Geology of the Imuruk Lake area, Seward Peninsula, Alaska

Abstract

The Imuruk Lake area is underlain by metamorphic rocks of Paleozoic age, granitic rocks of probable Late Jurassic or Early Cretaceous age, and sediments and lava flows of late Cenozoic age. The Paleozoic metamorphic rocks include schist of several types assignable to Eskola's green-schist facies, gneiss assignable to Eskola's amphibolite facies, and metalimestone. The schist unit includes rocks that are older and younger than the metalimestone unit. The rocks of the gneiss unit are believed to be stratigraphically equivalent to part of the rocks in the schist unit and to have acquired much of their present texture and mineralogy during the emplacement of the Kuzitrin Lake batholith. The granitic rocks of probable late Mesozoic age include the small Black Butte stock and the Kuzitrin Lake batholith, a large body of granitic rocks exposed in the northern Bendeleben Mountains south of Kuzitrin Lake that apparently extends northward beneath the Imuruk Lake lava plateau to the Asses Ears. The Kougarok gravel of late Tertiary and Pleistocene (?) age underlies the northwestern part of the Kuzitrin flats and consists of a basal gravel member locally more than 187 feet thick, a thin middle member composed of peaty lignite, and an upper gravel member 3 to 60 feet thick. The middle member contains a rich pollen and wood flora consisting of a mixture of coniferous and deciduous trees, including representatives of several genera that now reach.their northern limits in latitudes 5° to 15° south of Seward Peninsula. Similar pollen floras in the valley of the Aldan River, Yakutia, Siberia, and in the northern Alaska Range are of late Miocene or early Pliocene age. The upper member contains poplar and either spruce or larch wood and may be early Pleistocene in age. Pleistocene sediments include glacial deposits, windblown silt, stratified silt and peat, alluvial gravel, and lacustrine sediments. Till and outwash of the Nome River (Illinoian) and Salmon Lake (Wisconsin) glaciations are found in and along the north front of the Bendeleben Mountains. Windblown silt covers the pre-Cenozoic rocks, the Kougarok gravel, and the older lava flows throughout much of the Imuruk Lake area. The stratified silt and peat consists largely of windblown silt that has been washed downslope to form fans, aprons, and in places valley-fill deposits. The alluvial gravel underlies the.flood plains and low terraces of the larger streams. Cl C2 CONTRIBUTIONS TO GENERAL GEOLOGY Lake deposits are found in the many existing lakes and in the basins of several large filled or drained lakes. Imuruk Lake is adjoined by three systems of abandoned shoreline features consisting of wave-cut scarps, and of terraces composed of beach gravel, and lacustrine silt and peat. The two older systems, formed during the Nome River (Illinoian) and Salmon Lake (Wisconsin) glaciations, have been irregularly warped; the youngest system, formed in Recent time, is virtually horizontal. Imuruk Lake drained westward to the Noxapaga River when the older shorelines were occupied; its pr...