Figure 1. Paleogeographic reconstruction of the Late
Ordovician. Modified from Melchin et al., 2013. Study site localities
are marked as yellow stars, with the locations of other study areas
shown in black circles.
The first study site is a thick succession outcropping in the Monitor
Range, Nevada, where high-resolution
δ13Ccarb records have previously
documented the HICE (Finney et al., 1997, 1999; Jones et al., 2016).
This section consists of the Late Ordovician Hanson Creek Formation as
well as the early Silurian Roberts Mountain Formation (Finney et al.,
1997). These marine carbonates were deposited in an upper slope setting
on the western passive margin of Laurentia in the Panthalassic Ocean and
contain facies that range from deep water thinly laminated lime
mudstones and bedded cherts to wave agitated cross-laminated oolitic
grainstones (Finney et al., 1997). The second study site is a thick
succession recorded within the Kardla drill core, southern Estonia
consisting of the Late Ordovician Jonstrop, Halliku, Kuldiga, and Saldus
formations, and the early Silurian Õhne Formation (Kaljo et al., 2001).
These carbonates were deposited along the southern margin of Baltica
with direct connection to the Iapetus Ocean and contain facies ranging
from offshore argillaceous wackestones to shallow water oolitic
grainstones (Kiipli and Kiipli, 2020). Last, the Pointe Laframboise
section on Anticosti Island, Quebec, contains the Late Ordovician Ellis
Bay and early Silurian Becscie formations. This is a mixed siliciclastic
and carbonate succession deposited on a storm-dominated carbonate ramp
in a foreland basin on eastern Laurentia (Desrochers et al., 2010).
Facies range from distal bioturbated mudstones through shallow patch
reef limestones. Each of our study localities has been the subject of
extensive biostratigraphic and
δ13Ccarb chemostratigraphic studies,
allowing for correlation between the sections (supplemental material Fig
S1).
However, detailed correlations of stratigraphic sections from
hydrographically separate paleobasins remain difficult in any study,
especially when comparing sections with biostratigraphic zonation based
on different taxonomic groups. Here we base our Late Ordovician–Early
Silurian stratigraphic correlations on available graptolite, conodont,
and chitinozoan biostratigraphy, as well as the high-resolution
δ13Ccarb trends. The Monitor Range
section contains diagnostic Late Ordovician graptolites (e.g.,Metabolograptus extraordinarius and Metabolograptus
persculptus biozones) and conodonts (Amorphoganathus ordovicicusbiozone) (Murphy et al., 1979; Finney et al., 1997, 1999), which provide
direct and reliable high-resolution integration of this section with the
latest calibrated Ordovician Time Scale-2020 (Goldman et al., 2020). The
other sections, western Anticosti Island and the Kardla drill core,
contain diagnostic conodonts and chitinozoan biozones (Brenchley et al.,
2003). Both sections contain the chitinozoan Belonechitina
gamachiana and Spinachitina taugourdeaui biozones, allowing for
precise correlation between these two localities. Because of the lack of
a common biostratigraphic scheme, we also rely on the carbon isotope
stratigraphy previously documented from each study site to augment some
issues in integrating differing biostratigraphic schemes from these
three successions (see supplemental materials for more information).