OCEAN REDOX CONDITIONS IN A LESS OXYGENATED ORDOVICIAN WORLD: A CASE STUDY ON THE NON-EUXINIC KATIAN ORGANIC-RICH MUDROCKS OF SOUTHERN ONTARIO

Upper Ordovician (Katian) organic-rich mudrocks (ORM) in southern Ontario, the Collingwood Member of the upper Lindsay Formation and Rouge River Member of the lower Blue Mountain Formation, have received much attention due to their potential for hydrocarbon resources. The two ORM units, influenced by the concurrent Taconic Orogeny, were deposited in the foreland Appalachian Basin and intracratonic Michigan Basin, but the local paleoredox conditions are not well constrained. Here, we collected samples from the Collingwood (n = 53) and Rouge River members (n = 184) from multiple drillcores throughout southern Ontario. Redox-sensitive trace metals (RSTM) and total organic carbon (TOC) were analyzed to study spatiotemporal variations in local redox conditions.

The Collingwood Member (ca. 450 Ma) is a calcareous ORM (Ca > 10%). These rocks have higher average TOC in the St. Joseph (SJ, 6.8 wt.%) and Little Current (LC, 7.4 wt.%) cores compared with the Collingwood (CW, 3.8 wt.%) and Mount Forest (MF, 1.6 wt.%) cores. The Mo/U EF ratios (EF = Al-normalized enrichment factor) from SJ and LC are mostly close to the modern seawater ratio (Mo/U_SW) without pronounced Mo (EFs <30) and U (EFs <10) enrichments, suggesting a suboxic to anoxic/non-sulfidic environment with no clear evidence of water column euxinia. The Mo/U_SW ratios from CW and MF are 0.1-1 and <0.3, respectively, pointing to predominately oxic-suboxic and sporadic anoxic/non-sulfidic bottom waters. The overlying Rouge River Member (ca. 449 Ma) is a siliciclastic-dominated ORM (Ca < 6%) with an average TOC content of ~2 wt.% from six drillcores. The Mo/U_SW ratios (0.1-1) from the Port Stanley, Pickering, SJ, and CW cores suggest fluctuating bottom water redox conditions from oxic to anoxic (non-sulfidic) whereas a narrow range of Mo/U_SW ratios (0.3-0.7) suggests pervasive suboxic conditions at the Chatham and MF localities. These interpretations are consistent with Mo/Re EF ratios.

Overall, the RSTM data suggest regional spatiotemporal variations in redox conditions for the Collingwood Member and Rouge River Member. These units represent a case study of the less oxygenated Ordovician world, during which a larger extent of suboxic-anoxic seafloor was likely compared with today. Future work will examine ocean redox conditions from the perspective of metal isotopes.