Northern Appalachians: Avalon and Meguma Terranes

Possible correlatives of the West African Orogens in the northern Appalachians occur along the eastern seaboard of North America, where they comprise the Avalon Composite Terrane and the Meguma Terrane. The Avalon Composite Terrane is defined by the presence of a lithostratigraphically correlative, subaerial-shallow marine, Cambro-Ordovician overstep sequence containing an Acado-Baltic fauna (Keppie 1985). In general, these Cambro-Ordovician rocks are only preserved in outliers, so overstep relationships are inferred from their close lithostratigraphic similarity. Where the Cambro-Ordovician rocks have been removed by erosion, it is possible to use the presence of the lithostratigraphically correlative, Silurian-Gedinnian overstep sequence containing the distinctive Rhenish-Bohemian fauna (Boucot 1975). By this definition, the Avalon Zone includes New England southeast of the Hope Valley-Bloody Bluff fault zone; Maine and New Brunswick south of the Turtle Head and Honeydale faults; most, if not all of northern Nova Scotia, and Newfoundland east of the Dover-Hermitage fault (Fig. 1). Terranes with Late Precambrian rocks northwest of these faults, such as the Hope Valley and Nashoba Terranes (O’Hara and Gromet 1985), have not been included in the Avalon Composite Terrane because the distinctive, Avalonian, Early Paleozoic rocks are absent. Should future work show that the Early Paleozoic overstep sequence was present before erosion, these terranes would need to be added to the Avalon Composite Terrane. In the Avalon Composite Terrane, the Early Paleozoic rocks rest unconformably — disconformably — conformably upon a variety of Late Precambrian igneous rocks and turbidites overlying Middle-Late Proterozoic or older metasediments and gneisses.