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.
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.