Revised stratigraphy of the middle Simcoe Group (Ordovician, upper Sandbian–Katian) in its type area: an integrated approach

Abstract

The Upper Ordovician Bobcaygeon Formation of southern Ontario is a widespread unit that spans the Sandbian–Katian stage boundary and contains exceptionally preserved invertebrate fossil assemblages, including the famed ‘Kirkfield echinoderm fauna.’ However, the precise correlation of this interval remains poorly understood. This paper presents new data on high-resolution sequence and carbon isotope chemostratigraphy of the Bobcaygeon interval based on new quarry exposures and introduces refined definitions of unit boundaries based on allostratigraphic criteria. Sequence, chemo-, and biostratigraphic evidence indicate that the Bobcaygeon Formation represents a composite unit as it encompasses a major erosional unconformity. The Coboconk and Kirkfield formations, described in the early 20th century, were merged into a single unit, the Bobcaygeon, out of concern that the original lithostratigraphic divisions would be conflated with biostratigraphic zones of the same names. However, these biostratigraphic zones are no longer favoured, and the lower member of the Bobcaygeon is here elevated again to the status of formation (Coboconk Formation) and represents the uppermost portion of the Sandbian M4 sequence. The middle and upper members of the Bobcaygeon, herein reassigned to the Kirkfield Formation, represent the upper Sandbian to lower Katian M5A and M5B sequences recognized widely in the eastern and central United States. The term Bobcaygeon is retained and elevated to the rank of subgroup. The Kirkfield Formation is divided into three members and contacts are refined, placing a 1–2 m transgressive grainstone at the base of each sequence. These units are correlated with equivalent strata of New York and the Cincinnati Arch.

Biodiversity, systematics, and new taxa of cladid crinoids from the Ordovician Brechin Lagerstätte

“…Given the mercurial history of stratigraphic nomenclature of the region, we continue to follow the lithostratigraphic nomenclature of Armstrong (2000), which is also consistent with our previous work in this series (Ausich et al, 2018; Cole et al, 2018). However, we note that crinoid-bearing horizons considered Bobcaygeon herein are equivalent to the Kirkfield Formation sensu Paton and Brett (2019). Interested readers should see Cole et al (2018) and Paton and Brett (2019) for additional discussion on issues surrounding the current stratigraphic nomenclature, geological context, sedimentology, and paleoenvironmental interpretations of these strata.…”

“…However, we note that crinoid-bearing horizons considered Bobcaygeon herein are equivalent to the Kirkfield Formation sensu Paton and Brett (2019). Interested readers should see Cole et al (2018) and Paton and Brett (2019) for additional discussion on issues surrounding the current stratigraphic nomenclature, geological context, sedimentology, and paleoenvironmental interpretations of these strata.…”

“…As noted by Cole et al (2018), the stratigraphic nomenclature for the southern Ordovician of Ontario has been in a state of flux since Springer's (1911) classic publication and has yet to stabilize (Armstrong, 2000; Paton and Brett, 2019). Consequently, many species reported from these strata have previously been linked with a geological potpourri of named stratigraphic units, including the Hull, Kirkfield, and Cobourg formations.…”

“…Consequently, many species reported from these strata have previously been linked with a geological potpourri of named stratigraphic units, including the Hull, Kirkfield, and Cobourg formations. Paton and Brett (2019) recently redefined the Coboconk and Kirkfield formations, suggested the Bobcaygeon be subdivided into these units, and elevated “Bobcaygeon” to a subcategory within the Simcoe Group. Given the mercurial history of stratigraphic nomenclature of the region, we continue to follow the lithostratigraphic nomenclature of Armstrong (2000), which is also consistent with our previous work in this series (Ausich et al, 2018; Cole et al, 2018).…”

Paleocommunity composition, relative abundance, and new camerate crinoids from the Brechin Lagerstätte (Upper Ordovician)

“…Stratigraphic division and nomenclature for this region have been in flux for many years, and as a result, crinoids from the same or equivalent strata have been associated with multiple formation names, including the Hull, Kirkfield, Cobourg, Bobcaygeon, and Verulam formations (Liberty, 1969; Uyeno, 1974; Armstrong, 2000; see Swisher et al, 2015). Although the Bobcaygeon and Verulam formations have been the favored stratigraphic divisions in recent years (Armstrong, 2000), work by Paton and Brett (2019) suggested subdividing the Bobcaygeon into the Kirkfield and Coboconk formations and elevating the Bobcaygeon to a subcategory within the Simcoe Group. To maintain consistency with our previous work on the Brechin Lagerstätte, we continue to follow the stratigraphic nomenclature of Armstrong (2000) that uses the Bobcaygeon and Verulam formations.…”