ALASKA AND THE YUKON 



619 



clastic materials were probably derived from an uplifted shield north 

 of the present land area, according to Miller. Permian rocks in the western 

 Romanzof Mountains area become coarser toward the north. 



The Brooks Range geanticline began to rise in Jurassic time ( Miller, 

 1959). In one place mafic and ultramafic intrusions were emplaced 

 in Late Jurassic time. The main phase of orogeny occurred in Aptian 

 time (late Early Cretaceous) when the metamorphism of the rocks was 

 accomplished under deep burial, and an east-west structural pattern 

 took form. Uplift occurred throughout Late Cretaceous time and much 

 debris was shed to the Colville basin. A late (?) Paleocene phase of 

 deformation possibly resulted in the thrust faults, but these may have 

 formed earlier, and the east-west structural grain was intensified. Pene- 

 planation, and Quaternary uplift followed. 



Romanzof Uplift. The Romanzof uplift appears as a northern bulge 

 of the Brooks Range. Fold axes plunge westward in the Canning River 

 area, and strata primarily of Carboniferous, Devonian, and possibly Pre- 

 cambrian ages are exposed. Mesozoic rocks are preserved in certain 

 structural depressions. The general uplift started in mid-Cretaceous, or 

 possibly earlier, and continued in uplift during the Tertiary. 



Tigara Uplift. A small area of complexly folded and faulted rocks 

 pf Devonian, Carboniferous, and early Mesozoic age is exposed along 

 the coast line between Cape Lisburne and Point Hope, north of the 

 .De Long Mountains. These older rocks rise from the Southern Foothills 

 •belt (Index map of Fig. 39.6) and are called the Tigara uplift. It must 

 be a more extensive feature under the shallow water to die west. 



Seward Uplift. The Seward peninsula is made up largely of deformed 

 Paleozoic rocks with Cretaceous intrusions and three large areas of 

 Tertiary volcanic rocks. The most extensive area of Ordovician rocks in 

 Alaska is in the western part of Seward peninsula. The rock is domi- 

 nantly limestone, and the beds have been cast into broad open folds 

 and show little effects of dynamic metamorphism. Their exact thickness 

 [is not known but at least 5000 exist (Smith, 1939). 



There are also large thicknesses of Silurian, Devonian, and Carboni- 

 ferous limestones on Seward peninsula, but identities, correlations, and 

 thicknesses are not yet well known. Although the Ordovician strata of 



the western part of the peninsula are only gently folded, tin- strata <>l 

 other areas are intensely deformed. 



According to Payne (1955 and 1959) the dominant structural grain is 

 east-west and represents Early Cretaceous and possibly I. ate (post-Port- 

 landian) Jurassic phases of orogeny. Basic intrusions came in first and then 

 a number of large stocks or small batholiths of more acidic rocks. The 

 granitic intrusions with accompanying local metamorphism and miner- 

 alization occurred probably in Aptian time. The peninsula thereafter 

 remained mostly emergent and furnished sediments to adjacent basins, 

 particularly the Yukon. In early Tertiary time a second episode of 

 deformation produced a north to northeast grain superimposed on the 

 older east-west grain. Faulting was prominent. 



During the Tertiary, erosion was extensive but the peninsula remained 

 broadly above sea level. Considerable volcanism occurred in late Cenozoic 

 time and resulted in blankets of extrusive rocks over die deformed Pa- 

 leozoic complex. 



As portrayed on the map of Fig. 39.2, the Seward uplift included not 

 only the Seward peninsula but an approximately circular region under 

 the shallow water of Norton Sound and the Bering Sea. Although a 

 positive area in Mesozoic and Cenozoic times it is considered part of 

 the general Laramide belt. The phase of major deformation and intru- 

 sions, here as in the Brooks Range, appears to have been late Early 

 Cretaceous, and orogeny of this age is generally considered to be pre- 

 Laramide in the Rocky Mountains of the western United States. How- 

 ever, as previously explained, the Laramide belt is defined by physical 

 characteristics as well as time of orogeny and a phase of deformation 

 earlier than Late Cretaceous is a normal attribute of the Laramide belt. 



Arctic Foothills Belt 



A belt of "plateaus standing at different elevations" ( Mertie. 1930 ) 

 lies north of the Brooks Range, and much work incident to the explora- 

 tion of Naval Petroleum Reserve No. 4 has established clearly that this 

 is a foothills belt, both topographically and structurally. It is subdivided 

 into two sections, the southern foothills and the northern foothills. See 

 Fig. 39.6. 



