Figure 2. a, Geologic Map of the Hope Bay area (slightly modified from Montes et al., 2019) indicating the locations of the geochronology samples (stars) and of the main points for plant collection. b, Composite stratigraphic column of the Mount Flora Formation.
3 Materials and Methods
3.1 Stratigraphy and sample collection
Stratigraphic measurements were made along multiple transects on the northern slopes of Mount Flora overlooking Hope Bay and were assembled into a single stratigraphic column (Fig. 2b). Bed thickness, sedimentary/volcanic structures and rock facies were described in the field and recorded along with GPS coordinates (Text S1; Fig. S1) Samples from the tuffaceous interval of the lower member (Miembro Conglomerados) were collected for petrographic analysis in the lab (Fig. S2) and for U-Pb geochronology (Table S1). More than 1400 fossil plant specimens were collected from multiple locations throughout the Mount Flora area, from both in situ outcrops and drifted glacial blocks.
3.2 U-Pb Geochronology
Two samples from the tuffaceous interval (unidad 4a of the Miembro Conglomerados -Montes et al., 2019) of the Mount Flora Formation were chosen for U-Pb geochronology. Sample 22-1-18-5 was from a 16 meters-thick ignimbrite flow from the northeastern flank of Mount Flora, which exposes the most expanded section of the tuffaceous interval (Figs. 3; S1)). The tuffaceous interval thins out towards west and its lower part disappears laterally within a few tens of meters. Sample 24-1-18-1 is a crystal lithic lapilli tuff collected from approximately 290 meters to the west along strike from 22-1-18-5 and may be considered a lateral facies of the ignimbrite. Detailed stratigraphy of the tuffaceous interval and petrographic descriptions of the geochronology samples are given in the Supplementary Information.
High-precision U-Pb zircon analyses by the CA-ID-TIMS technique were conducted at the Massachusetts Institute of Technology Isotope Laboratory using procedures described in Ramezani et al. (2011). Samples were crushed and pulverized by standard techniques and heavy minerals were separated using a Frantz® isodynamic magnetic separator and high-density liquids. Final selection of zircons for analyses was made under a binocular microscope and based on morphological criteria including faceted prismatic habit, high aspect ratio and presence of elongated glass (melt) inclusions parallel to the crystallographic “C” axis (Fig. S3), which have proven effective in screening out reworked zircons. Selected grains were pretreated by a chemical abrasion technique modified after Mattinson (2005), which involved thermal annealing at 900oC for 60 hours before partial dissolution in 28M hydrofluoric acid at 210oC in a high-pressure digestion vessel for 12 hours. After thorough fluxing and rinsing to remove the leachates, the zircons were spiked with the EARTHTIME ET535 mixed U-Pb tracer solution (Condon et al., 2015; McLean et al., 2015) and completely dissolved in 28M HF at 210oC for 48 hours. Chemically purified Pb and U via anion-exchange column chemistry were subsequently analyzed on an Isotopx X62 thermal ionization mass spectrometer equipped with 9 Faraday detectors and a Daly ion counting system. Data reduction and error propagation were conducted using Tripoli and ET_Redux software (Bowring et al., 2011; McLean et al., 2011). Complete data are given in Supplementary Table S1.