Mixed siliciclastic-carbonate ramp sediments and coral bioherms of the Late Ordovician Lourdes Formation, western Newfoundland: Sedimentology, stratigraphy, and tectonic significance

The Late Ordovician Lourdes Formation (Long Point Group) of the Port au Port Peninsula, western Newfoundland, preserves a record of short-lived (∼5-7 m.y.), mixed siliciclastic-carbonate sedimentation on a narrow, high-energy ramp. The ca. 75 m-thick formation is divided into three members. In ascending order, they are; the dominantly peritidal Shore Point Member, the mostly shallow subtidal, carbonate Black Duck Member, and the more argillaceous Beach Point Member. The lower two members are capped by prominent paleokarst surfaces. Deposition of the Lourdes Formation post-dates a 6-8 m.y. hiatus; renewed deposition onlapped allochthonous strata on the orogen side of the local Taconic foreland basin. Facies across the platform range from peritidal carbonates and sandstones and shallow-ramp bioherms and skeletal shoals, to deeper calcareous shales. Cyclicity is evident on at least three scales, including m-scale shallowing-upward cycles, decameter-scale depositional sequences and larger-scale sequences. An overall deepening trend is identified, culminating with the deeper-water siliciclastic sediments of the overlying Winterhouse Formation. The demise of the carbonate ramp was likely caused by increased influx of sand and mud linked to tectonism along the basin margin. Coral bioherms within the uppermost Black Duck Member exhibit complex internal structures, including framework, boundstone, and suspension deposit elements. The bioherms exhibit classic stages of reef growth, including stabilization, colonization and diversification. With rigid frameworks built mostly by the stacking and intergrowth of large corals, bioherms of the Lourdes Formation are some of the oldest known reefs to show true dominance of a framework construction strategy over binding and baffling strategies that predominate in younger reefs.; This is the first detailed study of the Lourdes Formation, characterizing the initiation, development and demise of the mixed siliciclastic-carbonate ramp system. The unique depositional setting in a post-collisional Taconic remnant basin allows comparison of the Lourdes carbonate ramp with equivalent Late Ordovician epicontinental foreland platforms in eastern North America and offers a unique opportunity to explore the influence and timing of sea-level change related to ongoing local and regional tectonism and eustasy on patterns of platform development relative to patterns defined within epicontinental platform successions across the foredeep in eastern Laurentia.