigraphic and magnetobiostratigraphic relationships, and the introduction of a congruent astronomical/paleomagnetic chronology for the past 6 my (and concomitant adjustments to magnetochron age estimates).

Abstract The radiation of a diverse array of endemic marsupials, edentates, primates, rodents and "ungulates" has been exceedingly useful for developing a detailed biochronologic sequence of about 20 South American Land Mammal Ages (SALMAs), covering much of Cenozoic time. Independent chronologic controls on this terrestrial South American Cenozoic record have increased dramatically in the past 25 years. There now are numerous radioisotopic dates (including many new laser fusion 40 Ar/ 39 Ar analyses) and magnetochronologic studies, especially for the Neogene Period, leading to better resolution of the ages of the Riochican SALMA, the new Chilean Tinguiririca faunal interval, Deseadan SALMA, Santacrucian SALMA, "Friasian" SALMA, Colombian LaVenta sequence, Huayquerian SALMA, Ensenadan SALMA, and Uquian SALMA. 40 Ar/ 39 Ar dating and magnetostratigraphic studies are continuing actively, but current data indicate that there are significant temporal gaps in the SALMA sequence, most notably representing the early Paleocene, most of the Eocene, part of the early Oligocene, much of the early Miocene, and part of the late Miocene Epochs. In particular, the Tiupampan, Casamayoran, Mustersan, Divisaderan, and Colhuehuapian Cenozoic SALMAs (as well as the late Cretaceous [?Campanian] "Alamitian" SALMA), lack either magnetic polarity stratigraphies or radioisotopic dating, are temporally constrained only by the ages of superposed intervals or weakly justified "stage-of-evolution" arguments and thus remain relatively poorly constrained geochronologically. Best estimates for the approximate durations (question marks indicate very poor geochronologic control, and therefore highly provisional age estimates) of the SALMAs are: Lujanian, 10,000–800,000 years ago; Ensenadan, 0.8–1.2 Ma; Uquian, 1.5–3.0 Ma; Chapadmalalan, 3.4–4.0 Ma; Montehermosan, 4.0–6.8 Ma; Huayquerian, 6.8–9.0 Ma; Chasicoan, 9.0–10.0(?) Ma; Mayoan, (?)10.0–11.8 Ma; Laventan, 11.8–13.8 Ma; Colloncuran, 14.0–15.5 Ma; Friasian, uncertain; Santacrucian, 16.3–17.5 Ma; Colhuehuapian, (?)19–21(?) Ma; Deseadan, 24.5–29 Ma; New SALMA (“Tinguirirican”), 31.5–36 Ma; Divisaderan, (?)40–42(?) Ma; Mustersan, (?)45–48(?) Ma; Casamayoran, (?)51–54(?) Ma; Riochican 55.5–57 Ma; Itaboraian, 57.5–59 Ma; Peligran, 61–62.5 Ma; and Tiupampan, 63–64.5 Ma. The South American faunal and floral record can be combined with available geochronologic information to evaluate timing and pattern of major biotic and environmental changes and events. It is clear that Cenozoic terrestrial biotas responded to both global and regional, physical and biotic, changes and events, including major plate tectonic reorganizations and associated biogeographic events (e.g., Mesozoic-early Cenozoic Gondwanan [and subsidiary North American] continental connections and biogeographic relationships, final separation of Antarctica during the ?Eocene Epoch, formation of the Isthmus of Panama in the Pliocene Epoch, etc.), global Eocene-Oligocene boundary events (including climate change, initiation of oceanic deep water flow through the Drake Passage, onset of major Antarctic ice cap formation, expansion of open habitats [wooded grasslands and grasslands], increase in hypsodont mammalian taxa, and major clade extinction, origination, and diversification), phases of Andean uplift, Pliocene-Pleistocene Epoch glaciation/climatic changes, etc.

Abstract The generally well developed and understood stratigraphic record associated with fossil mammals in North America is combined with independent chronological data sets that foster the development of high-resolution geochronology in nonmarine sequences. An updated chronology for all North American mammal ages (or subdivisions) is utilized to examine the tempo and mode of overland mammal immigration/emigration episodes during the Cenozoic Era. In addition to the thirty or more "background" dispersals involving only a few taxa, ten major immigration/emigration episodes are recorded during the Cenozoic Era in North America. All are important for evaluating the dispersal pattern, as well as for mammal age boundary definition. For the Paleogene interval, major immigration/emigration episodes define the following mammal ages (or intervals): Clarkforkian, Wasatchian, late Uintan, and Chadronian, with the Wasatchian and late Uintan being especially noteworthy. The interval that embraces the late Arikareean mammal age is the first immigration episode of the Neogene interval, but the events recognized in the early and late Hemingfordian mammal ages, respectively, are the most impressive. An important, "medial" Clarendonian emigration is reflected in the North American basis for the Old World "Hipparion" Datum. The events that define the beginning of the Hemphillian and late Blancan mammal ages also are founded on important immigrant first occurrences, but for the first time in the Neogene interval involve taxa from South America as well as from Asia. At other times, either there is no effective immigration or, if present, it involves only a few (four or less) taxa (= "background"). In certain intervals, apparently lowered sea level had no effect on dispersal, but in an even larger number of cases immigration took place in spite of what appears to have been times of relative sea-level highstand. Thus tectonic, climatic, and other factors must be considered to account for North America’s dispersal history during the Cenozoic Era.