Rocky shoreline deposits in the Lower Silurian (upper Llandovery, Telychian) Chicotte Formation, Anticosti Island, Quebec

Abstract

The upper Llandovery (Telychian) Chicotte Formation is a regionally extensive crinoid-rich unit exposed in the south-central part of Anticosti Island in the Gulf of St. Lawrence. The Chicotte encrinites represent an inner ramp, crinoidal sand-shoal complex (about 80 m thick) that prograded over deeper middle to outer ramp facies of the underlying Jupiter Formation in response to a long-term sea-level fall. The typical depositional unit in the Chicotte Formation is a metre-scale subtidal cycle indicating that higher frequency sea-level changes were also present. Metre-scale cycles are typically characterized by coarsening-upward, locally cross-bedded encrinitic material capped by a sharp erosional surface. These bounding surfaces change progressively upsection from simple planar to low-relief scalloped erosional surfaces to complex polyphase surfaces formed by distinct but superimposed erosional events. Three-dimensional paleomorphological features with local relief up to 50 m associated with erosional surfaces are exposed along coastal and river sections and in places have been partially exhumed by modern erosion along coastal exposures to expose a prominent intraformational unconformity. Paleolandforms include large-scale irregular sea stacks and shallow cliffs, similar to those present along modern rocky shorelines. Reefal limestones with small pockets of well-washed crinoidal sand and brachiopod banks form a narrow shoreline sediment wedge piled against the intra-Chicotte unconformity. The sea-level lowstand recorded within the Chicotte Formation coincides with a major sea-level lowstand recognized elsewhere on several continents during the early to middle Telychian time, indicating a eustatic rather than tectonic origin.