Geologic Studies in Alaska by the U.S. Geological Survey, 1998

Introduction

The 14 papers that follows continue the series of U.S. Geological Survey (USGS) reports on investigations in the geologic sciences in Alaska. The series presents new and sometimes preliminary findings that are of interest to earth scientists in academia, government, and industry; to land and resource managers; and to the general public. Reports presented in Geologic Studies in Alaska cover a broad spectrum of topics from all parts of the State (fig. 1), which serves to emphasize the diversity of USGS efforts to meet the Nation’s needs for earth-science information in Alaska.

The papers in this volume are organized under the topics Geologic Framework, Environment and Climate, and Resources. Such an organization is intended to reflect the scope and objectives of USGS programs currently active in Alaska. Geologic framework studies provide background information that is the scientific basis for present and future studies of the environment, mineral and energy resources, paleoclimate, and hazards in Alaska. One paper presents ages of numerous igneous intrusive rocks in southern Alaska (Bradley and others). The authors demonstrate a systematic, along-strike diachronous age trend of near-trench magmatism in this region during the early Tertiary. Hildreth and others provide a map, rock descriptions, and chemical compositions of a geologically young volcanic center in Katmai National Park. Another paper describes the results of sedimentologic and paleontologic comparisons of lower Paleozoic, deep-water-facies rock units in central Alaska (Dumoulin and others). The authors show which of these scattered occurrences are likely to correlate with one another, suggest likely source regions, and provide a structural restoration of occurrences that have been fragmented by large fault motions. The paper by Bradley and Wilson provides the reconnaissance geology of the southeast part of the Kenai quadrangle. Another paper discusses the character, origin, and deformation of the major bedrock units within the Fourmile River area in east-central Alaska (Day and others). Further understanding the bedrock geology of this area is important for the assessment of potential environmental effects of placer-gold mining operations. The last Geologic Framework paper presents stratigraphic variations in composition within sandstones of the Nanushuk Group in the North Slope (Johnsson and Sokol). Studying the variations in the compositions of fluvial/deltaic sandstones of the Nanushuk Group is important in assessing climate change in northern Alaska during the Cretaceous.

Environmental studies are the emphasis of five papers. One study focuses on the environmental effects of gold-placer mining operations on the Fortymile River area of eastern Alaska (Wanty and others). The authors conclude that there is relatively little effect on metal content or turbidity of waters downstream from suction-dredge operations in this region. A second study presents an interesting synthesis and discussion of onshore geology, regional gravity and magnetic data, information about mineral deposits, and natural processes that disperse metals in the Bering Strait region (Hudson and Saltus). The source and dispersion of metals in this region is significant because elevated metal concentrations have been noted in tissues of some Pacific walrus in the Bering Sea. Two papers deal with climatic conditions during the Quaternary. Muhs and others present morphological and chemical criteria for identifying buried soils that developed in loess under a variety of different vegetation types. Loess sequences contain detailed records of Quaternary environmental conditions. The second paper presents the results of pollen analyses of two late Quaternary deposits in the southern Cook Inlet region (Ager). These results present the first dated postglacial vegetation histories for southern Cook Inlet in south-central Alaska and present useful examples of postglacial vegetation development under the partial influence of maritime climates. The fifth environmental paper presents major- and trace-element abundances, and Sr and Pb isotopic compositions of ground waters near Fairbanks (Farmer and others). The geochemical and isotopic compositions can be used to characterize these waters and assess the factors controlling variations in dissolved arsenic concentrations.

Resource papers include one that presents recently obtained ages of gold-mineralized dikes from the Stuyahok area in the Holy Cross quadrangle (Miller and others). The results suggest that the Stuyahok dikes are part of the Late Cretaceous and early Tertiay belt of gold-bearing deposits such as Donlin Creek and the Marshall-Kako Creek area in the Kuskokwim mineral belt. The last two resource papers focus on Ag-Pb-Zn (with or without Ba) occurrences and deposits in the Brooks Range of Alaska. Kelley and others present stable-isotope data that supports a genetic relationship between vein-breccia occurrences in clastic rocks and overlying shale-hosted massive sulfide deposits, such as the Red Dog deposit in northwestern Alaska. Soil and stream-sediment geochemical analyses and detailed descriptions of the vein-breccia occurrence at Whoopee Creek make up the subject matter of the last Resource paper (Schmidt).

Two bibliographies at the end of the volume present reports about Alaskan earth sciences in USGS publications during 1998, and reports about Alaska by USGS authors in non-USGS publications during the same period.

First posted August 26, 2000