216 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



Early Orodovician Phase 



With the beginning of Ordovician time, western Newfoundland started 

 to sink more rapidly and became the site of deposition of a thick clastic 

 series, and later of considerable limestone and dolomite. The central 

 area around Fortune Ray received much limestone, at least in places. East 

 Newfoundland also sank considerably and received over 6000 feet of fine 

 elastics and carbonates. It seems necessary to picture the western New- 

 foundland Lower Ordovician elastics coming from the Canadian Shield 

 where a rather sharp uplift set in (see Plates 2 and 3), but the source of 

 the shales in eastern Newfoundland is not clear. 



After early Ordovician time, the whole central part of Newfoundland 

 became a site of profound volcanic activity, much of it submarine, with 

 the passive emission of flows; but there was also abundant pyroclastic 

 activity, probably both submarine and subaerial. The Ordovician must 

 also have been a time of tumultuous crustal activity in the volcanic zone 

 because various elastics, such as graywacke, conglomerate, sandstone, 

 and shale, are commonly interbedded in the volcanics, or mixed with tuf- 

 faceous material, and they necessarily must have come from nearby up- 

 lifts. Chert and carbonate were also deposited, which with the above 

 lithologies are the common associates of volcanic orogenic belts. In places 

 upward of 20,000 feet of volcanics and sediments accumulated. 



Andesites are the most common of eruptive rocks in the orogenic belts, 

 but in the Relle Ray volcanic series of Fortune Ray, about 13,000 feet 

 thick, most of the volcanics are rhyolite (D. A. Bradley, personal com- 

 munication). This is indeed a great outpouring of rhyolite in an orogenic 

 belt. Hobbs ( 1944) has found that andesites are the first eruptives in new 

 orogenic belts in the southwest Pacific, but after a period of growth, other 

 less basic forms appear, with rhyolite one of the late entrants. Since vol- 

 canic activity continued long after the Belle Bay rhyolites in central New- 

 foundland, it appears that new volcanic cycles followed the early Ordo- 



vician one. 



Late Ordovician Phase (Taconic Orogeny) 



The Taconic orogeny is generally held to have been pronounced in 

 Newfoundland, not because of a great angular unconformity between 



Ordovician and Silurian rocks, but first, because the Ordovician sequences 

 are more metamorphosed than the younger ones ( Schuchert and Dunbar, 

 1934); second, because the Silurian has much conglomerate in it; and 

 third, because the Taconic orogeny of the Gaspe and Maritime Provinces 

 could not very well end abruptly without extension into Newfoundland. 

 Silurian beds are relatively not very abundant in Newfoundland, and 

 good exposures of their contact with the Ordovician sequences have so 

 far escaped detection. Twenhofel and Shrock wrote in 1937 that so far as 

 known there is no angular unconformity between the Ordovician and 

 Silurian systems. However, White ( 1940 and Ph.D. thesis, Princeton, 

 1939) recognized evidence of the Taconic orogeny in the Rencontre East 

 area of Fortune Ray, where the Long Harbour volcanics of Ordovician 

 age were folded and extensively eroded, he believes, before the Silurian 

 Rencontre series was deposited. 



The contention that the Ordovician sequences are more metamorphosed 

 than younger ones is correct only in so far as the "younger ones" are 

 Carboniferous sequences or, perhaps in a few places, Devonian. Some of 

 the granitic batholiths are now known to be Acadian, and most of the 

 metamorphism may be incident to them, in which both Silurian and 

 certain Devonian strata are altered as much as the Ordovician. Aside from 

 the Rencontre East area, it is difficult to find tangible evidence of a sharp 

 orogeny in Newfoundland at the close of the Ordovician. The Silurian se- 

 ries, with its volcanics and elastics, resembles the Ordovician of central 

 Newfoundland, and it seems more logical to regard the central belt as one 

 of continuing, but intermittent, orogenic and volcanic activity into and 

 through the Silurian. 



The ultrabasic intrusions of western Newfoundland are regarded as Late 

 Ordovician mostly by relation to those of the Gaspe and Quebec Taconic 

 belt (Snelgrove, 1934). Some of the ultrabasic plutons are known to in- 

 trude the Ordovician volcanic series and are therefore not older than the 

 Taconic. 



Late Silurian Phase (Caledonian Orogeny) 



The Clam Bank conglomerate of western Newfoundland and the Great 

 Bay de l'Eau conglomerate of Fortune Bay, both of early Devonian age, 



