The Long Rapids Formation: an Upper Devonian black shale in the Moose River Basin, northern Ontario

Abstract

The Long Rapids Formation, a sequence of interbedded black shales, green–grey mudstones, and carbonates in the Moose River Basin of northern Ontario, is Late Devonian in age and has counterpart equivalents in the Michigan, Appalachian, and Illinois basins. The Long Rapids Formation has a maximum known thickness of 85 m and can be divided into three informal members. Based on conodont studies, the formation spans the Frasnian–Famennian age boundary.The Long Rapids Formation is an organic-rich, laminated, marine black-shale sequence interbedded with green–grey mudstones and concretionary carbonate interbeds and nodules. The environment of deposition was principally reducing, allowing for preservation of large amounts of organic matter both originating in the overlying water column and derived from terrestrial sources. The depositional basin was stratified, and anoxic bottom waters and oxic surface waters were separated by a pycnocline. The position of the pycnocline (or the absence of it) dictated the type of sediment facies deposited.The Moose River Basin in the Late Devonian was located on the Laurasian Continent, proximal to the equator, in an area where tropical weather conditions prevailed. Conditions were warm and wet along the Appalachian highlands, and farther inland they were warm and dry as a result of the rain-shadow effect. The black shales in the Long Rapids Formation represent a period of transgression of the large epicontinental Catskill Sea, and the green–grey mudstones and carbonates represent periods of minor eustatic or tectonic perturbations.Dark-coloured, organic-rich shale facies (sometimes with abundant bioturbation) are succeeded by lighter coloured shale or mudstone facies and carbonate facies in a repetitive fashion. The facies sequences appear to represent a three-part sedimentary package, or "rhythm", that is very common in the lower two members of the Long Rapids Formation. Bioturbation and total organic carbon content are less in the green–grey shales or mudstones, as well as in the carbonate facies, than in the darker shale facies.