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.

During the summer of 1959, a geological reconnaissance of the Lochaber area in Antigonish County (Fig. 1) was made by several of the students at the Massachusetts Institute of Technology summer camp under Boucot’s supervision. Although the structural and stratigraphic results are less conclusive then those from nearby areas, this location is geologically strategic, and it is appropriate to list some of the facts observed. A suite of sedimentary, volcanic, and intrusive rock types lying to the west of Lochaber (PL 9) is similar to those assigned elsewhere in Antigonish and Pictou Counties to the Browns Mountain Group. To the east of the rocks assigned to the Browns Mountain Group (Pl. 10), there is a sequence of greenish siltstones and slates, together with a scattering of shelly limestones, which can best be referred to as “Arisaig Group undifferentiated.” These beds have provided enough diagnostic fossils to demonstrate that they include representatives of the following: French River Formation, McAdam Brook Formation?, Moydart Formation (green member only), and the Stonehouse Formation. Table 8 details the brachiopods found in these strata. East and southeast of the beds assigned to the Arisaig Group, a sequence of siltstones and fine-grained sandstones that are predominantly reddish are assigned to the lithologically similar Knoydart Formation. The contact with the underlying Arisaig Group is gradational on Cameron Brook (Pl. 10) as well as on the two unnamed brooks to the north of it. The gradational contact consists of an interbedding over a stratigraphic interval of . . .

Purpose and Scope of the Investigation The Arisaig area (Fig. 1; Pl. 1) has been selected for detailed study because it provides the most continuous and best exposed sections of marine Silurian and early Lower Devonian rocks in the Appalachian Mountain system. The earliest fossiliferous rocks present are lower Llandovery in age, and it is probable that the whole of the Silurian is represented in the 4,000 ft of beds that make up the Arisaig Group; but the underlying Bears Brook Volcanic Group has no known fossils, and the Silurian rocks are faulted against the Ordovician Browns Mountain Group. It is not clear, therefore, how large a stratigraphic break, if any, is present at the base of the Arisaig Group. The marine Silurian succession passes without a break up into marine Lower Devonian; the system junction comes near the top of the Stonehouse Formation. The basal marine Devonian beds are followed conformably by the nonmarine Knoydart Formation (about 1,000 ft thick) which contains Lower Devonian (upper Downtonian to middle Dittonian) fish remains. Special studies have also been made of the Bears Brook Volcanic Group and of the structure of the area; repeated movements along the Hollow Fault have resulted in complex folding and faulting in the inland parts of the area. The village of Arisaig (Fig. 1) lies on Northumberland Strait, 15 mi northeast of Antigonish, Nova Scotia. It is now easily accessible from New Glasgow or Antigonish by Nova Scotia Route 45. The area studied extends along the coast from McAras Brook (3 mi southwest . . .

The authors are indebted to C. W. Harper, Jr:, for the use of material from his doctor's thesis on the brachiopods of the Arisaig Group. In that study, he identified many of the small collections that have helped to make correlation possible between the shore and the inland areas. W.B.N. Berry and M. J. Copeland have greatly assisted by identifying the graptolites and the ostracods, respectively, found at Arisaig. R. K. Bambach has generously contributed his knowledge of the Arisaig bivalves; he has also permitted examination of Twenhofel's field notebook. The chapter on the Knoydart has greatly benefited from the work of R. H. Denison, D. J. MacNeil, A. Suparman, and M. Tauchid. John F. Dewey gratefully acknowledges financial support from the University of Manchester, the Royal Society of London, and the Nova Scotia Research Foundation. D. Kelley has been generous with information on the Cobequid Mountains. We are indebted for funds received by W. S. McKerrow from the Nova Scotia Research Foundation, the Royal Society of London, and the Geological Society of London Tyrrell Fund, and by D. L. Dineley from the Geological Survey of Canada and the National Museum of Canada. McKerrow spent the year 1964–1965 as a post-doctoral Research Fellow at the California Institute of Technology with support from the National Science Foundation (Grants GP-2290 and GP-3743). Acknowledgment is made in the list of maps to those Massachusetts Institute of Technology students who contributed to the brook maps presented in this report, but thanks are due