NEWFOUNDLAND APPALACHIANS 



219 



Reference to the map of Fig. 13.12 will indicate the position of the pre- 

 sumed fault. The structural front passes between Anticosti Island and the 

 Gaspe Peninsula and between Labrador and the Northern Peninsula of 

 Newfoundland under the Straight of Belle Isle (Figs. 13.1 and 13.6). Since 

 the front is entirely submerged, its position as shown on Fig. 13.12 is only 

 a guess. Nevertheless, the conclusion must be drawn that a deep recess in 

 the structural front exists between the Champlain-Gaspe salient and the 

 Newfoundland salient. Perhaps this is the result of horizontal offset along 

 a transcurrent fault. 



The submarine trough of the Gulf of St. Lawrence extends out under 

 Cabot Straight to the edge of the continental shelf. See Fig. 13.12. It has 

 a depth of over 600 feet for a distance of 750 miles, and from a point 

 midway south of Anticosti Island to the shelf rim is over 1200 feet deep. 

 At two places it is 1800 feet deep, and has a closed basin in this area 

 about 150 miles long below the 1320-foot contour. One large tributary of 

 the trough extends up toward the Straight of Belle Isle, and another ex- 

 tends along the north side of Anticosti Island. 



Six seismic profiles were shot on the extensive banks off Nova Scotia 

 and Newfoundland by Press and Beckmann ( 1954), and a combination of 

 three of them across the outer end of the Cabot Straight trough is shown 

 in Fig. 13.13. The position of the section is indicated on Fig. 13.12. 



The seismic section indicates for one thing that the trough is erosional 

 into the unconsolidated sediment layer, and this is the conclusion that 

 Shepard (1930) reached. From a study of the shape of the submarine 

 valley he concluded that it was first a subaerial stream valley and then 

 was modified by glaciers flowing seaward along it. Glacial striations and 

 roches moutonees on the southern tip of Newfoundland and on St. Paul 

 Island off the north end of Nova Scotia demonstrate the past ice flow. 

 The present depth of the trough is no greater than fiords elsewhere. The 

 trough walls do not resemble fault scarps — they are straight segments 

 with hanging valleys. 



In interpreting the seismic section, Press and Beckmann say that it sup- 

 ports the thesis that the trough is of fault origin, yet at the same time say 

 that the faulting occurred during the deposition of the sediments of the 

 3.80-km/sec layer. They regard the 3.8-km/sec layer under the north side 



Banquereau 

 Bank 



Cabot Strait Trough 





St Pierre 



Bank 





1 70 







2 94 



r . ■.-,•. 



— 10 ooo- 



'•.'•"■ .'•"•*• 3.0 V-V 





v.: 



— "' ' " ""* TIT 



4.6 



3.80 



1 

 I 

 / 



3.30 





15,000- 





MB****"^ 



/ 

 / 







20.000' 



1 











— 25,000' 



— 30.OO0- 















e S3 





Fig. 13.13. Seismic profile across Cabot Strait, Nova Scotia, and Newfoundland. See section 

 line A-A', Fig. 13.12. Figures are velocities in km/sec. 



of the trough (Fig. 13.13) as demonstrating the faulting. It is possible 

 that the wedge shape of this layer does indicate faulting, but not in post- 

 unconsolidated sediment time. The 3.80-km/sec layer is logically inter- 

 preted as consolidated sediment. Consolidated sedimentary rocks would 

 be either Triassic red-beds or Carboniferous of the nature of the basin 

 sediments of southwestern Newfoundland, and faults of this age are long 

 since dead, according to the history of the Piedmont and Greater Acadia. 



Mild earthquake activity is cited as evidence for the fault origin of the 

 Cabot Strait trough. Two earthquakes whose epicenters were on the shelf 

 slope immediately off the trough mouth have caused submarine land- 

 slides and numerous Trans-Atlantic cable breaks. Shepard questions the 

 presumed connection between these earthquakes and continuing displace- 

 ment along faults causing the trough. 



Both sides of the modern trough are about the same, yet the seismic 

 profile indicates the possibility of a fault on one side only. The conclusion 

 is reached that in Carboniferous or Triassic time a trough formed, pos- 

 sibly bv downfaulting, but that since then no further movement has 

 occurred. 



Now, to the original question; could the structural elements of the 

 Maritime Provinces and Newfoundland be offset appreciably by horizontal 

 motion along a transcurrent fault? The seismic profiles have demonstrated 

 the possibility of a late Paleozoic or Triassic fault along the outer stretch 

 of the Cabot Strait trough. If this fault is part of the Triassic fault system, 

 it would probably be one of vertical displacement. If like the fault that 



