Abstract

Belts of Devonian granitic rocks, having radiometric ages of 405 - 360 m.y., occupy the central core of the entire region. This Acadian orogeny was the major event in the structural development of the eastern part of the Appalachian fold system.

Lower Devonian volcanic rocks (chiefly rhyolite flows, welded tuffs and associated sediments) are widespread throughout the central granitic terrain. The Kineo volcanic sequence of Maine and west-central New Brunswick is Emsian in age (between Onondaga and Oriskany). This volcanic facies changes to a Lower Devonian carbonate facies which characterizes the Gaspe-Connecticut synclinorium. Sedimentary iron formations with a Rhenish faunal facies (Torbrook Formation, Nova Scotia) and carbonates with interbeds of fine-grained pyroclastics (Cape Bon Ami Formation — host rock for copper sulphides at Murdockville, Quebec) reflect stability of the Lower Devonian sedimentary environment both north and south of the Lower Devonian volcanic activity.

The first pulse of the Acadian orogeny, dated as pre-Early Givetian, is bracketed locally in Maine between the Chapman and Mapleton sandstones. Continued Lower and Middle Devonian subsidence and sedimentation allowed flysch (e.g. Fortin Formation) and molasse (e.g. Gaspe Sandstone Group) to accumulate in successor basins along the northern flank of the central granitic terrain.

Post-orogenic Upper Devonian clastic sediments blanketed the Acadian folded zone. Remnants of these clastics are now preserved in several widely separated regions, e.g., south-central Newfoundland (Terrenceville Formation), southern New Brunswick (Perry Formation), and southern Gaspe Peninsula (Escuminac and Fleurant Formations).

Palaeontologically the region is classic ground for study of Devonian plants (Psilophyton from Gaspe) and Devonian fish (Bothriolepis from Chaleur Bay). Brachiopod, ostracod and plant-microspore zones have recently been established in New Brunswick and Gaspe. Fossiliferous limestone blocks, remnants of platform carbonates deposited near the northwestward limit of the Lower Devonian marine transgression, occur as subsidence breccia-blocks in the Cretaceous intrusive at Ile Ste. Helene, in the St. Lawrence River at Montreal.