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).