Studies by the U.S. Geological Survey in Alaska, 2004

Summary

Introduction

By John P. Galloway and Peter J. Haeussler

The six papers in this volume cover a broad spectrum of Earth-science topics from many parts of Alaska. Houseknecht and Schenk describe Upper Cretaceous strata of the upper part of the Nanushuk Formation, the Seabee Formation, and the lower part of the Tuluvak Formation that are exposed along the Colville River on the east flank of Umiat Mountain in north-central Alaska. In the western Brooks Range, paleosols have been recognized in outcrops of the Endicott Group and Kingak Shale. Dumoulin and White present the first microscopic analysis of paleosols from these units, which provide the first evidence of subaerial exposure of the Siksikpuk and Ipewik Formations. An aeromagnetic study of the Lake Clark Fault by Haeussler reveals a north-trending band of magnetic anomalies that are right-laterally offset about 26 km across the fault in south-central Alaska. The magnetic anomalies correlate spatially with a belt of dated 34–39-m.y.-old granitic plutons. Therefore, the Lake Clark Fault apparently has had ~26 km of right-lateral offset in the past 34–39 m.y. The aeromagnetic data also define another previously unknown fault, which appears to have about 11 km of right-lateral offset. Calvert and others present a series of high-precision ⁴⁰Ar/³⁹Ar ages of selected lavas from Westdahl Volcano, in southwestern Alaska. A dike cutting an old pyroclastic flow and associated lahar deposits from a precursor volcano yields an age of 1,654 k.y., indicating that this precursor volcano is older than early Pleistocene. A total of 11 geographically distributed lavas with ages ranging from 47 to 127 k.y. date construction of the Westdahl volcanic center. Lava flows cut by an apparent caldera-rim structure yielded ages of 81±5 and 121±8 k.y., placing a maximum date of 81 ka on caldera formation.

Two other papers discuss the framework geology of southeastern Alaska. Himmelberg and Brew discuss thermobarometric constraints on mid-Cretaceous to Late Cretaceous metamorphic events in the western metamorphic belt of the Coast Mountains near Petersburg. The western metamorphic belt, which ranges from a few kilometers to several tens of kilometers in width, records a complex sequence of contact-metamorphic and regional metamorphic events. A second paper by Haeussler and others presents a laser-ablation/ICPMS study of detrital zircons from turbidites in the Sitka Graywacke. This monotonous unit was previously believed to be of essentially one age and to correlate with the Valdez Group, but another part is 25 m.y. older and correlates with rocks of the McHugh Complex in south-central Alaska.