Glen A. Izett

Revised ages for tuffs of the Yellowstone Plateau volcanic field: Assignment of the Huckleberry Ridge Tuff to a new geomagnetic polarity event

40 Ar/ 39 Ar ages were determined on the three major ash-flow tuffs of the Yellowstone Plateau volcanic field in the region of Yellowstone National Park in order to improve the precision of previously determined ages. Total-fusion and incremental- heating ages of sanidine yielded the following mean ages: Huckleberry Ridge Tuff—2.059 ± 0.004 Ma; Mesa Falls Tuff— 1.285 ± 0.004 Ma; and Lava Creek Tuff— 0.639 ± 0.002 Ma. The Huckleberry Ridge Tuff has a transitional magnetic direction and has previously been related to the Reunion Normal- Polarity Subchron. Dating of the Reunion event has been reviewed and its ages have been normalized to a common value for mineral standards. The age of the Huckleberry Ridge Tuff is significantly younger than lava flows of the Reunion event on Re union Island, supporting other evidence for a normal-polarity event younger than the Reunion event.

The Bishop ash bed, a Pleistocene marker bed in the Western United States

Biotite-bearing chalky-white rhyolitic ash, here called the Bishop ash bed, occurs in middle Pleistocene alluvial and lacustrine deposits at eight localities scattered from California to Nebraska and is correlated with the basal air-fall lapilli of the Bishop Tuff, an ash flow of eastern California, radiometrically dated about 0.7 million years. Correlation of the Bishop ash bed with the air-fall lapilli is made on the basis of similar petrography and on chemistry as determined by electron microprobe, atomic absorption, and emission spectrographic analyses. At five localities the Bishop ash bed lies stratigraphically below a Pearlette-like ash. As more occurrences of the Bishop ash bed are found, it should become an increasingly important dated stratigraphic marker relating middle Pleistocene deposits and events across several geomorphic provinces. Another biotite-bearing chalky-white ash, here called the ash of Green Mountain Reservoir, occurs at three other localities and is distinguishable from the Bishop ash bed by small differences in chemical composition of the glass. The ash of Green Mountain Reservoir is younger than the Bishop ash bed, as shown by the fact that at one locality it lies stratigraphically above the aforementioned bed of Pearlette-like ash.

Zircon Fission-Track Ages of Pearlette Family Ash Beds in Meade County, Kansas

Pearlette family volcanic ash beds at two faunally important late Cenozoic localities near Meade, Meade County, Kansas, are very similar in chemical and mineralogic composition, yet their zircon microphenocrysts have markedly different fission-track ages. Zircon microphenocrysts from type B Pearlette volcanic ash underlying sediments that contain the Borchers local fauna of Hibbard are various shades of pink and have a fission-track age of 1.9 ± 0.1 m.y. In contrast, zircon microphenocrysts from type O Pearlette volcanic ash overlying sediments that contain the Cudahy local fauna of Hibbard are colorless and have a fission-track age of 0.6 ± 0.1 m.y. These fission-track ages are in good agreement with K-Ar ages on the probable source material in Yellowstone National Park.

Correlation of a volcanic ash bed in pleistocene deposits near Mount Blanco, Texas, with the Guaje Pumice Bed of the Jemez Mountains, New Mexico☆

Abstract A rhyolitic volcanic ash bed about 0.3 m thick is exposed in a roadcut along Texas Highway 193 near Mount Blanco in the upper part of a sequence of Pleistocene sedimentary deposits at the type locality of the Blanco Formation, about 59 km northeast of Lubbock, Texas. This ash, here named informally the Guaje ash bed, has chemical and petrographic characteristics closely resembling those of the rhyolitic air-fall tephra (Guaje Pumice Bed) that directly underlies ash flows of Pleistocene age in the Jemez Mountains of northern New Mexico. The Guaje Pumice Bed and the ash flows belong to the Otowi Member of the Bandelier Tuff. Properties common to the Guaje ash bed and the Guaje Pumice Bed include: refractive index of glass, 1.497–1.498; microphenocrysts of quartz, sanidine (Or 42–44 ), ferrohedenbergite (Fe 51 Ca 42 Mg 7 ), chevkinite, allanite, zircon, and magnetite. Chemical composition of the glass of the Guaje ash bed matches that of the Guaje Pumice Bed for all major elements except K and Na and for trace elements determined by standard chemical analyses, atomic absorption, and neutron activation. Paleomagnetic measurements indicate that the ash has reverse depositional remanent magnetization. Glass shards of the ash have a fission-track age of about 1.4 ± 0.2 m. y. Sanidine from the Guaje Pumice Bed and its genetically related ash-flow sheet in the Jemez Mountains was KAr dated at about 1.4 m. y. by R. R. Doell and his colleagues in 1968. Correlation of the Guaje ash bed with the radiometrically dated Guaje Pumice Bed establishes a minimum age of about 1.4 m. y. for the Blanco Formation.

Age of the Bishop Tuff of eastern California as determined by the fission-track method

The zircon fission-track age of a sample of air-fall pumice of the Bishop Tuff (Pleistocene) of eastern California is 0.74 ± 0.05 m.y. This age is within the precision limits of the currently accepted age of the Bishop Tuff, about 0.7 m.y.

Authigenic “spherules” in K-T boundary sediments at Caravaca, Spain, and Raton Basin, Colorado and New Mexico, may not be impact derived

The K-T boundary interval at Barranco del Gredero, Caravaca, Spain, consists of three units: a light gray Cretaceous marl, a 1- to 3-mm-thick ferruginous clay called the “K-T boundary impact layer” because of its content of shock-metamorphosed quartz grains, and an overlying greenish-gray clay. A conspicuous feature of the impact layer is its content of potassium-feldspar particles called “sanidine spherules” by some authors. The particles consist of a porous mesh-work of prismatic potassium-feldspar crystals clearly of authigenic origin. Their shape, distribution, and texture suggest that they are not altered basaltic melt droplets formed during impact of an extraterrestrial object as advocated by some proponents of the impact-extinction theory of Alvarez. Moreover, authigenic spherules also occur in a thin kaolinitic claystone layer at the K-T boundary in continental sedimentary rocks of the Raton Basin, Colorado and New Mexico. This layer is overlain by another claystone (5–8 mm thick) that records the abrupt appearance of shock-metamorphosed quartz grains in the local stratigraphic section. Four compositional types of spherules (kaolinite, goyazite, layered goyazite/kaolinite, and jarosite) occur in the lower of the two claystone layers. Because the spherules are (1) not in a bed containing shock-metamorphosed minerals, (2) common at some and rare at other localities, and (3) hollow and form peculiarly shaped intergrown masses suggest that they are not altered microtektites or melt droplets.

Apatite fission-track dating of a sample from the type Caradoc (Middle Ordovician) Series in England

Fission-track dating of apatite from a bentonite in the type Caradoc Series (Middle Ordovician) of England provides a minimum average age of 451 ± 21 m.y. (external-detector and population methods). The conodont-color geothermometer of Epstein and others indicates that rock temperatures remained below 80°C. Therefore, there should have been little if any fading of fossil fission tracks in the apatite. Isotopic calibration of provincial stratotype sections can be done quickly and relatively inexpensively by the fission-track method, provided that bentonites are present, and will prove useful in understanding lower Paleozoic biostratigraphy.