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ABSTRACT

The Trenton Group (Late Ordovician), the youngest carbonate unit in the Taconic foreland basin of southern Quebec, is a tripartite unit with a distinctive coarse-grained middle part, the Deschambault Formation.

Lithofacies of the Deschambault Formation are dominated by coarse-grained bioclastic/intraclastic limestones; finer-grained lithofacies are ubiquitous but subordinate. The complete spectrum of lithofacies indicates sedimentation ranging from above fairweather- to below storm-wave base. Skeletal components are indicative of the modern temperate-water bryomol association. Non-skeletal elements are represented by peloids and intraclasts. Accretion rates from areas of continuous sedimentation were low (<14 cm/103 years). From sedimentological and faunal evidence, it is proposed that the Late Ordovician Deschambault ramp was bathed by temperate waters. The model compares favourably with modern cool-water shelves rimming the southern edge of the Australian continent.

Palaeomagnetic data locate southern Quebec in a low latitudinal setting during the Late Ordovician. Upper Ordovician facies distribution in eastern Canada and progressive disappearance of some faunal provinces through Late Ordovician time are used to conclude that the initiation of the Late Ordovician glaciation that covered most of Gondwana was instrumental in easing northward movement of cold oceanic currents. This resulted in the rapid contraction of the southern hemisphere warm-water tropical belt from a 30° latitudinal-wide zone in the early Caradoc to a 15° zone in the late Caradoc.