The Early Silurian–Early Devonian Arisaig Group, in the Avalon terrane of Nova Scotia, consists of a thick (∼1900 m) sequence of unmetamorphosed fossiliferous siliciclastic strata that unconformably overlies the 460 Ma bimodal Dunn Point Formation volcanic rocks and is unconformably overlain by basalts and red clastic rocks of the McArras Brook Formation. The Dunn Point volcanic rocks were deposited when Avalonia was a microcontinent, in a New Zealand–type arc setting ∼1800 km north of Gondwana and 1700–2000 km south of Laurentia. Geochemical, Sm-Nd, and U-Pb (detrital zircon) isotopic data of all Arisaig Group strata show fundamental differences from the underlying Avalonian rocks, indicating that they were not derived from Avalonian basement. These data are instead compatible with derivation from Baltica, implying that Avalonia had accreted to Baltica by the earliest Silurian and that the Arisaig Group is part of a clastic sequence that has overstepped Appalachian-Caledonide terrane boundaries. The lack of penetrative deformation and the approximately concordant nature of the contact between the Dunn Point Formation and the Arisaig Group suggest that this portion of Avalonia was located on the trailing edge of the Avalonia plate during the collision. Regional syntheses suggest that the basin was initiated by local transtension during oblique sinistral collision between Avalonia and Baltica. An overall trend toward increasingly negative ϵ Nd values in the clastic rocks toward the top of the Arisaig Group is thought to reflect increasing input from Laurentia by the time of deposition of the Early Devonian strata. The basin also preserves evidence of loading in the Late Silurian, which is thought to reflect the development of a foreland basin and the ongoing shortening across the orogen associated with the onset of the Acadian orogeny. The unconformity between the Arisaig Group and the overlying McArras Brook Formation is the local expression of the deformation associated with Acadian orogeny in the Antigonish Highlands. The orientation and style of Acadian deformation preserved in the Arisaig Group is compatible with dextral movement along major northeast-trending faults, consistent with evidence of regional dextral shear along the northern margin of the Rheic Ocean in the Middle Devonian. Late Devonian–early Carboniferous deposition of the predominantly continental clastic rocks of the Horton Group occurred around the Antigonish Highlands in a series of grabens and half-grabens, most notably represented by the St. Mary's basin, which originated by dextral shear along the boundary between the Meguma and Avalon terranes. Continued episodes of dextral shear in the late Carboniferous resulted in localized regions of transtension and basin development, and also in episodes of transpression, manifested by intense deformation, thrusting, and S-C fabric development. Taken together, Middle Devonian–late Carboniferous episodes of dextral shear reflect the local accommodation of oblique convergence and eventual collision between Gondwana and Laurussia.

The geological history of the Arisaig region begins with the poorly dated strata of the Browns Mountain Group. Based on fragmentary inarticulate brachiopods, the Browns Mountain has been tentatively dated as Lower Ordovician. The Browns Mountain beds were tightly folded and intruded by a variety of granitic rocks prior to Silurian time. At School Brook Cove, Cape George (Boucot and others, 1959) fossiliferous Middle or Upper Ordovician quartzites occur, which are also intruded by granitic rocks. Unfortunately, the relations of the fossiliferous rocks at School Brook Cove to the adjacent Browns Mountain Group are not established. BEARS BROOK VOLCANIC GROUP Dunn Point Formation The Bears Brook Volcanic Group was formed under predominantly terrestrial conditions, probably during the Upper Ordovician. Subaerial conditions are proved by the complete lateritic soil profiles and the welded tuffs, and it seems likely that the lavas and sediments were emplaced on a basement consisting of the Browns Mountain Group intruded by monzonite stocks. During Dunn Point Formation time, andesites were extruded and flowed southward from a hinterland to the north in a series of sheets and tongues to build up a tableland. The lateritic soil profiles developed on individual andesite flows give information on the time intervals between flows and on the climate of the period. Thickness of laterite profiles is a function of the time interval between flows, and the absence of a laterite on a flow indicates rapid extrusion of successive flows. The climatic factors indicated by the lateritic soils may be summarized as . . .