Some rocks of the central Saint Elias Mountains, Yukon Territory, Canada

Abstract

Brief observations were made on scattered bedrock outcrops in the heart of the St. Elias Mountains incidental to glaciological work on upper Seward Glacier, The principal rocks in this geologically unexplored area are metamorphic and representative of the amphibolite facies of medium-grade regional metamorphism. Four units are distinguished as follows: Unit I is chiefly a coarse biotite or hornblende gneiss with interlayers of micaceous quartzite and coarse mica schist. Unit II comprises a variety of schists, principally quartz-biotite schist, hornblende-biotite schist, and hornblende schist with some micaceous quartzite, amphibolite, lime-silicate rock, and at least one bed of metaconglomerate. Unit III is predominantly dark amphibolite and light foliated marble with minor amounts of chloritic hornblende schist, hornblende-biotite schist, quartz-biotite schist, and lime-silicate rock. Unit IV is a uniform medium- to coarse-grained hornblende gneiss. Units I, II and III are predominantly, if not wholly, of metasedimentary origin and comprise in each instance beds totaling at least 1000 to 2000 feet in thickness and possibly much more. Unit IV is possibly a pre-metamorphic diorite and quartz-diorite intrusive. These rocks are all definitely older than Early Cretaceous, with a Paleozoic age slightly favored by indirect evidence, although either an early Mesozoic or Precambrian age is almost as likely. At the east end of the Mt. Logan massif are some dark calcareous graywacke, sandy limestone, calcareous quartzite, and conglomerate beds which contain earliest Lower Cretaceous fossils. These rocks are younger than the metamorphics. The above rocks, especially the metamorphics, are intruded by small bodies of quartz diorite and by still younger lamprophyre dikes. These intrusives are younger than Early Cretaceous and by inference older than Paleocene. Northwesterly structural trends in the metamorphics are largely overshadowed in their influence on the present topography by larger and younger westward-trending structures which by analogy with better-known relations on the south face of the St. Elias Mountains probably involve Laramide (?) folding and later movements along steep thrust faults from late Pliocene to the present.