196 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



The Carboniferous strata make up extremely thick sequences, are 

 dominantly conglomerates, sandstones, and shales; they contain several 

 angular unconformities, and are particularly instructive of crustal unrest 

 and of the tectonic history of the region. The correlation chart of Fig. 12.4 

 gives the important formations of the Carboniferous rocks in the Maritime 

 Provinces. From it some idea of the numerous units, large thicknesses, 

 and unconformities can be gained. The sedimentary and tectonic history 

 is even more detailed than the chart indicates. For instance, the Missis- 

 sippian strata of Nova Scotia belong to two groups, the Horton and the 

 Windsor; and along the lower part of the Avon River, the Horton group 



... is made up of two formations, a lower known as the Horton Bluff, con- 

 sisting of some 3,400 feet of dark shale, sandstone, and conglomerate, and an 

 upper, the Cheverie, made up of 600 feet of red and grey shales, sandstone and 

 arkose. The Horton Bluff formation rests unconformably on pre-Carboniferous 

 metamorphic and igneous rocks; it contains plant remains, buried forests, and 

 soils, and has a fauna of ostracod, crustaceans, and fish remains. The Cheverie 

 rests with an angular unconformity on the Horton Bluff and is succeeded, also 

 unconformably, by the Windsor group of marine sediments. The latter comprise 

 limestone, gypsum, shale, sandstone and limestone conglomerate, the whole 

 having a thickness of about 1,550 feet. The limestone members are rich in fos- 

 sils and have yielded one hundred and twenty-seven species, chiefly molluscs 

 and brachiopods. 



The Mississippian rocks extend eastward through the lowland belt to the 

 Strait of Canso, and also occupy much of the lowlands of the southwestern part 

 of Cape Breton Island. In the Lake Ainslie area, the Horton group includes 

 about 6,000 feet of conformable, dominantly clastic sediments containing a 

 meagre flora and fauna. They are intruded by diabase dykes and sills. The suc- 

 ceeding Windsor beds have here a thickness of about 2,000 feet. In the Arisaig 

 region, diabase and basalt dykes and stocks intrude red conglomerate, sand- 

 stone, and sandy shale of the Mississippian McAras Brook formation, but are 

 apparently older than the limestone of the Ardness formation of Mississippian 

 age (Alcock, 1947). 



The Pennsylvanian rocks of Nova Scotia are wholly nonmarine, as far 

 as known, are dominantly clastic and red, and contain locally beds of coal 

 and thin limestones. Pennsylvanian rocks also cover much of the plain of 

 eastern New Brunswick, being made up of red and gray shales, sand- 

 stones, grits, and conglomerates. 



The north shore of Chaleur Bay is bordered for considerable distances by red 

 clastic beds of the Bonaventure formation, which takes its name from the Bona- 



venture Island at Perce. The strata consists chiefly of coarse conglomerates and 

 sandstones, with associated red shales, shaly sandstones, and locally limestone. 

 The beds for the most part lie horizontally, but are locally tilted and in places 

 faulted. 



For relations along the north shore of Chaleur Bay see Fig. 12.5. 



Magdalen Islands are composed of folded sedimentary and volcanic 

 rocks of Mississippian age, surrounded by flat-lying beds of red sandstone 

 of probable Pennsylvanian age. 



Triassic System 



Red sandstones, shales, and conglomerates of Triassic age occur in the 

 Bay of Fundy region. They are most extensive on the southeast side of 

 the bay, where a belt stretches along the entire length of the bay and 

 borders both sides of Minas Basin. See Fig. 11.31. They rest uncon- 

 formably on various Paleozoic and Precambrian formations and are 

 capped by about 1000 feet of basaltic lavas that form the North Moun- 

 tain upland. On the northwest side of the Bay of Fundy, patches of 

 similar red conglomerate and sandstone occur. The beds of all these 

 patches dip to the northwest and are in fault contact with the older 

 formations. It has been concluded that they are deposits in a down- 

 faulted basin similar to those of the Triassic red beds in the Connecticut 

 and New Jersey basins. This is the northeasternmost of the known Trias- 

 sic fault basins in the Appalachian mountain systems. It is believed to 

 extend under the Gulf of Maine nearly to Boston. See Fig. 11.31. 



IGNEOUS ROCKS 



Extrusive Rocks 



Interbedded volcanic rocks of various kinds have already been men- 

 tioned in the account of the stratigraphy. They are known in the Cam- 

 brian and Lower Ordovician of the Thetford-Beauceville region of Que- 

 bec, in the Middle Ordovician in the central Shickshock Mountains, and 

 on the south side of Chaleur Bay. They are also known in the Middle 

 Silurian in various places on the Gaspe Peninsula on the north side of 

 Chaleur Bay, along the New Brunswick side of the Bay of Fundy, and 



