Late Ordovician (Katian) conodont community analysis and anoxic shallow water origin of organic-rich black shales, Red Head Rapids Formation, Southampton Island, Canadian Arctic

Abstract

During the Late Ordovician, the Laurentian craton was characterized by extensive carbonate facies marking a major Katian transgression, which was followed by carbonates and evaporites associated with the Hirnantian/Gamachian regression and sea level drawdown in response to the Gondwana glaciation. Regionally, the Late Ordovician was also marked by periods of organic-rich black shale deposition indicating an anoxic environment, which many previous authors have interpreted as linked to transgression/highstand. This study examines three organic-rich black shale intervals (lower, middle and upper) in the lower Red Head Rapids Formation on Southampton Island, Canadian Arctic, which were deposited in the restricted Hudson Bay Basin during the late Katian. These shales represent the important hydrocarbon source rocks in the basin, and they are best exposed on the island, the northern margin of the basin during the early Paleozoic. This study uses a statistical analysis of conodont (microfossil) data from five stratigraphic sections on Southampton Island covering the three organic-rich black shale intervals and the strata below (Churchill River Formation) and above (middle and upper Red Head Rapids Formation) to establish the pattern of conodont communities before, during and after the deposition of these black shale intervals, and interprets their relationship with sea-level events. This pattern reveals that all three shale intervals were deposited during a phase of regression. Although the conodont community in the lower organic-rich black shale interval reveals a short-lived sea-level rise with an influx of Iapetus Ocean waters during this major regression, those in the middle and upper organic-rich intervals are linked to a regression/lowstand. Therefore, the three organic-rich black shale intervals of late Katian age on Southampton Island provide a definitive example of shallow water anoxia, rather than deposition within transgression/highstand; possible causes for the anoxia are discussed. • Three organic-rich black shales, Red Head Rapids Fm of late Katian, Southampton Is. • Their Total Organic Carbon (TOC) content (9–34%) made them important source rocks. • Cluster analysis identified five conodont communities from their relevant strata. • Conodont communities from the shales indicate a shallow water anoxic environment. • Anoxic conditions occurred during periods of regression and basin restriction.