Structure and stratigraphy of lower Paleozoic rocks, Doonerak Window, central Brooks Range, Alaska
Julian, Frances Elizabeth
Oldow, John S.
Doctor of Philosophy thesis
Deep structural levels of the Brooks Range thrust belt, and a unique sequence of lower Paleozoic rocks (informally named the Apoon assemblage) are exposed in a structural high near Mount Doonerak. Previous workers interpreted these rocks as autochthonous basement to the Brooks Range thrust belt, and correlated them with rocks of similar age in the North Slope and northeastern Brooks Range. The contact between the Apoon assemblage and the overlying Carboniferous rocks has been called an angular unconformity, with deformation in the lower Paleozoic rocks attributed to Devonian orogenesis. In this study, the Apoon assemblage is subdivided into four fault-bound lithologic units: (1) a fine-grained clastic unit (Pzp); (2) a mixed volcanic and volcaniclastic unit (Pzv); (3) a coarse clastic unit (Pzc); and (4) a pyroclastic volcanic unit (Pza). The rocks in the Doonerak Window contain three phases of folds: an early isoclinal phase overprinted by two less intense folding events. The last two phases of folds are parallel in the Apoon assemblage and the overlying Carboniferous rocks. The first phase of folds, however, is usually discordant. This discordance, and other data, suggest that the contact between the two units is a fault, not an angular unconformity. All three folding phases are probably related to Brookian deformation. A duplex model is proposed to explain the geometry of the structural high. The lack of Devonian structures within the Apoon assemblage affects correlation with other lower Paleozoic rocks in northern Alaska. The North Slope and northeastern Brooks Range show evidence of Devonian deformation, and hence, have a different structural history than the Apoon assemblage. The lithologies (especially volcanics) are also different in these area. The Apoon assemblage seems more closely related to lower Paleozoic rocks of the Skajit and Rosie Creek allochthons which contain related lithologies and show no evidence of Devonian deformation.