Tectonic Implications of Some New Mesozoic Stratigraphic Data on Alaska: ABSTRACT

Abstract

Increased geological exploration of Alaska during the past decade has produced a wealth of new stratigraphic data. Some of these data, particularly from Mesozoic rocks, are of fundamental importance in the interpretation of the tectonic history of Alaska. The U. S.:Geological Survey is compiling a comprehensive correlation chart of all known Mesozoic sedimentary, volcanic, and intrusive rocks. Preliminary work on the chart indicates the need for revising or refining present concepts of the stratigraphy and tectonics in several areas. For example, recent studies in northwestern Alaska suggest that the mid-Cretaceous Koyukuk geosyncline End_Page 274------------------------------ was not, as previously supposed, a single depositional trough stretching from the Brooks Range to the Yukon delta but consisted of a narrow east-trending trough along the Kobuk and upper Koyukuk Rivers and a larger southwest-trending trough that extended from the lower Koyukuk River to the Yukon delta. The two troughs were separated by an east-trending geanticline along Lat 66° N. The volcanics and granitic intrusives of Late Jurassic and Early Cretaceous age that are now exposed along this geanticlinal trend were an important source of the sediments in both troughs. Important data are also being developed by restudy of long-known Mesozoic terranes in southern Alaska. For example, the Talkeetna Formation of southeastern Talkeetna Mountains was thought to consist mainly of marine volcanics of Pliensbachian (Early Jurassic) age. However, it is now known to range in age at least from early Sinemurian to late Toarcian. Radioactivity dates indicate that, in the Talkeetna Mountains, plutons began to be intruded into the Talkeetna Formation shortly after it was deposited. Intrusion began at about the time the Matanuska epi-eugeosyncline was established in earliest Middle Jurassic time within the area of the more extensive eugeosyncline of the Talkeetna Formation. Intrusion probably continued into Late Jurassic and possibly Early Cretaceous time, although the adjacent Matanuska geosyncline was concurrently receiving a relatively complete sedimentary section. End_of_Article - Last_Page 275------------