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TitleSedimentology and stratigraphy of the Kettle Point Formation: implications for widespread marine anoxia and the extensive deposition of Upper Devonian black shales in eastern North America
AuthorBingham-Koslowski, N; Jin, J; Tsujita, C; Azmy, K
SourceAtlantic Geoscience Society, Special Publication (2017), 2017 p. 18-19
Alt SeriesNatural Resources Canada, Contribution Series 20170241
PublisherAtlantic Geoscience Society
Meeting43rd AGS Colloquium and Annual Meeting; Fredericton; CA; February 10-11, 2017
Mediapaper; on-line; digital
File formatpdf
Subjectssedimentology; sulphur isotope ratios; isotopic studies; mudstones; black shales; deposition; depositional environment; depositional history; organic residues; lithofacies; Kettle Point Formation; Devonian
ProgramBaffin Region Atlas, GEM2: Geo-mapping for Energy and Minerals
LinksOnline - En ligne
AbstractThe Upper Devonian Kettle Point Formation of southwestern Ontario is characterized by intervals of organic-rich interlaminated black shale interbedded with organic-poor greyish green mudstones and rare red beds, separated by metre-scale sequences of non-interlaminated black shale. The formation shows a largely consistent background value for the black shales around -20(permil)
d34S, punctuated by a substantial positive excursion of ~32(permil) (up to +12.87(permil)) coincident with a significant section of greyish green mudstone and red beds. Organic content, in conjunction with the sulphur isotope data, indicate that the black shales were deposited during periods of anoxia with thick intervals of non-interlaminated black shales representing the acme of anoxic conditions. Greyish green mudstones, with their lack of organic-content, increased bioturbation, and higher, more-positive, sulphur isotope values, therefore record deposition in more oxygenated environments. A new 4-stage, 2 cycle depositional model for the Kettle Point Formation is proposed with relative water depth interpreted as the key control on the vertical diffusion of oxygen in the water column, and therefore on the distribution of the Kettle Point lithofacies. Interbedded black shales and greyish green mudstones were deposited in relatively shallow waters where minor, short-lived falls in relative sea level promoted dysoxic to oxic conditions and the deposition of organic-poor lithologies. Non-interlaminated black shales are indicative of substantial rises in relative sea level, resulting in widespread anoxia and the deposition of thicker and more laterally extensive packages of organic-rich sediment. The sedimentology and stratigraphy of the Kettle Point Formation and other syndepositional black shales, suggests that the extensive deposition of organic-rich sediment across eastern North America during the Late Devonian was a product of widespread anoxia related to fluctuating water depth and restricted circulation in intracratonic and foreland basin depositional centers.
Summary(Plain Language Summary, not published)
The Upper Devonian (380-360 mya) Kettle Point Formation in southwestern Ontario is composed of alternating layers of black and green mudrocks. The change in colour reflects the presence (black) or absence (green) of organic material within the rocks. The preservation of organic material in shales is primarily controlled by oxygen content in the water column, as oxygen breaks down organic matter and therefore black shales represent deposition in an oxygen-free (anoxic) environment. The main factors influencing water column oxygen levels during the deposition of the Kettle Point Formation were determined to be fluctuating water depths and the development of restricted circulation. Upper Devonian black shales are not limited to southwestern Ontario and the presence of extensive, similar-aged, organic-rich deposits throughout eastern North America suggests that the conditions responsible for the deposition of the Kettle Point shales (anoxia, restricted circulation) were widespread.