626 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



bedded cherts; Middle Devonian, carbonates and shales; Upper Devo- 

 nian, elastics; Mississippian, carbonates; Upper Pennsylvanian and Lower 

 Permian, elastics; Triassic, shales and limestones; Jurassic and Cretaceous, 

 elastics; and Tertiary, elastics (Hume, 1954; Martin, 1959). A cross sec- 

 tion restored to the time of pre-Laramide deformation from west of the 

 Barn Mountains to the Mackenzie delta is shown in Fig. 39.12. Several 

 unconformities attest several times of crustal unrest with the formation 

 of various basins and uplifts. 



The first conspicuous disturbance occurred in the British and Barn 

 Mountains area, probably during late Middle Devonian or Late Devonian 

 time. Upper Devonian sediments derived from the uplift form a deposi- 

 tional body much like the Catskill delta (Martin, 1959). The area of 

 uplift was probably mountainous for a while. Judging from the Upper 

 Devonian elastics in the Brooks Range and the Barron arch under the 

 Coastal Plain the uplift extended westward through northern Alaska 

 as shown on Fig. 39.13 (Dutro, 1960). 



The second conspicuous uplift occurred in Pennsylvanian time in 

 the Richardson Mountains area. It seems to have proceeded in two im- 

 pulses, one before Late Pennsylvanian time and one during the Late 

 Pennsylvanian. The uplift was flanked by a complementary basin on 

 the northwest. A Pennsylvanian basin exists also under the Arctic Foot- 

 hills and Coastal Plain, whereas the rest of Alaska was emergent at the 

 time, so that a partial and approximate view of Pennsylvanian condi- 

 tions is shown in Fig. 39.13. 



An Upper Triassic and Jurassic basin subsided in a general north- 

 south direction in the Richardson Mountains area. Cretaceous beds are 

 absent in northwestern Yukon toward the Brooks Range, but reach con- 

 siderable thickness in the Mackenzie Mountains near Norman Wells. 



Laramide Orogeny 



The present mountains, plateaus, and plains are the aftermath of 

 Laramide deformation and some Cenozoic faulting, but the exact time 

 of disturbance or the number of phases have not been well fixed. The 

 Mackenzie and Franklin Mountains are foreland type, with gentle folds 

 the dominant structure. High-angle faults are reported in places but no 

 thrusts of typical Rocky Mountain fashion are known. The Franklin 

 Mountains are reported as narrow, flat-topped anticlines, generally 

 faulted on one side or both. 



General structures of the Mackenzie Mountains and of other ranges 

 in the region are shown in Fig. 39.14. As may be seen, folds dominate 

 the structural types, but along the eastern side of the Richardson Moun- 

 tains Jeletzky ( 1961 ) has mapped a fault pattern which he describes as 

 follows (see Fig. 39.15): 



Major faults split the area into a number of irregularly shaped and structural]} 

 disconnected fault blocks, which differ strongly in the degree of structural 

 complexity and age of their rocks. 



The structure of the area contrasts strongly with that of the central parts 

 of Richardson Mountains, which is dominated by symmetrical, large, mostly 



BARN BLOW 



MOUNTAINS PASS 



RICHARDSON MOUNTAINS 



WEST ED6E 

 MACKENZIE DELTA 



FAULT BELT OF LATE DEVONIAN 



Fig. 39.12. Cross section from 

 British Mountains to Mackenzie 

 delta, restored to pre-Laramide time. 

 After Martin, 1959. 



