Abstract
This paper documents the first U-Pb zircon ages for Ashfall Fossil Beds (Nebraska, USA), a terrestrial Konservat-Lagerstätte 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.
Highlights
Ashfall Fossil Beds State Historical Park (Fig 1), a U.S National Natural Landmark, is a terrestrial Konservat-Lagerstätte mass-death assemblage composed of Miocene amphibians, reptiles, birds, and mammals within an ~3-m-thick vitric ash deposit [1, 2]
AFB-00 yielded 136 zircon grains and the youngest concordant grouping of zircons with overlapping ages at 2σ is composed of 34 grains yielding a concordia age of 11.86 ± 0.13 Ma with a MSWD of 1.3 (Fig 6A and 6B). This is interpreted as a crystallization age and the maximum depositional age (MDA) of subsequent volcaniclastic deposition
The Cougar Point Tuff (CPT) VII is one of nine densely welded ignimbrites originating from the Bruneau-Jarbidge caldera that are best exposed in canyons along the Bruneau and Jarbidge rivers in southwestern Idaho [66]
Summary
Ashfall Fossil Beds State Historical Park (Fig 1), a U.S National Natural Landmark, is a terrestrial Konservat-Lagerstätte mass-death assemblage composed of Miocene amphibians, reptiles, birds, and mammals within an ~3-m-thick vitric ash deposit [1, 2]. K-bentonites as their weathered equivalents, are critical geochronological marker horizons because tephrochronologic correlations or the radiometric dating of volcanic minerals of airfall tephra constrain the depositional ages of host strata and are significant for the development of a regional chronostratigraphy Tephrochronology is a geochemical technique that correlates distal ashfall deposits to well-dated proximal volcanic tuffs through the electron probe microprobe analysis of glass shards Conventional wisdom maintains that preferred phenocrysts for the direct radiometric dating of ash beds, such as zircon and sanidine, are often rare or absent in distal airfall deposits [17]
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