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First U-Pb zircon ages for late Miocene Ashfall Konservat-Lagersta¨tte and Grove Lake ashes from eastern Great Plains, USA
Smith, Jon J. ; Turner, Elijah ; Moller, Andreas ; Joeckel, R. M. ; Otto, Rick E.
Smith, Jon J.
Turner, Elijah
Moller, Andreas
Joeckel, R. M.
Otto, Rick E.
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Abstract
This paper documents the first U-Pb zircon ages for Ashfall Fossil Beds (Nebraska, USA), a
terrestrial Konservat-Lagersta¨ tte mass-death assemblage that is arguably the most diverse
of its type and age. The Ashfall tephra was correlated with ignimbrites from the Bruneau-Jarbidge
volcanic field (12.7–10.5 Ma) in southwest Idaho based on geochemical analysis. The
methods and geochemical data supporting the original age assessment of the ash bed,
however, were never published, and there has been a persistent misconception that dateable
heavy minerals (e.g., zircon) are absent. Notwithstanding, we recovered abundant zircons
from Ashfall Fossil Beds, and from an ash bed ~6 km to the southeast at Grove Lake,
Nebraska, and analyzed them through LA-ICP-MS. Our new zircon U-Pb age of 11.86 ±
0.13 Ma substantiates correlation of the Ashfall Fossil Beds deposit to tuffs originating from
the Bruneau-Jarbidge caldera (~12.7–10.5 Ma). Our U-Pb zircon age of 6.42 ± 0.06 Ma for
the Grove Lake ash bed coincides with supervolcanic activity in the Heise volcanic field
(6.6–4.3 Ma) in eastern Idaho. These new dates improve age constraints of strata comprising
the Ogallala Group and the important paleontological site. Moreover, we find that detrital
and airfall zircons are unevenly distributed in the stratified ash beds we describe herein and
presumably in similar deposits worldwide. Therefore, a higher-resolution sampling scheme
is necessary in such cases.
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2018-11-08
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Public Library of Science
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Smith JJ, Turner E, Mo¨ller A, Joeckel RM,
Otto RE (2018) First U-Pb zircon ages for late
Miocene Ashfall Konservat-Lagersta¨tte and Grove
Lake ashes from eastern Great Plains, USA. PLoS
ONE 13(11): e0207103. https://doi.org/10.1371/
journal.pone.0207103