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Devonian of Alaska
Devonian rocks are widespread in Alaska but are incompletely known. Major structural features seem to be a eugeosynclinal province in southeastern Alaska, a miogeosynclinal province in Arctic Alaska, and a possible intervening shelf-like environment in the vicinity of the Porcupine and Kuskokwim Rivers.
In southeastern Alaska, clastic and volcanic rocks of Middle(?) through Late Devonian age, including the Gambier Bay and Freshwater Bay Formations, are at least 5,000 feet thick. Some limestones containing Pycnodesma, such as the Willoughby Limestone, heretofore assigned to the Late Silurian, are considered Middle Devonian in age.
In east-central Alaska, the Devonian System is thinner. Along the Yukon River a sequence of Lower through Upper Devonian graptolitic shale, minor limestone, shale, and chert, about 1,200 feet thick (Road River Formation and McCann Hill Chert), is overlain conformably by about 3,000 feet of Upper Devonian nonmarine clastic rocks (Nation River Formation). Along the Porcupine River to the north, Lower, Middle, and possibly Upper Devonian carbonate rocks and shale (Salmontrout Limestone and overlying unnamed units) are less than 1,500 feet thick. Middle Devonian limestone (parts of Holitna Group and Takotna Formation) and Middle to Upper Devonian unnamed clastic rocks have been identified locally in central and western Alaska.
No Lower Devonian rocks have been recognized in northern Alaska. In the Brooks Range, the Middle Devonian is represented by about 2,000 feet of limestone and shale (probably including most of the Skajit Limestone); locally it may include some mafic dykes and volcaniclastic rocks. Plant-bearing rocks of probable Middle Devonian age were found in Topagoruk Test Well No. 1 near Barrow. The Upper Devonian consists of a few thousand feet of unnamed Frasnian limestone and clastic rocks overlain by a Frasnian through Famennian sequence of shale, sandstone, and conglomerate (Hunt Fork Shale, Kanayut and Kekiktuk(?) Conglomerates) as much as 10,000 feet thick. In the western Brooks Range a predominantly carbonate sequence of Middle(?) and Upper Devonian rocks (Skajit and Kugururok Formations and unnamed limestones) has been thrust northward over the clastic sequence.
Palaeogeographic reconstructions suggest that in southeastern Alaska clastic and possibly some volcanic rocks were derived from emergent sources within the geosyncline and that carbonate banks were developed locally. Submarine volcanism took place in northwest-trending troughs. In Arctic Alaska a sequence of coarse clastic rocks was deposited from an emergent region near the Arctic Ocean; carbonate rocks accumulated on a probable shelf along the Porcupine and Kuskokwim Rivers. In the northeastern Brooks Range, evidence of pre-Mississippian folding and a granite at Mount Michelson with a Devonian lead-alpha age indicate middle Palaeozoic orogeny in the northern emergent area.
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