Abstract

The Long Point Group (Caradoc) is the only representative of Late Ordovician foreland basin deposits exposed onshore in the southwestern Newfoundland segment of the Appalachian Orogen. This paper revises the lithostratigraphy of the Long Point Group, necessitating a significant modification to the geological map of the Port au Port Peninsula. It also provides new graptolite-based age data for the Winterhouse Formation; new age data based on shelly fauna for the newly defined Misty Point Formation; and the first interpretation of the sedimentology of these units. The Winterhouse Formation of the Long Point Group (minimum 320 m thick, maximum 860 m) gradationally overlies the Lourdes Formation and is itself gradationally overlain by the Misty Point Formation. The Winterhouse Formation is interpreted as a storm-dominated shelf assemblage, and is characterised by thin- to medium-bedded fine-grained sandstone and siltstone, with minor bioclastic limestone and cobble to boulder limestone conglomerate. The Winterhouse Formation is commonly graptolitic, with species recovered including Climacograptus spiniferus Ruedemann, and Geniculograptus pygmaeus (Ruedemann). The species present and their associations indicate that the Winterhouse Formation is entirely middle to late Caradoc in age. The Misty Point Formation (minimum 390 m thick) is characterized by medium- to coarse-grained red sandstones, which were previously assigned to the Silurian Clam Bank Formation. Deposition took place in a marginal marine to terrestrial setting, which was dominated by unidirectional flow, suggesting a possible braid delta environment. The presence of brachiopods Sowerbyella sericea (J. C. de Sowerby) just below the base of the unit, and Rafinesquina deltoidea Conrad associated with species of Trigrammaria and the trilobite Isotelus walcotti Walcott at a new locality within the Misty Point Formation, indicate that this unit is also of Caradoc age. The large thickness of the Long Point Group has been hitherto unrecognised. Clearly sediment supply to the region was significant in the Late Ordovician, and the character of the deposits suggests that they were largely sourced by unroofed Taconic rocks, supporting suggestions of Late Ordovician tectonic activity in the region.

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