First U-Pb zircon ages for late Miocene Ashfall Konservat-Lagerstätte and Grove Lake ashes from eastern Great Plains, USA

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.

Chapter 8

“…The illite contents range from 3 to 5 %, with an exceptional value of approximately 16%, so the terrigenous contribution was generally very low, except for the wind-transported volcanic ash beds, which were dispersed on both sides of the basin. The U-Pb dating of distal volcanic ash zircons transported by wind has also been reported from the Cretaceous of Australia by Barham et al (2016) and from the Miocene of North America by Smith et al (2018).…”

Large-magnitude (VEI ≥ 7) ‘wet’ explosive silicic eruption preserved a Lower Miocene habitat at the Ipolytarnóc Fossil Site, North Hungary

“…While subsequent to these young eruptions the massive tephra cover has buried the paleotopography, at ancient settings the millions of years of erosion can bring buried habitats to the surface. This is the case, for example, with the Permian Chemnitz petrified forest in Germany 44 , the Cretaceous Jihol biota in Liaoning Province, China 45 , or the Miocene Ashfall Fossil Beds, Nebraska, USA 1 , 46 . Our study on Ipolytarnóc adds a poorly preserved but still reconstructible large Early Miocene explosive silicic eruption that preserves an important paleohabitat to the global picture.…”

A red bole zircon record of cryptic silicic volcanism in the Deccan Traps, India

“…The nearest documented convergent margins at the time are the intra-oceanic Trans-Tethyan subduction zone located ∼10°N, or ∼2000 km north of India, and subduction beneath the Eurasian continental margin at ∼20°N (Martin et al, 2020). Such distances are not entirely implausible for the deposition of zircon-bearing distal volcanic ash (e.g., ∼1400 km in Smith et al [2018]), but zircon delivery from these arcs to central India would have required repeated, large, explosive eruptions and a peculiar atmospheric circulation pattern. However, the observed low ε Hf in red bole zircons effectively excludes intra-oceanic subduction sources (in the Trans-Tethyan subduction zone or elsewhere) due to the youthfulness and isotopically juvenile nature of all components involved in island arc petrogenesis (ε Hf of +5 to +16 in the Kohistan-Ladakh Arc; Bouilhol et al, 2013).…”