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The depositional environment of the Kettle Point Formation, a southwestern Ontario black shale; Interpretations from core and sulphur isotope analyses


Despite the recent interest in black shales as alternative hydrocarbon reservoirs, the ancient depoformational environments of these rocks remains enigmatic due in large part to a lack of modern analogs. This study aims to help elucidate the mechanisms behind the deposition of black shales by using the Upper Devonian Kettle Point Formation, located in southwestern Ontario, as a case study. The Kettle Point Formation was deposited in an epeiric sea during the Acadian Orogeny and is paleogeographically situated in the Chatham Sag; a local depression formed by the downward plunging of the Algonquin and Findlay Arches and situated between the Appalachian and Michigan Basins. It is syndepositional with several mudstone units in the eastern United States including the Ohio, Antrim, New Albany, and Chattanooga shales.

Detailed core and thin section analyses resulted in the subdivsion of the Kettle Point Formation into three lithofacies: interlaminated black shales that occur interbedded with organic-poor greyish green mudstones, and are separated by thick intervals of non-interlaminated black shales. Subsurface correlation of the formation using gamma ray logs and detailed core descriptions, has revealed thickness variations of these lithofacies that suggest the influence of local tectonic features on their depositional patterns. Major differences in lithology within the succession are attributed to changes in the intensity and vertical extent of anoxia in the marine water column that culminated in the deposition of the thick packages of non-interlaminated black shales during the acme of anoxia. Interbedded interlaminated black shales and greyish green mudstones record lower intensities of oxygen deficiency overall, but fluctuating at a finer level between anoxic and dysoxic conditions respectively. Preliminary sulfur isotope data from over 50 samples spread throughout the formation show a largely consistent background value for the black shales around −20 δ34S. This is punctuated by a substantial positive excursion of ∼32 ‰ (up to 12.87) correlated with a significant interval of greyish green mudstones in the upper part of the Kettle Point. This positive shift in δ34S has also been noted in several other Upper Devonian deposits and is thought to be associated with the onset of the Frasnian-Famennian mass extinction event.