Fig 1.
Location of study areas of Ashfall Fossil Beds State Historical Park and Grove Lake, Nebraska.
Base map is DEM hillshade downloaded from the from USGS National Map Viewer (open access) at https://viewer.nationalmap.gov/viewer/.
Fig 2.
Stratigraphy of northern Antelope County, Nebraska.
(A) Distribution of Cretaceous and Cenozoic strata in northeastern Nebraska. (B) Composite stratigraphic section of units exposed at the Ashfall site and Grove Lake. Modified from Voorhies [1] and Tucker et al. [2].
Fig 3.
Outcrop photographs of the Ashfall Fossil Beds and Grove Lake ash localities.
(A) Ashfall Konservat-Lagerstätte from inside the Hubbard Rhino Barn showing ash pillars supporting (1) fully articulated fossils of Teleoceras major and (2) underlying medium-sized equid. (B) Ash bed in the Cap Rock Member exposed in the Hubbard Rhino Barn at Ashfall Fossil Beds showing sample locations for this study. (C) Ash bed in the Merritt Dam Member just west of Grove Lake and sample locations for this study. (D) Photograph of the dark colored basal horizon at Ashfall that contains dateable volcanogenic zircons (immediately above dotted line).
Fig 4.
(A) The Snake River Plain Volcanic Province and its constituent volcanic fields and their age ranges of magmatic activity; modified from Perkins and Nash [16]. (B) Apparent accumulation rates (cm/m.y.) of ash beds from Bruneau-Jarbidge and Heise volcanic fields; modified from Perkins [43]. Note approximate locations of source calderas (white triangles), recognized airfall tuff deposits (black dots), and the Ashfall Fossil Beds and Grove Lake study area (white star). Base maps downloaded and modified from the USGS National Map Viewer (open access) at https://viewer.nationalmap.gov/viewer/.
Fig 5.
Measured sections of ash bed deposits in this study.
(A) Ashfall Fossil Beds and (B) Grove Lake sections showing stratigraphic location of samples (AFB and GL, respectively). Some details of Ashfall Fossil Beds measured section (A) after Scheel and Rogers [10].
Fig 6.
Zircon U-Pb results for the AFB-00 sample from Ashfall Fossil Beds.
(A) U-Pb age ranges with 2σ error bars of the 61 youngest zircon grains plotted on a time axis showing 34 concordant analyses with an MSWD of ~1 (in red), used to calculate the MDA. (B) Tera-Wasserburg concordia diagram with MDA result of 11.86±0.13 Ma from 34 concordant analyses shown in 6A. (C) Kernel density estimate plot of AFB-00 showing 206Pb/238U (< 900 Ma) and 207Pb/206Pb (> 900 Ma) ages. MSWD = mean square weighted deviation; n = number of grains analyzed.
Fig 7.
Zircon U-Pb results for the GL-00 sample from Grove Lake.
(A) U-Pb ages ranges with 2σ error bars of the 65 youngest zircon grains plotted on a time axis showing 57 concordant analyses with an MSWD of 1 (in red), used to calculate the MDA. (B) Tera-Wasserburg concordia diagram with MDA result of 6.42±0.06 Ma from the 57 analyses with an MSWD of ~1 from 7A. (C) Kernel density plot of all GL-00 concordant results showing 206Pb/238U (< 900 Ma) and 207Pb/206Pb (> 900 Ma) ages. MSWD = mean square weighted deviation; n = number of grains analyzed.
Fig 8.
Ages and estimated volumes of known supervolcanic eruptions (ejecta volumes > 100 km3) from the Snake River Plain hotspot and reportedly correlated with ash beds on the Great Plains.
Eruption ages and ejecta volumes (estimated with large uncertainties; [49]) were derived from tuffs proximal to source calderas [15, 16, 48]. Colors indicate individual volcanic fields of origin, letters denote dated tuffs discussed in the text that fall within or are close in age with the U-Pb age ranges for sample AFB-00 and GL-00. See S2 Table in Supporting Information for the list of supervolcanic eruptions constituting this figure, modified and updated from Smith et al. [78].